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SELECTIVE ABSORPTION OF HYDROGEN SULFIDE IN AQUEOUS SODIUM CARBONATE

SOLUTIONS

by

Mohammad Al-Wohoush

A Thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the

requirements for the Degree of Master of Engineering

McGILL

Department of Chemical Engineering

McGiIl University

Montréal. Canada

Copyright© 1994

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ABSTRACT

The absorption of hydrogcn sullide and carbon dlo,ide in aqul'ollS sodium

carbonate solution was studied on a pilot scale pad .. ed column opelall:d cOllnter-cllllently

under atmospheric pressure. J'he ahsOIptlon COIUIllIl \\as 7.5 ('fil ~n diamder and ISO ('/11

in height and packcd randomly wlth 6 mm Intalox Saddlcs Il \\'ll!-. deslgncd to lemo\ c

hydrogen sultide from a gus mixture of 1.5 % hydrogen slIltide and 15 (} (} cm hon dio~ilk

at an cfficiency of 95 %. Rcliahiltty of the cxpcrimental setllp ha<; heen pelll<;ed hy

investigating the rcsidence time distribution of th...: gas phase in the colllllln and hy

studying thc absorption or carbon dioxidc 111 \\all'r.

In the first pDrt of the cxpcrimental \York. the lepcatahlhty or e~penl1lCllts and

material balances \Vere studicd and round to be within the acceptahle range I\lso. thl'

interactions betwecn tempcrature, carbonate concentration. initIai concentration or

hydrogen sulfide and carbon dioxide. and the gas/liqUld ratio wele Itllll1d to he negllgiblc.

Furthcrmore, it has been investigated how ail these parameter:-. eontnhllte to hydJ(\gen

sulfide and carbon dioxide absorption. Il was 1{)Und that carbonate conccntmtlOll has a

major cffect on the absorption of hydrogen sullide, whilc the ah!-.(lI pt ion or carboll

dioxide is atTectcd signilicantly by temperatllle. 1 Il the second part or the expenlllclltal

work, the mflucncc of ail parameters on the ab~orptlOn or hydlOgen <;ullide and t:arboll

dioxide has been invcstigatcd. Results ""cre in good agreement with tho~c rcported hy

other researchers; thcse rcsulls were analyzcd in terms or rCl1loyal cf'liciclleies, the (lyerall

mass transfer coefficients and the sclcctivity of the proccs<.; l'or hydrogell ~lIlfide. Il was

observed that hydrogen sullide ean he absorhed sclec\lvcly ill the presence 01 carboll

dioxide at low operating tcmperaturcs. high carbonatc conccntratloll and high gas to

liquid ratios. A rcmoval cfficicncy or hydrogen su 1 fide of ahout 92'X, at:compallled with

about 17% of initial amount of carbon dioxlde has been achicyed .

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Résumé

L'ahsorption d\: sulfure d'hydrogène et de dioxyde de carbone dans une solution

aqu\:use de carhonate de sodium a été étudiée sur une colonne garnie, à échelle pilote,

utilisée dans le ~cns opposé au courant sous pression atmosphérique. La colonne

d'ahsorptioll me~urait 7.5 cm de diamètre et ISO cm de haut ct était garnie de façon

arbitrair\: ù l'aide de "lntalox Saddles" de 6 I1Im. Elle était conçue pour extraire le sulfure

d'hydrogi:nc d'un mélange gazeux de 1.5 % d.! sulfure d'hydrogène ct de 15 % de

dioxyde de carhone avec un rendement de 95 %. La fiabilité du montage expérimental a

été sOigneusement vérifiée cn recherchant la répartition du temps de résidence de la phase

gazcuse dans la colonne ct en étudiant l'ahsorption du dioxyde de carhone dans l'eau.

Dans la première partie du travail expérimentaL la répétition des expériences et la

balance des maténaux fur\:nt étudiécs \:t se placèrent dans un intervalle acceptable. De

même Ics interactions entre la température. la concentration dc carbonate, la

concentration mitialc de sulfure d'hydrogène et de dioxyde de carbone et le ratio

gas/liqUIde s'avérèrent négligeables. De plus. la manière dont ces paramètres

contribuèrent il l'absorption du sulfure d'hydrogène ct du dioxyde de carbone fut étudiée.

II en ressortit que la concentration de carbonate a le plus d'influence sur l'absorption du

sulfure d'hydrogène tandis que la tempérrtarc a un effet maJeur sur l'absorption du

dioxyde de Cal bone. Dans la Jeuxième partie du travail expérimental. l'influence de tous

ces paramètres sur l'ahsorption du sulfure d'hydrogène et du dioxyde de carbone fut

étudiée. Les résultats correspondaient d'une manière satisfaisante à ceux mentionnés par

d'autres chercheurs: ces résultats furent analysés en fonction de l'efficacité d'extraction,

des coeflicients de translCrt de la masse totale ct de la selectivité du processus pour je

sulfure d'hydrogène. lia été observé quc le sulfure d'hydrogènc peut être absorbé

sélectivement en présence de dio:-.ydc de carbone à de basses températures, à haute

concentratIon de em bonatc ct à ratios gas/liquide élevés. Un rendement d'(:xtraction de

sulfure d'hydlOgène d'environ <)2 '% accompagné d'environ 17 % de la quantité initiale de

dioxyde de carbone fut atteint.

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ACKNOWLEDGMENTS

1 would like to e:~press Illy gratitude 10 Dr. G. Kuhcs IlH' hls \'aluahk guidance.

encouragemcnt and tinancial support throughout the period it took 10 eompktc this tlll~sis.

Special thanks go as weil to my co-supcrvisor. Dr. Adlian vall 1 il.'inillgcll (lINB)

for his valuablc comments.

AIso, J would like to sinccrely th<lI1k the InternatlOllal Energy Agelle)' and its

supporting members for providing mos! of the funding l'or this research. 1 am also

grateful to the Pulp and Paper Research Institute of Canada for the scholarships i1wardcd

and for building the equipment.

Many thanks go to the Chemical Engineering Ikpartl11ent. especially J Dumont

for his very kind hclp in the selcction and orderillg of malerials and suppllcs.

1 wish to thank ail members of thc CRIS» group. particularly i\kilcndra, Sabina.

Yujing, Stuart, Aliyc, Biao and Amir for thcir fricndship. A spccial word or thanks 10

François for his help with the expclÏmcntal design and Emmanuelle lor translating the

abstract to French.

Last, but not least, many thanks go tn my loving wi rc, Mariam. I(lr her sacri Iiccs,

much encouragement and mor:11 ~upport; a very special thanks go to our seven wceks old

son Anass .

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TABLE OF CONTENTS

ABSTRACT

Résumé

ACKNOWIJmGMENTS

LIST OF FIGURES

LIST OF TABLES

CHAPTERONE INTRODUCTION

1.1 IMPORTANCE OF THIS RESEARCH

1.2 BASIC OF RESEARCH

1.3 OUTLINE OF TIIESIS

CHAPTERTWO

LITERATURE REVIEW

2.1 ABSORPTION IN ALKANOLAMINE SOLUTIONS

2.2 ABSORPTION IN AÇùEOUS ALKALI SOLUTIONS

2.2.1 SIMULTANEOUS ABSORPTION OF H2S AND CO2

2.2.2 SELECTIVE ABSORPTION OF ll2S

2.3 CIIEMISTRY OF TI lE PROCESS

2.4 EQlIILlBRIU1\I1 DATA

2.4.1 V APOR LlQUID EQUILII3RIA

2.4.2 CARBONATl:-l3ICARBONA TE EQUILII3RIUM

2.5 CONCU ISIONS

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CHAPTER THREE

EXPERIMENTAL SET-lJP

~.I DESCRIPTION OF EXPFRIMEN rAI SE r-llp

3.2 CHARACTERIZA l'ION OF EXPFRIl\tFN l'AI. SI-' r-llP

3.2.1 GAS RESIDENCE T1MF DIS 1 RlBllllON

3.2.2 CARBON DIOXIDE ABSORP lION IN WA rFR

3.3 OPERATING PARAME rERS

3.4 CHEMICAL ANAL YSIS OF SAMPLES

3.4.1 GAS SAMPLES ANAL YSES

3.4.2 LIQUID SAMPLES ANALYSES

CHAPTER FOlJR

PRELIMINARY EXPERIMENTAL WORK

4.1 PRELIMINARY EXPERIMENTAL DESICiN

4.2 PRELIMINARY EXPERIMENTAL RESl ILTS

4.2.1 REPEATABILITY OF EXPERIMI:N rs

.., , --' l' --' "l.7

JO .1 1 ,1

4.2.2 INTERACTIONS & EFFECT OF PARAMETI':RS :'7

4.3 MATERIAL BALANCE 45

4.4 EFFECT OF BED IIEICiHT ON 11 2S & CO2 ABSORPTION 47

CHAPTER FIVE

RESULTS AND DISCUSSION

5.1 EXPERIMENTAL DESIGN

5.2 DISCUSSION OF RESUL 1 S

5.2.1 EFFECT ON THE RbMOVAL EFFICIENC'Y

5.2.2 OVERALL MASS TRANSFER COI:FFICIEN 1 S

5.2.3 SELECTIVITY OF TI lE PROCESS

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CHAPTER SIX

CONCLUSIONS & RECOMMENDATIONS

() 1 ('ONCLlJSIONS 69

() 2 RECOMMEND;\TIONS FOR FtrJ URL WORK 70

IU;:FERENCES 71

APPENDIX A : DESIGN OF TI lE ABSORPTION COLUMN 73

APPENDIX B: AXIAL DISPERSION PLUG FLOW MODEL 79

APPENDIX C: DATA AND CALCULATIONS 82

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LIST OF FIGlIRES

Figure 1./ Altclnatlve Inw tempel atule J..r,lft Ililld-bed chenllc,1I1 eUl\ el\ PIlILL''''

Figure 2./ Concenlratloll profile of rem:teu ga.,c~ ,1I1d lonlL "penc,

Figure 3.1 Schematlc dhlgJ am of the e\peflmcnl.ll ... cl-llp

Figure 3.2 The 1l0n-d"nenslOnal COnCel1tl.1t Ion 01 Ile al Ihe C\ Il 01 Ihe COIUlIlll ,l, IUIlL 1 11111

or Ume (L 1/'11/111" '1' -2:i"(')

Figllre 3.3 Residence "1 II1H! [)I~tllbulion 111 the pacJ..cd lllllllllil (1. 1 1111/1/. 1 .2 ,,"( ')

Figllre 3.4 E\.pcml1ental and Theoretical Re.,idellLe Illne nl,lllblillOll 01 thc !!.l' ph,\.,c

Figllre 3.5 Ab~orptioll of carbon dlo\lde III \V.ltel a, a 11IIlLlI01l 01 g.t., ,lIld liqllld velocilie~ at 25 ne

Figllre 4.1 InteractIOn bctween temperalure and c .. " bondte LOncenllallllll ,!Ild Ih cl kL\ 01 the absorptIon 01' 1 Ils and COl

Figure 4.2 Interactioll betwcen temperature and hydlOgcn ~lIllide mlet LOIlLCIlII.lllon ,lIld Its erfect of the absorptIon of II~S and CO2

Figllre 4.3 Interaction between tempcrature ,lIld cmboll dlo\,lde II1lel LOnL.entl,\llIlIl .llllilh

effcct of the ab~orptlon of 11 2S and cO2

flgllre 4.4 Interaction between tempclature ilnd ga~/liqllld lalm and 11\ clkLI III Ihe

absorptIon of 1 Ils and co} Figllre 4.5 Interaction bet\Vecn cmbonate conCCllIl,\IIOn and hydlOgell ~lIllidc IIIleI

concentration und Ils cffect of the ab~orptHlll of Il,S and co,

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Figure 4.6 InteractIon bctween carbonate concclltl,ltlOIl and c,uholl dlo\llie 111 IcI ·11

concentration and It~ clTect 01 the ab~orrtlOn ofll,S and co,

Figure 4.7 1 nteractlon between carbonate COllœntratlon and ga.,/lIqllld 1 at III ,lIltl Ih ellecl ,12

of the ab~orptlon of II~S and co} Figure 4.8 InteractIOn between mlet conœntratlon 01 hydlOgen .,lIllide .lIld miel ,12

concentratIOn and Il<, clTeet of the ab~orptlon 01 Il)S and ('(),

Figure 4.9 Intel action between mlet conccntratlon or hydlOgen ~lIllidc ,lIId !!a~/llqulll ·Il

ratIo and It<, efrect of the ab~orptlon 01 II~S and co!

Figllre 4.1 () InteractIOn between mlet conccntratlon of Cdl hon d IO\( Ide and g;I~!llqllld 1 atlO 'Il

and It., elTect orthe ab,>orption 01 Il,'') and co,

Fig.lre 4.// Relative elrect of parametcr., on the ah,orptlOl1 01 11 2 ~ and ((), Itl

Figure 4./2 Effect bed helght on the relnoval cflillency 01 Il, ~ tl()

Figure 4. /3 CITect bed helght on the relnoval ellk lency 01 CO, tl')

Figure 5./ Elrect oftemperaturc on the removal ellillency olll,~ and ( 0, 'if)

FigllTe 5.2 Effect of carbonate concentration on the relllovai cl fillcncy of Il, ~ and ( (), 'il)

Figure 5.3 Effeet or hydrogcn ,>ullidc miel C\lnecntratloll on thl! rCl110val dlitll!IICy 01

H}S and co} ')7

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l';~,,re 5.4 Llleet of l.arhon dloxlde ln let concentration on the removal efliclency of H2S 57 and co)

N~lIre 5.5 U lect 01 ga,> ',uperficlal veloclty on tlle rCllloval efliciency of H2S and CO2 58

N~,,,e 5.6 1.1 fCl.t 01 hqu Id <,uperfic lai veloclty on the removal efficlcncy of 11 2 Sand CO2 58

N~"re .;.7 Lffect 01 temper,lture on the overall l1las~ tran'ilèr coefficients of H2S and 61 ('°2

N~"re 5.11 I:flel.l 01 carbonate concentration on the over-all ll1a,>~ transfer coefficients of 61

11 2S and CO2

1';~lIre 5.9 Fffect 01 hydrogen ,>ulfide mlet concentration on the over-all mass transfer 62

coefficient,> or /IlS and cO2

N~lIre 5.m Effect 01 carbon dloxlde mlet concentration on the over-all mass transfer 62

coefficient,> of /IlS alld cO2

l-ï~lIre 5./1 1:lIèct ga~ ~uperlicJaI veloelty on the ovel-all mass transfer coefficients of 63

11 2S and CO2

N1:lIre 5.12 I:ffeet liquid ~tlperlieJaI veloclty on the over-allmass tran~fer roefficlents of 63

11 2S and CO2

N1:lIre 5. /3 I:flèct of tcmperature on the <,electlvlty factor 66

N~lIre 5.14 Effect 01 carbonate concentl atlon on the seleetlvlty factor 66

l-ï1:lIre 5./5 LI l'cL! of hydrogen <,ulfidc mlct concentratIon on the 'iclcctivity tàctor 67

l-ï~lIre 5.16 Lllcet 01 carbon dlo7\lde in let concentration on the selectlvlty factor 67

l-ï1:lIre 5./7 Elfect gas '>tlrerticral veloclty on the selcctivlty làctor

I-ï~IIre 5.111 ,. flect hqUld superlicial velocity on the selectivlty factor

I-ï1:IIre ,.1.1 Floodmg and pressure drop III random packed columns[J 9]

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LIST OF TABLES

Table 2.1 Dissocmtlon constant of carbonate m water Iio.,t .

Table 3.1 Charactenstics of the packed bed ....... .

Table 3.2 Ranges of operating parameter~

Table 4.1 Prehminary Expenmental design

Table 4.2 Repeatability of expenments ..

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T(Jble 4.3 Effect parameters and mteractions on the removal cl1icicncic ... of Il,S and {'() l • lK

TClble 4.4 Material balance ....

Table 5.1 Final experimental de'ilgn

Table 5.2 Effect of gas flow rate. ........ ..

Table 5.3 Effect of hquid flow rate ..................... .

Table A.I Composition and properties of the gas ..,tream at the in lei and at the out let 01

the column.. ...... ........ ........ .... ........ .. ...... . ...

Table CI Operating condition and removal efficlencle.., of hydrogen !'luI tide and C,II bon

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dlOxide m the tinal expenmcntal work ....... . .. . K3

T(Ible C2 Concentration of 10nlC !'pecies m the eXit hqulù ... tream and the CqlllllhllUIlI

partial pressures of hydrogen sulfide and carbon dloxlde .. .

T(Ible C3 Absorption rates and the overall mas~ tram fer coefficlen .... and the ... clcctlvlty

factor...... .. .............. .. . .. XH

Table C4 Effeet of temperature on the rcmoval efficicncy and the ovcr allllla~ ... trano.,fer

coeffiCient of 112S and CO2 and on the <,electlvlty factor .. 91

Table C5 Effect of carbonate concentratIOn on the rell10val efficlenl.y and the over ail

mass transfer coefticlCnt of 11 2S and CO2 ,md on the o.,elcctivlty factOl l) 1

Table C6 Effect of 112S mlet concentration on th.: rell10val efficlenl.y and the ovcr .dl

mass tran!.fer coefticlCnt of 11 2S and CO 2 and on the ... clel\lvlty lal\or . 92

Table C7 Effect of CO2 inlet concentJatlOn on the rell10val cl1iclelKy and the ovcr ail

mass tran~fer coefficient of 11 2S and CO] and on the '>Clel\lvlty fm.\or . 92

Table ca Effect of the gas superficlal veloclty on the rell10val efficlcncy and the ovcr ail

mass transfer coefficient of H2S and CO2 and on the ,>clettlvlty factor (data

obtumed wllh the expermlel1tul de\l~f1). .. ....... 93

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1t,hle C9 !:Ilect 01 the ga., ,>uperficml vcloclty on thc rcmoval cfficlcncy and the over ail

ma.,., tran.,fcr coefficlcnt or "2S and CO2 and on thc selcctivity factor (duta

ohill/lled wllh Ihe conventlOnal expel/II/cntal wrJrk).. . ................................... 93

lt/hle CH Enect of the ga., ~lIpcrficJaI veloclty on the rcmoval efficlcncy and the over ail

IlHl.,., lran.,fcr coerticlCnt or I/2S and COl and on the sdectivity factor .................. 94

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CHAPTERONE

INTRODlJCTION

Hydrogen sulfide is one of the undesircd gascs which IS prodm:cd hy Ihe

combustion or gasification of fuels that contain ~lIlfur compollnds. Il i ... I<HlIld gas 'ilrcall1s

especially in petroleum rdincries and 111 Ihe klaft (1l1lping 'iceluI' III the pulp and papci

industry.

For vanous reasons, such as the ollcn'ilve odm 01 hydrogcn ,,\lllidc, It:-.

corrosiveness and its toxicity, the presence of hydrogen sullidc has heen consldell:d to be

environmentally objectionable l'rom practically the beginning 01 the coal indllstry

Hydrogen sulfide is cspccially CorroSI vc if the Il lie gases arc cuolcd 10 li tcmperature

below their dew pomt. Its toxlcity has been 'itlldled ... crioll~ly and Il \Va ... round that the

maximum permlssible concentratIOn fi)r exposure not cxceedmg elghl hOlll 'i a day i'i

20 ppm. according to The .lmel'1Clll1 ,\"lIIulard .1.\.\(JcuIlI0I1 1 he IIllplcaSllnl Odor 01

hydrogen sultide can be detected al concentration., a ... Iowa., live pal h pel hllllOIl

For those rcasons, there has hcen a great cOl1ccrn over thc CIllI., ... IOI1 01 hydrogen

sulfide. Very shortly after coul ga~ was rntroulIccd a'i il dome'itlc t'ueL Il hecame apparent

that its sulfur content was a declded dbadvalltage. In 1~l1glalld, III 1 X()(), mrlk Illlle wa ...

utilized for the removal ofslIlfur compounds lrom fuel ga'i Iwo year:-. alter the lir:-.t use 01

gas for street lighting, a tcw years Iater scrubbers u ... ing .,olld lime \Vere elllployed.

Nowadays, many mdustnal proce:-.se'i, IIldudmg pctrolcu/ll Ic/inerre:-.. coal ga:-.

manutàcture and kraft pulping. generatc hydrogen :-.ullidc III large q\lantitics III thcir

operations. The gas is usually unwantcd. J\ ... a component 01 prOl:e ...... ga~ ... trcam'i,

hydrogen sultidc intcrfcres with important chcmical rcaetionc.;, while a., a compo/lcnt 01

wastc gases. It usually imparts an ohnoxi{)u~ odor to the nearhy atmo.,phcre. Ilowcvcr,

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Iho,>c indu~lne~ genemte carhon dioxide togcthcr \Vith hydrogen sulfidc 111 much larger

quuntltlc .... lor Ihis rcu ... on. eXI~tll1g procc,>~es arc inadequatc for hydrogen sulfide

rcmoval. hecau ... c Ihe ,>elcct!vlty f()r hydrogcn ~ultide ovcr carbon dim.ide is not high

cllough 10 achlevc a ... ati~lilctory hydrogen sul/ide removal efficiency without the

cO/lsumptÎon of Ul1eCOIlOmlC quantllle,> of chemlcal through carbon dloxlde remm al.

1.1 IMPORTANCE OF THIS RESEARCH

In the pulp and parer II1dustry, \\ood libers arc scparated from cach other by

mcchanical or chcmical mean~ or a combinatlon of the two. In the conventional kraft

pulping. the maJor chel11lcal pulpll1g proccss. \"ond libers arc liberated hy cooking the

wood chlP'i III whlle Ilqllor. a !--l)llltiol1 of ~otiIum ~ullidc and ~odium hydroxide. at

devated temperatures and pressures As a rcsult. the so-called. \\"eak h/ack !tC/IlO,. is

produccd. Il contains about 50 % of the dissolvcd wood and mo~t of the pulping

chemicals. In thc collventIOllal k.ralt chemical recovery process. this liquor is evaporated

tn a sol id content of ahout 70 0/r) to form a combustible liquor. which is then is burned in

the recovery furnacc. wherc several "unit operations" are performed In a single

compartl11ent 1 n the recovery fumace. a rcducmg atmosphere is maintmned in the lower

/one \\lhcre carbol1 IS gasIficd and the inorganic chel111cals are convcrtcd to a liquid smelt

of pnll1anly "'OdllIll1 ~ultidc and sodium carbonate \\ hich is then rcmoved and dlssoived

111 water 10 prnducc green liquor. (irccn 1 iquor i" processcd in a caustlclzing stcp to

producc licsh white IIquor Furthermore. the substantIaI heat of combustion IS rccovered

III Ihe uppcr part 01 the l'limace and high pressure stream is gem:rated.

f)c~rlte the facl that the objectives of the present chemical recovery proeess are

surticlcntly achic\'ed. therc are several drawback.s associated with the conventional kraft

chcl11lcal reco\'cry proccss. such as its hlgh li:-..ed capital cos1. the hazard of smclt-water

CXplO~IOI1. wrrosi\ enes~ of Ihe ..,mclt and the encrgy consumption charaeter of the

caustici/ing ~tcp.

For these leaSOIl~. therc \\:1S a need for an alternatl\,c chemical recovery proeess.

An altcrnatiw process for black. IIquor reeo\'cry based on low temperature proccssing in

tluidi/cd bed'i \\as proposcd tn the InternatiOnal Encrgy i\gency (lEA) [II. The proposed

prm:css j" the synergl"tlc integration of se\'eral black. Iiquor trcatments: superheated

... tcam dr~ 1I1g. tluid hed pyrnl~ SIS. leduction. gasitic:.lllOn and combustion. and gas liquid

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absorption. A now diagrmn of this proccss is sho\\ n 111 Figllle 1.1 1 hl' i'tllCC~,S IS IlHlll'

energy efficient and has a rclativdy small eapllal el)st compared tn the l'om l'ntInllal

recovery proccss and Jt docs Ilot sulTer from the smdt-\\ater e'plo<;\()11 h,I/:1I Li \10\\ CH'I.

this process is expectcd to 1 clease hydrogCll sullide 111 lm ger quantlt ICS \\ hlch I~ tl) hl'

recovered in the form green/ù/zlOr by thc absorptlnn in cal bOllatc ~llllltlon".

1.2 BASIS FOR RESEARCH

There have bccn several stlldie~ on the absorptlOll of h) dw/:!,cn sullidc 111 the

presence of carbon dioxide I\1to sodium carbonate ~olutions ,lI1d -;lldllll11 cal honate­

bicarbonate bulTer solution reported in the literatureI5-141. Ilowcvcl. thcle ha~ heen no

systematic study to absorb hydrogen sullide selectively l'rom gas stre<lI1lS that contam

carbon dioxide and to. simuItaneously, recovcr hydrogcn sullide in the lorlll of green

liquor .

The presence or carbon dioxide in the gas -;tream kad~ ft) tlle 1IlH.ksircd

conversion of sodium carbonate mto "lldium hicarbonate. l'herc!'(lIc It was dc","ed to

minimize the absorption of carbon dioxidc which, III rcturn, rc ... ult ... in optlllllling the

selective absorption of hydrogen ~ullide. hlrthermorc, the Slll1ultancoll-; ploductlon 01

green Iiquor 15 an important operalion in the alternallve procc,>" I(lr the n:covcry 01

pulping liquor. Thus, the basls of tlm re~earch wa,> to IllvcstIgak tlte ahsorptlon or

hydrogen sulfide in carbonate solutions 111 a packed colllllln reactor a~ 1 1I1lction of the

operating parameters .

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Air

Stearn

CHAR Fluid Bed ....... --, Combustor 700-750°C

Fluid Bed Gasifier

650-700 Oc

Heat Exchanger

... QJ

.D ~

2 u en

Flue Gas

!-Ieat Exchanger

Green Liquor to causticizing

Black Liquor

Figllre /./ Alternative low tcmpcrature "'raft fluid-bed chemical recovery process

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1.3 OUTLINE OF THESIS

This thesis IS concerned with thc l'undamcntal 111\ cstigation nt the seb:tl\'l' and

simultaneous absorption of hydrogen sultide and carbon dio\.H.lc 111 aqll\:olls solutions ot

sodium carbonate.

In Chapter Two. the literaturc of hydrngcn sullidc and carhon dill\.it\e ahsOlptlOJ1

as weIl as the chemistry of absorptIOn arc rc\'ie\\'t'tl.

In Chapter Threc. the cxpcrimcntal ~et-lip t .. descnhed. and the absorption colulllt1

is charactenzed by investigatmg the resldcnce time distribution and the ab~OIptlon of

carbon dioxidc in water. The analytical procedures for the gas and liquld phases arc abo

described.

In Chapter Four. results of the prclimmary expertmcntal \\'01 ~ alc presentcd 111

terms of interactions between the paramcters and their relattve eltect on the absorption of

both gases. Aiso. material balances for the ~pccics IOvolved arc estahlt~hed ln addition.

the repeatability of experiments and the ctTccl of the hed helghl on the ah~orpllOlI of holh

gases are discussed.

ln Chapter Five. the final results of this \York are presented and dlsc.:lI~sed. '1 hls

includes the int1uence of cach paramcter on the removal erticiencie ... the overall mass

transfer coefficients of both gases. and the proccss sclcctlvity fiJr hydrogcn sul tide.

Chapter Six comprises the gencral conclUSIOns and rccommcndattons li)r future

work .

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CHAPTERTWO

LITERA TURE REVIEW

The prohlem of the ab3orption of hydrogen sulfide and carbon dioxide was often

encollntereu in natllral gas processing plants and in synthetic ammonia plants. In

synthetic ammoma industry, wh en the synthesls gas is made by the partial oxidation of

petrolcul11 Ihu.:tlOl1S contammg signiticant quantitles of sulfur, the gas stream may contain

hydlOgen sultidc, carhon)'1 sultide and carbon dioxide.

Several processes have hecn proposcd and stlldicd for the removal of hydrogen

sullide and carhon dlOxide l'rom gasification gases. One of the most important gas

purilication techmques, is absorptIon. It involves the transfer of hydrogen sultide from

the gaseous to the liqUlu phase through the phase boundary. Hydrogen sulfide absorption

may he physical \"hen mercI)' dissolvcd in the absorbing solvent such as water, or it may

be chcmical \\hen hydrogen sultide reacts with the absorbing solution sueh as

al"'anola11lll1es and al"'ali solutions.

l'he ahsorhent solutions generally cmployed for treating a gaseous containing

hydrogcn sultiuc and carhon dioxide arc:

1. Sodium hydroxide solutions.

2. Solutions of 1110no- or di-ethanolamines or solutions of di-isopropanolamine

3. Aqucous ammonia solutions.

4. Potassium carhonate or sodium carbonate solutions.

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5. Potassium phosphate solutions

6. Tri-ethanolamine solutions.

The last threc solutions have a relatÏ\ e1y high selectivity \\ IIh 1 e~peet III hydrogcn

sulfide.

2.1 ABSORPTION IN ALKANOLAMINE SOLlJTIONS

Aqueous alkanolamincs solutions havc hecn utili/cd III Ihe ll1dU~lIy to ab",ol h

hydrogen sulfide and carbon dioxide from gas ... treums elther ... illlllllaneously 01

selectively cspecially 1I1 petroleum proccssmg plants and 111 Ihe 'i) nthetlc :t1ll11l01lla

industry. The alkanolamines have at Icast one hydroxyl group that scrvcs to rcduce Ihclr

vapor pressure and increase their solubility in water. rhey also have one aml11e group

which provides the necessary alkalinity for the ahsorptlOn of aCldic ga~es ... lIeh as

hydrogen sulfide and carbon dioxldc PI.

Hydrogen sultide reacts with alkanolamincs instantancously in accordancc to the

following reaction: ----

(2.1 )

There are several types of alkanolamines. the most Important hcing

methyldiethanolamme (MDEA). which can absorb hydrogen :-.ullidc rea~onably

selectively under proper operating conditions involving short contact tlllle~.

A study by 01lwerkerk 13] in which 'ideetlve ab~or;1tJ()n wlth c.,cvcral

ethanolamines was investigated on a pilot ~cale and a commercial plant dClllol1stratcd lhal

purified gas containing as )ittle as 5 ppm of hydrogen ~ulfidc could he obtained at a

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rcmoval cfticicncy of 91 % while only about 30 % of carbon dioxide that existed in the

Iccd gas was removcd.

A study hy Verstee~ et al r -II was carried out ta investigate the selective

absorptIon of hydrogcn sulfidc in the presence of carbon dioxide in several aqueous

solutions of albnnlamines. 'r heir study was donc using a co-cmrent down flow fixed

bcd reactor operated in the pulse now rcgime. They lound that this type of reactor is

more suitable for the selective absorption of hydrogen sultide l'rom aCld gases.

Ilowevcr, thcsc proccsses are not suitable when hydrogen sulfide IS to be

rccovcrcd as sodium sul/ide.

2.2 ABSORPTION IN AQUEOUS ALKALI SOLUTIONS

Sodium and potassium carbonate solutions have been utilized to absorb hydrogen

sulfidc and carbon dioxide because of their low cost and their ready availability.

Most nI' the proccsses fOlmd in the literature describe the phenomena of the

simultancous rcmoval of hydrogen sulfidc and carbon dioxide from gasification gases.

Howcvcr, thcrc arc only a tew processes in which hydrogen sultide is selectively

absorbcd in the presence of carbon dioxide.

One of t'le 1110st important processes, that have bcen used widely in the industry

I()r the rcmoval of hydrogen sultide, is The Seaboard Process [2]. ft is based on the

Icactlon hctween hydrogen sultidc and the carbonate ions. The etliciency of hydrogen

sulti~ic rcmoval in a s111glc stage packed column i'\ between 85 and 95 % for a solution

!low rate het\\ccn 60 to 150 gallIOOO L'li. /1 of gases depending on the concentration of

hydro~:cn sultidc and carhon dioxldc in the l'eed gas.

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~---- ------------------------.

2.2.1 SIMULTANEOUS ABSORPTION OF H2S AND ('°2

As/araa and GlOra [5.15]. studied the Chcllllstr) nI' thc Sll11ultallcnu" ahsOIptlllll nI'

hydrogen sul1ide in aqueous hydroxidc and C,lI honate-blcarblln·lh.' bul tl-r "nlut 1l1l1S ln thc

tirst paper, they derived an equatlOn for t:\C absorptIOn ratc ni h) d\(l!!.cn sullidc and

carbon dioxide. In addItion. they abo mcaslII'cd thc ahsorptlon 1 ate 01 h~ drogen ~lIllide 111

water and sodium hydroxide solutions \Vith a wctted-\\all collll11n ah~or her Illalk (lI' a

stainless steel tube which was I.u cm 111 diumctcr and ::!O ('11/ III helght at 1 X "(' 1 hcy

determined the rates of absorptIon \OlllIllctfll:all) hy Il1cans or" ,,('ap lillll tL'l'hlllqllc 1 hc

experimental results showed good agrccmcnt \\ 1 th th()~c ca\culatcd USIIll,!. t hL' pendl at Ion

theory. They also studied the absorptIon of hydrogcn ~ultidc and laI bon dioxlde III

potassium carbonatc-bicarbonate butTer solution 111 the saille ab~orhcl. 1 he)' also 100ll1d

that the absorption rate of carbon dloxide decreases !l' the concentratIOn ni the carbon.lte

bn is increased, while the absorptIOn rate of hydrogen sullide IIlcrca~c'i \VIth the

carbonate ion concentration.

In the second paper, they have glven a thcOlctical solution hascd on a lilm mode!

assuming that ail the ditfusivities of thc rcacting spccles arc l'quaI. and that two rl'actlon

planes exist in the liquid phase. the tirst one \Vilae hydrogen <,ulfidc l'i cOlllplctcly

consumed and the second plane whcre carbon dloxldc 1'1 partially IcacteJ. Abo they

studied the slmultaneous absorpt\()n of hydmgen ~lIl1ide and carbon dioxlde III aqucolls

hydroxide solutions experimentally using the ..,ume wetted-wall COlllll111. 1 he gas lIsed

was 50% H;!S and 50% CO;!. rhc Ilow rate of ga<; was hlgh 'iO thal the exit ga..,

composition was practically the same as the IIllet compo~ltion. 1 hey Illlllld that the rate

of absorption of hydrogen sulfide incrcases hy lI1crea~ing the concentratloll (lI hydroxlde

ions and that the ratio of the chemlcal ah<;orptlon ,.lI1d rhy~lcal ah..,orptlOll rate was

independent of the liquid tlow : .\tc. Ilowl'vcr. there wa~ no comparl'>ol) made 111 their

work, between thc experimcntal rcsults and their theoretical ..,olutlO!I'i l'urthermore, thelr

experimental date were not discusscd quantltativc\y

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A simdar study was pcrformcd by Onda el al. r 7]. They investlgated the kinetics

of hydrogcn 'iultidc ab~orption In a 'iodium carbonate-bIcarbonate butTer solutions. and

'ihowed that the rcactlOn hctwccn hydrogen ~ullide and carbonate Ion IS instantaneous.

Also, they studied the '>Imultancou~ absorptIon of h~ drogc'l sulfidc and carbon dloxlde at

25 0(. In an aqueou, ~od\Um hydroxlde ,>olutlon u~lI1g a \\ettcd-\\all column which agam

was 1 cm In dmmelcr and 20 cm In hcight. 1 hey round that the late of absorptIOn of

hydrogen ,>ulfidc 11lcrea,>c~ wlth increa~mg Ils partial pressure while it causes the rate of

ah~orptlon of carbon dloxlde to occrcasc. On the other hand. the y found that the rate of

ahsorptlon of carhon dloxloe IrIcreases slightly by II1crcasmg Its partIal pressure. In other

words. lhclr result'i 'iho\Vcd that ~olutions of sodium hydroxlde could be employed for

~c1ectlvc absorptIon of hydrogen ~ulfide m the presence of carbon dioxidc The reliability

of the expcnmcntal apparatu~ \Vas charactcriled hy ~tudymg the physical absorption of

carhon dlOXldc III water.

2.2.2 SELECTIVE ABSORPTION OF H2S

Oloman L'I al. [8]. studicd the ~cJecti\ c absorption of hydrogcn sulfidc in the

presence of carnon dIO:-"lde to reducc the odor a~socIated \Vith kraft pulp mill recovery

stnek gases. in \\'hich the concentratIon of hydrogen ~ul1idc is about 0.1 %. A buffer

'iolution of ~odlllln carbonate-hicarhonate havmg a pli or 9.5 and WlllCh is 2 molell in

sodIUm \Vas u~ed SIIlCC thls solution \Vas III cquilibnum \Vith the carbon dioxide III the

tced gas al an optImum temperaturc. the ahsorptlon or carbon dloxldc \\as minimized.

Ieadmg to sclectl\'c absorptIon of hydrogen ~ulfidc The study ",as carried out using a

laboratory :-.calc packcd column opcrated countcr-currently uSlIlg a gas mixture of 0 1 %

11 2S and 15% ('02 111 nitrogcn. 1 he ~lIltidc loadcd liquor leaving the absorption column

\Vas trcatcd 111 an o'\ldatlon lcactor in order 10 convcrt the absorbed hydrogcn sulfide to

lhiosulf;ltc (S20j ) '\0 that the butTer solutIon could be recycled. ft was found that an

dlicient absorptIOn :-.ystell1 for a rate of 500 IOl1s/day kraft pulp rnill \\,;ould reqUlre a

counler currcntl~ operatcd packed column rcactor of 20 Il diameter and 10ft height of

packing 10 .1cIllCVC a hydrogen sultide rcmo\'al cfticiency of 95 % with sodium carbonate-

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bicarbonate solution reclrculated at a r.lte nI' JO()l) gal da\' opel ated .It :1 templ.'latllll' nI' 160 oF.

(Jarner et al. [91. studied the ~eb:ti\'C absorptInn III h~ dmgl'n "li 1 li dl.' \\ IIh

carbonate solutions in the presence nf carbon din\.ide \\ Ith li \\ dll'lI diS,," and li \\ elled­

wall column. The feed gas stream conlained carbon dio:-"Ide and h~ dmgell "ultide ,It ,1

ratio of 30: 1. They II1vestigated the dTeet of temperatllle. concentlatHln or the ~nl\ltlnn.

gas and liquid tlow ratcs and hydrogen sullide IIlttlal eonceJ1t1 atlon 111 the g,as pha<;e lln

the rates of absorptIon of hydrogcn slIlfiùc and carbon dio'lde alld (ln tlll.' ')l.'b:tt\'ity

factor. They claimed that about 85°";) or 112S l11itlal .11110unt l'an be ahsorbed accol11pal\\I:d

\\ith about 10-15 % of thc IIlltial amount of CO,. Aiso. they 100llld that the 1 ate 01

hydrogen sulfide absorption can be increased hy II1creasing the "olutHln CIlIlCl.'ntmtloll

while thr rate of carbon dlOxide absorption \Va~ found to <.teneuse 1 he dkcl (lI' gas Ilow

rate was more signiticant on 112S absorption. because it is gas ~Ide èOnttolkd whllc Ihe

flow rate of liquid was !(mnd to IX' mOle sigmticanl in Ihe ca<;c of C()2 ahsorptlon.

because it is liqUld sldc controlled ln tcrms or sc1ectlvity. they lillllld thallhe "idCclivlly

for hydrogen sulfidc absorptIOn decreascs \\ Ith lIlcrca-ang thc IlqUlt! tlow 1 ate and

temperature. and it lI1creases \VIth 1I1creaslIlg the ga:-, Ilmv 1 ale and the coneentl alion 01

the carbonate solution. They also ~howcd that the <;elecllvity I~ II1depclldent 01 the

hydrogen suifide content in gas ahove 1 3 (%. hut It declca"e" whcn the concentration 01

hydrogen sulfidc is decreased below this value

Bent/al! el al [1 (JI studled the selcctlve absorption or hydlOgell ,>ullidc 1 rom lai ger

quantities of carbon dicxide by ahsorption and reaction in lin\.! spray'" U<;lI1g hullcred

solutions of carbonate-bicarbonate. rhey I()tllld Ihat the ahsorptron 01 hydrogcn sullidc 1 ...

virtually independent of the presence of l'arhlln dloxldc. On the other hand. they provcd

that hydrogen su\fidc reacts at the gas-liqllld Il1tcrluœ whilc carhon dloXldc 1 ... rcadcd in

the bulk of liquid. i\lso. thcy c1almed tliat thelr procc...... wa ... elliclcnt enough II)

selectively reduce the concentration 01 hydrogcn ... ullide in the nch gas 1 rom (J.!) 1 IYt, lo (1:-'

\ow as 0.0004 % (4 l'pm) l'ven If the concentration 01 carhon dloxlde III the ga~ mixture

is about 10 %. Their rcsults \Vere dlscll~sed in terms of the "'I/C 01 the dropleh and the

distance from the spray mg nozzle .

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ffohlfeld [11[. studicd thc selective absorption of hydrogen sulflde from gas

,>trcam~ contallling hydrogen sultide and carbon dioxide into a minimum amount of

,>odiulll hytlroxiuc <;olution. In ortler to reduce the total co st of the process they used a

hOrll'ontal ,>tatlc III 1 xer contactor operated co-currently. and It emptIed duectly into a

ga~/lIqUltl ~cparat()r. Part of the liquid was recycled to maintain an optimum 0a!1/Liquid

rallo \VIth thc LlIIll to utilllc. more completely. the unreacted sodium hydrmode. and to

gcnerate morc concentratetl eftluent streams. l'he expenmcnts \\ere carried out at room

temperatllre. Reslilts \\ere analY7ed t y means of the contact. detined as the ratio of void

volume III the contactor to the total gas llow rate. He found that if the contact lime is

decreaseu the consumption of sodium hydroxlde is deereased. Also the absorption of

carnon dioxluc uccreases ~lgJ1i ficantly \Vith decreasmg the contact time. beeause it reacts

\Vith hydroxldc Ions 111 a vcry ~Iow reactlon compared to the instantancous reaction of

hydrogen ~ullide wlth hydroxlde. rhis results in the preferential absorptIon of hydrogen

~ullide ovcr carnon dlOxlue absorption. The pressure drop in the contactor became severe

if the contact time \\as reuuced to 0.01 .,el'. He also tound that the ahsorption rate of

hydrogen sultiue lllcrcascs if its concentration at the inlet is increased. He showed that a

removal efliclelKy of hyurogen ~ultide of the order of 90 % was easy to be achieved by

l11eans of this t) pe of gas-liquid contactor.

NlIl1IlIclllll1drllll and .\ïlllrma [1':1 studied the absorption phenomena of hydrogen

... ullidc and cal hon dlo\lde in carbonate ~olutions. rhey employed a one-lOch diameter

and li"c I~cl long \\ctlcd \\all column. rhc mam lindings of thcir cxperiments was that

the rate of carhon dioxide absorption was not affected slgniticantly hy the gas tlow rate

",hile IIqllld Ihm rate had a larger erfeet. because the transtèr of carbon dioxide was

\:ontrolh:d hy the hqUld slde mass transter reslstance. On the other hand. they found that

the gas Ilo\\' rate had a marked effect on the rate of absorptton of hydrogen sulfide since it

\\a!'. largdy controlled hy the gas side mass transfer reslstance. They also found that

1lH:lea!'>ll1g the concentratIOn of carbonate n:sulted in the 1l1crcase in the ahsorption rate of

h~ dlOgen !'>llitide and it decreased absorption rate of carbon dioxide. The ciTeet of

IllcrcaslOg tempcrature \\as found to redllcc the absorption rate of hydrogen sulfide and to

incrcasc that of carhon dioxlde.

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A similar study \Vas performcd hy (]IOW 1/31 to imestigate th~ ~c\ectiH~

absorption of hydrogen sultide (from gas strcams that contalllcd h~ drngen sultide and

carbon dioxide at a ratio nI' 5: 1 0) in potassium carhonate-hicarbllnatc hulkr solutinns.

The solution \\-as fOlmd to have a Il1gh sclecti\'it~ \\ tth respect to h~ dlOgen ~llllïde. l'he

selectivity \Vas round to increase If either the concentratIon or carhonate or the partial

pressure ofhydrogen sultide \Vas increased.

Cir/io and Melchor r J-II investigated the selective ahsorption or hydrogen sultide

and carbon dioxide in potassium carbonate solutIOns in a wcttcd \\'all clllul11n at 70 "('

under atmospheric pressure. The found that hydrogcn ~llltidc coult! he ah-;orbcd

selectively at high gas t10w rates and ln\\' liquiJ no\\' lates. i\lsll. thcy IIm: ..... tlgaled the

effect of promoting the potassIUm solutions hy addlllg ammc. and Ihey Clllll'ludL'd Ihat Ihe

absorption of hydrogen suItidc \Vas Ilnproved slgl1lticantly \\hich rcsllited in 1I1lproVIIlg

the seleetivity of the proeess .

2.3 CHEMISTRY OF THE PROCESS

Hydrogen sulfidc and carbon dioxidc arc wcak aClds: llll:rc/ore. Ihey have Ihe

tendency to reaet \Vith basic solutions ~uch as aqucolls carhonate ,,()llIlJ()n~ whlch the

conjugate base of earbonie acid.

When hydrogen sulfidc and carbon dioxidc arc allowcd ln contact with aquC()ll~

carbonate solutions. the following cquilibrium reactlons take place:

co., + OH • .. He03 (2.2)

-H2S + OH • .. 112° + liS (2.3)

H.,S + C03 4 .. HC03 + liS (2.4 )

Il

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IICO; + 011 -

• ~ C03 + 1-1,0 (2.5)

liS + OH . ~ 112° + S (Z.6)

C03 j- 1 fS . ~ HeO; + S (2.7)

Experimental evidences lound in the literature [5.6.15]. suggests that reactions 2.6

and 2.7 can bc ncglectcd. bccausc at cquilibrium. those rcactions are shifted to the left. In

othcr words. It may hc assumcd Ihat no sulfide ions (S-2) are l'ormed.

/\Iso. as far as the kllletics IS concerned. reactions 2.3. 2.4 and 2.5 may be

considered as lI1~tantaneOllS rcactlons. regardless of the concentration of the reactant.

when they are compared to the rate of the diffusion process [6,15]. Thus. the reaction of

hydrogen slIlfide with carbonate ions and its reaction with the hydroxide ions take place

at or near the gas liqllid interface. This makes the concentration of hydrogen sulfide in

the liquid hulk l'cm. In other words. the absorption of hydrogen sulfide in the carbonate

solution may he said to be physically controlled. On the other hand. the reaction of

carbon dioxidc and hydroxide ions (reaction 2.2) IS fast and it might be considered

IIlstantaneous only If the concentratIOn of ol-f is substantial ( i.e. [OH-l > 1 0-2 mole//). At

slIch a Icvcl of 01 r concentratIOn. the equilibrium of reaction 2.5 is completely displaced

to the nght. sn that reaction 2.2 is followed by reaction 2.5. Th, refore. at low

concentrations of 01 r. that may be encountercd when CO; and HCOj ions coexist.

feaction 2.2 may be considered as a slow Iirst order reaction with respect to carbon

dioxide and Il has a ratc constant of 0.1 .\ec-Jat 25"C [I.J 1. Thereafter. the absorption of

carhon dioxide 111 carhonate solutions is said to be !iquid side controlled as it reaets in the

liquid huI\.... thus. the absorption of carbon dioxide is chemically controlled [6,12]. Figure

2.\ illustrates the concentration protiles of the gases being absorbed and the various ionie

specics i n\'ol\'ed in the reactions .

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•

-~ . ,

UAS FIL\(

, , , , , , ,

U(}I In "'1.11

() {l,

Concentration profile of reactcd ga,>c'i and IOnlC "pCCIC~

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2.4 EQUILIBRIUM DATA

2.4.1 VAPOR LIQUID EQUILIBRIA

As <.,œn. ,>everal proccs<; for the absorption of hydrogen sultide and carbon

dloxltle III alkall ,>olution have hcen stutlicd in the literaturc. However. 111 the literature.

very lew phy~lcal-chemical design data \Vere round concermng systems of H2S-C02-

NaIICOrNa2COl-NaIIS-Na:zS-II;!0.

An experirnental study \Vas pertormed by J'v/m et al. [/6] in order to develop a

mathematlcal expressIOn lor the cquilibrium constants which govern the above mentioned

system. I"hey dcvelopcd a method for the contmuos analysis of the vapor and liquid

phases. rhey il1vc~tlgated the clJuilibrium conditions at temperaturcs hetwecn 20 and

(l5 <1(., whlle the pressure \Vas betwecn 50 to 3500 mmHg. The ICSUItS of their

cxpenmental work were litted to a mathematical correlation for equilibrium partial

pressures of Il;!S and CO;! as a function of the concentration of the differcnt species in the

liquid phase and the opcrating temperature .

l'he se cOiTclations arc given in Equation 2.8 and Equation 2.9. and they were used

in this study to evaluatc the equilibrium partial pressure of H2S and CO2 above the absorhing solution.

whcn::

l'.: the equilihriul11 partial pressure abovc the solution, atm.

MN,,: the concentration of sodium ions in the solution, mo/ell.

(2.8)

(2.9)

t: the fraction of sodium ions as sodium bicarbonate in the sodium carbonate­

hicarhonatc solution.

r: the operating tcmperature, K .

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2.4.2 CARBONATE-BICARBONATE EQUILIBRIlJM

Data for the cquilibnum bct\\ccn carhonate and bicarbonalc ions arc IcqUll'cd in

ordcr to estimate the concentration ofthesc t\\O specics in the solution.

Carbonate is the conJugate base of cathOl1lc aCld Il dlssoclalcs ln \\,IICI ln

produce bicarbonate which is furthcr dissocwlcs \VIth \,\atcr ln plOduc': carbnnH: acid in

accordance to reaction 1.10 and Icaction 1.11 l'cspccti\'cly. l'hcoretlcal cvidcnee 1/-1. suggests that. rcaction 2.11 has a very small equilihrium constant. 1 e III Ihc ortler of 1 0 ~.

Therefore, it could be ncglectcd. llowcvcl. rcaction 2.10. \\ hil:h is the 1 C\ cr'\e of 1 cactu 1I1

2.5. is important determining the cquilihnum concentratIOn nI \1('()1 Valllcs of the

equilibrium constant of this rcaction at diflàcnt tempcratures aIl' Itstcd ln l'ahle 2.1 as

given by Lalifner el al. [f ï].

C03 + H20 Il • BCO; + 011 (2.10)

HCO; + H.,O· • 11 2C01 + OH (2 Il)

Table:2 1 DiSSOCiatIOn constant 01 carbonate III wa'cr . , -

Temperature, "e KI fI 'fcmpcnlturc, "( . K "1

5.0 4.463x Hl" 35.0 "\ 4))110 1

10.0 6.455x 10' 40 () 4721/10\

15.0 9 172-< 1 (j"' 450 () 4(1)". 1 0 '

20.0 1 2(2)( \(f' 50.0 X (,') 1,.. 10 '

15.0 1. 797x Hf' 55.0 1 14(,/ 10 \

30.0 1.506x 10-' 60 () l ')01/10 \

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2.5 CONCLUSIONS

As far as the sclectivity of the absorption process is concerned. most of the

previous studies a~rcc on the fàct that hydrogen sultidc could be absorbed selectively in

aqueous alkali ~olutlons under the followmg conditions:

1. lligh gas and liquid Ilow rates because the absorption of hydrogen sulfide is

considered to he gas side controlled and the absorption of carbon dioxidc is liquid

side controlled /5.9.//.12./-1).

2. Iligh concentratlon of carbonate in the liquid phase. because the absorption rate of

hydrogen sulfide is the highest in pure carbonate solutions [5.6.9.12.13].

3. Low opcratll1g temperatures. hecause the absorption rate of carbon dioxide

incrcases with temperature whereas the absorption rate of hydrogen sulfide is not

aflccted apprcciably 19.1.n

4. Ilighcr operating pressures are rccommended. because the absorption rate of

hydrogcn ~1I1fidc incrcascs significantly with ils increased partial pressure while

the absorptIon rate of carbon dioxide incrcases slightly if its partial pressure is

incrcascd 1 ~)

On the othcr hand. the effect of sodium sulfide (Na2S) is not wel1 documented.

For examplc. Olmall el al. 18] c1aimed that sodium sulfide formation hinders the

absorption of hydrogcn sulfide in carbonate-bicarbonate butTer solutions. This

contradicts the bdief that the reactions which produce sultide ions in the alkali solutions

arc ncgligible .

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CHAPTER THREE

EXPERIMENTAL SET-UP

A packed column has bcen selecteu as lhe equipment 10 absorb hydrogell slillide

in an aqueous sodium carbonate solution because of the fiJllowing reasons.

J. The specifie surface area of the packing material compared 10 \Vetted wall colllmns

is very high thcreby reducing the required Sl/.C of the absorptioll wlllllln sillCl! the

over ail mass transfer coefficient heeomes 11Igher

2. In terms of the complexily 01 cOl1slmclum paeked beu reactors ,Ile \'l:I)' '>lIl1ple since

there are no complicated internais IIke III tlay columns 1 his means Ihat

maintenance ofpacked columns IS easy.

3. In packed columns, the /tlfwd Iwld-up IS ~lIb~talltially ~l11allcr (han that ln other

types of contactors. This is an advantage sinec it reuuces the ah<;orptlol1 or ('C)2

which occurs in the bulk of the liquiu.

4. Packed columns can be operated in a "Indic hel/" \VIth ks'i huhhlrng (JI gm Ihrough

the liquid. thereby further rcdueing the Irc/uul lili/li-III' a", weil a", the chance 01

foaming.

5. Packed beds are casier to seule up than wettcd wall columns or lray colllmns.

6. In packed columns high valucs of liquuJ/gos IloH! raie.' l'olim are ca'iily achicvable.

1 C)

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3.1 DESCRIPTION OF EXPERIMENTAL SET-UP

A schemallc diagram of the expenmcntal set-up is shown in Figure 3.1. An

aqueous carhonate ~olution IS prepared hatch-wisc hy dissolving a suitable amount of

sodium carbonate mono-hydrate (Na2C01.lIi)) in 501 of de-ionized \Vater to obtain the

desired concentratIOn 111 the carhonate solution tced tank. An electric heater is installed

ln the tank ln pre-heat the solution to the desircd operating temperature. The solution is

pumped to the Ilquid thstributor at the top of the absorption column. The Iiquid

distrihuting system is deslgncd in such a way that the liquid is uniformly distributed over

the packing matenal and liqUid droplcts are prevented From being carried out hy the gas.

1 he ~()llItlOn kaves the column at the hottom through a siphon type tube. The tube

mamtams a certain hcad of liquid to prevent the gas l'rom leavmg at the bottom. A

'iample of this IIqUid is collccted for later analysis. while the rest of the solution is treated

with an ex cess 10% CuS0.t solutIOn to precipitate the absorbed H2S as CuS .

The ahsorption gas mixture is generated by mlxing two streams of pure hydrogen

sullide and pure carbon dioxide \VIth a nitrogen stream. Before mixing the nitrogen with

the two gascs. I1Itrogen IS bubbled through the saturator where it gets saturated with water

vapor. and al"o. prchcatcd to thc deSlrcd opcrating tcmperature. The Ilow rates of the

tlnce componcnts are ad,usted to obtain the rcquircd compOSition and total gas flow rate.

1 he gas mixturc cnters at the bottom of the absorption column where Il is uniformly

distriouted o\'cr the column cross section by passing through agas distributor. The gas

Icaving at the top of the column is further trcated ln a caustic scrubber where the

unreaetcd Il,,S is remm'ed completely l'rom the gas before heing released to the

atl1111sphere.

Th~~ absorption column \Vas designed to absorb hydrogen sulfide into carbonate

solutions at an dticiency of 95% l'rom a gas mixture of 1.5% H2S and 15% CO2 in

nitrogen (see appcndi~ :\ lor details). The three sections of the absorption column are

made of t\Hl concentric P/exlglll.\ tubes. The outer tube has an 0 cl. of 15 cm and a

thid.ncss nI' 6 mm "hile the inner tube is 7 5 ('/11 in 1 d. and has a thickness of 3 mm. The

clearance oet,,"een the tubes is uscd as a heatmg jacket in which hot water tlows upward

10

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•

to maintain an isothermal operation hy suhstitlltmg thc heat cnnsumcd by the

endothermic reactions. The column sections arc cqual III height (50 ('11/). rhc~ arc

separated from each other b~ a thick plate contammg a thcrmocoupk and sampling tube

as weil as a stainless steel scrccn ,\- hich supports the 6 mm llIfa/O\ ..... ·(/Cid/es pad,ing

material. and also it redistributes the liquid across the \m\'cr colul11l1 tll .I\'nid channding

in the liquid phase. This design \Vas adopted since it is rccommended to ledlstnhutc the

liquid across the height of packed co\umns evely 6-7 timcs Hs dWl11ctcl 1/8. jl)1.

For safety reasons, a hydrogcn sullide almm and control de\ l'>e \\as installed. The

device measures the concentration of 112S outslde the absorption clliumn hut IIlslde the

fume hood. The alarm gocs off when the 11 2S cllnœntration rcatls 1 () {1{1/I1 or 11Ighcr ami

automatically shuts off an electrical valve instalh.:d at the outlet ni the hydrogl'n "lIllide

cylinder. Since the expenmental set-up cou Id not lit \Il a stalldatd 11IIllC hom\. it was

surrounded by a heavy nylon plastic supported by a metallic structure. 1 he who le set-up

was connected to an induced draft fan placcd on the roof of the hllliding \'la li 110"e that IS

20 cm in diameter.

21

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Alorm

'" "'.Ier J)ryer

('au.hc ~crubh.r

lJquld DI'inbnlor

\\'ater Jacket C arbORai., ~o'ulion

~IS ,mil)!... ro nie AUlh/L.'f Intol", ~oddlc Feed Tank

.-------~==:::;----I~I. Pac~mg

~~~-,

WaterBath

11,<, co N l "IUld Qut ®

Va,lIIur !.aluralor .Jackel Feed Tank Ab\orphlln C. olumn

f'igllre 3./ Schematic diagram of the cxperimental set-up

22

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3.2 CHARACTERIZATION OF EXPERIMENTAL SET-tIP

The experimental set-up "as charactcrt/cd b~ mcasunng the Icsldencc time

distnbutlon of the gas phase in the packed column. and the ab~ol pt III Il phenomcnon nI'

carbon dioxide in \Vater \Vas im cstigated and comparL'd to that gl\'L'1l III the Ittcratule.

3.2.1 GAS RESIDENCE TIME DISTRIBLJTION

The residence time distnbutlon (RTD) or the gas 111 the racked wlumn \Vas

studied experimentally using stm1Ulus rcsponse e:-..periments 111 onk!' to ddel mine the

degree ofback mixing cffects m the column. l'he ga!:> phase R rD \\as Il1L'.I'atn:d SIllCC it is

reportcd f/81 that for H,S ahsorption the ga,> pha~e 1'> controlling thl.: ,Ihsorptlon

efficiency.

The RTD experiments wert' carried nut hy creatlllg a "tep up (,;hange in

concentration of helium at the inlet of the column. cu' (t'rom 0 10 50/'1'111 111 IlItrogcn )

The concentration ofhelium at the exit of the eolumn, \Vas mcasured as a fundlon 01' lime

(every 0.1 sec) using a Mass Spectrometcr The exit concentration. ('. which IS

nondimensionalized by the !ced concentration (' - LIe,,) I~ plotted agal\1"t tlllle, 1. III

Figure 3.1. The time has been corrcctcd by "uhtrm:ting the dcaJ timc III thc tubes. The

experiments were repeated at threc dIffèrent gas no\\' rates (20. j5 and 50//11/111 ) whde

water t10ws at a constant rate or II1111m under atl11o~plll:ric ple~"lIrC and loom

temperature.

From a mass balance, it is obvious that the area bctwecn the IlIle { . - 1 and Ihe

response curve in Figure 3.1 must be equal tn the me an residencc time as givcn hy

Equation 3.1

1=00

e = f (I - C)dl n.l ) 1=0

The mean residence time. 0, \Vas cvaluutcd for the thrcc gus Ilow rates. The time,

1. was nondimensionalized as

o.?)

21

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•

1 2

10

"" OX '" ".J

"';::: (,-"'O/mm è

~ OC!

:!! ~

Q 04 ~

02

(lO 00 180

t, SèC

Figllre 3.2 1 he l1on-dll11cn<;lonal concentration 01 Ile al thc exit of thc column as functlOn of III11e (1. -1 "'/lm. r -25"( Î

•

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The high frcquency noise \\as ICl11o\'l'd h;. liltl'ling thl' data uSlllg ~1.\ 1'1 :\H .

Subsequently the RTD l'un cs \\cre ohtaincd \1:- plottlllg the dll1lCnslOl1te"s hehlll1l

concentration \s. the dimenslOnlcss time as ~ho\\'n in Figure 3.3. rhese rL'slIlts ittustrates

that the degree of back 111lxing eftccts in the gas phase decrcasL's \\ tth IIH:reasmg Ihl\\'

rate.

The degree of bad.. mixing \\as charactcri/cd by litting thesc R rI) l'lII\ L'S to the

mathematical solution of thc axial dispcrsion modcl obtallled h;. /1,.('l/l/a 1': /1 dl'''Cllhcd

in Appendix (8). Figure 3 4. shows that thc c'Xperimcntal RTl) nhtalllcd at (i 20 1 1111/1 1:-'

weil represented by the RTO ohtall1cd \\Ith the malhcmulical model ,lIld a l'eckt (l't') or 32. The Pec1et numhers obtatned by liUing the I11casured R rD ClIl\l'-; 101 tlll' thll'C gas

tlow rates are tabulated 111 rable 3.1. l'he high \ ailles or the-;e Pœlct 1l1l1111K'i!-. IIldlcate

that RTO in thc cotumn is close tll plug Ihm 1 hl' colull1l1 \ old Il:\CIIOI1. 1 .. ha-; beell

cvaluated at each gas tlo\v rate us mg l:quattol1 -' 1 .• lI1d the le!-.lIlt-; aIl' al..,ll ...,howll 111

Table 3.1. The theoretical \Old rraction l'or the (l 111111 Int.llo'\ SaddIL-:-- (as l!-1\CIl hy Ihe

manufacturer. NORTON chemlcallJJ'oCI!.ls l'J'Oc/I/CII) 1..., () (l7. HuI. .. hout 1 11 (llll'a..,lIIcd

experimentally) of the total void volume II., equlpJ1ed by the IlqUIt! \\ hCIl \V.Ilcr Ilmv:-. al a

rate of 1.0 llmm. and at stcady state conditions. Iherdùre. the 1 rcc I.,pac~ 1 (lI l!-"" hCCllIlICI.,

about 0.48. Since this value is closc 10 lhat orthe three \'oid Iluctlonl., Il,,lcd III 1 ahlc 1 1.

and the se values are almost the salllc. 1 c. not u runetlon or the ga.., t1()\V ratc. It Gill he

concluded that the present expcrimcntal and analy-;i-; :--ystcl11 1.., ()rcrallll~ propclly

Ge f. =-

V

where:

G: gas flow rate.

e: rnean residcnce time.

V: total volume or packing

TlIhle J.I Charactemllc~ 01 the paLl ... cd heu

No. G.I/II1111 l'e :...Jo !:, \'()1l1 /1 {/CIlU/I

1 20 ~'1 \- () 42X

.., ~5 (lX () 414

" 50 X4 () 41() )

2'\

•

•

•

1 2 r---------------------------------------------------------~

" oK -..

~ , 06 -~ ~ O·, \"j (j=20 l'mm G=35/1mm (J=50 IImm

02

(1

0 0) 15 25 o ,D//1/t!Il.\·/(mlt!.\.\

NJ:llrf! 1.3 Residence Time Distribution III the packed column (L= II/mm. T=25°C)

1 2 ~--------------------------------------------------------~

.., os

~ .:: Oh

1 hcorctlcal RrD "~' PI! =32

-~ ::::

èS o , r ,pcnmcntal RTD ',ÎI 0=20 /, mm

~

02

Il 0 05 1 5 :2 :2 5

o . D/IIlenslOnle~s

1ïJ:llrl! 1.4 I:\penOlental and Theorellcal Residence Time Distribution of the gas phase

, .'

26

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3.2.2 CARBON DIOXIDE ABSORPTION IN WATER

Another \Vay to test the expenmental set-ur, \\.IS tn study the ahsllrptinn of carbon

dioxide in water bccause this problem has hœn !-ooh ed theoretically, and cxpcnmental

results are available in the literature.

The expcriments \Vere performed lIsing li gas stream of IOt!/" ('():! and 90°1c1 N:!

saturated \Vith \Vater. The gas stream tlows cOllntcr-currcntly to asti eam 01 distillcd watcr

running down l'rom the top of the column. l'he ~:xit concentratton 01 C~II hon dwxldc in the

gas phase was measured using a mass spcctrophotomctcr and a ('()2 IIlfra Icd analy:!cl

operated simultaneously, and an acid-base tltration procedure \\a'i Il'icd tll dC!':rmlllc Ihe

steady state concentratIOn of CO2 111 the liqllld phase at the hottom of thc colullln. 10

check the material balance for CO", the total amount or co, Ihat \Vas ahsol hcd \Vas - -detennined from the analysis of both phases, and it \Vas Itllllld that the dc\'wtion bclwcen

them was less than 5 % in most cases.

The absorption of carbon dioxide in \Vater has heen sludlcd a'! a lunclion of gas

and liquid flow rates at 25 "e. The rcsulls arc prescnted in the form of the (lverall mass

transfer coefficient. The overall mass transfcr cocrticlents have heen cvaluated hy llIeans

of the known carbon dioxide concentration in the inlel and outlet ga'- stream'! lI'!ing

Equation 3.4.

where:

9t K(j = ----=,--

J {l.A.h.IlP,.1I

KG: overall mass transfer coefficient, molelmm II11 210lm

a: specifie area of the packing material. 981 m'lm 1.

A: cross sectional arca of the bed, 4.56xH)"' m2•

Il: height of the packed bed, 1.50 m

(3.4 )

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The rate of absorption of carbon dioxide. ~H mole/min). IS calculated using

Equation 3.5

where:

G·!' ~n=--'x(y-y)

RT "'

G: gas flow rate. I/min.

l',: total operating pressure. 1 atm

R: gas constant. 0.082057 (! atm)j(mole.K).

T: operating temperature. 297 K

(3.5)

YII

and )', arc the measured inlet 111 outlet CO:! mole fraction in the gas

respectivcly.

The logarithmic mean pressure differencc. L\ P 'If • was calculated from Equation

3.6 assuming that the equilibrium partial pressure of carbon dioxide above water, P'. is

negligiblc comparcd to its partial pressure in the gas phase .

(3.6)

whcre:

!'" and P, are the inlet and out/et partial pressures respective/y.

It was found that the absorption rate of CO:! increases with increasing liquid or

gas /low rate. Morcover. the ctrect of the liquid now rate on the absorption is much

larger since the Iiquid film resistance is high. The results are summarized in Figure 3.5 as

a fllllctioll of the sllperticial liqUld veIocity. L' = LIA. which also. illustrates that the

present rcsults \Vere tound to be in a good agreement with those reported by Onda [7] .

28

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0010

- () 001! ~

~ -... -:: .:;-: 0006 ::::: ~

"-1 -0 0004 ~

,...:., " 0

U

~ 0002

o,ooe (/0

(/=201/1/11/1 - (;=35/1",1/1 (,~~() 111/1/11

Rc~ults Ilotamcd 0\ 0111'"

.. ' .•. j-'.

() 1

~.:. .. . -,' ',.' . . ", . --.-

,,, ------------

02 L·. /1/1111111

0.1 Il·'

Figure 3.5 Absorption of carbon dioxidc III waler a~ a tUl1ctlOI1 ot ga~ .lIld hqllld VciOCIIIC~ .11 25 Oc

29

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3.3 OPERATING PARAMETERS

r he paramctcr~ that have a malor effect on the absorption of hydrogen sulfide and

carhon Jioxidc arc tcmperature. concentration of carbonate ions in the solution. partial

pressures of l '2S and ("02' and gas and ItqUlJ flow rates.

The range of the parameters listed in Table 3.2 were determined based on the

prcliminary e'_penments. calculations and practlcal considerations. For example, the

maximum temperature was chosen to be 60 oC to avoid material problems since most

parts of the experimental apparatus are made of PleXiglas which does not stand

tempe ratures ahove 70 O( •

l'he operatmg lirlllts of the gas and liquid tlow rates were determined visually

during the study of the hydrodynamics of the packed column using water and air. The

minimum liquid flow rate. bclow which channcling of the liquid phase became apparent

was lound to he about 0.5 l/mm It was also observed that liquid started ta build up at the

hottol11 of each section of the column when the liqurd f10w rate was increased above 1.5

I/mm causing the gas to be hubbled through liquid The minimum gas tlow rate was

found to he 20 1/111/11 at the maximum liquid flow rate. rhe maximum gas flow rate was

50 1/111111 otherwise the pressure drop across the column became relatively high and

tlooding started to occur.

ln order 10 determine the working range of hydrogen sultide and carbon dioxide

wnccntrations. chemical equrlibrium calculations concerning black liquor gasification

\Vere made \VIth a module of the FACT computer pro gram by Thompson el a/.[22] which

is availahle on the McUlll mamframe. fhe results of this program show that about 50%

of the total sul fur 111 black liquor is converted to hydrogen sultide in the gaseous phase

\Vhile the lest remarns a~ Na,S 111 the smclt. It \Vas also found that the concentration of

II~S in the gas vunes approximate1y hetween 0.1 and 1.0 % depending on the conditions

nI' hlach. liqum gasllication. Similarly. the concentration of CO2 in the gas phase was

round to he hct\\'een 1.0 and 10.0 % .

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Since the concentration of carhonate in gn:en liqum 1" ahout 2 II/ole 1. Ihe

carbonate concentration in the solution \\as sekctcd 10 \ ar~ het\\ccn n.5 and 5.0 mole 1.

Ail the expcnments \\cre carru:d out \\ 1Ih thc cql11pmcnt c\haust opcncd to the

atmosphere so that the pressure inslde the co\umn \\as atmo~phcnc.

Table 3.2 Ranges of opcratmg paramctcr'i

Operating P~lrnmeter Minimum 1\ 1~"iltlu III

Temperature ("(1 25 () hO ()

Carbonate concentration (molell) ()j ')()

Hydrogen sultide concentration (mole 0'1l) n. \ JO

Carbon dioxide concentration (mol!! (~'o) \0 10 ()

Gas tlow rate (//min.) 20.0 50 ()

Liquid flow rate (//111111.) 0.5 1.5

3.4 CHEMICAL ANALYSIS OF SAMPLES

Chemlcal analysis was performed in order to cvaluatc thc ahsorptlon rate,; or hydrogen sulfidc and carbon dioxlde 111 the carhonate ~oll1liolls Il \Va" carrll:d out lin

samplcs from both the liquld and gas phases 111 ortlcr to check Ihe lotal and CO III pO Ill: n 1

material balances.

3.4.1 GAS SAMPLES ANALYSES

A Mass Spectrometer was employed fi.>r the analysis of the ga'i ~trcalll~ ft was

calibrated for H2S and CO2. rhe calihrallon fiH hydrogen ,>ulfidl! wa,> carned out by

measuring the response of the MS tn diffèrent '>t"lI1dard ga,> mixturl!'> of 11 2S 111 hehurn

(0.010.0.100 and 1.000 mole %). Slmtlarly. the calibration of the MS for carbon dloxldc

was done using threc diftercnt standard ga~ mixturcs of CO2 III nllrogcn (0500. 5.000

11

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and 10.000 mole !Yr,) simuitancously with an Infra Red (IR) (,02 analyzer to double check

the accuracy.

'1 he MS was used to dctermine the composition of the gaseous streams in terrns of

hydrogcn 'illitide and carbon dioxldc. A continuous gas strea,TI \Vas analyzed at a

rrequency or 10 /1=. The flow rate of this stream IS negligible comparcd to the total gas

Ilow rate. Watcr \Vas rcmovcd l'rom thls ~ample stream by hllbbling the gas through a

rclativcly large quantlty of conccntrated sulfuric acid and then passing it through a bed of

Si/ka (jel hcforc entering the MS.

At the hegmnll1g of cach cxpcnmcnl. the inlet composition of the gas stream

cntcring at the holtom or column was analyzed us mg the MS in order to adjust the tlow

rate of cach specics Lo obtain the dcsired gas composition and tlow late. Also. it \Vas used

to analyzc the cxhallst gas stream at the top of the colllmn after the steady state conditions

\Vere achicved. The amount ahsorbed of each gas \Vas calculated from the difference

hctwcen Ils concentration at thc inlet and thc outlet streams.

3.4.2 LIQUID SAMPLES ANALYSES

Alter steady state conditions v.ere reached. a sample of the liquid at the bottom of

thc ahsorption columl1 \Vas takcn to determine its composition in terms of HS". C01 and

. liCOl"' The procedurc or Uarnerl91 was rollowed and is described below.

1. Absorbe,/ "\',lrl1l:eII su/Ode: A sample or the liquld was neutralized by addir'g an

cxccss HI110unt of 1 () % cadmium chloride. Then a sufticicnt amount of

hydrochlonc acid \HlS addcd to hring the normality of the acid to 0.2 N. An

cxcess amount of standard iodine solution \Vas added. and the unconsumed iodine

\Vas detcrmined by titration \Vith a standard solution of sodium thiosulfate using

starch as an IIldicator. From thesc titration rcsults. the concentration of HS" in the

liquid phase \\as calculatcd.

•

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2. Absorbed carboll (lioxide: a) The total alkalil1lty of the solution \\as deternllned

by titration a sam pie of the liquid agamst a \ nlul11ctnc solution nI' sul furie ,Il'id

using methyl orange as an indicator. h) :\n c:-..ccss amount l)1 n.1 .\' ~tandard sodium hydroxidc solution \\ as added to anothcr sampk tn com ert hlcarhonate

into carbonate, which \Vas then prccipitated as barium carbonate hy adding a

sufficient amount of 10°;;) banul11 ehloride solution. l'he CllnœntlatlOll of

bicarbonate In the initial sample \Vas then determincd hy haek ttlwtlOIl of the

unreacted NaOH with a 0.1 N IICI standard solution Ih:n li "impie matenal

balance was applied to detcrminc the concentration or caeh ~pceic~ and the

amount of carbon dioxidc that was absorhcd.

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CHAPTER FOUR

PRELIMINARY EXPERIMENTAL WORK

The experimcntal work has bcen carried out in two main parts: The preliminary

cxpcrimcntal part, in which an cxpcrimental design proccdure was followed to investigate

the repeatability of the experiments, the interaction between parameters. the material

halancc of species bctween liqmd and gas phase and the effect of bed height: The final

cxperimental part l'rom which the tinal results were obtained and they are discussed in

Chapter Five.

4.1 11RELIMINARY EXPERIMENTAL DESIGN

ln order to minimiLe the total number of experiments. the interaction between

paramclcrs has hcen studicd by pcrforming an cxperimental design of 16 experiments at

lwn Icvcls as 11Illstrated hy Tagllclll el al. f23]. For each experiment the outlet

concentratIOn of hydrogen sultide and carbon dioxide at steady state conditions were

Illeasured. thcn the response of each cxpcrimcnt \Vas determined in terms of the removal

cfticiencles of Il:!S and CO:! . ll( Il;!S) and 1l( CO::!). as shown in Table 4.1.

ln Table 4.1. the values 1 and 2 correspond to the lowest and highest level of each

paral11ctcr .previollsly shown in Tahle 3.2. respectively. To simplify the expcrimental

design. gas and liquid !low ratcs \\cre combmed in a single parameter (GIL), where the

valuc 1 corresponds to the lo\Vest Icvel of G divided by the highest level of L while the

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value :2 is the highest lcvel of G dividcd by the hmest k\ d of L Also. \Il titis

experimental design. the follO\\ing t\\O important comhtions \\cre applted in 4.lnicr to

obtain a balanced expenmental design:

1. Any parameter appears eight tllnes at k"d and elght times at kwl 2. fllr the

whole set of experiments.

2. Any two parameters meet togctlter at the samc combination lIf le, ds (1-1. 1-2. 2-

1 or 2-2) only four times in the whole set or e:\periments.

The removal cfticiencies of hydrogcn sultidc and carbon dlOxide \Vere calculatcd

on the basis of the analyses of gas streams. The rel110val dliciency lIf the ahsOI'hed gas

(l')) is defined as the amount of gas being ahsorbed hy the carbonate solution dl"idcd hy

the total amount oflhat gas entcring thc cnlumn. I.C.

where:

Tl = Pli - PI X 100% PlI

(4.\ )

Po and PI are the inlct and thc oullet partial pressures or the ahsorhed gus and

the y are obtained by means of Equation 4.2

where:

y: is the mole fraction in the gas phase.

PI: is the total operating pressure. alm .

(4.2 )

1)

•

•

•

Tt,ble 4.1 Prchmmary L,,,pcrlmcntal dC~lgn

Run No. r ICO; 1 y (II,S) YjCO:!) G/L l1(H:!S).% 11 (CO:!),% " -

A-I 1 1 1 1 1 74.50 10.01

A-2 1 1 1 '1 ,., 78.90 9.50 -A-3 1 1 ,., 1 ,., 86.13 7.25

A-4 1 1 ,., ,., 1 80.23 11.13

A-5 1 2 1 1 2 86.80 5.10

A-6 1 ,., 1 ,., 1 81.09 9.30

A-7 1 ,., ,., 1 1 88.10 7.17 -A-X 1 ,., ,., , ,., 90.91 6.29 - -i\-9 ,., 1 1 1 '1 77.13 12.36

Â-IO ,., 1 1 '1 1 72.20 16.81

A-II ") 1 2 1 1 78.41 14.50

A-12 2 1 ") '1 ,., 81.90 13.34 -A-13 '1 ") 1 1 1 79.29 12.45

A-14 ") ,., 1 ") ") 82.80 11.39

A-15 , ") ,., 1 ") 89.88 9.39 -A-16 '1 '1 '1 '1 1 83.86 13.27

4.2 PRELIMINARY EXPERIMENTAL RESULTS

"'.2.1 REPEATABILITY OF EXPERIMENTS

'\'0 test the expenmcntal results some experiments listed in fable 4.1 were

repeated. rhe ùifferenccs in the r('moval efticiencies of hydrogen sultide and carbon

dioxiùe of the duplicate experiments. respectlvely Tl' and Tl". are very small as shown in

rahle 4.2. suggcsting that the rcpeatability of the experiments is very good.

36

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Table 4.2 Repeatabthty of e-..pertments

Run No. T] '( Il,S) 1l"(H2S) Dc,'. 11'(CO ~) '1"(('0,) DC\'. 0/

0 O{, III li II li II n II

n

A-5 86.80 86.21 o 6X 5.10 5 .\() 5 10

A-8 90.91 91.70 (U~6 () 29 (, 53 \ X2

A-15 89.88 89.10 I).X7 l) 39 1) 01 40)

4.2.2 INTERACTIONS & EFFECT OF PARAMETERS

The removal efticicncy of Il:!S and CO;! have also bcen ploltcd as functioll of the

operating parameters in Figure 4 1 through Figurc 4 10 l',ach pomt on tho<;e ligures is

calculated by taking the average valuc of thc four rcmoval cf liclcllcles where the two

parameters are set together at the samc comhlllation of Icvcb as glycn III 1 ahle ·1.1. I:adl

figure shows two straight lines onc IS hctwcen the avcraged n.:moval d liclcncles wherc

the two parameters are at the Icvel eOlllhinations ( 1-1 ) and (2-1 ). and the :-,ccolld IlIlc is

between (1-2) and (2-2),

The linding in Figures 4.8b and 4.11 a that the IlI1es arc alll1oo.;t hori/Olltal shows

that the efficiencies of H::!S and CO:! arc hardly intluenced hy the IIllet COI1(.:cntratloll 01

respectively CO:! and H::!S. Moreover, l'rom thosc li g ure 0.;. olle can "ce tllat tlle :-.traight

lines are almost parallel to caeh other, indieating that there i.., no IIltcracliOIl

The procedure of TlIglichl et al 1231 was followcd to calculate the IIlteraclions

and the eftèct of parameters on the absorptIOn of hydrogcn ... ullidc and carhon dloxJ<.lc

using the distances between the lines on those ligures and thcir ~Iopc<;.

The resuIts are listed in l'able 4.3 III terms of pcrcentagc valucs 01 how the

parameters and the interactions contributc in the removal eflicicm:y 01 1/2S and (,02 ft

shows that the ealculated IIlteractions are very ..,mall compared lo the cf leet or the

•

•

•

"

paramctcr~, thu,;, they mlght he ncglected. Figure 4 II also shows the percentage of the

an"olute relative cf tcct of each paramctcr on the remo\'al efficienc) of H:!S and CO:!. It

';lIggc~t'; that carhonate conccntration has the larger effcct on the of absorptIOn H2S. and

that tcmperature 1'; the mu,;t Important parameter ln the ease CO:! absorption

lilhle 431 ffcet p,lramctcr~ ,lIld II1tcr<lctlon~ on the rcmoval cfticlcnclcs of H2S and CO2

flanameterll and Interactions Effèet on T](I1 2S). % Erfeet on T](C02), %

'J emperature 6.50 57.61

IC'O~ 1 41.09 17.32

Y,,( 112~) 31.51 0.96

Y)C()~) 1.01 6.92

G/L 19.51 16.42

Temp * ICO, 1 001 0.01

Temp. * Y.,(11 2S) 0.03 0.06

Temp. * Y.,(C02) 0.00 0.05

Temp. * G/L 0.01 0.03

ICO~ -1 * J'J 112S) 0.02 0.02

ICO, 1 * Y.,(C02 ) 0.09 0.04

!COl 1 * G/L 001 007

Y.,(11 2S) * Y,,(<-'°2) 0.12 0.28

.v,,( 112S) * G/L 0.00 0.09

Y)('()2) * (l'lL 0.09 0.03

Tohll 100.00 100.00

38

•

•

•

lU) '(\

IU\I-I - 1\ Il,11 -!CO, I-~ 1" 1,1 è

X~ l'

;t:. 0 0' ,-:... ....:.

rJl III

" '"' :c ~

'-" ~ 1=" r=-

75

"1) ~ ____________ .-I

Temperature il) h)

Figllre 4. / Interaction bctween tcmpcraturc anù cMbonalc lOl1ccnll <111011

Il) Effect on H2S absorption. h) EtTcct on C()~ ab\OlpllOIl

'JO '0

\' III ~ 1 - 1 - 1 fil"" 1 ... \' III ~I - ~ 1 dl "0, ,

85 l'

;t:. :> ~'

,-:... ....:. rJl so " S :r: '-, "-"

1=" 1="

7~

70 '+ ____________ ~ "'-!-------------""":""

Temperature rcmpcraturc (/) h)

Figllre 4.2 Interaction bctween tcmperaturc and hydrogcn ,>ulfidc mlct lom.cntratlOI1

a) Effect on H2S absorptIOn. h) !.IfCll on CO~ Jb<,orptlon

•

•

•

0

'" ....: cr;

= -.-

,0 0'

....:.. 'Jl

"1,... ____________ ..... :1) ,... ____________ _

IllO )-1 ...

l'Il () )-:!

~< 15

,0 0'

....: ~II " III ,-.

'-' V -~

,<

'11 ..... ____________ ......

,,~------------~ 1 crnpcralurc Temperature

Il) h)

/'ï1:lIre 4.3 Intcr.JeliOn octwccn tcmperaturc and carbon dlO'lde mlct concentration

Il) Urccl on Il:S absorptIOn. h) Efreet on CO:! absorption

')fI :0 (,'11. - 1 ... (,'iL - 1 ... (,'lI. -:! (i/I. - 2

~< 15

?J( ,-..

~II " III '. 0 ::r: u ~ -~

'<

'11 ~ ____________ ~ Il~ _____________ ~

Icmpcraturc Temperature ,,) h)

/'ï1:lIrt! 4.4 InteractIOn oct\\cen tClllpcrature and gas'hqUld rallo

Il) UrCel on Il:S "bsorpllOn. h) Efreet on CO: absorption

40

•

•

•

'JO ~ll

y,(l15)-1 .. 1 III \1 1 --1',111 SI- 2 1 III \i

liS l'

'<f?, ~ " .-, -' r/l RO " III

" 0 J: U '-' 1=' 1='

75

70~ ______________________ -U I\~ ____________ .u

[CO~l lI) h)

Figu,e 4,5 Interaction betwecn carbonate concentrallon and hydrogcll ~lIl1idc III IcI COl1lcnt raI 1011

a) Effect on H~S absorptIOn. h) [ffccl 011 CO2 ,lb~O!pllOl1

90 20

J',llO,)-1 .. 1'1111 1 1 --Y.110 1-2 " 11111 1

!!5 l,

'<f?, ,0 0'

--:... ,-: r/l 80 " III " 0 J: ,."' U '-' .....

1=' 1='

75

70~ _____________________ ~

"'-!--____ -------~

a) [COi1

h) ICO Il

Figu,e4.6 Interaction betweel1 carbonate concentratIOn and carhon dlOxldc IIIlel LOIILcntratJOII

a) Effect on H2S absorptIOn. h) Effeet on c..02 Jh~orpllOn

41

•

•

•

f}f, 20

(,'lI - 1 - (i/I. - 1 --(,'lI. - 2 (in. -2 .-X~ 15

0 0

'" 0'

.--! .--! If) XI)

" 10 ., 0 :c ~

::-

~ ,...:. V1

u ~

::-

7~

71) L.!-------------~ 2 1) 1.!-------------~2 [COlI [C031

l') h)

l-ï1:ure 4.7 Interaction between carbonate concentration and ga~/hqllld ratio

Il) Efreet 011 11 2S ab~orptlon. h) Erfeet on CO~ absorption

tm ~()

l',tl Il 1 - 1 -l'III) 1

X~ 10;

~ -:

Kti " 10

l'ot< 0,)-1 --, I( 0 )-2

'" " . .. " 0 :r: ~

u ~

s::-

'~

"II L.J. ____________ ....

o ~------------~2

Il) h)

Fi1:IIre 4.8 Interaction bet\\'ccn mlet concentratIOn 01 hydrogen sullide and mlet concentration

of carbon dio'ode, Il) [ffect on H~S absorption, h) Erfect on CO~ absorption

42

•

•

•

'10 ~o

Gfl.-I - (. 1 - 1 ... G/L - ~ (;/1 - ~ ..

85 l'

";f.. ,0 • 0' .. --' -rJl RI! 1(\ " "" J: '-'

--" U ~

~

75

70~ ______________________ ~ IIL!---____________ _!_'

.l'JII,S) (1) h)

Figure 4.9 Interaction between mlct concentratIOn of hydrogcn ~ullidc .lI\d gol~/ilqllld 1.1\10

(/) Effect on H2S absorption, h) Effel:! on CO~ oIbsorplllln

'JO 20

(,ïf. - 1 ... !oÏl • 1

aiL - 2 (;// - 2

H5 o. l'

";f.. 0 0' -----' ,...:.

rJl HO 1_ " III ro 0 J: U '-"

~ ~

7,

70~ ______________________ ~

a) hl .1',,(( 0,)

Figure 4.10 Interaction between mlct concentration of carbon dloxldc and ga.,/llquld ratio

a) Effeet on H2S absorption, h) Effecl on CO; ao,>orpllon

... ..

tll

•

G/L

a) H2S absorption

• T

[CO)]

h) CO2 absorption

fïg"re 4.11 Relative cffect of parameters on the absorption of H2S and CO2

• 44

•

•

•

4.3 MATERIAL BALANCE

Material balance of the SpCCICS invol"cd in this system has bccn lI1\'Cstigatcd 111

order to contirm the rcliability of the experimental re~ults, the anal~ tkal I1Icthmls and the

calibration of the measuring instruments. RcslIlts of thls study 'lIe SlIll1m.lll1cd in l'ahk

4.4 based on the concentration of the ionic spccics (I1~'f, IICO,- and COl ) III thc outlet

liquid stream. The concentrations of thosc -;pccics wcre dctcrlllmcd analytlcally as

described earlier in Chapter Threc and then comparcd to thosc calclliated liulll the total

and component material balances bascd on the "-nO\\'11 composition of thc mlct and the

outlet gas streams and the inlet liqUld stream as described hellow, t,,"-ing mto account thal

the following two assumptions have bcen made tn calclliate the com:enll atlOn or IICO,-'

J. Hydrogen sultide is absorbed by l11eans ofrcaclion 2.3 onl)'.

2. The amount of HC03- produced by lhe ahsorptlOl1 of 11 2S and ('()2 is negliglhle

compared to that produced by the dissociation of the carbonate IOns according tn

Reaction (2.5) so that the mathematical expression of the eqllllihl illm constant I(lr

this reaction is given as:

Where:

(4.3 )

Ke" is the equilibrium constant

[HCO~ 1.'1 is the concentration of bicarbonate ions at equilihriull1, mo/ell.

[CO~]" is the inlet concentration of carbonate. lIlole/l.

The corresponding values of the cquilibrium constant K"II' lor rcaction 4.3 arc

given in Table 2.1 as a function of tempcraturc .

45

•

•

•

The concentrations of the spccies (ilS". HC03" and CO, = ) in the outlet liquid

stream were calculated using the following equatlons'

1 . l' Tolal moles 01 ahsorhed I!..,S

Ils = -V()lumelrtc Ilqu III flow raIe

(4.5)

fllco-]' = Tolal moles of lIhsorhed co:! + [I-IC03

],.,/ +[I-I..,sr :1 ('(Jlumelrie liquid flow raIe - -

(4.6)

[CO~ ( =[c071., -[IICO~L, (4.7)

The deviation is givcn as the ratio of the absolute deference between the measured

and the calclllatcd concentrations to the mcasured concentration .

I[ Measllred] -l ('alL'ulated]1 %Dev.= x J()()%

[ l'vIel/sured] (4.8)

Results of this ~tudy arc tabulated in Table 4.4. The deviation between the

l11easllred and calculated concentration for the three ionic species (HS". I-IC03" and

CO, ) was I()lmd to he Icss than 5 % in ail cases. Therefore these results shows that the

experimental set-up \Vas 111 good operation. the tlow meters were \Vell calibrated. and it

also suggcsts that the analytical instruments and procedures llsed to analyze gas and

liquid sumples \Vere accurate .

46

•

•

•

Table 4.4 Malenal balance

Run. (ns-)" IHS-)' Del'. IHCO~ )" l''c()~ l' De\'. I('u ,1" 1 CO, l' I>t"'. No. molell molell IYt. /IIoll.!d 11/01 ... 1 IVo. 1110'" 1 mole 1 Iy'.

A-\ 0.00046 0.00045 }.II () () 104 00101 ~ ()l) Il 4X(,7 Il 4l )4X 1 (,)

A-2 0.00341 0.00337 1.~2 ().05~7 0.0520 128 o 43llS Il 4529 ; ()l)

A-3 0.038\ 0 0.03776 089 0.0520 () 0501 .; (lh () ·n·H () 454X 474

A-4 0.00468 0.00455 2.63 () 020X 0.0200 'UN o 4hh 1 Il 4X49 402

A-5 0.00384 0.00370 3.80 0.0355 O.OJ49 174 478.17 4 I)(,(ln 1 (ll)

A-6 0.00051 0.00049 3.86 O.O}53 O.OJ49 106 47715 4 ,>(,(ln 1.'>5

A-7 0.00523 000501 409 0.0347 0.0344 () 74 48347 4 1)hO", ~ (,0

A-8 0.03917 0.03805 2.87 0.0957 0.0933 245 48471\ 4 l)O J(, III

A-9 0.00364 0.00359 1.44 0.0369 () 035() .\,(,1 0442", o ·.~X \ 1.59

A-IO 0.00043 0.00041 3.77 0.0372 O.OJ66 179 (4)02 (1 4~7\ I:W

A-Il 0.00447 0.00438 1.96 0.0329 003}7 1.66 04494 04h22 2 Xl)

A-12 0.03606 0.03467 3.86 0.1178 0.1172 055 () 1714 o 17()7 n. Kt)

A-13 0.00049 0.00049 0.69 0.0913 O.mnl 4.57 4 ~N11 4 lJl XX Il 56

A-14 0.00362 0.00344 4.77 0.1441 0.1184 3.95 475S') .. 8() 75 2.35

A-15 0.04028 0.03860 4.15 0.1334 0.1286 3.57 470()S .. 8771 3.63

A-16 0.00498 0.00489 1.90 n.IOIS 0.0985 2.()] 4 XX94 4.1)!)74 0.17

Nole 1 he sllper~CrlplS '\1 and c corre~p(llld, ln Ihe mc.t,urcd.md Ihc L.IILIII.IICd Llll1lClllr.llloll' rc,plLlIVd\

4.4 EFFECT OF BED HEIGHT ON H2S & C02 ABSOIU'TION

The cffcct of thc bcd hcight on thc rcmoval ctfieJ(.:ncy of' hythogcn <;ullidc and

carbon dioxide was invcsttgatcd by measunng the gas compo<;ttion alter caeh of the three

50 cm high sections orthe co\umn bec Figure 3 1). Bcfitrc !'>witching l'rom one "iumphng

point 10 another steady state condition,> wcrc al\owcd tn he rccovcred agum. 1 he removal

efficiencies H2S. CO:! at the diffcrent !-Jcctions of the column werc determmco. RC!-Jults lU

terms of the cffect of bcd height on the n:movul cfliciency of IlzS and ('():! arc .,hown in

Figure 4.12 and Figure 4.13 respcctivcly.

-17

•

•

•

Figure 4.12 shows that at the conditions of the present set of experiments which

gave the luwest hydrogen sultide removal efliciency (Run no. A-ID. where 11=72.2 %),

cf/cet of the hed height on the absorption of hydrogen sultide seems to be almost linear.

Ifowever, at conditions which gave hlgher rcmoval efficiencies (Run no. A-8. where

.,=909 %), this rclationship starts to take a power shape.

The results III FIgure 4.13 c1early show that the re1ationship bctween the removal

cfliciency of carbon dioxide and the hcight of the bed is 1 inear at the conditions where the

rcmoval efticiency is lowcst (Run no. 5 . .,=5.1 %). But at the conditions where the

highcr rcmoval cfticiency was obtaincd (Run no. 10. 11= 16.81 %). the relationship takes a

power shape strongcr than that of hydrogen sultide.

Thesc IWO ligures suggest that the packed bed was not high enough for hydrogen

sullide in the gas phase to reach the equilibrium conditions with the absorbed hydrogen

sullide in the liqllid phase taking into account its inlet concentration in the gas phase. On

the othcr hand. this indicates that carbon dioxide in the gas phase is not tàr from the

ct\uilibrillm conditions \Vith that in the liqllid phase.

Finally, an important conclusion of this study IS that the absorption of hydrogen

sullidc could approach 100 % if the height of the packed bed is increased ta

approximately 2.0 m. and it does not affect the absorption of carbon dioxide very much.

48

• 100

80

~ 60 ,-::; rJ:J .. -- 40 -~

20

20 .10 60 XO 100 120 l,Ill

l3ed Ilclght. ,'/II

Figure 4.12 EtTect bed height on the rcmoval clliclcncy of Il!S

• 20

Run no A-5

16

::R 12 0

r-. O~·

U !! r::

4

" ' () . ,

() 20 40 W XO 100 110 140 IW

Bcd Ilcight. cm

Figure 4,/3 Erreel bed height on the removal cfficlcncy of CO,

• 49

•

•

•

CHAPTER FIVE

RESUL TS & DISCUSSION

The absorption or hydrogen sultide and carbon dioxide have been investigated as

a limction of the opcratlllg parameters, I.e. temperaturc, carbonate concentration, gas and

1 iquid supcrlicial vclocltlcs, and thc inlet concentration of both gases.

RcsuIts are presentcd in tcnns of the influence of these parameters on the removal

cfficienclcs and the ovcrall mass transter coefficients of hydrogen sulfide and carbon

dioxidc. Furthermore, the process selcctivity for hydrogen sulfide removal has been

discussed.

5.1 EXPERIMENTAL DESIGN

For this cxpcrimcntal investigation, the same experimental design procedure

dcscribcd by Ta~lidl1 1231 was follo'Wed. Another set of sixteen cxperiments was

pcrli.mncd. hascd on thc prcliminury cxperimental resuIts (Chapter Four), where the

interactions between parametcrs arc negliglblc. In this sct of experiments, the liquid flow

rate was maintaincd constant at the avcrage valuc between its lowest and highest levels

(at 111cdllllll Ic\'cl. m). For thc olhcr paramelers, i.e. lcmperature, carbonatc concentration,

Il;!S and '-'()2 inlct concentratIOn, and the now rate of the gas, four equally spaced levels

\\erc in\'cstigatcd. rhus, thc value 1 corresponds to the lowest level of each parameter, 4

is the highest kwl. 2 is at the level one third of the ditference between the lowest and the

50

•

•

•

- - -----------------.

highest levels above the lowest kvel and 3 IS at the le\ cl t\\O thmls of the same

difference ubove the lowest levcl. The conditions ot"these e:-.pertlllellls arc gl\l.'1l in l'abk

5.1.

This expenmental design IS considered 10 be balalH.:ed "Illec Il "atl~,ilies lhl.'

follo\\-ing two conditions:

1. Each parametcr IS repcated at the saille levd only tour tl\l1eS III thl.' \\holt: set of

experirncnts.

2. Any cornbination oflevcls for any two parametcrs appems only ollce 1I11he who le

set of cxperiments.

Table 5./ Fmal cxpcnmental deSign

Run No. Temp. ICO;I Y,,(1I2S) Y.,((·02 ) GIL '1(I1.,"'i). no lllt'O,), n'i,

B- 1 1 1 1 1 1 7495 X hX

B- 1 1 2 ., ., ., ln 15 7 9(1

B-3 1 .,

3 ... ... X935 761 -' .) ,

B-4 1 4 4 4 4 () 2 ().~ 775

B-5 2 1 ., ... 4 XI (14 12 26 - )

B-6 ") '1 1 4 3 7() XI 1167

B-7 ") ., 4 1 ., X712 X 61 .)

8-8 ") 4 ... ., 1 X4 9() 1) 07 .' -8-9

.., 1 3 4 ., 7X X4 14.02 -' -

B-I0 ..,

Î 4 ..,

1 XII 'i 1220 -' .' B-ll 3

., 1 ., 4 X 1. 50 II.()() -'

B-12 3 4 ., 1 3 Xô 1 X 1 () 41

B-13 4 1 4 ., ... XI (J'i 1"\ (n - )

B-14 4 '1 3 1 4 X6 X5 Il ()5 .

B-15 4 3 ") 4 1 XO 5(1 1222

B-16 4 4 1 3 ., 7') 7() 12 10

'il

•

•

•

The removal efticiencics of H2S and CO2 have been determined based on the

analy'ils of the gas streams at thc inlct and the outlct of the column by means of Equation

4 1 and EquatIon 4.2.

"he Influence of gas Ilow rate on the absorption of hydrogen sultide and carbon

dioxldc has been ~tudled by running a set of expenments at tive levels (Table 5.2) where

aIl other parameters werc kept constant at thctr average level between the lowest and the

highest leveb (m,. rhis was done in order to compare the results obtained from the

TaguL'hl cxpenmental design wlth those obtained l'rom the ordinary cxpenmental design.

i\ simiJar ~ct of cxpeflments \Vas carned out to study the intluence of liqUld flow rate on

the absorplJon of 112S and CO2 ('rab le 5.3). The designation of those levels. 1 to 4, in

thcse two tables IS the ~amt.' as for that of the final cxperimental design given in

1 able 5.1.

Ttlhle 5.2 Erfeet 01 gas tlow rate

({un No. G ll(l-l:!S), % 1l(CÛ2),0/0

C- 1 1 80.41 10.54

('-2 ..., 82.25 10.72

C-3 3 84.08 10.91

('-4 4 85.92 1109

C-5 '" 83.06 10.71

Ttlhle J.3 Erfeet 01 IiqUid tlow rate

({un No. L 1l(H2S), % r1(,02)' %

D- 1 1 82.71 9.35

D-2 ...,

83.01 10.33 -))-3 ... 83.32 11.3/ -' D-4 4 83.61 12.28

1)-5 ln 83 n 10.93

52

•

•

•

5.2 DISCUSSION OF RESUL TS

The final rcsults \vcre evalualed \\'Ilh lespecl 10 the el'll'cl of the -;tudlcd

parameters on the rcmoval effieiencies of h)drogen sullide and carbon dloxlde, Oll the

overall mass transtèr coefticients and also on the selcctivity 1:lctor

5.2.1 EFFECT ON THE REMOV AL EFFICIENCY

The eITeet of tempcrature, concentration of carhonate, concentratlOll 01 hnlrogen

sulfide and concentration of carbon dioXH.ic on the rel110val dliciellcy 01 Il ~S ,lIId CO:,

has been estimated trom thc data shown in rahle 5,1. 1 he cllcet or cach pawlllctcr al ally

level has been cstimated by takll1g the average value or Ihe J'Olll Il'1I10\'al cl licIl'l1CICS

obtained wlth paramctcr at that Icvel. An ad<.htional data pOlIl1. al Ic\'el III. I~ ohlalllcd hy

taking the average of thc last two expenmental re~uIts in 1 ahlcs 5 2 .lIld 5 J (Ior IIIOIC

details see Appendix C), rhis gl ves the removal eflicH:ncy a~ a 11IIlCIlOll 01 each

parameter while thc rcst orthe parameters \Verc kepl constant at thell IIlcthUI11 Icvcl (III) 111

the form of five data points. Thc calculated data have hcen plotted 1\1 thl' 1;lgures 5 1 ln

5.6. Thus, each ligure shows the effect of a slllgle paramctcr Oll the lellloval eflielcllcy (lI

H;!S and CO:! at live levels while the five othcr parumeters are cOII~lallt al thclr medium

level.

Figure 5.1 shows that incrcasing the opcrating temperaturc dcercascs the

absorption rate of hydrogen sullidc and incrcases the absorption rate of carhon dlOxide.

According to theory, the solubility, or buth gases, ln water decrea<;c.., wlth increasing

temperature [18,19]. Although at non-equtlibrium c()ndition~, whcn chcllllcal rcael\()n~

between the gus phase and the liqll1d phase are 1I1volvcd. thl.., doc" Il()t alway.., apply

However. increasing the tcmpcraturc re~ulb ln a ~lgnJlicant I\1Cre,N! ln the retlctJon rate

of CO:! (reaction 2.2), Also, It decrcme,; the IIquld lilm re"l~tancc hccLllI"e of the lowcr

liquid vlscosity. Since thc absorption rate of ( ():! i.., controlled hy the IllJuld "Ide ma~s

transfer reslstance as was rcportcd by Garner el al, 1 f)1 thi,; might explmn the l\1f1ucnce of

temperaturc.

)1

•

•

•

()n the other hand. tempcraturc atfccts the absorption of H2S is an opposite way.

The ab'iorptlon "2S i .... control/cd by the gas side mass transter resistance: Therefore, the

c.lccreasc in the "quld film rcsistancc does Ilot increase the absorption of H2S in a

<;iglllficant all10unt Ilowever. an incrcasc in temperature shifts the cquilibrium

lh~""oclatlon reactlOn (rcaction 5 1) of COl in \\ater to the right side 118./6]; thus. more

IICO,' ionc;; arc produced Thcrcf(.)re. the cquilibrium reaction bet\\een H2S and C03

(reactlon 2 4) is <;hi flet! to the Idi slde. resulting in a dccrease in the absorption rate of

Il..,S. 1 hese reslIlts arc ln agreement \Vith those rcported by Ramachandran el al. [12].

CO 3 (5.1 )

rhe intlllencc of the mlet concentratIOn of carbonate Ions on the removal

cfficicncy of hydrogen ~ultide and carbon dioxide IS in good agreement \Vith those

rcportet! in literature 15.6,9,12,131 As illustrated in Figure 5.2. the removal etliciency of

"2S increases with II1creasing concentration of carbonate IOns. because the equilibrium

reaction hctwccn ('01-- and 112S (reaction 2.4) is shifted to the right side. However, an

increase in the carbonate ions concentratIon shifts reaclion 5.1 to the right side, thus,

incrcasing the concentration of 1 ICO) '. Therefore. reaction 2.2 becomes doser to

equilihnull1. \\hiLh resll/ts in redllcmg the absorption rate of CO2• as reported by AslarÎla

e( al 1 () 1·

From Ficure 5 3. it is dear that H,S removal efficiencv increases signiticantly ~ - .

when its concentration at the inlet of the column is increased. The concentration gradient

of Il,:!S het\\een the liquid and the gas phase becomes higher if its partial pressure is

incrcased. rherct~.)fe. a higher drivmg force is crcated. which reslllts in an increase in

ahsorptlon rate hy allo\\ ing more H2S to he absorbcd in accordance to reaction 2.3 and

n:actllln :2.4. (his linding is similar to those rcported in the literature [-.9, Il,13].

ln the saille \\av. increasinc the inlet concentration of CO, increases its removal - ~ -efticicnc~ (Figure 5.4). Hnwever. carbon dioxide is absorbed by means of a slow

reacttllll (reaction 2.2). rhus, the ciTect of CO, inlet concentration on its removal

efticicncy appcars to he sm aller than the cHect of H,S inlet concentration on the

54

•

•

•

absorption efficiency of H~S (Figure 5.3). Furtht:rmore. rmlll th~se 1"'0 ligures. lht:

removal efficiency of carbon dioxide and that of h~ drogl.'n ~ullidl.' arl' not alfl.'l.'ted

significantly by the inlet concentration of rt:spt:cti\ I.'ly. Il.!S and CO.!. l'lm linding.

corroborates the preliminary experimentallt:sults gi\1.'1l t:atlil.'r 111 l'ahll.' 4.3 and Figurl.'

4.11. and also agrees \Vith that rcportcd by Belldal! e{ al Il () 1.

The influence of gas and liquid supcrlicial velocitics is Illustratcd in Figure 5.5

and Figure 5.6. The results arc in excellent agrct:l1lent \\ith thost: round in lJterature

[-1,5,10,111. The removal efticiency of Il:!S incrcases signilicantl~ \\Ith incrcasing the

gas velocity. white the influencc of the gas vclocity on the rClllo\'al dlicil.'lll'Y or co:! is

very small as can be seen in Figure 5.5. On the otht:r hand. l'rom Figlln: .5 () Il is ohviolls

that increasing the liquid \'clocity. results in a signilicant IIlcrca'il.' 111 the Il.'llloval

efficieney of CO;!, while 1t has a sl1lall cffect on the rt:moval efliclcncy (lI' 11 2S 1 he main

reason for this phenomenon is that the absorption of Il;!S is agas "i<.k controllcd proccss

as il reaets at the gas-liquid intertace, while the absorption of ('{)2 is a litJuid 'iidc

eontrolled proeess; therefore. it reacts in the Itquid bulk. In thesc Iwo tigurl.'S, thc

relationship between the rcmoval elliciencics and the \'clocitlcs of ga'i and liquid appcars

to be linear. The only cxplanalion for thb hchavior is that thc hClght of the packed

eolumn was not high enough to reach the eqUllibriulll hetwccn the IWO pha~es as was

previously shown in Figure 4.12 and Figure 4.13

55

•

•

•

XX

X"

Q X4 Q'

,-:. 'fi

" - X2 -' r=

XO

7&

~ ____________________________________________ ~ 16

.'

14

12~

0" 10~ ~

8

~ ____________ ~ ____________________________ ~ ____ ~6

:W JO 40 50 Temperature. oC

60 70

fïg'ITe .'U Effeet oftemperature on the removal eftieiency of H2S and CO~

H8 18

II zS CO2 -!Ill 16

,0 N.t 14~ Q'

....:. """ [/J 0 " " :r:

X2 12~ r= ~

sn 10

7!! Il

l co~ 1. II/ole / 1

Fig"re .f.l [nect of carbonate concentration on the removal efficiency of H~S and CO2

56

•

•

•

R8

86

~ R" .-:., rJ)

rt

J: R2 '-' ç:-

RO

7R

r-------------------------------------------~18 ILS co, -

16

I.I~

0" 12 ~

ç:-

10

0 ~------~------------------------------------~8 0002 000" () O(}6 Il 008 () (JI 0012

Figure 5.3 Effect of hydrogcn sullidc mlet conccntratlon on Ihc 1 cl1\oval d IiCICIlCY lit Il!S and ('(),

R8 IH

R6 I(l

~ H4 110° .- -:-rJ)

rt

J: H2 '-'

~

0 "

12~ ~

RO 10

.'

7R H 000 002 o 114 () Of! (J OH () If) {J 12

Figure 5.4 Eftèct ot carbon dloxlde mlet concentration on the rcmoval efliucncy (lt " 1S and CO ..

'i7

•

•

•

XX

X"

0 H4 0

-.:. 'fJ .. ~ H2 ~

HO

7X

r---------------------------------------------~ IX JI,S ('0,

16

14~

0" 12~ ~

10

~----------------____________________________ 8 ,1 X 10 12

G'. /Il/mm

"Igllre 5.5 EfTect of ga~ ~uJlerfïclal velllclt)' on the removal efticiency of H2S and CO2

X8 r---------------------------------------------,IX II,S CO, -X6 16

,0 Hol 0'

-.:. r./l

" .....-

1ol~

-----------~ :I: ?:= HZ

HO 10

7H Il 1 Il 15 ~---------------11~2-------(-)2-5-------(-).-1-------()~3~s------~()1

L·. III/mm

"Ig"r#! J.6 l'nect of hqUld sUJll!rtictal veloclty on the removal efficiency of H2S and CO2

58

•

•

•

5.2.2 OVERALL MASS TRANSFER COEFFICIENTS

The overall mass translcr cocflicicnl of hydrogcn sullidc and cal bon dlll)o.ldc ha"c

been evaluated at the operating conditions of all cxpcflmcnls shO\\ Il in l'ahlcs ='.1. 5.2

and 5.3. i.e.

where:

9l KG =----=-­

(l' A· h· ô.P 1.\/

KG: overall mass transtcr cocfticicnt. mole/lI/lIl /111~/atll/

a: specifie area of the packing matcriaI. 98 1 II/~/m\.

A: eross seetional area orthe bed. m~.

Il: height of the packed bed. ni

(5.2)

The absorption rates of hydrogcn slilIidc ami carhon dioXllh:. ~H (II/oldmm ). arc

given by Equation 5.3:

where:

G·/', ( ) 9l=--lxy_y RT n 1

G: gas flow rate. 'Imm.

PI: total operating pressure. atm

R : gas constant. 0.082057 (/ atm)/(mole K).

T: operatmg tcmperaturc. K

(5.1 )

Yo and YI are the inlct in nul let 11 2S and ('°2 mole fractIOn III the gU!l ~tream

respecti vcl y.

The logarithmic mean pressure diffcrencc. Ill' 1\/ • was cakulated l'rom I·.quation

5.4 where the equilibrillm partial pressures of hydrogen 'iullide and carholJ dioxide anove

•

•

•

the carbonate solution. p' (/lJS) and p' (CO~), were estimated by means of Equations 2.8

and 2.9 respcctivcly.

whcrc:

(5.4)

Ji" and Ji 1 are the inlct and outlet partial pressures respectively and the y are given

as P = y.p,.

p' is thc cquilibrium partial pressure above the carbonate solution .

The cftect of the operating parameters on the overall mas transter coefficient, KG,

of both Il:!S and CO:! have been cvaluated using the same averaging technique followed

in the cvaluation ofthcir effect on the removal efficiencies.

The results of this study arc shown on Figure 5.7 through Figure 5.12. Those

ligures show that the influcnce of the operating parameters on the absorption of H2S and

CO:! \Vere in good agrcemcnt with the results reported by Garner el al. [9].

Ilowcvcr, sincc both the overall mass transtèr coefficient. and the removal

cfficiency. 'l, arc functions of the absorption rate. the effect of ail parameters on the

oyerall mass transfer coefficients nf hydrogen sulfide and carbon dioxide has the same

trend as the crleet on their removal efticicncles. Moreover. it was found that KdH2S) is

one order of magnitude higher than KdCO~). In terms of magnitudes. KG(H~S) values

l,btained l'rom this study \Vere much higher than those reported by (Jarner [9] for wetted

\\ ail columns .

60

•

•

•

o 22

1

1"" ------_________ ~ ______ ., 0016

1 LS CO, -(120 (l011 :::

"5 ...

Oll!~ tlOI:!~ ". :::

-:-~-----------1 ~ 016

.' ....

,,'

,,'

", ;:, () tl \tl :::

d' u

() OON '" ~

0,12 ,-=----~_._-..... ----------------...I 00116 20 JO 40 50 (,0 7()

rcmpcraturc. 0('

Figure 5.7 Effeet of temperaturc on the ovcrall ma.,., tramler coefliclCnh of Il!S ,lI1d CO!

0,22

S 020

~ -... -:::: OIS .... rI-

=-,'" .' ....:: 9 016 ::

A 00

N

:I: '--' 014 ~o

0,12

,.....------__________________ -,Olllll

" . ...

-----!!----I Il Il 12 ::

•• P'"

~

o 010 ~

0' U

OOON .~ <

0 ~-----------.... --------...... ---..... 1) 00(, 2 .• (,

1 COlI. mflle /1

Figure 5.8 Effeet of carbonate concentratIOn on the nver-all ma.,., tran.,lcr cocflicienh 01 IllS and CO2

61

•

•

•

() 22

- () 20 :::: ~ ... :::: () II!

'!... ::::

~ 0.16 :::: .-! rfl :é' -' () 14 '<..:-

o 12

,....-----------------------,0016

0014.§ ,l::! --. __ ----..... ---1 0 OIL~ "' -:: "" -.

o 010 ~ ,-.,

0' U

OOO!!~

L... ___ ...... ___ ...... ___ ..... ____ ..... __ ...... ___ .... 0006 () 1/ 002 () 004 0 006 () OO!! () 0 1 () 012

Y,,(H~S)

/<ïgllre 5.9 EITect of hydrogen ~ullide inlet concentration on the over-allmass transfer coefficients of H,S and CO, - -

0.22 ~--------------------------------~OOI6

02 :;:: ~ :::s

0.014 :;:: §

-:::: o II!

"!... ::::

....:: • ... .' ..... 0 () 16 :: r;:; :é'

,...:;,

0' U

'-" () 1·\ 'i<.:J 0008~

() 12 0 ---___ ~------__ ---___ ------.... 0006

002 0 04 () 06 () 08 () 1 () 12

Figllre 5. /(J EITcct of carbon dioxide mlet concentration on the over-all mass transfer coefficients of Il,S and CO, . -

62

•

•

•

-- ----------------------------.

030 li 010

- 025 Il OO! ~ ::: E: ... ... ~ , .... - ~ ~

o~ 020 Il o.'!o ~

r'.... ,'" :::: ~ ~ \:) o 15

~

~ Il Il l 'Ï :::

-- ...:. r/J d' r' ::r: u '-' 0,10 0010'0 ::.::'"' :.::

005 .. ' " 110115

4 6 X 10 12 (i'. III/mm

Figure 5. / / Effect gas superticml veloe Ity on the ovcr-alll11a,,>~ Inlll,,>lcr CllcllÏl:ICIII"> 01 Il ,S .m<l l '( ),

() 22 001" 1I 1S CO, -021 0011 - §

~ ':l " 0,20 0012 § -... - ,.. ,'"

N r.:

019 0011 -,~

~ " ..... ~ :::: l"" -

018 ", 0,01 (),.-;, ,-.. ", r/J 0 :t' ", u '-' .... -'

::.::~ 017 ';J

• - o ()()9:C:

-016 OOOK

01 () 15 02 Il 25 Il 1 Il 1-; (J,I

1. '. mlf/un

Figure 5./2 Effect hquld .,upcrlieml vcloclty on thc ovcr-all ma.,., lIan.,lcr LOclfkicnl., 01 1I 1S and CO2

63

•

•

•

5.2.3 SELECTIVITY OF THE PROCESS

ln ordcr to reduce the total cost of the process, and for the purpose of producing

green liquor, it is desircd to maximize the removal of hydrogen sulfide and to minimize

that of carbon dioxide, thus improving the efticiency of the absorption process which

il1volvcs the translcr of both gases simultaneously to the liquid phase. Selectivity of

carbonate lèJr hydrogen sultide absorption may be defined as the tendency for the ratio of

hydrogcn sullide to carbon dioxidc contents in the Iiquid phase to be larger than it is in

the gas phase. As a measure of this. the so-called sclectivity factor. S. \Vas introduced by

(il1rner el al 191. The seIectivity factor for hydrogen sultide is defined as the ratio of the

overall mass transler coefficient of hydrogen sultide to that of carbon dioxide. i.e.

S= K(j(H 2S)

Kc;{CO) (5.2)

The higher the selectivity tactor the more efficient is the absorption process. In

other words. to absorb hydrogen sulfide selectively in the presence of carbon dioxide a

high value of the sclectivity lactor IS needed. For each run, the selectivity factor was

calculatcd, and it was found 10 be bctwcen 13 and 20 which IS higher than those reported

by Garner el a/ 191 or those reported by Ramac/umdran et al. [12l The intluence of

cach paramcter on the selectivity factor was evaluated by the same averaging technique

describcd earlicr. and they ure shown in Figure 5 13 through Figure 5.18.

Figure 5.13 shows that if the tempe rature IS increased. the selectivity factor

dccreases because the rate of carbon dioxide absorption increases while the absorption

rate of hydrogen sultide decrcascs. This indicates that in order to improve the selectivity

of hydrogcn sultide absorption lower operating temperatures are required.

64

•

•

•

From Figure 5.14. it is clear that the s~kctJ, it~ t'actor IIlCleaSl'S \\ ith inca.'asing

the concentration of carbonate in the absorptton solution. 1 his nccurs hecause the ratl' ot'

absorption of H:!S II1creases hy increasing the concentratIOn nt' Ct)l \\'hich can he

attributed to the increases in the reaction rate b~t\\een cal ho na te ions ,md II,S and dll~ to

the decrease in the absorption rate of carbon dioxide as discusscd belole

Increasing the concentration of Il:!S at the ml et or the colull1n causes Ils rate or

absorption to increase, while it does not affect consldcrahly the absol ptlon late of ('( ):!.

Thus. the selectivity tàctor increases signilicuntly as Illustrated in Figure 5.15. Similarly,

the rate of absorption of CO ~ ~ltghtly increascs hy II1crcasing Its concentratloll at tl1l' miel.

Thus. it results in a decrcast~ in the sdcctivity Illctor as shown 111 l'Îgule .;; 16 1 hClcfore,

this behavior suggests that 111 ordcr to improvc the sclcctlvity of the plOCl.'SS. high partial

pressure of H2S is preferable which can be obtained by operatl11g the coluI1111 al highcl

total pressures .

Because the absorption of hydrogen sultide is controlled hy gas side mass translcr

resistance. and that of carbon dioxide is controlled by the Irquid "Ide mass transfcr

resistance. the selectivity factor appears to increase If the gas vcloclly 1" increased, and

nlso it decreases \Vith increasing the liquit: vclocity, as ~h()wn 1/1 l'Îgure 'i 17 and l'Îgulc

5.18. Thus. it is recommcndcd to operatc the ubsorption column at high gus/liquld latlos

in order to improve the selectivrty for hydrogen sul/ide absorptIOn .

•

•

•

2R~ ______ --__________________________ ~ ________________ ~

x~ ____________________________________________ ~

20 :;0 .tu 50 hO 70

Temperature. 0('

I-ïgure J./3 Effect of temperature on the selecuvity factor

2Xr---------------------------------______________________ ~

20

• • ----

16

L!

N~ ______________________________________________ ~

o 2:; 4 :; (,

[C03], mu/ell

Figure J./4 Effec! of carbonate concentration on the selectivity factor

66

•

•

•

28r-----------------------------------__________________ ~

20

16 • •

12

8~--------------------------------------------~ o On02 000 ... ()OOl!

J',,\II~S)

IIOOH 1101 () 01.'

Fig"re 5./5 Erreet of hydrogcn !>ullidc J11let concentr,ltloll 011 the \CleLlIVIt) 1,ILlm

28~----------------------------------------------------~

24

20

16 .

12

8L-__________________________________________ ~

o (J 02 (J 04 0 06 Il Ol! Il 1 Il 12

Fig"re 5./6 Effeet of carbon dloxidc mlet tollCentratlOn on the ~electlvlty f,lLtor

•

•

•

22

20

IX

~

16

14

12 .t h Il 10 12

G'. 111/111/11

/''i1:IIre 5./7 EfTect gas superticml velocity on the sclectlvtty factor

2Xr------------------------------------------------------,

20

16

12

() 15 02 025 L'. III/mm

03

H1:lIre J.18 Hfect hquid superticial veloclt)' on the selectivity làctor

() 35 04

68

•

•

•

CHAPTER SIX

CONCLUSIONS & RECOMMENDATI()NS

6.1 CONCLUSIONS

Based on the present expenmental work. it can he concluded Ihat a 11Igh rcrnoval

efficiency of hydrogen sulfide accompanicd \Vith a rclativcly low lel11o\'al efliciellcy of

carbon dioxide can be achieved by the absorption or gases contallling the"e Iwo species in

an aqueous sodium carbonate solution lIsing a packed collimn operatcd III Ihe cOllntcr­

current mode. The selectivity for hydrogen slilIidc absorption IS IlIcreased IInder the

following operating conditions:

J. Low operatmg temperatures, to minimize the absorption rate 01 carbon dioxide, by

reducing its reaction rate \Vith carbonate. and to maximi/.e Ihe ah"orption rate of

hydrogen sulfidc.

2. High concentration of sodium carbonate in the lilJuid stream to reôtlce the

absorption rate of carbon dioxide and to increase the ahsorption rate of hydrogell

sulfide by crcating a higher driving force hetween the two pha~e"

3. High total pressure to increasc the partial pre~sure of hydrogen ... ullide hecall~e ils

absorption rate increases by increasing il<.; partial pres ... ure. whrk the ah"orptroll

rate of carbon dioxide incrcases slightly hy illcrcasrng it~ parual prc~sure.

4. l-ligh gas flow rate to incrca~, he ab~orptlon mte of hydrogcn ... ullide hecause its

mass transter rate is gas side controllcd.

•

•

•

5. Low liquid Ilow rate to reduce the absorption rate of carbon dioxidc since it is

controlled by the liqUid side rcsistance.

Green liquor b a solution in which the concentration of sodium carbonate is about

2 mo/el/ and the concentration of sodium sultide is about 1 mo/ell. According to the

conditions of the expenrnents in this work. even if the removal cfficiency of hydrogen

')ul/ide is about 100%. the maximum concentration of sodium bisulfide that can be

produccd (atthe highe~t G/L ratio) is about 4.6xlO·2 M. Which I11cans that green liquor

can not be produced lI~ing the present experimental set-up.

6.2 RECOMMENDA TIONS FOR FUTURE WORK

Since we are concerned \Vith the removal of hydrogen sulfide. it is recommended

that the column hcight should be JI1creased. A theoretical extrapolation of the results

ohtailled l'rom this study indicates that tf'the column hcight is increased by 50 cm (total of

2.0 m) the rcmoval ellieiency could he increased up to about 98.5 % at G=50 /Imm., L=

0.5 //ml11 • J' (l1~S )=2.0 (Yc, and a carbonate concentration of 2.0 M. under these conditions • Il _

thc concentratIon of bisultide ion~ (HS') \Vould he around 0.1 mo/el 1. Moreover, to

rllrther II1Clca!:JC the concentration of IlS' il is neccssary to mcrease the GIL ratio. This

ean he donc hy gOIl1g to a triekle hed mode of operation by further decreasing the Iiquid

Ilow rate and/or hy II1crcasmg the SILe of packing which nllows highcr gas tlow rates.

Furthcnnore. another Iden to I\1crcase the concentration of bislIltidc ions in the

effluent. is to recycle some or the liquor back to the absorption column in order to utilize

mon: completcly the unrcaeted carbonates. This wIll improve the rcmoval efficiency of

hydrogcn sultide. pro\'lding that the equilibrium conditions between hydrogen sulfide in

the gas phase and blsultidc Ions in liquid phase are not achieved. On the other hand, the

rell1o\'al of carhon dio'\\(.lc will not he signiticantly affccted since its equilibrium

~nI1ccntration is rclati\ ely \ cr~ 11Igh comparcd to that of hydrogen sulfide .

70

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•

REFERENCES

1. Avedesian. M. M .. Kubes. G. J . and \'an 1 Icmingen. A. R. Il. "!>e\'c/opf11t'11I 0/1111

Alternalive Kraft Black Liquor ReL'O\'e,:v l'ro(.'e.\·\ l/a.\('eI 011 1 ou'- l',,ml,el'alw'(' Processlll~ in FllIldi=ed-Beds. .1 l'I'OIJO.\1I1 10 1 he 1-.'\'('('III1\'C ('ommlI1L'e loI' lire

InternatIOnal Ene,.~y AgenC.'y's Impleme11l111g 011 Fllcrgy ('011.\'-" mliOl11l11!rc /'111" l1/1d Paper InduSI1:l'''. Dept. of Chem. Eng .. Ivtcl J illl 1ni\'erslty. 1\ 1ontrcal. ('anada. 1 ()lN.

2. Kohl, A. and Reinscnfeld. F .. "Uas PunjiCal/ml". 2nd cd .. (Iull' Puhlishing. Ilouston. 1974.

3. Ouwerkerk. C., " De,\'lg1l jol' Se/eell\'e "'2.\' ,/hM"."IWII". Ilydrncarhnn PlOœssin~. 89-94, April. 1978.

4. Versteeg, G. F. and van Swaaij. W. P. M .. " ,1h.\orIJlltIIl 0/ ('()~ and Il'2.S /lllllflU'O/l.\'

a/kano/amine sol III/ons lIsmg a fixe" hed relll.:lol' Il'lI1t (O-CI/n ('1/1 dI/lin/loI\' 0IJ('rlll/lJll

in the pu/se /low f'e~ime". Chemical Engl\1ccring and Proccssing. 240). }(ll-17(,. 1988.

5. Astarita. G. and Gioia, F .. 1t 1 ~vdrogen ,""lIlfide ( 'hemICll/ .. 1 h,\'()/'l1 I/IJ/I " • ('he. Eng. Sc .. 19.963-971, 1964 .

6. Astarita. G. and Gioia. F., " Simliitaneous Ah.\Orpl/OI/ of III'drogeli Sill/icie mlll Carbon Dioxide in Aqueolis I~vdrox/(Ie Solu/ions". I&E(' Fundamcntals. 4( 1). j 17-320. 1965.

7. Onda. K .. Takenchi. IL Kuboyasi. T. and Yokola. K .. Il Simlllllll11!OIlS Ahsorp(lIm 01 Hydro~en Sul/ide and Carh(}n Dioxule ill //(/I/eOIlS Sod/llm f fI,d/'(/x/({c So/IIlW1/S". Jour. ofChem. Eng. of Japan. 5( 1),27-33. 1972

8. Oloman, c.. Murray. F. E. and Risk. J. IL Il Tite Se/eclll'e Ah.\'ol'/J/101l 01 IIvdrogl'11 Su/fide From StackGases". Pulp & Paper Mag .. 5. 69-74. Dcc. 11)(,9

9. Garner. F. 1-1.. Long, R. and Pcnnc11. A .... '/11e Se/ectl1'e Ah.\'(JIIJ/1011 0/ 1/\'(lrogl'l1 Sulfide 111 Carhonale S'OIWIOI1S ", Jour. Appl. Chl:m .. 325-)3(), May. 1 l)5X.

10. 8endall. E .. Aikcn, R. c.. ami Manda~. l, . " Selec/lve /lh,WJrpllOl1 01 "].\' lrom I.urger Quantilies of C02 and react/On 111 FmI.' .... iJf'(l.v\". AIChF Journal. 29( 1), M)-n. 1 ()'n.

II. Hohlfeld. R. W .. Il Selective /lh\'Orpl/011 of I/~,r..,' fmn7 Sour (;m" r()ur. Pet. 1 œh .. 32(6), 1083-1089. 1980 .

12. Ramachandran. P. A. and Sharma. M. M .. " SlIlluitaneOIl,\' Ah.\·of'/JI/OI1 of 'f'wo (ia.\e.l'".

Trans. Insl. Chcm. Eng., 49.253-280. 1971

71

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13. (iioia, F., " .<"'e/ecllve Ahsorption of H2'<'" in PotassIUm Carbonate-bicarbonate Bliffèr ,\'olutu)11s ".Chimica Ind. Milano. 49. 1287-1298. 1967.

14. Cirilo, R. and Mclchor. !\., "Carhon Dio:ade and I~vdmgen Sullide Absorption hy Prolnoted Potas.Hum Carhonate SolutIOns". Revista Technica INTEVEP, 8(2), 129-133, 19RH.

15. Doraiswamy, L. K. and Sharma M. M .. "Hetero~eneous ReactIOns", vol. 2. John Wiley & Sons. New York. 1984.

16. Mai, K. L. and Babb !\. L.. "Vapor-Liqllld Eqllilibria hy Radioactive Tracer Techniques .\:vstem Carhon Dioxide-Hydrogen Sulfide-Sodium Carbonate-Sodium lJiclirhonate-,\'odi /lm ,c;,'ulfide- Water". Ind. Ellg. Chcm .. 47(1749-1 757),1955.

17. Latimcr. W. M. and lIildebrand. 1. IL "Reference Book of Or~a11lC Chelnl.\·I1/" 3rd ed.,'lhe Macmillan Company. NcwYork. 1965.

1 H. Perry. ''l'ef'l)<'' 01(.'l11lcal Engll1eering Handbook". 5th ed., McGraw Hill Book Co., New York, 1985.

19. Treybal. R .. E.. "Alass Tran.\/er Operations", 3rd ed .. McGraw Hill Book Co., New York, ,1981.

20. Shah. Y. T .. Sticgcl. G . .1. and Sharrna, M. M., Il Back mlxing in Gas-Liquid Rel/ctors", AIChE Journal. 24(3),369-400.1978.

2 \. Brenner. II.. "7ïle dlflusion mm/el of longitudinal mixmg m hells of fimte length Numem:al l'ailles". L'hem. Eng. Sc .. 17. 229-243.1987.

22. Thompson. W. r.. Pc Iton. A. D .. and BaIes. C. W .. " Factlily loI' the Analysis of Chemu:al ThernwJynamics - GlIIde to Opera/IOn". McGill University, MTL., May, 1985.

23. Taguchi. G. and Konishi. S .. " TA GUCHI METI-IODS: Orthogonal Arrays and Linear Uraphs ". Arncrican Supplier Institute. Michigan. 1987.

T2

APPENDIXA

DESIGN OF THE ABSORPTION COLUMN

A preliminary desIgn procedure was followed in order tn estimate the sllitahle

height of packing that should be used to removc 95 °11) of hydrogcn sullidc h(lm li gas

mixture of 1.5 % H2S and 15 % CO:! in nitrogen. The optimum gas and liquid sllPcrlicial

velocities were estimated.

The design was carried out under the following assumptions:

1. The gas flow rate does not change significantly betwecn the intet

and the oudet since the amount \lI' gas absorbcd is ncgligihtc

compared to the amount of nitrogcn.

2. Isothermal operation.

3. The density of gas and tiquid strcams is constant.

4. Constant pressure ( 1 alm.).

5. Only 75 % of the initial amount of carbonate in the liqllid stream is

consumed during the absorption of hydrogen sullide.

A.t THEORY OF H2S ABSORPTION:

The absorption of hydrogcn sultide occurs according ln the lollowmg two

reactÏons:

(A.I)

73

(A.2)

Reaction A.I IS considered to b\! the controlling reaction in this process since

hydrogcn sul/ide is ahsorhed mostly by the reaction with carbonate ions in the Iiquid

phase. Since this reaction is considered to be instantaneous and it is gas side controlled,

the ahsorption of hydrogen sulfide takes place at the gas-liquid interface which means

that the equilibrium partial pressure of H2S is zero. Thus. the rate expression for the

absorption can be wriUen as f()lIows:

(A.3)

and the height of the packed bed may bl.! writtcn as:

G'~!- l',

11=- M ft/y K. ·u· P, y

C, 1 \'"

(A.4)

where:

li: packing average specitie area for mass transfer, m2/m\

G': superticial gas velocity, 1l1OIe/m~/min.

Il: height of packing. m

K(J: overallmass transt'er coetlicient in the ga~ phase. mole/m2flmin.

l'II~~, partial pressure of 112S in the gas bulk. atm,

l',: total pressure in the column, alm

~H : rate of dhsorption of H2S. mole/min.

YII . .,: mole fraction of Il,S in the gas bulk.

Y, andY~:ll1ole fraction of 11 2S in the gas phase at the inlet and at the outlet of the

column respccti"c\y.

74

A.2 COMPOSITION AND PROPERTIES OF STREAMS:

A.2.1 LIQUID STREAM:

The liquid stream cnters the colllmn at atmosphcric pressure and ~5 O( '. and il has

a concentration of 10 % 11'/" as Na::!C01 which corresponds to a Illolarily (Ill) of

0.095 r,wlell. The solution dcnslty (P,). and \ iscosity Ul,) as gi"en h)' Ila""I.:' 1 aIe

1007 kg/m'and 1.15x 10-1 kg/m/sec. respcctivcly.

A.2.2 GAS STREAM:

The propertics and tl1I' composition of the inlct und the oullet gas streams are gi ven in

Table A.I.

Table A.I Composition and propl'rtleS orthe ga~ stream at the II1lct and at the out Id 01 the LOhllllll

COMPONENT M wt.. ~/m()le Uensity. k~/m' .' • \1

" • 1

H2S 34 1.518 0.0150 O.()()O77

CO2 44 1.964 0.1500 O.1J910

N2 28 1 250 0.8350 0.86020

The average density (Pu) and molccular welght (fl.-/) allhe II1lcl of Ihe COIUIlIl1 arc '"

30.49 g/mole and 1.361 kwm'. respectivcly.

A.3 CALCULA TIONS

A.3.t LIQUID/GAS RATIO

The ratio of Iiquid slIpcrficial vclocity to that 01 the gas (L'/(i'). 15 calculated hy

means of Equation A.5 as:

L' L _=_= __ Yn-Y X p, (A.5) G' G 075 M M

7')

!:.:.. = L = __ 1 _ x 0.01500-1) 00077 x J.,907 = 6 596 G' G 0.75 0.095 30 49

A.3.2 (;AS AND LIQUID FLOW RATES:

lo determine the supcrficial gas vc10city (G1), figure A.l is used. First, the term

[( I .. /G)(p",/Cr, - pg) r 5] whieh corresponds to the x-axis was calculated. and under the

abovc conditions it has a value of (0243) , and then the corresponding y-axis value

[G,2 C, ~l/J /(rH/(p,- rJ)~.] was determined at 50 % of the tlooding line. which was

'()Und to be equivalcnt to (0.02). Then, the optimum superficial gas velocity, G', is

calculated, i.e.

G= (A.6)

Taking into aeeount that J=I. ~(=l and the packing factor (Cf) for 1/4" Inta/ox

.')'udlile.\' is 725, G' was found to he 0.2745 kg/m2/.\'ec. which is equivalent to a total

volumetrie flow rate of 55.18 //m;l1. irthe diameler of the column is assumed to he 3".

At a ratio of 6.596 the superticial liquid velocity is ea1culated l'rom Equation A.5

and \Vas found to he 1.838 kg/m21.\'t!c. which is equivalent to a totalliquid volumetrie flow

rate l)f 0.50 //mi".

76

A.3.3 ESTIMATION OFTHE BED HEIGHT:

To evaluatc the requircd bed hcight, Equation 1\.4 was solved ta~ing \1110 account

that a=981, PI = 1 atm., Pg= 1.36\ kg/m'. M anJ K(j=7 .26x Hr' nw/eIl1l2/mi1l lalm. as gÎ\'cll

by Garner[-I] and then, integrate fromY,,=O.O\50 to)\==0.00077. The value ol'heighl 01'

the packed bed, Il, was found to be 1,48 fil.

77

~ .. ~

o .•

010 0.01 006

00.

• : 002 l, ~ ... t i,)-

: 001 0001

0.006

000t

000 2

000 1

'- ' , ~~J ,..,..~

Ap,,",I,",II.

../ "_"'1 r.o;r-. ......

'" .!! '~/rt'. 6 J 7 • ur' N/ra'

.J- l'-,.. l'-- ro.... ~ Z ri III

i"HzG • 1 22 •• 10" NI.' fi •

~OO -" ~ " ........ .......

,~ ~ ,...... 1 ....... p..,. t"-~

r-... 1"'- ~ ""'" -~ """", .' l'-- ~o -....

~ "," L'-t\ GII ,rnau~ 1110'

" ~ ~

NI.' r"'!o. ... ~ '" - - ~ ~ l~ III

'" ~ " 1\ 1"

l' r\. ~~

'" "",1'1'

001 OO! 004 0 1 0 :! o. 1 0 2 • 10

!:. (2L)"1 Ci' "-"

fïgllre .'l.I Flnodmg and pressure drop in random packcd columns[l9]

78

APPENDIX B

AXIAL DISPERSION PLUG FLOW MODEL

This model describes the tlow of a non reactlve component thmug,h a hcd or incll

packing material wherc the effccts of aB mcchamsms th ut contrihutc!'> tn the axial mi.xl\1g

ure lumped together into a !>ingle clTcctive dispersion coefficient (0,,). Il \\'a~ tkvolped by

Brenner[J 2] in 1962. And it \Vas uscd in order to obtaill the l'eclct Ilumbcr values that lit

the experimental residence time distributions 111 the ga!> phase in the uhsorplion colull1ll.

The transport equation which is governing this modd ts:

oc oc El c -+U·-=D -, d at ax 8x'

( B.2)

where c: is the concentration of the non-reactive component. , is the tin1l': .. \ IS distance.:

along the direction of the f10w and U is the supcrtieial vclocity or the Iluid throllgh the

bed. In a dimensionless form, Equation B.2 may be written as:

ac ac 1 a2 C -+-=---Be ax Pe a.x2

(B.2)

Here. distances arc rendered dimentionless with respcct tn the hcght of the packed

bed (h), the concentration by the Înlet concentration (c,,) and the tl/ne hy the average

residcnce time of the tluid in the bed (0,,) while Pc is the P(,!Clet nllm ber whu,;h l~ defincd

by means of Equation B.3:

79

hU Pe=-

0" (8.3)

The initial and boundary conditions which reprcsent a step change in the feed

concentration arc:

at 0=0 ('=0 for 0::; X sI (B.4)

at .,Y = 0 ('=1+_1 Be lor e > 0 (B.5) Pe BX

at .\'=1 ae -=0 for e > 0 (B.6) ax

Equations B.5 and B.6 give the Dankwerts boundery conditions that arc written

gcnerally for packcd beds.

The analytical solution for this model was obtained by B/'enner[l2]. The

symptotic solution at large values of Pc was expressed in terms of the dimensionless

lime and Pcclct numbcr as shown below.

C,,=I--erjc - (1-0) +-- -- exp X\I/('t) (B.7) 1 [( P) 1 1 1 (e ) 5 [P(1- 0 )2] 2 0 0 + 1 ~1tP 0

whcrc 1" and 'l' arc dctincd by the following two equations

() "C=--

0+1 (8.8)

80

) ( ') t(l-t)( ') 'It(-t = 1-6t+4t" - \-lth+24t- t ... + 2P

( )J.[t(l-t)]J. ) ... + -1 P x (1)( t (B.9)

where k is a summntion index and P is equivalenl to (Pel 4 }and the rUl1cliol1 (Il( t )) IS given by Equation B. 10

<D(-r) == 1.3 ... (2 k+ \)(_1 -- 6t: + 4(k+ 1)-r2 ) 2k+1 (1l.IO)

The values of Peclct numbcr for an experimel1lal breakthrollgh 01 LI 1l0n-IC'H:tIVC

component can be obtained by comparing the cxpcrimcntal RTf) with li 1111111crical solution of Equation B. 7

XI

APPENDIX C

DATA AND CALCULATIONS

The following table shows the opcratmg conditions or each l'lill in the tinal

experimental part ofthis study. And it also shows the conccntrations of hydrogcll slIltilk

and carbon dioxide mcasured at the outlet of the columll. Ih: Icmoval crticlcncics \lf

both gases were calculated in aecordance to the !()llowlIlg equations.

where:

11 = P Il - [> 1 X 100 %

Pli (<.'.1 )

Po and PI are the inlet and the outlet partial pressures of the absOl hcd gas and

they are obtained by means of Equation 4.2

P=y'[> 1

({' 2)

where:

PI is the total pressure, 1.05 alm.

H2

Table ( . J ()pcratllll.! Londltlon and rcmoval ct ficicnCiC'i of hydrogen sulfide and carbon dJoxlde 111 the tinal c,<pcrlllIcnt.II work

NO. r IC03 1 Yjll~~) JI (CO.,) " - G L YI(II2S) )'I(C02) 11(H2S) 11 (C02)

"( . mo/eli //mm 1/111/11 /!/ //) %

B.I 24 o 4X7 (l (J009~ () 0101 20.19 1.02 0.00025 () 0092 74.95 8.68

B.2 24 2.019 () 00400 0.0399 29.84 1.03 0.00067 0.0366 83.15 7.96

B.3 24 1 479 000692 (J.()702 39.82 098 0.00074 0.0648 89.35 7.61

BA 24 5098 O.OIO()X 01008 50.46 0.99 0.00080 0.0929 92.04 7.75

B.5 36 o 4X7 () 004(15 () 0699 50.03 1.02 0.00074 0.0612 81.64 12.26

136 16 2.019 () 00103 (J.IOI3 4026 1 03 0.00021 00883 79.81 11.67

B7 )C) 3479 001021 () 0099 30.09 0.98 0.00129 0.0090 87.32 8.61

B.X 36 5.098 o (JO'/OO (J 0199 19.87 () 99 0.00105 00362 84.9Q 907

B.9 48 04";7 () 00709 () 0990 30.36 1.02 0.00150 0.0851 78.84 14.02

B.IO 48 2.019 0.00992 0.0697 20.33 1.03 000187 0.0612 81.15 12.20

B Il 48 3.479 o (J0102 () 0403 49.76 0.98 0.00017 00354 83.50 II.90

B.12 48 5.098 () 00392 0.0101 40.19 0.99 0.00054 0.0090 86.18 10.43

B.13 60 0.487 0.01032 0.0405 39.78 1.02 0.00196 0.0348 81.05 13.93

B.14 (,0 2.019 000703 0.0\02 50.29 1.03 0.00092 00089 86.85 Il.95

B 15 60 3.479 o O()W6 () 1009 20.33 0.98 0.00077 0.0885 80.56 12.22

B 16 60 .5 098 (),()O097 0.0703 30.11 099 0.00020 00616 79.70 12.30

CI 42 2.765 (J.00559 0.0554 2019 1.04 0.00109 00495 80.41 10.54

(',2 42 2765 () 00541 00552 29.92 1.04 0.00096 00493 82.25 10.72

CJ 42 2765 0.00553 0.0547 39.89 104 0.00088 00487 84.08 10.91

C.4 42 2.765 0.00561 00549 5016 1.04 0.00079 0.0487 85.92 Il 09

(',5 42 2.7(15 () 0052-+ () 0549 '14.89 0.99 0.00089 0.0490 83.06 10.71

D.I 42 .2 759 0.00556 0.0549 3489 0.52 0.00096 0.0497 82.71 9.35

)).2 42 2.759 0.00556 0.0549 34.89 084 0.00094 0.0492 83.01 10.33

lL\ 42 .2 759 0.00556 o 0549 34.89 1.\8 0.00093 0.0487 83.32 Il.31

D,4 42 2.759 000556 0.0549 34.89 1.61 0.00091 0.0481 83.61 12.28

1).5 ·~2 2.75C) 000556 0.0555 35.53 1.09 0.00093 0.0494 83.26 10.93

83

The concentration of HS-. IICO, - and COl I11casurcd 111 outle! liquid sllcam lilr

each mn are given in Table C 2. fhe t Iatio and thc eqllllihriul11 partial pn.'ssurcs llf

hydrogen sultide and carbon dioxide above the out let liqUld stream \\cre cakulatcd lIsing

Equations C.3. C.4 and C.5 respccti"cly. Data arc listel! in the samc table.

where:

where:

l= [HCO~] [ HCO~] + 2 x [C03]

(t '.3)

[ HCO;] and [CO;] arc the I11casured conccntrations or bicarbonate ilnd

carbonate at the bottom of the column.

Z ! l,62

p·(CO..,)=1.654xIO 1 MNol exp(-2729/Tl - (I-{)

.( s) -4 p·(c )M 125 [I1S'-] ("""75/7') PH., =9746x10 0') N-[ _]exp __ _ - - ., lIeo 1

(('.5 )

p.: the equilibrium partial prcssure ahovc the solution. atm.

MNa:the concentration ofsodium ions ln the solution and il is given as twice the

inlet concentration of carbonate "sted in Table C. L mo/elf.

j: the fraction of sodium ions as sodium bicarbonate in thc 'iOdlUlll carhonate­

bicarbonate solution,

T: the operatmg tcmpcraturc. "K.

Pu' and ~.

114

li/hIe C2 C ol1Lcntrallol1 of IOOle \pecle~ ln the eXit IIqUid stream and the cqUlllbrium partial prc<;\lIn!~ of hvdro!!t!n ~ulfide and t.arbon dlox Ide - ~

NO. (IIS-, (lIeO;, (CO;I f P'(II,S) () 4

P,,·(CO:.J

/IIo/ell molell m()lell atm, alm,

B,I 000063 (J,O 1 03 (J 477 (J,OIOR 5 1 7x 1 0") 1 91x 1 a'('

13,2 0.00415 0.0265 1992 0,0066 2.55x 10'x 4.97x 1 a'(' ,-lU OOlOX2 0,0443 3.435 0.0064 8.1 6x lO'x 9.82x 1 0'1'

BA 002037 0.0668 ~ 031 () 0066 I.96x 10,7 1.75xlO,5

13.5 O'()O671 00373 0.450 0,0399 8.79xlO,7 3.85x 1 0,5

B,6 000133 0.0473 1.91'2 0.0119 2.84x 10'x 2.30x 1 0"

B.7 001133 Il 0481 ] 431 0,0070 1.07x]((' 1.65x1O"

B,X 000494 (J,0512 5047 00051 2.94xlO'x IA6x 1 0"

B,l) 0.00663 0,0421 0.445 00452 1.20xlO'h 6.93x 1 0"

B.IO () 00633 00522 1.967 0.0131 I.74xl0,7 3.90x 1 0,5

B.II 0.00172 00629 3.416 0.0091 2.97xlO'x 3.96x 1 0.5

B.12 0.00547 0,069(1 5,028 0.0069 6.39x1O's 3.77x 10'5

B.I] 0.01253 00"173 0.440 0.051 1 3.IOxlO'll 1.21 x 1 O'~

B.14 001145 0.0680 1.951 0,01 ï2 5.76x1O,7 9.12x 1 0,5

B.15 () 00254 00839 3.395 0,0122 8.32xlO,l! 9.65x 10"

III Cl O,OOOl)O 00978 5000 0,0097 2.22xHr!! I.02x 1 O'~

CI O.(lO]54 0,0464 2.716 0,0084 458xlO'x 2.12xlfr'

C2 0.00520 0,0504 2712 0,0092 7.94xlO'!! 2.5lxl0"

CJ 0.00724 0,0548 2.707 0.0100 1.32xlO·7 2.97x 1 0,5

CA (),OO944 0,0596 2.702 0.0109 2.04x1O,7 3.53 xl 0,5

C5 000623 0.0524 2706 0,0097 I.05xIO,7 2.77x 1 0,5

D,I o (l1253 (),0648 2,694 0011 R 3.21xlO'7 4, 18x 10'5

D.2 0,00779 00556 2703 0,0101 1.46x 10,7 3.06x 1 0"

DJ 000556 0.0513 2.708 0,0093 8.82xlO'X 2.60x 1 0"

DA 0,00409 0,0483 2.711 0,0088 5.74xlO'K 2.30x 10'5

D.5 000613 00525 2.712 0,0097 1.02xlO,7 2.72x 1 0.5

85

From the data given in Table c.l. the absorption rates nI' hydrngen sultide and

carbon dioxidc \Vere calculated for each run b~ l11eans or Fquatilll1 C.6

where:

G·p ) 9t = __ 1 x (v -.' RT . n .1

G: gas now rate. IImil1.

PT: total opcrating pressure. 1 OS atm.

R :gas constant. 0082057 (! atlll)/(mo/e "K).

T: operating temperature. "K

j C.6)

Yo and YI are the inlet in nutlet 'I:;S and CO~ mole tiactwll in the gas sllCHm

respectively.

Then, using the data of Table C .2. the logarithmic mean prcssure dllïèl"l:ncc t\ I~,\I

for hydrogen sulfide and carbon dioxide \Vere calculatcd assuming that the equilihlllllll

partial pressures If, of bath gases ubove the inlct liquld stream 15 Icro. i.e.

(<.'.7)

The overall mass tnmstè:r cocrtlcicnts Kc,' lor hydrogen sul/iùe and carhon dio.xide

were caleulate using their absorption rates and thcir logarithmic Illcun rn:ssurc

differences as follows:

where:

~H K G =----=-

J (/. A. h· I!. P lAI

KG: overall mass transfer coefticicnt. mo/e/min.lm%l1m

a: specifie area of the packmg matcnaI. 9RI m'/ml

•

A: cross scctional arca orthe bcd, 4.56x\{f\ m2•

h: hcight of the packed bcd. 1.50 m

Then, the sclccflvity factor S, \Vas cvaluatcd as the ration of the overall mass

trans/cr coefficients, i .c.

K (Il,S) /}'= (, .

K (CO,) (,

(C.9)

Data ohtaincd from lhcse calculations are tabulated in Table C.3

87

Table CoI Absorption rates and the ovemll ma~s translel coetliclt:llh and tht: .,elecll\ 11\ t.lctllr

No. ~H(H2S) ~ P'1/(I1,S) K(, (II~S) ~H(C02 ) :\1'/1 ' ( CO: ) K(I(CO~ ) S

!nole/mm (//11/ II/oi/mm '(//m 'II/~ 11/ ni<' 1//1/1 ,am !IIO/IIIIII a/III 11/

8.1 0.00064 0.0012 0.0756 000076 o 022X () (}O4 l ) 15 Ihh r---

8.2 0.00428 0.0044 O.14J5 n.0040S 0.0901 () OOh 7 ~1 32X

B.3 0.01060 0.0065 0.2406 000917 0.1593 () OOX 5 2X Oh!)

B.4 0.02020 0.0087 03441 n.o 1700 () 2284 (l(l110 31 ()X9

8.5 0.00684 0.0046 0.2201 0.01770 () 15-l5 () () \70 12 X77

B.6 0.00137 0.0012 () 1673 001950 o 2220 Il 0130 12 XIX --

B.7 0.01110 00102 0.1611 0.00106 () 0222 () 0070 22 74~ -

8.8 0.004R9 0.0074 (J.097R 0.00298 () 0897 () O()4l) 1 () X'I')

8.9 0.00676 0.00R5 o 1179 0.01(;80 () 2 {(lX (lO115 1 () 223

8.10 0.00652 0.0114 0.0848 0.00689 0.1543 (). ()()( )() 12 7·H) --

. 8.11 0.00169 0.001 1 0.2241 0.00951 () 0892 o Ol5X 14.125

B.t2 0.00541 0.0040 0.1986 000169 0.0225 () 0 1 1 1 17 ~()6

8.13 0.01280 0.0119 0.1595 000863 O.08X7 o () 144 1 1 () JO

B.14 0.01180 00071 02456 000236 o 0225 ()O155 157'i7

B.15 0.00249 0.0046 00802 O.Ü0963 0.2232 () ()OM 12A87

B.16 0.00089 0.0011 0.1157 0.01000 0.1555 o 0(1)') 12071

C.l 0.00369 0.0065 o 083R 0.00479 o 1237 o O()57 14 548

C.2 0.00541 0.0061 0.1317 0.00719 () 1212 () OOS7 15.145

C.3 0.00753 0.0060 o 1867 0.00967 O.121!) (JOlIS J ') 71)!)

C.4 0.00982 0.0058 0.2503 0.01240 () 1221 ()O151 \() 555

C.5 0.00617 0.0058 () 1578 0.00833 o 1225 () 0101 15 'iM

D.l 0.00652 0.0062 () 1560 O.OO72S 0.1234 () 0087 17 7(,0

D.2 0.00654 00061 o 1575 O.OOS04 o 1227 O.O()<J7 1 h 150

D.3 0.00656 00061 0.1591 O.OOSSO o 1221 (J.O 1 07 14 S20

D.4 0.00659 0.0061 0.1607 0.00956 0.1215 o () 117 11 711

D.5 0.00669 0.0062 0.1616 0.00&77 0.1236 () 0105 15.344

The cf fect of the individual pararneters on the removal efficiency and the overall

I11USS trans/cr coefficient of hydrogen ~ulfide and carbon dIOXidc. and on the scleCl!vity

fuctor were evaluated l'rom the prcvious tahles. An averaging technique \Vas followed to

ohtain the corrc~ponding values 01 II ' K(, and S at euch level for ail the parameters,

exccpt 111 the cu,>c of the hqUld flow rate. where the results were obtained directly through

the sel of expenrnents. D.

ln the experimental design, cach parameter was repeated at the sarne level only

four tlllles in the wholc sel of cxpenrnents (set B). Theretor. the corresponding value of

II ,K(, and S (at level, 1 and for pararnctcr. A) arc given by the average of the tour values

where (1\) is at Icvel (i).

EXilmplc:

The rernoval efficicncy of hydrogen sulfide as a function of its inlet concentration,

may bc obtained l'rom Table CI at tour levels.

at levcl 1 JI . "' = (Yom. 1 )+Y,,(B.6)+Yo(B.ll )+Y()(B.16))/4

= (0.00098+0.00103+0.00102+0.00097)4 = 0.00100 at level 2 Y",2 =(Y,,(B.2)+Y,,(B.5)+YJB.12)+Y,,(B.15))/4

=(0.00400+0,00405+0.00392+0.00396)/4 == 0.00398 atlevel3 J'

- Il,' ~-::(Y,,(8.3 )+J',,( B,8)+)',,(R.9)+Y,,( 8.14»/4

=(0.00692+0,00700+0,00709+0,00703)/4 = 0.00701 at levcl4 )' . Il'' =(Yn~B.4)+J',,(B. 7)+YJR.I O)+Y.,(B, 13»/4

=(0.01008+0,001021 +0.00992+0,0 i OJ2)/4 = 0.01 013

similarly. at Icvcl 1 11, =(11(13.1 )+1l(B,6)+11(B.l1 )+11(B.16»/4

=( 74.95+ 79.81 +83.50 t 79.70)/4 =79.49 at Icvel 2 ll" =-( 11(B.2)+11(R.5)+11(B.12) tl1( 8.15))/4

=( 83 .15+81.64+86.18+80.56)/4 =82.88 at Icycl .3 '11 =( 11(B.3)+11(B.8)+11(B.9)+1l( 8.14 ))/4

=(89.35+84.99+78.84+86.85)/4 =85.01 al Icvel 3 111 =(11(B.4)+'1(B. 7)+11(B.1O)+11( 8.13))/4

=(92.04+87.32+81.15+81.05)/4 =85.39

89

In addition to these four data points. the removal efticiency llfhydwgcn sultidc al

level III (medium level)of is obtaincd by taking the average value nt" 1) nhla1l1cd fwm run

no. C.5 and 0.5, i.e.

at levelm 11 =(1)(C.5)+(11 0.5))/2

=(83.06+83.26)/2

Note that if the rest of pararnctcrs (T, COl ~,Y.,(CO:!'., (i and L) \Vcrc al ~.vcragcd

the different tive Icvels of Y,,(H:!S). cach parameter will show a constant valuc,

approximately at its medium levcl.

The above calculations gave the rcmoval cfticiency of hydrogcn sullidc as LI

function of the inlet concentration of 11 2S at live levcls. Similurly, ail thc data gl\'cn III

Table C.I and those in Table C.3 have avcraged using the same pl Ol:ctlure. And the

rernoval efficiencies and the mass transfcr coefficients as \vell as the seb:tlvlly ladol ail

function of the different operating parameters \Vere obtained. Urcel 01 liquld and gas

flow rates were expressed by means of the superticial velocities or the gas and the hquid

strcarns, L' and 6. i.e.

where:

L'= LI A

G'=GI A

Land G are the liquid and gas l10w rates respectively

A is the cross sectional area of the absorption column, 4.56x 1 0'\ /Il! .

(C.IO)

Results of these calculatlOns arc listcd in TabIc C.4 through Tahlc c.) O. Tahlc C.X

shows the effect of the superficial gas velocity ohtaincd l'rom this cxperimcntal design,

while Table C.9 shows the same crfeet obtatncd l'rom the conventlonal cxpcnlncntal

work. It is obvious that the data ln both tablc~ arc almost Idcntical. which mcans tl1at the

experimental design procedure followed in this ')tudy gave accuratc re~ult~

90

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Table C4 Errec! of temperature on the remO\ al efficlenc) and the over ail mass transfer coefficient of 11 2S and COl and on the selectivltv factor

NO. T ICO~I Y (H,S) Y.,(C02) G' L' llOI~S) 11(C02) K(,(1I2S) K(,(C02) S () -

Oc' l1Iole 1 111/111 III 111 1111111 % % mol, 1111/1 , atm /m~ mol/mm /atm ,'m2

1 24 2771 0.0055 0.0552 7.69 0.22 84.87 8.00 0.2010 0.0078 23.91

2 36 2.771 00056 0.0549 7.69 0.22 83.4-1 10.40 0.16/6 0.0/05 17.06

" 42 2.762 0.0054 00552 7.72 0.23 83.16 10.82 0.1598 0.0103 15.45 .)

4 -18 2771 0.0055 0.0548 7.71 0.22 82.42 12.14 0.1564 0.0113 13.73

5 60 2.771 0.0056 0.0555 7.70 l 0.22 82.04 12.6C 0.1503 0.0115 12.83 ---_.- ------- ----- ------ -

Table C" Efkct l,f ';.ubon:lle concl'ntrallon on the rell1O\al effïcil'nc) .1I1d the o\cr allll1d~s transrer coeffïclent of !lèS and CO2

and on the selectl\ Il:- factor

~o. T ICO;I )' (11,,\) )'(C(),) G' L' Tl(H,S) ll( CO,) K(,O I::!S) K(,(C02 ) S " - " -

C II/oh' i 11/ 11/ /11 1/1 11/ 11/ n 'u 11101 111"1 (/11/' 111' 11101 111111 alm f;j' '" " 1 -L2 O.4S7 o (lO56 00549 769 U 22 79./2 12.22 0.1433 0.0120 12.32 !

., f, 2.lH9 00055 00550 771 023 82.74 10.95 0.1604 0.0105 /5.66 - .... -

... 42 ~.76~ 00054 0055: 772 023 83./6 10.82 0./598 0.0/03 /5.45 -'

4 42 3.479 00055 00553 7.67 021 85.18 10.09 0./766 0.0095 19.36

5 42 5.098 0.0055 0.0553 7.71 0.22 85.73 9.89 0.1891 0.0092 20.19

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Tahle (,6 f.îf':Cl "f fI," ml.::t .:onc.:ntrJllc'll l'n rh" rè010\31 ètficlèllC; JnJ rh,' c'h'r ,III m,l" tr,1n~tè"r cl'dliclcm ,)fH,~ ,li1J CC), - ' ,

an,j on the ~elecu\ It\ factùr

~o. T {CO) 1 y (Il, <..,) Y(C(),)' G' L' 11(1--!:"»[ T1(Co:) 1\ (l1,S) l. _ 1\)1...'0: ) S

( /111)": 1 III m 1/1 mil/III " " 0) mol 1/1111 Li/ll/ 111= II/(l! IIlIII ,lflll /1/= '1

1 -t.2 :2 f7i 0.0010 00552 769 022 i9.49 Il. 14 O.145i O.OI09 /335

2 42 2 771 0.0040 U 0552 769 022 82.88 10.72 0.160-; 0.0104 16.12

.... 42 '2762 0.0054 00552 772 0.23 83.16 /0.82 0.1598 0.0103 /5.45 .)

i 4 41 2771 0.0070 0.0548 7.69 0.11 85.01 1 10.66 0.1755 0.0102 18.46

5 42 2771 0.0101 0.0552 7.71 0.22 85.39 10.62 0.1874 0.0098 19.40

7 able C 7 Effett of CO, mlet conccntratlon on the remo\ul effierenc\ and the 0\ er ,III tJl,I~S Iransfer eoeflierent 01 !l,S and CO, and on the selccliv It\ fac-tor J - -

NO. T ICO;I Y.,( Il, ~) )'jC():) G' L' l1(H,,,) ll(COJ K(,{ Il:S) K (CO,) (, - S

"c II/v/e / m II/Ill m 1111/1 ?~ IJ' /0 lIIo/mlll :allll 1Il~ mo/m/11 atm m~

1 42 2.771 00055 0.0101 7.72 0.22 83.82 9.92 0.1703 (}.0097 17.87

Î 42 2.771 00056 0.0401 7.63 0.22 83.17 10.72 0.1563 0.0105 16.57 .... 42 2.762 0.0054 0.0552 7.72 0.23 83.16 10.82 0.1598 0.0103 15.45 .)

4 42 2.771 0.0055 0.0700 7.69 0.22 82.96 11.09 0.1653 0.0105 16.44

i

5 42 2.771 0.0055 0.1002 7.75 0.22 82.81 11.42 0.1775 0.0105 16.65 !

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N =-

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Table CI~ Erfcct of the gas superficwl \cloclt) 011 the rem 0\ al eftïclenc) and the 0\ er ail mass tlansfer coeftïclent of H2S and

CO, and on the selectlvlty factor (duta abtllllled \1'/(11 the etpCI Imentul design) -NO. T ICO;I Y (1),';;) Y,,(0)2) G' L' l1(1I~S) 11«('°2) Kd H2S) K(,(C02 ) S

" -"( , mole / 1/1 111 1 1/ /11, 1/1//1

()/ 0/ mO/'IIll11 a[1II '/11' mO"'I1//1 atm Ill' /u ,ri

1 42 2771 00055 0.0552 4.43 0.22 80.41 10.54 0.0846 0.0058 15.05

'1 42 2771 00056 00548 6.60 022 82.25 /0.72 0./346 0.0087 /6.59 -3 42 2762 0.0054 0.0552 7.72 0.23 83.16 10.82 0.1598 0.0103 15.45

4 42 2 771 00055 00552 8.77 0.22 84.10 10.91 0.1916 0.OIl8 17.43

5 42 2.771 0.0055 0.0553 10.99 0.22 86.01 10.97 0.2585 0.0149 18.46 ------

Ji/hie C 9 l:tkct of the gJS ~llperlicJaI \c1uclt;. (Ill the rellll)\al efticllml;' JilL! the mer allma<;<; translà c(ldticlent uf H2~ and CO, .md l'Il the ~eh:ctl\ Il;' factor (d.i/Li ONd"I"'; 111111 tlll! COIll Cnl/o/1<..,/ l:\r-'rJ/III!I1f<l/ 11 ()/,,)

~o. r ICO~I 3 Y,(II:S) .r (( ( ), ) G' L' 11 (II: ~; 11(CO:) K(,I Il: S) K(,(l():.) S

C fIIO/,' / !II Ill/II III 1/1111 " 0 mol III/Il a(1II 111= mol mm 1.1(11/ 111= ~" , ,

1 42 2 Î62 o OÜ56 l) 0554 4.43 023 80.41 10.54 0.0839 0.0058 14.55

" 4:2 :2 76:2 () U054 0055:2 6.56 023 82.25 /O.i2 0.JJ/8 0.0087 /5.15 -~ 42 2 ï«~ 00054 Ü 0552 7.ï2 U 23 83.16 10.82 0.1598 0.0103 15.45 -'

4 4~ ~ 762 00055 o 0547 8.75 023 84.08 /0.91 0./867 0.01l8 /5.80

5 42 2.762 0.0056 00549 11.00 023 85.92 11.09 0.2503 0.0151 16.56

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l 1

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...., 0'

Table C8 EffLCt of the gas '-.Upèl fÏclal \ èloCIl~ (ln the remo\ a! èrticlenc~ and the L'\ er .111 111.155 transfer coeftïClènt l)f Il,~ .111..1 CO, and on the 'ielcctl\ It~ factor -

-

~O. T ICO;I .r(ll/; ) J' ,( ( ():) G' L' Tl(H:~) 11(COJ K[,(II:S) K t ,( C(2) s nc mole 1 n, 0

mn/III/Il ,11111 m' mO/1II111 ,UII1 /1/' 11/ III 1 Il /r/ mlll " <)

1 42 :. 759 () ()O56 00549 765- 0./1 82.71 9.35 0.1560 0.0088 1 Î. -r6

2 42 2,759 o 0056 o 0549 7,65 0.18 83.01 10.33 0.1576 0.0098 /6./5

.., 42 2 762 00054 o 0552 7.72 (J.23 83.16 10.82 0.1598 0.0/03 /i45 -'

-+ 42 2759 <L0056 0.0549 7.65 0.26 83.32 Il.31 0.1591 0.0107 14.82

5 42 2.759 0.0056 0.0549 7.65 0.35 83.61 12.28 0.1607 0.0117 13.71 ....

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