PSA modeling 1.pdf

7/27/2019 PSA modeling 1.pdf

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Simulation of Pressure Swing Adsorption for Air Separation

K.Chihara and Y.YonedaDept. of Ind. Chern., Faculty of Eng., Meiji Univers ity, Higas i-mi ta , Tama-ku,Kawasaki, 214, JAPANS.MorishitaToyo Soda Kogyo K.K. Iwasekoshimachi, Toyama, 931, JAPANM.SuzukiInstitute of Industrial Science, The University of Tokyo, Roppongi, Minato-ku,Tokyo, 106, JAPAN

Experimental data of a ir separation for oxygen enrichment bypressure swing adsorption (PSA) were obtained, using syntheticzeolite as adsorbent, at various conditions of flow rate, cycletime and production portion in half cycle time. PSA simulationmethod (called as Stop-Go method) was applied to this air separation. This Stop-Go method is a kind of cell model accounting forthe overall adsorption rate ( i ,e , the column is supposed to bedivided in axial dir ec tion to several tens of cells) and consistsof alternate calculation repetetion of adsorption or desorptionamount in cells and flow amount between cells fo r each small timeincrement at each pressurization, adsorption and evacuation step.Experimental results were well simulated by this Stop-Go method,that is , the effects of operational factors, such as flow rate andcycle time, on the performance of air separation PSA were wellexplained as well as the absolute values of yield and productpurity were estimated. Also, axial distributions of oxygen andnitrogen in the column were found to be resonably predicted.

INTRODUCTIONPressure swing adsorption is widely used fo r gas separation, but there had beenl i t t le published articles[6,S] concerned with the mathematical modelling and simulation of the pressure swing adsorption process except a linear equilibrium modelbased on the method of characteristics [2,10]. I t seems to be necessary fo r PSAsimulation to use numerical calculation by computer for accounting fo r non-linearadsorption isotherm of multi-components, velocity change in axia l d i rection, intraparticle diffusion, non-isothermal condition and complicated boundary condition incyclic operation, etc. Recently, such numerical simulations were reported fo r various types of PSA [1,4,5,9,11,12]. Among them, following two papers deal with thechange in mass flow. Fernandez and Kenney [5] proposed a cell model with theassumption of instantaneous equilibrium and applied i t to air separation PSA fo roxygen enrichment. Yang and Doong [12] reported a complicated mathematical modelaccounting for everything mentioned above and applied i t to H2-CHl separation.The object of this investization is the application of a rather simple PSAsimulation method called as Stop-Go method proposed by two of the authors to airseparation for oxygen enrichment by zeolite. This method accounts for multi component non-linear isotherm, overall adsorption rate, change in mass flow and variousboundary conditions in isothermal condition.

563


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564 (AD-6-4)EXPERIMENTALThe schematic diagram of the apparatus for PSA operation of O

2separation from

air is shown in Figure 1. Synthetic zeolite (provided by Toyo Soda KogyoK.K, Sample No.AC6184) was packed in two Pyrex glass tubes as the adsorbent columnof PSA. In cyclic operation, pressurized a ir flows into one of the columns, whichadsorbs nitrogen more than oxygen. While th e oxygen is allowed to pass through th ecolumn. After th e bed is fully util ized, th e feed a ir is automatically switchedover the second column, a llowing the f i rs t to be regenerated by evacuation. Thepressurized feed air is provided from compressor, passing through a sil icagelcolumn for drying, to th e adsorbent columns. The maximum pressure inside th e columnat adsorption step is controlled by the pressure regulator, which is set betweencompressor and the sil icagel column. Feed flow rate was set by a flow controlvalve. Effluent flow was controlled by needle valve. Both flow rate were measuredusing soap f low meter. Pressure change at pressurizat ion step, adsorption step andevacuation step was recorded. Effluent and evacuated gas volume were measured bywater-displacement. Oxygen concentrat ion of product and waste gas were measured byintroducing the t rapped gas into th e oxygen analyzer. All th e sequence of solenoidvalve swi tching in cyclic operation was controlled by keyboard programmer. In thisexperiment, repressure time plus production time is equal to evacuation time in onecycle. Experimental temperature was ambient temperature. Temperatures inside thecolumn were measured at two point on th e axial l ine. Properties of adsorbent andth e packed column, and experimental conditions are l isted in Table 1. The adsorpt ion charac te r is t ics of adsorbent ar e explained later .

STOP-GO METHOD FOR PSA SIMULATIONStop-Go method is descr ibed as follows (Fig.2). Each adsorption column isdivided to N complete mixed cells in a xial direction (40 cells in this study).While f low between cells is stopped for each small time increment, ~t , adsorbed ordesorbed amount of each component for this interval, ~q . is calculated by Eq.(1).n,~

yd t

(1 )

Here equilibrium amount adsorbed of each component is expressed as Eq.(2) for 0Z-Nsystem on zeolite. These equations are the loading ratio correlation (LRC equat~ont[12]

q. = (2 )

Adsorption equilibria can be changed corresponding to the adsorpt ion system studied. The gas phase amount remained is determined by mass balance in each cel l .After this Stop-calculation, this remaining gas amount of a l l the cells and inletand outlet gas for ~t are summed up to get th e column pressure after ~t. Flowamount between cells Q ~t in Eq.( J) , corresponding to this time interval, ~t , isobtained to compensae for the p re ssure sho rtage o r excess to thus obtainedcolumn pressure. (Go-calculation)mN d t +

tVN

den. ;d t = QnCn -L i - Qn+1Cn, i

(3)

For instance, a t adsorption step, flow amount from 1st cel l to 2nd cel l from th ein le t of the column is obtained as the surplus of the inlet quant ity for ~t, Q1~t ,subtracted the gas amount for f i l l ing the f i r s t cel l to th e column pressure after


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Table 1 Propert ies of adsorbent and the packedcolumn, and experimental conditionsparticle diameterapparent particle densitymacro pore volumemacro pore radiuscolumn sizecolumn volumeadsorbentpacked amount (one column)porosity in beddensi ty of packed bedfeed pressureevacuation ratefeed rateproduction ratehalf' cycle timeproduction portionin half cycle time

[m,] 2.74[g/Jm ] 1.169[cm /g] 0.24[Um] 0.18[c,] 4.0~ * 50[cm ] 507Toyo soda zeolum[g ] 375[j] 0.4[g/2m ] 0.74[kg/cm G] 0.765[ l / , in] 140[Ncm3/ s ] 41 - 108[Ncm /s ] 5 - 22[min] 2/3 - 5[- ] 1/6 - 3/4

K. Chihara et al. 565repressure stepS t o p ~GO~adsorption stepS t op~GO~evacuation stepS t o p ~GO~

Fig. 2 Concept of Stop-Gor-------~

l

[r

II

a oxygen analyzerb needle valvec solenoid valved keyboard programmere columnf pressure sensorg soap flowh pen recorderi compressorj silica gel columnk flow control valve1 rotary vacuum pumpm o il trapn thermocoupleo three way cockp pressure regulator

Fig. 1 Apparatus of PSA


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566 (AD-6-4)~t. Then th e change of gas phase concentration in each cell by this flow Q ~t, isdetermined to satisfy Eq.(3). These Stop and Go calculat ions for ~t are r~peated

alternately for a l l th e steps, i .e . pressurization, adsorption, blow-down and purgesteps. In case of the condition studied, there was no interaction between twocolumns. Simulation was executed fo r one column behavior.EQUILIBRIUM AND KINETIC PARAMETERS FOR SIMULATIONFigure 3 shows adsorption isotherm of oxygen and nitrogen on zeolite, obtainedby batch experiment. The sol id l in es are th e assumed isotherm of oxygen and nitrogen, respectively, for th e PSA simulation at 30C, which are expressed by Eq.(2),whose parameters ar e in Table 2. These isotherms supposed were used for a ll thesimulation in this study and gave a good results, although these ar e no t necessarily coincident with th e experimental isotherms. Before adopting Eq(2), Markam-Bentonequations were applied. However, th e results of simulations were no t so good.Adsorption rates ar e expressed as Eq.(1), using th e overall mass transfer coefficients, K a, for oxygen and nitrogen. These overall mass t ransfe r coef ficientincludes th~ vcontributions of axial dispersion in th e bed, 0d ' external masstransfer, of ' macropore diffusion, 0 , and micropore diffusion, o. [3,7]. Equation(4) is the relation between K a , 0d~ 0 and 0 . Here of was neglected because ofi ts small contr ibut ion~ 0d anB ~ were ~stimat~d accord~ng to the ref[3,7] . Adsorption coefficient, K and o. w~re determined us ing the chromatographic data fo r azeolite of similar s truc ture .~Thus estimated K a values are in Table 2.s v

=K.av {(l-f)/e* e.+p.K)2}RESULTS AND DISCUSSION1. Comparison of Experimental Data with Simulation

(4)

The comparison of experimental data with computer simulation by Stop-Go methodar e shown in Figure 4,5,6 and Table 3. These data and simulation were obtained indynamic steady state operations after several cycles from s tar t up for five 3 condit ions of half cycle time 1,2,3, j &5 min with feed flow rate of 77.5 Ncm /s andproduct gas flow rate of 10.4 Ncm /s at 30 C. Figure 4 shows th e oxygen concentrat ion change of product gas with time. Pressure change with time is shown in Figure5 fo r 2 min half cyc le t ime. Table 3 is th e comparison for mass balance, averagepurity and yield. Figure 6 is the relation between yield and aver age productpurity. Based on a l l of these comparison, i t could be said that th e experimental

1IlI% Table 2 Equilibrium & kineticparameters of adsorbent

111 1O2 N2

l>O k [1/atml 0.16 0.61.......C"\ [Ncm3/g]1IlI 1 q. . 20 23.2CJ:z:

A [-1 0.7 0.6e- n 1 Y/Ksav [sl 0.251 0.696 N2 25 C1IlI-% experimental111(% U-I 1IlI' 1IlI 1 1IlI% N2 35 Cpressure [atml O2 25 CFig. J Adsorption isotherm 30 CRC equation


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K. Chihara et al. 567behavior can be simulated by this simulation method a t this typical flow conditionwith five conditions of half cycle time. Difference for long half cycle timeconditions in Figure 4 could be corrected by accounting for the dead volume a t thecolumn outlet.Further, possibility of predict ion for the effect of other operational factorexcept cycle time on th e performance of PSA was checked. Figure 9 and 10 are thesame type of plot as Figure 6, al l of which show the relation between yield andaverage purity of product gas. In these plots, upper and right hand side is th ebetter condition, because higher yield and higher purity are desired. I t seems tobe from Figure 9 that th e predict ion of th e effect of product gas flow rate ispossible. However, the est imat ion of th e effect of th e portion of production timein. half cycle t ime has s t i l l some problem, although the absolute values are not sobad from the view point of optimization of PSA operation by simulation.

a 1.5...III~ l IM experimental::s I


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568 (AD-6-4)

t ion. These distributions at the end of each step ar e shown in Figure 7 and 8 forthree conditions of half cycle time 1,3 and 5 min with the same flow conditions inFigure 4 and 6. Such simulation of distribution might be usefull when the effectiveuse of total column l ength a re discussed fo r more sophisticated PSA operation.

--I

B3

I

73

simulation

I

experimental

oxygen concentrationof product gas (%]63

5 min" . .4 min~3min" -.2 min\\ -..1 min

211J l -

N 15 I--e lI1J... I -Ql.... 5 1-.

I1J

5353time (min]

1

CONCLUSIONA rather simple PSA simulation method called as Stop-Go method was applied toa ir separation PSA for oxygen enrichment by synthetic zeolite. The model is a kindof cell model accounting for mass transfer rate in th e bed and change in mass flow.The experimental results were well s imulated by this method, that is , th e absolutevalues of yield and average purity of product and waste gas and th e effect ofoperational factors, such as, flow rate and cycle t ime were found to be wellestimated and explained by th e Stop-Go method. 25133 min I-~ "-.c:!:!. .0 .. 2 min r... ~ B3..,'".. "..., '"llDQl 63...,c oo ""'"c ...

Ql "" 43lD ....>< 0023 simulation-- experimental

Fig. 4 Oxygen concentration changewith time in half cycleFig. 6 Effect of cycle time onthe relation of yield vsaverage product purity

33 I I I I 14 I I I I25 - .~. - 12 - -13 - (3/~23 - -..

"\. io.z B,.'/. :(1/2)

.. 8 - :A -'" 15 ... \ 1 ( 1 / 3 )....Ql .~ - '"... .... 6 - -Ql13 - .... experimental ~ 4 - -5.1 Ncm3/s_ experimental5 1-. simulation 2 - simulation -3 I I I I I I I343 53 63 73 83 93 133 75 B3 BS 93 95 lI?J3oxygen concentration oxygen concentrationof product gas (%] of product gas (%]

Fig. 9 Effect of product flow rate Fig. 10 Effect of production portionon the relation of yield vs in half cycle time on the relationaverage product purity of yield vs average product purity


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K. Chihara e t al. 569

a) 1min

.84 .6z [-1.2

evacuation

repressure--- ....'.a) 1min '\\..

42

68HI

.84 .6z [-1.2

2

~ 1.5";coco"o.....'"...~ .5:1

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- ..-,\\ .J....4 .6z [-1

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b) 3min

r":....'/0- - - - - - .=..:;;.-""'- --

.84 .6z [-1.2

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.5

1.5";coco"o.....'".... "...:1

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- - - - - - -=-:;;.... '~-=

c)

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570 (AD-6-4)NOMENCLATUREconstant in LRC equationgas phase concentrationadsorption equil ibrium constantconstant in LRC equationoverall mass transfer coefficientbased on amount adsorbedpacked weight of adsorbentNumber of cellsflow rateamount adsorbedequilibrium amount adsorbedsaturated amount adsorbedtime

column volumndensi ty o f packed bedbed porocitymacro porocityapparent particle density

REFERENCE1. J.W. Carter and D.J. Barrett, Trans. Instn. Chern. Engrs., 21, 75 (1973).2. Y.N.I. Chan, F.B. Hill and Y.W. Wong, Chern. Eng. Sci., 36, 243 (1981).3. K. Chihara, M. Suzuki and K. Kawazoe, AIChE J . , ~, 237 (1978).4. K. Chihara and M. Suzuki, J. Chern. Eng. Japan, 16, 53 (1983).5. G.F. Fernandez and C.N. Kenney, Chern. Eng. Sci.~38, 827 (1983).6. K. Kawazoe and T. Kawai, Kagaku kogaku, 37, 288 (1973).7. K. Kawazoe, Y. Takeuchi and S. Miyahara, "Adsorption and Ion Exchange," inKagaku Kogaku Binran, pp.847-910, Maruzen, Tokyo (1978).8. J.E. Mitchell and L.H. Shendalman, AIChE Symp. Ser., 69, No. 134, 25 (1973).9. N.S. Raghavan, M.M. Hassan and D.M. Ruthven, AIChE J . , 11, 385 (1985).10. L.H. Shendalman and J.E. Mitchell, Chern. Eng. Sci . , 27, 1449 (1972).11. M. Suzuki, AIChE Symp. Ser., l, No. 242, 67 (1985).12. R.T. Yang and S.J. Doong, AIChE J . , 11, 1829 (1985).

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PSA modeling 1.pdf