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System details

Figure 1

Column Configuration.

In the two-product reaction system, the column has both bottoms and distillate

products coming from the two ends of the column. The two reactant feed streams are

fed into the middle section of the column. With a one-product reaction system without

inerts, the column has only a bottoms or a distillate product. If the product component

C is heavier than the reactant components and !, there is a bottoms stream but no

distillate. The column operates at total reflu" with all of the overhead vapour

condensed and returned to the column as reflu". There is no need to have a rectifying

section because there is no distillate and no need to maintain any composition at the

top of the column. #o the column configuration in the one-product ternary system

without inerts is $uite different than the two-product configuration. Figure 1 shows

the flowsheet. There are only stripping and reactive sections.

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Chemistry and Phase Equilibrium Parameters.

1 gives %inetic and vapor-li$uid phase e$uilibrium parameters used in the numerical

case considered in this paper. The overall reaction rate on the nth tray depends on the

molar holdupM&', the specific forward and bac%ward reaction rates kFand k!, and

the li$uid mole fractions.

The e$uilibrium constant Ke$at ()) * is +. &elative volatilities are constant at +

between adacent components.

The product C is the high-boiling component, so it is removed from the bottom of the

column.

Design Parameters and Procedure. The ternary system without inerts has two feed

streams and a bottoms stream, but there is no distillate. In addition, the impurity in the

bottoms product will be mostly the heavier of the two reactants, component !. This

means that the flowrates of the two fresh feed streams FandF!will notbe e$ual.

In addition, the reaction is not e$uimolar. Two moles of reactants produce one mole

of product. Therefore, there is a decrease in the molar li$uid flowrates in the reaction

section due to the none$uimolar reaction. The design procedure used for this system is

to fi" the production rate of product C at 1+.) mol s-1 and the purity of the bottoms

product at ./ mole fraction C. This means that the bottoms flowrate is 1+.)0./

1+./23 mol s-1. The production rate re$uires that 1+.) mol s-1 of both and ! be

consumed. Therefore, at least this amount must be fed to the column. In addition,

however, there is a loss of reactants in the bottoms to account for the + mol 4

impurity. It is mostly !, but there is also a small amount of . The concentrations of

and ! change with the various designs. Therefore, the flowrates of the fresh feeds

are slightly different in each design. dynamic rela"ation method is used to find

steady-state conditions. t each point in time during the dynamic simulation, the fresh

feed flowrates are calculated from the fi"ed value of the bottoms flowrateB and the

present values of the bottoms compositions xB,j, which change with time until a

steady-state solution is achieved.

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#ince ! is heavier than , the fresh feed flowrate of ! is somewhat larger than that of

. The reflu" flowrate is changed to drive the bottoms composition to / mol 4 C.

The vapour boilup controls the level in the base. There is no distillate. The reflu"

drum level is not controlled. The base case considered has five stripping trays and

nine reactive trays. The column operates at / bar and has + mol of holdup on

reactive trays. The column diameter is 1. m, as calculated from vapor loading

limitations 5e$uation given in a previous paper2. This diameter gives a reasonable

li$uid height 5in terms of hydraulics on the reactive trays of .33 m with the +

mol of li$uid holdup. 6nder these conditions, the bottoms composition is .+2 mol 4

and 1.32 mol 4 !. The resulting fresh feeds are F 1+.)( mol s-1andF! 1+./+

mol s-1. The vapor boilup re$uired to achieve this purity of the product is )+.( mols -

1, and the reflu" flowrate is /.13 mol s-1, which is the overhead vapor rate. Table +

gives conditions for the base case. 7ote that the reflu" composition is mostly the

lightest component , but some of the other two components are also present. Figure

+ gives the composition profiles. 8$uimolal overflow is assumed, so the li$uid and

vapor rates are constant in the nonreactive section of the column. 9owever,

there are changes in the li$uid and vapor flowrates in the reactive section due to two

effects: 51 the reaction is not e$uimolar 51mole of product is produced by the

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consumption of + moles of reactants and 5+ the e"othermic reaction causes

vapori;ation of some of the li$uid. Therefore, vapor rates increase up the

column and li$uid rates decrease down the column. &emember the heat of reaction is

negative.

Figure +

Effect of Pressure.7ow that we have a design procedure and a base case, we are ready to see how

various parameters affect the ternary reactive column without inerts. #ince, vapor

boilup is a direct measure of energy consumption, the optimum economic pressure is

3 bar. There is an optimum pressure because there is an optimum temperature. 9igh

temperatures give low chemical e$uilibrium constants because the activation energy

of the bac%ward reaction is larger than that of the forward reaction in this e"othermic

reaction system.

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the vapor boilup by itself is not the only energy issue. What also matters is the cost of

the energy, which depends on its temperature level. 9igher column pressures mean

higher base temperatures and re$uire higher temperature energy sources. >ver the

pressure range )- bar, Figure = shows that the base temperature varies from =+ to

== *, which would change the re$uired pressure of the steam significantly.

Holdup on Reactive Trays.

Intuition would lead us to predict that increasing reactive tray holdup M&' should

improve performance and decreases energy consumption. This is indeed true. For the

base-case value of M&' 1 mol, vapor boilup V# is )+. mol s-1. ?ecreasingM&'

to / and ) mol raises V# to )=.2 and ).3 mol s-, respectively. IncreasingM&'to

+ mol decreases V# to ).3 mol s-1. There are no counterintuitive effects of

reactive tray holdup.

.!. "umber of Reactive Trays. dding more reactive trays improves the steady-

state design of the ternary system because vapor boilup decreases as reactive trays are

added. The composition profiles throughout the column. The sharp li$uid composition

changes that occur between the top of the column on tray NTand in the reflu" drum

5stageNT@ 1 for the values of N&'5+, 1 is caused by the total reflu" operation. The

vapor composition on trayNTis higher for the lighter component than the li$uid

composition. The reverse is true for the heavier component !. Therefore, the li$uid

leaving the total condenser is richer in than the top tray and leaner in ! .

#bout the reactants $ product%

Reactant #% &sobutene

&sobutylene 5or +-methylpropene is a hydrocarbon of significant industrial

importance. It is a four-carbon branched al%ene 5olefin.

'ses% It is reacted with methanol and ethanol in the manufacture of the gasoline

o"ygenates methyl tert-butyl ether 5AT!8 and ethyl tert-butyl ether 58T!8,

respectively. l%ylation with butane produces isooctane, another fuel additive.

Bolymeri;ation of isobutylene produces butyl rubber 5polyisobutylene. ntio"idants

such as butylated hydro"ytoluene 5!9T and butylated hydro"yanisole 5!9 are

produced.

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(anufacture% Isobutylene can be isolated from refinery streams by reaction with

sulfuric acid, but the most common industrial method for its production is by catalytic

dehydrogenation of isobutane.

Fig.(

Reactant )*+% (etahnol

(ethanol, also %nown as methyl alcohol, ,ood alcohol, ,ood naphtha or ,ood

spirits, is a chemical with the formula C9(>9. Aethanol is produced naturally in the

anaerobic metabolism of many varieties of bacteria.

'ses% Aethanol, a common laboratory solvent, is especially useful for 9B

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Fig.2

Result and Discussions%

Wor% ?one:

>bective: The effect of %inetic and design parameters on Ternary

&eactive ?istillation Columns and henceforth create a /4 efficient system. The

system here is without any inert.

Effect of ariation /f Equilibrium Constant%

Initially we too% a system at / bar pressure D *e$E+ with a reactive distillation

column having 1) trays. The feed we too% were isobutene and methanol and we got

AT!8 as our final product. We set /4 pure product as our target. For that purpose

we calculated the amount of heat re$uired 5r. fter that we varied the *e$for samepressure. 9ere, we observed that the process is feasible only within a certain pressure

range. lso the vapour boilup ratio decreases with increasing the pressure. Thus we

%now that as we increase the *e$ the reaction is easily possible but the limit of

increasing that is also fi"ed. Too much increase or decrease in the value of *e$will

disturb the desired concentration.

Bressure E / bar

*e$ apor boilup, mol s-1

*e$E+ )1.())1

*e$E1+ /).2+)1

*e$E1) 3.)(+(

*e$E+= 22.2()+

*e$E+/ 21.2)2

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&8#6

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Therefore

apour boilup in mol s-1E /).2+)1 by simulation

0c1

Initial pressure E / bar

?esired output E /4 product

*e$ E 1)

Terms to be calculated:

apour boilup

Corresponding we change the r value so as to obtain the desired product.

In the sub case we change the value of r to obtain the value of the product to be /

4.

Therefore

apour boilup in mol s-1E 3.)(+( by simulation

0d1



*e$ E +=


apour boilup



4.

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Therefore

apour boilup in mol s-1E 22.2()+ by simulation

0e1



*e$ E +/


apour boilup



4.

Therefore

apour boilup in mol s-1E 21.2)2 by simulation

Result in the tabular form

Table2

*e$ apor boilup, mol s-1

*e$E+ )1.())1*e$E1+ /).2+)1

*e$E1) 3.)(+(

*e$E+= 22.2()+

*e$E+/ 21.2)2

3ith graph as follo,s for the first result

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Fig.) It is the corresponding graph of the temperature against tray number for the

given initial condition.

keq v/s Vapor boilup, mol s-1

0

10

20

30

40

50

6070

80

90

100

0 5 10 15 20 25 30

keq

Vaporboilup,mols-1

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Fig.3 The above plot represents the variation of aopr boilup ratio with *e$

The effect of change of *e$as discussed in the theory is clearly reflected from the

graph i.e. with the increase in *e$ the boilup value decreases. For the

reactor0column0recycle process, optimum temperature of reactor increases with the

increase ofKEQvalue. 9igher temperatures give bigger specific reaction rates, which

results in smaller reactor holdup VR. #ince lower temperatures decrease specific

reaction rates, more zB leaves the reactor with decreasing value of KEQ. Therefore,

lower values ofKEQresult in higher amount of vapor boilup in the column to get the

specified product purity.

Case +

*eeping the *e$ to be the same

0a1

*e$ E +


B E / bar


apour boilup


4.

Therefore

apour boilup in mol s-1E )1.())1 by simulation

0b1

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*e$ E +


B E 3 bar


apour boilup


4.

Therefore

apour boilup in mol s-1E 22.2()+ by simulation

0c1

*e$ E +


B E ) bar


apour boilup


4.

Therefore

apour boilup in mol s-1E 2.1(( by simulation

0d1

*e$ E +

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Pressure v/s Qr

0

10

20

30

40

50

60

70

0 2 4 6 8 10

Pressure

Qr

Fig. The above figure ahows the variation of r when we change Bressure for a

given value of *e$

The above graphs have been plotted between

1. Tray number and temperature

+. tray number and mole fraction

The condenser wor%s at a temperature of (3 *. The bottom product which we are

getting has /4 purity. In the graph we also see that at two trays the change in

temperature in sudden. Those two trays are where we supply feed. This is because the

input feed ta%es latent heat for phase change or pre-heating before the reaction. These

trays are tray number 1 and . While simulating the program we actually put up the

feed trays as + and 1. GH is isobutene which is introduced at tray 1 and G!H is

methanol which is introduced at tray . The reaction is etherification. We are not

ta%ing any distillate here i.e. this is a situation of & E .

&E 5

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Fig./ is the graph of variation of mole fraction of the ( components of the system with

no. of trays. dding more reactive trays improves the steady-state design of the

ternary system because vapor boilup decreases as reactive trays are added. Fig.3 also

shows the composition profiles throughout the column for + values ofN&'51 and .

7ote. the sharp li$uid composition changes occur between the top of the column on

tray NT and in the reflu" drum 5stage NT@ 1. This is caused by the total reflu"

operation. The vapor composition on trayNTis higher for the lighter component

than the li$uid composition. The reverse is true for the heavier component !.

Therefore, the li$uid leaving the total condenser is richer in than the top tray and

leaner in !. Thus, the li$uid composition profiles ump up for and drop for ! at the

top of the column.

It is %nown that the e$uilibrium constantKEQdecreases with increasing temperature

for an e"othermic reaction, and higher e$uilibrium constant pushes the reaction to the

right. Thus, the column can operate at higher pressures whenKEQ is larger, and higher

values ofKEQ re$uire less vapor boilup. The optimum number of stripping trays does

not change significantly, because product purity is specified and relative volatilities

are constant. >n the other hand, since there is a need of e"tra trays to achieve to the

conversion in the reactive ;one, the optimum number of reactive trays increases

dramatically with the decrease ofKEQ.

The increase of vapor boilup and reactor si;e0number of reactive trays with smaller

KEQincreases both the capital and energy costs.

Conclusion%

AT!8 Ternary system having two reactants and one product has been e"plored. The

effects of a number of %inetic and design parameters have been e"plored and found to

change significantly from system to system.

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