multiple product solutions of tert-butyl alcohol dehydration in reactive distillation

Multiple Product Solutions of tert-Butyl Alcohol Dehydration in ReactiveDistillation

Zhiwen Qi† and Kai Sundmacher*,†,‡

Max-Planck-Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, D-39106 Magdeburg,Germany, and Process Systems Engineering, Otto-Von-Guericke-UniVersity Magdeburg, UniVersitatsplatz 2,D-39106 Magdeburg, Germany

The feasibility oftert-butyl alcohol (TBA) dehydration to selectively produce the high purity products isobuteneor diisobutene by using reactive distillation has been illustrated through process simulations. Correspondingly,different column configurations are proposed. The influence of the important operating parameters on thecolumn performance is analyzed by continuation methods, and the operating windows of the parameters aresuggested. Advantages of employing reactive distillation for TBA dehydration include the mild conditionsrequired, high TBA conversion per pass, and high selectivity of the desired products. Moreover, the excellentcoupling of TBA hydration and isobutene dimerization in one column leads this system to be a good examplefor studying the complex behaviors of reactive distillation integrating multiple chemical reactions.

1. Introduction

tert-Butyl Alcohol (TBA) production is of interest due to itsimportant use as a gasoline additive to meet part of theoxygenate requirement for reformulated gasoline. Since TBAis completely soluble in water, even small amounts originallypresent in gasoline can result in high levels in groundwater. Asindicated by its organic carbon partition coefficient, TBA issomewhat more retarded in its movement through soils thanmethyl tert-butyl ether (MTBE), but after TBA reaches ground-water, it is even more difficult to remove by carbon filters orair stripping. TBA has been detected at concentrations as greatas 18 000µg/L (much higher than the drinking water actionlevel of 12µg/L) in groundwater near source areas of ground-water plumes in the USA.1 From a toxicological point of view,TBA shows evidence of carcinogenicity. Therefore, it seemsthat TBA is not a good replacement of MTBE as a gasolineadditive and should be decomposed to manufacture otherproducts.

There has been an enormous technological interest in TBAdehydration during the past years, first, as a primary route toMTBE and, more recently, for the production of isobutene (IB)and polyisobutene. The valuable advantage of produced IB fromTBA dehydration is that no other C4 compound exists togetherwith IB, which is important for some IB downstream processing.Moreover, selectively produced dimerization products are ofinterest as a replacement for MTBE, which has attracted negativepublicity because of its water solubility. The ban on the use ofMTBE in California has spurred interest in new octane-enhancing components based on IB.2 Dimerization of IBproduces diisobutenes (DIB, i.e., 2,2,4-trimethyl-4-pentene and2,4,4-trimethyl-1-pentene), which can be used in gasoline asan additive or hydrogenated to isooctane. Another option is toetherify the DIBs to components such as 2-methoxy-2,4,4-trimethylpentane and 2-ethoxy-2,4,4-trimethyl-pentane.3

A number of commercializable TBA dehydration processeshave been developed, which typically involve either vapor phase

reaction over a silica-aluminum catalyst at 260-370°C or liquidphase reaction utilizing homogeneous or solid acid catalysis.4-7

In such conventional reactors, the TBA conversion is notcomplete due to the chemical equilibrium limitation and theselectivity is not high due to the IB oligomerizations under thehigh temperature.

The nature of the TBA dehydration (i.e., fast, water inhibited,and chemical equilibrium controlled) and the difference in thevolatilities of the species involved (difference in the normalboiling points between IB and others is more than 100 K) leadsto this reaction as a potential candidate for implementation ina reactive distillation column. The reactive distillation processof TBA dehydration has been demonstrated experimentally withreduced pressure8 and enhanced pressure.9

It is also a potential of reactive distillation technology forsimultaneous dehydration of TBA and further dimerization ofthe produced IB by high reflux of IB inside the column. Bothreactions are fast and use a common catalyst, for example, ion-exchange resin10,11can be applied. Though the DIB selectivityis favored by a polar component like TBA and water duringthe IB dimerization reaction, as suggested by Honkela andKrause,12 it is still the key concern in such a reactive distillationprocess that couples two reactions. This is because of therelatively high concentration of IB and the high reactiontemperature in part of the reactive zone, which may promotethe side reactions.

In this contribution, TBA dehydration for the products IBand DIB is analyzed based on simulation studies. The influenceof the most important parameters is investigated by continuationmethods. Special attention is given to the conversion of TBAand selectivities toward the desired products.

2. Kinetic Models for Reactions Involved

In this work, two main chemical reactions for producing IBand DIB are involved:

Beside the main reaction of IB dimerization, oligomerizationof IB to higher oligomers may occur theoretically. As argued

* To whom correspondence should be addressed. Phone:+49-391-6110351. Fax:+49-391-6110353. E-mail: [email protected].

† Max-Planck-Institute for Dynamics of Complex Technical Systems.‡ Otto-von-Guericke-University Magdeburg.

TBA S IB + H2O (1)

2IB f DIB (2)

1613Ind. Eng. Chem. Res.2006,45, 1613-1621

10.1021/ie0511027 CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 01/19/2006

by Honkela and Krause,11 tetramers and higher oligomers neednot to be considered because of their very low formation ratecompared to the dimer and trimer. So, only one side reactionfor triisobutene (TIB) formation is taken into account:

The kinetics of TBA dehydration or IB hydration have beenwidely studied. The solid catalysts are mainly classified asinorganic ones such as zeolite13 and ion-exchange resins.10,14-16

Though they are reversible, the hydration of IB on ion-exchangeresin is different from the dehydration of TBA because of alarge amount of water on the catalyst that leads to the resinbeing fully swollen. Since the concentration profile of water isvery different in reactive distillation, a model for TBA dehydra-tion is needed for the column simulation.

The recent presented kinetics of TBA dehydration and IBdimerization by Krause and co-workers are probably the mostsuitable ones when these two reactions are coupled in oneprocess. In their works, they took the TBA dehydration underIB dimerization conditions using the same catalyst of an ion-exchange resin. The kinetics were fitted using the same activitymodel. In this work, the kinetics and the parameters fromKrause’s works are adopted, as listed in Table 1. Accordingly,the temperature limitation of the catalyst ion-exchange resinshould be always taken into account in simulations (i.e.,<397K). One should note that, in the kinetics, the adsorption effectswere not treated consistently such that different adsorptionexpressions were adopted. Moreover, in the IB dimerization andtrimerization kinetics, the water adsorption is now consideredby modifying the original ones. The adsorption constant of wateris assumed as the same as TBA. Such an assumption seemsreasonable since the activity coefficients vary between 13 and42 for water and between 2 and 8 for TBA so that the stronginhibition effect of water can be compensated by the wateractivity coefficient.

3. System Thermodynamics

For the involved mixtures, liquid phase splitting may takeplace, which does not change the concentration of the vaporphase but also influences the reaction rate especially when thereaction takes place only in one liquid phase. To determine a

suitable phase equilibrium model for column simulations of thereaction systems, system thermodynamics is initially investi-gated. The liquid nonideality is described by the Dortmundmodified UNIFAC model20 as used in column simulations.

First, the phase equilibrium of the ternary system of IB, water,and TBA is studied. As indicated in Figure 1, at 1 atm, twoliquid phases are predicted from the vapor-liquid-liquidequilibrium (VLLE) model, a critical point ofx ) (0.0150,0.7401, 0.2449) is located on the liquid-liquid envelope. Whensuperposing the chemical equilibrium curve of eq 1 over themap, one can find that this curve is totally outside the liquid-liquid envelope. Therefore, for the IB formation system, ahomogeneous liquid mixture can be expected, which will beconfirmed in the later simulation.

For the DIB formation system, liquid phase splitting occursin a wide range of mixture concentration. As example, Table 2gives the VLLE prediction of typical mixtures in the DIBcolumn simulation (see Figure 7a). Therefore, the columnsimulation for DIB formation should generally consider aheterogeneous liquid mixture (i.e., the VLLE model).

Table 1. Kinetics of the Reactions Involved

reaction kinetics source

TBA S IB + H2O

r1 )kTBA(KaaTBA - aH2O

aIB)

aTBA + 1.5aH2O

10

kTBA ) 0.756 exp[- 18000R (1T - 1

343)] (kmol/(h kgcat))

Ka ) exp(7.6391- 3111.9T )

2IB f DIB

r2 )kDIBaIB

2

[aIB + 7.0(aTBA + aH2O)]2

11

kDIB ) 0.82 exp[- 30000R (1T - 1

373.15)] (kmol/(h kgcat))

DIB + IB f TIB

r3 )kTIBaIBaDIB

[aIB + 7.0(aTBA + aH2O)]3

11

kTIB ) 0.065 exp[- 1800R (1T - 1

373.15)] (kmol/(h kgcat))

DIB + IB f TIB (3)

Figure 1. VLLE prediction and chemical equilibrium curve of a ternarymixture of IB, water, and TBA at 1 atm.

1614 Ind. Eng. Chem. Res., Vol. 45, No. 5, 2006

4. Column Mathematical Model and Solution Method

The studied reactive distillation column with three zones isshown in Figure 2. The reactions occur only in the reactive zonewhere the catalyst is present, and the feed is introduced to areactive stage. For both IB and DIB formations, no distillate iswithdrawn from the top, i.e., total liquid reflux. Instead, a vaporoutlet from the partial condenser is set for IB formation;however, it is closed for DIB formation.

Since the TBA dehydration studies indicate that a masstransport limitation does not exist when macroporous ion-exchange resins are used as a catalyst,17 a dynamic equilibriumstage model considering mass and energy balances18,19is appliedto simulate the column. In the model, the solid catalyst is treatedas quasi-homogeneous with the liquid phase, i.e., the same liquidcomposition exists at the catalytically active site as in the bulkliquid composition. The energy balance is considered with themass balances. The vapor phase is assumed to be ideal at theinvestigated pressure, and the liquid activity coefficients aredescribed by the Dortmund modified UNIFAC model20 as wasdone for the kinetics fitting. For the IB formation column, onlythe VLE is applied, while the model considers the possibilityof liquid splitting on any stage of the column for the DIBformation column. Moreover, The pressure drop, heat lossesacross the column walls, and the effect of fluid dynamics areneglected.

The above model is formulated as differential algebraicequations, which are solved using the DIVA simulation environ-ment.21 DIVA contains robust solvers for dynamic and steadystate simulations as well as continuation solvers for bifurcationanalyses, which is especially suitable for large-scale systemssuch as reactive distillation processes.

In the following sections, the presented model will be appliedfor process analyses. All the three reactions are alwaysconsidered simultaneously. The effect of the important param-

eters, i.e., catalyst loading and column pressure as well as thereboil ratio, is analyzed. To quantify the column performance,the conversion of TBA and the selectivities are focused on. Sincethe amount of DIB and TIB in the vapor phase of the condenseris always very low in simulations, the selectivities of theproducts IB and DIB are defined according to their amounts inthe bottom as

whereB andVtop are the flow rates of the bottom product andtop product of vapor from the condenser, respectively.

5. Process for IB Formation

5.1. Steady State Behaviors.Due to the difference in boilingpoints between IB and other components, the concentration ofIB in the vapor phase is always higher than that in theequilibrated liquid phase. To obtain high purity IB, the productwithdrawn from the vapor phase of the condenser is designed

Table 2. VLLE Predictions of Typical Mixtures of IB, DIB, TIB, Water, and TBA

stage mole fractiona â T (K)

14 x ) (0.03099, 0.24317, 0.00105, 0.29889, 0.42590)b 0.06038 379.65xa ) (0.00065, 0.00019, 0.00000, 0.96357, 0.03560)xo ) (0.03294, 0.25878, 0.00112, 0.25618, 0.45098)

25 x ) (0.00318, 0.26617, 0.00080, 0.44049, 0.28936) 0.30095 388.34xa ) (0.00007, 0.00021, 0.00000, 0.96601, 0.03370)xo ) (0.00452, 0.38067, 0.00115, 0.21424, 0.39942)

29 x ) (0.00015, 0.40505, 0.00091, 0.58031, 0.01358) 0.54664 396.40xa ) (0.00000, 0.00011, 0.00000, 0.99707, 0.00282)xo ) (0.00032, 0.89332, 0.00201, 0.07781, 0.02655)

a Note: x ) mixture; xa ) aqueous phase;xo ) organic phase.b The sequence is IB, DIB, TIB, water, TBA.

Figure 2. Schematic of reactive distillation column for TBA dehydrationand DIB formation.

Table 3. Column Configurations and Performance

IB column DIB column

Configurationtotal stages 15 30reactive stages 3-13 2-28pressure (atm) 1.0 4.13reflux ratio infinitereboil ratio 5.0 6.1heat duty (MW) 1.61 2.12ratio of condenser vapor to feed 0.9999 0catalyst loading per stage (kg) 159.1 54.5feed flow rate (kmol/h) 100 100feed position 7 14

PerformanceTBA conversion (%) 99.69 99.59IB selectivity (%) 99.43DIB selectivity (%) 97.49

Top Product Concentration (Vapor)IB 0.9959DIB <1 × 10-5

TIB <1 × 10-6

Water 0.0036TBA 0.0004

Bottom Product ConcentrationIB <1 × 10-6 <1 × 10-5

DIB 0.0029 0.3249TIB 0.1 × 10-4 0.0056water 0.9944 0.6668TBA 0.0027 0.0027

SIB )VtopyIB,top

VtopyIB,top + B(2xDIB,bottom + 3xTIB,bottom)(4)

SDIB )2BxDIB,bottom

VtopyIB,top + B(2xDIB,bottom + 3xTIB,bottom)(5)

Ind. Eng. Chem. Res., Vol. 45, No. 5, 20061615

and the liquid from condenser is totally refluxed back to thecolumn, as shown in Figure 2. Since the duty for separating IBis light, both the rectifying and stripping sections contain onlyone nonreactive stage. Moreover, the ratio of the condenservapor to the feed is close to unity (i.e., 0.9999) and the columnoperating pressure is 1 atm. In brief, the configurations of thereactive distillation column for IB formation are listed in Table3.

The steady state column profiles are presented in Figure 3,and the corresponding column performance is given in Table3. As seen in Figure 3a, the IB concentration in the vapor phase(0.9959; product) is higher than that in the liquid phase (0.9623)of the condenser. TBA exists mainly inside the column (stages2-11). In the bottom, very small amounts of byproducts arefound, which hold a very low concentration in the reactivesection (the TIB concentration in the whole column is very lowsuch that one cannot distinguish in Figure 3a). From Table 3,both the TBA conversion and the IB selectivity are quite high.By checking the VLLE model, only in the liquid mixture ofthe condenser does phase splitting occur with a very low relativemolar holdup of the aqueous phase (â ) 0.0034).

Good column performance is favored by the coupled effectsof reaction and distillation in one unit: (1) The coproduct wateris immediately separated from the IB as it forms favoring theequilibrium of TBA dehydration (eq 1) to be shifted far to theright and, thus, high TBA conversion to be achieved under mild

operating conditions. (2) Once IB is generated, it is quicklyrectified to the upper part of the column, which leads to a verylow IB concentration in the most reactive zone and, thus, reducesthe rates of side reactions. (3) DIB is simultaneously removedfrom the reactive zone to the bottom, which further suppressesthe formation of the trimer and higher oligomers. (4) Lowerpressure results in a lower temperature profile in the column(Figure 3b), which is in favor of catalyst life.

In this work, the catalyst loading per stage is 159.1 kg andthe total catalyst loading in the column is 1750 kg for a feedflow rate of 100 kmol/h. Comparing these values to those ofthe literature with the same catalyst,8 i.e., a total of 0.05 kgcatalyst and a feed flow rate of 8.928× 10-4 kmol/h, thecapacity of the catalyst (i.e., the ratio of the feed flow rate tothe total catalyst loading) in this work is higher, i.e., 0.0571kmol/(h kgcat) vs 0.0179 kmol/(h kgcat)).

For process development, it is important to analyze theinfluence of the important parameters, which are the catalystloading, the operating pressure, and the reboil ratio for this IBformation process. In the following, the influence is analyzedby changing the investigated parameter while keeping othersthe same as in this base case. The continuation methods in DIVAare applied.

5.2. Effect of Catalyst Loading.The catalyst loading is oneof the most crucial design parameters for a reactive distillation

Figure 3. Steady state profiles in a TBA dehydration column: (a)concentration; (b) temperature. Figure 4. Bifurcation diagram of the effect of the catalyst loading on IB

column performance.


process. By changing the catalyst loading, the bifurcationdiagram of the influence of the catalyst loading per stage isgenerated, as shown in Figure 4. As can be seen, at lowercatalyst loading (less than 100 kg/stage), though the selectivityof IB is close to unity (due to the very weak side reactions),the TBA conversion is low due to the slow reaction rate.Therefore, the unreacted TBA should be withdrawn from thebottom. At catalyst loading between 101 and 3000 kg/stage,the TBA is almost completely converted and the IB conversionis high. This wide range is suitable for column design. At highercatalyst loading, the reaction rates of side reactions are promotedsuch that both IB purity and selectivity are reduced significantly.

5.3. Effect of Operating Pressure.TBA dehydration to IBis an endothermic reaction (heat of reaction is about 27-38kJ/mol),10,14,15and the forward reaction is theoretically favoredby a high operating temperature and low pressure, which canbe easily achieved by conventional reactors. However, in areactive distillation column, the temperature and pressurestrongly interact with each other. Therefore, one should carefullydetermine a suitable operating pressure generally consideringthree effects: (1) reaction rate, (2) separation efficiency, and(3) limitation of catalyst like resins. For this analysis, a rangeof pressure between 0.5 and 8 atm is suggested and thebifurcation diagrams are presented in Figure 5.

It is quite interesting that at reduced pressure (about 0.8 atm)the best column performance can be obtained with high purity

and high selectivity of IB. This is mainly attributed to tworeasons: (1) the enhanced distillation effect for IB as it is moreeasiliy evaporated to the top of column at reduced pressure dueto its very low boiling point, which leads to lower IBconcentration in column and increases the TBA dehydration rateand (2) the low temperature at the reactive zone due to thereduced pressure. Both result in slower rates of the sidereactions. Though a reduced pressure of 0.5-1 atm was alsoadopted in the literature experiments,8 it is not recommendableas the improvement is not so significant and more equipmentis needed to maintain such reduced pressure.

As the pressure increases, the temperature inside the columnincreases, which favors the side reactions and byproductformation. As a result, both the IB purity and selectivity aredecreasing. Unexpectedly, the reaction of TBA dehydration isnot enhanced, though the high temperature is helpful, whichimplies that the effect of pressure enhancement on the volatilityof IB (i.e., lower IB volatility) is stronger than the effect oftemperature enhancement (due to the increasing pressure) onthe reaction rate of TBA dehydration.

5.4. Effect of Reboil Ratio.The reboil ratio not only affectsthe column performance but also determines the heat duties ofthe column. As seen in Figure 6, the improvement of columnperformance, especially the TBA conversion (Figure 6b), issignificant as the reboil ratio increases in the lower range (<3).However, at higher reboil ratios (3-8), its influence is veryslight such that the column performance is stable. In other steady

Figure 5. Bifurcation diagram of the effect of the operating pressure onIB column performance.

Figure 6. Bifurcation diagram of the effect of the reboil ratio on IB columnperformance.


state studies with reboil ratios of 3-4, the column performanceis much more sensitive to other parameters. Therefore, the rangeof the reboil ratio between 4 and 5 is suggested for IB forma-tion.

6. Process for DIB Formation

6.1. Steady State Behaviors.Different from the IB formationcolumn, to generate DIB, the IB formed by TBA dehydrationshould remain totally inside the column. Therefore, no productto be withdrawn from the top is designed for this process.Moreover, higher pressure than the IB column is adopted toincrease the reaction rates. For the sake of better overall columnperformance, the feed is set at stage 14. The column configura-tions are listed in Table 3.

Under such conditions, the column profiles of concentration,temperature, reaction rates, and liquid phase splitting informationare illustrated in Figure 7 and the column performance is givenin Table 3. As can be seen, both the TBA conversion and DIBselectivity are high (Table 3), and the temperature in the reactivezone is below the limitation of the catalyst (Figure 7b). Thegood column performance and the column profiles againdemonstrate the advantages of reactive distillation technologyfor this process that couples the two reactions of TBAdehydration and DIB formation: (1) TBA dehydration is anendothermic reaction (27-38 kJ/mol), and DIB formation is ahighly exothermic reaction (-82.84 kJ/mol).22 Since 2 mol of

IB form 1 mol of DIB, coupling these two reactions effectivelymakes use of the heats of reaction without losses and signifi-cantly reduces the heat exchanger costs. (2) The remarkablebehavior is that the feed location plays an important role in theprofiles. From Figure 7a, IB dominates the upper part of thecolumn with very low concentrations of water and TBA at stages2-7 (<0.001 for TBA and<3 × 10-5 for water) while thehigher boiling components are condensed at the lower part.Correspondingly, the temperature below the feed stage is muchhigher than that at the upper part (Figure 7b). Moreover, thepredicted phase splitting in most stages (Figure 7c) shows verydifferent relative holdup of the aqueous phase, which varies fromhomogeneous between the condenser and stage 11 to 0.6413 inthe reboiler. (3) From the reaction rate profile (Figure 7d), theTBA dehydration mainly takes place below the feed location,while the DIB formation takes places above the feed position.Due to the difference in boiling points between IB and othercomponents, in the lower part of the column, the formed IB isimmediately condensed to the upper part, leading to a very lowIB concentration at the lower part. Moreover, the DIB formedat the upper part is simultaneously removed from the IB-enriched region to the lower part. The formations of IB andDIB in different column regions and the condensation of IBand DIB in different directions result in a very low rate of TIBformation (Figure 7c, hard to distinguish), and thus, a high DIBselectivity is achieved.

Figure 7. Steady state profiles in a DIB formation column: (a) concentration; (b) temperature; (c) relative molar holdup of the aqueous phase; (d) reactionrates.


It is worth noting that, at stages 9-13, TBA hydration ratherthan TBA dehydration occurs due to the low concentration ofTBA and much higher concentration of IB in this section, eventhough it is not recommended to feed TBA at the upper part,which can enhance the formation of TIB and, thus, reduce theDIB selectivity.

It is also worth noting that, in this process, the desired catalystloading (54.5 kg/stage and 1472 kg in total) is unexpectedlylower than that in the first studied IB formation column. Thereason is that, in the upper part of the reactive distillation columnfor IB formation, the reaction rate is quite slow due to therelatively high concentrations of isobutene and water. Therefore,part of the catalyst does not work efficiently. If high conversionof TBA is not required, the stages above the feed position couldbe nonreactive such that the catalyst loading can be reduced.This aspect also illustrates the valuable advantage of reactivedistillation gained by coupling multiple reactions, which has,to our best knowledge, not been studied so far.

6.2. Effect of Catalyst Loading. In the same way, theinfluence of catalyst loading on the column performance isanalyzed by continuation methods. The bifurcation diagram isshown in Figure 8. At low catalyst loading (<50 kg/stage), boththe reaction rates of TBA dehydration and DIB formation areslow, leading to low TBA conversion such that some TBAshould be drawn out from the bottom product. On the otherhand, the DIB selectivity is higher since the less formed IB ismostly converted to DIB. At higher catalyst loading (>50 kg/stage), the TBA conversion is close to unity. However, the DIBselectivity is slightly lower (around 4%), which is because the

side reaction for TIB formation is enhanced at higher catalystloading. With further increasing catalyst loading, the DIBselectivity is significantly reduced, which is not shown here.As a suggestion, the range of 50-100 kg/stage is recommended.

6.3. Effect of Pressure.In the base case for DIB formation,the pressure is 4.13 atm, which mainly considers the temperaturelimitation of catalyst. As illustrated in Figure 9, lower pressureleads to worse column performance due to the decreasedtemperature (and thus slow reaction rates) in the reactive zone.Though higher pressure (6-10 atm) has not much influence onthe column performance, it is not recommendable. One reasonis that more energy should be supplied as the reboiler temper-ature is increased. Another reason is that the temperature in thereactive zone exceeds the limitation of catalyst.

6.4. Effect of the Reboil Ratio.The bifurcation analysis ofthe influence of the reboil ratio on the column performanceshows similar behavior (Figure 10). Since the liquid of the topis fully refluxed back to the column, the liquid and vapor fluxinside the column is only determined by the reboil ratio. Atlow reboil ratios (<5.0), the flux inside the column is limitedsuch that it is difficult to generate a better concentration profile.As a result, TBA will mainly remain at the lower part of thecolumn, leading to lower conversion. Moreover, at lower reboilratios, the potential for rectifying IB to the top becomes weak,such that the IB concentration at the lower part is higher thanthat in the base case; thus, the TIB formation rate is enhancedand the DIB selectivity is reduced.

At higher reboil ratios (>6), TBA conversion and DIBselectivity are slightly improved. However, the improvement

Figure 8. Effect of the catalyst loading on DIB column performance. Figure 9. Effect of the operating pressure on DIB column performance.


in column performance may not compensate the cost of energyrequired. One should balance the overall cost and the productquality based on a detailed cost estimation.

7. Discussion

For process analysis, the kinetics model and activity coef-ficient method usually play crucial role, especially for systemsstudied in this work with the two strongly polar componentsTBA and water. Since no more suitable kinetics studies underthe same conditions are valuable, further investigation of theeffect of kinetics and activity models cannot be carried out. Eventhough, we believe that the general observations and conclusionsshould be the same, but the desired windows of the operatingparameters might be shifted.

Moreover, impurities in the crude TBA feed, most notablywater, methanol, acetone, and heavies, are not considered inthis work since they do not appear to significantly inhibit thedehydration process (eq 1), although the presence of methanolclearly leads to the formation of additional MTBE (eitherthrough etherification of the TBA feedstock or the isobutenecoproduct). However, the desired windows for the operatingparameters might be changed if they are taken into account.

Furthermore, for DIB production, the proposed process leadsto another problem of separating DIB from water due to theirclose boiling points (the difference of normal boiling pointsbetween water and two kinds of dimers is 1.5-5.0 K). However,if the DIB is further used for isooctane production by subsequenthydrogenation of DIB, the separation of isooctane from water

becomes much easier (the difference of normal boiling pointsbetween water and isoostane is 17.6 K).

As an alternative for the case of DIB as the final product, aprocess consisting of two reactive distillation columns issuggested (Figure 11). The dehydration of TBA (to IB) anddimerization of IB to DIB are carried out separately. Such anarrangement can avoid the separation problem of DIB fromwater. The concern is that the DIB selectivity might be lowerto a certain extent according to the recent study on thedimerization of isobutene.23 Moreover, the reboiler heat demandwill be significantly increased from 2.12 MW for one singlecolumn to 4.50 MW for two columns (the heat duty of thesecond column is referred to in the previous work23). The finaldecision of the flowsheet should be made based on theoptimization considering the product quality and the overall cost.

8. Conclusions

Reactive distillation favors the dehydration of TBA forselective formation of isobutene and diisobutene, and thefeasibility of the two processes is illustrated through simulationstudies. High purity isobutene and diisobutene with high TBAconversion can be obtained in different reactive distillations.Correspondingly, the column configurations are different forbetter column performance. The influence of the importantparameters on column performance is analyzed by usingcontinuation methods. On the basis of the bifurcation diagrams,suitable operating windows are suggested.

The coupling of the two reactions of TBA dehydration andDIB formation in one single reactive distillation column exploresnew features, which have not been reported so far. This reactionsystem could be a good candidate for studying the complicityof reactive distillation coupling multiple reactions in one column.

Acknowledgment

The financial support for this work from the VolkswagenFoundation in Germany (Project “Coupling of Chemical Reac-tions in a Reactive Distillation Process”, AZ.: I/79515) isgratefully acknowledged.

Nomenclature

ai ) liquid activity of componentik ) reaction rate constant, mol/(h gcat)S ) selectivity

Figure 10. Effect of the reboil ratio on DIB column performance.

Figure 11. Proposed alternative for DIB production.


r ) rate of reaction, kmol/(h gcat)xi ) liquid molar fractions of componentiyi ) vapor molar fractions of componentiâ ) relative molar holdup of aqueous phase to overall liquid

mixture

AbbreViations

DIB ) diisobutenesIB ) isobuteneTBA ) tert-butyl alcoholTIB ) triisobutenes

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ReceiVed for reView October 3, 2005ReVised manuscript receiVed December 14, 2005

AcceptedDecember 21, 2005

IE0511027


multiple product solutions of tert-butyl alcohol dehydration in reactive distillation

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