Modelling reactive distillation

More documents

Recommendations

Info

5212 R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229coe$cients being estimated using the correlations ofOnda, Takeuchi and Okumoto (1968). The model equationswere solved simultaneously. A predicted temperaturepro"le appears to be in good agreement with onedetermined experimentally. However, it must be rememberedthat the all important mass transfer coe$cientswere "tted to data obtained in their own column. Thus,the true predictive abilities of their model are untested. Itis reported that the amount of catalyst and the condensationrate are important design variables for this RDprocess, and that the DAA selectivity would have improvedif the liquid}catalyst mass transfer improved.Podrebarac et al. (1998b) modelled the reactive sectionas a plug-#ow reactor with radial dispersion. The set ofdi!erential and algebraic equations was solved using thegPROMS system. The authors were able to "t their ownexperimental data quite well. However, the model couldnot be used for predictive purposes due, it was claimed, tothe random nature of the #ow in a packed column.The recent study of Baur et al. (1999) comparing theEQ and NEQ models for hydration of ethylene oxide toethylene glycol throws a new light on the phenomena onMSS and the importance of using the NEQ model. Forthe Ciric and Gu (1994) con"guration for the ethyleneglycol column (see Fig. 24(a)), multiple steady-states arefound with both EQ and NEQ models. Three steadystatesSS-1 (high conversion), SS-2 (intermediate conversion)and SS-3 (low conversion) were found. The desiredhigh-conversion steady-state solution (SS-1) correspondsto high column temperatures and lowest molar #ow rateof the vapor up the column. Let us now consider theNEQ model simulations for the 1.7 m diameter columncon"guration. For this chosen column diameter, onlyone solution, SS-1, can be realised in the column. Theother solutions SS-2 and SS-3 could not be realised in theNEQ because for the higher vapor #ows, the column#oods in some (SS-2) or all (SS-3) of the stages; the#ooding boundaries are drawn in Fig. 24(c). Baur et al.(1999) also show that if the column diameter was chosento be 3 m, only the lowest conversion steady-state can berealised!5.4. NEQ cell modelAn issue that is not adequately addressed by mostNEQ models is that of vapor and liquid #ow patterns ondistillation trays or maldistribution in packed columns.Since reaction rates and chemical equilibrium constantsare dependent on the local concentrations and temperature,they may vary along the #ow path of liquid ona tray, or from side to side of a packed column. For suchsystems the residence time distribution could be veryimportant, as well as a proper description of mass transfer.On distillation trays, vapor will rise in plug-#owthrough a layer of froth. The froth will pass through theliquid more or less in plug-#ow, with some axial dispersiondue to the vapor jets and bubbles. In packed sections,maldistribution of internal vapor and liquid #owsFig. 24. (a) Con"guration of reactive distillation column for hydration of ethylene oxide to ethylene glycol used by Ciric and Miao (1994). (b)Equilibrium model calculations for the ethylene glycol process showing column pro"les for liquid-phase mole fraction, temperature and vapor-phasemolar #ow. (c) Non-equilibrium model calculations for the ethylene glycol process for a column of diameter 1.7 m showing the corresponding columnpro"les. Details of the simulations are available in Baur, Higler, Taylor and Krishna (1999).
R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229 5213Fig. 25. The non-equilibrium cell model of Higler, Krishna and Taylor(1999a).over the cross-sectional area of the column can lead toloss of interfacial area. This is known to be one of themain reasons for inadequate performance of packed columns.To deal with this shortcoming of earlier models,Higler, Taylor and Krishna (1999c) and Higler,Krishna and Taylor (1999a,b) developed an NEQ cellmodel. The distinguishing feature of this model is thatstages are divided into a number of contacting cells, asshown in Fig. 25. These cells describe just a small sectionof the tray or packing, and by choosing an appropriateconnection pattern, one can very easily study the in#uenceof #ow patterns and maldistribution on the distillationprocess.Flow patterns on distillation trays are modelled bychoosing an appropriate number of cells in each #owdirection. A column of cells can model plug-#ow in thevapor phase, and multiple columns of cells can modelplug-#ow in the liquid-phase as depicted in Fig. 26.Backmixing may also be taken into account by using anappropriate number of cells. This may be derived fromcalculating an equivalent number of cells from eddydi!usion models. Flow patterns in packed columns areevaluated by means of a natural #ow model. The #owsare split up according to the ratio of the cell surface areasbetween the cells. Various #ow patterns may be approximatedusing di!erent #ow splitting policies.A schematic diagram of the unit cell for a vapor}liquid}porous catalyst system is shown in Fig. 27. Eachcell is modelled essentially using the NEQ model forheterogeneous systems described above. The bulk of bothvapor and liquid-phases is assumed to be completelymixed. Mass transfer resistances are located in "lms nearthe vapor/liquid and liquid/solid interfaces, and the Maxwell}Stefanequations are used for calculation of themass transfer rates through each "lm. Thermodynamicequilibrium is assumed only at the vapor}liquid interface.Mass transfer inside the porous catalyst may bedescribed with the dusty #uid model described above.The unit cell for homogeneous systems (and for heterogeneoussystems modelled as though they were homogeneous)is as depicted in Fig. 23. The equations for eachcell are essentially as given above.For RD, the staging in the liquid-phase could havea signi"cant in#uence on the reaction selectivity. This isemphasised in the study by Baur et al. (1999) of thein#uence of hardware design on the formation of theby-product DEG in the hydration column (sieve tray)shown in Fig. 24(a). Their simulation results are presentedin Fig. 28. For "ve mixing cells in both vapor andliquid-phases (which corresponds closely to plug-#owconditions for either phase), the formation of by-productDEG is reduced while the conversion to EG is increased.Removing the mass transfer resistances, i.e. assuming theEQ model, gives the best performance with respect toconversion and selectivity; see the point towards thebottom right of Fig. 28. Reaction selectivity can be in-#uenced by tray hardware design. Decreasing the weirheight from 80 mm (base case) to 50 mm decreasesformation of EG and increases by-product DEG formation.This reduction is due to a lowering in the interfacialarea with decreasing weir height. Increasing the weirheight from 80 to 100 mm leads to improved conversionand improved selectivity. High weir heights, and operationin the froth regime, are generally to be preferred inRD operations. Increasing the number of passes from1 to 2 increases by-product formation; see the top pointin Fig. 28. This is because the liquid load per weir lengthis reduced by 50%. This reduction in the liquid load leadsto a reduction in the clear liquid height and lowering inthe total interfacial area, which has a detrimental in#uenceon both the conversion and selectivity. It appearsthat the usual design rules for conventional distillationcolumn design cannot be carried over to RD columnsbecause, for a column of 1.7 m diameter the conventionaldesign philosophy would be to use two passes for theliquid #ow.5.5. Pseudo-homogeneous vs. heterogeneous NEQmodellingHigler, Krishna and Taylor (2000) used the dusty #uidequations to model di!usion and reaction in porouscatalysts in RD. The MTBE process (following thecon"guration of Jacobs and Krishna (1993) shown in
Page 7 and 8: R. Taylor, R. Krishna / Chemical En
Page 9 and 10: R. Taylor, R. Krishna / Chemical En
Page 12 and 13: 5194 R. Taylor, R. Krishna / Chemic
Page 47: R. Taylor, R. Krishna / Chemical En

Modelling reactive distillation

Create successful ePaper yourself

Delete template?

Save as template?