Modelling reactive distillation

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5190 R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229Fig. 8. Counter-current vapor}liquid contacting in packed columns.Fig. 9. Flow regimes on trays.1.5.1. Catalytically packed RD columnsFor heterogeneously catalysed processes, hardware designposes considerable challenges. The catalyst particlesizes used in such operations are usually in the 1}3mmrange. Larger particle sizes lead to intra-particle di!usionlimitations. To overcome the limitations of #ooding thecatalyst particles have to be enveloped within wire gauzeenvelopes. Most commonly the catalyst envelopesare packed inside the column. Almost every conceivableshape of these catalyst envelopes has been patented; somebasic shapes are shown in Figs. 10}14. These structuresare:1. Porous spheres "lled with catalyst inside them (Buchholz,Pinaire & Ulowetz, 1995; Johnson, 1993); seeFig. 10(a).2. Cylindrical shaped envelopes with catalyst insidethem (Johnson, 1993); see Fig. 10(b).3. Wire gauze envelopes with various shapes: spheres,tablets, doughnuts, etc. (Smith, 1984); see Fig. 10(c).4. Horizontally disposed wire-mesh `guttersa, "lled withcatalyst (Van Hasselt, 1999); see Fig. 11(a).5. Horizontally disposed wire-mesh tubes containingcatalyst (Buchholz et al., 1995; Groten, Booker &Crossland, 1998; Hearn, 1993); see Fig. 11(b).6. Catalyst particles enclosed in cloth wrapped in theform of bales (Johnson & Dallas, 1994; Smith, 1985).This is the con"guration used by Chemical Researchand Licensing in their RD technology for etheri"cation,hydrogenation and alkylation of aromatic compounds(Shoemaker & Jones, 1987). The catalyst isheld together by "breglass cloth. Pockets are sewninto a folded cloth and then solid catalyst is loadedinto the pockets. The pockets are sewn shut afterloading the catalyst and the resulting belt or `catalystquilta is rolled with alternating layers of steel mesh toform a cylinder of `catalyst balesa as shown in Fig. 12.The steel mesh creates void volume to allow for vaportra$c and vapor/liquid contacting. Scores of thesebales are installed in the <strong>reactive</strong> zone of a typicalcommercial RD column. Bales are piled on top of eachother to give the required height necessary to achievethe desired extent of reaction. When the catalyst isspent the column is shut down and the bales aremanually removed and replaced with bales containingfresh catalyst. Improvements to the catalyst bale concepthave been made over the years (Johnson, 1993;Crossland, Gildert & Hearn, 1995). The hydrodynamics,kinetics, and mass transfer characteristics of baletypepackings have recently been published in theopen literature (Subawalla, Gonzalez, Seibert & Fair,1997; Xu, Zhao & Tian, 1997, 1999).
R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229 5191Fig. 10. Various `tea-baga con"gurations. Catalyst particles need to be enveloped in wire gauze packings and placed inside RD columns.Fig. 11. Horizontally disposed (a) wire gauze gutters and (b) wire gauze tubes containing catalyst.Fig. 12. Catalyst bales licensed by Chemical Research and Licensing.7. Catalyst particles sandwiched between corrugatedsheets of wire gauze (Stringaro, 1991, 1995; Gelbein& Buchholz, 1991; Johnson & Dallas, 1994); seeFig. 13. Such structures are being licensed by Sulzer(called KATAPAK-S) and Koch-Glitsch (calledKATAMAX). They consist of two pieces of rectangularcrimped wire gauze sealed around the edge, therebyforming a pocket of the order of 1}5 cm widebetween the two screens. These catalyst `sandwichesaor `wafersa are bound together in cubes. The resultingcubes are transported to the <strong>distillation</strong> column andinstalled as a monolith inside the column to the requiredheight. When the catalyst is spent, the columnis shut down and the packing is manually removedand replaced with packing containing fresh catalyst.Information on the #uid dynamics, mixing and masstransfer in such structures is available in the openliterature (Bart & LandschuK tzer, 1996; Ellenberger& Krishna, 1999; DeGarmo, Parulekar and Pinjala,1992; Higler, Krishna, Ellenberger & Taylor, 1999a;Moritz & Hasse, 1999). The important advantage ofthe structured catalyst sandwich structures over thecatalyst bales is with respect to radial distributionof liquid. Within the catalyst sandwiches, the liquidfollows a criss-crossing #ow path. The radialdispersion is about an order of magnitude higher thanin conventional packed beds (Van Gulijk, 1998).
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