Modelling reactive distillation

R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229 5189distillation is often passed over as a processing optionbecause the catalyst life would require frequent shutdowns.An RD device that allowed on-stream removal ofcatalyst would answer this concern.1.4.2. Ezcient contacting of liquid with catalyst particlesThe hardware design must ensure that the following`wish-lista is met.(a) Good liquid distribution and avoidance of channelling.Liquid maldistribution can be expected to havea more severe e!ect in RD than in conventionaldistillation (Podrebarac, Ng & Rempel, 1998a,b).(b) Good radial dispersion of liquid through the catalystbed. This is required in order to avoid reactor hotspotsand runaways and allow even catalyst ageing.The requirement of good radial mixing has an impacton the choice of the packing con"guration andgeometry. For example, frequent criss-crossing mixingpatterns may be desirable, as is realised in somehardware con"gurations discussed in Section 1.5.1.4.3. Good vapor/liquid contacting in the reactive zoneIf the reaction rate is fast and the reaction is equilibrium-limitedthen the required size of the reactive zone isstrongly in#uenced by the e!ectiveness of the vapor}liquid contacting. Vapor}liquid contacting becomes lessimportant for slower reactions. Commonly used devicesfor good vapor}liquid contacting are the same as forconventional distillation and include structured packing,random packing and distillation trays.1.4.4. `Lowa pressure drop through the catalyticallypacked reactive sectionThis problem arises because of the need to use smallcatalyst particles in the 1}3 mm range in order to avoidintra-particle di!usional limitations. Counter-currentoperation in catalyst beds packed with such small-sizedparticles has to be specially con"gured in order to avoidproblems of excessive pressure drop and `#oodinga.These con"gurations are discussed in Section 1.5.such devices have not been commercialised as yet. Catalystdeactivation is therefore accounted for in the designstage by use of excess catalyst. Besides adding excesscatalyst, the reaction severity can be increased by (a)increasing re#ux, leading to increased residence time and(b) increasing reaction temperature (by increase of columnpressure)1.5. Hardware aspectsBefore modelling aspects can be considered, carefulattention needs to be paid to hardware design aspects.Towler and Frey (2000) have presented an excellent summaryof hardware design aspects of RD columns. Someof the important issues are discussed below.For homogeneous RD processes, counter-current vapor}liquidcontacting, with su$cient degree of staging inthe vapor and liquid-phases, can be achieved in a multitraycolumn (cf. Fig. 7) or a column with random orstructured packings (cf. Fig. 8). The hardware designinformation can be found in the standard sources forconventional distillation design (Lockett, 1986; Stichlmair& Fair, 1998). The Hatta number for most RDapplications is expected to be smaller than about unity(Sundmacher, Rihko & Ho!mann, 1994) and the frothregime is usually to be preferred on the trays (cf. Fig. 9)because of the desire to maintain high liquid hold-up onthe trays. High liquid hold-ups could be realised by use ofbubble caps, reverse #ow trays with additional sumps toprovide ample tray residence time. In the Eastman processfor methyl acetate manufacture specially designedhigh liquid hold-up trays are used (Agreda, Partin& Heise, 1990).1.4.5. Suzcient liquid hold-up in the reactive sectionThe liquid hold-up, mean residence time, and liquidresidence time distribution are all important in determiningthe conversion and selectivity of RD. This is in sharpcontrast with conventional distillation where liquidhold-up and RTD are often irrelevant as the vapor}liquid mass transfer is usually `controlleda by the vaporside resistance. For trayed RD columns the preferredregime of operation would be the froth regime whereas forconventional distillation we usually adopt the spray regime.1.4.6. Designing for catalyst deactivationEven though, as discussed in Section 1.4.1, it is desirableto allow on-line catalyst removal and regeneration,Fig. 7. Counter-current vapor}liquid contacting in trayed columns.Animations of CFD simulations of #ows on the tray can be viewed onour web site: http://ct-cr4.chem.uva.nl/sievetrayCFD.