Combo reactors

Sometimes reaction rates can be enhanced by using multifunctional reactors, i.e., reactors in which more than one function (or operation) can be performed. Examples of reactors with such multifunctional capability, or combo reactors, are distillation column reactors in which one of the products of a reversible reaction is continuously removed by distillation thus driving the reaction forward extractive reaction biphasing membrane reactors in which separation is accomplished by using a reactor with membrane walls and simulated moving-bed (SMB) reactors in which reaction is combined with adsorption. Typical industrial applications of multifunctional reactors are esterification of acetic acid to methyl acetate in a distillation column reactor, synthesis of methyl-fer-butyl ether (MTBE) in a similar reactor, vitamin K synthesis in a membrane reactor, oxidative coupling of methane to produce ethane and ethylene in a similar reactor, and esterification of acetic acid to ethyl acetate in an SMB reactor. These specialized reactors are increasingly used in industry, mainly because of the obvious reduction in the number of equipment. These reactors are considered by Eair in Chapter 12. 

Looking back to the progress achieved in the areas of conventional separation and reactor design, it seems that major advances have now been made. This has led to increasing research into methods in which reaction and separation are combined in a single unit. The equipment in which this dual function is carried out is sometimes referred to as the combo reactor. 

Combo reactors can be of two types, separation oriented and reaction oriented. In the first, reaction is used to achieve efficient separation, such as in the separation of />-cresol from its mixture with /n-cresol. This method need not necessarily be restricted to separation, for it can also be attractive from the reaction point of view in the following scenario the required product from a process comes out with a byproduct of low value in a very difficultly separable mixture of the two. If the byproduct can be converted to a useful coproduct in an easily separable mixture with the primary product, then we would have an attractive process. Alternatively, there can be two unimportant by-products in a difficultly separable mixture which can be converted to an easily separable mixture of useful products. An interesting example of this is illustrated later in this chapter. 

Combo reactors can be of two types (1) reaction-oriented and (2) separation-oriented. In the first, distillation is used to enhance conversion beyond the equilibrium value, such as in esterification reactions. In the second, reaction is used to effect efficient separation, such as in the separation of p-cresol from its mixture with m-cresol. 

Irrespective of whether reaction or separation is of primary concern, three types of combo reactors are commonly used reaction-extraction, reaction-distillation, and reaction-crystallization. Each of these can, in theory, be either reaction- or separation-oriented. Among other, less conventional methods of combining reaction with separation are biphasing and the use of manbranes. Photochemistry, micelles, ultrasound, and microphases offer additional techniques/agents for enhancing the rate of a reaction, and a survey of the analysis and design of combo reactors involving these methods can be found in Doraiswamy (2001). 

The need for the membrane reactors also primarily stems from the equilibrium conversion limitation of a reversible reaction. These reactors are used when the boiling point differences are not sufficient to use distillation column (combo) reactors. The thermal sensitivity of the reactive domain may inhibit the use of boiling point differences for the product separation, hence the use of distillation column reactors. A perm selective membrane can be used within the reactor providing product separation. The other advantage of the membrane reactors is the possibility they offer to run the reaction either in the gas or in the liquid phase, thus... 

This chapter is primarily concerned with the distillation-reaction combo reactor (or distillation colnmn reactor (DCR) as it is usually called). We shall also briefly consider the case where reaction is imposed on a difficultly separable mixture to achieve complete separation. Membrane reactors are already treated separately in Chapter 13. 

Part III Beyond the Fundamentals presents material not commonly covered in textbooks, addressing aspects of reactors involving more than one phase. It discusses solid catalyzed fluid-phase reactions in fixed-bed and fluidized-bed reactors, gas-solid noncatalytic reactions, reactions involving at least one liquid phase (gas-liquid and liquid-liquid), and multiphase reactions. This section also describes membrane-assisted reactor engineering, combo reactors, homogeneous catalysis, and phase-transfer catalysis. The final chapter provides a perspective on future trends in reaction engineering. 

The separation of soluble PTC is a matter of concern in the industry not only due to environmental considerations, but also due to contamination of the product with the catalyst. Further research should be oriented towards development of novel catalyst separation techniques and of novel reactor-separator combo units. As outlined before, the development of a membrane reactor with PT catalyst immobilized on the membrane surface seems to be a novel and viable candidate for accomplishing PTC reactions on an industrial scale. Another aspect of PTC which needs urgent consideration is the development of engineering technology for immobilized PTC. This would require the development of supports with low diffusional limitations and with the right hydrophilic-lipohilic balance to ensure adequate contact of the aqueous and organic phases with the supported catalyst. 

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