Combined Reaction and Separation

The possibility of combining a reaction unit and a separation unit into one reactive separation unit has not been considered in this chapter. However, some of the textbooks, Coulson et al.2 and Seader and Henley,1 give a brief discussion of this type of equipment for distillation and absorption, respectively. A lot of research has also been devoted to this type of operation in recent years and it is generally believed that it will become more widely used in the future. 

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Catalytic Membrane Reactors Membrane reactors combine reaction and separation in a single vessel. By removing one of the... 

Figure 10.9 Combining reactions and separation steps leads to a loss in process design degrees of freedom.

Membrane reactors are known on the macro scale for combining reaction and separation, with additional profits for the whole process as compared with the same separate functions. Microstructured reactors with permeable membranes are used in the same way, e.g. to increase conversion above the equilibrium limit of sole reaction [8, 10, 11, 83]. One way to achieve this is by preparing thin membranes over the pores of a mesh, e.g. by thin-fihn deposition techniques, separating reactant and product streams [11]. 

It is useful to combine reaction and separation for equilibrium-limited reactions and also for consecutive reactions, particularly when the desired intermediate products undergo faster undesirable reactions. The concept of extractive reactions for equilibrium-limited and consecutive reactions has been covered in Section 4.2.1. Distillation column reactors provide yet another strategy. 

Catalysis plays a relevant role in several of these areas of development but, generally, an innovative effort to find new catalytic materials and their integration into advanced reactor technologies is required, e.g., to combine reaction and separation to reduce the overall costs of the process. More specific needs include ... 

Development of robust processes based on new catalytic reactor engineering solutions to improve energy and process efficiency and economics, and which combine reaction and separation. Special attention needs to be paid to cope with the natural variability in the quality of raw materials. 

In this chapter the three main modes of large-scale chromatographic operation, and combined reaction and separation. Many useful but small-scale chromatographic methods have been omitted, as well as allied separation techniques which combine aspects of chromatographic principles or practice with aspects of adsorption, extraction, sedimentation or electrophoresis. Such is the pace of invention that novel processes related to chromatography are still being developed and described in the literature. 

The best-known and (commercially) most successful example of combining reaction and separation is the reactive distillation. Reactive distillation has been investigated widely [3, 4 see also Chapters 3, 4, and 5] and is applied to many processes [5], However, the integration of more than one functionality in an apparatus leads to a loss in degrees of freedom. For a successful integration, the feasible window of operation concerning process conditions such as pressure and temperature must coincide for the reaction, the separation and the apparatus (Fig. 8.1). 

Combined Reaction and Separation Devices The continuous removal of ethanol from the reaction zone favors building up a higher yeast concentration and accordingly a higher reaction rate. The separation of ethanol can be achieved by a membrane, vacuum and solvent extraction. 

Figure 4. Units combining reaction and separation (a) reactor with countercurrent moving bed, (b) simulated moving-bed reactor with four catalyst layers (bold lines show gas streams during a quarter of cycle), (c) pressure-swing adsorption reactor separator (on the left i given time dependency of pressure during a cycle)...

Can the separation and reaction sections be suitably combined Combining reaction and separation in one unit can have some advantages. It often removes the equilibrium barrier, and, in some cases, complete recycle of unreacted feed is achieved in the same vessel. This question is more important in the design stage of new processes and is somewhat difficult to address in retrofitting existing processes. Membrane separations provide another area where reaction and separation can be combined. 

One of the most appealing methods for process intensification is the combination of more fimctions in a single unit. Reactive distillation— combining reaction and separation— is a prime example. However, reactive distillation can be applied only for processes in which the temperature window for reaction and separation coincides. In this respect, reactive stripping is more flexible. Another difference between the two is that reactive distillation is limited to countercurrent operation, whereas for reactive stripping both cocurrent and countercurrent operation are possible, because of the low degree of interaction between the two phases in this regime of separation (23). 

Interest in combining reaction and separation is growing. Thus industrial examples exist of reactive distillation and reactive crystallization. In this respect catalytic membranes [I] and membrane reactors come to the fore and are worldwide studied. 

The use of zeolitic membranes in separation or combined reaction and separation processes is very appealing. Advantages of using this type of membrane include not only their ability to discriminate between molecules based on molecular size but also their thermal stability. The large variety of zeolite types could provide a tailor-made separation medium for specific processes. Moreover, the properties of zeolites are often easily adjustable (ion exchange, Si/Al ratio, etc.). This makes zeolitic membranes also very promising for use as catalytic membranes. 

Membrane reactors are another type of system used for continuous reactions. They are unique in that they combine reaction and separation in a single operation with the membrane not only selectively removing some of the reactants or products fi-om the reacting area but also frequently acting as the catalyst or support for the catalyst used in the reaction. 6-29,30 xhe operation of such reactors is most easily described using a palladium membrane reactor for a... 

Extractive Reaction Extractive reaction combines reaction and separation in the same unit operation for the purpose of facilitating a desired reaction. To avoid confusion, the term extractive reaction is recommended for this type of process, while the term reaction-enhanced extraction is recommended for a process involving formation of reversible solute-extractant interactions and enhanced partition ratios for the purpose of facilitating a desired separation. The term reactive extraction is a more general term commonly used for both types of processes. 

One way to simplify processes is to eliminate equipment, by combining reaction and separation. The use of membranes is discussed in Chapter 4. Another relevant example is reactive distillation. Figure 1.16 compares the traditional methyl acetate process with that based on reactive distillation (Eastman Chemical) [97-99]. Eastman... 

An LM enzyme system, which can combine reaction and separation as well as maintain a concentration gradient between internal and external aqueous phases, is thus being investigated for the L-tyrosine to L-DOPA conversion reaction. This combined enzyme reactor / separation system... 

In all cases a reaction and a separation step are combined. Reaction and separation can be done in the same unit, i.e., simultaneously, or in separate units, i.e. successively. 

Catalytic distillation and other process configurations that combine reaction and separation in a single vessel are relatively new. Currently, only a few commodity chemicals are manufactured using catalytic distillation. This is not due to a lack of versatility of this design concept. Rather it is a reflection of the timing of process selection. The choice between process configurations that are as different as fixed bed reactors and catalytic... 

Catalytic distillation (CD) is an unit operation combining reaction and separation in a single reactor/distillation column. CD belongs to the general class of two-phase flow fixed-bed catalytic reactor. An upward flow of vapor and downward flow of liquid conqjrise the two flowing phases in the CD reactor. Solid catalyst packed in a distillation column not only accelerates a chemical reaction but also supplies a packing surface for vapor-liquid mass transfer to separate the reactants... 

Combining reaction and separation in the same device leads in principle to the most compact and economic design. For this reason Reactive Distillation (RD) has raised a high interest, both from industrial and scientific point of view. A synonym term is Catalytic Distillation, because only a catalyst can enhance the reaction rate at values compatible with the separation requirements. 

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