Reactor chromatographic

Reverse-flow reactors Reactive distillation Reactive extraction Reactive crystalization Chromatographic reactors Periodic separating reactors Membrane reactors Reactive extrusion Reactive comminution Fuel cells... 

Figure 7. Effect of methane conversion on C2 selectivity for some of the best state-of-the-art OCM catalysts (A, based on ref 4), the simulated chromatographic reactor of Aris and coworkers (A, ref. 10) and the present work. ( ) Ag electrocatalyst, single pass (O) Ag electrocatalyst with recycle and trapping (0) Sr/LagOg catalyst, single pass ( ) Sr/La20g catalyst with recycle and trapping. Open symbols, batch operation filled symbols, continuous-flow steady-state operation.

Transient reactors, such as pulse (chromatographic) reactors, temporary analysis of products (TAP) reactors, multitrack reactors, and temperature-programmed reactors have been developed mainly to study gas-solid (catalyst) reactions. These are rather sophisticated techniques used to study mechanisms of catalytic processes at the molecular level in great detail. Since this is rarely done in the development of processes for the manufacture of fine chemicals and pharmaceuticals, these reactors are not discussed further. The interested reader is referred to works by Anderson and Pratt (1985) and Kapteijn and Moulijn (1997). 

In the last decade there were many papers published on the study of enzyme catalyzed reactions performed in so-called chromatographic reactors. The attractive feature of such systems is that during the course of the reaction the compounds are already separated, which can drive the reaction beyond the thermodynamic equilibrium as well as remove putative inhibitors. In this chapter, an overview of such chromatographic bioreactor systems is given. Besides, some immobilization techniques to improve enzyme activity are discussed together with modern chromatographic supports with improved hydrodynamic characteristics to be used in this context. 

Keywords Chromatographic reactor, Chromatographic bioreactor, Immobilization, Chromatographic supports... 

The products leave the chromatographic reactor already separated and thus, no further purification steps are required. Therefore, two operations, namely reaction and separation, are combined in a single unit, which significantly reduces the costs of the whole process [131]. 

Fig. 3. Schematic presentation of the operating principle of a batch chromatographic reactor. A pulse of compound A is injected into the reactor. As the substance travels through the reactor it is converted into compounds B and C, which are continuously separated. (Reprinted with permission from [134])...

The full advantage of the chromatographic reactor (simultaneous reaction and separation) is fully realized only for selected types of reactions, which are briefly summarized below [132]. 

In this case the application of a chromatographic reactor leads to significantly higher conversion when B and C are eluted on either side of A. It means that the capacity factors should be as follows Kb>Ka>Kc or alternatively Kc>Ka>Kb. In the case of B and C eluting on the same side of A, even if they are separated, the improvement to be expected is much smaller [133]. 

Let R be the desired product. In this case, we have a competition between the rate of the second reaction and the rate of separation of R from B. Under these circumstances, the feeding mode determines the efficiency of the conversion. However, so far there are no published experimental results that would demonstrate the possibility of increasing yield of R by using a chromatographic reactor with simultaneous separation. 

Increased conversion and product purity are not the only benefits of simultaneous separation during the reaction. The chromatographic reactor was also found to be a very suitable tool for studying kinetics and mechanisms of chemical and biochemical reactions. Some recent publications describe the results on investigation of autocatalytic reactions [135], first-order reversible reactions [136], and estimation of enantioselectivity [137,138]. It is beyond the scope of this chapter to discuss the details, but the interested reader is referred to an overview published by Jeng and Langer [139]. 

Most publications dealing with chromatographic reactors focus on theoretical issues of this very complex system. Models of different complexity were derived and used to predict the behavior of chromatographic reactors. Such models typically take into consideration different types of mass transfer, adsorption isotherms, flow profiles, and reactions. A general scheme of these models, not including the reaction, is presented in Fig. 4. There are also several review papers... 

A chromatographic reactor can be realized with different configurations from a single fixed-bed reactor to multireactor arrangement enabling continuous operation. Here, a short description of the basic types together with some recent results are presented. 

Fig. 5. Complex behavior of a batch chromatographic reactor system. After an inlet step, three steady states were detected at the reactor outlet. Experimental data for acetic acid (filled circle), ethanol (x), water (+) and ethyl acetate (open circle) were successfully fitted by a mathematical model (solid and dashed lines). (Reprinted with permission from [159])... 

As already discussed, the enhanced conversion is due to the separation of the products from the reaction zone. This can be realized via different distribution coefficients of the compounds (and consequently, a separation of the components) or via (selective) adsorption on a support. Since in the first case the compound travels through the reactor with different speeds, a continuous feed would cause repeated mixing of the separated compounds. Therefore, no improvement can be expected. In the second case, a regeneration of the adsorbent is needed after a certain operative period. This is an inherent drawback of the discontinuous operation of the fixed-bed chromatographic reactor. 

For optimal performance, the feeding strategy for the chromatographic reactor should be carefully designed. In addition, the following criteria should be fulfilled [132] ... 

Recently, Falk and Seidel-Morgenstern [143] performed a detailed comparison between fixed-bed reactors and fixed-bed chromatographic reactors. The reaction studied was an equilibrium limited hydrolysis of methyl formate into formic acid and methanol using an ion-exchange resin as both the catalyst and the adsorbent. The analysis was based on a mathematical model, which was experimentally verified. The comparison was based on the following four assumptions ... 

The same amount of feed was introduced into both reactors i.e., pulses of higher concentration were injected into chromatographic reactor but due to the periods without feeding, the average concentration was equal to the concentration continuously feed into the fixed-bed reactor. 

Fig. 6. Schematic presentation of a continuous annular chromatographic reactor. The sample (big arrow) and the mobile phase (small arrows) are continuously introduced from the top of the column, which rotates with the constant angular velocity co. Passing through the column, compound A is converted into the compounds B and C. Due to their different retention they split in three streams and exit at different positions from the column. (Reprinted with permission from [144])... 

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