The Recycle Reactor Concept

Various experimental methods to evaluate the kinetics of flow processes existed even in the last centuty. They developed gradually with the expansion of the petrochemical industry. In the 1940s, conversion versus residence time measurement in tubular reactors was the basic tool for rate evaluations. In the 1950s, differential reactor experiments became popular. Only in the 1960s did the use of Continuous-flow Stirred Tank Reactors (CSTRs) start to spread for kinetic studies. A large variety of CSTRs was used to study heterogeneous (contact) catalytic reactions. These included spinning basket CSTRs as well as many kinds of fixed bed reactors with external or internal recycle pumps (Jankowski 1978, Berty 1984.) 

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The differential reactor is the second from the left. To the right, various ways are shown to prepare feed for the differential reactor. These feeding methods finally lead to the recycle reactor concept. A basic misunderstanding about the differential reactor is widespread. This is the belief that a differential reactor is a short reactor fed with various large quantities of feed to generate various small conversions. In reality, such a system is a short integral reactor used to extrapolate to initial rates. This method is similar to that used in batch reactor experiments to estimate... 

A new reactor concept for the study of catalyst deactivation is presented, it consists of the combination of an electrobalance and a recycle reactor. With the electrobalance, the coke content on the catalyst is measured continuously. The recycle reactor operates gradientlessly at high conversion, with on-line gas chromatographic analysis of the effluent. Thus, the catalyst activity and product selectivities may be coupled directly with the coke content and the coking rate on the catalyst. 

This concept was used for the study of the deactivation of n-hexane catalytic cracking on a US Y zeolite catalyst. The interpretation of the flow patterns in the recycle reactor, necessary for the quantification of the degree of mixing, was based upon tracer experiments. 

The general concept of balances, as explained in detail in Appendix 1, can be applied to a recycle reactor. Figure 3.6.1 shows the possibilities for balance calculations in a recycle reactor. 

The following flowsheet represents the simplest connections combined with good, inexpensive manual regulation required to execute valid experiments. This is the recommended minimum starting installation that can be expanded and made more sophisticated as need and budgets permit. The other extreme, a fully computer controlled and evaluated system that can be run without personnel will be shown later. The concepts, mentioned in Chapter 3, are applied here for the practical execution of experiments in recycle reactors. 

The application of a selective pyrolysis process to the recovery of chemicals from waste PU foam is described. The reaction conditions are controlled so that target products can be collected directly from the waste stream in high yields. Molecular beam mass spectrometry is used in small-scale experiments to analyse the reaction products in real time, enabling the effects of process parameters such as temperature, catalysts and co-reagents to be quickly screened. Fixed bed and fluidised bed reactors are used to provide products for conventional chemical analysis to determine material balances and to test the concept under larger scale conditions. Results are presented for the recycling of PU foams from vehicle seats and refrigerators. 12 refs. 

The process concept is shown in Fig. 26.7 where the recycle loop of the caustic scrubbing liquor passes through a fixed-bed reactor and then through the normal cooler. The blow-down of spent caustic and make-up with fresh caustic can be carried on in the same fashion as without the in-loop hypochlorite decomposition. Consideration of the optimum locations for removal and addition may, however, be slightly different. 

Conversion is defined as the percent of the feed that disappears in a chemical reaction. Take the pounds of benzene, for example, coming our of the ethylbenzene reactor, divide by the pounds of benzene going into the reactor, subtract that from 1.0 and multiply by 100, and you.get percent benzene conversion. Conversion is usually measured around a reactor, not around the whole plant. In the case of the ethylbenzene plant, since there is a benzene recycle, there is virtually no benzene leaving the EB plant. So plant conversion is often not a very helpful concept, but the conversion across the reactor is. 

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