Concentration management

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Concentration control is a term from chemical reaction engineering . This is understood to mean, through the choice of the residence time behavior of the apparatus, suitable feed and take-off points as well as possible recirculations, specifically to set stationary or dynamic concentration profiles that contribute to the desired behavior of the reactor. In addition, what happens in the reactor is influenced by suitable temperature control.

In the case of a simple reaction, the focus is on achieving the highest possible specific product output, i.e. achieving the desired conversion with a given throughput in a minimal volume. If, as in most cases, the reaction has a positive order, the optimum product performance is achieved in a reactor with flow tube characteristics (PFR, Plug Flow Reactor). With a reactor with stirred tank characteristics (CSTR, Continuously Stirred Tank Reactor), on the other hand, lower space-time yields are achieved, since the reaction takes place in the entire volume at the low concentration level of the outlet.

If, on the other hand, one is dealing with complex reactions, i.e. with the simultaneous occurrence of mostly undesirable side reactions, the focus of concentration management is usually on the control of selectivity. If the desired product continues to react in a subsequent reaction, the PFR is to be preferred, since here in the first part of the tube there is a low product concentration with a simultaneously high reactant concentration, so the reaction proceeds very selectively at least in this part. Only when very high conversions and thus residence times are required does the selectivity collapse in the rear part of the flow tube. In the CSTR, on the other hand, the high product concentration is present in the entire volume, which favors the side reaction. If, on the other hand, there is an undesirable side reaction parallel to the desired main reaction, the choice of the optimum concentration regime depends on the order of the reaction. If the reaction order of the secondary reaction is higher than that of the main reaction, the reaction proceeds more selectively at a lower concentration level, then a CSTR is the suitable type of reactor.

There are graphic methods for optimizing the concentration management of simple reactions; in the case of complex reaction networks, only simulation calculations generally help to find the optimal management of the reactions.

A well-known example of the coupled concentration and temperature management is exhaust gas recirculation during combustion. The increased proportion of exhaust gas in the combustion chamber reduces the combustion temperature and thereby suppresses the formation of nitrogen oxides .

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