Synergy, membrane reactors

The photocatalytic membrane reactors (PMRs) combine the advantages of classical photoreactors (catalyst in suspension) and those of membrane processes (separation at molecular level) with a synergy for both technologies. 

The presence of the membrane enhances the "per-pass" conversion and, in turn, enhanced "per-pass" conversions diminish the downstream separation requirements. Throughout this chapter, the reader should look for the need for this synergy. It is where a difficult separation problem exists, coupled to a "per-pass" conversion or selectivity or equilibrium limitation problem that the application of membrane reactors makes the best sense. 

Although there is no commonly accepted definition of a membrane reactor (MR), the term is usually applied to operations where the unique abilities of membranes to organize, compartmentalize, and/or separate are exploited to perform a (bio)chemical conversion under conditions that are not feasible in the absence of a membrane. In every MR, the membrane separation and the (bio)catalytic conversion are thus combined in such a way that the synergies in the integrated setup entail enhanced processing and improved economics in terms of separation, selectivity, or yield, compared to a traditional configuration with reactor and separation separated in time and space. When the membrane itself carries the catalytic functions, it is mostly referred to as a reactive membrane. ... 

In addition, a reactor may perform a function other than reaction alone. Multifunctional reactors may provide both reaction and mass transfer (e.g., reactive distillation, reactive crystallization, reactive membranes, etc.), or reaction and heat transfer. This coupling of functions within the reactor inevitably leads to additional operating constraints on one or the other function. Multifunctional reactors are often discussed in the context of process intensification. The primary driver for multifunctional reactors is functional synergy and equipment cost savings. 

Often the coupling of the membrane unit with the bioreactor results in significant synergy as in the study of O Brien et al. [6.15] on the application of PVMBR to ethanol production, which we discussed in Chapter 3. The required bioreactor volume for the PVMBR system was smaller than that of the conventional system by a factor of 12. Nevertheless, it turns out that the PVMBR-based process is still 25 % more expensive than the classical batch fermentation process in terms of capital costs despite the substantial reduction in the required reactor volume. This cost differential is not only due to the membrane costs, which are, themselves, substantial, but also due to the cost of the additional hardware associated with membrane operation. The application of MBR for the ethanol production by fermentation faces marginal economics, since ethanol is a relatively cheap commodity chemical. 

This led to higher C2 yields, while the CH4 permeability was also kept low, and the contact time was high enough. Removal of the desired products helped the distributed feed PBMR reactor more than the conventional co-feed PFR reactor, i.e. a synergy between the two membrane functions was observed. Unfortunately a membrane with the desired characteristics of selective removal of C2 species at high temperature is unknown. 

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