Membrane reactors drawbacks

A major drawback of surface diffusion for high-temperature membrane reactor application lies in the fact that adsorptive bonds l>etween molecules and surfaces become less and less strong as long as temperature increases, thus lowering the separation factors achievable. 

Concentration polarization phenomena, which are the main drawbacks of the aforesaid enzyme membrane reactors, can nevertheless be used to form, either in dynamic or in static conditions, a gel layer of enzyme proteins on a membrane.27 1 It is even possible to establish more than one enzyme layer and no coupling agent is needed to carry out the immobilization. Due to high protein... 

The structure of this chapter is as follows. In the next section a comparison of the FBR and PBMR will be presented using a simplified ID modeL Advantages and drawbacks of the PBMR will be illustrated. Subsequently, a more detailed 2D model will be developed to study important aspects of radial mass and heat transfer, as well as scale-up problems that might occur in a PBMR. In the last section a short outlook to more sophisticated 3D membrane reactor models is given. Such models are still not suitable for extensive parametric studies. However, they enable a deeper investigation of local velocity and concentration profiles that develop in such reactors. 

As a main scope, the present chapter will give an overview on the general classification of the membranes, paying particular attention to the palladium-based membranes and their applications, pointing out the most important benefits and the drawbacks due to their use. Finally, the application of palladium-based membranes in the area of the membrane reactors will be illustrated and such reaction processes in the issue of hydrogen production will be discussed. 

In spite of the drawbacks of ceramic membranes for DH processes, namely their low separation selectivity to hydrogen, coke deposition, and their mechanical strength which is often insufficient for high temperature applications, a significant amount of experimental work has been reported on the subject of DH in ceramic membrane reactors [21, 27-29, 32, 40, 41] and there are some recent reports on ceramic membranes with high selectivity to H2 (e.g., [26]). 

Theoretical aspects, advantages, and drawbacks of fixed beds versus monoliths, microreactors, and membrane reactors... 

The combination of these drawbacks has driven the research toward new reactor concepts such as micro-membrane reactors or fluidized bed membrane reactors as discussed in the following sections. 

Abstract Membrane reactor research has been focused on new membrane materials to be integrated in a compact configuration. Carbon membranes have scarcely been explored in the past because of mechanical drawbacks. For this reason, it is recommended that carbon membranes are supported. However, this can cause the formation of defects which are disadvantageous in membrane reactor (MR) applications. This chapter explores the main variables to be considered in the development of carbon membranes, mainly focusing on when the carbon material has to be supported. Some applications are revised for macro and micro reactors. 

The first applications of CMRs have concerned high temperature reactions. The employed inorganic membranes, characterized by higher chemical and thermal stability with respect to polymeric membranes, still today suffer from some important drawbacks high cost, limited lifetime, difficulties in reactor manufacturing (delamination of the membrane top-layer from the support due to the different thermal expansion coefficients). 

Most of the membrane segregated enzyme systems previously examined suffer some constitutive drawbacks which limit their yield and area of application. When enzymes are entrapped within the sponge of asymmetric membranes, product and substrate mass transfer occur mainly by a diffusive mechanism reactor performance is then controlled only by means of the amount and kind of charged enzyme, and the fluid dynamics of the solution in the core of the fibers. UF or RO fluxes, moreover, result in enzyme losses. Enzyme crosslinking in the membrane pores can reduce these losses, but it can lead to an initial activity loss, as compared to that of the native enzyme. Of course, once the enzyme is deactivated, it makes the reactor useless for further operation. Such immobilization techniques are seldom useful for microbial cells due to their large size. 

The light-off temperature increased from 150°C at 400 ppm to 190 °C at 4700 ppm. It was significant that these were among the lowest reported for VOC combustion. A range of 230-300 °C is more usual. The main drawback to this reactor configuration is the pressure drop across the membrane. The authors pointed out that increased pressure drop means increased operating cost, and that membrane optimization is needed to obtain the right balance between pressure drop and conversion. 

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