Membrane reactor permeability ratios

The values in the parentheses are the permeability ratios for the uncoated composite membrane. The location of substrate within a reactor is not the same. 

To measure the separation efficiency of a membrane reactor involving multiple reaction components, the extent of separation, briefly introduced in Chapter 7, was used to replace the more commonly used separation factor by Mohan and Govind [1988a]. This alternative index of separation performance is based on the flow quantities of the process streams involved while the separation factor is calculated from the compositions instead. The goals of a high conversion and a high separation sometimes contradict each other. The choice or, more often than not, compromise of the two goals depends, on one hand, on the downstream separation costs and, on the other, on the process parameters such as the ratio of the reactant permeation to reaction rate and the relative permeabilities of the reaction components. 

Figure 11.29 Conversion and separation index of a membrane reactor as a function of permeation to reaction rate ratio when reactant permeability is smaller than product permeabilities [Mohand and Govind, 1988a]...

A reference case for the C02-selective WGS membrane reactor was chosen with the C02/H2 selectivity of 40, the C02 permeability of 4000 Barrer, the inlet sweep-to-feed molar flow rate ratio of 1, the membrane thickness of 5jum, 52,500 hollow libers (a length of 61 cm, an inner diameter of 0.1 cm, and a porous support with a porosity of 50% and a thickness of 30jLon), both inlet feed and sweep temperatures of 140 °C, and the feed and sweep pressures of 3 and latm, respectively. With respect to this case, the effects of C02/H2 selectivity, C02 permeability, sweep-to-feed ratio, inlet feed temperature, inlet sweep temperature, and catalyst activity on the reactor behavior were then investigated. 

Pervaporation membrane reactors are not a recent discovery. The use of a PVMR was proposed in a U.S. patent dating back to 1960 [3.6]. Though the technical details on membrane preparation and experimental apparatus were rather sketchy, the basic idea was described there, namely, the use of a water permeable polymeric membrane to drive an esterification reaction to completion. A more detailed description of a PVMR can be found in a later European patent [3.7], which described the use of a flat membrane (commercial PVA or Nafion ) placed in the middle of a reactor consisting of two half-cells. The reaction studied was the acetic acid esterification reaction with ethanol. For an ethanol to acetic acid ratio of 2, liquid hourly space velocities (LHSV) in the range of 2-5, and a temperature of 90 °C complete conversion of the acetic acid was reported. The use of PVMR for this reaction shows promise for process simplification, as indicated schematically in Figure 3.2, which shows a side-by-side comparison of a conventional and a proposed PVMR plant for ethyl acetate production. 

Damle et al. [50] in 1992 developed a simplified process model to simulate catalytic membrane WGS reactor. They assumed the permeability ratios of different gases to be constant during membrane separation. The model further assumes that the WGS reaction is not limited by chemical kinetics and thus... 

Non-permselective membranes can also be used to provide a location for a reaction zone. One reactant is fed on the tube side of the membrane, and the other reactant is fed on the shell side. The partial-pressure gradients have to be chosen such that the two reactants permeate towards each other inside the membrane, where they can react. Usually, the membrane itself contains a suitable catalyst. In this type of membrane reactor, reactions are performed at a strict stoichiometric ratio. For fast reactions, this results in a reaction plane, whereas for slower reactions a reaction zone will be formed. This is shown schematically in Fig. 5.4. Balancing reaction rate and permeability can result in a reaction zone entirely located inside the membrane. When breakthrough of reactants can be avoided, and the product diffuses out on one side only, this can simplify the further separations required. 

Figure 5.30 illustrates the concentration distributions (circumferentially averaged) of the component dosed through a uniformly permeable membrane for Fts/Fss ratios 10 and 0.56. As expected, the concentration of the dosed component increases along the reactor length. However, the increase is not strictly monotonic, there are small tunnels , or, e.g., next to the outlet, even sharp concentration peaks visible. These fluctuations in the concentration profile are present for both simulated conditions. They result from the bed structure and from the simplifying assumption of impermeable particles. 

---