Membrane reactors permselective separation

Non-permselective membrane reactor with separate feed of reactants... 

Figure 5 Catalytic non-permselective membrane reactor with separated feed, showing transmembrane concentration profiles of the species and the direction of permeation...

The situation is somewhat different with porous membranes, where the permselectivities for all components do not equal zero but exhibit certain values determined in most cases by the Knudsen law of molecular masses. In general, when porous membranes are used as separators in a membrane reactor next to the catalyst or the reaction zone (Figure 7.2a), it has been shown experimentally (Yamada et al. 1988) and theoretically (Mohan and Govind 1986, 1988a, b, Itoh et al. 1984, 1985) that there is a maximum equilibrium shift that can be achieved. On the basis of simple mass balances one can calculate that this maximum depends on, besides the reaction mechanism, the membrane permselectivities (the difference in molecular weights of the components to be separated) and it corresponds to an optimum permeation to reaction-rate ratio for the faster permeating component (which is a reaction product). 

Cox et al. (1995) portray a new approach to thermochemical gasification of biomass to hydrogen. The process is based on catalytic steam gasification of biomass with concurrent separation of hydrogen in a membrane reactor that employs a permselective membrane to separate the hydrogen as it is produced. The process is particularly well-suited for wet biomass and may be conducted at temperatures as low as 575 K. 

Novel unit operations currently being developed are membrane reactors where both reaction and separation occur simultaneously. Through selective product removal a shift of the conversion beyond thermodynamic equilibrium is possible. The membrane itself can serve in different capacities including (i) a permselective diffusion barrier, (ii) a non-reactive reactant distributor and (iii) as both a catalyst and permselective membrane [44]. 

In a separate parametric study, Mohan and Govind(l)(9) analyzed the effect of design parameters, operating variables, physical properties and flow patterns on membrane reactor. They showed that for a membrane which is permeable to both products and reactants, the maximum equilibrium shift possible is limited by the loss of reactants from the reaction zone. For the case of dehydrogenation reaction with a membrane that only permeates hydrogen, conversions comparable to those achieved with lesser permselective membranes can be attained at a substantially lower feed temperature. 

When a reaction involves the evolution of a species while another reaction consumes the same species and a membrane can be found that is permselective to this species, it may be feasible to essentially combine the two membrane reactors involved into a compact single reactor configuration (see Figure 8.4). In this arrangement, a single membrane compartmentalizes the two reactor zones and keeps other reaction components separated. 

Permselectivity is crucial to the utility of any types of membranes. If the permselectivity toward a particular reaction species is high, the separation is quite clean and the need for further separation processing downstream of the membrane reactor is reduced. When a permeate of very high purity is required in some cases, dense membranes are preferred. While a high permselectivity is generally desirable, there may be situations where a high permeate flux in combination with a moderate permselectivity is a better alternative to a high permselectivity with a low permeability, particularly when recycle streams are used. 

Membrane surface contamination. Although not as hydrogen selective as Pd and its alloys, other metals such as niobium and vanadium in dense form also have moderate to high hydrogen permselectivity and potentially can be considered as membrane materials. Inevitably the membrane surface is contaminated with non-metal impurities prior to or during separation or membrane reactor applications. 

When combining the separator and the reactor functions into one compact physical unit, factors related to catalysis need to be considered in addition to those related to selective separation discussed in previous chapters. The selection of catalyst material, dispersion and heat treatment and the strategic placement of catalyst in the membrane reactor all can have profound impacts on the reactor performance. The choice of membrane material and its microstructure may also affect the catalytic aspects of the membrane reactor. Furthermore, when imparting catalytic activity to inorganic membranes, it is important to understand any effects the underlying treatments may have on the permeability and permselectivity of the membranes. 

There are, however, some studies demonstrating another concept of using porous membranes. In this concept, the permselective property of a membrane is immaterial and not utilized. Instead, the well sU uctured porous maU ix of the membrane serves to provide a well controlled reaction zone. Specifically, two reactants are fed separately to the opposing sides of a catalytic membrane. For those reactions the rate of which is faster than the diffusion rate of the reactants in the membrane, the reaction can take place inside the caialytically active membrane. This type of membrane reactor, where the membranes are catalytic but not selective, are called catalytic non-pcrmselective membrane reactors (CNMR). 

The coupling of a permselective membrane with a packed bed of catalyst pellets (Fig. 5b) has been one of the most widely studied membrane reactor setups. Generally, the catalyst fixed bed is enclosed on the tube side of a porous membrane, although several cases can be found in the literature in which permselective tubular membranes have been inserted at regularly spaced intervals into the packed bed of catalyst pellets (e.g.. Ref. 25). The most interesting property of this membrane reactor type is that the amount of catalyst and the membrane surface area can be varied almost independently within wide ranges, so as to optimize the coupling of reaction and separation. 

One is membrane reactor to enhance one pass conversion of equilibrium limited HI decomposition reaction in the gas phase using the hydrogen permselective membrane. The other is concern with the membrane application to enhance the HI molality of HI-I2-H2O mixture in order to facilitate the separation of pure HI. These second application is caused by the lack of capability to distill the Hix solution for the concentration HI more than 56%. To reach the HI concentration of 56% by conventional distillation, even consume a significant huge of heat, the distillation process would be decreased the over all thermal efficiency of the process. 

Membranes are classified by whether the thin permselective layer is porous or dense, and by the type of material (organic, polymeric, inorganic, metal, etc.) this membrane film is made from. The choice of a porous vs. a dense film, and of the type of material used for manufacturing depends on the desired separation process, operating temperature and driving force used for the separation the choice of material depends on the desired permeance and selectivity, and on thermal and mechanical stability requirements. For membrane reactor applications, where the reaction is coupled with the separation process, the thin film has also to be stable under the reaction conditions. 

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