Membrane reactors limit conversion

Applying an isothermal and plug>flow membrane reactor (on both sides of the membrane) to the above reactions, Itoh and Xu [1991] concluded that (1) the packed-bed inert membrane reactor gives conversions higher than the equilibrium limits and also performs better than a conventional plug-flow reactor without the use of a permselective membrane and (2) the co-current and counter-current flow configurations give essentially the same conversion. 

Fig. 9.5 Left side Pd-Ag membrane reactor isobutene conversion vs. feed space velocity, compared with equilibrium-limited and fixed-bed reactor (argon swept, T = 723 K, after [33]) right side carbon membrane reactor conversion, in the countercurrent sweep and vacuum modes, as a function of feed molar flows at 500°C also denoted are the conventional (non-membrane) reactor conversion and the simulated countercurrent sweep mode behavior (after [23])...

As most of the chemical reactions for hydrogen production are equilibrium-limited reactions, the integration of hydrogen separation through membranes in the reactor leads to equilibrium shift toward the products. Therefore, in membrane reactors higher conversions can be achieved compared with conventional reactors operated under the same conditions (pressures and temperatures) [1]. 

Membrane reactors are known on the macro scale for combining reaction and separation, with additional profits for the whole process as compared with the same separate functions. Microstructured reactors with permeable membranes are used in the same way, e.g. to increase conversion above the equilibrium limit of sole reaction [8, 10, 11, 83]. One way to achieve this is by preparing thin membranes over the pores of a mesh, e.g. by thin-fihn deposition techniques, separating reactant and product streams [11]. 

Membrane reactors can offer an improvement in performance over conventional reactor configurations for many types of reactions. Heterogeneous catalytic reactions in membrane reactors [1] and the membranes used in them [2,3] have been reviewed recently. One well studied application in this area is to remove a product from the reaction zone of an equilibrium limited reaction to obtain an increase in conversion [4-10]. The present study involves heterogeneous... 

It may be necessary to improve membrane selectivities, so that further purification of the produced hydrogen before re-use in the desulphurisation units can be limited as far as possible. Moreover the membrane reactor can be optimised for various variables, such as H2S conversion, hydrogen recovery, membrane area and temperature. In a techno-economic evaluation combined with advanced process design the impact of different operating parameters on the investment and operating costs should be studied. 

In the case of dense membranes, where only hydrogen can permeate (permselectivity for H2 is infinite), the permeation rate is generally much lower than the reaction rate (especially when a fixed bed is added to the membrane). Experimental conditions and/or a reactor design which diminishes this gap will have positive effects on the yield. An increase of the sweep gas flow rate (increase of the driving force for H2 permeation) leads to an increase in conversion and, if low reactant flow rates are used (to limit the H2 production), conversions of up to 100% can be predicted [55]. These models of dense membrane reactors explain why large membrane surfaces are needed and why research is directed towards decreasing the thickness of Pd membranes (subsection 9.3.2.2.A.a). 

The conditions are substantially more favorable for the microporous catalytic membrane reactor concept. In this case the membrane wall consists of catalyti-cally active, microporous material. If a simple reaction A -> B takes place and no permeate is withdrawn, the concentration profiles are identical to those in a catalyst slab (Fig. 29a). By purging the permeate side with an inert gas or by applying a small total pressure difference, a permeate with a composition similar to that in the center of the catalyst pellet can be obtained (Fig. 29b). In this case almost 100% conversion over a reaction length of only a few millimeters is possible. The advantages are even more pronounced, if a selectivity-limited reaction is considered. This is shown with the simple consecutive reaction A- B- C where B is the desired product. Pore diffusion reduces the yield of B since in a catalyst slab B has to diffuse backwards from the place where it was formed, thereby being partly converted to C (Fig. 29c). This is the reason why in practice rapid consecutive reactions like partial oxidations are often run in pellets composed of a thin shell of active catalyst on an inert support [30],... 

In membrane reactors, the reaction and separation processes take place simultaneously. This coupling of processes can result in the conversion enhancement of the thermodynamically-limited reactions because one or more of the product species is/are continuously removed. The performance of such reactors depends strongly on the membrane selectivity as well as on the general operahng conditions which influence the membrane permeability. 

Preliminary results obtained in an effort to model the dehydrogenation of ethylbenzene to styrene in a "membrane reactor" are described below. The unique feature of this reactor is that the walls of the reactor are conprised of permselective membranes through which the various reactant and product species diffuse at different rates. This reaction is endothermic and the ultimate extent of conversion is limited by thermodynamic equilibrium constraints. In industrial practice steam is used not only to shift the ec[uilibrium extent of reaction towards the products but also to reduce the magnitude of the ten erature decrease which accon anies the reaction when it is carried our adiabatically. 

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. 

In contrast to the studies on gas- and vapor-phase hydrogenation reactions utilizing dense Pd-based membrane reactors, dehydrogenation reactions have been consistently observed to benefit from the concept of a membrane reactor. In almost all cases the reaction conversion is increased. This is attributed to the well known favorable effect of equilibrium displacement applied to dehydrogenation reactions which are mostly limited by the equilibrium barrier. 

Similar to the case of dehydrogenation or other hydrogen-generating reactions, the use of a dense membrane reactor to remove oxygen from an oxygen-generating reaction such as decomposition of carbon dioxide displaces the reaction equilibrium and increases the conversion from 1.2% (limited by the equilibrium) to 22% [Nigara and Cales, 1986]. This has been confumed by Itoh et al. [1993]. 

Mohan and Govind [1988c] applied their isothermal packed-bed porous membrane reactor model to the same equilibrium-limited reaction and found that the reactor conversion easily exceeds the equilibrium value. The HI conversion ratio (reactor conversion to equilibrium conversion) exhibits a maximum as a function of the ratio of the permeation rate to the reaction rate. This trend, which also occurs with other reactions such as cyclohexane dehydrogenation and propylene disproportionation, is the result of significant loss of reactant due to increased permeation rate. This loss of reactant eventually negates the equilibrium displacement and consequently the conversion enhancement effects. 

Effects of space time under nonisothermal conditions. The above discussions around the effects of space time on a membrane reactor performance are limited to isothermal conditions. The behavior of the reaction conversion in response to space time can be further complicated under nonisothermal conditions. 

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