Permselective reactor

Permselective Reactor Model. This model was developed to size an experimental reactor system. Even though, some of our initial assun tions are being refined in our continuing efforts, some in ortant conclusions can be drawn from our early work. The first approach used was to idealize the permselectivity, and assume that only hydrogen diffuses through the membrane. This assun tion cannot be justified for the characteristics of the membranes studied experimentally. It does, however, permit one to place an upper limit on the expected performance of the system. 

Figure 10 contrasts the conversion obtained with a conventional reactor to that conversion obtained with a permselective reactor when the partial pressure of the hydrogen on the sweep side is 0.02 atm. It can be seen that the difference in conversion between the two systems increases as the temperature increases. An important operational limitation is imposed by the sintering temperature for the membranes. It should be noted that this factor is not considered since the membranes are assumed to be stable. 

In open fibers the fiber wall may be a permselective membrane, and uses include dialysis, ultrafiltration, reverse osmosis, Dorman exchange (dialysis), osmotic pumping, pervaporation, gaseous separation, and stream filtration. Alternatively, the fiber wall may act as a catalytic reactor and immobilization of catalyst and enzyme in the wall entity may occur. Loaded fibers are used as sorbents, and in ion exchange and controlled release. Special uses of hoUow fibers include tissue-culture growth, heat exchangers, and others. 

Catalytic A catalytic-membrane reactor is a combination heterogeneous catalyst and permselective membrane that promotes a reaction, allowing one component to permeate. Many of the reactions studied involve H9. Membranes are metal (Pd, Ag), nonporous metal oxides, and porous structures of ceran iic and glass. Falconer, Noble, and Speriy [in Noble and Stern (eds.), op. cit., pp. 669-709] review status and potential developments. 

Membranes can be applied to catalysis in different ways. In most of the literature reports, the membrane is used on the reactor level (centimeter to meter scale) enclosing the reaction mixture (Figure 10.3). In most cases, the membrane is used as an inert permselective barrier in an equilibrium-limited reaction where at least one of the desired products is removed in situ to shift the extent of the reaction past the thermodynamic equilibrium. 

Membrane reactors are defined here based on their membrane function and catalytic activity in a structured way, predominantly following Sanchez and Tsotsis [2]. The acronym used to define the type of membrane reactor applied at the reactor level can be set up as shown in Figure 10.4. The membrane reactor is abbreviated as MR and is placed at the end of the acronym. Because the word membrane suggests that it is permselective, an N is included in the acronym in case it is nonpermselective. When the membrane is inherently catalytically active, or a thin catalytic film is deposited on top of the membrane, a C (catalytic) is included. When catalytic activity is present besides the membrane, additional letters can be included to indicate the appearance of the catalyst, for example, packed bed (PB) or fluidized bed (FB). In the case of an inert and nonpermselective... 

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

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). 

It is obvious from the above discussion that porous and dense membranes form two different cases, each with its own advantages and disadvantages. Dense membranes, (permeable only to one component) operating at optimum conditions, can be used to obtain complete conversions. However, because the permeation rate is low, the reaction rate has also to be kept low. Porous membranes (permeable to all components but at different permselectivities) are limited under optimum conditions to a maximum conversion (which is not 100%) due to the permeation of all the components. The permeation rates through porous membranes are, however, much higher than those through dense membranes and consequently higher reaction rates or smaller reactor volumes are possible. 

Mohan, K. and R. Govind. 1988a. Analysis of equilibrium shift in isothermal reactors with a permselective wall. A.LCh.E. J. 34(9) 1493-1503. 

Sloot, H. 1991. A non-permselective membrane reactor for catalytic gas phase reactions. Thesis, University of Twcntc, Enschede. 

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]. 

For a packed-bed membrane reactor (PBMR) the membrane is permselective and removes the product as it is formed, forcing the reaction to the right. In this case, the membrane is not active and a conventional catalyst is used. Tavolaro et al. [45] demonstrated this concept in their work on CO2 hydrogenation to methanol using a LTA zeolite membrane. The tubular membrane was packed with bimetallic Cu/ZnO where CO2 and H2 react to form EtOH and H2O. These condensable products were removed by LTA membrane which increased the reaction yield when compared to a conventional packed bed reactor operating under the same conditions [45]. 

A catalytic membrane reactor (CMR) presents an alternate configuration where the membrane is both catalytically active and permselective. The reactant conver-... 

In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of... 

Figure 2. Effect of permselectivity of HCHO, Shcho membrane reactor performance at 873K.

In order for a membrane reactor to produce yields of HCHO greater than in a plug flow reactor, the membrane must be permselective for this species. The more permselective the membrane is to formaldehyde the better the membrane reactor performs until the formaldehyde is approximately one thousand times more permeable than methane. At this limit, the concentration of HCHO is essentially equal on both sides of the membrane at all times. No further improvement is possible by increasing the diffusivity of the formaldehyde further because there is... 

Membrane reactors tu. continuous operation Ho permeation sluggish > permeability vs. permselectivity thermal driving force not feasible no unwanted catalytic activity t sealing tu, operating life... 

E. Kikuchi and Y. Chen, Low-Temperature Syngas Formation by C02 Reforming of Methane in a Hydrogen Permselective Membrane Reactor , Stud. Surf. Sci. Catal. 107 547-53 (1997). 

The CVI-experiments with silane as precursor did not show any increase in permselectivity of the membranes. The reaction temperature was in all cases 275°C and several oxygen pressures were tried. In each experiment, however, white powder was obtained on the membrane surface, indicating the decomposition of silane at the surface of the membrane. Reaction conditions could not be chosen in such a way that a highly separative layer was obtained. This was probably related to with the fact that the reactor temperature or the concentration of silane in the precursor gas was too low. Safety regulations, however, prohibited an increase of the silane concentration in the precursor flow. 

The above-mentioned results on metal and Vycor glass membranes made us believe that the membranes which are developed for steam-reforming conditions, as described in this thesis, might be used as a starting point for developing a Th-permselective a H2S dehydrogenation membrane reactor. 

The results obtained for microporous silica membranes in the membrane steam-reforming project, described in this thesis, provide favourable perspectives to realise a Th-permselective membrane reactor for the dehydrogenation of H2S. Realisation of such a reactor, however, imposes significant scientific and technical challenges. 

From these two generic configurations, different variations can be found in the literature. In case (a) the membrane can be dense or porous, active or inactive. In case (b) the membrane can be dense or porous. Moreover, some applications (see Sections A9.3.3.2 and A9.3.3.3) do not require permselective membranes. A complete nomenclature of the different membrane reactors has been proposed by Tsotsis et al. [6],... 

Recent results on isobutane dehydrogenation have been reported, and a conventional reactor has been compared with membrane reactors consisting of a fixed-bed Pt-based catalyst and different types of membrane [51]. In the case of a mesoporous y-AKOi membrane (similar to those used in several studies reported in the literature), the observed increase in conversion could be fully accounted for simply by the decrease in the partial pressures due to the complete mixing of reactants, products and sweep gas. When a permselective ultramicroporous zeolite membrane is used, this mixing is prevented the increase in conversion (% 70%) can be attributed to the selective permeation of hydrogen shifting the equilibrium. 

Important parameters in a catalytic membrane reactor for dehydrogenation are the reaction rate, the permeability (i.e. permeation rate) and the permselectivity for hydrogen. It appears at first sight that good conditions are those where the permeation rate (removal of H2) and the reaction rate (formation of H2) are close to each other, but the role of the permselectivity is also important. 

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). 

Equilibrium-restricted reactions (Section A9.3.3.1) have until now been the main field of research on CMRs. Other types of application, such as the controlled addition of reactants (Section A9.3.3.2) or the use of CMRs as active contactors (Section A9.3.3.3), seem however very promising, as they do not require permselective membranes and often operate at moderate temperatures. Especially attractive is the concept of active contactors where the membrane being the catalyst support becomes an active interface between two non-miscible reactants. Indeed this concept, initially developed for gas-liquid reaction [79] has been recently extended to aqueous-organic reactants [82], In both cases the contact between catalyst and limiting reactant which restricts the performance of conventional reactors is favored by the membrane. 

The main problem with the integration of inert membranes in fixed bed reactors is the membrane itself. It has to be sufficiently stable against thermal and mechanical stress, inexpensive and of high selectivity and permeability. At present, Pd-based alloys are the only sufficiently temperature stable and permselective membranes for hydrogen permeation. Since permeation through homogeneous membranes is a rather slow process, large membrane areas are required for reasonable production quantities. 

Sorptive reactor concepts where periodic operation is used to temporarily store or remove educts or products in the fixed bed can be considered close to industrial realization, whereas membrane reactor concepts with permselective inert or catalytically active microporous membranes are still at the laboratory stage. They promise the highest potential for a further improvement of catalytic reactor technology and present the biggest challenges [54]. 

H J Sevot, G F Versteeg, W P M van Swaaij, A non-permselective membrane reactor for chemical processes normally requiring stnet stoichiometric feed rates of reactants, Chem Eng Set 1990,45,2415 2421... 

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