Dehydrogenation membrane reactor

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. 

Figure 11.41 Schematic diagram of a compact dehydrogenation membrane reactor preceded and followed by conventional reactors [adapted from Bitter, 1988]...

On behalf of KTI an experimental programme on these reactor concepts has been started at the University of Southern California (USC). Some of the experimental results, concerning the use of Knudsen diffusion membranes are available in the literature [32,40]. These data have been used to calculate the economics of an isothermal propane dehydrogenation membrane reactor concept and are compared with the commercial Oleflex and Catofin processes, based on an adiabatic concept. The experimental circumstances of these lab-scale experiments, especially residence time, pressures and gas composition are not the same as in commercial, large-scale processes. However, we do not expect these differences to have a great influence on the results of the work presented here. 

From a critical review by Armor [8] a number of problem areas can be defined for the industrial application of dehydrogenation membrane reactors. These are defects in metalHc membranes at elevated temperatures, phase transitions in metallic membranes, leakage, low surface area per volume, severe... 

Extractor-type membrane reactors Applied to PCMRs and PIMRs. This type of reactor is based on the selective removal of one or more reaction products, which could result in an increase of the conversion for equUibrium-limited reactions or in the improvement of the catalytic activity if the removed products are reaction-rate inhibitors. Dehydrogenation membrane reactors or pervaporation membrane reactors are examples of extractor-type membrane reactors. 

Membrane Reactor. Another area of current activity uses membranes in ethane dehydrogenation to shift the ethane to ethylene equiUbrium. The use of membranes is not new, and has been used in many separation processes. However, these membranes, which are mostly biomembranes, are not suitable for dehydrogenation reactions that require high temperatures. Technology has improved to produce ceramic and other inorganic (90) membranes that can be used at high temperatures (600°C and above). In addition, the suitable catalysts can be coated without blocking the pores of the membrane. Therefore, catalyst-coated membranes can be used for reaction and separation. 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

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]... 

In the isobutane dehydrogenation the catalytic membrane reactor allows a conversion which is twice the one observed in a conventional reactor operating under similar feed, catalyst and temperature conditions (and for which the performance corresponds to the one calculated from thermodynamics) [9]. 

Zheng, A., Jones, E., Eang, J., Cui, T., Dehydrogenation of cyclohexane to benzene in a membrane reactor, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4,... 

Dittmeyer, R., V. Hollein, and K. Daub, Membrane reactors for hydrogenation and dehydrogenation processes based on supported palladium, ]. Mol. Catal. A Chem., 173,135-184, 2001. 

T. Kokugan, A. Trianto, and H. Takeda, Dehydrogenation of pure cyclohexane in the membrane reactor and prediction of conversion by pseudo equilibrium model, J. Chem. Eng. Jpn. 31,596-603 (1998). 

Itoh, N., Y. Shindo, K. Haraya and T. Hakuta. 1988. A membrane reactor using microporous glass for shifting equilibrium of cyclohexane dehydrogenation. J. Chem. Eng. Japan 21(4) 399-404. 

There are certainly quite significant advantages that membrane reactor processes provide as compared to conventional reaction processes. The reactor can be divided by the membrane into two individual compartments. The bulk phases of the various components or process streams are separated. This is of importance for partial oxidation or oxidative dehydrogenation reactions, where undesirable consecutive gas phase reactions leading to total oxidation occur very often. By separating the process stream and the oxidant. 

INORGANIC MEMBRANE REACTORS TO ENHANCE PRODUCTIVITY 187 Table 7.3. Membrane Reactor Studies on Dehydrogenation Reactions... 

Table 7A Summarized Results on Inorganic Membrane Reactors Used for Dehydrogenation Reactions... 

The second type of membrane reactor, illustrated in Figure 13.16(b), uses the separative properties of a membrane. In this example, the membrane shifts the equilibrium of a chemical reaction by selectively removing one of the components of the reaction. The example illustrated is the important dehydrogenation reaction converting n-butane to butadiene and hydrogen... 

Figure 13.20 Methylcyclohexane conversion to toluene as a function of reactor temperature in a membrane and a nonmembrane reactor [45]. Reprinted with permission from J.K. Ali and D.W.T. Rippin, Comparing Mono and Bimetallic Noble Metal Catalysts in a Catalytic Membrane Reactor for Methyl-cyclohexane Dehydrogenation, Ind. Eng. Chem. Res. 34, 722. Copyright 1995, American Chemical Society and American Pharmaceutical Association...

Catalytic membrane reactors are not yet commercial. In fact, this is not surprising. When catalysis is coupled with separation in one vessel, compared to separate pieces of equipment, degrees of freedom are lost. The MECR is in that respect more promising for the short term. Examples are the dehydrogenation of alkanes in order to shift the equilibrium and the methane steam reforming for hydrogen production (29,30). An enzyme-based example is the hydrolysis of fats described in the following. 

Improved selectivity in the liquid-phase oligomerization of i-butene by extraction of a primary product (i-octene C8) in a zeolite membrane reactor (acid resin catalyst bed located on the membrane tube side) with respect to a conventional fixed-bed reactor has been reported [35]. The MFI (silicalite) membrane selectively removes the C8 product from the reaction environment, thus reducing the formation of other unwanted byproducts. Another interesting example is the isobutane (iC4) dehydrogenation carried out in an extractor-type zeolite CMR (including a Pt-based fixed-bed catalyst) in which the removal of the hydrogen allows the equilibrium limitations to be overcome [36],... 

Whilst the enhancement of unwanted side reactions through excessive distortion of the concentration profiles is an effect that has been reported elsewhere (e.g., in reactive distillation [40] or the formation of acetylenes in membrane reactors for the dehydrogenation of alkanes to olefins [41]), the possible negative feedback of adsorption on catalytic activity through the reaction medium composition has attracted less attention. As with the chromatographic distortions introduced by the Claus catalyst, the underlying problem arises because the catalyst is being operated under unsteady-state conditions. One could modify the catalyst to compensate for this, but the optimal activity over the course of the whole cycle would be comprised as a consequence. 

Several profound theoretical and experimental studies performed on the laboratory scale have been reported which focus on the use of various configurations of membrane reactors as a reactant distributor in order to improve selectivity-conversion performances. In particular, several industrially relevant partial oxidations have been investigated, including the oxidative coupling of methane [56], the oxidative dehydrogenations of propane [57], butane [58], methanol [59, 60], the epoxidation of ethylene [61], and the oxidation of butane to maleic anhydride [62]. 

Currently, several types of membrane reactors are under investigation for dehydrogenation reactions, for example, the dehydrogenation of propane to propene [122,123], or of ethylbenzene to styrene [124], In addition, the dehydrogenation of H2S has been studied in membrane reactors [125],... 

When considering membrane reactors for dehydrogenation and reforming reactions, three types of membrane are of most interest dense palladium or palladium composite membranes,... 

In chapter 8 a new project has been formulated for the use of membrane reactors for the thermal dehydrogenation of H2S. Compared to the conventional Claus process, the application of a membrane reactor in the thermal H2S might have some large advantages. 

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