Oxidative dehydrogenation membrane reactors

Novel Oxidative Membrane Reactor for Dehydrogenation Reactions Experimental Investigation... 

However, the addition of an oxidant such as oxygen is not without some trade-off. To help solve the problem of catalyst deactivation due to carbon deposit in an alumina membrane reactor for dehydrogenation of butane, oxygen is introduced to the sweep gas, helium, on the permeate side at a concentration of 8% by volume. The catalyst service life increa.scs from one to four or five hours, but the selectivity to butene decreases from 60 to 40% at 480 C [Zaspalis et al., 1991b]. If oxygen is added to the feed stream entering the membrane reactor in order to inhibit coke formation, the butene selectivity decreases even more down to 5%. 

Catalytic testings have been performed using the same rig and a conventional fixed-bed placed in the inner volume of the tubular membrane. The catalyst for isobutane dehydrogenation [9] was a Pt-based solid and sweep gas was used as indicated in Fig. 2. For propane oxidative dehydrogenation a V-Mg-0 mixed oxide [10] was used and the membrane separates oxygen and propane (the hydrocarbon being introduced in the inner part of the reactor). 

In the propane oxidative dehydrogenation, where the membrane separates the two reactants, a 20% increase in the yield was observed with respect to a conventional reactor working at isoconversion [10]... 

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. 

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]. 

The use of a membrane reactor for shifting equilibrium controlled dehydrogenation reactions results in increased conversion, lower reaction temperatures and fewer byproducts. Results will be presented on a palladium membrane reactor system for dehydrogenation of 1-butene to butadiene, with oxidation of permeating hydrogen to water on the permeation side. The heat released by the exothermic oxidation reaction is utilized for the endothermic dehydrogenation reaction. 

Recently, Itoh and Govind(ll.) have reported a theoretical study of coupling an exothermic hydrogen oxidation reaction with dehydrogenation of 1-butene in an isothermal palladium membrane reactor. 

A schematic of a palladium membrane reactor is shown in figure 1. The reversible reaction of 1-butene dehydrogenation occurs on the reaction side of the membrane in which the chrome-alumina catalyst is uniformly packed. The oxidation of hydrogen with oxygen in air occurs in the permeation or separation side on the palladium membrane surface. The... 

The feasibility of the palladium membrane system with an oxidation reaction on the permeation side and 1-butene dehydrogenation reaction on the reaction side in a membrane reactor has been successfully demonstrated. The palladium and its alloy membrane not only can withstand high temperature but also are selectively permeable to hydrogen... 

Enhancement in conversion by the usage of a membrane reactor has been demonstrated for many dehydrogenation reactions. Product selectivity of some hydrogenation and other reactions arc found to improve with a permselective membrane as part of the reactor. Several dense metal as well as solid elecu olyte membranes and porous metal as well as various oxide membranes have been discovered to be effective for the reaction performance. 

Relevant to this issue is dehydrogenation of ethylbenzene for the manufacture of styrene which uses alumina supported iron oxide as the preferred catalyst in most cases. Therefore, when an alumina membrane is used in conjunction with stainless steel piping or vessels as the membrane reactor, caution should be exercised. An estimate of the effects of their exposure to the reaction mixuire at the application temperature of 600 to 640 C is desirable. Wu et al. [1990b] estimated that the alumina membrane contributes to less than 5% conversion of ethylbenzene and the stainless steel tubing or piping could account for as much as 20% conversion. The high activity of the stainless steel is attributed to iron and chromium oxide layers that may form on the wetted surface. 

Itoh [1990] simulated a Pd membrane reactor coupling the cyclohexane dehydrogenation reaction on the feed side with oxidation of hydrogen on the permeate side. Given in Figure 11.35 is the predicted conversion of the dehydrogenation reaction as a function of the total flow rate of the sweep gas with the Damkbhler number for the permeate side as a parameter... 

The need to govern heat balances properly in membrane reactors will certainly become a major task if large-scale industrial units are ever to be put into operation. Whether the performed reaction is endothermic (dehydrogenation) or exothermic (oxidation), innovative means to supply or remove heat from large-scale membrane reactor modules will have to be designed. The isothermicity assumption valid for several lab-scale membrane reactors will not hold anymore, and much more complex modeling will certainly have to be developed. 

The major features of and application opportunities for inorganic-membrane reactors have been described in some detail. We can conclude that inorganic-membrane reactors actually show promise for improving either conversion of equilibrium-limited reactions (e.g., dehydrogenations) or selectivity toward some intermediates of consecutive reaction pathways (e.g., partial oxidations). 

Tiscareno-Lechuga F, Hill Jr. G.C. and Anderson M.A., Experimental studies of the non-oxidative dehydrogenation of ethylbenzene using a membrane reactor, Appl. Catal. 96 33 (1993). 

Pantazidis A, Dalmon JA, and Mirodatos C. Oxidative dehydrogenation of propane on catalytic membrane reactors. Catal Today 1995 25 403 08. 

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