Dehydrogenation reactions membrane reactors

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. 

Figure 6. Tubular membrane reactor (fixed-bed catalyst + inert porous membrane) for dehydrogenation reactions [51].

In this section some examples of inorganic gas separation membranes in membrane reactor applications will be discussed. A first indication of the technical and economic feasibility of these membranes in dehydrogenation reactions and in the water-gas shift reaction will be given. 

Figure 16.23 A set of fundamental equations for simulating the reaction progress in the palladium membrane reactor (a) reaction kinetics of ethylhenzene dehydrogenation and (b) the simultaneous differential equations.

Table 2.7 summarizes some of the dehydrogenation reactions studied in the literature using porous MRs. As can be seen, the membranes for dehydrogenation reactions are primarily microporous silica due to it high hydrogen perm-selectivity. Meso- or macroporous membranes are not suitable for dehydrogenation reactions because their low perm-selectivity may lead to much loss of the reactants. The catalysts for dehydrogenation may be packed in the reactor or loaded inside the membrane wall. All the studies demonstrate enhanced conversions or even beyond-equilibrium values. 

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]... 

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... 

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. 

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,... 

The catalytic activity of membrane catalyst, obtained by IR-pyrolysis of PAN and ammonium perrhenate, was studied in the flow membrane reactor in a model reaction of cyclohexane dehydrogenation at the temperatures from 500 to 700 K. 

In addition to this increase in conversion, other benefits can be expected when using a membrane reactor. The same yields can be achieved at lower temperatures, leading to energy savings and reduced catalyst deactivation (one of the major problems of alkane dehydrogenation), increased selectivities when temperature-promoted side reactions exist or when the permeating species arc involved in these side reactions. Moreover, the formation and separation of products in the same unit leads to a reduction in capital costs. 

The dehydrogenation of other hydrocarbons has also been studied in CMRs, generally with porous membranes. Conversions of ethane [47], propane [48], butane [49], and ethylbenzene [50] have been reported to be higher when membrane reactors were used. In the case of ethylbenzene dehydrogenation, the undesirable hydrodealkylation side reaction is slowed down due to the removal of H2, i.e. the membrane enables an increase in selectivity as well [50]. 

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 addition to a proper membrane, CMRs also need a good catalyst. Due to the specific conditions under which catalysts are placed in CMRs, conventional active phases could behave differently from when under classical conditions. For example, in dehydrogenation reactions, due to the removal of H2, the hydrogen hydrocarbon ratio is smaller in CMRs when compared to other reactors, which will probably affect the stability of the catalyst. The low oxygen partial pressure used in CMRs for selective oxidation (Section A9.3.3.2) could also lead to some changes in catalyst behavior. These aspects could necessitate the specific design of catalysts for CMRs. 

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