Introduction

The concept of completing both a reaction and separation in a single process unit has motivated research into the development of catalytic membrane reactors. For example, it has long been recognized that palladium metal has the capacity both to permeate hydrogen and to promote a variety of reactions. Thus, harnessing both of these features in a single device seemed a logical combination. In the mid 1960s, Wood and co-workers demonstrated that the dehyd-rogenation of cyclohexane to cyclohexene could be increased if the hydrogen produced was removed from the reaction vessel through semipermeable palladium walls. In this case, the palladium walls also acted to catalyse the dehydrogenation reaction. A membrane reactor of this type is illustrated in Figure 1.

In Russia, Gryaznov conducted much of the research that followed. Starting in the late 1970s, Gry-aznov began publishing his results on the use of palladium membrane reactors both to produce and to recover hydrogen from a myriad of dehydrogenation reactions. In the dehydrogenation reactions, hydrogen leaves the reactor by permeating through the semipermeable membrane. However, reactors can also be used in reactions where hydrogen or other reaction products enter the reaction chamber by penetration through the membrane. The commonest classes of reactions that have been successfully influenced by the use of membrane reactor technology are listed in Table 1. Details relating to the large volume of research reported are provided in the Further Reading section. None of these membrane reactors are in commercial use. But some - the selective oxidation of methane, for example - are the subject of a very large industrial research effort. If successfully developed, this process would change the feedstock basis of a number of petrochemical processes.

Most research on the development of membrane reactors involves the use of these devices to shift equilibrium-limited reactions (often dehydrogena-tions). The thermodynamic equilibrium of the react-ants and products at the temperature and pressure of the reaction determine the conversion achievable in any given reaction. For dehydrogenation reactions, increasing temperature and decreasing pressure promote an enhanced reaction. Unfortunately, each of these solutions has an associated cost. Increasing the reaction temperature typically results in a reduced

Figure 1 Schematic of a membrane reactor using hydrogen-permeable palladium membranes to shift the equilibrium of the dehydrogenation reaction cyclohexane to cyclohexene.

Table 1 Reaction classes that may be amenable to membrane reactor technology

Reaction class

Example

Role ofmembrane

Hydrogenation Hydrogenolysis

Dehydrogenation Partial oxidation

Esterifications Syn gas

Oxidative coupling

C2H2 # H2 p C2H4 in presence of C2H4

Cyclopentadiene # H2 p cyclopentene # cyclopentane

Cyclohexane pbenzene # 3H2 Butane # O2 p maleic anhydride

Controlled addition of hydrogen Controlled addition of hydrogen

Remove hydrogen to shift equilibrium limitation

Recovery of intermediate product of control reactant at addition rate to promote formation of intermediate product

Selective water removal to shift equilibrium limit without loss of reactant

Selective oxidation of methane Selective oxidation of methane catalytic selectivity for the desired product. Reducing pressure comes at the cost of adding a diluent to the reactor, paying for the additional capital to handle this component and paying the price of downstream separation.

Figure 2 provides a schematic representation of the behaviour of a conventional reactor and a theoretical membrane reactor. The conventional data are for a highly active butane dehydrogenation catalyst operating at 1 atm total pressure (pure normal butane feed). In the conventional system, the selectivity of the catalyst degrades rapidly at temperatures that are just beginning to promote reaction. Thus, the catalytic yield (defined as the product of conversion and cata-

500 700 900 1100

Temperature (K)

Figure 2 Influence of product hydrogen removal on the dehydrogenation of butane. Based on pure butane feed with 1.1 atm total pressure. Continuous line, conventional reactor; dashed line, membrane reactor.

500 700 900 1100

Temperature (K)

Figure 2 Influence of product hydrogen removal on the dehydrogenation of butane. Based on pure butane feed with 1.1 atm total pressure. Continuous line, conventional reactor; dashed line, membrane reactor.

lytic selectivity) goes through a pronounced maximum. Incorporation of an appropriately designed membrane into the reactor system results in the removal of hydrogen from the system. The catalytic selectivity does not appear to be influenced by this process, but the conversion of butane to butene is enhanced by the reduction in the hydrogen partial pressure. Thus, the yield of the membrane reactor system is markedly improved.

The ability to operate at acceptable conversions while maintaining very high catalytic selectivity is a strong driving force for the use of membrane reactor technology. By operating in a high selectivity region, the production of by-products that can act as catalyst poisons is minimized. This results in a longer catalyst life between regenerations and reduced waste production.

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