Combination step membrane reactors

The unusual interaction of hydrogen with palladium-based membrane materials opens up the possibility of oxidative hydrogen pump for tritium recovery from breeder blankets. The feasibility for this potential commercial application hinges on the hot-fusion and cold-fusion technology under development [Saracco and Specchia, 1994]. At first, Yoshida et al. [1983] suggested membrane separation of this radioactive isotope of hydrogen followed by its oxidation to form water. Subsequently, Hsu and Bauxbaum [1986] and Drioli et al. [1990] successfully tested the concept of combining the separation and reaction steps into a membrane reactor operation. 

While inorganic membrane reactors perform more efficiemly than conventional reactors in most cases, there are situations calling for the combined usage of these two types of reactors for reasons to be discussed. The conventional reactors in these special cases serve as either the pre-processing or post-processing step for the inorganic membrane reactor system to derive a maximum overall reaction conversion. These hybrid types of reactors consist of conventional reactors at the front end or tail end or both of the membrane reactor. 

When combining the membrane and the reactor into an integrated system, cost savings may be significant. This difference can be attributed to the convection and other transport modes in the separation steps by membranes in contrast to the traditional diffusion mode by other separation techniques. For example, the use of membrane reactors instead of the traditional reactors for bioengineered products can reduce the operating costs by as much as 25% [Chan and Brownstein, 1991]. 

The next step for application is the development of handsome modules that can be combined in larger units for separation or catalytic membrane reactors. In separation one strives to high surface-area-to-volume ratios in catalytic membrane reactors this ratio will depend on the volumetric catalytic rates compared to the required permeation fluxes. 

Catalytic membrane reactors (CMRs) are an interesting example of integrated system in which molecular separation and chemical conversions are combined in one step [8]. The heterogenization of catalysts in membrane is particularly suitable for catalyst design at the atomic and molecular... 

Moreover, Fig. 2.2 points out further statistics data on palladium membranes applied in the field of membrane reactors (MRs), devices combining the separation properties of the membranes with the typical characteristics of catalytic reaction steps in only one unit. In particular, this figure reports the number of publications on palladium-based membranes reactors with respect to the total number of publications in the membrane reactors area. 

Thermal catalytic decomposition of H2S to hydrogen and sulphur is a good candidate for application of a membrane reactor. The expected significant process simplification and intensification would capitalize on a new industrial paradigm offered by membrane reactors that allows combining reaction and separation in one step. A common feature of all catalytic membrane reactors having a hydrogen... 

Sometimes these results already allow a decision whether the realization of a planned chemical process is economically reasonable. For example, during process development, it can be decided if the planned project should be stopped or an alternative process should be applied, when from the economic point of view a minimum conversion of 35% is required, but only a conversion of 10% can theoretically be achieved. In an alternative process, the reaction could be directly combined with the separation step, as in the case of reactive distillation, membrane reactors, and so on, to increase the chemical conversion. 

A promising approach to overcoming these problems is the combined application of photocatalysis and membrane processes. Photocatalytic membrane reactors (PMRs) are useful for the catalyst separation and for the control of photo-oxidation products and/or by-products. TTie membrane may also ensure continuous operation in systems where the reaction of interest and the separation of product(s) can occur in a single step. The membrane process can be carried out without chemical additives and involves low energy costs. 

The expected significant process simplification and intensification offered by the membrane reactors due to combination of reaction and separation in one step would pave the way to a new industrial paradigm. 

Membrane reactors (MRs) for fuel processing combine the unit operation of membrane separation with catalytic reactions such as reforming and WGS. The membrane separation process is usually performed by hydrogen removal from the reformate by application of membranes made of ceramics or palladium and palladium alloys, while polymeric membranes are less convenient for systems of smaller than industrial scale, because several separation steps are required owing to their relatively low selectivity of the separation process. In MRs the equilibrium... 

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