Industrial Chemistry Of the many types of membrane processes available for separation, membrane reactors have been studied using reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), electrodialysis (ED), liquid membranes (LM), pervaporation (PV), gas permeation, vapor permeation, molecular sieving, Knudsen diffusion (and molecular diffusion), gas membrane, membrane solvent extraction, and membrane gas absorption/stripping.

Membranes are employed as flat films, hollow fibers, tubules, and tubes, while their physical structures can be as follows: microporous symmetric and asymmetric membranes, nonporous membranes, and composite membranes. Membranes can be of the polymeric variety or be inorganic in nature, which would include zeolitic, ceramic, and metallic membranes. Membranes can also conduct electrical charges and can be chosen from one of the following categories: ion-exchange membranes, bipolar membranes, mixed-conducting membranes, proton-conducting membranes, etc. In many cases, the membranes have catalysts incorporated in their porous structure or on the surfaces. The membranes in such cases are termed as catalytic membranes.

Functions of a Membrane in a Reactor

Of course, the membrane can be catalytic by itself without the addition of any catalyst materials from external sources. The term catalytic membrane reactor sometimes includes the above cases as well as a catalytic reactor enclosed by a membrane, which is noncatalytic. Figure 1 schematically identifies major generic functions performed by a membrane in a reactor. Of course, a given membrane in a given reactor is not capable of all functions illustrated in Fig. 1. However, a given membrane under appropriate circumstances can perform more than one generic function. The introduction of another membrane into the reactor can increase the number of generic membrane functions in the reactor or achieve the same generic membrane function in comparison to some other species. Fig. 1 also indicates other phenomena concurrently taking place in the so-called nonreactor- (or permeate-) side of the membrane as well as in the reactor-side of the membrane.

Generic functions performed by membranes in reactors are:

• Separation of products from the reaction mixture (Fig. 1a), in order to drive equilibrium-limited reactions to higher conversion, to suppress undesired side reactions, to increase the selectivity in consecutive reaction schemes, or to enhance reactions by removal of inhibiting products
• Separation of a reactant from a mixed stream for introduction into the reactor, in order to concentrate reactants prior to chemical reaction or to reject inhibiting species (Fig. 1b)
• Controlled addition of one reactant or two reactants, in order to enhance the yield of desired intermediate products, and to avoid thermal runaway of strongly exothermic reactions (Fig. 1c). This has been demonstrated in many works for gas-phase, for gas-liquid, and for liquid-phase reactions
• Nondispersive phase contacting with microporous/porous hydrophobic membranes (with reaction at the phase interface or in the bulk phases) (Fig. 1d). This technique has been employed in fermentor-extractor systems], enzymatic fat splitting, phase transfer catalysis, and extractive membrane bioreactors for enzymatic resolution of isomers
• Segregation of a mobile catalyst in a reactor via ultrafiltration or nanofiltration membranes, i.e. enzymes and cofactors in biocatalysis, and homogeneous catalysts in organic syntheses (Fig. 1e)
• Immobilization of a catalyst in (or on) a membrane, e.g., enzymes or whole cells for biocatalysis, and oxides or metals for chemical synthesis (Fig. 1f)
• Membrane is the catalyst, if the membrane material is inherently catalytically active such as cation-exchange membranes for esterification reactions or palladium membranes for dehydrogenation reactions (Fig. 1g)
• Membrane is the reactor, i.e. the bulk flow of a reaction mixture takes place through a porous/microporous membrane from one membrane surface to the other (Fig. 1h)
• Solid-electrolyte (SE) membranes (in particular conductors for H⁺ or O^2-) support electrodes, conduct ions, and achieve the reactions on their surfaces. This concept is the basis of fuel cells and electrolysers (Fig. 1i)
• Transfer of heat, e.g., when coupling endothermic and exothermic reactions(Fig. 1j)
• Immobilizing the liquid reaction medium in the pores of a membrane (often termed as supported liquid membrane, SLM (Fig. 1k)

Membrane Reactor Configurations

Membranes in a reactor existing as membrane laminates or physically separated membranes with a fluid phase in between have also been studied. They can provide particular combinations of the above functions sometimes with added and novel benefits including product separation and simultaneous concentration, separation of multiple products, reaction intensification, and physically containing the reaction medium in multiphase reaction systems.

Often the membrane is physically located in a device external to the reactor. The reaction medium is then circulated over the membrane and back to the reactor in a recycle mode. This configuration is frequently employed in reaction processes based on enzymes and whole cells; it is also being proposed for organic syntheses. The reactor vessel in such case is sometimes operated as a batch reactor or more frequently as a continuous stirred-tank reactor (CSTR). In many circumstances, the system behavior here can be considered to be equivalent to that with a membrane located directly inside the reactor. Major advantages of these different arrangements are:

• The mixing conditions and the flow velocities (and therefore the extent of consequent concentration polarization in membrane devices involving liquid-phase systems) can be maintained at different levels in the reactor and the membrane separator if recycle membrane reactors are employed; conditions can be optimized for each. The reactor may require long residence times whereas the membrane device may need a short residence time
• Building a reactor with a membrane in it or using a membrane device as the reactor can sometimes be very demanding on the membrane, especially for high-temperature systems. The recycle membrane reactor allows the reactor and the membrane unit to operate at two different temperatures by using heat exchangers in between
• Recycle membrane reactors allow the use of existing equipment, namely, a separate reactor and a separate membrane device
• For fast reactions, the ''membrane-in-a-reactor solution'' is likely to be a more desirable configuration
  

Membrane Reactor Applications

Many investigations on membrane reactors have been conducted in biochemical processing, petrochemical applications, and environmental pollution control. A few processes employing polymeric membranes have been commercialized. Utilizing more than one membrane in a reactor imparts additional functional capabilities. For effective large-scale utilization of the diverse functional capabilities of a membrane in a reactor, considerable research and development on membrane lifetime, available space-time, module design, membrane fouling, membrane poisoning, and membrane cost are essential. This is especially true for inorganic membranes and higher-temperature applications where the hurdles for industrial use are considerable.

Read more about this topic in Ullmann's

This article is an excerpt from the Ullmann's Encyclopedia of Industrial Chemistry which celebrates its 100^th anniversary in 2014. More about the topic can be found in the encyclopedia article on Process Intensification, 4. Plant Level. More concept articles on general interest topics in industrial chemistry and chemical engineering can be found on the Ullmann's Academy homepage