Membrane reactor

A membrane reactor is a piece of chemical equipment that combines a catalyst-filled reaction chamber with a membrane to add reactants or remove products of the reaction.

Chemical reactors making use of membranes are usually referred to as membrane reactors. The membrane can be used for different tasks:

• Separation
  • Selective extraction of reactants
  • Retention of the catalyst
• Distribution/dosing of an reactant
• Catalyst support (often combined with distribution of reactants)

Membrane reactors are an example for the combination of two [unit operation|unit operations]in one step e.g. membrane filtration with the chemical reaction.

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Examples

Biological systems

In biological systems membranes fulfil a number of essential functions. The compartmentalisation of biological cells is achieved by membranes. The semi-permeability allows to separate reactions and reaction environments.A number of enzymes is membrane bound and often mass transport through the membrane is active rather than passive as in artificial membranes allowing the cell to keep up gradients for example by using active transport of protons or water.

The use of an natural membrane is the first example of the utilisation for a chemical reaction. By using the selective permeability of a pigs bladder water could be removed from a condensation reaction to shift the equilibrium position of the reaction towards the condensation products according to the principle of Le Châtelier.

Size exclusion: Enzyme Membrane Reactor

As enzymes are macromolecules and often differ greatly in size from reactants they can be separated by size exclusion membrane filtration with ultra- or nanofiltration [artificial membranes]. This is used on industrial scale for the production of enantiopure amino acids by kinetic racemic resolution of chemically derived racemic amino acids. The most prominent example is the production of L-methionine on a scale of 400t/a.^[1] The advantage of this method over other forms of immobilisation of the catalyst is that the enzymes is not altered in activity or selectivity as it remains solubilised.

The principle can be applied to all macromolecular catalysts which can be separated from the other reactants by means of filtration. So far, only enzymes have been used to an significant extent.

Reaction combined with pervaporation

In P. dense membranes are used for separation. For dense membranes are the separation is governed by the difference of the chemical potential of the reactants in the membrane. The selectivity of the transport through the membrane is given by the different solubility of the materials in the membrane and their diffusivity in the membrane. For example for the selective removal of water by using lipophilic membranes. This can be used to overcome thermodynamic limits of condensation e.g. esterification reactions by removing water.

Dosing: Partial oxidation of methane to methanol

In the STAR process for the catalytic conversion of from methane from natural gas and air oxygen to methanol by the partial oxidation
2CH₄ + O₂ --> 2CH₃OH.

The partial pressure of oxygen has to low to prevent the formation of explosive mixtures and to suppress the successive reaction to carbon monoxide, carbon dioxide and water. This is achieved by using a tubular reactor with an oxygen-selective membrane. The membrane allows the uniform distribution of oxygen as the driving force for the permeation of oxygen through the membrane is the difference in partial pressures on the air side and the methane side.

Selective removal: Hydrogen

A number of metal membranes are highly hydrogen selective at higher temperatures. Especially palladium and platinium can therefore be used for the production of highly purified hydrogen from steam reforming off gases. The equilibrium limited reaction gives:
CH₄ + H₂O <--> 3H₂ + CO
CO + H₂O <--> H₂ + CO₂

Hydrogen is removed with high selectivity by use of thin dense metallic membranes. The mechanism of the transport is the separation of hydrogen into protons and electrons at the surface and recombination on the filtrate or raffinate side.

Benefits

Making a gaseous product in a membrane reactor generally affects the way that pressure affects the extent of reaction at thermodynamic pseudo-euilibrium. In an ordinary flow reactor, the composition of the exhaust gas is determined by the composition of the feed gas and the extent of reaction. As a result, at pseudo-equilibrium, the extent of reaction is entirely determined by the feed composition and the exhaust equilibrium constant, the latter being determined by the temperature and pressure of the exhaust. In a membrane reactor, the partial pressure of the components at psueudo-equilibrium are not uniquely determined by the total pressure, exit temperature, and feed composition. There is also a significant (and beneficial) effect that derives from the controlled removal of a product or addition of reactant.