||This article needs attention from an expert in Chemistry. (November 2008)|
Chemical reactors making use of membranes are usually referred to as membrane reactors. The membrane can be used for different tasks:
- Selective extraction of reactants
- Retention of the catalyst
- Distribution/dosing of a reactant
- Catalyst support (often combined with distribution of reactants)
Membrane reactors are an example for the combination of two unit operations in one step e.g. membrane filtration with the chemical reaction.
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 are 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 a natural membrane is the first example of the utilisation for a chemical reaction. By using the selective permeability of a pig's 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. The advantage of this method over other forms of immobilisation of the catalyst is that the enzymes are 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 a significant extent.
Reaction combined with pervaporation
In P. dense membranes are used for separation. For dense membranes the separation is governed by the difference of the chemical potential of the components in the membrane. The selectivity of the transport through the membrane is dependent by the difference in solubility of the materials in the membrane and their diffusivity through the membrane. For example for the selective removal of water by using lipophilic membranes. This can be used to overcome thermodynamic limitations of condensation e.g. esterification reactions by removing water.
Dosing: Partial oxidation of methane to methanol
The partial pressure of oxygen has to be 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 platinum can therefore be used for the production of highly purified hydrogen from steam reforming of gases. The equilibrium limited reaction gives:
CH4 + H2O <--> 3H2 + CO
CO + H2O <--> H2 + CO2 or
CH3OH + H2O <--> CO2 + 3 H2
Ultra pure hydrogen, generated from these reactions, is extracted by use of thin dense metallic membranes that are 100% selective to hydrogen. 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. Other high temperature membranes are being considered for hydrogen generation where the purity requirements are not as great; for example for clean coal power generation. Hydrogen, produced from coal gas in the membrane reactor would be used for power generation, while the carbon dioxide would remain at high pressure for carbon capture and storage.
As with the other reactions, hydrogen extraction drives the conversion forward, but for this reaction, the desired product is the benzene, and not the hydrogen.
Making a gaseous product in a membrane reactor generally affects the way that pressure affects the extent of reaction at thermodynamic pseudo-equilibrium. 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.
- Industrial Biotransformations, 2nd, Completely Revised and Enlarged Edition Andreas Liese (Editor), Karsten Seelbach (Editor), Christian Wandrey (Editor) ISBN 978-3-527-31001-2
- Article on the beneficial results that can be derived, Separation Science and Technology, 34 (1999) 2113-2123.
- The University Laval Study on deriving benzene from methane
- Environmentally Friendly Gas Technologies 2nd Canadian-Korean joint WORKSHOP Feb. 28 to Mar. 2, 2000 Montreal / Boucherville / Varennes / Bells Corners Canada
- Catalytic Membranes and Membrane Reactors
Copyright © 2002 Wiley-VCH Verlag GmbH Author(s): Dr. José G. Sanchez Marcano, Professor Theodore T. Tsotsis