Membrane reactor
From Wikipedia, the free encyclopedia
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.
Contents |
[edit] Examples
[edit] Cells
A natural example of a membrane reactor is a biological cell, where the membrane serves to keep the catalyst enzymes concentrated in space, while allowing a great deal of material to be processed efficiently. In the biological cell material is added and removed through the membrane, while in the chemical industry material (generally) passes only one way: in or out.
[edit] Methanol
A prominent example in the chemical industry where a reactant is added is the STAR process for making methanol from natural gas and air by the reaction
2CH4 + O2 --> 2CH3OH.
In this process, air is made to flow through tubular oxygen-specific membranes that pass through a high temperature reactor fed with natural gas. The low partial pressure of oxygen in the reforming side of the reactor draws oxygen out of the air, and induces it to move across the membrane. Once in the reformer, the oxygen combines with the natural gas to form the product methanol. Without the membrane present in the reactor purifying oxygen for this process would be more cumbersome, less safe, and less efficient.
[edit] Hydrogen
A prominent example where a product is removed from the reactor is in the use of a membrane to promote the production of ultrapure hydrogen from methanol and water by the reaction
CH3OH + H2O --> 3 H2 + CO2.
In this process, vaporized methanol and water (steam) are made to pass through a catalytic bed containing metallic, hydrogen extraction membranes. Hydrogen formed in the reaction passes through the membranes while the leftovers (raffinate) exits separately. Extraction of hydrogen drives the reaction forward and simplifies the process for making pure hydrogen. This is important since, in an ordinary reactor the reaction is equilibrium limited at typical pressures and temperatures. The raffinate stream can be burnt to provide heat for the endothermic reforming reaction.
[edit] 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 or addition of reactant.
