A membrane reactor is a physical device that combines a chemical conversion process with a membrane separation process 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 products 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. The integration of reaction section with selective extraction of a reactant allows an enhancement of the conversions compared to the equilibrium value. This characteristic makes membrane reactors suitable to perform equilibrium-limited endothermic reactions.
Benefits and critical issues
Selective membranes inside the reactor lead to several benefits: reactor section substitutes several downstream processes. Moreover, removing a product allows to exceed thermodynamics limitations. In this way, it is possible to reach higher conversions of the reactants or to obtain the same conversion with a lower temperature.
Reversible reactions are usually limited by thermodynamics: when direct and reverse reactions, whose rate depends from reactants and product concentrations, are balanced, a chemical equilibrium state is achieved. If temperature and pressure are fixed, this equilibrium state is a constraint for the ratio of products versus reactants concentrations, obstructing the possibility to reach higher conversions.
This limit can be overcome by removing a product of the reaction: in this way, the system cannot reach equilibrium and the reaction continues, reaching higher conversions (or same conversion at lower temperature).
Nevertheless, there are several hurdles in an industrial commercialization due to technical difficulties in designing membranes with long stabilities and due to the high costs of membranes. Moreover, there is a lack of a process which lead the technology, even if in recent years this technology was successfully applied to hydrogen production and hydrocarbon dehydrogenation.
Reactor configurations
Generally, membrane reactors can be classified based on the membrane position and reactor configuration. Usually there is a catalyst inside: if the catalyst is installed inside the membrane, the reactor is called catalytic membrane reactor (CMR); if the catalyst (and the support) are packed and fixed inside, the reactor is called packed bed membrane reactor; if the speed of the gas is high enough, and the particle size is small enough, fluidization of the bed occurs and the reactor is called fluidized bed membrane reactor. Other types of reactor take the name from the membrane material, e.g., zeolite membrane reactor.
Among these configurations, higher attention in recent years, particularly in hydrogen production, is given to fixed bed and fluidized bed: in these cases the standard reactor is simply integrated with membranes inside reaction space.
Membrane reactors for hydrogen production
Today hydrogen is mainly used in chemical industry as a reactant in ammonia production and methanol synthesis, and in refinery processes for hydrocracking. Moreover, there is a growing interest in its use as energy carrier and as fuel in fuel cells.
More than 50% of hydrogen is currently produced from steam reforming of natural gas, due to low costs and the fact that it is a mature technology. Traditional processes are composed by a steam reforming section, to produce syngas from natural gas, two water gas shift reactors which enhance hydrogen in syngas and a pressure swing adsorption unit for hydrogen purification. Membrane reactors make a process intensification including all these sections in one single unit, with both economic and environmental benefits.
Membranes for hydrogen production
To be suitable for hydrogen production industry, membranes must have a high flux, high selectivity towards hydrogen, low cost and high stability. Among membranes, dense inorganic are the most suitable having a selectivity orders of magnitude bigger than porous ones. Among dense membranes, metallic ones are the most used due to higher fluxes compared to ceramic ones.
The most used material in hydrogen separation membranes is palladium, particularly its alloy with silver. This metal, even if is more expensive than other ones, shows very high solubility towards hydrogen.