Membrane reactor
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.
Chemical reactors making use of membranes are usually referred to as membrane reactors. The membrane can be used for different tasks:
Membrane reactors are an example for the combination of two unit operation
s in one step e.g. membrane filtration with the chemical reaction.
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 membrane
s 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
.
s are macromolecule
s 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 acid
s 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.
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.
from natural gas
and air oxygen
to methanol
by the partial oxidation
2CH4 + O2 --> 2CH3OH.
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.
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
.
An alternative application of membrane reactors, developed at University Laval was to convert methane
into benzene
by the following reaction:
6 CH4 --> C6H6 + 9 H2
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.
Membrane technology
The membrane technology covers all process engineering measures for the transport of substances between two fractions with the help of permeable membranes...
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 a reactant
- Catalyst support (often combined with distribution of reactants)
Membrane reactors are an example for the combination of two unit operation
Unit operation
In chemical engineering and related fields, a unit operation is a basic step in a process.Unit operation involves bringing a physical change such as separation, crystallization, evaporation, filtration etc.. For example in milk processing, homogenization, pasteurization, chilling, and packaging are...
s in one step e.g. membrane filtration with the chemical reaction.
Biological systems
In biological systems membranes fulfil a number of essential functions. The compartmentalisation of biological cellsCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
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 membrane
Artificial membrane
An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A...
s 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
Le Châtelier's principle
In chemistry, Le Chatelier's principle, also called the Chatelier's principle, can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently...
.
Size exclusion: Enzyme Membrane Reactor
As enzymeEnzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s are macromolecule
Macromolecule
A macromolecule is a very large molecule commonly created by some form of polymerization. In biochemistry, the term is applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...
s 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 acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s by kinetic racemic resolution of chemically derived racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...
amino acids. The most prominent example is the production of L-methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
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 solubilitySolubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
of the materials in the membrane and their diffusivity
Diffusivity
Diffusivity can refer to:*Diffusivity of heat*Diffusivity of mass:** Molecular diffusivity ** Eddy diffusivity*Momentum diffusivity...
through the membrane. For example for the selective removal of water by using lipophilic
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...
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
In the STAR process for the catalytic conversion of from methaneMethane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
from natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
and air oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
to methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
by the partial oxidation
2CH4 + O2 --> 2CH3OH.
The partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of oxygen has to be low to prevent the formation of explosive mixtures and to suppress the successive reaction to carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. This is achieved by using a tubular reactor with an oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
-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 palladiumPalladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
and platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
can therefore be used for the production of highly purified hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
from steam reforming
Steam reforming
Fossil fuel reforming is a method of producing hydrogen or other useful products from fossil fuels such as natural gas. This is achieved in a processing device called a reformer which reacts steam at high temperature with the fossil fuel. The steam methane reformer is widely used in industry to...
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
Clean coal
Historically used to refer to technologies for reducing emissions of ash, sulfur, and heavy metals from coal combustion; the term is now commonly used to refer to carbon capture and storage technology...
power generation. Hydrogen, produced from coal gas
Coal gas
Coal gas is a flammable gaseous fuel made by the destructive distillation of coal containing a variety of calorific gases including hydrogen, carbon monoxide, methane and volatile hydrocarbons together with small quantities of non-calorific gases such as carbon dioxide and nitrogen...
in the membrane reactor would be used for power generation, while the carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
would remain at high pressure for carbon capture and storage
Carbon capture and storage
Carbon capture and storage , alternatively referred to as carbon capture and sequestration, is a technology to prevent large quantities of from being released into the atmosphere from the use of fossil fuel in power generation and other industries. It is often regarded as a means of mitigating...
.
An alternative application of membrane reactors, developed at University Laval was to convert methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
into benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
by the following reaction:
6 CH4 --> C6H6 + 9 H2
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.
Benefits
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.External links
- 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