Hybrid technologies: examples of applications
Membranes in the production of alternative fuels
Membranes are helping to produce environmentally friendly systems to generate energy. Especially gas separation membranes, both polymeric and inorganic, are playing an important role in the search towards alternatives for fossil fuels.
On one hand, membranes are involved in the production of alternative fuels. On the other hand, they are used for the purification of those fuels to make them suitable for certain applications. Commercial membranes are already available for:
• hydrogen production;
• hydrogen separation and purifi cation;
• natural gas conditioning;
• fuell cells.
Hydrogen is not available in its free molecular form on earth. To obtain molecular hydrogen as fuel, separating hydrogen from carbon, oxygen, nitrogen and other elements to which it is chemically bound is necessary. The most widely used method of hydrogen production is steam reforming of light hydrocarbons (mainly methane).
The process consists of:
• the initial reforming step (methane and steam react to form CO and hydrogen);
• the water-gas shift reaction (CO reacts with steam to form CO2 and hydrogen).
After the overall steam methane reforming process, CO2 has to be removed from the process stream. Traditionally this separation is performed with an amine-based acid gas scrubber or with pressure swing adsorption, but membranes can offer a better solution. Because methane is a stable hydrocarbon, high temperatures and pressures are required. To make the production of hydrogen energy efficiently, the purification of hydrogen should be run at or close to the reforming temperatures. Therefore, inorganic membranes are the most suitable.
Instead of purification of hydrogen afterwards, also a membrane reactor can be used. In that case, reaction and separation can happen simultaneously. As soon as hydrogen is formed, it is transported through the membrane.
By removing hydrogen selectively from the reaction system, the initial reforming reaction and the water-gas shift reaction are shifted to the product sides. That way, highly efficient conversion of methane to CO2 and hydrogen can be attained. Membrane reactors with palladium alloy membranes are already commercial available.
Another way of producing hydrogen is electrolysis. Electrolysis takes place when an electric current flows through an electrolyte (water) from an anode to a cathode. Water molecules split into hydrogen and oxygen. Two types of water electrolysers are available: alkaline electrolysers and proton exchange membrane (PEM) electrolysers. Dupont’s fluorocarbon-based membrane, Nafion, is the most used membrane.
The third method to produce hydrogen is coal gasification. Coal gasification breaks down the coal into smaller molecular weight molecules, usually by subjecting it to high temperature and pressure. This leads to the production of syngas, a mixture mainly consisting of carbon monoxide and hydrogen. Membranes can be used to separate hydrogen from the mixture.
Hydrogen recovery from refinery, petrochemical and chemical process streams
The fi rst large-scale application of membrane gas separation was the separation of hydrogen from nitrogen, methane and argon in ammonia purge-gas streams. This application is ideal for membrane separation because
• hydrogen is highly permeable;
• the ammonia purge gas is already at high pressure;
• the gas is clean and free of higher hydrocarbons (no plasticization or fouling).
Nowadays, hydrogen can also be recovered from refi nery, petrochemical and chemical process streams and a range of polymeric membranes are commercially available.
Natural gas conditioning
In many locations, natural gas contains CO2, H2S, heavy carbons and tom much N2. These components have to be removed to produce pipeline-acceptable gas. Membranes can offer therefore a simple and low-cost solution. In the natural gas industry, the principal application for membranes is the separation of CO2 from natural gas. Polymeric hollow fibre and spiral wound cellulose acetate membranes are commercially available for this application.
One of the cleanest way of producing power is the use of fuel cells. Fuel cells use non-depleting fuels and can produce energy without the formation of polluting compounds. They offer a unique method to convert chemical energy into electrical energy. A fuel cell is composed of two electrodes (anode and cathode) and an electrolyte. The fuel is oxidized at the anode (with the help of a catalyst), the oxidant moves through the electrolyte and is reduced at the cathode. The electrons released in these reactions can be used to produce the desired electrical energy.
There are several types of fuel cells. They can be distinguished by type of electrolyte material used as a medium for the internal transfer of ions (protons). The electrolyte can be for example an alkaline solution, but also a membrane.
In a proton exchange membrane fuel cell (PEMFC), a proton-conducting polymer membrane is used as electrolyte. The fuel can be pure hydrogen (PEMFC), reformed hydrogen or direct methanol (direct methanol fuel cell or DMFC). Currently, perfluorinated polymer electrolyte membranes are used in PEMFCs. Nafion-type materials, also used for electrolyses, have very favourable characteristics but they are not suitable for large-scale DMFC applications due to their high methanol crossover and high costs. Therefore, research is know focused on the development of alternative materials.
The electrolyte in a solid oxide fuel cell (SOFC) can be an oxygen ion conducting electrolyte membrane (OSOFC) or a proton conducting electrolyte membrane (HSOFC). The membrane is a solid, nonporous metal oxide. In OSOFC, the main used electrolyte is made from zirconia doped with yttria. In HSOFC, perovskite materials are used.
In microbiological fuel cells, organic substrates are used as fuel. Microorganisms or enzymes are used as catalyst. They are attached to the anode and break down the organic substrate producing electrons and protons. A polymeric membrane is used as electrolyte, especially sulfonated poly (ether ether ketone) membranes are suitable.