The initial application of supercritical fluids, and probably still its most productive, is extraction with carbon dioxide. It makes most economic sense for high-value materials, such as pharmaceutical precursors. For low-value materials it is only viable if carried out on a very large scale. The most prominent applications are the decaffeination of coffee and the extraction of hops as part of the beer-making process (although this is often carried out with liquid carbon dioxide). A related process is the ‘cleaning’ of seeds, such as rice, which is carried out in the Far East. Dry-cleaning is also carried out commercially. The advantages of supercritical carbon dioxide in these processes are speed due to rapid diffusion, less pollution in the working and general environment, less solvent residues in products, less solvent disposal costs.
Chromatography using supercritical carbon dioxide has been carried out on an analytical scale in niche applications and it has also been carried out on a small production scale for high value products and chiral separations. Efficient simulated simulated bed units are available. Because of more rapid diffusion, greater chromatographic efficiencies are possible.
A number of other processes have been and are being researched, with the hope of useful applications, which have not yet been notably achieved. These include fractionation of liquid mixtures, chemical reactions, metals processing, impregnation and dyeing of polymers and synthetic fibres, particle formation and the fabrication of minute electronic devices.
Other substances have been researched as supercritical fluids. One example of possible importance is the production of biodiesel using supercritical methanol.
Carbon dioxide under pressure is used to enhance the extraction of gas and oil from wells. This is related to the use of carbon dioxide to accomplish carbon capture and storage.
decaffeination of coffee
In a plant in Houston Texas, coffee is decaffeinated by supercritical carbon dioxide using a semi-continuous process in a plant shown schematically in the figure above. A large vessel has two smaller vessels above and below it connected to it by large-bore valves. Large-bore valves are also in place to fill the upper small vessel and empty the lower small vessels. The green coffee beans are extracted, i.e. before roasting, with pure carbon dioxide. These are mostly located in the central vessel and carbon dioxide passes up through it. Periodically, decaffeinated coffee beans are taken out via the smaller lower vessel and new coffee beans are put into the system via the upper small vessel by operating the valves and filling and emptying the small vessels with carbon dioxide. Caffeine is removed from the carbon dioxide after extraction by scrubbing with water. Carbon dioxide from all vessels is recycled to the maximum extent.
Coffee is not a very valuable commodity and the only way the process can be made viable is by economies of scale. The plant is therefore very large and the large vessel is more than 60 m3 in volume. The production volume is estimated to be around 25,000 tonnes per year. A picture of the large vessel being manoeuvred into position is shown below.
production of biodiesel using supercritical methanol
When Rudolf Diesel envisaged his Diesel engine it was intended to run on vegetable oils. Since then, the availability of cheap petroleum diesel has resulted in the engines being designed to run on this form of fuel. The major problem with vegetable oils in modern engines is high viscosity. Vegetable oils consist mainly of glycerol tri-esters of fatty acids (triglycerides). If the glycerol esters are converted to methyl esters (a process called transesterification), the viscosity is much closer to that of diesel, and the fatty acid methyl esters (FAMEs) can be used as a replacement for diesel.
The most common method for transestrification is to stir the oil with methanol and a caustic catalyst for several hours. This produces a FAMEs layer and a glycerol layer. The glycerol is tapped off, the FAMES washed and then are ready to use. Free fatty acids and water are a problem in this method. The free fatty acid produces soap, which hinders the separation of the layers, and also uses up catalyst. The level of acid in the feedstock needs to be determined, and the level of catalyst adjusted. If the level of acid is too high, the feedstock cannot be used. Recycled cooking oils, the most environmentally friendly oil source, tend to contain a lot of free fatty acid, so this is a problem for the catalysed method. Energy is needed to continuously mix the immiscible oil and methanol. The method lends itself well to small scale batch production, although large scale continuous or semi-continuous commercial plants exist.
An alternative method is to mix the oil and methanol under supercritical conditions. The two components are then miscible, and react spontaneously without a catalyst. A schematic diagram of a system for carrying out the reaction is shown below. Work has so far been carried out in laboratories and on a pilot or small production scale.
After reaction, the FAMEs and glycerol are then separated without further need of washing. Free fatty acids are converted to FAMEs also (instead of soap), increasing yield and allowing recycled oil to be used with no extra consideration. Fatty acid sources such as soap stock could also be used directly. Yields are high (98%) reaction times fast (300 secs) and the process is easy to operate continuously (rather than batch). The plant design is relatively simple, requiring a tubular reactor, pumps for oil and methanol and a separator. Flash distillation recovers the methanol for re-cycling back to the process.
The critical temperature of methanol is 239°C, but the temperature needs to be higher than this as it is the critical temperature of the mixture that is required. For a methanol : triglyceride mixture in a weight ratio of 1.5 : 1 the critical temperature is between 320 and 350°C and the pressure required is under 100 bar. Lower temperatures can be used with higher ratio of methanol to oil, and using CO2 as a co-solvent may also reduce the critical temperature of the mixture, and hence the reaction temperature. Depending on the feedstock, some unsaturated fatty acids may be slightly degraded at the higher temperatures. Hydrolysis of the oil to free fatty acids by superheated water, followed by esterification with supercritical methanol has also been suggested. The temperature required in this two stage process is reduced to 275°C, so there is less risk of degradation.
Currently, nearly all biodiesel is produced by the catalytic method. With the increase in scale of production of biodiesel the supercritical route to production will deliver many advantages.
carbon capture and storage
Carbon capture and storage (CCS) has been put forward as one way of reducing emissions of the greenhouse gas carbon dioxide. Carbon dioxide from the use of fossil fuels is stored underground in closed geological formations such as aquifers or depleted oil and gas wells. A number of schemes have been announced and in at least one case CCS is already being carried out on a large scale. Because these schemes are in early stages their long-term viability is not yet established. There are some doubts about the desirability of CCS. For example, Greenpeace, while it does not object to CCS in principle, feels that it is diverting our attention from the superior goal of renewable energy. It feels that the subsidies, which will be spent on CCS, would be better spent on renewables.
Many schemes are proposed in a number of countries. For example, a process patented by BP and GE, is proposed in which natural gas, extracted with the assistance of carbon dioxide, is brought ashore and converted into hydrogen and CO2. The hydrogen is then burnt to produce electricity. The CO2 is returned to the gas field to extract more gas. Eventually the gas field will consist mainly of CO2 with a small amount of residual natural gas and will be sealed. There was a plan to carry out this scheme in the North Sea, but this has now been abandoned. However there are a number of possible sites for the scheme worldwide and some of these will be pursued.
The most advanced scheme is at the Sleipner field in the North Sea between Norway. and Scotland, which has been running since 1996. This is shown schematically below. Natural gas, which contains a high proportion of CO2, is extracted from a gas field and brought up to a rig, where the CO2 is separated from the rest of the gas. The CO2 is then compressed and stored in an aquifer, which lies, enclosed in impervious rock between the gas field and the surface. The form the CO2 takes in the aquifer is being studied and the storage is monitored for stability. Of course, you could argue that this project is limited because it does not store CO2 emissions from combustion, but merely does not extract the CO2 from the gas field. Nevertheless, it is an important stage in the development of CCS.