SUPERCRITICAL FLUIDS

Traditionally liquids and gases are assumed to be distinct and different phases, exhibiting different properties, such as density and diffusivity. However it has been found that increasing the temperature and pressure of a fluid beyond a certain value (depending on the fluid), it becomes harder to describe the properties as simply liquid or gaseous. This point is called the Critical Point. The bizarre fluids that occur beyond this point have a combination of properties both liquid and gaseous. As such, a variety of industrial and chemical uses have been found for these fluids, which exploit the unusual combination of properties. The fluids are given the name Supercritical fluids.

Supercritical fluids have low viscosities and are highly compressible, like gases, and are relatively dense and can dissolve a wide range of solid compounds, like liquids.

This essay will first attempt to describe how these properties lead to the widespread uses of supercritical fluids, primarily as solvents, in processes such as extraction, chromatography and reaction chemistry. Secondly I will describe in a Chemical Engineering context how supercritical fluid reactors are being developed and how the 'scale up' or 'scale out' considerations affect the industrial outcome of SCF (supercritical fluid) research. I will include details of the environmental factors that are driving the research into supercritical technology.

THE HISTORY (top)

The development in supercritical fluid technology has been slower than would have been expected from the early date that supercritical phase were discovered. This can be explained by the difficulty of experimenting at high pressures and at high temperatures. The early discovery was around the beginning of the Nineteenth century, by Cagniard de la Tour. He was the first to notice that the gas/liquid phase boundary disappeared when a certain temperature was exceeded. Supercritical Carbon Dioxide was one of the first substances looked into by Andrews in 1869. However the interesting solubility power that the fluids exhibited was demonstrated by Hannay and Hogarth in 1880 [Hannay J.B. and Hogarth J., Proc. Roy. Soc. (London) ser. A 1879 29 324]. They noticed that the solubility of Cobalt Chloride in supercritical ethanol was greater than expected by extrapolating subcritical data. Van der Waals then went on to look into the Thermodynamics of supercritical fluids. Even though the natural physical effects of supercritical fluids, as described above, were known by early researchers, it is only in the last 30 years that a rapid development in supercritical knowledge has been achieved. The early patents that some American engineers suggested were mostly ignored. However by the time the end of the fifties arrived, a Russian, Zhuze [Zhuze T.P., Vesnik Akad. NaukS.S.S.R 1959 29 47]. described a procedure for fractionating crude oil, extracting earth waxes and obtaining Lanolin from wool fat with the aid of a dense (supercritical gas). It was not however enough to spur great enthusiasm into research into this area. This was achieved by researchers at the Max-Planck Institute for coal research, who were looking into supercritical extraction. Their development in the area resulted in other researchers gathering enough information for there to be the first 'Symposium' in 1978 inn Essen, with the topic: 'Extraction with Supercritical gases'.

THE CRITICAL POINT (top)

The Critical point is the temperature and pressure at which the liquid and vapour phases are no longer distinguishable. It is best visualised on a phase diagram.

The lines that separate the liquid and vapour and solid phases are phase boundaries. Crossing a phase boundary causes a change in phase. They all cross at the Triple point. The liquid/vapour phase boundary comes to a stop at a particular temperature and pressure. This is the critical point. Beyond this is the critical region. Taking a vapour whose temperature is beyond the critical point but whose pressure is not, and increasing the pressure isothermally, we find we end up with a supercritical fluid. Doing the same with a liquid whose pressure is beyond the critical point and increasing the temperature isobarically, a supercritical fluid is achieved. However in both cases a phase boundary was not crossed. Therefore, since supercritical fluids can be accessed from both routes, it can be seen that the properties of a SCF can only be described as a combination of liquid and vapour properties.

Beyond the critical point there is no phase boundary, however it is possible to see what happens in this region if we 'extrapolate' the liquid/vapour phase boundary [J.A.Banister]. Increasing the pressure, approaching the extrapolated phase boundary, causes the density to increase rapidly. Therefore before the extrapolated phase boundary, the fluid behaves more like a dense gas, and after the extrapolated boundary, more like a dense diffuse liquid. Therefore by varying the pressure the density can be changed.

The 'phase' that exists beyond the critical point can therefore be 'accessed' from both the liquid region and the gaseous region. Therefore if a reaction is taking place between two substances in a different phase (liquid and gas), which is called a heterogeneous reaction, the use of supercritical properties implies that the reaction can be made to take place in the same phase, i.e. a homogeneous reaction. This is called phase homogenisation. A homogenous reaction is often faster because there are fewer mass transfer resistances. Homogenisation can also ease product separation.

The critical point can be measured by several methods.[www.nottingham.ac.uk/supercritical/scacoust.htm]. One of these is the Acoustic method, which can be used to locate the critical point of reaction mixtures. The general principle is that the speed of sound reaches a minimum at the critical point. Therefore if a source of sound whose speed within the reaction mixture can be measured is used and recorded against changes in temperature and pressure.

THE PROPERTIES OF SUPERCRITICAL FLUIDS (top)

Certain 'Thermophysical' properties exist which can be manipulated to selectively direct the progress of chemical reactions. [Sundaresh Ramayya et al].

Variable Solubility

M. Jobling reported in her 1992 Thesis (Organometallic Photochemical Reactions in Supercritical Fluids) that Hannay and Hogarth first observed the pressure dependence of solubility of inorganic salts in ethanol. They observed that increasing the pressure on a fluid caused more salts to dissolve, and decreasing the pressure on a fluid caused more salts to precipitate. They noted that beyond the critical point, there was a dramatic increase in the amount of solute that would dissolve in the fluid. Only a small increase of pressure was needed to increase the solubility after the critical point was reached. Conversely decreasing the pressure by increasing the volume of what is essentially a gas with dissolved constituents, and the dissolved substances precipitate. This phenomenon can be explained quite simply by imagining the increase in mobility of the particles in the fluid as it becomes supercritical. The greater the mobility the larger the ability to attract and hold ions within the structure of the fluid. Also at supercritical pressures fluids tend to retain their ionic properties when raised to higher temperature, for example, water retains its ionic properties (high dielectric constant and ion product) to temperatures of 400C or more when the density is raised to 0.4 gcm^-3. [Sundaresh Ramayya et al].

This ability to change the solubility and the density of a substance simply by changing the pressure is very useful and can be applied to many separation techniques such as chromatography and liquid extraction, as I will describe below.

Ion Product

For chemical Reactions involving polar

These properties cause some well known physical effects in the environment. The solvent power of supercritical gases is involved in geological processes through the influence of water on rock formation [Ingerson E., Econ. Geol. 1934 29 454]. In this reference Ingerson noted that solutions above their critical temperature may effect a considerable transfer of material. He described how both volatile and non-volatile compounds are transported under high temperatures and pressures. The volatile compounds (having a lower critical point) often 'dissolve' the less volatile compounds (higher critical points) and transport them to a point where the pressure on the mixture is released and the compounds can be deposited. For example SiF4 (Tc -1.5C), SiCl4(Tc230C), SnCl4(319) and TiCl4(358C) are all compounds with a low critical point. These have been found to transport the less volatile compounds found in magma, e.g. (FeCl2 (Tc 1450C), AgCl (Tc 2460C), Cu₂Cl₂ (Tc 2185C) etc. The transport occur when a confined gaseous solution, which is under considerable pressure from overlying rock, moves through the pore space and channels in the rock so slowly that there is no appreciable pressure relief as pressure due motion away from its source. Water is the most important solvent from the geolgic point of view,. Water above its critical temperature can carry at least 5% of its weight of alkali halides and appreciable amounts of silica [George C. Kennedy Econ. Geo. 1950 45 pg 629]. This reference details the possibility of silica dissolving in water under high pressures and temperatures. This effect also has a bearing on methane in petroleum formation and migration [ref.3]. Knopf [Knopf A. U.S. Geol. Surv. Prof. Paper 114 pp44-45 1918] wrote a paper describing the ore deposits of the Yerington District, Nevada, saying that aqueous solutions in the supercritical state may have been responsible for the formation of the deposits.

The high solvent ability and tuneable density of supercritical fluids such as carbon dioxide and water means that they can be used as industrial solvents and reaction media that are non-toxic. Non-toxic solvents are preferred because toxic solvents need substantial investment in safe disposal mechanisms. Toxic solvents can get into the body where they can affect the nervous system, the respiratory system, liver and the skin (particularly after prolonged and repeated contact the solvents will dissolve the protective skin lipids). They may get in the body by inhalation of a volatile solvent. Another possibility is for the solvent to pass through the skin and into the blood stream. When the solvent is in the body the solvent undergoes a metabolic change. Toxic solvents include trichloroethylene, perchlorethylene and trifluroethane. Obviously water and carbon dioxide will not cause these effects to the human body and are much cheaper to produce, and are therefore much preferred.

TYPES OF SUPERCRITICAL FLUID (top)

D.F. Williams notes that Carbon Dioxide is a commonly used supercritical fluid. Supercritical Carbon dioxide is often used as a replacement for organic solvents. It has many properties that make it a good choice: it is very non-reactive; non-toxic; non-flammable; cheap (since it is available in the atmosphere) and therefore environmentally friendly. Carbon dioxide has therefore become increasingly considered as the man alternative to the problems of recycling or disposal of organic solvents in industry. The non-toxic nature makes it ideal for use in the food-industry, where contamination of the food produced by the organic solvents used can be a problem. The cost of the carbon dioxide is not, however the only expense to be considered since operating the system at high pressure is always expensive. Operating at supercritical conditions increases the extraction rate by 2.5 times compared to using liquid carbon dioxide [D.F. Williams].

An example of a synthesis reaction that can be carried out in supercritical carbon dioxide, is the Friedel Crafts Alkylation reactions (Addition of an alkyl group to a benzene ring). These traditionally require long reaction times and low temperatures, using dirty catalysts, for example AlCl3 or H2SO4. Using supercritical CO2 allows reaction conditions to be tuned to get high product selectivity. Solvent removal is also easy using supercritical CO2. [www.nottingham.ac.uk/supercritical/scwater.html].

In order to improve the solubility of carbon dioxide it is sometimes modified to make it more polar. This can be done with small amounts of ethanol.

Another example is demonstrated by researchers at the Technical Research Centre, Finland, who are researching into the extraction of organic pollutants from soil using supercritical CO2 as a solvent. This is a simple process in which the scCO2 is pumped through the soil, which is kept in a pressure vessel. The pollutants are recovered by pressure reduction. (process 100 to 200 bar, temp, 30-70C). Chemical Engineering Progress July 1995 - pg. 47 reports that supercritical fluids can also be used to regenerate carbon. The scCO2 is used in place of the desorbing solvent, extracting the adsorbate from the surface of the Carbon. The solvent is recovered by reducing the pressure so that it is sub-critical. The adsorbates being insoluble in the sub-critical fluid are separated as a liquid from the sub-critical vapour. This is still not a widely used method due to the cost of high-pressure equipment.

Water is also a widely used supercritical fluid. It is a common solvent in everyday chemistry, but supercritical water is a much better solvent. It has the effect of becoming a reaction causing solvent, and since great control over the solubility is achieved by altering the pressure reactions can be controlled as never before. Therefore supercritical water is applied to areas of chemistry such as organic substance reactions, where many steps are involved. The critical point of water is 374C and 218 atm. There is an additional change in water when it reaches its critical point, it turns from the traditional polar liquid to a non-polar one. The acidity also increases in line with the other changes that I have already detailed above. This happens because the dissociation of ions within the water increases dramatically up to and after the critical point. A term often used to describe the amount of dissociation is the ion product. The ion product of water is as follows: Kw=[H+][OH-]. The concentration under the conditions of room temperature and pressure will be 1*10(-14). As the pressure and temperature is increased the value of the ion product increases rapidly. Conditions of 34.5Mpa and 300C the value is 1*10(-11). This is an increase in the hydrogen ion concentration by about 30 times that of RTP. Thus supercritical water can be used as an 'acid catalyst'. This leads to scientists researching into ways that supercritical water can be used in the total oxidation of toxic organics in supercritical water.

THE DEVELOPMENT OF INDUSTRIAL APPLICATIONS

The uses of supercritical fluids are increasing rapidly. Much research is being done into new areas and into making existing supercritical fluid technology more affordable. Below I will detail some of the technologies that use supercritical fluids. The concept was first noted by Hannay and Hogarth (ref. 2).

Supercritical Fluid Extraction (SFE) (top)

Supercritical gas extraction is a technique that makes use of the solvent power of supercritical fluids at temperatures and pressures near the critical point. [D.F. Williams (ref.1)]. It requires high pressures, and therefore densities for the solvent to have a sufficient capacity. The advantage of solvent extraction with supercritical fluids, according to Micheal E. Mackay et al [Michael E. Mackay and Michael E. Paulaitis Industrial and Engineering Chemistry Vol.18 No.2 1979 pp149] is that the enhanced solubility effect is completely reversible and very sensitive to changes in temperature and pressure. Consequently, the separation process can be controlled closely and is capable of highly selective.

D.F. Williams [Chemical Engineering Science pp1769 1981] says the main advantage over distillation is that it can take place at moderate temperatures, and therefore it is suitable for use in the recovery of heat-labile substances such as foods and petroleum products. D.F. Williams goes onto describe the various uses of supercritical fluids. He says that their uses include the breaking of azeotropes and the selective extraction of stright chain paraffins or of aromatics from other hydrocarbons. He goes onto list the following:

* ScCO2 and scPropane are used in the extraction of spices and flavours. The flavour and colour of such extracts tend to be much better than for other techniques.

* Carbon dioxide has advantages over other gaseous or liquid solvents, because it introduces no health hazards.

* Decaffeination of coffee. [D.F. Williams] This went into commercial production in Germany in 1978. The coffee beans having previously been soaked in water, are palced in a pressure vessel and are extracted with carbon dioxide at 16-22 Mpa, 363K. The carbon dioxide is continuously recycled. The caffeine diffuses out of the beans into the supercritical phase, in which it is carried into a washing tower where it is washed with water at 343-363K. After 10 hrs, all the caffeine has been transferred from the beans to the wsh water, which is then degassed and the caffeine is recovered by distillation. The caffeine is reduced from around 3% to about 0.02%. It is found that the caffeine is selectively removed from the coffee, no substances contributing to the aroma are removed.

* Nutraceuticals (such as ginger) and vegetable oil are also extracted by this technique.

* Organics can be removed from wastewater, for example NG and DNT.

* The production of void free particles can be achieved using SFE to remove the gas from the powder.

* Carbon dioxide has been used to extract anti-cancer drugs such as taxol, vincristine sulfate, AIDS drug such as Michellamine B, and other compounds. Polymers are separated in the pharmaceutical industry. [sc-times.com:applications].

[Chemical Engineering Progress October 1995 pg.36.] Fluid extraction as a process is used in different circumstances to distillation. It has the advantage that moderate temperatures can be used, so it can be used with thermally sensitive materials. It can also be used where low relative volatilities and azeotropes are involved and where energy savings are possible. It is however a slow process, requiring long reaction times. Supercritical extraction can be used because of the solvent abilities and variable properties of the supercritical fluid. SFE uses a fluid such as scCO2 in place of a liquid solvent. In contrasts to liquid/liquid extraction, which runs at atmospheric pressure SFE is normally run around 1000-2000 psig. This process is well beyond the research stage and is used commercially for the extraction of caffeine from coffee and tea, and for the extraction of hops. Supercritical pentane to recover liquid fuels from heavy oils, in a process called Residue Oil Supercritical Extraction (ROSE). SFE's main advantages lie in the non-toxic nature of the supercritical solvents used. This means that it can find applications where there are concerns about the environment or worker over-exposure. However the big problem is the cost associated with high-pressure equipment.

Jobling noted in her thesis that SFE has disadvantages, namely:

* Once the extraction is complete the solvent must then be removed.

* The choice of supercritical fluid is restricted to those substances that have a low critical point.

* SFE is carried out at high pressures which makes the process more dangerous.

* The expense of high-pressures.

And the main disadvantages are:

* Supercritical fluids are usually gaseous at room temperature and atmospheric Pressure. Therefore recovery of the extract is relatively straight forward, by decreasing the pressure in a controlled manner; the extracted material can be deposited.

* Solvent strength can be changed by changing density.

* Higher diffusion coefficients found in supercritical fluids give rise to higher mass transfer, and much faster extraction.

* Because SFE is carried out at room temperature, thermally unstable compounds can be extracted.

Chromatography

Supercritical fluids as reaction media (top)

Supercritical fluids can be used as reaction media, making use of their property of greater diffusivity and tuneable density and thus solubility properties. However the reaction processes may be different under supercritical conditions. This becomes an important consideration particularly when organic synthesis reactions take place because these often involve many complex stages.

Supercritical water oxidation (top)

[Chemical Engineering Progress April 1995 pg18] Organic materials can be oxidised with great efficiency in supercritical water. SCWO is compared to a similar process called wet oxidation. WO has the disadvantage that it leaves acetic acid and ammonia in the waste stream, which have to be removed by biological means and the exhaust gases have high levels of carbon monoxide. SCWO allows ammonia production to be reduced to about 5% and lowers carbon monoxide levels to an acceptable level. The main point of concern about the SCWO method is solids handling within the system. It has been found by researchers at the University of Tulsa that the addition of a catalyst makes the supercritical oxidation more effective. Catalysts such as V2O5 and MnO2/CeO are being tried. NATO is developing the SCWO for use in destroying various chemical weapons, particularly in the Iraqi chemical weapon stockpile. This contains such items as sulphur mustard gases and nerve agents. NATO engineers have developed mobile SCWO reactors with capacities of 400l/day. The oxidant used is sometimes compressed air, but carries the penalty of 80% inert Nitrogen. Liquid oxygen can be cryogenically pumped and vaporised into the reactor and may be economical for large systems. Hydrogen peroxide is more expensive but is relatively safe to handle.

Pipe flow reactors are the simplest but may be more subject to plugging by salts and erosion by solids. The vertical vessel is most convenient for salt/solid separation from the reaction medium and may be best for high solids feeds or when neutralisation of the waste

Supercritical fluids as reactant solvents (top)

[M. Jobling] Hydrocarbons have such a wide variety of applications in synthetic chemistry that it seems such a waste that a significant proportion of the constituents of crude oil, the alkanes, are very unreactive. As a result a huge research effort has taken place in response to the massive growth in synthetic chemistry in industry since the war. It was found by the end of the 1960s that transition metal complexes could be made to activate C-H bonds (alkanes have no pie-bond to react with the metal centre, so it has been suggested that the unbroken C-H bond co-ordinates to the metal centre, donating some electron density and thus weakening the C-H bond). One problem however still remained in solving this problem, and that was to find an appropriate solvent for the activation. The solvent must have reasonable solvating power (capable of dissolving the activation complex), there can be no C-H bonds in the solvent and the solvent must be chemically inert. Supercritical fluids were suggested a s an appropriate solvent. Supercritical Xenon. ScXenon has been used at Nottingham as a solvent for organometallic photochemical reactions, so it can dissolve organometallic compounds and it obviously does not have any C-H bonds. Supercritical fluids have the advantage of being completely miscible with reactant gases. Conventional solvents give rise to an additional problem in C-H activation reactions, once the reaction has taken place, recovering the product is often accompanied with complete or total decomposition. With supercritical fluids the products can be removed by reducing the pressure. However the process must be developed so that when the pressure is reduced the product does not just coat the reaction vessel.

Polymer Synthesis (top)

Flow Reactors [J. Banister ORGANOMETALLIC REACTIONS IN SUPERCRITICAL FLUIDS: Development of a Flow Reactor 1994 pg.66-80]

Flow reactors are the most efficient way of dealing with supercritical reactions. A flow reactor is a continuous, with feed flowing into the reactor and product removed from the reactor all the time. The design of such reactors concentrates on keeping the volume within the reactor to a minimum. This is because it is cheaper and easier to keep only a small section of the apparatus at the required high pressure. Flow reactors have the following advantages:

* Solution, reaction and product removal can occur simultaneously.

* Only a small proportion of the solution is involved n the reaction (associat4d with keeping the volume down).

* Since it can be continuous, the overall volume of the reactor can be less than for a batch reactor. This makes economic sense, since a smaller volume will require less cost on high-pressure equipment.

* Since supercritical fluids have a high viscosity, small diameter, highly flexible steel tubing can be used, which keeps the overall volume of the reactor to a minimum.

* Deposition of solid product can be controlled under inert condition. By having two valves, one at the inlet and one at the outlet, high-pressure 'stabilising' conditions can be maintained at the point of precipitation when the product is deposited by the rapid expansion of the fluid.

DETAILS OF A FLOW REACTOR (top)

A continuous flow reactor will have a pump at one end, which supplies the fluid at high pressure, maintaining a small pressure gradient across the reactor. The type of pump used may be very different depending on whether or not the reactor is laboratory scale or industrial scale. In a laboratory it makes sense to use a syringe pump because a syringe can maintain a constant pressure, but it has a limited volume. In a larger scale a compressor is an alternative, however the pressure fluctuates with time as the valves poem and close, which must be damped out.

In working reactors a lot of consideration is put into how the volume is to be kept as small as possible. All individual components of a flow reactor have a low volume. Even the taps are kept so there is no dead volume. The connection between the tubing and tap body is also designed to keep internal volumes to a minimum. At the start of the flow reactor, the reactants must be dissolved into the supercritical fluids. This can be done by a supercritical extraction vessel (removes substances into supercritical fluid). The solvent can be collected by either bubbling the effluent through another solvent or by passing the solvent over a nn adsorbent. This provides additional problems to industry in finding a solvent that the product can easily be removed from. At the other end there will be a valve that will reduce the pressure to atmospheric in a controlled manner.

Polymer Impregnation [M Jobling 5.1-5.4][sc-times.com David L. Tomasko 'sorption processes in Supercritical Fluids']

Supercritical fluids can be made to dissolve into polymers with very interesting effects. The fluid will pass into the polymeric matrix as a result of a high-pressure gradient supplied. Once the pressure is removed the internal pressure will exceed the external pressure of the atmosphere and the polymer will expand. The polymer eventually returns to its original size. However any dissolved substances that existed within the supercritical gas will have become deposited within the polymer. This can become very useful if the dissolved substances have some useful properties, such as colour. The 'wettability' or and density of a polymer can be changed by similar impregnation. Liquid organic swelling agents do exist, but are hard to separate after impregnation.

THE ENVIRONMENT

Industry is under constant pressure from government organisations and pressure groups to increase their environmental awareness and limit wastes released into the surrounding as much as possible. Dr. Lalit Chordia of the supercritical Times says:

'As a result of the Montreal Protocol and the Kyoto Conference, there is a sense of urgency and need to develop industrial processes that are energy efficient and environmentally-compatible'.

' Research of supercritical fluids as reaction medium and as a solvent medium has seen recent resurgence, driven by needs to satisfy environmental regulations using efficient processing and separation techniques.'

ALTERNATIVES TO SUPERCRITICAL FLUIDS (top)

Supercritical technology is developing so fast, with numerous lists of users for supercritical fluids. However I have noticed that there is an understandable enthusiasm by the supercritical researchers to apply their technique to everything even when it appears that such technology is not required. An Article in 'The Chemical Engineer' (July 94 pg. 3) brought to my attention that alternatives to supercritical operation exist and are more practicable in some circumstances. James Green suggests that WAO (Wet Air Oxidation) is essentially a lower energy alternative to SCWO. WAO uses milder process conditions (280-330C and 62-120 bar). The efficiency is not quite as high (95% compared to 99% for SCWO). He suggests that if the milder conditions do not produce the required purification of the water, it can be considered with another technology such as biotreatment. He suggests that using the SCWO process could sometimes be, like using "A Sledgehammer to Crack a Nut!"

CONCLUSION (top)

There are clearly many new and varied technologies available making use of supercritical fluids. The development of these techniques is encouraged by the environmental advantages of using non-toxic solvents for example. However the cost of such equipment is inhibiting the technologies use at its full potential. We live in a World where individual companies have to fit the bill for environmentally friendly technologies. The high capital costs are sometimes weighed against the low operating costs that exist when the critical point is near ambient. However the operating costs can be significant if the pressures required are high and the temperatures are equally large. Equipment costs have been traditionally high for supercritical technologies because the equipment is adapted from other areas of chemical processing rather than addressing specific equipment needs of supercritical fluids. European companies are selling the equipment as custom equipment instead of standard designs[sc.times.com:Editorial]. A few American firms have developed equipment specifically for supercritical work and are able to produce it much more cheaply. I believe that as the technology become more widespread and engineers develop experience and confidence in its use and design it will become a technology that will influence everything from the dry cleaning of our clothes to the production of the polymers in our drink bottles.

Appendix 2 (top)

Other Uses of Supercritical Fluids

Activation of the C-H bond in supercritical solution.

Hydrocarbons have such a wide variety of applications in synthetic chemistry that it seems such a waste that a significant proportion of the constituents of crude oil, the alkanes, are very unreactive. [M. Jobling20 ]. As a result a huge research effort has taken place in response to the massive growth in synthetic chemistry in industry since the war. It was found by the end of the 1960s that transition metal complexes could be made to activate C-H bonds (alkanes have no p-bond to react with the metal centre, so it has been suggested that the unbroken C-H bond co-ordinates to the metal centre, donating some electron density and thus weakening the C-H bond). One problem however still remained in solving this problem, and that was to find an appropriate solvent for the activation. The solvent must have reasonable solvating power (capable of dissolving the activation complex), there can be no C-H bonds in the solvent and the solvent must be chemically inert. Supercritical fluids were suggested as an appropriate solvent. Supercritical Xenon has been used at Nottingham as a solvent for organometallic photochemical reactions, so it can dissolve organometallic compounds and it obviously does not have any C-H bonds. Supercritical fluids have the advantage of being completely miscible with reactant gases. Conventional solvents give rise to an additional problem in C-H activation reactions, once the reaction has taken place, recovering the product is often accompanied with complete or total decomposition. With supercritical fluids the products can be removed by reducing the pressure. However the process must be developed so that when the pressure is reduced the product does not just coat the reaction vessel.

Polymer Impregnation

Supercritical fluids can be made to dissolve into polymers with very interesting effects (M Jobling) 20. The fluid will pass into the polymeric matrix as a result of a high-pressure gradient supplied. Once the pressure is removed the internal pressure will exceed the external pressure of the atmosphere and the polymer will expand. The polymer eventually returns to its original size. However any dissolved substances that existed within the supercritical gas will have become deposited within the polymer. This can become very useful if the dissolved substances have some useful properties, such as colour. The 'wettability' or and density of a polymer can be changed by similar impregnation. Liquid organic swelling agents do exist, but are hard to separate after impregnation.

Liquefaction of Coal

Amestica. L.A. et al1 describes the use of supercritical water/CO/solvent media in the catalytic liquefaction of coal. CoMo and Tetralin sulphided catalysts have been used, and it was noted that the supercritical conditions enhance the solubility and diffusivity of the solvent, and minimize mass transfer effects by operating in a single-phase system. A similar process is described by Townsend et al27, where they note that the observed extract yields correlate well with the solvent critical temp. Both the importance of thermal fragmentation of the coal during extraction at high tempts and the significance of the supercritical region on solubility were included in the article.

Supercritical fluids can be used as cleaning solvents. Cleaning narrow tubes and other small apparatus can be very difficult with conventional solvents. Supercritical fluids provide much better cleaning. The synthesis reactions also include polymer Synthesis among the vast other varieties, only a few of which I have mentioned.

12. References

Key:

· The date of publication is in brackets.

· The volume number is underlined

· Where possible I have included the publisherand the place of publication.

1. [Amestica L.A. and Wolf E.E. Catalytic Liquefaction of coal with supercritical water/CO/solvent media Fuel (1986) 65, pp.1226 University of Notre Dame, IN USA, Butterworth & Co.]

2. [Banister J.A, Ph.DOrganometallic Reactions In Supercritical Fluids: Development of a Flow Reactor, pp. 7]

3. [Barton P. Supercritical separation in aqueous coal liquefaction with impregnated catalyst, Ind.Eng.Chem.Process Des. Dev (1983) 22 pp.589 The Pennsylvania State University, American Chemical Society ]

4. [Brennecke. J.F, Chemistry and Industry New applications of supercritical fluids Chemistry and Industry (1996) pp.831 University of Notre Dame, IN, USA ]

5. [Brock E.E., Savage PE and Barker J.R., A reduced mechanism for methanol oxidation in supercrit Chem.Eng.Sci, 53 No.5 pp 857 (1998) University of Michigan, Peragmon Publishers Ltd.]

6. [Caignard de la Tour, C Ann. Chim.(1822), 22 pp.410]

7. [Caruana.C.M. Supercritical water oxidation aims for wastewater cleanup Chemical Engineering Progress April 1995 pp. 10 New York, NY 10017 American Institute of Chemical Engineers (AIChE)]

8. [Caruana C.M. Technologies breaking new ground for soil remediation, Chemical Engineering Progress October 1995 pp.23. New York NY 10017 American Institute of Chemical Engineers (AIChE)]

9. [Chen Shaw-Horng Rough-Hard-Sphere theory for diffusion in supercritical carbon dioxide, Chemical Engineering Science Supercritical Fluid Extraction, (1983) 38 pp.655 University of Rochester, NY, USA, Pergamon Press Ltd. ]

10. [Deshpand GV, Holder GD, Bishop AA, Gopal J and Wender I, Fuel Extraction of coal using supercritical water (1984) 63 , University of Pittsburgh, PA, USA, Butterworth & Co.]

11. [Diepen G.A.M.; Scheffer, F. E. C. J. Am. Chem. Soc., 1948, 70, 4084.]

12. [Edward W. Cook, Oil-shale technology in the USA Fuel July 1974, 53, pp146 Rocky Flats Reasearch center, Colorado].

13. [George C. Kennedy, A portion of the system silica-water Econ. Geo. (1950) 45 pp 629 Division of Geological Sciences, Harvard University].

14. [Green J, author of a letter in 'The Chemical Engineer' (1994) pp. 3. 122 Kiln Road, Benfleet, Essex]

15. [Hannay J.B. and Hogarth J., Proc. Roy. Soc. (London) ser. A (1879) 29 pp.324]

16. [Harmon T.C. and Jackson D.Y, Dispersion and diffusion in porous media under supercritical conditions, Chem.Eng.Sci., (1999) 54 pp.357, University of California, Pergamon Press Ltd.]

17. [Houser T.J., Tiffany D.M., Li Z, McCarville M E. and Houghton M.E. Fuel Reactivity of some organic compounds with supercritical water (1986) 65 pp827 Western Michigan University, Butterworth & Co.]

18. [Humphrey JL. Separation Processes playing a critical role, Chemical Engineering Progress (1995) pp..36. New York 10017 American Institute of Chemical Engineers (AIChE)]

19. [Ingerson E., Relations of critical and supercritical phenomena of solutions to geologic processes, Econ. Geol. (1934) 29 pp. 454, Yale University, New Haven, CONN.]

20. [Jobling M, Ph.D Organometallic Photochemical Reactions in Supercritical Fluids (1992) Chapters 1-5].

21. [Knopf A. U.S. Geol. Surv. (1918) Prof. Paper 114 pp44 ]

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