Supercritical Fluid Processing: Application Fields

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Ministry of Transport, Public Works and Water Management Directorate-General for Public Works and Water Management Institute for Inland Water Management and Waste Water Treatment, RIZA P.O. Box 17 8200 AA Lelystad Telephone +31 320 298411 Telefax +31 320 249218

SUPERCRITICAL FLUID PROCESSING: APPLICATION FIELDS

RIZA Werkdocument No. 96.072X drs.ing. L. Motamedi

January 1996 Content Page

Summary

Backgrounds 4

1. Introduction 5

2. Application fields of supercritical fluid processes 10

2.1 Rate process: Crystallization 10 2.2 Extraction processes in Food, Pharmaceutical and 11 Environmental applications 2.3 Chemical reactions 15 2.4 Polymer processing technology 20 2.5 Fractionation 22 2.6 Analytical Supercritical Extraction 25 2.7 Supercritical Water Oxidation 25

3. Methods for modelling of solubilities of compounds and influence of co- solvent in supercritical fluid 26

4. Conclusions 28

Literature 29

Appendix List of solubility of selected compounds in near critical CO2 40 Summary

This report contains a survey on the application fields of the SuperCritical (SC) technique available in open literature. This with the intention to explore those areas of the technique that can be implemented as an alternative for traditional processes which have a negative impact on the environment. This project is a part of the programme "Clean Technology" at RIZA which is concerned with the development of new or improved processes and products that have less negative impact on the environment. The programme "Clean Technology" focuses on processes which may be implemented within the next 10-20 years.

In recent years SuperCritical Fluids (SCF) have gained considerable importance as media in various fields of application such as extraction, fractionation, chemical and enzymatic reactions, etc. The special physical properties of SCF distinguish it from liquids and gases. An SCF has a liquid like density but its viscosity is more like that of a gas, resulting in diffusion coefficients that are much higher than diffusion coefficients in liquids. The solubilities of SCF appear to be virtually exponential in density, which means small pressure changes can result in enormous solubility variations. This gives the opportunity to put chemicals into the solution or drop them out very selectively. For this reason SCF is an excellent candidate as:

i. an alternative for replacing organic solvents in extraction processes; ii. a media for suppression of undesired by-products in chemical and enzymatic reactions which results in higher selectivity; iii. a media for the ultimate destruction of organic materials which are difficult to oxidize by conventional methods; iv. a technique in analytical gas chromatography; v. a technique in polymer processing technology.

Most of the application fields are in the developmental and/or research stage and there is a lack of information on the cost aspect. There is no general rule to evaluate the process viability of an SCF application. For each case in which one considers use of this technique, an evaluation should be done to investigate if the process has additional (environmental and economical) advantages compared to the traditional process.

Backgrounds

The programme "Clean Technology" is concerned with the development of new or improved processes and products that have less negative impact on the environment. This programme focuses on processes which may be implemented within the next 10-20 years. The programme Clean Technology in his turn is a part of the SPA development programme (SPA = Clean Technology, Prevention and Wastewater Treatment) which started in 1991 in the Institute for Inland Water Management and Waste Water Treatment, RIZA1, the Netherlands (SPA programme plan, 1995, Senhorst, 1995).

One of the topics within the programme "Clean Technology" is the replacing of wet processes with dry processes. In this framework a feasibility study has been done on the dyeing of polyester in SuperCritical (SC) CO2 (Van Asselt et al., 1994a, 1994b). The results of this study show that the SC-CO2 dyeing of polyester is a good alternative for the conventional (water based) process, considering the impact on the environment. It appears that the environment related costs are sub- stantially lower for SC-CO2 dyeing than for the conventional dyeing process. A number of questions though have not yet been answered, such as solubilities and influence of entrainer on the solubility of dyes in SC-CO2. To find an answer to these questions and in general to make an inventory of the use of the SC Fluid (SCF) technique as an alternative for the existing water based processes a literature survey has been done, as is presented here.

This literature survey may be used as a guideline for further work on the use of the SCF technique for the replacing of wet processes with dry processes.

1 Rijksinstituut voor Integraal Zoetwaterbeheer en Afvalwaterbehandeling

4 1. Introduction

During the past 20 years SuperCritical Fluid (SCF) processing has developed from a laboratory scale to commercial processes. The relatively new processes include coffee decaffeination, hops extraction, catalyst regeneration, extraction of organic wastes from water and soil and SCF chromatography. These applications complement older technologies such as Residuum Oil Supercritical Extraction process (ROSE), propane deasphalting and reaction processes for the production of polyethylene and primary alcohols in SCF ethylene. Table 1 shows a short list of SCF processes which are constructed by several companies and have been taken in operation during the last 10 years. Uhde is by far the major supplier of SCF systems.

Table 1: List of selected SCF systems that have been constructed during the last ten years (Brunner et al., 1994; Moore et al., 1994)

Year Vessel Operator Materials processed Supplier volume [m3]

1984 0.2 Fuji Flavour Co. Tobacco Uhde 0.5 Barth Co. Hops Uhde 2 Natal Cane Byproducts Hops, red pepper Uhde

1986 4 SKW/Trostberg Unknown Uhde 3 Fuji Flavour Co. Unknown Uhde 0.006 CEA Pharmaceuticals Muller

1987 8 Bath & Co. Hops Uhde 0.4 Messer Griesheim Various Uhde 0.1 Sumitomo Seika Chemicals Sugar cane Sumitoma

1988 0.4 Nippon Tobacco Tobacco Uhde 1.2 Takeda Acetone from antibiotics Sumitoma 0.4 CAL-Pfizer Aromas Muller

1989 0.6 Hasegawa Unknown Uhde 9 HACO/AG Unknown Uhde 2 Clean Harbors Waste water CF Systems 6 Ensco Solid waste CF Systems

1990 5 Jacobs Suchard Coffee Uhde 0.62 SKW/Trostberg Various Uhde 2 Barth & Co. Various Uhde 1.5 Raps & Co. Spices Uhde 8 Barth & Co. Unknown Uhde 12 Pitt-Des Moines Hops Uhde

1991 0.3 Fuji Flavour Co Unknown Uhde 8 Barth Co Unknown Uhde 6 Texaco Refinery wastes Uhde

1993 1 Hasegawa Unknown Uhde 0.9 Agrisana Pharmaceuticals Separex ? Bioland Bone Separex ? CF Technologies Cleaning of metal parts ?

5 1994 1.4 Unknown Aromas Separex 0.6 Nan Fang Flour Mill Unknown Uhde 0.4 Barth & Co Unknown Uhde 0.07 AT&T Fiber Optics Rods Autoclave ? Union Carbide Spray painting ?

There are many other smaller and larger plants which are not mentioned in table 1 because of the confidential state of the process or a lack of information in the open literature, for example the tobacco denicotinization plant of Philip Morris in U.S. or the tea decaffeination plant of SKW Trostberg.

The very special physical properties of SCF distinguish it from liquid and gases. An SCF has a liquid-like density but its viscosity is more like that of a gas, resulting in diffusion coefficients that are much higher than those in liquids. Table 2 shows a comparison of these characteristics for a gas, liquid and SCF. Table 3 shows the critical pressure and the temperature of various compounds. The solubilities of SCF appear to be virtually exponential in density, which means small pressure changes can result in enormous solubility variations. This gives the opportunity to put chemicals into the solution or drop them out very selectively. Bartle et al., 1990, provide a review of solubilities of low volatility substances (88 compounds) in pure CO2.

Table 2: Comparison of some physical properties for a gas, liquid and SCF

Density [kg/m3] Diffusion coefficient [m2/s] Viscosity [Pa.s]

Gas (1 bar, 20 °C) 0.6-2.0 1-4 10-5 0.01-0.03

Liquid (20 °C) 600-1200 0.2-2 10-9 0.2-3.0

SCF 200-900 2-7 10-7 0.01-0.09

Table 3: Critical temperature and pressure for various compounds

Critical pressure [bar] Critical temperature [ °C]

Carbon dioxide 73.8 31.1

Ethane 48.8 32.2

Water 220.5 374.2

Benzene 48.9 289.0

Ammonia 111 133

Methanol 81 240

Although the first papers regarding the use of SCF relate from 1879 (Hannay et al.) and since then a huge amount of research and effort has been done in this area the industry has not shown much interest in this technology yet. Brunner et al., 1994 explain this disinterest of industry by one simple word: "motivation". For example a prohibition of the use of methylene chloride for decaffeinating coffee puts a company in the situation that it has to switch to another component such as

6 benzene. The application of a component such as benzene is of course not an alternative and thus the need for evaluating SC-CO2 as an alternative process becomes alive. This "motivation" plays an important role in the last 10 to 15 years and its reflex may be found in an exponential number of research, patents and published papers on the application of SCF in almost all areas such as food, environment, pharmaceutical, chemical and enzymatic reaction engineering, analytical techniques, etc.

Among the gases which may come in consideration for use in supercritical processing, CO2 plays an important role. CO2 is an attractive organic solvent because it is non-flammable, inexpensive and exhibits low toxicity. Further, materials solubilized in CO2 can be easily recovered from the solution via a simple pressure quench. Despite its many favourable properties, the low solvent power of CO2 limits its application. To overcome this solvent power limitation not only in the case of CO2 but in general, a new aspect of SCF processing technique has in recent years got a lot of attention: the use of a second compound, a so- called cosolvent, modifier or entrainer, to upgrade the solvent power of the solvent. Many investigators have established that the addition of a small amount of a cosolvent to an SCF can dramatically alter its solvent properties (Lemert et al., 1991; Ting et al., 1993; Roop et al., 1989). The majority of information in the literature focuses on the increased solubility of solute in SCF containing a small amount of cosolvent. Nonpolar cosolvents are the first studied and are shown to be useful in increasing solute solubilities. This increase in solubility has been attributed to the enhancement of the polarization of solvent molecules which lead to increased dispersion forces in the solution. In spite of the appearance of greater dispersion forces, no improvement in the selectivity of the process has been obtained. In the contrary, the use of polar cosolvents usually results into an improvement of the selectivity and causes much larger solubility enhancement. In the recent years a lot of effort has been done for prediction of the influence of different types of cosolvents in SCF (Johnston et al., 1982, 1987; Economou et al., 1992; Lemert et al., 1991; Sheng et al., 1992).

As is mentioned earlier the first paper dealing with the application of SCF dates from 1879 (Hannay et al. 1879). They discussed the ability of an SCF to dissolve low vapour pressure solid materials. Since then a substantial amount of work has been done by many investigators to understand the basic fundamentals of a fluid in the supercritical region. However the work of Francis, 1954, by far surpasses the work of others when one considers the extent of his work. In a single paper he presents an extensive, quantitative study on the solvent properties of liquid CO2 with hundreds of compounds. His work is primarily concerned on the phase behaviour of ternary systems containing liquid CO2, collected data for 464 ternary phase diagrams and determined solubilities of 261 compounds in near critical CO2 (appendix 1 shows a selected compound of his work). Nearly half of these compounds is reported to be miscible with liquid CO2. He included many classes of organic compounds, e.g. aliphatics, aromatics, heterocyclics and compounds with a large variety of functional groups.

7 Although Francis studied solubility behaviour in near critical liquid CO2 (25 °C, 655 bar), his results are generally applicable. For example a compound that is soluble in liquid CO2 will also be soluble in SC-CO2. Therefore from the listed data of his work (McHugh et al., 1994, see appendix 1) it is possible to extract some general rules of functional groups.

1. The lower weight aromatic compounds such as benzene and toluene are completely soluble in near critical liquid CO2, however by increasing the molecular weight the solubility will drop (e.g. bibenzyl of biphenyl).

2. Aliphatic components are completely soluble in near-critical liquid CO2, however by increasing the molecular weight the solubility will drop (e.g. n- octadecane).

3. The unbranched heterocyclic components are soluble in near critical CO2 (e.g. furfural, pyridine and thiophene). The existence of a group such as methyl in a heterocyclic compound like pyrrole (2,5-dimethyl pyrrole) causes the compound to be not completely soluble in near critical liquid CO2.

4. The low molecular weight carboxylic acids are soluble in near-critical CO2 (e.g. acetic acid, caproic acid). The solubility of aliphatic acid will be marginal when both carboxyl and hydroxyl groups are present in the molecule or when the molecular weight increases (e.g. oleic acid).

5. The halogenated substitution shows no negative influence on solubility of a compound (aliphatic or aromatic). In general, metha position of substitution in aromatic compounds shows higher solubility than para or ortho position.

6. The low molecule weights nitriles such as acetonitrile and acrylonitrile are soluble in near-critical CO2. In the molecules wherein hydrogens of the amide group are substituted with an alkyl group (e.g. n,n-dimethylaceta- mide) the molecules become completely miscible in near-critical CO2.

7. The low molecular weight aliphatic substitution seems to enhance the solubility of components which would otherwise not be soluble. For example the solubility of n,n-dimethylaniline, n,n-diethylacetamide and n,n- dimethylformamide show increased to complete miscibility.

To be able to handle systematically the large amount of information available in literature, this information will be categorized under the following application fields of SCF.

1. Rate processes: Crystallization; 2. Extraction processes in Food, Pharmaceutical and Environmental applications; 3. Chemical reactions;

8 4. Polymer processing technology; 5. Fractionation; 6. Analytical Supercritical Extraction; 7. Supercritical Water Oxidation.

Each application field will be treated in a separate paragraph and a review will be given of so far explored information in open literature. Because most of the information and research work found in literature is limited to technical aspects and/or possible new application fields of SCF and does not treat the cost or industrial experiences, this aspect is not included unless it was mentioned.

Finally (in chapter 3) an overview of possible mathematical methods will be presented and discussed regarding the estimation and simulation of technical aspects of SCF such as the solubility, influence of cosolvent, etc. which are available in literature.

9 2. Application fields of Supercritical Fluid processes

2.1 Rate process: Crystallization

The particle size and size distribution of solid materials formed in industrial processes is frequently not the size that is desired for subsequent reaction or the use of these materials. Crushing, grinding, ball milling and precipitation from the solution are examples of the methods for particle size redistribution applied to chemicals, pharmaceuticales, dyes and polymers. There are many solids that are difficult to process by grinding or by solution techniques for one reason or another. For example certain dyes and explosive compounds. The use of SCF in this field has recently been under the attention, especially the GAS and RESS process as will be described below.

Gas Anti-Solvent (GAS) Recrystallization

Gallagher et al., 1989, describe a new process to recrystallize compounds insolu- ble in SCF, so called GAS (Gas Anti-Solvent) Recrystallization. In their study this process has been used for recrystallization of nitroguanidine from n-methyl- pyrrolidone and n,n-dimethyl formamide using SCFs. The type of SCF used is not mentioned in the paper. The investigation was directed to a specific explosive, nitroguanidine, but the process has been reported to be general in its application. In this process, gases that are soluble in liquid can be admixed through complete expansion which causes an extremely high level of supersaturation and nucleation rate resulting in the formation of extremely small particles which are not readily achievable by other processes. This process can not be used to recrystallize solids which are "too" soluble in the gas.

Crystallization through Rapid Expansion of Supercritical Solvent (RESS)

In the RESS process supersaturation is caused by a mechanical perturbation, contrary to the conventional process wherein supersaturation is caused by a thermal perturation. The speed with which perturation propagates in the RESS process gives rise to a narrower particle size distribution compared to the conventional method. Halverson et al., 1991, investigate the influence of pre- and post-expansion condition on the crystallinity and particle size distribution. Their studies concern the continuous expansion of mixtures of naphthalene in SC-CO2. It seems that the particle size and shape are a sensitive function of pre and post- expansion temperature and solute concentration in SC mixture.

Although the Gas Recrystallization and the RESS process is in developing status, it seems to be an effective process for (re)crystallizing of solid material within a controlled particle size distribution. There is no information reported regarding the process conditions. Application of these techniques for a new process should be subjected to a careful case-by-case evaluation for its economic and environmental factors compared with other recrystallization processes. 2.2 Extraction processes in Pharmaceutical, Food and

10 Environmental applications

In recent years much research has been done on the application of SCF in the food, pharmaceutical and environmental field. Schaeffer et al., 1989, report extraction of pyrrolizidine alkaloids (anti-cancer agent) from seeds of Crotalaria spectabilis which is difficult to extract and isolate from the plant material without degradation or use of toxic solvents. In this process oil of crushed seeds of Crota- laria is extracted using SC-CO2 with addition of ethanol and water as co-solvents. There are no data reported regarding the process conditions and/or the physical parameters such as solubilities. Extraction of chemotherapeutic agents from the plant materials like maytansine and rollinia papillionela with SC-CO2 also has been mentioned (McHugh et al., 1994).

Use of SC-CO2 as a replacement of hexane in soybean-oil extraction recently is being considered. Data on the extraction and oil composition of soybean oil have been described by Friedrich et al., 1984. He showed that the separation of oil from CO2-oil stream at 800 bar can be carried out by dropping the pressure by only 150 or 200 bar at 70 °C.

Other development work on a SC-CO2 process for extracting oils from potato chips and extension of seed- and fish-oil extraction has been reported (Hannigan, 1981). It is reported that potato chips containing about 45-50 wt% oil can be extracted of about 50% of their oils while retaining the original flavour and texture.

Robey et al., 1984, describe a SC-CO2 process to concentrate aromatic constituents in lemon oil. The conventional process which is based on either steam distillation or liquid-liquid extraction suffers from certain drawbacks such as product degradation and requirement of subsequent removal of solvent. It is reported that a tenfold concentration of aromatics can be achieved in a single extraction stage with a superior result concerning the flavour compared to the conventional method.

The extraction of alcohol from water using SCF has been described in some detail by Kuk et al., 1983. It has been suggested that the compressibility of SCF solvent and the differences in the liquid phase nonideality exhibited by the binary alcohol- SCF and alcohol-water mixtures are two major factors for controlling the selectivity and loading of the solvent. These two factors determine whether the alcohol- water-SCF mixture will split into an LLV mixture.

Novak et al., 1989, discuss the preliminary design and economic of SCF extraction of flavours from a large number of spices and herbs, such as thyme, nutmeg, coriander. The estimated average extract yield for spices is 17% with an average process rate of 103 kg/h. The estimated average production cost of the extract is $2 7/kg. The estimated average extract yield for herbs is 5.6% with an average

2 1 $ = Nfl 1.7

11 process rate of 60 kg/h. The estimated average production cost of the extract is $ 35/kg. The key parameters applied for the preliminary design are: - spices and herbs in solid state as feedstock; - preliminary design for multiproduct plant; - extraction pressure of 60-300 bar and temperature of 20-80 EC; - separation pressure of 45-150 bar and temperature of 15-40 °C; - Use of two 973 l extractors; - CO2 as extraction solvent with a flow rate of 75.8 kg/min.; - annual capacity of 800 ton based on 8,000 h and a max. availability of 85% operation which is required for a multiproduct plant.

The capital cost for this design base is estimated to be $M 2.8. The operating cost for this design base has been estimated to be $ 115/hr.

Böhm et al., 1989, describe some guidelines for the design of a multipurpose SC extraction plant. The following separation problems were chosen as basis for the designing plant: - deacidification of palm oil with ethane as extractant and acetone as entrainer; - separation of monoglycerides from a mixture with Di- and triglycerides with CO2 as extractant and propane as entrainer; - de-oiling of lecithin with a mixture of CO2 and propane. In order to achieve the requirements of such a plant a modular concept was developed so that the plant could be adopted easily to a given feedstock. No data regarding the solubility of compounds and the influence of entrainer were given in the paper. The design characteristics considered were: explosion-proof installation; sandwich membrane pumps to avoid leakage; installation of a flare; sufficient air circulation and installation of gas sensors. Estimation of heat transfer area has been reported to be a problem in case of use of an entrainer. Therefore the authors recommend use of a safety factor up to 100% in such cases. Application of a total automation in multipurpose plant is dissuade, because the operation conditions for various feedstocks can be quite different.

Use of SC-CO2 for the extraction of flavours from milkfat is reported by De Haan, 1991. About 98% of the constituents of milkfat are triglycerides with a carbon number varying from 26 to 54. In the remaining 2% a large number of other compounds, lactones, ketones, etc., in small concentrations (ppm level) contribute to the characteristic flavour of milkfat, which are of great interest in the food industry. De Haan, 1991, describes a process for the production of a flavour extract from milkfat in which the flavour components are concentrated over 1000 times. The economic attraction of the process is discussed for a process with a capacity of extracting 10 kton milkfat/year.

The estimated economical evaluation shows that the produced flavour extract costs around $ 145/kg which is encountered in other SC processes where flavours of spice extracts are produced. A comparsion with the conventional method of extraction of flavours from milkfat is missing in the paper. However from this

12 price level clearly SC solvents can from an economic point of view only be used for the production of high value products.

Use of SC-CO2 with addition of ethanol as co-solvent for fractionation of fish oil ethyl ester has been reported by Nilsson et al., 1979. Fractionation of triglycerides from mixed glycerides has been obtained with purities higher than 92 wt% with addition of 4 wt% ethanol. No further data regarding the process are available.

Extraction of phenol from soil and water using near critical and SC-CO2 has been reported by Roop et al., 1989. They report extraction of phenol from water using SC-CO2 at pressures up to 310 bar for isotherms 25 and 50 °C. Benzene has been reported as suitable co-solvent since its solubility in water is very small and it enhances the distribution of phenol into the SC phase. The presence of methanol was found to have no effect; since methanol is polar and completely soluble in water, it favours the aqueous phase and therefore does not change the characteristics of the SC phase. The two soil systems they have investigated were contaminated dry and wetted soil. They found that the SC-CO2 was able to remove phenol from both systems equally effectively. For contaminated soil both co-solvents, benzene and methanol, increased the distribution coefficient of phenol, whereas methanol has been reported by far the most effective. The presence of moisture in the soil was found to have no effect on the extraction of phenol using pure CO2. While the influence of water has been shown to have a dramatic impact on the effectiveness of the entrainers. The benzene/CO2 has been reported to be able to remove all of the phenol from the wetted soil while the methanol/CO2 offered no enhancement over that of the pure CO2. From the results it can be concluded that the choice of a co-solvent is not independent of the contaminated matrix or in the case of soil, its moisture content. Data regarding the process conditions and physical parameters were missing in this report.

Oudkerk et al., 1994, estimated the cost for the handling of 10 tons of sludge per hour, based on 5000 h/a, to be $ 26-53/ton with investment costs of about $M 3,-. It concerns a continuous SC-CO2 extraction process where no co-solvent has been used. The sludge is assumed to be polluted with polycyclic aromatic hydrocarbons (PAH's), polychloric biphenyls and pecticides. Figure 1 shows the simplified flowsheet of the process. Table 4 shows for the major process streams, the pressure, temperature and enthalpies. The residence time of slurry in the extractor is assumed to be 20 minutes and a ratio of the CO2/slurry of 2 has been used.

Figure 1: Simplified flowsheet for the purification of contaminated sludge V=pre-separation (hydrocyclon, filter), M=mixture, B=CO2 buffer vessel, E=Extration vessel, S=separator, R=Flash vessel

Table 4: Pressure, temperature and enthalpies of the major process streams

P [bar] T [ °C] Enthalpy [kJ/kg] Slurry CO2

Inlet slurry 1 10 1185 -

Extractor 150 40 1310 510

Separator 60 20 - 625

CO2 buffer vessel 60 20 - 470

Raffinate sep. vessel 1 40 1310 -

Use of SC-CO2 for extraction and recovery of heavy metals by using metal chelating agents from soil matrices has been reported by several investigators (Bartle et al., 1994; Yazdi et al., 1994). The molecular design of materials has been applied for the design of chelating agents which have a high solubility in CO2. The presence of CO2-phillic groups such as fluoroether, silaxane and fluoroalkyl has been reported to increase the solubility in CO2. Design of a specific head and tail group in chelating agents makes the extraction and recovery of specific metal possible.

14 2.3 Chemical reactions

In the recent years much investigation has been done to understand the influence of SCF on the reaction rate, conversion and reaction pathway (Tiltscher et al., 1987; Yokota et al., 1991; Dooly et al., 1987; Johnston et al., 1987 and McHugh et al, 1994).

The major reasons for carrying out chemical processes in SCF phase are: - The availability of an extended pressure and temperature range provides the opportunity to operate the reaction under conditions which are optimal with respect to kinetics and selectivity; - The Solvent properties are easily varied and may be adjusted to special requirements, e.g. homogeneity is often easily reached. This make it possible to precipitate the product from the reaction mixture as the reaction proceeds. In this manner unwanted side-reactions may be avoided if the product species is immediately removed from the reacting system as it precipitates from solution. - The product recovery may be achieved by simply decompressing the system; - The heat and mass transfer are very efficient in SCF state.

These advantages have been reported for example for solid-catalyzed hydrocarbon reactions where catalyst deactivation due to coke and/or heat removal is a problem. Examples include catalyst dehydration of toluene (Gabbito et al., 1988), Fischer-Tropsch synthesis on Fe and Co-based catalysts (Fan et al., 1992) and esterification of oleic acid by methanol on acid catalysts (Vieville et al., 1993).

Occhiogrosso et al., 1987, and Suppes et al., 1989, studied the oxidation of cumene in a homogeneous phase in SC-CO2. The oxidation reaction which is normally carried out in the liquid phase suffers from the occurrence of side reactions. Application of SC-CO2 has been reported to decrease the formation of byproducts and enhance the selectivity.

The application of SC-CO2 for the production of furfural from xylose has been reported by Sako et al., 1991, 1992 and 1994. The conventional method for the production of furfural has a low selectivity due to the high reactive character of furfural and so the appearance of side reactions such as decomposition and poly- merization. Use of SC-CO2 makes it possible to extract the furfural out of the reactor as soon as possible and so increase the selectivity. The authors carried out experiments at 150 °C and 200 bar.

Srinivas et al., 1994, studied the oxidation of cyclohexane in SC-CO2 for production of cyclohexanone and cyclohexanol. In general the oxidation of cyclohexane is carried out in the liquid phase at 393-413 K and pressures up to 20 bar with air or oxygen as coreactant under several oxidation conditions, such as uncatalyzed, catalyzed by transition metals or promoted by initiators. The vapour phase oxidation requires high temperatures in the range of 320-340 °C, which leads to

15 degradative oxidation and formation of by-products. The mechanism of oxidation reaction in a SC-CO2 is proceeded by free radical as in the case of liquid phase oxidation. The temperature and pressure effects on the reaction rate constant are described by Arrhenius type of equation. The conversion of oxidation reaction in SC-CO2 is low compared to the liquid phase oxidation because of dilute concentrations of the reactants. The cyclohexanone is more selectively formed and favoured by increasing of pressure than cyclohexanol. An increase of 20% in pressure results in reduction of the induction period by 50%, an increase in activation energy, an increase in the preexponential factor by 5 orders of magni- tude and an increase in the first order rate constant by about 70%.

These studies show that the reaction rates can be manipulated by adjusting the operating conditions of temperature, pressure and feed composition near the mixture critical point.

Use of an SCF reaction medium to lower the operating temperature of pyrolysis reaction is mentioned by McHugh et al., 1994. Use of SCF reaction medium suppresses the formation of carbon that occurs at the high temperatures in pyrolysis reactions. Therefore an improved yields, selectivity and product separation can be attained as compared to conventional pyrolysis methods. Koll et al., 1978, report the pyrolysis reaction of cellulose in the presence of SC acetone (Tc=235.7 °C, Pc=47 bar) operating at 250 atm and 150-290 °C. At SCF conditions, the yield of glucosan is about 38.8% which is 72% higher than the yield obtained with conventional pyrolysis. This increase in yield is explained by the increase of density of the reacting mixture with pressure. At higher densities the intermolecular reaction proceeds at a much higher rate.

Titscher et al., 1984, describe the influence of an SCF reaction medium on the activity of a heterogeneous catalyst. They studied the catalytic isomerization of 1- hexene (Tc=231 °C and Pc=30.7 bar) on σ-Al2O3 with 2-chlorohexane as a co- catalyst whereby cis and trans-2-hexene are the desired products. The catalyst normally is deactivated after 1.5 hours (operating conditions: 250 °C and 14.8 bar) and a maximum conversion of 20% has been reached. They show that the conversion slowly drops to 12% after 12 hours. The deactivation of the catalyst is a result of accumulation of oligomeric compounds (C12-C30) on the surface of the catalyst. By increasing the pressure up to 493 bar the authors have observed that the conversion increases up to 40% after 1.5 hours, and also remains at the same level after 12 hours reaction time.

Alexander et al., 1984, describe a reaction/separation scenario for the Diels-Alder reaction of isoprene with maleic anhydride in SC-CO2. They find that the product precipates as a solid from the reaction mixture as the reaction proceeds. In this case the reaction is run at fairly low concentrations of reactants in SC-CO2.

Use of SC-CO2 for the neutralization and strengthening of old paper has been reported by Perre et al., 1994. The fabricated paper from wood pulp deteriorates with age through degradation of cellulose molecules by oxidation and hydrolysis reaction which results into formation of acid substances. To treat deteriorated

16 paper documents and reverse the damage, SC-CO2 has been showed to offer many advantages compared to the available processes based on the use of organic solvents. This not only from the environmental point of view but also from the quality of printed document (damage of inks or bindings) and cost of operation. In the process suggested by the authors the books (closed) are stacked in the autoclave and submitted to SC-CO2 at a pressure of 200 bar and a temperature of 40 °C. The flowrate of SC-CO2 is reported to be 25 kg CO2 per kg of books. The acidification treatment has been carried out using dissolved methylethyl magne- sium carbonate in SC-CO2. Residence time of 4 hours has been reported to be sufficient to increase the pH level to alkaline range (> 8). Strengthening of the paper has been reported through impregnation of paper with components like grafted silica or polymersntransported by SC-CO2. Further details regarding this technique are missing in the paper.

Bhise, 1983, describes a multistep process for the production of ethylene glycol in near-critical to SC-CO2, whereby much less high glycols have been produced compared to the conventional method.

Poliakoff et al., 1991, explored several organometallic systems in SC fluids. Organometallic complexes are used in many homogeneous reactions, and volatile organometallic components may be used in chemical vapour deposition processes for the production of thin films for microelectrconic applications. The inertness of CO2 and Xe and the complete miscibility with other permanent gases such as N2 and H2 offer significant benefits. Both N2 and H2 are used in organometallic chemistry. The limited solubility of these gases in liquids limits the generation of potential useful organometallic components and limits the rate of reaction. In addition since Xe is completely transparent for UV, visible and IR radiation, it is pre-eminently useful in spectroscopic reaction studies. By performing the reactions in SC-Xe, high concentrations of both N2 and H2 could be obtained facilitat- ing and stabilising the formation of new complexes. The progress of the reaction was studied using time-resolved IR-spectroscopy. The UV photolysis of η5- C5H5M(CO)3 with M being Mn or Re, was studied in a supercritical Xe / H2 mixture. Both complexes reacted differently. The Mn complex formed a dihydrogen complex whereas the Re complex formed a dihydride complex. The Mn-H2 complex was stabilised by the presence of the hydrogen in the reaction medium. Replacing H2 with N2 leads to a rapid H2-N2 exchange in the metalcomplex.

17 Biomass applications

Interest in the application of SC media for the treatment of biomass emerged from the wish to extract lignin from wood with a SCF. Traditional lignin extraction proceeds with chemicals and under severe conditions (cooking conditions). The use of SC fluids for the delignification may offer milder conditions. Lignin is a crosslinked phenolic polymer. Spruce wood was lignified with a water/methylamine mixture at moderate temperatures of 150 - 200 oC. Two first order rate constants were observed by Beer et al., 1986. The initial constant is high and shifts to a lower value when over 90 % was extracted. These results were confirmed by Reyes et al., 1989. He observed two different constants during the investigation of continuous extraction of lignin from wood with supercritical tertiair butylalcohol. As indicated by others, the reaction rate constant increased with increasing temperature and pressure. No inter particle diffusion limitation were observed under the experimental laboratory conditions. Diffusion limitation under industrial conditions is more likely to occur due to increased particle size. Lignin extraction with NH3 / water mixtures was also studied because ammonia is less expensive than methylamine. Over 70 % lignin was extracted within one hour at 200 oC and 270 bar with 20 % water.

Both the use of SC fluids for the biomass conversion to liquid fuels and the production of fine chemicals from biomass have been studied by various authors (Li et al., 1988; Beer et al., 1986, Reyes et al., 1989). Over the past few years the interest in the use of renewable resources such as wood, cellulose and lignin as high value products has grown steadily. Lignin depolymerisation for example could become an interesting route for the production of phenol or other aromatic derivates. Also biomass consists of hydrocarbons rich in oxygen. Upgrading the biomass to oil and liquid fuels is a feasible option. Application of supercritical fluids could be an effective conversion process to unlock the intrinsic fuel and chemical value of biomass. Pyrolysis is not really an attractive alternative. The thermal character of this route for the conversion of the macro molecules to small molecules leads to undesired light ends. Also the formation of potentially useful products occurs with the formation of undesired char.

Porier et al., 1987, investigated the reactions of wood with supercritical methanol at temperatures up to 350 oC and a pressure of 170 bar. The objective was to convert wood to a liquid fuel mixture that could be used either for automotive applications or as chemical feedstock. Several process parameters were studied like yield, extract composition and molecular weight distribution of the mixture. Also wood conversion levels and char production were studied. The highest obtained yields were 63 %, with 95 % wood conversion. The applied supercritical conditions and residence time were the major factors that influenced the extraction. High pressures increased both the wood conversion and oil yield and decreased the char formation. The reduction of char formation with increasing pressure was attributed to the increase in solvent strength of methanol with increasing pressure. Primary thermal degradation products which could eventually lead to char were extracted from the wood before char forming reactions occurred.

18 Modell, 1985, reports on the liquefaction of biomass in SC water. Glucose was converted to liquid organic and gases rich in H2 and CO (syngas) when reacted at near critical conditions of water. No char formation was observed at supercritical conditions contrary to processing at sub-critical conditions. Char formation was prevented because char precursors were highly solvated and maintained in the supercritical phase. The prevention of char formation was also observed by Goto et al., 1990a and 1990b. They studied the extraction of volatiles and other constituents from cellulose at temperatures between 220 and 300 oC.

The role of SC water in biomass conversion was studied by Antal et al., 1987b. He argued that by choosing different reaction conditions two pathways may be distinguished. At lower temperatures the dissociation product of water was 10-14, while at higher temperatures this value was much smaller, 10-20. At lower temperatures (< 400 oC) an ionic pathway was preferred, while at temperatures exceeding 500 oC a free radical mechanism was favoured in both a catalysed and uncatalysed ethanol conversion. Ethanol was converted to ethylene by the ionic path, while the selectivity to ethylene decreases significantly at higher temperatures when the free radical mechanism dominates. Methane, ethane, hydrogen and CO were the formed products at higher temperatures.

Enzymatic reactions

In the recent years the catalytic enzyme reaction in non-aqueous solvents is becoming an active research area. The impetus for this activity is that enzymes show high activity and selectivity in non-aqueous media such as organic solvents. Also enhanced substrate solubility in a non-aqueous solvent may give higher reaction rates. The SCF constitute a promising class of alternate non-aqueous solvents. SC-CO2 has been reported as an attractive medium for biocatalysis reactions. For example enzymatic esterification, hydrolysis and oxidation. In SC- CO2 enzymes show high stability and activity. In particular the stereoselectivity of enzyme reactions in SC-CO2 may be changed by adjusting pressure and temperature, which results into production of high optical pure products. The application of SC-CO2 for sterilization, recovery of intracellular proteins and selective inactivation of enzymes has been investigated. For example enzymatic esterification of monoglycerides, triglycerides, flavour esters and vinyl butyrate.

The use of CO2 as SCF in enzymatic reactions shows, despite of a large number of advantages like low critical temperature and nontoxic, some potential disadvan- tages like decreasing of pH during the hydration layer of enzyme due to the formation of carbonic acid and modification of enzyme by forming of amine complexes. The use of other types of gases like ethane, propane and ethylene is under investigation.

19 2.4 Polymer processing technology

The major activities relating use of SCF in polymers relate from the 1980s with one exception: polyethylene. The development of the high pressure polymerizati- on process in the 1940s represents the first commercialized SCF process. In the recent years a growth of activities in SCF-polymer processing technology can be observed, as in other fields of application of SCF processing. This in the light of increasingly stringent regulations and bans of organic solvent use ("motivation").

The dissolution of an SCF into a polymer melt results in modification of both thermodynamic and rheological properties.

Recent activities on the use of SCF in polymer lie in two fields: fundamental studies and applied process development studies. The fundamental studies recover the following areas. - thermodynamics (phase equilibria, polarity effects, reaction kinetics, cosolvent effects, etc); - transport properties (diffusion, viscosity, etc.); - predictive modelling (binary diffusion coefficient, solubility, distribution coefficient, etc.); - equipment related (chromatography, mass transfer, etc.).

The applied process development studies lie in the field SCF fractionation of polymers and copolymers by molecular weight and chemical composition. Bangert et al., 1977, describe a process for spinning polymer fibers from a supercritical solution. This process entails dissolving a polymer in a high pressure fluid and extruding the gaseous solution through a die. Polymers studied are polypropylene, polybutene-1 and Nylon-6. The solubilities of these materials in CO2 and n-butane are shown below.

Polymer SCF Pres. Temp. Solubility Polymer melting point [bar] [°C] [wt %] [°C]

Polypropylene CO2 450-850 163-208 6-38 162-176

Nylon-6 CO2 400-510 233-241 13-16 212-225

Polybutene-1 CO2 300-900 131-150 6-38 126

Polybutene-1 n-butane 120-170 167-190 5-21 126

Polypropylene n-butane 130-190 166-186 5-20 162-176

Diameters ranging up to 25 microns have been reported to spun from polypropyelen dissolved in supercritical propylene. The pressure levels needed to dissolve a polymer are directly related to the molecular weight of the polymer. Polymer solubility in SCF is also related to polymer tacticity (McHugh et al., 1994).

20 The fractionation of a high molecular-weight silicone oil, a poly(dimethyl)siloxane using SC-CO2 has been reported by McHugh et al., 1994. The polymer was extracted at 80 °C over a pressure range from 121 to 440 bar. Six fractions have been reported to be separated from low molecular weight cyclics and liners to high molecular weight oligomers. The replacement of methyl group in poly(dimethyl)siloxane with phenyl group lowers the solubility dramatically. In the same article, the fractionation of a perfluoroalkylpolyether, ([CF(CF3)- CF2O]nC2F5), in SC-CO2 has been reported. Perfluoroalkylpolyether is resistant to corrosive or oxidizing materials and is being used as a diaphragm fluid, in computer disc lubricant and as a seal fluid for computer disc drives. The fractionation of perfluoroalkylpolyether is carried out at 80 °C over a pressure range from 81 to 271 bar. Further the fractionation of polystyrene, photoresist polymer (polysilastyrene, (CH3)3Si[C6H5SiCH3]x[CH3SiCH3]ySi(CH3)3), polymeric surfactant (carboxylic acid-terminated perfluoralkyl polyether, HOOC[CF(CF3)CF2O]nC2F5) and polyisobutylene-succinic have been reported (McHugh et al., 1994). More data regarding these processes are missing in the articles.

Reactive monomers which are used in applications that require high purity but which are difficult to process by distillation because of their sensitivity to temperature, can be purified by SCF processing techniques (McHugh et al., 1994). One of the examples is silicon monomer for the use in soft contact lenses or BisGMA monomer for the use in hard dental structures. These monomers have been purified with SC-CO2 at 60 °C and 94.6 bar. More data regarding these processes are missing in the article.

Another field of application of SCF in polymers are the RESS and the GAS recrystallization techniques. These techniques offer precipitation of polymers which are difficult to handle by traditional means or provide the possibility to alter the morphologies of polymers which can have advantage on further processing (e.g. in membranes). Within this concept use of SCF for modification of polymer surfaces can be mentioned.

21 2.5 Fractionation

Use of SCF for regeneration of activated carbon is reported by McHugh, 1994. The advantages of SCF regeneration of activated carbon are reported to be lower energy requirements and carbon loss as compared to the thermal regeneration process.

Design procedure and economic attraction of the use of SC-CO2 for the fractionation of mixtures containing a homogenous series of hydrocarbons at low temperatures (25-100 °C) is mentioned by De Haan, 1991. As a case model he took a process with a feed capacity of 10 kton/year for the fractionation of a mixture containing 25 wt% tetradecane, hexadecane, octadecane and eicosane into its 99 wt% pure constituents. The fractionation column used was equipped with Sulzer BX gauze packing. The required vapour-liquid equilibrium data were either measured or calculated by Peng-Robinson equation of sate. The extraction of the alkane mixture is carried out through three extraction units whereby tetradecane, hexadecane and octadecane are obtained as top products respectively and eicosane as bottom product of the last unit. The extraction temperature was set up on 80 °C and pressures obtained were 170 bar, 190 bar and 210 bar respectively in the first, the second and the third column. After extraction the CO2 is being separated by isothermal pressure reduction to 70 bar and recycled. Figure 5 shows the flowsheet of the process (De Haan, 1991). In the first column the feed is pumped up to the extraction pressure (170 bar) and contacted counter currently with the SC-CO2. After leaving the top of the column the pressure is reduced isothermally to 70 bar to separate the CO2 and the tetradecane. The gaseous CO2 is recompressed to 170 bar and after being cooled to 80 °C with cooling water, recycled to the bottom of the column. Most of the produced tetradecane, which contains 20 wt% CO2 is returned to the top of the column as a reflux. The remaining raffinate which contains less than 1 wt% tetradecane and 40 wt% CO2 is drawn off from the bottom and pumped to the second column. In this column the hexadecane is separated from the alkane mixture at 190 bar. In the third column a pressure of 210 bar is used to separate the octadecane and eicosane. The produced eicosan is first expanded to 70 bar to recover the major part of the dissolved CO2 and is expanded to 1 bar before further use. The gaseous CO2 from both stages was recompressed and recycled to the first column. Table 5 shows the pressures, temperatures, flow rates and compositions of the most important streams in figure 2. The cost of fractionation of alkane mixture into 99 wt% pure constituents comes from $ 0.65/kg to $ 0.71/kg which is much higher compared to for instance high vacuum distillation (0.02 bar) of approximately from $ 0.09 to 0.12/kg. However the detailed design and cost evaluation of high vacuum distillation are missing in the paper of De Haan, he concluded that SC fractionation of hydrocarbon mixtures is an expensive technique. The only advantage of this technique upon the distillative workup is reported to be lower operating temperatures, 80 °C compared with the range of 100-400 °C, which can be important for the components integrity.

Figure 2: Flowsheet for the fractionation of an alkane mixture with SC-CO2 (De Haan, 1991)

Table 5: Pressures, temperatures, flowrates and composition of the important streams in figure 2 (De Haan, 1991)

Use of SC-CO2 for extraction of high boiling petroleum fractions has been reported by Severin et al., 1994. The influence of a series of entrainers (n-hexane,

23 toluene, chloroform, acetonitrile, methanol and diethyl-sulfide) has been tested on the extraction power of SC-CO2. The experiments have been carried out with 5 mol% entrainer and at a pressure range of 150-300 bar and at a temperature range of 70-120 °C. They have shown that the phase behaviour between entrainer and SC-CO2 has a strong influence on the yield and selectivity. The phase behaviour depends strongly on the molecular weight and the polarity of the entrainer. In the case of a homogenous phase the entrainer influences the solvent strength by increasing the density. If the entrainer and SC-CO2 form a two phase system, the entrainer shows only a slight effect on the extraction. At 20 bar and 55 °C, use of diethyl-sulfide as solvent shows the highest yield (25.9) followed by hexane (21), chloroform (20.9), acetonitrile (18.8), methanol (16.4) and at last pure CO2 (14). Use of n-hexane increases the yield twofold without changing the extract composition. With the use of pure CO2 the extract showed the highest selectivity towards the saturated components. The selectivity toward the saturated components decreases by using polar cosolvents ( methanol and acetonitrile). Use of an aromatic cosolvent results into an increasing of the amount of aromatics in the extract. Table 6 shows a review of the results. The compositions in the table 6 are averaged over all fractions with different experiments (Severin et al., 1994).

Table 6: Influence of cosolvent on selectivity of fractionation of high boiling petroleum (Severin et al., 1994)

Entrainer Saturates Aromatics Polar

non 41.3 47.4 11.2

methanol 36.6 43.9 19.5

acetonitrile 35.5 51.2 13.3

n-hexane 40.4 45.6 13.9

toluene 36.6 52.3 11.5

chloroform 38.3 48.9 12.7

diethylsulfide 37.1 51.5 11.5

24 2.6 Analytical Supercritical Extraction

In the recent years many papers and books (Smith et al., 1988; White et al., 1988) have been published on application of SCFC (Chromatography). The SCFC uses compressed gases in the critical temperature range as mobile phases in combina- tion with packed or capillary columns that contain stationary phase. This technique supplements well-known techniques such as GC and high pressure liquid chromatography (HPLC) and makes possible the analytical and preparative separation of thermally unstable and/or low volatility compounds. Several commercial SCFC are already available on the market. However use of SCFC is not limited to analytical purposes but is also used for the determination of thermodynamic quantities. A typical application of this use is the investigation of the dilute region (activity coefficients, Henry's constants, second virial coefficients, etc).

2.7 Supercritical Water Oxidation

Supercritical water oxidation (SCWO) is a waste treatment technology being developed for the ultimate destruction of organic materials. The technology involves the complete oxidation of organic compounds in an aqueous phase at conditions that exceed the critical point of water (Tc=374 °C, Pc=218 bar). Supercritical conditions are attractive because organic compounds, oxygen and water can exist in a single homogeneous phase. Consequently the rapid oxidation reactions can proceed unaffected by the transport limitations that may occur at subcritical conditions. While SCW is a strong solvent for organic compounds, it is a poor solvent for inorganic salts. The oxidation process produces CO2 and water but no NOx, SOx or dioxins. Compounds containing halogens, sulphur, phospho- rus and nitrogen produce their corresponding inorganic acids (corrosion!). However nitrogen compounds can also produce ammonia, nitrogen or N2O. The SCWO can handle a wide range of aliphatic and aromatic organic compounds. For example polychlorinated biphenyls (PCBs), dioxins, the pesticides DDT and aldrin, etc. The SCWO can proceed autogenously with no need for an external energy supply at COD values of 5-10 g/l.

Despite of two major drawbacks of the process, corrosion and precipitation of inorganics, this year Texaco Chemical Company decided to start the first SCWO plant worldwide (Svensson, 1995) in Texas. However MODEC (Modell Environ- mental Cooperation's) claims a new type of reactor which has no zones where solid deposits can build up. The reactor (Texaco) which is 4 meter tall and 2 meter wide handles 1.5 m3/h of aqueous waste containing some 10% organic compounds. The degree of purification has been reported to be higher than 99.5%. The new SCWO plant is expected to have a payback time of 2 years. The main application field of SCWO is expected to be for aqueous wastes that are too difficult to handle by conventional methods or wastes that are too expensive or hazardous to be incinerated or landfilled. 3. Methods for modelling of solubilities of compounds

25 and influence of co-solvent in supercritical fluid

There exist a large variety of models in the literature for the prediction of solubility of a compounds in SCF and the influence of co-solvent (Johnston et al., 1987; Williams, 1981; Mukhopadhyay et al., 1993; Roop et al., 1989; Shing et al., 1987; Brennecke et al., 1989; Johnston et al., 1982; Allada, 1984; Bartle et al., 1990; Sheng et al., 1992; Ting et al., 1993; Lembert et al., 1991; Campanella; Kramer et al., 1988; Economou et al., 1992). In this paper only a review of different models will be presented (for more information see the references). The important aspects for predicting of phase equilibria in the SCF state are 1. the vapour pressure which is the most important indicator of solubility of compound in SCF; 2. the equation of state (EOS) which should predict densities accurately in the critical region; 3. the molecular thermodynamics of highly nonideal mixtures (binary interaction parameters); 4. the high compressible mixture which may lead to solvent condensation or clustering about the solute.

In order to predict cosolvent effects on solubilities, the important step is to determine the strength of the solute-cosolvent attraction constant. This constant will be influenced by dispersion, orientation (dipole-dipole), induction (dipole- induced-dipole) and acid-base forces.

Table 7 shows a summary of models available in literature for prediction of solubility of compounds and influence of co-solvent in SCF.

Table 7: Summary of thermodynamic models for solid-Fluid, liquid-fluid and for multiphase systems

Model type Reference No. of systems studied No. of adjustable parameters

solid-fluid systems

cubic Deiters et al., 1976 2 2

cubic Mackay et al., 1979 2 1-2

cubic Kurnik et al., 1981 9 1

cubic Kurnic et al., 1982 8 3

cubic Debebedetti et al., 1986 3 1

cubic Moradinia et al., 1987 5 1

cubic Rao et al., 1988 16 0

CSVDW Johnston et al., 1981 4 1

HSVDW Wong et al., 1985 16 0

AVDW Johnston et al., 1982 12 0-1

PHCT Mart et al., 1986 6 0-1

lattice Leblans et al., 1985 1 2-4

lattice Nielson et al., 1987 3 7

26 lattice Kumar et al., 1987 2 1

lattice Van der Haegen et al., 1988 1 2-4

lattice Bamberger et al., 1988 2 1

Kirkwood Pfund et al., 1988 19 2-3

M-C Shing et al., 1987 1 0

solid-fluid systems including co-solvent

HSVDW Dobbs et al., 1986 6 0

HSVDW Dobbs et al., 1987 11 0

APACT Walsh et al., 1987 2 2

liquid-fluid and multiphase systems

cubic Hong et al., 1983 4 1

cubic De Loos et al., 1984 1 3

cubic McHugh et al., 1986 4 1-3

cubic Benmekki et al. 1988 1 10

cubic De Azevedo et al., 1988 3 1

cubic Nagahama et al., 1988 2 2-9

cubic Lemert et al., 1989 2 3

HS/cubic Deiters et al., 1984 2 3

HSVDW Wu et al., 1988 5 1

HS/Soave Ashour et al., 1988 5 1

PHCT Radosz et al., 1987 2 2 cubic = cubic equation of state; CSVDW = Carnahan-Starling van der Waals; HSVDW = hard-sphere van der Waals; AVDW = augmented van der Waals; DDLC = density dependent local composition mixing rules; PHCT = perturbed hard chain theory; APACT = associating perturbed chain theory; HS = hard sphere

27 4. Conclusions

A large number of articles and patents exist in literature regarding the different application fields of the SCF technique and methods for the prediction of solubility of compounds and influence of co-solvent in the solubility of a compound in an SCF. Use of co-solvent can drastically enhance the solubility of a compound in an SCF. Most of the application fields are in developmental and/or research stage and there is a lack of information on the cost aspect of the process.

SCF can be applied in a number of chemical and enzymatic reactions to increase the selectivity while maintaining high conversions and/or to suppress the deactivation of the catalyst. In pyrolysis reaction the SCF can solubilize reacted products and remove them from the high temperature zone, so avoiding further thermal decomposition.

SCF can be applied in place of organic solvents in extraction processes, such as extraction of anti-cancer agent from special seeds, oil from potato chips, organic materials from soil and water, alcohol from beer and wine, cholesterol from milk fat and egg, flavours from herbs and spices, etc.

A large number of examples exists about which because of the confidential state of research no further data have been published. Among them are cleaning of metal surfaces by SCF solvent, use of SCF in plastics recycling and production of metal oxides.

There is no general rule to evaluate the process viability of an SCF application. For each case in which one considers use of this technique, an evaluation should be done to investigate if the process has additional (environmental and economical) advantages compared to the traditional process. Several parameters influence the cost of an SCF process such as pressure level, solubility level, pressure reduction ratio and amount of material to be extracted.

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39 Appendix: List of solubilities of selected compounds in liquified CO2

Table ap1: Solubilities of selected compounds in liquid CO2 at 25 C and 65.5 bar (McHugh et al., 1994)

Substance wt% Substance wt%

Esters Amines & Heterocyclics Benzyl benzoate 10 Aniline 3 Butyl oxalate M o-Chloroaniline 5 Butyl phthalate 8 m-Chloroaniline 1 Butyl stearate 3 N,N-Diethylaniline 17 Ethyl acetate M N,N-Dimethylaniline M Ethyl acetoacetate M Diphenylamine 1 Ethyl benzoate M N-Ethylaniline 13 Ethyl chloroformate M N-Methylaniline 20 Ethyl maleate M α-Naphthylamine 1 Ethyl oxalate M 2,5-Dimethyl-pyrrole 5 Ethyl phthalate 10 Pyridine M Methyl salicylate M o-Toluidine 7 Phenyl phthalate 1 m-Toluidine 15 Phenyl salicylate 9 p-Toluidine 7 Ethylene diformate M Furfural M Ethyl formate M Thiophene M

Alcohols Phenols t-Amyl alcohol M o-Chlorophenol M Benzyl alcohol 8 p-Chlorophenol 8 Cinnamyl alcohol 5 o-Cresol 2 Cyclohexanol 4 m-Cresol 4 1-Decyl alcohol 1 p-Cresol 2 Methyl alcohol M 2,4-Dichlorophenol 14 Ethyl alcohol M p-Ethylphenol 1 2-Ethylhexanol 17 o-Nitrophenol M Furfuryl alcohol 4 Phenol (mp 41 C) 3 Heptyl alcohol 6 β-Methoxyethanol M Hexyl alcohol 11 Phenylethanol 3

Carboxylic Acids Nitriles & Amides Acetic acid M Acetonitrile M Caproic acid M Acrylonitrile M Caprylic acid M Phenylacetonitrile 13 Formic acid M Succinonitrile 2 Isocaroic acid M Tolunitriles (mixed) M Latic acid 0.5 Acetamide 1 Lauric acid 1 N,N-Diethylacetamide M Oleic acid 2 Formamide 0.5

M = miscible

Butyl oxalate:HOOC-COOC4H9; Butyl phthalate:COOH-C6H5-COOC4H9; Butyl stearate:CH3(CH2)16COOC4H9; Ethyl acetoacetate:CH3COCH2COOC2H5; Oleic acid: CH3(CH2)7CH=CH(CH2)7COOH; Caprylic acid: CH3(CH2)6COOH; Caproic acid:CH3(CH2)4COOH; Lauric acid:CH3(CH2)10COOH; Latic acid:CH3CHOHCOOH; Stearic acid:CH3(CH2)16COOH; Cinnamyl alcohol:C6H5CH=CHCH2OH; Methyl salicylate:C6H5OHCOOCH3; Phenyl salicylate:C6H5OHCOOC6H5 a-Naphthylaniline:C6H5- C6H5NHC6H5