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 pro- cesses 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 environ- ment. 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 literatu- re 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, envi- ronment, 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.