Supercritical water industrial processes

The destruction of hazardous chemical wastes by oxidation in supercritical water is a promising new technology which has several advantages over conventional methods of toxic chemical waste disposal. Although the feasibility of the supercritical water oxidation process has been demonstrated, there is little kinetic information available on the underlying reaction mechanisms. We have recently determined the oxidation kinetics of several model compounds in supercritical water, and now report on our results of the oxidation of methanol, a conunon industrial solvent, in supercritical water. Globd kinetic expressions are presented and our attempts to model the reaction using a free-radical mechanism with 56 elementary reactions are discussed. The inability of the elementary reaction model to represent oxidation in supercritical water is demonstrated and future model modifications are discussed. 

Two Other chemical processes that rely on hydrothermal processing chemistry are wet oxidation and supercritical water oxidation (SCWO). The former process was developed in the late 1940s and early 1950s (3). The primary, initial appHcation was spent pulp (qv) mill Hquor. Shordy after its inception, the process was utilized for the treatment of industrial and municipal sludge. Wet oxidation is a term that is used to describe all hydrothermal oxidation processes carried out at temperatures below the critical temperature of water (374°C), whereas SCWO reactions take place above this temperature. 

Gasafi, E., Meyer, L., Schebek, L. (2004) Using Life-Cycle Assessment in Process Design Supercritical Water Gasification of Organic Feedstocks. Journal of Industrial Ecology, 7(3M), 75-91. 

A flow chart of a generic SCWO process is shown in Figure 10.4. It illustrates the feed stream of a typical aqueous waste. Oxidants such as air, oxygen, or hydrogen peroxide must be provided unless the waste itself is an oxidant. A supplemental fuel source should also be available for low-heat-content wastes. The streams entering the SCWO reactor must be heated and pressurized to supercritical conditions. Influent streams are frequently heated by thermal contact with the hot effluent. Both influent pressure and back pressure must be provided. The influent streams are then combined under supercritical conditions where oxidation occurs. Certain properties of supercritical water make it an excellent medium for oxidation. Acetic acid is generally considered one of the most refractory by-products of the SCWO process of industrial waste. 

The rest of this chapter describes some industrial processes that use water, carbon dioxide or ionic liquids as solvents. In some cases, such as supercritical water oxidation and catalytic ionic liquids, the solvent is also a reagent. 

Most research work on the use of supercritical water has been conducted batchwise and involved non-analytical determinative applications. Thus, supercritical water oxidation (SCWO) was proposed as an alternative treatment for hazardous waste disposal [191] and also as a commercial tool for decomposing trichloroethylene, dimethyl sulphoxide and isopropyl alcohol on a pilot plant scale [192]. Current commercially available equipment (the aqua Critox" system) is usable with industrial and municipal sludge, mixed (radioactive and organic, liquid and solid) waste and military waste. This commercially available treatment has a number of advantages, namely (a) because it uses an on-site treatment method, it avoids the need to transport hazardous materials (b) it ensures complete destruction of organic wastes and allows reuse of the effluent as process water with results that meet the regulations for drinking water and (c) no licence for effluent or air emissions is needed. 

The use of ion exchange resins combined with supercritical extraction results in an attractive isolation technique. Since the pyrrolizidine alkaloids are generally basic, the process described here could be used for the isolation of other members of this class, and for other basic alkaloids. In designing a process using ion exchange resins, it is important that the co-solvent not deactivate the resin. In our example, water and ethanol are acidic in carbon dioxide and, therefore, do not deactivate the acidic resin. An industrial process based on this concept could be quite efficient and inexpensive since pressure reduction and subsequent solvent recompression are unnecessary (Figure 11). 

J. Abeln, M. Kluth, G. Petrich and H. Schmieder, Supercritical Water Oxidation (SCWO) A Process for the Treatment of Industrial Waste Effluents, High Pressure Research, 20, 537-547 (2001). 

The industrial research efforts on coffee decaffeination, spice extraction, and flavors concentration are, to a great extent, shrouded by the cloak of proprietary security, but the investigations of the use of supercritical fluids to treat various waste streams is reasonably well publicized. Most familiar, perhaps, is the supercritical waste water detoxification process developed by Modar Inc. This is potentially attractive for detoxifying refractory chemicals such as polychlorinated biphenyls, dioxin, and other toxic materials (Anon., 1982 Modell, 1982). In the Modar process, the toxic chemicals are homogeneously reacted with oxygen in supercritical water, the solvent for the organics and the oxygen. The main feature of the process is a chemical reaction discussed in more detail in chapter 11. 

The same equipment can be used as in PLE, but for temperatures above 200°C, there is no commercial equipment available and specially designed or home-built insmiments are therefore used. Chematur in Sweden manufactures industrial-scale supercritical water oxidation plants operating at temperatures above 400°C (Aqua Critox ). It is possible that such equipment can be modified to subcritical water extraction processes. Uhde in Germany probably also has industrial-scale solutions for PHWE. 

This chapter first explains enzyme nomenclature, describes enzymatic, supercritical reactor configurations, and gives a compilation of published experimental results. The- most important topics concerning enzymatic reactions in SCFs are then covered. These are factors affecting enzyme stability, the role of water in enzymatic catalysis, and the effect of pressure on reaction rates. Studies on mass transfer effects are also reviewed as are factors that have an effect on reaction selectivities. Finally, a rough cost calculation for a hypothetical industrial process is given. 

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