Supercritical fluids processing, hazards

In a supercritical fluid process, advantages in process performance must exceed the penalties from the requirement for elevated pressures. It is pmdent to undertake a formal hazard analysis of the SCF process to identify unknown and potentially dangerous design conditions (see Hazard analysis and risk assessment). 

The two fluids most often studied in supercritical fluid technology, carbon dioxide and water, are the two least expensive of all solvents. Carbon dioxide is nontoxic, nonflammable, and has a near-ambient critical temperature of 31.1°C. CO9 is an environmentally friendly substitute for organic solvents including chlorocarbons and chloroflu-orocarbons. Supercritical water (T = 374°C) is of interest as a substitute for organic solvents to minimize waste in extraction and reaction processes. Additionally, it is used for hydrothermal oxidation of hazardous organic wastes (also called supercritical water oxidation) and hydrothermal synthesis. 

Supercritical fluids also find application in the areas of pollution prevention and remediation, and supercritical carbon dioxide is used as a replacement solvent for many hazardous solvents in both extraction and separation processes and also as a reaction medium and in materials processing. Although carbon dioxide is considered as a greenhouse gas , there is actually no net increase in the amount of the gas if it is removed from the environment, used as the solvent instead of a hazardous substance, and returned to the environment. In this way, most of the uses of supercritical carbon dioxide may be considered as environmentally friendly. Because the solubilities of oils and greases in carbon dioxide are high, it is particularly suited to the cleaning of machinery 47 and, as discussed in the literature 48, it is used as a solvent in textile dyeing operations where it is used to treat any dye-laden... 

Equipments handling supercritical fluids and liquefied gases present important hazards that must be taken into account both for equipment design and construction and for operation and maintenance. Safety considerations must influence any technical choice and operation and a detailed analysis of potential hazards must be specifically conducted for any case. In this paper, we would try to list the different classes of hazards and how to cope with them, so that both the process designer and the operator be informed. 

Clavier J-Y, Allemand A-V. Equipment for SCF processing of hazardous biologically active powders. Proceedings of the 6th International S5unposium on Supercritical Fluids, 2003 1843-1848. 

The Clean Air Act has set strict limitations on the emission of certain VOCs, so new safer solvents are needed to replace them. One new spray system has been developed that yields a high-quality coating while emitting as much as 80% fewer VOCs of all types and none of the VOCs that are considered hazardous air pollutants. This system is called the supercritical fluid spray process. The solvent mixture for this process still contains some of the slow evaporating solvents that allow the coating to spread evenly but replaces the fast evaporating solvents with high pressure CO2. 

By the late 1970s evidence had accumulated on the potential hazards of certain extraction solvents, especially chlorinated hydrocarbons. Increased scrutiny of traditional industrial solvents is responsible for spawning another large body of R D programs on SCF processes. Increased consumer awareness of potential chemical hazards coupled with the uncertainty of future governmental regulatory action motivated an examination of supercritical fluids as extraction solvents for foods, beverages, and spices. 

The examples discussed demonstrate that the unique nature of SCFs provides a means of dialing up the selectivity of a chemical process in a manner that is impossible using conventional solvents (i.e. by manipulation of temperature and pressure). SCF solvents such as CO2 and H2O, the latter of which has not yet been exploited as a solvent for organic photochemistry, are especially attractive as they are environmentally benign alternatives to a number of classical solvents which pose hazards to either health or the environment. Coupled with the tunable properties of a supercritical fluid, these solvents emerge not only as viable alternatives to conventional organic solvents, but in some cases at least, superior alternatives. Finally, a point which has not been fully emphasized in this chapter is that SCF solvents are superb tools for probing... 

Supercritical processing, allowing the use of less hazardous solvents such as carbon dioxide or water in chemical reactions. This benefit must be balanced against the high temperatures and pressures required for handling supercritical fluids. Johnston... 

The committee also believes that commercially available hazardous waste incinerators should be suitable for final treatment of neutralents, although test burns may be necessary. Some neutralents are high in sodium, which tends to shorten the life of the refractory brick used to line incinerators, but wastes of similar composition have been treated satisfactorily. Commercial hazardous waste facilities are available that offer other technologies that might be better for aqueous wastes. These technologies include biological treatment, supercritical fluid extraction (not to be confused with supercritical water oxidation, discussed later in this chapter) followed by incineration of the smaller volume of extracted organics, and chemically based proprietary processes. 

Nine hazardous waste incinerators that are operating commercially in the United States might be available, two each in Texas and Ohio, and one each in Arkansas, Illinois, Kentucky, Nebraska, and Utah. The largest commercial hazardous aqueous waste treatment facility in the United States is managed by DuPont in Deepwater, New Jersey. It provides a combination of physical, chemical, and biological treatment. Clean Harbors, in Baltimore, uses supercritical fluid extraction to treat aqueous wastes. Perma-Fix, with facilities in the Southeast and Midwest, uses proprietary aqueous treatment processes tailored to specific waste streams. 

Modification of polymers in supercritical fluids is an attractive alternative for more conventional approaches such as solution modification and melt modification. Neither conventional technique is economically or ecologically attractive because of the hazardous waste in the form of organic solvents, together with left-over monomer(s) and initiator(s). Furthermore, much energy is required to remove the solvents at the end of the solution process, or undesired side reactions may occur at elevated temperatures necessary for melt modification. Modification of polymers in supercritical fluids is therefore an attractive alternative, since very mild reaction conditions can be applied, and side reactions are expected to be limited, if not avoided. After modification is completed, the solvent can be easily released by reducing the pressure, and non-reacted chemicals can be supercritically extracted with pure supercritical fluid. 

It was also reported that PPy could be polymerized using nontoxic supercritical fluids as solvents. In conventional chemical polymerization, incorporation of the oxidant into the polymerization process and/or washing to remove unwanted byproducts generates a large amount of environmentally hazardous solvent. In the process using supercritical fluid, which is nontoxic, nonflammable, and environmentally acceptable, PPy was polymerized within preformed polyurethane (PU) foam using supercritical carbon dioxide as solvent [41]. 

---