Carbon dioxide, liquified

A supercritical fluid (SCF) is a substance above its critical temperature and critical pressure. The critical temperature is the highest temperature at which a substance can exist as a gas. The critical pressure is the pressure needed at the critical temperature to liquify a gas. Above the critical temperature and critical pressure, a substance has a density characteristic of a liquid but the flow properties of a gas, and this combination offers advantages as a reaction solvent. The liquidlike density allows the supercritical fluid to dissolve substances, while the gaslike flow properties offer the potential for fast reaction rates. Supercritical carbon dioxide (scC02) has a critical temperature of 31°C and critical pressure of 73 atm. 

A number of compressed and liquified gases are used as refrigerants and aerosol propellants. These include nitrous oxide, nitrogen, carbon dioxide, propane, and butane. The use of chlorofluorocarbons (CFCs) is restricted because of environmental pollution leading to health hazards. These have been replaced by hydrogenated fluorocarbons (HFCs), which are less likely to cause environmental pollution. 

Absorption of carbon dioxide in a suspension of lime and thermal coal liquefaction are examples of Type I reactions. In the first example, calcium carbonate is produced by carbonation of suspensions of lime, whereas, in the second example, coal is liquified in the presence of hydrogen and oil to produce a host of products. These and several other examples of this type of reaction are summarized in Table 1-1. 

Water has been shown to be an effective solvent in some chemical reactions such as free radical bromination. Supercritical fluids such as liquified carbon dioxide are already commonly used in coffee decaffeination and hops extraction. However, supercritical carbon dioxide can also be used as a replacement for organic solvents in polymerization reactions and surfactant production. Future work may involve solventless or neat reactions such as molten-state reactions, dry grind reactions, plasma-supported reactions, or solid materials-based reactions that use clay or zeolites as carriers. 

The most important extraction technique nowadays is simple solvent extraction. The traditional solvent for extraction was benzene, but this has now been superseded by other solvents because of concern over possible toxic effects of benzene on those working with it. Petroleum ether, acetone, hexane and ethyl acetate, together with various combinations of these, are typical solvents for extraction. Recently, there has been a great deal of interest in the use of carbon dioxide as an extraction solvent. Both supercritical carbon dioxide and subcritical liquid carbon dioxide are used, depending on circumstances. The pressure required to liquify carbon dioxide at ambient temperature is considerable and thus the necessary equipment is expensive. This is reflected in the cost of the oils produced but carbon dioxide has the advantage that it is easily removed and there are no concerns about residual solvent levels. The major applications of liquid carbon dioxide extraction are in decaffeination of coffee and extraction of hops. 

Hydrogen, Liquified Not pertinent Not pertinent 4 75 Let fire burn shut off gas supply Carbon dioxide Bum with almost invisible flame 1065 Ambient temperature of water will cause vigorous vaporization of hydrogen ... 

Ammonia is used in a variety of forms as a fertilizer. It can be liquified or dissolved in water and sprayed on land, or it can be converted into any one of a number of compounds, such as ammonium nitrate, ammonium phosphate, or ammonium sulfate. In these forms, it is spread as dry granules on the land. Urea, made from ammonia and carbon dioxide, is also used as a feed supplement for cattle. Plants and animals use the... 

Propiolic acid is readily prepared in a one-step process using liquified acetylene, carbon dioxide and sodium, potassium or lithium metal as the starting materials [96]. Yields are substantially increased by adding a small amount of a tertiary amine such as trimethyl amine... 

The principle of corresponding states, extended as above to mixtures of acentric molecules, has been applied to the calculation of many of the properties needed for the design of separation equipment. The examples reviewed briefly here are taken from our own work on cryogenic fluids, liquified natural gas (LNG), mixtures of hydrocarbons, and mixtures of carbon dioxide with hydrocarbons. In all this work methane was used as the reference substance. 

The previous sections have given a very brief overview of the PPDS-2 calculation program. The examples given in the Appendix illustrate typical outputs obtained from the system. Example 1 shows the calculated pure component properties of tert-amyl methyl ether, which is commonly used as a lead-free additive to petrol to increase its octane number. Example 2 gives the results of a phase envelope calculation, illustrated in Figure 17.7, for a liquified natural gas mixture, typical of those which are pumped ashore from the North Sea. It contains a large amount of dissolved carbon dioxide. These examples have only scratched the surface of the available calculations which, due to the lack of space, are not possible to review here. 

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