Critical pressure and temperature

If the volume of a gas (or vapour) is reduced by compressing it (as in a piston), or cooling it, or both, the gas may condense to a liquid. Experiments show that a gas cannot he liquefied by pressure alone unless it is at (or below) a temperature known as the critical temperature. Put another way, the critical temperature is the highest temperature at which a liquid can exist. 

At a molecular level, molecules in a sample at a temperature above the critical temperature simply have too much energy to stick together - no matter what the applied pressure. 

The pressure needed to liquefy a gas at its critical temperature is called its critical pressure. The critical temperatures and pressures for some gases are given in Table 10.3. For example, if ethanol vapour is above 243°C, no amount of pressure will convert the vapour to liquid. If the ethanol vapour were exactly at a temperature of 243 C, a pressure of 63 atm would need to be applied to the vapour in order to force it to condense to a liquid. 

Gas Normal boiling point/°C Critical temperature/°C Critical pressure/atm 

In an experiment (in which aU gases were at the same temperature and pressure before and after reaction) it was found that 1.21 dm of hydrogen gas reacted completely with 1.21dm of chlorine gas to produce 2.42 dm of hydrogen chloride. Using Avogadro s law, explain whether or not these volumes confirm the equation 

A gas normally liquefies at some point when pressure is applied. Suppose we have a cylinder fitted with a piston, and the cylinder contains water vapor at 100 °C. If we increase the pressure on the water vapor, liquid water will form when the pressure is 760 torn However, if the temperature is 110 °C, the liquid phase does not form until the pressure is 1075 torn At 374 °C the liquid phase forms only at 1.655 X 10 torr (217.7 atm). Above this temperature no amount of pressure causes a distinct liquid phase to form. Instead, as pressure increases, the gas becomes steadily more compressed. The highest temperature at which a distinct liquid phase can form is called the critical temperature. The critical pressure is the pressure required to bring about liquefaction at this critical temperature. 

TABLE 1 1. 5 Criti cal Temperatures and Pressures of Selected Substances 1 

Several critical temperatures and pressures are listed in A TABLE 11.5. Notice that nonpolar, low-molecular-weight substances, which have weak intermoiecuiar attractions, have lower critical temperatures and pressures than substances that are polar or of higher molecular weight Notice also that water and ammonia have exceptionally high critical temperatures and pressures as a consequence of strong intermoiecuiar hydrogen-bonding forces. 

Because they provide information about the conditions under which gases liquefy, critical temperatures and pressures are often of considerable importance to engineers and other people working with gases. Sometimes we want to liquefy a gas other times we want to avoid liquefying it. It is useless to try to liquefy a gas by applying pressure if the gas is above its critical temperature. For example, O2 has a critical temperature of 154.4 K. It must be cooled below this temperature before it can be liquefied by pressure. In contrast, ammonia has a critical temperature of 405.6 K. Thus, it can be liquefied at room temperature (approximately 295 K) by applying sufficient pressure. 

When the temperature exceeds the critical temperature and the pressure exceeds the critical pressure, the liquid and gas phases are indistinguishable from each other, and the substance is in a state called a supercritical fluid. Like liquids, supercritical fluids can behave as solvents dissolving a wide range of substances. Using supercritical fluid extraction, the components of mixtures can be separated from one another. Supercritical fluid extraction has been successfully used to separate complex mixtures in the chemical, food, pharmaceutical, and energy industries. Supercritical CO2 is a popular choice because it is relatively inexpensive and there are no problems associated with disposing of solvent, nor are there toxic residues resulting from the process. 

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The average error is about 2% for tbe critical temperatures and pressures. The error increases with molecular weight and can reach 5%. 

Beyond propane, it is possible to arrange the carbon atoms in branched chains while maintaining the same number of hydrogen atoms. These alternative arrangements are called isomers, and display slightly different physical properties (e.g. boiling point, density, critical temperature and pressure). Some examples are shown below ... 

A state of matter where a substance is held at a temperature and pressure that exceeds its critical temperature and pressure. 

The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

Conventional nitrocellulose lacquer finishing leads to the emission of large quantities of solvents into the atmosphere. An ingeneous approach to reducing VOC emissions is the use of supercritical carbon dioxide as a component of the solvent mixture (172). The critical temperature and pressure of CO2 are 31.3°C and 7.4 MPa (72.9 atm), respectively. Below that temperature and above that pressure, CO2 is a supercritical fluid. It has been found that under these conditions, the solvency properties of CO2 ate similar to aromatic hydrocarbons (see Supercritical fluids). The coating is shipped in a concentrated form, then metered with supercritical CO2 into a proportioning airless spray gun system in such a ratio as to reduce the viscosity to the level needed for proper atomization. VOC emission reductions of 50% or more are projected. 

Critical temperature and pressure are reqmred and can be estimated from the methods of this section. Vapor pressure is predicted by the methods of the next section. Experimental values should be used if available. The acentric factor is used as a third parameter with and... 

Table 4.5 Critical temperature and pressure data for common gases ...

The gas pseudo critical pressures and temperatures can be approximated from Figure 2-16 or they can be calculated as weighted averages of the critical temperatures and pressures of the various components on a... 

TABLE 6.1. Boiling Point, Critical Temperature and Pressure, and Measured... 

Figure 7.6 The Supercritical Fluid Clnomatography range, above both the critical temperature and pressure at aU compositions. (Reprinted with peimission from reference 17. Copyright 1997 American Chemical Society.) Reproduced by permission of the American Chemical Society.

The second method can be applied to mixtures as well as pure components. In this method the procedure is to find the final temperature by trial, assuming a final temperature and checking by entropy balance (correct when ASp t, = 0). As reduced conditions are required for reading the tables or charts of generalized thermodynamic properties, the pseudo critical temperature and pressure are used for the mixture. Entropy is computed by the relation. See reference 61 for details. ... 

Critical temperature and pressure for pure liquid methane. 

Above the critical temperature and pressure, a substance is referred to as a supercritical fluid. Such fluids have unusual solvent properties that have led to many practical applications. Supercritical carbon dioxide is used most commonly because it is cheap, nontoxic, and relatively easy to liquefy (critical T = 31°C, P = 73 atm). It was first used more than 20 years ago to extract caffeine from coffee dichloromethane, CH2C12, long used for this purpose, is both a narcotic and a potential carcinogen. Today more than 10s metric tons of decaf coffee are made annually using supercritical C02. It is also used on a large scale to extract nicotine from tobacco and various objectionable impurities from the hops used to make beer. 

The dense fluid that exists above the critical temperature and pressure of a substance is called a supercritical fluid. It may be so dense that, although it is formally a gas, it is as dense as a liquid phase and can act as a solvent for liquids and solids. Supercritical carbon dioxide, for instance, can dissolve organic compounds. It is used to remove caffeine from coffee beans, to separate drugs from biological fluids for later analysis, and to extract perfumes from flowers and phytochemicals from herbs. The use of supercritical carbon dioxide avoids contamination with potentially harmful solvents and allows rapid extraction on account of the high mobility of the molecules through the fluid. Supercritical hydrocarbons are used to dissolve coal and separate it from ash, and they have been proposed for extracting oil from oil-rich tar sands. 

There are two distinct conditions that have been used above the critical temperature and pressure (374°C and 218 atm) water becomes a supercritical fluid in which the distinction between the liquid and gaseous states disappears. Since supercritical water can dissolve nonpolar compounds, it has been examined for the degradation of such contaminants. Subcritical water in which the liquid state is maintained by the pressure of the containing vessel has also achieved attention. 

Figure 6.6 Estinated critical temperature and pressure for binary nixtures of 2-propanol in carbon dioxide as a function of the mole fraction of organic modifier.

In the above equations, co is the acentric factor, and Tc, Pc are the critical temperature and pressure respectively. These quantities are readily available for most components. 

Figure 3.7) [241], Some consider the SCF state to be more extended and comprising the area of the phase diagram above Tc independent of p0 [242], Critical temperature and pressure are usually defined as the maximum temperature at which a gas can be converted to a liquid by an increase in pressure, and the maximum pressure at which a liquid can be converted to a gas by an increase in temperature, respectively. In a PT diagram the vaporisation curve ends at the critical point. At a temperature above the critical point, the vapour and liquid have the same density. The critical parameters for some common fluids in analytical studies are listed in Table 3.11, but others may be found elsewhere [243], in particular, rc = 31.3 °C and pc = 7.38MPa for the most common SCF (C02). Supercritical C02 (scC02) is widely used because of its convenient critical parameters, low cost, and safety aspects (low toxicity, nonexplosive). 

If the normal boiling point (vapour pressure = 1 atm) and the critical temperature and pressure are known, then a straight line drawn through these two points on a plot of log-pressure versus reciprocal absolute temperature can be used to make a rough estimation of the vapour pressure at intermediate temperatures. 

Values of the critical temperature and pressure will be needed for prediction methods that correlate physical properties with the reduced conditions. Experimental values for many substances can be found in various handbooks and in Appendix C. Critical reviews of the literature on critical constants, and summaries of selected values, have been published by Kudchadker et al. (1968), for organic compounds, and by Mathews (1972), for inorganic compounds. An earlier review was published by Kobe and Lynn (1953). 

The major differences between behavior profiles of organic chemicals in the environment are attributable to their physical-chemical properties. The key properties are recognized as solubility in water, vapor pressure, the three partition coefficients between air, water and octanol, dissociation constant in water (when relevant) and susceptibility to degradation or transformation reactions. Other essential molecular descriptors are molar mass and molar volume, with properties such as critical temperature and pressure and molecular area being occasionally useful for specific purposes. A useful source of information and estimation methods on these properties is the handbook by Boethling and Mackay (2000). 

This equation requires only knowledge of the critical temperature and pressure for its use and gives accurate results in the vicinity of room temperature for unassociated substances at moderate pressures. 

The fundamental properties of SCFs and their relation to organometallic catalysis have been reviewed extensively in recent years, and will not be re-iterated here [1, 7]. The term supercritical indicates that the substance used as reaction medium or solvent is heated and compressed beyond its critical temperature and pressure. For C02, which is the most widely used SCF in hydrogenation reactions, these values are Tc=31.04°C and pc=73.83 bar. Owing to the complex... 

IUPAC defines supercritical chromatography as a separation technique in which the mobile phase is kept above (or relatively close to) its critical temperature and pressure. 

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