Water, critical point, temperature

An article in Chemical cmd Engineering News (Sept. 28, 1987) descn bes a hydrothermal autoclave. This device is of constant volume, is evacuated, and then water is added so that a fraction. r of the total volume is filled with liquid water and the remainder is filled with water vapor. The autoclave is then heated so that the temperature and pressure in the sealed vessel increase. It is observed that if x is greater than a critical fill value, x, the liquid volume fraction increases as the temperature increases, and the vessel becomes completely filled with liquid.at temperatures below the critical temperature. On the other hand, if. c < Xc, the liquid evaporates as temperature is increased, and the autoclave beconres completely filled with vapor below the water critical temperature. If, however, x = Xc, the volume fraction of liquid in the autoclave remains constant as the temperature increases, and the temperature-pressure trajectory passes through the water critical point. Assuming the. hydrothermal autoclave is to be loaded at 25° C, calculate the critical fiU Xc... 

Many pairs of partially miscible liquids possess neither a lower nor an upper C.S.T. for reasons outlined in the previous paragraph. Thus consider the two liquid phases from the two components water and diethyl ether. Upon cooling the system at constant pressure, a point will be reached when a third phase, ice, will form, thus rendering the production of a lower C.S.T. impossible, likewise, if the temperature of the two layers is raised, the critical point for the ether rich layer will be reached while the two liquid phases have different compositions. Above the critical point the ether-rich layer will be converted into vapour, and hence the system will be convert into a water rich liquid and an ether rich vapour the upper C.S.T. cannot therefore be attained. 

Va.por Pressure. Vapor pressure is one of the most fundamental properties of steam. Eigure 1 shows the vapor pressure as a function of temperature for temperatures between the melting point of water and the critical point. This line is called the saturation line. Liquid at the saturation line is called saturated Hquid Hquid below the saturation line is called subcooled. Similarly, steam at the saturation line is saturated steam steam at higher temperature is superheated. Properties of the Hquid and vapor converge at the critical point, such that at temperatures above the critical point, there is only one fluid. Along the saturation line, the fraction of the fluid that is vapor is defined by its quaHty, which ranges from 0 to 100% steam. 

Density. The density of saturated water and steam is shown in Figure 2 as a function of temperature on the saturation line. As the temperature approaches the critical point, the densities of the Hquid and vapor phase approach each other. This fact is cmcial to boiler constmction and steam purity because the efficiency of separation of water from steam depends on the density difference. 

Solvent. The solvent properties of water and steam are a consequence of the dielectric constant. At 25°C, the dielectric constant of water is 78.4, which enables ready dissolution of salts. As the temperature increases, the dielectric constant decreases. At the critical point, the dielectric constant is only 2, which is similar to the dielectric constants of many organic compounds at 25°C. The solubiUty of many salts declines at high temperatures. As a consequence, steam is a poor solvent for salts. However, at the critical point and above, water is a good solvent for organic molecules. 

Gases. At low temperatures and pressures, most gases are relatively insoluble in water and tend to appear in the steam phase. Only those gases that ionize to some extent violate this rule. However, as the pressure approaches the critical point, the solubiUty of gases increases. 

Along the saturation line and the critical isobar (22.1 MPa (3205 psi)), the dielectric constant of water declines with temperature (see Fig. 10). In the last 24°C below the critical point, the dielectric constant drops precipitously from 14.49 to 4.77 in the next 5°C, it further declines to 2.53 and by 400°C it has declined to 1.86. In the region of the critical point, the dielectric constant of water becomes similar to the dielectric constants of typical organic solvents (Table 6). The solubiHty of organic materials increases markedly in the region near the critical point, and the solubiHty of salts tends to decline as the temperature increases toward the critical temperature. 

A. Ciach, J. S. Hoye, G. Stell. Microscopic model for microemulsion. II. Behavior at low temperatures and critical point. J Chem Phys 90 1222-1228, 1989. A. Ciach. Phase diagram and structure of the bicontinuous phase in a three dimensional lattice model for oil-water-surfactant mixtures. J Chem Phys 95 1399-1408, 1992. 

Some volatile fluids are used once only, and then escape into the atmosphere. Two of these are in general use, carbon dioxide and nitrogen. Both are stored as liquids under a combination of pressure and low temperature and then released when the cooling effect is required. Carbon dioxide is below its critical point at atmospheric pressure and can only exist as snow or a gas. Since both gases come from the atmosphere, there is no pollution hazard. The temperature of carbon dioxide when released will be - 78.4°C. Nitrogen will be at - 198.8°C. Water ice can also be classihed as a total loss refrigerant. 

Curve AB is a portion of the vapor pressure-temperature curve of liquid water. At any temperature and pressure along this line, liquid water is in equilibrium with water vapor. At point A on the curve, these two phases are in equilibrium at 0°C and about 5 mm Hg (more exactly, 0.01°C and 4.56 mm Hg). At B, corresponding to 100°C, the pressure exerted by the vapor in equilibrium with liquid water is 1 atm this is the normal boiling point of water. The extension of line AB beyond point B gives the equilibrium vapor pressure of the liquid above the normal boiling point. The line ends at 374°C, the critical temperature of water, where the pressure is 218 atm. 

Phase diagram of water (not to scale). The curves and line represent the temperatures and pressures at which phases are in equilibrium. The triple point is at 0.0rC, 4.56 mm Hg the critical point is at 374°C. 

Carbon dioxide and water are the most commonly used SCFs because they are cheap, nontoxic, nonflammable and environmentally benign. Carbon dioxide has a more accessible critical point (Table 6.13) than water and therefore requires less complex technical apparatus. Water is also a suitable solvent at temperatures below its critical temperature (superheated water). Other fluids used frequently under supercritical conditions are propane, ethane and ethylene. 

Thermal expansion causes liquid water to become less dense as the temperature increases. At the same time, the liquid vapor becomes more dense as the pressure rises. For example, the density of water varies from 1.0 g/cm3 at room temperature to 0.7 g/cm3 at 306°C. At the critical point, the densities of the two phases become identical and they become a single fluid called supercritical fluid. Its density at this point is only about 0.3 g/cm3 (Figures 1.4 and 1.5). 

As the temperature increases from ambient to the critical point, the electrolytic conductance of water rises sharply and is almost independent of the pressure. Macroscopically, this is due to the decrease in water viscosity over this range. The primary cause for the fall in viscosity is a disintegration of water clusters. 

Most hydrophobic substances have low solubilities in water, and in the case of liquids, water is also sparingly soluble in the pure substance. Some substances such as butanols and chlorophenols display relatively high mutual solubilities. As temperature increases, these mutual solubilities increase until a point of total miscibility is reached at a critical solution temperature. Above this temperature, no mutual solubilities exist. A simple plot of solubility versus temperature thus ends at this critical point. At low temperatures near freezing, the phase diagram also become complex. Example of such systems have been reported for sec-butyl alcohol (2-butanol) by Ochi et al. (1996) and for chlorophenols by Jaoui et al. (1999). 

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