Saturated Gas-Vapor Mixtures

In the discussion that follows, the term vapor will be applied to that substance, designated as substance A, in the gaseous state which is relatively near its condensation temperature at the prevailing pressure and, therefore, could easily condense. The term gas will be applied to substance B, which is a highly superheated gas and would never condense under the prevailing conditions. 

When operations involve changes in the vapor content of a gas-vapor mixture without changes in the gas content, it is convenient to use concentration units based on the unchanging amount of gas. The mass of vapor/mass of gas ratio is the absolute humidity, Y. If the quantities are expressed in moles, the ratio is the molal absolute humidity, Y. Under conditions where the ideal gas law applies, 

If an insoluble gas B is brought into contact with sufficient liquid A, the liquid will evaporate into the gas until ultimately, at equilibrium, the gas will be saturated with the vapor and the partial pressure of the vapor in the saturated mixture will be the vapor pressure PA of the liquid at the prevailing temperature. Then, at equilibrium, 

A gas(B)-benzene (A) mixture is saturated at 1 bar and 320 K. Calculate the absolute humidity if B is (a) nitrogen and (b) carbon dioxide. 

Since the solution is saturated, the partial pressure of benzene equals the equilibrium vapor pressure of liquid benzene at 320 K. From equation (6-5) and using the data for benzene from Example 6.4, at 320 K, Pa - 0.32 bar. The molecular weight of benzene is Ma - 78 g/mol. 

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Equation (8-3) shows that the enthalpy of a gas-vapor mixture depends upon the temperature and humidity at a given pressure. If the mixture is saturated, the humidity is determined by the temperature therefore, the enthalpy of saturated gas-vapor mixtures depends only upon the temperature at a given pressure. ... 

A computer program developed hy Volta handles the problem of condensing in the presence of a noncondensable gas for down-flow of either a saturated or superheated gas-vapor mixture vertical tubes. The program is based on a modification of Colburn-Hougen and Bras and is certainly more accurate and easier to use than the lengthy manual calculations. Although the program was written for vertical tubes, it can be used to approximate the results in a horizontal unit, and if the correction factor between vertical and horizontal tube condensation is applied, the compari-... 

The method is applicable only to gas-vapor mixtures with the vapor at saturation. However, systems involving superheated mixtures and subcooling can be handled as separate problems and added to the cooler-condenser area requirements to form a complete unit. 

For the case in which the exiting gas-vapor mixture is saturated, we have the term T, Jfas, Has. The liquid enters at T and the gas is humidified by evaporation of liquid and cooled. By expansion of the humid heat terms in the equation derived for problem 3.9,... 

Finally, is it possible to have the water evaporate into air and saturate the air, and yet maintain both a constant temperature, volume, and pressure in the cylinder Hint What would happen if you let some of the gas-vapor mixture escape from the cylinder )... 

Saturation vapor pressure p. Pressure exerted by pure vapor at a given temperature. When the vapor partial pressure p in the gas-vapor mixture at a given temperature equals the saturation vapor pressure ps at the same temperature, the air is saturated and the absolute humidity is designated the saturation hiunidity Y,. 

Erocess almost exactly follows a constant-enthalpy line, as the sensi-le heat given up by the gas-vapor mixture exactfy balances the latent heat of the liquid that evaporates back into the mixture. The only difference is due to the sensime heat added to the water to take it from the datum temperature to The adiabatic saturation line differs from the constant-enthalpy line as follows, where Cpi is the specific heat capacity of the liquid ... 

Associated saturated Water vapor . Total gas-vapor mixture ... 

Calculate the concentration (in terms of absolute humidity and molal absolute humidity) and enthalpy per unit mass of dry gas of saturated and unsaturated gas-vapor mixtures. 

If the partial pressure of the vapor in a gas-vapor mixture is less than the equilibrium vapor pressure of the liquid at the same temperature, the mixture is unsaturated. This condition is called partial saturation. In this case, to completely define the thermodynamic state of the mixture, the degree of saturation of the mixture must... 

Relative saturation (relative humidity for the system air-water), expressed as a percentage, is defined as 100pA/PA, where PA is the vapor pressure at the dry-bulb temperature of the mixture. Therefore, if the relative saturation, temperature, and pressure of a gas-vapor mixture are specified, all of its thermodynamic properties can be calculated. 

Another way of fixing the state of an unsaturated mixture is to specify its dew point. This is the temperature at which a gas-vapor mixture becomes saturated when cooled at constant total pressure. In this case, the partial pressure of the vapor in the unsaturated mixture equals the vapor pressure of the liquid at the specified dew-point temperature. 

In the special case where the gas-vapor mixture leaves saturated, and therefore at conditions Tas, YJ, Hj, and the liquid enters at temperature Tas, equation (8-7) can be written as (Treybal, 1980)... 

Consider a drop of liquid immersed in a rapidly moving stream of unsaturated gas-vapor mixture. If the liquid is initially at a temperature higher than the gas dew point, the vapor pressure of the liquid will be higher at the drop surface than the partial pressure of the vapor in the gas, and the liquid will evaporate and diffuse into the gas. The latent heat required for the evaporation will at first be supplied at the expense of the sensible heat of the liquid drop, which will then cool down. As soon as the liquid temperature drops below the dry-bulb temperature of the gas, heat will flow from the gas to the liquid, at an increasing rate as the temperature difference becomes larger. Eventually the rate of heat transfer from the gas to the liquid will equal the rate of heat required for the evaporation, and the temperature of the liquid will remain constant at some low value, the wet-bulb temperature Tw. The mechanism of the wet-bulb process is essentially the same as that governing the adiabatic saturation process, except that in the wet-bulb process the humidity of the rapidly moving gas is assumed to remain constant. 

Now, if a further restriction is made that the gas and the liquid phases reach equilibrium when they leave the system (i.e., the gas-vapor mixture leaving the system is saturated with liquid), then = T s, /g , = IGS, and Kgs where T s is the adiabatic saturation temperature and GS is the absolute humidity saturated at GS- Still further, if the liquid enters at the adiabatic saturation temperature Tq, that is, Tl = Tq, Equation 1.36 becomes... 

Sc should be calculated for mean conditions between those of the gas-vapor mixture and the wet-bulb saturation conditions. However, for the dilute mixture considered here, the bulk-gas value of Sc is satisfactory and is essentially independent of temperature... 

In Figure 2 the lines, volume, m /kg dry air, indicate humid volume, which includes the volume of 1.0 kg of dry gas plus the volume of vapor it carries. Enthalpy at saturation data are accurate only at the saturation temperature and humidity however, for air—water vapor mixtures, the diagonal wet bulb temperature lines are approximately the same as constant-enthalpy adiabatic cooling lines. The latter are based on the relationship ... 

If the gas to be compressed contains water vapor (saturated or only partially saturated), this water content must be determined by (1) test of the mixture or (2) calculation. Then the properties of the gas-water vapor mixture must be determined by the usual gas calculations for weighted aver-... 

Ewald22 studied this system at 150° and 155°K. These temperatures are above the critical temperature of pure nitrogen, 126°K, but he found that they are below the lower critical end point of the mixture. The saturated vapor pressure of the system was 50 atm at 150°K and 57 atm at 155°K. The mole fraction of xenon in the saturated gas (X in Figs. 5 and 9) was 0.035 and 0.045 at these temperatures, respectively. 

Desorption of the dissolved gases formed bubbles of gas and a limited amount of bubbles containing gas-water vapor mixture. Under these conditions, during flow boiling of water boiling incipience occurred at channel wall temperatures below that of saturation temperature. Addition of surfactants led to an increase in wall temperature. In this case the boiling occurred in the range of heat flux of 5.2-21 W/cm at wall temperatures of 107-121 °C. 

The conditions that apply for the saturated liquid-vapor states can be illustrated with a typical p-v, or (1 /p), diagram for the liquid-vapor phase of a pure substance, as shown in Figure 6.5. The saturated liquid states and vapor states are given by the locus of the f and g curves respectively, with the critical point at the peak. A line of constant temperature T is sketched, and shows that the saturation temperature is a function of pressure only, Tsm (p) or psat(T). In the vapor regime, at near normal atmospheric pressures the perfect gas laws can be used as an acceptable approximation, pv = (R/M)T, where R/M is the specific gas constant for the gas of molecular weight M. Furthermore, for a mixture of perfect gases in equilibrium with the liquid fuel, the following holds for the partial pressure of the fuel vapor in the mixture ... 

In the laboratory work of general chemistry, we have important applications of partial pressures. Gases (such as oxygen) that are not very soluble in water are collected in bottles by displacement of water. As the gas bubbles rise through the water, tney become saturated with vapor, and the collected gas is a mixture of water vapor and the original gas. When the bottle is filled, it is at atmospheric pressure, or... 

Suppose we take a sample of bone-dry air at some temperature, Ti, and directly contact it with water until it becomes saturated at the same temperature. The water vapor that enters into the air contains with it its latent heat of vaporization. The vapor pressure of water out of the liquid will be greater than it is in the saturated air, causing vaporization to occur and subsequently increasing the humidity of the air-water-vapor mixture. The process of vaporization ends when the vapor pressure of the water in the air becomes equal to that of the liquid. At this condition the air is saturated. During the air saturation process, isothermal conditions for the water can be maintained if heat is supplied to replace the heat lost from it to the gas as latent heat of vaporization. Thus, heat transfer during the saturation of a gas with a liquid can be accomplished without a temperature differential (although this is rarely encountered). This type of heat transfer phenomenon, better known as diffusional heat transfer, is different from conduction, convection or radiation. 

Another frequently used unit is dew point (saturation) or wet-bulb temperature. Dew point is the saturation temperature of a gas-water vapor mixture (the temperature at which water condenses as the gas is cooled). Finally, moisture is also expressed in volume or mass ratio as parts per million (ppm) volume or weight. 

If the temperature of a liquid is below its flash point, flammable concentrations of vapor cannot exist, but conditions still can exist for flammability if mists or foams are formed. A suspension of finely divided drops of a flammable liquid in air has many of the characteristics of a flammable gas-air mixture and can bum or explode. A mist may be produced by condensation of a saturated vapor or by mechanical atomization. Normally, the diameter of drops in a condensed mist is less than 0.01mm, whereas in a mechanical spray it usually is greater than 0.1 mm. 

Sup eat of a pore vapor is removed at the same coeffidrat as fOT condensation of the sahirated vapOT if the exit coolant temperature is less than the saturation temperature (at the pressure existing in the v x phase) and if the (constant) saturation temperature Is used in calculating the mean temperature differmce. But see note k tor vapor mixtures with or without noncooidensable gas. 

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