Partial pressure of water vapor

Phase relationships ia the system K O—B2O2—H2O have been described and a portion of the phase diagram is given ia Figure 8. The tetrahydrate, which can be dried at 65°C without loss of water of crystallisation, begias to dehydrate between 85 and 111°C, depending on the partial pressure of water vapor ia the atmosphere. This conversion is reversible and has a heat of dehydration of 86.6 kj/mol (20.7 kcal/mol) of H2O. Thermogravimetric curves iadicate that two moles of water are lost between 112 and 140°C, one more between 200 and 230°C and the last between 250 and 290°C (121). 

Absolute humidity H equals the pounds of water vapor carried by 1 lb of diy air. If ideal-gas behavior is assumed, H = M p/[M P — p)], where M,, = molecular weight of water = molecular weight of air p = partial pressure of water vapor, atm and P = total pressure, atm. 

Percentage relative humidity is defined as the partial pressure of water vapor in air divided by the vapor pressure of water at the given temperature. Thus RH = lOOp/p,. 

Dew point, or saturation temperature, is the temperature at which a given mixture of water vapor and air is saturated, for example, the temperature at which water exerts a vapor pressure equal to the partial pressure of water vapor in the given mixture. 

Partial pressure of water vapor is calculated from Eq. (4.84) ... 

The partial pressure of water vapor in air cannot be higher than the vapor pressure of saturated water ft (T) corresponding to air temperature T. If it were higher, condensation of water vapor would occur until the equilibrium state corresponding to the saturated vapor pressure was achieved. 

The wet bulb temperature can be solved for from Eq. (4.116) when the state of the air, the temperature t, and the partial pressure of water vapor pf, are known. Inversely, if the temperature t and the wet bulb temperature 6, 4 are known, the partial pressure and consequently the humidity of air can be found from Eq. (4.116). 

We will now derive an approximation for Eq. (4.116) that can be used when the partial pressure of water vapor in air is low compared with the total pressure. 

If, instead, the air is damped adiabatically with the wet cloth, so that the state of the air varies, the cloth will settle to a slightly different temperature. Each state of air (0, x) is represented by a certain wet bulb temperature 6, which can be calculated from Eq. (4.116) or its approximation (4.123), when the partial pressures of water vapor are low compared with the total pressure. When the state of air reaches the saturation curve, we have an interesting special case. Now the temperatures of the airflow and the cloth are identical. This equilibrium temperature is called the adiabatic cooling border or the thermodynamic wet bulb temperature (6 ). 

Equation (4.137) is almost exactly the same as the approximation equation (4.123) derived for wet bulb temperature. When the partial pressure of water vapor is low compared with the total pressure—in other words when the humidity x is low—the specific heat of humid air per kilogram of humid air, Cp, and the specific heat of humid air per kilogram of dry air, Cp, are al most the same Cp = Cp. Therefore, in a situation where the humidity is low and Le s 1, the thermodynamic wet bulb temperature is very nearly the same as the technical wet bulb temperature dy... 

The partial pressure of water vapor can be calculated as a function of the dry-bulb and wet-bulb temperatures, Eq. (12.23), and the relative humidity from its definition ... 

The humidity can be determined using either charts or equations provided by the psychrometer manufacturer. The partial pressure of water vapor provides a ni(2re general approach and can be calculated from the psychrometer equation ... 

Relative humidity The ratio of the partial pressure of the water vapor in moist air at a given temperature to the partial pressure of water vapor in saturated air at the same temperature. 

In this case, Ptot is the measured pressure. The partial pressure of water vapor, Ph2o, is equal to the vapor pressure of liquid water. It has a fixed value at a given temperature (see Appendix 1). The partial pressure of hydrogen, PH2, can be calculated by subtraction. The number of moles of hydrogen in the wet gas, h2, can then be determined using the ideal gas law. 

The total pressure can be regarded as a sum of the parts furnished by the individual pressures exerted by each of the components of the gas mixtures. The pressure exerted by each of the gases in a gas mixture is called the partial pressure of that gas. The partial pressure is the pressure that the gas would exert if it were alone in the container. In the example of Figure 4-3, the total pressure in the third bulb is 113 mm. The partial pressure of water vapor in this bulb is 20 mm and the partial pressure of air is 93 mm. 

This problem asks about the partial pressure of water vapor in the atmosphere. Fog forms when that partial pressure exceeds the vapor pressure. Partial pressures are not given among the data, but relative humidity describes how close the partial pressure of water vapor is to its vapor pressure at the given temperature. Because vapor pressure varies strongly with temperature, we must use the information... 

C05-0037. A weather report gives the current temperature as 18 °C and sets the dew point at 10 °C. Using data from Table 5A, determine the partial pressure of water vapor in the atmosphere and calculate the relative humidity. 

The boundary conditions for the system are (1) that at the surface of the hygroscopic material the partial pressure of water is determined by that of the saturated salt solution (Ps) and (2) that at a characteristic distance from the surface (8) the partial pressure of water vapor is given by the chamber pressure (Pc). 

The important variables in Eq. (44) are the thickness of the diffusion layer (8) and the partial pressures of water vapor in the chamber and above the solid surface Pc and Ps). 

Preliminary studies into a third variable, the partial pressure of water vapor in the system, are discussed in Part 3 of the Results and Calculations section. Each calorimetric sample ( 1 g, 13.47 mass % bitumen) came from a large sample of "reconstructed" oil sand consisting of Athabasca bitumen loaded onto a chemically inert solid support material (60/80 mesh acid washed Chromosorb W) of well-defined particle size. 

Further to our preliminary studies at 225°C subsequent wet oxidation experiments have been carried out at temperatures of 250 and 285°C. Partial pressures of oxygen were varied from 19 to 289 kPa whilst the partial pressure of water vapor in the calorimetric system was maintained at approximately 15 kPa. 

Here Wx represents the initial rate of heat production for some process (or processes) that is dependent on the partial pressure of the water vapor in the calorimetric system and gives a positive contribution to the initial rate of heat production by wet oxidation (unlike the steam distillation process envisaged for wet oxidation at 225°C, which aids in the retardation of the initial rate of heat production). If this relationship were valid, then a plot of the logarithm of the initial rate Wx against the logarithm of the ratio of the partial pressure of water vapor in the system to the total pressure in the system would be linear. However, as demonstrated by Figure 3, no such simple relationship is found. 

Figure 3. Variation of the difference between the wet and the dry initial rates of heat production as a function of the partial pressure of water vapor in the system at 285°C.

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