Water partial pressure

The relative humidity is the ratio of the partial pressure of the water vapour in the air to the partial pressure of water vapour in the air when saturated at the same temperature. This ratio is usually expressed as a p>ercentage. 

Deliquescence and efflorescence. A substance is said to deliquesce (Latin to become liquid) when it forms a solution or liquid phase upon standing in the air. The essential condition is that the vapour pressure of the saturated solution of the highest hydrate at the ordinary temperature should be less than the partial pressure of the aqueous vapour in the atmosphere. Water will be absorbed by the substance, which gradually liquefies to a saturated solution water vapour will continue to be absorbed by the latter until an unsaturated solution, having the same vapour pressure as the partial pressure of water vapour in the air, is formed. In order that the vapour pressure of the saturated solution may be sufficiently low, the substance must be extremely soluble in water, and it is only such substances (e.g., calcium chloride, zinc chloride and potassium hydroxide) that deliquesce. 

Vacuum flash processes, which operate under the atmospheric boiling point of the solution, include the Uhde—LG. Farbenindustrie process and the closely related Kestner process (22). In these, ammonia, nitric acid, and recirculated ammonium nitrate solution are fed into the neutralizer. Hot solution overflows to an intermediate tank and then to a flash evaporator kept at 18—20 kPa (0.18—0.2 atm) absolute pressure. Partial evaporation of water at this point cools and concentrates the solution, part of which is routed to evaporation. The rest is circulated to the neutralizer. 

The physical properties of vinyl chloride are Hsted in Table 1 (12). Vinyl chloride and water [7732-18-5] are nearly immiscible. The equiUbrium concentration of vinyl chloride at 1 atm partial pressure in water is 0.276 wt % at 25°C, whereas the solubiUty of water in vinyl chloride is 0.0983 wt % at 25°C and saturated pressure (13). Vinyl chloride is soluble in hydrocarbons, oil, alcohol, chlorinated solvents, and most common organic Hquids. 

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). 

Partial Pressures of Water over Aqueous Solutions of HCl, , 2-76... 

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. 

When the partial pressure p of water vapor in the air at a given temperature equals the vapor pressure of water p, at the same temperature, the air is saturated and the absolute humidity is designated the saturation humidity H,. 

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. 

Relative humidity and dew point can be determined for other than atmospheric pressure from the partial pressure of water in the mixture and from the vapor pressure of water vapor. The partial pressure of water is calculated, if ideal-gas behavior is assumed, as... 

The relative humidity is obtained by dividing the calculated partial pressure by the vapor pressure of water at the diy-bulb temperature. Thus ... 

I. The wet-bulb or saturation temperature line gives the maximum weight of water vapor that I kg of dry air can cariy at the intersecting dry-bulb temperature shown on the abscissa at saturation humidity. The partial pressure of water in air equals the water-vapor pressure at that temperature. The saturation humidity is defined by... 

Dehydration of CUSO4 pentahydrate at 53 to 63°C (127 to 145°F) and of the trihydrate at 70 to 86°C (158 to 187°F) obey the Avrami-Erofeyev equation. The rate of water loss from Mg(OH)9 at lower temperatures is sensitive to partial pressure of water. Its decomposition above 297°C (567°F) yields appreciable amounts of hydrogen and is not reversible. 

The percent relative humidity is defined as lOOp/P, where p is the actual partial pressure of the water vapor and P is the vapor pressure of water at the same temperature. The total pressure is taken as normal barometric, unless otherwise stated. Note that since the per cent relative humidity is defined as 100(p/Pj) and the per cent absolute humidity equals 100[p/(760-p) -h P,/(760-Pj)], the factor by which the former must be multiplied to convert it to the latter is (760 -P ) / (760-p), where p and Pj can be expressed in units of millimeters of mercury. 

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). 

Temperature rc) Humidity kg HjO/kg dry air) Water vapor partial pressure (kPa) Water v K>r partial density (kg/m ) Water vaporization heat M/kg) Mixture enthalpy (kj/kg dry air) Dry air partial density (lKinematic viscosity (I0< mJ/s) Specific heat (kJ/K kg) Heat conductivity (W/m K) Diffusion factor water air (1 O mJ/s) Temperature rc)... 

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... 

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