Humidification Operations

As in all mass-transfer problems, it is necessary for a complete understanding of the process to be familiar with the equilibrium characteristics of the systems. Moreover, since the mass transfer in these cases will invariably be accompanied by simultaneous transfer of energy as well, consideration must also be given to the enthalpy characteristics of the systems. 

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For the air—water system, the Lewis relation shows that r = 1. Under these conditions, the two parenthetical terms on the right-hand side of equation 33 ate enthalpies, and equation 33 becomes the design equation for humidification operations ... 

Deliberate atomization in some humidification operations, gas scrubbing, spray drying, spray painting... 

This result (which is far from obvious) allows us to perform adiabatic cooling calculations with relative ease using the psychrometric chart. First locate the initial state of the air on the chart then locate the final state on the constant wet-bulb temperature line that passes through the initial state (or on the 100% humidity curve if cooling below the adiabatic saturation temperature takes place) and finally perform whatever material and energy balance calculations are required. Example 8.4-7 illustrates such a calculation for an adiabatic humidification operation. 

In gas absorption a soluble vapor is absorbed by means of a liquid in which the solute gas is more or less soluble, from its mixture with an inert gas. The washing of ammonia from a mixture of ammonia and air by means of liquid water is a typical example. The solute is subsequently recovered from the liquid by distillation, and the absorbing liquid can be either discarded or reused. When a solute is transferred from the solvent liquid to the gas phase, the operation is known as desorption or stripping. In dehumidification a pure liquid is partially removed from an inert or carrier gas by condensation. Usually the carrier gas is virtually insoluble in the liquid. Removal of water vapor from air by condensation on a cold surface and the condensation of an organic vapor such as carbon tetrachloride out of a stream of nitrogen are examples of dehumidification. In humidification operations the direction of transfer is from the liquid to the gas phase. In the drying of solids, a liquid, usually water, is separated by the use of hot, dry gas (usually air) and so is coupled with the humidification of the gas phase. 

DEFINITIONS. In humidification operations, especially as applied to the system air-water, a number of rather special definitions are in common use. The usual basis for engineering calculations is a unit mass of vapor-free gas, where vapor means the gaseous form of the component that is also present as liquid and gas is the component present only in gaseous form. In this discussion a basis of a unit mass of vapor-free gas is used. In the gas phase the vapor will be referred to as component A and the fixed gas as component B. Because the properties of a gas-vapor mixture vary with total pressure, the pressure must be fixed. Unless otherwise specified, a total pressure of 1 atm is assumed. Also, it is assumed that mixtures of gas and vapor follow the ideal-gas laws. 

In humidification or dehumidification (depending upon the direction of transfer) the liquid phase is a pure liquid containing but one component while the gas phase contains two or more substances. Usually the inert or carrier gas is virtually insoluble in the liquid. Removal of water vapor from air by condensation on a cold surface and the condensation of an organic vapor such as carbon tetrachloride out of a stream of nitrogen are examples of dehumidification. In humidification operations the direction of transfer is from the liquid to the gas phase. 

The Lewis number plays an important role in problems of simultaneous heat and mass transfer, such as humidification operations. 

It has been observed that for the system air-water vapor at near-ambient conditions, Le = 1.0 (Treybal, 1980). This observation, called the Lewis rela -tion, has profound implications in humidification operations, as will be seen later. Based on the Lewis relation, estimate the diffusivity of water vapor in air at 300 K and 1 atm. Compare your result with the value predicted by the Wilke-Lee equation. For air at 300 K and 1 atm C = 1.01 kJ/kg-K, k = 0.0262 W/m-K, x = 1.846 x 10-5 kg/m-s, and p = 1.18 kg/m3. 

Figure 20.6 Polarization curves of a Hj/Oj fuel cell using an SPI-2.15(75) composite membrane without humidification ( , ) operated at 30 °C, ,0) operated at 120°C, (A,A) operated at 140 C. The flow rates of the reaction gases (Hj and Oj) are 12 ml min" for 120 and and...

Operations of multicomponents by stages Diffusion and mass transfer between stages Gas absorption Humidification operations Solids drying Filtration... 

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