Humidification processes dehumidification

Figure 4 shows the general arrangement and nomenclature for a humidification or dehumidification process, where the subscript 1 refers to the bottom of the column, and subscript 2 to the top. Steady state is assumed. Flow rates and compositions are given in molar terms because this simplifies the results. 

This chapter deals with the basic air-handling processes filtration of particles and gaseous substances from the supply and recirculated air, air heating and cooling, heat-recovery processes, and humidification and dehumidification. It also describes fans and ductwork. 

Figure 9.14 shows the processes of humidification and dehumidification in a skeleton psychrometric chart, while Table 9.5 explains the cycle. 

The reverse of the humidification process is that of dehumidification. In this process the water content of air, gases, or fluids is reduced. 

Define the dry-bulb temperature, wet-bulb temperature, and humid volume of humid air. Given values of any two of the variables plotted on the psychrometric chart (dry-buib and wet-bulb temperatures, absolute and relative humidity, dew point, humid volume), determine the remaining variable values and the specific enthalpy of the humid air. Use the psychrometric chart to carry out material and energy balance calculations on a heating, cooling, humidification, or dehumidification process involving air and water at 1 atm. 

Besides the obvious processes of humidification and dehumidification of air for control of environment, interaction of air and water is a major aspect of the drying of wet solids and the cooling of water for process needs. Heat and mass transfer then occur simultaneously. For equilibrium under adiabatic conditions, the energy balance is... 

Fig. 4. Arrangement and nomenclature for general humidification—dehumidification process.

In the processing of materials it is often necessary either to increase the amount of vapour present in a gas stream, an operation known as humidification or to reduce the vapour present, a process referred to as dehumidification. In humidification, the vapour content may be increased by passing the gas over a liquid which then evaporates into the gas stream. This transfer into the main stream takes place by diffusion, and at the interface simultaneous heat and mass transfer take place according to the relations considered in previous chapters. In the reverse operation, that is dehumidification, partial condensation must be effected and the condensed vapour removed. 

Choose an inlet air temperature that is high enough to negate weather effects (outside air humidity or inside room conditions). The air temperature should not be detrimental to the product being granulated. (To achieve consistent process year round, a dehumidification/humidification system is necessary, which provides the process air with constant dew point and, hence, constant drying capacity.)... 

The key properties of mixtures of air and water vapor are described in Section 9.1. Here the interactions of air and water in packed towers under steady flow conditions will be analyzed. The primary objectives of such operations may be to humidify or dehumidify the ait as needed for particular drying processes or other processes, or to cool process water used for heat transfer elsewhere in the plant. Humidification-dehumidification usually is accomplished in spray towers, whereas cooling towers almost invariably are filled with seme type of packing of open structure to improve contacting but with minimum pressure drop of air. 

Systems containing several components, of which only one is capable of existing as a liquid at the process conditions, are common in industrial processes. Separation processes that involve such systems include evaporation, drying, and humidification—all of which involve transfer of liquid into the gas phase—and condensation and dehumidification, which involve transfer of the condensable species from the gas to the liquid phase. 

Bergero S and Chiari A. Experimental and theoretical analysis of air humidification/dehumidification processes using hydrophobic capillary contactors. Appl. Then Eng. 2001 21 1119-1135. 

In processes where a condensing vapor or vapor from a liquid phase moves through an inert gas, eg, condensation in the presence of air, drying, humidification, crystallization (qv), and boiling of a multicomponent liquid, mass-transfer as well as heat-transfer effects are important (see Air conditioning Distillation Evaporation). Such processes are discussed elsewhere in the Encyclopedia, but the primary emphasis is on either the heat transfer or the mass transfer taking place. Herein the interactions between heat and mass transfer in such processes are discussed, and applications to humidification, dehumidification, and water cooling are developed. These same principles are applicable to other operations. 

When the gas is pure air and the liquid is pure water, the process is called humidification. Dehumidification involves removal of water vapor from air. 

---