Humidification processes water-cooling

Packed columns are used primarily in gas absorption and liquid extraction and in air-water contact operations such as humidification and water cooling, which we take up in Chapter 9. They are found less frequently in distillation operations where their use is confined mostly to small-scale processes involving high-efficiency packing. 

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. 

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. 

Secondly, the humidification process involves evaporating water in the incoming gas. This will cool the gas, as the energy to make the water evaporate will come from the air. In pressurised systems this is positively helpful. It will be seen in Chapter 9, when compressors are considered, that a result of compressing the air is considerable heating, and the humidification process can be an ideal way of bringing the air temperature to a suitable value. 

In addition to taking the heat away from the fuel cell stack, the cooling loop also cools down the reformate gas coming from the fuel processor and provides heat, if needed, in the air humidification process. Moreover, temperature control of the preferential oxidation process is needed (as shown in Figures 9-37 and 9-38). The heat is then rejected from the system to the environment by the heat exchanger, typically of a radiator type. Some heat may be rejected from the system in the condenser just before the exhaust gases leave the system. The condenser is needed to condense and save the water in the exhaust to maintain a neutral water balance in the system. 

Nucleation A cleaning process using a humidification and cooling cycle, causing water or another fluid to condense on sub-micrometer particles. This process increases particle size until impingement on packing is possible. 

Besides the obvious processes of humidification and dehumidifica-tion 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... 

The use of these relationships in constructing and applying humidity charts is best illustrated by examining a simplified case, that of adiabatic cooling or humidification. Figure 5.4 illustrates this process between air and water that is recycled through the cooling tower. In this operation air is both cooled and... 

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. 

Another type of process of some importance occurs when adiabatic cooling or humidification takes place between air and water that is recycled as illustrated in Fig. 4.25. In this process the air is both cooled and humidified (its water content rises) while a little bit of the recirculated water is evaporated. At equilibrium, in the steady state, the temperature of the air is the same as the temperature of the water, and the exit air is saturated at this temperature. By making an overall energy balance around the process Q = 0), we can obtain the equation for the adiabatic cooling of the air. 

In Chapter 9 the fundamentals of humidity and adiabatic humidification were discussed. In this section the performance and design of continuous air-water contactors is considered. The emphasis is on cooling of water, since this is the most important type of process in the process industries. There are many cases in industry in which warm water is discharged from heat exchangers and condensers when it would be more economical to cool and reuse it than to discard it. 

Cooling towers dissipate tremendous quantities of heat into the atmosphere through the process of humidification. Water circulated counter-currently to a stream of air is reduced in temperature owing to the fact... 

Water balance requirement results in additional cooling loads [6,22], Although water is produced in a fuel cell, water is needed for humidification of reactant gases and for fuel processing (in case of an onboard fuel processor), and it has to be reclaimed from the exhaust gases. 

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