Continuous Humidification Processes

5 CONTINUOUS HUMIDIFICATION PROCESSES 10.5A Introduction and Types of Equipment for Humidification 

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. 

10 Stage and Continuous Gas-Liquid Separation Processes 

The water cannot be cooled below the wet bulb temperature. The driving force for the evaporation of the water is approximately the vapor pressure of the water less the vapor pressure it would have at the wet bulb temperature. The water can be cooled only to the wet bulb temperature, and in practice it is cooled to about 3 K or more above this. Only a small amount of water is lost by evaporation in cooling water. Since the latent heat of vaporization of water is about 23(K) kJAcg, a typical change of about 8 K in water temperature corresponds to an evaporation loss of about 1.5%. Hence, the total flow of water is usually assumed to be constant in calculations of tower size. 

In humidification and dehumidification, intimate contact between the gas phase and liquid phase is needed for large rates of mass transfer and heat transfer. The gas-phase resistance controls the rate of transfer. Spray or packed towers are used to give large interfacial areas and to promote turbulence in the gas phase. 

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All the systems described above require liquid water to operate. This means that the exit air must be treated so that a good deal of the water content is condensed out as liquid water, stored, and then pumped to where it is needed for the humidification process. This clearly adds to the system size, cost, and complexity. There are some systems that use the water in the exit gas to humidify the inlet air, but without actually condensing it out as liquid water. One way to do this is to use a rotating piece of water-absorbing material. It is put into the path of the exit air, and it absorbs water. It is then rotated so that it is in the path of the dry entry air, which will at least partly dry it out. If the piece is made circular this can be done on a continuous basis - constantly transferring water from exit to entry gases. This method has the disadvantage that it will still be fairly bulky and will require power and control to operate. 

The transfer of mass as well as heat from one material phase to another is quite commonly encountered in chemical process flow sheets. The same physical laws, rate equations, and design principles can be applied to mass-transfer operations as occurring in absorption, adsorption, crystal-lization, distillation, drying, extraction, jluidization, and humidification Equipment is designed to obtain intimate contact between phases, in either a stagewise or continuous manner, and many special types of equipment have been developed for any given operation. This discussion will be limited to the conventional types of equipment. 

This part, on applications, covers the following unit operations 8. Evaporation 9. Drying of Process Materials 10. Stage and Continuous Gas-Liquid Separation Processes (humidification, absorption) 11. Vapor-Liquid Separation Processes (distillation) 12. Liquid—Liquid and Fluid-Solid Separation Processes (adsorption, ion exchange, extraction, leaching, crystallization) 13. Membrane Separation Processes (dialysis, gas separation, reverse osmosis, ultrafiltration) 14. Mechanical-Physical Separation Processes (filtration, settling, centrifugal separation, mechanical size reduction). 

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