Absorption and Humidification

Mass transfer between liquids and gases depends on the vapor pressure of the components as functions of temperature. Thus appropriate selection of operating temperature and pressure allows the reverse (desorption or stripping, and dehumidification) to be performed. The purpose of absorption and stripping operations is to remove and recover the maximum amount of a particular component from a feed stream. It is most efficiently accomplished in multiple stages, as in tray or packed columns. Humidification and dehumidification arc similar in principle, but are directed toward control of an environment short of equilibrium (e.g., 100 percent humidity) for them, a single stage is ordinarily sufficient. 

Each component in a vapor mixture exerts a partial pressure pi relative to its concentration i/,  

It can be seen that since the concentrations total 100 percent, the sum of the partial pressures is the total static pressure p exerted by the system. 

According to Raoult s law, each component in an ideal liquid solution generates a partial pressure relative to its concentration Xi in the liquid  

The coefficient p in Eq. (12.2) is the vapor pressure of component i at the prevailing temperature. Unfortunately, wide departures from the ideal situation are encountered in typical solutions nonetheless, linearity prevails over certain ranges, allowing p to be replaced with an equilibrium constant Kii 

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Refs 1)H.C.Carlson et al, IEC 38, Jan 1948 and following years under Unit Operations "Absorption and Humidification" 2)Kirk Othmer 1(1947)14—32(25 references) 3)Perry (1950),667-711 4)G.G.Brown, Edit/ Unit Operations Wiley,NY(1950) 5)T.K.Sherwood R.L. Pigford, "Absorption and Extraction, McGraw-Hill,NY(1952),pp 115-390... 

Refs 1)H.C.Carlson et al, lEC 38, Jan 1948 and following years under Unit Operations "Absorption and Humidification 2)Kirk Ochmer 1(1947)14-32(25 references) 3)Perry... 

Packed column Countercurrent, cocurrent Differential Liquid and/or gas Distillation, absorption, stripping, humidification, dehumidification, DCHT, washing... 

The transfer of mass from one phase to another is involved in the operations of distillation, absorption, extraction, humidification, adsorption, drying, and crystallization. The principal function of the equipment used for these operations is to permit efficient contact between the phases. Many special types of equipment have been developed that are particularly applicable for use with a given operation, but finite-stage contactors and continuous contactors are the types most commonly encountered. A major part of this chapter, therefore, is devoted to the design aspects and costs of stagewise plate contactors and continuous packed contactors. 

Packed columns are often used for distillation, liquid-liquid extraction, and humidification as well as for gas absorption. The design can be based on overall transfer coefficients or on the number of transfer units and the height of a transfer unit. For distillation or humidification, where the gas phase is continuous and the liquid flows in rivulets over the packing, the mass-transfer coefficients and flooding characteristics are similar to those for gas absorption, and the same generalized correlations would apply. 

Humidification and dehumidification involve the transfer of material between a pure liquid phase and a fixed gas that is insoluble in the liquid. These operations are somewhat simpler than those for absorption and stripping, for when the liquid contains only one component, there are no concentration gradients and no resistance to transfer in the liquid phase. On the other hand, both heat transfer and mass transfer are important and influence one another. In previous chapters they have been treated separately here and in drying of solids (discussed in Chap. 24) they occur together, and concentration and temperature change simultaneously. 

The operations which include humidification and dehumidification, gas absorption and desorption, and distillation all have in common the requirement that a gas and a liquid phase be brought into contact for the purpose of diffusional interchange between them. The equipment for gas-liquid contact can be broadly classified according to whether its principal action is to disperse the gas or the liquid, although in many devices both phases become dispersed. In principle, at least, any type of equipment satisfactory for one of these operations is suitable for the others, and the major types are indeed used for all. For this reason, the main emphasis of this chapter is on equipment for gas-liquid operations. 

Selection of equipment—many equipment design problems require trial and error or complex mathematical solution examples are absorption, distillation, humidification—dehumidification, evaporation, extraction, fluid flow, and heat exchange (Chap. 4). 

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. 

Contacting Columns. A tall cylindrical column or tower can be filled with packing for continuous contact of two or more phases or fabricated with a number of trays at fixed distances apart for stagewise contact operation. Such columns, either alone or in series, are commonly specified for separations-in gas absorption, distillation, extraction, and humidification. 

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 operations which include humidification and dehuroidification, gas absorption and desorption, and distillation in its various forms all have in common the requirement that a gas and a liquid phase be brought into contact for the purpose of a diffusional interchange between them. 

Direct-contac t condensers involve the simultaneous transfer of heat and mass. Design procedures available for absorption, humidification, cooling towers, and the like may be apphed with some modifications. 

Gas absorption/desorption, stripping, scrubbing Humidification and water cooling Dehumidification and air conditioning Drying Adsorption... 

Distillation and gas absorption are the prime and most common gas-liquid mass-transfer operations. Other operations that are often performed in similar equipment include stripping (often considered part of distillation), direct-contact heat transfer, flashing, washing, humidification, and dehumidification. 

The nose is equipped with a unique cellular architecture to perform several functions, including filtration of inspired particles, humidification of inspired air, olfaction, and some immunological functions [18,19], The nose is not specifically designed for nutrient or peptide drug absorption. However, the large absorptive capacity of the nasal epithelium has now been fully appreciated because of the extremely high bioavailability of nasally applied peptide drugs observed under certain experimental conditions (described below). 

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