Air dehumidification

Figure 264. Air Dehumidification and energy storage capacity in an ideal absorption process as a function of the air to solution mass ratio (cooling temperature 24°C, inlet humidity ratio 14.5 g/kg, LICL-H20 solution)...

In summer the ventilation air has to be dehumidified to keep the required comfort and to prevent from condensation at cold ceilings. The air dehumidification is done by a liquid desiccant dehumidification and cooling system, sketched in Figure 271. Warm and humid outside air is cooled and dried in a special dehumidifier by a concentrated Lithium Chloride salt solution (LiCl-H20) before it is blown into the atrium of the building. From there several air handling units draw the air into the offices and provide additional cooling on demand. 

Figure 12-8.3. Schematic drawing of (a) basic sorption dehumidification process and (b) compressor-assisted air dehumidification process.

The design employs filters for particulate removal, a desiccant-VOCs adsorbent wheel for simultaneous air dehumidification and purification. The catalyst was coated on a separate sleeve directly across the hot air stream. This design eliminates the need for separate wheels and is both cheaper and simpler to manufacture. The higher temperature and pollutant concentration in the regeneration stream mean higher catalyst activity. However, the small flow rate and high moisture content of the regeneration stream can result in a poorer overall performance. 

Flat-membrane contactors have been specifically designed and commercialized by GVS SpA (Italy) for air dehumidification processes [23]. 

In Table 22.1 the characteristics of a membrane contactor housing 1 m2 of flat membrane and used for the demonstration tests related to air dehumidification are... 

Air Dehumidification Results of Demonstration Tests with Refrigerated Storage Cells 505... 

Wagner B, Jehle W, Steinwandel J, Staneff T, and Herczog J, Continuous membrane supported cabin air dehumidification process. Euromembrane 97, University of Twente, Enschede, The Netherlands, June 26, 1997. 

Isetti C, Nannei E, and Magrini A. On the application of a membrane air-hquid contactor for air dehumidification. Energ. Build. 1997 25 185-193. 

As many processes require both hot and cold drying, frost on the evaporator surface builds up rapidly in low-temperature applications. Once frosting occurs, heat transfer between the evaporator and drying air decreases dramatically due to the additional resistance to conductive heat transfer. Some research has been initiated to study the ice build-up process and its influence on air dehumidification with the objective of eventually coming up with better mechanisms to reduce the build-up rates (Sanders, 1985). 

M.A. Grolmes and M. Epstein, Property and Composition Effects on the Cyclic Performance of Fixed Bed Desiccant Air Dehumidification, ASME/AIChE National Heat Transfer Conference, Denver, CO, August,... 

The main applications of PSA are to be found in the production of oxygen from air, dehumidification of gases and purification of hydrogen. Other applications include removal of carbon dioxide, recovery of radioactive waste gas, enrichment recovery of rare gases, purification of helium, purification of natural gases, separation of isomers and separation of carbon monoxide. Separation of iso-parafllns from normal paraffins is accomplished by using a shape-selective adsorbent such as a molecular sieve. Separation of carbon monoxide involves chemical adsorption on complex adsorbents. 

If the liquid phase is a pure liquid containing but one component while the gas contains two or more, the operation is humidification or dehumidification, depending upon the direction of transfer (this is the exception mentioned earlier). For example, contact of dry air with liquid water results in evaporation of some water into the air (humidification of the air). Conversely, contact of very moist air with pure liquid water may result in condensation of part of the moisture in the air (dehumidification). In both cases, diffusion of water vapor through air is involved, and we include these among the mass-transfer operations. 

The heal of water absorption is of importance in low-pressure gas dehydration and air dehumidification. In such applications, it is common practice to use cooling coils in the absorption zone to remove heat which is liberated. In units used for air conditioning dehumidification, the cooling coils may also serve as packing in the air-glycol contact zone as illustrated in Figure 11-13. 

Figure 11-53. Effect of air velocity on heat transfer and absorption coefficients for air dehumIdIfication with lithium chloride solutions. Column is equipped with a rotating distributor and 2-in. clay raschig-ring packing. Packed height 1.33 ft, cross-sectional area 2.4 sq ft, packing surface area 27 sq ft/cu ft. Data of Bichowsky and Kelley (1935)...

Figure 11-55. Flow arrangement and design specifications for air-dehumidification plant employing lithium chloride solution. Anon., 1951)...

Separation of a gas from the gas mixture is a key issue in the various industrial fields hydrogen recovery in petroleum refinery process, oxygen removal to prevent flame, air dehumidification to prevent moisture absorption, dehydration of fine chemical products, for example. Although there are various methods to perform gas separation [membrane separation, pressure swing adsorption (PSA) separation, cryogenic separation], the method using membranes attracts much attention. 

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