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266 CHAPTER 17 1965 Guide And Data Book percent relative humidity. This single curve can;normally be used fur temperatures ranging from 0 F to approxi mately 180 F. fig. 7 contains equilibrium data for several common desiccants plotted on the relative humidity basis. Silica gel and alumina gel are so-called kigh<apacity desiccants because of their large equilibrium capacities at high relative humidities. It should be remembered, however, that the ca pacities given in figs. 6.and 7 are equilibrium values and should not be used for the design of dynamic systems. Under certain operating conditions; low-capacity desiccants actually outperform high-capacity desiccants. The process of adsorption by solid desiccants is reversible. If tiie vapor pressure of the adsorbed water becomes greater than the partial pressure of the vapor in the surrounding atmos phere, water will be released by the adsorbent. In most cases, this desorption is accomplished by the application of heat to drive off the adsorbate. Reactivation temperatures for com mon desiccants are generally within the range 2(X) to 500 F. Since no physical or chemical change occurs to the adsorbent in this process, it is again ready to extract water from a wet gas after the so-called reactivation. It is this property of re versibility which makes adsorption an economical process. Most solid adsorbents are able to operate for thousands of adsorption-reactivation cycles. It is not necessary to apply heat to reactivate solid desic cants, since these materials will come to equilibrium with the relative humidity, of the gas stream with which they are in contact. The new heailesa dryers use this system for reactiva tion. After the adsorbent becomes wet in the drying opera tion, a portion of the dried gas is expanded and used for reac tivation. This expanded gas will have a very low relative hu midity and moisture will be removed from the desiccant in order to approach a new equilibrium. This type of reactivation is usually restricted to units operating at pressures greater than 60 psig. A suitable vacuum can also be used to reactivate solid adsorbents. In order to be satisfactory for commercial dehumidification purposes, an adsorbent should have. the following .charac teristics: -1. High adsorptive capacity. 2. Chemical stability, resisting contamination from impurities. 3. Physical ruggedness to resist breakdown from handling and use. 4. Capability of reactivation at temperatures generally, ob tainable. 5. Heat stability at reactivation temperatures. 6. Relatively high density to avoid excessive bulk. 7. Reasonable cost. ' A number of adsorbents are available for use in-solid ad sorption systems, including activated aluminas and bauxites, silica gels, and alumino-silicates marketed under various trade namea. The reader is referred to the various manufac turers for detailed information regarding performance of the various desiccants. SOLID ADSORPTION SYSTEMS Dehumidification by a solid desiccant such as silica gel or activated alumina may be performed under either static or dynamic operation. In the static method, there is.no forced .circulation of the gas tobe dried into, or through, the desic cant. Instead, the air immediately surrounding the adsorbent is initially dried and subsequently, through convection , and diffusion,.water vapor from spaces and objects farther away pass into the air surrounding the desiccant and then to the .desiccant where it is adsorbed. Since considerable tlnw may be required for the air and the desiccant to establish equilib rium, this type of dehumidification is best suited for small containers. This method was used extensively by the armed services during World War II to preserve overseas shipments and domestic storage. Some foods sensitive to humidity such as potato chips, are now packaged with a small container of solid adsorbent to retain freshness. On the other hand, dynamic dehumidification is operated with forced passage of the air being .treated through the desiccant bed. This method is used in installations such as de humidified warehouses'and'ships of the "Mothball fleet," where large volumes of air must be dried. . The only requisites for a dynamic dehumidifier are a desic cant bed, a fan to force the humid air through this bed, and a heater or other means to periodically reactivate the adsorbent. As the air passes into the activated desiccant, it surrenders a certain amount of. its water vapor. The rate' of moisture pickup and the humidity condition of the leaving air are functions of a great many variables, some of which will be discussed later. The ratio of the amount of water adsorbed by the desiccant in a given time'to the amount of water vapor in the air entering tire desiccant bed during that time is known as adsorption efficiency. A characteristic of adsorbents in dynamic use is that this adsorption efficiency remains con stant and at a relatively high level from the beginning of an adsorption cycle until some later point in the cycle at which time the efficiency begins to drop. This point is known as tie breakpoint, and the amount of water adsorbed on the desiccant at this point is known as the breakpoint capacity. In the ideal case the breakpoint capacity would coincide with the equi librium capacity. In actual operation, however, breakpoint capacity can be a small proportion of the equilibrium capacity, depending on operating conditions. For isothermal operation with low flow rates and deep beds, breakpoint capacity will approach equilibrium capacity. High inlet temperature and humidity, small bed depths, and high flow rates will all tend to decrease the breakpoint capacity. Although additional drying can be effected beyond the breakpoint, good commer cial practice.requires that the desiccant be regenerated at or near this point. Adsorptioa carried beyond the breakpoint continues at an increasingly slower rate until the adsorbent is completely saturated. This point is known as completion. When regeneration of the adsorbent is desired, the heater is energised and the direction of air flow through the bed is usually reversed. The dry-bulb temperature of the effluent air rises rapidly at first, and then virtually levels off for a period of time. This period of level or slowly increasing temperature represents the period during which the major portion of the heat input is being used to boil off the adsorbed water. This temperature pattern continues until most of the water con tained in or on the desiccant is released.When the latent heat requirements begin to diminish, the heat input goes into sensi ble heat gain to the passing air stream. This is reflected in a rather sharp increase in-the. dry-bulb temperature of the effluent air. This period, measured from the beginning of de sorption, has been designated temperature-rise time. Although additional regeneration (at a slower rate) can be attained by continuing the heat addition process beyond the temperaturerise time, once again commercial practice calls for reactivation to be ended near this point unless very dry effluent air is re quired in .the following adsorption cycle. Regeneration past this point until the adsorbent.is in moisture equilibrium with the air stream is known as complete desorption or desorption to completion. The energy expended in the heater per unit weight of water desorbed for any. given time is called economy of desorption and usually is expressed in kilowatt hours per pound of water desorbed. During the process of adsorption there is a liberation of heat which results in an elevation of the effluent air tempera- Dehumidification By Sorbent Materials 267 This heat is equivalent to the latent heat of.vaporisation oTtbe adsorbed liquid -plus an added'quantity.knqwn as the /teat of xoetting, which is defined as the heat developed when the liquid and the solid surface contact one another. The heat of wetting is relatively large when adsorbing the first, water molecules on a freshly reactivated desiccant and tapers off to a very Tow value as the desiccant approaches saturation. As the adsorbed vapor condenses, the latent heat,is converted to sensible. All of the released heat, known cumulatively as the heat of adsorption,' is dissipated into the desiccant, the enclosure, and the passing air stream.. In comfort air con ditioning it is often necessary to cool theeffluent air prior to its introduction to the conditioned space, but in most other dehumidification applications this heat is not-objectionable, in which no provisions-arc made for its removal. An operation of this type is called adiabatic/ meaning that the released heat is taken'up for the most part by the'passing air stream, that the adsorbent chamber is insulated, and that ho attempt is made to cool either the adsorbent or the effluent air. Although ordinary air-conditioning dehumidification only approaches the adiabatic process, it is called adiabatic to differentiate it. from isothermal adsorption' in which the bed is cooled by cooling coils. Even then only part of-the heat of adsorption-can be removed. Under :trne -adiabatic condi tions at atmospheric pressure, temperature in the desiccant bed can rise as high as 245 F when drying air oMugh moisture content.* As would be expected, the useful concentration of the ad sorbent will be quite low for this type of operation and, consequently, the volumetric heat capacity of the adsorbent will be the controlling factor for design. Some of the many variables that influence the results of a dynamic dehumidification operation are as follows: A. Variables concerning the desiccant bed: L Typo of desiccant. 2. Dry weight of desiccant. 3. Particle size. 4. Bulk density. 6. Shape of bed. 6. Area of bed normal to gas flow. 7. Depth of bed. 8. Packing of the desiccant in the bed. 9. Pressure drop through bed. B. Variables concerning the gas to be dried: 1. Flow rate. 2. Temperature. 3. Moisture content. 4. Pressure. 5. Contact time between and adsorbent; a function of inlet face velocity and bed depth. C. Variables concerning reactivation: 1. Reactivation temperature. S*** magnitude of heat supply. 3. Heat storage capacity of the bed. 4. Temperature gradient of the bed. 5. Amount of insulation. 6. Amount of Bweep g**. D. Miscellaneous: 1- Cycle time. 2. Leakage from the apparatus. Solid adsorption dehumidifiers are usually of the stationary dual-bed type in which one bed is adsorbing while the other being reactivated. Some air or gas flow should be provided during the reactivation cycle to sweep out the moisture which released by the heat. Fig. 8 shows a schematic flow diagram for a typical dual-bed dehumidification unit using heat for reactivation. Moist air is passing through Bed A from top to bottom while Bed B is being reactivated. When the effluent air from Bed A is no longer dry, the flow to the two towers is fwitehed so that Bed B goes on the drying cycle while Bed A is reactivated. Cycle time is normally specified for the unit by -the manufacturer and is controlled by a timer. Larger units may have adjustable time cycles that can be changed for various operating conditions. Still other units are operated from either manual or automatic readings of the effluent moistture content: 'Many variations in the ample design given in Fig. 8 are available from (he various manufacturers of dual-bed units. Valves may be one-way, two-way or four-way. Flow of air may be up' or down in either the drying or the reactivation cycle. 'Beds may be vertical, horizontal or radial, the'latter to give a larger cross-sectional- area for large volumes of air'to be dried. Reactivation can be by steam, electricity, or gas, or by the newer heatless methods -previously mentioned. Some manufacturers install cooling coils in the adsorbent beds to Fig. 8 .... Typical Duat-Bed Dehumidification Unit remove some of the heat of adsorption while others divide the adsorbent beds and install coolers between the sections to accomplish this purpose. Beds may be relatively shallow for partial drying, or deep for complete drying. Relatively low dew points can be obtained in a closed room with a machine having a thin desiccant bed by recirculating the room air through the machine a number of times, pro gressively lowering the room dew point as increasingly drier air enters the machine. The machine capacity must be greater than the internal moisture load and the infiltration of mois ture from external sources. Deep desiccant beds-are used when extreme dryness is required in one pass through the dehumidifier, and the air is not recirculated. Beds as deep as 15 ft have been used in some large units. Such beds can pro duce effluent dew points below --100 F in continuous opera tion. Typical performance data for dry desiccant units will be found in Chapter 85 of the 1964 Guide And Data Book. SORPTION DEHUMIDIFIERS FOR ELEVATED PRESSURES The same sorption principles which pertain to low-pressure (approximately atmospheric) dehumidification- apply to drying `of high-pressure air, process, or other gases. The