1、32.1CHAPTER 32SORBENTS AND DESICCANTSDesiccant Applications 32.1Desiccant Cycle. 32.1Types of Desiccants. 32.3Desiccant Isotherms 32.5Desiccant Life 32.5Cosorption of Water Vapor and Indoor Air Contaminants 32.5ORPTION refers to the binding of one substance to another.SSorbents are materials that ha
2、ve an ability to attract and holdother gases or liquids. They can be used to attract gases or liquidsother than water vapor, which makes them very useful in chemicalseparation processes. Desiccants are a subset of sorbents; they havea particular affinity for water.Virtually all materials are desicca
3、nts; that is, they attract and holdwater vapor. Wood, natural fibers, clays, and many synthetic mate-rials attract and release moisture as commercial desiccants do, butthey lack holding capacity. For example, woolen carpet fibers attractup to 23% of their dry mass in water vapor, and nylon can take
4、upalmost 6% of its mass in water. In contrast, a commercial desiccanttakes up between 10 and 1100% of its dry mass in water vapor,depending on its type and on the moisture available in the environ-ment. Furthermore, commercial desiccants continue to attract mois-ture even when the surrounding air is
5、 quite dry, a characteristic thatother materials do not share.All desiccants behave in a similar way: they attract moisture untilthey reach equilibrium with the surrounding air. Moisture is usuallyremoved from the desiccant by heating it to temperatures between 50and 205C and exposing it to a scaven
6、ger airstream. After the desic-cant dries, it must be cooled so that it can attract moisture once again.Sorption always generates sensible heat equal to the latent heat of thewater vapor taken up by the desiccant plus an additional heat of sorp-tion that varies between 5 and 25% of the latent heat o
7、f the watervapor. This heat is transferred to the desiccant and to the surroundingair.The process of attracting and holding moisture is described aseither adsorption or absorption, depending on whether the desiccantundergoes a chemical change as it takes on moisture. Adsorptiondoes not change the de
8、siccant, except by addition of the mass ofwater vapor; it is similar in some ways to a sponge soaking up water.Absorption, on the other hand, changes the desiccant. An exampleof an absorbent is lithium chloride, which changes from a solid to aliquid as it absorbs moisture.DESICCANT APPLICATIONSDesic
9、cants can dry either liquids or gases, including ambient air,and are used in many air-conditioning applications, particularlywhen theLatent load is large in comparison to the sensible loadEnergy cost to regenerate the desiccant is low compared to the costof energy to dehumidify the air by chilling i
10、t below its dew pointand reheating itMoisture control level for the space would require chilling the airto subfreezing dew points if compression refrigeration alone wereused to dehumidify the airTemperature control level for the space or process requires contin-uous delivery of air at subfreezing te
11、mperaturesAir delivered to a space or ductwork must be at less than 70% rhIn any of these situations, the cost of running a vapor compressioncooling system can be very high. A desiccant process may offer con-siderable advantages in energy, initial cost of equipment, and main-tenance.Because desiccan
12、ts can attract and hold more than simply watervapor, they can remove contaminants from airstreams to improveindoor air quality. Desiccants have been used to remove organicvapors and, in special circumstances, to control microbiologicalcontaminants (Battelle 1971; Buffalo Testing Laboratory 1974).Hin
13、es et al. (1991) also confirmed their usefulness in removingvapors that can degrade indoor air quality. Desiccant materials canadsorb hydrocarbon vapors while collecting moisture from air.These cosorption phenomena show promise of improving indoor airquality in typical building HVAC systems.Desiccan
14、ts are also used in drying compressed air to low dewpoints. In this application, moisture can be removed from the desic-cant without heat. Desorption uses differences in vapor pressurescompared to the total pressures of the compressed and ambient pres-sure airstreams.Finally, desiccants are used to
15、dry the refrigerant circulating inair-conditioning and refrigeration systems. This reduces corrosionin refrigerant piping and prevents valves and capillaries from be-coming clogged with ice crystals. In this application, the desiccantis not regenerated; it is discarded when it has adsorbed its limit
16、 ofwater vapor.This chapter discusses the water sorption characteristics of des-iccant materials and explains some of the implications of those char-acteristics in ambient pressure air-conditioning applications.Information on other applications for desiccants can be found inChapter 36 of this volume
17、; Chapters 7, 8, 18, 39, and 44 of the 2010ASHRAE HandbookRefrigeration; Chapters 1, 2, 6, 10, 18, 20, 23,30, and 46 of the 2011 ASHRAE HandbookHVAC Applications;and Chapters 24 and 26 of the 2012 ASHRAE HandbookHVACSystems and Equipment.DESICCANT CYCLEPractically speaking, all desiccants function t
18、he same way: bymoisture transfer caused by a difference between water vapor pres-sures at their surface and of the surrounding air. When the vaporpressure at the desiccant surface is lower than that of the air, the des-iccant attracts moisture. When the surface vapor pressure is higherthan that of t
19、he surrounding air, the desiccant releases moisture.Figure 1 shows the moisture content relationship between a des-iccant and its surface vapor pressure. As the desiccants moisturecontent rises, so does the water vapor pressure at its surface. At somepoint, the vapor pressure at the desiccant surfac
20、e is the same as thatof the air: the two are in equilibrium. Then, moisture cannot move inThe preparation of this chapter is assigned to TC 8.12, Desiccant Dehumid-ification Equipment and Components.32.2 2013 ASHRAE HandbookFundamentals (SI)either direction until some external force changes the vapo
21、r pressureat the desiccant or in the air.Figure 2 shows the effect of temperature on vapor pressure at thedesiccant surface. Both higher temperature and increased moisturecontent increase surface vapor pressure. When surface vapor pres-sure exceeds that of the surrounding air, moisture leaves the de
22、sic-cant (reactivation or regeneration). After the desiccant is dried(reactivated) by the heat, its vapor pressure remains high, so it hasvery little ability to absorb moisture. Cooling the desiccant reducesits surface vapor pressure so that it can absorb moisture again. Thecomplete cycle is illustr
23、ated in Figure 3.The economics of desiccant operation depend on the energy costof moving a given material through this cycle. Dehumidifying air(loading the desiccant with water vapor) generally proceeds withoutenergy input other than fan and pump costs. The major portion ofenergy is invested in rege
24、nerating the desiccant (moving from point2 to point 3) and cooling the desiccant (point 3 to point 1).Regeneration energy is equal to the sum of the heatNecessary to raise the desiccant to a temperature high enough tomake its surface vapor pressure higher than that of the surround-ing airNecessary t
25、o vaporize the moisture it contains (about 2465 kJ/kg)From desorption of water from the desiccant (a small amount)The cooling energy is proportional to the (1) mass of desiccantcycled and (2) difference between its temperature after regenerationand the lower temperature that allows the desiccant to
26、remove waterfrom the airstream again.The cycle is similar when desiccants are regenerated using pres-sure differences in a compressed air application. The desiccant issaturated in a high-pressure chamber (i.e., that of the compressedair). Then valves open, isolating the compressed air from the mate-
27、rial, and the desiccant is exposed to air at ambient pressure. The sat-urated desiccants vapor pressure is much higher than ambient air atnormal pressures; thus, moisture leaves the desiccant for the sur-rounding air. An alternative desorption strategy returns a small por-tion of dried air to the mo
28、ist desiccant bed to reabsorb moisture, thenvents that moist air to the atmosphere.Table 1 shows the range of vapor pressures over which the desic-cant must operate in space-conditioning applications. It converts therelative humidity at 21C to dew point and the corresponding vaporpressure. The great
29、er the difference between the air and desiccantFig. 1 Desiccant Water Vapor Pressure as Function of Moisture Content(Harriman 2003)Fig. 2 Desiccant Water Vapor Pressure as Function of Desiccant Moisture Content and Temperature(Harriman 2003)Table 1 Vapor Pressures and Dew-Point Temperatures Correspo
30、nding to Different Relative Humidities at 21CRelative Humidity at 21C, %Dew Point,CVapor Pressure,kPa10 10.5 0.2520 2.5 0.5030 2.8 0.7540 6.9 1.0050 10.2 1.2460 13.0 1.4970 15.3 1.7480 17.4 1.9990 19.3 2.24100 21.0 2.49Fig. 3 Desiccant Cycle(Harriman 2003)Sorbents and Desiccants 32.3surface vapor pr
31、essures, the greater the ability of the material toabsorb moisture from the air at that moisture content.The ideal desiccant for a particular application depends on therange of water vapor pressures likely to occur in the air, tempera-ture of the regeneration heat source, and moisture sorption andde
32、sorption characteristics of the desiccant within those con-straints. In commercial practice, however, most desiccants can bemade to perform well in a wide variety of operating situationsthrough careful engineering of mechanical aspects of the dehu-midification system. Some of these hardware issues a
33、re discussedin Chapter 24 of the 2012 ASHRAE HandbookHVAC Systemsand Equipment.TYPES OF DESICCANTSDesiccants can be liquids or solids and can hold moisturethrough absorption or adsorption, as described earlier. Most absor-bents are liquids, and most adsorbents are solids.Liquid AbsorbentsLiquid abso
34、rption dehumidification can best be illustrated bycomparison to air washer operation. When air passes through an airwasher, its dew point approaches the temperature of water suppliedto the machine. Air that is more humid is dehumidified, and air thatis less humid is humidified. Similarly, a liquid a
35、bsorption dehumid-ifier brings air into contact with a liquid desiccant solution. The liq-uids vapor pressure is lower than water at the same temperature,and air passing over the solution approaches this reduced vaporpressure; it is dehumidified.A liquid absorption solutions vapor pressure is direct
36、ly propor-tional to its temperature and inversely proportional to its concen-tration. Figure 4 illustrates the effect of increasing desiccantconcentration on the water vapor pressure at its surface. The figureshows the vapor pressures of various solutions of water and triethyl-ene glycol, a commerci
37、al liquid desiccant. As the mixtures glycolcontent increases, its vapor pressure decreases. This lower pressureallows the glycol solution to absorb moisture from the air wheneverthe airs vapor pressure is greater than that of the solution.Viewed another way, the vapor pressure of a given concentra-t
38、ion of absorbent solution approximates the vapor pressure valuesof a fixed relative humidity line on a psychrometric chart. Highersolution concentrations give lower equilibrium relative humidities,which allow the absorbent to dry air to lower levels.Figure 5 illustrates the effect of temperature on
39、vapor pressures ofvarious solutions of water and lithium chloride (LiCl), another com-mon liquid desiccant. A solution that is 25% lithium chloride has avapor pressure of 1.25 kPa at a temperature of 21C. If the same 25%solution is heated to 37.8C, its vapor pressure more than doubles to3.3 kPa. Exp
40、ressed another way, the 21C, 25% solution is in equi-librium with air at a 10.5C dew point. The same 25% solution at37.8C is at equilibrium with an airstream at a 26C dew point. Thewarmer the desiccant, the less moisture it can attract from the air.In standard practice, behavior of a liquid desiccan
41、t is controlledby adjusting its temperature, concentration, or both. Desiccant tem-perature is controlled by simple heaters and coolers. Concentrationis controlled by heating the desiccant to drive moisture out into awaste airstream or directly to the ambient.Commercially available liquid desiccants
42、 have an especiallyhigh water-holding capacity. Each molecule of LiCl, for example,can hold two water molecules, even in the dry state. Above twowater molecules per molecule of LiCl, the desiccant becomes a liq-uid and continues to absorb water. If the solution is in equilibriumwith air at 90% rh, a
43、pproximately 26 water molecules are attachedto each molecule of LiCl. This represents a water absorption ofmore than 1000% on a dry-mass basis.As a practical matter, however, the absorption process is limitedby the exposed surface area of desiccant and by the contact timeallowed for reaction. More s
44、urface area and more contact time allowthe desiccant to approach its theoretical capacity. Commercial des-iccant systems stretch these limits by flowing liquid desiccant ontoan extended surface, much like in a cooling tower.Fig. 4 Surface Vapor Pressure of Water/Triethylene Glycol Solutions(from dat
45、a of Dow 1981)Fig. 5 Surface Vapor Pressure of Water/Lithium Chloride Solutions(from data of Foote Mineral 1988)32.4 2013 ASHRAE HandbookFundamentals (SI)Solid AdsorbentsAdsorbents are solid materials with a tremendous internal surfacearea per unit of mass; a single gram can have more than 4600 m2of
46、surface area. Structurally, adsorbents resemble a rigid sponge, and thesurface of the sponge in turn resembles the ocean coastline of a fjord.This analogy indicates the scale of the different surfaces in an adsor-bent. The fjords can be compared to the capillaries in the adsorbent.Spaces between the
47、 grains of sand on the fjord beaches can be com-pared to the spaces between the individual molecules of adsorbent, allof which have the capacity to hold water molecules. The bulk ofadsorbed water is contained by condensation into the capillaries, andthe majority of the surface area that attracts ind
48、ividual water mole-cules is in the crystalline structure of the material itself.Adsorbents attract moisture because of the electrical field at thedesiccant surface. The field is not uniform in either force or charge,so specific sites on the desiccant surface attract water molecules thathave a net op
49、posite charge. When the complete surface is covered,the adsorbent can hold still more moisture because vapor condensesinto the first water layer and fills capillaries throughout the material.As with liquid absorbents, an adsorbents ability to attract moisturedepends on the difference in vapor pressure between its surface andthe air.Capacity of solid adsorbents is generally less than the capacity ofliquid absorbents. For example, a typical molecular sieve adsorbentcan hold 17% of its dry mass in water when the air is at 21C and20% rh. In contrast, LiCl can hold 130%
