ASHRAE FUNDAMENTALS IP CH 32-2017 Sorbents and Desiccants.pdf

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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 weight in water vapor, and nylon can tak

4、e upalmost 6% of its weight in water. In contrast, a commercial desiccanttakes up between 10 and 1100% of its dry weight 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

5、air is 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 between120 and 400F and exposing it to a

6、 scavenger airstream. After thedesiccant dries, it must be cooled so that it can attract moisture onceagain. Sorption always generates sensible heat equal to the latentheat of the water vapor taken up by the desiccant plus an additionalheat of sorption that varies between 5 and 25% of the latent hea

7、t ofthe water vapor. This heat is transferred to the desiccant and to thesurrounding air.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

8、 desiccant, except by addition of the weight 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.1. DESICCANT APPLICATI

9、ONSDesiccants 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 ch

10、illing it 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 subfre

11、ezing temperaturesAir 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

12、desiccants 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 1

13、974).Hines 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.

14、Desiccants 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

15、used to 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 i

16、ts limit 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 thi

17、s volume; Chapters 7, 8, 18, 39, and 44 of the 2014ASHRAE HandbookRefrigeration; Chapters 1, 2, 6, 10, 18, 20, 23,30, and 46 of the 2015 ASHRAE HandbookHVAC Applications;and Chapters 24 and 26 of the 2016 ASHRAE HandbookHVACSystems and Equipment.2. DESICCANT CYCLEPractically speaking, all desiccants

18、 function the 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 highertha

19、n that of the 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 desic

20、cant surface 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 2017 ASHRAE HandbookFundamentals either direction until some external force changes t

21、he vapor 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

22、 the desic-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

23、illustrated 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

24、in regenerating 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 airNece

25、ssary to vaporize the moisture it contains (about 1060 Btu/lb)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 desic

26、cant to 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 t

27、he mate-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 t

28、o the moist 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 70F to dew point and the corresponding vaporpressure. T

29、he greater the difference between the air and desiccantsurface vapor pressures, the greater the ability of the material toabsorb moisture from the air at that moisture content.Fig. 1 Desiccant Water Vapor Pressure as Function of Moisture Content(Harriman 2003)Fig. 2 Desiccant Water Vapor Pressure as

30、 Function of Desiccant Moisture Content and Temperature(Harriman 2003)Table 1 Vapor Pressures and Dew-Point Temperatures Corresponding to Different Relative Humidities at 70FRelative Humidity at 70F, %Dew Point, FVapor Pressure,in. Hg10 12 0.0720 28 0.1530 37 0.2240 45 0.3050 51 0.3760 55 0.4470 60

31、0.5280 64 0.5990 67 0.67100 70 0.74Fig. 3 Desiccant Cycle(Harriman 2003)Sorbents and Desiccants 32.3The 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 anddesorpt

32、ion 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 are di

33、scussedin Chapter 24 of the 2016 ASHRAE HandbookHVAC Systemsand Equipment.3. 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 absorp

34、tion 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 abs

35、orption 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 directly

36、 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 trieth-ylene glycol, a commercial

37、 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-tio

38、n 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 va

39、por pressuresof various solutions of water and lithium chloride (LiCl), anothercommon liquid desiccant. A solution that is 25% lithium chloridehas a vapor pressure of 0.37 in. Hg at a temperature of 70F. If thesame 25% solution is heated to 100F, its vapor pressure more thandoubles to 0.99 in. Hg. E

40、xpressed another way, the 70F, 25% solu-tion is in equilibrium with air at a 51F dew point. The same 25%solution at 100F is at equilibrium with an airstream at a 79F dewpoint. The warmer the desiccant, the less moisture it can attract fromthe air.In standard practice, behavior of a liquid desiccant

41、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 h

42、ave 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, app

43、roximately 26 water molecules are attachedto each molecule of LiCl. This represents a water absorption ofmore than 1000% on a dry-weight 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 2017 ASHRAE HandbookFundamentals Solid AdsorbentsAdsorbents are solid materials with a tremendous internal surfacearea per unit of mass; a single gram can have more than 50,000 ft2ofs

46、urface 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 indi

48、vidual 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 opp

49、osite 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 weight in water when the air is at 70F and20% rh. In contrast, LiCl can hold 130%