A PRELIMINARY EVALUATION of ALTERNATIVE LIQUID DESICCANTS for a HYBRID DESICCANT AIR CONDITIONER JOSEPH W. STUDAK Graduate Stude

ESL-HH-88-09-25

A PRELIMINARY EVALUATION OF ALTERNATIVE LIQUID DESICCANTS FOR A HYBRID DESICCANT AIR CONDITIONER

JOSEPH W. STUDAK Graduate Student JOHN L. PETERSON Research Associate Center for Energy Studies The University of Texas at Austin Austin. TeXms

This pnper presents the results of a 1 was initiated. This paper presents the results preliminary investigation at The University of of this investigation to identify the most Texas at Auatin of alternative liquid desiccants promising liquid desiccant from among the following for use in a hybrid desiccant air-conditioning candidates: lithium chloride (LiCl), lithium system in which a desiccant is circulated between bromide (LiBr), calcium chloride (CaCl2), and the evaporator and the condenser of a vapor- triethylene glycol (TEG). compression air conditioner. The liquid desiccants studied were lithium chloride, lithium bromide, calcium chloride, and triethylene glycol. Bach candidate desiccant was subjected to a screening Each candidate desiccant wae subjected to a process which weighed the merits of the desiccant screening process that weighs the merits of the in tema of selected characteristics. The best desiccant in terns of the characteristics shown in liquid desiccant for the anticipated application Table 1. A maximum weighting, or weighting factor, was found to be calcium chloride. was assigned to each characteristic depending upon the importance of the characteristic to the ------INTRODUCTION researchers. An actual value, or relative weight, is assigned depending upon an objective figure of Desiccant dehumidification air-conditioning merit for the characteristics being considered, systema have excellent potential for coet-effective e.g., LD50 for safety. In the final analysis, the application in commercial buildings located in hot, product of weighting factors and relative weight humid climates. Hybrid liquid desiccant/vapor- are summed for each characteristic and the compression air-conditioning systems, in candidate with the highest total is selected aa the particular, are promising because they take best desiccant for the anticipated application. , advantage of the high efficiency for heat transfer A literature search was conducted to obtain inherent in vapor-compression systems and the high relevant data so that figures of merit could be mama transfer potential of the liquid desiccants. developed for the screening of candidate desiccant However, to be cost effective, hybrid systems muat eolutions. The figures of merit used are shown in View metadata,provide citation and operational similar papers savings at core.ac.uk over competing vapor- Table 1. The sources of the data are referencedbrought in to you by CORE compression air-conditioning systema and/or be less the following discussion. provided by Texas A&M University costly to manufacture. In the hybrid air conditioning system shown in Figure 1, a Table 1 Weighting Factors and Figures of Merit desiccant is circulated between the evaporator and condenser of a vapor- compression air conditioner Characteristic Weighting Figure of ~erit! providing both operational and first-cost savings. Factors I A preliminary evaluation of the thermodynamic ...... -- potential with a slight variation of the system Safety 1.0 Lethal dose (LDSO) sham in Figure 1 was conducted by Howell and Corroeion 0.8 Corrosion rate Peterson1. Considerable potential was shown for Mass transfer potential 0.8 Equilibrium vapor! saving energy and reducing peak electric demand pressure over conventional vapor-compression systems when Heat of mixing 0.6 Energy/lb water providing the same outlet air conditions. absorbed Continuing analytical work at The University Cost of desiccant 0.5 $/lo0 lbs eolution of Texas has shown potential first-coat savings Heat transfer potential 0.6 Thermal conductivity resulting in a 34% reduction in the exchange area Paraaitic power losses 0.3 Viscosity for both the evaporator and condenser and a 25% reduction in the required compressor capacity. Additionally, electric power drawn by the compressor is expected to drop by 25%, resulting in savings on electric utility bills. These savings SAFETY are possible without sacrificing cooling efficiency or thermal comfort. Since the liquid desiccants will be in direct Given the promising economics, a preliminary contact with conditioned air, it is important to investigation of alternative liquid desiccants for investigate any possible effects of ingestion, uae in the hybrid desiccant system shown in Figure inhalation, or skin contact. One source of

Proceedings of the Fifth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, September 12-14, 1988 ESL-HH-88-09-25

information is the Material Safety Data Sheet have similar behavior in terma of inhalation (MSDS) available for moat chemicals produced in the concerns. Air blown over solutions of lithium United States, whose form and content are chloride, lithium bromide, and calcium chloride designated by the Occupational Safety and Health contain no hazardous constituents; however, aalt Administration. The MSDS is provided by the solutions may irritate eyes or lungs if a mist is supplier of each chemical and contains information generated21317. Since lithium compounds do not such as physical data, fire and explosion hazard accumulate in the body, only chronic overexposure data, health hazard data, reactivity data, epill or to lithium would produce adverse side effectse. leak procedures, and special handling or protection Effective meana of removing entrained mist should information. Review of the MSDS for TEQ, LiCl, be included in any system using one of these CaC12, and LiBr indicates that none of these desiccants. chemicals poses a health hazard when used in the Skin contact with saline solutions or TBQ system to dehumidify air. There are adverse could cause minimal irritation. The possibility of effects such as nausea due to ingestion of these solutions drying out and leaving residue on the chemicals, but the desiccants are considered toxic equipment end diotribution ductwork is a small only in large amounts. problem because these materials readily absorb The candidate desiccants have toxicity ratings water, thereby maintaining the deaiccant in from moderate (LiC1) to low (CaC12) to relatively solution. nontoxic (TEG). One measure of toxicity is the ID50 dose, which is the lethal dose that will Table 2 Figures of Merit for Safety result in fatality 60% of the time. Lithium chloride has an LD50 of approximately 1.23 oz (35 ID50 Relative g) of the pure chemical for an average person when Desiccant (02) Weight taken orally2. Lithium bromide hee an LDSO for mice of 1.68g/kg, which would correspond to LiCl 1.23 7 approximately 4.03 oz (114 g) for an average LiBr 4.03 8 person3. The LD50 for calcium chloride is approximately the same as that for table salt, 5.30 oz (160 g)4. The LD6O for TEG is reported to be 52.7 oz (1,496 g) for an average person5. The next most likely method of ingestion of one of The saline desiccants are described aa these chemicals, after oral ingestion, is the odorless, while trietheylene glycol is described as inhalation of a mist containing one of these having a mild odor. chemicals. The possibility of inhalation of a mist CORROSION containing one of these chemicals could exist due to the direct exposure of a liquid desiccant to the Corrosive liquid desiccants should be avoided air stream. No evidence of adverse effects exists for long life and reliable operation of the hybrid for inhalation of TBQ vapors at nomal system. The saline solutions represent one family temperatures. Overexposure to TEG vapors generated of corrosion problem due to the presence of the at high temperaturee may result in respiratory electrolytic halide ions, end WG represents tract irritation and in the inhalation of harmful another. cm~untsof the materiale. The three salt solutions

SUPPLY AIR MIXED AIR OUTSlDE AIR oe. sar WB 1

F(EFRIGERAN~ d F(EFRIGERAN~ FLw

DE8CCANr FLOW

AIR FLOW

Figure 1 The Proposed Hybrid Vapor Compression/Liquid Desiccant Air Conditioning System

156

Proceedings of the Fifth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, September 12-14, 1988 ESL-HH-88-09-25

The open deeign configuration of the absorber achieved. Thua a potential for mama trans:fer unit allowa atmospheric oxygen and carbon dioxide exists to dehumidify air or to regenerate a to be dissolved into the deaiccant eolution aa it desiccant whenever a nonequilibrium condition i is flows over the abeorber. Dissolved oxygen and created. carbon dioxide increase the corrosion rates of A desirable candidate liquid desiccant most metals *. One method of corrosion control is solution is one that has eufficiently low the we of inhibitors in the molution. Uninhibited equilibriua vapor preaeure when sorbing and solutione of calcium chloride have surface sufficiently high equilibrium vapor preasure when corrosion raten in the 2 to 20 miln/year (mpy) being regenerated. Since the equilibrium vapor range for aluminum, steel, and copperlo. pressure of a deaiccant solution ie a function of Stainlees ateele are ueually considered poor concentration and temperature, either of these can choices for chloride envirormenteQ and are eeen be varied to improve the potential for mass to have corrosion rates greater than 20 npy and in transfer. However, for the hybrid system being some casee greater than 50 mpylo. A eatiafactory studied at The University of Texas, the rated corroeion rate would be leas than 2 mpy. A sorbing and desorbing temperatures are ,specified as manufacturer of lithium chloride dehumidifiers 410 F (50 C) and 1220 F (500 C), reepectively. reports that the use of hot-dipped galvanized steel Consequently, only eolution concentration can be components haa resulted in 25 years of useful varied independently to promote mass transfer service in these dehumidifiersl1. A aupplier of during operation at deeign conditions. lithium chemicale recommends the use of lithium The limiting concentration for any application chromate and lithium molibdate as inhibitors for of malt solutions is the crystallization point. copper/nickel alloys used in some dehumidifiers Thie maximum allowable concentration decreases with that apply lithium chloride eolution aa a temperature and, for the system under study, the desiccant. Lithium chromate ie also an effective temperature of interest ia 410F. A 5% inhibitor for LiBr aolutione, as eeen in one study concentration aafety margin is desired to ensure where the corroeion rate of plain carbon steel in that crystallization will not occur. an inhibited 64% LiBr eolution was 0.55 mpy, The maximum concentration for the desiccant approximately one-tenth of the uninhibited value of solutione, along with the concentration used for 5.7 mpylz. all property evaluations in this study, ie shown in Corrosion rates in TEQ solutione are low for Table 4. These concentration8 were specified in most metals, including aluminum, copper, and steel. the deeign to obtain an equilibrium vapor preesdre Reported raten of corrosion are leae than 2 mpy for of 0.13 inches (3 m) of mercury, i.e., the rated these materials1=. Copper used in TEG air- sorbing equilibrium vapor presaure of the hybrid conditioning syatems hes reported rates of system studied at The University of Texas at corrosion from 0.1 to 1 mpy at 3200F (1600C). Austin. Theee concentrations provide the requisite Aerated solutione of TEQ at room temperature are 5% aafety margin. reported to have ineignificant corroeion rates for copper1 Table 4 Maximum Concentration for Desiccant , Solutions at 410 F (50 C) Table 3 Figures of Merit for Corroaion Crystallization Point Maximum Concentration 1 Corrosion Rate Relative Desiccant (weight %) (weight %) Demiccant ~PY) Weight ------+- LiCl 42 15 25 I LiBr 57 16 40 CaClz 39 17 34

TEG N/A 85 I

Note that TEG ham no such cryatallizatibn point, although it does glass over at temperatures and concentrations outside those of the expected MASS TRANSFER POTENTIAL eystem operating envelope. Assuming the aolutiops are regenerated to the maximum concentration, the The property that moat clearly identifies the equilibrium vapor pressures associated with the potential of a desiccant eolution to remove water abeorber and regenerator temperaturea will indicate vapor from a process air stream is the equilibrium the potential for mass trannfer. These equilibrium vapor preeeure of water in eolution. When the vapor preenurea are ehown in Tablea 5 and 6. vapor preeaure of a solution in less than that of the air, water vapor will be sorbed into eolution. As more water vapor ie sorbed, the eolution is diluted and the vapor preseure increaaes until the The heat of mixing (differential heat bf water vapor being eorbed from the air equals the eolution) is the amount of heat liberated by the water vapor desorbed from the desiccant solution. absorption of water into a solution of desiccant at i.e., until equilibrium and an equilibrium vapor a fixed concentration. The differential heat of preseure is achieved. Conversely, when the vapor solution of water can be calculated from integrgl pressure of the solution ie greater than that of heat of eolution data. The addition of 1 lb of the air, water ie deeorbed until equilibrium and an water to a very large quantity of deeiccant equilibrium vapor preseure with the proceee air ia aolution will cause no detectable change in

Proceedings of the Fifth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, September 12-14, 1988 ESL-HH-88-09-25 solution concentration. The heat that ie released Table 8 Figures of Merit for Cost of Desiccant when 1 lb of water is absorbed into a solution at the maximum concentration specified in Table 4 is Cos t shown in Table 7. In addition to the latent heat of Desiccant ($/ 100 lbs Relative vaporization, this heat must be removed by the solution) Weight cooling system. Therefore, a relatively low heat ...... of mixing is desirable. LiCl 77 7 LiBr 174 5 Table 6 Figuree of Merit for Mase Transfer CaCln 15 10 Potential at Absorber (410 F) TEG 38 9

Equilibrium Vapor Pressure Relative Table 9 Figures of Merit for Them1 Conductivity Desiccant (in. of Hg) Weight ...... Thermal Conductivity Relative LiCl 0.13 15 10 Desiccant (Btu/h-ft-OF) Weight LiBr 0.13 le 10 CaC12 0.13 17 10 LiCl 0.301 15 10 TEG 0.13 5 10 LiBr 0.260 18 9 CaC12 0.306 le 10 ma 0.162 21 5

Table 6 Figures of Merit for Mass Transfer Potential at Regenerator (12%' F) PARASITIC POWER U)SSES

Equilibrium Vapor The viscosity of the solution8 at 41° F and Pressure Relative maximum concentration represents the maximum Des iccant (in. of Hg) Weight viscosity within the eystem. This viecoeity is important in determining the pumping power required for circulation of the desiccant. Aesuming all LiCl 2.01 l5 10 LiBr 1.97 le 10 other factors are equal, the pumping power required is proportional to the viecoeity of the solution. CaC12 1.97 l7 10 TEG 1.97 10 Table 10 Figures of Merit for Viscosity

Table 7 Figures of Merit for Heat of Mixing Vimcomi ty Relative Desiccant (centipoise) Weight Heat of Mixing Relative Desiccant (Btu/lb water) Weight LiCl 4 15 10 LiBr 3 18 10 CaCln 8 l7 9 LiCl 31 15 7 TECl 66 5 6 LiBr 17 16 9 CaC12 26 10 8 TEG 12 7.0 10

COST OF DESICCANT The ecreening procees is concluded by summing the weights of the characterietice for each Since the four candidate liquid desiccants candidate as ehown in Table 11. The weighting ie have the same maes tranefer potential when the product of the relative weight and the regenerated to their maximum concentration, the weighting factor given in Table 1. reeulting solutions will absorb the same amount of Table 11 Weighting Summary water in the absorber. Therefore, the cost of deeiccant to create 100 lbs of eolution at maximum Characteristic (Max Weight) LiCl LiBr CaCln TEG concentration, considering the figure of merit for cost, ie shown in Table 8 for the four candidates. Safety(l.0) 7.0 8.0 9.0 10.0 Prices are telephone quotes taken during the spring Corroeion(0.8) 8.0 8.0 7.2 8.0 of 1988. Mass transfer potential at absorber(0.8) 8.0 8.0 8.0 8.0 HEAT TRANSFER WTENTIAL Mass tranefer potential at regenerator(0.8) 8.0 8.0 8.0 8.0 The thermal conductivity of the solutions Heat of mixing(0.6) 4.2 5.4 4.8 6.0 represent5 an important factor in limiting the Coet(O.6) 3.5 2.5 6.0 4.5 performance of the hybrid desiccant air conditioner Heat tranefer potential because maes tranefer is accompanied by (0.5) 5.0 4.6 6.0 2.6 a jmultaneous heat tranefer. The thermal Parasitic power losses conductivity of the four candida;te solutions at (0.3) 3.0 3.0 2.7 1.6 41° F is given in Table 9. ------Total 46.7 47.4 49.7 48.6

Proceedings of the Fifth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, September 12-14, 1988 ESL-HH-88-09-25

According to the weighting systm developed in 13. Hamner, N. E., Corrosion Data Survey, 5,th this paper, the best liquid desiccant for the Ed., National Association of Corrosion Engineers, anticipated application is CaClz. The high heat p. T4, Houston, Texaa, 1974. transfer potential and low cost are the primary advantages of using calcium chloride. Further 14. American Society of Metals, "Corrosion," research plans include construction of an in Metals Handbook, 9th Ed., Vol. 13, Metals Park, experimental absorber/regenerator apparatus and Ohio, 1985. improved computer modeling of the hybrid liquid desiccant/vapor-compression system. 15. Midland Ross Incorporated, Kathabar Systems Division, Kathene Solution Standarde, New Brunewick, New Jersey, 1983. The authors wish to thank the American Public 16. Foote Mineral Company, Technical Data: Power Association for the Energy Services ------Lithium Bromide, Bulletin 145, Exton, Pennsylvania, Scholarship and the Center for Energy Studies for n.d. its support. 17. Dow Chemical Company, Calcium Chloride: --Properties and Forma Handbook, Midland, Michigan, 1983. 1. Howell, John R., and John L. Peterson, "Preliminary Performance Evaluation of a Hybrid 18 ASHRM AS!BM--FlE!db&i----1BSS Vapor Compremsion/Liquid Desiccant Air Fundamentals, American Society of Heating, Conditioning System," ASMB Paper 86-WA/Sol. 9, Refrigerating and Air-conditioning Engineers, Inc., Anaheim. California. December 1986. Atlanta, Georgia, 1985.

2. Matcrid--Safst~-Ea2_a~S_hest-~IMSBSli--LiC1~ 19. Wagman, D. D., W. H. Evans, and V. B. Foote Mineral Company, Exton, Pennsylvania, 1986. Parker, "NBS Tables of Thernodynamic Properties," JoucnaLnf~~Phraics~~and~C_hemic~L~e~e~~nc_e_D&s~ 3. Material--Se_f~tr--!!ete~sheet--fMsDSli-LiB_. Vol. 11, Supplement 2, 1982. Qenim Publishing Corp., Schenectady, New York, 1986. 20. Konnecke, H.Q., H. Steinert, and E. Keibnitz, "Uberdie Miachungswarmen Binarer 4. Qraymon, M., and W. L. Shearer, "Calcium Flusinger System am Beispeil Benzol/Glykole und Chloride," _E_n_cy_cccpsdia of Cheaical Technolotzy, 3rd ~asser/~lykole," 2. F. ~h&ikal ische Cheik, ABT 'Ed., Wiley and Sons, New York, 1978. A., Vol. 208, 1958. 6. A Quid? to Qlycole, Dow Chemical, Midland, 21. Union Carbide Corporation, ~riethylehe Michigan, 1981. Glycol: Product Infomation Bulletin, Danbury, Connecticut, 1981. 6. Materi!l--Safet~--DataSh~~t-fMsnSli~~BQ, Union Carbide Corp., Danbury, Connecticut. 1986.

7. Material-Safel~-E_at_a-S_h_e_eI-~.&DS~L-_C_~C_~Z , Dow Chemical, Midland, Michigan, 1988.

8. Mstecirl--Saf~ty--!!ata~Sheet~fMSDSli~LiC1. Lithco (subsidiary of Fm: Corporation), Besseaer City, North Carolina, 1985.

9. Fontana, M. Q., and N. D. Qreene, Corrosi on Enj&_egI_i_ng, Mcarsw-Hi 11, New York, 1978. 10. Perry, R. H., and C. H. Chilton, Chwical Engineering-_Handbook,------7th Ed., McGraw- Hill, New York, 1984. 11. Personal cmication between. John Peterson, Center for Energy Studies, The University of Texaa at Amtin, and Bill Qriffiths, Kathabar Syatema, on Auguet 20, 1987. 12. Cohen, A., and R. V. Jelinek. "Corromion Raten of Mild Steel in Alkaline Lithium Braride Solutions by the Polarization Resistance Method," Ccrrfilcn, Vol. 22, p. 43, February 1966.

Proceedings of the Fifth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, September 12-14, 1988