250
Ind. Eng. Chem. Res. 1989, 28, 250-253
Do = Arrhenius preexponential factor E = fraction conversion efficiency of gaseous reactant E' = apparent activation energy taken as 36 600 cal/mol F = moles of reactant solid fed/mole of gas reactant fed I , = integral defined by eq 11 Id = integral defined by eq 8 I , = integral defined by eq 14 k = ((1- S ) / S ) ' I 3 k , = constant defined by eq 12 kd = constant defined by eq 9 k , = constant defined by eq 15 kG = gas-phase coefficient in eq 13, cm/s ko = constant defined by eq 7 k , = reaction rate constant in eq 10, cm/s M = molecular weight of reactant solid m = constant defined by eq 7 p A = partial pressure of gas A, Pa p A i n = partial pressure of gas A at inlet conditions, Pa pSol = partial pressure of SO2, Pa pSOzin= partial pressure of SO2 at inlet conditions, Pa r = radius of the unreacted core, cm R = initial grain radius defined by eq 3, cm; the gas constant in eq 17, 18, 19, and 21, 1.987 cal/(mol K) S = Fa/b Sa = BET area of solid, cm2/g S,= BET area of solid, m2/g T = temperature, K t = time, s x = r / R where R is the initial grain radius x = r / R at the end of the reaction time
Greek L e t t e r = mass per unit volume of grain, g/cm3 Registry No. SOz, 7446-09-5.
p
L i t e r a t u r e Cited Boni, A. A.; Gorman, A. R.; Simons, G. A.; Parker, T. E. Sorption of Fuel-Bound Contaminants for Coal-Fired Gas Turbine Systems. DOE/MC/213388-2227, Sept 1986; DOE, Washington, DC. Borgwardt, R. H.; Bruce, K. R. Effect of Specific Surface Area on the Reactivity of CaO with SOz. AIChE J. 1986, 32, 239. Borgwardt, R. H.; Roache, N. R.; Bruce, K. R. Surface Area of Calcium Oxide and Kinetics of Calcium Sulfide Formation. Enuiron. Prog. 1984, 3, 129. Bortz, S.; Roman, V. P.; Yang, R. J.; Offen, G. R. Dry Hydroxide Injection at Economizer Temperatures for Improved SOz Control. Presented a t the EPA/EPRI Dry SOz and Simultaneous SOz/ NO, Control Technology, Raleigh, NC, 1986; paper 6C. Levenspiel, 0. Chemical Eacting Engineering; Wiley: New York, 1962.
Bruce Weinstein Research Cottrell, Inc. Route 22 W e s t Branchburg, N e w Jersey 08876 Receioed f o r review February 5, 1988 Revised manuscript received September 30, 1988 Accepted October 17, 1988
Performance of an Experimental Absorption Cooler Using Aqueous LiCl and LiCl/LiBr Solutionst Performance data have been obtained on a small absorption cooler operating on the following working pairs: (i) water-lithium chloride and (ii) water-lithium chloride/lithium bromide (1:l by weight). For a similar range of evaporating temperatures, the water-LiBr/LiCl pair requires lower generator temperature than the water-LiC1 pair. By comparing the performance data reported in this work with the published data for t h e water-lithium bromide pair on the same unit, it has been found that, for a given cooling duty, t h e mixture LiBr/LiCl appears to require t h e least generator heat load. Most of the technologies for absorption systems have been developed for chilling or cooling using ammoniawater as the working pair. This technology has been well developed and accepted in the European countries such as France, Germany, and Sweden. However, water-lithium bromide is the pair actively pursued in countries such as Japan and the US. If one wishes to utilize waste heat or low grade heat to produce chilling in the range -60 to -40 O C , the obvious pair choice is ammonia-water. However, if the objective is to recycle heat a t above ambient conditions, the choice is currently limited to water-lithium bromide. The heat input to produce cooling or heating can be any waste heat. In a typical chemical or process industry, the heat sources can be low-pressure steam, vapor, and hot gases such as flue gases. Direct firing with natural gas, fuel oil, or heat from incinerators or flare gases can also be used for this purpose. It appears that there is a lack of basic and applied thermodynamic data for absorption systems. Most of the published data have been for the ammonia-water and water-lithium bromide pairs. Perhaps, the scope of absorption heat pumps and heat transformers could be broadened if a few more working pairs are identified. Some of the inorganic salts that may be chosen are LiCl, LiI, LiSCN, ZnBr2, MgC12,and NaOH. These can be used
as single- or double-salt mixtures. Grover et al. (1988) have theoretically evaluated the performance of an absorption cooling system operating on water-lithium chloride and compared the performance data with those for water-lithium bromide. A comparison of the derived thermodynamic data obtained for identical operating temperatures indicates that a system with aqueous lithium chloride can operate a t much lower flow ratios than with aqueous lithium bromide. Flow ratio (FR) is defined as the ratio of mass flow rate of the solution from the absorber to the generator to the mass flow rate of the refrigerant. This can be written in terms of concentrations as FR =
XGE XGE- XAB
Also the enthalpy-based ideal coefficient of performance is higher (though marginally) for the water-lithium chloride pair. On comparison it appears that lithium chloride is a better choice of absorbent. However, other physical properties like viscosity should also be considered. Aqueous solutions of lithium chloride have higher viscosities than lithium bromide solutions, which could lead to a higher load on the solution circulation pump and
0888-5885/89J 2628-0250$01.50/Q C 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 251
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Condensate accumalator
I # EvaDorator
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Figure 1. Schematic diagram of the experimental absorption cooler.
reduced heat-transfer coefficients. The derived data also indicate that, in order to attain a lower flow ratio, the difference between the concentrations in the generator and absorber is quite large, which may be difficult to achieve in practice. Hence, the final choice of the absorbent should be a compromise between the efficiency and operational viability. Alternatively, a compromise may be possible by using a solution of lithium bromide and lithium chloride mixtures. Therefore, it would be interesting to theoretically analyze the performance of an absorption system operating on water-lithium bromide/lithium chloride mixtures. However, the thermodynamic data for such mixtures are not readily available. But it will also be useful to assess the performance of a specific unit operating on different solutions within the operational limits set by the properties of the solution under consideration. In this work, the operating characteristics of the watel-lithium chloride and water-lithium bromide/lithium chloride (1:l mixture) pairs have been investigated in an experimental glass absorption cooler, to assess the suitability of these working pairs. The water-LiBr/LiCl pair has also been used to study its performance in an absorption heat pump (Patil et al., 1988) and in a heat transformer (Adyanthaya, 1987; Pataskar, 1987). The addition of LiCl to LiBr reduces the corrosivity and health hazards of the pure LiBr solutions. The solubility range for the solution is also increased, which reduces the risk of crystallization. As LiCl is relatively inexpensive, its addition reduces the overall cost of the solution. However, the mixture solution has a slightly higher viscosity than the LiBr solution.
Experimental Section An absorption system which was originally designed and
0
0.2 0
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I
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I
I
I
10
20
30
40
50
60
FLOW RATIO I F R ) , DIMENSIONLESS
Figure 2. Temperatures and (COP)* against FR at T,, = 45 and 50 "C for water-LiBr/LiCl (1:l mixture) pair.
operated by Landauro-Parades et al(1983) was used, with only minor modifications, for this study. A schematic diagram of the experimental setup is shown in Figure 1. The unit was made with glass as the material of construction. Bayonet-type heaters of 1kW were used as the heat source in the generator and evaporator. These (not shown in Figure 1) were enclosed in quartz tubes over which the circulating liquids were sprayed from sparge pipes. In operation, the energy inputs were adjusted so as to maintain a constant water level in the evaporator, as this implied constant concentrations of solutions in the system and overall steady-state operation. Solution samples could be taken through the sampling points in the generator and absorber, respectively. Thermocouples connected to a 30-point data logger were used to measure the temperature at the main flow points. The anhydrous lithium chloride and hydrated lithium bromide used to make the solutions were of purity greater than 99%. The concentration of the solutions was determined by refractive index measurement. The viscosity of aqueous solutions of lithium chloride is higher than that of lithium bromide solutions. Hence, in order to avoid damage to the circulating pumps, it was decided to do the experimental study in two stages. In the first stage, the performance of the water-LiBr/LiCl pair was studied, followed by the water-LiC1 pair. This approach gave the operational confidence to handle a new system. A safe temperature margin for a given concen-
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Figure 3. Heat loads against FR for water-LiBr/LiCl (1:l mixture) pair.
tration was decided by the crystallization characteristics of the respective solutions.
Results and Discussion The actual coefficient of performance was calculated according to QEV/QGE
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(COP)A=
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Figure 2 shows the variation of T G E , TAB, and T E V as a function of the flow ratio (FR), a t constant condenser temperatures of 45 and 50 "C, respectively, for the water-LiBr/LiCl pair. It is seen that with increasing flow ratio, though the generator temperature (TGE) records a marginal decrease, the absorber temperature (TAB)rises, while the evaporator temperature (TEV) is more or less constant. The coefficient of performance ((COP),) shows a downward trend with increasing flow ratio. Although it is desirable to operate an absorption cooler a t lower generating temperatures so as to utilize any low-temperature waste heat, the COP values are relatively low for these conditions. In addition, the flow ratios are higher, leading to higher energy inputs for the circulation pumps and higher heat losses. Figure 3 shows the variations of the respective heat loads as a function of the flow ratio for the water-LiBr/LiCl pair. It can be seen that the heat loads for the generator and
Figure 4. Temperatures and (COP)* against FR a t T,-o = 46 and 50 "C for water-LiC1 pair.
absorber increase with FR, while the condenser load falls, even though the evaporator load is more or less constant. The heat balance with the heat loads of the four units indicates that the heat loss was in the range 10-239'0 a t the highest flow ratio. Similar trends are also observed for the water-LiC1 pair, as can be seen from Figures 4 and 5. For a similar range of TEv,the water-LiBr/LiCl pair requires lower T G E than the water-LiC1 pair. It is interesting to compare the performance data of this cooler using the water-LiBr/LiCl and water-LiC1 pairs with those obtained using the water-LiBr pair (Eisa et al., 1985) under nearly identical conditions. For the conditions of TAB in the range 30-40 "C and T C O = 50 "C, the ranges of TGE required for the water-LiBr, water-LiBrlLiCl, and water-LiC1 pairs are 104-95,78-73, and 88-65 "C for the corresponding T E V ranges of 9-12, 12-13, and 12-14 "C, respectively. The addition of LiCl brings down the T G E values significantly. However, the TEv values are higher. The corresponding FR and (COP)Aranges are 20-50, 11-51, and 11-55 and 0.4-0.35, 0.55-0.37, and 0.4-0.3, respectively. For a given cooling duty, the mixture LiBr/LiCl appears to require the least generator heat load.
Conclusions It appears that it is practicable to use aqueous LiCl and LiBr/LiCl solutions in absorption coolers. The performance of well-lagged units constructed of metal would be expected to be better than the values reported.
Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 253
T = temperature, "C X = concentration by weight percent
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Registry No. HzO,7732-18-5; LiC1,7447-41-8; LiBr, 7550-35-8.
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Literature Cited Adyanthaya, S. D. Performance studies of aqueous halides in a heat transformer. MSc. Thesis, University of Salford, U.K., 1987. Eisa, M. A. R.; Sane, M. G.; Devotta, S.; Holland, F. A. Experimental studies to determine the optimum flow ratio in a water-lithium bromide absorption cooler for high absorber temperatures. Chem. Eng. Res. Des. 1985, 63(4), 267. Grover, G. S.; Eisa, M. A. R.; Holland, F. A. Thermodynamic design data for absorption heat pump systems operating on water-lithium chloride. Part 1: Cooling. Heat Recouery Syst. CHP 1988, 8(1), 33-41. Landauro-Parades, J. M.; Watson, F. A.; Holland, F. A. Experimental study of the operating characteristics of a water-lithium bromide absorption cooler. Chem. Eng. Res. Des. 1983,61(6), 362. Pataskar, S. G. An experimental evaluation of aqueous salt solutions in an absorption heat pump system. MSc. Thesis, University of Salford, U.K., 1987. Patil, K. R.; Kim, M. N.; Eisa, M. A. R.; Holland, F. A. Experimental evaluation of aqueous lithium halides as single and double salt systems in absorption heat pumps. Appl. Energy 1988, in press.
0.1
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*Author to whom all correspondence should be addressed. NCL Communication No. 4388.
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0.2
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Gursharn S. Grover, Sukumar Devotta* Chemical Engineering Division National Chemical Laboratory Pune 411 008, India
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20 30 40 so F L O W RATIO ( F R ) , OIMENSIONLESS
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Figure 5. Heat loads aeainst FR for water-LiC1 Dair. I
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Nomenclature (COP)* = actual coefficient of performance, dimensionless FR = flow ratio, dimensionless Q = heat load, kW
F. Antony Holland Department o f Chemical and Gas Engineering University o f Salford Salford, U.K. Received for review February 29, 1988 Revised manuscript received July 12, 1988 Accepted October 21, 1988
