I n d . E n g . C h e m . Res. 1990, 29, 1658-1662
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Performance of an Experimental Absorption Heat Transformer Using Aqueous LiBr, LiCl, and LiBr/LiCl Solutionst Suryakant G. Pataskar,i Suguna D. Adyanthaya,§ and Sukumar Devotta*i* Chemical Engineering Division and Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India
F. Anthony Holland Department of Chemical and Gas Engineering, University of Salford, Salford M5 4WT, U.K.
Performance data have been obtained on a small absorption heat transformer operating with the following working fluid/absorbent pairs: (i) water-lithium bromide, (ii) water-mixture of lithium chloride and lithium bromide (1:lby weight), and (iii) watel-lithium chloride. It is possible to deliver heat a t temperatures beyond 100 “C. It is demonstrated that water-LiC1 or water-LiBr/LiCl pairs can be advantagenously used as alternatives to the well-known water-LiBr pair. T h e actual coefficients of performance obtained by using the water-LiC1 pair are the best among the three pairs studied, under comparable conditions.
Introduction
COP =
Many industrial sectors such as chemical, petroleum, food, and process industries consume large amounts of energy in the form of heat, which is then released mostly a t low tempertures. A fraction of the waste heat can be upgraded to useful temperature levels by means of absorption heat tranformers or reversed absorption heat pumps. As the heat transformer has the unique capability of raising the temperature of low-grade heat to a more useful level, it is also known as a “temperature booster”.
Absorption Heat Transformer The general principle of heat transformers was first proposed by Altenkirch (1914). A heat transformer is shown schematically in Figure 1. Heat from a waste heat source is supplied to the generator to vaporize the working fluid from a solution of an absorbent in the working fluid at temperature TGE.The vaporized working fluid is condensed in the condenser at temperature Tco, rejecting the heat to a cooling medium. The liquefied working fluid leaving the condenser is pumped to the evaporator at higher pressure, where it evaporates by extracting heat from the waste heat source at temperature TEV. The working fluid vapor is then absorbed by absorbent at a higher temperature, TAB, in the absorber. The heat generated in the absorber is delivered to a heat sink to do some useful heating. The working fluid rich solution from the absorber is throttled and returned to the generator through an economizer, where it exchanges heat to preheat the working fluid lean solution pumped from the generator to the absorber. Similar to the conventional absorption heat pump, the electrical energy requirement of a heat transformer is marginal when compared to various heat loads in the system. The heat transformer cycle can be considered to be a combination of a Rankine power cycle operating between TGEand Tco driving a Rankine heat pump cycle operatin TAB and TEV The efficiency of the system can be assessed through the coefficient of performance (COP), which is conventionaly defined as the ratio of useful heat delivered by the system to the heat suppled to the system. For the absorption heat transformer, COP is given by the equation
* Author
to whom all correspondence should be addressed.
‘NCL Communication No. 4680. 3 Chemical Engineering Division. 5 Physical Chemistry Division.
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QAB/(QGE
+
QEV)
(1)
From standard thermodynamic principles, an ideal Carnot coefficient of performance can be derived as
(C0P)c = (TAB/TGE)(TGE - Tco)/(TAB- Tco) (2) Typically, industrial systems are reported to approach COP values in the range 0.4-0.48when they are operated close to the design conditions. With further developments, heat transformers are likely to find wider applications than those found for the mechanical vapor heat pumps, owing to their low primary energy demand and low consumption of high-grade (electrical)energy. The possible applications are district heating, process (waste) heat recovery, evaporation, and distillation.
Working Pair The desirable properties of the working pair (working fluid/absorbent) can be briefly stated as (1)the working fluid should have a high latent heat of vaporization, (2) the solute (absorbent) should have a low vapor pressure, (3) the solute should be soluble in the working fluid over a wide range of concentrations, (4) the mixture should have a moderate heat of mixing, (5) the solution should exhibit negative deviation from Raoult’s law, and (6) the solution should not be too viscous, corrosive, or toxic. An ideal working pair should provide the best performance in a system with minimum cost. In most of the early studies reported on heat transformers, the water-ammonia pair has been used. With this pair, the system has to operate under relatively high pressures. The water-LiBr system is conventionally used in absorption refrigeration and heat pumps, and the technology is fairly well established. Some commerical models of absorption heat transformers operating with water-LiBr, capacities ranging up to 50 MW, are currently in use (Berntsson et al., 1988). The disadvantages of this pair are the high cost, corrosiveness, and toxicity. The scope for the absorption heat transformer could be broadened if alternative working pairs were identified and evaluated. Some potential working pairs presently being investigated in laboratorysize units involve chlorofluorocarbons (CFCs). With the current controversy over CFCs and ozone layer depletion, there is likely to be only limited scope for pairs involving CFCs. The choice, is, therefore, limited to aqueous electrolytes. Salts generally have the advantage of negligible vapor pressure, and their solutions exhibit large negative deviations from Raoult’s law. However, some salts have 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 1659 condenser
generator QGE
TC 0
T~ E
waste heat
R - Rolomitcr Siphlpl~ir
SG
pump
7 i
OEV
waste heat
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"
economiser
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*
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'A 0
TAB
Figure 2. Schematic diagram of the experimental absorption heat transformer.
heat d e K e r e d
different pairs within the operational limits set by the properties of the pair under consideration. The present work was undertaken to study the comparative performance of an experimental absorption heat transformer with aqueous solutions of lithium bromide, lithium chloride, and a mixture (1:lby weight) of lithium bromide and lithium chloride. The water-LiBr/LiCl pair has been used to study the peformance in an absorption cooler (Grover et al., 1989) and in an absorption heat pump (Patil et al., 1989). The corrosivity and health hazards of the pure LiBr solutions are reduced by the addition of LiC1. As LiCl is relatively inexpensive, its addition reduces the overall cost of the solution. The solubility range of LiBr solution is also extended, reducing the risk of crystallization. However, the mixture solution has a slightly higher viscosity than does pure LiBr solution. Experimental Section The experiments were carried on a heat transformer unit constructed of glass. A simplified diagram of the experimental unit is presented in Figure 2. The unit originally designed and operated by Siddig Mohammed et al. (1983) was used with minor modifications. A detailed description of the unit can be found elsewhere (Adyanthaya, 1987; Pataskar, 1987). Bayonet type heaters of 1 kW were used as the heat source in the generator and evaporator. These heaters (not shown in Figure 2) were enclosed in quartz tubes over which the circulating liquids were sprayed through sparge pipes. Thermocouples connected to a 30-point data logger were used to measure the temperatures at the required points. The salts used tu prepare the solutions had a purity greater than 99%. The variables measured or controlled in the experiments were temperature, pressure, electric power input, solution and cooling media flow rates, and salt concentration. Experiments were carried out by varying the flow ratio, absorber temperature, and solution concentration. Since it was necessary to carry out some experiments with the absorber temperature in excess of 100 "C, aqueous glycerol was used as the cooling medium in the absorber. Concentrations of the solution (XAB)along with the temperature ( T A B ) in the absorber fix the pressure in the absorber/evaporator loop and thereby determine the evaporator temperature (TEv).The concentration of the strong solution in the generator ( X G E ) and the pressure in the absorber/evaporator loop fix the generator temperature ( TCE).For each initial concentration of the solution, a set of readings was taken by varying the absorber temperature while keeping the solution recirculation ratio approximately constant. In order to maximize the effi-
1660 Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990
500y
6001
450 W W
c
50i 3.
30
0
In CI a
0
s
SET
I
0 S E T II
t
a W
I
I
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'
15
I
I
I
I
I
20
25
30
35
J 40
FLOW RATIO (FR), DIMENSIONLESS
Figure 3. Temperatue and (COP)Aagainst FR for the water-LiBr pair.
ciency of the system, the generator temperature was maintained as close as possible to the evaporator temperature. The energy inputs were adjusted by using dimmerstats so that a constant water level in the evaporator was maintained. This implied constant concentrations of solutions in the system and overall steady-state operation. Samples of the strong and dilute solutions were taken, through the sampling ports in the generator and absorber, during each run, and their concentrations were determined by using an Abbe refractometer. The heat loads of the condenser and absorber were estimated from the mass flow rates of cooling media, measured by calibrated rotameters, and their temperatures at the inlet and outlet conditions. Wattmeters were used to measure the power input to the heaters in the evaporator and generator. Experimenal measurements were repeated for each run to check the steady state and the consistency of the results. As mentioned earlier, the viscosity of aqueous solutions of LiCl is higher than that of LiBr solutions. Hence, in order to avoid damage to the circulating pumps, it was decided to perform the experimental study in stages. The performance of the water-LiBr pair ws evaluated first, followed by the water-LiBr/LiCl pair and then by the water-LiC1 pair. This approach provided the operational confidence for working with a new system. The safe temperature and concentration ranges were selected after a review of the crystallization characteristics of the respective solutions.
Results and Discussion Water-LiBr. Two sets of experimental data are presented for the water-LiBr pair in Figures 3 and 4. Set I represents the conditions of 48.0% C XAB< 51.0%, 50.3% C X G E < 53.8%, and 92.8 "C C TAB < 94.0 "C. The corresponding ranges for set I1 are 53.0% C XAB < 54.4%, 56.0% C X G E C 57.1%, and 103.0 OC C TAB < 109.0 "C. It is obvious from these data that rather high concentration
150
I
I
I
15
20
25
FLOW R A T I O ( F R )
e 30
35
, DIMENSIONLESS
Figure 4. Heat loads against FR for the water-LiBr pair.
ranges are required to attain relatively high delivery temperatures. The effect of flow ratio (FR) on the actual coefficient of performance is shown in Figure 3. This also shows that high flow ratios are not favorable since the (C0P)A decreases rapidly with increasing flow ratio. Any increase in the flow ratio affects the performance in the following ways: the load on the economizer increases, the heat losses from the system increase, and the power required for the solution pumps increases (Eisa et al., 1985). Therefore, the flow rate in the heat transformer needs to be optimized with respect to the above factors. Temperatures TAB, TEV, T G E , and Tco and the temperature lift (TAB- TEV) have also been plotted against the flow ratio in Figure 3. This gives a clearer indiction of the temperature levels required in the various units of the system for the two sets of conditions specified earlier. Figure 4 shows the variation of various heat loads with FR. It can be seen that with increasing flow ratio the generator and condenser heat loads increase, the absorber heat load decreases, and the evaporator heat load remains almost constant. These effects have resulted in the variation of (COP)Awith FR presented in Figure 3. The plots imply that higher delivery temperatures can be achieved with relatively high temperature lift but at the cost of poorer coefficients of performance. It can also be seen that for a given temperature lift higher delivery temperatures are possible but with relatively high flow ratios. Therefore, based on the temperature of the heat source, one has to optimize between the delivery temperature and (COP)Aby choosing an optimal flow ratio. Water-LiBr/LiCl. Experimental data for the waterLiBr/LiCl pair are presented in Figures 5 and 6. In general, the trends of these plots are similar to those presented for the water-LiBr pair. However, it should be emphasized that the (COP)Avalues obtained with the water-LiBr/LiCl pair are higher than those obtained with water-LiBr under comparable conditions but the temperature lifts obtained are slightly lower. Therefore, the addition of LiCl to LiBr has resulted in some improvement
Ind. Eng. Chem. Res., Vol. 29, No. 8, 1990 1661 0-SET I 0 - S E T I[
110
90
P a W P
I
I
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I
I
I
I
15
20
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I
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Figure 5. Temperature and (COP)* against FR for the waterLiBr/LiCl (1:l mixture) pair.
FLOW
RATIO ( F R )
,
DIMENSIONLESS
Figure 7. Temperature and (COP)* against FR for the water-LiC1 pair.
3
."
0
500
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I
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0
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Figure 6. Heat loads against FR for the water-LiBr/LiCl (1:l mixture) pair.
in the performance efficiency of the unit. Water-LiC1. The results for the water-LiC1 pair are shown in Figures 7 and 8. The experiments were carried out for two sets of conditions. Set I represents the conditions of 35.2% < X m < 36.8%, 37.4% < X G E < 40.5%, and 91.1 "C < TAB < 97.6 "C. The corresponding ranges for set I1 are 3.92% < X m < 40.8%, 41.6% < X G E < 43.4%, and 98.7 "C < TAB< 108.5 "C. Here again, the trends of the plots are similar to the ones obtained earlier for water-LiBr and water-LiBr/LiCl, but the (C0P)A values under comparable conditions are the best of the three pairs studied. This has confirmed the theoretical conclusions of Grover et al. (1988).
300
1801
IO
I
13
I
16
I
19
I
22
I
25
I
28
FLOW RATIO (FR), DIMENSIONLESS
Figure 8. Heat loads against FR for the water-LiC1 pair.
It may be noted that the operation of the experimental unit with water-LiC1 did not warrant any additional care compared to the operation with water-LiBr.
I n d . Eng. C h e m . Res. 1990,29, 1662-1668
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Literature Cited
Conclusion It has been demonstrated that it is possible to use the water-LiC1 pair or the water-LiBr/LiCl pair advantageously as alternatives to the well-known water-LiBr pair for absorption heat transformer operations. Lithium chloride does not possess some of the unfavorable characteristics of LiBr, such as toxicity, corrosivity, and high cost. The performance of specifically designed units for the water-LiC1 pair or the water-LiBr/LiCl pair would be expected to be better than those presented in this work. Further work, however, would be necessary to establish the optimum operating conditions using these solutions. It would be worth charging a commercial unit with water-LiC1 and evaluating its performance.
Adyanthaya, S. D. Performance studies of aqueous halides in a heat transformer. M.Sc. Thesis, University of Salford, U.K., 1987. Altenkirch, E. Reversible absorption maschiess. 2. Gas Kailternd 1915, 21, 2.
Berntsson, T.; Holmberg, P.; Wimby, M. Research activities on heat transformer at Chalmers University of Technology, Sweden. Proceedings of the workshop on "Absorption Heat Pumps"; Zegers, P., Miriam, J., Eds.; Directorate-General, Research, Science and Development, Commission of the European Communities: London, 1988. Eisa, M. A. R.; Devotta, S.; Holland, F. A. A study of the emnomiser performance in a water/LiBr absorption cooler. Int. J.Heat Mass Transfer 1985,23, 2323-2329. Grover G. S.; Devotta, S.; Holland, F. A. Thermodynamic design data for absorption heat transformers, Part 111: Operating on waterlithium chloride. Heat Recovery Syst. CHP 1988,8 (5) 425-432. Grover, G. S.; Devotta, S.; Holland, F. A. Performance of an experimental absorption cooler using aqueous LiCl and LiCl/LiBr solutions. Ind. Eng. Chem. Res. 1989, 28 (2), 250-253. Pataskar, S. G. An experimental evaluation of aqueous salt solutions in absorption heat pump systems. 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 1989, 34 (2), 99-105. Siddig Mohammed, B. E.; Watson, F. A.; Holland, F. A. Study of the operating Characteristics of a reversed absorption heat pump (heat transformer). Chem. Eng. Res. Des. 1983, 61 (5), 283-289. Smith, . E.; Carey, C. 0. B. Thermal transformers for upgrading industrial waste heat. Proc. Int. Symp. on Industrial Heat Pumps, Coventry, England, 1982; BHRA, U.K.; paper H1.
Nomenclature
(COP)* = coefficient of performance, dimensionless FR = recirculation flow ratio, dimensionless MAB= mass flow rate of solution from absorber, kg/s MWF= mass flow rate of working fluid ciculated, kg/s Q = heat load, k W T = temperature, "C X = concentration, wt 70 Subscripts AB = absorber C = Carnot
CO = condenser EV = evaporator GE = generator
Received for review May 5, 1989 Revised manuscript received March 19, 1990 Accepted April 4, 1990
Registry No. LiBr, 7550-35-8; LiC1, 7447-41-8; water, 773218-5.
SEPARATIONS Recovery of Uranium from Seawater by Composite Fiber Adsorbent Yoshiaki Kobuke,* Takao Aoki, Hiromitsu Tanaka, and Iwao Tabushi' Department of Synthetic Chemistry, Faculty of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606, Japan
Tadami Kamaishi and Ikuo Hagiwara Polymers Research Laboratories, Toray Industries Inc., Sonoyama 3-2-1, Ohtsu 520, Japan
We developed a composite fiber adsorbent (CFA) to entrap finely powdered amidoxime into fibrils of supporting material with silica in a previous report. This was further tested for uranyl recovery directly from seawater. The adsorption rate showed a flow rate dependence with almost a saturation value of -100 pg of U/g of CFA a t a mean flow rate of sea current. Chemical as well as physical deterioration was overcome by using 1 N NaHC03 and 0.72 M NaCl as liberating and washing agents, to keep the p H and ionic strength, respectively, constant, and the initial adsorption rate was maintained even after a recycle time of 50. A continuous passage of seawater showed a linear increase of the adsorption to afford 1560 p g of U/g of CFA after 3 weeks. In the preceding paper (Kobuke et al., 1988), we reported on a composite fiber adsorbent (CFA) that exhibited superior adsorption characteristics, especially in view of the adsorption rate, i.e., the most important criterion *To whom correspondence should be addressed. Deceased.
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for establishing economical uranyl recovery from seawater. An adsorption rate as high as 200 (pg of U/g of Ads)/day was obtainable by incorporating fine particles (10-102 pm) of amidoxime resin into a fiber matrix of supporting polymer such as polyethylene. The same resin gave an adsorption rate of only 20 (pg of U/g of Ads)/day, when used in the form of conventional particles (Egawa and 0 1990 American Chemical Society
