Cristalizacao de Brometo de Litio-Agua

7/27/2019 Cristalizacao de Brometo de Litio-Agua

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Crystallization Temperature of Aqueous Lithium Bromide Solutions at LowEvaporation Temperature

Padmaja Kisari, Kai Wang, Omar Abdelaziz*, and Edward Allan Vineyard

Whole Building and Community Integration, Energy & Transportation Science Division, Oak Ridge National

Laboratory

P.O. Box 2008, MS-6070, One Bethel Valley Road, Oak Ridge, TN, United States, 37831-6070

Key Words: Absorption chiller; Water/Lithium Bromide; Crystallization

Abstract

Water- aqueous Lithium Bromide (LiBr) solutions have shown superior performance as working fluid

pairs for absorption refrigeration cycles. Most of the available literature (e.g. ASHRAE Handbook of

Fundamentals, etc.) provide crystallization behavior down to only 10°C. The typical evaporating

temperature for an absorption chiller system is usually lower than 10°C. Hence, it is essential to have an

accurate prediction of the crystallization temperature in this range in order to avoid crystallization during

the design phase. We have therefore conducted a systematic study to explore the crystallization

temperatures of LiBr/Water solutions that fall below an evaporating temperature of 10°C. Our preliminary studies revealed that the rate of cooling of the sample solution influences the crystallization

temperature; therefore we have performed a quasi steady test where the sample was cooled gradually by

reducing the sample temperature in small steps. Results from this study are reported in this paper and can

be used to extend the data available in open literature.

* Corresponding author. Tel: +01-865-574-2089; fax: +01-865-574-9392; Email: [email protected]

IntroductionConcentrated lithium bromide solutions are used in absorption heat pumps for heating and cooling

purposes. To increase the Carnot efficiency of heat pumps, it is necessary to decrease the lowest

temperature of the cycle while keeping the highly concentrated LiBr solution from freezing. Both LiBr

and water are eco-friendly and do not cause ozone depletion and hence devoid of global warming hazards.

Therefore, this environmentally friendly working fluid has gained enormous popularity in recent years.

Absorption system could provide cooling and/or heating driven by heat and not by electricity. Nowadays,

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absorption systems play an important role in applications such as Combined Heating and Cooling,

residential and commercial building space heating and cooling as well as water heating. These include

refrigerating machines, heat pumps and heat transformers.

Many theoretical and experimental studies have been performed on LiBr/H 2O based working fluid for

absorption chillers and heat pumps (Biermann, W.J., 1978; Ally, M. R., 1988; De Lucas, A., et.al 2004

and 2007; Rosiek, S. and Batlles, F. J., 2009). Biermann, W.J. (Biermann, W.J., 1978) reported an

alternate working fluid based on LiBr called Carrol, which comprises of LiBr and ethylene glycol. Carrol

had been tested extensively in solar-powered, water cooled (Biermann and Reimann, 1981b) and air

cooled (Biermann and Reimann, 1981a) absorption application both in the laboratory and in the field. A

Japan company (Yazaki Corporation, 2000) developed and patented a LiBr/LiCl/LiI solution for air-

cooled applications that increases allowable absorber and condenser operating temperatures. Lee, H. R.

et.al. (Lee, H. R., et.al. 2000) have calculated the thermodynamic design data and performance evaluation

of H2O/LiBr/LiI/LiNO3/LiCl system and the simulation results show that the proposed working fluid is

applicable to air-cooled absorption chiller with no crystallization problem at higher absorber temperature.

For the development of advanced absorption refrigeration system or improvements in primary energyefficiency, accurate thermodynamic property data of LiBr aqueous solution are highly desirable. Most of

the available literature (e.g. 2009 ASHRAE Handbook of Fundamentals, etc.) provide crystallization

behavior down to only 10°C. But the typical evaporating temperature for an absorption chiller system is

usually lower than 10°C. Hence, it is essential to have an accurate prediction of the crystallization

temperature in this range in order to avoid crystallization during the design phase. We noticed that there is

a difference in crystallization temperatures as reported in ASHRAE handbook and other literature

(Murakami, K. and Kondo, N., 2003). We have therefore conducted a systematic study to explore the

crystallization temperatures of LiBr/Water solutions that fall below an evaporating temperature of 10°C.

Results from this study are reported in this paper and can be used to extend the data available in open

literature.

Crystallization Experimental Setup

The experimental setup consists of a supporting stand with a clamp, a hot plate stirrer, a crystallizing dish

containing water, a controlled temperature water bath, and a test flask. The test flask was first immersed

in water inside the crystallizing dish and held in place by the clamp fixed to the stand. The hot plate

stirrer has the ability to heat the water in the crystallizing dish as well as stir the test solution that enables

complete dissolution of the salt to form a homogeneous solution. As the solution becomes homogeneous,

the test flask is moved to the controlled temperature water bath and a T-type thermocouple is immersed in

the solution to continuously monitor the temperature. The experimental setup is shown in Fig. 1.

LiBr with 99.9% in purity and distilled water were used for sample preparation. The sheathed T type

thermocouple was calibrated with NIST traceable fractional degree calibrated thermometers for the range

from 273 to 373 K. A precision electric balance within the uncertainty of ±0.1 mg was used to measure

the weight of the salt and water.



Fig.1 Schematic diagram of (a) salt solution preparation and (b) crystallization temperature measurement

The experimental setup was modified to enable to reproduction of Dühring plots (pressure-temperature-

concentration curves). These plots can be used to verify the accuracy of the experimental procedure since

such curves have been widely reported and validated. A 3-necked round bottom flask was used instead of

in order to maintain the required vacuum level while measure pressure and temperature simultaneously.

The flask is fitted to a condenser (cooled by circulating cold water), thermocouple and a pressure sensor.

The other end of the condenser is connected to the cooling trap that in turn is connected to the vacuum

pump. The condenser consists of two stop valves fitted at each end to gain a better control during the

evacuation process. The experimental setup is shown in Fig. 2. During the initial stage of the evacuation,

the valve at the bottom of the condenser is closed and that at the top is opened. This allows the evacuationof the system only up to the condenser. Now closing the valve on the top and opening the one at the

bottom enables partial evacuation of the reaction flask. Repeating such process would enable evacuation

of the flask reasonably until the sample in the flask attains equilibrium. Such procedure results in

minimum loss of water from the sample thereby preventing the change in concentration of the sample.

The solution should be homogeneous at all times. The temperature and pressure are monitored as a

function of time. The pressure-temperature-concentration data for LiBr aqueous solution was plotted and

the data were in good agreement with that reported in ASHRAE handbook as shown in Fig 3. These

results indicate accurate test procedure.



Fig. 2 Setup for the LiBr aqueous solution vapor pressure experiments

0.1

1

10

35 45 55 65 75 85 95

T (°C)

P r e s s u r e ( k P a )

Expt. 65% LiBr-Water Lit. 65% LiBr-Water

Fig. 3 Experimental and literature data comparison of saturation temperature and saturation pressure for

65% LiBr aqueous solution



Effect of Cooling Rate on the Crystallization Temperature

The crystallization temperatures of various concentrations of LiBr-water solutions were measured and it

was found that the rate of cooling influences the crystallization temperature. Fig. 4 shows the

crystallization temperature of 65% Lithium-Bromide solution at various cooling rates. As shown, in Fig.

4, higher cooling rate resulted in lower crystallization temperatures. The highest crystallization

temperature found for this solution was 310.66 K while the lowest was 296.48 K. This indicates that the

highest crystallization temperature would reach an asymptotic value as the cooling rate approach zero.

This would provide consistent results for different samples. As such, a systematic Quasi-steady procedure

was developed and the crystallization temperature of various concentrations of LiBr solutions in the range

of 65-68% were studied according to the procedure described below.

Fig 4. Crystallization temperatures for 65% LiBr-water solutions at various cooling rates.

Quasi Steady Test Procedure

LiBr-water samples of various concentrations (65-68%) were prepared using the required

amounts of LiBr (weighed in a closed flask) and water. The water was introduced into the test flask and

the required amount of LiBr was added to it. The exact amount of LiBr/water introduced into the reaction

flask was noted. The flask was immediately closed with a rubber stopper and was heated using the setup

described above to achieve a homogeneous solution. Once the test solution became homogeneous, the stir

bar was removed and the stopper of the flask was replaced with a one holed rubber stopper through which

a thermocouple was inserted into the solution. The temperature of the solution was monitored as a

function of time. The crystallizing dish and the hot plate stirrer were then replaced by a cooling bath for

accurate temperature control. The temperature of the cooling bath was raised until the solution in the

flask reached 90 °C. The temperature was held for 5 min. The solution was then gradually cooled in steps

of 5 °C holding the temperature at each step for 5 min until the solution has reached 70 °C, thereafter the

reaction flask was cooled in steps of 2 °C (maintaining the temperature for 5 min at each step). The

solution was closely monitored for the appearance of crystals. The temperature of the solution as a



function of time was recorded and the graph was plotted to note the accurate crystallization temperature.

The crystallization temperature was determined from the graph at the point of sudden rise in temperature.

At the beginning of crystallization the temperature of the solution rises very rapidly because of the heat

generated by solidification as shown in Fig. 4 for 65 % of LiBr-Water solution.

Results and Discussion

The development of reliable LiBr-Water chillers requires accurate prediction of the crystallization

temperatures for the range of operating evaporation pressures. Water being the working fluid in these

chillers, there is a limit on the minimum operating pressure which is the triple point. The upper limit of

evaporating pressure is determined by the maximum possible evaporating temperature that would provide

cooling effect at room temperature. In the current research we tried to verify the crystallization

temperature of LiBr-Water solutions for water evaporating temperatures between 273.15 K and 293.15 K.

According to the LiBr-Water ASHRAE Dühring plots, crystallization takes effect for concentration ratios

of 65% to 68% by weight for this range of evaporating temperatures. As such, 4 different samples were

prepared and tested with a 1% solution concentration increment.

Results of the LiBr aqueous solutions are summarized in Table 1. The difference between the measured

values and publically available values range between 18.3 and 12.2 K. However, Our test procedure

reproduced the same Dühring plots compared to publically available data; this reflects accurate

experimental results. Overlaying these results on publically available plots, as shown in Fig. 5, shows that

there is a small deviation in the crystallization temperature for a given evaporating temperature. At

constant evaporating temperature both 67% and 68% LiBr-water solutions have a crystallization

temperature deviation in the range of 2-3 K in comparison with the data reported in ASHRAE handbook.

Table 1. Quasi Steady Crystallization Temperatures for LiBr aqueous solutions for various concentrations

Concentration(LiBr-Water wt%)

CrystallizationTemperature (K)

68 348.46

67 336.3266 318.83

65 310.66

LiBr-Water absorption chillers are designed for a specific evaporating temperature. As such, the absorber

design is limited by the maximum solution temperature for the evaporating temperature. Assuming an

evaporating temperature of 10˚C, previously reported crystallization behavior showed a maximum

solution temperature of 62.5˚C, as shown in Fig. 5. The experimental results reported in this paper

suggest that the maximum solution temperature is ~65˚C at 10˚C evaporating temperature. This would

allow absorption chiller designs that reject heat at higher temperatures. It would also enable designers to

more accurately avoid the occurrence of crystallization during system operation.



Fig. 5 Updated crystallization limits overplayed on ASHRAE data.

Conclusion

A new systematic experimental procedure was devised to accurately measure crystallization temperatures

of LiBr aqueous solutions. The study focused on the range of water evaporating temperatures relevant to

absorption chiller application; namely 0 to 20°C. Initial studies showed a strong dependence of

crystallization temperature on the cooling rate; higher cooling rates resulted in lower crystallization

temperatures. As such, a quasi-steady experimental procedure was devised to provide consistent results

for different LiBr concentrations.

Quasi-steady test results showed an average 14.4 K lower crystallization temperatures compared to

publically available data for similar LiBr concentrations. However, the impact of these variations on the

absorption chiller design was shown to be less significant. Experimental results showed only 2 to 3 K difference in crystallization temperatures at constant evaporating pressure. The results showed that more

aggressive absorption chillers designs could be developed without risking crystallization.

References

Ally, M.R., 1988. Computer Simulation of Absorption Heat Pump Using Aqueous Lithium Bromide and

Ternary Nitrate Mixture. Oak Ridge National Laboratory Report: ORNL/TM-10392.



American Society of Heating Refrigerating and Air-conditioning Engineers, 2009. 2009 ASHRAEhandbook fundamentals. ASHRAE, Atlanta, GA.

Biermann, W.J., Reimann, R., 1981a. Air Cooled Absorption Chillers for Solar Cooling Applications

Proceedings of the DOE Heat Pump Contractors' Program Integration Meeting, pp. 59-61.

Biermann, W.J., Reimann, R., 1981b. Water Cooled Absorption Chillers for Solar Cooling Applications,

Proceedings of the DOE Heat Pump Contractors' Program Integration Meeting, pp. 62-64.

Biermann, W.J., 1978. Candidate chemical systems for air cooled, solar powered, absorption air conditioner design. Part III. Lithium salts with anti-freeze additives, Report Number: DOE/CS/31587-T1.

De Lucas, A., Donate, M., Molero, C., Villaseñor, J., Rodríguez, J.F., 2004. Performance evaluation and

simulation of a new absorbent for an absorption refrigeration system. International Journal of

Refrigeration 27, 324-330.

De Lucas, A., Donate, M., Rodriguez, J.F., 2007. Absorption of Water Vapor into New Working Fluids

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Lee, H.-R; Koo, K.-K,; Jeong, S.; Kim, J.-S.; Lee, H.; Oh, Y.-S.; Park, D.-R.; Baek, Y.-S. 2000.

Thermodynamic design data and performance evaluation of the water + lithium bromide + lithium iodide

+ lithium nitrate + lithium chloride system for absorption chiller. Applied Thermal Engineering 20, 707-

720.

Murakami, K. and Kondo, N. 2003. Density and Crystallization temperature of Lithium-Bromide aqueous

solution. Fifteenth symposium on thermophysical properties, Boulder, CO, USA, June 22-27.

Rosiek, S. and Batlles, F.J. 2009. Integration of the solar thermal energy in the construction: Analysis of

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Cristalizacao de Brometo de Litio-Agua