Cristalizacao de Brometo de Litio-Agua
Transcript of 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.
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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.
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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
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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
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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.
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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
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