The Latent Application of Ionic Liquids in Absorption Refrigeration

The absorption refrigeration technology, which went through more than 100 years, has attracted much attention all over the world, for the reason that it is environmental friendly and could make use of the low-grade energy, which refers to the ignored energy embedded in the exhaust steam of low pressure and low temperature. The absorption refrigeration is widely used in many fields, such as military, air conditioning, electric power, steelmaking, chemical industry, drugs manufacture and so on. In this section, the absorption refrigeration cycle and its working pairs are the key contents that will be introduced to the readers.

Here we consider refrigerant as solvent. The resulting low concentration solution, called weak solution, is pumped to the generator, where the refrigerant is boiled off. The remaining strong solution flows back to the absorber and, thus, completes the cycle [1].  The another common cycle in the actual application, called double-effect absorption refrigeration cycle, is shown in Figure 2, which is mainly composed of a high pressure generator, low pressure generator, absorber, condenser and evaporator. Compared with the single one, the double effect absorption refrigeration system has two generators. The solution in high pressure generator, indirectly heated by the heat source, emits high pressure refrigerant vapor and forms strong solution. The solution in low pressure generator is heated by high pressure refrigerant vapor from the high pressure generator. The solution is concentrated in some degree and passes into high pressure generator. The energy is used twice, directly in the high pressure generator and indirectly in the low one. The double one is more complex and costly than the single one, but its efficiency is higher than the latter. Besides above, there are some other absorption refrigeration cycles in order to make use of the energy more efficiently and completely, for example, the triple-effect absorption refrigeration cycle, the GAX cycle and the absorption/compression refrigeration cycle.

Working pairs
Many working fluids are suggested to be used as working pairs for absorption refrigeration in literature, but there is no ideal absorbent-refrigerant pair by now. Currently, the binary systems of NH 3 -H 2 O and LiBr-H 2 O are well known as working fluid pairs to be applied in absorption refrigerators, but they both present advantages and disadvantages. The advantage for refrigerant NH 3 is that it can evaporate at lower temperatures (i.e. from -10 to 0°C) compared to H 2 O (i.e. from 4 to 10°C). Therefore, for refrigeration, the NH 3 -H 2 O cycle is used. However, the NH 3 /H 2 O system is high-pressure and explosive, and the refrigerant NH 3 is poisonous, and its solution is alkaline and corrosive. The disparity in boiling point between NH 3 and H 2 O is not large, which makes it necessary to utilize the distillation equipment. The coefficient of the performance for the H 2 O-LiBr system is much higher than that for the NH 3 -H 2 O system. The only disadvantage is that H 2 O-LiBr solution is corrosive to metal and easily crystallized, in addition, the working temperature and pressure of the H 2 O-LiBr system are too low. The refrigerant for the application being investigated should have the following properties: high latent heat of vaporization and low saturation pressures at normal operating temperature. Ammonia, water, methanol, and fluorocarbon refrigerants are at the top of the choice list. The important considerations influencing the choice of a suitable absorbent are: higher boiling point than refrigerant, strong ability to absorb the refrigerant, high thermal and chemical stability, low mass flow rate and heat capacity, non-poisonous, non-corrosive, non-flammable and so on. Ionic liquids are organic salts with a melting point below some arbitrary temperature, such as 100°C. Comparing with frequently-used solvents, ionic liquids exhibit distinctive properties, such as negligible vapor pressure, low combustibility, excellent thermal stability, wide liquid regions, and favorable solvating properties for a range of polar and non-polar compounds. If used as absorbents in absorption refrigeration system, their good solvating properties will make them useful in absorption of large amount of refrigerant under low temperature conditions to yield good COP, and their involatile will ensure them not contaminating with refrigerant stream when desorbed. Therefore, in recent years, RTILs are regarded as the potential candidates of absorbent in absorption refrigeration system [2,3]. In the following sections, we'd like to introduce to readers a number of studies on the application of ionic liquids in absorption refrigeration system. Whether ionic liquids can be www.intechopen.com used as refrigeration absorbent, there are two issues needed to be focused on. Firstly, we should make sure whether the physicochemical properties of ionic liquid and refrigerant's binary solution can meet the requests of absorption refrigerator's working pairs. Secondly, we should make sure whether the selected ionic liquid working pairs have competitive advantages in terms of performance compared with those traditional ones.   [7] in 2007. Six mixture compositions of each binary system were involved from about 30 to 85 mole% of ammonia. Pressure-temperature-composition (P-T-x) data were claimed at isothermal conditions of 283, 298, 323, 348, and 373 K. The observed solubility of ammonia in ionic liquids is very high, and all cases show negative deviations from ideal solution behavior. Experimental P-T-x data were successfully correlated with the equation-of-state (EOS) model [8]. The experimental data and fitting results are shown in Figure 5 ~ 8. The opportunity for the absorption cycle application using the ammonia-RTIL system, replacing the traditional ammonia-water system, has been discussed [7]. S.P. Verevkin et al. [9] studied the vapor-liquid equilibria (VLE) of binary mixtures containing methanol, ethanol, 1-propanol and benzene in the ionic liquid [BMIm]NTf2 by using a static method. VLE measurements were carried out over the whole concentration range at four different temperatures in the range from 298.15 K to 313.15 K. Activity coefficients γi of these solvents in the ionic liquid have been determined from the VLE data and are described formally by using the NRTL equation.      17.8% and 30.0% is predicted. As the low concentration solutions, the relationships between the saturated vapor pressure of the medium to high concentrations and the temperature are similar to the pure solvent, which obey the Antoine equation. The saturated vapor pressure of the [MMIm]DMP-methanol solutions when the mole fraction of [MMIm]DMP is 12.6% and 30.0% is also calculated by the NRTL model with three parameters by J. Zhao [5]. All the above T-P-x data are shown in Figure 9.  As shown in Figure 10, with the increase of the mole fraction of [MMIm]DMP, the saturated vapor pressure was significantly reduced. The saturated vapor pressure of the [MMIm]DMP-methanol solution at normal temperatures is lower than that of pure methanol at 5°C. This means that the normal temperatures solution can absorbs the lowtemperature methanol vapor, which makes the [MMIm]DMP-methanol solution suitable for absorption refrigeration as the working pair. The solution of working pair can absorb the refrigerant vapor when the saturated vapor pressure of solution is lower than that of the pure refrigerant at the evaporating temperature. As shown in The refrigerant can be separated from the solution of working pair when the saturated vapor pressure of solution is higher than that of the pure refrigerant at the condensing temperature. As shown in  Table 4. Maximum concentration of absorption solution.

Viscosity
Viscosity is an important physical parameter of the binary system containing ionic liquids, and it can largely influence the application of ionic liquids in absorption refrigeration.  Figure 12 and Figure 13.  As shown in Figure 12 and Figure 13

Heat capacity
Heat capacity is a physical quantity which is the important bridge connecting the macroscopic observable thermodynamics quantity and the microscopic molecular structure.
On the basis of heat capacity, the h-ω diagram is mapped, from which we can analyze COP of the whole absorption refrigeration cycle. Fredlake [18] where C P is heat capacity, t is temperature, A=A 1 ·ω 2 +A 2 ·ω+A 3 , B =B 1 ·ω 2 +B 2 ·ω+B 3 . The fitting parameters and average absolute deviation is shown in Table 5. The experimental data and correlate results are shown in Figure 14 and Figure 15.

Summary
This section investigates the application potential in binary system containing ionic liquids and refrigerant as working pairs for absorption refrigeration from three aspects: vapor liquid equilibrium, viscosity and heat capacity. The saturated vapor pressure and heat capacity in plenty of these binary systems are relatively low. The density and viscosity in these systems are moderate. Compared with the traditional working pairs, the new ones have their own advantages. They are non-corrosive and non-crystalline. Since ionic liquids are non-evaporative, this new kind of absorption refrigeration system is no need to set up distillation equipment. The physical and chemical properties in binary system containing ionic liquids and frequently-used refrigerants are still not perfect, and the studies on the application of ionic liquids in absorption refrigeration are still inadequate. With the increasing of the quantity of ionic liquids and the deepening of studies on their physical and chemical properties, ionic liquid absorption refrigeration must become an important part of refrigeration in near future.

Single-effect absorption refrigeration system
A. Yokozeki [7,8] measured the solubility, vapor-liquid equilibrium of binary solutions containing ionic liquids, and computed the specific heat capacity at constant pressure, enthalpy, Gibbs energy, and entropy based on the EOS equation. According to the results conducted, various parameters of ideal single-effect absorption refrigeration system were computed. The quality circulation rate f, solution concentration in generator X g , solution concentration in absorber X a , and COP are shown in Table 6 when the temperatures of the generator, absorber, condenser, and evaporator are 373 K, 313 K, 303 K, and 283 K respectively. The results present that when ionic liquids are used as working pairs with NH 3 , Freon, water, and CO 2 respectively, COP of absorption refrigeration systems are higher than that systems using NH 3 /H 2 O or H 2 O/LiBr. Wang et al. [24] used TFE-[BMIm]Br in double-effect parallel absorption refrigeration system, and the effect of the effectiveness of solution heat exchanger η on COP was analyzed. In the Figure 16, ω 1 , ω 2 , ω 3 , ω 4 stand for COP in four kinds of process respectively. The results show that COP increases markedly with the improvement of η. Therefore, it is important to use high effect solution heat exchanger for double-effect parallel absorption refrigeration. Considering COP of the system and actual heat transfer performance of the heat exchanger, η is chosen to be 0.9. The effects of evaporating temperature, absorbing temperature, high pressure generating temperature on COP, solution circulation ratio, and operating pressure of the system are presented.  Table 6. Parameters of single-effect absorption refrigeration system.
From Figure 17, it can be concluded that double-effect parallel absorption refrigeration using TFE-[BMIm]Br is more suitable for air conditioning. However, COP is relatively low when evaporation temperature is below zero. As shown in Figure 18, COP increases with the high pressure generating temperature. Considering the heat stability of TFE-[BMIm]Br, it is not suitable for the operation when the high pressure generating temperature is more than 500 K.   Liang et al. [25] used [MMIm]DMP-CH 3 OH to analyze the effectiveness of absorption refrigeration system. In order to analyze the effect of various temperatures on the effectiveness of the system, different values were set, and loads of heat equipment, solution concentration, circulation ratio, gas-emission scope, and COP were computed.
As shown in Figure 19, for single-effect absorption refrigeration system, COP increases with the improvement of heat source temperature and evaporating temperature. When heat source and evaporating temperature are 120°C and 10°C respectively, COP gets 0.895. However, it is not suitable for evaporating temperature to be too low. For example, if evaporating temperature is 5°C, and heat source temperature is below 80 °C, solution concentration will not meet the requirements of solution circulation. Fig. 19. The effect of heat resource temperature on COP. Figure 20 and 21 show the variation tendency of gas-emission scope, circulation ratio with heat resource temperature and evaporating temperature. With the improvement of heat source temperature and evaporating temperature, gas-emission scope increases, while circulation ratio decreases. The smaller the gas-emission scope, the bigger the circulation ratio, and the more the heat absorbed by solution liquid, the lower COP. When the evaporating temperatures are 5°C, 7°C, and 10°C respectively, the heat source temperatures should be above 60°C, 70°C and 80°C respectively. Otherwise, the gas-emission scopes will be too small and the circulation ratios will be too big, which will lead to lower effectiveness of the system, huge equipment, and high operating cost. Figure 22, 23 and 24 show the changes of COP, gas-emission scope, and circulation ratio with heat source temperatures at various evaporation temperatures. COP of the system increases with the reduction of the following parameters: condensing temperature, absorbing temperature and circulation ratio, while increases with the improvement of gas-emission scope.       Table 7. Comparison of COP in the system with different working pairs.

Double-effect absorption refrigeration system
Single-effect absorption refrigeration system, with simple structure, convenient operation, and low heat source requirement, is suitable for the situation that the heat source temperature and operating pressure is not too high. However, while the heat source temperature is high, there are several drawbacks of the single system, such as low COP and low utilization rate of energy. In order to utilize energy more efficiently and fully, some other systems are developed, such as double-effect system, three-effect system and multiple    Single-effect absorption refrigeration system, with simple structure, convenient operation, and low heat source requirement, is suitable for the situation that the heat source temperature and operating pressure is not too high. However, while the heat source temperature is high, there are several drawbacks of the single system, such as low COP and low utilization rate of energy. In order to utilize energy more efficiently and fully, some other systems are developed, such as double-effect system, three-effect system and multiple  Figure 25, 26 and 27. Figure 25 shows that COP of the system increases with the improvement of the heat source temperature and evaporating temperature. For example, COP gets 1.06 when the heat source temperature and evaporating temperatures are 160°C and 10°C respectively. However, the evaporating temperature should not be too low. When the evaporating temperature is 5°C, and heat source temperature is under 120°C, the solution concentration of [MMIm]DMP-CH 3 OH will be too low, the circulation ratio will be too high, and the double-effect system will lose its predominance in comparing with the single-effect one. Figure 26 and Figure 27 show the variation tendency of gas-emission scope and circulation ratio with heat source temperature and evaporating temperature for double-effect absorption refrigeration system. With the improvement of heat source temperature and evaporating temperature, gas-emission scope increases, while circulation ratio decreases. The smaller the gas-emission scope, the bigger the circulation ratio, and the more the heat absorbed by solution liquid, the lower COP. When the evaporating temperatures are 5°C, 7°C, and 10°C respectively, the heat source temperatures should be above 120°C, 110°C and 100°C respectively. Otherwise, gas-emission scope is too small and circulation ratio is too big, so that compared with single-effect absorption refrigeration system, double-effect absorption refrigeration system will not have any advantage. Furthermore, the structure double-effect absorption refrigeration system is huger, and the cost is higher. In order to compare the difference between the system using [MMIm]DMP-CH 3 OH and that using the traditional working pairs, such as NH 3 Table 8. Comparison of COP in the system with different working pairs. They also analyzed the flux characteristics of some fundamental parameters with different heat source temperature, condensing temperature, evaporating temperature and absorption temperature. The pressure of the whole cycle is moderate, and there is no distillation equipment, no corrosion and no crystallization in the new type of absorption refrigeration. Compared with the double effect cycle, the single effect cycle is more economical and practical when this new working pairs containing ionic liquids is used. Since the working pairs used by different researchers are not the same, COP of different cycles is different from each other. Further research will focus on the practical applications of the new type absorption refrigeration.

Conclusions and outlook
Based on the low-grade heat source, absorption refrigeration, which owns many advantages such as simple and quiet, along with energy-saving and environmental protection, has huge spaces for development. It is revealed that the binary system, containing ionic liquids and refrigerants as working pairs of absorption refrigeration, has large application potential through the researches on the physical and chemical properties. The new type ionic liquid absorption refrigeration can overcome same defects belonging to the traditional type, such as corrosion, crystallization and requirement of distillation equipment. However, the largescaled industrial application of the new technology is still restricted by several factors as follows. Firstly, COP of some working pairs containing ionic liquids is still not high, requiring that better working pairs with high COP should be screened out. Secondly, the researches are still at the academic stage in defect of the studies on practical design and structure optimization. The experimental units are all theoretical simulation ones, and until now the suitable absorption refrigeration units in practice have not been produced. In addition, the manufacturing costs for the new type of absorption refrigeration are high, for the reason that the price of ionic liquids is high, which prevents the market from popularizing this technology. Overall, the ionic liquid absorption refrigeration has a huge space for development and good market prospect, and it will certainly bring innovative promotion and ground-breaking progress for the absorption refrigeration technology.