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Influences of additives on the gas hydrate cool storage process in a new gas hydrate cool storage system
Energy Conversion and Management 47 (2006) 2974–2982 www.elsevier.com/locate/enconman
Influences of additives on the gas hydrate cool storage process in a new gas hydrate cool storage system Yuehong Bi a
a,b
, Tingwei Guo c, Tingying Zhu c, Liang Zhang c, Lingen Chen
b,*
Institute of Civil and Architectural Engineering, Beijing University of Technology, Beijing 100022, PR China b Postgraduate School, Naval University of Engineering, Wuhan 430033, PR China c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China Received 24 August 2005; received in revised form 1 March 2006; accepted 22 March 2006 Available online 22 May 2006
Abstract Experimental research on the crystallization process of the gas hydrate HCFC141b is performed for this paper. The influences of different proportions of calcium hypochlorite or benzenesulfonic acid sodium salt on the crystallization process are studied. The results show that the degree of subcooling of formation is obviously decreased, and the formation rate of the gas hydrate is greatly accelerated by adding reasonable proportions of the additives. The degree of subcooling of formation decreases 0.78 °C by adding benzenesulfonic acid sodium salt of 0.03%, and the formation rate of the gas hydrate increases 0.2 g/s by adding calcium hypochlorite of 0.08%. In the cool storage system, clathrate hydrates can be formed effectively, and thermal energy can be stored efficiently. When adding benzenesulfonic acid sodium salt of 0.03%, the cold energy stored is 4.74 MJ, and the cool storage density is 206.07 MJ/m3. The performance of this cool storage system can meet the needs of practical air conditioning engineering. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Gas hydrate; Additive; Crystallization process; Cool storage; Cool storage materials; Experimental study
1. Introduction Cool storage technology gradually developed with the appearance of air conditioning systems using mechanical refrigeration, and the development history of cool storage air conditioning technology is the development history of cool storage materials [1–6]. The cool storage materials in common use include water, ice, eutectic salt etc. Water as a cool storage medium utilizes sensible heat, whereas ice and eutectic salt utilize the latent heat of phase change. The phase change temperature of ice is lower, the latent heat of phase change of the eutectic salt is smaller, and the latter causes serious corrosion. These shortcomings urge research and development of new types of high temperature phase change cool storage materials. In 1982, American scientists suggested taking freon and forming a hydrate as a cool storage medium in air conditioning systems, and *
Corresponding author. Tel.: +86 27 83615046; fax: +86 27 83638709. E-mail addresses: [email protected], [email protected] (L. Chen).
0196-8904/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.03.027
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thereafter, the so called ‘‘gas hydrate warm ice’’ technology has been rapidly developed in the USA and Japan [7]. A gas hydrate, also called clathrate, is an ice like crystalline compound that occur when water molecules form a cage like structure around smaller ‘‘guest molecules’’ of gas or easily volatile liquid at certain temperature and pressure [8]. Hydrate crystals can form from the interaction between most refrigerants and water under a hydrate formable temperature (5–12 °C) condition. The phase change latent heat of the gas hydrate is similar to that of ice [9]. Therefore, most refrigerants are considered as the most ideal cool storage medium in air conditioning systems. Chatti et al. [10] reviewed the benefits and drawbacks of clathrate hydrates in their areas of interest in detail. The ideal critical decomposition temperature and pressure of alternative refrigerant R141b are 8.44 °C and 43.0 kPa, respectively, and the latent heat of phase change is 344 kJ/kg. It is adapted to engineering applications because of its low saturated vapor pressure and low cost, and its negative pressure character avoids the difficulty of a pressure vessel. However, the degree of subcooling of the gas hydrate R141b formation is relatively large, and the rate of crystallization is slow. In recent years, a new type of gas hydrate cool storage system has been built by Guo et al. [11] and Lv et al. [12]. In the system, the inner heat exchange/outer crystallization technology and the integrated condenser/evaporator structure design were adopted. By experimental research with this system, one has found that the outer crystallizer can provide a larger cool storage density and higher efficiency of the charge and discharge processes only in a certain range of volume flow rate of the crystallizer, and the influence of additives and surfactants is one of the most important research works in the field of gas hydrate cool storage [13–16]. In this paper, the influences of different proportions of calcium hypochlorite or benzenesulfonic acid sodium salt on the crystallization process of the gas hydrate HCFC141b are experimentally studied. 2. Experimental system and experimental process Gas hydrate R141b cool storage experiments are performed using an efficient gas hydrate cool storage experimental system, which is illustrated in Fig. 1. This experimental system includes cold/hot reservoirs, a gas hydrate cool storage tank and a data acquisition system. The cold/hot reservoirs are composed of a chiller and cold/hot water tanks. A 30% glycol solution is used as the secondary refrigerant. Its volume flow rate is measured through a LZB-15 rotameter (range ability: 40– 400 L/h, precision degree: 2.5). It supplies the cooling solution at the charge stage or the warm solution at the discharge stage for the cool storage tank. The cool storage tank is a flat cube of net volume 23.52 L. The inner and outer shells of the tank are stainless steel. The interval between the inner and outer shell is 50 mm and is filled with polyurethane as the insulation material. Two double layer glass observation windows of 50 mm width and 600 mm height at the
Fig. 1. Schematic of gas hydrate cool storage experimental system. (1) Inlet of second refrigerant; (2) vacuumizing joint; (3) sealed pins for connecting thermocouples; (4) sprinkler vertical pipe; (5) sealed pins for connecting thermocouples; (6) joint of vacuum pressure gauge; (7) outlet of second refrigerant; (8) liquid ejection pipe; (9)/(10) drop pipe; (11) sprinkler pipe; (12) crystallizer; (13) flow meter; (14) circumfluence pipe; (15) cool storage coil; (16) window; (17) cool storage tank; (18) ice bottle; (19) scan card; (20) digital voltmeter; (21) computer; (22) chiller; (23) cold/hot water tank; (24) second refrigerant pump and (25) electric heater.
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position of the midline of the cool storage tank are used to observe and video record the gas hydrate crystallization and decomposition processes. Parallel connection heat exchanger coils with the same configuration are installed in the tank. The vertical pipes and horizontal branch pipes of the coil are copper with outer diameters of 20 mm and 8 mm, respectively, and a wall thickness of 1.0 mm. The total outer heat transfer surface area of the heat exchanger is 0.326 m2. There are 20 custom made Cu–CuNi (copper–constantan) thermocouples to measure the temperatures of the cool storage medium in the tank, the pipe walls and the secondary refrigerant. The thermocouples are made of copper and constantan wire with the diameter of 0.1 mm. The National Institute of Metrology of China calibrated to guarantee the accuracy of the thermocouples. The inaccuracy of the thermocouples is up to ±0.05 °C. The measured temperature data of 10 thermocouples are automatically collected by the data acquisition system. The data acquisition system includes a KEITHLEY2000 digital voltmeter, an inserted KEITHLEY2000 scan card, a computer with CEC488 card and an IEEE488 cable conductor. The other 10 thermocouples, which connect with the digital voltmeter (KEITHLEY2000) using a manual switch of type B385 without thermoelectric potential, assist in measuring temperature. The resolution of the KEITHLEY2000 digital voltmeter is 0.1 lV, the corresponding measuring accuracy is 0.005 °C. The structure of the cool storage tank and the distribution of the thermocouples in the tank are shown in Fig. 2. Thermocouples T1 and T16 are used for measuring the temperatures of the vapor layer, T17 for the temperature of the interface between the vapor and liquid, T2 and T10 for the temperatures of the upper water, T3, T4, T12, T5, T9 and T13 for the temperatures of water or clathrate, T6 and T15 for the upper and lower temperatures near the interface between the water and R141b liquid, T18 for the temperature of the gas hydrate or R141b liquid, T7, T8, T11 and T14 for the temperatures of the pipe wall near the R141b liquid, clathrate and water and T19 and T20 for the inlet and outlet temperatures of the secondary refrigerant. Water and R141b have been filled in the tank according to the experimental procedure before starting the experiment. R141b liquid is located at the bottom of the tank, and the height of R141b liquid is 140 mm above which is water with the height of 450 mm. The remaining space in the tank is for R141b vapour and a lot of water vapour. In the first four experiments, calcium hypochlorite of 0.01%, 0.03%, 0.08% and 0.16% were
Fig. 2. The structure of the cool storage and the distribution of thermocouples in the tank.
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added according to mass proportions. In two later experiments, benzenesulfonic acid sodium salt of 0.03% and 0.08% were added according to mass proportions. The refrigeration cycle, secondary refrigerant cycle and the pipelines cycle of crystallizer are synchronously turned on, the volume flow rate of secondary refrigerant is regulated to 400 L/h and the volume flow rate of the crystallizer is regulated to 300 L/h. 3. Functions of additives 3.1. Functions of calcium hypochlorite Calcium hypochlorite belongs to a sort of chlorite salt. It exists in the solution in the form of Ca(ClO)2 Æ xCa(OH)2 Æ yH2O, which is a nucleation substance. It can rapidly form a crystal nucleus. In addition, calcium hypochlorite is an extremely versatile cleaning chemical to clear away impurities and a very strong oxidizer that can break down and destroy organic debris in the water and R141b. The impurities can be adsorbed on the surface of the crystal nucleus or the growing crystal, which reduces the nucleation rate and crystal growth rate in case the impurities cannot become the nucleus of heterogeneous nucleation. Therefore, adding calcium hypochlorite can reduce the degree of subcooling of the formation and accelerate the process of hydrate reaction. The function is more evident when the proportion of calcium hypochlorite is suitably increased in the range of its solubility (20/10000). 3.2. Function of benzenesulfonic acid sodium salt Benzenesulfonic acid sodium salt belongs to a sort of non-ion type surfactant. Being added in R141b and water, on the one hand, it can decreases the surface tension of the water, decrease the diameter of bubbles and increase the rising frequency of bubbles, thereby decreasing the resistance of mass transfer of the gas molecules. On the other hand, it can make the R141b liquid emulsify to become tiny liquid drops. Thus, the effective interface area between the two phases is greatly increased, and a great deal of hydrate nuclei is produced. In short, by means of decreasing the surface tension and producing emulsification between the two insoluble phases, benzenesulfonic acid sodium salt can make liquid drops of the hydrate medium R141b suspend in the water or make drops of water suspend in the liquid of the hydrate medium R141b. This leads to a certain quantity of water molecules getting the chance of surrounding the molecules of the hydrate medium. Thereby, the required time of forming the reticulated molecule arrangement structure is shortened, the hydrate reaction rate is increased and the degree of subcooling of hydrate formation is decreased. 4. Experimental results and discussion 4.1. Experimental results of gas hydrate formation process It is found by experimental observation that the boundary between the water and R141b is evident, and the layer of water is clear and transparent with adding calcium hypochlorite. However, with adding benzenesulfonic acid sodium salt, the liquid is emulsified to a nontransparent creaming, the layer of water becomes cloudy, the diameter of rising bubbles obviously decreases and the rising frequency of the bubbles observably increases. The crystallization process obviously changes with adding calcium hypochlorite or benzenesulfonic acid sodium salt. Flocculent gas hydrates, at first, are quickly formed in the field of the top five rows of pipe. Then, they gradually fall flakily and are sedimentated in the fields of the middle and bottom rows of the pipes, and finally, the total cool storage tank is fulfilled with relatively homogeneous gas hydrates. This phenomenon is not the same as that without adding any additives. Most of the gas hydrates surround the pipe wall if no additives are added. The gas hydrates formation temperature and rate obviously increase with adding the additives. The temperature variations at the measuring points in the cool storage tank during cold charging with the volume flow rate of the crystallizer of 300 L/h without adding any additives is shown in Fig. 3. The temperature variations at the measuring points in the cool storage tank during cold charging with adding calcium hypochlorite of 0.08% is shown in Fig. 4. The temperature variations at the measuring points in the cool storage tank during cold charging with adding benzenesulfonic acid sodium salt of 0.03% is shown in Fig. 5. One can see from
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Fig. 3. Temperature variation of measuring points in the cool storage tank during cold charging without adding any additives and with the volume flow rate of the crystallizer 300 L/h.
Fig. 4. Temperature variation at measuring points in the cool storage tank during cold charging with adding calcium hypochlorite of 0.08%.
Figs. 3–5 that at the initial stage, the temperatures of all points conformably decrease to the lowest temperature, which is called the hydrate reaction temperature. Because of the parallel connection heat exchanger coils
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Fig. 5. Temperature variation at measuring points in the cool storage tank during cold charging with adding benzenesulfonic acid sodium salt of 0.03%.
with the same configuration in the designed device, which are composed of 16 branch pipes with the same resistance, the flow rates of the second refrigerant in the branch pipes are the same. In addition, the relatively larger flow rate of the second refrigerant and the spraying and agitation of the crystallizer assure that the temperatures of the measuring points in the cool storage medium are almost identical. One can see from the temperature variation curves that the hydrate reaction temperature is 6.07 °C in Fig. 3, 6.2 °C in Fig. 4 and 6.85 °C in Fig. 5. Beyond this point, the temperatures of all points rise again simultaneously, which indicates that the gas hydrate begins to form. Because of the differences in the places of formation and retention of the gas hydrates, the temperatures of the measuring points in the cool storage tank during the crystallization process are different. When the temperatures of the entire cool storage tank decrease, the process of cool storage is finished. The required time of the cool storage process in Fig. 4 is 68% of that of the process without adding any additives and that in Fig. 5 is 92%. 4.2. Influences of additives on the degree of subcooling of crystallization (DTsc) The phase diagram of R141b/water [17] indicates that the critical decomposition point is at 8.4 °C and 0.043 MPa. In the experiment, the temperature at which gas hydrates are formed is called the hydrate reaction temperature. The mean value of all point temperatures is used as the hydrate reaction temperature in this paper. The difference between the critical decomposition temperature and the hydrate reaction temperature is called the degree of subcooling of the gas hydrate formation. Both calcium hypochlorite and benzenesulfonic acid sodium salt can decrease the degree of subcooling of hydrate formation. The function of benzenesulfonic acid sodium salt is more notable. The degree of subcooling of gas hydrate formation decreases from 2.33 °C when no additives are used to 1.55 °C when adding benzenesulfonic acid sodium salt of 0.03%. Fig. 6 shows the degree of subcooling of gas hydrate formation at different conditions. 4.3. Influences of additives on the formation rate of gas hydrate (W) The mass of gas hydrate formed in the hydrate reaction process per unit time is called the formation rate of gas hydrate. Both calcium hypochlorite and benzenesulfonic acid sodium salt can increase the formation rate of gas hydrate. The function of calcium hypochlorite is more notable. The formation rate of gas hydrate
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Fig. 6. The degree of subcooling of gas hydrate formation at different conditions (with volume flow rate of the crystallizer 300 L/h).
Fig. 7. The calculated formation rates of gas hydrate versus time (with volume flow rate of the crystallizer 300 L/h).
increases from 0.39 g/s when no additives are used to 0.59 g/s when adding calcium hypochlorite of 0.08%. Fig. 7 shows the calculated formation rates of gas hydrate versus time. The corresponding mean formation rates of gas hydrate are also shown in Fig. 7.
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Fig. 8. Comparison of the cold energy stored at different condition (with volume flow rate of the crystallizer 300 L/h).
4.4. Influences of additives on the cold energy stored (Q) If the cool storage process has finished, the total latent energy of phase change of the R141b gas hydrates is called the cold energy stored. The cold energy stored per unit volume of cool storage tank is called the cool storage density. The comparison of the cold energy stored at different conditions is shown in Fig. 8. With adding the additives, the hydrate reaction becomes more perfect, and the entire volume of the cool storage tank is filled with more homogeneous gas hydrates. Therefore, both the cold energy stored and the cool storage density increase. When adding benzenesulfonic acid sodium salt of 0.03%, the cold energy stored is 4.74 MJ, and the cool storage density is 206.07 MJ/m3. They are obviously higher than those without adding any additives [12], and the performance of this gas hydrate cool storage system is closer to that of the commercial ice cool storage system. 5. Conclusion The experimental study has confirmed that both calcium hypochlorite and benzenesulfonic acid sodium salt can notably influence the cool storage processes. Both additives can shorten the required time of the cool storage process. In this aspect, the effect of calcium hypochlorite is better than that of benzenesulfonic acid sodium salt. The required time of the cool storage process with adding calcium hypochlorite of 0.08% (cold energy stored is 3.94 MJ) is shortened to 2/3 of that without adding any additives (cold energy stored is 3.87 MJ). Both additives can decrease the degree of subcooling of the gas hydrate formation. In this aspect, the function of benzenesulfonic acid sodium salt is more notable than that of calcium hypochlorite. The degree of subcooling of gas hydrate formation decreases from 2.33 °C when no additives are used to 1.55 °C when adding benzenesulfonic acid sodium salt of 0.03%. By adding the additives, the formation rate of gas hydrate and cool storage density obviously increase. When adding benzenesulfonic acid sodium salt of 0.03%, the cold energy stored is 4.74 MJ and the cool storage density is 206.07 MJ/m3. The performance of this cool storage system can meet the needs of practical air conditioning engineering. This experimental research helps to popularize and utilize gas hydrate cool storage technology in the practice of engineering. Acknowledgement This paper is supported by The National Natural Science Foundation of the PR China (Project No. 59836230).
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References [1] Dincer I, Rosen MA. Thermal energy storage systems and applications. New York: Wiley; 2001. [2] Dincer I, Rosen MA. Energetic, environmental and economic aspects of thermal energy storage systems for cooling capacity. Appl Therm Eng 2001;21(10):1105–17. [3] Dincer I. On thermal energy storage systems and applications in buildings. Energy Buildings 2002;34(4):377–88. [4] Dincer I. Thermal energy storage systems as a key technology in energy conservation. Int J Energy Res 2002;26(7):567–88. [5] Saito A. Recent advances in research on cold thermal energy storage. Int J Refrig 2002;25(2):177–89. [6] Hasnain SM. Review on sustainable thermal energy storage technologies, part I: heat storage materials and techniques; part II: cool thermal storage. Energy Convers Manage 1998;39(11):1127–53. [7] Tomlinson JJ. Heat pump cool storage in a clathrate of freon. In: Proc 17th IECEC, Los Angeles, USA. American Institute of Chemical Engineers 1982 (4): 2060–4. [8] Makogon YF. Hydrates of hydrocarbons. Oklahoma: Penn Well Books; 1997. [9] Tanino M, Kozawa Y. Ice-water two-phase flow behavior in ice heat storage systems. Int J Refrig 2001;24(7):639–51. [10] Chatti I, Delahaye A, Fournaison L, Petitet JP. Benefits and drawbacks of clathrate hydrates: a review of their areas of interest. Energy Convers Manage 2005;46(9–10):1333–43. [11] Guo K, Shu B, Meng Y, Zhao Y, Zeng L. A research on warm ice cool storage technology. In: Proc 3rd Int Symp HVAC, 1999(1): 565–72. [12] Lv C, Guo T, Zhu T, Guo K. Experimental study of gas hydrates cool storage. J Refrig 2000;2:14–6 (in Chinese). [13] Wan S, Li F, Xiao L. Effect of alcoholic additives on the characteristics of R134a gas hydrate. In: Proc 3rd Int Conf on Gas Hydrate. Salt Lake City, USA, July, 1999: 158–62. [14] Guo Y, Fan S, Guo K, Shi L, Chen Y. Methane storage via hydrate formation using calcium hypochlorite as additive. J Chem Industry Eng 2002;53(5):452–3 (in Chinese). [15] Sun Z, Wang R, Ma R, Guo K, Fan S. Effect of surfactants and liquid hydrocarbons on gas hydrate formation rate and storage capacity. Int J Energy Res 2003;27(8):747–56. [16] Bi Y, Guo T, Zhu T, Fan S, Liang D, Zhang L. Influence of volumetric-flow rate in the crystallizer on the gas-hydrate cool-storage process in a new gas-hydrate cool-storage system. Appl Energy 2004;78(1):111–21. [17] Ohmura R, Shigetomi T, Mori YH. Formation, growth and dissociation of clathrate hydrate crystals in liquid water in contact with a hydrophobic hydrate-forming liquid. J Cryst Growth 1999;196(1):164–73.
Influences of additives on the gas hydrate cool storage process in a new gas hydrate cool storage system Yuehong Bi a
a,b
, Tingwei Guo c, Tingying Zhu c, Liang Zhang c, Lingen Chen
b,*
Institute of Civil and Architectural Engineering, Beijing University of Technology, Beijing 100022, PR China b Postgraduate School, Naval University of Engineering, Wuhan 430033, PR China c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China Received 24 August 2005; received in revised form 1 March 2006; accepted 22 March 2006 Available online 22 May 2006
Abstract Experimental research on the crystallization process of the gas hydrate HCFC141b is performed for this paper. The influences of different proportions of calcium hypochlorite or benzenesulfonic acid sodium salt on the crystallization process are studied. The results show that the degree of subcooling of formation is obviously decreased, and the formation rate of the gas hydrate is greatly accelerated by adding reasonable proportions of the additives. The degree of subcooling of formation decreases 0.78 °C by adding benzenesulfonic acid sodium salt of 0.03%, and the formation rate of the gas hydrate increases 0.2 g/s by adding calcium hypochlorite of 0.08%. In the cool storage system, clathrate hydrates can be formed effectively, and thermal energy can be stored efficiently. When adding benzenesulfonic acid sodium salt of 0.03%, the cold energy stored is 4.74 MJ, and the cool storage density is 206.07 MJ/m3. The performance of this cool storage system can meet the needs of practical air conditioning engineering. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Gas hydrate; Additive; Crystallization process; Cool storage; Cool storage materials; Experimental study
1. Introduction Cool storage technology gradually developed with the appearance of air conditioning systems using mechanical refrigeration, and the development history of cool storage air conditioning technology is the development history of cool storage materials [1–6]. The cool storage materials in common use include water, ice, eutectic salt etc. Water as a cool storage medium utilizes sensible heat, whereas ice and eutectic salt utilize the latent heat of phase change. The phase change temperature of ice is lower, the latent heat of phase change of the eutectic salt is smaller, and the latter causes serious corrosion. These shortcomings urge research and development of new types of high temperature phase change cool storage materials. In 1982, American scientists suggested taking freon and forming a hydrate as a cool storage medium in air conditioning systems, and *
Corresponding author. Tel.: +86 27 83615046; fax: +86 27 83638709. E-mail addresses: [email protected], [email protected] (L. Chen).
0196-8904/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.03.027
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thereafter, the so called ‘‘gas hydrate warm ice’’ technology has been rapidly developed in the USA and Japan [7]. A gas hydrate, also called clathrate, is an ice like crystalline compound that occur when water molecules form a cage like structure around smaller ‘‘guest molecules’’ of gas or easily volatile liquid at certain temperature and pressure [8]. Hydrate crystals can form from the interaction between most refrigerants and water under a hydrate formable temperature (5–12 °C) condition. The phase change latent heat of the gas hydrate is similar to that of ice [9]. Therefore, most refrigerants are considered as the most ideal cool storage medium in air conditioning systems. Chatti et al. [10] reviewed the benefits and drawbacks of clathrate hydrates in their areas of interest in detail. The ideal critical decomposition temperature and pressure of alternative refrigerant R141b are 8.44 °C and 43.0 kPa, respectively, and the latent heat of phase change is 344 kJ/kg. It is adapted to engineering applications because of its low saturated vapor pressure and low cost, and its negative pressure character avoids the difficulty of a pressure vessel. However, the degree of subcooling of the gas hydrate R141b formation is relatively large, and the rate of crystallization is slow. In recent years, a new type of gas hydrate cool storage system has been built by Guo et al. [11] and Lv et al. [12]. In the system, the inner heat exchange/outer crystallization technology and the integrated condenser/evaporator structure design were adopted. By experimental research with this system, one has found that the outer crystallizer can provide a larger cool storage density and higher efficiency of the charge and discharge processes only in a certain range of volume flow rate of the crystallizer, and the influence of additives and surfactants is one of the most important research works in the field of gas hydrate cool storage [13–16]. In this paper, the influences of different proportions of calcium hypochlorite or benzenesulfonic acid sodium salt on the crystallization process of the gas hydrate HCFC141b are experimentally studied. 2. Experimental system and experimental process Gas hydrate R141b cool storage experiments are performed using an efficient gas hydrate cool storage experimental system, which is illustrated in Fig. 1. This experimental system includes cold/hot reservoirs, a gas hydrate cool storage tank and a data acquisition system. The cold/hot reservoirs are composed of a chiller and cold/hot water tanks. A 30% glycol solution is used as the secondary refrigerant. Its volume flow rate is measured through a LZB-15 rotameter (range ability: 40– 400 L/h, precision degree: 2.5). It supplies the cooling solution at the charge stage or the warm solution at the discharge stage for the cool storage tank. The cool storage tank is a flat cube of net volume 23.52 L. The inner and outer shells of the tank are stainless steel. The interval between the inner and outer shell is 50 mm and is filled with polyurethane as the insulation material. Two double layer glass observation windows of 50 mm width and 600 mm height at the
Fig. 1. Schematic of gas hydrate cool storage experimental system. (1) Inlet of second refrigerant; (2) vacuumizing joint; (3) sealed pins for connecting thermocouples; (4) sprinkler vertical pipe; (5) sealed pins for connecting thermocouples; (6) joint of vacuum pressure gauge; (7) outlet of second refrigerant; (8) liquid ejection pipe; (9)/(10) drop pipe; (11) sprinkler pipe; (12) crystallizer; (13) flow meter; (14) circumfluence pipe; (15) cool storage coil; (16) window; (17) cool storage tank; (18) ice bottle; (19) scan card; (20) digital voltmeter; (21) computer; (22) chiller; (23) cold/hot water tank; (24) second refrigerant pump and (25) electric heater.
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position of the midline of the cool storage tank are used to observe and video record the gas hydrate crystallization and decomposition processes. Parallel connection heat exchanger coils with the same configuration are installed in the tank. The vertical pipes and horizontal branch pipes of the coil are copper with outer diameters of 20 mm and 8 mm, respectively, and a wall thickness of 1.0 mm. The total outer heat transfer surface area of the heat exchanger is 0.326 m2. There are 20 custom made Cu–CuNi (copper–constantan) thermocouples to measure the temperatures of the cool storage medium in the tank, the pipe walls and the secondary refrigerant. The thermocouples are made of copper and constantan wire with the diameter of 0.1 mm. The National Institute of Metrology of China calibrated to guarantee the accuracy of the thermocouples. The inaccuracy of the thermocouples is up to ±0.05 °C. The measured temperature data of 10 thermocouples are automatically collected by the data acquisition system. The data acquisition system includes a KEITHLEY2000 digital voltmeter, an inserted KEITHLEY2000 scan card, a computer with CEC488 card and an IEEE488 cable conductor. The other 10 thermocouples, which connect with the digital voltmeter (KEITHLEY2000) using a manual switch of type B385 without thermoelectric potential, assist in measuring temperature. The resolution of the KEITHLEY2000 digital voltmeter is 0.1 lV, the corresponding measuring accuracy is 0.005 °C. The structure of the cool storage tank and the distribution of the thermocouples in the tank are shown in Fig. 2. Thermocouples T1 and T16 are used for measuring the temperatures of the vapor layer, T17 for the temperature of the interface between the vapor and liquid, T2 and T10 for the temperatures of the upper water, T3, T4, T12, T5, T9 and T13 for the temperatures of water or clathrate, T6 and T15 for the upper and lower temperatures near the interface between the water and R141b liquid, T18 for the temperature of the gas hydrate or R141b liquid, T7, T8, T11 and T14 for the temperatures of the pipe wall near the R141b liquid, clathrate and water and T19 and T20 for the inlet and outlet temperatures of the secondary refrigerant. Water and R141b have been filled in the tank according to the experimental procedure before starting the experiment. R141b liquid is located at the bottom of the tank, and the height of R141b liquid is 140 mm above which is water with the height of 450 mm. The remaining space in the tank is for R141b vapour and a lot of water vapour. In the first four experiments, calcium hypochlorite of 0.01%, 0.03%, 0.08% and 0.16% were
Fig. 2. The structure of the cool storage and the distribution of thermocouples in the tank.
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added according to mass proportions. In two later experiments, benzenesulfonic acid sodium salt of 0.03% and 0.08% were added according to mass proportions. The refrigeration cycle, secondary refrigerant cycle and the pipelines cycle of crystallizer are synchronously turned on, the volume flow rate of secondary refrigerant is regulated to 400 L/h and the volume flow rate of the crystallizer is regulated to 300 L/h. 3. Functions of additives 3.1. Functions of calcium hypochlorite Calcium hypochlorite belongs to a sort of chlorite salt. It exists in the solution in the form of Ca(ClO)2 Æ xCa(OH)2 Æ yH2O, which is a nucleation substance. It can rapidly form a crystal nucleus. In addition, calcium hypochlorite is an extremely versatile cleaning chemical to clear away impurities and a very strong oxidizer that can break down and destroy organic debris in the water and R141b. The impurities can be adsorbed on the surface of the crystal nucleus or the growing crystal, which reduces the nucleation rate and crystal growth rate in case the impurities cannot become the nucleus of heterogeneous nucleation. Therefore, adding calcium hypochlorite can reduce the degree of subcooling of the formation and accelerate the process of hydrate reaction. The function is more evident when the proportion of calcium hypochlorite is suitably increased in the range of its solubility (20/10000). 3.2. Function of benzenesulfonic acid sodium salt Benzenesulfonic acid sodium salt belongs to a sort of non-ion type surfactant. Being added in R141b and water, on the one hand, it can decreases the surface tension of the water, decrease the diameter of bubbles and increase the rising frequency of bubbles, thereby decreasing the resistance of mass transfer of the gas molecules. On the other hand, it can make the R141b liquid emulsify to become tiny liquid drops. Thus, the effective interface area between the two phases is greatly increased, and a great deal of hydrate nuclei is produced. In short, by means of decreasing the surface tension and producing emulsification between the two insoluble phases, benzenesulfonic acid sodium salt can make liquid drops of the hydrate medium R141b suspend in the water or make drops of water suspend in the liquid of the hydrate medium R141b. This leads to a certain quantity of water molecules getting the chance of surrounding the molecules of the hydrate medium. Thereby, the required time of forming the reticulated molecule arrangement structure is shortened, the hydrate reaction rate is increased and the degree of subcooling of hydrate formation is decreased. 4. Experimental results and discussion 4.1. Experimental results of gas hydrate formation process It is found by experimental observation that the boundary between the water and R141b is evident, and the layer of water is clear and transparent with adding calcium hypochlorite. However, with adding benzenesulfonic acid sodium salt, the liquid is emulsified to a nontransparent creaming, the layer of water becomes cloudy, the diameter of rising bubbles obviously decreases and the rising frequency of the bubbles observably increases. The crystallization process obviously changes with adding calcium hypochlorite or benzenesulfonic acid sodium salt. Flocculent gas hydrates, at first, are quickly formed in the field of the top five rows of pipe. Then, they gradually fall flakily and are sedimentated in the fields of the middle and bottom rows of the pipes, and finally, the total cool storage tank is fulfilled with relatively homogeneous gas hydrates. This phenomenon is not the same as that without adding any additives. Most of the gas hydrates surround the pipe wall if no additives are added. The gas hydrates formation temperature and rate obviously increase with adding the additives. The temperature variations at the measuring points in the cool storage tank during cold charging with the volume flow rate of the crystallizer of 300 L/h without adding any additives is shown in Fig. 3. The temperature variations at the measuring points in the cool storage tank during cold charging with adding calcium hypochlorite of 0.08% is shown in Fig. 4. The temperature variations at the measuring points in the cool storage tank during cold charging with adding benzenesulfonic acid sodium salt of 0.03% is shown in Fig. 5. One can see from
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Fig. 3. Temperature variation of measuring points in the cool storage tank during cold charging without adding any additives and with the volume flow rate of the crystallizer 300 L/h.
Fig. 4. Temperature variation at measuring points in the cool storage tank during cold charging with adding calcium hypochlorite of 0.08%.
Figs. 3–5 that at the initial stage, the temperatures of all points conformably decrease to the lowest temperature, which is called the hydrate reaction temperature. Because of the parallel connection heat exchanger coils
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Fig. 5. Temperature variation at measuring points in the cool storage tank during cold charging with adding benzenesulfonic acid sodium salt of 0.03%.
with the same configuration in the designed device, which are composed of 16 branch pipes with the same resistance, the flow rates of the second refrigerant in the branch pipes are the same. In addition, the relatively larger flow rate of the second refrigerant and the spraying and agitation of the crystallizer assure that the temperatures of the measuring points in the cool storage medium are almost identical. One can see from the temperature variation curves that the hydrate reaction temperature is 6.07 °C in Fig. 3, 6.2 °C in Fig. 4 and 6.85 °C in Fig. 5. Beyond this point, the temperatures of all points rise again simultaneously, which indicates that the gas hydrate begins to form. Because of the differences in the places of formation and retention of the gas hydrates, the temperatures of the measuring points in the cool storage tank during the crystallization process are different. When the temperatures of the entire cool storage tank decrease, the process of cool storage is finished. The required time of the cool storage process in Fig. 4 is 68% of that of the process without adding any additives and that in Fig. 5 is 92%. 4.2. Influences of additives on the degree of subcooling of crystallization (DTsc) The phase diagram of R141b/water [17] indicates that the critical decomposition point is at 8.4 °C and 0.043 MPa. In the experiment, the temperature at which gas hydrates are formed is called the hydrate reaction temperature. The mean value of all point temperatures is used as the hydrate reaction temperature in this paper. The difference between the critical decomposition temperature and the hydrate reaction temperature is called the degree of subcooling of the gas hydrate formation. Both calcium hypochlorite and benzenesulfonic acid sodium salt can decrease the degree of subcooling of hydrate formation. The function of benzenesulfonic acid sodium salt is more notable. The degree of subcooling of gas hydrate formation decreases from 2.33 °C when no additives are used to 1.55 °C when adding benzenesulfonic acid sodium salt of 0.03%. Fig. 6 shows the degree of subcooling of gas hydrate formation at different conditions. 4.3. Influences of additives on the formation rate of gas hydrate (W) The mass of gas hydrate formed in the hydrate reaction process per unit time is called the formation rate of gas hydrate. Both calcium hypochlorite and benzenesulfonic acid sodium salt can increase the formation rate of gas hydrate. The function of calcium hypochlorite is more notable. The formation rate of gas hydrate
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Fig. 6. The degree of subcooling of gas hydrate formation at different conditions (with volume flow rate of the crystallizer 300 L/h).
Fig. 7. The calculated formation rates of gas hydrate versus time (with volume flow rate of the crystallizer 300 L/h).
increases from 0.39 g/s when no additives are used to 0.59 g/s when adding calcium hypochlorite of 0.08%. Fig. 7 shows the calculated formation rates of gas hydrate versus time. The corresponding mean formation rates of gas hydrate are also shown in Fig. 7.
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Fig. 8. Comparison of the cold energy stored at different condition (with volume flow rate of the crystallizer 300 L/h).
4.4. Influences of additives on the cold energy stored (Q) If the cool storage process has finished, the total latent energy of phase change of the R141b gas hydrates is called the cold energy stored. The cold energy stored per unit volume of cool storage tank is called the cool storage density. The comparison of the cold energy stored at different conditions is shown in Fig. 8. With adding the additives, the hydrate reaction becomes more perfect, and the entire volume of the cool storage tank is filled with more homogeneous gas hydrates. Therefore, both the cold energy stored and the cool storage density increase. When adding benzenesulfonic acid sodium salt of 0.03%, the cold energy stored is 4.74 MJ, and the cool storage density is 206.07 MJ/m3. They are obviously higher than those without adding any additives [12], and the performance of this gas hydrate cool storage system is closer to that of the commercial ice cool storage system. 5. Conclusion The experimental study has confirmed that both calcium hypochlorite and benzenesulfonic acid sodium salt can notably influence the cool storage processes. Both additives can shorten the required time of the cool storage process. In this aspect, the effect of calcium hypochlorite is better than that of benzenesulfonic acid sodium salt. The required time of the cool storage process with adding calcium hypochlorite of 0.08% (cold energy stored is 3.94 MJ) is shortened to 2/3 of that without adding any additives (cold energy stored is 3.87 MJ). Both additives can decrease the degree of subcooling of the gas hydrate formation. In this aspect, the function of benzenesulfonic acid sodium salt is more notable than that of calcium hypochlorite. The degree of subcooling of gas hydrate formation decreases from 2.33 °C when no additives are used to 1.55 °C when adding benzenesulfonic acid sodium salt of 0.03%. By adding the additives, the formation rate of gas hydrate and cool storage density obviously increase. When adding benzenesulfonic acid sodium salt of 0.03%, the cold energy stored is 4.74 MJ and the cool storage density is 206.07 MJ/m3. The performance of this cool storage system can meet the needs of practical air conditioning engineering. This experimental research helps to popularize and utilize gas hydrate cool storage technology in the practice of engineering. Acknowledgement This paper is supported by The National Natural Science Foundation of the PR China (Project No. 59836230).
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