- Email: [email protected]
R134a pair
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
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Experimental study on adsorptionedesorption characteristics of granular activated carbon/R134a pair Ahmed A. Askalany a,*, M. Salem a, I.M. Ismail b, Ahmed Hamza H. Ali c, M.G. Morsy b a
Mechanical Department, Faculty of Industrial Education, Sohag University, Sohag, Egypt Department of Mechanical Engineering, Faculty of Engineering, Assiut University, Assiut, Egypt c Department of Energy Resources and Environmental Engineering, Egypt-Japan University of Science and Technology (E-JUST), Egypt b
article info
abstract
Article history:
Experimental runs were done to estimate the adsorption characteristics of granular acti-
Received 3 February 2011
vated carbon (GAC)/R134a pair. A laboratory scale test rig was designed and built to run the
Received in revised form
experiments. The adsorption capacity of the GAC was studied at different temperatures
25 March 2011
25 C, 35 C, 45 C and 65 C. Pressure and time were recorded during the experiments. The
Accepted 1 April 2011
maximum adsorption capacity was found to be 1.68 kgR134a kgcarbon1 at 25 C after 1000 s.
Available online 8 April 2011
The activation energy and the exponential constant were estimated to be 9575 J mol1 and 1.83 respectively. ª 2011 Elsevier Ltd and IIR. All rights reserved.
Keywords: Adsorption Activated carbon Cooling R134a
Etude expe´rimentale sur les caracte´ristiques d’adsorption / de´sorption d’un couple actif charbon actif granule´ / R134a Mots cle´s : adsorption ; charbon actif ; refroidissement ; R134a
1.
Introduction
Adsorption heat pumps consume low-grade energy to achieve a cooling effect. This makes the development of these systems become an attractive research topic (Jung-Yang San and WeiMin Lin, 2008; Wang et al., 2006). The adsorption cooling and refrigeration systems have the advantages of being compact, free or nearly free of moving parts, efficiently driven by lowtemperature waste heat or renewable energy sources, free of
toxic and environmentally harmful substances as these systems can use natural refrigerants such as water, ethanol, methanol, ammonia etc. and do not require any synthetic lubricants (Habib et al., 2011; Kashiwagi et al., 2002; Saha et al., 2001). Applications of adsorption refrigeration are very limited. More efforts are necessary for development its performance and extend its area of application. The adsorption working pairs are vital in enhancing the performance of adsorption refrigeration systems. New adsorption pairs should be
* Corresponding author. Tel.: þ20 125680400. E-mail address: [email protected] (A.A. Askalany). 0140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2011.04.002
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
Nomenclature A a b E hfg n P Pi Pf Ps
Area of adsorbent particle, m2 Constant Constant Characteristic energy, J mol1 Latent heat, kJ kg1 Exponential constant Pressure, bar Pressure before desorption, bar Pressure after desorption, bar Saturation pressure, bar
investigated, and their characteristics in adsorption cooling should be defined. The selection of any pair of adsorbent/adsorbate for refrigeration applications depends on certain desirable characteristics of their constituents. These characteristics range from their thermodynamic and chemical properties to their physical properties and even to their costs or availability. The characteristics of refrigerants and adsorbent were classified by Alghoul et al. (2007) and Wang et al. (2010). The adsorption characteristics of R134a on three types of activated carbons were measured by Akkimaradi et al. (2001). El-Sharkawy et al. (2006) evaluated adsorption parameters of ethanol on activated carbon fiber of type (A-20). By using renewable energy source, Pons and Guilleminot (1986) studied activated carbon/methanol system for ice production. Chan et al. (1984) measured experimental data of hydrogen, helium, neon and nitrogen on activated charcoal. Riffat et al. (1997) determined adsorption blends of HFC-32, HFC-125 and R-134a with AX-21. In this research, the adsorption capacity rates of R134a onto granular activated carbon (GAC) have been measured within the temperatures ranging from 25 to 65 C for adsorption cooling and refrigeration applications purposes. Using the constant volume, constant temperature and variable pressure, the instantaneous capacity of R134a has been recorded at each 60 s. Desorption characteristics the pair were measured at different temperatures.
2.
Experimental section
2.1.
Experimental test rig
A test rig was designed and built as shown in Fig. 1a and b to run the experiments. The test rig consists of a water tank containing an adsorption bed, heater and a refrigerant bottle. The bed is a cylinder made of carbon steel contains 500 g of GAC. Properties of GAC based on bituminous coal are tabulated in Table 1. A refrigerant grader glass bottle has a capacity of 1.5 0.001 L. An electric heater of 2 kW was used to heat the water inside the water tank. Type J thermocouples were established inside the bed and the water tank to measure temperatures. The adsorption cylinder and the bottle are connected to a pressure gauge to measure the pressure. The connection between the bed and the bottle is a non-return
qst R Sd Sd0 T Tcr Tdes t V W W0
495
Isosteric heat of adsorption, J mol1 Universal gas constant, 8.314 J mol1 K1 Surface diffusion, m2 s1 Pre-exponential coefficient Temperature, K Critical temperature of R134a, K Desorption temperature, K Time, s Volume of adsorbent particle, m3 Adsorption mass capacity, kgR134a kgcarbon1 Maximum adsorption capacity, kgR134a kgcarbon1
valve fixed at bed side and connected directly to a needle valve on the other side. When the bed is separated from the system to be weighted, the needle valve is closed and it is opened again after rejoining the bed. The non-return valve prevents outside air to enter the bed during weighing process. This mechanism was designed to be easy disassembly and reassembly the bed to avoid the effect of leakage on the system. The test rig was in a conditioned room at 25 C during the runs.
2.2.
Procedure
First of all the bed was evacuated using a vacuum pump to 0.1 bar. The bed was then heated gradually up to 100 C during 4 h while the vacuum process is still running. This process was done to ensure that there are no residual gases in the carbon. The bed was weighed before and after this vacuum process. The pressure at the end of this process was recorded to be 25 Pascal.
Fig. 1 e (a) Schematic diagram of the experimental test rig. 1, Water tank; 2, adsorption bed; 3, heater; 4, refrigerant glass bottle; 5, valve; 6, granular activated carbon; 7, thermocouple; 8, pressure gauge. (b) Cross-section in the adsorption cylinder.
496
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
1.8
Table 1 e Properties of the granular activated carbon.
25°C
1.6
900 0.88 510 850 2
35°C
1.4
45°C
1.2 W(kg/kg)
Surface areae (m2 g1) Total pore volume (cm3 g1) Apparent density (kg m3) Iodine number (mg g1) Mean particle diameter (mm)
65°C
1 0.8 0.6
All over the adsorption runs the temperature of the bottle was about 25 C as the system is in the conditioned room. The adsorption process starts by opening the needle valve between the bed and the bottle. While, the refrigerant was entering the bed, the pressure of the bed was rising. The pressure does not allowed exceeding the saturation pressure at such a temperature (25 C) to avoid condensation. The valve between the bottle and the bed was closed every 60 s and the bed was disassembled and weighed. After the bed reaches to the stability, the bed is then heated gradually up to 100 C. The needle valve is then opened and the refrigerant collected again in the bottle. Finally the bed is evacuated to pressure 0.1 bar and be ready to start a new run. This process was repeated four times at different four temperatures 25 C, 35 C, 45 C and 65 C. Desorption characteristics of the pair were measured using adsorption isotherm at 25 C. The desorption process was done at different temperatures 80 C, 90 C, 100 C and 110 C and let the adsorbate to run away from the bed gradually and record the pressure.
2.3.
Instrumentation
The experimental setup comprises (i) thermocouples type J with an uncertainty of 0.7 C , (ii) gauge pressure with an uncertainty of 0.15% of full scale and pressure ranging from 0 to 2 Mpa, (iii) balance with an uncertainty of 0.03% of full scale and maximum capacity of 15 kg, (iv) adsorption bed with a volume of 1.121 0.001 L.
0.4 0.2 0 0
1
2
3
4
5
6
7
P(bar)
Fig. 2 e Pressure of adsorption capacity at different temperatures.
pressure and temperature of the bed which free most of the adsorbate in the bed. After that the pressure decrease slowly and return again to decrease rapidly.
4.
Mathematical section
4.1.
Governing equation
The governing equation for the adsorption pairs is DubinineAstakhov (DeA) which usually used to estimate the equilibrium adsorption/desorption quantity in physical adsorption (El-Sharkawy et al., 2008; Wang et al., 2009; Wood, 2001). n RT Ps ln W ¼ W0 exp P E
(1)
n n RTdes Ps RTdes Ps exp ln ln DW¼W0 exp E Pi E Pf (2)
Results and discussion
The measured values at the runs were the weight of the bed, the temperatures and pressure at every 60 s. Fig. 2 shows experimental results for the adsorption mass capacity of R134a onto the GAC against pressure at four different temperatures 25 C, 35 C, 45 C and 65 C. It is found that the maximum adsorption capacity could be reached is 1.68 kg kg1 at 25 C adsorption temperature with an interval time of 1050 s. It is clear from the figure that, raising the adsorption temperature decreases the maximum adsorption capacity. The figure shows that the adsorption of GAC of R134a has a same trend. The adsorption capacity rises rapidly in the first 100 s and then has a slow progress. This flying start is because of the vacuum pressure in the bed. Fig. 3 shows the relation between the adsorption capacity during desorption process and the pressure of the bed. The desorption process at different temperatures has a same attitude. The pressure and the adsorption uptake decrease rapidly at the first of desorption because of the relatively high
1.8
110°C 100°C 90°C 80°C
1.6 1.4 1.2 W(kg/kg)
3.
1 0.8 0.6 0.4 0.2 0
10
9
8
7
6
5 P(bar)
4
3
2
1
Fig. 3 e Adsorption capacity at different temperatures during desorption process.
0
497
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
1.8 30°C
20°C
40°C
1.6
50°C
60°C 70°C
1.4
80°C 90°C
1.2
W
1 0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
6
7
8
9
10
P
Fig. 4 e Adsorption isotherms of granular activated carbon/R134a.
Eq. (2) used for desorption process while, the left-hand side is measured. The maximum adsorption capacity (W0) was experimentally estimated to be 1.68 kg kg1. Using the experimental results, characteristic energy (E) and exponential constant (n), Eqs. (1) and (2) could be solved. E and n were determined to be 9575 J mol1 and 1.83 respectively. By knowing n, E and W0 Eq. (1) could be in form of Eq. (3). 1:83 Ps W ¼ 1:68 exp 0:000868 T ln P
(3)
Using Eq. (3) one can draw an adsorption isotherms figure for the adsorption of R134a in the GAC. The adsorption isotherms figure as shown in Fig. 4 is a relation between adsorption capacity and pressure at different temperatures (El-Sharkawy et al., 2009). A relation between pressure and temperature at different adsorption capacities could be drawn using Eq. (3) which known as PeTeW diagram (El-Sharkawy et al., 2009). The PeTeW diagram for the GAC with R134a is shown in Fig. 5.
100
ra satu
tion
1.6 1.5 1.4 1.3 1.2 1.1
10
P
1
0.9 0.8 0.7 0.6 0.5 0.4
1
0.3 0.2
0.1 kg kg-1
0.1
0.01 -20
-10
0
10
20
30
40
50
60
70
80
t (°C)
Fig. 5 e PeTeW diagram of the granular activated carbon/R134a.
90
100
498
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
and 1.83 respectively. It can be concluded that the GAC and R134a could be used as adsorption pair in an adsorption cooling system.
350 300
qst (kJ/kg)
250
references 200 150 100 50
20°C
30°C
40°C
50°C
60°C
70°C
80°C
90°C
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
W(kg/kg)
Fig. 6 e Isosteric heat at different isotherms adsorption.
4.2.
Isosteric heat
The isosteric heat is the heat of desorption under constant adsorption capacity. This amount of heat per kg of adsorbent is determined by Eq. (5) which was derived from DubininAstakhov (Eq. (1)) and ClausiuseClapeyron relation (Eq. (4)) (El-Sharkawy et al., 2007; Habib et al., 2011). qst ¼
vln p vð1=TÞ
1=n b ! W0 T ln þa qst ¼ h fg þ E W Tcr 0:55 6:25 1:68 T þ1:81 qst ¼ h fg ðTR134a Þ þ 93:85 ln W 373:9
(4)
(5)
(6)
where E is in kJ kg1 which is equal 93.85 kJ kg1. The values of the constants a and b for R134a are 1.81 and 6.25 respectively (El-Sharkawy et al., 2007; Habib et al., 2011). The critical temperature for R134a is 100.9 C so Eq. (4) could be in the form of Eq. (6). From Eq. (6), the relation between the adsorption capacity and the isosteric heat was plotted at different temperatures in Fig. 6. It is clear from the figure that the changing temperature has no significant effect on the isosteric heat. The increasing in the adsorption capacity decreases the isosteric heat.
5.
Conclusion
Adsorption characteristics of R134a onto GAC with temperature varying from 25 C to 65 C have been experimentally studied. A test rig was designed and built to do the experiments. The maximum adsorption capacity is 1.68 kg kg1 at 25 C after 1000 s. By increasing the temperature of the adsorbent, the maximum adsorption capacity decreases until reaches to 0.53 kg kg1 at 60 C at 450 s. The maximum adsorption capacity occurs quickly at higher temperatures at it is 1 kg kg1 at 65 C in 500 s. The characteristic energy and the exponential constant were estimated to be 9575 J mol1
Akkimaradi, B.S., Prasad, M., Dutta, P., Srinivasan, K., 2001. Adsorption of 1,1,1,2-tetrafluoroethane on activated charcoal. J. Chem. Eng. Data 46, 417e422. Alghoul, M.A., Sulaiman, M.Y., Azmi, B.Z., AbdWahab, M., 2007. Advances on multi-purpose solar adsorption systems for domestic refrigeration and water heating. Appl. Therm. Eng. 27, 813e822. Chan, C.K., Tward, E., Boudale, K.I., 1984. Adsorption isotherms and heat of adsorption of hydrogen, helium, neon and nitrogen on activated charcoal. Cryogenics 24, 451e459. El-Sharkawy, I.I., Kuwahara, K., Saha, B.B., Koyama, S., Ng, K.C., 2006. Experimental investigation of activated carbon fibers/ ethanol pairs for adsorption cooling system application. Appl. Therm. Eng. 26, 859e865. El-Sharkawy, I.I., Saha, B.B., Koyama, Shigeru, Kandadai Srinivasan, 2007. Isosteric heats of adsorption extracted from experiments of ethanol and HFC 134a on carbon based adsorbents. Int. J. Heat Mass Transf. 50, 902e907. El-Sharkawy, I.I., Saha, B.B., Koyama, S., He, J., Ng, K.C., Yap, C., 2008. Experimental investigation on activated carboneethanol pair for solar powered adsorption cooling applications. Int. J. Refrigeration 31, 1407e1413. El-Sharkawy, I.I., Hassan, M., Saha, B.B., Koyama, S., Nasr, M.M., 2009. Study on adsorption of methanol onto carbon based adsorbents. Int. J. Refrigeration 32, 1579e1586. Habib, K., Saha, B.B., Anutosh Chakraborty, Shigeru Koyama, Kandadai Srinivasan, 2011. Performance evaluation of combined adsorption refrigeration cycles. Int. J. Refrigeration 34, 129e137. Jung-Yang San, Wei-Min Lin, 2008. Comparison among three adsorption pairs for using as the working substances in a multi-bed adsorption heat pump. Appl. Therm. Eng. 28, 988e997. Kashiwagi, T., Akisawa, A., Yoshida, S., Alam, K.C.A., Hamamoto, Y., 2002. Heat driven sorption refrigerating and air conditioning cycle in Japan. In: Proceedings of The International Sorption Heat Pump Conference, Shanghai, China. Science Press, New York, pp. 50e62. Pons, M., Guilleminot, J.J., 1986. Design of an experimental solar powered, solid adsorption ice maker. J. Solar Eng. Trans. ASME 103 (4), 332e337. Riffat, S.B., Williams, M.D., Corr, S., 1997. Adsorption heat pump using HFC refrigerants. Int. J. Energy Res. 21, 481e495. Saha, B.B., Alam, K.C.A., Hamamoto, Y., Akisawa, A., Kashiwagi, T., 2001. 2001. Sorption refrigeration/heat pump cycles. Trans. JSRAE 18 (1), 1e14 (in Japanese). Wang, D.C., Xia, Z.Z., Wu, J.Y., 2006. Design and performance prediction of a novel zeoliteewater adsorption air conditioner. Energy Convers. Manage. 47, 590e610. Wang, L.W., Wang, R.Z., Oliveira, R.G., 2009. A review on adsorption working pairs for refrigeration. J. Renewable Sustainable Energy Rev. 13, 518e534. Wang, D.C., Li, Y.H., Li, D., Xia, Y.Z., Zhang, J.P., 2010. A review on adsorption refrigeration technology and adsorption deterioration in physical adsorption systems. Renew. Sust. Energ. Rev. 14, 344e353. Wood, G.O., 2001. Affinity coefficients of the Polanyi/Dubinin adsorption isotherm equations: a review with compilations and correlations. Carbon 39, 343e356.
Available online at www.sciencedirect.com
w w w . i i fi i r . o r g
journal homepage: www.elsevier.com/locate/ijrefrig
Experimental study on adsorptionedesorption characteristics of granular activated carbon/R134a pair Ahmed A. Askalany a,*, M. Salem a, I.M. Ismail b, Ahmed Hamza H. Ali c, M.G. Morsy b a
Mechanical Department, Faculty of Industrial Education, Sohag University, Sohag, Egypt Department of Mechanical Engineering, Faculty of Engineering, Assiut University, Assiut, Egypt c Department of Energy Resources and Environmental Engineering, Egypt-Japan University of Science and Technology (E-JUST), Egypt b
article info
abstract
Article history:
Experimental runs were done to estimate the adsorption characteristics of granular acti-
Received 3 February 2011
vated carbon (GAC)/R134a pair. A laboratory scale test rig was designed and built to run the
Received in revised form
experiments. The adsorption capacity of the GAC was studied at different temperatures
25 March 2011
25 C, 35 C, 45 C and 65 C. Pressure and time were recorded during the experiments. The
Accepted 1 April 2011
maximum adsorption capacity was found to be 1.68 kgR134a kgcarbon1 at 25 C after 1000 s.
Available online 8 April 2011
The activation energy and the exponential constant were estimated to be 9575 J mol1 and 1.83 respectively. ª 2011 Elsevier Ltd and IIR. All rights reserved.
Keywords: Adsorption Activated carbon Cooling R134a
Etude expe´rimentale sur les caracte´ristiques d’adsorption / de´sorption d’un couple actif charbon actif granule´ / R134a Mots cle´s : adsorption ; charbon actif ; refroidissement ; R134a
1.
Introduction
Adsorption heat pumps consume low-grade energy to achieve a cooling effect. This makes the development of these systems become an attractive research topic (Jung-Yang San and WeiMin Lin, 2008; Wang et al., 2006). The adsorption cooling and refrigeration systems have the advantages of being compact, free or nearly free of moving parts, efficiently driven by lowtemperature waste heat or renewable energy sources, free of
toxic and environmentally harmful substances as these systems can use natural refrigerants such as water, ethanol, methanol, ammonia etc. and do not require any synthetic lubricants (Habib et al., 2011; Kashiwagi et al., 2002; Saha et al., 2001). Applications of adsorption refrigeration are very limited. More efforts are necessary for development its performance and extend its area of application. The adsorption working pairs are vital in enhancing the performance of adsorption refrigeration systems. New adsorption pairs should be
* Corresponding author. Tel.: þ20 125680400. E-mail address: [email protected] (A.A. Askalany). 0140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2011.04.002
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
Nomenclature A a b E hfg n P Pi Pf Ps
Area of adsorbent particle, m2 Constant Constant Characteristic energy, J mol1 Latent heat, kJ kg1 Exponential constant Pressure, bar Pressure before desorption, bar Pressure after desorption, bar Saturation pressure, bar
investigated, and their characteristics in adsorption cooling should be defined. The selection of any pair of adsorbent/adsorbate for refrigeration applications depends on certain desirable characteristics of their constituents. These characteristics range from their thermodynamic and chemical properties to their physical properties and even to their costs or availability. The characteristics of refrigerants and adsorbent were classified by Alghoul et al. (2007) and Wang et al. (2010). The adsorption characteristics of R134a on three types of activated carbons were measured by Akkimaradi et al. (2001). El-Sharkawy et al. (2006) evaluated adsorption parameters of ethanol on activated carbon fiber of type (A-20). By using renewable energy source, Pons and Guilleminot (1986) studied activated carbon/methanol system for ice production. Chan et al. (1984) measured experimental data of hydrogen, helium, neon and nitrogen on activated charcoal. Riffat et al. (1997) determined adsorption blends of HFC-32, HFC-125 and R-134a with AX-21. In this research, the adsorption capacity rates of R134a onto granular activated carbon (GAC) have been measured within the temperatures ranging from 25 to 65 C for adsorption cooling and refrigeration applications purposes. Using the constant volume, constant temperature and variable pressure, the instantaneous capacity of R134a has been recorded at each 60 s. Desorption characteristics the pair were measured at different temperatures.
2.
Experimental section
2.1.
Experimental test rig
A test rig was designed and built as shown in Fig. 1a and b to run the experiments. The test rig consists of a water tank containing an adsorption bed, heater and a refrigerant bottle. The bed is a cylinder made of carbon steel contains 500 g of GAC. Properties of GAC based on bituminous coal are tabulated in Table 1. A refrigerant grader glass bottle has a capacity of 1.5 0.001 L. An electric heater of 2 kW was used to heat the water inside the water tank. Type J thermocouples were established inside the bed and the water tank to measure temperatures. The adsorption cylinder and the bottle are connected to a pressure gauge to measure the pressure. The connection between the bed and the bottle is a non-return
qst R Sd Sd0 T Tcr Tdes t V W W0
495
Isosteric heat of adsorption, J mol1 Universal gas constant, 8.314 J mol1 K1 Surface diffusion, m2 s1 Pre-exponential coefficient Temperature, K Critical temperature of R134a, K Desorption temperature, K Time, s Volume of adsorbent particle, m3 Adsorption mass capacity, kgR134a kgcarbon1 Maximum adsorption capacity, kgR134a kgcarbon1
valve fixed at bed side and connected directly to a needle valve on the other side. When the bed is separated from the system to be weighted, the needle valve is closed and it is opened again after rejoining the bed. The non-return valve prevents outside air to enter the bed during weighing process. This mechanism was designed to be easy disassembly and reassembly the bed to avoid the effect of leakage on the system. The test rig was in a conditioned room at 25 C during the runs.
2.2.
Procedure
First of all the bed was evacuated using a vacuum pump to 0.1 bar. The bed was then heated gradually up to 100 C during 4 h while the vacuum process is still running. This process was done to ensure that there are no residual gases in the carbon. The bed was weighed before and after this vacuum process. The pressure at the end of this process was recorded to be 25 Pascal.
Fig. 1 e (a) Schematic diagram of the experimental test rig. 1, Water tank; 2, adsorption bed; 3, heater; 4, refrigerant glass bottle; 5, valve; 6, granular activated carbon; 7, thermocouple; 8, pressure gauge. (b) Cross-section in the adsorption cylinder.
496
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
1.8
Table 1 e Properties of the granular activated carbon.
25°C
1.6
900 0.88 510 850 2
35°C
1.4
45°C
1.2 W(kg/kg)
Surface areae (m2 g1) Total pore volume (cm3 g1) Apparent density (kg m3) Iodine number (mg g1) Mean particle diameter (mm)
65°C
1 0.8 0.6
All over the adsorption runs the temperature of the bottle was about 25 C as the system is in the conditioned room. The adsorption process starts by opening the needle valve between the bed and the bottle. While, the refrigerant was entering the bed, the pressure of the bed was rising. The pressure does not allowed exceeding the saturation pressure at such a temperature (25 C) to avoid condensation. The valve between the bottle and the bed was closed every 60 s and the bed was disassembled and weighed. After the bed reaches to the stability, the bed is then heated gradually up to 100 C. The needle valve is then opened and the refrigerant collected again in the bottle. Finally the bed is evacuated to pressure 0.1 bar and be ready to start a new run. This process was repeated four times at different four temperatures 25 C, 35 C, 45 C and 65 C. Desorption characteristics of the pair were measured using adsorption isotherm at 25 C. The desorption process was done at different temperatures 80 C, 90 C, 100 C and 110 C and let the adsorbate to run away from the bed gradually and record the pressure.
2.3.
Instrumentation
The experimental setup comprises (i) thermocouples type J with an uncertainty of 0.7 C , (ii) gauge pressure with an uncertainty of 0.15% of full scale and pressure ranging from 0 to 2 Mpa, (iii) balance with an uncertainty of 0.03% of full scale and maximum capacity of 15 kg, (iv) adsorption bed with a volume of 1.121 0.001 L.
0.4 0.2 0 0
1
2
3
4
5
6
7
P(bar)
Fig. 2 e Pressure of adsorption capacity at different temperatures.
pressure and temperature of the bed which free most of the adsorbate in the bed. After that the pressure decrease slowly and return again to decrease rapidly.
4.
Mathematical section
4.1.
Governing equation
The governing equation for the adsorption pairs is DubinineAstakhov (DeA) which usually used to estimate the equilibrium adsorption/desorption quantity in physical adsorption (El-Sharkawy et al., 2008; Wang et al., 2009; Wood, 2001). n RT Ps ln W ¼ W0 exp P E
(1)
n n RTdes Ps RTdes Ps exp ln ln DW¼W0 exp E Pi E Pf (2)
Results and discussion
The measured values at the runs were the weight of the bed, the temperatures and pressure at every 60 s. Fig. 2 shows experimental results for the adsorption mass capacity of R134a onto the GAC against pressure at four different temperatures 25 C, 35 C, 45 C and 65 C. It is found that the maximum adsorption capacity could be reached is 1.68 kg kg1 at 25 C adsorption temperature with an interval time of 1050 s. It is clear from the figure that, raising the adsorption temperature decreases the maximum adsorption capacity. The figure shows that the adsorption of GAC of R134a has a same trend. The adsorption capacity rises rapidly in the first 100 s and then has a slow progress. This flying start is because of the vacuum pressure in the bed. Fig. 3 shows the relation between the adsorption capacity during desorption process and the pressure of the bed. The desorption process at different temperatures has a same attitude. The pressure and the adsorption uptake decrease rapidly at the first of desorption because of the relatively high
1.8
110°C 100°C 90°C 80°C
1.6 1.4 1.2 W(kg/kg)
3.
1 0.8 0.6 0.4 0.2 0
10
9
8
7
6
5 P(bar)
4
3
2
1
Fig. 3 e Adsorption capacity at different temperatures during desorption process.
0
497
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
1.8 30°C
20°C
40°C
1.6
50°C
60°C 70°C
1.4
80°C 90°C
1.2
W
1 0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
6
7
8
9
10
P
Fig. 4 e Adsorption isotherms of granular activated carbon/R134a.
Eq. (2) used for desorption process while, the left-hand side is measured. The maximum adsorption capacity (W0) was experimentally estimated to be 1.68 kg kg1. Using the experimental results, characteristic energy (E) and exponential constant (n), Eqs. (1) and (2) could be solved. E and n were determined to be 9575 J mol1 and 1.83 respectively. By knowing n, E and W0 Eq. (1) could be in form of Eq. (3). 1:83 Ps W ¼ 1:68 exp 0:000868 T ln P
(3)
Using Eq. (3) one can draw an adsorption isotherms figure for the adsorption of R134a in the GAC. The adsorption isotherms figure as shown in Fig. 4 is a relation between adsorption capacity and pressure at different temperatures (El-Sharkawy et al., 2009). A relation between pressure and temperature at different adsorption capacities could be drawn using Eq. (3) which known as PeTeW diagram (El-Sharkawy et al., 2009). The PeTeW diagram for the GAC with R134a is shown in Fig. 5.
100
ra satu
tion
1.6 1.5 1.4 1.3 1.2 1.1
10
P
1
0.9 0.8 0.7 0.6 0.5 0.4
1
0.3 0.2
0.1 kg kg-1
0.1
0.01 -20
-10
0
10
20
30
40
50
60
70
80
t (°C)
Fig. 5 e PeTeW diagram of the granular activated carbon/R134a.
90
100
498
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 4 9 4 e4 9 8
and 1.83 respectively. It can be concluded that the GAC and R134a could be used as adsorption pair in an adsorption cooling system.
350 300
qst (kJ/kg)
250
references 200 150 100 50
20°C
30°C
40°C
50°C
60°C
70°C
80°C
90°C
0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
W(kg/kg)
Fig. 6 e Isosteric heat at different isotherms adsorption.
4.2.
Isosteric heat
The isosteric heat is the heat of desorption under constant adsorption capacity. This amount of heat per kg of adsorbent is determined by Eq. (5) which was derived from DubininAstakhov (Eq. (1)) and ClausiuseClapeyron relation (Eq. (4)) (El-Sharkawy et al., 2007; Habib et al., 2011). qst ¼
vln p vð1=TÞ
1=n b ! W0 T ln þa qst ¼ h fg þ E W Tcr 0:55 6:25 1:68 T þ1:81 qst ¼ h fg ðTR134a Þ þ 93:85 ln W 373:9
(4)
(5)
(6)
where E is in kJ kg1 which is equal 93.85 kJ kg1. The values of the constants a and b for R134a are 1.81 and 6.25 respectively (El-Sharkawy et al., 2007; Habib et al., 2011). The critical temperature for R134a is 100.9 C so Eq. (4) could be in the form of Eq. (6). From Eq. (6), the relation between the adsorption capacity and the isosteric heat was plotted at different temperatures in Fig. 6. It is clear from the figure that the changing temperature has no significant effect on the isosteric heat. The increasing in the adsorption capacity decreases the isosteric heat.
5.
Conclusion
Adsorption characteristics of R134a onto GAC with temperature varying from 25 C to 65 C have been experimentally studied. A test rig was designed and built to do the experiments. The maximum adsorption capacity is 1.68 kg kg1 at 25 C after 1000 s. By increasing the temperature of the adsorbent, the maximum adsorption capacity decreases until reaches to 0.53 kg kg1 at 60 C at 450 s. The maximum adsorption capacity occurs quickly at higher temperatures at it is 1 kg kg1 at 65 C in 500 s. The characteristic energy and the exponential constant were estimated to be 9575 J mol1
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