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Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Available online at www.sciencedirect.com
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Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons Nessrine Soli*, Nabil Ben Hafsia, Bechir Chaouachi Department of Chemical-Processes Genius, UR11ES83.Environnement, Catalyzes and Analyzes Processes, National School of Engineers of Gabes, Omar Ibn El Khattab Street, Zrig, 6029, Gabes, Tunisia
article info
abstract
Article history:
We propose in this work a thermodynamic feasibility study of absorptiondiffusion refrig-
Received 11 March 2016
erating machine working with hydrocarbon mixtures. We used a machine of low power
Received in revised form
(300 W) that operates with generator temperatures lower than 150 C (fossil energy or solar
19 June 2016
energy can be used) and where the condenser and absorber temperatures are taken equal
Accepted 21 June 2016
to 42 C. The inert gas used is helium and the total functioning pressure is about 17.5 bars.
Available online xxx
A modeling on suitable software was made to simulate the machine functioning for four binary mixtures which are: propylene/hexane, propylene/heptane, propylene/octane and
Keywords:
propylene/nonane. The validation of our model was made by comparison with the results
Absorption diffusion machine
taken from literature and the optimal operating conditions are determined.
Propylene
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hexane Heptane Octane Nonane
Introduction The human tendency consists continuously in search of the comfort state and the field of the production of cold does not escape the rule, especially if we remember that 15% of the worldwide production of electricity is intended for the refrigeration [10]. In addition, today refrigerating industry is touched of full whip by the consecutive directives with the protocol of Montreal (1987) and with the agreements of Kyoto (1997) on the use of the refrigerants.
Indeed, the use of CFC (chlorofluorocarbons) is prohibited and the HCFC refrigerants (hydro chlorofluorocarbons) are subjected to an increasingly severe regulation because of their contribution to the destruction of the layer of ozone. They are also shown to contribute to the greenhouse effect and must be consequently used with prudence. It is in this general context some alternative research solutions are developed. The development of the absorption refrigeration technology that uses new fluids witch respect the environment presents an efficient solution to resolve this problem.
* Corresponding author. E-mail addresses: [email protected] (N. Soli), [email protected] (N. Ben Hafsia), [email protected] (B. Chaouachi). http://dx.doi.org/10.1016/j.ijhydene.2016.06.184 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Karamangil et al. [4] presented a literature review on the absorption refrigeration systems and the pairs of cooling agent currently used with their substitutes. Moreover, they carried out a thermodynamic analysis of the absorption refrigeration systems using refrigerants usually met in the literature. The numerical results showed that the system working with the LiBrH2O mixture has the higher energetic performance. In other hand, Starace et al. [12] provided a parametric study of the diffusion absorption refrigerator without any assumption regarding the purity of the refrigerant. The study gives a better prediction of the real operation of the refrigerator. Also, Ersoz [13] had given an experimental study of the effect of three different generators, heat inputs (62, 80 and 115 W) on the energy performance of the DAR cycle. In another study, Zhenlong et al. [5] developed a numerical model for the simulation of TFE e TEGDME couple used as working fluid in an absorptionediffusion refrigeration system with two refrigeration vectors witch are water (with 32 C) and the air (with 35 C). It is noted that, the optimal temperature for the generation of TFE e TEGDME system (DAR) is about 170 C and the coefficient of performance is up to 0, 45. On the other hand for a water cooling, the optimal temperature of generator is lower than 130 C and a COP reaching to 0, 56. By comparing the performances of TFE e TEGDME system and NH3eH2O system, Zhenlong et al. [5] concluded that mixture TFE e TEGDME is a good working fluid for DAR cycle. Ben Ezzine et al. [7] carried out an experimental study of absorptiondiffusion machine cooled by the air and working with a binary of light hydrocarbons butane e nonane and helium as inert gas. The capacity of cooling is respectively in the order 0.40e0.47 kW and the temperature of cold water is about between 9 and 11 C. According to Ben Ezzine et al., the cold is produced at temperatures between 10 and 10 C for a temperature of control ranging between 120 C and 150 C. In addition, they affirmed that the lowest temperature attack at the inlet of evaporator is about 10 C; this value corresponds to a temperature of generator of 138 C and for a machine power of 260 W. Moreover the COP of the machine reaches a maximum of 0, 14 for a temperature of water equal to 9 C and a power of 275 W. Sayadi et al. [8] studied theoretically an absorptionediffusion machine cooled by water for a refrigerating capacity equals to 1 kW. This study is made by HYSYS software and the used fluids are the mixtures of light hydrocarne/cC5, propyle ne/iC4, propyle ne/ bons (C3/nC6, C3/cC6, propyle iC5) in combination with helium like inert gas. The heat is supposed to be provided by a field of vacuum solar collector where its temperature reaches 121 C and an evaporator exit temperature of 0 C. The rate of flow of cooling water circulating between the coolers and the tower of cooling is of 140 L/h. In another study, Bouaziz et al. [14] presented an energy and exergy analysis of a novel configuration of absorption
cooling system operating. In their work, authors presented a double stage cycle operating with water-ammonia. Also, a thermodynamic model based on the energy and exergy balances is developed. The results of the study presented the performance of the novel configuration witch compared with the two stage conventional configuration. Mbarek et al. [11] carried out an experimental study on a refrigerating machine functioning with absorptionediffusion cycle by using light hydrocarbons. The used couple is Nbutane/octane with helium like an inert gas. According to the authors, for a power value of 270, 6 W, the evaporator temperature was about of 10 C. Dixita et al. [15] studied thorough analysis of waterammonia generator-absorber-heat exchanger (GAX) and hybrid GAX (HGAX) absorption refrigeration cycles based on energy and exergy. In the aim to study the effect of generator temperature, condenser temperature and evaporator temperature, Dixita et al. [15] calculated the coefficient of performance (COP) and exergetic efficiencies at various operating conditions. It is observed that the increase in approach temperature from 0 C to 14 C causes decrease in COP of GAX cycle by 30% and of HGAX cycles by 40%e45%. Hamed et al. [16] had predicted, analyzed and optimized the performance of the double-effect absorption system. Also it is required to predict the coefficient of performance (COP) and exergetic efficiency (hex) of the system, and to formulate them in equations as functions of the operating parameters of the cycle. In this study, authors presented the energetic and exergetic performance of the double-effect absorption cycle for evaporator temperature range of 2e10 C, absorber and condenser temperature range of 28e45 C, and HPG temperature range of 100e200 C, two equations, which are functions of the operating temperatures, have been developed to predict the performance of the cycle. Dardour et al. [9] carried out the study of the absorptionediffusion machine performance using the mixture/ n-nonane propane as working fluid and hydrogen as inert gas. The results of this study showed that in mode of cooling by the water and in an interval driving heat temperature ranging between [110 Ce125 C], the coefficient of performance is about 0.51, this value makes this couple competitive compared to certain working liquids used in marketed machines with absorption and allows him to be presented it as a form of a possible substitute. In this context, our study aims to exanimate the use of mixtures of hydrocarbons like working fluids in the refrigerating machines operating according to an absorptionediffusion cycle. All the combinations of mixtures formed by the couples refrigerant/absorbent, following Table 1, are considered.
Table 1 e Binary of studied hydrocarbons. Solvent Solute
n-C6H14 C3H6
n-C7H16 C3H6
n-C8H18 C3H6
n-C9H20 C3H6
Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
In this study, the absorber and the condenser are cooled by the ambient air. These mixtures are selected according to:
Selection criteria of the mixture for an absorptionediffusion refrigerating machine
3
➢ Absence of solid phase: the fluid couple refrigerantabsorbent should not form a solid phase in the range of composition and operating temperature. Indeed, the formation of the solids can stop the flow and pose problems to the equipment. Moreover mixtures considered must be characterized by:
The selection criteria of an absorptionediffusion refrigerating machine mixture are multiple and concern the refrigerant, the absorbent and the mixture. The principal criteria of the refrigerants for an absorptionediffusion machine are the following:
➢ A strong solubility of the refrigerant in the absorbent to reduce dimensions of the installation. ➢ To be non-polluting ➢ To respect the regulations on the toxic and dangerous products.
1) Criteria of the refrigerant choice [9] The refrigerant must have: ➢ ➢ ➢ ➢ ➢ ➢
A high latent heat and low density; A highest possible criticizes temperature; A lowest possible specific heat capacity of the liquid (Cpm); A high stability at the temperatures of the cycle; A great energy efficiency associating fluid and system; A good compatibility with metallic materials and elastomers. 2) Criteria of the absorbent choice [7,8] The absorbent must have:
➢ A great affinity with the refrigerant, ➢ A definitely higher normal boiling point than that of the refrigerant, ➢ A high chemical stability, ➢ A low heat capacity to minimize the losses, ➢ A raised thermal conductivity, ➢ A low viscosity and surface tension to support the heat transfers. 3) Selection criteria of the mixture [1e3] In refrigerating, for absorptionediffusion machines, the choice of the mixture of work must fulfill some physicochemical, thermodynamic and transport properties. Consequently the choice of certain combination refrigerantabsorbent must satisfy certain criteria which are primarily: ➢ A normal functioning of the machine in fields of fixed pressure and temperature. ➢ A big difference between the boiling point of the refrigerating fluid and that of the absorbent; ➢ The properties of transport (viscosity, thermal conductivity, coefficient of diffusion) must be favorable; ➢ The mixture must be chemically stable, not poison and nonexplosive. It must also be noncorrosive and inexpensive.
The study of the liquidevapor equilibrium of the mixture, represented in the diagram of Oldham, concerning the considered working fluid is the base of the discussions which will follow. Indeed, examination of this diagram allows the determination of the interval of the temperatures in which the machine's functioning is possible.
Thermodynamic model [8] The choice of adequate thermodynamic model for calculation and/or prediction of the refrigerating mixtures properties are a crucial importance for the reliability of results. For the systems with low pressures, the two approaches symmetrical (ff) and dissymmetrical (g-f) make it possible to carry out this kind of calculation with a good precision. For the systems with high pressure the symmetrical method is recommended.
Model of Peng-Robinson [8,10] The thermodynamic model of Peng-Robinson uses: - The cubic equation of state of Peng-Robinson for all the thermodynamic properties except liquid molar volume. - API method for calculation of liquid molar volume of the pseudo compounds and the model of Rackett for the real compounds. The cubic equation of state of Peng-Robinson connecting the temperature, the pressure and the molar volume (T, P, v) of a pure substance is written: P¼
RT a v b v2 þ 2vb b2
With:
8 sffiffiffiffiffiffi !#2 " > > R2 T2c T 2 > > a ¼ 0; 45724 1 1 þ 0; 37464 þ 1; 54226w þ 0; 26992w < T p > > > RT > : b ¼ 0; 0778 c pc
c
(1)
c
(2)
Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Tc, PC and W are respectively the critical temperature, the critical pressure and the acentric factor of the fluid.
The considered couples are: C3H6/n-C6H14, C3H6/n-C7H16, C3H6/n-C8H18 and the C3H6/n-C9H20.
ðwi ≡ log Pri at Tri ≡0:7Þ 1Þ
Traced cycle
(3)
The Peng-Robinson method allows us to obtain satisfactory results whatever the values of the temperature and the pressure. This method is consistent in the critical area [8], it doesn't present an abnormal behavior like the methods using the activities coefficients.
Thermodynamic feasibility study using propylene as a refrigerant This part was devoted to determine the functioning limits of the refrigerating absorptionediffusion machines working with various mixtures of alkanes by observing the fixed operating conditions. Fundamental data and assumptions are: - The installation has a refrigerating power of 300 W. - Cooling with the ambient air at an average temperature of 35 C, - Exit temperature of refrigerant is equal to 0 C. - Condenser and absorber temperatures are taken equal to 42 C - Generator: Driving heat provided to the generator can have several origins: solar collector, thermal discharge, steam, combustion gas. The maximum temperature of this source is fixed at 130 C. - The steady state is established and the pressure losses are considered negligible. - The vapor refrigerant purity at the outlet generator side is equal to 99, 6%. - The two heat exchangers are supposed adiabatic and characterized each one by a thermal pinch equal to 10 C for the vapor/gas exchanger and 5 C for Solution/solution one [6]. - There is equilibrium at the levels of the various apparatuses, - The evaporator and absorber pressures are equal to the low pressure and the generator and condenser pressures are equal to the high pressure
By using suitable software and taking into account the operating conditions we can determine the low and the high pressure of this cycle. The stages of Oldham diagram tracing concerning the couples propylene/absorbents follow the following steps: The two temperatures of vaporization and condensation are identified on the diagram within the intersection of the isotherms corresponding to 0 C and 42 C with a propylene molar fraction of 99.6%. The intersection points represent the evaporator and the condenser allows us to read directly on the diagram the values of low and high pressure (P evaporator; P condenser). The absorber and the generator work under pressure of the evaporator and the condenser respectively. To determine the maximum temperature of the generator, the molar fraction is fixed equal to the propylene-poor fraction (X ¼ Xp ¼ 0) and the pressure is equal to the high pressure (P ¼ Pcond). To determine the minimum temperature of the generator, the mole fraction is fixed equal to the propylene rich fraction (X ¼ Xr) and the pressure is equal to the low pressure (P ¼ Pevap). On the following figures, we traced the Oldham diagram for different couples (see Figs. 1e4). In the following table, we summarized the various values determined starting from the Oldham diagram of different binary of hydrocarbons (see Table 2). According to this table, one notices that the C3H6/nC9H20 couple presents the broadest range of temperatures what encourages its use in these cycles.
Validation of result To validate our model, we compared our results for the couple propane/nonane, with those of H. Dardour et al. [9].These results are given in Table 3. According to the comparison of these results, one can affirm that our simulation shows a good agreement. In order to determine the optimum operating conditions of the
Fig. 1 e Diagram of Oldham relating to the couple propylene/n-hexane. Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
5
Fig. 2 e Diagram of Oldham relating to the couple propylene/n-heptane.
Fig. 3 e Diagram of Oldham relating to the couple propylene/n-octane.
Fig. 4 e Diagram of Oldham relating to the couple propylene/nonane.
Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
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Table 2 e Functioning limits of studied binary hydrocarbons. Driving temperature ( C)
C3H6/nC6H14 C3H6/nC7H16 C3H6/nC8H18 C3H6/nC9H20
Tmin Tmax Tmin Tmax Tmin Tmax Tmin Tmax
Possible enrichment Poor solution composition (XP)
Rich solution composition (Xr)
0.3015 0 0.3475 0 0.3556 0 0.3615 0
0.3715 0.3715 0.385 0.385 0.3867 0.3867 0.39 0.39
106 196 110 232 113 265 113 300
Table 3 e Functioning limits of the couple propane/ nonane. Driving temperature ( C)
Propane/n-C9H20 Dardour and Al [9] Propane/n-C9H20 (our work)
Possible enrichment Composition poor solution
Composition rich solution
Tmin Tmax
159 305
0.297 0
0.297 0.297
Tmin Tmax
159 305
0.295 0
0.2975 0.2975
Fig. 5 e COP according to generator temperature for the binary propylene/n-nonane.
refrigerating absorptionediffusion machine, we plotted on Fig. 5, the COP as a function of the generator temperature and on Fig. 6, the COP as a function of DX. From Fig. 5, we notice that the COP increases with Tgen, passes through a maximum in the vicinity of Tgen ¼ 130 C and then decreases. Fig. 6 shows that the COP increases with DX, passes through a maximum in the vicinity of DX ¼ 0.113 and then decreases.
Fig. 6 e COP according to DX for the binary propylene/nnonane, at Tgen ¼ 130 C.
Difference of composition (DX) 0.07 0,0375 0,0311 0,0285
Hence, the optimal conditions for this machine operating with propane/n-C9H20 e Helium mixture are: Xr ¼ 0.408; Xp ¼ 0.296; DX ¼ 0.113 and Tgen ¼ 130 C.
Conclusion In this work, we studied the thermodynamic feasibility of hydrocarbon binary mixtures faintly used until now as working fluids for the absorption diffusion refrigerating machines. These environmental fluids constitute an interesting alternative since the machines operating with such mixtures needed only moderate thermal source temperatures. It is the case for example of a solar collector. Moreover, this installation works under a moderate pressure (17 bar) with propylene as refrigerant and hexane, heptane, octane and nonane as absorbents. The study shows that the minimal operative generator temperature is about of 106 C and the maximum temperature is in order of 300 C. These proposed hydrocarbon mixtures are promising refrigerants since they present some advantages such as a moderate heat source temperature used for the functioning of the absorptionediffusion refrigeration machine that is not exceeding 150 C. This study is a first stage for later contribution which will consider the evaluation and performances analyzes that could carry out of the machines with absorption using these mixtures.
Nomenclature COP P Pabs Pevap Pgen T Xp Xr Tmin Tmax Tcond Tgen Tabs DX
coefficient of performance pression, bar absorber pressure, bar evaporator pressure, bar generator pressure, bar temperature, K composition of the poor solution composition of the rich solution minimal temperature maximal temperature condenser temperature generator temperature absorber temperature difference of composition
Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
references [10] [1] Srikhirin P, Aphornratana S, Chungpaibulpatana S. A review of absorption refrigeration technologies. Renew Sustain Energy Rev 2001;5:343e72. [2] Wang R, Li Y. Review perspectives for natural working fluids in China. Int J Refrig 2007;30:568e81. [3] Sun J, Fu L, Zhang S. A review of working fluids of absorption cycles. Renew Sustain Energy Rev 2012;16:1899e906. [4] Karamangil M, Coskun S, Kaynakli O, Yamankaradeniz N. A simulation study of performance evaluation of single-stage absorption refrigeration system using conventional working fluids and alternatives. Renew Sustain Energy Rev 2010;14:1969e78. [5] Zhen L, Yong L, Huashan L, Xianbiao B, Weibin M. Performance analysis of a diffusion absorption refrigeration cycle working with TFEeTEGDME mixture. Energy Build 2013;58:86e92. [6] Chekir N, Mejbri Kh, Bellagi A. Simulation d’une machine absorption fonctionnant avec des me langes frigorifique a d’alcanes. Int J Refrig 2006;29:469e75. [7] Ben Ezzine N, Garma R, Bourouis M, Bellagi A. Experimental studies on bubbles pump operated diffusion absorption machine based on light hydrocarbons for solar cooling. Renew Energy 2010;35:464e70. [8] Sayadi Z, Ben Thameur N, Bourouis M, Bellagi A. Performance optimization of solar driven small-cooled absorptionediffusion chiller working with light hydrocarbons. Energy Convers Manag 2013;74:299e307. moire de the se: [9] Dardour H, Mhiri H, Gabsi S, Bellagi A. me Etude des machines frigorifiques a absorption diffusion
[11]
[12]
[13]
[14]
[15]
[16]
7
lange d’alcanes: e tude syste matique et utilisant un me lisation rigoureuse de l’absorbeur. 2012. p. 45e60. mode Batoul B, Kaabi A, Khattib Y, Belhamri A, Gomri R. Etude langes des fluides frigorige nes et thermodynamique des me moire de leur utilisation dans les machines trithermes. me se. 2012. p. 16e22. the rimentale d’une machine Mbarek W, Bellagi A. Etude expe absorption-diffusion fonctionnant aux frigorifique a me Congre s International sur hydrocarbures (C4/C8). In: Ve les Energies Renouvelables et l’Environnement; 04e06 Novembre, 2010. Sousse, Tunisie. Starace G, De Pascalis L, Carluccio F. The operation and performance. Handbook of research on advances and applications in refrigeration systems and technologies. IGI Global; 2015. p. 36e84 [Chapter 2]. Ersoz MA. Investigation the effects of different heat inputs supplied to the generator on the energy performance in diffusion absorption refrigeration systems. Int J Refrig 2015;45:10e21. Bouaziz N, Lounissi D. Energy and exergy investigation of a novel double effect hybrid absorption refrigeration system for solar cooling. Int J Hydrogen Energy 19 October 2015;40(39):13849e56. Dixita M, Arorab A, Kaushika SC. Thermodynamic analysis of GAX and hybrid GAX aqua-ammonia vapor absorption refrigeration systems. Int J Hydrogen Energy 14 December 2015;40(46):16256e65. Hamed A, Kaseb A, Abdalla S. Prediction of energetic and exergetic performance of double-effect absorption system. Int J Hydrogen Energy 26 November 2015;40(44):15320e7.
Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons Nessrine Soli*, Nabil Ben Hafsia, Bechir Chaouachi Department of Chemical-Processes Genius, UR11ES83.Environnement, Catalyzes and Analyzes Processes, National School of Engineers of Gabes, Omar Ibn El Khattab Street, Zrig, 6029, Gabes, Tunisia
article info
abstract
Article history:
We propose in this work a thermodynamic feasibility study of absorptiondiffusion refrig-
Received 11 March 2016
erating machine working with hydrocarbon mixtures. We used a machine of low power
Received in revised form
(300 W) that operates with generator temperatures lower than 150 C (fossil energy or solar
19 June 2016
energy can be used) and where the condenser and absorber temperatures are taken equal
Accepted 21 June 2016
to 42 C. The inert gas used is helium and the total functioning pressure is about 17.5 bars.
Available online xxx
A modeling on suitable software was made to simulate the machine functioning for four binary mixtures which are: propylene/hexane, propylene/heptane, propylene/octane and
Keywords:
propylene/nonane. The validation of our model was made by comparison with the results
Absorption diffusion machine
taken from literature and the optimal operating conditions are determined.
Propylene
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hexane Heptane Octane Nonane
Introduction The human tendency consists continuously in search of the comfort state and the field of the production of cold does not escape the rule, especially if we remember that 15% of the worldwide production of electricity is intended for the refrigeration [10]. In addition, today refrigerating industry is touched of full whip by the consecutive directives with the protocol of Montreal (1987) and with the agreements of Kyoto (1997) on the use of the refrigerants.
Indeed, the use of CFC (chlorofluorocarbons) is prohibited and the HCFC refrigerants (hydro chlorofluorocarbons) are subjected to an increasingly severe regulation because of their contribution to the destruction of the layer of ozone. They are also shown to contribute to the greenhouse effect and must be consequently used with prudence. It is in this general context some alternative research solutions are developed. The development of the absorption refrigeration technology that uses new fluids witch respect the environment presents an efficient solution to resolve this problem.
* Corresponding author. E-mail addresses: [email protected] (N. Soli), [email protected] (N. Ben Hafsia), [email protected] (B. Chaouachi). http://dx.doi.org/10.1016/j.ijhydene.2016.06.184 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
2
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Karamangil et al. [4] presented a literature review on the absorption refrigeration systems and the pairs of cooling agent currently used with their substitutes. Moreover, they carried out a thermodynamic analysis of the absorption refrigeration systems using refrigerants usually met in the literature. The numerical results showed that the system working with the LiBrH2O mixture has the higher energetic performance. In other hand, Starace et al. [12] provided a parametric study of the diffusion absorption refrigerator without any assumption regarding the purity of the refrigerant. The study gives a better prediction of the real operation of the refrigerator. Also, Ersoz [13] had given an experimental study of the effect of three different generators, heat inputs (62, 80 and 115 W) on the energy performance of the DAR cycle. In another study, Zhenlong et al. [5] developed a numerical model for the simulation of TFE e TEGDME couple used as working fluid in an absorptionediffusion refrigeration system with two refrigeration vectors witch are water (with 32 C) and the air (with 35 C). It is noted that, the optimal temperature for the generation of TFE e TEGDME system (DAR) is about 170 C and the coefficient of performance is up to 0, 45. On the other hand for a water cooling, the optimal temperature of generator is lower than 130 C and a COP reaching to 0, 56. By comparing the performances of TFE e TEGDME system and NH3eH2O system, Zhenlong et al. [5] concluded that mixture TFE e TEGDME is a good working fluid for DAR cycle. Ben Ezzine et al. [7] carried out an experimental study of absorptiondiffusion machine cooled by the air and working with a binary of light hydrocarbons butane e nonane and helium as inert gas. The capacity of cooling is respectively in the order 0.40e0.47 kW and the temperature of cold water is about between 9 and 11 C. According to Ben Ezzine et al., the cold is produced at temperatures between 10 and 10 C for a temperature of control ranging between 120 C and 150 C. In addition, they affirmed that the lowest temperature attack at the inlet of evaporator is about 10 C; this value corresponds to a temperature of generator of 138 C and for a machine power of 260 W. Moreover the COP of the machine reaches a maximum of 0, 14 for a temperature of water equal to 9 C and a power of 275 W. Sayadi et al. [8] studied theoretically an absorptionediffusion machine cooled by water for a refrigerating capacity equals to 1 kW. This study is made by HYSYS software and the used fluids are the mixtures of light hydrocarne/cC5, propyle ne/iC4, propyle ne/ bons (C3/nC6, C3/cC6, propyle iC5) in combination with helium like inert gas. The heat is supposed to be provided by a field of vacuum solar collector where its temperature reaches 121 C and an evaporator exit temperature of 0 C. The rate of flow of cooling water circulating between the coolers and the tower of cooling is of 140 L/h. In another study, Bouaziz et al. [14] presented an energy and exergy analysis of a novel configuration of absorption
cooling system operating. In their work, authors presented a double stage cycle operating with water-ammonia. Also, a thermodynamic model based on the energy and exergy balances is developed. The results of the study presented the performance of the novel configuration witch compared with the two stage conventional configuration. Mbarek et al. [11] carried out an experimental study on a refrigerating machine functioning with absorptionediffusion cycle by using light hydrocarbons. The used couple is Nbutane/octane with helium like an inert gas. According to the authors, for a power value of 270, 6 W, the evaporator temperature was about of 10 C. Dixita et al. [15] studied thorough analysis of waterammonia generator-absorber-heat exchanger (GAX) and hybrid GAX (HGAX) absorption refrigeration cycles based on energy and exergy. In the aim to study the effect of generator temperature, condenser temperature and evaporator temperature, Dixita et al. [15] calculated the coefficient of performance (COP) and exergetic efficiencies at various operating conditions. It is observed that the increase in approach temperature from 0 C to 14 C causes decrease in COP of GAX cycle by 30% and of HGAX cycles by 40%e45%. Hamed et al. [16] had predicted, analyzed and optimized the performance of the double-effect absorption system. Also it is required to predict the coefficient of performance (COP) and exergetic efficiency (hex) of the system, and to formulate them in equations as functions of the operating parameters of the cycle. In this study, authors presented the energetic and exergetic performance of the double-effect absorption cycle for evaporator temperature range of 2e10 C, absorber and condenser temperature range of 28e45 C, and HPG temperature range of 100e200 C, two equations, which are functions of the operating temperatures, have been developed to predict the performance of the cycle. Dardour et al. [9] carried out the study of the absorptionediffusion machine performance using the mixture/ n-nonane propane as working fluid and hydrogen as inert gas. The results of this study showed that in mode of cooling by the water and in an interval driving heat temperature ranging between [110 Ce125 C], the coefficient of performance is about 0.51, this value makes this couple competitive compared to certain working liquids used in marketed machines with absorption and allows him to be presented it as a form of a possible substitute. In this context, our study aims to exanimate the use of mixtures of hydrocarbons like working fluids in the refrigerating machines operating according to an absorptionediffusion cycle. All the combinations of mixtures formed by the couples refrigerant/absorbent, following Table 1, are considered.
Table 1 e Binary of studied hydrocarbons. Solvent Solute
n-C6H14 C3H6
n-C7H16 C3H6
n-C8H18 C3H6
n-C9H20 C3H6
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In this study, the absorber and the condenser are cooled by the ambient air. These mixtures are selected according to:
Selection criteria of the mixture for an absorptionediffusion refrigerating machine
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➢ Absence of solid phase: the fluid couple refrigerantabsorbent should not form a solid phase in the range of composition and operating temperature. Indeed, the formation of the solids can stop the flow and pose problems to the equipment. Moreover mixtures considered must be characterized by:
The selection criteria of an absorptionediffusion refrigerating machine mixture are multiple and concern the refrigerant, the absorbent and the mixture. The principal criteria of the refrigerants for an absorptionediffusion machine are the following:
➢ A strong solubility of the refrigerant in the absorbent to reduce dimensions of the installation. ➢ To be non-polluting ➢ To respect the regulations on the toxic and dangerous products.
1) Criteria of the refrigerant choice [9] The refrigerant must have: ➢ ➢ ➢ ➢ ➢ ➢
A high latent heat and low density; A highest possible criticizes temperature; A lowest possible specific heat capacity of the liquid (Cpm); A high stability at the temperatures of the cycle; A great energy efficiency associating fluid and system; A good compatibility with metallic materials and elastomers. 2) Criteria of the absorbent choice [7,8] The absorbent must have:
➢ A great affinity with the refrigerant, ➢ A definitely higher normal boiling point than that of the refrigerant, ➢ A high chemical stability, ➢ A low heat capacity to minimize the losses, ➢ A raised thermal conductivity, ➢ A low viscosity and surface tension to support the heat transfers. 3) Selection criteria of the mixture [1e3] In refrigerating, for absorptionediffusion machines, the choice of the mixture of work must fulfill some physicochemical, thermodynamic and transport properties. Consequently the choice of certain combination refrigerantabsorbent must satisfy certain criteria which are primarily: ➢ A normal functioning of the machine in fields of fixed pressure and temperature. ➢ A big difference between the boiling point of the refrigerating fluid and that of the absorbent; ➢ The properties of transport (viscosity, thermal conductivity, coefficient of diffusion) must be favorable; ➢ The mixture must be chemically stable, not poison and nonexplosive. It must also be noncorrosive and inexpensive.
The study of the liquidevapor equilibrium of the mixture, represented in the diagram of Oldham, concerning the considered working fluid is the base of the discussions which will follow. Indeed, examination of this diagram allows the determination of the interval of the temperatures in which the machine's functioning is possible.
Thermodynamic model [8] The choice of adequate thermodynamic model for calculation and/or prediction of the refrigerating mixtures properties are a crucial importance for the reliability of results. For the systems with low pressures, the two approaches symmetrical (ff) and dissymmetrical (g-f) make it possible to carry out this kind of calculation with a good precision. For the systems with high pressure the symmetrical method is recommended.
Model of Peng-Robinson [8,10] The thermodynamic model of Peng-Robinson uses: - The cubic equation of state of Peng-Robinson for all the thermodynamic properties except liquid molar volume. - API method for calculation of liquid molar volume of the pseudo compounds and the model of Rackett for the real compounds. The cubic equation of state of Peng-Robinson connecting the temperature, the pressure and the molar volume (T, P, v) of a pure substance is written: P¼
RT a v b v2 þ 2vb b2
With:
8 sffiffiffiffiffiffi !#2 " > > R2 T2c T 2 > > a ¼ 0; 45724 1 1 þ 0; 37464 þ 1; 54226w þ 0; 26992w < T p > > > RT > : b ¼ 0; 0778 c pc
c
(1)
c
(2)
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Tc, PC and W are respectively the critical temperature, the critical pressure and the acentric factor of the fluid.
The considered couples are: C3H6/n-C6H14, C3H6/n-C7H16, C3H6/n-C8H18 and the C3H6/n-C9H20.
ðwi ≡ log Pri at Tri ≡0:7Þ 1Þ
Traced cycle
(3)
The Peng-Robinson method allows us to obtain satisfactory results whatever the values of the temperature and the pressure. This method is consistent in the critical area [8], it doesn't present an abnormal behavior like the methods using the activities coefficients.
Thermodynamic feasibility study using propylene as a refrigerant This part was devoted to determine the functioning limits of the refrigerating absorptionediffusion machines working with various mixtures of alkanes by observing the fixed operating conditions. Fundamental data and assumptions are: - The installation has a refrigerating power of 300 W. - Cooling with the ambient air at an average temperature of 35 C, - Exit temperature of refrigerant is equal to 0 C. - Condenser and absorber temperatures are taken equal to 42 C - Generator: Driving heat provided to the generator can have several origins: solar collector, thermal discharge, steam, combustion gas. The maximum temperature of this source is fixed at 130 C. - The steady state is established and the pressure losses are considered negligible. - The vapor refrigerant purity at the outlet generator side is equal to 99, 6%. - The two heat exchangers are supposed adiabatic and characterized each one by a thermal pinch equal to 10 C for the vapor/gas exchanger and 5 C for Solution/solution one [6]. - There is equilibrium at the levels of the various apparatuses, - The evaporator and absorber pressures are equal to the low pressure and the generator and condenser pressures are equal to the high pressure
By using suitable software and taking into account the operating conditions we can determine the low and the high pressure of this cycle. The stages of Oldham diagram tracing concerning the couples propylene/absorbents follow the following steps: The two temperatures of vaporization and condensation are identified on the diagram within the intersection of the isotherms corresponding to 0 C and 42 C with a propylene molar fraction of 99.6%. The intersection points represent the evaporator and the condenser allows us to read directly on the diagram the values of low and high pressure (P evaporator; P condenser). The absorber and the generator work under pressure of the evaporator and the condenser respectively. To determine the maximum temperature of the generator, the molar fraction is fixed equal to the propylene-poor fraction (X ¼ Xp ¼ 0) and the pressure is equal to the high pressure (P ¼ Pcond). To determine the minimum temperature of the generator, the mole fraction is fixed equal to the propylene rich fraction (X ¼ Xr) and the pressure is equal to the low pressure (P ¼ Pevap). On the following figures, we traced the Oldham diagram for different couples (see Figs. 1e4). In the following table, we summarized the various values determined starting from the Oldham diagram of different binary of hydrocarbons (see Table 2). According to this table, one notices that the C3H6/nC9H20 couple presents the broadest range of temperatures what encourages its use in these cycles.
Validation of result To validate our model, we compared our results for the couple propane/nonane, with those of H. Dardour et al. [9].These results are given in Table 3. According to the comparison of these results, one can affirm that our simulation shows a good agreement. In order to determine the optimum operating conditions of the
Fig. 1 e Diagram of Oldham relating to the couple propylene/n-hexane. Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184
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Fig. 2 e Diagram of Oldham relating to the couple propylene/n-heptane.
Fig. 3 e Diagram of Oldham relating to the couple propylene/n-octane.
Fig. 4 e Diagram of Oldham relating to the couple propylene/nonane.
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Table 2 e Functioning limits of studied binary hydrocarbons. Driving temperature ( C)
C3H6/nC6H14 C3H6/nC7H16 C3H6/nC8H18 C3H6/nC9H20
Tmin Tmax Tmin Tmax Tmin Tmax Tmin Tmax
Possible enrichment Poor solution composition (XP)
Rich solution composition (Xr)
0.3015 0 0.3475 0 0.3556 0 0.3615 0
0.3715 0.3715 0.385 0.385 0.3867 0.3867 0.39 0.39
106 196 110 232 113 265 113 300
Table 3 e Functioning limits of the couple propane/ nonane. Driving temperature ( C)
Propane/n-C9H20 Dardour and Al [9] Propane/n-C9H20 (our work)
Possible enrichment Composition poor solution
Composition rich solution
Tmin Tmax
159 305
0.297 0
0.297 0.297
Tmin Tmax
159 305
0.295 0
0.2975 0.2975
Fig. 5 e COP according to generator temperature for the binary propylene/n-nonane.
refrigerating absorptionediffusion machine, we plotted on Fig. 5, the COP as a function of the generator temperature and on Fig. 6, the COP as a function of DX. From Fig. 5, we notice that the COP increases with Tgen, passes through a maximum in the vicinity of Tgen ¼ 130 C and then decreases. Fig. 6 shows that the COP increases with DX, passes through a maximum in the vicinity of DX ¼ 0.113 and then decreases.
Fig. 6 e COP according to DX for the binary propylene/nnonane, at Tgen ¼ 130 C.
Difference of composition (DX) 0.07 0,0375 0,0311 0,0285
Hence, the optimal conditions for this machine operating with propane/n-C9H20 e Helium mixture are: Xr ¼ 0.408; Xp ¼ 0.296; DX ¼ 0.113 and Tgen ¼ 130 C.
Conclusion In this work, we studied the thermodynamic feasibility of hydrocarbon binary mixtures faintly used until now as working fluids for the absorption diffusion refrigerating machines. These environmental fluids constitute an interesting alternative since the machines operating with such mixtures needed only moderate thermal source temperatures. It is the case for example of a solar collector. Moreover, this installation works under a moderate pressure (17 bar) with propylene as refrigerant and hexane, heptane, octane and nonane as absorbents. The study shows that the minimal operative generator temperature is about of 106 C and the maximum temperature is in order of 300 C. These proposed hydrocarbon mixtures are promising refrigerants since they present some advantages such as a moderate heat source temperature used for the functioning of the absorptionediffusion refrigeration machine that is not exceeding 150 C. This study is a first stage for later contribution which will consider the evaluation and performances analyzes that could carry out of the machines with absorption using these mixtures.
Nomenclature COP P Pabs Pevap Pgen T Xp Xr Tmin Tmax Tcond Tgen Tabs DX
coefficient of performance pression, bar absorber pressure, bar evaporator pressure, bar generator pressure, bar temperature, K composition of the poor solution composition of the rich solution minimal temperature maximal temperature condenser temperature generator temperature absorber temperature difference of composition
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Please cite this article in press as: Soli N, et al., Thermodynamic feasibility study of absorption diffusion machine working with hydrocarbons, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.184