- Email: [email protected]
Adsorption of Difluoromethane (HFC-32) onto phenol resin based adsorbent: Theory and experiments
International Journal of Heat and Mass Transfer 127 (2018) 348–356
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Adsorption of Difluoromethane (HFC-32) onto phenol resin based adsorbent: Theory and experiments Muhammad Sultan a,⇑, Takahiko Miyazaki b,c, Bidyut B. Saha c, Shigeru Koyama b,c, Hyun-Sig Kil b, Koji Nakabayashi d, Jin Miyawaki d, Seong-Ho Yoon d a
Department of Agricultural Engineering, Bahauddin Zakariya University, Bosan Road, Multan 60800, Pakistan Faculty of Engineering Sciences, Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan d Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan b c
a r t i c l e
i n f o
Article history: Received 29 August 2017 Received in revised form 8 July 2018 Accepted 18 July 2018
Keywords: Adsorption isotherm Adsorption heat Phenol resin Difluoromethane HFC-32
a b s t r a c t Adsorption and desorption of difluoromethane (HFC-32) onto newly developed phenol resin based adsorbent (SAC-2) have been measured experimentally for the isotherm temperatures ranging from 30 °C to 130 °C and pressure up to 3 MPa. A magnetic suspension balance based adsorption measurement unit is used to measure adsorption uptake gravimetrically. The presented SAC-2/HFC-32 pair has adsorption uptake as high as 2.23 kgref/kgads (excess adsorption) and 2.34 kgref/kgads (absolute adsorption) at 30 °C and 1.67 MPa. To the best of our knowledge, it is the highest HFC-32 adsorption capacity onto any adsorbent available in the literature. The experimental data of adsorption/desorption isotherms show that there is no hysteresis for the studied pair. The data have been fitted with Tóth; Dubinin–Astakhov (D–A); and Guggenheim, Anderson, De-Boer (GAB) adsorption isotherm models. The parameters of adsorption isotherm models are optimized by nonlinear optimization technique. The D–A model fits the experimental data precisely as compared to other models. In addition, numerical values of isosteric heat of adsorption have also been extracted by means of Clausius–Clapeyron equation using adsorption isotherm models. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Difluoromethane (CAS# 75-10-5) is a hydrofluorocarbon refrigerant and also known as HFC-32 and R-32. It is an organic compound of the dihalogenoalkane variety and has the chemical formula of CH2F2. It is slightly flammable and possesses relatively smaller global warming potential (GWP) of 675 as compared to other HFCs e.g. HFC-134a, HFC-407a, HFC-404A, HFC-410A, HFC507A, and HFC-502 [1–3]. It enables zero ozone depletion potential (ODP) and therefore has been used as a pure fluid for low temperature refrigeration. It has been used as an ingredient of 400 series nonazeotropic refrigerants. For example, the HFC-410A is a zeotropic mixture of HFC-32 and pentafluoroethane (HFC-125) of equal mass fraction. The HFC-410A is considered as the common replacement of various CFCs in the recent refrigerant systems particularly in the air-conditioning field. Similarly, the zeotropic mixture of HFC-32 with tetrafluoroethane (HFC-134a) and HFC-125 yields HFC-407A to HFC-407E based on the composition [4]. ⇑ Corresponding author. E-mail address: [email protected] (M. Sultan). https://doi.org/10.1016/j.ijheatmasstransfer.2018.07.097 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved.
In this study, adsorption data of HFC-32 onto newly developed adsorbent are presented which will be useful for adsorption based distinct applications e.g. adsorption cooling [5–7], gas storage [8,9], and gas purification and separation [10,11]. Over the past few decades, adsorption heat pump systems have received massive attention due to the environment-friendly technology. These systems are operated by the heat source; therefore, bring the opportunity to utilize low-grade waste heat and solar thermal energy options. Consequently, physical adsorption of various refrigerant–adsorbent pairs has been extensively studied in the literature at pressurized and subatmospheric conditions. However, this study presents the adsorption of HFC-32 at pressurized conditions which will help to reduce the footprint of the system. Some of the studied refrigerant–adsorbent pairs are: HFC32–ACs [1,12], HFC152a–ACs [13], HCF410A–ACs [2], HFO1234ze(E)–AC [5], HFC134a–AC [14], nbutane–AC [15], CO2–ACs [16,17], CO2–zeolite [18], ammonia– ACs [19], methanol–ACs [20], ethanol–ACs [21], ethanol–ACF [22,23], ethanol–MOF [24], water–silica gels [25], water–zeolite [26], water–MOFs [27], and water–polymers [28,29]. In view of that, the present study is another contribution to measure the HFC-32 adsorption isotherms and heat of adsorption, which will
349
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
Nomenclature A AC ACF ARE b bo C C Co Cm E GWP hfg hst k K1, K2 MOF n ODP P Pc
adsorption potential [J/mol] activated carbon activated carbon fiber average relative error [%] adsorption affinity [kPa1] adsorption affinity at T = 1 [kPa1] absolute adsorption uptake [kgref/kgads] excess adsorption uptake [kgref/kgads] saturated adsorption uptake [kgref/kgads] GAB monolayer adsorption uptake [kgref/kgads] characteristic energy [J/mol] global warming potential heat of HFC-32 vaporization [kJ/mol] isosteric heat of adsorption [J/mol] D-A model constant for pseudo-saturated vapor pressure [-] GAB model constants related to adsorption heat [-] metal organic framework structural heterogeneity parameter [-] ozone depletion potential pressure [kPa] critical pressure [kPa]
lead towards the development of sustainable technologies for adsorption heat pump systems as well as for the storage, purification, and separation of gases. Adsorption equilibrium uptake by the refrigerant–adsorbent pairs is considered an important parameter, by which the thermodynamic analysis can be performed for the performance evaluation of adsorption cooling cycle. Accordingly, thermodynamic analyses for the refrigerant–adsorbent pairs are performed in the literature and some of them are as follows: CO2–AC [30], HFC134a–AC [31], ethanol–ACs [32], ethanol–ACFs [31,33], water–silica gels [31,34] and water–AC/ACF [34]. In this regard, present study experimentally investigates the adsorption equilibrium of HFC-32 onto newly developed phenol resin based adsorbent. To the best of our knowledge, the studied adsorbent possesses highest HFC-32 adsorption uptake available in the literature. Consequently, adsorption isotherms have been measured gravimetrically using magnetic suspension adsorption measurement unit for the wide range of adsorption temperature (i.e. 30–130 °C) and pressure up to 3 MPa. The adsorption isotherm data have been analyzed for various adsorption models. Furthermore, isosteric heat of adsorption are estimated by means of adsorption isotherm data. 2. Experimental section 2.1. Materials The present study uses newly developed adsorbent named as SAC-2 [35] for the adsorption measurement of HFC-32. The studied absorbent belongs to the SAC adsorbent series [35,36], and HFC-32 adsorption onto any SAC adsorbent has not reported in the literature. The adsorbent is based on spherical phenol resin treated with potassium hydroxide (KOH). The spherical phenol resin sample has been used as a raw material with average particle diameter of 17 mm, as provided BEAPSÒ that is a registered trademark of ASAHI YUKIZAI CORPORATION. Under the flow of nitrogen, the BEAPSÒ has been carbonized at 600 °C for an hour followed by a heat treatment process at 900 °C for an hour. Potassium hydroxide with purity of more than 85% is used as an activating agent. A brief
Ps Q st R R2 T Tc Tt t
v
Vm W Wo
a q u
DHCC
saturated vapor pressure [kPa] temperature-independent adsorption heat [kJ/mol] universal gas constant [J/(mol.K)] coefficient of determination [-] temperature [K] critical temperature [K] triple point temperature [K] Tóth model heterogeneity constant [-] pore volume [cm3/g] molar volume [cm3ref/gref] volumetric adsorption amount [cm3ref/gads] maximum volumetric adsorption capacity [cm3ref/gads] thermal expansion coefficient [K1] gas density [g/cm3] relative pressure [–] temperature-dependent adsorption heat [kJ/mol]
Subscripts ads adsorbent cal calculation exp experiment ref refrigerant
description of SAC-2 preparation is given here for the sake of completeness whereas the detail procedure can be found from reference [35,36]. The porous properties and elemental composition of SAC-2 is given in Table 1 [35]. HFC-32 and Helium cylinders are used with purity of more than 99.5%. 2.2. Description of experimental apparatus Fig. 1 shows the schematic diagram of the magnetic suspension adsorption measurement unit used for the measurement of adsorption isotherms of SAC-2/HFC-32 pair. The adsorption measurement unit (MSB-GS-100-10M) is provided by BEL Japan Inc., which measures the adsorption uptake gravimetrically using Rubotherm Magnetic Suspension Balance (MSB) i.e. ElectroMagnetic Balance: MSB-CP-S-MP-250-S-D-AC100V. The experimental unit consists of a sample cell with MSB; an evaporator with efficient temperature control unit; oil circulation bath/jacket for controlling adsorption and evaporation temperatures; isothermally controlled air baths which prevent condensation within the system; three distinctive vacuum pumps i.e. diaphragm, rotary and turbo molecular types; set of pressure gauges; thermocouples; and data recording unit along with PC and respective software. Absolute pressure transducer (10,000 kPa) of type PAA-35X KELLER is used for pressure measurements. The pressure sensor possesses the full scale of 10 MPa whereas the resolution and accuracy of the
Table 1 Porosity and elemental composition of newly developed adsorbent, SAC-2 [35]. SAC-2
Properties
Values
Porosity
Total surface area [m2/g] Micropore volume [cm3/g] Total pore volume [cm3/g] Average pore width [nm]
2992 2.29 2.52 1.62
Elemental composition
C [wt.%] H [wt.%] N [wt.%] Odiff [wt.%] Ash [wt.%]
95.18 0.22 0.26 4.36 –
350
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
Fig. 1. Schematic diagram of magnetic suspension adsorption measurement used for HFC-32 adsorption measurements.
sensor is ±0.002% and ±0.05% of full scale, respectively. Maximum sample weight of 15 g can be used for adsorption/desorption equilibrium/kinetics experiments. Resolution of the MSB for recording the adsorbed/desorbed mass is 10 mg whereas the repeatability of weight measurement is ±30 mg with relative error of ±0.002% of the reading. Furthermore, when adsorption temperature is lower, the overall relative error is smaller because the adsorption uptake is relatively higher. It is worth mentioning that the MSB unit considers the buoyancy effect during the adsorption measurements, and consequently provides the corrected adsorption amount. Furthermore, MSB measures the weight of sinker under vacuum and pressurized conditions, and thereby measures the gas density of the refrigerant precisely. In this regard, Jribi et al. [16] did a comparison between the refrigerant density measured by MSB and calculated by Refprop software (NIST Standard Reference Database 23, v. 9.1), which shows a precise agreement between both methods.
Thus, saturated HFC-32 pressure in the vaporizer at the set temperature reflect the adsorption pressure. Consequently, experiments have been conducted for the pressure up to 3 MPa whereas the minimum temperature of oil bath is 10 °C i.e. 0.58 MPa. 3. Data reduction The magnetic suspension adsorption measurement unit (Fig. 1) used in this study performs buoyancy corrections and provides excess adsorption data in kgref/kgads. However, the absolute adsorption amount [kgref/kgads] can be estimated from the excess adsorption data using refrigerant density and adsorbent pore volume [16,37]. Consequently, absolute adsorption amount is calculated using Eq. (1).
C ¼ C þ qref :v
ð1Þ
2.3. Experimental procedure The MSB unit (Fig. 1) has been used for the measurement of adsorption and desorption isotherms of SAC-2/HFC-32 pair at 30 °C, 50 °C, 70 °C, 90 °C, 110 °C, and 130 °C, and pressure up to 3.0 MPa. Key points of the experimental procedure can be explained as follows: - For each adsorption/desorption isotherm measurement, SAC-2 sample is placed into the sample bin and regenerated at 130 °C under vacuum condition for several hours. - The sample temperature is then allowed to cool down at adsorption isotherm temperature where it is maintained precisely by temperature control unit. - For low and medium pressure ranges, HFC-32 is directed to the MSB unit from the tube that is charged from HFC-32 cylinder. In this regard, control valve (CV) is used to regulate the pressure in the connecting tubes. It is worthwhile to mention that the pressure cannot exceed the saturated vapor pressure of HFC-32 cylinder at ambient condition i.e. 1.47 MPa at 20 °C. - For high pressure range, HFC-32 is charged in the liquid reservoir and directed to the MSB unit. Oil bath shown in Fig. 1 controls the temperature of liquid reservoir accurately.
where C and C represent absolute adsorption amount [kgref/kgads] and excess adsorption amount [kgref/kgads] at equilibrium conditions, respectively. qref is gas density [g/cm3] of refrigerant (HFC-32) which is determined from Refprop software using adsorption temperature and pressure. v represents the total pore volume [cm3/g] of SAC-2 which is equal to 2.52 cm3/g. Adsorption isotherms data have been correlated with (i) Tóth, (ii) D–A, and (iii) GAB adsorption isotherm models which are quite consistent in the literature to fit adsorption equilibrium data [16,17,28]. Tóth model can fit the experimental data exclusively based on pressure, P (without considering the relative pressure, Ps), whereas, D–A and GAB models can consider the effect of Ps specially when the adsorption temperature is higher than critical temperature. 3.1. Tóth adsorption isotherms The Tóth adsorption isotherm model is well-known in adsorption field [1,16,17,38] and can be expressed by Eq. (2).
C ¼ Co
!
bP t 1=t
ð1 þ ðbPÞ Þ
ð2Þ
where C and C o are equilibrium adsorption amount [kgref/kgads] and saturated adsorption amount [kgref/kgads], respectively. P is
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
equilibrium pressure [kPa]. t is Tóth model constant [–] and represents system heterogeneity which in other words is the characteristic of adsorbent and/or refrigerant. It is noteworthy that the Tóth equation expresses Langmuir equation for t = 1. The parameter b represents adsorption affinity [kPa1] which is given by Eq. (3).
b ¼ bo exp
hst RT
ð3Þ
where bo is adsorption affinity [kPa1] at T = 1; R is the universal gas constant [J/(mol K)]; hst is isosteric heat of adsorption [J/mol]; and T is adsorption temperature [K]. 3.2. D–A adsorption isotherms
n Wo A exp E Vm
ð5Þ
where the molar volume V m has been calculated as [39]:
V m ¼ V t expðaðT T t ÞÞ
ð6Þ
where V t is molar volume of liquid HFC-32 at triple point temperature (T t ) which is calculated from Refprop software i.e. V t = 0.69966 cm3ref/gref at T t = 136.34 K. The thermal expansion coefficient (a) has been considered as 0.0025 K1 in the literature [16,17,30] as suggested by Ozawa et al. [39]. However, according to Saha et al. [40], a is supposed to embody the isosteric coefficient of expansion of adsorbed volume. Therefore, it has been taken as 1=T in this study according to references [40,41], which is also equal to 0.0025 K1 at T = 400 K.The adsorption potential, A used in D–A equation is given by Eq. (7).
A ¼ RT ln
Ps P
Cm K 1K 2 C¼u ð1 uK 2 Þð1 uK 2 þ uK 1 K 2 Þ
ð9Þ
model (C m ; K 1 ; K 2 ) have physical meanings and thereby expressed as function of adsorption temperature by Eqs. (10)–(12), respectively [28].
C m ¼ C om exp
q
ð8Þ
where Pc represents critical pressure of HFC-32 i.e. 5.782 MPa. The parameter k [–] is an additional fitting constant that is a measure of refrigerant–adsorbent interactions. Thus, four constants of D–A model (i.e. W o , E, n, k) are determined from the fitting of experimental data.
m
ð10Þ
RT
K 1 ¼ K o1 exp
Dq1 RT
ð11Þ
K 2 ¼ K o2 exp
Dq2 RT
ð12Þ
where C om [kgref/kgads], K o1 [–], K o2 [–], qm [J/mol], Dq1 [J/mol], and Dq2 [J/mol] are six adjustable constants of the GAB model for the effects of adsorption temperature and adsorption heat [28]. Above the critical temperature of HFC-32, the saturation vapor pressures are estimated by pseudo-saturated vapor pressure (i.e. Eq. (8)) like the D–A adsorption isotherms. Consequently, parameter k of Eq. (8) is also fitted with the experimental data altogether with the six GAB model constants. 3.4. Isosteric heat of adsorption Isosteric heat of adsorption is the difference between activation energy of adsorption and desorption, therefore, represents the strength of refrigerant–adsorbent interactions [51]. It is commonly expressed as function of adsorption uptake [3,28,52] whereas many studies have shown its dependency on adsorption temperature [36,53,54]. Therefore, in this study, it has been measured from adsorption isotherm equations with and without temperature dependency. Temperature-independent isosteric heat of adsorption (Q st ) has been estimated using Clausius–Clapeyron relationship as given by Eq. (13) [3,28].
ð7Þ
where Ps is saturated vapor pressure [kPa]. It is worthwhile to mention that the concept of liquid is nonexistent at T > T c (T c = 351.26 K), therefore, Ps are calculated by means of pseudosaturated vapor pressure as given by Eq. (8) [42].
k Ps T ¼ Tc Pc
The Guggenheim [43], Anderson [44], De-Boer [45] (GAB) adsorption isotherm model is an improved version of Brunauer–E mmett–Teller (BET) theory [46] and can be expressed by Eq. (9). In comparison with BET model, it gives flexibility for the analysis of adsorption equilibrium data by introducing an additional constant i.e. K 2 . Consequently, it has been extensively used in the literature for various adsorbents e.g. bentonite [47], nanoparticles [48], lignite [49], and food products [50]. The GAB equation will reduce to BET equation for K 2 = 1 [28].
ð4Þ
where W and W o are volumetric adsorption amount [cm3ref/gads] and maximum volumetric adsorption capacity [cm3ref/gads], respectively. The parameters A, E, and n represent adsorption potential [J/mol], characteristic energy [J/mol], and structural heterogeneity parameter [–], respectively. As the volumetric adsorption amount [cm3ref/gads] is equal to equilibrium adsorption amount [kgref/kgads] multiply by molar volume of the adsorbed phase [cm3ref/gref] i.e. W ¼ CV m , therefore, the Eq. (4) can be rewritten as follows:
C¼
3.3. GAB adsorption isotherms
where u [–] is relative pressure i.e. u ¼ PPs . All the constants of GAB
The Dubinin–Astakhov (D–A) adsorption isotherm model has been successfully used in the literature for the analysis of adsorption equilibrium data of various refrigerant–adsorbent pairs [1,16,17,36,38]. The governing equation of D–A model for volumetric adsorption uptake is given by Eq. (4).
n A W ¼ W o exp E
351
Q st ¼ R
@ ln P @ð1=TÞ
ð13Þ C
where Q st is temperature-independent adsorption heat [kJ/mol] at adsorption uptake of C. Isosteric lines i.e. linear plots between ln P versus ð1=TÞ are established, consequently, numerical value of Q st are calculated from the slope of isosteric lines. The ln P is estimated from D–A equation as given by Eq. (14).
ln P ¼ ln P s
1=n E CV m ln RT Wo
ð14Þ
Temperature-dependent isosteric heat of adsorption (DHCC ) has been determined using Clausius–Clapeyron equation in the following form as given by Eq. (15) [38]. It has been extensively used in the literature in order to assess the degree of energetic heterogeneity for various refrigerant–adsorbent pairs [17].
352
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
DHCC
@ ln P ¼ RT 2 @T C
ð15Þ
where DHCC is temperature-dependent adsorption heat [kJ/mol] at adsorption uptake of C. The DHCC can be derived from D–A equation and consequently given by Eq. (16) [17,30,38].
"
DHCC
@ ln Ps En Að1nÞ ¼ RT þAþ ðaTÞ @T n
#
2
ð16Þ C
The numerical value of ðaTÞ will be one for a ¼ 1=T. For the isosteric behavior, the adsorption potential (A) can be written from D–A equation in the following form:
1=n CV m A ¼ E ln Wo
ð17Þ
hysteresis. Therefore, with the highest adsorption uptake and no hysteresis, the studied SAC-2/HFC-32 pair can be a promising adsorbent/refrigerant pair for the development of efficient adsorption systems for various applications e.g. adsorption cooling, gas storage, purification and separation. Absolute adsorption is estimated from the excess adsorption data by means of Eq. (1) as presented in Fig. 3. It can be noticed that the studied adsorption pair possess absolute adsorption uptake as high as 2.34 kgref/kgads at 30 °C and 1.67 MPa. The absolute adsorption data have been analyzed for the best fit with wellknown adsorption isotherm models of (i) Tóth, (ii) D–A, and (iii) GAB, as shown by Fig. 3(a)–(c), respectively. Consequently, the optimized values of fitting parameters of the studied adsorption models are furnished in Table 2. A nonlinear optimization technique is adopted in order to optimize the parameters of adsorption
The first term of Eq. (16) can be further explained from the viewpoint of critical temperature as [17,55]:
@ ln Ps ¼ hfg @T
RT 2
@ ln Ps ¼ kRT @T
for ðT < T C Þ
ð18Þ
for ðT > T C Þ
ð19Þ
where hfg is heat of HFC-32 vaporization [kJ/mol], which is determined from the Refprop software. Moreover, numerical values of adsorption heat derived from Tóth isotherm model (Eq. (2)) by means of Clausius–Clapeyron equation are independent from the effects of adsorption temperature and uptake. It is simply equal to the parameter (hst ) used in Eq. (3).
(a)
2.5 Lines: Tóth model Points: Experiments
Adsorption uptake [kg/kg]
RT 2
30 °C
50 °C
2 70 °C
1.5 90 °C 110 °C
1 0.5
130 °C
0 0
4. Results and discussion
(b)
50 °C
2 70 °C
1.5 90 °C
110 °C
1 0.5
130 °C
0
1.2
90°C
0.8 Filled symbol: Adsorption Open symbol: Desorption 1500
2000
2500
3000
Adsorption uptake [kg/kg]
Excess adsorption [kg/kg]
70°C
1.6
1000
3000
500
1000
1500 P [kPa]
Lines: GAB model
2000
30 °C
2500
3000
50 °C
Points: Experiments
2
500
30 °C
2500
0
30°C
0
2000
Points: Experiments
(c) 2.5
0
1500 P [kPa]
2.5
2.4
0.4
1000
Lines: D–A model
Adsorption uptake [kg/kg]
Excess adsorption of HFC-32 onto newly developed phenol resin based adsorbent named as SAC-2 is measured gravimetrically using magnetic suspension adsorption measurement unit. The schematic diagram of the experimental unit is shown in Fig. 1 whereas porosity and elemental composition of the adsorbent is given in Table 1. Series of experiments have been conducted for adsorption and desorption isotherms of SAC-2/HFC-32 pair at 30 °C, 50 °C, 70 °C, 90 °C, 110 °C, and 130 °C, and pressure up to 3 MPa. To the best of our knowledge, the presented adsorbent possesses highest HFC-32 adsorption capacity available in the literature due to the optimum pore size and high surface area. The maximum recorded excess adsorption is 2.23 kg of HFC-32 per kg of SAC-2 at 30 °C and 1.67 MPa. Fig. 2 shows the comparison between excess adsorption and excess desorption isotherms, which is evident that the studied pair possess no adsorption
500
2 70 °C
1.5 90 °C
1
110 °C
0.5
130 °C
0 0
500
1000
1500 P [kPa]
2000
2500
3000
P [kPa] Fig. 2. Comparison between excess adsorption and excess desorption isotherms at 30 °C, 70 °C and 90 °C.
Fig. 3. Absolute adsorption isotherms of SAC-2/HFC-32 pair at 30–130 °C. Points represent absolute adsorption (experimental) and lines represent the best fit using: (a) Tóth, (b) D–A and (c) GAB adsorption models.
353
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356 Table 2 Fitting parameters of Tóth, D–A and GAB adsorption isotherm models. To´th model
D–A model
GAB model
Parameter
Value
Parameter
Value
Parameter
Value
C o [kgref/kgads] bo [kPa1] hst [kJ/mol] t [–] R = 8.3144598 J mol1 K1 RHFC-32 = 0.1598196 J g1 K1 Molar mass (HFC-32) = 52.024 g mol1
3.9981 2.20E-07 19.067 1.8873
W o [cm3ref/gads] E [kJ/mol] n [–] k [–]
3.1344 3.4981 1.0217 3.65
C om [kgref/kgads] K o1 [-] K o2 [–] qm [kJ/mol] Dq1 [kJ/mol] Dq2 [kJ/mol] k [–]
0.2897 0.0903 48.8268 7.7888 15.3647 20.0476 4.1397
models to fit with the experimental data at all isotherm temperatures. In this regard, average relative error (ARE) between experimental and calculated values has been minimized, as defined by Eq. (20). In addition, coefficient of determination (R2 ) is calculated using commonly known expressions given by Eqs. (21)–(24).
ARE ¼
N 100 X jC exp;i C cal;i j N i¼1 C exp;i
R2 ¼ 1
SSres ¼
ð20Þ
SSres SStot
ð21Þ
n X ðC exp;i C cal;i Þ2
ð22Þ
i¼1
SStot ¼
n X 2 ðC exp;i C exp Þ
ð23Þ
i¼1
C exp ¼
n 1X Cexp;i n i¼1
ð24Þ
where C exp;i and C cal;i are ith terms of experimental and calculated values, respectively. Accordingly, the numerical values of ARE and R2 are presented in Table 3 for each adsorption temperature. It can be noticed from Fig. 3 and Table 3 that the Tóth adsorption model is unable to represent the experimental data of SAC-2/HFC32 pair as compared to other studied adsorption models. On the other hand, D–A and GAB adsorption models can fit the experimental data accurately with reasonable levels of ARE and R2 . On the basis of combined effect of ARE and R2 , both adsorption models are equally effective to represent the experimental data of SAC-2/ HFC-32 pair. The failure of Tóth model as compared to D–A and GAB models can be explained as follows: The Tóth model i.e. Eq. (2) is solely based on pressure (P) irrespective of saturation pressure (Ps ). However, the concept of liquid is nonexistent above the critical temperature of HFC-32 (i.e. T c = 351.26 K), therefore, numerical values of P s are calculated for D–A and GAB models by means of pseudosaturated vapor pressure as defined by Eq. (8). The parameter k in Eq. (8) is a measure of refrigerant–adsorbent interactions;
consequently, it has been fitted with the experimental data in case of D–A and GAB models. It is noteworthy that the proposed Eq. (8) presents better fitting for both D–A and GAB models as compared to the one proposed initially by Dubinin [56] with k = 2. Similarly, it has been successfully used in many studies [16,17,30,57]. The optimized values of k for SAC-2/HFC-32 pair are 3.65 and 4.14 for D–A and GAB adsorption models, respectively. Fig. 4(a) and (b) show the plots of adsorption uptake versus adsorption potential, A (i.e. Eq. (7)), for D–A and GAB adsorption models, respectively. The plots shown in Fig. 4 are commonly known as characteristic curve in the literature. It should not be temperature-dependent and supposed to follow a single line [17,28]. Accordingly, it can be seen from Fig. 4 that the characteristic curve of SAC-2/HFC-32 pair obtained from D–A equation is more reasonable than the GAB model, therefore, confirms the goodness of the fit of D–A equation. However, in the case of Dubinin [56] with k = 2, the presented SAC-2/HFC-32 adsorption data do not give single-line characteristic curve for all adsorption models. Furthermore, considering the operating range of adsorption systems, Appendix A presents precise-fit of D–A equation for shorter range of HFC-32 pressure i.e. up to 1.0 MPa. The GAB model is relatively complex as it has seven fitting constants as compared to D–A model that possesses only four, therefore, D–A model has been used for the determination of adsorption heat. Temperature-independent isosteric heat of adsorption (Q st ) has been estimated by means of D–A model using Clausius–Clapeyron relationship as given by Eq. (13). In this regard, linear plots of ln P versus ð1=TÞ are established as shown in Fig. 5(a). Accordingly, numerical values of Q st have been determined from the slope of isosteric lines and presented by Fig. 5(b). It can be seen from Fig. 5(b) that the Q st is decreasing with the increase in adsorption uptake as reported by many studies [3,28,53]. Numerical values of Q st are ranging from 19.52 kJ/mol to 37.7 kJ/mol for the adsorption uptake ranging from 2.30 kgref/kgads to 0.01 kgref/kgads, respectively. Thus, the Q st can be written as a function of adsorption uptake using the logarithmic equation as follows:
Q st ¼ 22:331 3:35 ln C
ð25Þ
In addition, adsorption heat derived from Tóth model i.e. equivalent to parameter hst of Eq. (3), is presented on Fig. 5(b) for comparison point of view. It can be seen that the adsorption
Table 3 Values of ARE and R2 for Tóth, D–A and GAB adsorption isotherm models. Temperature [°C]
30 50 70 90 110 130
Tóth model
D–A model
GAB model
ARE [%]
R2 [–]
ARE [%]
R2 [–]
ARE [%]
R2 [–]
5.9150 5.0169 9.6096 8.2702 5.6712 4.8395
0.9741 0.9907 0.8999 0.9271 0.9736 0.9900
5.0491 4.2823 2.5788 3.2837 2.9878 5.6482
0.9917 0.9979 0.9981 0.9942 0.9928 0.9867
3.4914 1.7045 3.1085 2.6758 1.4071 1.0022
0.9865 0.9978 0.9934 0.9959 0.9979 0.9999
354
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
(a)
(a) 2.5
48
0 °C
10 °C
20 °C
30 °C
40 °C
50 °C
60 °C
70 °C
40
2
30 °C
HCC [kJ/mol]
Adsorption uptake [kg/kg]
D–A model
50 °C
1.5
70 °C 1
90 °C
32 24 16
110 °C
0.5
8
130 °C
0
0 0
(b)
5000
10000 A [J/mol]
15000
0
20000
48
2
40
50 °C
1.5
70 °C 90 °C
1
130 °C
5000
10000 A [J/mol]
15000
20000
ln(P) [kPa]
6
2
-6 0.0024
0.0027
0.1 kg/kg 0.7 kg/kg 1.9 kg/kg 0.003
0.3 kg/kg 1.1 kg/kg 2.3 kg/kg 0.0033
0.0036
1/T [K-1]
(b)
40
Qst [kJ/mol]
35
Qst (D–A, Eq. 13) = 22.331-3.35ln(C) R² = 1
30 25
Hst (Tóth, Eq. 3) = 19.067 R² = 1
10 0
0.5
1
T > Tc 0.35
0.45
0.55
24
0 °C
80 °C 110 °C
90 °C 120 °C
70 °C
100 °C 130 °C
0 0
0.5
1
1.5
2
2.5
1.5
Fig. 6. Temperature-dependent isosteric heat of adsorption of SAC-2/HFC-32 adsorption pair for: (a) T < T C and (b) T > T C .
respectively. Thus, with the increase in adsorption temperature, numerical values of hfg decreases whereas kRT increases. It is noteworthy that the DHCC values at T > T C has relatively less deviation with respect to temperature as compared to the one obtained at T < T C . Furthermore, the numerical values of DHCC at T > T C exist within the DHCC range at 0–70 °C i.e. T < T C , as shown in Fig. 6(b).
20 15
19 0.25
heat from Tóth model is lower than the one obtained from D–A equation as reported by [3,17,30], and it is nearly equal to the D– A adsorption heat at saturation conditions. Many studies have shown the dependency of adsorption heat on adsorption temperature [53,54]. Therefore, temperature-dependent isosteric heat of adsorption (DHCC ) has been determined by means of D–A model using Clausius–Clapeyron equation in the form as given by Eq. (16). Fig. 6(a) and (b) show the resulted DHCC for the adsorption temperature below and above the critical temperature of HFC-32, respectively. Referring to Fig. 6(a) for T < T C , the DHCC values are decreasing with the increase in adsorption uptake and adsorption temperature. Consequently, it is ranging from 10.52 kJ/mol (at T = 70 °C and C = 2.30 kgref/kgads) to 38.60 kJ/mol (at T = 0 °C and C = 0.01 kgref/kgads). On the other hand, for T > T C , the DHCC values are decreasing with the increase in adsorption uptake and decrease in adsorption temperature as shown in Fig. 6(b). Accordingly, it has been ranging from 14.56 kJ/mol (at T = 80 °C and C = 2.30 kgref/kgads) to 33.90 kJ/mol (at T = 130 °C and C = 0.01 kgref/kgads). This behavior of DHCC is due to the first term of Eq. (16) i.e. Ps RT 2 @ ln , which is equal to hfg and kRT at T < T C and T > T C , @T
10
0.01 kg/kg 0.5 kg/kg 1.5 kg/kg
32
Adsorption uptake [kg/kg]
Fig. 4. Insights of characteristic curve for the SAC-2/HFC-32 pair using: (a) D–A and (b) GAB adsorption models.
-2
2.5
21
8
0 0
2
16
110 °C 0.5
1.5
23
30 °C
HCC [kJ/mol]
Adsorption uptake [kg/kg]
GAB model
1
Adsorption uptake [kg/kg]
(b)
2.5
(a)
0.5
2
2.5
Adsorption uptake [kg/kg] Fig. 5. (a) Plots of ln P versus ð1=TÞ showing isosteric lines for SAC-2/HFC-32 adsorption pair. (b) Temperature-independent isosteric heat of adsorption for SAC-2/HFC-32 adsorption pair.
5. Conclusions Adsorption and desorption of Difluoromethane (HFC-32) onto newly developed phenol resin based adsorbent (named as SAC-2)
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
has been measured gravimetrically using magnetic suspension adsorption measurement unit. Series of experiments have been performed for a wide range of adsorption/desorption isotherm temperatures i.e. 30 °C, 50 °C and 70 °C for T < T C ; and 90 °C, 110 °C and 130 °C for T > T C . During the experiments, the pressure of HFC-32 was ranging from 0.04 MPa to 3.0 MPa. The experimental data is novel, and the proposed adsorbent–refrigerant pair possess adsorption uptake as high as 2.23 kgref/kgads (excess adsorption) and 2.34 kgref/kgads (absolute adsorption) at 30 °C and 1.67 MPa. To the best of our knowledge, it is the highest HFC-32 adsorption capacity onto any adsorbent available in the literature. Adsorption and desorption data have been compared at various temperatures which is evident that there is no hysteresis for the studied pair. Adsorption data have been analyzed for well-known adsorption isotherm models of (i) Tóth, (ii) D–A and (iii) GAB. According to the results from optimization and characteristic curve analysis, it is found that the D–A adsorption model can successfully represent the adsorption data of SAC-2/HFC-32 pair. Consequently, the optimized parameters of studied adsorption models are presented. In addition, isosteric heat of adsorption has been measured by means of Clausius–Clapeyron relationship using D–A model. The numerical values of adsorption heat are ranging from 10.52 kJ/mol (at T = 70 °C and C = 2.30 kgref/kgads) to 38.60 kJ/mol (at T = 0 °C and C = 0.01 kgref/kgads). The results show that the presented adsorbent–refrigerant pair has promising adsorption characteristics, which will lead towards the development of advance adsorption systems for various applications. Conflict of interest Authors declare no conflict of interest. Acknowledgements The research was partially supported by Japan Science and Technology Agency (JST), Core Research for Evolutional Science and Technology (CREST). Authors acknowledge Mr. Tsutomu Sumii for the help in the experiments. Appendix A See Fig. A1.
1.6
(Characteristic Curve)
1.2
C [kg/kg] vs A/1000 [J/mol]
0.8
Points: Experiments Lines: D–A Equation
Adsorption uptake [kg/kg]
1.6
0.4 0
(Best fit parameters)
0
Wo = 2.6673 E = 3.946 kJ/mol n = 1.1724 k = 1.80 R2 = 0.995
5
10
15
20
cm3/g
1.2
0.8
50 °C
30 °C
70 °C 90 °C
0.4
110 °C 130 °C
0
0
200
400
600
800
1000
P [kPa] Fig. A1. Precise-fit analysis of D–A equation for SAC-2/HFC-32 pair for pressure range up to 1.0 MPa.
355
Appendix B. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ijheatmasstransfer. 2018.07.097.
References [1] A.A. Askalany, B.B. Saha, K. Uddin, T. Miyzaki, S. Koyama, K. Srinivasan, I.M. Ismail, Adsorption isotherms and heat of adsorption of difluoromethane on activated carbons, J. Chem. Eng. Data 58 (2013) 2828–2834, https://doi.org/ 10.1021/je4005678. [2] A.A. Askalany, B.B. Saha, I.M. Ismail, Adsorption isotherms and kinetics of HFC410A onto activated carbons, Appl. Therm. Eng. 72 (2014) 237–243, https://doi.org/10.1016/j.applthermaleng.2014.04.075. [3] B.B. Saha, I.I. El-Sharkawy, R. Thorpe, R.E. Critoph, Accurate adsorption isotherms of R134a onto activated carbons for cooling and freezing applications, Int. J. Refrig. 35 (2012) 499–505, https://doi.org/10.1016/j. ijrefrig.2011.05.002. [4] ASHRAE, Handbook—Fundamentals, 2009. [5] S. Jribi, B.B. Saha, S. Koyama, A. Chakraborty, K.C. Ng, Study on activated carbon/HFO-1234ze(E) based adsorption cooling cycle, Appl. Therm. Eng. 50 (2013) 1570–1575, https://doi.org/10.1016/j.applthermaleng.2011.11.066. [6] M. Sultan, T. Miyazaki, S. Koyama, Z.M. Khan, Performance evaluation of hydrophilic organic polymer sorbents for desiccant air-conditioning applications, Adsorpt. Sci. Technol. 36 (2017) 311–326, https://doi.org/ 10.1177/0263617417692338. [7] M. Sultan, I.I. El-Sharkawy, T. Miyazaki, B.B. Saha, S. Koyama, An overview of solid desiccant dehumidification and air conditioning systems, Renew. Sustain. Energy Rev. 46 (2015) 16–29, https://doi.org/10.1016/j.rser.2015.02.038. [8] J.A.A. Gibson, E. Mangano, E. Shiko, A.G. Greenaway, A.V. Gromov, M.M. Lozinska, D. Friedrich, E.E.B. Campbell, P.A. Wright, S. Brandani, Adsorption materials and processes for carbon capture from gas-fired power plants: AMPGas, Ind. Eng. Chem. Res. 55 (2016) 3840–3851, https://doi.org/10.1021/ acs.iecr.5b05015. [9] I.I. El-Sharkawy, M.H. Mansour, M.M. Awad, R. El-Ashry, Investigation of natural gas storage through activated carbon, J. Chem. Eng. Data. 60 (2015) 3215–3223, https://doi.org/10.1021/acs.jced.5b00430. [10] M. Yates, J. Blanco, P. Avila, M.P. Martin, Honeycomb monoliths of activated carbons for effluent gas purification, Microporous Mesoporous Mater. 37 (2000) 201–208, https://doi.org/10.1016/S1387-1811(99)00266-8. [11] S. Sircar, Applications of gas separation by adsorption for the future, Adsorpt. Sci. Technol. 19 (2001) 347–366, https://doi.org/10.1260/0263617011494222. [12] A.A. Askalany, B.B. Saha, Experimental and theoretical study of adsorption kinetics of Difluoromethane onto activated carbons, Int. J. Refrig. 49 (2015) 160–168, https://doi.org/10.1016/j.ijrefrig.2014.10.009. [13] M. Ghazy, K. Harby, A.A. Askalany, B.B. Saha, Adsorption isotherms and kinetics of activated carbon/Difluoroethane adsorption pair: theory and experiments, Int. J. Refrig. 70 (2016) 196–205, https://doi.org/10.1016/j. ijrefrig.2016.01.012. [14] B.B. Saha, K. Habib, I.I. El-Sharkawy, S. Koyama, Adsorption characteristics and heat of adsorption measurements of R-134a on activated carbon, Int. J. Refrig. 32 (2009) 1563–1569, https://doi.org/10.1016/j.ijrefrig.2009.03.010. [15] B.B. Saha, A. Chakraborty, S. Koyama, S.-H. Yoon, I. Mochida, M. Kumja, C. Yap, K.C. Ng, Isotherms and thermodynamics for the adsorption of n-butane on pitch based activated carbon, Int. J. Heat Mass Transf. 51 (2008) 1582–1589, https://doi.org/10.1016/j.ijheatmasstransfer.2007.07.031. [16] S. Jribi, T. Miyazaki, B.B. Saha, A. Pal, M.M. Younes, S. Koyama, A. Maalej, Equilibrium and kinetics of CO2 adsorption onto activated carbon, Int. J. Heat Mass Transf. 108 (2017) 1941–1946, https://doi.org/10.1016/j. ijheatmasstransfer.2016.12.114. [17] B.B. Saha, S. Jribi, S. Koyama, I.I. El-Sharkawy, Carbon dioxide adsorption isotherms on activated carbons, J. Chem. Eng. Data. 56 (2011) 1974–1981, https://doi.org/10.1021/je100973t. [18] B.K. Sward, M. Douglas LeVan, Examination of the performance of a compression-driven adsorption cooling cycle, Appl. Therm. Eng. 19 (1999) 1–20, https://doi.org/10.1016/S1359-4311(98)00013-1. [19] Z. Tamainot-Telto, R.E. Critoph, Adsorption refrigerator using monolithic carbon-ammonia pair, Int. J. Refrig. 20 (1997) 146–155, https://doi.org/ 10.1016/S0140-7007(96)00053-9. [20] I.I. El-Sharkawy, M. Hassan, B.B. Saha, S. Koyama, M.M. Nasr, Study on adsorption of methanol onto carbon based adsorbents, Int. J. Refrig. 32 (2009) 1579–1586, https://doi.org/10.1016/j.ijrefrig.2009.06.011. [21] I.I. El-Sharkawy, K. Uddin, T. Miyazaki, B.B. Saha, S. Koyama, J. Miyawaki, S.-H. Yoon, Adsorption of ethanol onto parent and surface treated activated carbon powders, Int. J. Heat Mass Transf. 73 (2014) 445–455, https://doi.org/10.1016/ j.ijheatmasstransfer.2014.02.046. [22] I.I. El-Sharkawy, K. Kuwahara, B.B. Saha, S. Koyama, K.C. Ng, Experimental investigation of activated carbon fibers/ethanol pairs for adsorption cooling system application, Appl. Therm. Eng. 26 (2006) 859–865, https://doi.org/ 10.1016/j.applthermaleng.2005.10.010. [23] I.I. El-Sharkawy, B.B. Saha, S. Koyama, K.C. Ng, A study on the kinetics of ethanol-activated carbon fiber: theory and experiments, Int. J. Heat Mass
356
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39]
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356 Transf. 49 (2006) 3104–3110, https://doi.org/10.1016/j. ijheatmasstransfer.2006.02.029. B.B. Saha, I.I. El-Sharkawy, T. Miyazaki, S. Koyama, S.K. Henninger, A. Herbst, C. Janiak, Ethanol adsorption onto metal organic framework: theory and experiments, Energy 79 (2015) 363–370, https://doi.org/10.1016/j. energy.2014.11.022. K.C. Ng, H.T. Chua, C.Y. Chung, C.H. Loke, T. Kashiwagi, A. Akisawa, B.B. Saha, Experimental investigation of the silica gel–water adsorption isotherm characteristics, Appl. Therm. Eng. 21 (2001) 1631–1642, https://doi.org/ 10.1016/S1359-4311(01)00039-4. S. Kayal, S. Baichuan, B.B. Saha, Adsorption characteristics of AQSOA zeolites and water for adsorption chillers, Int. J. Heat Mass Transf. 92 (2016) 1120– 1127, https://doi.org/10.1016/j.ijheatmasstransfer.2015.09.060. A. Rezk, R. Al-Dadah, S. Mahmoud, A. Elsayed, Characterisation of metal organic frameworks for adsorption cooling, Int. J. Heat Mass Transf. 55 (2012) 7366–7374, https://doi.org/10.1016/j.ijheatmasstransfer.2012.07.068. M. Sultan, I.I. El-Sharkawy, T. Miyazaki, B.B. Saha, S. Koyama, T. Maruyama, S. Maeda, T. Nakamura, Insights of water vapor sorption onto polymer based sorbents, Adsorption 21 (2015) 205–215, https://doi.org/10.1007/s10450-0159663-y. M. Sultan, I.I. El-Sharkawy, T. Miyazaki, B.B. Saha, S. Koyama, T. Maruyama, S. Maeda, T. Nakamura, Water vapor sorption kinetics of polymer based sorbents: theory and experiments, Appl. Therm. Eng. 106 (2016) 192–202, https://doi.org/10.1016/j.applthermaleng.2016.05.192. A. Pal, I.I. El-Sharkawy, B.B. Saha, S. Jribi, T. Miyazaki, S. Koyama, Experimental investigation of CO2 adsorption onto a carbon based consolidated composite adsorbent for adsorption cooling application, Appl. Therm. Eng. 109 (2016) 304–311, https://doi.org/10.1016/j.applthermaleng.2016.08.031. W.S. Loh, I.I. El-Sharkawy, K.C. Ng, B.B. Saha, Adsorption cooling cycles for alternative adsorbent/adsorbate pairs working at partial vacuum and pressurized conditions, Appl. Therm. Eng. 29 (2009) 793–798, https://doi. org/10.1016/j.applthermaleng.2008.04.014. A. Pal, M.S.R. Shahrom, M. Moniruzzaman, C.D. Wilfred, S. Mitra, K. Thu, B.B. Saha, Ionic liquid as a new binder for activated carbon based consolidated composite adsorbents, Chem. Eng. J. 326 (2017) 980–986, https://doi.org/ 10.1016/j.cej.2017.06.031. B.B. Saha, I.I. El-Sharkawy, A. Chakraborty, S. Koyama, Study on an activated carbon fiber–ethanol adsorption chiller: part I – system description and modelling, Int. J. Refrig. 30 (2007) 86–95, https://doi.org/10.1016/j. ijrefrig.2006.08.004. M. Sultan, T. Miyazaki, B.B. Saha, S. Koyama, Steady-state investigation of water vapor adsorption for thermally driven adsorption based greenhouse airconditioning system, Renew. Energy 86 (2016) 785–795, https://doi.org/ 10.1016/j.renene.2015.09.015. T. Miyazaki, J. Miyawaki, T. Ohba, S.-H. Yoon, B.B. Saha, S. Koyama, Study toward high-performance thermally driven air-conditioning systems, AIP Conf. Proc. 1788 (2017) 020002, https://doi.org/10.1063/1.4968250. I.I. El-Sharkawy, K. Uddin, T. Miyazaki, B. Baran Saha, S. Koyama, H.-S. Kil, S.-H. Yoon, J. Miyawaki, Adsorption of ethanol onto phenol resin based adsorbents for developing next generation cooling systems, Int. J. Heat Mass Transf. 81 (2015) 171–178, https://doi.org/10.1016/j.ijheatmasstransfer.2014.10.012. A.L. Myers, P.A. Monson, Physical adsorption of gases: the case for absolute adsorption as the basis for thermodynamic analysis, Adsorption 20 (2014) 591–622, https://doi.org/10.1007/s10450-014-9604-1. D.D. Do, Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London, 1998. S. Ozawa, S. Kusumi, Y. Ogino, Physical adsorption of gases at high pressure. IV. An improvement of the Dubinin—Astakhov adsorption equation, J. Colloid
[40]
[41]
[42]
[43] [44] [45] [46]
[47]
[48]
[49]
[50]
[51] [52]
[53]
[54]
[55]
[56]
[57]
Interface Sci. 56 (1976) 83–91, https://doi.org/10.1016/0021-9797(76)901491. B.B. Saha, S. Koyama, I.I. El-Sharkawy, K. Habib, K. Srinivasan, P. Dutta, Evaluation of adsorption parameters and heats of adsorption through desorption measurements, J. Chem. Eng. Data 52 (2007) 2419–2424, https:// doi.org/10.1021/je700369j. K.A. Rahman, W.S. Loh, H. Yanagi, A. Chakraborty, B.B. Saha, W.G. Chun, K.C. Ng, Experimental adsorption isotherm of methane onto activated carbon at sub- and supercritical temperatures, J. Chem. Eng. Data 55 (2010) 4961–4967, https://doi.org/10.1021/je1005328. K.A.G. Amankwah, J.A. Schwarz, A modified approach for estimating pseudovapor pressures in the application of the Dubinin-Astakhov equation, Carbon 33 (1995) 1313–1319, https://doi.org/10.1016/0008-6223(95)00079-S. E.A. Guggenheim, Applications of Statistical Mechanics, Clarendon, Oxford, 1966. R.B. Anderson, Modifications of the Brunauer, Emmett and Teller equation, J. Am. Chem. Soc. 68 (1946) 686–691, https://doi.org/10.1021/ja01208a049. J.H. De Boer, The Dynamical Character of Adsorption, Clarendon, Oxford, 1953. S. Brunauer, P.H. Emmett, E. Teller, Adsorption of gases in multimolecular layers, J. Am. Chem. Soc. 60 (1938) 309–319, https://doi.org/ 10.1021/ja01269a023. D. Mihoubi, A. Bellagi, Thermodynamic analysis of sorption isotherms of bentonite, J. Chem. Thermodyn. 38 (2006) 1105–1110, https://doi.org/ 10.1016/j.jct.2005.11.010. Q. Ribeyre, G. Grévillot, A. Charvet, C. Vallières, D. Thomas, Modelling of water adsorption–condensation isotherms on beds of nanoparticles, Chem. Eng. Sci. 113 (2014) 1–10, https://doi.org/10.1016/j.ces.2014.03.027. Z. Pakowski, R. Adamski, M. Kokocin´ska, S. Kwapisz, Generalized desorption equilibrium equation of lignite in a wide temperature and moisture content range, Fuel 90 (2011) 3330–3335, https://doi.org/10.1016/j.fuel.2011.06.044. C.T. Akanbi, R.S. Adeyemi, A. Ojo, Drying characteristics and sorption isotherm of tomato slices, J. Food Eng. 73 (2006) 157–163, https://doi.org/10.1016/j. jfoodeng.2005.01.015. J. Szekely, J.W. Evans, H.Y. Sohn, Gas-Solid Reactions, Academic Press, New York, 1976. T. Miyazaki, A. Akisawa, B.B. Saha, I.I. El-Sharkawy, A. Chakraborty, A new cycle time allocation for enhancing the performance of two-bed adsorption chillers, Int. J. Refrig. 32 (2009) 846–853, https://doi.org/10.1016/j.ijrefrig.2008.12.002. K. Uddin, I.I. El-Sharkawy, T. Miyazaki, B.B. Saha, S. Koyama, H.-S. Kil, J. Miyawaki, S.-H. Yoon, Adsorption characteristics of ethanol onto functional activated carbons with controlled oxygen content, Appl. Therm. Eng. 72 (2014) 211–218, https://doi.org/10.1016/j.applthermaleng.2014.03.062. A. Pal, K. Thu, S. Mitra, I.I. El-Sharkawy, B.B. Saha, H.-S. Kil, S.-H. Yoon, J. Miyawaki, Study on biomass derived activated carbons for adsorptive heat pump application, Int. J. Heat Mass Transf. 110 (2017) 7–19, https://doi.org/ 10.1016/j.ijheatmasstransfer.2017.02.081. S. Jribi, Performance investigation of adsorption chillers utilizing low GWP refrigerants, PhD thesis, Kyushu University, 2011.
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Adsorption of Difluoromethane (HFC-32) onto phenol resin based adsorbent: Theory and experiments Muhammad Sultan a,⇑, Takahiko Miyazaki b,c, Bidyut B. Saha c, Shigeru Koyama b,c, Hyun-Sig Kil b, Koji Nakabayashi d, Jin Miyawaki d, Seong-Ho Yoon d a
Department of Agricultural Engineering, Bahauddin Zakariya University, Bosan Road, Multan 60800, Pakistan Faculty of Engineering Sciences, Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan d Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan b c
a r t i c l e
i n f o
Article history: Received 29 August 2017 Received in revised form 8 July 2018 Accepted 18 July 2018
Keywords: Adsorption isotherm Adsorption heat Phenol resin Difluoromethane HFC-32
a b s t r a c t Adsorption and desorption of difluoromethane (HFC-32) onto newly developed phenol resin based adsorbent (SAC-2) have been measured experimentally for the isotherm temperatures ranging from 30 °C to 130 °C and pressure up to 3 MPa. A magnetic suspension balance based adsorption measurement unit is used to measure adsorption uptake gravimetrically. The presented SAC-2/HFC-32 pair has adsorption uptake as high as 2.23 kgref/kgads (excess adsorption) and 2.34 kgref/kgads (absolute adsorption) at 30 °C and 1.67 MPa. To the best of our knowledge, it is the highest HFC-32 adsorption capacity onto any adsorbent available in the literature. The experimental data of adsorption/desorption isotherms show that there is no hysteresis for the studied pair. The data have been fitted with Tóth; Dubinin–Astakhov (D–A); and Guggenheim, Anderson, De-Boer (GAB) adsorption isotherm models. The parameters of adsorption isotherm models are optimized by nonlinear optimization technique. The D–A model fits the experimental data precisely as compared to other models. In addition, numerical values of isosteric heat of adsorption have also been extracted by means of Clausius–Clapeyron equation using adsorption isotherm models. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Difluoromethane (CAS# 75-10-5) is a hydrofluorocarbon refrigerant and also known as HFC-32 and R-32. It is an organic compound of the dihalogenoalkane variety and has the chemical formula of CH2F2. It is slightly flammable and possesses relatively smaller global warming potential (GWP) of 675 as compared to other HFCs e.g. HFC-134a, HFC-407a, HFC-404A, HFC-410A, HFC507A, and HFC-502 [1–3]. It enables zero ozone depletion potential (ODP) and therefore has been used as a pure fluid for low temperature refrigeration. It has been used as an ingredient of 400 series nonazeotropic refrigerants. For example, the HFC-410A is a zeotropic mixture of HFC-32 and pentafluoroethane (HFC-125) of equal mass fraction. The HFC-410A is considered as the common replacement of various CFCs in the recent refrigerant systems particularly in the air-conditioning field. Similarly, the zeotropic mixture of HFC-32 with tetrafluoroethane (HFC-134a) and HFC-125 yields HFC-407A to HFC-407E based on the composition [4]. ⇑ Corresponding author. E-mail address: [email protected] (M. Sultan). https://doi.org/10.1016/j.ijheatmasstransfer.2018.07.097 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved.
In this study, adsorption data of HFC-32 onto newly developed adsorbent are presented which will be useful for adsorption based distinct applications e.g. adsorption cooling [5–7], gas storage [8,9], and gas purification and separation [10,11]. Over the past few decades, adsorption heat pump systems have received massive attention due to the environment-friendly technology. These systems are operated by the heat source; therefore, bring the opportunity to utilize low-grade waste heat and solar thermal energy options. Consequently, physical adsorption of various refrigerant–adsorbent pairs has been extensively studied in the literature at pressurized and subatmospheric conditions. However, this study presents the adsorption of HFC-32 at pressurized conditions which will help to reduce the footprint of the system. Some of the studied refrigerant–adsorbent pairs are: HFC32–ACs [1,12], HFC152a–ACs [13], HCF410A–ACs [2], HFO1234ze(E)–AC [5], HFC134a–AC [14], nbutane–AC [15], CO2–ACs [16,17], CO2–zeolite [18], ammonia– ACs [19], methanol–ACs [20], ethanol–ACs [21], ethanol–ACF [22,23], ethanol–MOF [24], water–silica gels [25], water–zeolite [26], water–MOFs [27], and water–polymers [28,29]. In view of that, the present study is another contribution to measure the HFC-32 adsorption isotherms and heat of adsorption, which will
349
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
Nomenclature A AC ACF ARE b bo C C Co Cm E GWP hfg hst k K1, K2 MOF n ODP P Pc
adsorption potential [J/mol] activated carbon activated carbon fiber average relative error [%] adsorption affinity [kPa1] adsorption affinity at T = 1 [kPa1] absolute adsorption uptake [kgref/kgads] excess adsorption uptake [kgref/kgads] saturated adsorption uptake [kgref/kgads] GAB monolayer adsorption uptake [kgref/kgads] characteristic energy [J/mol] global warming potential heat of HFC-32 vaporization [kJ/mol] isosteric heat of adsorption [J/mol] D-A model constant for pseudo-saturated vapor pressure [-] GAB model constants related to adsorption heat [-] metal organic framework structural heterogeneity parameter [-] ozone depletion potential pressure [kPa] critical pressure [kPa]
lead towards the development of sustainable technologies for adsorption heat pump systems as well as for the storage, purification, and separation of gases. Adsorption equilibrium uptake by the refrigerant–adsorbent pairs is considered an important parameter, by which the thermodynamic analysis can be performed for the performance evaluation of adsorption cooling cycle. Accordingly, thermodynamic analyses for the refrigerant–adsorbent pairs are performed in the literature and some of them are as follows: CO2–AC [30], HFC134a–AC [31], ethanol–ACs [32], ethanol–ACFs [31,33], water–silica gels [31,34] and water–AC/ACF [34]. In this regard, present study experimentally investigates the adsorption equilibrium of HFC-32 onto newly developed phenol resin based adsorbent. To the best of our knowledge, the studied adsorbent possesses highest HFC-32 adsorption uptake available in the literature. Consequently, adsorption isotherms have been measured gravimetrically using magnetic suspension adsorption measurement unit for the wide range of adsorption temperature (i.e. 30–130 °C) and pressure up to 3 MPa. The adsorption isotherm data have been analyzed for various adsorption models. Furthermore, isosteric heat of adsorption are estimated by means of adsorption isotherm data. 2. Experimental section 2.1. Materials The present study uses newly developed adsorbent named as SAC-2 [35] for the adsorption measurement of HFC-32. The studied absorbent belongs to the SAC adsorbent series [35,36], and HFC-32 adsorption onto any SAC adsorbent has not reported in the literature. The adsorbent is based on spherical phenol resin treated with potassium hydroxide (KOH). The spherical phenol resin sample has been used as a raw material with average particle diameter of 17 mm, as provided BEAPSÒ that is a registered trademark of ASAHI YUKIZAI CORPORATION. Under the flow of nitrogen, the BEAPSÒ has been carbonized at 600 °C for an hour followed by a heat treatment process at 900 °C for an hour. Potassium hydroxide with purity of more than 85% is used as an activating agent. A brief
Ps Q st R R2 T Tc Tt t
v
Vm W Wo
a q u
DHCC
saturated vapor pressure [kPa] temperature-independent adsorption heat [kJ/mol] universal gas constant [J/(mol.K)] coefficient of determination [-] temperature [K] critical temperature [K] triple point temperature [K] Tóth model heterogeneity constant [-] pore volume [cm3/g] molar volume [cm3ref/gref] volumetric adsorption amount [cm3ref/gads] maximum volumetric adsorption capacity [cm3ref/gads] thermal expansion coefficient [K1] gas density [g/cm3] relative pressure [–] temperature-dependent adsorption heat [kJ/mol]
Subscripts ads adsorbent cal calculation exp experiment ref refrigerant
description of SAC-2 preparation is given here for the sake of completeness whereas the detail procedure can be found from reference [35,36]. The porous properties and elemental composition of SAC-2 is given in Table 1 [35]. HFC-32 and Helium cylinders are used with purity of more than 99.5%. 2.2. Description of experimental apparatus Fig. 1 shows the schematic diagram of the magnetic suspension adsorption measurement unit used for the measurement of adsorption isotherms of SAC-2/HFC-32 pair. The adsorption measurement unit (MSB-GS-100-10M) is provided by BEL Japan Inc., which measures the adsorption uptake gravimetrically using Rubotherm Magnetic Suspension Balance (MSB) i.e. ElectroMagnetic Balance: MSB-CP-S-MP-250-S-D-AC100V. The experimental unit consists of a sample cell with MSB; an evaporator with efficient temperature control unit; oil circulation bath/jacket for controlling adsorption and evaporation temperatures; isothermally controlled air baths which prevent condensation within the system; three distinctive vacuum pumps i.e. diaphragm, rotary and turbo molecular types; set of pressure gauges; thermocouples; and data recording unit along with PC and respective software. Absolute pressure transducer (10,000 kPa) of type PAA-35X KELLER is used for pressure measurements. The pressure sensor possesses the full scale of 10 MPa whereas the resolution and accuracy of the
Table 1 Porosity and elemental composition of newly developed adsorbent, SAC-2 [35]. SAC-2
Properties
Values
Porosity
Total surface area [m2/g] Micropore volume [cm3/g] Total pore volume [cm3/g] Average pore width [nm]
2992 2.29 2.52 1.62
Elemental composition
C [wt.%] H [wt.%] N [wt.%] Odiff [wt.%] Ash [wt.%]
95.18 0.22 0.26 4.36 –
350
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
Fig. 1. Schematic diagram of magnetic suspension adsorption measurement used for HFC-32 adsorption measurements.
sensor is ±0.002% and ±0.05% of full scale, respectively. Maximum sample weight of 15 g can be used for adsorption/desorption equilibrium/kinetics experiments. Resolution of the MSB for recording the adsorbed/desorbed mass is 10 mg whereas the repeatability of weight measurement is ±30 mg with relative error of ±0.002% of the reading. Furthermore, when adsorption temperature is lower, the overall relative error is smaller because the adsorption uptake is relatively higher. It is worth mentioning that the MSB unit considers the buoyancy effect during the adsorption measurements, and consequently provides the corrected adsorption amount. Furthermore, MSB measures the weight of sinker under vacuum and pressurized conditions, and thereby measures the gas density of the refrigerant precisely. In this regard, Jribi et al. [16] did a comparison between the refrigerant density measured by MSB and calculated by Refprop software (NIST Standard Reference Database 23, v. 9.1), which shows a precise agreement between both methods.
Thus, saturated HFC-32 pressure in the vaporizer at the set temperature reflect the adsorption pressure. Consequently, experiments have been conducted for the pressure up to 3 MPa whereas the minimum temperature of oil bath is 10 °C i.e. 0.58 MPa. 3. Data reduction The magnetic suspension adsorption measurement unit (Fig. 1) used in this study performs buoyancy corrections and provides excess adsorption data in kgref/kgads. However, the absolute adsorption amount [kgref/kgads] can be estimated from the excess adsorption data using refrigerant density and adsorbent pore volume [16,37]. Consequently, absolute adsorption amount is calculated using Eq. (1).
C ¼ C þ qref :v
ð1Þ
2.3. Experimental procedure The MSB unit (Fig. 1) has been used for the measurement of adsorption and desorption isotherms of SAC-2/HFC-32 pair at 30 °C, 50 °C, 70 °C, 90 °C, 110 °C, and 130 °C, and pressure up to 3.0 MPa. Key points of the experimental procedure can be explained as follows: - For each adsorption/desorption isotherm measurement, SAC-2 sample is placed into the sample bin and regenerated at 130 °C under vacuum condition for several hours. - The sample temperature is then allowed to cool down at adsorption isotherm temperature where it is maintained precisely by temperature control unit. - For low and medium pressure ranges, HFC-32 is directed to the MSB unit from the tube that is charged from HFC-32 cylinder. In this regard, control valve (CV) is used to regulate the pressure in the connecting tubes. It is worthwhile to mention that the pressure cannot exceed the saturated vapor pressure of HFC-32 cylinder at ambient condition i.e. 1.47 MPa at 20 °C. - For high pressure range, HFC-32 is charged in the liquid reservoir and directed to the MSB unit. Oil bath shown in Fig. 1 controls the temperature of liquid reservoir accurately.
where C and C represent absolute adsorption amount [kgref/kgads] and excess adsorption amount [kgref/kgads] at equilibrium conditions, respectively. qref is gas density [g/cm3] of refrigerant (HFC-32) which is determined from Refprop software using adsorption temperature and pressure. v represents the total pore volume [cm3/g] of SAC-2 which is equal to 2.52 cm3/g. Adsorption isotherms data have been correlated with (i) Tóth, (ii) D–A, and (iii) GAB adsorption isotherm models which are quite consistent in the literature to fit adsorption equilibrium data [16,17,28]. Tóth model can fit the experimental data exclusively based on pressure, P (without considering the relative pressure, Ps), whereas, D–A and GAB models can consider the effect of Ps specially when the adsorption temperature is higher than critical temperature. 3.1. Tóth adsorption isotherms The Tóth adsorption isotherm model is well-known in adsorption field [1,16,17,38] and can be expressed by Eq. (2).
C ¼ Co
!
bP t 1=t
ð1 þ ðbPÞ Þ
ð2Þ
where C and C o are equilibrium adsorption amount [kgref/kgads] and saturated adsorption amount [kgref/kgads], respectively. P is
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
equilibrium pressure [kPa]. t is Tóth model constant [–] and represents system heterogeneity which in other words is the characteristic of adsorbent and/or refrigerant. It is noteworthy that the Tóth equation expresses Langmuir equation for t = 1. The parameter b represents adsorption affinity [kPa1] which is given by Eq. (3).
b ¼ bo exp
hst RT
ð3Þ
where bo is adsorption affinity [kPa1] at T = 1; R is the universal gas constant [J/(mol K)]; hst is isosteric heat of adsorption [J/mol]; and T is adsorption temperature [K]. 3.2. D–A adsorption isotherms
n Wo A exp E Vm
ð5Þ
where the molar volume V m has been calculated as [39]:
V m ¼ V t expðaðT T t ÞÞ
ð6Þ
where V t is molar volume of liquid HFC-32 at triple point temperature (T t ) which is calculated from Refprop software i.e. V t = 0.69966 cm3ref/gref at T t = 136.34 K. The thermal expansion coefficient (a) has been considered as 0.0025 K1 in the literature [16,17,30] as suggested by Ozawa et al. [39]. However, according to Saha et al. [40], a is supposed to embody the isosteric coefficient of expansion of adsorbed volume. Therefore, it has been taken as 1=T in this study according to references [40,41], which is also equal to 0.0025 K1 at T = 400 K.The adsorption potential, A used in D–A equation is given by Eq. (7).
A ¼ RT ln
Ps P
Cm K 1K 2 C¼u ð1 uK 2 Þð1 uK 2 þ uK 1 K 2 Þ
ð9Þ
model (C m ; K 1 ; K 2 ) have physical meanings and thereby expressed as function of adsorption temperature by Eqs. (10)–(12), respectively [28].
C m ¼ C om exp
q
ð8Þ
where Pc represents critical pressure of HFC-32 i.e. 5.782 MPa. The parameter k [–] is an additional fitting constant that is a measure of refrigerant–adsorbent interactions. Thus, four constants of D–A model (i.e. W o , E, n, k) are determined from the fitting of experimental data.
m
ð10Þ
RT
K 1 ¼ K o1 exp
Dq1 RT
ð11Þ
K 2 ¼ K o2 exp
Dq2 RT
ð12Þ
where C om [kgref/kgads], K o1 [–], K o2 [–], qm [J/mol], Dq1 [J/mol], and Dq2 [J/mol] are six adjustable constants of the GAB model for the effects of adsorption temperature and adsorption heat [28]. Above the critical temperature of HFC-32, the saturation vapor pressures are estimated by pseudo-saturated vapor pressure (i.e. Eq. (8)) like the D–A adsorption isotherms. Consequently, parameter k of Eq. (8) is also fitted with the experimental data altogether with the six GAB model constants. 3.4. Isosteric heat of adsorption Isosteric heat of adsorption is the difference between activation energy of adsorption and desorption, therefore, represents the strength of refrigerant–adsorbent interactions [51]. It is commonly expressed as function of adsorption uptake [3,28,52] whereas many studies have shown its dependency on adsorption temperature [36,53,54]. Therefore, in this study, it has been measured from adsorption isotherm equations with and without temperature dependency. Temperature-independent isosteric heat of adsorption (Q st ) has been estimated using Clausius–Clapeyron relationship as given by Eq. (13) [3,28].
ð7Þ
where Ps is saturated vapor pressure [kPa]. It is worthwhile to mention that the concept of liquid is nonexistent at T > T c (T c = 351.26 K), therefore, Ps are calculated by means of pseudosaturated vapor pressure as given by Eq. (8) [42].
k Ps T ¼ Tc Pc
The Guggenheim [43], Anderson [44], De-Boer [45] (GAB) adsorption isotherm model is an improved version of Brunauer–E mmett–Teller (BET) theory [46] and can be expressed by Eq. (9). In comparison with BET model, it gives flexibility for the analysis of adsorption equilibrium data by introducing an additional constant i.e. K 2 . Consequently, it has been extensively used in the literature for various adsorbents e.g. bentonite [47], nanoparticles [48], lignite [49], and food products [50]. The GAB equation will reduce to BET equation for K 2 = 1 [28].
ð4Þ
where W and W o are volumetric adsorption amount [cm3ref/gads] and maximum volumetric adsorption capacity [cm3ref/gads], respectively. The parameters A, E, and n represent adsorption potential [J/mol], characteristic energy [J/mol], and structural heterogeneity parameter [–], respectively. As the volumetric adsorption amount [cm3ref/gads] is equal to equilibrium adsorption amount [kgref/kgads] multiply by molar volume of the adsorbed phase [cm3ref/gref] i.e. W ¼ CV m , therefore, the Eq. (4) can be rewritten as follows:
C¼
3.3. GAB adsorption isotherms
where u [–] is relative pressure i.e. u ¼ PPs . All the constants of GAB
The Dubinin–Astakhov (D–A) adsorption isotherm model has been successfully used in the literature for the analysis of adsorption equilibrium data of various refrigerant–adsorbent pairs [1,16,17,36,38]. The governing equation of D–A model for volumetric adsorption uptake is given by Eq. (4).
n A W ¼ W o exp E
351
Q st ¼ R
@ ln P @ð1=TÞ
ð13Þ C
where Q st is temperature-independent adsorption heat [kJ/mol] at adsorption uptake of C. Isosteric lines i.e. linear plots between ln P versus ð1=TÞ are established, consequently, numerical value of Q st are calculated from the slope of isosteric lines. The ln P is estimated from D–A equation as given by Eq. (14).
ln P ¼ ln P s
1=n E CV m ln RT Wo
ð14Þ
Temperature-dependent isosteric heat of adsorption (DHCC ) has been determined using Clausius–Clapeyron equation in the following form as given by Eq. (15) [38]. It has been extensively used in the literature in order to assess the degree of energetic heterogeneity for various refrigerant–adsorbent pairs [17].
352
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
DHCC
@ ln P ¼ RT 2 @T C
ð15Þ
where DHCC is temperature-dependent adsorption heat [kJ/mol] at adsorption uptake of C. The DHCC can be derived from D–A equation and consequently given by Eq. (16) [17,30,38].
"
DHCC
@ ln Ps En Að1nÞ ¼ RT þAþ ðaTÞ @T n
#
2
ð16Þ C
The numerical value of ðaTÞ will be one for a ¼ 1=T. For the isosteric behavior, the adsorption potential (A) can be written from D–A equation in the following form:
1=n CV m A ¼ E ln Wo
ð17Þ
hysteresis. Therefore, with the highest adsorption uptake and no hysteresis, the studied SAC-2/HFC-32 pair can be a promising adsorbent/refrigerant pair for the development of efficient adsorption systems for various applications e.g. adsorption cooling, gas storage, purification and separation. Absolute adsorption is estimated from the excess adsorption data by means of Eq. (1) as presented in Fig. 3. It can be noticed that the studied adsorption pair possess absolute adsorption uptake as high as 2.34 kgref/kgads at 30 °C and 1.67 MPa. The absolute adsorption data have been analyzed for the best fit with wellknown adsorption isotherm models of (i) Tóth, (ii) D–A, and (iii) GAB, as shown by Fig. 3(a)–(c), respectively. Consequently, the optimized values of fitting parameters of the studied adsorption models are furnished in Table 2. A nonlinear optimization technique is adopted in order to optimize the parameters of adsorption
The first term of Eq. (16) can be further explained from the viewpoint of critical temperature as [17,55]:
@ ln Ps ¼ hfg @T
RT 2
@ ln Ps ¼ kRT @T
for ðT < T C Þ
ð18Þ
for ðT > T C Þ
ð19Þ
where hfg is heat of HFC-32 vaporization [kJ/mol], which is determined from the Refprop software. Moreover, numerical values of adsorption heat derived from Tóth isotherm model (Eq. (2)) by means of Clausius–Clapeyron equation are independent from the effects of adsorption temperature and uptake. It is simply equal to the parameter (hst ) used in Eq. (3).
(a)
2.5 Lines: Tóth model Points: Experiments
Adsorption uptake [kg/kg]
RT 2
30 °C
50 °C
2 70 °C
1.5 90 °C 110 °C
1 0.5
130 °C
0 0
4. Results and discussion
(b)
50 °C
2 70 °C
1.5 90 °C
110 °C
1 0.5
130 °C
0
1.2
90°C
0.8 Filled symbol: Adsorption Open symbol: Desorption 1500
2000
2500
3000
Adsorption uptake [kg/kg]
Excess adsorption [kg/kg]
70°C
1.6
1000
3000
500
1000
1500 P [kPa]
Lines: GAB model
2000
30 °C
2500
3000
50 °C
Points: Experiments
2
500
30 °C
2500
0
30°C
0
2000
Points: Experiments
(c) 2.5
0
1500 P [kPa]
2.5
2.4
0.4
1000
Lines: D–A model
Adsorption uptake [kg/kg]
Excess adsorption of HFC-32 onto newly developed phenol resin based adsorbent named as SAC-2 is measured gravimetrically using magnetic suspension adsorption measurement unit. The schematic diagram of the experimental unit is shown in Fig. 1 whereas porosity and elemental composition of the adsorbent is given in Table 1. Series of experiments have been conducted for adsorption and desorption isotherms of SAC-2/HFC-32 pair at 30 °C, 50 °C, 70 °C, 90 °C, 110 °C, and 130 °C, and pressure up to 3 MPa. To the best of our knowledge, the presented adsorbent possesses highest HFC-32 adsorption capacity available in the literature due to the optimum pore size and high surface area. The maximum recorded excess adsorption is 2.23 kg of HFC-32 per kg of SAC-2 at 30 °C and 1.67 MPa. Fig. 2 shows the comparison between excess adsorption and excess desorption isotherms, which is evident that the studied pair possess no adsorption
500
2 70 °C
1.5 90 °C
1
110 °C
0.5
130 °C
0 0
500
1000
1500 P [kPa]
2000
2500
3000
P [kPa] Fig. 2. Comparison between excess adsorption and excess desorption isotherms at 30 °C, 70 °C and 90 °C.
Fig. 3. Absolute adsorption isotherms of SAC-2/HFC-32 pair at 30–130 °C. Points represent absolute adsorption (experimental) and lines represent the best fit using: (a) Tóth, (b) D–A and (c) GAB adsorption models.
353
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356 Table 2 Fitting parameters of Tóth, D–A and GAB adsorption isotherm models. To´th model
D–A model
GAB model
Parameter
Value
Parameter
Value
Parameter
Value
C o [kgref/kgads] bo [kPa1] hst [kJ/mol] t [–] R = 8.3144598 J mol1 K1 RHFC-32 = 0.1598196 J g1 K1 Molar mass (HFC-32) = 52.024 g mol1
3.9981 2.20E-07 19.067 1.8873
W o [cm3ref/gads] E [kJ/mol] n [–] k [–]
3.1344 3.4981 1.0217 3.65
C om [kgref/kgads] K o1 [-] K o2 [–] qm [kJ/mol] Dq1 [kJ/mol] Dq2 [kJ/mol] k [–]
0.2897 0.0903 48.8268 7.7888 15.3647 20.0476 4.1397
models to fit with the experimental data at all isotherm temperatures. In this regard, average relative error (ARE) between experimental and calculated values has been minimized, as defined by Eq. (20). In addition, coefficient of determination (R2 ) is calculated using commonly known expressions given by Eqs. (21)–(24).
ARE ¼
N 100 X jC exp;i C cal;i j N i¼1 C exp;i
R2 ¼ 1
SSres ¼
ð20Þ
SSres SStot
ð21Þ
n X ðC exp;i C cal;i Þ2
ð22Þ
i¼1
SStot ¼
n X 2 ðC exp;i C exp Þ
ð23Þ
i¼1
C exp ¼
n 1X Cexp;i n i¼1
ð24Þ
where C exp;i and C cal;i are ith terms of experimental and calculated values, respectively. Accordingly, the numerical values of ARE and R2 are presented in Table 3 for each adsorption temperature. It can be noticed from Fig. 3 and Table 3 that the Tóth adsorption model is unable to represent the experimental data of SAC-2/HFC32 pair as compared to other studied adsorption models. On the other hand, D–A and GAB adsorption models can fit the experimental data accurately with reasonable levels of ARE and R2 . On the basis of combined effect of ARE and R2 , both adsorption models are equally effective to represent the experimental data of SAC-2/ HFC-32 pair. The failure of Tóth model as compared to D–A and GAB models can be explained as follows: The Tóth model i.e. Eq. (2) is solely based on pressure (P) irrespective of saturation pressure (Ps ). However, the concept of liquid is nonexistent above the critical temperature of HFC-32 (i.e. T c = 351.26 K), therefore, numerical values of P s are calculated for D–A and GAB models by means of pseudosaturated vapor pressure as defined by Eq. (8). The parameter k in Eq. (8) is a measure of refrigerant–adsorbent interactions;
consequently, it has been fitted with the experimental data in case of D–A and GAB models. It is noteworthy that the proposed Eq. (8) presents better fitting for both D–A and GAB models as compared to the one proposed initially by Dubinin [56] with k = 2. Similarly, it has been successfully used in many studies [16,17,30,57]. The optimized values of k for SAC-2/HFC-32 pair are 3.65 and 4.14 for D–A and GAB adsorption models, respectively. Fig. 4(a) and (b) show the plots of adsorption uptake versus adsorption potential, A (i.e. Eq. (7)), for D–A and GAB adsorption models, respectively. The plots shown in Fig. 4 are commonly known as characteristic curve in the literature. It should not be temperature-dependent and supposed to follow a single line [17,28]. Accordingly, it can be seen from Fig. 4 that the characteristic curve of SAC-2/HFC-32 pair obtained from D–A equation is more reasonable than the GAB model, therefore, confirms the goodness of the fit of D–A equation. However, in the case of Dubinin [56] with k = 2, the presented SAC-2/HFC-32 adsorption data do not give single-line characteristic curve for all adsorption models. Furthermore, considering the operating range of adsorption systems, Appendix A presents precise-fit of D–A equation for shorter range of HFC-32 pressure i.e. up to 1.0 MPa. The GAB model is relatively complex as it has seven fitting constants as compared to D–A model that possesses only four, therefore, D–A model has been used for the determination of adsorption heat. Temperature-independent isosteric heat of adsorption (Q st ) has been estimated by means of D–A model using Clausius–Clapeyron relationship as given by Eq. (13). In this regard, linear plots of ln P versus ð1=TÞ are established as shown in Fig. 5(a). Accordingly, numerical values of Q st have been determined from the slope of isosteric lines and presented by Fig. 5(b). It can be seen from Fig. 5(b) that the Q st is decreasing with the increase in adsorption uptake as reported by many studies [3,28,53]. Numerical values of Q st are ranging from 19.52 kJ/mol to 37.7 kJ/mol for the adsorption uptake ranging from 2.30 kgref/kgads to 0.01 kgref/kgads, respectively. Thus, the Q st can be written as a function of adsorption uptake using the logarithmic equation as follows:
Q st ¼ 22:331 3:35 ln C
ð25Þ
In addition, adsorption heat derived from Tóth model i.e. equivalent to parameter hst of Eq. (3), is presented on Fig. 5(b) for comparison point of view. It can be seen that the adsorption
Table 3 Values of ARE and R2 for Tóth, D–A and GAB adsorption isotherm models. Temperature [°C]
30 50 70 90 110 130
Tóth model
D–A model
GAB model
ARE [%]
R2 [–]
ARE [%]
R2 [–]
ARE [%]
R2 [–]
5.9150 5.0169 9.6096 8.2702 5.6712 4.8395
0.9741 0.9907 0.8999 0.9271 0.9736 0.9900
5.0491 4.2823 2.5788 3.2837 2.9878 5.6482
0.9917 0.9979 0.9981 0.9942 0.9928 0.9867
3.4914 1.7045 3.1085 2.6758 1.4071 1.0022
0.9865 0.9978 0.9934 0.9959 0.9979 0.9999
354
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
(a)
(a) 2.5
48
0 °C
10 °C
20 °C
30 °C
40 °C
50 °C
60 °C
70 °C
40
2
30 °C
HCC [kJ/mol]
Adsorption uptake [kg/kg]
D–A model
50 °C
1.5
70 °C 1
90 °C
32 24 16
110 °C
0.5
8
130 °C
0
0 0
(b)
5000
10000 A [J/mol]
15000
0
20000
48
2
40
50 °C
1.5
70 °C 90 °C
1
130 °C
5000
10000 A [J/mol]
15000
20000
ln(P) [kPa]
6
2
-6 0.0024
0.0027
0.1 kg/kg 0.7 kg/kg 1.9 kg/kg 0.003
0.3 kg/kg 1.1 kg/kg 2.3 kg/kg 0.0033
0.0036
1/T [K-1]
(b)
40
Qst [kJ/mol]
35
Qst (D–A, Eq. 13) = 22.331-3.35ln(C) R² = 1
30 25
Hst (Tóth, Eq. 3) = 19.067 R² = 1
10 0
0.5
1
T > Tc 0.35
0.45
0.55
24
0 °C
80 °C 110 °C
90 °C 120 °C
70 °C
100 °C 130 °C
0 0
0.5
1
1.5
2
2.5
1.5
Fig. 6. Temperature-dependent isosteric heat of adsorption of SAC-2/HFC-32 adsorption pair for: (a) T < T C and (b) T > T C .
respectively. Thus, with the increase in adsorption temperature, numerical values of hfg decreases whereas kRT increases. It is noteworthy that the DHCC values at T > T C has relatively less deviation with respect to temperature as compared to the one obtained at T < T C . Furthermore, the numerical values of DHCC at T > T C exist within the DHCC range at 0–70 °C i.e. T < T C , as shown in Fig. 6(b).
20 15
19 0.25
heat from Tóth model is lower than the one obtained from D–A equation as reported by [3,17,30], and it is nearly equal to the D– A adsorption heat at saturation conditions. Many studies have shown the dependency of adsorption heat on adsorption temperature [53,54]. Therefore, temperature-dependent isosteric heat of adsorption (DHCC ) has been determined by means of D–A model using Clausius–Clapeyron equation in the form as given by Eq. (16). Fig. 6(a) and (b) show the resulted DHCC for the adsorption temperature below and above the critical temperature of HFC-32, respectively. Referring to Fig. 6(a) for T < T C , the DHCC values are decreasing with the increase in adsorption uptake and adsorption temperature. Consequently, it is ranging from 10.52 kJ/mol (at T = 70 °C and C = 2.30 kgref/kgads) to 38.60 kJ/mol (at T = 0 °C and C = 0.01 kgref/kgads). On the other hand, for T > T C , the DHCC values are decreasing with the increase in adsorption uptake and decrease in adsorption temperature as shown in Fig. 6(b). Accordingly, it has been ranging from 14.56 kJ/mol (at T = 80 °C and C = 2.30 kgref/kgads) to 33.90 kJ/mol (at T = 130 °C and C = 0.01 kgref/kgads). This behavior of DHCC is due to the first term of Eq. (16) i.e. Ps RT 2 @ ln , which is equal to hfg and kRT at T < T C and T > T C , @T
10
0.01 kg/kg 0.5 kg/kg 1.5 kg/kg
32
Adsorption uptake [kg/kg]
Fig. 4. Insights of characteristic curve for the SAC-2/HFC-32 pair using: (a) D–A and (b) GAB adsorption models.
-2
2.5
21
8
0 0
2
16
110 °C 0.5
1.5
23
30 °C
HCC [kJ/mol]
Adsorption uptake [kg/kg]
GAB model
1
Adsorption uptake [kg/kg]
(b)
2.5
(a)
0.5
2
2.5
Adsorption uptake [kg/kg] Fig. 5. (a) Plots of ln P versus ð1=TÞ showing isosteric lines for SAC-2/HFC-32 adsorption pair. (b) Temperature-independent isosteric heat of adsorption for SAC-2/HFC-32 adsorption pair.
5. Conclusions Adsorption and desorption of Difluoromethane (HFC-32) onto newly developed phenol resin based adsorbent (named as SAC-2)
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356
has been measured gravimetrically using magnetic suspension adsorption measurement unit. Series of experiments have been performed for a wide range of adsorption/desorption isotherm temperatures i.e. 30 °C, 50 °C and 70 °C for T < T C ; and 90 °C, 110 °C and 130 °C for T > T C . During the experiments, the pressure of HFC-32 was ranging from 0.04 MPa to 3.0 MPa. The experimental data is novel, and the proposed adsorbent–refrigerant pair possess adsorption uptake as high as 2.23 kgref/kgads (excess adsorption) and 2.34 kgref/kgads (absolute adsorption) at 30 °C and 1.67 MPa. To the best of our knowledge, it is the highest HFC-32 adsorption capacity onto any adsorbent available in the literature. Adsorption and desorption data have been compared at various temperatures which is evident that there is no hysteresis for the studied pair. Adsorption data have been analyzed for well-known adsorption isotherm models of (i) Tóth, (ii) D–A and (iii) GAB. According to the results from optimization and characteristic curve analysis, it is found that the D–A adsorption model can successfully represent the adsorption data of SAC-2/HFC-32 pair. Consequently, the optimized parameters of studied adsorption models are presented. In addition, isosteric heat of adsorption has been measured by means of Clausius–Clapeyron relationship using D–A model. The numerical values of adsorption heat are ranging from 10.52 kJ/mol (at T = 70 °C and C = 2.30 kgref/kgads) to 38.60 kJ/mol (at T = 0 °C and C = 0.01 kgref/kgads). The results show that the presented adsorbent–refrigerant pair has promising adsorption characteristics, which will lead towards the development of advance adsorption systems for various applications. Conflict of interest Authors declare no conflict of interest. Acknowledgements The research was partially supported by Japan Science and Technology Agency (JST), Core Research for Evolutional Science and Technology (CREST). Authors acknowledge Mr. Tsutomu Sumii for the help in the experiments. Appendix A See Fig. A1.
1.6
(Characteristic Curve)
1.2
C [kg/kg] vs A/1000 [J/mol]
0.8
Points: Experiments Lines: D–A Equation
Adsorption uptake [kg/kg]
1.6
0.4 0
(Best fit parameters)
0
Wo = 2.6673 E = 3.946 kJ/mol n = 1.1724 k = 1.80 R2 = 0.995
5
10
15
20
cm3/g
1.2
0.8
50 °C
30 °C
70 °C 90 °C
0.4
110 °C 130 °C
0
0
200
400
600
800
1000
P [kPa] Fig. A1. Precise-fit analysis of D–A equation for SAC-2/HFC-32 pair for pressure range up to 1.0 MPa.
355
Appendix B. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ijheatmasstransfer. 2018.07.097.
References [1] A.A. Askalany, B.B. Saha, K. Uddin, T. Miyzaki, S. Koyama, K. Srinivasan, I.M. Ismail, Adsorption isotherms and heat of adsorption of difluoromethane on activated carbons, J. Chem. Eng. Data 58 (2013) 2828–2834, https://doi.org/ 10.1021/je4005678. [2] A.A. Askalany, B.B. Saha, I.M. Ismail, Adsorption isotherms and kinetics of HFC410A onto activated carbons, Appl. Therm. Eng. 72 (2014) 237–243, https://doi.org/10.1016/j.applthermaleng.2014.04.075. [3] B.B. Saha, I.I. El-Sharkawy, R. Thorpe, R.E. Critoph, Accurate adsorption isotherms of R134a onto activated carbons for cooling and freezing applications, Int. J. Refrig. 35 (2012) 499–505, https://doi.org/10.1016/j. ijrefrig.2011.05.002. [4] ASHRAE, Handbook—Fundamentals, 2009. [5] S. Jribi, B.B. Saha, S. Koyama, A. Chakraborty, K.C. Ng, Study on activated carbon/HFO-1234ze(E) based adsorption cooling cycle, Appl. Therm. Eng. 50 (2013) 1570–1575, https://doi.org/10.1016/j.applthermaleng.2011.11.066. [6] M. Sultan, T. Miyazaki, S. Koyama, Z.M. Khan, Performance evaluation of hydrophilic organic polymer sorbents for desiccant air-conditioning applications, Adsorpt. Sci. Technol. 36 (2017) 311–326, https://doi.org/ 10.1177/0263617417692338. [7] M. Sultan, I.I. El-Sharkawy, T. Miyazaki, B.B. Saha, S. Koyama, An overview of solid desiccant dehumidification and air conditioning systems, Renew. Sustain. Energy Rev. 46 (2015) 16–29, https://doi.org/10.1016/j.rser.2015.02.038. [8] J.A.A. Gibson, E. Mangano, E. Shiko, A.G. Greenaway, A.V. Gromov, M.M. Lozinska, D. Friedrich, E.E.B. Campbell, P.A. Wright, S. Brandani, Adsorption materials and processes for carbon capture from gas-fired power plants: AMPGas, Ind. Eng. Chem. Res. 55 (2016) 3840–3851, https://doi.org/10.1021/ acs.iecr.5b05015. [9] I.I. El-Sharkawy, M.H. Mansour, M.M. Awad, R. El-Ashry, Investigation of natural gas storage through activated carbon, J. Chem. Eng. Data. 60 (2015) 3215–3223, https://doi.org/10.1021/acs.jced.5b00430. [10] M. Yates, J. Blanco, P. Avila, M.P. Martin, Honeycomb monoliths of activated carbons for effluent gas purification, Microporous Mesoporous Mater. 37 (2000) 201–208, https://doi.org/10.1016/S1387-1811(99)00266-8. [11] S. Sircar, Applications of gas separation by adsorption for the future, Adsorpt. Sci. Technol. 19 (2001) 347–366, https://doi.org/10.1260/0263617011494222. [12] A.A. Askalany, B.B. Saha, Experimental and theoretical study of adsorption kinetics of Difluoromethane onto activated carbons, Int. J. Refrig. 49 (2015) 160–168, https://doi.org/10.1016/j.ijrefrig.2014.10.009. [13] M. Ghazy, K. Harby, A.A. Askalany, B.B. Saha, Adsorption isotherms and kinetics of activated carbon/Difluoroethane adsorption pair: theory and experiments, Int. J. Refrig. 70 (2016) 196–205, https://doi.org/10.1016/j. ijrefrig.2016.01.012. [14] B.B. Saha, K. Habib, I.I. El-Sharkawy, S. Koyama, Adsorption characteristics and heat of adsorption measurements of R-134a on activated carbon, Int. J. Refrig. 32 (2009) 1563–1569, https://doi.org/10.1016/j.ijrefrig.2009.03.010. [15] B.B. Saha, A. Chakraborty, S. Koyama, S.-H. Yoon, I. Mochida, M. Kumja, C. Yap, K.C. Ng, Isotherms and thermodynamics for the adsorption of n-butane on pitch based activated carbon, Int. J. Heat Mass Transf. 51 (2008) 1582–1589, https://doi.org/10.1016/j.ijheatmasstransfer.2007.07.031. [16] S. Jribi, T. Miyazaki, B.B. Saha, A. Pal, M.M. Younes, S. Koyama, A. Maalej, Equilibrium and kinetics of CO2 adsorption onto activated carbon, Int. J. Heat Mass Transf. 108 (2017) 1941–1946, https://doi.org/10.1016/j. ijheatmasstransfer.2016.12.114. [17] B.B. Saha, S. Jribi, S. Koyama, I.I. El-Sharkawy, Carbon dioxide adsorption isotherms on activated carbons, J. Chem. Eng. Data. 56 (2011) 1974–1981, https://doi.org/10.1021/je100973t. [18] B.K. Sward, M. Douglas LeVan, Examination of the performance of a compression-driven adsorption cooling cycle, Appl. Therm. Eng. 19 (1999) 1–20, https://doi.org/10.1016/S1359-4311(98)00013-1. [19] Z. Tamainot-Telto, R.E. Critoph, Adsorption refrigerator using monolithic carbon-ammonia pair, Int. J. Refrig. 20 (1997) 146–155, https://doi.org/ 10.1016/S0140-7007(96)00053-9. [20] I.I. El-Sharkawy, M. Hassan, B.B. Saha, S. Koyama, M.M. Nasr, Study on adsorption of methanol onto carbon based adsorbents, Int. J. Refrig. 32 (2009) 1579–1586, https://doi.org/10.1016/j.ijrefrig.2009.06.011. [21] I.I. El-Sharkawy, K. Uddin, T. Miyazaki, B.B. Saha, S. Koyama, J. Miyawaki, S.-H. Yoon, Adsorption of ethanol onto parent and surface treated activated carbon powders, Int. J. Heat Mass Transf. 73 (2014) 445–455, https://doi.org/10.1016/ j.ijheatmasstransfer.2014.02.046. [22] I.I. El-Sharkawy, K. Kuwahara, B.B. Saha, S. Koyama, K.C. Ng, Experimental investigation of activated carbon fibers/ethanol pairs for adsorption cooling system application, Appl. Therm. Eng. 26 (2006) 859–865, https://doi.org/ 10.1016/j.applthermaleng.2005.10.010. [23] I.I. El-Sharkawy, B.B. Saha, S. Koyama, K.C. Ng, A study on the kinetics of ethanol-activated carbon fiber: theory and experiments, Int. J. Heat Mass
356
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39]
M. Sultan et al. / International Journal of Heat and Mass Transfer 127 (2018) 348–356 Transf. 49 (2006) 3104–3110, https://doi.org/10.1016/j. ijheatmasstransfer.2006.02.029. B.B. Saha, I.I. El-Sharkawy, T. Miyazaki, S. Koyama, S.K. Henninger, A. Herbst, C. Janiak, Ethanol adsorption onto metal organic framework: theory and experiments, Energy 79 (2015) 363–370, https://doi.org/10.1016/j. energy.2014.11.022. K.C. Ng, H.T. Chua, C.Y. Chung, C.H. Loke, T. Kashiwagi, A. Akisawa, B.B. Saha, Experimental investigation of the silica gel–water adsorption isotherm characteristics, Appl. Therm. Eng. 21 (2001) 1631–1642, https://doi.org/ 10.1016/S1359-4311(01)00039-4. S. Kayal, S. Baichuan, B.B. Saha, Adsorption characteristics of AQSOA zeolites and water for adsorption chillers, Int. J. Heat Mass Transf. 92 (2016) 1120– 1127, https://doi.org/10.1016/j.ijheatmasstransfer.2015.09.060. A. Rezk, R. Al-Dadah, S. Mahmoud, A. Elsayed, Characterisation of metal organic frameworks for adsorption cooling, Int. J. Heat Mass Transf. 55 (2012) 7366–7374, https://doi.org/10.1016/j.ijheatmasstransfer.2012.07.068. M. Sultan, I.I. El-Sharkawy, T. Miyazaki, B.B. Saha, S. Koyama, T. Maruyama, S. Maeda, T. Nakamura, Insights of water vapor sorption onto polymer based sorbents, Adsorption 21 (2015) 205–215, https://doi.org/10.1007/s10450-0159663-y. M. Sultan, I.I. El-Sharkawy, T. Miyazaki, B.B. Saha, S. Koyama, T. Maruyama, S. Maeda, T. Nakamura, Water vapor sorption kinetics of polymer based sorbents: theory and experiments, Appl. Therm. Eng. 106 (2016) 192–202, https://doi.org/10.1016/j.applthermaleng.2016.05.192. A. Pal, I.I. El-Sharkawy, B.B. Saha, S. Jribi, T. Miyazaki, S. Koyama, Experimental investigation of CO2 adsorption onto a carbon based consolidated composite adsorbent for adsorption cooling application, Appl. Therm. Eng. 109 (2016) 304–311, https://doi.org/10.1016/j.applthermaleng.2016.08.031. W.S. Loh, I.I. El-Sharkawy, K.C. Ng, B.B. Saha, Adsorption cooling cycles for alternative adsorbent/adsorbate pairs working at partial vacuum and pressurized conditions, Appl. Therm. Eng. 29 (2009) 793–798, https://doi. org/10.1016/j.applthermaleng.2008.04.014. A. Pal, M.S.R. Shahrom, M. Moniruzzaman, C.D. Wilfred, S. Mitra, K. Thu, B.B. Saha, Ionic liquid as a new binder for activated carbon based consolidated composite adsorbents, Chem. Eng. J. 326 (2017) 980–986, https://doi.org/ 10.1016/j.cej.2017.06.031. B.B. Saha, I.I. El-Sharkawy, A. Chakraborty, S. Koyama, Study on an activated carbon fiber–ethanol adsorption chiller: part I – system description and modelling, Int. J. Refrig. 30 (2007) 86–95, https://doi.org/10.1016/j. ijrefrig.2006.08.004. M. Sultan, T. Miyazaki, B.B. Saha, S. Koyama, Steady-state investigation of water vapor adsorption for thermally driven adsorption based greenhouse airconditioning system, Renew. Energy 86 (2016) 785–795, https://doi.org/ 10.1016/j.renene.2015.09.015. T. Miyazaki, J. Miyawaki, T. Ohba, S.-H. Yoon, B.B. Saha, S. Koyama, Study toward high-performance thermally driven air-conditioning systems, AIP Conf. Proc. 1788 (2017) 020002, https://doi.org/10.1063/1.4968250. I.I. El-Sharkawy, K. Uddin, T. Miyazaki, B. Baran Saha, S. Koyama, H.-S. Kil, S.-H. Yoon, J. Miyawaki, Adsorption of ethanol onto phenol resin based adsorbents for developing next generation cooling systems, Int. J. Heat Mass Transf. 81 (2015) 171–178, https://doi.org/10.1016/j.ijheatmasstransfer.2014.10.012. A.L. Myers, P.A. Monson, Physical adsorption of gases: the case for absolute adsorption as the basis for thermodynamic analysis, Adsorption 20 (2014) 591–622, https://doi.org/10.1007/s10450-014-9604-1. D.D. Do, Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London, 1998. S. Ozawa, S. Kusumi, Y. Ogino, Physical adsorption of gases at high pressure. IV. An improvement of the Dubinin—Astakhov adsorption equation, J. Colloid
[40]
[41]
[42]
[43] [44] [45] [46]
[47]
[48]
[49]
[50]
[51] [52]
[53]
[54]
[55]
[56]
[57]
Interface Sci. 56 (1976) 83–91, https://doi.org/10.1016/0021-9797(76)901491. B.B. Saha, S. Koyama, I.I. El-Sharkawy, K. Habib, K. Srinivasan, P. Dutta, Evaluation of adsorption parameters and heats of adsorption through desorption measurements, J. Chem. Eng. Data 52 (2007) 2419–2424, https:// doi.org/10.1021/je700369j. K.A. Rahman, W.S. Loh, H. Yanagi, A. Chakraborty, B.B. Saha, W.G. Chun, K.C. Ng, Experimental adsorption isotherm of methane onto activated carbon at sub- and supercritical temperatures, J. Chem. Eng. Data 55 (2010) 4961–4967, https://doi.org/10.1021/je1005328. K.A.G. Amankwah, J.A. Schwarz, A modified approach for estimating pseudovapor pressures in the application of the Dubinin-Astakhov equation, Carbon 33 (1995) 1313–1319, https://doi.org/10.1016/0008-6223(95)00079-S. E.A. Guggenheim, Applications of Statistical Mechanics, Clarendon, Oxford, 1966. R.B. Anderson, Modifications of the Brunauer, Emmett and Teller equation, J. Am. Chem. Soc. 68 (1946) 686–691, https://doi.org/10.1021/ja01208a049. J.H. De Boer, The Dynamical Character of Adsorption, Clarendon, Oxford, 1953. S. Brunauer, P.H. Emmett, E. Teller, Adsorption of gases in multimolecular layers, J. Am. Chem. Soc. 60 (1938) 309–319, https://doi.org/ 10.1021/ja01269a023. D. Mihoubi, A. Bellagi, Thermodynamic analysis of sorption isotherms of bentonite, J. Chem. Thermodyn. 38 (2006) 1105–1110, https://doi.org/ 10.1016/j.jct.2005.11.010. Q. Ribeyre, G. Grévillot, A. Charvet, C. Vallières, D. Thomas, Modelling of water adsorption–condensation isotherms on beds of nanoparticles, Chem. Eng. Sci. 113 (2014) 1–10, https://doi.org/10.1016/j.ces.2014.03.027. Z. Pakowski, R. Adamski, M. Kokocin´ska, S. Kwapisz, Generalized desorption equilibrium equation of lignite in a wide temperature and moisture content range, Fuel 90 (2011) 3330–3335, https://doi.org/10.1016/j.fuel.2011.06.044. C.T. Akanbi, R.S. Adeyemi, A. Ojo, Drying characteristics and sorption isotherm of tomato slices, J. Food Eng. 73 (2006) 157–163, https://doi.org/10.1016/j. jfoodeng.2005.01.015. J. Szekely, J.W. Evans, H.Y. Sohn, Gas-Solid Reactions, Academic Press, New York, 1976. T. Miyazaki, A. Akisawa, B.B. Saha, I.I. El-Sharkawy, A. Chakraborty, A new cycle time allocation for enhancing the performance of two-bed adsorption chillers, Int. J. Refrig. 32 (2009) 846–853, https://doi.org/10.1016/j.ijrefrig.2008.12.002. K. Uddin, I.I. El-Sharkawy, T. Miyazaki, B.B. Saha, S. Koyama, H.-S. Kil, J. Miyawaki, S.-H. Yoon, Adsorption characteristics of ethanol onto functional activated carbons with controlled oxygen content, Appl. Therm. Eng. 72 (2014) 211–218, https://doi.org/10.1016/j.applthermaleng.2014.03.062. A. Pal, K. Thu, S. Mitra, I.I. El-Sharkawy, B.B. Saha, H.-S. Kil, S.-H. Yoon, J. Miyawaki, Study on biomass derived activated carbons for adsorptive heat pump application, Int. J. Heat Mass Transf. 110 (2017) 7–19, https://doi.org/ 10.1016/j.ijheatmasstransfer.2017.02.081. S. Jribi, Performance investigation of adsorption chillers utilizing low GWP refrigerants, PhD thesis, Kyushu University, 2011.