R507A pairs

R507A pairs

international journal of refrigeration 33 (2010) 706–713 available at www.sciencedirect.com w w w . i i fi i r . o r g journal homepage: www.elsevi...

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international journal of refrigeration 33 (2010) 706–713

available at www.sciencedirect.com

w w w . i i fi i r . o r g

journal homepage: www.elsevier.com/locate/ijrefrig

Experimental study on adsorption kinetics of activated carbon/R134a and activated carbon/R507A pairs Khairul Habib a, Bidyut B. Saha b,*, Kazi A. Rahman c, Anutosh Chakraborty c, Shigeru Koyama a, Kim Choon Ng c a

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga-shi, Fukuoka 816-8580, Japan Mechanical Engineering Department, Kyushu University, 744 Motooka, Fukuoka-shi, Fukuoka 819-0395, Japan c Mechanical Engineering Department, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 b

article info

abstract

Article history:

The objective of this article is to evaluate adsorption kinetics of R134a and R507A on pitch

Received 7 April 2009

based activated carbon experimentally by a constant volume variable pressure method at

Received in revised form

different adsorption temperatures ranging from 20 to 60  C. These data are useful for the

24 November 2009

design of adsorption cooling and refrigeration systems and are unavailable in the litera-

Accepted 25 January 2010

ture. Data obtained from the kinetic studies were analyzed with various kinetic models and

Available online 1 February 2010

the Fickian diffusion model is found to be the most suitable overall. Guided by the experimental measurements, the surface diffusion is also estimated and is found that it

Keywords:

follows the classical Arrhenius law within the experimental range. ª 2010 Elsevier Ltd and IIR. All rights reserved.

Adsorption system Activated carbon R134a R507A Experiment Adsorption

Etude expe´rimentale sur la cine´tique d’adsorption des couples actifs charbon actif/R134a et charbon actif/R507A Mots cle´s : Syste`me a` adsorption ; Charbon actif ; R134a ; R507A ; Expe´rimentation ; Adsorption

1.

Introduction

The physical adsorption process occurs mainly within the pores and surface of the adsorbent. It required the knowledge of adsorption characteristics when the temperatures and

pressures are varying. In the fields of adsorption cooling (Saha et al., 2003, 2009a; Wang et al., 2008; Chua et al., 2004; Anyanwu and Ezekwe, 2003), purification of gas (Hagiwara et al., 2005; Huo et al., 2005; Hidayat et al., 2004), separation process (Zhu et al., 2005; Xu et al., 2001), natural gas storage

* Corresponding author. Tel.: þ81 92 802 3101; fax: þ81 92 802 3125. E-mail address: [email protected] (B.B. Saha). 0140-7007/$ – see front matter ª 2010 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2010.01.006

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international journal of refrigeration 33 (2010) 706–713

Nomenclature Symbols Ds Dso miref riref Tich cell Pich cell Vch_cell f mref Vch_cell Vads_cell Vvoid

surface diffusion, m2/s pre-exponential coefficient initial mass of refrigerant before adsorption, kg initial charging density, kg/m3 initial charging cell temperature, K initial charging cell pressure, Pa volume of charging cell, m3/kg remaining mass of refrigerant after adsorption, kg volume of adsorption cell, m3/kg volume of adsorption cell, m3/kg void volume, m3/kg

(Biloe´ et al., 2002; Barbosa Mota et al., 1997), adsorption on a solid adsorbent is a fundamental process. Generally, the thermo-physical properties (surface area, pore volume, particle diameter, etc.) of an adsorbent play a vital role in the adsorption characteristics of adsorbent–refrigerant pair. Pitch based activated carbon (type Maxsorb III) can become a potential adsorbent in the field of refrigeration and some studies in Maxsorb III have already been performed (Saha et al., 2008a; Hamamoto et al., 2006). Comparing with the palletized or granular activated carbon, Maxsorb III has many intrinsic characteristics that make it superior over other activated carbons. For example, Maxsorb III has large surface area and fast inter-particle adsorption kinetics (Saha et al., 2008a). It is essential to estimate the kinetics of adsorbent– refrigerant pair for designing an adsorption system. The design codes of the adsorption chiller must be equipped with the correct isotherms, isosteric heat of adsorption and the coefficients of uptake model. Using these key data, the numerical modeling of the processes of the chiller operation can be performed with a high level of confidence (Buzanowski and Yang, 1989). Usually the well known linear driving force (LDF) correlation is used to calculate the coefficients of uptake model. In the LDF method, the key parameter is the overall mass transfer coefficient. Both the diffusion time constant and the overall mass transfer coefficient are computed by tracking the experimental vapor uptake, where the particle mass transfer coefficient could be determined (Ruthven, 1984). Another model known as Fickian diffusion model is also used to evaluate the kinetics parameters. In this model, the relative uptake is considered as a function of square root of time. In this article, the adsorption uptake rates of R134a and R507A onto Maxsorb III have been measured within the temperatures ranging from 20 to 60  C for adsorption cooling and refrigeration applications purposes. Using the constant volume variable pressure or volumetric method, the instantaneous uptake of R134a and R507A has been recorded at each second. Fickian diffusion model has been adapted to estimate kinetics parameters of R134a and R507A on Maxsorb III and they are found to be fairly consistent within acceptable uncertainties.

mvoid mads a0 A0 C C0 t Rp A V r M r NA

mass of refrigerant in the void space, kg mass of adsorbed refrigerant, kg molecular surface area of adsorbate, m2 surface area of adsorbate, m2 instantaneous adsorption uptake, kg/kg limiting adsorption uptake, kg/kg time, s adsorbent particle radius, m area of adsorbent, m2 volume of adsorbent, m3 radial coordinate, m molecular weight, kg/kmol density, g/cm3 Avogadro number

2.

Experimental section

2.1.

Materials

In this study, we have used pitch based activated carbon (type Maxsorb III) for kinetics experiments. It is well known that pitch based activated carbon is produced by a direct chemical activation route in which oxidative stabilized pitch derived from ethylene tar oil is reacted with potassium hydroxide under various activation conditions. Abundant oxygencontaining functional groups (C–OH, C–O–C, C]O, COOR, etc.) are found to exist on its surface. The adsorbent (type Maxsorb III) used in the present study is labeled as MSC-30, was supplied by Kansai Coke & Chemical Company Ltd., Japan (Batch No. 03-07061) with a measured surface area of 3150 m2/g and a micropore volume (vm) of 17.0  104 m3/kg. It has a mean particle diameter of 72 mm, an ash content of no more than 0.1%, and moisture of no more than 0.8%. The thermo-physical properties of Maxsorb III specimen are furnished in Table 1. The parameters such as limiting amount of adsorbate uptake, characteristic energy and the surface-structural heterogeneity factor for the adsorption of R134a and R507A on Maxsorb III specimen are tabulated in Table 2. These parameters are calculated by using experimentally measured adsorption isotherms data and Dubinin–Astakhov model. The skeletal density of Maxsorb III is about 2200 kg/m3 (Saha et al., 2007). The samples for both R134a and R507A are of 99.999% pure and are supplied by Daikin Asia Servicing Pte Ltd, Singapore and

Table 1 – Thermo-physical properties of Maxsorb III (Source: Saha et al., 2008a). BET surface area (m2/g) Micropore volume (ml/g) Total pore volume (ml/g) Apparent density (g/ml) Residual heat (%) pH (–) Mass reduction during preparation from carbon (%) Average particle diameter (mm) Mean pore diameter (nm)

3140 1.70 2.01 0.31 0.1 4.1 0.8 72 2.008

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international journal of refrigeration 33 (2010) 706–713

Table 2 – Adsorption parameters of R134a and R507A on Maxsorb III. Refrigerants

Maximum volumetric uptake, W0 (m3/kg)

Characteristic energy, E (J/mol)

Heterogeneity constant, n (–)

Source

1.649  103 1.175  103

8460 5740

1.3 1.47

Saha et al. (2009b) Saha et al. (2008b)

R134a R507A

GALCO s.a., Germany, respectively. All properties of R134a were evaluated using the generalized equation of state proposed by Tillner-Roth and Baehr (1994). Fig. 1 shows the SEM picture of Maxsorb III at 3700 magnification.

2.2.

Experimental apparatus

Fig. 2 shows the schematic diagram of the constant volume variable pressure (CVVP) experimental test rig. In the CVVP setup, the charging tank and the adsorption tank are connected through a capillary tube. The Maxsorb III specimen with a mass of 19 g was kept wrapping with the copper coil and put inside the adsorption cell. The copper coil was used to ensure rapid cooling inside the adsorption tank. The dry mass of Maxsorb III was determined by the calibrated moisture balance of type Satorious MA40 moisture analyzer with an uncertainty of 0.05%. Three identical filters having pore diameter of 5 mm were used to stop any migration of activated carbon particles during evacuation and desorption and these filters were fitted at three different exits of the adsorption tank.

2.3.

Instrumentation

The experimental setup comprises (i) a charging tank with a volume of 2570  10 cc, (ii) an adsorption tank with a volume of 2427  10 cc, (iii) two constant temperature water baths of model HAAKE-F8-C35 on which both the charging and adsorption tanks are immersed to control the temperatures of both charging and adsorption tanks, (iv) a KYOWA PGA-10KA pressure transducer with an uncertainty of 0.1% of full scale and a pressure ranging from 0 to 1 MPa, (v) Class-A type Pt 100 U resistance temperature detector (diameter ¼ 1/16 inch) with an

Fig. 1 – SEM picture of activated carbon specimen (3700 magnification).

uncertainty of 0.1 K for temperature measurement, (vi) four thermisters (Model: Omega 4407) with a resistance of 5000 U at 25  C, with an uncertainty of 0.2  C and maximum operating temperature of 150  C connected with the activated carbon for direct temperature measurement, (vii) a BOC Edwards direct drive vane vacuum pump that achieves vacuum level of 0.05 mbar, and (viii) Agilent data logger to record the data. The volume of both charging and adsorption tanks are inclusive of the volumes of related piping and valves.

2.4.

Procedure

Prior to each experiment, the entire test rig is evacuated for 24 h using the vacuum pump to a vacuum level of 0.05 mbar. During the evacuation, the adsorbent sample is regenerated in situ 120– 130  C for 12 h to desorb any residual gases. At the end of regeneration process, the test system is purged with helium gas with a purity of 99.9995% and evacuated further to achieve low vacuum conditions. The evacuation and helium purging are continued several times to ensure that there is no residual gas left in the system. Based on the measurements, there is no measurable interaction between the inert gas and the adsorbent. After evacuation, the charging cell is pressurized with the assorted refrigerant and left until it reaches an equilibrium state (with the ball valve closed). During charging, it is necessary to keep the charging pressure lower than the saturation pressure of the refrigerant to ensure no condensation is occurred. At this state the initial pressure and temperature in the charging cell are measured before adsorption. Once equilibrium is achieved, the needle valve between the charging and adsorption tank is opened. The pressure and temperature in the adsorption tank are recorded. With the known initial mass of dry Maxsorb III, the temperature of the test system is varied to calculate the uptake of the assorted refrigerant varying with time. The kinetic uptake data of R134a and R507A are furnished in Tables 3–8. The temperature profiles of Maxsorb III are shown in Figs. 3 and 4, respectively when refrigerant (R134a and R507A) is charged to the adsorption tank from the charging tank. It is observed from Figs. 3 and 4 that the temperature rises inside the adsorption tank are not significant at the beginning of adsorption and temperature is maintained constant throughout the experiments. The pressures as a function of time at the temperature of 40  C for both R134a and R507A are depicted in Figs. 5 and 6, respectively. Prior to the experiments, the initial pressure of the adsorption tank is kept as vacuum pressure (z0.1 Pa). It can be seen from Figs. 5 and 6 that after charging the refrigerant (R134a/R507A) to the adsorption tank, the pressure of the adsorption tank starts increasing (t < 50 s) and reaches equilibrium. At that time, the amount of adsorbate uptake also increases. This indicates a natural phenomenon. It is also found from Figs. 5 and 6 that,

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international journal of refrigeration 33 (2010) 706–713

13

3

8

2

12

2 3

1 14

5 4 6

9

11

7

1 1 1

10

1: Thermistor; 2:Pressure transducer; 3: Filter; 4: Copper coil; 5:Activated carbon; 6: Adsorption tank; 7: Charging tank; 8: Needle valve; 9: Helium gas cylinder; 10: Water circulator; 11: Refrigerant cylinder; 12: Pressure regulator; 13: Ball valve and 14: To vacuum pump

Fig. 2 – Schematic diagram of the constant volume variable pressure (CVVP) experimental test rig.

the pressure increases abruptly at the beginning period and decreases slightly (t ¼ 500 s) due to adsorption of R134a or R507A on the porous structures of Maxsorb III.

2.5.

Data reduction

At initial stage of the experiment, adsorbate (R134a or R507A vapor) is introduced into the charging cell of a known volume. In the absence of the adsorbent, the initial mass of refrigerant  from the following  is calculated equation, (1) miref ¼ riref Pich cell ; Tich cell Vch cell where riref is the initial refrigerant density at charging temperature, Tich cell and Vch_cell is the charging cell volume. When the adsorption and charging cells are connected, adsorption occurs in the pores of the adsorbent and the void volume in the adsorption cell is given by;

Vvoid ¼ Vads

cell



mac  vm mac rs

(2)

where Vads_cell is the adsorption cell volume, mac is the mass of activated carbon of type Maxsorb III in the adsorption cell, rs is the solid density of activated carbon, and vm is the micro pores volume of activated carbon. The analysis presented here assumes that the total micro pores volume of adsorbent is constant. The cell of 2427cm3 was packed with Maxsorb III of 19 g and the void volume of adsorption cell is equal to 2387 cm3. This volume is inclusive of the vapor space of the adsorption cell and the adsorbate mass is calculated using the generalized equation of state proposed by Tillner-Roth and Baehr (1994) at respective adsorption temperatures, and pressures, i.e.; mvoid ¼ rref ðP; Tads ÞVvoid

(3)

Table 3 – Experimental uptake of R134a on Maxsorb III at 25 8C.

Table 4 – Experimental uptake of R134a on Maxsorb III at 40 8C.

t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg)

t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

0 20 40 60 80 100 120 140 160 180 200 220 240 260

0 0.71 0.86 0.94 1.00 1.06 1.10 1.15 1.18 1.21 1.24 1.26 1.28 1.30

280 300 320 340 360 380 400 420 440 460 480 500 520 540

1.32 1.34 1.35 1.36 1.38 1.39 1.40 1.41 1.42 1.42 1.43 1.44 1.45 1.45

560 580 600 620 640 660 680 700 720 740 760 780 800 820

1.46 1.47 1.47 1.48 1.48 1.49 1.49 1.50 1.50 1.51 1.51 1.51 1.52 1.52

840 860 880 900 920 940 960 980 1000

1.53 1.53 1.53 1.54 1.54 1.54 1.54 1.55 1.55

0 0.65 0.78 0.85 0.91 0.96 1.00 1.03 1.06 1.09 1.11 1.13 1.15 1.16

280 300 320 340 360 380 400 420 440 460 480 500 520 540

1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.25 1.26 1.27 1.27 1.28 1.28

560 580 600 620 640 660 680 700 720 740 760 780 800 820

1.29 1.29 1.30 1.30 1.31 1.31 1.31 1.32 1.32 1.32 1.33 1.33 1.33 1.33

840 860 880 900 920 940 960 980 1000

1.34 1.34 1.34 1.34 1.35 1.35 1.35 1.35 1.35

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Table 5 – Experimental uptake of R134a on Maxsorb III at 60 8C.

Table 7 – Experimental uptake of R507A on Maxsorb III at 30 8C.

t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg)

t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

0 20 40 60 80 100 120 140 160 180 200 220 240 260

0 0.55 0.65 0.70 0.74 0.78 0.81 0.83 0.85 0.87 0.89 0.91 0.93 0.93

280 300 320 340 360 380 400 420 440 460 480 500 520 540

0.95 0.95 0.96 0.97 0.98 0.98 0.98 0.99 0.99 0.99 1.00 1.00 1.02 1.03

560 580 600 620 640 660 680 700 720 740 760 780 800 820

1.03 1.04 1.04 1.04 1.04 1.04 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05

840 860 880 900 920 940 960 980 1000

1.06 1.06 1.06 1.06 1.06 1.06 1.06 1.06 1.06

where rref is the density of refrigerant in the adsorption cell. The remaining amount of refrigerant present in the charging cell is calculated from f

f

mref ¼ rref ðP; Tch

(4)

cell ÞVch cell

0 0.63 0.75 0.81 0.86 0.90 0.93 0.96 0.99 1.01 1.03 1.04 1.06 1.07

280 300 320 340 360 380 400 420 440 460 480 500 520 540

1.08 1.09 1.10 1.11 1.12 1.13 1.13 1.14 1.14 1.15 1.15 1.16 1.16 1.17

560 580 600 620 640 660 680 700 720 740 760 780 800 820

1.17 1.17 1.18 1.18 1.18 1.19 1.19 1.19 1.19 1.20 1.20 1.20 1.20 1.21

840 860 880 900 920 940 960 980 1000

1.21 1.21 1.21 1.21 1.21 1.22 1.22 1.22 1.22

due to the mathematical regression. It is expected that the overall uncertainty will be within 1%.

3.

Mathematical modeling

3.1.

Semi-infinite model

f rref

where, is the refrigerant density at respective temperature. Therefore, the amount of adsorbed mass can be calculated from, f

mads ¼ miref  mvoid  mref

(5)

Finally the specific uptake value or the loading, is determined as x¼

mads mac

(6)

Ruthven (1984) reported that during the initial phase of adsorption, surface diffusion coefficient follows the uptake behavior of a semi-infinite medium of any particle shape and can be expressed as,  1=2 C 2A Ds t z (7) C0 V p

There are some uncertainties associated with instrumentation, average adsorption cell temperature during adsorption and the void correction. Moreover, certain errors introduced

where C denotes instantaneous adsorption uptake, C0 stands for limiting adsorption uptake, A is the area of the adsorbent particle, V represents particle volume and Ds stands for surface diffusion. Maxsorb III samples have been assumed to be spherical. A plot of the relative uptake and square root of pffiffiffiffiffiffiffiffiffiffiffiffi time for each isotherm yields with a slope of ð2A=VÞ Ds =p. Surface diffusion, Ds can be expressed as,

Table 6 – Experimental uptake of R507A on Maxsorb III at 20 8C.

Table 8 – Experimental uptake of R507A on Maxsorb III at 60 8C.

t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg)

t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg) t (s) C (kg/kg)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

0 20 40 60 80 100 120 140 160 180 200 220 240 260

2.6.

Assessment of overall uncertainty

0 0.63 0.77 0.78 0.89 0.93 0.97 1.00 1.04 1.06 1.08 1.10 1.12 1.13

280 300 320 340 360 380 400 420 440 460 480 500 520 540

1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.21 1.22 1.22 1.23 1.24 1.24

560 580 600 620 640 660 680 700 720 740 760 780 800 820

1.24 1.25 1.25 1.26 1.26 1.26 1.27 1.27 1.27 1.28 1.28 1.28 1.28 1.29

840 860 880 900 920 940 960 980 1000

1.29 1.29 1.29 1.29 1.30 1.30 1.30 1.30 1.30

0 0.43 0.59 0.63 0.67 0.69 0.72 0.74 0.76 0.77 0.79 0.80 0.81 0.81

280 300 320 340 360 380 400 420 440 460 480 500 520 540

0.82 0.83 0.83 0.84 0.85 0.85 0.85 0.86 0.86 0.86 0.87 0.87 0.87 0.87

560 580 600 620 640 660 680 700 720 740 760 780 800 820

0.88 0.88 0.88 0.88 0.88 0.88 0.89 0.89 0.89 0.89 0.89 0.90 0.90 0.90

840 860 880 900 920 940 960 980 1000

0.90 0.90 0.90 0.90 0.91 0.91 0.91 0.91 0.91

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60

300 57°C

250 40°C

50

P(kPa)

Temperature (°C)

55

45 40

40°C

35

35°C

200 150 100

30

50 25°C

25

0 0

20 0

50

100

150

200

500

1000

1500

2000

2500

t(sec)

250

Time (sec)

Fig. 5 – Pressure profile of R134a experiments.

Fig. 3 – Temperature profile of Maxsorb III of R134a experiments.

Ds ¼ Dso

  Ea exp  RT

(8)

! (11)

where n is an integer and its value varies from 1 to infinity, Rp is the radius of the particles and C0 stands for limiting uptake and can be estimated from the following equation.

Eq. (8) can be arranged as, lnðDs Þ ¼ 

N C 6 X 1 n2 p2 Ds t ¼1 2 exp  C0 p n¼1 n2 R2p

Ea 1 þ lnðDso Þ R T

(9)

By plotting ln(Ds) against 1/T, one can get the numerical values of activation energy, Ea and the pre-exponential coefficient, Dso. This plot is popularly known as Arrhenius plot. There exists a linear relationship between ln(Ds) and 1/T. The slope yields – Ea/R and the intercept provide the preexponential constant, Dso from Eq. (9). The Fickian diffusion model is used to evaluate adsorption kinetics of various types of adsorbate pairs. The Fickian model is valid when the diffusivity is independent of adsorbate concentration and when the system is thermodynamically ideal (Ruthven, 1984). The diffusion equation is used to express adsorption rate in spherical shaped adsorbent,   vC 1 v 2 vC ¼ 2 r Ds vt r vr vr

(10)

Considering a constant diffusivity and applying the appropriate initial and boundary conditions, Eq. (10) can be written as (Crank, 1975),

C0 ¼ ðBET surface area of the adsorbentÞ=A0 Here A0 is the adsorbate surface area and can be defined as A0 ¼ NA a0

(12)

where NA stands for Avogadro constant, and a0 is the molecular surface area of adsorbate and is calculated by (Chakraborty et al., 2008), 2=3  M a0 ¼ 1:091 rlb NA

(13)

where, M is the molecular weight of adsorbate, rlb is the density of liquid phase of adsorbate at boiling point temperature in g/cm3. Putting the values of A0, C0 and a0 into Eq. (11), we can obtain, ! N 1:091CNA ðM=rlb NA Þ2=3 6 X 1 n2 p2 Ds t ¼1 2 exp  Z p n¼1 n2 R2p

(14)

65 450 400

54°C

40°C

350

45

P(kPa)

Temperature (°C)

60°C

55

38°C

35

30°C

300 250 200 150

25 20°C

100 50

15 0

50

100

150

200

Time (sec) Fig. 4 – Temperature profile of Maxsorb III of R507A experiments.

250

0 0

200

400

600

800

1000

1200

1400

t(sec) Fig. 6 – Pressure profile of R507A experiments.

1600

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international journal of refrigeration 33 (2010) 706–713

1.8

1.4 20˚C

1.6

25˚C 32˚C

1.4

40˚C

1.2

1

52˚C

C(kg/kg)

C(kg/kg)

30˚C

1.2

60˚C

1 0.8 0.6

54˚C 60˚C

0.8 0.6 0.4

0.4 0.2

0.2 0

0 0

200

400

600

800

1000

1200

1400

0

1600

200

400

600

t(sec)

Relative uptake, C/C0 (-)

1.20 1.00 0.80

60C-Fickian model 40C-Fickian model

0.60

25C-Fickian model

0.40

40C-Experiemntal 60C-Experimental

0.20

25c-Experimental

0.00 10

20

30

0.5

40

50

0.5

t (sec ) Fig. 8 – Plot of relative uptake vs square root of time of R134a experiments.

Results and discussion

4.1.

Maxsorb III/R134a pair

1200

can adsorb R134a as high as 1.6 kg/kg within an adsorption time interval of 1200 s at an adsorption temperature of 25  C. When the adsorption temperature is increased to 60  C, equilibrium uptake falls to 1.0 kg/kg. However, only 600 s is required to reach the equilibrium condition. Fig. 8 depicts plots of relative uptake vs square root of time for Maxsorb III/R134a pair. In Fig. 8, the solid lines represent the fitting of Eq. (11) where the diffusion time constant is calculated assuming the semi-infinite model. It can be noticed from Fig. 8 that an accurate estimation of relative uptake can be evaluated using the Fickian diffusion model which indicates the goodness of fit. A linear relationship between ln(Ds) and 1/T for Maxsorb III/ R134a pair is shown in Fig. 9. One can notice from Fig. 9 that the diffusivity of Maxsorb III increases with the adsorption temperature and thus agrees well with the Arrhenius trend.

4.2.

4.

1000

Fig. 10 – Kinetic uptakes of R507A on Maxsorb III at different temperature.

Fig. 7 – Kinetic uptakes of R134a onto Maxsorb III at different adsorption temperature.

0

800

t(sec)

Fig. 7 shows the variation of uptake mass against time for R134a onto Maxsorb III at five different temperatures of 25, 32, 40, 52 and 60  C. It can be observed from Fig. 7 that Maxsorb III

Maxsorb III/R507A pair

The kinetics of R507A on Maxsorb III at four different temperatures is shown in Fig. 10. It can be observed from Fig. 10 that when the adsorption temperature is 20  C, the Maxsorb III can adsorb R507A as high as 1.3 kg/kg within an adsorption time interval of 1100 s. However, when the 1.20

-28.2

Relative uptake, C/C0 (-)

-28.1 y = -1110.4x - 24.965

lnDs (m2/sec)

-28.3 -28.4 -28.5 -28.6 -28.7 -28.8 -28.9 0.0029

1.00 0.80 60C-Fickian model

0.60

40C-Fickian model 20C-Fickian model 60C-experimental

0.40

40C-experimental 20C-experimental

0.20 0.00 0

0.003

0.0031

0.0032

0.0033

0.0034

1/T (K-1)

Fig. 9 – Plot of ln(Ds) vs 1/T for R134a experiments.

5

10

15 0.5

20

25

30

35

0.5

t (sec ) Fig. 11 – Plot of relative uptake of R507A vs square root of time.

international journal of refrigeration 33 (2010) 706–713

-28.1 -28.2 y = -1420x - 24.1

lnDs (m2/sec)

-28.3 -28.4 -28.5 -28.6 -28.7 -28.8 -28.9 -29 -29.1 -29.2 0.0029

0.003

0.0031

0.0032

0.0033

0.0034

0.0035

1/T (K-1) Fig. 12 – Plot of ln(Ds) vs 1/T for R507A experiments.

adsorption temperature is 60  C, only 800 s is required to achieve equilibrium uptake of 0.91 kg/kg. Fig. 11 shows plots of relative uptake vs square root of time for Maxsorb III/R507A pair. In Fig. 11, solid lines represent the fitting of Eq. (11) where the diffusion time constant is calculated assuming the semi-infinite model. It is also noticeable from Fig. 11 that an accurate estimation of relative uptake can be evaluated using the Fickian diffusion model which indicates the goodness of fit. A linear relationship between ln(Ds) and 1/T for Maxsorb III/ R507A pair is presented in Fig. 12 and the trend follows the classical Arrhenius trend.

5.

Conclusions

Adsorption rates of R134a and R507A onto pitch based activated carbon of type Maxsorb III have been measured by CVVP method with temperatures varying from 20 to 60  C. The data obtained from the experiments are analyzed using the Fickian diffusion model and are found to be fairly matched. The surface diffusions for both pairs are also evaluated and they follow the classical Arrhenius trend. The kinetics data of R134a and R507A are essential and useful in designing adsorption cooling and refrigeration systems.

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