Engineering Sciences Reports, Kyushu University
Vol.31,No.2,pp.5-9, SEP.2009

九 州 大 学 大 学 院 総 合 理 工 学 報 告第 31 巻 第 2 号 5-9 頁 平成 21 年 9 月

Experimental Study of Adsorption Rate of R507A on
Activated Carbon
Khairul HABIB*1,† Bidyut Baran SAHA*2
Ibrahim Ibrahim El-SHARKAWY*2 and Shigeru KOYAMA*2
E-mail of corresponding author: habib@phase.cm.kyushu-u.ac.jp†(Received July 31, 2009)This article deals with the experimental study of adsorption rate of R507A on highly porous activatedcarbon of type Maxsorb III by constant volume variable pressure (CVVP) method over a temperature ranging
from 20 to 60ºC. These data are useful for the operation of adsorption chillers and these data are unavailable
in the literature. Data obtained from the kinetic studies were treated according to the various kinetic models
with the Fickian diffusion model being the most suitable overall. Guided by the experimental measurements,
the surface diffusion is also estimated and is found that it follows the classical Arrhenius law within theexperimental range.

Key words: Adsorption rate, Maxsorb III, R507A, CVVP

1.

Introduction

Highly porous activated carbons have many
intrinsic characteristics that are superior ascompared with the palletized or granular
activated carbon fiber, for example, the large
surface area, fast inter-particle adsorption
kinetics. It could be used in applications suchas gas separation, gas storage and catalytic
reactions1-3). The uptake behaviour of highly
porous activated carbon/R507A pair seems to
be a promising pair for low temperature
cooling applications.
The refrigerant R507A is an azeotropic blend
consisting of 50% R125 (pentafluoroethane)and 50% R143a (1,1,1-trifluoroethane) byweight. The refrigerants R125 and R143a arehydrofluorocarbons and they do notcontribute to the stratospheric ozone layer. It
has relatively smaller GWP compared with
the CFCs and the ODP is zero. It is also
non-flammable and the level of acidity issignificantly small (below 1.0 ppm).
In order to develop adsorption based systems,
it is inevitable to determine adsorption
isotherms,
kinetics
oftheassorted
adsorbent-refrigerant pair. It is essential toestimate the kinetics of adsorbent-refrigerantpair accurately for design purposes andimproving understanding of chillers. Design*1*2

Department of Energy and Environmental Engineering,
Graduate student
Department of Energy and Environmental Engineering

codes of the chiller must be equipped with thecorrect isotherms, isosteric heat of adsorption
and the coefficients of uptake model. With
these key data available, only then thenumerical modeling of the processes of thechiller operation can be estimated accurately
with a high level of confidence4).
In this article, adsorption kinetics of R507A
on Maxsorb III specimen has beenexperimentally evaluated by constant volume
variable pressure (CVVP) method when thetemperatures are varying from 20 to 60ºC. To
analyze the experimental data, the Fickian
diffusion model is used and found to be fairly
consistentwithin
theacceptable
experimental uncertainties.

2. Experimental
2.1 Materials
The highly porous activated carbon (MaxsorbIII specimen) labeled as MSC-30 has been
supplied by the Kansai Coke and ChemicalCo.
Ltd.,
Japan.
IthasaBrunauer−Emmet−Teller (BET) surface areaof 3,140 m2/g and micropore volume of 1.7 cc/g.
The mean particle diameter of Maxsorb-III
specimen is 72 µm, ash content is less than0.1%, moisture of no more than 0.8% and pHvalue is 4.1.

6Experimental Study of Adsorption Rate of R507A on Activated Carbon

2.2 Experimental setup
Figures 1 and 2 show the photograph andschematic diagram of the experimental test
rig, respectively.The experimental setup
comprises (i) a charging tank made ofstainless steel (SS 304) with a volume of 2570
cc whose volume was inclusive of related
piping and valves, (ii) an adsorption tankmade of stainless steel (SS 304) with avolume of 2427 cc, whose volume was again
inclusive of related piping and valves , (iii)
two constant temperature baths of model
HAAKE-F8-C35 and Polyscience on which theboth the charging and adsorption tanks are
immersed to control the temperatures ofboth charging and adsorption g tanks, (iv) aKYOWA PGA-10KA
pressure transducer
with anuncertainty of 0.1% of full scale
and a pressure range of 0-1 MPa, (v) Class-A
type Pt 100 Ω resistance temperaturedetector (diameter = 1/16 inch) with anuncertainty of±0.1 K for temperature
measurement, (vi) four thermistors (Model:Omega 4407) with a resistance of 5000 Ωat 25˚C with an uncertainty of ±0.2˚C andmaximum operating temperature of 150˚C
connected with the activated carbon for direct
temperature measurement , (vii) a vacuum
pump a BOC Edwards direct drive vane
vacuum pump to a vacuum level of 0.05 mbar,and (vii) Agilent data logger to record thedata. Three filters (diameter = 5 µm) that arecapable of stopping migration of activatedcarbon particles during evacuation anddesorption were fitted at three different exitsof the adsorption tank.
2.3 Experimental procedure
Prior to experiment, the entire test rig isevacuated for 24 hours using a BOC Edwards
direct drive vane vacuum pump to a vacuum
level of 0.05 mbar. During the evacuation, thetest system is regenerated in situ 120-130 ˚C
for 12 h to desorb any residue gas in thesystem. At the end of regeneration process,the test system is purged with helium gas
with a purity of 99.9995% and evacuatedfurther to achieve low vacuum conditions.
The evacuation and helium purging iscontinued 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 theadsorbent, as the effect if the partial pressureof helium in the tanks is found to be small.

After evacuation, the charging cell ispressurized with the assorted refrigerant andleft to reach an equilibrium state (with theball valve closed). During charging, it isnecessary that the charging pressure shouldbe less than the saturation pressure of therefrigerant at that temperature to ensure nocondensation is occurred. At this state theinitial pressure and temperature in thecharging cell are measured before adsorption.
Once equilibrium is achieved, the ball valve
between the charging and adsorption tank isopened. The pressure and temperature in theadsorption tank are recorded. With theknown initial mass of dry Maxsorb III, thetemperature of the test system is varied to
calculate uptake of the assorted refrigerant
varying with time.

Fig. 1 Photograph of the experimental test rig.

2.4 Data reduction
At initial stage of the experiment, R507A isintroduced into the charging cell of a known
volume. In the absence of the adsorbent, theinitial mass of refrigerant is calculated fromthe following equation,()iimref= ρ ref
Pchi _ cell , Tchi _ cell Vch _ cell

(1)

When the adsorption and charging cells areconnected, adsorption occurs in the pores of theadsorbent and the void volume in the adsorption
cell given by;

Vvoid = Vads _ cell −macρs− vµ mac(2)

平成 21 年

九 州 大 学 大 学 院 総 合 理 工 学 報 告The adsorbate mass in the adsorption cell canbe calculated from,mvoid = ρ ref (P , Tads )Vvoid(3)

The remaining amount of refrigerant present
in the charging cell is calculated from,mreff = ρreff (P, Tch _ cell )Vch _ cell

(4)

Therefore, the amount of adsorbed mass canbe calculated from,imads = mref
− mvoid − mreff(5)

Finally the specific uptake value or theloading, is determined as,

x=madsmac(6)

3. Mathmatical modeling
3.1 Semi-infinite model
I.I. El-Sharkawy et al. 5) reported that duringthe initial phase of adsorption surface
diffusion coefficient follows the uptake
behavior of semi-infinite medium of any
particle shape and is expressed by,1C2 A ⎛ Ds t ⎞ 2=⎜⎟C0V ⎝ π ⎠(7)

where 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 rootof time for each isotherm yields with a slopeof2AVDsπ7

Ea 1+ ln(Dso )R T(9)

By plotting ln(Ds) against 1/T, one can get thenumerical values of activation energy, Ea andthe pre-exponential coefficient, Dso. This plot
is popularly known as Arrhenius plot. Thereexists a linear relationship between ln(Ds)and 1/T. The slope yields –Ea/R and theintercept
providethepre-exponential
constant, Dso from eq. (9).
The Fickian diffusion model is used toevaluate adsorption kinetics of various typesof adsorbate pairs. The Fickian model is valid
when the diffusivity is independent of sorbate
concentrationwhenthesystemisthermodynamically ideal5). The diffusion
equation is used to express adsorption rate inspherical shaped adsorbent,1 ∂ ⎛ 2 ∂C ⎞∂C=⎜ r Ds⎟∂r ⎠∂t r 2 ∂r ⎝( 10)

C is the adsorbate concentration, Ds is thediffusion coefficient; r is the radial coordinate.
Considering a constant diffusivity andapplying the appropriate initial and boundary
conditions, the eq. (10) can be written as5),∞⎛ n 2π 2 D t ⎞C61s ⎟⎜−exp= 1− 22⎟⎜C0π n=1 n 2Rp⎠
⎝

∑(11)

where n is an integer from 1 to infinity, Rp isthe radius of the particles and C0 stands forlimiting uptake and can be estimated fromthe following equation.

C0 = (BET surface area of Maxsorb III)/A0
where A0 is the adsorbate surface area and is
calculated by6),
A0 = NAa0
Where, a0adsorbate6),(12)
is molecular surface area of⎛ M ⎞3⎟
a0 = 1.091⎜⎜⎟⎝ ρ lb N A ⎠Surface diffusion, Ds can be expressed as,Eq. (8) can be re-arranged as,ln(Ds ) = −第2号2.

⎛ E ⎞Ds = Dso exp⎜ − a ⎟⎝ RT ⎠第 31 巻(8)(13)where, M is the molecular weight ofadsorbate, ρlb is the density of liquid phase ofadsorbate at boiling point temperature ing/cm3.

8Experimental Study of Adsorption Rate of R507A on Activated Carbon

8

122

13

2

3
3

1

11
7

14

10

54
6

9

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 experimental test rig.

4. Results and Discussion

Fig. 4 depicts plots of relative uptake vssquare root of time for Maxsorb III/R507A
pair. It is observable from Fig. 4 that solidlines represent the fitting of eq.(11) where thediffusion time constant is calculated
assuming semi-infinite model. It is also
noticeable from Fig. 4 that an accurate
estimation of relative uptake can beevaluated using the Fickian diffusion model
which indicates the goodness of fit.

A linear relationship between ln (Ds) and 1/T
is observed in Fig. 5. It is also visible from Fig.
5 that the diffusivity of Maxsorb III increases
with the adsorption temperature which
ultimately agrees well with the Arrhenius
trend.
1.4
20˚C

1.2
30˚C

1Uptake (kg/kg)

Fig. 3 shows the kinetic uptake of R507A onMaxsorb III at four different temperatures. Itis observable from Fig. 3 that when theadsorption temperature is 20ºC, the Maxsorb
III can adsorb R507A as high as 1.2 kg/kgwithin an adsorption time interval of 1100s.
However, when the adsorption temperature is60ºC, only 800 s is required to achieve
equilibrium uptake of 0.76 kg/kg.

54˚C

0.860˚C

0.6
0.4

0.2
00200

400

600

800

1000Time, t(sec)Fig. 3 Kinetic uptake of R507A on activatedcarbon.

1200

平成 21 年第2号

9CC0AVadsorption uptakekg/kg

limiting adsorption uptakekg/kg

area of particle

m2

volume of particle

m3im refinitial refrigerant mass before kg

九 州 大 学 大 学 院 総 合 理 工 学 報 告第 31 巻

Nomenclature
1.20

Relative uptake, C/C0 (-)

60˚C20˚C

1.0040˚C

0.80
Fickian model

0.60
Fickian model

adsorption

Fickian model

0.40

ρ

experimental
experimental

initial density of refrigerant kg/m3

mass of activated carbon

kgsolid carbon density

kg/m3

micropore volume

m3

surface diffusion

m2/sec-28.5

Vch_cell
Vvoid
mvoid
Vads_cellmac
ρsVµ
Dsttimesec-28.6

rradial coordinate

m-28.7Eaactivation energy

J/mol-28.8

Dso

pre-exponential coefficient

-28.9

M

molecular weight-29a0surface area of adsorbate

0.20
experimental

0.00
05

101520

0.5

0.5

2530

35t (sec )Fig. 4 Relative uptake vs square root of time.

-28.3
-28.4
y = -1420x - 24.1
lnDs (m2/sec)

i
ref

-29.1
0.00290.003

0.00310.00320.00330.00340.0035charging cell volume

m3

void volume

m3

adsorbate mass in void spacem3

adsorption cell volume

m3

kg/kmolof 1 moleculem2

1/T (1/K)

A

surface area of adsorbate

m2

Fig. 5 Plot of lnDs vs 1/T.

NA

Avogadro’s number-4. Conclusions
Adsorption kinetics of Maxsorb III-R507A
pair has been evaluated by CVVP method
over a wide range of temperatures of 20 to60˚C. The Fickian diffusion model is used toanalyze the experimental data and are foundto be fairly consistent within the acceptable
uncertainties. The present kinetics data ofR507A are useful in designing adsorption
chiller.

References
1) S. Sircar et al., Carbon, 34 (1) (1996) 1.
2) K. R. Matranga et al., Chem. Engg. Sci.,
47(7) (1992) 1569.
3) D. Mehandjiev et al., J. Coll. and Inter. Sci.,
192(2) (1997) 440.
4) B.B. Saha et al., Int. J. Heat and Mass
Trans., 51 (2008) 1582.
5) I.I. El-Sharkawy et al., J. Chem. Eng. Data,
53 (2008) 41.
6) A. Chakraborty et al., Appl. Phys. Lett., 92(2008) 201911.