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Adsorption of carbon dioxide by sodium hydroxide-modied granular

coconut shell activated carbon in a  xed bed

Y.L. Tan  a , Md. Azharul Islam  a , b, M. Asif  c , B.H. Hameed  a ,  *

a School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysiab Foretsry and Wood Technology Discipline, Khulna University, Khulna 9208, Bangladeshc Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia

a r t i c l e i n f o

 Article history:

Received 2 May 2014

Received in revised form

19 August 2014

Accepted 29 September 2014

Available online 29 October 2014

Keywords:

Fixed-bed

Adsorption

CO2Breakthrough

NaOH-modied activated carbon

a b s t r a c t

In the present work, commercial coconut shell activated carbon was impregnated with alkaline NaOH to

investigate the ef ciency of modied activated carbon for CO2   adsorption in a   xed-bed column

adsorption system. The modication parameters, such as the NaOH concentration (24e48%) and

dwelling time (1e4 h), were also investigated. The results showed that a 32% NaOH concentration with a

3 h dwelling time provided the best CO2  adsorption capacity. Later, the modied activated carbon was

characterized by nitrogen adsorptionedesorption, scanning electron microscopy and Fourier transform

infrared spectroscopy. The effects of the CO2 % in the feed, the adsorption temperature, the feed  ow rate

and the amount of adsorbent in the column were investigated in the adsorption experiments. The

maximum CO2  adsorption capacity in this study was 27.10 mg/g at 35  C. This study also suggests that

NaOH-modied activated carbon is a state-of-the-art adsorbent for CO2  adsorption.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

GHGs (global warming and greenhouse gases) are highly dis-

cussed topics around the world. Of all the GHGs, CO2   has been

regarded as a major contributor to the present global warming

trend and is mainly emitted from anthropogenic sources. Thus,

global awareness about CO2 emissions has increased during the last

decades, leading to increasing efforts to reduce their environmental

impact, including preventive and remediation methods. To combat

CO2, innovative CO2   capturing technologies are needed. Several

techniques have been proposed to capture CO2, including chemical

absorption, physical adsorption and membrane separation   [1e5].

Of these techniques, chemical absorption with an aqueous solution

of alkanolamines, such as monoethanolamine and diethanolamine,is the most popular   [6e8]. However, there are some problems

associated with this technique, such as high corrosion, oxidative

degradation of absorbents, and foaming in the gaseliquid interface

[9]. Adsorption is a widely used technology for gas treatment due to

its versatility and ef ciency. For this method, solid porous adsor-

bents, such as activated carbon, can act as suitable alternatives for

capturing CO2   because activated carbons are insensitive to

moisture, have large surface areas and have distinct porosities, ul-

timately leading to high adsorption capacities. Moreover, the

regeneration of ACs (activated carbons) is possible at a low cost.

The surface chemistry of the activated carbon can be modied

by chemical activation to increase the adsorption capacity.

Recently, several excellent research initiatives have been taken to

removal of CO2   by modied activated carbon as an adsorbent.

Caglayan and Aksoylu   [10]   observed the substantial adsorption

capacity of CO2  by commercial activated carbon through modied

with HNO3  oxidation, air oxidation, alkali impregnation and heat

treatment under helium gas atmosphere. In another study, by Yin

and other coauthors  [11]  reported that the effect of N2  has little

inuence of the CO2 adsorption on activated carbon. Three types of 

sterically hindered amines were successfully impregnated onto thesurface of palm shell-based activated carbon and found higher

adsorption capacity than virgin activated carbon   [12]. Activated

carbonwas modied by washing with acid mixture of concentrated

HNO3 and H2SO4, and it was found that the adsorption capacities of 

the modied samples were increasedupto 70% [13]. Inonestudyby

Shafeeyan et al.  [14]  reported that the commercial GAC (granular

activated carbon) became an ef cient adsorbent for CO2   when

modied by using thermal treatment with ammonia. But activated

carbon modied with NaOH for CO2   adsorption is scarce in the

literature. Because CO2 is an acidic gas, treatment with an alkaline

compound can provide more active functional sites for CO2.*  Corresponding author. Tel.:  þ60 45996422; fax:  þ60 45941013.

E-mail address:  chbassim@usm.my (B.H. Hameed).

Contents lists available at  ScienceDirect

Energy

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m/ l o c a t e / e n e r g y

http://dx.doi.org/10.1016/j.energy.2014.09.079

0360-5442/©

 2014 Elsevier Ltd. All rights reserved.

Energy 77 (2014) 926e931

mailto:chbassim@usm.myhttp://www.sciencedirect.com/science/journal/03605442http://www.elsevier.com/locate/energyhttp://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://www.elsevier.com/locate/energyhttp://www.sciencedirect.com/science/journal/03605442http://crossmark.crossref.org/dialog/?doi=10.1016/j.energy.2014.09.079&domain=pdfmailto:chbassim@usm.my

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Activation with sodium hydroxide (NaOH) is more effective in the

preparation of activated porous carbon because NaOH is inexpen-

sive, minimally corrosive and environmentally friendly [15,16].

In the present work, granular coconut shell activated carbonwas

modied with NaOH for CO2  adsorption. The effects of different

NaOH concentrations, contact times, gas  ow rates, temperatures,

and CO2   concentrations on the adsorption process were also

studied in a  xed-bed adsorption mode.

2. Materials and methods

 2.1. Materials

Coconut shell-based activated carbon was purchased from Laju

Carbon Products Sdn. Bhd., Malaysia. Granular activated carbons

with particle sizes ranging from 0.85 to 0.425 mmwere used in this

study. Sodium hydroxide (NaOH) was purchased from Merck

Chemical Company, and puried carbon dioxide (99.98%) and ni-

trogen (99.995%) as carrier gases were supplied by Wellgas Sdn.

Bhd., Malaysia.

 2.2. Preparation of the NaOH e AC adsorbent 

Commercial coconut shell-based activated carbon was modied

by sodium hydroxide (NaOH) impregnation. For this purpose, 10 g

of AC was soaked in 100 mL of different strength of aqueous NaOH

solutions (24e48%) and shaken in a water bath shaker for a pre-

determined dwelling time (1e4 h) at room temperature and

40 rpm. The sample was then  ltered and washed with deionized

water. After washing, the sample was dried overnight in an oven at

105  C. The prepared samples were named according to the usage

of NaOH% and dwelling time as 24ACSH3, 32ACSH3, 40ACSH3 and

48ACSH3 for 24%, 32%, 40% and 48% NaOH, respectively.

 2.3. Carbon dioxide  xed-bed column adsorption procedures

A known amount of adsorbent was placed into the   xed-bedcolumn (l  ¼  42 cm and  d  ¼  1.1 cm), and the column was heated

to 110   C and held for 1 h under a constant nitrogen  ow rate of 

90 mL/min to remove excess moisture. At the same time, nitrogen

gas and carbon dioxide gas were mixed in a steel mixer and owed

through the analytical system. The inow of each gas from the

cylinders were controlled by a calibrated mass   ow controller

(AALBORG, model AFC26 NY, USA). After heating, the column was

cooled to 35   C. The nitrogen gas purging was stopped, and the

adsorption columnwas vacuumed. Then, the mixed gas waspurged

upward into the column. The CO2 concentration was measured and

recorded every 10 s by an online carbon dioxide analyzer model

906 (Quentek instrument, USA). A blank experiment was also

performed following the same procedure as described above

without the adsorbent in the column and found no CO 2  adsorptionby the column.

The experiment was repeated with different parameters, such as

varying reaction temperatures, total gas   ow rates, adsorbent

loadings and carbon dioxide concentrations, to optimize the reac-

tion conditions. All experiments were conducted under conditions

of 0.1 Mpa (atmospheric) pressure and the standard deviation was

used as the value of the experimental error.

 2.4. Regeneration

The regeneration study was carried out by performing the car-

bon dioxide adsorption with the sample that gave the optimum

result. The   rst adsorption cycle was performed as mentioned in

section   2.3. When the 

rst cycle was completed, the furnace

temperature was increased to 110   C and held for a half hour with

nitrogen gas purging. After heating, the temperature was reduced

to 35   C and vacuumed. Then, a gas mixture with the same con-

centration and  ow rate as the  rst cycle was purged upward from

the bottom of the column. The concentration of the outlet gas was

determined and recorded. The procedures were repeated for a few

cycles to determine the reusability of the adsorbent.

 2.5. Characterization of the adsorbents

The surface area and porosity properties of the AC and NaOH-

modied AC were determined by nitrogen (N2) adsorp-

tionedesorption at  196   C (77 K) with a saturation pressure of 

106.65 kPa using an automated gas sorption system (Micromeritics,

Model ASAP 2020, USA). The BET surface area, Langmuir surface

area, cumulative pore volume, micropore volume and average pore

width were obtained from the N2   adsorption isotherms. The

external surface area, micropore surface area and micropore vol-

ume were evaluated using the t-plot method. The cumulative pore

volume was calculated by measuring the amount of N2 adsorbed at

a relative pressure of 0.984. The BJH (Barrette JoynereHalenda)

method was used to determine the average pore width and pore

size distribution.FT-IR (Fourier transform infrared) spectroscopy analysis was

performed using an FT-IR spectrometer (Perkin Elmer, Model 2000

FTIR, USA) to identify the surface functional groups on the AC and

NaOH-modied ACs. The surface morphologies of the unmodied

and modied AC were examined, and their porosities were veried

using a scanning electron microscope (SEM, JEOL JSM- 6460LV,

 Japan).

3. Results and discussion

 3.1. Characterization of the adsorbent 

The BET surface areas and average pore sizes of selected ad-

sorbents are presented in Table 1. The values of the BETsurface areaand micropore area after modication by 32% NaOH decreased

drastically compared to the unmodied AC. This observation is

attributed to the structural changes of the adsorbents due to the

entrapment of NaOH in the micropore area, which reduced the

ultimate surface area of the adsorbents during modication with

high temperatures [17]. On the other hand, after the modication,

the poresizeof AC increased from2.89 nm to 4.12nm. The probable

mechanism behind the large pore size due to NaOH activation is as

follows.

2Cþ 6NaOH/2Naþ 2Na2CO3 þ 3H2   (1)

with elevated temperature, Na2CO3 degraded into CO2 and H2O,

which were entrappedand created wider holes and surface become

less acidic in presence of Naþ by replacing carboxylic and phenolsions [18].

Fig. 1 shows the SEM images of the unmodied and modied

granular activated carbon. The morphological structures appear as

 Table 1

Surface area and average pore size of the prepared adsorbent.

Sample BET surface

areaa (m2/g)

Pore sizeb

(nm)

Micropore areac

(m2/g)

AC 787.65 2.89 555.80

32ACSH3 378.23 4.12 277.42

a Obtained by BET measurement.b Calculated by the BJH (desorption) method using N2 adsorption isotherms.c

Obtained by the t-plot method.

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wavy ripple marks (Fig.1a) beforeNaOH modication, but after 32%

NaOH modication with a 3 h dwelling time, the morphological

structure changed. The 32ACSH3 adsorbents showed highly

cracked surfaces with a distinct pore size, indicating the suitability

of CO2 adsorption onto these pores.

The FT-IR spectrum of the adsorbent is important for evaluating

the active functional groups on the surface of the raw material and

on the adsorbent. The spectra of the different NaOH-modied ACs

are displayed in   Fig. 2. All of the FT-IR curves were similar. The

broad band at approximately 3500 cm1 is usually ascribed to

hydroxyl groups (eOH). The band located at approximately

1650 cm1 corresponds to amide (eNeH) groups. Some distinct

strong peaks at 1400 cm1 were attributed to the presence of some

carboxylates (C]O) and asymmetric nitro functional groups on all

modied adsorbents.

 3.2. Parameters affecting the modi ed adsorbents

The granular coconut shell activated carbon was modied by

different strengths of 24%, 32%, 40% and 48% NaOH at a constant

dwelling time (3 h). The prepared NaOH-modied activated car-

bons were later used as adsorbents to investigate their perfor-

mance for CO2   adsorption. As shown in   Fig. 3   the 32% NaOH-modied adsorbent (32ACSH3) showed the best performance for

CO2   adsorption. The unmodied AC (without NaOH loading) can

adsorb 17.52 mg/g CO2, whereas the 24%, 32%, 40% and 48% NaOH-

modied adsorbents can adsorb 24.10, 27.10, 21.69, and 23.24 mg/g

CO2, respectively. Again, the 32% NaOH-enriched activated carbon

modication was performed with various dwelling times (1e4 h).

As shown in   Fig. 4   a 3 h dwelling time provides better CO2adsorption compared to the other contact times. Thus, the

32ACSH3 adsorbent was used for further studies. Moreover, the

longest breakthrough time corresponds to the 32ACSH3, indicating

the porous structure of the adsorbent. The CO2 adsorption was also

increased due to the formation of carbonates between the carbon

dioxide molecules and the sodium on the surfaces of the modied

activated coconut shell (32ACSH3) [19].

Fig. 1.  SEM micrographs of the prepared adsorbents: (a) unmodied AC (magnication   ¼   1000), (b) 32ACSH3 (magnication   ¼   1000), (c) unmodied AC

(magnication  ¼  10000) and (d) 32ACSH3 (magnication  ¼  10000).

40080012001600200024002800320036004000

   %    T  r  a  n  s  m   i   t   t  a  n  c  e

wavenumber (cm-1)

(a)

(b)

(c)

(d)

(e)

Fig. 2.  FT-IR spectra for (a) unmodied AC, (b) 24ACSH3, (c) 32ACSH3, (d) 40ACSH3,

and (e) 48ACSH3.

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 3.3. Factors affecting   xed-bed column study

 3.3.1. Effect of total gas  ow rate on CO 2  adsorption

Three gas   ow rates of 90, 120 and 150 mL/min into the

adsorption column were compared against the 32ACSH3 adsorp-

tion capacity. As shown in Fig. 5, the slowest ow rate of 90 mL/min

produced a longer breakthrough time compared with the highest

ow rate of 150 mL/min. The longer dwelling times at lower  ow

rates allow for slower CO2   diffusion, which leads to a higher

adsorption capacity of 27.10 mg/g in comparison with 25.38 and

25.73 mg/g for 120 and 150 mL/min, respectively. The high   ow

rate of gases saturated the adsorption column very quickly, which

was associated with higher mass transfer coef cients [20].

 3.3.2. Effect of the adsorption temperature on CO 2  adsorption

The effects of different adsorption temperature, including 35  C,

45 

C and 55 

C, on CO2  adsorption by 32ACSH3 are presented inFig. 6. The adsorption capacity at 35   C is 27.10 mg/g, and it grad-

ually decreases from 24.3 mg/g to 16.62 mg/g with increasing

temperature from 45   C to 55   C. Shorter breakthrough times and

consequently lower breakthrough values at higher adsorption

temperatures may be attributed to the fact that the adsorption

capacity of the adsorbent is reduced at elevated temperatures. This

trend of lower adsorption capacity with high temperature indicates

that the adsorption of CO2 on 32ACSH3 is physisorption. Moreover,

according to the FT-IR analysis, the surface of the 32ACSH3

contained some N2 functional groups, which may be responsible for

the CO2   adsorption at low temperature  [14]. The presence of so-

dium molecules on the surface of 32ACSH3 might absorb CO2  and

form Na2CO3 at low temperature, but if the temperature increases,

then Na2CO3   decomposes and converts into CO2  and H2O, which

ultimately reduce the CO2 adsorption.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

   C   /   C   0

Time (s)

32ACSH1

32ACSH2

32ACSH3

32ACSH4

Fig. 4.  Effect of the contact time of the 32% NaOH solution with AC on CO 2 adsorption

(Reaction condition: gas  ow rate: 90 mL/min; reaction temperature: 35   C; concen-

tration of CO2: 10%; adsorbent loading: 3 g).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

   C   /   C   0

Time (s)

90 mL/min

120 mL/min

150 mL/min

Fig. 5.   Effect of the total gas   ow rate on CO2   adsorption by 32ACSH3. (Reaction

condition: adsorption temperature   ¼  35   C; concentration of CO2   ¼  10%; adsorbent

loading  ¼ 3.0 g).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

   C   /   C   0

Time (s)

35 C

45 C

55 C

Fig. 6.  Effect of the adsorption temperature on CO2 adsorption by 32ACSH3 (reaction

condition: gas   ow rate   ¼   90 mL/min; concentration of CO2   ¼   10%; adsorbent

loading  ¼ 3.0 g).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

   C   /   C   0

Time (s)

Unmodified AC

24ACSH3

32ACSH3

40ACSH3

48ACSH3

Fig. 3.   Effect of the NaOH concentration on the CO2  adsorption (Reaction condition:

gas   ow rate: 90 mL/min; reaction temperature: 35   C; concentration of CO2: 10%;

adsorbent loading: 3 g).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800 900

   C   /   C   0

Time (s)

3.0 g

4.5 g

6.0 g

Fig. 7.  Effect of adsorbent loading on CO2 adsorption by 32ACSH3 (reaction condition:

gas   ow rate   ¼   90 mL/min; adsorption temperature   ¼   35   C; concentration of 

CO2 ¼

10%).

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 3.3.3. Effect of adsorbent loading on CO 2 adsorption

The effects of different 32ACSH3 loadings on CO2 adsorption are

illustrated by the breakthrough curve shown in   Fig. 7.   Three

different amounts of 32ACSH3 adsorbents, 3, 4.5 and 6 g, were

selected for CO2   adsorption. According to the data presented in

Table 2, increasing the adsorbent loading did not increase the

adsorption capacity of CO2. The highest adsorption capacity of 

27.01 mg/g was found for the lowest 32ACSH3 loading of 3 g, and

further loading from 4.5 to 6 g did not substantially increase the

CO2   adsorption capacity (17.33 and 14.07 mg/g). The higher

adsorbent loading means higher total surface area of 32ACSH3, but

fewer CO2

 molecules were adsorbed onto the surface per gram unit

of 32ACSH3, which led to lower adsorption capacity at high

adsorbent loading.

 3.3.4. Effect of the CO 2 concentration on CO 2 adsorption

The effect of various CO2 concentrations (10, 15 and 20%) in the

feed with N2 were studied to determine the adsorption capacity of 

32ACSH3. The breakthrough curves for different CO2 concentration

in the adsorption are presented in Fig. 8. The percentage increase in

the CO2   feed   ow also increased the adsorption capacity. For

instance, the 10, 15 and 20% CO2   concentrations contributed to

27.10, 29.69 and 34.18 mg/g CO2  adsorption, respectively. The as-

prepared and modied 32ACSH3 adsorbents have highly porous

active sites that can easily accommodate the higher number of CO2molecules on its surface. Moreover, an increase in the CO 2 feed also

increases the concentration gradient, which overcomes the mass

transfer resistance and allows higher adsorption capacity on the

32ACSH3 [9].

 3.4. Regeneration of adsorbents

The possibility of regenerating the adsorbent after the rst cycle

of CO2  adsorption was studied. For this purpose, the spent adsor-

bent (32ACSH3) was regenerated by heating the reactor at 110   C,

which was similar to the adsorbent pretreatment column temper-

ature (110   C) for 1 h under N2  gas purge at a  ow rate of 90 mL/

min. The breakthrough curve for the cyclic CO2   adsorption by32ACSH3 is shown in  Fig. 9. The desorption of the attached CO2molecules on the surface of 32ACSH3 was attributed to initial

physical adsorption into the pore volume of the CO2. According to

the experimental data presented in Fig. 10, the adsorption capacity

of fresh sample was 27.10 mg/g, and the adsorption capacity later

decreased to 19.30 mg/g and 17.33 mg/g at the second and third

cycles of regeneration. Similar performance of the adsorbents

continues until ten cycles. The variation in the adsorption capacity

of the 32ACSH3 adsorbent at the  rst cycle compared to the other

 Table 2

Breakthrough time and adsorption capacity for the different investigated factors.

Parameters Values Adsorption capacity,

mg/g

NaOH loading,% strength 0 17.52 (±0.310)

24 24.10 (±0.302)

32 27.10 (±0.362)

40 21.69 (±0.320)

48 23.24 (±0.354)Contact time, h 1 21.53 (±0.319)

2 22.03 (±0.319)

3 27.10 (±0.362)

4 21.91 (±0.330)

Adsorption temperature,   C 35 27.10 (±0.362)

45 24.03 (±0.266)

55 16.62 (±0.329)

Flow rate, mL/min 90 27.10 (±0.306)

120 25.38 (±0.440)

150 25.73 (±0.567)

Adsorbent loading, g 3.0 27.10 (±0.217)

4.5 17.33 (±0.206)

6.0 14.07 (±0.156)

CO2 concentration (feed), % 10 27.10 (±0.008)

15 29.69 (±0.016)

20 34.18 (±0.047)

Figure in parentheses is the respective standard deviation.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

      C      /      C      0

Time (s)

10%

15%

20%

Fig. 8.   Effect of the CO2  concentration on CO2  adsorption by 32ACSH3 (reaction con-

dition: gas   ow rate   ¼  90 mL/min; adsorption temperature   ¼   35   C; adsorbent

loading ¼

 3.0 g).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50

   C   /   C   0

Time (min)

Fig. 9.  Breakthrough curve for cyclic CO2 adsorption by 32ACSH3 (reaction condition:

gas  ow rate ¼ 90 mL/min; adsorption temperature ¼ 35   C; CO2 concentration: 10%;

adsorbent loading  ¼  3.0 g).

0

10

20

30

1 2 3 4 5 6 7 8 9 10

   A   d  s  o  r  p   t   i  o  n  c  a  p  a  c   i   t  y   (  m  g   /  g   )

No. of cycles

AC

32ACSH3

Fig. 10.   Comparison of the CO2   adsorption capacity on the regenerated 32ACSH3

adsorbent with non-modied AC (reaction condition: gas   ow rate   ¼   90 mL/min;

adsorption temperature ¼

 35

 

C; CO2  concentration: 10%; adsorbent loading ¼

 3.0 g).

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cycles was attributed to the occupying of the 32ACSH3 pores by

some chemically bonded CO2  molecules with existing residual so-

dium on the surface of the adsorbent. The performance of the

adsorbent after different cycles of regeneration also depends on

different adsorption parameters, such as the   ow rate, CO2   con-

centration, and temperature. Therefore, this study suggests that the

adsorbent can be reused for several cycles successfully under mild

regeneration conditions.

4. Conclusion

The coconut shell activated carbon was modied by NaOH

(32ACSH3) with 3 h of dwelling time and was studied for CO2adsorption in a  xed bed under different experimental conditions,

including   ow rate, adsorption temperature and feed concentra-

tion. The   xed-bed column adsorption experiment revealed that

the feed  ow rate of 90 mL/min, adsorbent loading of 3.0 g, CO2concentration (feed) of 15%, and adsorption temperature of 35   C

were optimum for CO2  adsorption. Furthermore, the 32ACSH3 can

be completely regenerated under moderate conditions (i.e., 1 h at

110   C under N2   purge at a   ow rate of 100 mL/min). Therefore,

activated carbon modied with alkaline NaOH, i.e., 32ACSH3, offers

outstanding properties in terms of high CO2   capacity and regen-eration, and this system can provide further insight into other

modied activated carbons for ef cient CO2  adsorption.

 Acknowledgments

The support of Distinguished Scientist Fellowship Program

(DSFP) at the King Saud University is greatly appreciated by Prof. M.

Asif.

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