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Absorption of Hydrogen Sulfide and Carbon Dioxide in Water Based Nanofluids seyyed hamid Esmaeili Faraj, and Mohsen Nasr Esfahany Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04816 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 5, 2016

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Industrial & Engineering Chemistry Research

Absorption of Hydrogen Sulfide and Carbon Dioxide in Water Based Nanofluids

Seyyed Hamid Esmaeili-Faraj, Mohsen Nasr Esfahany* Department of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran, 8415683111.

Abstract Absorption of hydrogen sulfide and carbon dioxide in Exfoliated Graphene Oxide (EGO)-water and Synthesized Silica (SS)-water nanofluids in a bubble column was investigated.Oxygen group functionalities and silanol groups were detected on the surface of EGO and SS nanoparticles, respectively. Due to the adsorption of H2S by these functionalities in EGO and SS nanoparticles,the mass transfer coefficient, kL,increased more than 500% and 200% relative to the based fluid, respectively. EGO-water nanofluid significantly absorbs H2S while CO2 absorption is diminished to zero in nanoparticle mass fractions higher than 0.02%wt.,because oxygen groups on EGO surface attract H2S but repel CO2 molecules. However, SS-

*

To whom correspondence should be addressed. Tel.: +98 (31) 33915631, Fax: +98 (31) 33912677, E-mail: [email protected]. + Department of chemical engineering ++ Department of chemistry

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nanoparticles attract both CO2 and H2S due to hydrogen bonding between gas molecules and hydrogen atoms in the silanol groups. Keywords:

absorption;

mass

transfer;exfoliated

graphene

oxide

(EGO);synthesized silica (SS) nanofluid

1. Introduction Urban wastewater treatment plants are the main source of biogas. Biogas contains methane (CH4, about 60% by volume), carbon dioxide (CO2, about 40%), and other components such as hydrogen sulfide (H2S, between 10 to 10,000 ppmv).1-3 In fact, the presence of H2S,as the main impurity in biogas, intensifies thecorrosion of combustion engines and generation of sulfur oxide components (SOx) in exhaust gases.4 Various methods are implemented to removeH2Sfrom biogas. The conventional techniques for removal of acidic gases such as H2S and CO2arebased on physical and chemical absorption.5 Common absorbents such as di-glycolamine (DGA), mono-ethanolamine (MEA) andmethyl-di-ethanolamine (MDEA)have been used in absorptionof acidic gases.6 The drawbacks of these absorbentsarehigh energy consumptionandoperating costs for theregeneration of the absorbents.3 Although there are several cheap absorbents (NaOH, KOH and Ca(OH)2) for biogas purification, theydo not perform selectivelyand efficiently forabsorption 2 ACS Paragon Plus Environment

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ofH2S,rather than CO2which is abundant in biogas.7-9Water is a natural non-toxic absorbent which can absorbH2Sand CO2physically. However, thesolubility of these components in water is limited. Researchers found that the presenceof nano particles inwater can significantly enhance the absorption of gases into the nanofluids.10-18 Ashrafmansouri and Nasr Esfahany (2014) reviewed mass transfer in nanofluids, recently.10Xuan(2009) and King (2006) studied mass transfer enhancement in nanofluids.

They

developed

a

model

for

equating

mass

transfer

in

nanofluids.19,20Zhou et al. (2003) reviewed the absorption rate enhancement into an aqueous solution by the presence of fine particles.21Ma et al. (2009) investigated the absorption of ammonia in carbon nanotube (CNT)-water as binary nanofluid. Their experimental results showed that 0.23 %wt. of nano-tubes in water cause enhancementin absorption by 16.2%.22 Also, Park et al. (2006) have studied the absorption of carbon dioxide into silica nanofluidsand have calculated the mass transfer coefficient.24Effect of different types of nano-particles for ammonia and carbon dioxide absorption have been studied.23Esmaeili-Faraj et al. (2014) studied the absorption of hydrogen sulfide into graphene oxide-water nanofluid and showed that grazing effect (adsorption of gas molecules on the nanoparticle surfaces) is dominant mechanism for mass transfer enhancement by these nanofluid.25Although several studies have recently discussed on the absorption of 3 ACS Paragon Plus Environment


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nanofluids, there is notany investigation focuson thecomparison of various gases in the absorption process with nanofluids as an absorbent. The aims of the previous paper were investigation of dominant mechanism for mass transfer enhancement by EGO-water nanofluid and characterization of the suitable nanofluid for absorption of H2S.25In this paper, the mass transfer of H2S and CO2 in two nanofluids (EGO-water and synthesized silica (SS)water) have beenstudied.Although the volume fraction of CO2in biogas stream is significantly higherthan H2S, the removal of H2S is more important than CO2due to more toxic and corrosive properties of H2S. Then, mass transfer parameters such as diffusivity coefficient and liquid mass transfer coefficient of H2S have been calculatedto comparethe absorptionof H2Sin two various nanofluids.

2. Methods and materials 2.1.

Reagents

In the experimental investigation, graphite powder with 70 µm in size and a purity of 99.999%, hydrochloric acid, sulfuric acid, nitric acid and potassium chloratewere used from Merck Company topreparegraphene oxideaccording to Staudenmaier method.26To enhanced the measurement of sulfide concentration by iodometeric method,27required componentssuch assodium thiosulfate, iodine and 4 ACS Paragon Plus Environment

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potassium iodinewere purchased from Merck,andpotassium bi-iodate is obtained from Samchun Pure Chemical Company (Korea). Moreover, sodium hydroxide and phenolphthalein for titrimetric method were purchased from the Merck Company as well. Also, carbon dioxide was obtained from the high pressure capsule with a purity greater than 99.0% that was prepared from ArdestanIndustrial and Medical Gases Company (Isfahan). Hydrogen sulfide was producedthrough thereaction of ferric sulfide with hydrochloric acid according to the followingreactionin a cylindrical reactor as shown in figure 1. + 2 → +

(1)

For preparation of synthesized silica (SS) nanoparticles, sol-gel process based on Stöber method24 was applied.In addition, siloxane groups (Si-O-Si) beside of silanol groups (Si-O-H) are presented in solid lattice. In this method, Tetra Ethyl Ortho Silicate (TEOS), ethanol and aqueous ammonia solution (25%wt.) were used as silica source, starting material and catalyzing agent, respectively.24All of these chemicals were purchased from the Merck Company. 2.2.

Apparatus

Figure 1 represents the schematic view of asemi-continuous polyethylene bubble column with 15 mm diameter and650 mm high, for gas absorption. A biogasis injectedin frequent single bubbles from the bottom of the column into the stagnant 5 ACS Paragon Plus Environment


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liquid. After gas injection into the column is completed, the liquid is discharged to a flask and the amount of absorbed gas is measured.Syringe-pump model Viltechmeda Plus SEP21S is used for gas injection inthe column.Ultrasonic homogenizer model Hielscher (Germany) with amplitude 70%and cycle 0.5 is utilizedfor nanofluid sonication. Atomic Force Microscopy (AFM) and Dynamic Light Scattering (DLS) was respectively applied for the observation of the shape and size distribution of the obtained SS-nanoparticles.Fourier Transform Infrared Spectroscopy (FT-IR) have been performed to illustrate thefunctional group bondson theEGO- and SS-nanoparticles. pH values have been evaluated with pHmeter model PHM 350 manufactured by Dostmann Electronic (Germany). 2.3.

Nanofluid preparation

2.3.1. EGO nanofluid preparation The modiﬁed Staudenmaier method is used for preparation of EGO nanoparticle.26This method is completely explained in previous paper.25The EGOwaternanofluids with variousnanoparticle concentrations between 0 and 0.025 %wt. were prepared throughdilution of master suspension. Then,they were sonicated for 20 min in three times. This cause to nanoparticles dispersed in nanofluid stably.


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Figure 1. Schematic of bubble column for gas absorption. Dash line streams are not continue.

2.3.2. Silica nanofluid preparation Synthesized silica (SS) nanoparticles with mean particle diameters of 20.0 nm were synthesized through the following procedure. First, 6.3mol ethanol was added to 1mol pure TEOS. Then, 1.8mol aqueous ammonia (25 %wt.) was separately added to the other 6.3mol ethanol. After a rigorous stirring is performed for 1 hour, these two mixtures were mixed together. Then, the mixture was dried to achieve nanoparticles as powders. Sequentially, 0.5 grams of the powder weredispersed in 7 ACS Paragon Plus Environment


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distilled water until the volume of the mixture becomes 500 ml. The suspension then was sonicated for 20 min with cycle 0.5 and amplitude 70% for three sequences.SS-nanofluids with nanoparticle mass fractions less than 0.5 %wt. were prepared.

2.4.

Experimental methods

2.4.1. Gas absorption by nanofluids In the first step, absorption of H2S or CO2in both nanofluids (EGO-water and SSwater) arestudied. In this part, EGO-water nanofluid with nanoparticle mass factions 0, 0.005, 0.015, 0.02 and 0.025 %wt. and SS-water nanofluids with 0, 0.005, 0.01, 0.05 and 0.1 %wt. have been prepared.100 mlnanofluids are charged into the bubble column as liquid absorbent. Then 3 mlpure H2S or CO2 wasseparately injected from the bottom of the bubble column (providing ~150 s gas-liquid contact time). Column contents discharged into a flask after each experiment.Absorbed H2S is measured by the iodometeric method (4500-S2-)27and absorbed CO2 is definedby titrimetric Method (4500-CO2 C).27

2.4.2. Gas absorption with EGO-nanofluid in optimum point


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The performance of EGO-nanofluid is highly varied on absorption of CO2 and H2S.In this section, the extensive study is performed to obtain the best sense ofthe behavior ofEGO-water nanofluid withhigh nanoparticle mass fraction inH2S and CO2 absorption. For this purpose, EGO-water nanofluid with 0.02% wt. was used asabsorbent in the bubble column. In each test, 6 mlgas as frequent single bubblesare injected into the column by syringe-pump to provide 255.75 stotal gasliquid contact time.After the gas injection is done, the column is discharged to a flask that included 1 ml0.5 M NaOH solution and then titrationis done by HCl solution 0.1 M. In fact, H2S and CO2 readily reacted with aqueous hydroxides as below.28 + 2 → + 2

+ 2 → +

(2) (3)

During the titration process, Na2CO3 and Na2S react with 0.1 M HCl in two steps as described in table 1,28and they are correspond to two equivalent points. These two equivalent points are clearly noticed in distribution curves for CO2 and H2S presented in figure 2. The difference of the volume of HCl used between two equivalent points defines the concentration of H2S and CO2 by following equation: or =

×

(4)

∆



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Where M is an acid molarity, ∆ is an absorbent volume in bubble column (100 ml for present setup), and V1 and V2 are the HCl solution volume consumed for first and second equivalent points, respectively. Table 1. Reaction equations during the titration by HCl solution.

Reaction number 1 2 3 4

Reaction equations + → +

Ione consumed during the titration

+ → + +

+ → + +

+ → +


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Figure 2. Ion distribution curve vs. pH value for (A) CO2 and (B) H2S (Data from James et al. (2002)29).

2.4.3. Determination of mass transfer parameters Mass transfer parametersplay significant role in the evaluation of H2S absorption by

nanofluids

in

our

experiments.

In

this

section,

these

parametersareimplementedby reorganization and fitting of themathematical model that was developed earlier by Esmaeili-Faraj et al. (2014).25First, the column contained 100 mlabsorbent fluidsis prepared. The absorbent fluidscan be either the 11 ACS Paragon Plus Environment


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nanofluidsin optimum nanoparticle mass fraction orthe base fluid. Then different amounts of H2Sare injected into the column withfrequent single bubbles to provide different contact time between the gas and the liquid.In the next step, the amount of absorbed sulfide is measured as it was described earlier. In this experiment,molar flux transferred from gas to liquid is computed through the sulfide concentration in the absorbent by the following equation.

=

! ×∆ "×#$%& ×'

(5)

( is the absorbed H2Sconcentration into the fluid. In present study, the volume of H2Sinjected into the column varied from 1 mlto 5 ml that equals 1.15 mg to 5.75 mg. The number of single bubbles (n) injected into the column were calculated from the total injected gas volume dividing on one single bubble volume. Bubble diameters at the top and bottom of the column were measured through photographic analysis. The experiments show that the average bubble diameters are 0.005 m and significant changes do not occur in the bubble diameter of the base and nanofluids.t0 is the contact time of one single bubble with the liquid that measured as bubble rising time in the column. The total gas-liquid contact time, θ, is calculated with ) = *+, . 3. Results and Discussion 3.1.

Nanofluid Characterization 12 ACS Paragon Plus Environment

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FE-SEM images of EGO nano-sheets in the previous paper clearly shows that the size of graphite oxide sheets is in the micron (600 nm). However, nano-sheets of EGO have been prepared with sheet thickness below 20 nm.AFM image for SSnanoparticlesis presented in figure 3a. This imageshows that SS-nanoparticles are highlyclose to nanometric spheres.In addition, an average diameter of SSnanoparticles according to DLS examination is 20 nm (figure 3b).Furthermore, results of zeta potential test for SS nanofluid (figure 3c)confirm that SS nanofluid has high stability. Marqueset al. showed that EGO-nanofluid is very stable in higher pH values26 and it was regulated in pHabout 9. In this condition, synthesized EGO nanoparticles donot transform to sediment after several days. Figure 4 presents the results of FT-IR test which indicates the functionalities ofEGO and SSnanoparticles.The plot of the SS shows that both siloxane and silanol groups are existed. On the other side the results of EGO showseveral oxygen-containing functionalities such as carbonyl, epoxy and carboxyl groups, and there is no hydrogen-containinggroup (such as hydroxyl) on the surface of EGO-nanoparticle.Moreover, hydroxyl group in the silanol group in SSnanoparticles and oxygen-containing functional groups in EGO-nanoparticles can involve in hydrogen bonding with H2O and stabilize the nanofluids.Also, it is



expected that hydrogen bonding forms between the above functional groups on the surface of the nanoparticles and hydrogen and oxygen atoms in H2S and CO2.

18

B

16 14

Distribution, %

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12 10 8 6 4 2 0 0

20

40

60

Particle size diameter


80

100

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2.5

C

- 45.2

2

Total Counts*10-5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1 0.5 0 0

-50

-100

-150

-200

Zeta Potential, mV

Fig 3. AFM image (A), DLS test result (B) and zeta potential test (C)forSS- nanoparticles

Fig 4. FT-IR result for EGO and SS-nanoparticles



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3.2.

Absorption experiments

In this section,an absorption of CO2 and H2S in EGO and SS nanofluids with different nanoparticle mass fractionispresented. Figure 5comparesthe results of H2S and CO2 absorption as an effective absorption ratio for nanofluids.The effective absorption ratio is definedby the ratio of the amount ofgas absorption in nanofluid than base fluid. Absorption of H2S in EGO-water nanofluid is investigated previously.25Figure5ashows that the effective absorption ratio of H2S for SS-waternanofluids increases with nanoparticle mass fraction until achieving maximum value.This behavior is similar to H2S absorption by EGO-water nanofluid that was presented in previous paper.25As fraction of nanoparticle increases more than optimum value, the absorption ratio fall down sharply.Indeed extra particles in the vicinity of the saturated bubble hinders the grazing with the other particles resulting in decreasing mass transfer.25In fact, EGO-nanofluid with 0.02%wt.25 and SS-nanofluid with 0.1%wt. have optimum nanoparticle mass fraction for H2S absorption.Similarly, results of CO2 absorption for two nanofluids have beendepicted in figures5b and 5c. Figure 5b shows thatSS-nanofluid absorb CO2 more than base fluid similar to what explained for H2S absorption. However,the maximum absorption ratio for SS-nanofluids occurs in 0.01%wt. In addition,the absorption of CO2inEGO-nanofluidfor all nanoparticle mass fractions is lower than base fluid and decrease with increase of nanoparticle mass fraction 16 ACS Paragon Plus Environment

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(Fig. 5c).Figure 5c clearly show thatCO2 absorption in EGO-water nanofluid with nanoparticle mass fraction greater than 0.02 %wt. is not achieved.Since this characteristic of EGO-nanofluid in CO2absorption is highly significant and valuable, further investigations will onlyfocus on EGO-nanofluid with 0.02%wt.

1.42

Effective Absorption Ratio, Reff

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A 1.40 1.38 1.36 1.34 1.32 1.30 1.28 0.001

0.01

0.1

SS- nanoparticle mass fraction, %wt.


1


Effective absorption ratio, Reff

1.5

B 1.4 1.3 1.2 1.1 1.0 0.9 0.00

0.02

0.04

0.06

0.08

0.10

0.12

SS-nanoparticle mass fraction, %wt.

1.2

Effective absorption ratio, Reff

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C

1.0 0.8 0.6 0.4 0.2 0.0 -0.2 0.000

0.005

0.010

0.015

0.020

0.025

0.030

EGO-nanoparticle mass fraction, %wt.

Fig 5.Effective absorption ratio of (A) H2S by SS-nanofluid, (B) CO2 by SS-nanofluid and (C) CO2 by EGO-nanofluid.


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In order to evaluate this characteristic of EGO for absorption of H2S and CO2, the new examination for EGO nanofluids (0.02%wt) with different detection methods is performed. In this investigation, equivalent points are determined in a plot of pH vs. volume of HCl added in titration. Figure 6 shows that detection of equivalent points isclearly visible in the plot of minus differential of pH values vs. amount of added HCl. Figure 6a and 6binvestigate the influence of added HCl on the performance oftitration of absorbed H2S in the base fluid and EGO-water nanofluid, respectively. In these figures, dash line indicates the titration result for the fluids without H2Sinjection andthe maximum point of the plot shows the equivalent point that indicated the neutralization of NaOH with HCl.

14

A

Without H2S Injection

12

-(pH Differential)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

With H2S Injection

〖2-〗^−

8 6

-^/ ^/−

4 2 0 0

2

4

6

Volume of Acid Added, ml


8

10


14

-(pH Differential)

B

Without H2S Injection

12

With H2S Injection

10 8 6

-^/ ^/−

4

〖2-〗^−

2 0 0

2

4

6

8

10

12


14

Without CO2 Injection

12

-(pH Differential)

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10

C

〖245〗_7^−

With CO2 Injection

8 6

〖45〗_7^/ ^/−

4 2 0 0

2

4

6



8

10

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14

Without CO2 Injection

D

12

-(pH Differential)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


With CO2 Injection

10 8 6 4 2 0 0

2

4

6

8

10

12

Volume of Acid Added, ml Fig 6.Minus differential of pH value vs. amount of added HCl in titration process: (A) H2S absorption in base fluid, (B) H2S absorption in nanofluid, (C) CO2 absorption in base fluid, (D) CO2 absorption in nanofluid.

Also, two maximum points are discerned in the variation of pH whenH2S is injected (solid lines)in boththe base fluid and nanofluid.In fact, this means that both anions ( and )are detected. Results of CO2absorption in base fluid and EGO-water nanofluid are presented in figure 6c and d. In figure 6c, two peaks in solid line are detected which are related to and anions, just like what explained in H2S absorption. However, only one single peak isnoticedin titration of the nanofluid in contact with CO2gas(fig.6d). The precise evaluation of this figure shows that this peak exactly



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occurs at the specificvolume where the peak is noticed in fresh nanofluid withoutany contact with CO2gas. It shows that CO2 absorption in nanofluid is negligible, and and ions do not present as two separate peaks. These items are closeto results of the last section.The amounts of absorbed H2S are calculated by Eq.4 as 2 and 2.5 mmol/L for the base fluid and EGO-water nanofluid. In the other hand, CO2 is absorbed in the base fluid by 1.25 mmol/L but due to the presence of only one peak in Fig6d, the amount of the absorbed CO2 in EGO-nanofluid cannot be calculated by Eq. (4). It is concluded that hydrogen bonding between and oxygen-functionalities, is the main reason forthe enhancement of H2S absorption by nanofluids.According to the results, CO2 is not absorbed in EGO-nanofluidwhen nanoparticle mass fraction is higher than 0.02%wt. Three factorshighly influence ontheabsorption of CO2 in EGO-water nanofluid. The first is a low solubility of CO2 in water and water based nanofluids.In fact, thesolubility of H2S in water is approximately three times greater than CO2at ambient temperature.28The second one is reduction of CO2solubility in the nanofluid whennano particle volume fraction is increased. This reason is extensively discussed by Park et al. (2006).24Last but not least, the difference of chemical potential for mass transfer from gas phase to liquid reduces when the concentration ofEGO nanoparticles is high. Electrostatic repulsion between electron cloudof oxygen-containing groups on the surface of EGO 22 ACS Paragon Plus Environment

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particles30 and absorbed CO2 ledto decreasethe chemical potential in EGO-water nanofluid. In addition, the reduction ondifference of chemical potential between gas

phase

and

EGO-water

nanofluiddecreasesthe

CO2absorption

in

nanofluidespecially whenthe mass fraction of the nanoparticle is high.Thus,these factors cause to no CO2 absorption in EGO-nanofluid with high nanoparticle mass fractions. Therefore,it is deduced that EGO-water nanofluid with nanoparticle mass fractions higher than 0.02%wt.can absorb H2S selectively. However, hydrogen bonding between CO2 and hydrogen atoms in the silanol group will be formed in SS-nanofluid(Figure 5b). Figure 7a and 7bschematically illustrates the interaction between oxygen group functionalities on the surface ofEGO-nanoparticles and gas molecules(CO2and H2S). It is shown that oxygen groups on EGO surface attractH2Smolecules, but repelCO2 ones. However, SS-nanoparticles attract both CO2 and H2S as it is schematically shown in figure 7 (c and d).



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Fig 7. Schematic of interaction between carbon dioxide and hydrogen sulfide with EGO-sheets (A and B) and SS-nanoparticles (C and D), R=Repulsion, A=Attraction.

3.3.

Characterization of mass transfer parameters

Mass transfer parameters highly important in investigationof the performance of the absorbent. Asingle bubble that rose in liquid column is prepared to define mass transfer parameters such as diffusivity coefficient, D,and mass transfer coefficient,


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8( ,in the gas-liquid system,. In the previous work, for this system, a correlation was obtained as follows:25 = 9 :!;'

(6)

Where is a molar flux of H2S in nanofluids or a base fluid (mol/m2.s), θ is total

gas-liquid contact time, 8( ; is Gas-liquid volumetric mass transfer coefficient (s-1)

and m is a constant parameter that definesas:25 >

A

A

9 = < = + ?@ coth =E@ FF % > > &

(7)

Andliquid mass transfer coefficient was obtained as:21 >

A

A

8( = = + ?@ coth =E@ FF % > > &

(8)

Where s is a surface renewal constant (s-1), ? is a gas diffusion coefficient in the nanofluids or a base fluid (m2/s), E is liquid film thickness (m), G, is bubble radius (m), < is gas concentration in the gas-liquid interface (mol/m3) and 8( is a liquid

mass transfer coefficient (m/s). Substituting parameters m and 8( from equations 7 and 8 into 6 gives a correlation for with contact time, θ, as: ?

J

J

?

J

J

= < HG + ?@? coth HE@?KK exp H− HG + ?@? coth HE@?KK )K 0

0


(9)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Equation9 include three model parameters D, E and s. Since thevariation of surface renewal constant in base fluids and nanofluids are subtle (according to Zhao et al. (2003)), it can be fixed at0.001 s-1 for both the base and the nanofluid.21Othertwo parameters can be calculated throughcurve fitting of equation 9 on experimental data. As nanofluids have optimum nanoparticle mass fractionin these experiments, SS-nanofluid with 0.1%wt. and EGO-nanofluid with 0.02%wt. have been applied. Results of experiments for the calculation of model parameters are presented in table 2. Figure8 and 9 illustrate the variation of molar flux valuesofthe base fluid and EGO- and SS-waternanofluid versus measured contact time, respectively. Also, equation 9has been fitted on experimental data as it is noticed in figures 8 and 9. Curve fitting was done by“curve fitting toolbar” ofMATLAB software. Table 2. Results of absorption experiments for characterization of mass transfer parameters


Page 26 of 36

Page 27 of 36

Total contact time,), (s)

Average H2S molar flux in EGO, OPQ ×104

Average H2S molar flux in SS, OPQ ×104

Average H2S molar flux in BF, OPQ ×104

1

51.2±0.01

11.70±1.29

6.83±0.65

3.34±0.54

2

105.7±0.01

4.70±0.42

4.53±0.32

2.15±0.21

3

150.0±0.01

2.77±0.24

3.67±0.20

1.85±0.21

4

187.6±0.01

1.93±0.16

2.81±0.18

1.74±0.15

5

218.2±0.01

1.58±0.13

2.98±0.16

1.56±0.13

Volume of H2S injected (mL)

O .A

O .A

O .A

-4

12

x 10

A 10

Molar Flux, mol/m2.s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BF Experimental 8

BF Theoretical EGO 020 Experimental

6

EGO 020 Theoretical 4

2 50

100

150

200

Total Contact Time, s

Fig 8. Experimental data and model curve for average H2S molar flux in both the base fluid and EGO-water nanofluid versus contact time.



-4

x 10 6.5

B

6

Molar Flux, mol/m2 .s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

SS 100 Experimental

5.5

SS 100 Theoretical

5

BF Experimental

4.5 4

BF Theoretical

3.5 3 2.5 2 1.5 50

100

150

200

Total Contact Time, s

Fig 9. Experimental data and model curve for average H2S molar flux in both base fluid and SSwater nanofluid versus contact time.

Figures8and 9 show thatmodel equationsaresignificantly congruous with experimental data for the base fluid, SS-nanofluid and EGO-nanofluidwith Rsquared 0.9229, 0.9563 and 0.9444, respectively. Two model parameters (D and E)areobtained astable 3. Diffusivity coefficient ofboth nanofluidsdoes not presentremarkable difference with each other,but the diffusivity coefficient of EGO-nanofluid and SS-nanofluid is approximately 42% and 52%more than base fluid, respectively. As it is shown in Fig. 5, theequal absorption ratio implies on equal diffusion coefficient, and it is independent of thetype of nanoparticles.The film of mass transfer in the base fluid is four times thicker than the EGO-nanofluid and 1.5 times more than the SS-nanofluid. It seems that Brownian motion of nano28 ACS Paragon Plus Environment

Page 29 of 36

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particles and micro-convection in a mass transfer layer of nanofluid decrease the film thickness.20 Furthermore,due to larger particle sizein EGO case, this microconvection effect is more effective than SS-nanoparticle.Thus,smaller liquid film thickness producesin EGO nanofluid. Ashrafmansouri et al. (2014) investigated self-diffusion coefficient of water in water-based silica nanofluid and their measurements show that the diffusivity ofthe nanofluid only enhanced up to 10% more than the base fluid.31 Therefore, 42% and 52% increasing in diffusivity coefficient for EGO-nanofluid and SS-nanofluid and difference in liquid film thickness for nanofluids and base fluid are not completeddue tochange of these parameters. In fact,other effects such as grazing effect and thermodynamic properties probably affectthese parameters. The mass transfer coefficient of H2S absorption in the nanofluids and the base fluid will be calculated by substitutingof model parameters from table 3 into equation 8. The liquid mass transfer coefficient for the nanofluids and the base fluid arecalculated from table 3.The liquid mass transfer coefficient for H2S absorption in SS-nanofluid and EGO-nanofluid is about twoand fivefold of thebase fluid, respectively. Table 3. Model parameters obtained from curve fitting. Parameter

Symbol

base fluid

EGO nanofluid


SS nanofluid

unit


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

Diffusivity coefficient

D

1.9e-9

2.7e-9

2.9e-9

m2/s

Liquid film thickness

E

6e-4

1.5e-4

4e-4

m

ST

4.12e-6

1.91e-5

8.54e-6

m/s

Calculated liquid mass transfer coefficient

3.4.

Sensitivity analysis

Sensitivity analysis was done to evaluatethe model sensitivity with model parameters. For this purpose, molar flux byoriginal parameters presented in table 3 were calculatedfor three typical total contact time 50, 100 and 150 s withthe model equation (eq. 9). Sensitivity analysis results have been presented in table 4. According to the table 4, it is concluded that: 1- In all cases,hydrogen sulfide molar fluxis directly proportional to the variation ofthe diffusivity coefficient and it is inversely proportional to liquid film thickness changes. 2- EGO-nanofluid is highly sensitive to variations of model parameters and then followed by SS-nanofluid and base fluid. 3- In all cases, model is more sensitive with diffusivity coefficientrather than liquid film thickness. 4- In all cases, molarflux decreased by increasing contact time. Reduction of driving force in large amount of contact time is the main reason of this decreasing. 30 ACS Paragon Plus Environment

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Table 4. Results of sensitivity analysis Original Contact absorbent time, s

OPQ

10.39

EGO 5.70

Nanofluid

150

50

3.13

6.53

SS 100

4.99

Nanofluid

150

50

Base fluid

3.82

3.21

2.51 100

150

values

values

Parameters

10# ×

O .A

100

10% decrease in parameters original

values

,

50

10% increase in parameters original

1.96

Parameters

,

value

)

10# ×

OPQ

O .A

)

Change

Parameters

percent

value

10# × ,

OPQ

O .A

Change )

percent

?*W

2.97e-9

11.429

9.971

2.43e-9

9.356

-9.975

δ

1.67e-4

9.507

-8.522

1.36e-4

11.476

10.422

?*W

2.97e-9

6.2725

9.974

2.43e-9

5.1348

-9.973

δ

1.67e-4

5.2176

-8.521

1.36e-4

6.298

10.421

?*W

2.97e-9

3.4424

9.973

2.43e-9

2.818

-9.974

δ

1.67e-4

2.8635

-8.521

1.36e-4

3.4564

10.420

?"X

3.19e-9

6.987

6.998

2.61e-9

6.046

-7.412

δ

4.4e-4

6.161

-5.650

3.6e-4

6.962

6.616

?"X

3.19e-9

5.204

4.288

2.61e-9

4.747

-4.870

δ

4.4e-4

4.808

-3.647

3.6e-4

5.193

4.068

?"X

3.19e-9

3.876

1.465

2.61e-9

3.727

-2.435

δ

4.4e-4

3.752

-1.780

3.6e-4

3.874

1.414

?YW

2.1e-9

3.46

7.452

1.7e-9

2.98

-7.322

δ

6.6e-4

3.06

-4.932

5.4e-4

3.41

6.014

?YW

2.1e-9

2.63

4.778

1.7e-9

2.38

-5.128

δ

6.6e-4

2.43

-3.409

5.4e-4

2.61

3.892

?YW

2.1e-9

2.00

2.170

1.7e-9

1.91

-2.882

δ

6.6e-4

1.93

-1.860

5.4e-4

2.00

1.812



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

4. Conclusions Absorption of hydrogen sulfide (H2S) and carbon dioxide (CO2) as main compounds that contaminate anaerobic biogaswere studiedby using of Exfoliated Graphene

Oxide

(EGO)-water

and

Synthesized

Silica

(SS)-

waternanofluids.CO2absorption in EGO-water nanofluid was negligible in nanoparticle mass fractionshigher than 0.02%wt.ButH2S was significantly absorbeddue to the existence of large amounts of oxygen atoms that functionalized on the EGO particle surface. SS-nanofluid absorbed both H2S and CO2 better than base fluiddue to hydrogen bonding between gas molecules and silanol group (Si-OH).Two parameters: diffusivity coefficient and liquid film thicknesswere calculated for the nanofluidsand the base fluidby reorganization of mathematical models.The diffusivity coefficient of EGO-nanofluid is 42% more than that of the base fluid while it was 52% for SS-nanofluid. Mass transfer coefficient of the EGO-water nanofluid and SS-water nanofluidare respectively five and twofold of that for the base fluid, respectively. Finally, it is concluded that the mass transfer coefficient in nanofluids have been enhanced by grazing effect (adsorption of gas molecules on nanoparticle surface).


Page 33 of 36

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Absorption of Hydrogen Sulfide and Carbon ... - ACS Publications

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