Article pubs.acs.org/IECR
Hydrogen Sulfide Bubble Absorption Enhancement in Water-Based Nanofluids Seyyed Hamid Esmaeili Faraj,† Mohsen Nasr Esfahany,*,† Mehdi Jafari-Asl,‡ and Nasrin Etesami† †
Department of Chemical Engineering, and ‡Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran ABSTRACT: Nanoparticles addition is a novel technique for enhancement of mass transfer in absorption process. A bubble column was used for investigation of hydrogen sulfide (H2S) absorption process. Silica and exfoliated graphene oxide (EGO) were used for preparation of nanofluids used as hydrogen sulfide absorbent. The absorption rate of hydrogen sulfide deteriorated by addition of 14 nm silica nanoparticles to water, while EGO−water nanofluid augmented hydrogen sulfide absorption relative to the base fluid. EGO−water nanofluid with 0.02% wt EGO enhanced absorption rate by 40%. For greater mass percentages of EGO in water, the absorption rate was reduced. It is concluded that EGO−water nanofluid is suitable for H2S absorption. Grazing is the main reason for mass transfer enhancement in EGO−water nanofluids. Mathematical modeling was developed for predicting the effective absorption ratio. The maximum deviation of model predictions from the measurements is 13%.
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Grazing Effect. Some other researchers report that grazing is the main reason for mass transfer enhancement in ultrafine particle slurries. This effect (also called shuttle mechanism) is defined as the gas component adsorption on the particle surface at the gas−liquid interface or from the liquid film and its desorption from the particles into the liquid bulk.13,14 Bubble Breaking Effect. Pineda et al.15 and Kim et al.12 suggested that the bubble breaking mechanism may affect the mass transfer enhancement in nanofluids. According to this mechanism, nanoparticles collide with the gas bubbles, breaking them into smaller bubbles. Smaller bubbles cause increased interfacial area that enhances volumetric mass transfer coefficients.12,15 The effects of nanoparticles on absorption of ammonia,5,16−19 carbon dioxide,6,12,15,20,21 and oxygen22,23 in nanofluids have been investigated. Several industrial water-based techniques for removal of H2S from a gas stream were reviewed by Rene et al.24 Possible potentials of nanofluids in H2S removal have remained unattended so far. Because of interaction between sulfur compounds and carbon surfaces (graphene and CNTs),25,26 it is expected that exfoliated graphene oxide (EGO) is a good candidate to be used for H2S absorption enhancement. The authors’ literature review showed that no investigation about absorption and mass transfer enhancement by EGO nanofluid has been published yet.7 H2S absorption into silica−water nanofluids as a common absorbent for CO26,12,21 was also investigated. The aims of this study are investigation of the dominant mechanism for mass transfer enhancement by nanofluids and characterization of the suitable nanofluid for absorption of hydrogen sulfide.
INTRODUCTION Hydrogen sulfide (H2S) is present in most of the industrial emission gases. H2S is a major air pollutant that causes acidic rains.1 H2S removal can be achieved via physical, chemical, and biological methods.2,3 Physical and chemical methods are commonly applied for removal of H2S from valuable gases such as natural gas and biogas.2 Absorption is a physical process in which molecules transfer from gas to liquid absorbent. Absorption is a widespread operation for H2S removal from gas streams but is limited by H2S solubility in absorbent.3 The addition of proper surfactants to the absorbent is one of the chemical treatment methods that increases absorption rate.4,5 Nanofluid is defined as a suspension of nanosize solid particles in liquid.6 Several studies have reported the effects of nanoparticles in heat transfer enhancement.7−10 Nanofluid can be applied in the gas absorption process to enhance the rate of absorption.11 Ashrafmansouri et al. (2014) have recently reviewed mass transfer in nanofluids.11 Kim et al. (2008) investigated the effect of nanoparticle concentration on mass transfer enhancement in CO2 absorption. They observed a 24% increase in CO2 absorption with 0.021% wt silica water nanofluid. They concluded that absorption rate increases with nanoparticles’ concentration.12 The exact mechanism that explains mass transfer enhancement by nanofluid is still unknown. The main mechanisms suggested by researchers are as follows. Hydrodynamic Effects. Some researchers believe that hydrodynamic effects are responsible for mass transfer enhancement.13This mechanism was mainly used to justify mass transfer enhancement in bubbly two-phase flows. Suspended nanoparticles may cover the bubbles, preventing the coalescence of the bubbles, resulting in smaller bubbles and an increased specific surface.14 On the other hand, nanoparticles, due to the Brownian motions, induce microconvection that results in increased kL.15 Nanoparticles therefore affect the volumetric gas−liquid mass transfer coefficient (kLa). The volumetric mass transfer coefficient is readily measured.13 © 2014 American Chemical Society
Received: Revised: Accepted: Published: 16851
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Figure 1. Schematic of bubble column setup.
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METHODS AND MATERIALS Reagents. The silicon dioxide (SiO2) nanoparticles with purity of 99.8% and 14 nm size (Plasma Chem GmbH) and graphite powder (Merck) were used for making the nanofluid. Hydrochloric acid, sulfuric acid, nitric acid, potassium chlorate, ferric sulfide, sodium thiosulfate, iodine, potassium iodine, and potassium bi-iodate were purchased from Merck. Hydrogen sulfide was produced by reaction of ferric sulfide with hydrochloric acid. Apparatus. A bubble column 15 mm in diameter and 65 cm long, made from poly ethylene, was used for H2S absorption. A schematic of experimental setup is presented in Figure 1. The process is continuous for gas and batch for liquid. Syringe-pump model Viltechmeda Plus SEP21S was used for gas injection to the column. Hydrogen sulfide concentration in the liquid phase can be determined by the iodometeric standard method.27 According to this method, the total sulfide includes dissolved H2S and HS−. The S2− is neglected, especially in the presence of the acid solution. Iodine oxidizes sulfide in acid solution. A titration-based iodometric method is an accurate method for determining hydrogen sulfide. Ultrasonic processor model Hielscher was used for nanofluid sonication. Scanning electron microscopy (SEM) was performed on a PHILIPS XL-30 ESEM at an accelerating voltage of 20 kV. Energy dispersive X-ray analysis (EDXA) was performed for elemental characterization of EGO nanoparticles. Also, transmission electron microscopy (TEM) was used because of high resolution and the depth of focus in the surface. Preparation of Nanofluids. In the first step, 2.5 g of nanosilica was mechanically dispersed in distillated water and
diluted in a 500 mL vessel to achieve 5000 mg/L suspension. The suspension then was sonicated for 20 min with cycle 0.5 and amplitude 65%. The sequence was repeated three times. Other suspensions with 1000, 500, 250, 200, and 100 mg/L concentration of nanoparticles (equal to 0.1, 0.05, 0.025, 0.02, and 0.01% wt) were made by dilution of the concentrated suspension. Graphene oxide (GO) were used to make nanofluid in the second step. Graphene is defined as a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice.28 GO exhibits a range of unusual properties, such as exceptionally high crystal and electronic quality, and, despite its short history, holds great promise in many applications.28 Graphene oxide was prepared using a modified Staudenmaier method29 explained by Ensafi et al.30 According to that, natural graphite powder with a particle size of up to 100 μm and a purity of 99.999% was chemically oxidized to make graphite oxide (GO). The graphite (1 g) was continuously stirred in a solution of sulfuric acid (20 mL), nitric acid (10 mL), and potassium chlorate (10 g) for approximately 100 h. The resulting GO was rinsed with 5% wt hydrochloric acid aqueous solution and then frequently washed with deionized water until the pH of the filtrate was raised to neutral, and then was dried in air. The sheets of graphite oxide (GO) are still on the micrometer scale. GO was dispersed in water with concentration of 0.5 mg/mL and was put in the ultrasonic bath for 2 h for conversion to exfoliated graphene oxide (EGO).30 EGO− water nanofluid was prepared following the method used for silica−water nanofluid. 16852
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Experimental Methods. According to Figure 1, liquid charged from the top of the column at the beginning of each experiment, and discharged from the bottom at the end of the experiment. pH and temperature of the liquid were measured at the beginning and the end of each test. Gas was injected from the bottom of the column and rose along the liquid column. Hydrogen sulfide was produced from the following reaction and collected from reactor by a syringe: FeS + 2HCl → H 2S + FeCl 2
percentage of carbon and oxygen in EGO particles is reported in Table 1. According to this test, the amount of oxygen atoms functionalized on carbon is 38%, which is a large amount and shows the high oxidation of graphene. Oxygen exists in oxygenated functional groups, such as epoxy, hydroxyl (−OH), carboxyl (−COOH), and carbonyl (−CO) groups.30These oxygen functionalities can stabilize the dispersion of EGO nanoparticles in water. Also, these groups can bond by hydrogen atoms in H2S as a hydrogen bond. Absorption Experiment Results. The parameter that explains the absorption performance by nanofluid is the effective absorption ratio. This parameter is defined as mass absorbed by nanofluid divided by mass absorbed by the base fluid and displayed by Reff.15,31
(1)
In the first set of experiments, 3 mL of H2S was injected at the bottom of the column as sequential single bubbles using a syringe pump. The gas bubbles vented to the atmosphere exiting the absorbent. The column contained 100 mL of absorbent fluid. After each test, the solution was discharged to the flask, and H2S content was determined by iodometry. The aim of the first set of experiments was comparing the performance of silica and EGO−water nanofluids in H2S absorption and determining the optimum concentration of the proper nanofluid type. Different concentrations of nanofluids were examined. Therefore, absorbent is included the base fluid (distillated water), EGO nanofluid, and silica nanofluid, and the concentration of nanofluid was changed, but the amount of H2S injection was fixed at 3 mL that equals about 3.46 mg. The second set of experiments was performed to obtain the model parameters and validate the model. Both sets of experiments were performed following the same procedure, but in the second set of the experiments the amount of injected hydrogen sulfide was varied between 1 and 5 mL (1.15−5.75 mg), while the EGO nanofluid concentration was kept at optimum.
R eff =
NA,nf NA,bf
m. = nf. = mbf
( C θ× V )nf ( C θ× V )bf AL
AL
=
CAL,nf CAL,bf
(2)
where CAL is the H2S content in the liquid phase measured by the iodometric method. Figure 6 shows the effective absorption ratio for silica−water nanofluid in different nanoparticle fractions. It is shown that nanofluid absorbs H2S less than the base fluid. Also, by increasing the nanoparticle fraction in the nanofluid, the absorption ratio decreases, and addition of silica nanoparticles decreases the mass absorbed as compared to the base fluid. The reason for reduction of mass transfer after addition of nanoparticles may be due to that (1) increasing the viscosity of suspension causes the reduction of diffusivity coefficient, and (2) reducing the effective fraction of the gas−liquid contact area can reduce the effective diffusivity.14 Absorption of H2S in EGO nanofluid was studied in the second step. Figure 7 shows the effective absorption ratio for various EGO nanoparticle fractions 0−0.025% wt. It is shown that up to the 0.02% wt EGO nanoparticles in the nanofluid, the effective absorption ratio increases with the particle fraction. In the optimum concentration, 0.02% wt particle fraction, the absorption enhancement was measured to be about 40%. However, after this optimum concentration, the effective absorption ratio decreases with the particle concentration in nanofluid. By increasing the particle concentration in the nanofluid, more particles reach the bubble interface and take part in the grazing mechanism, until the surface of the bubble is saturated. Increasing the particle concentration beyond this concentration will not produce any more grazing effect. Indeed, extra particles in the vicinity of the saturated bubble hinder the grazing, with the other particles resulting in decreasing mass transfer. Mechanism of Mass Transfer Enhancement. The main mechanism that enhanced absorption in EGO−water nanofluid is believed to be grazing. According to this mechanism, the nanoparticle used for enhancement of mass transfer should be a good adsorbent for gas component. In Figure 8, the grazing effect is explained schematically. According to this figure, the bubbles containing H2S are strongly adsorbed into EGO nanoparticles close to the gas−liquid interface, and the sequent desorption and redistribution of H2S in the liquid bulk is the reason for mass transfer enhancement by EGO nanofluid. Also, according to results obtained by EDXA test for EGO nanoparticle characterization, because of the existence of oxygen functionalities, hydrogen bonding takes place between these oxygen atoms and two hydrogen atoms available in H2S
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RESULTS AND DISCUSSION Characterization of Nanofluid. The TEM image of silica nanoparticles is presented in Figure 2.
Figure 2. TEM image of silica nanoparticle.
Figure 3 shows the SEM image for synthesized EGO nanoparticles. In Figure 3A and B (with scales of 1 μm and 500 nm, respectively), the graphene sheets are shown that are not in nanoscale. In Figure 3C and D (with scales of 1 μm and 500 nm, respectively), the exfoliated graphene oxide (EGO) with nanoscale sheets is shown. Figure 4A and B shows the TEM image of EGO nanoparticles. Figure 4 shows that GO was transformed to exfoliated form by the sonication treatment. The result of the EDXA test is presented in Figure 5, and the 16853
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Figure 3. SEM images of nanoparticles (A and B) GO, (C and D) EGO.
Figure 4. TEM images of EGO nanoparticle, which show exfoliated GO sheets.
molecules. Because of hydrogen bonding between H2S and EGO nanoparticles, it is concluded that in a mixture of H2S and CO2, EGO nanofluid is selective for H2S. Modeling Results. In the bubble column, it is assumed that nanofluid is continuous phase and single bubbles are disperse phase. The gas phase is assumed pure H2S; therefore, there is no mass transfer resistance in the gas phase, and mass transfer is taking place in a thin liquid film surrounding the bubble. There is mass transfer resistance only in liquid film with thickness δ. Because of the rising of the bubbles in the liquid phase, the gas−liquid interface experiences an age distribution and liquid film is constantly renewed. Two characteristic times as microscopic time (t) and total contact time (θ) are defined. Microscopic time (t) determines the time that one bubble is contact with liquid in the column, and the total contact time (θ) is the time for the determination of all bubbles crossing into
the liquid column. Steady-state diffusion of gas into the liquid is considered as the sole mechanism for transfer of gas molecules.32 The mass transfer rate was obtained as follows:32
(
D sinh δ NA,av =
) + Dr cosh(δ ) r sinh(δ )
s D
0
0
(CAi − CAL)
s D
s D
s D
(3)
where NA is the mass transfer rate of H2S (mol/m2·s), s is the surface renewal constant (s−1), D is the gas diffusion coefficient in the nanofluid or the base fluid (m2/s), and r0 is the bubble radius (m). CAi is the gas concentration in the gas−liquid interface (mol/m3), and CAL is the concentration of component 16854
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Figure 5. EDXA image test for EGO nanoparticle.
Table 1. Percentage of Carbon and Oxygen Elements in EGO Particle According to EDXA Test element
line
intensity (c/s)
atomic %
units
carbon oxygen total
C Ka O Ka
66.81 18.79
61.59 38.41 100.00
wt % wt % wt %
Figure 7. Effective absorption ratio for various EGO nanofluids.
(
D sinh δ m = CAi
) + Dr cosh(δ ) r sinh(δ )
s D
0
0
A (H2S) in liquid bulk. This concentration is continuously changed along the total contact time and described as below:32
NA,av = m e−kLaθ
(4)
( Dδ
NA,av = CAi
( Dδ
)
sD + r0 sD cosh
(
r0 sinh
δ D
sD
sD
)
(6)
(7)
Equation 7 can be used for any absorption process from single bubble in a bubble column. Figure 9 shows the experimental data fitted by model equation. In this figure, the average molar flux of gas to liquid (both base fluid and 0.02% wt EGO nanofluid) versus total contact time (θ) is plotted. The value of this parameter was measured as in Table 2. According to the curve fitting of experimental data by eq 7, for nanofluid and base fluid, we obtained two correlations for mass transfer rate by R-squared
kLa is the gas−liquid volumetric mass transfer coefficient (s−1). Substituting eq 4 into 3 gives D sinh
s D
s D
Equation 6 shows that coefficient m is independent of concentration and time and is dependent only on physical properties such as equilibrium concentration, diffusivity coefficient, liquid film thickness, surface renewal constant, and bubble radius. Substituting eq 6 into eq 5 gives
Figure 6. Effective absorption ratio for various silica nanofluids.
CAL = CAi(1 − e−kLaθ )
s D
) e − k aθ L
(5)
For simplification of this equation, we defined m as 16855
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Figure 8. Schematic of grazing effect, dominant mechanism for mass transfer enhancement: (1) gas bubble internal, (2) liquid film around the bubble, (3) EGO nanoparticles in the gas−liquid interface, and (4) H2S molecule.
R eff,Sth =
0.9735 and 0.9145, respectively. These values show good conformity of model and experimental results. Equations 8 and 9 represent the mass transfer rate for EGO−water nanofluid and base fluid, respectively. (8)
NA,bf = 0.0004 e−0.004θ
(9)
NA,bf
= 4.75 e−0.008θ (10)
A recent equation gives semitheoretical effective absorption ratio for 0.02% wt EGO nanofluid. For 3 mL of gas injected, θ equals 150.04 s (refer to Table 2), and eq 10 gives the value Reff,Sth = 1.43 that has 2.1% deviation from the experimental results (Figure 7). The validity of eq 10 was examined by several measurements at different conditions. Two experiments were performed at constant θ with variable amounts of gas injected into the column filled with different depths of liquid, while in the other two experiments variable θ without trying to keep other parameters constant with 0.02% wt EGO−water nanofluid and the base fluid was performed. In constant θ experiments, the amount of gas injection was changed inversely by height of the fluid in the column to achieve constant θ. Figure 10 shows the validation of eq 10 with these measurements (also see Table 3). The average and maximum deviations of the predicted effective absorption ratio from the new measurements are about 7% and 13%, respectively. It is concluded that eq 10 can predict the effective absorption rate for 0.02% wt EGO−water nanofluid.
Figure 9. Hydrogen sulfide molar flux versus total contact time for 0.02% wt EGO−water nanofluid and the base fluid.
NA,nf = 0.0019 e−0.012θ
NA,nf
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CONCLUSION In this research, the effect of the presence of silica and exfoliated graphene oxide (EGO) nanoparticles in water on H2S absorption was experimentally investigated. Results show that silica−water nanofluid has a deteriorating effect on H2S absorption, while EGO−water nanofluid enhances mass
The semitheoretical effective absorption ratio can be obtained by use of eqs 2, 8, and 9 as
Table 2. Measurement of Model Experiment Parameters for Finding Total Contact Time and H2S Molar Flux for Nanofluid and Base Fluid volume of H2S (mL)
liquid bulk concentration in nanofluid (CAL,nf, mol/m3)
liquid bulk concentration in base fluid (CAL,bf, mol/m3)
number of bubbles (n)
total contact time (θ = nt, s)
average molar flux of H2S in nanofluid (NA,nf = ((CAL,nf × V)/(n × 4πr20 × θ)), mol/(m2 s))
average molar flux of H2S in base fluid (NA,bf = ((CAL,bf × V)/(n × 4πr20 × θ)), mol/(m2 s))
1 2 3 4 5
0.7052 1.2089 1.4356 1.5615 1.7378
0.2015 0.5541 0.9571 1.4104 1.7126
15 31 44 55 64
51.15 105.71 150.04 187.55 218.24
0.001170 0.000470 0.000277 0.000193 0.000158
0.000334 0.000215 0.000185 0.000174 0.000156
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Figure 10. Validation of semitheoretical effective absorption ratio (Reff) correlation.
Table 3. Parameters Used for Validation of Semitheoretical Effective Absorption Ratio Correlation test no.
gas vol (mL)
fluid height (cm)
total contact time, θ, s
Reff exp.
Reff theor.
error, %
1 2 3 4
3.6 2.4 2 1.4
50 80 50 38.5
147.9 149.04 92.8 49.98
1.416 1.611 2.307 3.655
1.455 1.441 2.261 3.185
2.75 10.55 1.99 12.86
transfer relative to the base fluid up to 40% in 0.02% wt nanoparticle concentration. It was shown that EGO−water nanofluid is a strong and selective absorbent for H2S. Grazing is shown to be the dominant mechanism for mass transfer enhancement by EGO−water nanofluid due to the hydrogen bonding between oxygen functionalities on EGO particles and hydrogen atoms in H2S molecules. EGO−water nanofluid can selectively absorb H2S in competition with CO2 from a gas stream. A mathematical model was developed for prediction of mass transfer in EGO−water nanofluid and the base fluid. The model predicts the mass transfer rate of H2S in 0.02% wt EGO nanofluid with good accuracy.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +98 (31) 33915631. Fax: +98 (31) 33912677. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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