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Influence of non-wetting, partial wetting and complete wetting modes of operation on hydrogen sulfide removal utilizing monoethanolamine absorbent in hollow fiber membrane contactor
Sustainable Environment Research 28 (2018) 186e196
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Original Research Article
Influence of non-wetting, partial wetting and complete wetting modes of operation on hydrogen sulfide removal utilizing monoethanolamine absorbent in hollow fiber membrane contactor Ali Taghvaie Nakhjiri a, Amir Heydarinasab a, *, Omid Bakhtiari b, Toraj Mohammadi c a b c
Department of Chemical Engineering, Science and Research Branch of Islamic Azad University, Tehran 1477893855, Iran Membrane Research Center, Razi University, Kermanshah 6714414971, Iran Research Center for Membrane Separation Processes, Iran University of Science and Technology, Tehran 1684613114, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 December 2017 Received in revised form 10 January 2018 Accepted 28 February 2018 Available online 7 March 2018
Hydrogen sulfide is a highly poisonous acidic gas which is regarded as one of the major causes of corrosion and odorous problems. For this reason, the efficient separation of hydrogen sulfide from various gaseous streams is mandatory. This paper aims to evaluate the removal of hydrogen sulfide pollutant from hydrogen sulfide/methane gaseous flow using monoethanolamine liquid absorbent inside the hollow fiber membrane contactor. As the novelty, a mechanistic modelling and a two dimensional numerical simulation are developed under non-wetting (0% wetting of membrane pores), partial wetting (50% wetting of membrane pores) and complete wetting (100% wetting of membrane pores) modes of operation to predict the effects of various operational parameters such as module length, tortuosity and porosity of membrane, initial concentrations of hydrogen sulfide and monoethanolamine on the removal of hydrogen sulfide. The results of mechanistic model prediction under non-wetting mode are in an excellent agreement with the experimental data with average absolute relative error less than 5%. On the basis of the results, monoethanolamine provides the highest hydrogen sulfide sequestration while being used under non-wetting mode of operation in comparison with partial wetting and complete wetting conditions. Membrane pore wettability has a detrimental influence on the removal efficiency of hydrogen sulfide due to creating a considerable mass transfer resistance along the way of hydrogen sulfide gas transport inside the membrane contactor. © 2018 Chinese Institute of Environmental Engineering, Taiwan. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Keywords: Hydrogen sulfide sequestration Gaseous stream Model prediction Wettability
1. Introduction Industrial gaseous mixture often consists of various contaminants such as hydrogen sulfide and carbon dioxide which must be removed in order to decrease their environmental and operational detriments such as the greenhouse phenomenon and corrosion [1]. The numerous treatment techniques such as biofilters and packed beds columns can be used to remove toxic H2S from gaseous streams [2,3]. Due to the existence of various limitations in conventional technologies such as packed towers and mist scrubbers,
* Corresponding author. E-mail address: [email protected] (A. Heydarinasab). Peer review under responsibility of Chinese Institute of Environmental Engineering.
hollow fiber membrane contactor (HFMC) was introduced as a promising technology to eliminate the disadvantages associated with conventional techniques and increase the efficiency of acid gas removal [4,5]. Matson et al. expressed that the surface contact area which is created by a HFMC may attain up to 8000 m2 m3 while in conventional absorption towers, the surface contact area is ten times lower than membrane contactors [6]. The application of HFMC as a promising technology was the focal point of numerous investigators for efficient sequestration of various gases [7e10]. Apart from the existence of nondispersive contact between gas and liquid flows, this novel technique enjoys an easier and larger interfacial contact area and also easy linear scale up [7]. There are a number of studies with the aim of presenting mathematical modelling and numerical simulations of different gas separation processes inside the HFMCs [10e12]. The principal purpose of the modelling and simulation papers was to determine the
https://doi.org/10.1016/j.serj.2018.02.003 2468-2039/© 2018 Chinese Institute of Environmental Engineering, Taiwan. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
A.T. Nakhjiri et al. / Sustainable Environment Research 28 (2018) 186e196
concentration of acid gases such as CO2 and SO2 inside the membrane contactor [10,12]. However, the existence of a membrane segment may append another mass transfer resistance that is not present in the absorption columns. This resistance significantly deteriorates the overall mass transfer and subsequently decreases the selectivity. The minimization of the membrane resistance can be implemented by decreasing the thickness of membrane or by enhancing its inherent gas permeability [12e14]. Microporous membranes such as polyimide, polysulfone, polypropylene and other dense polymeric membranes such as polydimethylsiloxane (silicone rubber) can be used to provide high gaseliquid contact area. The micro-porous membranes have higher permeability and dense membranes are more appropriate where a special selectivity is desired [15e20]. The utilization of HFMC for the separation of acid pollutants has been recently reviewed [5,21]. Due to high toxicity of H2S, few investigations reported the data of H2S removal from gaseous mixtures applying HFMCs [21e23]. In the recent years, the use of finite element method (FEM) to evaluate the sequestration efficiency of contaminants such as CO2, H2S and SO2 from gaseous flows and also simulation of governing equations such as momentum, mass and heat transfer have been developed by several investigators with good results in comparison with experimental results [17,24e26]. For instance, Shirazian et al. confirmed that UMFPACK is regarded as an appropriate numerical solver due to its ability of solving non-stiff and stiff non-linear boundary value problems [27]. Additionally, numerical simulation of CO2 separation from different gaseous streams was performed by numerous authors using COMSOL software with excellent results in comparison with experimental researches [18,28]. Based on the abovementioned characteristics, COMSOL was used in this paper to develop a numerical simulation of H2S removal from a gaseous mixture. The principal motivation of this investigation is to describe the removal efficiency of H2S from a gaseous mixture consisting of 98% CH4 and 2% H2S using monoethanolamine (MEA) liquid solvent and expanded polytetrafluoroethylene (ePTFE) membrane in a HFMC. For this reason, a mechanistic modelling and a numerical simulation which consider the governing mechanisms (diffusion and convection) in both axial and radial coordinates were developed and solved. As the novelty, effect of membrane wettability in different percentages (0%, 50% and 100%) is investigated to evaluate the removal performance of H2S from gaseous mixture. The effects of important operational parameters such as tortuosity and porosity of the membrane, initial concentrations of H2S and MEA absorbent and module length on the sequestration efficiency of H2S from gaseous mixtures in different percentages of membrane pores wetting are compared with each other. Finally, to validate the results of mechanistic modelling and numerical simulation, the model predictions are compared with the experimental data performed by Marzouk et al. [29].
2. Materials and methods The reaction mechanism of H2S with MEA is defined by Cornelissen as follows [30]: þ
MEA þ H2 S4MEAH þ HS
(1)
MEA þ HS 4MEAHþ þ S2
(2)
However, he perceived that reaction (2) is negligible from the ionization constant of HS and the amines. Numerous authors emphasized that the reaction of different amines with H2S is fast [30e32]. The reaction rate equations for H2S-MEA system are presented by Eq. (3). as follows [10]:
187
! HS MEAþ ½H2 S½MEA Keq
RMEA ¼ RH2 S ¼ kH2 SMEA
(3)
where, Keq denotes the equilibrium constant of H2S-MEA system. 3. Mechanistic model development The present paper aims to perform a mechanistic modelling and a two dimensional simulation for describing the transport of H2S inside membrane contactor applying MEA absorbent. The mechanistic model is generally flexible to consider for all non-wetting, partial wetting and complete wetting modes. To achieve the best removal performance of H2S from gaseous stream, non-wetting mode of operation is more appropriate in comparison with the other conditions. A HFMC contactor contains three main compartments: shell section, membrane side and tube segment. Liquid absorbent MEA flows in the tube compartment of HFMC (at z ¼ 0) while in a counter-current formation, gaseous stream consisting of 98% CH4 and 2% H2S is fed through the shell section (at z ¼ L). The scheme of a HFMC considering non-wetting, partial wetting and complete wetting modes of operation is plotted in Fig. 1. Free surface model of Happel denotes that only part of the gas surrounding the fiber of expanded ePTFE membrane contactor is considered which may be predicted as circular cross section [33]. The cross sectional region along with the circular approximation of ePTFE membrane contactor is plotted in Fig. 2. The development of the mechanistic model was based on the major assumptions include: 1) isothermal and steady state modes; 2) similar pore size distribution; 3) similar membrane wall thickness; 4) ideal gas behaviour inside the shell compartment; 5) use of Henry's law in the interface of liquid and gas flows; 6) existence of Newtonian liquid absorbent in the tube compartment; 7) counter-current formation of liquid and gas streams; 8) Laminar and parabolic velocity profile inside the fibers; and 9) utilization of Happel's free surface model with the goal of anticipating the velocity profile of components in the liquid phase. The specifications of hollow fiber module and operating conditions used for modelling and numerical simulation are listed in Table 1. Also, physicochemical properties of H2S and MEA are presented in Table 2. 3.1. Governing equations inside the shell compartment of membrane contactor Eq. (4) describes the continuity equation inside the shell compartment of membrane contactor as follows [10]:
vCi ¼ VNi þ Ri vt
(4)
In this equation Ci, Ri and Ni are respectively defined as concentration, rate of reaction and flux of species i along the membrane length. The flux of species i is defined by Fick's law of diffusion as follows [10]:
Ni ¼ Di VCi þ Ci VZ
(5)
where Vz and Di are velocity of axial direction and diffusion coefficient of components i along the length of membrane contactor, respectively. The steady state continuity equation in the shell compartment of porous ePTFE HFMC can be derived by combining Eqs. (4) and (5) as follows [10]:
" # v2 CH2 S;s 1 vCH2 S;s v2 CH2 S;s vCH2 S;s þ DH2 S;s þ ¼ Vz;s r vr vz vr 2 vz2
(6)
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CH4/H2S gaseous stream
ePTFE Porous membrane
Liquid Absorbent MEA
a)
b)
r=0
r=0
r = r1
r = r1
r = r2
ar = rw
r = rw
r = r3
r = r2
r = r3
r = r1 r = r3
r = r2 = rw
c) r=0 r = r1 r = r2 = rw
r = r3
Fig. 1. Diagrammatic scheme of a hollow fiber membrane contactor under a) non-wetting b) partial wetting (50%) and c) complete wetting modes of operation.
In this equation, DH2 S;s and CH2 S;s are denoted as the diffusion coefficient and concentration of H2S in the shell compartment of membrane contactor. The velocity profile in the shell compartment (Vz;s ) can be correlated using Happel's free surface model as follows [10]: Fig. 2. Cross sectional region and circular approximation of the porous ePTFE membrane contactor.
A.T. Nakhjiri et al. / Sustainable Environment Research 28 (2018) 186e196 Table 1 The specifications of hollow fiber membrane module and operating conditions used for modelling and numerical simulation. Parameter
Value
Membrane type Fiber inner diameter (m) Fiber outer diameter (m) Module inner diameter (m) Porosity (ε) Tortuosity (t) Module length (m) Number of fibers T (K) Gas flow rate (Qg) (mL min1) Liquid flow rate (Ql) (mL min1) Rg (J mol1 K1)
ePTFE 1 103 2 103 20.5 103 0.18 10 0.75 50 298.15 1000 25 8.314
Value
DH2 S;S ðm2 s1 Þ
2.01 105
[36]
DH2 S;S ðε=tÞ
[37]
1.52 109
Calculated from Ref. [38]
9.32 1010
[39]
3.6 1012
[30]
DMEA;t
kMEA ðm3 kmol1 h1 Þ Keq mH2 S
295 2.515
Reference
"
[30] [9]
# 2 # " r ðr=r3 Þ2 ðr2 =r3 Þ2 þ 2lnðr2 =rÞ ¼ 2V 1 2 r3 3 þ ðr2 =r3 Þ4 4ðr2 =r3 Þ2 þ 4lnðr2 =r3 Þ (7)
In this equation, V is the average velocity inside the shell compartment of membrane module. Also as can be apparently seen in Fig. 1, r2 is denoted as the outer radius of tube. r3 is defined as the effective radius of each shell which is calculated considering the active area and hexagonal shape of shell unit around each fiber as follows [10]:
r3 ¼ r2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1=ð1 4Þ
(8)
In Eq. (8), 4 is the volume fraction of void.
v2 Ci;gm vr 2
þ
# 1 vCi;gm v2 Ci;gm þ ¼0 r vr vz2
Eq. (9) shows the steady state material balance equation which defines the transport of gas molecules from gaseous mixture. Due to the existence of non-wetting condition, the governing mechanism of transport through the membrane compartment is molecular diffusion. Besides, no chemical reaction takes place in the gas filled pores [10].
(11)
3.3.2. Material balance inside the liquid filled compartment The left side of Fig. 3 demonstrates the liquid filled section of porous ePTFE membrane where diffusion and reaction are the governing mechanisms describing the transport of H2S along with MEA liquid absorbent. Hence, the material balance under the steady state mode can be derived as follows [10]:
" Di;lm
v2 Ci;lm vr 2
þ
# 1 vCi;lm v2 Ci;lm þ þ Ri;m ¼ 0 r vr vz2
(12)
Due to the use of porous membrane (ePTFE), those chemical reactions taking place inside the membrane compartment only occur inside the pores of membrane. Therefore, the reaction inside the liquid section of porous ePTFE membrane is as follows [10,34]:
Ri;m ¼ Ri ε
3.2. Governing equations inside the membrane compartment under non-wetting mode
" # v2 CH2 S;m 1 vCH2 S;m v2 CH2 S;m þ DH2 S;m þ ¼0 r vr vr 2 vz2
(10)
3.3.1. Material balance inside the gas filled compartment The gas filled section of porous ePTFE membrane is depicted in the right side of Fig. 3. In the gas filled portion of membrane, the only governing mechanism which describes the transport of H2S is diffusion. Hence, the steady state material balance is presented as follows [10]:
Di;gm
"
Vz;s
t
It is illustrated in Fig. 3 that due to substantial decrease in the surface tension, a percentage or even all of the membrane pores can be wetted by liquid absorbent. Therefore, in order to derive the steady state continuity equation for the transport of H2S inside the membrane compartment along with MEA liquid absorbent, two sections (the gas filled and the liquid filled) are considered as follows:
Parameter
ðm2 s1 Þ
DH2 S;s ε
3.3. Governing equation inside the membrane compartment considering membrane pore wetting
Table 2 Physicochemical properties of H2S and MEA used for mechanistic modelling and simulation.
DH2 S;MEA ðm2 s1 Þ
in the membrane compartment. Therefore, diffusion coefficient of H2S in the membrane compartment is presented as follows [10]:
DH2 S;m ¼
All data are achieved from Ref. [29].
DH2 S;m ðm2 s1 Þ
189
(13)
r = r1 r = rw
r = r2
(9)
where DH2 S;m and CH2 S;m are interpreted as the diffusion coefficient and concentration of H2S inside the porous ePTFE membrane. Tortuosity (t) and porosity (ε) of membrane are two principal operational parameters which affect the diffusion coefficient of H2S
Fig. 3. A diagrammatic scheme of membrane thickness under partial wetting mode.
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Table 3 Boundary conditions under non-wetting mode of operation. Boundary Tube compartment
Membrane compartment Shell compartment
z ¼0
Insulated
vCH2 S;s vr
Insulated
CH2 S;s ¼ Cinitial
z ¼L r ¼0 r ¼ r1
CH2 S;t ¼ 0 Csolvents;t ¼ Cinitial
¼0
3.5. Numerical solution
vCH2 S;t vr
¼0 CH2 S;t ¼ mH2 S CH2 S;m C H2 S;m ¼
r ¼ r2 r ¼ r3
CH2 S;t mH2 S
CH2 S;m ¼ CH2 S;s
CH2 S;s ¼ CH2 S;m vCH2 S;s vr
¼0
3.4. Governing equations inside the tube compartment Eq. (14) is the steady state continuity equation for H2S and MEA transport through the tube compartment of porous ePTFE HFMC. This governing equation is derived due to diffusion, reaction and convection mechanisms as follows [10]:
Di;t
" # v2 Ci;t 1 vCi;t v2 Ci;t vCi;t þ Ri þ ¼ Vz;t r vr vz vr 2 vz2
(14)
where “i” shows H2S acid gas and MEA liquid absorbent. The velocity distribution in the tube compartment of membrane contactor is assumed to follow Newtonian laminar flow and as follows [35]:
"
Vz;t
membrane and shell compartments of membrane contactor considering non-wetting, partial wetting and complete wetting modes of operation are listed in Tables 3e5, respectively.
2 # r ¼ 2V 1 r1
(15)
In Eq. (15), V is s the average axial velocity of gas through the tube compartment. The boundary conditions of tube, porous
This paper aims to make a two dimensional mechanistic model to predict the removal efficiency of H2S in a porous ePTFE HFMC under non-wetting, partial wetting and complete wetting conditions. The feed gas contains 98% CH4 and 2% H2S and MEA with pH of 12.5 is applied as a liquid absorbent. In pursuit of this goal, the governing equations of the mechanistic model along with their suitable boundary conditions were numerically solved using COMSOL software via FEM. By comparing several numerical solvers, the UMFPACK solver was selected due to its ability in solving nonstiff and stiff non-linear boundary problems [27]. The convergence status of numerical simulation is plotted in Fig. 4. Fig. 4 indicates that the system is considered as a non-stiff system which provides confidence and stability in the solution process. As can be seen from the figure, in the first iteration, the convergence error is 100 but after the 9th iteration, convergence error decreases to 109 which implies the excellent convergence of system (see Fig. 5). 3.6. Mesh independence As can be seen in Fig. 6, mapped meshing is applied in order to study and evaluate the simulation results using finite element analysis. The effect of mesh number on H2S concentration in the outlet of shell compartment under non-wetting mode of operation is illustrated in Fig. 6. Although increase in the mesh number leads in higher accuracy of calculations, it can dramatically improve the duration of calculation.
Table 4 Boundary conditions under partial wetting mode of operation. Boundary
Tube compartment
Gas filled side of membrane
Liquid filled side of membrane
Shell compartment
z ¼0
CH2 S;t ¼ 0 Csolvents;t ¼ Cinitial
Insulated
Insulated
vCH2 S;s vr
Insulated
Insulated
CH2 S;s ¼ Cinitial
z ¼L r ¼0 r ¼ r1
vCH2 S;t vr
¼0 CH2 S;t ¼ mH2 S CH2 S;m
¼0
CH2 S;lm ¼ CH2 S;t
vCs;t vr
¼0 (s ¼ other species than H2S) r ¼ rw
CH2 S;gm ¼
r ¼ r2 r ¼ r3
CH2 S;lm ¼ mH2 S CH2 S;gm
CH2 S;lm mH2 S
vC
s;m vr ¼ 0 (s ¼ other species than H2S)
CH2 S;gm ¼ CH2 S;s
CH2 S;s ¼ CH2 S;m vCH2 S;s vr
¼0
Table 5 Boundary conditions under complete wetting mode of operation. Boundary
Tube compartment
Liquid filled side of membrane
Shell compartment
z ¼0
CH2 S;t ¼ 0 Csolvents;t ¼ Cinitial
Insulated
vCH2 S;s vr
Insulated
CH2 S;s ¼ Cinitial
z ¼L r ¼0 r ¼ r1
vCH2 S;t vr
¼0 CH2 S;t ¼ mH2 S CH2 S;m
¼0
CH2 S;lm ¼ CH2 S;t
vCs;t vr
¼0 (s ¼ other species than H2S) r ¼ r2 ¼ rw
CH2 S;lm ¼ mH2 S CH2 S;m
CH2 S;s ¼ CH2 S;m
vCs;m vr
¼0 (s ¼ other species than H2S) r ¼ r3
vCH2 S;s vr
¼0
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Fig. 4. Convergence status of numerical simulation.
191
Fig. 6. The influence of mesh number on H2S concentration in the outlet of shell compartment.
Fig. 5. Mapped meshing for the simulation of H2S removal from gaseous mixture in HFMC.
It is understood from the figure that increment in the number of mesh causes a better convergence in the numerical simulation results but after the 260th mesh, no significant change in the concentration of hydrogen sulfide is seen which implies the result convergence of the numerical simulation. Hence, the accuracy of calculations is independent of the number of mesh for values more than 260. 4. Results and discussion 4.1. Model validation The simulation outcomes for the sequestration of H2S from H2S/ CH4 gaseous mixture using MEA considering non-wetting mode of operation were compared with the experimental data presented by Marzouk et al. [29] with the purpose of validating mechanistic model predictions proposed in this study. Fig. 7 demonstrates that there is a rational agreement between the results of two dimensional simulation and experimental data with an average absolute relative error (AARE) less than 5%.
Fig. 7. Model validation for the flux of hydrogen sulfide as a function of feed gas pressure under non-wetting mode of operation using MEA absorbent solvent, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1. Experimental data were provided by Marzouk et al. [29].
4.2. Concentration gradient of H2S in the shell compartment of HFMC under non-wetting, partial wetting and complete wetting modes Fig. 8 shows the mechanistic model simulation for concentration gradient of H2S inside the shell compartment of HFMC considering different membrane pores wettability (0, 50 and 100%). On the basis of the counter-current arrangement of gaseliquid flows, gaseous mixture containing 2% H2S and 98% CH4 moves inside the shell compartment of porous ePTFE membrane module and MEA absorbent flows inside the tube compartment of HFMC. By streaming H2S/CH4 gaseous mixture inside the shell compartment of membrane contactor, this mixture moves toward the membrane segment because of concentration gradient and is
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membrane contactor considering non-wetting mode is 26.1 mol m3 while the minimum concentrations of H2S under 50% partial wetting and complete wetting modes of operation are 28.6 and 30.4 mol m3, respectively, which confirms the negative influence of membrane pores wetting on the sequestration efficiency of H2S from H2S/CH4 gaseous flow.
Fig. 8. Concentration gradient of H2S inside the shell compartment of membrane contactor using MEA liquid absorbent under a) non-wetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.2 K, P ¼ 100 kPa.
absorbed by moving liquid absorbing MEA. It is worth noting that inside the tube compartment of porous ePTFE membrane contactor, only diffusional mass transfer is the main cause of H2S transport. However, the principal mechanisms of H2S transport through the shell compartment of membrane module are both axial convection and radial diffusion. According to the figure, the minimum concentration of H2S pollutant inside the shell compartment of
Fig. 9. Effect of module length on H2S removal under a) non-wetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 1 bar.
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4.3. Influence of module length on H2S removal considering nonwetting, partial wetting and complete wetting modes The sequestration efficiency of H2S from H2S/CH4 gaseous flow as a function of module length using MEA absorbent under non-wetting, partial wetting, and complete wetting modes is illustrated in Fig. 9. Eq. (16) is derived to evaluate the removal performance of H2S from H2S/CH4 gaseous flow as follows [10]:
%Removal ¼ 100
Coutlet ðyCÞinlet ðyCÞoutlet ¼ 100 Cinlet ðyCÞinlet
(16)
Increase in the module length results in a significant increment in the sequestration performance of H2S from H2S/CH4 gaseous flow. This increase is due to improving the gas/liquid contact area and consequently residence time. Liquid penetration into the pores of membrane which is defined as membrane wettability deteriorates the sequestration efficiency of H2S from gaseous stream. This deterioration is due to increasing mass transfer resistance which causes decrement in the effective diffusivity of H2S inside the membrane compartment. Consequently, by decreasing the diffusivity of H2S inside the membrane compartment, the sequestration efficiency of H2S in comparison with non-wetting mode declines dramatically. 4.4. Influence of H2S initial concentration on H2S removal considering non-wetting, partial wetting and complete wetting modes The removal percentage of H2S as a function of H2S initial concentration using MEA absorbent under non-wetting, partial wetting, and complete wetting modes of operation is plotted in Fig. 10. As expected, increment of H2S concentration enhances the availability of H2S contaminant in the membrane module. Increase of H2S pollutant in H2S/CH4 gaseous flow (assuming constant absorbent concentration) increases the amount of un-absorbed H2S gas in the membrane module which leads in a considerable decrement in the removal efficiency of H2S. The wettability of ePTFE porous membrane negatively affects the capture rate of H2S from H2S/CH4 gaseous mixture due to enhancing mass transfer resistance and effective diffusion coefficient of H2S pollutant inside the membrane portion of HFMC. 4.5. Influence of membrane porosity on H2S removal considering non-wetting, partial wetting and complete wetting modes The removal of H2S as a function of membrane porosity applying MEA absorbent under non-wetting, partial wetting, and complete wetting modes of operation is illustrated in Fig. 11. Eq. (17) implies the direct relation of H2S effective diffusion coefficient inside the membrane compartment (DH2S-mem) with porosity of membrane. Whenever the membrane porosity increases, diffusion coefficient of hydrogen sulfide in the membrane compartment improves substantially. Consequently, increase of the diffusion coefficient of H2S encourages the mass transfer of H2S contaminant inside the membrane compartment which positively affects the sequestration efficiency of H2S from gaseous flow [10].
DCO2 mem ¼ DCO2 Shell
ε
t
(17)
As expected, non-wetting mode of operation can be regarded as the most ideal condition for maximizing the removal percentage of H2S gas from H2S/CH4 gaseous stream. This is due to the minimum mass transfer resistance in the non-wetting condition.
Fig. 10. Effect of H2S initial concentration on H2S acid gas removal under a) nonwetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 100 kPa.
4.6. Influence of membrane tortuosity on H2S removal considering non-wetting, partial wetting and complete wetting modes The removal performance of H2S as a function of membrane tortuosity applying MEA liquid absorbent under non-wetting, partial wetting, and complete wetting modes of operation is illustrated in Fig. 12. As expected, whenever the membrane tortuosity
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Fig. 12. Effect of membrane tortuosity on H2S acid gas removal under a) non-wetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 100 kPa.
Fig. 11. Effect of membrane porosity on H2S acid gas removal under a) non-wetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 100 kPa.
increases, it increases the total mass transfer resistance of H2S which causes a considerable deterioration in the diffusion coefficient of hydrogen sulfide inside the membrane compartment (DH2Smem). As a result, decrease in the sequestration percentage of H2S from H2S/CH4 gaseous mixture occurs by decreasing the diffusion coefficient of hydrogen sulfide in the membrane compartment.
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195
4.7. Influence of MEA concentration on H2S removal considering non-wetting, partial wetting and complete wetting modes The removal percentage of H2S as a function of MEA initial concentration under non-wetting, partial wetting, and complete wetting modes of operation is demonstrated in Fig. 13. Increase of MEA initial concentration causes a significant increase in the active absorption of H2S at the boundary layer which positively affects the removal efficiency of H2S from gaseous flow. The evaluation of H2S sequestration efficiency under various wettability of membrane pores again confirms that the wetting of membrane pores by liquid absorbent has a detrimental influence on the removal of H2S due to creating additional mass transfer resistance along the way of H2S acid gas transport inside the HFMC. 5. Conclusions The evaluation of H2S removal from a gaseous mixture containing 98% CH4 and 2% H2S considering different percentages of membrane pores wetting (0%, 50% and 100%) using MEA absorbent agent in the porous ePTFE HFMC is the main novelty of this paper. For this reason, a mechanistic modelling and a two dimensional simulation are developed with the aim of validating the experimental data. Comparison of model predictions with the experimental data illustrates a reasonable agreement with AARE less than 5%. A counter-current arrangement of liquid and gas flows inside the tube and shell compartments of HFMC is implemented in the numerical simulation. Non-wetting mode of operation exhibits the highest H2S sequestration due to the minimum mass transfer resistance in the membrane compartment. The membrane pore wetting significantly deteriorates the removal of H2S contaminant due to increasing mass transfer resistance along the way of H2S transport inside the membrane compartment. It is understood from the simulation results that the removal efficiency of H2S decreases significantly as initial concentration of H2S and membrane tortuosity increase. However, increase in the module length, membrane porosity and MEA concentration positively enhances the sequestration efficiency of H2S from H2S/CH4 gaseous flow. Nomenclature r1 r2 r3 rw L DH2 Sshell DH2 Smem DH2 SMEA
Fig. 13. Effect of MEA initial concentration on H2S acid gas removal under a) nonwetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 1 bar.
mH2 SMEA n P CH2 S;0 Vl Vg T V V Keq ε
Inner radius of tube, m Outer radius of tube, m Radius of Inner shell, m Wetting segment of the membrane, m Module length, m Diffusion coefficient of H2S inside the shell compartment, m2 s1 Diffusion coefficient of H2S inside the membrane compartment, m2 s1 Diffusion coefficient of H2S in MEA inside the tube compartment, m2 s1 H2S solubility in MEA, dimensionless Number of fibers, dimensionless pressure, Pa Initial H2S concentration in the gas phase, mol m3 Liquid velocity, m s1 Gas velocity, m s1 Gas velocity, K Average axial velocity of the liquid inside the shell region, m s1 Average axial velocity of gas inside the tube region, m s1 Reaction rate constant of MEA, dimensionless Membrane porosity, dimensionless
196
4
t
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Volume fraction of the void inside the HFMC, dimensionless Membrane tortuosity, dimensionless
References [1] Mavroudi M, Kaldis SP, Sakellaropoulos GP. Reduction of CO2 emissions by a membrane contacting process. Fuel 2003;82:2153e9. [2] Lee EY, Cho KS, Ryu HW. Simultaneous removal of H2S and NH3 in biofilter inoculated with Acidithiobacillus thiooxidans TAS. J Biosci Bioeng 2005;99: 611e5. [3] Lu JG, Zheng YF, He DL. Selective absorption of H2S from gas mixtures into aqueous solutions of blended amines of methyldiethanolamine and 2tertiarybutylamino-2-ethoxyethanol in a packed column. Separ Purif Technol 2006;52:209e17. [4] Wang D, Teo WK, Li K. Removal of H2S to ultra-low concentrations using an asymmetric hollow fibre membrane module. Separ Purif Technol 2002;27: 33e40. [5] Li JL, Chen BH. Review of CO2 absorption using chemical solvents in hollow fiber membrane contactors. Separ Purif Technol 2005;41:109e22. [6] Matson SL, Lopez J, Quinn JA. Separation of gases with synthetic membranes. Chem Eng Sci 1983;38:503e24. [7] deMontigny D, Tontiwachwuthikul P, Chakma A. Comparing the absorption performance of packed columns and membrane contactors. Ind Eng Chem Res 2005;44:5726e32. [8] Atchariyawut S, Jiraratananon R, Wang R. Separation of CO2 from CH4 by using gas-liquid membrane contacting process. J Membr Sci 2007;304:163e72. [9] Dindore VY, Brilman DWF, Versteeg GE. Modelling of cross-flow membrane contactors: mass transfer with chemical reactions. J Membr Sci 2005;255: 275e89. [10] Faiz R, Al-Marzouqi M. Mathematical modeling for the simultaneous absorption of CO2 and H2S using MEA in hollow fiber membrane contactors. J Membr Sci 2009;342:269e78. [11] Nakhjiri AT, Heydarinasab A, Bakhtiari O, Mohammadi T. Modeling and simulation of CO2 separation from CO2/CH4 gaseous mixture using potassium glycinate, potassium argininate and sodium hydroxide liquid absorbents in the hollow fiber membrane contactor. J Environ Chem Eng 2018;6:1500e11. [12] Fasihi M, Shirazian S, Marjani A, Rezakazemi M. Computational fluid dynamics simulation of transport phenomena in ceramic membranes for SO2 separation. Math Comput Model 2012;56:278e86. [13] Qi Z, Cussler EL. Microporous hollow fibers for gas-absorption: I. Mass transfer in the liquid. J Membr Sci 1985;23:321e32. [14] Prasad R, Sirkar KK. Dispersion-free solvent extraction with microporous hollow-fiber modules. AIChE J 1988;34:177e88. [15] Zhang Y, Sunarso J, Liu S, Wang R. Current status and development of membranes for CO2/CH4 separation: a review. Int J Greenh Gas Control 2013;12: 84e107. [16] Yeo ZY, Chew TL, Zhu PW, Mohamed AR, Chai SP. Conventional processes and membrane technology for carbon dioxide removal from natural gas: a review. J Nat Gas Chem 2012;21:282e98. [17] Ghadiri M, Shirazian S. Computational simulation of mass transfer in extraction of alkali metals by means of nanoporous membrane extractors. Chem Eng Process 2013;69:57e62. [18] Marjani A, Shirazian S. Mathematical modeling and CFD simulation of hydrocarbon purification using membrane technology. Orient J Chem 2012;28: 123e9.
[19] Ghadiri M, Marjani A, Shirazian S. Mathematical modeling and simulation of CO2 stripping from monoethanolamine solution using nano porous membrane contactors. Int J Greenh Gas Control 2013;13:1e8. [20] Karoor S, Sirkar KK. Gas absorption studies in microporous hollow fiber membrane modules. Ind Eng Chem Res 1993;32:674e84. [21] Gabelman A, Hwang ST. Hollow fiber membrane contactors. J Membr Sci 1999;159:61e106. [22] Li K, Wang D, Koe CC, Teo WK. Use of asymmetric hollow fibre modules for elimination of H2S from gas streams via a membrane absorption method. Chem Eng Sci 1998;53:1111e9. [23] Boucif N, Favre E, Roizard D, Belloul M. Hollow fiber membrane contactor for hydrogen sulfide odor control. AIChE J 2008;54:122e31. [24] Sheikholeslami M, Lohrasbi S, Ganji DD. Numerical analysis of discharging process acceleration in LHTESS by immersing innovative fin configuration using finite element method. Appl Therm Eng 2016;107:154e66. [25] Lohrasbi S, Gorji-Bandpy M, Ganji DD. Thermal penetration depth enhancement in latent heat thermal energy storage system in the presence of heat pipe based on both charging and discharging processes. Energy Convers Manag 2017;148:646e67. [26] Lohrasbi S, Miry SZ, Gorji-Bandpy M, Ganji DD. Performance enhancement of finned heat pipe assisted latent heat thermal energy storage system in the presence of nano-enhanced H2O as phase change material. Int J Hydrogen Energy 2017;42:6526e46. [27] Shirazian S, Moghadassi A, Moradi S. Numerical simulation of mass transfer in gas-liquid hollow fiber membrane contactors for laminar flow conditions. Simulat Model Pract Theor 2009;17:708e18. [28] Eslami S, Mousavi SM, Danesh S, Banazadeh H. Modeling and simulation of CO2 removal from power plant flue gas by PG solution in a hollow fiber membrane contactor. Adv Eng Software 2011;42:612e20. [29] Marzouk SAM, Al-Marzouqi MH, Abdullatif N, Ismail ZM. Removal of percentile level of H2S from pressurized H2S-CH4 gas mixture using hollow fiber membrane contactors and absorption solvents. J Membr Sci 2010;360: 436e41. [30] Cornelissen AE. Simulation of absorption of H2S and CO2 into aqueous alkanolamines in tray and packed-columns. T I Chem Eng Lond 1980;58: 242e50. [31] Haimour N, Sandall OC. Selective removal of hydrogen-sulfide from gases containing hydrogen-sulfide and carbon-dioxide using diethanolamine. Separ Sci Technol 1983;18:1221e49. [32] Hamour N, Bidarian A, Sandall OC. Simultaneous absorption of H2S and CO2 into aqueous methyldiethanolamine. Separ Sci Technol 1987;22:921e47. [33] Happel J. Viscous flow relative to arrays of cylinders. AIChE J 1959;5:174e7. [34] Keshavarz R, Fathikalajahi J, Ayatollahi S. Analysis of CO2 separation and simulation of a partially wetted hollow fiber membrane contactor. J Hazard Mater 2008;152:1237e47. [35] Bird RB, Stewart WE, Lightfoot EN. Transport Phenomena. New York: John Wiley & Sons; 1960. [36] Cussler EL. Diffusion: Mass Transfer in Fluid Systems. 3rd ed. Cambridge, UK: Cambridge University Press; 2009. [37] Ghasem N, Al-Marzouqi M. Modeling and experimental study of carbon dioxide absorption in a flat sheet membrane contactor. J Membr Sci Res 2017;3: 57e63. [38] Wilke CR, Chang P. Correlation of diffusion coefficients in dilute solutions. AIChE J 1955;1:264e70. [39] Paul S, Ghoshal AK, Mandal B. Removal of CO2 by single and blended aqueous alkanolamine solvents in hollow-fiber membrane contactor: modeling and simulation. Ind Eng Chem Res 2007;46:2576e88.
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Sustainable Environment Research journal homepage: www.journals.elsevier.com/sustainableenvironment-research/
Original Research Article
Influence of non-wetting, partial wetting and complete wetting modes of operation on hydrogen sulfide removal utilizing monoethanolamine absorbent in hollow fiber membrane contactor Ali Taghvaie Nakhjiri a, Amir Heydarinasab a, *, Omid Bakhtiari b, Toraj Mohammadi c a b c
Department of Chemical Engineering, Science and Research Branch of Islamic Azad University, Tehran 1477893855, Iran Membrane Research Center, Razi University, Kermanshah 6714414971, Iran Research Center for Membrane Separation Processes, Iran University of Science and Technology, Tehran 1684613114, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 December 2017 Received in revised form 10 January 2018 Accepted 28 February 2018 Available online 7 March 2018
Hydrogen sulfide is a highly poisonous acidic gas which is regarded as one of the major causes of corrosion and odorous problems. For this reason, the efficient separation of hydrogen sulfide from various gaseous streams is mandatory. This paper aims to evaluate the removal of hydrogen sulfide pollutant from hydrogen sulfide/methane gaseous flow using monoethanolamine liquid absorbent inside the hollow fiber membrane contactor. As the novelty, a mechanistic modelling and a two dimensional numerical simulation are developed under non-wetting (0% wetting of membrane pores), partial wetting (50% wetting of membrane pores) and complete wetting (100% wetting of membrane pores) modes of operation to predict the effects of various operational parameters such as module length, tortuosity and porosity of membrane, initial concentrations of hydrogen sulfide and monoethanolamine on the removal of hydrogen sulfide. The results of mechanistic model prediction under non-wetting mode are in an excellent agreement with the experimental data with average absolute relative error less than 5%. On the basis of the results, monoethanolamine provides the highest hydrogen sulfide sequestration while being used under non-wetting mode of operation in comparison with partial wetting and complete wetting conditions. Membrane pore wettability has a detrimental influence on the removal efficiency of hydrogen sulfide due to creating a considerable mass transfer resistance along the way of hydrogen sulfide gas transport inside the membrane contactor. © 2018 Chinese Institute of Environmental Engineering, Taiwan. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Keywords: Hydrogen sulfide sequestration Gaseous stream Model prediction Wettability
1. Introduction Industrial gaseous mixture often consists of various contaminants such as hydrogen sulfide and carbon dioxide which must be removed in order to decrease their environmental and operational detriments such as the greenhouse phenomenon and corrosion [1]. The numerous treatment techniques such as biofilters and packed beds columns can be used to remove toxic H2S from gaseous streams [2,3]. Due to the existence of various limitations in conventional technologies such as packed towers and mist scrubbers,
* Corresponding author. E-mail address: [email protected] (A. Heydarinasab). Peer review under responsibility of Chinese Institute of Environmental Engineering.
hollow fiber membrane contactor (HFMC) was introduced as a promising technology to eliminate the disadvantages associated with conventional techniques and increase the efficiency of acid gas removal [4,5]. Matson et al. expressed that the surface contact area which is created by a HFMC may attain up to 8000 m2 m3 while in conventional absorption towers, the surface contact area is ten times lower than membrane contactors [6]. The application of HFMC as a promising technology was the focal point of numerous investigators for efficient sequestration of various gases [7e10]. Apart from the existence of nondispersive contact between gas and liquid flows, this novel technique enjoys an easier and larger interfacial contact area and also easy linear scale up [7]. There are a number of studies with the aim of presenting mathematical modelling and numerical simulations of different gas separation processes inside the HFMCs [10e12]. The principal purpose of the modelling and simulation papers was to determine the
https://doi.org/10.1016/j.serj.2018.02.003 2468-2039/© 2018 Chinese Institute of Environmental Engineering, Taiwan. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
A.T. Nakhjiri et al. / Sustainable Environment Research 28 (2018) 186e196
concentration of acid gases such as CO2 and SO2 inside the membrane contactor [10,12]. However, the existence of a membrane segment may append another mass transfer resistance that is not present in the absorption columns. This resistance significantly deteriorates the overall mass transfer and subsequently decreases the selectivity. The minimization of the membrane resistance can be implemented by decreasing the thickness of membrane or by enhancing its inherent gas permeability [12e14]. Microporous membranes such as polyimide, polysulfone, polypropylene and other dense polymeric membranes such as polydimethylsiloxane (silicone rubber) can be used to provide high gaseliquid contact area. The micro-porous membranes have higher permeability and dense membranes are more appropriate where a special selectivity is desired [15e20]. The utilization of HFMC for the separation of acid pollutants has been recently reviewed [5,21]. Due to high toxicity of H2S, few investigations reported the data of H2S removal from gaseous mixtures applying HFMCs [21e23]. In the recent years, the use of finite element method (FEM) to evaluate the sequestration efficiency of contaminants such as CO2, H2S and SO2 from gaseous flows and also simulation of governing equations such as momentum, mass and heat transfer have been developed by several investigators with good results in comparison with experimental results [17,24e26]. For instance, Shirazian et al. confirmed that UMFPACK is regarded as an appropriate numerical solver due to its ability of solving non-stiff and stiff non-linear boundary value problems [27]. Additionally, numerical simulation of CO2 separation from different gaseous streams was performed by numerous authors using COMSOL software with excellent results in comparison with experimental researches [18,28]. Based on the abovementioned characteristics, COMSOL was used in this paper to develop a numerical simulation of H2S removal from a gaseous mixture. The principal motivation of this investigation is to describe the removal efficiency of H2S from a gaseous mixture consisting of 98% CH4 and 2% H2S using monoethanolamine (MEA) liquid solvent and expanded polytetrafluoroethylene (ePTFE) membrane in a HFMC. For this reason, a mechanistic modelling and a numerical simulation which consider the governing mechanisms (diffusion and convection) in both axial and radial coordinates were developed and solved. As the novelty, effect of membrane wettability in different percentages (0%, 50% and 100%) is investigated to evaluate the removal performance of H2S from gaseous mixture. The effects of important operational parameters such as tortuosity and porosity of the membrane, initial concentrations of H2S and MEA absorbent and module length on the sequestration efficiency of H2S from gaseous mixtures in different percentages of membrane pores wetting are compared with each other. Finally, to validate the results of mechanistic modelling and numerical simulation, the model predictions are compared with the experimental data performed by Marzouk et al. [29].
2. Materials and methods The reaction mechanism of H2S with MEA is defined by Cornelissen as follows [30]: þ
MEA þ H2 S4MEAH þ HS
(1)
MEA þ HS 4MEAHþ þ S2
(2)
However, he perceived that reaction (2) is negligible from the ionization constant of HS and the amines. Numerous authors emphasized that the reaction of different amines with H2S is fast [30e32]. The reaction rate equations for H2S-MEA system are presented by Eq. (3). as follows [10]:
187
! HS MEAþ ½H2 S½MEA Keq
RMEA ¼ RH2 S ¼ kH2 SMEA
(3)
where, Keq denotes the equilibrium constant of H2S-MEA system. 3. Mechanistic model development The present paper aims to perform a mechanistic modelling and a two dimensional simulation for describing the transport of H2S inside membrane contactor applying MEA absorbent. The mechanistic model is generally flexible to consider for all non-wetting, partial wetting and complete wetting modes. To achieve the best removal performance of H2S from gaseous stream, non-wetting mode of operation is more appropriate in comparison with the other conditions. A HFMC contactor contains three main compartments: shell section, membrane side and tube segment. Liquid absorbent MEA flows in the tube compartment of HFMC (at z ¼ 0) while in a counter-current formation, gaseous stream consisting of 98% CH4 and 2% H2S is fed through the shell section (at z ¼ L). The scheme of a HFMC considering non-wetting, partial wetting and complete wetting modes of operation is plotted in Fig. 1. Free surface model of Happel denotes that only part of the gas surrounding the fiber of expanded ePTFE membrane contactor is considered which may be predicted as circular cross section [33]. The cross sectional region along with the circular approximation of ePTFE membrane contactor is plotted in Fig. 2. The development of the mechanistic model was based on the major assumptions include: 1) isothermal and steady state modes; 2) similar pore size distribution; 3) similar membrane wall thickness; 4) ideal gas behaviour inside the shell compartment; 5) use of Henry's law in the interface of liquid and gas flows; 6) existence of Newtonian liquid absorbent in the tube compartment; 7) counter-current formation of liquid and gas streams; 8) Laminar and parabolic velocity profile inside the fibers; and 9) utilization of Happel's free surface model with the goal of anticipating the velocity profile of components in the liquid phase. The specifications of hollow fiber module and operating conditions used for modelling and numerical simulation are listed in Table 1. Also, physicochemical properties of H2S and MEA are presented in Table 2. 3.1. Governing equations inside the shell compartment of membrane contactor Eq. (4) describes the continuity equation inside the shell compartment of membrane contactor as follows [10]:
vCi ¼ VNi þ Ri vt
(4)
In this equation Ci, Ri and Ni are respectively defined as concentration, rate of reaction and flux of species i along the membrane length. The flux of species i is defined by Fick's law of diffusion as follows [10]:
Ni ¼ Di VCi þ Ci VZ
(5)
where Vz and Di are velocity of axial direction and diffusion coefficient of components i along the length of membrane contactor, respectively. The steady state continuity equation in the shell compartment of porous ePTFE HFMC can be derived by combining Eqs. (4) and (5) as follows [10]:
" # v2 CH2 S;s 1 vCH2 S;s v2 CH2 S;s vCH2 S;s þ DH2 S;s þ ¼ Vz;s r vr vz vr 2 vz2
(6)
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CH4/H2S gaseous stream
ePTFE Porous membrane
Liquid Absorbent MEA
a)
b)
r=0
r=0
r = r1
r = r1
r = r2
ar = rw
r = rw
r = r3
r = r2
r = r3
r = r1 r = r3
r = r2 = rw
c) r=0 r = r1 r = r2 = rw
r = r3
Fig. 1. Diagrammatic scheme of a hollow fiber membrane contactor under a) non-wetting b) partial wetting (50%) and c) complete wetting modes of operation.
In this equation, DH2 S;s and CH2 S;s are denoted as the diffusion coefficient and concentration of H2S in the shell compartment of membrane contactor. The velocity profile in the shell compartment (Vz;s ) can be correlated using Happel's free surface model as follows [10]: Fig. 2. Cross sectional region and circular approximation of the porous ePTFE membrane contactor.
A.T. Nakhjiri et al. / Sustainable Environment Research 28 (2018) 186e196 Table 1 The specifications of hollow fiber membrane module and operating conditions used for modelling and numerical simulation. Parameter
Value
Membrane type Fiber inner diameter (m) Fiber outer diameter (m) Module inner diameter (m) Porosity (ε) Tortuosity (t) Module length (m) Number of fibers T (K) Gas flow rate (Qg) (mL min1) Liquid flow rate (Ql) (mL min1) Rg (J mol1 K1)
ePTFE 1 103 2 103 20.5 103 0.18 10 0.75 50 298.15 1000 25 8.314
Value
DH2 S;S ðm2 s1 Þ
2.01 105
[36]
DH2 S;S ðε=tÞ
[37]
1.52 109
Calculated from Ref. [38]
9.32 1010
[39]
3.6 1012
[30]
DMEA;t
kMEA ðm3 kmol1 h1 Þ Keq mH2 S
295 2.515
Reference
"
[30] [9]
# 2 # " r ðr=r3 Þ2 ðr2 =r3 Þ2 þ 2lnðr2 =rÞ ¼ 2V 1 2 r3 3 þ ðr2 =r3 Þ4 4ðr2 =r3 Þ2 þ 4lnðr2 =r3 Þ (7)
In this equation, V is the average velocity inside the shell compartment of membrane module. Also as can be apparently seen in Fig. 1, r2 is denoted as the outer radius of tube. r3 is defined as the effective radius of each shell which is calculated considering the active area and hexagonal shape of shell unit around each fiber as follows [10]:
r3 ¼ r2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1=ð1 4Þ
(8)
In Eq. (8), 4 is the volume fraction of void.
v2 Ci;gm vr 2
þ
# 1 vCi;gm v2 Ci;gm þ ¼0 r vr vz2
Eq. (9) shows the steady state material balance equation which defines the transport of gas molecules from gaseous mixture. Due to the existence of non-wetting condition, the governing mechanism of transport through the membrane compartment is molecular diffusion. Besides, no chemical reaction takes place in the gas filled pores [10].
(11)
3.3.2. Material balance inside the liquid filled compartment The left side of Fig. 3 demonstrates the liquid filled section of porous ePTFE membrane where diffusion and reaction are the governing mechanisms describing the transport of H2S along with MEA liquid absorbent. Hence, the material balance under the steady state mode can be derived as follows [10]:
" Di;lm
v2 Ci;lm vr 2
þ
# 1 vCi;lm v2 Ci;lm þ þ Ri;m ¼ 0 r vr vz2
(12)
Due to the use of porous membrane (ePTFE), those chemical reactions taking place inside the membrane compartment only occur inside the pores of membrane. Therefore, the reaction inside the liquid section of porous ePTFE membrane is as follows [10,34]:
Ri;m ¼ Ri ε
3.2. Governing equations inside the membrane compartment under non-wetting mode
" # v2 CH2 S;m 1 vCH2 S;m v2 CH2 S;m þ DH2 S;m þ ¼0 r vr vr 2 vz2
(10)
3.3.1. Material balance inside the gas filled compartment The gas filled section of porous ePTFE membrane is depicted in the right side of Fig. 3. In the gas filled portion of membrane, the only governing mechanism which describes the transport of H2S is diffusion. Hence, the steady state material balance is presented as follows [10]:
Di;gm
"
Vz;s
t
It is illustrated in Fig. 3 that due to substantial decrease in the surface tension, a percentage or even all of the membrane pores can be wetted by liquid absorbent. Therefore, in order to derive the steady state continuity equation for the transport of H2S inside the membrane compartment along with MEA liquid absorbent, two sections (the gas filled and the liquid filled) are considered as follows:
Parameter
ðm2 s1 Þ
DH2 S;s ε
3.3. Governing equation inside the membrane compartment considering membrane pore wetting
Table 2 Physicochemical properties of H2S and MEA used for mechanistic modelling and simulation.
DH2 S;MEA ðm2 s1 Þ
in the membrane compartment. Therefore, diffusion coefficient of H2S in the membrane compartment is presented as follows [10]:
DH2 S;m ¼
All data are achieved from Ref. [29].
DH2 S;m ðm2 s1 Þ
189
(13)
r = r1 r = rw
r = r2
(9)
where DH2 S;m and CH2 S;m are interpreted as the diffusion coefficient and concentration of H2S inside the porous ePTFE membrane. Tortuosity (t) and porosity (ε) of membrane are two principal operational parameters which affect the diffusion coefficient of H2S
Fig. 3. A diagrammatic scheme of membrane thickness under partial wetting mode.
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Table 3 Boundary conditions under non-wetting mode of operation. Boundary Tube compartment
Membrane compartment Shell compartment
z ¼0
Insulated
vCH2 S;s vr
Insulated
CH2 S;s ¼ Cinitial
z ¼L r ¼0 r ¼ r1
CH2 S;t ¼ 0 Csolvents;t ¼ Cinitial
¼0
3.5. Numerical solution
vCH2 S;t vr
¼0 CH2 S;t ¼ mH2 S CH2 S;m C H2 S;m ¼
r ¼ r2 r ¼ r3
CH2 S;t mH2 S
CH2 S;m ¼ CH2 S;s
CH2 S;s ¼ CH2 S;m vCH2 S;s vr
¼0
3.4. Governing equations inside the tube compartment Eq. (14) is the steady state continuity equation for H2S and MEA transport through the tube compartment of porous ePTFE HFMC. This governing equation is derived due to diffusion, reaction and convection mechanisms as follows [10]:
Di;t
" # v2 Ci;t 1 vCi;t v2 Ci;t vCi;t þ Ri þ ¼ Vz;t r vr vz vr 2 vz2
(14)
where “i” shows H2S acid gas and MEA liquid absorbent. The velocity distribution in the tube compartment of membrane contactor is assumed to follow Newtonian laminar flow and as follows [35]:
"
Vz;t
membrane and shell compartments of membrane contactor considering non-wetting, partial wetting and complete wetting modes of operation are listed in Tables 3e5, respectively.
2 # r ¼ 2V 1 r1
(15)
In Eq. (15), V is s the average axial velocity of gas through the tube compartment. The boundary conditions of tube, porous
This paper aims to make a two dimensional mechanistic model to predict the removal efficiency of H2S in a porous ePTFE HFMC under non-wetting, partial wetting and complete wetting conditions. The feed gas contains 98% CH4 and 2% H2S and MEA with pH of 12.5 is applied as a liquid absorbent. In pursuit of this goal, the governing equations of the mechanistic model along with their suitable boundary conditions were numerically solved using COMSOL software via FEM. By comparing several numerical solvers, the UMFPACK solver was selected due to its ability in solving nonstiff and stiff non-linear boundary problems [27]. The convergence status of numerical simulation is plotted in Fig. 4. Fig. 4 indicates that the system is considered as a non-stiff system which provides confidence and stability in the solution process. As can be seen from the figure, in the first iteration, the convergence error is 100 but after the 9th iteration, convergence error decreases to 109 which implies the excellent convergence of system (see Fig. 5). 3.6. Mesh independence As can be seen in Fig. 6, mapped meshing is applied in order to study and evaluate the simulation results using finite element analysis. The effect of mesh number on H2S concentration in the outlet of shell compartment under non-wetting mode of operation is illustrated in Fig. 6. Although increase in the mesh number leads in higher accuracy of calculations, it can dramatically improve the duration of calculation.
Table 4 Boundary conditions under partial wetting mode of operation. Boundary
Tube compartment
Gas filled side of membrane
Liquid filled side of membrane
Shell compartment
z ¼0
CH2 S;t ¼ 0 Csolvents;t ¼ Cinitial
Insulated
Insulated
vCH2 S;s vr
Insulated
Insulated
CH2 S;s ¼ Cinitial
z ¼L r ¼0 r ¼ r1
vCH2 S;t vr
¼0 CH2 S;t ¼ mH2 S CH2 S;m
¼0
CH2 S;lm ¼ CH2 S;t
vCs;t vr
¼0 (s ¼ other species than H2S) r ¼ rw
CH2 S;gm ¼
r ¼ r2 r ¼ r3
CH2 S;lm ¼ mH2 S CH2 S;gm
CH2 S;lm mH2 S
vC
s;m vr ¼ 0 (s ¼ other species than H2S)
CH2 S;gm ¼ CH2 S;s
CH2 S;s ¼ CH2 S;m vCH2 S;s vr
¼0
Table 5 Boundary conditions under complete wetting mode of operation. Boundary
Tube compartment
Liquid filled side of membrane
Shell compartment
z ¼0
CH2 S;t ¼ 0 Csolvents;t ¼ Cinitial
Insulated
vCH2 S;s vr
Insulated
CH2 S;s ¼ Cinitial
z ¼L r ¼0 r ¼ r1
vCH2 S;t vr
¼0 CH2 S;t ¼ mH2 S CH2 S;m
¼0
CH2 S;lm ¼ CH2 S;t
vCs;t vr
¼0 (s ¼ other species than H2S) r ¼ r2 ¼ rw
CH2 S;lm ¼ mH2 S CH2 S;m
CH2 S;s ¼ CH2 S;m
vCs;m vr
¼0 (s ¼ other species than H2S) r ¼ r3
vCH2 S;s vr
¼0
A.T. Nakhjiri et al. / Sustainable Environment Research 28 (2018) 186e196
Fig. 4. Convergence status of numerical simulation.
191
Fig. 6. The influence of mesh number on H2S concentration in the outlet of shell compartment.
Fig. 5. Mapped meshing for the simulation of H2S removal from gaseous mixture in HFMC.
It is understood from the figure that increment in the number of mesh causes a better convergence in the numerical simulation results but after the 260th mesh, no significant change in the concentration of hydrogen sulfide is seen which implies the result convergence of the numerical simulation. Hence, the accuracy of calculations is independent of the number of mesh for values more than 260. 4. Results and discussion 4.1. Model validation The simulation outcomes for the sequestration of H2S from H2S/ CH4 gaseous mixture using MEA considering non-wetting mode of operation were compared with the experimental data presented by Marzouk et al. [29] with the purpose of validating mechanistic model predictions proposed in this study. Fig. 7 demonstrates that there is a rational agreement between the results of two dimensional simulation and experimental data with an average absolute relative error (AARE) less than 5%.
Fig. 7. Model validation for the flux of hydrogen sulfide as a function of feed gas pressure under non-wetting mode of operation using MEA absorbent solvent, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1. Experimental data were provided by Marzouk et al. [29].
4.2. Concentration gradient of H2S in the shell compartment of HFMC under non-wetting, partial wetting and complete wetting modes Fig. 8 shows the mechanistic model simulation for concentration gradient of H2S inside the shell compartment of HFMC considering different membrane pores wettability (0, 50 and 100%). On the basis of the counter-current arrangement of gaseliquid flows, gaseous mixture containing 2% H2S and 98% CH4 moves inside the shell compartment of porous ePTFE membrane module and MEA absorbent flows inside the tube compartment of HFMC. By streaming H2S/CH4 gaseous mixture inside the shell compartment of membrane contactor, this mixture moves toward the membrane segment because of concentration gradient and is
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membrane contactor considering non-wetting mode is 26.1 mol m3 while the minimum concentrations of H2S under 50% partial wetting and complete wetting modes of operation are 28.6 and 30.4 mol m3, respectively, which confirms the negative influence of membrane pores wetting on the sequestration efficiency of H2S from H2S/CH4 gaseous flow.
Fig. 8. Concentration gradient of H2S inside the shell compartment of membrane contactor using MEA liquid absorbent under a) non-wetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.2 K, P ¼ 100 kPa.
absorbed by moving liquid absorbing MEA. It is worth noting that inside the tube compartment of porous ePTFE membrane contactor, only diffusional mass transfer is the main cause of H2S transport. However, the principal mechanisms of H2S transport through the shell compartment of membrane module are both axial convection and radial diffusion. According to the figure, the minimum concentration of H2S pollutant inside the shell compartment of
Fig. 9. Effect of module length on H2S removal under a) non-wetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 1 bar.
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4.3. Influence of module length on H2S removal considering nonwetting, partial wetting and complete wetting modes The sequestration efficiency of H2S from H2S/CH4 gaseous flow as a function of module length using MEA absorbent under non-wetting, partial wetting, and complete wetting modes is illustrated in Fig. 9. Eq. (16) is derived to evaluate the removal performance of H2S from H2S/CH4 gaseous flow as follows [10]:
%Removal ¼ 100
Coutlet ðyCÞinlet ðyCÞoutlet ¼ 100 Cinlet ðyCÞinlet
(16)
Increase in the module length results in a significant increment in the sequestration performance of H2S from H2S/CH4 gaseous flow. This increase is due to improving the gas/liquid contact area and consequently residence time. Liquid penetration into the pores of membrane which is defined as membrane wettability deteriorates the sequestration efficiency of H2S from gaseous stream. This deterioration is due to increasing mass transfer resistance which causes decrement in the effective diffusivity of H2S inside the membrane compartment. Consequently, by decreasing the diffusivity of H2S inside the membrane compartment, the sequestration efficiency of H2S in comparison with non-wetting mode declines dramatically. 4.4. Influence of H2S initial concentration on H2S removal considering non-wetting, partial wetting and complete wetting modes The removal percentage of H2S as a function of H2S initial concentration using MEA absorbent under non-wetting, partial wetting, and complete wetting modes of operation is plotted in Fig. 10. As expected, increment of H2S concentration enhances the availability of H2S contaminant in the membrane module. Increase of H2S pollutant in H2S/CH4 gaseous flow (assuming constant absorbent concentration) increases the amount of un-absorbed H2S gas in the membrane module which leads in a considerable decrement in the removal efficiency of H2S. The wettability of ePTFE porous membrane negatively affects the capture rate of H2S from H2S/CH4 gaseous mixture due to enhancing mass transfer resistance and effective diffusion coefficient of H2S pollutant inside the membrane portion of HFMC. 4.5. Influence of membrane porosity on H2S removal considering non-wetting, partial wetting and complete wetting modes The removal of H2S as a function of membrane porosity applying MEA absorbent under non-wetting, partial wetting, and complete wetting modes of operation is illustrated in Fig. 11. Eq. (17) implies the direct relation of H2S effective diffusion coefficient inside the membrane compartment (DH2S-mem) with porosity of membrane. Whenever the membrane porosity increases, diffusion coefficient of hydrogen sulfide in the membrane compartment improves substantially. Consequently, increase of the diffusion coefficient of H2S encourages the mass transfer of H2S contaminant inside the membrane compartment which positively affects the sequestration efficiency of H2S from gaseous flow [10].
DCO2 mem ¼ DCO2 Shell
ε
t
(17)
As expected, non-wetting mode of operation can be regarded as the most ideal condition for maximizing the removal percentage of H2S gas from H2S/CH4 gaseous stream. This is due to the minimum mass transfer resistance in the non-wetting condition.
Fig. 10. Effect of H2S initial concentration on H2S acid gas removal under a) nonwetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 100 kPa.
4.6. Influence of membrane tortuosity on H2S removal considering non-wetting, partial wetting and complete wetting modes The removal performance of H2S as a function of membrane tortuosity applying MEA liquid absorbent under non-wetting, partial wetting, and complete wetting modes of operation is illustrated in Fig. 12. As expected, whenever the membrane tortuosity
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Fig. 12. Effect of membrane tortuosity on H2S acid gas removal under a) non-wetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 100 kPa.
Fig. 11. Effect of membrane porosity on H2S acid gas removal under a) non-wetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, CMEA ¼ 1 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 100 kPa.
increases, it increases the total mass transfer resistance of H2S which causes a considerable deterioration in the diffusion coefficient of hydrogen sulfide inside the membrane compartment (DH2Smem). As a result, decrease in the sequestration percentage of H2S from H2S/CH4 gaseous mixture occurs by decreasing the diffusion coefficient of hydrogen sulfide in the membrane compartment.
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4.7. Influence of MEA concentration on H2S removal considering non-wetting, partial wetting and complete wetting modes The removal percentage of H2S as a function of MEA initial concentration under non-wetting, partial wetting, and complete wetting modes of operation is demonstrated in Fig. 13. Increase of MEA initial concentration causes a significant increase in the active absorption of H2S at the boundary layer which positively affects the removal efficiency of H2S from gaseous flow. The evaluation of H2S sequestration efficiency under various wettability of membrane pores again confirms that the wetting of membrane pores by liquid absorbent has a detrimental influence on the removal of H2S due to creating additional mass transfer resistance along the way of H2S acid gas transport inside the HFMC. 5. Conclusions The evaluation of H2S removal from a gaseous mixture containing 98% CH4 and 2% H2S considering different percentages of membrane pores wetting (0%, 50% and 100%) using MEA absorbent agent in the porous ePTFE HFMC is the main novelty of this paper. For this reason, a mechanistic modelling and a two dimensional simulation are developed with the aim of validating the experimental data. Comparison of model predictions with the experimental data illustrates a reasonable agreement with AARE less than 5%. A counter-current arrangement of liquid and gas flows inside the tube and shell compartments of HFMC is implemented in the numerical simulation. Non-wetting mode of operation exhibits the highest H2S sequestration due to the minimum mass transfer resistance in the membrane compartment. The membrane pore wetting significantly deteriorates the removal of H2S contaminant due to increasing mass transfer resistance along the way of H2S transport inside the membrane compartment. It is understood from the simulation results that the removal efficiency of H2S decreases significantly as initial concentration of H2S and membrane tortuosity increase. However, increase in the module length, membrane porosity and MEA concentration positively enhances the sequestration efficiency of H2S from H2S/CH4 gaseous flow. Nomenclature r1 r2 r3 rw L DH2 Sshell DH2 Smem DH2 SMEA
Fig. 13. Effect of MEA initial concentration on H2S acid gas removal under a) nonwetting b) partial wetting (50%) and c) complete wetting modes, feed gas ¼ 2/98 H2S/CH4, CH2 S0 ¼ 0.04 M, Qg ¼ 1000 mL min1, Ql ¼ 25 mL min1, T ¼ 298.15 K, P ¼ 1 bar.
mH2 SMEA n P CH2 S;0 Vl Vg T V V Keq ε
Inner radius of tube, m Outer radius of tube, m Radius of Inner shell, m Wetting segment of the membrane, m Module length, m Diffusion coefficient of H2S inside the shell compartment, m2 s1 Diffusion coefficient of H2S inside the membrane compartment, m2 s1 Diffusion coefficient of H2S in MEA inside the tube compartment, m2 s1 H2S solubility in MEA, dimensionless Number of fibers, dimensionless pressure, Pa Initial H2S concentration in the gas phase, mol m3 Liquid velocity, m s1 Gas velocity, m s1 Gas velocity, K Average axial velocity of the liquid inside the shell region, m s1 Average axial velocity of gas inside the tube region, m s1 Reaction rate constant of MEA, dimensionless Membrane porosity, dimensionless
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Volume fraction of the void inside the HFMC, dimensionless Membrane tortuosity, dimensionless
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