2. Waste and Biomass Valorization
1 3
is considered as a better alternative for the treatment of both
ÂH2S and Â
CO2 [10]. Membranes used in a gasâliquid contact-
ing process act as barrier between the gas and the liquid
phases without dispersing one phase into another. In the
membrane contactor, gas diffuses from the gas side across
the membrane and reaches into the gasâliquid interface,
where gas is absorbed in the liquid. Compared with conven-
tional gas absorption devices, the use of membrane contactor
offers a number of advantages such as high surface area per
unit contact volume, ease of adjusting the flowrate of liquid
and gas phases independently, high compactness and easy
to scale up or down [10â12]. Because of the advantages
offered by membrane contactors, numerous studies have
been carried out. Qi and Cussler [13, 14] first used micropo-
rous hollow fiber membranes for gas absorption. Follow-
ing their innovative findings, many others have focused on
gasâliquid membrane contactors for removal of Â
H2S, ÂCO2
and other gases from gas streams [15, 16]. The prolonged
contact of microporous membrane with the liquid phase can
generate detrimental effects on the membrane material due
to swelling or gradual wetting of the hydrophobic membrane
pores, which then may decrease the absorption flux signifi-
cantly [17â19]. Similarly, other researchers also attributed
the performance deterioration of the membrane contactors
due to wetting problem. Wetting significantly affects the
mass transfer coefficients of the membrane module, thus
membrane resistance increases sharply and operation perfor-
mance declines soon [20, 21]. The possibility to ensure non-
wetting conditions for long operational period is of drastic
importance. Hence, to overcome the wetting problem and
avoid any penetration of the liquid into the membrane, a
material which represents a barrier for penetration of liquid
into the membrane is suggested. One possible solution is
to use a nonporous or dense polymeric membrane, which
is highly permeable to Â
H2S and absolutely impermeable to
the liquid permeation. Besides, the nonporous membrane
can enhance selectivity for Â
H2S over Â
CO2 and Â
CH4 since
separation is mediated by diffusion through the polymeric
membrane material [22]. Nevertheless, it was concluded in
a limited number of studies that the nonporous membrane
layer could not provide sufficient mass transfer compared to
microporous membranes [23, 24]. Therefore, a high mem-
brane permeability is necessary to increase the overall mass
transfer capacity of the nonporous membrane contactors. As
a result, reactive liquid absorbent such as quicklime, slaked
lime, sodium hydroxide, or amine solutions are preferably
used to increase the driving force for better absorption rate
and capacity. The high concentrations of Â
CO2 in the biogas
make the Â
H2S removal difficult because the Â
CO2 consumes
the alkalinity in the liquid phase. Due to its low cost and
rapid reaction rate with Â
H2S compared to other gases, a
sodium hydroxide solution was ideally used as an absorbent
liquid. In addition, hydrophobic nonporous membranes like
polydimethylsiloxane (PDMS) is suitable for use in mem-
brane contactors due to its low cost and availability. So
far, many studies investigated the effects of different liquid
absorbents and membrane materials on the performance of
ÂH2S and Â
CO2 absorption. However, studies evaluating the
effects of different operating parameters on the selective Â
H2S
removal performance of membrane contactor was very lim-
ited. In this study, the effects of gas retention time (GRT),
membrane thickness and pH of the liquid absorbent on the
membrane contactor based biogas desulfurization process
were investigated experimentally.
Experimental
Materials
A gas cylinder containing Â
H2S (1%) and Â
CO2 (39%) in
balance of Â
CH4 (60%) was purchased from Hat Industrial
Gases PLC, Kocaeli, Turkey. Sodium hydroxide was pur-
chased from a local supplier (TEKKÄ°M Kimya Sanayi,
Bursa, Turkey) and diluted as required with distilled water
to prepare the stock alkaline solution. All chemicals were
of the highest available purities and used without further
purification. A commercial tubular PDMS (polydimethylsi-
loxane) membrane having 1Â mm thickness and 7Â mm inter-
nal diameter was purchased from EUROFLEX (Germany),
while the PDMS having thickness of 1.5 and 2Â mm with
internal diameter of 7Â mm were purchased from DEUTSCH
& NEUMANN (Germany).
Membrane Contactor Setup
A laboratory scale gasâliquid membrane contactor was
designed and manufactured to perform biogas desulfurization
experiments under different operational conditions. The sche-
matic diagram and the details of the desulfurization setup was
shown in our previous paper [25]. The internal diameter and
height of the glass reactor were 120 and 200Â mm, respectively.
The reactor active working volume and the tubular membrane
length immersed in the liquid were 1.5Â L and 3Â m, respectively.
The reactor was totally filled with tap water to minimize the
volatilization of the sulfur compounds. To reduce the oxy-
gen concentration inside the reactor the liquid absorbent was
flushed with nitrogen gas. Due to the high toxicity of Â
H2S, all
experiments were performed in a fume hood to confine any
accidental leakage of Â
H2S gas. The gas flow rate was adjusted
by a stainless steel mass flow controller on the line between
the gas cylinder and the set-up. Digital gas counter was used
to measure the inlet and outlet biogas volumetric flow rate
(MGC, Ritter, Germany). A pH range of 7â10 was maintained
by automated addition of 1N NaOH into the liquid absorbent
as needed. Almost all of Â
H2S and some of Â
CO2 were removed
3. Waste and Biomass Valorization
1 3
from the simulated gas mixture by diffusing through the mem-
brane and then absorbing/reacting with the mildly alkaline
solution. For uniform distribution of gas and alkaline solu-
tion in the liquid phase magnetic stirrer was used throughout
the experiment. The experiments were carried out at ambient
pressure (approximately 1Â atm), and the liquid temperature
was kept at 20â±â1 °C using a heating blanket equipped with
electric coils wrapped on the wall of the reactor. The con-
ductivity, oxidation reduction potential (ORP) and dissolved
oxygen (DO) were monitored with electrodes on-line using a
digital multimeter (Multi 9430, WTW, Germany).
Techniques and Test Methods
Scanning Electron Microscopy with Energy Dispersive X-ray
Spectroscopy (SEM-EDS) analyses were carried out at Yildiz
Technical University Science and Technology Application and
Research Center (Istanbul, Turkey). The changing morpholo-
gies of the membrane were directly observed using a SEM
after AuâPd coating. The semi quantitative elemental analyses
of the inorganics deposited on the external surface of the mem-
brane were performed using an EDS coupled with SEM at the
end of the experiment. The concentration of sulfide in the liq-
uid phase was determined spectrometrically (WTW PhotoLab
6100VIS) following the method described by Cord-Ruwisch
[26]. Sulfate (4500-SO4
2â
E) concentration was measured
according to standard methods [27]. The contents of Â
H2S,
ÂCO2 and Â
CH4 gas in the entrance and exit of the membrane
contactor were analyzed by gas chromatography (GC-2014;
Shimadzu, Tokyo, Japan) equipped with a thermal conductiv-
ity detector and a stainless steel column. Argon gas was used
as the carrier gas. The temperatures of injection port, column
and detector were 150, 40 and 150 °C, respectively. When
the ÂH2S concentration of the biogas in the exit was below the
detection limit of the GC-TCD, a second scrubbing liquid was
used at the end of the gas line to absorb all traces of Â
H2S before
venting, the detail of the measurement on the second scrubber
was described in a former study [28]. All samples were col-
lected and analyzed in triplicate following each change in the
process operation conditions and the averages were reported.
Calculations
In this study, Â
H2S, ÂCO2 or Â
CH4 removal efficiencies (RE) from
the gas phase were calculated according to Eq. 1.
where ÂQg
in
and Â
Qg
out
are inlet and outlet biogas flowrates
Â(m3
/days), respectively. Â
Cg
in
and Â
Cg
out
are the inlet and
outlet concentrations in gas phase (mg/l). In the case of
(1)
RE (%) =
(
Qg
in
Ă Cg
in
)
â
(
Qg
out
Ă Cg
out
)
(
Qg
in
Ă Cg
in
) Ă 100
simultaneous removal of Â
H2S and Â
CO2, selectivity of the
desulfurization could be represented by selectivity factor
or separation factor (S), which could be expressed by Eq. 2.
where ÂXi,j are the mole fraction of components i and j in the
liquid phase and Â
Yi,j are the mole fraction of components i
and j in the feed gas. Surface removal rate or flux (J) of the
membrane contactor is another performance indicator, which
can be estimated as Eq. 3.
where J is the flux of the gas components (g/m2
/days) and A
is the membrane surface area Â
(m2
).
Results and Discussions
Impact of GRT and Membrane Thickness
on Desulfurization Performance
The gas retention time (GRT) is one of the crucial param-
eters determining the biogas desulfurization efficiency. The
influence of GRT on the removal efficiency (RE) of each
gas component is presented in Table 1. The pH of the liq-
uid absorbent was constant (pH 10) in the experiments. The
result shows that, the decrease in GRT reduces the mem-
brane surface contact per unit volume of gas, which in turn
reduces RE. The Â
H2S RE of the process was superior, espe-
cially when the GRT laid in the range of 19â10.4Â min. The
ÂH2S was completely removed at that GRT ranges. Moreover,
at GRT of over 10Â min, an effluent Â
H2S concentration below
300 ppmv was achieved, which is safe to use in cogenera-
tion units [29]. However, lower GRT resulted in higher
effluent ÂH2S concentrations which needs further treatment
before using the gas for various applications. In the same
way, ÂCO2 removal declined significantly with the decreased
GRT. At all GRT tested, the RE of Â
H2S was much higher
than that of Â
CO2. Because Â
H2S has relatively high critical
temperature or gas condensability and expected to permeate
faster through the dense membrane than Â
CO2. Baker [30]
stated that the permeation of gas components through rub-
bery polymers depends mainly on the gas condensability.
Over all, the RE of Â
H2S and Â
CO2 improved by more than
2.5 and 5.2 times, when the GRT was raised from 3.4 to
19 min, respectively (Table 1). This finding is in agreement
with the results presented by Wang et al. [21]. They reported
that gas removal capacity decreased as the GRT declined
because of the shorter contact between the gas and liquid
(2)
S =
(
XiâXj
)
(
YiâYj
)
(3)
J =
(
Qg
in
Ă Cg
in
)
â
(
Qg
out
Ă Cg
out
)
A
4. Waste and Biomass Valorization
1 3
phases. Nevertheless, Â
CH4, having a low diffusivity through
the PDMS membrane and low solubility in the liquid absor-
bent, could not permeate across the membrane. Figure 1 also
illustrated that, Â
CH4 content in the effluent stream increased
along with the GRT and enriched from 60% to a maximum
of 87% with only 4.68% loss. Heile et al. [31] performed a
similar study for upgrading of a biogas containing 20% of
ÂCO2 and 80% of Â
CH4 using a PDMS membrane and they
improved the Â
CH4 content up to 88% in the outlet.
In addition, experiments with varying membrane thick-
nesses of 1, 1.5 and 2Â mm were also performed. According
to the results shown in Table 1, thicker membrane reduces
the ÂH2S and Â
CO2 transfer rates across the membrane due to
longer diffusion time. The negative impact of higher mem-
brane thickness in overall mass transfer of other permeates in
silicone membranes also reported by Raghunath and Hwang
[32]. They found significant mass transfer resistance when
the thickness of a silicone membrane was above 1.16Â mm.
In a similar manner, Brookes and Livingston [33] used sili-
cone based membranes and they reported a reduction by
a factor of 1.5 in the overall mass transfer coefficient for
phenol when the membrane thickness was increased from
0.3 to 0.5Â mm. Therefore, with thinner membranes, higher
loading rates could be attained. As a general trend, when the
membrane thickness increases, the Â
CO2 removal efficiency
decreases more significantly than that of Â
H2S even at high
GRT. The reduction was much more pronounced for lower
GRT. Nii and Takeuchi [34] also used PDMS hollow fıber
modules for the removal of Â
CO2 and achieved a higher per-
formance with thinner membranes. It can be concluded that
thicker membrane had introduced a considerable resistance
to the gas diffusion, which decreased the absorption flux
inevitably.
In the previous studies, separation factor has been exten-
sively used to indicate gas separation efficiency of mem-
brane based processes [35, 36]. In the gasâliquid membrane
contacting process used here Â
H2S in the feed gas diffused
through the membrane and was absorbed in the mildly alka-
line absorbent. For that reason, the permeate selectivity was
used to describe the process efficiency. As shown in Eq. 2,
the permeate selectivity depends on the difference between
the inlet and outlet gas concentrations which are influenced
by the membrane thickness, GRT, and pH of the absorbent
used. In order to examine the effect of different membrane
thicknesses and GRT on the selectivity of the membrane
contactor, the overall mass transfer ratio of Â
H2S to Â
CO2 and
ÂCH4 were calculated. During each experimental tests, pH
of the liquid was adjusted to 10. As presented in Table 1,
a higher selectivity for the separation of Â
H2S from the gas
mixture was observed at lower GRT. A change in membrane
thickness from 1 to 2Â mm had a positive influence on the
selectivity due to the higher permeability of Â
H2S in thicker
membrane compared to those of other gases. However, in
that case a lower RE was observed as a result of additional
TableâŻ1ââThe membrane
contactor performance at
different GRT and membrane
thicknesses at liquid pH of 10
Thickness
(mm)
Gas retention time
(GRT) (min)
Removal efficiency (RE) (%) Selectivity (S)
H2S CO2 CH4 H2S/CH4 H2S/CO2
1 19 100 79.3 4.68 21.4 1.26
10.4 98.3 56.4 3.44 28.6 1.74
5.6 63.6 34.9 1.84 34.6 1.82
3.4 40.0 15.3 0.69 59 2.6
1.5 19 99.9 68.7 4.12 24.2 1.45
10.4 97.8 43.0 2.86 34.2 2.27
5.6 60.6 22.8 1.56 38.8 2.66
3.4 38.2 12.1 0.63 60.6 3.06
2 19 99.7 60.5 3.58 27.8 1.65
10.4 97.0 35.8 2.4 40.2 2.72
5.6 56.9 18.0 1.23 46.3 3.16
3.4 36.2 8.92 0.58 62.4 4.06
GRT (min)
3 6 9 12 15 18 21
Gas
effluent
(%)
0
20
40
60
80
100
CH4
CO2
H2S
Fig.âŻ1ââImpacts of GRT on the effluent gas contents at pH 10. (Color
figure online)
5. Waste and Biomass Valorization
1 3
resistance of the membrane. Consequently, for a gas liquid
membrane contactor process used here, these two parameters
can considerably influence the Â
H2S selectivity and RE. Spe-
cific to the experimental results of this work, the maximum
separation factor for Â
H2S/CO2 and Â
H2S/CH4 were reached up
to 4 and 62, respectively, in the case of using thicker mem-
brane (2Â mm). Stern and Bhide [37] pointed out that Â
H2S is
more permeable than Â
CO2 through PDMS by approximately
a factor of 1.8. Chatterjee et al. [38] also used cellulose ace-
tate membrane to clean the biogas having 6% of Â
H2S, 29%
of ÂCO2 and 65% of Â
CH4 at 10Â bar, and they reported the
ÂH2S/CH4 separation factor as only 19. In another work on
separation of gases using Poly (ether urethane) membrane,
separation factor of Â
H2S/CO2 and Â
H2S/CH4 were reported as
3 and 21, respectively [38].
Impact of Liquid Absorbent pH on Desulfurization
Performance
The gas-liquid-membrane contactor was also tested with
liquid absorbent having different pH values for the removal
of ÂH2S from the biogas stream. As displayed in Table 2,
the reduction of GRT negatively affected the RE of the gas
components. However, when the pH of the liquid absorbent
increased, the permeation of Â
H2S across the membrane
improved due to the increased concentration differences
between the two sides of the membrane, i.e. the driving
force. According to the report of earlier studies charged
ions existing in the liquid absorbent are unable to permeate
through the silicone rubber membrane [22, 39, 40]. Indeed,
when the pH decreased, the amount of undissociated Â
H2S
increased, resulting in an increase of the Â
H2S back diffusion
[25]. Hence, the pH is a key parameter in controlling Â
H2S
mass transfer through the membrane. The mildly alkaline
solutions give the greater fractional removal of Â
H2S com-
pared to Â
CO2 and Â
CH4. This is owing to the instantaneous
reaction rates Â
OHâ
with Â
H2S rather than the other gases [13,
14]. At the highest GRT (19Â min) changing of pH from 7
to 10 were not significantly affect Â
H2S RE (Table 2). To
establish the limiting process conditions, the transition in
GRT from 19 to 3.4Â min was examined, and upon decreasing
of the GRT, Â
H2S RE dropped sharply. In general, at higher
liquid pH (>â8.5) a better RE was achieved. Because at high
pH values almost all of the dissolved Â
H2S was dissociated
to hydrosulfide ion, which increased the RE by continually
re-establishing the higher Â
H2S concentration gradient. Smet
et al. [41] also observed a sharp increase in hydrosulfide ion
concentration when the pH was higher than 7.04. Similarly,
Gonzålez-Sånchez et al. [42] reported that a slightly alkaline
pH could improve the Â
H2S mass transfer from the gas phase
to the liquid phase.
According to the result shown in Table 2, the absorption
of ÂCO2 into the liquid absorbent was considerably higher
than that of Â
CH4, as it chemically reacted with Â
OHâ
. The
absorption of both Â
H2S and Â
CO2 in alkaline liquid was
assisted by agitation. The turbulence in the liquid increases
the diffusion of the molecules into the liquid phase due to
reduction of boundary layer resistance between membrane
and liquid interface. Under the tested conditions, the highest
RE of Â
CO2 was achieved at GRT of 19Â min as 69.9, 71.1 and
79.3% at liquid pH of 7, 8.5 and 10, respectively. On the con-
trary, at the lowest GRT (3.4Â min), the RE of Â
CO2 at liquid
pH of 7 and 8.5 were 12 and 14.8%, respectively, while as
the liquid pH raised to 10 its RE increased to 15.31%, which
is due to the high concentration of Â
OHâ
ion at higher liquid
pH values. Likewise, the positive effect of absorbent con-
centration on the mass transfer rate were formerly reported
[43, 44]. In general, the pH of the liquid in this study was
not increased above 10, to avoid much carbonate formation
Â(CO2 absorption) and Â
CH4 loss.
The conductivity of a solution depends on the concen-
tration of all the ions present, the greater their concen-
trations, the higher the conductivity. During operation
of the system used here, the major cations are, Â
H+
, ÂNa+
,
ÂCa2+
, and Â
Mg2+
. The major anions are, Â
OHâ
, ÂHSâ
, ÂS2â
,
ÂHCO3
â
and Â
CO3
2â
. Hence, acidic or basic solution resulted
with high conductivity. Moreover, the conductivity is the
sum of the contribution of all ions present in the solu-
tion. For soft water samples, a pH of 7 will have the least
conductivity. On other hand, samples with a pH above 7
are likely to be higher conductivity. Figure 2 also con-
firmed that when the pH of the liquid absorbent increased
from 7 to 10, proportionally the conductivity raised up.
This happened due to higher dissociation of Â
H2S and Â
CO2
into sulfide and carbonate. Moreover, to kept constant pH
sodium hydroxide has been supplied to the liquid phase.
TableâŻ2ââImpact of different liquid pH on gas RE at 1 mm membrane
thickness
pH Gas retention time
GRT (min)
Removal efficiency (RE) (%)
H2S CO2 CH4
10 19.0 100 79.3 4.68
10.4 98.3 56.4 3.44
5.6 63.6 32.9 1.84
3.4 40.0 15.3 0.69
8.5 19.0 99.9 71.1 4.44
10.4 91.5 51.6 3.25
5.6 52.8 28.9 1.55
3.4 32.0 14.8 0.59
7.0 19.0 97.9 69.9 3.96
10.4 82.0 45.8 3.01
5.6 41.8 27.1 1.15
3.4 20.0 12.0 0.50
6. Waste and Biomass Valorization
1 3
Our results were consistent with the finding of leveling
[45]. Figure 2 demonstrated that ORP decreased as the
pH of the liquid absorbent increased, which was due to
higher accumulation of sulfide. ORP values was around
ââ200Â mV at low sulfide accumulation, whereas at higher
loadings significant sulfide accumulation caused ORP to
drop below ââ380Â mV. Besides, at pH 10, in particular with
a GRT of 3.4Â min, the ORP value decreased sharply as an
indication of sulfide accumulation. This behavior proves a
correlation between the ORP and Â
H2S absorption capacity
of the slightly alkaline liquid absorbent.
Membrane Morphology and Inorganics Deposition
The surface morphology of PDMS membrane was examined
using a scanning electron microscope (SEM). SEM images
of virgin membrane and dirty membrane are presented in
Fig. 3a and b, respectively. The SEM images confirms there
is no visible variation between the surface morphologies of
the two membranes samples. However, when the surface of
the membrane was magnified, the layer of deposits on the
membrane surface could be clearly realized. As illustrated
in Fig. 3b, it is observed that after each experimental work
a white crystal substances appeared on the surface of used
Fig.âŻ2ââORP and conductivity
observation at different pH and
GRT. (Color figure online)
ORP
(mV)
-400
-300
-200
-100
Biogas retention time (min)
5 10 15 20
Conductivity
(”S/cm)
0
500
1000
1500
2000
pH 7 pH 8.5 pH 10
pH 7 pH 8.5 pH 10
7. Waste and Biomass Valorization
1 3
membrane, which should be inorganic matters as confirmed
by EDS analysis, whereas the virgin membrane surface
viewed clean and smooth (Fig. 3a). The SEM images also
attributed to the fact that the pattern of inorganic matters is
an unevenly distributed over the membrane surface. Despite
minor deposition of inorganics on the membrane surface,
the structure was not suffered after being exposed to sodium
hydroxide aqueous solutions. In addition, the membrane
used here was less sensitive to wetting since there is no pores
that supports the liquid to penetrate through the membrane.
Thus, membrane wetting was not observed in our PDMS
membrane. However, other researchers attributed perfor-
mance deterioration of the microporous membrane contac-
tors due to wetting and blockage of the membrane pores.
Fig.âŻ3ââSEM image of the mem-
brane surface a before (virgin)
and b after the experiments
(used)
8. Waste and Biomass Valorization
1 3
They reported significant drop in mass transfer capacity of
the contactors due to the developed membrane resistance
[20, 46]. Keshavarz et al. [19] used a microporous hollow
fiber membrane contactor to investigate the simultaneous
absorption of Â
CO2 and Â
H2S into the aqueous solution of
diethanolamine and they found that the RE of both gases
significantly decreased due to membrane wetting compared
with the non-wetted mode. Wang et al. [21] also studied
on ÂCO2 absorption using a diethanolamine solution as an
absorbent in a polypropylene microporous membrane con-
tactor. They reported that with only 5% membrane pores
wetting the overall mass transfer coefficient of the contactor
may reduce by 20%.
With the use of scanning electron microscopy (SEM)
associated with an energy dispersive X-ray spectrometer
(EDS) often focus on the top surface deposits as indi-
cated in Fig. 4a and b, and it is possible to detect the exist-
ence of metal sulfide and carbonate salts deposition. The
Fig.âŻ4ââThe semi quantitative elemental analyses of the inorganics deposited on the membrane surface a SEM image b EDS analysis after the
experiment
9. Waste and Biomass Valorization
1 3
semi-quantitative EDS composition analysis indicated cer-
tain amounts of inorganic elements were accumulated on the
membrane surface. The presence of multivalent metal ions
in the tap water, which used in the system as an absorbent
liquid, was the origin of inorganics deposition. Thus, in the
presence of metal ions such as Â
Ca2+
and Â
Mg2+
that act as
cationic effects, there would be a strong interaction with
anionic sulfide, and carbonate, which were generated dur-
ing the dissociation of Â
H2S and Â
CO2, to form precipitates.
Furthermore Fig. 2 also approves that, when the pH of the
liquid phase increased the conductivity of the liquid absor-
bent linearly increased proportionally. The reason for this
observation was the greater accumulation of sulfide and car-
bonate in the reactor as the pH of the liquid raised up (when
the pH increased to 10). It can be observed that carbon (C)
and oxygen (O) were the major elements present in the spot
samples. The C and O peaks were likely due to the chemi-
cal structure of the membrane and entrapment of inorganic
matters. As discussed above, the multivalent ions have been
shown to form precipitates with sulfide and carbonates could
contribute to the calcium (Ca), magnesium (Mg) peaks and
their weight percent were about 36.1 and 0.5%, respectively.
The sulfur (S) peaks possibly indicating that S containing
compounds were able to penetrate through the membrane
with some retained on the membrane surface. Silicon (Si)
detected on the used membrane surface was mainly due to its
presence in the polydimethylsiloxane membrane layer. The
EDS spectra of the virgin membrane has also strong Si and
O peaks which originated from the membrane itself (data
not shown). Gold (Au) and palladium (Pd) were used dur-
ing coating procedures, due to its irrelevant to the elemental
analysis of the membrane foulants their peaks were removed.
In the present study regardless of inorganics deposition
discussed above, the abiotic experiments show selective
removal ÂH2S than Â
CO2 and Â
CH4. Besides, in this gasâliquid
membrane contactor applications, membrane fouling and
clogging were not observed, which resulted in almost stable
flux during the operation of the membrane. Previous studies
also demonstrated nonporous membrane was more resistant
to fouling [47]. However, in the long run operation, an excess
deposition may have reduced the mass transfer efficiency of
the PDMS membrane. It was assumed that the accumulation
of inorganics on the membrane surface may decreased the
cross sectional area, subsequently reduced the gas retention
time and increased the pressure drop of the gas permea-
tion. Moreover, it may create an additional film resistance
for the gas to reach the liquid phase, which likely resulted
in limitation of Â
H2S transfer. Chuichulcherm et al. [40] used
silicone membrane combined with biological sulfide pro-
duction for the treatment of metal-containing wastewater.
They reported that the chemical reaction between sulfide
and multivalent ions in the wastewater enhanced the sulfide
mass transfer. However, accumulation of metal precipitates
on the membrane surface limited sulfide transfer and the
resistance due to the metal sulfide precipitates even exceeded
the membrane resistance. Other studies also demonstrated
the mass transfer reduction of membrane based reactors due
to the blockage of the membrane pores by the organic and
inorganics accumulation [46, 48, 49].
Conclusions
Results of the study performed to evaluate the effects of
operating parameters on Â
H2S removal performance of a
gasâliquid PDMS membrane contactor showed that the
ÂH2S removal declined severely, when GRT decreased below
10Â min, due to mass transfer limitation. In addition, increas-
ing membrane thickness reduced the rate of Â
H2S diffusion
through the membrane. By using a slightly alkaline absor-
bent, more Â
H2S was removed with respect to Â
CO2 and Â
CH4.
On other hand, lower GRT and higher membrane thickness
significantly increased the mass transfer resistance against
ÂCO2, but showed a marginal influence on Â
H2S removal;
hence it favors a higher selectivity for Â
H2S. The maximum
selectivity for Â
H2S/CO2 and Â
H2S/CH4 were 4 and 62, respec-
tively. Despite some loss, the Â
CH4 content of the biogas
enriched up to 87%. Results of SEM-EDS analysis showed
that at higher pH values, sulfide and carbonate salts of Ca,
Mg, and Si deposit were observed on the membrane surface.
In spite of inorganics deposition, no membrane clogging and
fouling problems were observed. However, it is supposed
that in long run, the chemical precipitates on the outer sur-
face of membrane may behave as a secondary barrier and
reduce the diffusion of gasses. Finally, it is concluded that
the gasâliquid PDMS membrane contactor process tested in
this study is a promising alternative to conventional biogas
desulfurization processes.
Acknowledgementsâ This study was financially supported by YTB
(Presidency for Turks Abroad and Related Communities) and Mar-
mara University Scientific Research Committee BAPKO (Project No.
FEN-C-DRP-070317-0109).
References
1. Holm-Nielsen, J.B., Al Seadi, T., Oleskowicz-Popiel, P.: The
future of anaerobic digestion and biogas utilization. Bioresour.
Technol. 100, 5478â5484 (2009). httpsâ://doi.org/10.1016/j.biortâ
ech.2008.12.046
2. Zhang, R., Brown, R.C., Suby, A., Cummer, K.: Catalytic destruc-
tion of tar in biomass derived producer gas. Energy Convers.
Manag. 45, 995â1014 (2004). httpsâ://doi.org/10.1016/j.enconâ
man.2003.08.016
3. Marzouk, S.A.M., Al-Marzouqi, M.H., Teramoto, M., Abdul-
latif, N., Ismail, Z.M.: Simultaneous removal of Â
CO2 and H2S
from pressurized Â
CO2âH2SâCH4 gas mixture using hollow fiber
10. Waste and Biomass Valorization
1 3
membrane contactors. Sep. Purif. Technol. 86, 88â97 (2012).
httpsâ://doi.org/10.1016/j.seppuâr.2011.10.024
4. Poloncarzova, M., Vejrazka, J., Vesely, V., Izak, P.: Effec-
tive purification of biogas by a condensing-liquid membrane.
Angew. Chem. Int. Ed. 50, 669â671 (2011). httpsâ://doi.
org/10.1002/anie.20100â4821
5. Panza, D., Belgiorno, V.: Hydrogen sulphide removal from land-
fill gas. Process Saf. Environ. Prot. 88, 420â424 (2010). httpsâ://
doi.org/10.1016/j.psep.2010.07.003
6. Asri, O., Hafidi, I.E., Afilal, M.E.: Comparison of biogas purifi-
cation by different substrates and construction of a biogas purifi-
cation system. Waste Biomass Valoriz. 6, 459â464 (2015). httpsâ
://doi.org/10.1007/s1264â9-015-9378-z
7. Kurchania, A.K., Panwar, N.L., Pagar Savita, D.: Improved
biogas stove with scrubbing unit for household use. Waste Bio-
mass Valoriz. 2, 397â402 (2011). httpsâ://doi.org/10.1007/s1264â
9-011-9080-8
8. Awe, O.W., Zhao, Y., Nzihou, A., Minh, D.P., Lyczko, N.: A
review of biogas utilisation, purification and upgrading tech-
nologies. Waste Biomass Valoriz. 8, 267 (2017)
9. Ryckebosch, E., Drouillon, M., Vervaeren, H.: Techniques
for transformation of biogas to biomethane. Biomass Bioen-
erg. 35, 1633â1645 (2011). httpsâ://doi.org/10.1016/j.biombâ
ioe.2011.02.033
10. Gabelman, A., Hwang, S.-T.: Hollow fiber membrane contac-
tors. J. Memb. Sci. 159, 61â106 (1999). httpsâ://doi.org/10.1016/
S0376â-7388(99)00040â-X
11. Klaassen, R., Feron, P.H.M., Jansen, A.E.: Membrane contactors
in industrial applications. Chem. Eng. Res. Des. 83, 234â246
(2005). httpsâ://doi.org/10.1205/cherdâ.04196â
12. Belaissaoui, B., Claveria-Baro, J., Lorenzo-Hernando, A.,
Albarracin Zaidiza, D., Chabanon, E., Castel, C., Rode, S.,
Roizard, D., Favre, E.: Potentialities of a dense skin hollow
fiber membrane contactor for biogas purification by pressurized
water absorption. J. Memb. Sci. 513, 236â249 (2016). httpsâ://
doi.org/10.1016/j.memscâi.2016.04.037
13. Qi, Z., Cussler, E.L.: Microporous hollow fibers for gas absorp-
tion: Iâmass transfer in the liquid. J. Memb. Sci. 23, 321â332
(1985). httpsâ://doi.org/10.1016/S0376â-7388(00)83149â-X
14. Qi, Z., Cussler, E.L.: Microporous hollow fibers for gas absorp-
tion: IIâmass transfer across the membrane. J. Memb. Sci. 23,
333â345 (1985). httpsâ://doi.org/10.1016/S0376â-7388(00)83150â
-6
15. Karoor, S., Sirkar, K.K.: Gas absorption studies in microporous
hollow fiber membrane modules. Ind. Eng. Chem. Res. 32, 674â
684 (1993). httpsâ://doi.org/10.1021/ie000â16a01â4
16. Poddar, T.K., Majumdar, S., Sirkar, K.K.: Membrane-based
absorption of VOCs from a gas stream. AIChE J. 42, 3267â3282
(1996)
17. Mavroudi, M., Kaldis, S.P., Sakellaropoulos, G.P.: A study of mass
transfer resistance in membrane gas-liquid contacting processes. J.
Memb. Sci. 272, 103â115 (2006). httpsâ://doi.org/10.1016/j.memscâ
i.2005.07.025
18. Keshavarz, P., Fathikalajahi, J., Ayatollahi, S.: Analysis of Â
CO2
separation and simulation of a partially wetted hollow fiber mem-
brane contactor. J. Hazard. Mater. 152, 1237â1247 (2008). httpsâ
://doi.org/10.1016/j.jhazmâat.2007.07.115
19. Keshavarz, P., Fathikalajahi, J., Ayatollahi, S.: Mathematical
modeling of the simultaneous absorption of carbon dioxide and
hydrogen sulfide in a hollow fiber membrane contactor. Sep. Purif.
Technol. 63, 145â155 (2008). httpsâ://doi.org/10.1016/j.seppuâ
r.2008.04.008
20. Atchariyawut, S., Jiraratananon, R., Wang, R.: Separation of Â
CO2
from ÂCH4 by using gas-liquid membrane contacting process. J.
Memb. Sci. 304, 163â172 (2007). httpsâ://doi.org/10.1016/j.memscâ
i.2007.07.030
21. Wang, R., Zhang, H.Y., Feron, P.H.M., Liang, D.T.: Influence of
membrane wetting on Â
CO2 capture in microporous hollow fiber
membrane contactors. Sep. Purif. Technol. 46, 33â40 (2005).
httpsâ://doi.org/10.1016/j.seppuâr.2005.04.007
22. Tilahun, E., Sahinkaya, E., Ăalli, B.: A hybrid membrane gas
absorption and bio-oxidation process for the removal of hydro-
gen sulfide from biogas. Int. Biodeterior. Biodegrad. 127, 69â76
(2018). httpsâ://doi.org/10.1016/j.ibiodâ.2017.11.015
23. Al-Marzouqi, M.H., Marzouk, S.A.M., El-Naas, M.H., Abdul-
latif, N.: Â
CO2 removal from Â
CO2âCH4 gas mixture using differ-
ent solvents and hollow fiber membranes. Ind. Eng. Chem. Res.
48, 3600â3605 (2009). httpsâ://doi.org/10.1021/ie800â977z
24. Al-saffar, H.B., Ozturk, B., Hughes, R.: A comparison of porous
and non-porous gas-liquid membrane contactors for gas sepa-
ration. Chem. Eng. Res. Des. 75, 685â692 (1997). httpsâ://doi.
org/10.1205/02638â76975â24182â
25. Tilahun, E., Bayrakdar, A., Sahinkaya, E., Ăalli, B.: Perfor-
mance of polydimethylsiloxane membrane contactor process for
selective hydrogen sulfide removal from biogas. Waste Manag.
1â8 (2017). httpsâ://doi.org/10.1016/j.wasmaân.2017.01.011
26. Cord-Ruwisch, R.: A quick method for the determination of dis-
solved and precipitated sulfides in cultures of sulfate-reducing
bacteria. J. Microbiol. Methods 4, 33â36 (1985). httpsâ://doi.
org/10.1016/0167-7012(85)90005â-3
27. AWWA A.: WEF, standard methods for the examination of
water and wastewater. (2005)
28. Bayrakdar, A., Tilahun, E., Calli, B.: Biogas desulfurization
using autotrophic denitrification process. Appl. Microbiol. Bio-
technol. 100, 939â948 (2016). httpsâ://doi.org/10.1007/s0025â
3-015-7017-z
29. Ramos, I., Fdz-Polanco, M.: Microaerobic control of biogas sul-
phide content during sewage sludge digestion by using biogas
production and hydrogen sulphide concentration. Chem. Eng. J.
250, 303â311 (2014). httpsâ://doi.org/10.1016/j.cej.2014.04.027
30. Baker, R.W.: Membrane Technology and Application. Wiley, New
York (2004)
31. Heile, S., Rosenberger, S., Parker, A., Jefferson, B., McAdam,
E.J.: Establishing the suitability of symmetric ultrathin wall poly-
dimethylsiloxane hollow-fibre membrane contactors for enhanced
CO2 separation during biogas upgrading. J. Memb. Sci. 452,
37â45 (2014). httpsâ://doi.org/10.1016/j.memscâi.2013.10.007
32. Raghunath, B., Hwang, S.T.: General treatment of liquid-phase
boundary layer resistance in the pervaporation of dilute aqueous
organics through tubular membranes. J. Memb. Sci. 75, 29â46
(1992). httpsâ://doi.org/10.1016/0376-7388(92)80004â-4
33. Brookes, P.R., Livingston, A.G.: Aqueous-aqueous extraction of
organic pollutants through tubular silicone rubber membranes. J.
Memb. Sci. 104, 119â137 (1995). httpsâ://doi.org/10.1016/0376-
7388(95)00020â-D
34. Nii, S., Takeuchi, H., Takahashi, K.: Removal of Â
CO2 by gas
absorption across a polymeric membrane. J. Chem. Eng. Jpn. 25,
67â72 (1992)
35. Stern, S.A.: Polymers for gas separations: the next decade. J.
Memb. Sci. 94, 1â65 (1994)
36. Mudler, M.: Basic Principles of Membrane Technology. Springer,
New York (1996)
37. Stern, A., Bhide, B.D.: Permeability of silicone polymers to
ammonia and hydrogen sulfide. J. Appl. Polym. Sci. 38, 2131â
2147 (1989)
38. Chatterjee, G., Houde, A.A., Stern, S.A.: Poly(ether urethane) and
poly(ether urethane urea) membranes with high Â
H2S/CH4 selectiv-
ity. J. Memb. Sci. 135, 99â106 (1997)
39. Livingston, A.G.: Extractive membrane bioreactors: a new pro-
cess technology for detoxifying chemical industry wastewaters.
J. Chem. Technol. Biotechnol. 60, 117â124 (1994). httpsâ://doi.
org/10.1002/jctb.28060â0202