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An experimental study for H2S and CO2 removal via caustic scrubbing system
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
An experimental study for H2 S and CO2 removal via caustic scrubbing system a ˘ Ersin Üresin a,∗,1 , Halil I˙brahim Sarac¸ b , Alper Sarıoglan , S¸iringül Ay a , Fehmi Akgün a a b
TÜBI˙TAK Marmara Research Center Energy Institute, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey Kocaeli University, Mechanical Engineering Department, Kocaeli, Turkey
a b s t r a c t In this study, removal of hydrogen sulfide (H2 S) and carbon dioxide (CO2 ) from simulated syngas has been studied on one column scrubbing system. Gas flow rate as a measure of gas residence time and superficial gas velocity, gas composition, inlet H2 S load, flow modes (countercurrent and cocurrent) and packing geometry were the parameters in the design and/or operation of an acid gas scrubber system. Better H2 S scrubbing efficiencies have been obtained in countercurrent flow mode than that of cocurrent flow mode. When accordingly designed, static mixer with its superior performance on H2 S removal overweighed to structured packings. The coexistence of CO2 and H2 S has been shown to increase the sodium hydroxide (NaOH) consumption along the scrubber column thereby decreasing the H2 S removal efficiency at higher H2 S loads. The gas residence time as changing with the gas velocity was found to be more dominant on acid gas removal efficiency than the effect of superficial gas velocity within the experimented range. A gas residence times of equal or above 3 s were seemed to be closer to the optimum point. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Gas clean up; Packed bed gas scrubbers; Hydrogen sulfide removal; CO2 removal; NaOH; Static mixer
1.
Introduction
Particulate, ammonia (NH3 ) and hydrogen sulfide (H2 S) comprise the main impurities of synthetic raw gas produced by coal gasification. Removal of these contaminants is an essential step in coal and biomass gasification process to avoid its detrimental effects on materials and environment. Hydrogen sulfide and ammonia can cause corrosion in pipelines and thus limit plant lifetime. These are also a well-known catalyst poisoner. Therefore, utilization of gasification products either as a fuel for gas turbines, gas engines or fuel cells or as a syngas for Fischer–Tropsch fuel, ammonia or methanol productions requires an effective gas clean-up technology (Chen et al., 2001; Moussavi et al., 2008; Panza and Belgiorno, 2010). For the abatement of H2 S and NH3 , packed towers are widely used for chemical and physical scrubbing. The choice of the solution is critical in determining dissolution or chemical
reaction rate of pollutant in the liquid. In physical scrubbing, the pollutants in basic nature can be passivated by the acidic solutions and vica-versa. Currently, the processes of choice in the commercial integrated gasification combined cycle (IGCC) facilities for the removal of acid gases are both the chemical solvent acid gas removal processes based on aqueous methyldiethanolamine (MDEA) and the physical solvent-based Selexol process which uses mixtures of dimethyl ethers and polyethylene glycol (Couvert et al., 2008; Turpin et al., 2008; Ohtsuka et al., 2009; Wang et al., 2004; Wallin and Olausson, 1993). Alkaline hypochlorite, hydrogen peroxide or caustic solutions are economicaly alternative solvents to be used in physical solvent-based processes and show good results for decades. Sodium hydroxide solution is a very effective, but non-regenerable absorbent for CO2 and H2 S. Therefore the use of caustic is usually limited to the removal of trace amounts of
∗
Corresponding author. Tel.: +90 262 677 27 80; fax: +90 262 641 23 09. E-mail address: [email protected] (E. Üresin). 1 Present address. http://dx.doi.org/10.1016/j.psep.2014.06.013 0957-5820/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Üresin, E., et al., An experimental study for H2 S and CO2 removal via caustic scrubbing system. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.06.013
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these impurities. For the disposal of the spent caustic, different considerations are applied such as simple neutralization or its use in pulp and paper mills after some certain quality control analysis (Couvert et al., 2006; Miltner et al., 2012; Mamrosh et al., 2008; Bontozoglou and Karabelas, 1991). For the improvement of scrubbing efficiencies, design of the scrubber system is as important as the choice of the solvent. Multiple gas sorption columns packed with structured materials are used in order to increase the active surface area and residence time for an efficient gas–liquid interaction. High efficiencies of commercial structured packing materials for H2 S removal with an alkaline washing solution was attributed to a high contact area existing between the gas and the liquid phases. The major drawback in the use of structured packing is their excessive price per volume unit. Static mixers, if properly designed, can be acceptable in terms of investment costs and have higher resistance to pluggings. Nevertheless, high mass transfer performances cannot be reached by static mixers as structured packing materials can be. Hence, the design of a static mixer has crucial importance in generating high interfacial areas and improving mass transfer rates (Couvert et al., 2008; Mamrosh et al., 2008; Godini and Mowla, 2008). Basic design parameters of the structured packing columns are hydrodynamic flow characteristics and packing surface area as determined by the packing material geometry, residence time, liquid to gas ratio, pH and reactivity of the solution. To avoid high investment and operation costs, the scrubber system should be designed by taking into consideration of these parameters (Bhave et al., 2008; Biard et al., 2010). Although design studies and commercial applications on structured packings were found in the literature extensively (Miltner et al., 2012; Godini and Mowla, 2008; Bhave et al., 2008; Aliabad and Mirzaei, 2009; Dang et al., 1998; Lu et al., 2006;
Ballaguet et al., 2003) there were only a sufficient number of comparison studies differentiating the performances of both the static mixer and the structured packing for the acid gas removal. Static mixers have been known with their excellent gas–liquid contact due to a high turbulence rate inside and their potential on well mixed gas distribution giving high mass transfer rates (Couvert et al., 2008; Sanchez et al., 2008). Consequently, the objective of this study is to design a static mixer being able to generate high interfacial areas and improved mass transfer rates for acid gas alkali washing towers and compare the results with those of commercial packings. In order to elucidate the advantages of static mixers over structured packings, the removal of H2 S has been studied on one column scrubbing system on both of the designed static mixer and commercial structured packing. The effect of gas composition has been investigated by using H2 S in nitrogen and H2 S in simulated refinery gas mixtures. For these two different gas atmospheres, inlet H2 S concentration was increased in stepwise. The gas flow rate as a measure of gas residence time was changed to monitor the effect of superficial gas velocity and gas residence time. The scrubber has been operated in both countercurrent and cocurrent modes.
2.
Materials and methods
2.1.
Experimental apparatus
Fig. 1 illustrates the experimental set-up used in this study. The hydrogen sulfide (H2 S), carbon dioxide (CO2 ), methane (CH4 ) and nitrogen (N2 ) inlet concentrations were adjusted by mass flow controllers (3) at the outlet of standard gas cylinders respectively, and mixed with a manifold (4) for preparing simulated refinery gas mixtures before entering to scrubber
Fig. 1 – Scheme of the experimental set-up. Please cite this article in press as: Üresin, E., et al., An experimental study for H2 S and CO2 removal via caustic scrubbing system. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.06.013
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column. There were two different scrubber columns (8) used for the experiments in this study. The first one was equipped with packing materials. The internal and external diameters of the packed column were 20.10 mm and 25.4 mm, respectively. The column was constructed from 316 grade stainless steel and had a total height of 85 cm. The total packed bed height and volume of column were 64 cm and 0.2 L, respectively. In the second column static mixer was located. The second column was made of 316 grade stainless steel and had a inner diameter of 30.15 mm, a external diameter of 33.7 mm and a total height of 45 cm. The total static mixer height was 29 cm. A 30 L tank (5) allowing to store the scrubbing liquid was placed downstream the column. The liquid phase was dispersed by means of a spray nozzle located at the top of the both columns, whereas the gas phase was sparged at the bottom of the column in countercurrent experiments. The concentration of NaOH in the scrubbing solution was controlled by a pH meter (6). The flowmeter (2) used for measuring liquid flow rates was made from stainless steel to reduce the corrosion problem. The spent scrubber liquid-gas mixture was stored in a waste tank (10) and drained by a valve which was mounted at the bottom of the tank. A water trap (11) was placed in the column outlet to remove entrained liquid droplets from the gas stream.
2.2.
Experimental procedure
The experiments were carried out at 20 ◦ C. H2 S, CO2 , CH4 and N2 gases were injected from gas cylinders and the volumetric mixed gas flow rate QG , controlled with mass flow controllers, ranged from 0.18 to 0.30 m3 /h. Packing material and static mixer have a diameter of 20 mm and 30 mm respectively. In the experiments, commercial metal sheet structured pack was used to generate a packing bed in the column. This kind of commercial structured pack has been manufactured with series of channels. The channels are consist of corrugated stainless steel sheets which are parallel with each other and located vertically. Corrugated sheets are arranged in a crisscrossing relationship to create flow channels for the vapor shape. The intersections of the corrugated sheets create mixing points for the liquid and vapor phases. Designed each static mixer was used in the experiments have slightly venturi shape, an internal diameter of 30 mm, length of 96.5 mm and 11 mixing baffles. The baffles were located equidistant from each other in the vertical direction and consecutive ones
Fig. 2 – Schematic representation of the static mixer.
Table 1 – Main characteristics of the static mixer. Operating parameters
Case 1
Internal diameter (mm) Element thickness (mm) Total length of static mixer (mm) Number of internal elements
30 0.5 290 33
Table 2 – Geometric characteristics of packing material and static mixer used in the present study.
Internal diameter (mm) Packed bed height (mm) Void fraction,
Packing material
Static mixer
20 640 0.86
30 290 0.90
are attached to the opposite sides of the static mixer wall in step-wise manner (Fig. 2). Table 1 lists the main characteristics of the static mixer. The liquid stream passes into the throat sections of the static mixer, where the high gas velocity and the associated low pressure region causes atomization of the liquid into small droplets to occur. Also, when liquid is introduced at the throat the high turbulence zone is created. Small droplets formed by atomization join together by hydrostatic force to form droplets cloud which would move as a single system and have a much larger effective diameter. So, these atomized liquid droplets improve the gas–liquid interaction and increase the surface area tremendously which allows enhanced absorption of desired components of interest. Nevertheless, the contact surface area increase gradually at the upper and the lowest region of the static mixer due to its designed shape. The geometric characteristics of the commercial packing and static mixer used are shown comparatively in Table 2. The scrubbing liquid solution was stored fed to the column by spraying the solution with the nozzle at a constant rate of 2.5 L/h. During the experiments the pH value of the liquid solution was measured continuously and the caustic (NaOH aqueous solution) was added if it is necessary for adjusting the pH as 11. The operating parameters are shown in Table 3. By-pass line valves were mounted to appropriate locations on the systems and the scrubber was operated in countercurrent and cocurrent modes. At the first stage of experiments, direction was countercurrent with mixed gas entering from the bottom of the column and flowing upward through the column. Cocurrent direction was studied in the second stage of experiments where mixed gas enters from the top of the packed bed as with the same direction of the sprayed liquid. Dry outlet gas stream is fed to a gas chromatography. Gas analysis system consists of two gas chromatographs. Sulfide containing gases are analyzed with a gas chromatography instrument equipped with a flame photometric (FPD) and a pulsed flame photometric detector (PFPD). Permanent gases (H2 , CO, CO2 , CH4 , N2 ) are detected by a gas chromatography
Table 3 – Operating parameters of the system for each experiment. Operating parameters 3
Gas flow rate (m /h) Liquid flow rate (L/h) Liquid/gas ratio (L/m3 ) NaOH concentration (g/L) Operating temperature (K) Operating absolute pressure (Pa)
Case 1
Case 2
Case 3
0.18 2.50 8.30 0.25 293 ≈105 Pa
0.24 2.50 10.40 0.25 293 ≈105 Pa
0.30 2.50 13.80 0.25 293 ≈105 Pa
Please cite this article in press as: Üresin, E., et al., An experimental study for H2 S and CO2 removal via caustic scrubbing system. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.06.013
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3.
Results and discussion
Absorption experiments have been realized in two different gas compositions. The first set of experiments, the simulated gas composed of H2 S in balance nitrogen was used. The level of H2 S has been adjusted by using a secondary nitrogen diluent gas. Thereby, four different inlet conditions for H2 S concentration in nitrogen have been adjusted as namely 250 ppmv, 500 ppmv, 750 ppmv and 1000 ppmv in balance nitrogen. In the second set of experiments, the effect of CO2 , which is in competition with H2 S to react with NaOH, was investigated. To do this, a simulated gas composed of 25% CO2 , 25% CH4 and balance nitrogen was prepared. A separate H2 S in balance nitrogen stream was used to adjust H2 S levels to 250 ppmv, 500 ppmv, 750 ppmv and 1000 ppmv, respectively. Each of the reactant gas streams were scrubbed with a caustic solution, prepared by NaOH. At every set of experiments, pH of the caustic solution was maintained as 11. Countercurrent and cocurrent modes of operation were tested as well. In the scrubbing processes, sodium hyroxide (NaOH) is known commercially as caustic solution and NaOH reacts with H2 S dissolved in aqueous solution. Hydrogen sulfide and caustic reaction is in the water phase, thus hydrogen sulfide is diffused from organic phase to the interface and then entered to the water phase. Sodium sulfide (Na2 S) and sodium hydrosulfide (NaHS) are reaction products of the reaction as specified by the Eqs. (1) and (2). Sodium sulfide has partial miscibility in water. To maintain removal quality, controlling the pH of the spent NaOH solution is required otherwise NaHS can occur as predominant sulfide species. So, if caustic is added, H2 S is removed, and if the pH of NaOH decreases, it becomes acidic and H2 S is produced. The following equations describe the reaction mechanism of H2 S and caustic solution: NaOH + H2 S → NaHS + H2 O
(1)
2NaOH + H2 S → Na2 S + 2H2 O
(2)
Also, the assessment of system efficiency had to take into account NaOH parasitical consumption due to carbon dioxide (CO2 ) absorption. Carbon dioxide in gas streams complicates using NaOH solution for H2 S scrubbing, because CO2 is readily scrubbed into NaOH solution as well (Eq. (3)). Consequently, the presence of CO2 concentration increases the consumption of NaOH solution, thus decreases the H2 S removal. 2NaOH + CO2 → Na2 CO3 + H2 O
(3)
The results of caustic scrubbing have been seen in Fig. 3. It is apparent that near 100 percent of H2 S capture was succeeded in the case of nitrogen atmosphere, whatever of the inlet H2 S level was and without depending on the flow mode. On the other hand, the level of H2 S removal affected in a negative way when a competitive acid gas presented for both of the flow modes. In spite of the better H2 S scrubbing efficiencies have been obtained in countercurrent flow mode, the efficiency values were decreased with increasing the inlet H2 S levels. The decrease in H2 S removal efficiencies with increasing the inlet H2 S concentration can be explained by an important decrease
100
H2S removal efficiency, %
with two thermal conductivity detectors (TCD). The outlet gas was sent to both of these two instruments and so H2 S, CO2 , CH4 and N2 concentrations were measured simultaneously.
80
60
40
20
0
200
400
600
800
1000
Inlet H2S concentration, ppmv
countercurrent; mixture
countercurrent; N2
cocurrent; mixt ure
cocurrent; N2
Fig. 3 – H2 S removal efficiency (%) in N2 atmosphere and CO2 , CH4 and N2 mixture in cocurrent and countercurrent modes on commercial packing. in the pH all along the reactor, possibly due to the presence of CO2 in excess concentration. The superficial gas velocity and residence time play important roles in scrubbing processes. To determine the effects of superficial gas velocity and residence time, three variable flow rates (0.18, 0.24, 0.30 m3 /h) were used for both packed bed column and static mixer column during our experiments. As the gas flow rate increases, the superficial gas velocity increases through the column which has a constant diameter. By the way, with the increase in the superficial gas velocity, the residence time decreased. So, three variable superficial gas velocity and residence time values were obtained for each two columns of different diameters. These also provided to compare performances of static mixer column and packed column in terms of superficial gas velocity. In order to investigate the influence of gas residence time on the H2 S removal, the gas residence times were changed in between 2.4 s and 4 s. In the experimented range, it was shown in Fig. 4 that the H2 S removal efficiency decreased with decreasing the gas residence times. This is reasonable since one would expect a decrease in H2 S removal efficiency by limiting the contact time. On the other hand, as claimed by Turpin et al. the degree of H2 S removal might be increased with the gas velocity, since an increase in the superficial gas velocity would bring an increase in the volumetric mass transfer coefficient. According to them, when the superficial gas velocity increases, the gas prevents the droplets from falling inside the column and the droplets residence time in the column increases. So, the exchange area available for the mass transfer gets bigger. It may be proposed here that the residence time in the column might be more effective on the H2 S removal efficiency than the effect of volumetric mass transfer coefficient in the experimented range. In other words, all superficial gas velocities in the experiments were high enough providing less variations of the mass transfer coefficients (Turpin et al., 2008). To investigate the effect of the geometry of structured packing material, a static mixer has been designed, manufactured and used in the experiments. The properties of commercial structured packing and in house static mixer have been given in Table 1. The diameter of the static mixer and the commercial packing were 30 mm and 20 mm, respectively. To maintain
Please cite this article in press as: Üresin, E., et al., An experimental study for H2 S and CO2 removal via caustic scrubbing system. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.06.013
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100 90
H2S-CO2 removal efficiencies, %
H2S
CO2 removal at
250 ppmv inlet H2S CO2 removal at
80
500 ppmv inlet H2S
70
CO2 removal at
60
CO2 removal at
50
H2S removal at
750 ppmv inlet H2S 1000 ppmv inlet H2S 250 ppmv inlet H2S H2S removal at
40
500 ppmv inlet H2S H2S removal at
30
750 ppmv inlet H2S CO2
20
H2S removal at
1000 ppmv inlet H2S
10 0
2.5
3
3.5
4
4.5
Gas residence time, sec
Fig. 4 – H2 S removal efficiency (%) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing.
the same volume of the packing column, the height of the static mixer has been adjusted accordingly. Hence, while the superficial gas velocities were changed for each point of experiment, the gas residence times were constant for the same experimental conditions of two different absorber units (commercial packing materials and static mixer) geometries. As seen in Fig. 5, better H2 S removal efficiencies have been obtained with the static mixer than the commercial structured packing. Although the H2 S removal efficiencies were affected more at lower gas residence time of ∼2 s for commercial structured packing, no remarkable change was observed with the static mixer (Fig. 5). At every experimented gas residence time, the quite stable results have been obtained with the use of static mixer. Solely, at the lowest inlet H2 S concentration of 250 ppmv and high residence times of 3 and 4 s, the H2 S removal performances of both packings were merged. Fig. 6 shows the results for the higher inlet H2 S loads of 500, 750 and 1000 ppmv. At the higher inlet H2 S loads,
the band between the H2 S removal efficiency plots were broadened showing remarkable performance improvements with the static mixer. Relatively stable H2 S removal performances were obtained with the static mixer for all the experimented gas residence times. To investigate the origin of the H2 S removal performances with the use of static mixer, the results were re-evaluated according to the changes with superficial gas velocities (Fig. 7). High cross-sectional area of static mixer located column presented lower superficial gas velocities when the volume of the packing column and the gas flow rate were kept constant for each of the absorber units for every gas residence time points. Hence while the gas residence time was changing, the superficial gas velocities also changed. As seen in Fig. 7, the H2 S removal performances were changed with the superficial gas velocities as well. The less the superficial gas velocity was, the more the H2 S removal performance was meaning that
100
100
90
H2S removal efficiency, %
H2S removal efficiency, %
90
80
70
60
80
70 500 ppmv; commercial packing 750 ppmv; commercial packing 1000 ppmv; commercial packing 500 ppmv; static mixer 750 ppmv; static mixer 1000 ppmv; static mixer
60
50 250 ppmv
40 2
2.5
3
3.5
4
Gas residence time, sec static mixer
50
commercial pack ing
Fig. 5 – H2 S removal efficiency (%) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing and static mixer at 250 ppmv inlet H2 S.
40 2.5
3
3.5
4
Gas residence time, sec
Fig. 6 – H2 S removal efficiency (%) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing and static mixer at 500, 750 and 1000 ppmv inlet H2 S.
Please cite this article in press as: Üresin, E., et al., An experimental study for H2 S and CO2 removal via caustic scrubbing system. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.06.013
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100
100 90
CO2 removal at
250 ppmv inlet H2S
H2S-CO2 removal efficiencies, %
H2S removal efficiency, %
90
80
70
60
80
CO2 removal at
500 ppmv inlet H2S
70
CO2 removal at
60
750 ppmv inlet H2S
50
1000 ppmv inlet H2S
CO2 removal at H2S removal at
250 ppmv inlet H2S
40
H2S removal at
500 ppmv inlet H2S
30
H2S removal at
20
50 static mixer
750 ppmv inlet H2S
commercial packing
H2S removal at
10
1000 ppmv inlet H2S
40 0
0.05
0.1
0.15
0.2
0.25
0
0.3
2.5
Superficial gas velocity, m/sec 250 ppmv
500 ppmv
750 ppmv
3
3.5
4
4.5
Gas residence time, sec 1000 ppmv
Fig. 9 – H2 S removal efficiency (%) in CO2 , CH4 and N2 mixture with countercurrent modes on static mixer.
Fig. 7 – H2 S removal efficiency (%) versus superficial gas velocity (m/s) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing and static mixer.
4.
Conclusions
40
CO2 removal efficiency, %
35
30
25
20
15
10
5
comm ercial packing
static mixer
0 0
0.05
0.1
0.15
0.2
0.25
0.3
Superficial gas velocity, m/sec at 250 ppmv inlet H2S
at 500 ppmv inlet H2S
at 750 ppmv inlet H2S
at 1000 ppmv inlet H2S
Fig. 8 – CO2 removal efficiency (%) versus superficial gas velocity (m/s) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing and static mixer at 250, 500, 750 and 1000 ppmv inlet H2 S.
the superficial gas velocity is a very important parameter in determining the overall scrubbing performance. The same effect was also examined for the CO2 removal via caustic scrubbing. Fig. 8 presents the results of CO2 removal efficiencies with the superficial gas velocities. A change in the CO2 removal efficiency with the superficial gas velocity was observed individually for each of the packing geometry. For each of the packed material, the CO2 removal efficiencies were increased with the superficial gas velocity. This might mean that the CO2 removal efficiency was seen to depend on the packing material geometry as well, besides its dependency on the superficial gas velocity. Fig. 9 gives the change of H2 S and CO2 removal efficiencies with the gas residence time for the static mixer type packing. In agreement with the scrubber literature, the selective H2 S removal was accomplished by limiting the contact time between the gas and the liquid in a static mixer, as was shown in Fig. 9.
The key parameters to design a packed bed scrubber unit were the gas residence time, superficial gas velocity, flow mode and packing geometry. The better H2 S scrubbing efficiencies have been obtained in the countercurrent flow mode. It was shown that the lower the superficial gas velocity was, the higher the removal efficiency for both acid gases, namely H2 S and CO2 . The effect of superficial gas velocities was interpreted that the gas residence time as changing with the gas velocity was more effective on acid gas removal efficiency than its effect on liquid droplets residence time within the experimented range. On the other hand, although both of the acid gas removal efficiencies were decreased with decreasing the gas residence time, the degree of decrease for CO2 , due to its relatively weak reactivity with NaOH, was more noticeable in relation to H2 S. Coexistence of CO2 and H2 S has been shown to affect the design of the scrubber. The presence of CO2 leads to increase in NaOH consumption along the scrubber column thereby decreasing the H2 S removal efficiency. This effect was more remarkable at higher H2 S loads. In order to find the best design values, gas residence time should be optimized regarding the lowest CO2 removal at the highest H2 S removal. The results showed that a gas residence time of 3 s and more seemed to be close to the optimum point. The static mixer might be preferable over the structured packings not only due to its cost effective nature and resistance to plugging but also its superior performance in H2 S removal when accordingly designed.
Acknowledgement We greatly acknowledge The Scientific and Technological Research Council of Turkey (TÜBI˙TAK) for supporting of “Fuel Production from Biomass and Coal Blends” project (Contract code: 108G043) in which this study was carried out.
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Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
An experimental study for H2 S and CO2 removal via caustic scrubbing system a ˘ Ersin Üresin a,∗,1 , Halil I˙brahim Sarac¸ b , Alper Sarıoglan , S¸iringül Ay a , Fehmi Akgün a a b
TÜBI˙TAK Marmara Research Center Energy Institute, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey Kocaeli University, Mechanical Engineering Department, Kocaeli, Turkey
a b s t r a c t In this study, removal of hydrogen sulfide (H2 S) and carbon dioxide (CO2 ) from simulated syngas has been studied on one column scrubbing system. Gas flow rate as a measure of gas residence time and superficial gas velocity, gas composition, inlet H2 S load, flow modes (countercurrent and cocurrent) and packing geometry were the parameters in the design and/or operation of an acid gas scrubber system. Better H2 S scrubbing efficiencies have been obtained in countercurrent flow mode than that of cocurrent flow mode. When accordingly designed, static mixer with its superior performance on H2 S removal overweighed to structured packings. The coexistence of CO2 and H2 S has been shown to increase the sodium hydroxide (NaOH) consumption along the scrubber column thereby decreasing the H2 S removal efficiency at higher H2 S loads. The gas residence time as changing with the gas velocity was found to be more dominant on acid gas removal efficiency than the effect of superficial gas velocity within the experimented range. A gas residence times of equal or above 3 s were seemed to be closer to the optimum point. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Gas clean up; Packed bed gas scrubbers; Hydrogen sulfide removal; CO2 removal; NaOH; Static mixer
1.
Introduction
Particulate, ammonia (NH3 ) and hydrogen sulfide (H2 S) comprise the main impurities of synthetic raw gas produced by coal gasification. Removal of these contaminants is an essential step in coal and biomass gasification process to avoid its detrimental effects on materials and environment. Hydrogen sulfide and ammonia can cause corrosion in pipelines and thus limit plant lifetime. These are also a well-known catalyst poisoner. Therefore, utilization of gasification products either as a fuel for gas turbines, gas engines or fuel cells or as a syngas for Fischer–Tropsch fuel, ammonia or methanol productions requires an effective gas clean-up technology (Chen et al., 2001; Moussavi et al., 2008; Panza and Belgiorno, 2010). For the abatement of H2 S and NH3 , packed towers are widely used for chemical and physical scrubbing. The choice of the solution is critical in determining dissolution or chemical
reaction rate of pollutant in the liquid. In physical scrubbing, the pollutants in basic nature can be passivated by the acidic solutions and vica-versa. Currently, the processes of choice in the commercial integrated gasification combined cycle (IGCC) facilities for the removal of acid gases are both the chemical solvent acid gas removal processes based on aqueous methyldiethanolamine (MDEA) and the physical solvent-based Selexol process which uses mixtures of dimethyl ethers and polyethylene glycol (Couvert et al., 2008; Turpin et al., 2008; Ohtsuka et al., 2009; Wang et al., 2004; Wallin and Olausson, 1993). Alkaline hypochlorite, hydrogen peroxide or caustic solutions are economicaly alternative solvents to be used in physical solvent-based processes and show good results for decades. Sodium hydroxide solution is a very effective, but non-regenerable absorbent for CO2 and H2 S. Therefore the use of caustic is usually limited to the removal of trace amounts of
∗
Corresponding author. Tel.: +90 262 677 27 80; fax: +90 262 641 23 09. E-mail address: [email protected] (E. Üresin). 1 Present address. http://dx.doi.org/10.1016/j.psep.2014.06.013 0957-5820/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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these impurities. For the disposal of the spent caustic, different considerations are applied such as simple neutralization or its use in pulp and paper mills after some certain quality control analysis (Couvert et al., 2006; Miltner et al., 2012; Mamrosh et al., 2008; Bontozoglou and Karabelas, 1991). For the improvement of scrubbing efficiencies, design of the scrubber system is as important as the choice of the solvent. Multiple gas sorption columns packed with structured materials are used in order to increase the active surface area and residence time for an efficient gas–liquid interaction. High efficiencies of commercial structured packing materials for H2 S removal with an alkaline washing solution was attributed to a high contact area existing between the gas and the liquid phases. The major drawback in the use of structured packing is their excessive price per volume unit. Static mixers, if properly designed, can be acceptable in terms of investment costs and have higher resistance to pluggings. Nevertheless, high mass transfer performances cannot be reached by static mixers as structured packing materials can be. Hence, the design of a static mixer has crucial importance in generating high interfacial areas and improving mass transfer rates (Couvert et al., 2008; Mamrosh et al., 2008; Godini and Mowla, 2008). Basic design parameters of the structured packing columns are hydrodynamic flow characteristics and packing surface area as determined by the packing material geometry, residence time, liquid to gas ratio, pH and reactivity of the solution. To avoid high investment and operation costs, the scrubber system should be designed by taking into consideration of these parameters (Bhave et al., 2008; Biard et al., 2010). Although design studies and commercial applications on structured packings were found in the literature extensively (Miltner et al., 2012; Godini and Mowla, 2008; Bhave et al., 2008; Aliabad and Mirzaei, 2009; Dang et al., 1998; Lu et al., 2006;
Ballaguet et al., 2003) there were only a sufficient number of comparison studies differentiating the performances of both the static mixer and the structured packing for the acid gas removal. Static mixers have been known with their excellent gas–liquid contact due to a high turbulence rate inside and their potential on well mixed gas distribution giving high mass transfer rates (Couvert et al., 2008; Sanchez et al., 2008). Consequently, the objective of this study is to design a static mixer being able to generate high interfacial areas and improved mass transfer rates for acid gas alkali washing towers and compare the results with those of commercial packings. In order to elucidate the advantages of static mixers over structured packings, the removal of H2 S has been studied on one column scrubbing system on both of the designed static mixer and commercial structured packing. The effect of gas composition has been investigated by using H2 S in nitrogen and H2 S in simulated refinery gas mixtures. For these two different gas atmospheres, inlet H2 S concentration was increased in stepwise. The gas flow rate as a measure of gas residence time was changed to monitor the effect of superficial gas velocity and gas residence time. The scrubber has been operated in both countercurrent and cocurrent modes.
2.
Materials and methods
2.1.
Experimental apparatus
Fig. 1 illustrates the experimental set-up used in this study. The hydrogen sulfide (H2 S), carbon dioxide (CO2 ), methane (CH4 ) and nitrogen (N2 ) inlet concentrations were adjusted by mass flow controllers (3) at the outlet of standard gas cylinders respectively, and mixed with a manifold (4) for preparing simulated refinery gas mixtures before entering to scrubber
Fig. 1 – Scheme of the experimental set-up. Please cite this article in press as: Üresin, E., et al., An experimental study for H2 S and CO2 removal via caustic scrubbing system. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.06.013
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column. There were two different scrubber columns (8) used for the experiments in this study. The first one was equipped with packing materials. The internal and external diameters of the packed column were 20.10 mm and 25.4 mm, respectively. The column was constructed from 316 grade stainless steel and had a total height of 85 cm. The total packed bed height and volume of column were 64 cm and 0.2 L, respectively. In the second column static mixer was located. The second column was made of 316 grade stainless steel and had a inner diameter of 30.15 mm, a external diameter of 33.7 mm and a total height of 45 cm. The total static mixer height was 29 cm. A 30 L tank (5) allowing to store the scrubbing liquid was placed downstream the column. The liquid phase was dispersed by means of a spray nozzle located at the top of the both columns, whereas the gas phase was sparged at the bottom of the column in countercurrent experiments. The concentration of NaOH in the scrubbing solution was controlled by a pH meter (6). The flowmeter (2) used for measuring liquid flow rates was made from stainless steel to reduce the corrosion problem. The spent scrubber liquid-gas mixture was stored in a waste tank (10) and drained by a valve which was mounted at the bottom of the tank. A water trap (11) was placed in the column outlet to remove entrained liquid droplets from the gas stream.
2.2.
Experimental procedure
The experiments were carried out at 20 ◦ C. H2 S, CO2 , CH4 and N2 gases were injected from gas cylinders and the volumetric mixed gas flow rate QG , controlled with mass flow controllers, ranged from 0.18 to 0.30 m3 /h. Packing material and static mixer have a diameter of 20 mm and 30 mm respectively. In the experiments, commercial metal sheet structured pack was used to generate a packing bed in the column. This kind of commercial structured pack has been manufactured with series of channels. The channels are consist of corrugated stainless steel sheets which are parallel with each other and located vertically. Corrugated sheets are arranged in a crisscrossing relationship to create flow channels for the vapor shape. The intersections of the corrugated sheets create mixing points for the liquid and vapor phases. Designed each static mixer was used in the experiments have slightly venturi shape, an internal diameter of 30 mm, length of 96.5 mm and 11 mixing baffles. The baffles were located equidistant from each other in the vertical direction and consecutive ones
Fig. 2 – Schematic representation of the static mixer.
Table 1 – Main characteristics of the static mixer. Operating parameters
Case 1
Internal diameter (mm) Element thickness (mm) Total length of static mixer (mm) Number of internal elements
30 0.5 290 33
Table 2 – Geometric characteristics of packing material and static mixer used in the present study.
Internal diameter (mm) Packed bed height (mm) Void fraction,
Packing material
Static mixer
20 640 0.86
30 290 0.90
are attached to the opposite sides of the static mixer wall in step-wise manner (Fig. 2). Table 1 lists the main characteristics of the static mixer. The liquid stream passes into the throat sections of the static mixer, where the high gas velocity and the associated low pressure region causes atomization of the liquid into small droplets to occur. Also, when liquid is introduced at the throat the high turbulence zone is created. Small droplets formed by atomization join together by hydrostatic force to form droplets cloud which would move as a single system and have a much larger effective diameter. So, these atomized liquid droplets improve the gas–liquid interaction and increase the surface area tremendously which allows enhanced absorption of desired components of interest. Nevertheless, the contact surface area increase gradually at the upper and the lowest region of the static mixer due to its designed shape. The geometric characteristics of the commercial packing and static mixer used are shown comparatively in Table 2. The scrubbing liquid solution was stored fed to the column by spraying the solution with the nozzle at a constant rate of 2.5 L/h. During the experiments the pH value of the liquid solution was measured continuously and the caustic (NaOH aqueous solution) was added if it is necessary for adjusting the pH as 11. The operating parameters are shown in Table 3. By-pass line valves were mounted to appropriate locations on the systems and the scrubber was operated in countercurrent and cocurrent modes. At the first stage of experiments, direction was countercurrent with mixed gas entering from the bottom of the column and flowing upward through the column. Cocurrent direction was studied in the second stage of experiments where mixed gas enters from the top of the packed bed as with the same direction of the sprayed liquid. Dry outlet gas stream is fed to a gas chromatography. Gas analysis system consists of two gas chromatographs. Sulfide containing gases are analyzed with a gas chromatography instrument equipped with a flame photometric (FPD) and a pulsed flame photometric detector (PFPD). Permanent gases (H2 , CO, CO2 , CH4 , N2 ) are detected by a gas chromatography
Table 3 – Operating parameters of the system for each experiment. Operating parameters 3
Gas flow rate (m /h) Liquid flow rate (L/h) Liquid/gas ratio (L/m3 ) NaOH concentration (g/L) Operating temperature (K) Operating absolute pressure (Pa)
Case 1
Case 2
Case 3
0.18 2.50 8.30 0.25 293 ≈105 Pa
0.24 2.50 10.40 0.25 293 ≈105 Pa
0.30 2.50 13.80 0.25 293 ≈105 Pa
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3.
Results and discussion
Absorption experiments have been realized in two different gas compositions. The first set of experiments, the simulated gas composed of H2 S in balance nitrogen was used. The level of H2 S has been adjusted by using a secondary nitrogen diluent gas. Thereby, four different inlet conditions for H2 S concentration in nitrogen have been adjusted as namely 250 ppmv, 500 ppmv, 750 ppmv and 1000 ppmv in balance nitrogen. In the second set of experiments, the effect of CO2 , which is in competition with H2 S to react with NaOH, was investigated. To do this, a simulated gas composed of 25% CO2 , 25% CH4 and balance nitrogen was prepared. A separate H2 S in balance nitrogen stream was used to adjust H2 S levels to 250 ppmv, 500 ppmv, 750 ppmv and 1000 ppmv, respectively. Each of the reactant gas streams were scrubbed with a caustic solution, prepared by NaOH. At every set of experiments, pH of the caustic solution was maintained as 11. Countercurrent and cocurrent modes of operation were tested as well. In the scrubbing processes, sodium hyroxide (NaOH) is known commercially as caustic solution and NaOH reacts with H2 S dissolved in aqueous solution. Hydrogen sulfide and caustic reaction is in the water phase, thus hydrogen sulfide is diffused from organic phase to the interface and then entered to the water phase. Sodium sulfide (Na2 S) and sodium hydrosulfide (NaHS) are reaction products of the reaction as specified by the Eqs. (1) and (2). Sodium sulfide has partial miscibility in water. To maintain removal quality, controlling the pH of the spent NaOH solution is required otherwise NaHS can occur as predominant sulfide species. So, if caustic is added, H2 S is removed, and if the pH of NaOH decreases, it becomes acidic and H2 S is produced. The following equations describe the reaction mechanism of H2 S and caustic solution: NaOH + H2 S → NaHS + H2 O
(1)
2NaOH + H2 S → Na2 S + 2H2 O
(2)
Also, the assessment of system efficiency had to take into account NaOH parasitical consumption due to carbon dioxide (CO2 ) absorption. Carbon dioxide in gas streams complicates using NaOH solution for H2 S scrubbing, because CO2 is readily scrubbed into NaOH solution as well (Eq. (3)). Consequently, the presence of CO2 concentration increases the consumption of NaOH solution, thus decreases the H2 S removal. 2NaOH + CO2 → Na2 CO3 + H2 O
(3)
The results of caustic scrubbing have been seen in Fig. 3. It is apparent that near 100 percent of H2 S capture was succeeded in the case of nitrogen atmosphere, whatever of the inlet H2 S level was and without depending on the flow mode. On the other hand, the level of H2 S removal affected in a negative way when a competitive acid gas presented for both of the flow modes. In spite of the better H2 S scrubbing efficiencies have been obtained in countercurrent flow mode, the efficiency values were decreased with increasing the inlet H2 S levels. The decrease in H2 S removal efficiencies with increasing the inlet H2 S concentration can be explained by an important decrease
100
H2S removal efficiency, %
with two thermal conductivity detectors (TCD). The outlet gas was sent to both of these two instruments and so H2 S, CO2 , CH4 and N2 concentrations were measured simultaneously.
80
60
40
20
0
200
400
600
800
1000
Inlet H2S concentration, ppmv
countercurrent; mixture
countercurrent; N2
cocurrent; mixt ure
cocurrent; N2
Fig. 3 – H2 S removal efficiency (%) in N2 atmosphere and CO2 , CH4 and N2 mixture in cocurrent and countercurrent modes on commercial packing. in the pH all along the reactor, possibly due to the presence of CO2 in excess concentration. The superficial gas velocity and residence time play important roles in scrubbing processes. To determine the effects of superficial gas velocity and residence time, three variable flow rates (0.18, 0.24, 0.30 m3 /h) were used for both packed bed column and static mixer column during our experiments. As the gas flow rate increases, the superficial gas velocity increases through the column which has a constant diameter. By the way, with the increase in the superficial gas velocity, the residence time decreased. So, three variable superficial gas velocity and residence time values were obtained for each two columns of different diameters. These also provided to compare performances of static mixer column and packed column in terms of superficial gas velocity. In order to investigate the influence of gas residence time on the H2 S removal, the gas residence times were changed in between 2.4 s and 4 s. In the experimented range, it was shown in Fig. 4 that the H2 S removal efficiency decreased with decreasing the gas residence times. This is reasonable since one would expect a decrease in H2 S removal efficiency by limiting the contact time. On the other hand, as claimed by Turpin et al. the degree of H2 S removal might be increased with the gas velocity, since an increase in the superficial gas velocity would bring an increase in the volumetric mass transfer coefficient. According to them, when the superficial gas velocity increases, the gas prevents the droplets from falling inside the column and the droplets residence time in the column increases. So, the exchange area available for the mass transfer gets bigger. It may be proposed here that the residence time in the column might be more effective on the H2 S removal efficiency than the effect of volumetric mass transfer coefficient in the experimented range. In other words, all superficial gas velocities in the experiments were high enough providing less variations of the mass transfer coefficients (Turpin et al., 2008). To investigate the effect of the geometry of structured packing material, a static mixer has been designed, manufactured and used in the experiments. The properties of commercial structured packing and in house static mixer have been given in Table 1. The diameter of the static mixer and the commercial packing were 30 mm and 20 mm, respectively. To maintain
Please cite this article in press as: Üresin, E., et al., An experimental study for H2 S and CO2 removal via caustic scrubbing system. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.06.013
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100 90
H2S-CO2 removal efficiencies, %
H2S
CO2 removal at
250 ppmv inlet H2S CO2 removal at
80
500 ppmv inlet H2S
70
CO2 removal at
60
CO2 removal at
50
H2S removal at
750 ppmv inlet H2S 1000 ppmv inlet H2S 250 ppmv inlet H2S H2S removal at
40
500 ppmv inlet H2S H2S removal at
30
750 ppmv inlet H2S CO2
20
H2S removal at
1000 ppmv inlet H2S
10 0
2.5
3
3.5
4
4.5
Gas residence time, sec
Fig. 4 – H2 S removal efficiency (%) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing.
the same volume of the packing column, the height of the static mixer has been adjusted accordingly. Hence, while the superficial gas velocities were changed for each point of experiment, the gas residence times were constant for the same experimental conditions of two different absorber units (commercial packing materials and static mixer) geometries. As seen in Fig. 5, better H2 S removal efficiencies have been obtained with the static mixer than the commercial structured packing. Although the H2 S removal efficiencies were affected more at lower gas residence time of ∼2 s for commercial structured packing, no remarkable change was observed with the static mixer (Fig. 5). At every experimented gas residence time, the quite stable results have been obtained with the use of static mixer. Solely, at the lowest inlet H2 S concentration of 250 ppmv and high residence times of 3 and 4 s, the H2 S removal performances of both packings were merged. Fig. 6 shows the results for the higher inlet H2 S loads of 500, 750 and 1000 ppmv. At the higher inlet H2 S loads,
the band between the H2 S removal efficiency plots were broadened showing remarkable performance improvements with the static mixer. Relatively stable H2 S removal performances were obtained with the static mixer for all the experimented gas residence times. To investigate the origin of the H2 S removal performances with the use of static mixer, the results were re-evaluated according to the changes with superficial gas velocities (Fig. 7). High cross-sectional area of static mixer located column presented lower superficial gas velocities when the volume of the packing column and the gas flow rate were kept constant for each of the absorber units for every gas residence time points. Hence while the gas residence time was changing, the superficial gas velocities also changed. As seen in Fig. 7, the H2 S removal performances were changed with the superficial gas velocities as well. The less the superficial gas velocity was, the more the H2 S removal performance was meaning that
100
100
90
H2S removal efficiency, %
H2S removal efficiency, %
90
80
70
60
80
70 500 ppmv; commercial packing 750 ppmv; commercial packing 1000 ppmv; commercial packing 500 ppmv; static mixer 750 ppmv; static mixer 1000 ppmv; static mixer
60
50 250 ppmv
40 2
2.5
3
3.5
4
Gas residence time, sec static mixer
50
commercial pack ing
Fig. 5 – H2 S removal efficiency (%) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing and static mixer at 250 ppmv inlet H2 S.
40 2.5
3
3.5
4
Gas residence time, sec
Fig. 6 – H2 S removal efficiency (%) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing and static mixer at 500, 750 and 1000 ppmv inlet H2 S.
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100
100 90
CO2 removal at
250 ppmv inlet H2S
H2S-CO2 removal efficiencies, %
H2S removal efficiency, %
90
80
70
60
80
CO2 removal at
500 ppmv inlet H2S
70
CO2 removal at
60
750 ppmv inlet H2S
50
1000 ppmv inlet H2S
CO2 removal at H2S removal at
250 ppmv inlet H2S
40
H2S removal at
500 ppmv inlet H2S
30
H2S removal at
20
50 static mixer
750 ppmv inlet H2S
commercial packing
H2S removal at
10
1000 ppmv inlet H2S
40 0
0.05
0.1
0.15
0.2
0.25
0
0.3
2.5
Superficial gas velocity, m/sec 250 ppmv
500 ppmv
750 ppmv
3
3.5
4
4.5
Gas residence time, sec 1000 ppmv
Fig. 9 – H2 S removal efficiency (%) in CO2 , CH4 and N2 mixture with countercurrent modes on static mixer.
Fig. 7 – H2 S removal efficiency (%) versus superficial gas velocity (m/s) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing and static mixer.
4.
Conclusions
40
CO2 removal efficiency, %
35
30
25
20
15
10
5
comm ercial packing
static mixer
0 0
0.05
0.1
0.15
0.2
0.25
0.3
Superficial gas velocity, m/sec at 250 ppmv inlet H2S
at 500 ppmv inlet H2S
at 750 ppmv inlet H2S
at 1000 ppmv inlet H2S
Fig. 8 – CO2 removal efficiency (%) versus superficial gas velocity (m/s) in CO2 , CH4 and N2 mixture with countercurrent modes on commercial packing and static mixer at 250, 500, 750 and 1000 ppmv inlet H2 S.
the superficial gas velocity is a very important parameter in determining the overall scrubbing performance. The same effect was also examined for the CO2 removal via caustic scrubbing. Fig. 8 presents the results of CO2 removal efficiencies with the superficial gas velocities. A change in the CO2 removal efficiency with the superficial gas velocity was observed individually for each of the packing geometry. For each of the packed material, the CO2 removal efficiencies were increased with the superficial gas velocity. This might mean that the CO2 removal efficiency was seen to depend on the packing material geometry as well, besides its dependency on the superficial gas velocity. Fig. 9 gives the change of H2 S and CO2 removal efficiencies with the gas residence time for the static mixer type packing. In agreement with the scrubber literature, the selective H2 S removal was accomplished by limiting the contact time between the gas and the liquid in a static mixer, as was shown in Fig. 9.
The key parameters to design a packed bed scrubber unit were the gas residence time, superficial gas velocity, flow mode and packing geometry. The better H2 S scrubbing efficiencies have been obtained in the countercurrent flow mode. It was shown that the lower the superficial gas velocity was, the higher the removal efficiency for both acid gases, namely H2 S and CO2 . The effect of superficial gas velocities was interpreted that the gas residence time as changing with the gas velocity was more effective on acid gas removal efficiency than its effect on liquid droplets residence time within the experimented range. On the other hand, although both of the acid gas removal efficiencies were decreased with decreasing the gas residence time, the degree of decrease for CO2 , due to its relatively weak reactivity with NaOH, was more noticeable in relation to H2 S. Coexistence of CO2 and H2 S has been shown to affect the design of the scrubber. The presence of CO2 leads to increase in NaOH consumption along the scrubber column thereby decreasing the H2 S removal efficiency. This effect was more remarkable at higher H2 S loads. In order to find the best design values, gas residence time should be optimized regarding the lowest CO2 removal at the highest H2 S removal. The results showed that a gas residence time of 3 s and more seemed to be close to the optimum point. The static mixer might be preferable over the structured packings not only due to its cost effective nature and resistance to plugging but also its superior performance in H2 S removal when accordingly designed.
Acknowledgement We greatly acknowledge The Scientific and Technological Research Council of Turkey (TÜBI˙TAK) for supporting of “Fuel Production from Biomass and Coal Blends” project (Contract code: 108G043) in which this study was carried out.
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Please cite this article in press as: Üresin, E., et al., An experimental study for H2 S and CO2 removal via caustic scrubbing system. Process Safety and Environmental Protection (2014), http://dx.doi.org/10.1016/j.psep.2014.06.013