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Biogas purification performance of new water scrubber packed with sponge carriers

Roslan Noorain, Tomonori Kindaichi, Noriatsu Ozaki, Yoshiteru Aoi, Akiyoshi Ohashi

PII: S0959-6526(18)33929-5
DOI: https://doi.org/10.1016/j.jclepro.2018.12.209
Reference: JCLP 15267

To appear in: Journal of Cleaner Production

Received Date: 8 August 2018

Revised Date: 3 December 2018
Accepted Date: 19 December 2018

Please cite this article as: Noorain R, Kindaichi T, Ozaki N, Aoi Y, Ohashi A, Biogas purification
performance of new water scrubber packed with sponge carriers, Journal of Cleaner Production (2019),
doi: https://doi.org/10.1016/j.jclepro.2018.12.209.

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1 Biogas purification performance of new water scrubber packed with sponge carriers
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3 Roslan Nooraina,b, Tomonori Kindaichia, Noriatsu Ozakia, Yoshiteru Aoic, and Akiyoshi
4 Ohashia,*
5

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a
6 Department of Civil and Environmental Engineering, Graduate School of Engineering,
7 Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan

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b
8 Section of Environmental Engineering Technology, Malaysia Institute of Chemical &
9 Bioengineering Technology, University Kuala Lumpur, Lot 1988 Kawasan Perindustrian Bandar

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10 Vendor Taboh Naning, 78000, Alor Gajah, Melaka, Malaysia
c
11 Department of Molecular Biotechnology, Graduate School of Advanced Sciences of Matter,

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12 Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima 739-8530 Japan
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14 *Corresponding author:
15 Akiyoshi Ohashi
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16 Department of Civil and Environmental Engineering, Graduate School of Engineering,

17 Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan
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18 Phone & Fax: +81-82-424-7823

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19 E–mail: ecoakiyo@hiroshima-u.ac.jp
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21 Nomenclature
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22 Alk = alkalinity
∗
23 = dissolved gas concentration in liquid in equilibrium with partial pressure Pj atm in gas
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24 phase for gas j mol L

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25 Cj = dissolved concentration of gas j mol L

26 , = concentration of each dissolved gas in influent mol L

27 = Henry’s constant atm L mol

28 = mass transfer coefficient (day

29 Kw = water dissociation constant

30 = molar flux of gas j mol L dy

31 Pj = partial pressure atm

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32 , = biogas concentration in influent mol L

33 = biogas flow rate L dy

34 = water flow rate L dy

35 RG = biogas loading rate (L m-3 h-1)

36 RL = water loading rate (L m-3 h-1)
υL = liquid velocity m h

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37

38 ∆Vs = sponge volume L

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39 Abstract

40 Water scrubbing technology is applied to biogas purification to obtain useful gas. However, it is

41 difficult to produce highly purified gas of sufficient quality under typical operational conditions

42 without imposing external pressure. Here, we propose a new water scrubber packed with sponge

carriers instead of conventional packing materials, which has the advantage of increased

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43

44 hydraulic retention time for the scrubbing water. The results of biogas purification experiments

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45 indicate that the proposed scrubber can perform high purification even under atmospheric

46 conditions. An artificial biogas of 60% methane is purified to more than 90% methane with no

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47 hydrogen sulfide detected; this quality level is acceptable for use as city gas. In addition, a

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48 mathematical model to simulate the purification phenomenon is constructed. Simulation
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49 experiments reveal that a high hydraulic retention time is very effective for good performance.

50 We also found that the flow ratio of biogas to scrubbing water is the most crucial among the
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51 various operational parameters governing the purification performance.

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52 Keywords: Biogas purification, hydrogen sulphide, methane upgrading, polyurethane sponge

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53 carrier, water scrubbing technology

54
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55 1. Introduction
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56 Biogas can be biologically produced in anaerobic digestion processes for treating wastewater or
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57 solid organic waste such as animal manure, sewage sludge, agricultural residue, and food scraps

58 (Bauer et al., 2013). Generally, biogas is composed of 50−75% methane (CH4), 25−50% carbon

59 dioxide (CO2), 0−10% nitrogen gas (N2), 0−3% hydrogen sulphide (H2S), 0−1% hydrogen gas

60 (H2), and traces of other gases (Goswami et al., 2016). The main component, CH4, is very

61 valuable as a clean and renewable energy source, which renders biogas a potential candidate to

62 replace fossil fuels.

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63 Biogas is similar to natural gas in most physical and chemical properties. The main

64 differences between these two gases are the CH4 content and their applications. Natural gas

65 normally consists of 75−98% CH4 and has higher burning energy; for instance, a lower heating

66 value of 38.6 MJ Nm-3 is generated from 94% CH4 content (Agarwal et al., 2014). In contrast, a

67 lower heating value of biogas in the range of approximately 15−30 MJ Nm-3 results from much

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68 lower CH4 content (Tippayawong and Thanompongchart, 2010), and this value is far less than

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69 that of natural gas owing to the CO2 content. The large amount of noncombustible CO2 gas

70 present in biogas reduces not only its calorific value, but also the flame velocity and

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71 flammability limit (Wylock and Budzianowski, 2017). Therefore, biogas application is restricted

72 to vehicle engines and city gas. In addition, if biogas is used as a transport vehicle fuel, CO2 gas

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occupies additional space in the storage cylinder tanks, generating additional energy
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73

74 consumption for biogas compression and indirect increase in operational cost (Shah et al., 2016).
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75 Apart from the issue of CO2, even the small amount of H2S present in biogas is

76 problematic as H2S is one of the most harmful environmental pollutants and causes severe
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77 corrosion of equipment such as pipes, engines, pumps, compressors, gas storage tanks, and
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78 valves (Ryckebosch et al., 2011). Moreover, sulfur dioxide (SO2) is formed by H2S combustion,
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79 and this gas is considered more dangerous than H2S. The product of the reaction between SO2

80 and water vapor in the atmosphere induces smog formation and acid rain problems (Wichitpan et
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81 al., 2012).
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82 Various countries have different standards and recommendations for upgrading biogas to

83 vehicle fuel. For example, the Swiss regulations for biogas use in vehicle engines or as city gas

84 state that the standard CO2 and H2S concentrations must be lower than 6% and 5 mg m-3,

85 respectively (Margareta et al., 2006). Nagaoka City, Japan, has purified biogas produced by the

86 municipal sewage treatment plant for use as city gas, satisfying the quality requirements that the

87 heat value must be more than 35.56 MJ Nm-3 (equivalent to 90% CH4 content) and the CO2 and

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88 H2S concentrations must be less than 4% and 2 ppm, respectively (Tatsuo and Shojiro, 2012).

89 Thus, it is essential to remove CO2 and H2S from biogas to attain a feasible energy source.

90 There are several methods of biogas purification: physicochemical absorption, pressure

91 swing adsorption (PSA), membrane separation, cryogenic separation, and/or use of biological

technologies (Paolo et al., 2017). As these purification methods have individual characteristics,

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93 the appropriate technology should be selected by considering the purification efficiency,

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94 operational conditions, investment, and maintenance cost (Olumide et al., 2017). PSA and wet

95 scrubbing with water (water scrubbing) are popular. The PSA process consists of several steps to

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96 separate CH4 and CO2 from biogas under pressure according to the molecular characteristics of

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97 the gases and their affinity to the adsorbent material; however, this process is complex compared
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98 to water scrubbing (Rafael et al., 2016). Indeed, the most difficult aspect of PSA operation is

99 controlling the high temperature and pressure, which has limited the application of this method
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100 on a wider scale (Shang et al., 2012).

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101 In contrast, water scrubbing at high pressure is the most commercially feasible
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102 technology for biogas purification owing to its simplicity and performance reliability (Karim and

103 Fatima, 2018). However, according to Cozma et al. (2013), the disadvantage of this technology
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104 is its higher electricity cost (0.34 kWh m-3 raw biogas). Therefore, water scrubbing performed at

105 near atmospheric pressure has been proposed, which requires a lower specific electricity of 0.24
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106 kWh m-3 for raw biogas (Budzianowski et al., 2017). However, water scrubbing at atmospheric
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107 pressure is limited to very small installations and is not usually offered by commercial vendors

108 because a higher liquid-to-biogas flow ratio is required (Budzianowski et al., 2017). Moreover,

109 high purification performance is difficult to achieve. Geng et al. (2015) and Walozi et al. (2016)

110 reported that they achieved maximum purified CH4 concentrations of only 77 and 80%,

111 respectively under atmospheric pressure conditions using the water scrubbing method.

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112 Packing carriers are usually installed in water scrubbers to enhance the mass transfer

113 efficiency between the gas and liquid phases (Tan et al., 2012). The most commonly used

114 packing carriers are pall ring, intalox metal, berl saddles, tellerette, tri-packs and raschig ring

115 (Yasin et al., 2018). The packing carrier configuration is selected considering the porosity,

116 specific surface area, and water holding capacity, which are directly related to the mass transfer

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117 efficiency (Dorado et al., 2009). However, even if packing carriers with high efficiency were

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118 used, improved purification performance under low-pressure conditions cannot be expected, as

119 discussed above. General packing carriers provide minimal water retention, yielding a very short

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120 actual hydraulic retention time (HRT), which in turn results in a short contact time for water and

121 biogas and a short absorption time. Therefore, the present authors surmised that high purification

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performance would be possible for a water scrubber even at low pressures if the HRT was
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123 increased. Sponge has very high water retention characteristic. Thus, high biogas purification
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124 performance was predicted for a water scrubber with a sponge material as the packing carrier

125 owing to the expected increase in the HRT.

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126 In this study, a new water scrubber packed with sponge carriers is proposed. To evaluate
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127 the performance of the proposed water scrubber, biogas purification experiments were conducted
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128 under atmospheric conditions. In addition, a mathematical model to simulate the purification

129 phenomenon was constructed, and simulation experiments were conducted to determine the
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130 water scrubber operational conditions that yield highly purified gas satisfying the quality
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131 requirements of city gas (CH4 ≥ 90%, CO2 < 4%, H2S < 2 ppm in Nagaoka).

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133 2. Materials and methods

134 2.1 Experimental setup

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135 A column with a height of 1.2 m and working volume of 2.3 L was used as a lab-scale water

136 scrubber in this study. A string of 23 carriers, which were connected to each other in series, was

137 hung in the column (Fig. 1a). Each carrier consisted of polyurethane sponge (volume: 18.5 cm3)

138 and a plastic frame (framed sponge). An artificial biogas made in a gas bag was provided to the

139 scrubber from the lower end, at atmospheric pressure. The purified gas was collected from the

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140 top. The scrubbing water supplied from the top was down-flowed through and onto the sponges

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141 in contact with the biogas, and was discharged from the bottom, primarily with absorbed CO2.

142 The artificial biogas was composed of CH4 (60−67%), CO2 (32−40%), and H2S

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143 (0−3.0%). The scrubbing water was made by adding sodium bicarbonate (NaHCO3) to tap water

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144 to set the alkalinity to approximately 0.411−0.769 meq L-1. The scrubbing water pH was
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145 adjusted by adding hydrochloric acid (HCl).

146
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147 2.2 Operational conditions

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148 Biogas purification experiments were conducted under various conditions for 16 runs (Runs 1 to
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149 16, Table 1). For all runs, the ratio of the water flow rate L dy to the biogas flow rate
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150 L dy , namely / , was varied as detailed in Table 1, and the effects were investigated.

151 Runs 1 and 2 were conducted to investigate the effects of the pH at a constant of 21.2 ml min-
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152
1
(corresponding to 20 min HRT based on the sponge volume) and 20 °C. Runs 3 and 4 were
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153 conducted at 5 and 30 °C, respectively, to investigate the influence of temperature compared to

154 Run 1. Runs 5 to 7 were conducted at three different HRTs of 5, 10, and 15 min, respectively.

155 The H2S removal performance was investigated in Runs 8 and 9 by changing the gases that make

156 up the artificial biogas. To evaluate and verify the mass transfer coefficient ( ) inherent in the

157 used water scrubber, Runs 10 to 16 were separately conducted under various operational

158 conditions (Table 1).

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159

160 2.3 Sampling and analyses

161 The biogas purification performance should attain steady state after several HRTs or gas

162 retention times (GRTs) from the start of operation because the flow used in the scrubber is

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163 almost a plug flow. Therefore, the effluent gas and water were sampled and analyzed at 6, 8 and

164 24 h after adjustment to achieve each of the operational mode listed in Table 1. We confirmed

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165 that the performance attained steady state by 24 h as expected. The purification gas was

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166 collected into a gasbag (Smart Bag PA, GL SCIENCE/CEK 3008-97720) and the CH4, CO2, O2,

167 and N2 concentrations were measured using a gas chromatograph equipped with a thermal

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168 conductivity detector (GC-TCD, Shimadzu GC-8A). Kitagawa detector tubes (Tube Nos.
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169 120SM, 120SB, and 120SH) were used to measure the H2S gas concentration. Triplicate

170 measurements were conducted, and some of the raw measurement values are presented in the
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171 supplementary material section. The effluent pH was measured.

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172
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173 3. Mathematical model concept

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174 In this study, a mathematical model was constructed to describe the gasification and absorption

175 phenomena in the water scrubber and simulate the biogas purification performance. The CH4 and
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176 CO2 in the biogas are absorbed into the scrubbing water of the liquid phase as dissolved gas,
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177 whereas dissolved gases in the scrubbing water such as N2 and O2 are gasified into the gas phase

178 and mixed with the purified gas. Each mass transfer at the liquid–gas interface is governed by

179 Fick’s law; however, the transfer phenomenon is very complex. Therefore, we adopted the

180 following widely used simplified-overall mass transfer expression.

∗
181 = ! − , (1)

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∗
182 where mol L dy is the molar flux of the gas # , mol L is the dissolved gas

183 concentration in a liquid in equilibrium with partial pressure Pj atm in the gas phase for gas j,

184 and Cj mol L is the dissolved concentration of gas j. Here, gas j can correspond to CH4, CO2,

185 H2S, N2, and O2.

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186 Further, C*j can be determined using Henry’s law, as follows:

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187 = ∙ , (2)

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188 where atm L moL is the Henry’s constant (Table S-1 in the supplementary material). The

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189 dissolved CO2 and H2S are dissociated into ions, and the concentrations of the dissociated ionic

compounds depend on the pH. The charge balance of the ions in water is expressed as follows:

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191 %H ' ( + %*+,( − - ⁄%H
'(
− 2%CO32 ( − %HCO2 ( − %HS ( − 2%S 3 ( = 0, (3)

192 where Alk represents the alkalinity and Kw is the water dissociation constant (Table S-2 in the
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193 supplementary material). The pH can be calculated using the charge balance equation, which
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194 was adopted in our model. In other words, the dissociated ionic concentrations are determined
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195 using the H+ concentration with the CO2 and H2S dissociation constants (Table S-2 in the

196 supplementary material).

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197 A conceptual schematic of the mass transfer model, which was constructed by assuming
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198 that the biogas and scrubbing water have a plug flow in the column, is shown in Fig. S1. The
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199 following mass balances over an infinitesimal distance dz at position i were established for the

200 gas components:

33.>?
201 6 78 ,9 − 78:; ,9' <A3@2. = !6 ,9' − ,9 < =− ,9 ∆BC , (4)

33.>E
202 ∑9 78 ,9 − ∑9 78:; ,9' = ∆BC ∑ ,9 ∙ 3@2. , for # = CH4, CO2, H2S, N2, and O2, (5)
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203 where ∆Vs L is the sponge volume in dz and is the molar flux based on the sponge volume.

204 As the biogas concentration , and the concentration of each dissolved gas , in the influent

205 (shown in Fig. S1) are known for the provided biogas and water, respectively, the concentration

206 profile of each gas along with the column height at steady state can be numerically calculated

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207 using the above formulas. The biogas concentration profiles are shown in Fig. 1b as a sample

208 simulation result. Details of the calculation method are described in the supplementary material.

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209

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210 4. Results

4.1 Purification performance

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211 AN
212 Before conducting the purification experiment, we did not know the exact amount of biogas that

213 can be treated by a scrubbing column containing sponge carriers with capacities of 0.42 L, and
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214 the required amount of scrubbing water to obtain purified gas of 90% CH4. Therefore, at the
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215 beginning of the experiment (Run 1), scrubbing water of pH 8 was supplied randomly at of

21.2 ml min-1 (corresponding to water loading rate RL of 332 L m-3 h-1, based on the sponge
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216

217 volume) and approximately 20 °C. Artificial biogas composed of 60% CH4 and 40% CO2 was
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218 first fed at of 42.3 ml min-1 (corresponding to biogas loading rate RG of 166 L m-3 h-1, based

219 on the sponge volume). Under this operational condition, which corresponds to / of 0.5 (=
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220 21.2/42.3), 75.2% CH4 was observed in the purified gas. Unfortunately, the target CH4
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221 concentration was not achieved (Fig. 2a). Then, we gradually reduced . Hence, the purified

222 CH4 content was increased to 85.1 and 91.3% at of 21.2 and 14.1 ml min-1, respectively

223 (corresponding to / of 1.0 and 1.5, respectively). However, the purified CH4 concentration

224 decreased slightly to 90.5% at / of 3.0, suggesting that a much higher / ratio slightly

225 degrades the purification performance. Note that this is the first time biogas of 60% CH4 was

226 concentrated to ≥90% (v/v) using water scrubbing technology under normal pressure conditions.

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227 In Run 2, water of pH 7 was supplied under the same conditions as in Run 1 (except for the pH

228 difference). Surprisingly, the respective CH4 concentrations in the purified gas were very close

229 to those obtained in Run 1. Thus, almost identical purification performance was observed even

230 when water of different pH values was used (Fig. 2a). Therefore, it was found that the /

231 ratio is one of the crucial factors that affect biogas purification performance compared to the

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232 scrubbing water pH value.

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233 To investigate the effect of temperature on biogas purification, Runs 3 and 4 were

234 conducted at 5 and 30 °C respectively, with the operational conditions being the same as in Run

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235 1 (except for the temperature changes). The tendency for the CH4 concentration of the purified

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236 gas to increase with increased / was similar to that at 20 °C for Run 1 (Fig. 2b). However,
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237 the effluent CH4 concentrations differed significantly. A CH4 concentration ≥90% (v/v) was

238 obtained at a lower / of 1.0 in Run 3, compared to the result for Run 1. This difference in
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239 the purification performance indicates that the purification water required to obtain a highly
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240 purified gas is strongly dependent on temperature.

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241 In Runs 1 to 4 described above, the experiments were conducted at a constant of 21.2

242 ml min-1, corresponding to 20-min HRT based on the sponge volume. The HRT should influence
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243 the purification performance. Then, Runs 5−7 were conducted at various HRTs of 5, 10, and 15

244 min, respectively. Interestingly, the results exhibited very little difference in CH4 concentration
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245 for the respective HRTs at the same / (Fig. 2c). Thus, the purification performance was
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246 found to be mainly governed by the / ratio at the same temperature regardless of the HRT.

247 This means that it is only necessary for to be changed proportional to the change in the

248 provided to the scrubbing column in order to obtain the same performance.

249 The effect of H2S on the purification performance was investigated in Runs 8 and 9,

250 where the provided artificial biogases with approximately 60% CH4 contained H2S at

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251 concentrations of 0.5 and 2.5%, respectively. This differed from Run 1, which had 0% H2S. The

252 CH4 concentrations in the purified gas were almost the same regardless of the input H2S

253 concentration (Fig. 2d). Thus, it was found that the H2S in the biogas had a very small impact on

254 the CH4 concentration, even at a high H2S concentration. On the other hand, the H2S removal

255 was influenced by the provided H2S concentration. H2S concentrations of 400 and 700 ppm in

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256 the purified gas were detected for the cases of 0.5 and 2.5% influent H2S, respectively, under

/ /

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257 of 1.0 (data not shown). However, as the was increased to 2.5, H2S was

258 sufficiently removed within the scrubbing column. That is, the H2S concentration in the purified

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259 gas was below the detection limit (ppm), even though a very high H2S of 2.5% was provided

260 (data not shown). Therefore, the described experimental results indicate that the artificial biogas

261
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was successfully purified using the water scrubbing column to produce ≥90% (v/v) CH4 gas with
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262 simultaneous H2S removal under operation with appropriate / ratios, even under
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263 atmospheric pressure conditions.

264
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265 4.2 Mass transfer coefficient determination and validation

266 The experiment discussed above demonstrate that biogas purification is possible using the
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267 proposed water scrubber. However, the operational conditions suitable for attaining high

268 performance were not revealed. It is difficult to determine appropriate water scrubber
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269 operational conditions from insufficient experimental data. Therefore, we attempted to determine
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270 these conditions using a mathematical model to simulate the water scrubbing phenomenon.

271 In the model, the only unknown parameter is the overall , which is generally strongly

272 dependent on the packing material configuration in the scrubbing column. Therefore, we first

273 attempted to estimate the value. For an effective model, the observed performance should

274 be coincident with the simulated results; here, we found the that yielded a simulated

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275 purified-gas CH4 concentration corresponding to the measured concentration through trial and

276 error. Runs 10−13 (Table 1) were specifically conducted to determine for a wide range of

277 liquid velocities (υL). As shown in Fig. 3, the determined values (based on the sponge

278 volume, not the column volume) were inconstant, and tended to linearly increase with increasing

279 υL. This observed trend was also reported in a previous study (Sherwood and Holloway, 1939).

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280 In addition, the evaluated values of 8.5−34.2 L h-1 for the tested υL of 0.09−0.36 m h-1 were

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281 in the same order as those reported in previous studies, even though different packing materials

282 such as Raschig rings (Evren et al., 1999), Hiflow rings (Biard et al., 2017), and polyurethane

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283 sponges (Almenglo et al., 2016; Dorado et al., 2009; Kim et al., 2008) were used. The

determined h was expressed as a function of υL m h as follows:

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284 AN
285 = 95.0I . (6)

286 To check the validity of the simulation performed based on the constructed model and
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287 using the determined , the simulated performance was compared with experimental data. Fig.
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288 4 shows a graph of the simulated purified gas concentrations versus the measured results for
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289 Runs 14−16; the data were plotted almost on a straight line, which indicates that the simulated

290 values were almost coincident with the measured values. Further, the simulations were in
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291 agreement with the measured results not only for CH4, but also for the other gas compositions,

292 namely, CO2, O2, N2, and H2S. In terms of the effect of the / ratio on biogas purification,
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the measured concentrations and effect were well simulated (Figs. 2a−d). This good agreement
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294 between simulation and measurement indicates that the constructed model and determined

295 are acceptable for simulating the purification phenomena. Note that, although should be

296 dependent on the temperature, we neglected its influence in the simulation owing to its minimal

297 value. Further, the simulated data agreed well with measurement data even under different

298 temperature conditions (Fig. 4).

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299

300 4.3 Simulation performance

301 4.3.1 Appropriate JK /JL ratio

302 The biogas purification performance was significantly affected by temperature and the /

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303 ratio, as mentioned above. Temperature is an uncontrollable parameter; however, the /

ratio can be controlled. Numerical simulations using the proposed mathematical model were

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305 used to assess the appropriate / . The range of predicted / ratios for achieving purified

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306 gas with ≥90% (v/v) CH4 concentration at any temperature is shown in Fig. 5, where the two

307 lines correspond to 90% CH4. The area bounded by the lines corresponding to the minimum and

308 maximum /
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ratios (hatching) indicates CH4 ≥ 90% (v/v). For example, if the scrubber is
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309 operated in the range of 1.0−3.0 / at 20 °C and HRT is 20 min, the CH4 concentration of
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310 the purified gas exceeds 90%. Thus, the appropriate / ratio must be increased with

311 increasing temperature to achieve the target purification (Fig. 5).

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312 The GRT was thought to strongly influence the biogas purification performance. However,
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313 simulations show that although / it is a crucial factor, the effect of GRT is not significant.
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314 That is, the / ratio required to achieve 90% CH4 concentration is almost constant even if

315 GRT is changed (Fig. 6). Although the minimum / should increase with decreasing GRT,
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316 the necessary increment is minimal for a wide range of GRTs (10−60 min) at any temperature.
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317 Even if the scrubber is operated for a short GRT of 10 min, good performance can be achieved

318 by increasing . Thus, the numerical simulations performed in this study were very useful for

319 determining the appropriate operational conditions of the water scrubber.

320

321 4.3.2 Column height

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322 In this study, a column height of 1.2 m was used. However, we were not certain that this is the

323 optimum height for biogas purification. Theoretically, a higher column yields higher purification

324 performance. The effect of column height on performance was investigated through simulation

325 using the mathematical model. As predicted, the simulation results show that the CH4

326 concentration of the purified gas increased with increasing column height (Fig. 7). Surprisingly,

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327 a very small column height of approximately 0.6 m is sufficient under operation with / of

of 8.5 ml min-1 at 20 °C. In other words, the simulation indicates that the purification

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328 2.5 and

329 is not enhanced even if a higher column (>0.6 m) is used. For a lower / of 1.5, a higher

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330 column is required; however, a column height of only 1.3 m is sufficient and the CH4

331 concentration of the purified gas increases (Fig. 7). Fortunately, the column with a height of 1.2

332
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m used in this study is appropriate for the purification experiment. There is a negative relation
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333 between the column height required to achieve 90% CH4 and the implemented / ratio (Fig.
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334 8). This indicates that increasing the column height while decreasing / is necessary to

335 obtain the same purification performance. In terms the influence of temperature, the column
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336 height should increase with increasing temperature for any / . In addition, Fig. 8 suggests
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337 that, under conditions with excessively low / , good purification is impossible even if a

338 very tall column is used.

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339
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340 5. Discussion
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341 In this study, artificial biogas purification through water scrubbing was successfully performed

342 using a column packed with original carriers, that is, sponge with plastic-ringed frames (framed

343 sponge), even under atmospheric pressure. The purification performance was excellent

344 compared with previous reports (Geng et al., 2015; Walozi et al., 2016), with purified gas with

345 more than 90% CH4 successfully produced, even for a very short column. Our experiment differs

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346 completely from other studies involving packed carriers. The framed sponge has a water

347 retention characteristic, which facilitates longer actual HRT. Note that the interphase mass

348 transfer performance of the water scrubber should be affected by the actual HRT owing to its

349 relation to the time in contact with the gas and liquid. That is, the interphase mass transfer

350 performance is expected to increase with increased actual HRT. To investigate the manner in

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351 which the water retention of the framed sponge affects the purification performance, simulations

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352 were conducted under different water retention conditions, which can be established by virtually

353 changing the sponge volume in the ringed frame in an actual experiment. For constant , the

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354 calculated CH4 concentration of the purification gas clearly declined with decreased sponge

355 volume (Fig. 9). This means shortening of the actual HRT, suggesting that the carrier water

356
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357 concentration was achieved at a / of 2. However, the CH4 concentration was improved to
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358 90% by increasing / under the operational conditions shown in Fig. 10. However,

359 improved purification performance cannot be expected for a framed sponge of 10% sponge
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360 volume, even if / is increased significantly (Fig. 10). Previously, Läntelä et al. (2012)
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361 reported that purified gas with 88.1% CH4 was achieved in a purification experiment using a pall

362 ring as packed carrier for value almost identical to that used in the present study. However,
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363 in a study reported by William et al. (2014), good purification was not achieved even though a
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364 pall ring was used with a reactor height of less than 3 m. These packed carriers should have low
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365 water retention capability, yielding a brief period of time in contact with the gas and liquid

366 because of the short actual HRT. If framed-sponge carriers were used in the experiments

367 (Läntelä et al., 2012; William et al., 2014), excellent purification performance similar to our

368 results could be expected under suitable operational conditions.

369 Before conducting the experiment, we had predicted that the scrubbing water pH would

370 have some effect on the purification performance because the amount of dissolved CO2 is

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371 dependent on the pH. However, both the experiment and simulation results indicate that the pH

372 has minimal impact on the purification. The scrubbing water pH should decrease along with the

373 down-flow in the scrubbing column through absorption of the CO2 of the provided biogas. When

374 all the CO2 is absorbed, that is, for 100% CO2 removal, the decrease in the pH should stop.

375 However, no limitless pH drop occurred. As the dissolved CO2 concentration achieved

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376 equilibrium with the provided biogas at the bottom of scrubber, the pH became constant. This

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377 corresponds to a minimum pH for the effluent, which depends on the alkalinity and biogas CO2

378 concentration. Assuming 100% CO2 removal efficiency, the pH in the effluent can be estimated

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379 based on the CO2 balance. The pH calculation is described in the supplementary document. As

380 an example, Fig. S2 (in supplementary document) shows the calculated pH for the effluent

381
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382 / ratio of 2, the pH in the effluent became 5.30 and 5.01 for influent pH values of 8.00 and
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383 5.32, respectively, with 100% CO2 removal. This simulation result indicates that, even if

384 scrubbing water samples in a very wide pH range (e.g., spanning a range >5.32) are supplied,
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385 complete CO2 removal is possible. When / is increased to 3, a wide applicable pH range
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386 (spanning >5.19) is attained. Thus, the pH has minimal impact on the purification performance

387 because water has high CO2 absorption capacity for a wide range of pH values. At a low /
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388 of 1, the effluent pH reaches a minimum value of 5.01 for any influent pH (<8), meaning that
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389 100% CO2 removal is not attained. Therefore, the / ratio is of notable significance in
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390 biogas purification without considering the scrubbing water pH.

391 As discussed above, we confirmed the theoretical relationship between the / ratio

392 and CO2 gas removal. Previously, Rasi et al. (2008) also reported that a higher / yields

393 greater CO2 removal. In Fig. 11, 100% CO2 removal is theoretically attained for / ratios of

394 2 and 3 and scrubbing water of pH 8. This performance shows that purified gas with 100% CH4

395 can be produced. However, different results were observed in experiment (Fig. 2a). The gas

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396 purified at / of 2.5 did not consist of 100% CH4; rather, 90.5% CH4 was detected.

397 Moreover, as / was increased to 3, the CH4 concentration declined. Water contains 6−8

398 and 10−20 mg L-1 of dissolved O2 and N2 at normal temperatures and equilibrium conditions,

399 respectively (Díaz and Breitburg, 2009). This dissolved O2 and N2 should be gasified in the

400 scrubbing column, so that the purified gas does not contain O2 and N2. In fact, O2 and N2 were

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401 detected in our experiments; Table S-3 lists sample measured values for the purified gas

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402 obtained in Run 10. A simulation was conducted under the conditions shown in Fig. 12 to

403 elucidate the manner in which / affects the purified gas composition. As expected, the CO2

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404 concentration in the produced gas decreased steadily with increasing / , corresponding to a

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405 large amount of water supply. Eventually, no residual CO2 was attained at of

/
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406 approximately 3. In contrast, increased induced increments in the O2 and N2

407 concentrations due to gasification of the dissolved gases. Therefore, although the CH4
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408 concentration initially increases with increasing / , it subsequently decreases. It is

409 impossible to produce a perfectly purified gas. However, the simulation revealed that there is an
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410 optimum / ratio for biogas purification using a water scrubber, which can be determined
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411 using the mathematical model proposed in this study.

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412

413 6. Conclusions
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414 We experimentally demonstrated that a water scrubber can purify biogas to have a methane

415 concentration exceeding 90% without imposing external pressure. The developed mathematical

416 model and numerical simulations indicate that framed-sponge carriers are effective in biogas

417 purification and essential for high performance because the use of a sponge with high water

418 retention increases the HRT.

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419 The / ratio was found to be the most crucial factor determining the purification

420 performance. Excessively high / values deteriorate the performance because the dissolved

421 nitrogen and oxygen in the scrubbing water are incorporated into the recovered purified gas,

422 which decreases the methane concentration. Although the proposed water scrubber can be

423 applied to a wide range of pH and temperature values, it should be operated at an appropriate

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424 / ratio depending on the temperature.

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425

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426 Declarations of interest: none.

427 Funding: This research was supported by the Japan Society for the Promotion of Sciences as a

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429
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430 References

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518 pressurized water scrubbing via modelling and simulation. Chemical Engineering Science 170,

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520 Yasin, P., Ramesh, A., Ramana, M.V., Rahul, V., 2018. CVR J. of Sci. and Technol. 14, 86 – 90.
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521
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522 Figure legends

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523 Fig. 1 (a) Schematic diagram of water scrubber packed with framed sponge and (b) simulated
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524 biogas purification performance according to column height for specific operational conditions.
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525 Fig. 2 Measured (symbols) and simulated (lines) CH4 concentrations of purified gas at different

526 ⁄ ratios in Runs 1−9. Effects of (a) pH, (b) temperature, (c) HRT, and (d) input H2S

527 concentration.

528 Fig. 3 Determined at different water-scrubber liquid velocities (υL).

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529 Fig. 4 Relationship between measured and simulated concentrations using determined of

530 purified biogas.

531 Fig. 5 Temperature and ⁄ operational conditions to obtain purified gas with minimum 90%

532 CH4 content at 20 min HRT. The two lines correspond to 90% CH4. The circles indicate the

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533 measured CH4 concentrations.

534 Fig. 6 Relationship between GRT and minimum ⁄ ratio for 90% CH4 in simulation at

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535 different temperatures.

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536 Fig. 7 Effect of column height on biogas purification at 20 °C.

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537 Fig. 8 Column height required to achieve 90% CH4 in purified gas at 20 min HRT.
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538 Fig. 9 Effect of framed-sponge water retention (assumed sponge volume in percentages) on

539 biogas purification at 20 °C and 20 min HRT.

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540 Fig. 10 Effect of ⁄ ratio on CH4 concentration of purified gas for different framed-sponge
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541 water retentions (10, 50, and 100%) at 20 °C and 20 min HRT. The lines and circles correspond
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542 to simulation and experiment data, respectively.

⁄ on CO2 removal simulated for biogas with 40% CO2 at 20 °C, pH 8,

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543 Fig. 11 Effect of

544 and 20 min HRT.

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545 Fig. 12 Composition of purified gas simulated for 60% CH4 biogas operated at 20 °C, pH 8, and
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546 20 min HRT.

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Table 1 Experimental conditions.

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Run Input biogas (%) HRT pH Temperature ⁄ Objectives
(CH4:CO2:H2S:N2) (ml min-1) (min) (˚C)
1 60:40:0:0 21.2 20 8 20 0.5,1,1.5,2.5,3 Effect of pH

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2 21.2 20 7 20 0.5,1,1.5,2.5,3
3 21.2 20 8 5 0.5,1,1.5,2.5,3 Effect of temperature

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4 21.2 20 8 30 0.5,1,1.5,2.5,3
5 84.0 5 8 20 0.5,1,1.5 Effect of HRT
6 42.0 10 8 20 0.5,1,1.5

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7 28.0 15 8 20 0.5,1,1.5
8 60:39.5:0.5:0 21.2 20 8 20 1,3 Effect of H2S
9 60:37.5:2.5:0 21.2 20 8 20 1,3

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10 67:32:0:1 21.2 20 8 20 0.5,1,1.5
11 28.0 15 8 20 0.5,1,1.5 evaluation

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12 42.0 10 8 20 0.5,1,1.5
13 84.0 5 8 20 0.5,1,1.5
14 67:33:0:0 21.2 20 8 10 0.3,0.5,0.7,0.9,

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15 60:39.5:0.5:0 21.2 20 8 20 1,3

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16 60:37:3:0 21.2 20 8 10 1,3
*
HRT is based on sponge volume.

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Highlights
• A new water scrubber packed with sponge carriers is proposed.
• The technology proposed is feasible for biogas usage as city gas or vehicle fuel.
• Purification experiments using artificial biogas yield >90% CH4 concentration.
• Optimal operating conditions are determined based on experiment and simulation.
• High concentration of CO2 and H2S eliminations from biogas.

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