Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent

Applied Energy xxx (2015) xxx–xxx

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent q Zhi-Ming Xia, Xiao-Sen Li ⇑, Zhao-Yang Chen, Gang Li, Ke-Feng Yan, Chun-Gang Xu, Qiu-Nan Lv, Jing Cai Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Purifying simulated biogas by hydrate

process with synergic additives.  Synergic additives comprise DMSO

and TBAB or THF.  Microcosmic structure analysis,

kinetic and separation efficiency were studied.  Synergic additives can enhance the hydrate-based separation process due to DMSO.

a r t i c l e

i n f o

Article history: Received 6 November 2014 Received in revised form 8 January 2015 Accepted 1 February 2015 Available online xxxx Keywords: Biogas Purification Gas hydrate Gas solvent Synergic additive

a b s t r a c t Treatment and subsequent use of biogas are garnering huge interest for both energy recovery and mitigation of environmental impact. However, separation process is pivotal for increasing its calorific value and removing CO2. This work presents the kinetic and separation efficiency study as well as microcosmic structure analysis for purifying simulated biogas (45.0 mol% CO2/CH4 binary mixture) through hydrate crystallization approach. Particularly, synergic additives comprise gas solvent (dimethyl sulfoxide (DMSO)) and traditional hydrate promoter (tetrahydrofuran (THF) or tetra-n-butyl ammonium bromide (TBAB)) were proposed to enhance the hydrate-based separation process. The promotion mechanism was explored through in-situ Raman spectroscopy. The residual gas phase and the decomposition gas phase from the hydrate slurry were sampled and analyzed. Based on the experimental data, the gas storage capacity, unit system gas consumed rate, gas selectivity and separation efficiency were calculated for evaluating the separation process. It was found that, the synergic additives could promote the mixture hydrate formation process due to DMSO (acid gas solvent) could improve both rate and selectivity of CO2 during the dissolution and diffusion processes. In addition, the Raman analysis reveals that the simulated biogas forms structure II hydrate and semiclathrate framework with THF–DMSO and TBAB– DMSO respectively, and CH4 molecules are only found in the smaller (512) cages of the mixture hydrates. It is inferred that DMSO just performs as an acid gas solvent during the gas dissolution and diffusion

q This article is based on a short proceedings paper in Energy Procedia Volume 161 (2014). It has been substantially modified and extended, and has been subject to the normal peer review and revision process of the journal. This paper is included in the Special Issue of ICAE2014 edited by Prof. J Yan, Prof. DJ Lee, Prof. SK Chou, and Prof. U Desideri. ⇑ Corresponding author at: Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China. Tel.: +86 20 87057037; fax: +86 20 87034664. E-mail address: [email protected] (X.-S. Li).

http://dx.doi.org/10.1016/j.apenergy.2015.02.016 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Xia Z-M et al. Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.016

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processes but not participate in the hydrate framework formation. It will be of practical interest in relation to resolving the bottleneck of hydrate-based biogas purification technology and of potential importance for the industry application of gas hydrate. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction It is generally accepted that worldwide resources of fossil fuels would be depleted in the near future [1]. Exploring adequate alternatives of new and renewable energy source is significant for both energy and environment issues [2]. Recently, purification of raw biogas has attracted a great attention due to the environmental and economical benefits for possibly production of cheap biofuel from waste or biomass [1]. Biogas typically refers to mixture gases produced through biological degradation of recycled waste (such as animal wastes, household wastes, crop residues, sewage sludge, wastewater and landfill) and mainly consists about 50–75% methane (CH4) and 25–50% carbon dioxide (CO2) [3,4]. It is acknowledged that both CH4 and CO2 are greenhouse gases (GHG) which could induce climate problems. Intergovernmental Panel on Climate Change (IPCC) reported that CH4 and CO2 components in the atmospheric have increased respectively by 151% and 31% since 1750 [5], and biogas contributes to about 5–20% of the methane emission derived from anthropogenic source [6]. Therefore, reducing emission of CH4 and CO2 from biogas is significant for stabilizing global temperature and eliminating climate problem [7]. Meanwhile, it should be noted that CH4 is also a clean fuel. For instance, the higher calorific value of a purified biogas (consists about 97% methane) is in the region of 36– 40 MJ/m3 [8]. Accordingly, rather than being released directly into the atmosphere, biogas can be purified and subsequently used as natural gas or vehicle fuel. Of course, it is prerequisite to upgrade the biogas through removing carbon dioxide to enhance the calorific values. Therefore, separation of CO2 from CH4 is very important in these cases. Numerous ways can be applied to separate CO2 and obtain purified CH4 from biogas [4], such as water wash [9], absorption [10], adsorption [11], cryogenic fractionation [12], and membrane separation [13]. However, new processes should be considered due to the energy cost. Clathrate hydrate crystallization has attracted great attention as a prerequisite process for reducing greenhouse effect and obtaining clean energy resources [14–34]. Clathrate hydrates are non-stoichiometric crystalline inclusion compounds, which formed by different types guest molecules enclathrated in the hydrogen bond network of water molecules at moderate conditions (low temperature and high pressure). Moreover, the thermodynamic condition for the hydrate formation with guest mainly depends on the difference of the guest molecules (such as size and shape) [35]. In other words, during the hydrate formation from gaseous mixture, the component possessing milder hydrate formation condition will be enriched more easily in the hydrate phase. Therefore, a certain component can be separated selectively from the gaseous mixture whose components have large difference in the hydrate formation conditions. It is known that CO2 can form hydrate under the milder condition, for instance, which is at higher temperature or lower pressure than CH4 [36]. In fact, the precious researches have proved that CH4 can be separated with high selectivity from the mixtures containing CO2 [17,37–39]. Furthermore, compared with the conventional gas separation technologies, the hydrate-based gas separation method can consume the lower energy potential due to the following factors: (1) The hydrate operating temperature is relative moderate and the temperature difference between the formation and the

decomposition sectors is only about 10 K, which is remarkably lower than that of conventional separation methods [23]; (2) the hydrate operating process can be actualized isobarically and has lower pressure loss [23]; (3) theoretically, gas hydrate has a preeminent potential for gas storage that will be beneficial to the separation process once it is improved [31,36]; and (4) the aqueous solution for hydrate formation can be re-circulated used and almost without any material loss [40]. Despite these advantages, a commercially viable hydrate-based CO2 separation process has been challenging due to it must satisfy the following factors [41– 44]: (1) Moderate operating condition; (2) high selectivity of CO2; (3) rapid hydrate formation rate; and (4) excellent gas storage capability. For the first factor, some outstanding thermodynamic additives such as tetrahydrofuran (THF) [27], Tetra-b-utyl ammonium/phosphonium salts [17,20,26] and cyclopentane (CP) [28,45] have been proved. For instance, when the simulated landfill gas hydrate formed with 0.0234 mol fraction tetra-n-butyl ammonium bromide (TBAB), the equilibrium hydrate formation pressure could reduce by approximately 90% [17], it is very close to the realistic temperature and pressure condition for separation operating. However, it has been reported that even though the quaternary ammonium salts hydrate could offer greatly enhanced thermal stability, the hydrate formation rate and gas storage capability were very low [21]. Hence, the last three essentials have been the major bottlenecks for the gas hydrate application technologies. The precious researchers proposed that a higher solubility of guest gas molecules in water and larger contact area between the guest molecules and water can reduce the mass transfer resistance and obtain a faster hydrate formation rate [18,42,46]. Consequently, it is possibly that to reduce the interstitial water, enhance the hydrate growth rate, and improve gas selectivity and gas storage capability through improving the gas dissolution and diffusion processes by physical gas solvent coupling with traditional hydrate promoter. In order to confirm the feasibility of improving the hydratebased process for biogas purification by synergic additives based on gas solvent, this work focus on microcosmic structure analysis, gas storage capability, unit system hydrate formation rate, gas selectivity and separation efficiency of the hydrate formation process. The CO2/CH4 mixture gas containing 45 mol% CO2 was simulated as the biogas due to it mainly consists of CO2 and CH4 in practice. While it is recognized that a real biogas will also contain trace amounts of H2S whose hydrate crystallization condition is similar to CO2. Thus, the ternary systems containing H2S was simplified as binary system of CO2 and CH4 in this work. 2. Experimental section 2.1. Materials The simulated biogas (45.0 mol% CO2/CH4) with precisely components was supplied by Guangdong South China Special Gases Technology Institute Ltd, China. Tetrabutyl ammonium bromide (TBAB), tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO) with 99.9% purity were supplied by Aladdin Industrial Corporation (Shanghai agent, China). The deionized water (with 18.25 mXcm1 resistivity) was produced in the laboratory through an ultra-pure water system.

Please cite this article in press as: Xia Z-M et al. Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.016

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2.2. Apparatus The experimental apparatus used in this work (as Fig. 1 shown) was just the same as that used in our previous research [23,25], except an additional in situ apparatus for Raman analysis. The apparatus mainly consists of high-pressure crystallizer (CR) and supply vessel (SV). CR has 336 cm3 inner volume and 25 MPa maximum working pressure. There are a couple circular viewing windows on the front and the back of CR. During the reaction, the contents in CR were mixed by a magnetic agitator (450 r/ min). The temperatures of both gas phase and hydrate slurry phase inside CR were measured by two Pt1000 thermoprobes (JM6081) with ±0.1 K accuracy. All pressure parameters were detected with Setra smart pressure transducers with 0–25 MPa pressure range and ±0.02 MPa accuracy. The gas phase and the hydrate slurry phase in CR were sampled and measured through in situ gas chromatography (GC). In addition, the hydrate structure was investigated by an in situ Raman spectrometer (LabRam, Jobin Yvon), and the Raman spectra were obtained through a 50 tele lens. Both CR and SV were immersed in a temperature-controlled bath.

2.3. Experimental procedure The kinetic and separation experimental procedures used in this work were described elsewhere [14,25], except for an additional in situ Raman analyzing for the mixture hydrate framework. For every experimental run, CR was firstly filled with 140 mL aqueous additive solution and pressured with the simulated biogas to the desired pressure. Then, once CR was stabilized at the experimental temperature, the agitator was actuated to form hydrate, and all the experimental data began to be recorded with time. During the experimental process, CR was still controlled at a certain pressure through supplying gas from SV with a PID controller. Based on the pressure difference of SV, the total mixture gas or individual gas component consumed can be calculated through gas equation of state in associate with phase composition. After the hydrate fully formed (there is almost no change in SV pressure), the PID controller and the agitator were stopped. Then the residual gas phase was directly sampled and analyzed by GC. Subsequently, the gas phase was quickly purged. After that, CR was closed and subsequently warmed to 25 °C to cause the hydrate decompose completely. Finally, the composition of the decomposition gas (represents the component of the hydrate phase) was determined through GC. During the whole process, the gas hydrate was analyzed through in-situ Raman spectroscopy with a multichannel air-cooled CCD detector. It should note that the single

PC

2.4. Gas storage capacity and hydrate formation rate The gas storage capacity (GSC) and unit system gas consumed rate (USGCR) used for evaluating the kinetic process in this work are described as follows:

GSC ¼

¼

nH;t  nH;0 V system h   PV   PV   PV  i PV  þ  zRT G;0 zRT G;t zRT SV;0 zRT SV;t

USGCR

V system ¼ ¼

ð1Þ ;

Dngas consumed Dt DV system  PV   PV   PV   PV   zRT þ zRT  zRT zRT G;0 G;t SV;0 SV;t DtV system

ð2Þ ;

Subscript t and subscript 0 refers to ‘‘t’’ time point and ‘‘0’’ time point during the experimental process, respectively; subscript G and subscript SV refers to the residual gas phase in CR and SV, respectively. Dt refers to the time interval from ‘‘0’’ time point to ‘‘t’’ time point, Vsystem refers to the total reaction volume of the system. For instance, if the system only contains aqueous solution, Vsystem just is the volume of the aqueous solution; while if the system of gels or sands containing water, Vsystem should includes the volume of the gels or sands. 2.5. Gas selectivity and separation efficiency The selectivity of CO2 over CH4 (GSCO2 =CH4 ) for quantifying the selectivity of the hydrate process is defined as follow:

GSCO2 =CH4

¼

CO2 2 nCO H;t  nH;0

CH4 4 nCH H;t  nH;0  CO PV        PV PV PV y 2 zRT G;0  yCO2 zRT þ yCO2 zRT  yCO2 zRT G;t SV;0      SV;t ; ¼  CH PV  PV PV PV y 4 zRT G;0  yCH4 zRT þ yCH4 zRT  yCH4 zRT G;t SV;0 SV;t

ð3Þ The split fraction (S.Fr.) of CO2 for evaluating the separation efficiency of the hydrate-based separation process is given as follows:

S:Fr: ¼

nHCO2

nFeed CO  CO2 PV        PV PV PV y 2 zRT G;0  yCO2 zRT þ yCO2 zRT  yCO2 zRT G;t SV;0 SV;t       ¼ ; PV CO2 PV CO2 PV yCO2 zRT þ y  y zRT SV;0 zRT SV;t G;0

GC

PT

PT

monochromatic grating is 1800 grooves/mm, and the Ar-ion laser source emitting is 532 nm line with a power of 100 mW. Moreover, the subtractive spectrograph is calibrated through the silicon (Si) crystal standard of 520.7 cm1.

ð4Þ

PID

‘‘z’’ refers to the compressibility factor for the mixture gas. Vent

Heat/Cooling

Feed gas

3. Results and discussion

CR

SV

TP

Raman

TP

Agitator Fig. 1. Schematic diagram of the experimental apparatus.

For investigating the feasibility of hydrate-based biogas purification process through synergic additives, the influence of gas solvent (DMSO) as well as the thermodynamic condition should be determined firstly. Coincidentally, the thermodynamic conditions of simulated landfill gas hydrates formation with the synergic additives have been studied in our previous research [17]. For convenience, some representative data were selected and plotted in Fig. 2 for analysis. As shown in Fig. 2, both the two synergic additives (THF–DMSO and TBAB–DMSO) have prominent thermodynamic effect on the CH4/CO2 equilibrium hydrate

Please cite this article in press as: Xia Z-M et al. Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.016

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8

Gas Consumed (mol)

0.20

Pressure (MPa)

6

4

2

0.15

0.10

0.05

0.00

0

0 273

276

279

282

285

288

291

294

20

297

Temperature (K) Fig. 2. Equilibrium hydrate formation conditions of the simulated biogas (45.0 mol% CO2/CH4 binary mixture) formation with pure water and synergic additives [17]: j, pure water; , 0.0234 TBAB; , 0.0556 THF; , synergic additives of 0.0556 THF and 0.0165 DMSO; , synergic additives of 0.0234 TBAB and 0.0165 DMSO.

formation conditions, although DMSO has slightly influence on the thermodynamic effect of the two synergic additives may be due to that the addition of DMSO lowers the activity of water and simultaneously lowers the chemical potential of aqueous water [47]. Furthermore, for CH4/CO2 equilibrium hydrate formation in the high temperature region, the thermodynamic effect of THF– DMSO is better than that of TBAB–DMSO [17]. It provides the thermodynamic validity of the hydrate-based biogas purification process. Moreover, the previous research has demonstrated that DMSO and the synergic additives with DMSO could enhance the solubility of CO2 [17], which is pivotal to improve the hydrate formation process. Hence, it is expected that the synergic additives have a potential kinetic effect on the simulated biogas hydrate formation process. Based on the thermodynamic conditions, the simulated biogas hydrate formation processes with different synergic additives were carried out. Driving force, defined as the pressure difference between the actual hydrate formation condition and its equilibrium hydrate formation condition, was given individually for each experiment. All the experimental conditions and results were listed in Table 1. In view of practical application scenario, this work focus on the key kinetic parameters such as induction time, unit system gas hydrates formation rate and gas storage capability.

40

60

Time (min) Fig. 3. Kinetic curves of the simulated biogas hydrate formation with different additives: j, 0.0234 TBAB; , 0.0556 THF; , 0.0234 TBAB and 0.0165 DMSO; , synergic additives of 0.0556 THF and 0.0165 DMSO.

Particularly, unit system gas consumed rate, a creative evaluation standards, was firstly used as gas consumed rate (or water conversion ratio in elsewhere) to estimate the hydrate formation process. The critical kinetic parameters were listed in Table 1 and the kinetic curves were plotted in Fig. 3. It can be seen that, for the same simulated biogas, the unit system gas consumed rate and gas storage capability of the mixture hydrate formed with TBAB is 0.0356 mol/L/min and 0.7791 mol/L respectively, and that of the mixture hydrate formed with THF is 0.0339 mol/L/min and 0.8254 mol/L respectively. While the unit system gas consumed rate and gas storage capability of the mixture hydrate formed with TBAB–DMSO can reach 0.0791 mol/L/min and 1.2965 mol/L respectively, and that of the mixture hydrate formed with THF– DMSO can reach 0.0729 mol/L/min and 1.3433 mol/L respectively. It illustrates that both unit system hydrate formation rate and gas storage capability of the simulated biogas hydrate formation with synergic additives TBAB–DMSO or THF–DMSO are higher than that with signal TBAB or THF additive, respectively. Furthermore, the hydrate formation rate of TBAB–DMSO is higher than that of THF–DMSO, but the latter performs a higher gas storage capability. It also be worthy noted that the induction time of the simulated biogas hydrate formation with TBAB or THF was 3 or 8 min respectively, while it is not over 1 min in the case of with TBAB–DMSO or THF–DMSO. It is mainly due to that, DMSO as a acid gas solvent, can enhance the rate of gas molecules dissolution and diffusion process, and consequently cause more gas molecules fast

Table 1 Experimental conditions and results of simulated biogas mixture hydrate formation with different additives.

a b c d e f g h i j

TBABa

THFb

DMSOc

Td K

DFe MPa

ITf min

USGCRg mol/min/L

GSCh mol/L

GSi

S.Fr.j %

0.0234 – 0.0234 –

– 0.0556 – 0.0556

– – 0.0165 0.0165

285.95 284.75 284.45 283.25

2.50 2.50 2.50 2.50

3.0 8.0 0.8 1.0

0.0356 0.0339 0.0791 0.0729

0.7791 0.8254 1.2965 1.3433

6.89 7.51 14.16 13.45

40.85 43.61 59.22 57.36

Tetra-n-butyl ammonium bromide mole concentration. Tetrahydrofuran mole concentration. Dimethyl sulfoxide mole concentration. Experimental temperature. Driving force (Pexp–Peq). Induction time. Unit system gas consumed rate for the first 10 min after hydrate nucleation. Unit system gas storage capability for the first 60 min. Gas selectivity of CO2 over CH4 for the first 10 min after hydrate nucleation. Split fraction of CO2.

Please cite this article in press as: Xia Z-M et al. Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.016

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Additives

DMSO-THF

DMSO-TBAB

TBAB

THF

0

10

20

30

40

50

60

70

80

90

CO2 composition (mol %) Fig. 4. Phase component changes for the simulated biogas hydrate formation process with different additives: Red slant, hydrate slurry phase; Green slant, residual gas phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

S.Fr. (%)

70 12

60

10

50

8

40 30

GS (nCO2/nCH4)

6

THF

TBAB

DMSO-TBAB

DMSO-THF

GSC (mol/L)

0.9

Additives

0.8 0.7 0.6 0.5 THF

TBAB

DMSO-TBAB DMSO-THF

Additives Fig. 5. The gas storage capability, gas selectivity and separation efficiency for the simulated biogas hydrate formation process with different additives.

Intensity (a.U.)

dissolving and diffusing in the solution. Therefore, the synergic additive with DMSO can improve the hydrate formation process and performs the shorter induction time, the higher unit system gas consumed rate and gas storage capability, which is significant for the practical hydrate-based biogas purification process. After the simulated biogas mixture hydrate fully formed, the residual gas and the decomposition gas from the hydrate slurry were sampled and analyzed, and the results are plotted in Fig. 4. As Fig. 4 shown, the CO2 component in both residual gas and gas decomposed from the hydrate slurry was obviously changed. Moreover, the synergic additives both TBAB–DMSO and THF– DMSO can richen more CO2 in the hydrate slurry and richen more CH4 in the residual gas phase than single TBAB and single THF, respectively. For instance, the simulated biogas hydrate formation with the synergic additives can richen 83 mol% CO2 in hydrate slurry and richen 76 mol% CH4 in residual gas, while the single TBAB or THF additive can only richen 76 mol% CO2 in hydrate slurry and richen 71 mol% CH4 in residual gas. Furthermore, the separation effect of TBAB–DMSO is slightly better than that of THF–DMSO. In addition, based on the above experimental data, the gas selectivity and separation efficiency also were calculated for evaluating the hydrate processes. As shown in Fig. 5, the gas selectivity and separation efficiency of the simulated biogas hydrate formation with TBAB–DMSO, THF–DMSO, TBAB, and THF are 14.16% and 59.22%, 13.45% and 57.36%, 7.51% and 43.61%, and 6.89% and 40.85%, respectively. It is possible that, during the dissolution process, CO2 relative to CH4 is more easier to dissolve in the synergic additive aqueous solution due to DMSO is an acid gas solvent, resulting a higher dissolution selectivity of CO2. On the other hand, in the diffusion process, CO2 molecules can be carried fast to anywhere for hydrate formation in the system due to that DMSO can act as a carrier for CO2 after dissolved CO2, resulting a higher kinetic selectivity of CO2. Therefore, the synergic additives with DMSO have the higher gas selectivity of CO2 over CH4 and consequently perform the better separation efficiency. Moreover, the TBAB– DMSO performs the higher CO2 selectivity and separation efficiency. It demonstrates that the synergic additives perform higher CO2 recovery efficiency and higher gas selectivity of CO2 over CH4 for the hydrate-based biogas purification process. Of course, it is important that have an insight into the microcosmic guest occupancy for the promote mechanism. Raman spectroscopy, known to be simpler and less resource intensive, was used to examine the structure and guest occupancy of the simulated biogas mixture hydrate formed with synergic additives in this work. The characteristic Raman spectra of the

0

1000

2000

3000

4000

-1

Raman Shift (cm ) Fig. 6. Raman spectra of the simulated biogas mixture hydrate formed in systems with different synergic additives: Red line, TBAB–DMSO synergic additive; Blue line, THF–DMSO synergic additive. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

simulated biogas mixture hydrates formed with TBAB–DMSO and THF–DMSO synergic additives is shown in Fig. 6. As seen from Fig. 6, for the simulated biogas mixture hydrate formed with THF–DMSO, the Raman peaks at around 800 cm1 to 980 cm1 are correspond to the overlap of the characteristic C–C–C–C stretching (ring breathing) mode in clathrate phase and THF aqueous phase [37]. On the other hand, for the simulated biogas mixture hydrate formed with TBAB–DMSO, the Raman peaks at around 700–1500 cm1, 2800–3000 cm1 and 3080–3660 cm1 correspond to the synergic additives solution, the TBAB molecules as hydrates and the O–H vibration of the host water lattice, respectively. The Raman peaks obtained in Fig. 6 are similar to the studies reported by Hashimoto et al., Kim et al. and Xu et al. [14,48,49]. However, the Raman peaks correspond to the synergic additives aqueous solution are obvious stronger than that correspond to the guest gas in the full range of Raman spectra. In order to insight the peaks shift due to structure transition or guest occupancy of CO2 and CH4 in the mixture hydrate, the Raman spectra correspond to CO2 region and CH4 region in both hydrate phase and vapor phase is partial enlarged as Figs. 7 and 8 shown, respectively. As

Please cite this article in press as: Xia Z-M et al. Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.016

Z.-M. Xia et al. / Applied Energy xxx (2015) xxx–xxx

Intensity (a.U.)

6

1200

1250

1300

1350

1400

Raman Shift (cm-1) Fig. 7. Raman spectra for CO2 gas in vapor or in hydrate of the simulated biogas mixture hydrate systems with different synergic additives: Red line, vapor of TBAB– DMSO system; Green line, vapor of THF–DMSO system; Blue line, hydrate of TBAB– DMSO system; Blue line, hydrate of TBAB-DMSO system; Pink line, hydrate of THFDMSO system. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Intensity (a.U.)

Fig. 7 shown, for the vapor phase, the characteristic Raman peaks at around 1286 cm1 and 1389 cm1 correspond to the CO2 gas. It is well agree with the results reported by Sum et al. [50]. On the other hand, for the hydrate phase formed from the simulated biogas and THF–DMSO, the Raman peaks at 1274 cm1 and 1381 cm1 are correspond to CO2 gas enclathrated in the structure II hydrate, which is attributed to the inclusion of THF into the hydrate lattices [15]. While in the case of the simulated biogas mixture hydrate formed with TBAB–DMSO, the peak at 1286 cm1 was observed shift to 1278 cm1 except that the peak at 1381 cm1 was the same position as that of the THF–DMSO case. It is well known that Raman peaks at 1278 cm1 and 1381 cm1 should correspond to CO2 occupies both 51262 and 51263 cages of the semiclathrate hydrate framework [14,49,51]. It should be noted that the relative intensity of peak splitting at 1307 cm1 and 1326 cm1 are likely corresponding to a change in the structural

2880

environment of the alkyl groups and can be attributed to CH2bending modes in this spectral region [52]. Moreover, as seen in Fig. 8, the Raman peak at around 2917 cm1 corresponds to the m1 symmetric band of CH4 in vapor, while the Raman peak of CH4 shift to lower frequency side in the hydrate formed with the synergic additives. It is noted that only one Raman peak of CH4 (at around 2913 cm1) can be found in hydrate phase formed with both TBAB–DMSO and THF–DMSO, although the Raman peak of CO2 in case of THF–DMSO is weak due to the overlaps with the Raman peak of THF aqueous solution. It indicated that the simulated biogas hydrate formed with the synergic additives only the smaller (512) cages encaged CH4. With the above analysis, it seems reasonable to conclude that, the simulated biogas forms structure II hydrate and semiclathrate framework with THF–DMSO and TBAB–DMSO, respectively. Moreover, CH4 molecules are only found in the smaller (512) cages of the mixture hydrates formed with the two synergic additives, which is pivotal to the selectivity of CO2 over CH4. It is should be noted that DMSO just performs as a gas solvent during the gas dissolution and diffusion process but not participate in the hydrate framework formation. 4. Conclusion In this work, creativeness synergic additives were proposed to enhance the hydrate-based simulated biogas purification process based on the kinetic, separation efficiency experimental study and microcosmic structure analysis. The synergic additives comprise physical acid gas solvent (dimethyl sulfoxide) and traditional hydrate promoter (tetra-n-butyl ammonium bromide or tetrahydrofuran). It is worthy noted that compared with the pure water or signal THF or TBAB, the synergic additives (DMSO–THF or DMSO–TBAB) can not only remarkably accelerate the hydrate formation rate and enhance the CO2 selectivity, but also improve the gas storage capacity of simulated biogas mixture hydrate. It is mainly due to that, DMSO (an acid gas solvent) can enhance the capacity, rate, and selectivity of CO2 dissolution and diffusion process. In addition, the Raman spectroscopy analysis indicates that, the simulated biogas forms structure II hydrate and semiclathrate framework with THF–DMSO and TBAB–DMSO respectively, and CH4 molecules are only found in the smaller (512) cages of the mixture hydrates formed with the two synergic additives. Moreover, it is inferred that DMSO just performs as a gas solvent during the gas dissolution and diffusion processes but not participate in the hydrate framework formation. It will be of practical interest in relation to the development of hydrate-based biogas purification technology and of potential importance for the industry application of gas hydrate. Acknowledgements

2900

2920

Raman Shift

2940

2960

This work was supported by National Science Fund for Distinguished Young Scholars of China (51225603), National Natural Science Foundation of China (21306194, 51376184 and 21106144), Guangdong Province Natural Science Foundation (S2013040011997), and Science and Technology Program of Guangzhou City (2012J5100012), which are gratefully acknowledged.

(cm-1) References

Fig. 8. Raman spectra for CH4 gas in vapor or in hydrate of the simulated biogas mixture hydrate systems with different synergic additives: Red line, vapor of TBAB– DMSO system; Green line, vapor of THF–DMSO system; Blue line, hydrate of TBABDMSO system; Blue line, hydrate of TBAB–DMSO system; Pink line, hydrate of THF– DMSO system. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Please cite this article in press as: Xia Z-M et al. Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.02.016