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N2 on modified coal-based carbon molecular sieve
Separation and Purification Technology 218 (2019) 130–137
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Adsorption separation of CH4/N2 on modified coal-based carbon molecular sieve
T
Zhiyuan Yanga, , Dechao Wanga,b, Zhuoyue Menga, Yinyan Lia ⁎
a b
College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, PR China College of Science, Northwestern Polytechnical University, Xi’an 710129, PR China
ARTICLE INFO
ABSTRACT
Keywords: Coal-bed methane Carbon molecular sieve Modification Adsorption separation Breakthrough curve
Coal-based carbon molecular sieves (CMS) were modified by organic reagents with affinities for hydrocarbons including tetracosane (C24), lauryl sodium sulfate (SDS), and polyethylene imine (PEI), and low temperature plasma treatment in a CH4 and N2 atmosphere, respectively. The modified CMS samples were characterized by FT-IR analysis, low-temperature N2 adsorption tests and SEM. In addition, the adsorption capacities of pure CH4 and N2 were measured by the static volume method and the CH4/N2 separation coefficients of the modified samples were also compared. The saturated adsorption capacities qm after modification and separation coefficients were investigated. CH4 was more strongly adsorbed on all the modified samples. The sample of CMS-P-N has the best modification effect and its saturated adsorption capacities qm were 6.76 mmol/g for CH4 and 5.56 mmol/g for N2, respectively. The separation coefficient of CH4/N2 of CMS-P-N was 3.32. Besides, CH4 breakthrough tests were performed using a fixed bed apparatus. The breakthrough curves confirmed that low temperature plasma treatment of CMS in an N2 atmosphere was beneficial for the adsorption separation of CH4/ N2.
1. Introduction Coal-bed methane (CBM), with methane (CH4) as the main component, is a non-renewable resource [1]. Generally, The methane volume concentration of extracted CBM in China is lower than 30% [2]. To ensure coalmine safety, the methane cannot be used if its volume concentration is below 30%. Therefore, it is usually discharged directly into the air [3,4]. On one hand, there is a remarkable waste of resources due to direct methane emissions, which totals 200 million tons from standard coal per year [5]. On the other hand, as a greenhouse gas, methane is seven times more destructive to the ozone than CO2 [6]. Therefore, the exploitation and utilization of low concentration CBM is important for ensuring coalmine safety, improving the energy structure, reducing energy waste and environmental pollution, and developing a low-carbon, circular economy [7–10]. Pressure swing adsorption (PSA) is an energy efficient gas separation technology [11], and the adsorption performance of the adsorbent material plays a decisive role. However, the separation of CH4 and N2 in CMB would be a major issue because of their similar physicochemical properties [12]. Therefore, the development of suitable adsorbent for the separation of methane and nitrogen has been attracting the
attention of many researchers, which is a bottleneck for the application of PSA to the separation of CH4 and N2 [13]. Development of adsorbents that have high capacities, high selectivities, and good regenerabilities for adsorption/desorption is critical for the success of methane enrichment processes via PSA [14,15]. Typically, two types of porous carbon have been widely studied and used in PSA: carbon molecule sieves (CMS) and activated carbon (AC) [16]. The CMS is a carbonaceous material with narrow pores, which has a high resistance to alkali and basic media, strong hydrophobicity, low cost, and high hydrothermal stability. However, the relatively low separation efficiency of CMS as adsorbent materials for methane enrichment is a major challenge. Many researchers have focused on modified molecular sieves to improve the adsorption separation performance for other gases [17,18]. Modification methods include impregnation methods, oxidation-reduction modifications, plasma modifications, and metal-loading modifications [19]. However, there are few reports on the modification of CMS adsorbents for CH4/N2 separation. The purpose of this work was to modify CMS samples using organic reagents with affinities for hydrocarbon and low temperature plasma treatment, respectively. These reagents included tetracosane (C24),
Abbreviations: CBM, coal-bed methane; CMS, carbon molecular sieve; DBD, dielectric barrier discharge ⁎ Corresponding author. E-mail address: [email protected] (Z. Yang). https://doi.org/10.1016/j.seppur.2019.02.048 Received 19 November 2018; Received in revised form 15 February 2019; Accepted 25 February 2019 Available online 26 February 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram of diagnostic equipment for the DBD plasma treatment.
lauryl sodium sulfate (SDS) and polyethylene imine (PEI). The low temperature plasma modification was performed in a methane and nitrogen atmosphere, respectively. The adsorption capacities and methane and nitrogen separation coefficients of the modified CMS samples were measured. Finally, breakthrough tests were also performed.
aqueous KOH solution with a 3:1 mass fraction of KOH: carbonized sample, and the mixture were stirred thoroughly. The activated samples were heated to 1073 K and held for 2 h with a ramp rate of 5 K·min−1 in the flowing stream of nitrogen (200 mL·min−1). After cooling under nitrogen flow, the samples were washed with 5 mol·L-1 HCl until the pH was 7 and dried at 393 K for 12 h in a vacuum oven. Because it was difficult to obtain highly uniform pores through the carbonization-KOH activation process, a benzene vapor deposition process (chemical vapor deposition, CVD) was used. This process was carried out in a reactor under a N2 atmosphere (200 mL·min−1). The quartz jacketed reactor was purged with N2 for 30 min at 298 K to remove air from the system. The activated sample was heated to a deposition temperature of 873 K, after which benzene liquid was pumped into the reactor using a metering pump at 1.2 mL·min−1, and this process was held for 10 min. The prepared samples, denoted as CMS, were cooled to room temperature.
2. Experimental section 2.1. Materials Anthracite was supplied by Shenhua Ningxia Coal Industry Group Co., Ltd. The textural characteristic of the anthracite measured by N2 adsorption at 77 K was shown in Fig. S1. Pore structure parameters of Anthracite were presented in Table S1. Tetracosane, lauryl sodium sulfate (SDS, CH3(CH2)11OSO3Na) and polyethylene imine (PEI) were obtained from Tianjin Fuchen Organics. Concentrated nitric acid (HF, 49%) and hydrochloric acid (HCl, 36.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. High-purity (> 99.99%) of N2 and CH4 were purchased from Shaanxi Hongwei Gas Co. Ltd, respectively. Deionized water was used in all the experiments and other solvents and chemicals were used without further purification.
2.3. Modification of CMS 2.3.1. Impregnation modification with organic reagents Tetracosane, sodium dodecyl sulfate, and polyethylene imine were chosen to modify the CMS respectively. The detailed steps were as follows: Tetracosane modification: First, 1 g of CMS was impregnated with a tetracosane solution (1 g of tetracosane in 100 mL of anhydrous ether) and stirred at 313 K until the anhydrous ether evaporated completely. Subsequently, the sample was washed with boiling water to remove any excess tetracosane and dried at 393 K for 12 h in vacuum. The prepared sample was denoted as CMS-C24. Sodium dodecyl sulfate modification: First, 1 g of CMS was impregnated with lauryl sodium sulfate solution (1 g of sodium dodecyl sulfate in 100 mL of deionized water) and stirred at 303 K for 5 h. Subsequently, the sample was washed with deionized water, filtered, and dried at 393 K for 12 h in vacuum. The prepared sample was denoted as CMS-SDS. Polyethylene imine modification: First, 1 g of CMS was immersed in a polyethylene imine solution (1 g of polyethylene imine in 100 mL of ethyl alcohol) and was stirred for 6 h at 353 K to remove ethanol. Subsequently, it was washed with water to remove excess polyethylene
2.2. Preparation of CMS Taixi anthracite was used as precursor material for the preparation of CMS. The precursor was firstly ground and sieved to a particle size less than 74 µm, after which it was washed with 15 wt% HF, 45 wt% HCl, and 40 wt% deionized water to eliminate ash impurities, then dried at 373 K for 12 h to remove any moisture, and stored in zipper storage bags. The ultimate analysis of Taixi anthracite are as follows: N content of 0.84 wt%, C content of 88.61 wt%, and H content of 3.17 wt %, respectively. The ash content of Taixi anthracite is 5.48 wt%. The preparation process was performed in a custom-made carbonization-physical/chemical activation-deposition integrated tube furnace. The schematic of the setup is shown in Fig. S2. The carbonization of the precursor material described above was carried out by heating at 673 K for 1 h with a ramp rate of 5 K·min−1 in a flowing stream of nitrogen (200 mL·min−1). The activation process was performed as follows: the carbonized sample was mixed in an 131
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Fig. 2. Schematic diagram of the breakthrough test for the CH4/N2 mixture.
imine. Finally, the sample was dried at 373 K for 12 h in vacuum. The prepared sample was denoted as CMS-PEI.
instrument analyzer is shown in Fig. S3. And the detailed measurement principle and procedures were presented in Supporting Information. All the adsorption isotherms of CH4 and N2 are obtained based on the Langmuir equation Eq. (1) [22]:
2.3.2. Low temperature plasma modification The CMSs was treated by a dielectric barrier discharge (DBD) plasma under a CH4 or N2 atmosphere. The DBD plasma setup is shown in Fig. 1. First, 0.2 g of CMS sample was loaded in the barrier discharge cell. The dielectric barrier discharge was generated by applying 45 V to the electrodes using a DC voltage generator with methane or nitrogen as the plasma-forming gas. The plasma conditions were maintained for 1 min and each sample was treated five times. The samples prepared under methane and nitrogen were denoted as CMS-P-C and CMS-P-N, respectively.
q=
qm bp 1 + bp
(1)
where q is the gas uptake (mmol·g−1), qm the maximum gas uptake (mmol·g−1), b is a fitting constant that reflects the adsorption strength, and p is the adsorption pressure. It is possible to predict the co-adsorption of CH4 and N2 on the modified CMS from the single-component isotherms using a simple multi-component Langmuir equation Eq. (2) [23]:
qmi bi pyi
2.4. Characterization
qi =
The surface functional groups of the samples were characterized using a Nicolet 20SXB spectrometer. A disk of pure KBr was used as a reference sample for the background measurements. Samples were mixed with 99.99% KBr at a ratio of 1:500, and the spectra were recorded in the range of 400–4000 cm−1. The specific surface areas and pore structures of the samples were measured using N2 adsorptiondesorption at 77 K with a Micromeritics ASAP 2020 instrument. The Brunauer-Emmett-Teller (BET) method was used to determine the specific surface areas at relative pressures in the range of P/ P0 = (0.05–0.30) [20]. The t-plot method was applied to calculate the microspore volumes [21]. The specific total pore volumes (Vtotal) were determined directly from the amount of gas adsorbed at a relative pressure of P/P0 = 0.99.
For a binary mixture of two components (i and j), the separation coefficient can be calculated using Eq. (3) [24]: ij
=
1 + bCH4 pyCH4 + b N2 p (1
x i / yi = x j / yj (1
x i /yi x i )/(1
yi )
yCH4 )
(2)
(3)
where xi and xj are the mole fractions of component i and j in the adsorbed-phase, respectively, xi = qi/(qi + qj), and yi and yi are the mole fraction of component i and j in the gas-phase, respectively. Using the fitting parameters, qm and b in Eq. (2) and (3), the separation coefficient of CH4 and N2 was calculated with Eq. (4) [14]: CH4 / N2
2.5. Adsorption isotherms of pure CH4 and N2
=
qmCH4 bCH4 qmN2 b N2
(4)
2.6. Breakthrough curve tests
CH4 and N2 adsorption isotherms were measured using a hightemperature and high-pressure adsorption instrument analyzer (3H2000PH, Beishide Instrument Technology, Beijing Co., Ltd, China) using the static volumetric technique at 298 K in a pressure range of 0–2 MPa. Prior to the adsorption measurements, the samples were evacuated at 523 K for 12 h to desorb moisture and gases. The schematic diagram of high-temperature and high-pressure adsorption
The breakthrough curves of CH4 in the mixed gases were measured in a fixed single-bed unit as shown in Fig. 2. The sample was degassed at 393 K for 12 h to remove water prior to the breakthrough experiments. The adsorption bed was a stainless-steel tube with length of 150 mm and internal diameter of 10 mm. The adsorbent samples (9 ± 0.001 g) were loaded into the bed at 298 K. The breakthrough 132
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Fig. 3. FT-IR spectra of modified samples. (a) CMS, CMS-SDS, CMS-PEI, and CMS-C24. (b) CMS, CMS-P-N, and CMS-P-C.
time was defined as the time required to detect CH4 at the outlet of the adsorption column with a volume fraction of 1.25%. More detailed measurement information is shown in the Supporting Information.
3.3. Porous structures The N2 adsorption-desorption isotherms and pore size distribution of the modified samples are shown in Fig. 5. According to the IUPAC classification, all the samples exhibited type I isotherms, with significant nitrogen adsorption amounts at P/P0 > 0.1 and no obvious hysteresis loop at P/P0 > 0.4, which are characteristics typical of microporous materials. According to Fig. 5(b), the pore size distribution (PSD) of these modified samples was larger than that of other CMS samples in previous literatures [30–32]. Compared with the unmodified CMS, the CMS-P-N adsorbed more nitrogen. Table 1 lists the BET specific surface areas (SBET) and the total pore (Vtotal) and micropore (Vmicro) volumes of the samples, which can be used to analyze changes in the pore characteristics of these samples. As seen from the Table 1, the modification of CMS samples did not considerably change their textural properties. After impregnation modification, the ratios of the total pore to micropore volumes (Vmicro/ Vtotal) of the modified CMS samples decreased slightly, while they increased after low temperature plasma modification. These results indicate that the low temperature plasma modification promoted the formation of new micropores and the impregnation modification blocked partial micropores in the CMS, but the blockage was very slight and could be neglected. The variations in the CH4/N2 selective adsorption on the modified samples should primarily be related to the surface chemistry changes.
3. Results and discussion 3.1. Surface chemical properties The FT-IR spectra of the samples are shown in Fig. 3. The main characteristic absorption peaks are similar. The broad bands around 3445 cm−1 are attributed to O-H stretching vibration due to the adsorbed water on the CMS surfaces [25]. The bands at around 2986 and 2930 cm−1 are assigned to aliphatic -CH stretching. The band at around 1630 cm−1 is attributed to the aromatic C]C [26], the band in the range of 900–700 cm−1 is attributed to the vibrations of the aromatic structure. After C24 modification, the weak bands observed around 751, 2851, and 1631 cm−1 were caused by v(C-H) in -CH, -CH3, and -CH2, respectively. After SDS modification, the band at around 1051 cm−1 is attributed to stretching modes of S]O group and the weak peaks at 621 cm−1 to the stretching modes of S-O in the SDS molecules [27]. After PEI modification, the band around 1403 and 1647 cm−1 are attributed to vibration of C-H and N-H of the amine group, respectively. The peaks at 1577 cm−1 are assigned to deformation vibration of -NH2 groups [28,29]. These observed variations in the functional groups indicate that the modification of the biomass by PEI was successful. Thus, after impregnation modification with the above organic reagents, corresponding characteristic groups were present on the surfaces of the CMS. After treatment of the CMS with N2 plasma, the adsorption peak at 1112 cm−1 is attributed to the presence of tertiary nitrogen species (CN stretching vibrations). There were nitrogen-containing groups on the surface of the CMS-P-N sample. After treatment of CMS with CH4 plasma modification, there was not significant change to the band at around 1630 cm−1.
3.4. Pure gas adsorption capacities of CH4 and N2 The adsorption capacities of pure CH4 and N2 of the modified CMS samples were investigated at 298 K, as depicted in Fig. 6. More detailed absolute adsorption capacity data measured at 298 K and pressures from 0 to 2 MPa are shown in Tables S1 and S2. The adsorption capacities of CH4 and N2 were different on the modified CMS samples compared with the pristine CMS samples. The adsorption capacities of CMS-C24 and CMS-P-C were similar to that of the unmodified CMS. The adsorption capacities of CMS-P-N were slightly higher than that of the CMS, while adsorbed amounts on CMS-SDS and CMS-PEI were much lower.
3.2. SEM Fig. 4 shows the SEM images of precursor material and modified samples. The surface of anthracite precursor was smooth and tight, no obvious pores were observed (Fig. 4(a)). Whereas the modified samples showed rugged surface and plenty of pores with different size and shapes on the surface. These numerous pores were beneficial to the adsorption of CH4 and N2. Meanwhile, compared with Fig. 4(b–f), no obvious changes were observed.
3.5. Separation coefficients The linear Langmuir equation was used to fit the above experimental adsorption capacity data, as shown in Fig. S4. The fitting parameters are listed in Table 2. After modification, the saturated adsorption capacities qm of CH4 for the modified CMS were larger than those of N2. However, the 133
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Fig. 4. SEM images of (a) anthracite precursor material, (b) CMS-C24, (c) CMS-SDS, (d) CMS-PEI, (e) CMS-P-C and (f) CMS-P-N.
adsorption capacities differed between samples. The saturated adsorption capacities qm of CH4 and N2 for CMS-C24, CMS-P-N, and CMS-P-C were larger than those of the pristine CMS, while those of CMS-SDS and CMS-PEI were lower than that of the pristine CMS. Comparison of the saturated adsorption capacities qm of CH4 and N2 obtained in this work and other adsorbent were presented in Table S4. The adsorption strength b of CH4 was larger than that of N2 on the modified CMS because the polarizability of CH4 (26 × 10-25 cm−3) molecules is larger than that of N2 molecules (17.6 × 10-25 cm−3) [33]. This created stronger interactions between CH4 molecules and the CMS compared to those of N2 and the CMS. Thus, CH4 molecules were preferentially adsorbed. The separation coefficients of CH4 and N2 prepared in this study and previously reported values are in Table 3. After modification, CMS-SDS has the largest separation coefficient, and the separation coefficients of all the modified samples were larger than that of pristine CMS. The saturated adsorption capacity of CMS-SDS was the lowest. To
Table 1 BET Surface area and pore structure parameters of modified CMS samples. Sample
SBET (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmicro/Vtotal
CMS CMS-C24 CMS-SDS CMS-PEI CMS-P-N CMS-P-C
1134 1140 1073 1081 1241 1137
0.548 0.426 0.437 0.525 0.591 0.534
0.372 0.264 0.267 0.342 0.406 0.369
0.678 0.620 0.612 0.651 0.703 0.691
achieve a high adsorption capacity and separation coefficient simultaneously, the best method is low temperature plasma modification. Plasma is a pollution-free technology with a low energy consumption and short processing time, and it has great potential for use in the modification of porous CMS for PSA [41,42].
Fig. 5. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of adsorbents. 134
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Fig. 6. Adsorption capacities of CH4 and N2 of (a) CMS, (b) CMS-C24, (c) CMS-SDS, (d) CMS-PEI, (e) CMS-P-N, and (f) CMS-P-C at 298 K.
3.6. Adsorption breakthrough curves
The breakthrough time of the unmodified CMS was 112 s at the breakthrough point (1.25 vol% CH4) at the outlet, and the methane concentration increased quickly after 112 s. The breakthrough times of CMSC24, CMS-SDS, CMS-PEI, CMS-P-N, and CMS-P-C were 159, 144, 140, 168, and 160 s, respectively. The order of CH4 breakthrough times was: CMS-P-N > CMS-P-C > CMS-C24 > CMS-SDS > CMS-PEI > CMS. A larger breakthrough time corresponds to a larger adsorption capacity. Therefore, the largest adsorption capacity of CH4 CMS-P-N was obtained. The breakthrough curves agreed well with the Langmuir equation fitting parameter b, which reflects the adsorption strength. This
The CH4 adsorption breakthrough curves of the modified CMS in the simulated CBM were shown in Fig. 7. In all the breakthrough curves, CH4 was adsorbed at the adsorption column inlet at the beginning of the experiments. The concentration of CH4 was zero in the outlet early in the experiment. Over time, the mass transfer zone moved up the column, and the adsorption bed reached saturation when mass transfer zone reached the outlet. When the CH4 concentration was equal to the inlet, the breakthrough curves no longer increased. 135
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Table 2 Parameters of Langmuir equation fitting to CH4 and N2 adsorption isotherms at 298 K. Langmuir fitting results CH4
N2
Samples
qm (mmol/g)
b (MPa−1)
R2
qm (mmol/g)
b (MPa−1)
R2
Separation coefficient
CMS CMS-C24 CMS-SDS CMS-PEI CMS-P-N CMS-P-C
5.2751 7.5182 2.2051 2.8977 6.7632 5.7290
1.4513 0.7818 0.8667 1.5040 1.8790 1.5421
0.9993 0.9991 0.9883 0.9967 0.9951 0.9979
3.9638 4.6328 1.6291 2.2460 5.5661 4.6025
1.0160 0.5981 0.3480 0.6461 0.6875 0.6062
0.9959 0.9888 0.9871 0.9970 0.9922 0.9966
1.90 2.12 3.37 3.00 3.32 3.16
4. Conclusions
Table 3 Comparison of separation coefficients of CH4 and N2 obtained in this work and other adsorbents. Sample
Equilibrium separation coefficient of CH4/N2
Testing condition
Anderson AX-21 [34] Carbon (PVDC) [34] Carbon (FFAD) [34] Carbon (G2X7/12) [35] 13X Zeolite [36] Vulcan carbon [24] AC/X-G-3 [37] Zeolite 13X [38] Activated Carbon Norit RB3 [39] CMS [40] This work
3.0 2.3 2.3 3.3 1.8 3.5 3.0 1.9–2.3 3.9
298 K, 0.10 MPa 298 K, 0.10 MPa 298 K, 0.10 MPa 298 K, 0.10 MPa 303 K, 0.19 MPa 293 K, 0.10 MPa 298 K, 0.10 MPa 273 K, 0.106–0.903 MPa 303.3 K, 0.994 MPa
3.08 1.9–3.3
303.3 K 298 K
In summary, coal-based carbon molecular sieves (CMS) were modified by organic reagents including tetracosane, lauryl sodium sulfate, and polyethylene imine. In addition, CMS samples were modified by low temperature plasma treatment in CH4 and N2 atmospheres, respectively. The pore structures of the modified CMS samples were similar. However, the surface chemical properties were different by the FT-IR analysis. The adsorption capacities of pure CH4 and N2 on the modified CMS samples were measured by static volume method. Meanwhile, separation coefficients of CH4/N2 were compared. The saturated adsorption capacities (qm) were in the following order: CMSC24 > CMS-P-N > CMS-P-C > CMS > CMS-PEI > CMS-SDS, and separation coefficients of all the modified samples were larger than that of the pristine CMS. In all the modified CMS samples, CH4 was more strongly adsorbed. The sample of CMS-P-N has the best modification effect and its saturated adsorption capacities qm were 6.76 mmol/g for CH4 and 5.56 mmol/g for N2, respectively. Finally the separation coefficient of CH4/N2 of CMS-P-N was 3.32. The breakthrough time of CMS-P-N was the largest, which confirmed that low temperature plasma treatment in an N2 atmosphere was beneficial for the adsorption separation of CH4/N2. This may be due to a large number of nitrogen atoms being inserted into the carbon chain and nitrogen groups formation or the plasma treatment in an N2 atmosphere etching the surfaces of the CMS. Therefore, it is reasonably expected that low temperature plasma modification for the CMS may supply a promising approach towards adsorption separation of CH4/N2.
indicated the CMS-P-N had the largest CH4 adsorption ability after low temperature plasma modification and good adsorption separation of CH4/N2. Under the nitrogen atmosphere, electrons in the non-equilibrium plasma were accelerated to become high-energy electrons in an electronic field, which collided with gas molecules, leading to cracking, ionization, and excitation reactions. The dissociation degree of nitrogen molecules and the energy of active particles were relatively high. The low temperature plasma treatment in an N2 atmosphere etched the surfaces of the CMS and pores formed. However, many nitrogen atoms were inserted into the carbon chains, and nitrogen-containing groups formed, such as nitro, amidogen, and acylamino. In summary, the CMS modified by low temperature plasma treatment in a N2 atmosphere exhibited improved adsorption capacity and separation coefficient.
Conflicts of interest The authors declare no competing financial interest.
Fig. 7. Breakthrough curves of CH4 on samples: (a) CMS, CMS-C24 and CMS-SDS; (b) CMS-PEI, CMS-P-C, and CMS-P-N.
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This work was supported by the National Natural Science Foundation of China (41772166), the Key Industry Chain Innovation Project, Shaanxi province, China (2017ZDCXL-GY-10-01-02), the Independent Research and Development Fund from the Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Land and Resources, China (ZKF2016-1), and Xi'an Science and Technology Project (201805036YD14CG20(6)). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.02.048. References [1] Q. Fu, H. Yan, Y.H. Shen, D.H. Zhang, Y. Zhou, J. Cleaner Prod. 170 (2018) 704–714. [2] G. Wang, Contemp Chem. Ind. 37 (2008) 526–528. [3] D.S. Yuan, Y.N. Zheng, Q.Z. Li, B.Q. Lin, G.Y. Zhang, J.F. Liu, Powder Technol. 333 (2018) 377–384. [4] L. Wang, S.M. Liu, Y.P. Cheng, G.Z. Yin, D.M. Zhang, P.K. Guo, Int. J. Min. Sci. Technol. 27 (2017) 277–284. [5] L.H. Nie, S.P. Xu, Y.L. Su, Q. Shu, Chem. Ind. and Eng. Prog. 27 (1505–1511) (2008) 1521. [6] Y. Yang, D. L. Qu, P. Li, J.G. Yu, 69 (2018) 4518-4529. [7] S. Rasi, J. Läntelä, J. Rintala, Fuel 115 (2014) 539–543. [8] I. Karakurt, G. Aydin, K. Aydiner, Renew. Sust. Energy Rev. 15 (2011) 1042–1049. [9] Y.J. Wu, Y. Yang, X.M. Kong, P. Li, J.G. Yu, A.M. Ribeiro, A.E. Rodrigues, J. Chem. Eng. Data 60 (2015) 2684–2693. [10] J.A. Delgado, V.I. Águeda García, J. García, S. Álvarez-Torrellas, Sep. Purif. Technol. 209 (2019) 550–559. [11] W.N. Sun, Y.H. Shen, D.H. Zhang, H.W. Yang, H. Ma, Ind. Eng. Chem. Res. 54 (2015) 7489–7501. [12] S. Effendy, C. Xu, S. Farooq, Ind. Eng. Chem. Res. 56 (2017) 5417–5431. [13] S. Cavenati, C.A. Grande, A.E. Rodrigues, Chem. Eng. Sci. 61 (2006) 3893–3906. [14] M. Gu, B. Zhang, Z.D. Qi, Z.J. Liu, S. Duan, X.D. Du, X.F. Xian, Sep. Purif. Technol.
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Adsorption separation of CH4/N2 on modified coal-based carbon molecular sieve
T
Zhiyuan Yanga, , Dechao Wanga,b, Zhuoyue Menga, Yinyan Lia ⁎
a b
College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, PR China College of Science, Northwestern Polytechnical University, Xi’an 710129, PR China
ARTICLE INFO
ABSTRACT
Keywords: Coal-bed methane Carbon molecular sieve Modification Adsorption separation Breakthrough curve
Coal-based carbon molecular sieves (CMS) were modified by organic reagents with affinities for hydrocarbons including tetracosane (C24), lauryl sodium sulfate (SDS), and polyethylene imine (PEI), and low temperature plasma treatment in a CH4 and N2 atmosphere, respectively. The modified CMS samples were characterized by FT-IR analysis, low-temperature N2 adsorption tests and SEM. In addition, the adsorption capacities of pure CH4 and N2 were measured by the static volume method and the CH4/N2 separation coefficients of the modified samples were also compared. The saturated adsorption capacities qm after modification and separation coefficients were investigated. CH4 was more strongly adsorbed on all the modified samples. The sample of CMS-P-N has the best modification effect and its saturated adsorption capacities qm were 6.76 mmol/g for CH4 and 5.56 mmol/g for N2, respectively. The separation coefficient of CH4/N2 of CMS-P-N was 3.32. Besides, CH4 breakthrough tests were performed using a fixed bed apparatus. The breakthrough curves confirmed that low temperature plasma treatment of CMS in an N2 atmosphere was beneficial for the adsorption separation of CH4/ N2.
1. Introduction Coal-bed methane (CBM), with methane (CH4) as the main component, is a non-renewable resource [1]. Generally, The methane volume concentration of extracted CBM in China is lower than 30% [2]. To ensure coalmine safety, the methane cannot be used if its volume concentration is below 30%. Therefore, it is usually discharged directly into the air [3,4]. On one hand, there is a remarkable waste of resources due to direct methane emissions, which totals 200 million tons from standard coal per year [5]. On the other hand, as a greenhouse gas, methane is seven times more destructive to the ozone than CO2 [6]. Therefore, the exploitation and utilization of low concentration CBM is important for ensuring coalmine safety, improving the energy structure, reducing energy waste and environmental pollution, and developing a low-carbon, circular economy [7–10]. Pressure swing adsorption (PSA) is an energy efficient gas separation technology [11], and the adsorption performance of the adsorbent material plays a decisive role. However, the separation of CH4 and N2 in CMB would be a major issue because of their similar physicochemical properties [12]. Therefore, the development of suitable adsorbent for the separation of methane and nitrogen has been attracting the
attention of many researchers, which is a bottleneck for the application of PSA to the separation of CH4 and N2 [13]. Development of adsorbents that have high capacities, high selectivities, and good regenerabilities for adsorption/desorption is critical for the success of methane enrichment processes via PSA [14,15]. Typically, two types of porous carbon have been widely studied and used in PSA: carbon molecule sieves (CMS) and activated carbon (AC) [16]. The CMS is a carbonaceous material with narrow pores, which has a high resistance to alkali and basic media, strong hydrophobicity, low cost, and high hydrothermal stability. However, the relatively low separation efficiency of CMS as adsorbent materials for methane enrichment is a major challenge. Many researchers have focused on modified molecular sieves to improve the adsorption separation performance for other gases [17,18]. Modification methods include impregnation methods, oxidation-reduction modifications, plasma modifications, and metal-loading modifications [19]. However, there are few reports on the modification of CMS adsorbents for CH4/N2 separation. The purpose of this work was to modify CMS samples using organic reagents with affinities for hydrocarbon and low temperature plasma treatment, respectively. These reagents included tetracosane (C24),
Abbreviations: CBM, coal-bed methane; CMS, carbon molecular sieve; DBD, dielectric barrier discharge ⁎ Corresponding author. E-mail address: [email protected] (Z. Yang). https://doi.org/10.1016/j.seppur.2019.02.048 Received 19 November 2018; Received in revised form 15 February 2019; Accepted 25 February 2019 Available online 26 February 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram of diagnostic equipment for the DBD plasma treatment.
lauryl sodium sulfate (SDS) and polyethylene imine (PEI). The low temperature plasma modification was performed in a methane and nitrogen atmosphere, respectively. The adsorption capacities and methane and nitrogen separation coefficients of the modified CMS samples were measured. Finally, breakthrough tests were also performed.
aqueous KOH solution with a 3:1 mass fraction of KOH: carbonized sample, and the mixture were stirred thoroughly. The activated samples were heated to 1073 K and held for 2 h with a ramp rate of 5 K·min−1 in the flowing stream of nitrogen (200 mL·min−1). After cooling under nitrogen flow, the samples were washed with 5 mol·L-1 HCl until the pH was 7 and dried at 393 K for 12 h in a vacuum oven. Because it was difficult to obtain highly uniform pores through the carbonization-KOH activation process, a benzene vapor deposition process (chemical vapor deposition, CVD) was used. This process was carried out in a reactor under a N2 atmosphere (200 mL·min−1). The quartz jacketed reactor was purged with N2 for 30 min at 298 K to remove air from the system. The activated sample was heated to a deposition temperature of 873 K, after which benzene liquid was pumped into the reactor using a metering pump at 1.2 mL·min−1, and this process was held for 10 min. The prepared samples, denoted as CMS, were cooled to room temperature.
2. Experimental section 2.1. Materials Anthracite was supplied by Shenhua Ningxia Coal Industry Group Co., Ltd. The textural characteristic of the anthracite measured by N2 adsorption at 77 K was shown in Fig. S1. Pore structure parameters of Anthracite were presented in Table S1. Tetracosane, lauryl sodium sulfate (SDS, CH3(CH2)11OSO3Na) and polyethylene imine (PEI) were obtained from Tianjin Fuchen Organics. Concentrated nitric acid (HF, 49%) and hydrochloric acid (HCl, 36.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. High-purity (> 99.99%) of N2 and CH4 were purchased from Shaanxi Hongwei Gas Co. Ltd, respectively. Deionized water was used in all the experiments and other solvents and chemicals were used without further purification.
2.3. Modification of CMS 2.3.1. Impregnation modification with organic reagents Tetracosane, sodium dodecyl sulfate, and polyethylene imine were chosen to modify the CMS respectively. The detailed steps were as follows: Tetracosane modification: First, 1 g of CMS was impregnated with a tetracosane solution (1 g of tetracosane in 100 mL of anhydrous ether) and stirred at 313 K until the anhydrous ether evaporated completely. Subsequently, the sample was washed with boiling water to remove any excess tetracosane and dried at 393 K for 12 h in vacuum. The prepared sample was denoted as CMS-C24. Sodium dodecyl sulfate modification: First, 1 g of CMS was impregnated with lauryl sodium sulfate solution (1 g of sodium dodecyl sulfate in 100 mL of deionized water) and stirred at 303 K for 5 h. Subsequently, the sample was washed with deionized water, filtered, and dried at 393 K for 12 h in vacuum. The prepared sample was denoted as CMS-SDS. Polyethylene imine modification: First, 1 g of CMS was immersed in a polyethylene imine solution (1 g of polyethylene imine in 100 mL of ethyl alcohol) and was stirred for 6 h at 353 K to remove ethanol. Subsequently, it was washed with water to remove excess polyethylene
2.2. Preparation of CMS Taixi anthracite was used as precursor material for the preparation of CMS. The precursor was firstly ground and sieved to a particle size less than 74 µm, after which it was washed with 15 wt% HF, 45 wt% HCl, and 40 wt% deionized water to eliminate ash impurities, then dried at 373 K for 12 h to remove any moisture, and stored in zipper storage bags. The ultimate analysis of Taixi anthracite are as follows: N content of 0.84 wt%, C content of 88.61 wt%, and H content of 3.17 wt %, respectively. The ash content of Taixi anthracite is 5.48 wt%. The preparation process was performed in a custom-made carbonization-physical/chemical activation-deposition integrated tube furnace. The schematic of the setup is shown in Fig. S2. The carbonization of the precursor material described above was carried out by heating at 673 K for 1 h with a ramp rate of 5 K·min−1 in a flowing stream of nitrogen (200 mL·min−1). The activation process was performed as follows: the carbonized sample was mixed in an 131
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Fig. 2. Schematic diagram of the breakthrough test for the CH4/N2 mixture.
imine. Finally, the sample was dried at 373 K for 12 h in vacuum. The prepared sample was denoted as CMS-PEI.
instrument analyzer is shown in Fig. S3. And the detailed measurement principle and procedures were presented in Supporting Information. All the adsorption isotherms of CH4 and N2 are obtained based on the Langmuir equation Eq. (1) [22]:
2.3.2. Low temperature plasma modification The CMSs was treated by a dielectric barrier discharge (DBD) plasma under a CH4 or N2 atmosphere. The DBD plasma setup is shown in Fig. 1. First, 0.2 g of CMS sample was loaded in the barrier discharge cell. The dielectric barrier discharge was generated by applying 45 V to the electrodes using a DC voltage generator with methane or nitrogen as the plasma-forming gas. The plasma conditions were maintained for 1 min and each sample was treated five times. The samples prepared under methane and nitrogen were denoted as CMS-P-C and CMS-P-N, respectively.
q=
qm bp 1 + bp
(1)
where q is the gas uptake (mmol·g−1), qm the maximum gas uptake (mmol·g−1), b is a fitting constant that reflects the adsorption strength, and p is the adsorption pressure. It is possible to predict the co-adsorption of CH4 and N2 on the modified CMS from the single-component isotherms using a simple multi-component Langmuir equation Eq. (2) [23]:
qmi bi pyi
2.4. Characterization
qi =
The surface functional groups of the samples were characterized using a Nicolet 20SXB spectrometer. A disk of pure KBr was used as a reference sample for the background measurements. Samples were mixed with 99.99% KBr at a ratio of 1:500, and the spectra were recorded in the range of 400–4000 cm−1. The specific surface areas and pore structures of the samples were measured using N2 adsorptiondesorption at 77 K with a Micromeritics ASAP 2020 instrument. The Brunauer-Emmett-Teller (BET) method was used to determine the specific surface areas at relative pressures in the range of P/ P0 = (0.05–0.30) [20]. The t-plot method was applied to calculate the microspore volumes [21]. The specific total pore volumes (Vtotal) were determined directly from the amount of gas adsorbed at a relative pressure of P/P0 = 0.99.
For a binary mixture of two components (i and j), the separation coefficient can be calculated using Eq. (3) [24]: ij
=
1 + bCH4 pyCH4 + b N2 p (1
x i / yi = x j / yj (1
x i /yi x i )/(1
yi )
yCH4 )
(2)
(3)
where xi and xj are the mole fractions of component i and j in the adsorbed-phase, respectively, xi = qi/(qi + qj), and yi and yi are the mole fraction of component i and j in the gas-phase, respectively. Using the fitting parameters, qm and b in Eq. (2) and (3), the separation coefficient of CH4 and N2 was calculated with Eq. (4) [14]: CH4 / N2
2.5. Adsorption isotherms of pure CH4 and N2
=
qmCH4 bCH4 qmN2 b N2
(4)
2.6. Breakthrough curve tests
CH4 and N2 adsorption isotherms were measured using a hightemperature and high-pressure adsorption instrument analyzer (3H2000PH, Beishide Instrument Technology, Beijing Co., Ltd, China) using the static volumetric technique at 298 K in a pressure range of 0–2 MPa. Prior to the adsorption measurements, the samples were evacuated at 523 K for 12 h to desorb moisture and gases. The schematic diagram of high-temperature and high-pressure adsorption
The breakthrough curves of CH4 in the mixed gases were measured in a fixed single-bed unit as shown in Fig. 2. The sample was degassed at 393 K for 12 h to remove water prior to the breakthrough experiments. The adsorption bed was a stainless-steel tube with length of 150 mm and internal diameter of 10 mm. The adsorbent samples (9 ± 0.001 g) were loaded into the bed at 298 K. The breakthrough 132
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Fig. 3. FT-IR spectra of modified samples. (a) CMS, CMS-SDS, CMS-PEI, and CMS-C24. (b) CMS, CMS-P-N, and CMS-P-C.
time was defined as the time required to detect CH4 at the outlet of the adsorption column with a volume fraction of 1.25%. More detailed measurement information is shown in the Supporting Information.
3.3. Porous structures The N2 adsorption-desorption isotherms and pore size distribution of the modified samples are shown in Fig. 5. According to the IUPAC classification, all the samples exhibited type I isotherms, with significant nitrogen adsorption amounts at P/P0 > 0.1 and no obvious hysteresis loop at P/P0 > 0.4, which are characteristics typical of microporous materials. According to Fig. 5(b), the pore size distribution (PSD) of these modified samples was larger than that of other CMS samples in previous literatures [30–32]. Compared with the unmodified CMS, the CMS-P-N adsorbed more nitrogen. Table 1 lists the BET specific surface areas (SBET) and the total pore (Vtotal) and micropore (Vmicro) volumes of the samples, which can be used to analyze changes in the pore characteristics of these samples. As seen from the Table 1, the modification of CMS samples did not considerably change their textural properties. After impregnation modification, the ratios of the total pore to micropore volumes (Vmicro/ Vtotal) of the modified CMS samples decreased slightly, while they increased after low temperature plasma modification. These results indicate that the low temperature plasma modification promoted the formation of new micropores and the impregnation modification blocked partial micropores in the CMS, but the blockage was very slight and could be neglected. The variations in the CH4/N2 selective adsorption on the modified samples should primarily be related to the surface chemistry changes.
3. Results and discussion 3.1. Surface chemical properties The FT-IR spectra of the samples are shown in Fig. 3. The main characteristic absorption peaks are similar. The broad bands around 3445 cm−1 are attributed to O-H stretching vibration due to the adsorbed water on the CMS surfaces [25]. The bands at around 2986 and 2930 cm−1 are assigned to aliphatic -CH stretching. The band at around 1630 cm−1 is attributed to the aromatic C]C [26], the band in the range of 900–700 cm−1 is attributed to the vibrations of the aromatic structure. After C24 modification, the weak bands observed around 751, 2851, and 1631 cm−1 were caused by v(C-H) in -CH, -CH3, and -CH2, respectively. After SDS modification, the band at around 1051 cm−1 is attributed to stretching modes of S]O group and the weak peaks at 621 cm−1 to the stretching modes of S-O in the SDS molecules [27]. After PEI modification, the band around 1403 and 1647 cm−1 are attributed to vibration of C-H and N-H of the amine group, respectively. The peaks at 1577 cm−1 are assigned to deformation vibration of -NH2 groups [28,29]. These observed variations in the functional groups indicate that the modification of the biomass by PEI was successful. Thus, after impregnation modification with the above organic reagents, corresponding characteristic groups were present on the surfaces of the CMS. After treatment of the CMS with N2 plasma, the adsorption peak at 1112 cm−1 is attributed to the presence of tertiary nitrogen species (CN stretching vibrations). There were nitrogen-containing groups on the surface of the CMS-P-N sample. After treatment of CMS with CH4 plasma modification, there was not significant change to the band at around 1630 cm−1.
3.4. Pure gas adsorption capacities of CH4 and N2 The adsorption capacities of pure CH4 and N2 of the modified CMS samples were investigated at 298 K, as depicted in Fig. 6. More detailed absolute adsorption capacity data measured at 298 K and pressures from 0 to 2 MPa are shown in Tables S1 and S2. The adsorption capacities of CH4 and N2 were different on the modified CMS samples compared with the pristine CMS samples. The adsorption capacities of CMS-C24 and CMS-P-C were similar to that of the unmodified CMS. The adsorption capacities of CMS-P-N were slightly higher than that of the CMS, while adsorbed amounts on CMS-SDS and CMS-PEI were much lower.
3.2. SEM Fig. 4 shows the SEM images of precursor material and modified samples. The surface of anthracite precursor was smooth and tight, no obvious pores were observed (Fig. 4(a)). Whereas the modified samples showed rugged surface and plenty of pores with different size and shapes on the surface. These numerous pores were beneficial to the adsorption of CH4 and N2. Meanwhile, compared with Fig. 4(b–f), no obvious changes were observed.
3.5. Separation coefficients The linear Langmuir equation was used to fit the above experimental adsorption capacity data, as shown in Fig. S4. The fitting parameters are listed in Table 2. After modification, the saturated adsorption capacities qm of CH4 for the modified CMS were larger than those of N2. However, the 133
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Fig. 4. SEM images of (a) anthracite precursor material, (b) CMS-C24, (c) CMS-SDS, (d) CMS-PEI, (e) CMS-P-C and (f) CMS-P-N.
adsorption capacities differed between samples. The saturated adsorption capacities qm of CH4 and N2 for CMS-C24, CMS-P-N, and CMS-P-C were larger than those of the pristine CMS, while those of CMS-SDS and CMS-PEI were lower than that of the pristine CMS. Comparison of the saturated adsorption capacities qm of CH4 and N2 obtained in this work and other adsorbent were presented in Table S4. The adsorption strength b of CH4 was larger than that of N2 on the modified CMS because the polarizability of CH4 (26 × 10-25 cm−3) molecules is larger than that of N2 molecules (17.6 × 10-25 cm−3) [33]. This created stronger interactions between CH4 molecules and the CMS compared to those of N2 and the CMS. Thus, CH4 molecules were preferentially adsorbed. The separation coefficients of CH4 and N2 prepared in this study and previously reported values are in Table 3. After modification, CMS-SDS has the largest separation coefficient, and the separation coefficients of all the modified samples were larger than that of pristine CMS. The saturated adsorption capacity of CMS-SDS was the lowest. To
Table 1 BET Surface area and pore structure parameters of modified CMS samples. Sample
SBET (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmicro/Vtotal
CMS CMS-C24 CMS-SDS CMS-PEI CMS-P-N CMS-P-C
1134 1140 1073 1081 1241 1137
0.548 0.426 0.437 0.525 0.591 0.534
0.372 0.264 0.267 0.342 0.406 0.369
0.678 0.620 0.612 0.651 0.703 0.691
achieve a high adsorption capacity and separation coefficient simultaneously, the best method is low temperature plasma modification. Plasma is a pollution-free technology with a low energy consumption and short processing time, and it has great potential for use in the modification of porous CMS for PSA [41,42].
Fig. 5. (a) N2 adsorption-desorption isotherms and (b) pore size distribution of adsorbents. 134
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Fig. 6. Adsorption capacities of CH4 and N2 of (a) CMS, (b) CMS-C24, (c) CMS-SDS, (d) CMS-PEI, (e) CMS-P-N, and (f) CMS-P-C at 298 K.
3.6. Adsorption breakthrough curves
The breakthrough time of the unmodified CMS was 112 s at the breakthrough point (1.25 vol% CH4) at the outlet, and the methane concentration increased quickly after 112 s. The breakthrough times of CMSC24, CMS-SDS, CMS-PEI, CMS-P-N, and CMS-P-C were 159, 144, 140, 168, and 160 s, respectively. The order of CH4 breakthrough times was: CMS-P-N > CMS-P-C > CMS-C24 > CMS-SDS > CMS-PEI > CMS. A larger breakthrough time corresponds to a larger adsorption capacity. Therefore, the largest adsorption capacity of CH4 CMS-P-N was obtained. The breakthrough curves agreed well with the Langmuir equation fitting parameter b, which reflects the adsorption strength. This
The CH4 adsorption breakthrough curves of the modified CMS in the simulated CBM were shown in Fig. 7. In all the breakthrough curves, CH4 was adsorbed at the adsorption column inlet at the beginning of the experiments. The concentration of CH4 was zero in the outlet early in the experiment. Over time, the mass transfer zone moved up the column, and the adsorption bed reached saturation when mass transfer zone reached the outlet. When the CH4 concentration was equal to the inlet, the breakthrough curves no longer increased. 135
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Table 2 Parameters of Langmuir equation fitting to CH4 and N2 adsorption isotherms at 298 K. Langmuir fitting results CH4
N2
Samples
qm (mmol/g)
b (MPa−1)
R2
qm (mmol/g)
b (MPa−1)
R2
Separation coefficient
CMS CMS-C24 CMS-SDS CMS-PEI CMS-P-N CMS-P-C
5.2751 7.5182 2.2051 2.8977 6.7632 5.7290
1.4513 0.7818 0.8667 1.5040 1.8790 1.5421
0.9993 0.9991 0.9883 0.9967 0.9951 0.9979
3.9638 4.6328 1.6291 2.2460 5.5661 4.6025
1.0160 0.5981 0.3480 0.6461 0.6875 0.6062
0.9959 0.9888 0.9871 0.9970 0.9922 0.9966
1.90 2.12 3.37 3.00 3.32 3.16
4. Conclusions
Table 3 Comparison of separation coefficients of CH4 and N2 obtained in this work and other adsorbents. Sample
Equilibrium separation coefficient of CH4/N2
Testing condition
Anderson AX-21 [34] Carbon (PVDC) [34] Carbon (FFAD) [34] Carbon (G2X7/12) [35] 13X Zeolite [36] Vulcan carbon [24] AC/X-G-3 [37] Zeolite 13X [38] Activated Carbon Norit RB3 [39] CMS [40] This work
3.0 2.3 2.3 3.3 1.8 3.5 3.0 1.9–2.3 3.9
298 K, 0.10 MPa 298 K, 0.10 MPa 298 K, 0.10 MPa 298 K, 0.10 MPa 303 K, 0.19 MPa 293 K, 0.10 MPa 298 K, 0.10 MPa 273 K, 0.106–0.903 MPa 303.3 K, 0.994 MPa
3.08 1.9–3.3
303.3 K 298 K
In summary, coal-based carbon molecular sieves (CMS) were modified by organic reagents including tetracosane, lauryl sodium sulfate, and polyethylene imine. In addition, CMS samples were modified by low temperature plasma treatment in CH4 and N2 atmospheres, respectively. The pore structures of the modified CMS samples were similar. However, the surface chemical properties were different by the FT-IR analysis. The adsorption capacities of pure CH4 and N2 on the modified CMS samples were measured by static volume method. Meanwhile, separation coefficients of CH4/N2 were compared. The saturated adsorption capacities (qm) were in the following order: CMSC24 > CMS-P-N > CMS-P-C > CMS > CMS-PEI > CMS-SDS, and separation coefficients of all the modified samples were larger than that of the pristine CMS. In all the modified CMS samples, CH4 was more strongly adsorbed. The sample of CMS-P-N has the best modification effect and its saturated adsorption capacities qm were 6.76 mmol/g for CH4 and 5.56 mmol/g for N2, respectively. Finally the separation coefficient of CH4/N2 of CMS-P-N was 3.32. The breakthrough time of CMS-P-N was the largest, which confirmed that low temperature plasma treatment in an N2 atmosphere was beneficial for the adsorption separation of CH4/N2. This may be due to a large number of nitrogen atoms being inserted into the carbon chain and nitrogen groups formation or the plasma treatment in an N2 atmosphere etching the surfaces of the CMS. Therefore, it is reasonably expected that low temperature plasma modification for the CMS may supply a promising approach towards adsorption separation of CH4/N2.
indicated the CMS-P-N had the largest CH4 adsorption ability after low temperature plasma modification and good adsorption separation of CH4/N2. Under the nitrogen atmosphere, electrons in the non-equilibrium plasma were accelerated to become high-energy electrons in an electronic field, which collided with gas molecules, leading to cracking, ionization, and excitation reactions. The dissociation degree of nitrogen molecules and the energy of active particles were relatively high. The low temperature plasma treatment in an N2 atmosphere etched the surfaces of the CMS and pores formed. However, many nitrogen atoms were inserted into the carbon chains, and nitrogen-containing groups formed, such as nitro, amidogen, and acylamino. In summary, the CMS modified by low temperature plasma treatment in a N2 atmosphere exhibited improved adsorption capacity and separation coefficient.
Conflicts of interest The authors declare no competing financial interest.
Fig. 7. Breakthrough curves of CH4 on samples: (a) CMS, CMS-C24 and CMS-SDS; (b) CMS-PEI, CMS-P-C, and CMS-P-N.
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