N2 Separation

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Preparation of Carbon Molecular Sieves Used for CH4/N2 Separation Jinhua Zhang,*,†,‡,§ Sijian Qu,‡,§ Lanting Li,‡,§ Peng Wang,‡,§ Xuefei Li,‡,§ Yongfang Che,‡,§ and Xiaoliang Li‡,§ †

School of Chemical and Environmental Engineering, China University of Mining and TechnologyBeijing, Beijing 100083, China Beijing Research Institute of Coal Chemistry, China Coal Research Institute, Beijing 100013, China § State Key Laboratory of Coal Mining and Clean Utilization, Beijing 100013, China ‡

ABSTRACT: In this study, carbon molecular sieves (CMS) were prepared by chemical vapor deposition method using benzene as depositing agent based on activated carbon. The influences of deposition temperature, time, and flow rate of benzene on adsorption capacity and separation coefficient were investigated. The results show that the CH4 equilibrium adsorption capacity and CH4/N2 separation coefficient of CMS-G, which is prepared at the deposition temperature of 1023 K, time of 60 min, and flow rate of 4 mL·min−1, are at a maximum value (parametric investigation of this study is based on the parameter ranges in the text). The deposition temperatures had an obvious effect on the textural modification of CMS. CMS-G was characterized by mercury intrusion, CO2 permeation, analysis of kinetics, and pressure swing adsorption (PSA). Both kinetic and PSA results demonstrate a good CH4/N2 separation performance for the CMS-G. A very high kinetic selectivity was observed, which reached 35.26. An enrichment of 30.20% for methane in pressure swing adsorption experiment was also observed. The data demonstrate that CMS-G is a good adsorbent suitable for separating CH4 from methane and nitrogen gas mixture.

1. INTRODUCTION Use of carbon molecular sieves in pressure swing adsorption (PSA) gas separation technology has been known for many years. Carbon molecular sieves (CMS) are a special class of activated carbon (AC) which is often used as adsorbent in gas separation processes.1 It is mainly composed of micropores that are less than 1 nm and a few macropores. Due to the relatively narrow and uniform pore size distribution, using CMS as adsorbent has successfully separated nitrogen from air or carbon dioxide and methane from their mixtures.2−4 CH4 is both a greenhouse gas and an important chemical raw material.5 While it is recognized that the methane gas in coal bed methane is abundant, the development of CMS used for CH4/N2 separation has attracted international attention6−8 because the main gas composition of coal bed methane are methane and nitrogen after the oxygen is removed. However, the kinetic diameter of nitrogen (0.364 nm) is slightly smaller than that of methane (0.382 nm), and their kinetic diameters are so similar that commercial CMS cannot separate the CH4/N2 mixture sufficiently. Developing a new type of CMS special for CH4/N2 separation is needed. MacElroy et al.9 reported that the pore width is of fundamental importance in the selectivity of CMS. © XXXX American Chemical Society

In this paper, CMS was prepared by modification the pore size of activated carbon with chemical carbon deposition. Benzene was used as an agent because it did not produce intermediate species in the cracking process.10−13 An effective CMS is defined by two properties: selectivity and adsorption capacity.14 Selectivity determines whether one gas can be separated from gas mixtures or not; adsorption capacity is related to the micropore volume. The higher the selectivity and adsorption capacity, the better its adsorption properties will be. In this paper, CMS was obtained through the modification of activated carbon by deposition of carbon from benzene, the influences of deposition temperature, times, and benzene flow rates on adsorption properties of CMS were investigated. Equilibrium adsorption isotherms of CMS prepared at different conditions were first characterized. The sample of CMS-G was prepared in this study. The kinetic selectivity and pore size of CMS-G for CH4/N2 separation were evaluated. To evaluate the Received: January 14, 2018 Accepted: April 16, 2018

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DOI: 10.1021/acs.jced.8b00048 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Proximate and Ultimate Analysis of Raw Coal proximate analysis (%)

ultimate analysis (%)

Mad

Ad

Vd

FCd

Cad

Had

Nad

Oad

St,ad

1.10

2.11

7.88

90.01

88.18

3.54

0.81

4.06

0.22

Table 2. Attributes of Coal Tar C/%

H/%

N/%

S/%

M/%

A/%

viscosity/Pa·s

density/kg·m−3

89.13

4.64

1.07

0.59

4.4

0.09

668

1225.1

separation properties of CMS used in PSA equipment, PSA experiments of CMS-G in this work were investigated.

Table 3. Properties of Material Used in the Adsorption Experiment

2. .EXPERIMENTAL METHODS 2.1. Preparation Procedures of Carbon Molecular Sieves. 2.1.1. Materials and Chemicals. Anthracite was used as raw material for preparation of activated carbon. The proximate and ultimate analysis are shown in Table 1. The binder used for agglomeration was coal tar. Its attributes are shown in Table 2. Benzene with purity of 99.99% supplied by Sinopharm Chemical Reagent Beijing Co. Ltd. (China) was used as a deposition agent in the chemical vapor deposition (CVD) process. The purities of all gases used in this work are greater than 99.999%. 2.1.2. Preparation of Activated Carbon. Anthracite coal was dried and crushed to particles having the size of 1−3 mm and then ground and sorted to particles having the size under 0.074 mm. Anthracite coal powders were mixed with coal tar and water, the mixing ratio of the coal powder, coal tar, and water was 10:4:1. The homogeneous paste was extruded in a hydraulic press under the form of pellets with a diameter 1.6 mm and a length of 6−8 mm. After drying at 373 K for 4 h, 100 g of these pellets were sent into the reactor and heated up to the carbonization temperature in a flowing stream of nitrogen (200 mL·min−1). The temperature of the reactor was increased at the rate of 5 K·min−1 until it reached the final carbonization temperature of 1073 K, and this temperature was maintained for 1.0 h. After that, the product was cooled under nitrogen flow. The sample is known as activated carbon, which was used as a precursor for production of CMS; detailed information about the preparation and characterization of the AC was given elsewhere.15 2.1.3. Preparation of CMS. CMS was prepared by pore size modification of AC with benzene by CVD methods.16 The preparation of CMS was carried out in a reactor under N2 atmosphere (100 mL·min−1). At the beginning, the reactor was purged with N2 at the rate of 1L· min−1 for about 20 min to drive out the air inside the reactor, and then 100 g of AC sample was put into the reactor and heated to the deposition temperature at the rate of 15 K·min−1 until it reach the final deposition temperature. After that, the metering pump was switched on to inject the benzene vapor. The benzene vapor was driven into the reactor by the N2 flow. The influence of different preparation conditions on the equilibrium adsorption of the material was analyzed: (1) the deposition temperature from 923 to 1123 K, (2) time from 20 to 80 min, and (3) flow rate of benzene from 1.0 to 10.0 mL·min−1. The samples of carbon molecular sieves prepared in this study were supplied in extruded pellet form of cylindrical shape. The properties of samples are shown in Table 3. 2.2. Characterization of CMS. 2.2.1. N 2 and CH 4 Adsorption. The kinetic adsorption experiments were carried out in a gravimetric adsorption apparatus (Cahn TherMax500). About 0.5 g of samples were used in the adsorption apparatus.

sample

diameter

length

bulk density

CMS

1.6 mm

2−4 mm

650 g·L−1

The samples were outgassed at 473 K for 2 h every time. Then, the samples were exposed to helium at a flow rate of 80 mL·min−1. After the samples cooled, a stable baseline was obtained; the gas flow was switched to pure N2 or CH4, also flowing at 80 mL·min−1, and the weight uptake with time would be monitored. The balance control unit was connected to a computer. All of the adsorption experiments were conducted at 303 K. The equilibrium adsorption experiments were also carried out using Cahn TherMax500 balance. The equilibrium adsorption isotherms of N2 and CH4 were determined at 303 K with relative pressure of 0−1.0. The mass changes were monitored by a stepwise increase of the pressure of N2 or CH4. The flow rate of gas and temperature were the same as in the kinetic adsorption experiment. If the equilibrium adsorption isotherms belong to typical type I (IUPAC), the ideal Langmuir model (eq 1) is often used to correlate isotherm data, as reported by Rutheven.17 bp V = Vs 1 + bp

(1)

Where Vs is the saturated amount adsorbed, V is the amount adsorbed, b is the Langmuir constant, and p is the pressure. The values of Vs and b for different samples were calculated by linear regression with an R2 of 0.99. The equilibrium separation coefficient (eq 2) is used to estimate the static separation ability.

αij =

Vsibi Vsjbj

(2)

Where αij is the equilibrium separation coefficient, i/j represents different gas. 2.2.2. Textural Characterization. Textures of CMS prepared at different deposition temperatures were characterized using N2 adsorption at 77 K using a Micromeritics ASAP 2010 surface area analyzer. The macroporous structure of CMS was assessed by mercury porosimetry. Microporous properties of CMS was characterized using CO2 adsorption at 273 K using Quantachrome NOVA4200e automated surface area and pore size analyzer. 2.3. Technical Applications of CMS. At present, technology of PSA is successfully applied for gas separation. The separation ability of adsorbents could be characterized directly. In this study, three methane−nitrogen feed mixtures with different concentrations were separated in a four-bed PSA unit (Figure 1) using the carbon molecular sieve adsorbent selected. The main equipment consists of four stainless steel vessels, connected with tubes, which can be opened or closed by automatically controlled valves. The size of the adsorption vessel is Φ80 × 400 mm; the flow of raw gas is 10 L·min−1, and the B



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adsorption pressure is 0.6 MPa. The desorption pressure is −80 KPa; adsorption time = 60 s, and the flow of product gas = 4.2 L·min−1. The PSA process cycle is composed of the following five operations: adsorption stage (60 s), equalizing pressure stage (5 s), pathwise release (35 s), counter-current release (20 s), desorption (40 s), and repressurization stage (60 s). A/B/C/D, adsorption column. In the adsorbed reactor, N2 is more quickly absorbed by CMS when the raw gas passes through the adsorbed vessel upward; CH4 product will continuously release to the buffer vessel from the top of the A/B/C/D adsorbed vessel. N2, CMS adsorbed, will be evacuated from the vessel at desorption stage using a vacuum pump.

3. RESULTS AND DISCUSSION 3.1. Study of the CMS Preparation Process. 3.1.1. Influence of Deposition Temperature. In this experiment, the adsorption isotherms of CH4 and N2 on the CMS prepared at different deposition temperatures are shown in Figure 2, and the actual experimental values of Figure 2 are shown in Tables 4 and 5. It is apparent that the equilibrium adsorption isotherms belong to typical type I. Adsorption data were used to characterize the CMS by the Langmuir equation. Results by linear regression are shown in Table 6. It is observed that the adsorption capacity of nitrogen or methane increases with the deposition temperature to reach a maximum for a deposition temperature of 1023 K and then decreases as the deposition temperature exceeds 1023 K. The saturated adsorption volume of CH4 and N2 are 1.41 and 0.58 mmol·g−1, respectively. The maximum equilibrium separation coefficient obtained is 4.74 when the deposition temperature is 1023 K. This is because the macropore and mesopore can be converted to micropore effectively when the temperature is between 923 and 1023 K. However, many pore mouths of CMS may be covered by the depositing carbon when the temperature is higher than 1023 K, and the adsorption performance decreases quickly. These results suggest that to obtain an optimal separation performance, the deposition temperature should be around 1023 K. 3.1.2. Influence of Deposition Time. Figure 3 shows the influence of different deposition times on the adsorption amount of the CMS at a deposition temperature of 1023 K and benzene flow rate of 4 mL·g−1. The actual experimental values of Figure 3 are shown in Tables 7 and 8. The same method was used to fit the adsorption data. The results are given in Table 9. It can be seen that the adsorption amount increases with increasing deposition time from 20 to 60 min. However, above 60 min, it reduces apparently. This is explained by the blockage of

Figure 1. A four-bed PSA unit for CH4/N2 separation using CMS.

Figure 2. Adsorption isotherms of CH4 and N2 on the CMS prepared at different deposition temperatures, deposition time of 60 min, and benzene flow rate of 4 mL·min−1: (a) N2 and (b) CH4.

Table 4. Actual Experimental Values of Figure 2a: N2 P/P0

CMS-923K

CMS-973K

CMS-1023K

CMS-1073K

CMS-1123K

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.262 0.3174 0.3668 0.402 0.42 0.4302 0.4319 0.4326 0.4334 0.4341

0 0.311 0.3674 0.413 0.4532 0.4776 0.4896 0.4906 0.4913 0.4921 0.4927

0 0.3038 0.3766 0.4413 0.4746 0.4931 0.5132 0.524 0.5267 0.5281 0.5296

0 0.3074 0.372 0.4271 0.4639 0.4854 0.5014 0.5073 0.509 0.5101 0.5111

0 0.1117 0.1427 0.1706 0.2053 0.2378 0.2615 0.2809 0.2977 0.3114 0.3185

C



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Table 5. Actual Experimental Values of Figure 2b: CH4 P/P0

CMS-923K

CMS-973K

CMS-1023K

CMS-1073K

CMS-1123K

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.7283 0.9521 1.0358 1.1028 1.1354 1.1563 1.1617 1.163 1.1643 1.1668

0 0.9072 1.0308 1.1173 1.1685 1.2007 1.2141 1.2193 1.222 1.2233 1.2247

0 0.9545 1.1075 1.2102 1.257 1.2978 1.3208 1.3263 1.3273 1.3286 1.33

0 0.9309 1.0692 1.1637 1.2128 1.2492 1.2674 1.2728 1.2746 1.276 1.2773

0 0.1126 0.1294 0.1404 0.1574 0.1744 0.188 0.2089 0.2102 0.216 0.2174

Table 6. Analysis of Adsorption Isotherms by Means of the Langmuir Equation N2

CH4

samples

Vs1 (mmol·g−1)

b1

Vs2 (mmol·g−1)

b2

equilibrium separation coefficient α12

CMS-923K CMS-973K CMS-1023K CMS-1073K CMS-1123K

0.48 0.54 0.58 0.56 0.35

11.63 13.20 10.46 1.05 4.21

1.29 1.29 1.41 1.35 0.22

13.36 23.20 20.59 21.77 9.10

3.08 4.23 4.74 4.48 1.35


CMS-20min

CMS-40min

CMS-60min

CMS-80min

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.2136 0.2836 0.3542 0.4039 0.4366 0.4678 0.4839 0.4881 0.4907 0.4917

0 0.311 0.3674 0.413 0.4532 0.4776 0.5042 0.5098 0.514 0.5151 0.5159

0 0.3038 0.3766 0.4413 0.4746 0.4931 0.5132 0.524 0.5267 0.5281 0.5296

0 0.2566 0.3342 0.3923 0.4354 0.4598 0.4811 0.4831 0.4893 0.4901 0.4908


CMS-20min

CMS-40min

CMS-60min

CMS-80min

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.0634 0.0666 0.0744 0.095 0.1281 0.1693 0.2067 0.2456 0.2782 0.341

0 0.9072 1.0308 1.1173 1.1685 1.2007 1.2141 1.2193 1.222 1.2233 1.2247

0 0.9545 1.1075 1.2102 1.257 1.2978 1.3208 1.3263 1.3273 1.3286 1.33

0 0.1634 0.2205 0.2829 0.3481 0.4038 0.4531 0.4885 0.5342 0.561 0.5989

the micropores may be blocked by the excessive deposition carbon. Table 9 indicates that the CMS-60min has the highest adsorption amount and equilibrium separation coefficient, so the best deposition time should be controlled around 60 min. 3.1.3. Influence of Benzene Flow Rate. The adsorption isotherms of CH4 and N2 on the CMS prepared at different deposition flow rates are shown in Figure 4, and the actual experimental values of Figure 4 are shown in Tables 10 and 11. The adsorption data were fitted by the Langmuir equation, the results are shown in Table 12. It can be seen that the adsorption

Figure 3. Adsorption isotherms of CH4 and N2 on the CMS prepared at different deposition times, deposition temperature of 1023 K, and benzene flow rate of 4 mL·min−1: (a) N2 and (b) CH4.

the pores during CVD processing; one possible reason is that the benzene molecules in the vapor phase are first adsorbed onto the inner surface and then pyrolyzed into carbon. At first, some of the macropores and mesopores are converted into micropores when the deposition time is less than 60 min, and the width of pores is reduced effectively. While the deposition time is longer, some of D



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Table 9. Analysis of Adsorption Isotherms by Means of the Langmuir Equation N2 −1

CH4 −1

samples

Vs1 (mmol·g )

b1

Vs2 (mmol·g )

b2


CMS-20min CMS-40min CMS-60min CMS-80min

0.55 0.54 0.58 0.56

5.48 13.20 10.46 8.24

0.63 1.29 1.41 0.70

1.98 23.20 20.59 2.81

0.39 4.23 4.74 0.43

isotherm curve. Figure 5 shows the adsorption isotherms of N2 on the CMS prepared at different deposition temperatures at 77 K. The amount of N2 adsorbed at P/P0 = 1.0 increases initially with the deposition temperature to reach a maximum at 1023 K and then decreases with the deposition temperature. This shows that raising the deposition temperature is beneficial to the formation of micro porous channels, but high temperature will lead to the pore shrinkage and even collapse. In all isotherms, the N2 adsorption isotherm on CMS prepared at 1023 K is more similar to Type I isotherm which indicate the presence of micropores. The adsorption and desorption isotherms of samples prepared at other deposition temperatures show a significant hysteresis loop which is caused by the presence of mesopores. This indicates that mesopore of AC can be transferred into micropore effectively at the deposition temperature of 1023 K. Different deposition process variables led to the textural modification of CMS. Textural characterization of these samples prepared at different deposition temperatures based on the adsorption data of nitrogen is showed in Table 13. SBET is the surface area measured by Brunauer−Emmett−Teller (BET) method. Vmic is the micropore volume calculated by Dubinnin− Radushkevich (DR) method. Da is the mean pore size. The results show that the carbon deposition are effective in producing constrictions at the pore entrances when the temperature is 1023 K and SBET and Vmic reach a maximum, 389.00 m2·g−1 and 0.1821 cm3·g−1, respectively. However, it has the smallest mean pore size (0.1922 nm). 3.2.2. Adsorption Isotherms of CO2. To assess the micropore properties of carbon materials, adsorption with nitrogen at 77 K provides an effective means for pore size characterization in general; however, the technique has limitations when dealing with small micropores,18 especially pores smaller than 0.5 nm. The kinetic energy of nitrogen is so low that it is hard to diffuse into the pores and be adsorbed. In this experiment, supercritical gas adsorption can be effective, and CO2 at 273 K is employed to characterize the CMS-G sample in this experiment. The macroporous structure of the sample CMS-G was analyzed by mercury porosimetry. Macropore size distribution of CMS-G is given in Figure 6. The CO2 adsorption isotherm at 273 K and micropore size distribution using the DFT method19 are shown in Figure 7. Equilibrium adsorption data of CO2 are used to characterize the CMS by the DR equation20−22

Figure 4. Adsorption isotherms of CH4 and N2 on the CMS prepared at different flow of benzene, deposition temperature of 1023 K, and deposition time of 60 min: (a) N2 and (b) CH4.

amount and separation coefficient increase with increasing benzene flow rate from 1 to 4 mL·min−1 and then decrease when the flow rate is above 4 mL·min−1. This can be explained that the deposition process may be due to diffusion control at low benzene flow rate, and as a result, more mesopores and macropores are converted to micropores. At a higher flow rate, the adsorption performance become worse, and some of the micropores may be covered and blocked by deposition coke. According to the experiment, the benzene flow rate should be controlled at 4 mL·min−1. To sum up, the CMS sample which is prepared at 1023 K, deposition time of 60 min, and benzene flow rate of 4 mL·min−1 has the best performance in equilibrium adsorption. To narrate conveniently, the CMS sample is defined as CMS-G. 3.2. Texture Characterization of CMS. 3.2.1. Adsorption Isotherms of N2. One of the effective methods for estimating the type of pores present in a porous material is by analyzing its

W = W0exp[− (A/βE0)2 ]

(3)

where W is the amount adsorbed at relative pressure P/P0, W0 is the micropore volume, A = RTln (P0/P) is the adsorption potential, β is the affinity coefficient taken as 0.39 for CO2 at 273 K, and E0 is the characteristic adsorption energy. Eq 3 can also be written in the style of eq 4 as follows: ⎛ RT ⎞2 ⎛ P0 ⎞2 log W = log W0 − 2.3026⎜ ⎟ ⎜log ⎟ ⎝ βE 0 ⎠ ⎝ P ⎠ E

(4)



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CMS-1mL/min

CMS-4mL/min

CMS-7mL/min

CMS-10mL/min

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.262 0.3174 0.3668 0.402 0.42 0.4302 0.4319 0.4326 0.4334 0.4341

0 0.3038 0.3766 0.4413 0.4746 0.4931 0.5132 0.524 0.5267 0.5281 0.5296

0 0.1117 0.1427 0.1706 0.2053 0.2378 0.2615 0.2809 0.2977 0.3114 0.3185

0 0 0 0 0 0 0 0 0 0 0


CMS-1mL/min

CMS-4mL/min

CMS-7mL/min

CMS-10mL/min

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.7283 0.9521 1.0358 1.1028 1.1354 1.1563 1.1617 1.163 1.1643 1.1668

0 0.9545 1.1075 1.2102 1.257 1.2978 1.3208 1.3263 1.3273 1.3286 1.33

0 0.1126 0.1294 0.1404 0.1574 0.1744 0.188 0.2089 0.2102 0.216 0.2174

0 0 0 0 0 0 0 0 0 0 0

Table 12. Analysis of Adsorption Isotherms by Means of the Langmuir Equation N2

CH4

sample

Vs1 (mmol·g−1)

b1

Vs2 (mmol·g−1)

b2


CMS-1mL/min CMS-4mL/min CMS-7mL/min CMS-10mL/min

0.48 0.58 0.35 0.00

5.82 10.46 4.21 0.00

1.29 1.41 0.22 0.00

6.68 20.59 9.09 0.00

3.08 4.74 1.34 0.00

Table 13. Textural Properties of the Samples Measured by N2 Adsorption at 77 K sample

SBET (m2·g−1)

Vmic (cm3·g−1)

Da (nm)

CMS-923K CMS-973K CMS-1023K CMS-1073K CMS-1123K

232.80 295.30 389.00 346.40 157.80

0.1095 0.1383 0.1821 0.1613 0.0755

0.2122 0.2036 0.1922 0.1948 0.2481

these results, the textural parameters (Table 14) of CMS-G can be estimated. 3.3. Kinetic Study of CH4 and N2 Adsorption. In this work, individual adsorption kinetics of nitrogen and methane on CMS-G were carried out at 303 K, as explained in the Experimental Methods. The kinetics of adsorption of nitrogen and methane are presented in Figure 9. The kinetics of adsorption can be described by the eq 6:

Figure 5. Adsorption isotherms of N2 on the CMS prepared at different deposition temperatures at 77 K.

E0 can be determined from the intercept of the logW vs log2(P0/P). E0 can be further related to the mean micropore pore width L. K L= E0 (5)

mt 6 =1− π m∞

∞

∑ n=1

⎛ −Dn2π 2 ⎞ 1 exp⎜ t⎟ 2 ⎝ r2 ⎠ n

(6)

where mt and m∞ are the weight of gas absorbed at time t and at equilibrium, respectively; D is the diffusivity, r is the radical coordinate for the particles, and t is the adsorption time.23−26

with K = 12 KJ·nm·mol−1. DR linear regression using carbon dioxide equilibrium adsorption data is shown in Figure 8. Using F



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Figure 6. Macropore size distribution of the CMS-G obtained by mercury intrusion.

Figure 9. Kinetics adsorption of N2 and CH4 for the CMS-G.

evaluated by fitting equation to the curves in Figure 9. The results are as follows: Dr2−(N2) = 1.25 × 10−2s−1, Dr2− (CH4) = 3.55 × 10−4s−1. It can be observed that the diffusion time constants for nitrogen are about two orders in magnitude higher than those for methane. The selectivity can be calculated as the ratio D(N2)/ D(CH4), which is 35.26. The results indicate that the material CMS-G is suitable for separating the mixture of nitrogen and methane. 3.4. PSA Applied Experiment. In this study, purity, recovery, and productivity were used as measures of PSA performance for evaluation of the CMS-G adsorbent. Purity of CH4 is defined as the average volume fraction of methane leaving the bed during the high-pressure adsorption step. The recovery of CH4 is defined as the volumes of CH4 withdrawn as product during the high-pressure adsorption step divided by the volumes of CH4 fed to the PSA process. Productivity is defined as the volumes of CH4 produced per unit mass of adsorbent per unit time. The results for PSA process of adsorbent (CMS-G) in separation from CH4/N2 feed mixtures with different purities are shown in Table 15. The results were computed after 10 cycles

Figure 7. CO2 adsorption isotherm at 273 K on the CMS-G and obtained micropore size distribution.

Table 15. Results for PSA Process of Adsorbent (CMS-G)

Table 14. Analysis of CO2 Adsorption Isotherm at 273 K Using the DR Equation

a

W0 cm3·g−1

E0 KJ·mol−1

L nm

Smia m2·g−1

CMS-G

0.165

25.325

0.47

702.128

Smi = micropore surface area.

mt 6 ⎛ −Dπ 2 ⎞ ≈ exp⎜ 2 t⎟ π ⎝ r m∞ ⎠

recovery/%

productivity/Nm3·t−1·h−1

27.00 50.50 75.00

57.20 79.50 90.8

90.90 89.80 85.40

84.96 67.65 59.53

4. CONCLUSIONS In this study, carbon molecular sieves were prepared by deposition and carbonization of benzene on activated carbon. The adsorption capacities and equilibrium separation coefficient of CH4/N2 for CMS-G were at maximum values at a deposition temperature of 1023 K, time of 60 min, and benzene flow rate of 4 mL/min (parametric investigation of this study is based on the parameter ranges in the text; the interrelated nature of different

At long adsorption time, the higher terms of the summation become very small, and eq 6 can simplify to 1−

product purity/%

when the methane concentration stabilized each time. It can be obviously observed that CMS-G has a better performance in separation feed mixtures with low concentration of CH4, a higher methane product purity of 57.20% with 90.90% recovery can be achieved, and the adsorbent productivity is 84.96 N m3·t−1·h−1. However, for the 75/22 (v/v) CH4/N2 feed mixture, the performance of PSA process has an obvious decline, which may be related to the dynamics separation mechanism of CMS. In conclusion, the absorbent (CMS-G) has a good performance in the PSA process, especially for the feed mixtures with medium and low concentration of CH4.

Figure 8. DR plot of adsorption of CO2 in the CMS-G.

sample

feed purity/%

(7)

Figure 9 showed that diffusion of nitrogen is clearly much faster than that of methane. The diffusion time constants were G



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fabrication process variables will be researched later). When the deposition temperature, time, and flow rate of benzene were changed, the adsorption properties of CH4/N2 changed significantly. The CMS-G adsorbent was comprehensively characterized by means of mercury intrusion, CO2 permeation, kinetics adsorption, equilibrium adsorption, and PSA applied experiment. The CMS-G was shown to have a bimodal pore size distribution, including significant macro- and micropores. The DR equation calculated that the mean micropore width was at 0.47 nm. The adsorption kinetics and PSA applied experiment showed that the CMS-G produced has a good selectivity for separating CH4 from N2 and could be used for the PSA application.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-13426193487; Fax:86-010-84262097; E-mail address:[email protected]. ORCID

Jinhua Zhang: 0000-0002-7851-7697 Funding

J.-H.Z. is grateful for the financial support for this work from National Science and Technology Major Project of China (2016ZX05045-005) and the research coworkers in this project. Notes

The authors declare no competing financial interest.

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REFERENCES

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N2 Separation

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