CH4 Gas Mixture on Carbon

Article pubs.acs.org/jced

Adsorption Separation of CO2/CH4 Gas Mixture on Carbon Molecular Sieves Modified by Potassium Carbonate Dingding Liu, Honghong Yi,* Xiaolong Tang, Shunzheng Zhao, Zhixiang Wang, Fengyu Gao, Qian Li, and Bin Zhao Civil and Environmental Engineering School, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: In order to improve the separation performance of CO2/CH4 on carbon molecular sieves (CMS), CMS was modified by metal in this study. The surface of CMS had −OH, which could provide a great support for the chemical modification. In this paper, different metal ions, precursors of potassium, loading concentration of potassium carbonate (K2CO3), and calcination temperature were investigated for CO2/CH4 separation. Results showed that CMS loaded with K2CO3 and calcined at 773−873 K was the best for CO2/ CH4 separation. Compared with the untreated CMS, the adsorption capacity of CO2 for the best adsorbent increased from 1.15 to 1.72 mmol·g−1 and the value of separation factor increased from 1.9 to 2.75 mmol·g−1. The K2CO3 treatment was effective in generating metal oxide K2O and KO2, which could facilitate the adsorption of CO2 after K2CO3/CMS being calcined. However, the reaction between CH4 and metal was weak. In summary, K2CO3/CMS was proved to be one of the candidates for CO2/CH4 separation.

1. INTRODUCTION As a major component of biogas, methane (CH4) has cleanerburning properties than other traditional fuels like petroleum and coal due to its higher hydrogen-to-carbon ratio.1,2 Thus, the replacement of fossil fuels by clean and renewable source has been a matter of core concern.3 Unfortunately, the coexistence of carbon dioxide (CO2) in biogas will not only reduce its calorific values but also seriously lead to pipeline and equipment corrosion.4−6 Besides, the excess emission of CO2 in the atmosphere will cause climate change and global warming.7 Therefore, the removal of CO2 from CO2/CH4 mixture is imperative. Technologies including membrane separation, cryogenic distillation, adsorption separation, chemical conversion, and chemical absorption have been applied to separate CO2 from CO2/CH4 mixture.8 Currently, adsorption separation by microporous adsorbents is the most widely applied technology owing to simple operation, cheap regeneration cost, and weak environmental impact,9 and an excellent adsorbent is the core of this technology. So far, metal organic frameworks (MOFs),10−12 zeolites,3,13 carbon molecular sieves (CMS),14,15 and activated carbon (AC)16 have been studied. Besides, many kinds of metal compounds supported on carrier adsorbents have been researched for improving the separation performance of CO2/CH4 due to the differences of electronic properties of gases (CO2 has a strong quadropole moment and CH4 is nonpolar). CMS is a kind of carbonaceous material with narrow pores, which possesses a selective adsorption capacity of CO2 in biogas. Nabasis17 found the pores of CMS are slit-shaped, which may increase the adsorption capacities of linear © XXXX American Chemical Society

molecules such as CO2. As an adsorbent, CMS has its superiority: strong resistance to alkali and basic medium, strong hydrophobicity, low cost, and high hydrothermal stability. Thus, researching the modification of CMS to improve the separation of CO2/CH4 is worthwhile. Guo18 studied CO2 (in the confined spaces) adsorption and reaction kinetic performance of K2CO3/AC in low temperature). The CO2 adsorption capacity of the desired K2CO3/AC was 1.015 mmol·g−1. Upender19 prepared Na and K titanate nanotubes (Na−Ti−NT, K−Ti−NT) for CO2 adsorption. The modified materials were proved to be superior to their raw ones. Their results were in line with the data reported by Walton, wherein it was demonstrated that Na-NT owned more adsorption capacity than K−NT because of the stronger ionquadruple interaction with CO2.20 However, few studies use it as the precursor for CMS. In this paper, metal modification of CMS was researched. We studied the metal species, different metal precursors, loading concentration of metal, and different calcinations temperatures by means of characterizations.

2. MATERIALS AND METHODS 2.1. Adsorbent Preparation. CMS was provided by Zhejiang Changxing Haihua Chemical Co., Ltd. The adsorbent preparation was divided into three steps: (i) the CMS was crushed and sieved to 40−60 mesh size, washed with deionized water to remove fines and dirt, and then dried at 373 K for 12 h Received: September 1, 2015 Accepted: June 14, 2016

A

DOI: 10.1021/acs.jced.5b00742 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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in a drying oven; (ii) the cleaned materials were modified by metal compounds using ultrasonic-assisted impregnation method. The impregnation progress continued for 60 min at 303 K. Also, the wet samples were dried in a drying oven at 373 K for 24 h; (iii) the modified samples were calcined for 3 h at a specific temperature in a tube furnace in nitrogen atmosphere. 2.2. Activity Test. Activity test of CO2/CH4 separation was operated at constant pressure in a fixed-bed quartz tube reactor of 10 mm inner diameter at 298 K. 1g of sample was loaded into the reactor. The feed gas was 20 mL·min−1, containing 50% N2, 20% CO2, and 30% CH4. Also, the space velocity was 764.33 h−1. Concentrations of CO2 and CH4 at the inlet and outlet of the reactor were measured by gas chromatography (GC4000A). The adsorption tendency of CO2 and CH4 were estimated by the breakthrough curves. Adsorption capacities of binary gases were calculated as the outlet concentration of CO2 achieved 90% of the feed. To facilitate the discussion, the adsorption capacities of CO2 or CH4 (AC) and the separation factor of CO2/CH4 (SF) were defined as follows:

AC(mmol/g) =

SF =

Qvkts − ∫ 0

ts

Figure 1. FT-IR spectra of CMS.

( y )dt V0 wt k , t

22.4m

(x /y)i (x /y)j

where Q is the flow of the feed gas (ml·min−1); vk is the concentration of a component in the inlet (%); ts is the time when the effluent concentration of CO2 achieves 90% of the feed (min); V0 is the flow of N2 (ml·min−1); wt is the concentration of N2 in the outlet (%); yk,t is the concentration of a component in the outlet at time t (%); m is the weight of the adsorbent (g); x is the mole fraction of a component in the adsorbed phase; y is the mole fraction of a component in the gaseous phase; i is CO2; j is CH4. 2.3. Characterization. Fourier transform infrared (FT-IR) spectra were obtained in 4000−400 cm−1 wavenumber range and using the KBr pressed pellet method with a Nicolet IS50 Fourier transform infrared spectrometer. X-ray diffraction (XRD) was analyzed by D/max-2200 X-ray diffractometer with Cu Ka radiation, 36 kV voltage, λ= 0.15406 nm, 30 mA electric current and 5° min−1 scanning speed from 2θ = 10− 90°. The data were analyzed with MDI Jade 5.0 software.

Figure 2. Breakthrough curves of CO2 and CH4 on CMS modified by different metal ions.

Table 1. AC and SF on CMS Modified by Different Metal Ionsa AC (mmol/g)

3. RESULTS AND DISCUSSION 3.1. Effect of Different Metal Ions. In order to detect whether CMS was suitable for chemical modification, FT-IR spectra, in the 4000−400 cm−1 wavenumber range at ambient temperature, were carried out on the raw CMS in Figure 1. The peak at 3430 cm−1 was ascribed to the presence of −OH, which could provide a great support for the chemical modification of CMS.21 In addition, many studies have reported that metal and metal oxides, including alkali metals, alkaline-earth metals, and transition metal, are good for decarbonization because of the acidity and strong quadrupole moment of CO2. Thus, part of the metal ions (K+, Ba2+, Ni2+, Ca2+) were chosen to modify CMS. These samples were referred to as K/CMS, Ba/CMS, Ni/CMS, and Ca/CMS, respectively. The breakthrough curves of CO2 and CH4 on the modified CMS were presented in Figure 2. AC and SF were shown in Table 1.

a

adsorbent

CH4

CO2

SF

CMS K/CMS Ba/CMS Ni/CMS Ca/CMS

0.95 1 1 0.99 1.05

1.15 1.62 1.4 1.36 1.18

1.9 2.52 2.3 2.25 1.94

Standard uncertainties u are u(AC) = 0.01 and u(SF) = 0.01.

As shown in Figure 2, the differences in CH4 adsorption resulting from different metal ions were not very apparent. But the differences were huge in CO2 adsorption. The order of CO2 breakthrough time were: K/CMS > Ba/CMS > Ni/CMS > Ca/ CMS > CMS. In Table 1, because of AC−K/CMS > AC−Ba/ CMS > AC−Ni/CMS > AC−Ca/CMS > AC−CMS(CO2 adsorption) and SF−K/CMS > SF−Ba/CMS > SF−Ni/CMS > SF−Ca/CMS > SF−CMS, K/CMS was the best. In short, potassium supported on the CMS was good for CO2/CH4 separation. CO2 was acidic gas and had stronger permanent quadrupole moment than CH4. So the introduction of metal B



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from the analysis of Figure 3 and Table 2 showed that the adsorption capacity of CO2 did not increase with the alkaline of the modifying agent, although the basicity of KOH was the strongest among three precursors. Also, literature showed metal oxide K2O generated from K2CO3 after being calcined could improve the adsorption capacity of CO2.23,24 However, KCl and KOH did not decompose at current conditions. Li got the result that the adsorption type of CH4 on the adsorbent modified by K2CO3 was physical adsorption.25 So, adsorption capacity of CH4 had nothing to do with loading species. In order to explain better the influence of K2CO3, the fresh K2CO3/CMS and saturated K2CO3/CMS were examined by XRD. The phase and crystalline orientation of two samples were exhibited in Figure 4. Figure 4 showed that the fresh sorbent of K2CO3/CMS shows a lot of K2O, KO2 and a small amount of K2CO3. This may be explained that K2O and KO2 were produced after the sample was calcined. After reaction in the condition of CO2, the XRD pattern of saturated K2CO3/CMS shows phases including K2O, K2CO3, KHCO3, and K2CO3·1.5H2O. H2O was probably adsorbed from the air by K2CO3/CMS. This indicated that K2O and KO2 can be converted to K2CO3, KHCO3, and K2CO3·1.5H2O in the presence of CO2, then the separation performance of CO2/CH4 was improved. 3.3. Effect of Different Loading Concentration of K2CO3. In order to determine the optimal loading concentration of K2CO3, the dynamic experiments of CMS modified with different concentrations (13.8, 27.6, 41.4, 55.2, and 69 g· L−1) were operated in this part. Samples were named 0.1/K/ CMS, 0.2/K/CMS, 0.3/K/CMS, 0.4/K/CMS, 0.5/K/CMS, respectively. All of the samples were calcined at 773 K. The breakthrough curves of CO2 and CH4 on the modified CMS were presented in Figure 5. AC and SF were shown in Table 3. As shown in Figure 5, it may be observed that the differences in CH4 adsorption were not apparent and the CO2 adsorption was better with the increased concentration of K2CO3. The value achieved optimum at 41.4 g·L−1. However, there was a gradual decrease in the CO2 adsorption as loading concentration continued to increase. In Table 3, because of AC−0.3/ K/CMS > AC−0.2/K/CMS > AC−0.1/K/CMS > AC−0.4/K/ CMS > AC−0.5/K/CMS (CO2 adsorption) and SF−0.3/K/ CMS > SF−0.2/K/CMS > SF−0.1/K/CMS > SF−0.4/K/ CMS > SF−0.5/K/CMS, 41.4 g·L−1 K2CO3 was the best loading concentration for CO2/CH4 separation. K 2 CO 3 supported on CMS was good for CO 2 /CH 4 separation, but more K2CO3 supported was not necessarily better. The reasons for the above phenomenon mightbe when the load concentration increased, the number of adsorption activity sites on the surface of CMS were increased. As a result, the adsorption capacity of CMS for CO2 was increased. However, excessive amounts of potassium leads to microporous jams. Therefore, the adsorption capacity of CMS for CO2 was decreased. 3.4. Effect of Calcination Temperature. Different calcination temperatures have great effects on the chemical and physical structure of the carrier and loading component, which have a great impact on the CO2/CH4 separation. 0.3/K/ CMS were calcined at different temperatures. Samples were recorded as 673/K/CMS, 773/K/CMS, 873/K/CMS, and 973/K/CMS, respectively. The results were showed in Figure 6 and Table 4. The results in Figure 6 and Table 4 showed that too low (673 K) or too high (973 K) calcination temperature was bad

ions could improve the selective adsorption of CO2 to some extent. Also, the reactions between CH4 and metal ions were weak due to the nonpolarity of CH4. Thus, the values of SF increased. In addition, the different metal ions had different effects for CO2/CH4 separation. This might be that stronger the metallicity was, the more active sites were generated. The metallicity of K was strongest among these metals. Besides, Ba was divalent with high charge density and polarizability, which led that electron clouds of CO2 were easily deformed, then adsorbed.22 So, the adsorption ability of Ba is stronger than Ni and Ca. Thus, K+ was chosen to as the active component. 3.2. Effect of Different Precursors of Potassium. K was chosen to as the active component for CO2/CH4 separation according to the experiment above. However, different precursors of potassium had different effects on the adsorption separation. We compared the separation performance of three precursors of potassium (K2CO3, KOH, and KCl), which were recorded as K2CO3/CMS, KOH/CMS, KCl/CMS. The potassium concentration was 27.6 g·L−1 and all samples were calcined at 773 K. The breakthrough curves of CO2 and CH4 on the modified CMS were presented in Figure 3. AC and SF were shown in Table 2.

Figure 3. Breakthrough curves of CO2 and CH4 on CMS modified by different precursors of potassium.

Table 2. AC and SF on CMS Modified by Different Precursors of Potassiuma AC (mmol/g)

a

adsorbent

CH4

CO2

SF

K2CO3/CMS KOH/CMS KCl/CMS

1 0.94 0.96

1.62 1.31 1.28

2.52 2.25 2.1


As shown in Figure 3, three samples had the effect of CO2/ CH4 separation. The trends of CH4 adsorption resulting from different precursors of potassium were almost the same. But the differences were very apparent in CO2 adsorption. CO2 adsorption by K2CO3/CMS was much better than KOH/ CMS and KCl/CMS. In Table 2, because of AC−K2CO3/CMS > AC−KOH/CMS > AC−KCl/CMS (CO2 adsorption) and SF−K 2 CO 3 /CMS > SF−KOH/CMS > SF−KCl/CMS, K2CO3/CMS was the best. In summary, K2CO3 was the most suitable precursor for CO2/CH4 separation. The results C



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Figure 4. XRD patterns of fresh K2CO3/CMS (a) and saturated K2CO3/CMS (b): □ K2O,

△

KO2, ○ K2CO3, ☆ KHCO3, ◇ K2CO3·1.5H2O.

Figure 5. Breakthrough curves of CO2 and CH4 on CMS modified by different loading concentrations of K2CO3.

Figure 6. Breakthrough curves of CO2 and CH4 on K2CO3/CMS at different calcination temperatures.

Table 3. AC and SF on CMS Modified by Different Loading Concentration of K2CO3a

Table 4. AC and SF on K2CO3/CMS at Different Calcination Temperaturesa

AC (mmol/g)

a

AC (mmol/g)

adsorbent

CH4

CO2

SF

0.1/K/CMS 0.2/K/CMS 0.3/K/CMS 0.4/K/CMS 0.5/K/CMS

0.99 1 1 0.95 0.93

1.3 1.62 1.73 1.42 1.26

2.21 2.52 2.75 2.31 1.92

a


adsorbent

CH4

CO2

SF

400/K/CMS 500/K/CMS 600/K/CMS 700/K/CMS

0.98 1 0.99 0.87

1.37 1.7 1.72 1.25

2.27 2.7 2.74 2.17


maximum when temperature was 873 K. However, the CO2 adsorption began to decline after 873 K. It may be that high temperature caused the collapse of pore structure of modified CMS so that CO2 adsorption changed weaker.

for CO2/CH4 separation. The range from 773 to 873 K was appropriate. The CO2/CH4 separation performance increased at first and then decreased as calcinations temperature was further increasing. Some authors mentioned that its decomposition temperature was from 773 to 973 K after K2CO3 was loaded on the carbonaceous materials.26,27 Roasting product was metal oxide K2O, which was in favor of the adsorption of CO2.24 So, the adsorption capacity of CO2 was weak at the low calcination temperature (673 K) almost due to the less K2O generated on the surface of CMS. With calcination temperature increasing, there were more K2O which could facilitate the CO2 adsorption generated. The adsorption capacity of CO2 reached

4. CONCLUSION The surface of CMS had −OH proved by FT-IR and −OH could provide a great support for the chemical modification. Thus, CMS was modified by metal to improve the separation performance of CO2/CH4. In this study, CO2/CH4 separation performances of modified CMS were investigated under different conditions by varying the metal ions, precursors of D



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potassium, loading concentration of K2CO3, and calcination temperature. Results showed that CMS loaded with 41.4 g·L−1 K2CO3, and calcined at 773−873 K was the best for CO2/CH4 separation. Compared with the untreated CMS, the adsorption capacity of CO2 for the best adsorbent increased from 1.15 to 1.72 mmol·g−1 and the value of SF increased from 1.9 to 2.75 mmol·g−1. The K2CO3 treatment was effective in generating metal oxide K2O which could facilitate the adsorption of CO2 after K2CO3/CMS being calcined. However, the reaction between CH4 and metal was weak. In the summary, this study indicated that K2CO3/CMS was one of candidates for CO2/CH4 separation. The regeneration performance is an important indicator for commercial materials. Therefore, in our further study, effort will be devoted to the subject of regeneration performance and recycling use of the adsorption and separation material.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00742. Tables detailing the export concentration changes. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported by the Fundamental Research Funds for the Central Universities (06101046). Notes

The authors declare no competing financial interest.

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REFERENCES

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CH4 Gas Mixture on Carbon

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