CYCLIC ADSORPTION AND DESORPTION OF METHANE AND CARBON DIOXIDE ON COCONUT SHELL ACTIVATED CARBON Suwadee Uttaraphat a, Pramoch Rungsunvigit *,a,b, Boonyarach Kitiyanan a,b, Santi Kulprathipanja c a) The Petroleum and Petrochemical College, Chulalongkorn University b) Center of Excellence on Petrochemical and Materials Technology c) UOP, A Honeywell Company, Des Plaines, Illinois, U.S.A. Keywords: Adsorbed natural gas, Methane adsorption, Coconut shell activated carbon ABSTRACT Adsorbed Natural Gas (ANG) technology based on natural gas adsorption in porous materials at relatively 3.5-4 MPa and at room temperature is challenging. In this work, cyclic adsorption and desorption of carbon dioxide and methane by using coconut shell activated carbon (CSAC) was investigated. For every cycle, pure carbon dioxide was fed into the packed bed column for one hour in the adsorption step. After that, the desorption step started by using pure methane for one hour. The cycle was repeated for ten times. The same results were shown in all cycles. In the adsorption step, carbon dioxide was adsorbed on the CSAC, and the breakthrough time was around 20 minutes. For the desorption step, methane slowly adsorbed on the CSAC while carbon dioxide continually desorbed out. After one hour of desorption step, carbon dioxide remained on the CSAC around 1-2 percent of the initial concentration. It can be concluded that methane can desorb carbon dioxide on the CSAC in many repetitions of adsorption and desorption. *pramoch.r@chula.ac.th INTRODUCTION Natural gas is a popular fuel and has been promoted for use in automobile field because of its environmental advantages, and it is a cheaper fuel than diesel or gasoline. It can be used as a fuel for many vehicles such as cars, trucks, and jet engines (Wang and Huang, 1999). However, natural gas has low-energy density (heat of combustion/volume), which constitutes a limitation for some applications (Solar et al., 2010). To make adsorbed natural gas (ANG) systems competitive to gasoline or diesel fuel, the cost of its storage and refueling systems must be reduced drastically. ANG technology based on natural gas adsorption on porous materials at relatively low pressures 3.5-4 MPa and at room temperature is a challenge to the liquid fuel application (Vasiliev et al., 2000). In this work, the appropriate desorption time of adsorption and desorption of carbon dioxide and methane by using coconut shell activated carbon (CSAC) was repeated. First, pure carbon dioxide was fed into the packed bed column for one hour in the adsorption step. After that, the desorption step started by using pure methane for many hours. Moreover, the cyclic adsorption and desorption of carbon dioxide and methane was investigated. For every cycle, pure carbon dioxide was fed into the packed bed column for one hour in the adsorption step followed by the desorption step, using pure methane at the appropriate desorption time. The cycle was repeated for ten times. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1
EXPERIMENTAL A. Adsorbent Preparation CSAC was grinded and sieved to obtain a particle size of 20-40 mesh. It was dried at 120 C for 24 hours to remove moisture. B. Adsorbent Characterization The surface area, total pore volume, and pore size distribution of the adsorbents was measured by a Quantachrom/Autosorb1-MP instrument. The adsorbents were first out gassed to remove the humidity on its surface under vacuum at 300 C for 16 hours prior to the analysis. After that, nitrogen was purged to adsorb on its surface. The volumepressure data was used to calculate the BET surface area, total pore volume, and pore size distribution. The morphology of the adsorbents was investigated by using the FE- SEM, Hitachi S-4800, with an accelerating voltage of 2 kv and varying magnifications of 500, 2,000, and 25,000. The adsorbents were coated with platinum under vacuum condition before observation. C. Adsorption Measurement Thermo-volumetric apparatus was constructed to study the gas-solid interaction between methane/carbon dioxide and potential solid adsorbents. The schematic of the experimental set-up for the dynamic adsorption of methane and carbon dioxide is shown in Figure 1. The set-up consisted of a high pressure stainless steel reactor, which held the sample and part of stainless steel tube as a gas reservoir. A pressure transducer was used to measure pressure of the system in the range of 0-3,000 psig with a 0.13% global error. A pressure regulator with 4,000 psig maximum limit was installed to control a gas flow rate into the whole system. Figure 1 Schematic of the experimental set-up for the dynamic adsorption of CH 4 and CO 2. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2
RESULTS AND DISCUSSION A. Adsorbent Characterizations Table 1 shows the summary of the BET surface area, total pore volume, micropore volume, and average pore diameter of the adsorbents. The averages of BET surface area, total pore volume, micropore volume, and average pore diameter of CSAC are 1,015 m 2 /g, 0.560 cm 3 /g, 0.540 cm 3 /g, and 22.1 Å, respectively. The micropore volume of CSAC is almost similar with the total pore volume, which could be confirmed by the pore size distribution, shown in the range of micropore. Table 1 BET surface area, total pore volume, micropore volume, and average pore diameter of investigated adsorbents Adsorbent BET surface area (m 2 /g) Physical characterization Total pore volume (cm 3 /g) Micropore volume (cm 3 /g) Average pore diameter (Å) CSAC (1) 1,010 0.551 0.533 21.9 CSAC (2) 1,020 0.568 0.547 22.2 Average 1,015 0.560 0.540 22.1 The SEM micrographs show that the CSAC has a high degree of porosity and contains the largest possible number of randomly distributed pores of various shapes and size, that cause the CSAC to be a porous material with an extended and extremely high surface area. Moreover, the SEM micrographs also show that the CSAC exhibits dust or impurity in the pores, which may block the pores, and the adsorbates (methane and carbon dioxide) cannot reach into the pores, resulted in the decrease in the adsorption capacity. (a) (b) (c) Figure 2 SEM micrographs for (a) magnification of 500, (b) magnification of 2,000, and (c) magnification of 25,000. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3
B. Desorption time by using methane as a desorbent Investigation on one cycle adsorption of pure methane and carbon dioxide by using coconut shell activated carbon (CSAC) was carried out in a stainless steel packed bed column with an inside diameter of 10.2 mm at atmospheric pressure and room temperature. Pure carbon dioxide was fed into packed bed column at the flow rate of 50 ml/min for one hour in the adsorption step after that the desorption step started by using pure methane with the same flow rate. To determine the proper desorption time, pure methane was fed into pack bed column until virtually no carbon dioxide was detected. Figure 3 Pure carbon dioxide adsorption and desorption by using pure methane at room temperature for CO 2 adsorption ( ), CO 2 desorption ( ), CH 4 adsorption in the desorption step ( ). From the results in Figure 3, CO 2 adsorption line shows that carbon dioxide is adsorbed on the CSAC, and the breakthrough time is around 20 minutes. For the desorption step, carbon dioxide is desorbed out by the time when methane is fed into the packed bed column. CO 2 desorption line shows all carbon dioxide comes out around 60 minutes after the desorption starts. Hence, the proper desorption time of the cyclic adsorption and desorption of carbon dioxide on the CSAC by methane as a desorbent should be one hour, and the desorption time that more than one hour does not play an important role on the carbon dioxide desorption by using pure methane at room temperature. On the other hand, methane is adsorbed on the CSAC, and the breakthrough time is less than 2.5 minutes. It can be seen that the breakthrough time of carbon dioxide is longer than methane because the activated carbon reaches its saturation slower, so the adsorption capacity of carbon dioxide on the CSAS is higher than methane. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4
C. Effect of methane adsorption on the cyclic adsorption and desorption of carbon dioxide on CSAC The cyclic adsorption and desorption of methane and carbon dioxide on the CSAC was followed and applied. For every cycle, pure carbon dioxide was fed into the packed bed column with the flow rate of 50 ml/min for one hour in the adsorption step. After that, the desorption step started by using pure methane with the same flow rate for one hour. The cycle was repeated for ten times. Figure 4 Adsorption and desorption of carbon dioxide and methane for 10 cycles, (Blue) CH 4, (Green) CO 2 for adsorption step, (Pink) CH4, and (Red) CO2 for desorption step. Figure 4 shows the same results in all cycles. In the adsorption step, carbon dioxide is adsorbed on the CSAC, and the breakthrough time is around 20 minutes. For the desorption step, methane is slowly adsorbed on the CSAC, while carbon dioxide continually desorbs out. After one hour of desorption step, carbon dioxide remains on the CSAC around 1-2 percent of the initial concentration. It can be concluded that methane can adsorb on the CSAC that fully saturated with carbon dioxide in many repetition of adsorption and desorption. CONCLUSIONS This research investigated on the cyclic adsorption and desorption of methane and carbon dioxide on coconut shell activated carbon (CSAC) by using methane as a Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5
desorbent. Carbon dioxide is desorbed out by the time when methane is fed into the packed bed column. All carbon dioxide comes out around 60 minutes after the desorption starts. For ten repetitions, carbon dioxide is replaced by methane and remains on the CSAC only 1-2 percent, confirming methane can be used as a desorbent for the adsorption and desorption of methane and carbon dioxide on CSAC. ACKNOWLEDGEMENTS The authors would like to sincerely thank the National Research Council of Thailand; The 90 th Anniversary of Chulalongkorn University Fund and Grant for International Integration: Chula Research Scholar, Ratchadapiseksomphot Endowment Fund, Chulalongkorn University, Thailand; Center of Excellence on Petrochemicals and Materials Technology; The Petroleum and Petrochemical College, Chulalongkorn University, Thailand; UOP, A Honeywell Company, USA for providing support for this research work. REFERENCES Solar, C., Blanco, A.G., Vallone, A., and Sapag, K. (2010). Natural Gas, 205-244. Vasiliev, L.L., Kanonchik, L.E., Mishkinis, D.A., and Rabetsky, M.I. (2000). International Journal of Thermal Sciences, 39(9 11), 1047-1055. Wang, M.Q. and Huang, H.S. (1999). Center for Transportation Research, U.S. Department of Energy. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6