Modification of coconut shells by polyether ether ketone for high-presure adsorption of methane and natural gas

Abstract

Application of natural gas (NG) as a transportation fuel introduces the possibility of reducing the dependency of liquid based petroleum fuel and emissions of greenhouse gases. At present, compression and liquefaction are the most used technology used for transportation system. For transportation use, compression requires high pressure (200–300 bar) while liquefaction is impractical. A relatively low pressure of 30-40 bar is achievable by adsorbed natural gas to store nearly compressed natural gas. In this study, adsorbents for high-pressure adsorption of methane (CH4) and NG were prepared from coconut shells and polyether ether ketone (PEEK) using potassium hydroxide (KOH) and microwave-assisted activation. The role of KOH was to modify the properties and structure of the adsorbent to suffice better adsorption applications. Design expert software version 7.1.6 was used for optimization and prediction of preparation conditions of the adsorbents for CH4 and NG adsorption. Effects of microwave power, activation time and quantity of PEEK on the adsorbents performance toward CH4 and NG adsorption were investigated. The adsorbents were characterized by pH, Fourier transform infrared spectroscopy, thermogravimetric and derivative thermogravimetric, mechanical property, nitrogen adsorption, bulk density, scanning electron microscopy, and ultimate and proximate analyses. The ideal CH4 and NG adsorption capacities of adsorbents were determined using volumetric method at pressures of 5, 7.5, 11, 17, 25, 30 and 35 bar at ambient temperature, while at 5 oC, the adsorption was carried out at 5, 17 and 35 bar. Isotherm and kinetics models were used to validate the experimental results. The optimum preparation conditions were found to be 15 wt % of PEEK, 3 minutes activation time and 300 W microwave power. The highest CH4 uptake of 9.7045 mmol CH4 adsorbed/g adsorbent was recorded by adsorbent (M33P15) (300 W of microwave power, 3 minutes activation time and 15 wt % amount of PEEK) among the sorbents at ambient temperature and 35 bar. Similarly, the highest NG uptake of 9.9432 mmol NG adsorbed/g adsorbent was also achieved by the same sample under the same adsorption conditions. The CH4 and NG equilibrium data were well correlated with Sips, Toth, Freundlich and Langmuir. Isotherms revealed that the Sips isotherm has the best fit, while the kinetics studies revealed that the pseudo-second-order kinetic model best describes the adsorption process. In all scenarios studied, a decrease in temperature led to an increase in adsorption of both gases. The M33P15 maintained its stability even after seven adsorption/desorption cycles. The findings revealed the potential of coconut shell-PEEK as CH4 and NG adsorbent.