Carbon Nanoporous Adsorbents Prepared from Walnut Shell for Liquefied Natural Gas Vapor Recovery in Cryogenic Storage Systems

A one-step notion of synthesis was developed to prepare microporous activated carbons from walnut shell by physical gas activation in a CO2 atmosphere for a low-temperature methane accumulation system operating at 120, 160, and 178 K. The raw material was carbonized within a temperature range from 240 to 950 degrees C. Temperatures close to 900 degrees C were found to be optimal for the development of microporosity in the adsorbent in a CO2 atmosphere. Activation under these conditions made it possible to achieve a burnoff degree up to 70% and form an optimal porous structure for adsorption accumulation of liquefied natural gas (LNG) vapors. The adsorbent thus obtained exhibits a high micropore volume W-0 = 0.59 cm(3)/g, mesopore volume W-\ME = 0.33 cm(3)/g, specific surface S-BET = 1490 m(2)/g, and half-width of micropores of 0.59 nm, which provided a high methane adsorption capacity. The presence of mesopores can make additional contribution to the adsorption process due to capillary condensation. The theoretical assessment of the methane adsorption capacity of the adsorbent showed that at temperatures of 120, 160, and 178 K and pressures up to 6 bars, the values of equilibrium adsorption were 15, 13.5, and 12 mmol/g, respectively.