Hydrocarbon Adsorption in Nanoporous Materials

DOS SANTOS G.; CARLOS WEXLER; ZACHARY VALLEROY

San Luis

Congreso; Congreso Regional de Física Estadística y Aplicaciones a la Materia Condensada, TREFEMAC; 2019

Understanding the interactions between adsorbed natural gas (NG) and ?nano-sponges? ? materials with pores in the nanometer scale ? have become extremely important in the engineering of energy storage materials. Methane, the primary component of NG, produces significantly lower CO2 emissions per unit energy than any other hydrocarbon, making it an attractive alternative fuel. Because of the low density of NG, typical storage tanks require a series of high-pressure compressions that significantly increase the costs of operation and the bulk of storage tanks. Physisorption in a porous substrate could substitute as a more feasible means of storage at significantly lower pressures. Many porous materials have been previously studied for pure methane adsorption; however, it has been largely ignored that NG contains a significant portion of heavier hydrocarbons. This mixture is problematic in the sense that heavier molecules such as ethane and propane have stronger van der Waals interactions with most adsorbents. As a result, nanoporous substrates may accumulate heavy hydrocarbons and not be suitable for sustained use as a storage medium. In our work, we are modeling various ethane-methane and propane-methane mixtures inside graphene nanopores using molecular dynamics simulations at various temperatures and pressures ranging from 0 to 500 atmospheres. We have observed the predicted higher adsorption affinity of the heavier hydrocarbons. However, in all cases, we also note that the heavier molecules remain quite mobile within the pores. We are able to observe the thermal desorption of these molecules at higher temperatures (400 K) and derive an Arrhenius relationship to predict the partial pressures at lower temperatures. We conclude that the presence of propane or ethane is unlikely to permanently ruin the adsorption characteristics of carbonaceous adsorbents.