The effect of coal grain size on the sorption of hydrocarbons from gas mixtures

The flow of gas mixtures through coal, accompanied by sorption, is one of the natural processes occurring in the spontaneous heating of coal in underground mines. So far, studies on the sorption of mine gases have been mostly performed with regard to single components. In turn, sorption measurements with the use of gaseous mixtures were performed only on coals with grain size at the narrow range of 0.5 to 0.7 mm. In this paper, a dynamic sorption of a mixture of propylene, ethylene, propane, and ethane in a fixed-bed column was investigated. Coal samples of various grain classes, ie, 0.25 to 0.50, 0.7 to 1.00, and 1.00 to 2.00 mm, were used as an adsorbent. The sorption tests were conducted at a constant gas flow rate of 2.17 center dot 10(-7) m(3)/s and the pressure of approximately 1 atm. Next, the results of the sorption tests were compared with parameters characterizing the porous structure of the materials used. The total amount of sorbed gases decreases as the grain size becomes larger. The samples characterized by a lower carbon content (w/w), a slightly higher oxygen content, and a larger surface area dominated by micropores at the range of 0.6 to 2.0 nm and mesopores with diameter of 2.0 to 10.0 nm had a higher sorption capacity than samples with the structure determined mainly by mesopores. It has been noticed that a high sorption ability of ethane results from its highest concentration in the mixture at the inlet of the sorption column. In most cases, propylene was sorbed in larger amount than ethylene, independently of grain size of coal and pore size distribution.