Suitability of Thermal Swing Adsorption for the Treatment of Siloxanes and Toluene in Biogas: An Experimental Study

Abstract

The use of biogas has been proven to be an interesting alternative energy source. However, biogas usually contains trace compounds, which can lead to damage in the valorization installation. Consequently, their treatment is an important issue for the suitability of biogas valorization. Although thermal swing adsorption (TSA) has already been implemented, there is still a lack of knowledge about the deactivation of the adsorbent. This work examined under dynamic conditions, the treatment of three typical trace compounds from landfills (L2, D4, and toluene) via the TSA process using activated carbon cloth (ACC). The process was implemented according to temperature programs: one with a moderate desorption temperature (180 °C) and the other with a high desorption temperature (220 °C). Both experiments involved several cycles of adsorption and desorption (60 cycles). In the single-compound adsorption experiment, the adsorption capacities were 24%, 19%, and greater than 15% weight basis for toluene, L2, and D4, respectively. From TSA experiments, it was found that the adsorption capacities of the ACC decreased rapidly over the first four cycles for the two temperature programs. This decrease was lower at high temperatures: no significant decrease was found between the 4th cycle and the 50th cycle. On the contrary, a constant decrease was measured at moderate temperatures. Both the surface analysis (infrared measure) and the analysis of the gas from the desorption step indicate that siloxanes were deposited on the ACC and polymerized to a high-molecular-weight polymer. This reaction could cause ACC deactivation by pore clogging. Mass balances and gas analysis also suggested that toluene could be thermally degraded during the desorption step. The high temperatures promoted desorption of high-molecular-weight compounds, such as D5, L5, and D6, formed during adsorption/desorption processes. Thus, a high-temperature program was found to be more effective than a moderate-temperature program.