Kinetic study of the thermal decomposition of octamethylcyclotetrasiloxane on activated gamma alumina

Abstract

Biogas produced at wastewater treatment plants and landfills is contaminated with siloxanes that impedes its use as fuel. Before being safe to use in solid oxide fuel cells or engines, siloxane contamination must be reduced to below 0.01 and 9.1ppm (vol/vol) Si equivalent respectively. The literature has identified thermal decomposition in a packed bed of heated gamma alumina as a method that can remove siloxanes to the requisite levels Finocchio (2008) & Urban (2009). Missing from the literature is a kinetic study of the decomposition reaction where a theoretical model is fitted to experimental breakthrough curves so that breakthrough times can be predicted. The present work is this kinetic study.Experiments with synthetic biogas (0 to 100% methane and carbon dioxide mixtures) contaminated with octamethylcyclotetrasiloxane (D4) at concentrations between 32.3 and 72.7ppm (vol/vol) Si equivalent were conducted. Beds of alumina were exposed to the gas at temperatures between 307 and 384°C for contact times between 5.0 and 8.0ms. The effluent D4 concentration was measured by GC-FID.The breakthrough curves obtained were fitted with a kinetic model that assumed a first order surface reaction, shrinking core particle kinetics, and plug flow in the reactor. For temperatures between 333 and 384°C, the model fit the data well, most especially near the breakthrough point. Experimental conditions had no effect on quality of fit. At 307°C, the model was inadequate; possibly due to D4 physisorption or shrinking core kinetics assumptions not holding true anymore.