Deactivation of HDS catalysts in deoxygenation of vegetable oils

Several rapeseed oils with different degree of upgrading were used as feedstocks for deoxygenation over sulfided CoMo/gamma-Al2O3 catalyst. The rapeseed oil samples differed in the concentration of inorganic impurities, water, free fatty acids and phospholipids. The catalytic experiments were carried out in a fixed bed reactor at constant reaction conditions (310 degrees C. WHSV = 2 h(-1), hydrogen pressure 3.5 MPa). Refined rapeseed oils were converted to hydrocarbons more efficiently than neat rapeseed oil, trap grease and waste oil. The high concentration of phospholipids in trap grease was the most likely cause of catalyst deactivation by coking. Decomposition of phospholipids as well as oligomerization of rapeseed oil by phosphoric acid at 310 degrees C was confirmed by separate laboratory experiments. The high concentration of alkalis in waste rapeseed oil was the main reason for increased deactivation in comparison with neat rapeseed oil. Sulfur removal from catalyst active sites affected adversely the deoxygenation of triglycerides. The catalyst deoxygenation activity was partially restored by pulse addition of dimethyldisulfide (DMDS) to the feedstock, however the degradation of active sites by loss of sulfur was not fully reversible. In contrast, continually added sulfur agent (DMDS) ensured significantly slower catalyst deactivation. Moreover, presence of H2S lowered considerably hydrodeoxygenation/decarboxylation ratio. (C) 2010 Elsevier B.V. All rights reserved.