Hydro-processing of cottonseed oil for renewable fuel production : effect of catalyst type and reactor operating parameters

Abstract

The production of liquid bio-hydrocabons from cottonseed oil for the biofuel industry was the main focus of this study. Cottonseed oil is a by-product from the cottonwool industry. The liquid bio-hydrocarbons were produced in a batch reactor by means of hydrotreatment using three different hydroteating catalysts. The effect of reaction parameters on the conversion, liquid product yield, reaction pathways and fuel product distribution was evaluated. The reaction temperature was varied from 390°C to 410°C with a 10°C increment at a fixed initial hydrogen pressure. The initial hydrogen pressure was varied from 9 to 11 MPa with a 1 MPa increment at a constant reaction temperature. Three different catalysts, Ni/SiO2-Al2O3, NiMo-Al2O3 and CoMo-Al2O3, were utilised for every set of reaction conditions. The reaction time and catalyst-to-oil ratio were kept constant at 120 minutes and 0.088, respectively, throughout the investigation. All three catalysts were activated either by pre-sulphiding or reduction prior to their use. The highest conversion was obtained at similar reaction conditions for the NiMo-Al2O3 and CoMo-Al2O3 catalysts, but not for the Ni/SiO2-Al2O3 catalyst. For the CoMo-Al2O3 and NiMo-Al2O3 catalysts, conversions of 98.5% and 99.7% respectively were obtained at 410°C reaction temperature and intial hydrogen pressure of 11 MPa, while 99.71% was obtained for Ni/SiO2-Al2O3 at 400°C and initial hydrogen pressure of 9 MPa. The hydrotreating conversion order at a temperature of 410°C, catalyst-to-oil ratio of 0.08 and initial hydrogen pressure of 9 MPa was found to be sulphided NiMo-Al2O3 (99.86%)> sulphided CoMo-Al2O3 (98.9%)> reduced Ni/SiO2-Al2O3 (96.8%). The highest liquid product yield was obtained at the lowest temperatures and pressures for all the catalysts investigated. The highest liquid product yield of 811g.kg-1 was obtained with the NiMo-Al2O3 catalyst at 390°C reaction temperature and initial hydrogen pressure of 9 MPa. On the other hand, the highest diesel yield of 493g.kg-1 was obtained with NiMo-Al2O3 and CoMo-Al2O3 at 390°C reaction temperature and initial hydrogen pressure of 9 MPa. The liquid product contained more n-heptadecane relative to n-octadecane, which is an indication that the decarboxylation/decarbonylation pathways were dominant. These reaction pathways were less favoured with increases in temperature when using Ni/SiO2-Al2O3 as compared to NiMo-Al2O3 and CoMo-Al2O3.