(420d) A Catalytic Hydrotreatment Model for Plant Oils and Bio-Oils to Green Diesel

Mullins, M., Michigan Technological University
Shonnard, D. R., Michigan Technological University

Plant oils are triglycerides which are complex oxygenated mixtures of fatty acids. They may be reacted with alcohols in the presence of a base catalyst to yield fatty acid methyl ester (FAME) fuels; however, FAME fuels can only be blended in limited amounts with pure hydrocarbon fuels, may cause coking and have poor cold flow properties.  Direct catalytic hydrotreatment (HDT) of plant oils to paraffinic hydrocarbons yields hydrocarbon components with a higher heating value, which are readily blended with traditional petroleum fuels in a product commonly referred to as green diesel.  Catalytic hydrotreatment of plant oils is an exothermic process that can follow either a hydro-deoxygenation or decarboxylation route to lower the oxygen content, but both processes require substantial hydrogen.  A product distribution similar to diesel fuel is usually desired, but the actual distribution is a strong function of the feed composition, reactor process conditions, and the type of catalysts employed.  Unfortunately, the catalytic routes by which plant oils react with hydrogen and the reaction intermediates are not well understood.  In our study, a multi-phase reactor model incorporating gas/liquid ratios, mass transfer, pressure, temperature, reactor residence time, and intrinsic reaction kinetics has been developed.  This model, combined with the catalyst-specific kinetics, yields the stoichiometry, energy requirements and product distribution for a given feed material.  The effects of temperature, pressure, hydrogen/oil ratio and reactor space velocity have received particular attention.   As reaction temperature increases the yield of n-paraffins decreases, while the ratio of iso-paraffins increases.  Lower operating pressures generally favor green diesel production, since higher pressures also promote cracking reactions. Similarly, conversion efficiency decreases with increasing hydrogen/oil ratio and cracking reactions are triggered, lowering paraffin yields.  Higher reactor space velocities are preferable since they also promote C8 – C30 production.   This model allows us to assess the process efficiency and economics and perform a realistic LCA for different plant oil feedstocks