Converting Algal Triglycerides to Diesel and HEFA Jet Fuel Fractions

Over 2 L of algal triglycerides were first converted to n-alkanes using a 3% Pd/carbon catalyst in a fixed bed reactor at 350 degrees C under 800 psig of H-2. The starting triglyceride was composed of 10.5% C-16 and 85.2% C-18 fatty acids. The Pd/C catalyst exhibited primarily decarbonylation and total reduction deoxygenation pathways, which changed in relative contribution over the nearly 200 h of continuous operation. Both the C-18/(C-18+C-12) and C-16/(C-16+C-15) product ratios changed from 0.3 at the start of the deoxygenation run to 0.6 at its conclusion. Overall, the catalyst became more active during the run. After a first pass deoxygenation, over 85% of the product was composed of n-alkanes divided nearly equally between the C-n and Cn-1 chain lengths corresponding to the starting fatty acid chain lengths. The alkanes along with a few percent 1-alcohols were separated from the remaining incompletely converted components by distillation and subjected to a polishing hydrogenation using a 0.5% Pt/alumina catalyst. A portion of this first pass n-alkane composite was hydroisomerized to improve its cold flow properties using a 0.5% Pt/US-Y zeolite catalyst. Even with an iso/normal ratio of just over 1 in the product stream, the mixture solidified near ambient temperature. To further improve the cold flow properties of this diesel composition, it was subjected to a solvent dewaxing procedure, which yielded a product that remained liquid at -20 degrees C. The incompletely converted components remaining after the first pass deoxygenation were subjected to a second deoxygenation pass under the same conditions used for the fresh triglycerides. The product alkanes were again separated by distillation, alcohols were reduced to alkanes, and a single composite alkane mixture was prepared by combining the first and second pass alkanes. Overall, more than 95% of the product stream produced by the two deoxygenation passes was composed of n-alkanes. This final composite mixture was processed further to a HEFA (hydrotreated esters and fatty acids) jet composition using the same 0.5% Pt/US-Y zeolite catalyst at three different conversion severities. With single pass cracking conversions of 43%, 59%, and 93%, yield losses of 41%, 44%, and 75% to a C8- naphtha fraction were observed. Such high yield losses argue against a strategy where HEFA jet is the primarily targeted product. We suggest a strategy in which the yield loss to naphtha can be limited to less than 10% based on making principally a renewable diesel fuel from which a HEFA jet can be extracted as a minority product.