Researchers at the University of Dayton Research Institute (UDRI) investigating the conversion of algal triglycerides to renewable diesel and HEFA (hydrotreated esters and fatty acids) renewable jet fuel have concluded that a renewable aviation turbine fuel strategy that preserves the overall highest liquid fuel yield from the renewable feedstocks would target the production of primarily diesel fuel.
Renewable aviation fuel would be recovered from the cracked fraction that naturally accompanies the hydroisomerization of the original n-alkanes derived from the algal triglycerides to the extent required for meeting an appropriate diesel fuel pour point specification. Such an approach would limit the loss of algal alkane fuel value to less than 10%, according to their paper published in the ACS journal Energy & Fuels.
To convert renewable triglycerides to liquid transportation fuels, either diesel or HEFA (hydrotreated esters and fatty acids) jet, a number of chemical transformations must be undertaken. First, the triglycerides must be converted to normal alkanes. This can be accomplished by catalytic deoxygenation of (1) triglycerides; (2) free fatty acids derived from triglycerides; or (3) secondary esters produced by the transesterification of triglycerides with an inexpensive alcohol. Next, the normal alkanes must be catalytically isomerized and hydrocracked to a distribution of alkane isomers and the fractions appropriate for diesel and HEFA jet recovered.
Hydrocracking is required for producing HEFA jet because the naturally occurring distribution of fatty acid chain lengths found in the triglycerides yields alkanes with boiling points near or above the high temperature limit of the boiling point distribution in both commercial and military aviation fuels. Similarly, when considering the low temperature requirements for these fuels, any remaining normal alkanes in a very highly isomerized mixture of the initial alkane distribution will have a freezing point considerably higher than the −40 °C required by the Jet-A commercial specification and further still from the −47 °C required for military JP-8.
On the other hand, the native distribution of fatty acid chain lengths yields alkanes that are quite suitable for use as a diesel fuel. To improve cold flow properties, the normal paraffins would require only a relatively mild hydroisomerization treatment. Consequently, productionof diesel fuel would result in a much higher yield to a commercial product. However, the European renewable fuels initiative has set targets for renewable fuel use by energy consuming sector. Consequently, it will be necessary to produce aviation fuel from renewable sources.
...While much effort has been focused on the production of alkanes from the triglycerides, much less has been published with regard to the further conversion of these alkanes to actual fuel compositions.
-Robota et al.
In their study, the UDRI team converted algae-derived triglycerides to a mixture of normal alkanes using a 3% Pd/carbon catalyst in a hydrogen stream with an approximate H2/triglyceride molar feed ratio of 30. The starting triglyceride was composed of 10.5% C16 and 85.2% C18 fatty acids. Rather than targeting complete conversion to alkanes in a single reactor pass, they selected operating conditions which gave a product alkane content between 70 and 85 mass percent. The alkane yield increased as the run progressed with an increasing fraction of the even-numbered alkanes.
These first pass alkanes were concentrated by distillation into a composite in which the alkane concentration was nearly 95%. The remaining high boiling liquids were then converted in a second catalytic pass to produce additional alkanes, which were again concentrated by distillation and aggregated with the first pass alkanes for further conversion into fuel compositions.
They then examined three different bifunctional catalytic cracking strategies for producing HEFA jet using a composite of first and second pass normal alkanes. In the first hydrocracking approach, the single pass conversion is higher than that used for mildly isomerizing the feed alkanes to a diesel-type of composition, with net cracking targeted near 50%.
The feed n-alkanes become substantially isomerized and can be separated from the jet fraction and used directly as a diesel fuel composition. This heavier fraction could also be recombined with fresh feed in a recycle to extinction strategy and eventually wholly converted to jet and naphtha fractions. Under these mildest of cracking conditions, the cracked product distribution would remain unchanged by further, secondary cracking of the initial product distribution, the team found.
The second strategy was designed such that secondary cracking of products could just be clearly detected. Again, the heavier-than-jet fraction could be separated and either used directly as diesel or recycled to extinction.
The third strategy, the most aggressive, resulted in near 100% cracking of the feed alkanes in a single reactor pass; only a negligible portion of the composition heavier than jet remained. This would result in no diesel and require only the separation of the naphtha fraction from the jet fraction.
The three conditions result in about 43%, 59%, and 93% net cracking at temperatures of 268 °C, 272 °C, and 278 °C, respectively.
Only under the most aggressive single pass conditions are the heaviest molecules sufficiently reduced in abundance that no recycle of the insufficiently converted fraction would be needed in a continuous conversion process. Under the least aggressive conditions, it is doubtful that the amount of remaining n-C14 and n-C15 is low enough that the HEFA jet fraction would meet the −47 °C freezing point requirement under MIL-DTL-83133G for JP-8. Under these three conditions, losses to the C8−naphtha fraction when normalized to the C9−C15 fraction comprise 41%, 44%, and 75%, respectively.
Because of these high losses, a renewable aviation turbine fuel strategy that preserves the overall highest liquid fuel yield would target the production of primarily diesel fuel. The aviation fuel would then be recovered from the cracked fraction that naturally accompanies the hydroisomerization of the original n-alkanes to the extent required for meeting an appropriate diesel fuel pour point specification. Such an approach would limit the loss of algal alkane fuel value to less than 10%.
-Robota et al.
Resources
Heinz J. Robota, Jhoanna C. Alger, and Linda Shafer (2013) Converting Algal Triglycerides to Diesel and HEFA Jet Fuel Fractions. Energy & Fuels doi: 10.1021/ef301977b
