(42b) The Process Chemistry of Lipid Hydrodeoxygenation

Commercial production of Renewable Diesel (RD) from fats and oils began in 2007, and has grown to about 1.5 billion gal/y in just over ten years. The growth is driven not only by national and local regulatory regimes promoting low carbon fuels, but also quality. With a composition made entirely of n-paraffins and methyl-branched iso-paraffins, RD has all the desirable molecules of conventional hydrocarbon diesel without the undesirable polyaromatics present in petroleum middle distillates. As a high cetane, clean burning fuel with good low temperature properties, RD is not only a drop-in biofuel, but a premium diesel.

The key step in production of RD is hydrodeoxygenation (HDO). This is the reaction that converts fatty acid/glycerides into paraffins. Although the HDO paraffin product is typically hydroisomerized for RD production, it is also an excellent steam cracker feed for olefins. Depending on feedstock, HDO paraffins may also be used as phase change material or in other specialty paraffin applications.

Multiple HDO reaction pathways are possible, each influenced by type of catalyst and reactor conditions (and to some extent, the lipid feed itself). In this paper, we will review the various reaction pathways and discuss the corresponding mechanisms. Two reaction pathways in particular, one involving an aldhehyde intermediate and the other based on a carboxylic acid intermediate, are presented and discussed in terms of observations from commercial and pilot plant reactor systems.

Role of catalyst type in promoting various reaction pathways is discussed, including type of hydrogenation metals (active sites) and support porosity. HDO catalyst deactivation mechanisms are reviewed based on actual commercial feeds and fixed-bed reactor performance data. Side-reactions and their kinetics relative to desirable HDO pathways are also reviewed.

Additionally, "hydrogen free" approaches to deoxygenation of lipids to hydrocarbons are discussed. Recommendations for future research in view of RD production needs are also presented.