Renewable diesel can be produced by hydroprocessing of triglyceride feedstocks. While several process licensers offer proven technologies for this process with a variety of feedstocks, there’s a lot to understand before beginning a project and selecting a feedstock for your renewable diesel production process.
The Nature of Triglycerides
All triglycerides consist of three fatty acids on a glycerin backbone. Triglycerides include plant oils and animal fats and can be sourced as either virgin or recycled materials. Each source is unique in terms of the fatty acid profile and the pretreatment steps necessary for use as a feedstock for the renewable diesel production process.
Each plant or animal source produces triglycerides with their own unique blend of fatty acids as defined by the number of carbon atoms in each of the three chains, as well as the number and location of double bonds between carbons in the chains. These triglyceride sources also provide characteristic trace components, which are present in the fat or oil due to its growing conditions or any processing it has undergone between the farm and the renewable diesel processing unit.
The renewable diesel production process is typically made up of three steps:
- Saturation of the double bonds in the fatty acid
- Decarboxylation to free the fatty acids from the glycerin backbone
- Isomerization to reduce carbon chain lengths and provide branching to improve flow and combustion characteristics
Catalysts are used to facilitate the reactions necessary to create renewable diesel. The process consumes hydrogen and liberates water as the three fatty acids in the triglyceride feedstock are converted to renewable diesel molecules. The processing conditions also result in conversion of the glycerin backbone to propane.
The renewable diesel product is fractionated to separate propane as a separate product and to remove water from the renewable diesel as a waste stream.
The cost of catalysts to facilitate the conversion of triglycerides to renewable diesel is significant to the process economics. In turn, a catalyst’s costs are a function of its service life. The service life is a function of catalyst poisons and inhibitors present in the triglyceride feedstock. The presence and concentrations of poisons and inhibitors in the feedstock are strongly related to the triglyceride source and any pretreatment the feedstock has undergone. It’s important to note that the cost of the triglyceride feedstock is the largest contributor to the cost of production.
Demands for triglycerides have been historically satisfied by supplies that result from the production of other materials. For instance, animal triglycerides — including tallow from beef production, lard from pork production, and poultry fat from chicken and turkey processing — are fats available exclusively as a result of demand for those primary meat products. Similarly, plant triglyceride sources — which include cotton, flax, corn and soy — have been exploited as sources for vegetable oil only when a primary demand for other plant products exists.
Notable current examples include soybean oil as a coproduct of soybean meal for animal feed and distillers corn oil as a coproduct of ethanol production via the dry milling process. Efforts to identify dedicated plant sources for triglycerides have focused on jatropha, palm and algae. Oil production from algae and jatropha have yet to demonstrate suitable yield or scale to make any significant contribution to the triglyceride supply. Palm oil production is scalable, but acceptance is limited by environmental concerns.
The result is that three primary sources furnish essentially all the triglyceride feedstocks available in the U.S. for conversion to renewable diesel. These sources are soybeans, corn and animal fats. Other contributing sources might include reclaimed fats and oils like yellow grease and used cooking oil recovered from restaurants, but these are secondary sources limited to reuse of triglycerides from the primary sources. This argument could even be extended to animal fats as secondary to soy and corn production, but the link is less direct due to protein production during the life cycle of the animal.
Choosing an effective and cost-efficient feedstock requires that operators understand these issues before making an informed decision for their renewable diesel production process.
As refiners consider renewable, low-carbon alternatives, renewable diesel — refined from agricultural products using petroleum refinery processes — is gaining traction.