Multiple processes exist for triglyceride feedstock purification. These processes have been developed primarily to permit the production of edible fats and oils for human consumption. Application of selected steps optimizes the refining process for a given source to produce the required level of purity. This optimization is important as each refining step results in some loss of oil yield.
Economic considerations dictate that refining yield loss becomes the controlling factor once the standard of purity for commodity trade has been achieved. Clear understanding of both the triglyceride source and the end use are necessary to determine the best application of purification processes. Two common refining practices can be used to illustrate this point.
Pretreatment for Plant-Based Feedstocks
Once-refined or water-degummed, soybean oil can be further refined with the application of acid degumming, bleaching and deodorizing to produce the commodity known as refined, bleached and deodorized (RBD) soy oil, which is often used as a feedstock for biodiesel.
Water degumming is effective in removing soluble phosphorus. Acid degumming is useful for removal of insoluble phosphorus. Bleaching is required for reduction of metals. Deodorizing eliminates free fatty acids, which is necessary for most biodiesel production processes. In the renewable diesel process, however, it is possible to convert these free fatty acids to valuable product. Deodorization of renewable diesel feedstock will result in a yield loss equal to the quantity of free fatty acids in once-refined soybean oil.
Pretreatment for Animal-Based Feedstocks
Tallow can be refined and bleached to improve Lovibond color and is suitable for industrial uses as a technical fat. One of the methods used to meet this commodity specification is the application of another preliminary bleaching step during the rendering process. Application of sodium chlorite and activation with sulfuric acid to produce chlorine dioxide is effective in reducing the observed color of the tallow.
This reduction in color is beneficial for some uses, but adds little value for production of renewable diesel, particularly in comparison to the increased chemical burden that must be removed in the pretreatment process through additional water wash with associated loss of triglycerides.
Putting Pretreatment to Use
Successful projects for production of renewable diesel require a thorough understanding of feedstock sources and characteristics in order to select and implement the proper pretreatment steps for successful conversion of triglycerides to hydrocarbon via hydroprocessing. Feedstocks and pretreatment processes must be matched effectively to achieve expected product yields and hydroprocessing catalyst life while controlling processing costs. Successful hydroprocessing projects require solid experience with process safety for hydrogen and high-pressure catalysis with process licensers. For the added complications of triglyceride feedstocks, look for a project partner with knowledge and experience in all aspects of renewable diesel production, including knowledge of pretreatment and the associated wastewater processes.
As refiners consider renewable, low-carbon alternatives, renewable diesel — refined from agricultural products using petroleum refinery processes — is gaining traction.