Liquid fuel components are mainly based on crude oil. There is an ever growing demand for liquid fuels with lower CO2 emissions compared to crude oil based fuels. Various renewable sources have been used as alternatives for crude oil fuels. Methods for converting biomass to liquid fuels include transesterifications of triglycerides to alkylester, hydrotreatment of oils and fats to paraffins, and renewable diesel obtained through gasification and the Fischer-Tropsh process. One of the most promising and furthest developed method is hydrogenation of vegetable oil (HVO) or animal fats to produce paraffins, which can further be refined, e.g., through isomerisation reactions to renewable diesel with excellent properties.
Vegetable oils and animal based fats can be processed to decompose the ester and/or fatty acid structure and to saturate the double bonds of the hydrocarbon backbone thus obtaining about 80-85% of n-paraffins relative to the mass of the starting material. This product can be directly used as a middle distillate fuel component. The cold flow properties of n-paraffins can be enhanced in an isomerisation step where iso-paraffins are formed. A method of producing iso-paraffins from vegetable oils and animal fats is presented in European Patent Document No. EP 1 396 531.
Certain impurities present in the renewable oil can be harmful for the performance of the hydrotreatment/deoxygenation catalyst. Triglycerides can be converted to hydrocarbons through a hydrodeoxygenation pathway using classical hydrodesulphurisation (HDS) catalyst such as NiMo and CoMo HDS catalysts. However, the catalyst has been shown to be deactivated as a result of phosphorous in the feedstock. In publication Kubicka et al (2010) it is shown that elimination of phosphorous from the feedstock is crucial to prevent rapid catalyst deactivation.
U.S. Patent Application Publication No. 2007/0010682 discloses a process for the production of diesel range hydrocarbons from bio oils and fats, which method includes hydrodeoxygenation and hydroisomerisation to achieve fuel components with excellent performance. It is also acknowledged that in order to avoid catalyst deactivation and undesired side reactions the amount of alkali metals and alkaline earth metals are less than 1 w-ppm calculated as elemental metal and phosphorus content less than 5 w-ppm. Degumming and bleaching are suggested to remove the impurities from the feed. In bleaching the refined oil feed is heated and mixed with natural or activated bleaching clay to remove impurities such as chlorophyll, phosphoric compounds and metals remaining in the oil after degumming.
There exist numerous commercial bleaching clays, which can be used in the bleaching of renewable oils. European Patent Document No. EP 0 507 217 discloses a porous base-treated inorganic adsorbent for removal of contaminants such as free fatty acids and metals from glyceride oils. The adsorbent is capable of reducing the levels of phosphorus to below 0.3 ppm and the level of metals to below 0.1 ppm.
In previous laboratory scale experiments it has been shown that triglycerides, e.g., in vegetable oils can be converted to hydrocarbons in hydrotreatment reactions. However, in large scale production of hydrotreated vegetable oil some difficulties have occurred over time. A significant increase in the pressure drop over the hydrodeoxygenation catalyst bed was observed. In normal running conditions there is a higher pressure at the top of the catalyst bed compared to the pressure at the bottom of the catalyst bed and this difference is called the pressure drop. The pressure drop drives the feed stream forward in the reactor. Over time, continuous plugging of the catalyst increases the pressure drop and could lead to complete plugging of the flow in the catalyst bed. An increase in the pressure drop was observed after some production time even if the feed contained only trace amounts of phosphorous and metal impurities such as Na, Ca and Mg.
These findings indicate that the hydrotreatment of glyceride oils from renewable sources still needs improvements and especially large scale production encounters problems which could not have been foreseen from the early laboratory scale experiments.