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17/08/2015
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Fie Wilbek, Haldor Topsoe A/S, fahw@topsoe.dk
Haldor Topsoe A/S (Topsoe) is a catalyst provider for hydrotreating of vegetable oils. It is well know that diesel fuel can be produced from renewable organic material, such as vegetable oil in a refinery hydrotreating unit over a conventional sulfided hydrodesulfurization catalyst. Hence, the vendors of conventional sulfided hydrodesulfurization catalyst could all be considered: however, Topsoe has over the last manyyears optimized a line of their hydrodesulfurization catalysts for use in the production of diesel and jet fuel from renewable feedstocks (HydroFlex™).
Although many types of renewable feeds exist, the chemistry of vegetable oil or animal fat hydrotreating to produce diesel/jet molecules is somewhat simplified by the fact that most of such feedstocks, almost independent of seed type, are supplied as triglycerides, having a C-3 backbone with three carboxylic acids (fatty acids). The fatty acid chain length and degree of saturation vary depending on seed type. Upon hydrotreating, the unsaturated bonds on the fatty acids chains will be hydrogenated followed by breakage of the bond between the fatty acids and C-3 backbone.
This breakage takes place via two competing pathways. The first pathway involves complete hydrogenation to form 6 moles of water, 1 mole of propane, and 3 moles of n-paraffins with same chain length as the fatty acid chains. This pathway is called HDO or hydrodeoxygenation pathway. The other pathway involves a decarboxylation step where 3 moles of CO2, 1 mole of propane, and 3 moles of normal paraffins with a chain length, which is one carbon atom shorter than the fatty acid chains, are produced. In the hydrotreating catalyst portfolio from Topsoe, you can find catalyst for maximizing the HDO reactions and thereby maximizing your diesel yield – or, in case you are limited in hydrogen consumption, catalysts for maximization of decarboxylation.
Two additional reactions also need to be taken into consideration. Hydrotreating catalysts are known to be active for both reverse water gas shift (CO2 + H2 -> CO + H2O) and methanation (CO + 3 CO -> CH4 + H2O). The CO2 formed from the decarboxylation reactions will thus partly be converted to CO and CH4.
The hydrotreated vegetable oil results in n-paraffins (linear alkenes), which has good cetane index, but poor cold flow properties. Depending on the grade of diesel or jet fuel, dewaxing of the hydrotreated vegetable can be required.
Regarding the second questions about precautions, there are two major issues: the impurities found in vegetable oils and the exothermic reactions.
The impurities (especially phosphor) in the feed, found in practically any vegetable oil, are an important factor for the design of a pretreat unit and/or guard catalyst. If not handled correctly, the impurities will affect the catalyst performance and may cause pressure drop build-up during operation, eventually resulting in reactor shutdown for skimming. The phosphor is mainly present as phosphatides. Phosphatides are prone to cause reactor plugging problems, even in well-designed catalyst systems containing guard catalysts tailored for maximum phosphor pick-up. At elevated concentrations, the phosphatides will form a layer of gum on top of the first catalyst bed instead of being captured in the pores of the guard catalyst. It is therefore important to load a comprehensive grading system, tailored to properties of the vegetable oil to be processed. The grading system also serves the purpose of controlling the exotherm by spreading the exothermic reactions of a large catalyst volume.
If you want to know more, I can recommend the following two articles:
Hydroprocessing of Bio-Oils and Oxygenates to Hydrocarbons. Understanding the Reaction Routes.
B. Donnis, R. G. Egeberg, P. Blom, K. G. Knudsen
Bigger is Better: Industrial-scale Production of Renewable Diesel.
R. G. Egeberg, K. G. Knudsen, S. Nyström, E. L. Grennfelt, K. Efraimsson
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29/08/2007
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Amarjit Bakshi, Refining Hydrocarbon Technologies LLC, abakshi@rhtgulfcoast.com
The most common catalyst for vegetable oil hydrogenation are precious metals platinum, palladium, rhodium and ruthenium and one can also use non precious metal nickel based catalysts or dual catalyst system as described in the Selective Hydrogenation article (includes Total hydrogenation) in PTQ Q3 2007.
The process should be operated at reasonable temperature and pressure consistent with Hydrogenation conditions, as high temperature will effect the vegetable oils adversely.
Please see the www.rhtgulfcoast.com web site for biodiesel and ETBE production which provides enhancement in Biofuels technology by rejecting water from the wet ethanol feed rather than using expensive dry ethanol option.
Refining Hydrocarbon Technologies LLC provides answers in these and other Refinery and Petrochemical technology areas together with complete Refining optimisation, heat conservation and Safety audits.
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