![]() In the reactor effluent train, liquid water and CO2 may form carbonic acid, which must be handled properly to avoid increased corrosion rates. The CO, which cannot be removed by an amine wash unit, will build up in the treat gas, requiring a high purge rate or another means of treat gas purification. Further problems with CO and CO2 may occur due to competitive adsorption of sulphur and nitrogen-containing molecules on the hydrotreating catalyst. If not handled properly, the gases formed will give a decreased hydrogen partial pressure, which will reduce the catalyst activity. These gases must be removed from the loop either through chemical transformation by a gas cleaning step such as an amine wash or, more simply, by increasing the purge gas rate. In contrast to conventional hydrotreating, high amounts of propane, water, carbon monoxide (CO), carbon dioxide (CO2) and methane (CH4) are formed. In this way, it is possible to achieve a gradual conversion without affecting the cycle length and still meeting product specifications. Control of these factors would require the use of tailor-made catalysts and a careful selection of unit layout and reaction conditions. The depletion of hydrogen combined with high temperatures may lead to accelerated catalyst deactivation and pressure drop build-up. Thus, the refinery hydrogen balance must be checked, and the unit capacity may be lower than when processing fossil diesel only. As the reactions also consume large amounts of hydrogen (for a 100% renewable feed, a hydrogen consumption of 300–400 Nm3/m3 is not unusual), higher make-up hydrogen and quench gas flows are needed even when co-processing quite small amounts. This means that the problem of industrial operation will typically not be to achieve full conversion, but rather to be able to control exothermic reactions when using an adiabatic reactor. When considering the conversion of most naturally occurring, oxygen-containing species, it is evident that these are much more reactive than refractory sulphur compounds, which must be removed to produce diesel with less than 10 ppm sulphur. Thus, before introducing even minor amounts of new feedstocks into a diesel hydro-treater, it is important to know the implications and how to mitigate any potential risks. Hydrotreating is a vital part of fuel production, and the economy of the refinery depends on the on-stream factor of these units. This gives rise to a series of challenges relating to catalyst and process design.Ĭhallenges of hydrotreating renewable feeds In either case, the new feed components mean that completely new reactions occur and new products are formed. The hydrotreating may also take place in a dedicated standalone unit that processes 100% renewable diesel. Thus, a co-processing scheme where fossil diesel and renewable feedstocks are mixed and co-processed is possible, producing a clean and green diesel meeting all EN 590 specifications. The same types of catalysts are used in the hydrotreating of renewable feeds as are presently used for the desulphurisation of fossil diesel streams to meet environmental specifications. The clear advantage of hydrotreating seed oils (or fatty acid methyl ester, FAME) relative to the use of FAME biodiesel is the fact that the final products from this simple hydroprocessing process (simple paraffins) are the same components as those present in normal fossil diesel. In this process, the renewable organic material is reacted with hydrogen at elevated temperature and pressure in a catalytic reactor. One well-established method for this purpose is the conversion of vegetable oils into normal paraffins in the gasoline or diesel boiling range by employing a hydrotreating process. Before feedstocks derived from renewable organic material can be used in conventional car engines and distributed using existing fuel infrastructure, it is desirable to convert the material into hydrocarbons similar to those present in petroleum-derived transportation fuels.
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