Hydrotreating in the production of green diesel
A novel scheme enables co-processing of light gas oil and tall diesel to produce a renewable diesel meeting EN 590 specifications
Rasmus Egeberg, Niels Michaelsen, Lars Skyum and Per Zeuthen
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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. 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. In this process, the renewable organic material is reacted with hydrogen at elevated temperature and pressure in a catalytic reactor. 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.
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. 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 hydrotreating may also take place in a dedicated standalone unit that processes 100% renewable diesel. In either case, the new feed components mean that completely new reactions occur and new products are formed. This gives rise to a series of challenges relating to catalyst and process design.
Challenges of hydrotreating renewable feeds
Hydrotreating is a vital part of fuel production, and the economy of the refinery depends on the on-stream factor of these units. 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.
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. 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. 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. Thus, the refinery hydrogen balance must be checked, and the unit capacity may be lower than when processing fossil diesel only.
The depletion of hydrogen combined with high temperatures may lead to accelerated catalyst deactivation and pressure drop build-up. Control of these factors would require the use of tailor-made catalysts and a careful selection of unit layout and reaction conditions. In this way, it is possible to achieve a gradual conversion without affecting the cycle length and still meeting product specifications.
In contrast to conventional hydrotreating, high amounts of propane, water, carbon monoxide (CO), carbon dioxide (CO2) and methane (CH4) are formed. 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. If not handled properly, the gases formed will give a decreased hydrogen partial pressure, which will reduce the catalyst activity. Further problems with CO and CO2 may occur due to competitive adsorption of sulphur and nitrogen-containing molecules on the hydrotreating catalyst. 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. In the reactor effluent train, liquid water and CO2 may form carbonic acid, which must be handled properly to avoid increased corrosion rates.
When processing other feed types, such as tall oil or vegetable oils with a high content of free fatty acids, severe corrosion of pipes and other equipment upstream of the reactor will take place, which is also the case when processing high-TAN fossil crudes.
Finally, the main products from this process are normal paraffins with high cloud and pour points, and they may be problematic in harsh climates. However, in contrast to the FAMEs, the n-alkanes produced can be transformed into iso-alkanes with excellent cold flow properties in dewaxing refinery processes without compromising other improved properties of the diesel product. Such isomerising dewaxing may take place over a base-metal sulphidic catalyst with high diesel yields and be controlled separately to provide different grades of product quality, such as summer and winter diesel fuels.
These challenges impose restrictions on current industrial practice involving the hydrotreatment of a feed comprising oil and renewable organic material with respect to how much of the organic material can be used in the process, normally below 5 vol%. In order to achieve better economy in the co-processing scheme, it would be desirable to increase the proportion of renewable organic material in the feed up to 25 vol% or more.
In this article, the fundamental reactions taking place when processing renewable feeds are investigated and resolved in detail. Based on this information, special catalyst formulations have been developed and are currently running in industrial operation. These are designed to have a high activity and stability under the harsh conditions prevailing in this operation. Finally, we will describe how process innovations have led to a new technology that mitigates the challenges mentioned above and enables Preem to co-process up to 30% tall oil-derived material in a revamped hydrotreating unit.
Reaction pathways in renewable diesel hydroprocessing
The industrial goal of hydrogenating biologically derived (renewable) feedstocks is to produce hydrocarbon molecules with boiling points in the diesel range, which are directly compatible with existing fossil-based diesel and meet all current legislative specifications. With the introduction of feedstocks stemming from renewable sources, new types of molecules with a significant content of oxygen are present and must be treated properly by both the hydrotreating process and catalysts.
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