Triglyceride molecule
Better cetane index Higher cloud point
Para ns
O
H C O
O
Backbone – propane formation
Higher H consumption Formation of CO, CO & H O
Oxygen
HC O
O
9
12
H C O
3
Ole-sat gives higher H consumption Higher heat release
Ole ns
Figure 1 First-generation renewable feedstocks mainly consist of triglycerides that consist of a propane backbone linking the paraffin chains with oxygen bonds. Breaking of the oxygen bonds will result in yields of paraffins, propane, and oxygen-containing molecules like water, CO and CO 2
amounts of hydrogen, and the reactions are highly exo- thermic. The product will be boiling in the middle distillate range and suitable for diesel. If jet fuel is needed, the lon- ger normal paraffin chains need to be converted to lower boiling material by either cracking or isomerisation. Normal paraffins have low density and high cetane index, but also high cloud point. Isomerisation provided by a hydrocrack - ing catalyst is therefore sometimes needed to achieve cloud point specifications. Hydrotreating of aromatics and oxygen-containing ring structures produce naphthenes with higher density and boiling range than what is suitable for diesel. Selective hydrocracking reactions involving ring opening will there - fore be needed for processing these more demanding second- and third-generation feedstocks to produce high yields of high-quality jet and diesel products. Overcoming operational challenges Operational challenges are better overcome in hydrocrack - ers. Considering the high content of oxygen in renewable feeds, the H 2 consumption associated with these reactions is in the range of 300-500 Nm 3 /m 3 , which is orders of mag- nitude greater than what is normally seen in hydrotreaters. However, compared to hydrocrackers, the difference is less, and the impact from processing in a hydrocracker would
therefore be less. Likewise, the associated heat release originating from these reactions is also more in the same order of magnitude as in hydrocrackers designed to handle high temperature rises by having multiple beds. Contaminants present in the renewable feeds pose a challenge to the hydrotreaters, and significant amounts of bulk catalysts need to be replaced to manage the con- taminants. In hydrocrackers, the VGOs and HCGOs often processed also have considerable amounts of contami - nants, and hydrocrackers are therefore normally designed to include volume for grading catalysts. They may therefore allow greater quantities of renewables to be processed without compromising catalyst cycle length. Different types of renewable feeds may produce differ- ent renewable products, and the fractionation section in hydrocrackers makes it possible to separate these differ- ent products. The products from renewables often require more than hydrotreating, either to achieve better cold flow properties by isomerisation or, in the case of second- and third-generation feedstocks, hydrogenation of rings and ring opening. Hydrocracking catalysts are multifunctional and may be selected to include both isomerisation and selective ring opening activity to ensure the fulfilment of all required end-product properties. Real-world examples Several hydrocrackers worldwide are processing renewable feedstocks. Some hydrocrackers are part of standalone units comprising hydrotreating, isomerisation, and hydrocracking sections, and others are conventional fossil fuel hydrocrack - ers co-processing renewable feed batches. Topsoe has supported refineries in preparing for and conducting test runs, and the following discussion gives examples of the observations from one of these test runs showing industrial data supporting the theoretical basis previously discussed, confirming learnings from pilot plant studies. Test run co-processing vegetable oil in a European refinery The refinery operates a single-stage once-through hydro - cracking unit, designed for 80 wt% gross conversion of various low-value gas oil streams to high-quality products. The primary objective of the unit is to maximise middle dis - tillate yield. In 2021, the refinery considered co-processing bio-based vegetable oil feedstocks in the hydrocracking unit.
Vegetable oil, 0%
Vegetable oil, 5%
Feed C, 3%
Feed C, 3%
Feed A, 36%
Feed A, 36%
Planned throughput co-processing run, vol%
Planned throughput during fossil run, vol%
Feed B, 56%
Feed B, 61%
Figure 2 Unit throughput was kept stable during the test run processing typical feedstock for baseline comparison. 5 vol% of the feedstock was slowly replaced with vegetable oil and then all unit operational parameters were kept stable to evaluate performance during co-processing
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Catalysis 2023
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