Question
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How can hydroprocessing operators better manage the high heat release during O2 and olefin hydrogenation (for example, when processing lipid feeds)?
Oct-2024
Answers
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Joris Mertens, KBC (A Yokogawa Company), joris.mertens@kbc.global
Most vegetable oils and animal fats are triglycerides consisting of fatty acid structures containing 16, 18 and 20 carbon atoms (C16, C18, C20), rich in oxygen and olefins. Hydrodeoxygenation of 100 tonnes of an oil/fat triglyceride containing only tristearin (saturated C18 chains, C18:0) will generate 25 Gcal/100 MMBTU of heat, five times more than the exotherm resulting from hydrotreating a similar amount of fossil diesel. Most of that heat (70%) is generated during the cracking of triglycerides into fatty acids and propane rather than by oxygen removal from the fatty acid using hydrogen (hydrodeoxygenation). Oxygen removal from fatty acids through the removal of a carbon atom and production of CO₂ (decarboxylation) is even endothermic.
The exotherm will further increase to 30 Gcal/120 MMBTY per 100 tonnes of feed if it contains pure unsaturated trilinolein (C18:2). With exotherms at least five times higher than those of conventional mid-distillate hydrotreating and higher than full conversion hydrocracking, design and operational precautions are needed to avoid temperature runaway. Special attention should also be paid to maximising the recovery of the heat generated.
As in conventional hydrotreating and hydrocracking, the hydrodeoxygenation (HDO) catalyst will be distributed over different catalyst beds, and gas quench is applied to control exotherms and allow catalyst grading. Also, as with conventional hydrotreating and hydrocracking, there is the need for efficient feed mixture distribution over the catalyst beds. HDO will use three-to-six catalyst beds in one or two reactors, depending on licensor preferences and the need for pretreatment catalyst to remove catalyst poisons.
Providing an additional heat sink by recycling the (heavier fraction of the) liquid product is the most important additional handle to avoid excessive HDO exotherms. The amount of liquid recycled will be higher than the fresh feed rate, possibly twice as high. Some licensors design the HDO section with a split feed routed to the first two or three catalyst beds, which distributes the heat generation more evenly.
Similar to conventional hydrotreating/hydrocracking, heat generated is recovered from the effluent in the feed/effluent exchanger, the effluent of which can be used to generate medium-pressure steam or to preheat the isomerisation or cracker section feed. Some designs use the HDO effluent heat to preheat the downstream iso-dewaxing or cracker sections.
The design of hydrotreaters/hydrocrackers can be made more energy efficient by using a hot high-pressure separation. In the case of triglyceride feeds, the HDO section is normally followed by a sulphur-intolerant iso-dewaxing step to improve the cold flow properties of the paraffinic product. Therefore, a hot high-pressure stripper will need to be used rather than a simple separator.
Oct-2024
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