4 Deoxygenation of free fatty acids
1
2
Phosphate removal from phospholipids
Saturation of fatty acid ole ns
H
H
C
C O
O R
H O C O
O C O C O
Phosphate
C O
H
O
P O
O
C O
H
CH CH CH
H
O C O
O C
Glycerol
O
3 Separation of propane from glycerol backbone
5 Cracking and/or isomerisation of normal para ns
H
H
C H
H O C O H O C O H O C O
Fatty acids
C H
H
C H
H
H
Figure 2 HEFA unit reactions
greatly simplifies gas and water treatment processes, if not eliminating the need entirely. However, since HEFA units utilise supported metal sulphide catalysts, the lack of sul - phur in the feedstock introduces a new challenge not found in fossil feed processing: retaining catalytic metals in their sulphidic state while operating in a reducing environment. Although metal sulphides reduction under HEFA process - ing conditions is slow, continuous operation in a sulphur- free atmosphere will eventually lead to sulphur and activity losses. Small H2S amounts in the process gas are sufficient to counterbalance reduction reactions and mitigate catalyst sulphur loss. Continuous injection of a small amount of a readily decomposable sulphur compound, such as dimethyl disulphide (DMDS), with the HEFA feed ensures the cata - lysts remain fully sulphided. Like all metal sulphide catalysts, catalysts for HEFA pro - cessing units are manufactured as metal oxides and must be activated (sulphided) prior to use. In-situ activation of the oxidic HEFA catalysts is an option, but there are several issues which make in-situ activation problematic. In-situ activation requires copious amounts of H₂S, including a fin - ishing step with more than 1 vol% in the recycle gas. This H₂S has to be removed from the gas stream, but unlike fossil units, which have an amine scrubber to remove H₂S, HEFA units have no need for one. Purging the H2S- rich recy - cle gas to flare may be the only option to remove it, but that runs the risk of exceeding permit limits. Another issue with in-situ activation of HEFA catalysts is input heat limitations. Considering the high heats of reaction, HEFA units are usually highly heat integrated and may only have a small furnace. The unit’s normal heat of reaction cannot be accessed to supplement the furnace heating capacity because the catalyst has no activity yet. In addition, in-situ sulphiding should not be performed with the reactive HEFA feedstock because it could lead to a less-than-optimal acti - vation. This combination of factors makes in-situ activation of HEFA catalysts very challenging. Considering the infre - quency of catalyst activation, correcting the issues with
added hardware or even maintaining existing hardware is not justified. Ex-situ activated catalyst provides a more cost-effective solution to these problems. Advantages The previously described limitations are generally sufficient to justify the modest cost premium of ex-situ activation over in-situ activation for HEFA unit catalysts. However, additional benefits may also make ex-situ activation an economically advantageous choice. The biggest economic advantage of ex-situ activated catalyst is the short time needed to return the unit to normal operation. Depending on the unit’s operating margin, the time savings from ex-situ activated catalyst can be substantial. The margin from the increased production is usually much more than the cost difference for ex-situ activation. Ex-situ catalyst activation can also be considered a type of insurance since it mitigates risk from unforeseen start-up events. Mechanical breakdowns and turnaround delays do not affect the cost of activation or the resulting catalyst activity. Finally, the very simple start-up procedure requires no addi - tional staffing or expertise. Eurecat’s proprietary Totsucat ex-situ activation applications in both fossil and HEFA units have been well documented. A well-activated catalyst is not the last step in a HEFA unit’s start-up procedure. Freshly activated catalyst has pristine active and support phases, free of adsorbed spe - cies, which would moderate its catalytic behaviour. This condition is often referred to as hyperactivity, referring to the period before the catalyst reaches equilibrium with adsorbed reaction inhibitors. During this period, catalyst activity may exceed the ability of the unit to deliver a stable environment for the catalyst to operate. It is standard practice to break in catalysts by operating under mild conditions with a feedstock free of high reac - tivity components until hyperactivity is reduced. During the break-in period, catalyst activity is tempered by slow
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Catalysis 2025
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