Mar-2025

Value of pre-activated catalyst in HEFA units

Hydroprocessing units designed for fossil feedstocks cannot easily switch to HEFA feedstock processing without a solution that addresses the significant challenges.

Steve Mayo
Eurecat

Article Summary

The rapid increase in processing capacity for hydrotreated esters and fatty acids (HEFA) to produce renewable diesel (RD) and sustainable aviation fuel (SAF) has been nothing short of phenomenal. Starting from less than 50 KBPD capacity 10 years ago, global capacity is now approaching 500 KBPD. Despite recent setbacks in unit profitability, strong growth in HEFA processing capacity is expected to continue through 2030, with a projected doubling of current capacity. To date, North America has seen the largest increase in HEFA processing capacity, driven by strong incentives and subsidies for RD. By 2030, HEFA processing capacity is projected to shift from RD in favour of SAF, with all regions expected to show substantial capacity gains compared to the present level (see Figure 1).

While similar in some respects to the hydroprocessing of fossil feedstocks in which refiners are well versed, hydrotreating HEFA introduces significant challenges:
• The feedstock is sulphur-free.
• High olefin and oxygen content make the feedstock very reactive.
• Very high hydrogen consumption.
• Large reaction exotherms.
• Copious quantities of reaction byproducts (C₃H₈, H₂O, CO, CO₂).
• Phospholipids may cause catalyst activity loss and pressure drop build-up.
• Feedstock corrosivity may increase.
• Long-chain n-paraffins require catalytic dewaxing and/or hydrocracking to meet cloud point (RD) or freeze point (SAF) specifications.
• Undesired conversion to lighter products (gas, naphtha) significantly reduces unit profitability.

Grassroot units licensed from technology providers address all these challenges with bespoke process design and catalysts. Hydroprocessing units designed for fossil feedstocks are usually incapable of directly switching from fossil to HEFA feedstock processing. Even small amounts of a renewable feedstock (<10%) processed together with fossil feedstock in an existing unit will see compromised operation and cycle length.

However, those same units can often be revamped to process 100% renewable feedstocks. Technology providers can reuse much of an existing unit’s hardware along with a new catalyst system to switch the unit from fossil to HEFA service. This is particularly true of hydrocracking units, which already have the robust hardware needed to accommodate the HEFA feedstock’s high reactivity and hydrogen consumption as well as improve the resulting cold flow properties.

Figure 2 shows the five general reactions that take place in a HEFA unit:
- Phosphate removal from phospholipids.
- Saturation of olefins in fatty acids.
- Propane removal from the glycerol backbone of the triglyceride.
- Deoxygenation of free fatty acids.
- Cracking and/or isomerisation of normal paraffins.

Reaction one depends on the feedstock origin as well as pretreatment steps completed ahead of the HEFA unit. Reactions two to four are common among most HEFA units and are often referred to as hydrodeoxygenation (HDO) or hydrotreated vegetable oil (HVO) reactions. The objectives of Reaction five depend upon the desired product split between RD and SAF.

Ex-situ catalyst activation
The sulphur-free HEFA feedstock initially seems like a positive aspect of HEFA processing compared to processing fossil feeds. No sulphur and no hydrogen sulphide (H₂S)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 sulphur 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 processing 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 catalysts remain fully sulphided.

Like all metal sulphide catalysts, catalysts for HEFA processing 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 finishing 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 recycle 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 activation. This combination of factors makes in-situ activation of HEFA catalysts very challenging. Considering the infrequency 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.