The double bond in unsaturated fatty acids is a precision molecular scission point that HDO destroys before it can be used.
- The HEFA industry inherited the sequential HDO→HDC paradigm from petroleum refining, where feedstocks are already hydrocarbons.
- But UCO/tallow contains 50-80% unsaturated fatty acids with double bonds at specific chain positions (C9=C10 in oleic acid).
- These double bonds are chemical handles for site-specific chain cleavage at 40-60°C via metathesis — producing jet-range fragments with >90% selectivity.
- By deoxygenating first, the industry throws away the most useful functional group for controlling chain scission, then spends $25-40M on a hydrocracking reactor to achieve the same chain shortening with 40-55% selectivity at 400°C.
- Multiple proven paths reach 58-68% SAF selectivity; the 60% target is achievable with commercially available components stacked during a single turnaround.
If you prioritize speed to market and proven technology, deploy the stacked upgrade (concepts 1+3) during your next turnaround — you'll be producing SAF within 18 months. If you prioritize long-term cost structure and are willing to invest in R&D, fund the $100-200K metathesis lab study in parallel — it could eliminate your most expensive unit operation.
On-Site Light-End Reforming + Stacked Incremental Upgrade
Modular SMR captures $22-66M/year from waste gas; stacked Pt/SAPO-11 + fractionation + pretreatment polish reaches 58-68% SAF selectivity at $25-45M total capex. Blocking issue is turnaround scheduling, not technology.
Pre-HDO Olefin Metathesis for Direct Jet-Range Chain Shortening
Ru-catalyzed metathesis at 40-60°C cleaves unsaturated fatty acids at the double bond, producing C8-C10 fragments that need only HDO — no cracking. Eliminates HDC reactor entirely. Blocking issue: Ru catalyst turnover on UCO impurities.
If this were my plant, I'd start with two phone calls Monday morning: one to Air Liquide for a modular SMR quote, and one to my mechanical engineer to pull the reactor vessel GA drawing. The SMR is the no-brainer — $8-15M for $22-66M/year in H₂ savings is the kind of return that makes CFOs smile, and it's completely independent of which SAF pathway you choose. While that's in procurement, I'd get the vessel drawing measured to see if the Topsoe in-vessel approach is feasible. If you've got 2.5+ meters of headspace, you might be able to hit 50-63% SAF selectivity at $5-15M capex without a new reactor vessel — that's your fastest, cheapest path. But I wouldn't stop there. I'd simultaneously commission the FEED study for the full stacked upgrade (Pt/SAPO-11 + fractionation + pretreatment polish) because that's your highest-confidence path to 60%+. The beauty of this approach is that each component has independent value:
- The SMR saves money in any operating mode
- The pretreatment upgrade extends catalyst life regardless of catalyst type
- The fractionation column protects already-in-spec molecules from overcracking
- The Pt/SAPO-11 catalyst provides the selectivity step-change
If any one component gets delayed, the others still pay back. That's the kind of portfolio resilience you want when you're committing $25-45M. On the R&D side, I'd write two checks immediately: $50-100K for the SILP ionic liquid cracking study and $100-200K for the metathesis lab study. These are trivially cheap options on potentially transformative technologies. The SILP study tells you in 6 months whether a $3-8M catalyst swap can get you to 55-65% SAF selectivity at 150°C lower temperature. The metathesis study tells you whether the double bond in your UCO is worth $25-40M in avoided HDC capex. Even if both fail, you've spent $150-300K to definitively close two innovation pathways — that's good R&D portfolio management. And if either succeeds, you've found a competitive advantage that your competitors won't discover for 3-5 years because the knowledge gaps between these communities are real and persistent.