Decarbonisation Technology November 2026 issue

local variation will still produce a mixed slate spanning SAF, diesel, and wax simultaneously. A reactor that holds the same average temperature within a tight band, enabled by even heat distribution from the liquid catalyst phase, produces a product slate concentrated in the target SAF carbon-number range. Hydrocracking and hydroisomerisation: Converting heavy wax into final SAF FT synthesis produces a broad carbon-chain distribution, including a significant fraction of heavy wax above the C16 cut-off for the jet-fuel specification. This heavy wax fraction is not a waste stream; it is a high-value intermediate that contains exactly the carbon inventory needed for SAF, but in chains that are too long. Two sequential catalytic reactors unlock it: a hydrocracking unit, followed by a hydroisomerisation unit that reshapes the resulting molecules into the branched structures that define on-specification jet fuel. “ It is the operating conditions and catalyst performance in the hydrocracking and hydroisomerisation reactors that establish the final fuel properties required for SAF certification ” Hydrocracking reactor: 320-360°C, 50-100 bar In the hydrocracking reactor, clean hydrogen reacts with the heavy wax fraction over a bifunctional catalyst, typically platinum or palladium on a silica-alumina or zeolite support. The catalyst provides both acid sites (for chain cracking) and metal sites (for hydrogenation), and the combination selectively breaks C-C bonds in long wax chains while simultaneously saturating the resulting fragments to prevent olefin formation and coke deposition. ZCT operates this reactor in the temperature range of 320-360°C at pressures of 50-100 bar. Within this window, chain cracking is selective towards the C 8 -C 16 range rather than over-cracking into light gas or naphtha. Temperature control is again decisive: above 360°C, secondary cracking reactions accelerate,

converting valuable jet-range intermediates into light fractions that require recycle or rejection. Below 320°C, conversion of the heaviest wax fractions (C 30 +) becomes incomplete, leaving unconverted wax that loads the downstream fractionation system. The 320-360°C band is therefore not an arbitrary range; it is the kinetic window within which selectivity towards jet-fuel carbon numbers is maximised for the specific wax distributions ZCT’s FT reactor produces. Hydroisomerisation reactor: 340-380°C, 30-70 bar Hydrocracking produces a jet-range hydrocarbon mixture, but predominantly comprising straight-chain (normal paraffins). Straight-chain hydrocarbons in the C 8 -C 16 range have high freeze points. This is a critical disqualifier for jet fuel, which must remain pumpable at the low temperatures encountered at cruising altitude (typically down to -47°C for Jet A-1 specification). Hydroisomerisation solves this by rearranging the carbon skeleton of normal paraffins into branched isomers (isoparaffins), which have significantly lower freeze points while retaining the energy density and combustion characteristics of the parent molecule. ZCT’s hydroisomerisation reactor operates at 340-380°C and 30-70 bar over a selective isomerisation catalyst, typically platinum on a shape-selective molecular sieve (SAPO-11 or similar), which directs isomerisation towards mono-branched products rather than over- branching into species that would degrade fuel quality. The temperature ceiling of 380°C is critical: above this threshold, the shape-selective pore structure of the molecular sieve begins to admit secondary cracking pathways, reducing isomer yield and generating unwanted light ends. Below 340°C, the isomerisation rate over the platinum sites drops, leading to incomplete conversion of straight-chain paraffins and freeze-point non-compliance in the final product. Why these two reactors define the path to final SAF The hydrocracking and hydroisomerisation reactors represent the critical quality gate within ZCT’s process design for converting a FT