Reaction Chemistry

Chapter from the Renewable Fuels Processing Guide.

RENEWABLE FUELS REACTION CHEMISTRY

Primary Reaction Pathways

Triglycerides and fatty acids are converted to paraffinic hydrocarbons via three primary pathways:

1. HYDRODEOXYGENATION (HDO):
R-COOH + 3H2 → R-CH3 + 2H2O
• Removes oxygen as water
• Preserves carbon chain length (C18 fatty acid → C18 paraffin)
• Consumes the most H2 (3 mol H2 per carboxyl group)
• Dominant at high H2 pressure and with CoMo/NiMo catalysts
• Produces water — must manage water in reactor and separator

2. DECARBOXYLATION (DCO):
R-COOH → R-H + CO2
• Removes oxygen as CO2
• Loses one carbon (C18 fatty acid → C17 paraffin)
• Consumes no H2 for the decarboxylation step itself
• Produces CO2 (and CO via reverse water-gas shift)

3. DECARBONYLATION (DCO'):
R-COOH → R-H + CO + H2O
• Removes oxygen as CO and water
• Also loses one carbon
• CO is a temporary catalyst inhibitor (reversible)

Product Carbon Number Signature:
• HDO pathway: even carbon numbers (C16, C18) from C16 and C18 fatty acids
• DCO/DCO' pathway: odd carbon numbers (C15, C17)
• Ratio of even/odd carbons indicates dominant pathway

Additional Reactions:
• ISOMERIZATION: n-paraffins → iso-paraffins (improves cold flow properties)
Requires bifunctional catalyst (metal + acid sites)
Critical for meeting cold flow specs (cloud point, pour point, CFPP)
• CRACKING: C18 → lighter products (naphtha, jet, lighter diesel)
Increases with temperature and acidity; controlled for SAF production

Source: Renewable_Fuels_Processing_Guide_v1.xlsx · sheet “Reaction Chemistry”

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