Enzymatic biodiesel from high-FFA waste oil (lipase)

Immobilised-lipase (e.g. Novozym 435) production of FAME from a waste feedstock -- used cooking oil / rendered fat carrying ~17 mol% free fatty acid. This is the feed slate a renewable-fuels producer actually buys, and it is exactly where the conventional alkali route breaks down: NaOH or methoxide saponifies free fatty acid into soap (the databank even carries sodium palmitate), which destroys yield and emulsifies the glycerol settling step, so a high-FFA feed needs an acid-esterification pretrain before it can be base-catalysed at all. A lipase does both reactions at once and at 40 C: it transesterifies the triglyceride to esters plus glycerol, and it esterifies the free fatty acid into more product rather than losing it to soap. Both reactions are carried explicitly here, so the FFA ends up as methyl palmitate instead of as a loss.

The result: 97.9 wt% ester content (EN 14214 requires >= 96.5 wt%) with the free fatty acid taken down to under 0.1 mol% of the product, i.e. a low acid value, from a feed the alkali route could not process directly. Esterification also makes one mole of water per mole of FFA converted, which is why the vacuum flash takes methanol and water overhead together -- water inhibits the enzyme and has to leave the loop.

Honesty note: as with the alkali example, the deliverable is the material balance; the energy balance is not, for the same reason (no ideal-gas Cp correlation for the triglyceride/ester records, and acentric factors far outside Peng-Robinson's fitted range), so every unit is temperature-specified. Two things this deliberately does not claim: the reactors are specified-conversion models standing in for an immobilised-lipase packed bed -- no enzyme kinetics are solved, because lipase methanolysis follows ping-pong bi-bi kinetics with methanol inhibition, which the in-tree enzymatic_reactor (a single-substrate, pH-branching Michaelis-Menten CSTR) cannot express; and the recovered methanol/water overhead would need a methanol recovery column before recycle, which is out of scope here. The glycerol/ester split is a specified settling recovery rather than a rigorous liquid-liquid flash -- now a modelling choice, not a data limit, since the unconverted triglyceride and the residual free fatty acid gained predictive-UNIFAC group decompositions alongside the methyl esters and glycerol.