Crude oils bearing elevated free fatty acid concentrations enter biodiesel production from used cooking oil, distillers corn oil, crude palm oil, tallow, and grease trap waste. The acid value of these feedstocks, measured by ISO 660:2020, frequently exceeds 10 mg KOH/g; palm fatty acid distillate and high-acid trap grease may exceed 150 mg KOH/g. Direct alkali-catalyzed transesterification converts the free fatty acid fraction into sodium or potassium carboxylates rather than methyl esters. These soaps raise viscosity, stabilize emulsions, and consume alkali in direct proportion to the acid value. Acid esterification is therefore inserted upstream of alkali transesterification to convert free fatty acids to fatty acid methyl esters in the presence of an acid catalyst and excess methanol. The reaction is equilibrium limited and produces one mole of water per mole of fatty acid converted. That water is not an inert diluent; it suppresses catalyst activity and shifts the equilibrium toward hydrolysis. Feedstock moisture above 0.2 wt% by ISO 12937:2000 has the same effect, making pre-drying or low-moisture feed selection an operational boundary rather than a recommendation. Phospholipids, polyethylene polymers, and particulate solids from used cooking oil also interfere with acid esterification by coating heat-transfer surfaces, stabilising emulsions, and blocking decanter sight legs. Published data for acid esterification of raw trap grease without dewatering and solids removal is limited; industrial practice inserts screening, settling, and vacuum drying before the acid reactor.
Operation of a continuous acid esterification reactor at atmospheric pressure is constrained by the boiling point of methanol, 64.7 °C at 101.3 kPa, not solely by the intrinsic reaction rate. The practical temperature window is 60–65 °C, which is a narrow 5 K processing band. At temperatures below 55 °C, the homogeneous sulfuric acid-catalysed esterification of high-acid crude oils becomes residence-time limited, and batch reactors require more than 8 h to reduce free fatty acid content below 2 mg KOH/g. At temperatures above 67 °C, methanol vapourisation increases, liquid-phase contact falls, and reflux condenser duty destabilises pressure control. The esterification stoichiometry is R-COOH + CH3OH ⇌ R-COOCH3 + H2O. Sulfuric acid at 1.0–5.0 wt% of the free fatty acid fraction is the most widely reported industrial catalyst for this pretreatment. Methanol-to-FFA molar ratios from 20:1 to 40:1 are used to shift the equilibrium toward methyl ester formation. Under these conditions, batch residence times of 1–4 h are commonly reported for feedstocks with initial free fatty acid contents between 10% and 40% by mass. The reaction is approximately pseudo-first order in free fatty acid at high methanol excess, with reported apparent activation energies ranging from 45 kJ/mol to 62 kJ/mol for sulfuric acid-catalysed esterification of oleic acid. The exact endpoint is feedstock specific; rendered fats containing unsaponifiable matter and oxidation products may retain an acid value 0.5–1.5 mg KOH/g under identical conditions. Residual water from the raw oil and water produced during esterification accumulate in the polar methanol phase and must be removed continuously or intermittently to preserve conversion. Table 1 summarises the comparative behaviour of acid catalyst systems used for this pre-treatment.
Table 1. Comparative behaviour of acid catalyst systems for free fatty acid esterification in high-acid crude oils.
| Catalyst system | Typical loading | Temperature range | Methanol-to-FFA molar ratio | Reported FFA reduction | Equipment constraints |
|---|---|---|---|---|---|
| Sulfuric acid 98% | 1.0–5.0 wt% of FFA | 60–65 °C | 20:1–40:1 | Above 90% for FFA 15–40% | Glass-lined or Hastelloy C276 reactor; neutralisation and water washing required; sulfur carryover possible |
| Methanesulfonic acid 70% | 1.0–3.0 wt% of FFA | 60–65 °C | 15:1–30:1 | Similar to sulfuric acid with lower charring | Higher cost; lower corrosivity permits 316L in low-chloride service |
| p-Toluenesulfonic acid | 1.0–4.0 wt% of FFA | 60–80 °C | 20:1–35:1 | Above 85% reduction reported | Solid dosing required; thermal decomposition above 100 °C |
| Amberlyst 15 or 70 | 5.0–20.0 wt% of oil | 60–120 °C | 10:1–25:1 | Above 80% under dry feed conditions | Pressure reactor needed above methanol boiling point; deactivation by water; regeneration with methanol |
At production scale, the choice of acid catalyst is determined less by laboratory conversion than by post-reactor neutralisation demand, corrosion management, and the ability to separate the polar acid-methanol-water phase from the nonpolar oil phase. Sulfuric acid remains dominant because its cost per tonne of free fatty acid converted is lower than sulfonic acids, despite the need for acid-resistant materials and salt disposal from neutralisation. Methanesulfonic acid is selected when chloride contamination is low and the plant is designed around 316L stainless steel rather than glass-lined equipment. p-Toluenesulfonic acid may be used when sulfuric acid charring of oxidised used cooking oil is unacceptable. Ion-exchange resin catalysts avoid soluble acid neutralisation but lose activity when water exceeds 2–3 wt% in the reacting phase. In fixed-bed operation over sulfonated resin, liquid hourly space velocity is typically limited to 0.5–2.0 h⁻¹ for high-acid feeds; published data for higher throughput in high-moisture trap grease is limited. Batch-to-batch variance in rendered animal fats changes the endpoint acid value more than the residence time if the feedstock is not blended to a stable free fatty acid load. High-acid distillers corn oil from ethanol plants may contain residual free fatty acids above 15%, requiring a second esterification stage or methanol-water interstage removal to reach a target below 1.0 mg KOH/g before alkali transesterification.
In a continuously operated acid pretreatment module processing 100 t/day of high-acid feedstock, the reactor discharge is cooled to 50–55 °C and sent to a decanter or disc-stack centrifuge. The upper oil phase contains methyl esters, unreacted triglycerides, and residual free fatty acids. The lower phase contains methanol, water, acid catalyst, and polar oxidation products. Decanter sizing is sensitive to phase-density differences; at 50 °C, the methanol-water-acid phase has a density near 0.95–0.99 g/cm³, while the oil-methyl ester phase is below 0.90 g/cm³. Insufficient settling time produces an oil phase that carries acid into the downstream neutralisation section, increasing caustic consumption and soap formation. Production failures have been observed where steam tracing of decanter sight glasses causes local methanol boiling and disrupts the interface layer. The methanol-water-acid phase is routed to a stripping column for methanol recovery. The recovered methanol is recycled to the esterification reactor, but water and acid traces accumulate in the recycle loop. A side draw or periodic blowdown is required to keep recycled methanol water content below 0.1 wt%. In batch plants, the acid esterification reactor is frequently a glass-lined vessel of 10–25 m³ working volume with a retreat-blade impeller or turbine agitator operating at 45–65 rpm. Agitator speed above this range does not improve conversion but increases shear on the methanol-water-oil dispersion and can stabilise emulsions in rendered fats with high monoglyceride content. Heating is supplied by an external half-pipe jacket using low-pressure steam at 2–3 barg; overheating the jacket above 130 °C causes local sulfonation and darkening. The acid catalyst is dosed through a PTFE-lined lance into the circulating methanol-oil mixture rather than added as a concentrated slug to the oil. Concentrated sulfuric acid addition into hot high-FFA oil without adequate methanol circulation produces local charred material and increases sulfur-bound impurities that are difficult to remove downstream.
Water participates in the esterification equilibrium as a reaction product and in the acid catalysis mechanism as a solvating species. In homogeneous sulfuric acid systems, the proton is transferred to the fatty acid carbonyl oxygen, and the presence of water reduces the proton activity by hydrating the acid species. At water concentrations above 2.0 wt% in the polar methanol phase, the apparent rate constant falls measurably, and the reverse hydrolysis reaction becomes significant. The equilibrium constant for fatty acid methyl ester formation is not strongly favourable; therefore, water removal or high methanol excess is required to obtain high conversion. Methanol-to-FFA molar ratios above 40:1 are technically effective but economically unfavourable because the excess methanol must be vaporised, condensed, and refluxed at 64.7 °C. Methanol recovery from the aqueous phase consumes distillation energy and increases the distillation column diameter. Reactive distillation concepts couple esterification and water-methanol stripping in a single vessel, but the acidic methanol-water mixture challenges conventional packing. Structured PTFE packing is specified to prevent corrosion and plugging. Molecular sieves are not generally applied to the main esterification loop because the water content can exceed their adsorption capacity; they are more suitable for drying recycled methanol. Vacuum drying of the incoming oil at 105–110 °C and 20–30 kPa absolute is used to reduce moisture below 0.1 wt%. This pre-drying is mandatory for greases and waste oils with initial moisture above 0.5 wt%. The combination of pre-drying, methanol excess, and interstage water removal permits a two-stage batch esterification to reduce free fatty acid content from 35% to below 0.5% without exceeding 5 wt% sulfuric acid loading.
Table 2 presents typical feedstock entry conditions and the corresponding acid esterification approach. The values are ranges reported in industrial literature and standard test methods; site-specific feedstocks may fall outside these ranges, and published data for specific configurations is limited in the case of heavily oxidised trap grease and high-solids waste oils.
| Feedstock | Free fatty acid range | Moisture threshold | Acid esterification configuration | Target acid value after pretreatment |
|---|---|---|---|---|
| Degummed soybean or rapeseed oil | 0.5–2.0% | Below 0.2 wt% | Usually not required unless acid value exceeds 2 mg KOH/g | Below 0.5 mg KOH/g |
| Used cooking oil | 2–20% | 0.5–2.0 wt% | Single-stage H2SO4 esterification after filtration and drying | Below 1.0 mg KOH/g |
| Crude palm oil | 3–8% | Below 0.2 wt% after degumming | Single-stage acid esterification after degumming and bleaching earth treatment | Below 0.5 mg KOH/g |
| Rendered animal fat | 5–30% | Variable, often 0.2–1.0 wt% | Two-stage esterification or one stage plus interstage water draw | Below 0.5–1.0 mg KOH/g |
| Palm fatty acid distillate | 70–95% | Below 0.5 wt% | Continuous esterification with high methanol-to-FFA ratio and methanol-water recovery | Below 2 mg KOH/g |
| Trap grease | 50–100% | Often above 1.0 wt%, requiring dewatering | Multi-stage acid esterification after solids removal and vacuum drying | Published data for this specific configuration is limited |
Methanolic sulfuric acid service at 60–65 °C is more aggressive than nominal corrosion tables for dilute sulfuric acid suggest because methanol reduces the solution dielectric constant and can increase the activity of hydrogen ions at metal surfaces. Carbon steel is not acceptable for the reactor, piping, or heat exchangers. 316L stainless steel may be used for short campaigns in low-chloride feedstock with sulfuric acid concentrations below 1 wt%, but pitting and weld attack have been observed in production units operating above 2 wt% acid when chloride contamination from used cooking oil is present. Glass-lined carbon steel is preferred for reactor vessels because the lining resists methanolic sulfuric acid up to the design temperature of 200 °C, but thermal shock must be limited to 80 K differential between vessel wall and pumped stream. PTFE, PFA, and PVDF linings are specified for acid-dosing lines and methanol-water transfer lines. Hastelloy C276 is selected for valve trim, instruments, and pump internals that cannot be lined. Centrifugal pumps in acid-methanol service are specified with silicon carbide faces and PTFE or Kalrez O-rings. Gaskets in flanged joints exposed to warm methanolic sulfuric acid should be flexible graphite or PTFE; EPDM and nitrile gaskets degrade and extrude within weeks. Copper and copper-based alloys must be excluded because acid attack and copper-catalysed oxidation of methyl esters produce high sediment and oxidation instability. Titanium is generally avoided in methanolic sulfuric acid service because the protective oxide film is destabilised by the mixed solvent and can lead to localised corrosion. At temperatures above 65 °C, pressure-rated equipment is required, and the corrosion rates in pressure reactors must be validated by specific corrosion coupons rather than extrapolated from atmospheric service. Plants that switch from refined oil to high-FFA crude oil without a materials review often find that the original carbon steel methanol stripper and decanter are attacked within months. The failure mode is not uniform thinning but localised attack at the liquid-liquid interface, welds, and stagnant regions inside flange crevices.
Countercurrent stripping of the methanol-water phase recovered from acid esterification cannot be treated as a simple distillation cut because water, methanol, and traces of sulfur compounds form a polar mixture. The stripper reboiler operates at 75–85 °C under atmospheric pressure; the overhead methanol vapour is condensed at 30–40 °C using cooling water. Water of reaction must be removed from the bottom stream, and the bottom stream may still contain 0.1–0.5 wt% methanol plus sulfuric acid. Discharge of this acidic water to a municipal treatment plant requires neutralisation to pH 6–9 before release. Thermal energy for methanol recovery is a major contributor to the operating cost of acid esterification. The energy demand is approximately 0.3–0.5 MJ/kg of methanol recovered in single-stage systems, but column inefficiency, fouling from organic residues, and sulfuric acid in the feed push real plants toward the upper end of this range. The reflux ratio must be held high enough to avoid sulfur compounds reaching the recovered methanol stream. If recovered methanol contains even 10 mg/kg sulfate, the recycled sulfate accumulates in the esterification reactor and raises the acid number after washing. A caustic scrubber or an ion-exchange guard bed is installed on the recycle methanol line in plants that have experienced sulfate accumulation. The stripper reboiler may be a plate-and-frame or shell-and-tube exchanger with Hastelloy C276 tubes and a steam pressure of 2–4 barg. Fouling occurs when oxidised lipids polymerise on the hot surfaces and is more severe with used cooking oil than with crude palm oil. Cleaning cycles of 30–60 days are common in high-FFA used cooking oil service. Published data for extended fouling behaviour in trap grease distillates is limited.
Residual acid value, sulfur, and methanol are quantified under ASTM D664-18e2, ASTM D6751-23a, and EN 14110:2019 after downstream alkali transesterification, washing, and vacuum drying. The acid esterification pre-treatment must reduce free fatty acids to a level that allows sodium methoxide or potassium hydroxide dosing to be used for triglyceride transesterification without excessive soap formation. For a high-FFA feedstock entering at 20% free fatty acid, acid esterification must not be considered successful if the intermediate oil acid value remains above 1.0 mg KOH/g. Neutralisation of residual sulfuric acid from the pre-treatment is performed with sodium hydroxide or sodium carbonate to pH 6–7 before alkali transesterification. Water washing at 50–60 °C removes sodium sulfate and residual methanol; washing below this temperature increases emulsion stability in feeds containing monoglycerides. Vacuum drying at 10–20 kPa absolute and 110–120 °C reduces moisture below 500 mg/kg. Final biodiesel must meet the ester content limit of EN 14103:2020, free and total glycerin limits of ASTM D6584-21, and oxidative stability of EN 14112:2020. Acid esterification does not remove oxidation products, polymers, or trace metals; these species pass into the downstream biodiesel and may require distillation or antioxidant addition. High-sulfur feedstocks from rendered animal fat may require distillation or additional polishing to meet the sulfur limits in ASTM D6751-23a and EN 14214:2012+A2:2019. Operational boundaries include avoiding feedstock with phospholipid-derived phosphorus above 30 mg/kg without degumming, pre-drying all oils with moisture above 0.2 wt%, and excluding chlorinated solvents because chloride stress corrosion cracking in 316L is accelerated in warm methanolic sulfuric acid. These boundaries are set by esterification equilibrium, materials of construction, downstream neutralisation demand, and final biodiesel specification compliance.
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