1. The transesterification process: overview
Biodiesel is chemically defined as a mixture of methyl or ethyl esters of fatty acids (FAME or FAEE, respectively), obtained by the reaction of an oil or fat with an alcohol (typically methanol) in the presence of an alkaline catalyst (sodium or potassium hydroxide).
The reaction is transesterification: three molecules of fatty acid linked to one molecule of glycerol (which is crude vegetable oil) react with three molecules of methanol, releasing three molecules of methyl ester (biodiesel) and one molecule of free glycerol.
In industrial practice, the reaction is carried out in reactors with excess methanol to shift equilibrium toward products, followed by glycerin separation by decantation or centrifugation, biodiesel washing with water to remove soaps and methanol traces, vacuum drying and final purification.
From a pumping standpoint, the process goes through at least ten distinct points: feeding vegetable oil or tallow to the reactor; methanol dosing; alkaline catalyst dosing (methanol solution); reactor recirculation; transferring reaction product to the decanter or centrifuge; transferring crude glycerin to storage; transferring washed biodiesel to drying; methanol recovery by distillation and return to the process; transferring finished biodiesel to storage and loading tank; and effluent drainage (wash water, discarded glycerin).
Each point has its own material, flow, temperature and compliance requirements.
2. Feedstock feeding: heated vegetable oil and tallow
The feedstock feed pump is the first critical point of the process. Soybean oil at ambient temperature has viscosity of approximately 60 cP, relatively low, and can be pumped with FBCN without major difficulty. Bovine tallow, however, has a solidification point near 40 °C, which forces heating the lines to at least 50 °C before the pump to keep tallow liquid.
Some lower-quality tallows have even higher solidification points (45-50 °C), requiring heating to 60 °C. Once heated, tallow has similar viscosity to vegetable oil and can be pumped with the same technology.
The standard recommendation for feeding vegetable oil and heated tallow is FBE (external gear), not FBCN. The reasoning is: although viscosity is moderate, oils and fats are oily fluids with good natural lubricity for gears, and FBE offers constant flow independent of discharge pressure — an important characteristic in batch reactors where the operating point changes throughout the cycle.
In addition, FBE accepts reduced rotation (1,150-1,450 rpm) without significant flow loss, allowing precise feed control via VFD without vibration or motor heating problems. FBE pumps from 1.1/2" to 3" serve most Brazilian medium-size plants (50 to 500 thousand liters per day capacity).
3. Methanol and catalyst: volatility and ATEX compliance
Methanol is the second critical ingredient of the transesterification process and also the most dangerous. It is a short-chain alcohol with a flash point of just 11 °C — well below Brazilian ambient temperature — which means any leak immediately forms explosive atmosphere. In addition, it is toxic by ingestion, skin absorption and inhalation, capable of causing blindness and death in uncontrolled exposures.
The specification for methanol pumps is therefore the most rigorous of the entire biodiesel process: mandatory INMETRO compliance for Zone 1 operation, minimum Ex d IIB T3 motorization, pressurized double sealing with continuous monitoring, and mechanical design that minimizes leak points.
The typical methanol pump is FBCN in 316L with cast CF8M casing, Plan 53B double mechanical seal with inert barrier fluid (glycol or heavy mineral oil that does not react with methanol), silicon carbide against carbon seal faces, PTFE or Kalrez elastomers for chemical compatibility, WEG Ex d IIB T3 motorization with anti-spark coupling, periodically verified equipotential grounding, and Ex ia instrumentation (barrier reservoir pressure and temperature).
FB Bombas supplies the complete set pre-certified INMETRO on a skid ready for installation, eliminating multi-vendor certification management by the end customer.
4. Crude glycerin and washing: alkaline chemistry and emulsions
After the transesterification reaction, the product is a two-phase mixture: biodiesel (methyl ester) in the upper phase and crude glycerin in the lower phase, containing most of the soap salts, residual catalyst and excess methanol. Separation is done by decantation in large tanks or continuous centrifugation.
The resulting crude glycerin is extremely alkaline (pH 10-12 due to residual NaOH), contains 30-40% pure glycerol, 20-30% soap, 10-15% residual methanol and water, and has viscosity varying between 100 and 1,000 cP depending on temperature. The crude glycerin transfer pump is typically external gear FBE in 316L to resist alkalinity, with reduced rotation (1,150 rpm) to minimize aeration and secondary emulsification.
Washing crude biodiesel is done with industrial water (slightly acidified with citric or sulfuric acid to neutralize soap traces) in tanks with gentle agitation followed by secondary decantation. The wash water pump can be FBCN in cast iron or 316L depending on acid chemistry; the washed biodiesel transfer pump is preferably FBE (although biodiesel has low viscosity, it may contain soap traces that crystallize at cold points of the pump — positive displacement is more tolerant).
After washing, the vacuum drying phase removes water traces and residual methanol, and the final transfer pump of dried biodiesel to the storage tank is 316L FBE with ATEX/INMETRO motor.
5. Methanol recovery: distillation and return to process
The economy of a biodiesel plant is heavily dependent on recovery of excess methanol used in the reaction. The standard process uses two distillation columns: the first recovers methanol from the biodiesel phase after washing, and the second recovers methanol from the crude glycerin phase. Both columns operate at moderate temperature (methanol boils at 64.7 °C at atmospheric pressure) and with partial vacuum to further reduce operating temperature.
Recovered methanol is condensed, stored in a dedicated intermediate tank, and re-injected into the process along with fresh makeup methanol.
Pumps in this methanol recovery section fall under the same rigorous ATEX/INMETRO compliance specification discussed in the previous section, with the difference that they operate with purer methanol (after distillation) and at elevated temperature during condensate transfer. The standard configuration is 316L FBCN with Plan 53B double seal, Ex d IIB T3 motor, and condensate recirculation system operating between 30 and 60 °C.
For the recovery column bottom (hottest point), using FBOT may be necessary if operating temperature exceeds 100 °C, but in practice most Brazilian plants remain below this limit to maintain operational safety.
6. Storage and loading of finished biodiesel
Finished biodiesel after drying and purification has specifications controlled by ANP (updated ANP Resolution 906/2022) in parameters such as ester content (minimum 96.5%), free glycerin (maximum 0.02%), residual methanol (maximum 0.2%), water content (maximum 350 ppm), iodine value, minimum flash point 100 °C, oxidation stability, and others. This specification must be maintained throughout the storage and loading process for distribution to the station chain.
Pumps at this final stage are simpler than those of the chemical process: they operate with already purified biodiesel, at ambient temperature, and do not have the aggressive chemistry of previous phases.
The recommendation for storage and loading pumps is FBE 2" or 3" in 316L, with simple cartridge mechanical seal (finished biodiesel is no longer classified as highly flammable, having flash point above 100 °C, but still requires Ex d IIA T3 motorization for operation in Zone 2 classified areas), VFD-controlled rotation to allow truck-tank loading without water hammer, and integrated flow meter for commercial transaction control.
This configuration is identical to that used in conventional S10 diesel distribution bases, discussed in the petrochemical and fuels cluster.
