Biodiesel can be produced on industrial scale using a batch or continuous process. The most suitable oils are produced from soybean (USA, Latin America), rapeseed (EU) and palm (Southeast Asia) oils. Refined vegetable oils are the best resource due to the high conversion of triglycerides into esters in a short time of reaction. Nearly only methanol is used to the lowest price and the easiest production process is employed. The most commonly used catalysts are NaOH, KOH, NaOCH3 with a loading in the range between 0.3% and 1%. The operating temperature is 60-70°C and molar ratio of CH3OH:oil is 6:1. In a batch process, the oil is charged in the reactor followed by the catalyst and methanol addition. After stirring, the reaction mixture is settled, centrifuged or pumped to another vessel in order to separate the glycerol layer from the biodiesel. The methanol is recovered from the ester layer and glycerol by flash evaporation.

The ester is neutralized by diluted acid, washed with water and dried under vacuum. The glycerol is neutralized, the FFAs separated and eventually refined for further use. In many cases the batch process is carried out in a two-step reactor.

An example of this is the Lurgi technology, in which most of the glycerin is separated at the first reactor supplied with esterification column for the separation of the excess of methanol and glycerin. Biodiesel produced after the second reactor is treated in a wash column to remove the glycerin and methanol.

Biodiesel production can also be carried out in a continuous process using tubular system as in the Desmet Ballestra biodiesel technology. This technology is characterized by its integrated feedstock pre-treatment and transesterification. Crude oils and fats are first pretreated to meet certain preset quality standards and are then processed in the standard transesterification process. This approach allows the processing of a whole range of feedstocks, including traditional biodiesel feedstocks (rapeseed, soybean, palm oil and sunflower) but also alternative and/or lower quality feedstocks (animal fat, used cooking oil, jatropha oil, etc.). A typical Desmet Ballestra biodiesel plant configuration (including pre-treatment section) is outlined in Fig. 5.17.

The Desmet Ballestra biodiesel process technology uses three reactors in series which operate under mild conditions (temperature of 55°C and atmospheric pressure) (Fig. 5.18). Pre-treated feedstock is continuously fed to the first loop reactor 1 together with methanol and catalyst (NaOCH3). Methanol is added in a proper excess compared to the required stoichiometric amount in order to maximize the degree of transesterification and to minimize soap formation. Loop reactor 1 has a settling zone in the bottom part from which spent glycerin is continuously discharged. The reacted light phase overflows to the second loop reactor 2 where fresh methanol and catalyst are added. Loop reactor 1 and 2 are identical and operate under the same conditions. The light phase leaving the

5.17 Desmet Ballestra biodiesel process.

second loop reactor consists of almost fully converted biodiesel. It is transferred to a third stirred-tank (safety) reactor in which the final conversion takes place.