The reaction that is used for the synthesis of bio-diesel is known as transesterification, a process in which oils react with methanol, in the presence of a catalyst, to form methyl ester (bio-diesel) and raw glycerine, as a secondary product. In other words, the reaction induces the breakdown of the triglycerides molecules that make up the vegetable fats to obtain the methyl ester of the fatty acids mixture. The main result of this process is the reduction in the viscosity of the starting oils, making them compatible with certain energy uses and particularly for use of the bio-diesel as a auto traction fuel.

triglyceride glycerol

The simplified representation of the entire process is as follows:

1000 kg of refined oil + 100 kg of methanol = 1000 kg of bio-diesel

+ 100 kg of glycerine

There are several plant solutions for the realization of bio-diesel from vegetable oils, where the medium yield of conversion is equal to 98%. The factors that affect the choice of the technology to be adopted are the quantity to be treated, the perio­dicity with which the raw materials are available and the quantity of oils at the plant entrance [2, 3].

There are three main technologies that are distinguished in terms of the process temperature and pressure [2, 14]:

• Environment temperature plant: The process takes place at environmental tem­perature and atmospheric pressure, using sodium or potassium hydroxide as the catalyst. It is appropriate for batch treatment (discontinuous), with a bio-diesel production capacity of up to 3,000 t/year. The time of reaction is 8 hours.

• Medium-high temperature plant: The process takes place at atmospheric pres­sure and at a temperature of 70°C, using sodium or potassium hydroxide as the
catalyst. It is appropriate for continuous or batch treatment, with a production capacity of up to 25,000 t/year. The reaction time is 1 hour.

• High temperature and pressure plant: The transesterification takes place at a pressure of 50 MPa and a temperature of 200°C. This technology, involving higher installation and management costs, is justified for production capacity higher than 25,000 t/year, both in continuous and in batch treatments. This technique, com­pared to the other two techniques, allows the treatment of high acidity oils (up to 4%) because it uses phosphoric acid for acid catalysis.

All the technological solutions described above involve the recovery of excess methanol for the vacuum distillation (stripping) and its reuse at the start of the plant.

The main by-product of the transesterification process is a glycerol that has a high economic value in pharmaceutical and cosmetic applications.