Bio-diesel can be produced by using as raw materials both oils extracted from oil cultures (vegetable oil extraction process: soy, sunflower, rape, etc.) and oils that are recovered (regeneration process of vegetable oils) from alimentary uses through separate collection systems. The products that are obtained upstream of the extraction and the regeneration can be directly applied as a combustible or directed to the transesterification process to obtain bio-diesel [1-3, 30].

2.4.1 Vegetable oil extraction

The steps involved in the extraction of vegetable oils from oil cultures are:

1. cleaning (with electrovalent elements or magnets);

2. grinding;

3. heating and conditioning (80-90°C, humidity 7%);

4. mechanical extraction (hydraulic press or strew-shaped) or chemical (solvents);

5. purification for depuration or refining (neutralization of the free fatty acids) if the oils are to be used for bio-diesel production.

The seeds derived from oil cultures are first cleaned, using magnets or electrovalent elements to remove additional materials or collect gross residuals, and are then decorticated. In the following grinding step, there is an outburst of the oils from the cells. Heating and conditioning, in the temperature range 80-90°C and at a humidity of 7-10%, promote the lysis of the cells, the diffusion of the seeds’ fat material and the separation of the proteic components. Oil extraction from milled seeds can be done by mechanical or chemical techniques. Mechanical extraction uses a screw or hydraulic press, and it leaves an unextracted residual fat content equal to 5-12%. Chemical extraction, characterized by an unextracted residual fat content equal to 1% (with a seed-solvent ratio equal to 1:18, reaction environment temperature of 50°C and contact times equal to 2 hours for rape seeds and 1 hour for sunflower seeds), involves the use of organic solvents (such as trichloroethylene, hexane, carbon sulphur). Chemical extraction can be done in a discontinuous man­ner (batch), which is the preferred option for plants that treat at least 250-500 t/day. Chemical and thermochemical extraction can be integrated with each other. Despite a high investment cost, the yield is close to 100%. In this technique, the milled material is first subjected to mechanical extraction, which leaves a residual fat content of 20-24%, and, subsequently, chemical extraction is carried out. The yield of raw oil obtained from the extraction process is variable, from rape and sunflower seeds 36-38% of oil weight is extracted. If we want to convert the raw oil obtained into bio-diesel, it must be subjected to a purification step that can be performed depending on two modalities: purification and refining. The two proc­esses, which are preceded by a centrifugation step, are diverse in terms of the qualitative levels, which is higher in the refining step. Depuration is directed to the removal of impurities (waxes, resins, pigments and mucilages) present in the raw oil and it is carried out using sulphuric acid, salt water solutions or by percolation using absorbing grounds. Refining removes impurities working in salt solutions with sulphuric acid or citric acid. Furthermore, refining reduces the acidity of the raw oils for the neutralization, which can be done in a physical (at 240-260°C and in conditions of vacuum at -1 mbar) or chemical manner (working with sodium hydroxide at a temperature of 60-80°C and at atmospheric pressure). The best quality of the refined oil, compared to the depured oil, is the reduction of the acid­ity of the raw oils. At the end of the purification step, the vegetable oil yield is about 34.4%. The main by-product of the vegetable oil extraction is the proteic panel, which is used in zootechny as animal feeds [2, 14, 24].