FAMEs production that meet specifications and standards can be easily obtained using refined vegetable oils or animal fats and the appropriate processing conditions. However, when alternative feedstocks are utilized including cooking oils or non-refined oils/fats, a post-treatment is required in order to reach standard properties.

Three different post-treatments are most generally employed in this regard: distillation, adsorption and filtration. The most effective is biodiesel distillation in order to remove non-volatile contaminants such as steryl glucosides, phospholipids, soaps, dimeric and polymeric materials and inorganic salts. However, the distillation has to be performed at ca. 200°C at 1 Mbar which is not an energy-friendly operation. The refining of biodiesel via vacuum distillation is illustrated by using used cooking oils (Ensungur, 2008; Zyaykina et al., 2009). In the refining step, a number of polar dimeric, polymeric and oxidation compounds have been formed together with the FFAs, di — and monoglycerides and trans fatty acids, while dimeric and polymeric FAMEs are generated during transesterification. Accordingly, the biodiesel obtained in this process will have a high viscosity and a very low oxidative stability. Used cooking oil containing 24% polar compounds was transesterified producing a biodiesel with a viscosity of 6.4 mm2/s. After distillation, the viscosity was reduced to 3.7 mm2/s. However, the oxidative stability was further reduced from 3.3 hours (before distillation) to 1.5 hours (after distillation).

Distillation is lowering the viscosity below the maximum limit but at the same time the oxidative stability is decreased as the natural anti-oxidants (tocopherols) are not distilled and remain in the distillation residue. Addition of synthetic anti­oxidants is necessary to reach to oxidative stability of 6 hours.

Distillation has however a favorable influence on the cold filtration properties of biodiesel (cold soak filter test and filter plugging point) due to removal of mono-, diglycerides and steryl glucosides during distillation.

In future, due to the expected high stringent standards, distillation is looking the most favorable process to produce biodiesel in compliance with all the standards and quality demands. Adsorption by magnesium silicate was also reported as an efficient procedure in order to upgrade the biodiesel quality (Bertram et al., 2009).

Table 5.4 shows the results of dry washing with 1% Magnesol®R60, at 70°C for 20 minutes, followed by filtration.

The most important result is that the heated biodiesel contained lower soap content and had higher oxidative stability. Similar results have been obtained for rapeseed oil and yellow grease feedstock. Biodiesel can also be purified by cellulose derivatives produced by Rettenmaier. Filtracel®EFC plus are silica gel encapsulated fibers combining the excellent filterability of cellulose filter aids with the excellent adsorption properties of silica gel in just one product.

It works as filter aid and adsorbent to remove soaps, trace metals, phospholipids and other polar oil contaminants, being more efficient than bleaching earth. Some Filtracel grades have been additionally activated by citric acid which chelates non-hydratable soaps, phospholipids and metals by converting them into a hydratable form for adsorption. These cellulose derivatives are acting as desiccant and also remove hazy cloudiness. Biodiesel is chilled to crystallize free sterol glucosides, which are removed by filtration.