A review analyzing the industrial biodiesel production practice starting from vegetable oil refining

One of the most promising renewable fuels proposed as an alternative to fossil diesel is biodiesel. The competitive potential of biodiesel is limited by the price of vegetable oils, which strongly influences the final price of this biofuel. On the other hand, extensive use of vegetable oils may cause other significant problems such as starvation in developing countries. Appropriately planning and designing the whole production process, from the seed to the biodiesel end-product, is essential to contain the influence of energy inefficiencies on the high price of the end-product. The present study reviews the technologies currently used in the production of biodiesel. We first discuss the technologies for extracting the vegetable oil from the seed, and its subsequent refining and conversion into biodiesel. This study focuses on the characteristics of the production processes currently used in the sector, illustrating the technological options and emphasizing the drawbacks of certain practices and the best choices available. The vegetable oils tend to be processed using procedures that are well established, but oriented more towards obtaining products suitable for the foodstuffs industry, and that consequently use technologies that are sometimes excessive for energetic purposes. The processes for extracting the vegetable oil from the seed generally include a set of steps, the complexity of which depends on the raw material. Basically, the two extraction technologies involved rely on the use of pressure or solvents. In practice, the two systems are often combined. Using the vegetable oils as a source of energy makes some of these steps superfluous and enables technologies to be used that would be unsuitable for foodstuffs production. This study focuses on feasible technological improvements that would give rise to oil that is still suitable for use as a source of energy, but at a lower cost. The refined vegetable oil can subsequently be converted into biodiesel by means of a great variety of technologies, many of which are still not suitable for applications on an industrial scale. The solution that has met with the greatest favor is homogeneous alkaline transesterification with KOH and methanol. Even when dealing with this type of conversion alone, it is impossible to establish a universal schema to describe the conversion or purification stages because there are numerous possible different solutions. When we then look more closely at the state of the art in industrial biodiesel production plants, we encounter the potential problems introduced by the type and characteristics of the original raw material. Comparing some of the reference solutions that have inspired numerous installations, a sensitivity analysis is conducted on the main elements involved in the process, focusing on their behavior in different working conditions to obtain products with the characteristics required by the international standards (EN 14214:2008, ASTM D 6751 07b).