The biodiesel production process

1. Extraction.

Rapeseed contains an average 40% oil content. 98-99% of this can be removed and utilised. The extraction of vegetable oils is a well-established industry. 

There are two main processes for the extraction of vegetable oil.

(a)          Mechanical Extraction using a series of presses. 

(b)          Mechanical extraction/solvent extraction - the mechanical extraction being an initial crushing process before removing oil in the solvent extraction process.

(a) Mechanical Extraction.

This is a process of mechanical separation of the oil from the oil seed. This process produces a crude oil and a cake meal, which contain approximately 10% of the oil content.

The seeds are firstly cleaned to remove stones and pieces of metal, which may be present. Once the seeds have been cleaned, they are broken open to aid in the mechanical extraction. 

The prepared seeds are crushed under great pressures using a screw press, which causes the cells within the seed to rupture, allowing the release of the oil.

Depending on the intensity of the pressure 50-75% of the oil content can be removed by this method.

(b) Solvent Extraction.

This is a process where by a solvent (usually Hexane) is used to remove the oil. Prior to solvent extraction, most processes use a mechanical extraction process to remove around 20% of the oil content. The remaining cake is then fed into a solvent extractor on a moving bed. The solvent is sprayed over the cake in a counter-current clockwise stream and the oil is removed, as it is soluble in the solvent. The solution is then taken off and the solvent is distilled off leaving the crude oil. From this process around 1-2% oil content remains in the meal.

2. Refining.

Once the crude oil has been extracted, it then must go through a refining process. The impurities present in the oil need to be removed as they can effect the transesterification process. For example if there is a high percentage of phosphorous in the oil, then the methanol and oil could form an emulsion so strong that the process would have to be stopped and the product rejected. The process would then have to be restarted.

3. Degumming.

This process is used to remove the phospholipids, minerals, chlorophylls and colloidal proteins [ AAEOI using phosphoric acid. The phosphoric gums settle out, as they are denser and can be removed by centrifuging the solution.

4. Bleaching.

Bleaching is a process used to remove the colour pigments in the oil. These pigments are absorbed into the bleaching earth. The bleaching earth also removes trace metals, soaps and oxidation products

5. Neutralisation.

The addition of a alkali (Caustic Soda) in a centrifuge removes the free fatty acids. As well as this, the process also removes heavy metals, which would start the oxidation of the oil.

6. Esterification.

This is a chemical process where methanol is mixed with the refined oil to produce Rape Methyl Ester (RME) and glycerine. This process is carried out with the aid of a catalyst to speed up the reaction. Excess methanol/catalyst solution is used in this process to ensure all the oil is converted to esters. The oil and methanol are mixed at a ratio of 1 methanol to 10 of oil.

This is a slow process, which takes up to 8 hours under vigorous agitation. Once all the oil has been converted to RME/glycerine the separation can be carried out.

The methanol is then removed from the mixture by distillation. This methanol can then be recycled back into the process.

The remaining RME, glycerine mixture must then be separated. This can be carried out by simple use of gravity as the glycerine has a higher density than the RME. Alternatively, a centrifuge can be used to separate out the mixture.

The RME then needs to be washed with warm water to remove any catalyst or soap which may remain.

At this stage, the RME (Biodiesel) is around 98% pure, which can be used directly in diesel engines. If the Biodiesel does not meet required standards at this stage it may need to go through further purification in order to meet specification set by the engine manufacturers.

The esterification process can be carried out in one of two ways. It can be carried out in:

(a) Batches

Batch processing is a processing method where by a given amount of oil/methanol is put into the reactor. Once the reaction is complete the batch is moved on to the next stage and another batch is fed into the reactor.

Or

(b) Continuous

A simple flow diagram of biodiesel production

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