Transesterification

1) Mixing of Alcohol and Catalyst

The catalyst is typically sodium hydroxide (caustic soda) or potassium hydroxide (potash). It is dissolved in the alcohol using a standard agitator or mixer. Reaction. The alcohol/catalyst mix is then charged into a closed reaction vessel and the oil or fat is added. The system from here on is totally closed to the atmosphere to prevent the loss of alcohol. The reaction mix is kept just above the boiling point of the alcohol (around 160 °F) to speed up the reaction and the reaction takes place. Recommended reaction time varies from 1 to 8 hours, and some systems recommend the reaction take place at room temperature. Excess alcohol is normally used to ensure total conversion of the fat or oil to its esters. Care must be taken to monitor the amount of water and free fatty acids in the incoming oil or fat. If the free fatty acid level or water level is too high, it may cause problems with soap formation and the separation of the glycerine by-product downstream.

2) Separation

Once the reaction is complete, two major products exist: glycerine and biodiesel. Each has a substantial amount of the excess Methanol that was used in the reaction. The reacted mixture is sometimes neutralized at this step if needed. The glycerine phase is much denser than biodiesel phase and the two can be gravity separated with glycerine simply drawn off the bottom of the settling vessel. In some cases, a centrifuge is used to separate the two materials faster.

3) Alcohol Removal

Once the glycerine and biodiesel phases have been separated, the excess alcohol in each phase is removed with a flash evaporation process or by distillation. In other systems, the alcohol is removed and the mixture neutralized before the glycerine and esters have been separated. In either case, the alcohol is recovered using distillation equipment and is re-used. Care must be taken to ensure no water accumulates in the recovered alcohol stream.

4) Glycerine Neutralization

The glycerine by-product contains unused catalyst and soaps that are neutralized with an acid and sent to storage as crude glycerine. In some cases, the salt formed during this phase is recovered for use as fertilizer. In most cases the salt is left in the glycerine. Water and alcohol are removed to produce 80-88% pure glycerine that is ready to be sold as crude glycerine. In more sophisticated operations, the glycerine is distilled to 99% or higher purity and sold into the cosmetic and pharmaceutical markets.

5) Methyl Ester Wash

Once separated from the glycerine, the biodiesel is sometimes purified by washing gently with warm water to remove residual catalyst or soaps, dried, and sent to storage. In some processes this step is unnecessary. This is normally the end of the production process resulting in a clear amber-yellow liquid with a viscosity like Petro diesel. In some systems the biodiesel is distilled in an additional step to remove small amounts of colour bodies to produce a colourless biodiesel.

6) Product Quality

Prior to use as a commercial fuel, the finished biodiesel must be analysed using sophisticated analytical equipment to ensure it meets any required specifications. The most important aspects of biodiesel production to ensure trouble free operation in diesel engines are:

Methanol Rectification

Process Description

The Methanol rectification unit is designed to distillate water and Methanol starting from wet Methanol streams coming from upstream process units. Methanol vapor is recovered from the top of distillation column, condensed, and further cooled, while water is discharged from the column bottom to the recovery heat exchanger where it is cooled with the column feed. Both streams are then reprocessed into biodiesel plant.

Advantages

The advantages of Fostechno Methanol Rectification Unit are mainly two:

1) The recovered Methanol is so deeply dried that it is totally re-used as reagent of transesterification reaction with a consequent important reduction of its consumption as OSBL required chemical.

2) The distilled water contains Methanol in so negligible traces that it is totally recovered as process water to prepare citric acid solution reducing the OSBL utilities consumption. Distillation tray column design is well optimized to ensure a zero-waste unit for both liquid and gaseous effluents.