PRODUCTION: Production Process
There are three basic routes to BioDiesel production from oils and fats: (i) base catalyzed transesterification of the oil, (ii) direct acid catalyzed transesterification of the oil, and (iii) conversion of the oil to its fatty acids and then to BioDiesel. Most of the BioDiesel produced today is done with the base catalyzed reaction for several reasons, including it is low temperature and pressure, it yields high conversion (98%) with minimal side reactions and reaction time, it is direct conversion to BioDiesel with no intermediate compounds, and no exotic materials of construction are needed.
One hundred pounds of fat or oil (such as soybean oil) are reacted with 10 pounds of a short chain alcohol in the presence of a catalyst to produce 10 pounds of glycerin and 100 pounds of BioDiesel. The short chain alcohol is charged in excess to assist in quick conversion. The catalyst is usually sodium or potassium hydroxide that has already been mixed with methanol.
The base catalyzed production of BioDiesel generally occurs using the following steps:
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 fact 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 one to eight hours, and some systems recommend the reaction take place at room temperature. Excess alcohol is normally used to ensure total conversion of the fact 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 glycerin by-product downstream.
Separation - once the reaction is complete, two major products exist: glycerin and BioDiesel. Each has a substantial amount of excess methanol that was used in the reaction. The reacted mixture is sometimes neutralized at this step if needed. The glycerin phase is denser than the BioDiesel phase and the two can be gravity separated with the glycerin simply drawn off the bottom of the settling vessel. In some cases, a centrifuge is used to separate the two materials faster.
Alcohol Removal - once the glycerin 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 glycerin 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.