The feedstock comprising a mixture of various waste plant oils recovered from restaurants and other sources is fed into a vessel with metal filter to remove undesirable materials. The filtered oil accumulates at the bottom of the vessel and is transferred by a gear pump to the Feed Tank up to the level of about 80% of the tank. The Feed Tank is then heated to 500-600C. A pre-treatment agent and 5-10% crude glycerin are added to prevent solidification of the oil and hasten the precipitation and sedimentation of soap particles. The mixture is agitated for about 20 minutes and then settled for about one hour. The soap sediment is removed from the bottom of the tank and the pre-treated oil fed into the esterification reactor.
Methanol and caustic soda, as catalyst, are mixed together in the Methanol Tank. Caution is necessary to ensure that not too much catalyst is added since the reaction is exothermic. Normally, the amount of caustic soda is about 4.7% of that of methanol, which is sufficient to raise the temperature of the mixture in the Methanol Tank to 500C, still below the boiling temperature of methanol of 650C. This mixture is pumped to the continuous esterification reactor to mix and react with the pre-treated feed oil.
Feed oil is first preheated to 700C by passing it through a tube coil heater before it enters the esterification reactor. The preheated feed oil and the solution from the Methanol Tank are mixed through a line mixer and then reacted inside the continuous tube esterification reactor where the retention time is around 30 minutes and the temperature is kept under 600C. The reaction products are sent to the Glycerin Separator where crude glycerin, which contains residual soap and methanol, is separated. The crude biodiesel is sent to the Warm Water Tanks and the Warm Water Wash Tank for further treatment and refining.
The Warm Water Wash Tank is heated by a steam jacket and equipped with an agitator. Zinc sulfate is added to the crude biodiesel in the Warm Water Wash Tank to destroy the emulsion and precipitate residual soap. The mixture then goes to the Warm Water Separator where the impurities are separated from the supernatant crude biodiesel. At this point the crude biodiesel still looks whitish and muddy due to residual water contaminants in emulsion. This crude product is sent to the Settling Tank to remove the contaminants. The recovered water from the Settling Tank is treated under vacuum to remove contaminants and then recycled back into the Warm Water Wash Tank.
The crude biodiesel is further treated in the Crude Biodiesel Tank by the addition of 0.5% glycerin as de-watering agent and 0.3% activated white earth as sedimentation agent. The solution is agitated for 15 minutes and settled overnight. The sediments are removed from the bottom and the treated supernatant is further refined through filtration in the Filtration Tank. Further cleaning is attained by a second stage filtration in a Paper Filter equipped with a float type level regulator. The refined biodiesel product is then placed in regular drums ready for delivery and use.
Glycerin processing and recovery
The crude glycerin from the Glycerin Separator is stored in the Neutralization Tank, which is glass-lined and equipped with a closed-type agitator. The crude glycerin is heated under normal pressure to distill out the methanol, which is recovered from the condenser located at the top of the Neutralization Tank. The remaining crude glycerin is neutralized by the addition of hydrochloric acid and the residual soap is converted into oil, which is removed from the upper layer. Crude glycerin is taken from the bottom of the Neutralization Tank and pumped first to the Crude Glycerin Tank and then to the Crude Glycerin Concentrator for further treatment. The crude oil that is skimmed off from the Neutralization Tank is burned as fuel for heating the Vacuum Distillation Unit. The Crude Glycerin Concentrator is equipped with steam-heated jacket and vacuum ejector to remove water from crude glycerin.
The crude glycerin is then sent to a two-stage Vacuum Distillation Unit where dissolved solids such as sodium chloride are removed (first stage) and 99.5% glycerin with a light yellow color is produced. This product is further refined in the Activated Carbon Treatment Tank to remove residual color. The amount of activated carbon needed to produce a product of acceptable quality is 2-3% of the amount of glycerin. The activated carbon is mixed with glycerin for 15 minutes at a temperature of 900-1000C. The mixture is then passed through the Glycerin Filter to produce the final product of clear 99.5% glycerin.
The wastewater effluents from the Warm Water Separator and Warm Water Wash Tank, which contain soap, sodium soap, and residual biodiesel, are accumulated in the Waste Water Storage Tank. Once the level in the tank reaches a certain point, the wastewater is automatically pumped to the Acid Treatment Tank where dilute hydrochloric acid is added to keep the pH at 2-3. After mixing for about 10 minutes, the mixture is allowed to stand to separate the oily phase (upper layer) from the aqueous phase (bottom layer). The treated water is withdrawn from the bottom of the tank and sent to the pH Control Tank where the pH is automatically adjusted to around pH 9 with the addition of dilute caustic soda before being discharged to the public sewerage system. The oily materials from the Acid Treatment Tank and the pH Control Tank are recovered and used as heating fuel for the production of process steam.
Source: Tanimoto, S., Production Process of Vegetable Diesel Oil, Kanazawa Institute of Technology, Japan, 2004.