The Degumming of biodiesel feedstocks process is necessary prior to the esterification and transesterification processes. Typical crude degummed oils have had their Hydratable Phospholipids (HPLs) removed by a simple water wash process. We typically accomplish this at the crushers facility. However, a typical soybean oil contains both hydratable and non-hydratable phospholipids (NPL’s). The water wash process does not remove these NPLs and they remain in the oil. The typical concentration of NPLs in water degummed oil is 200 ppm.
The presence of these NPLs in the oil feed to the Transesterification may potentially cause two problems. First, the finished biodiesel may not pass the ASTM specification for phosphorous. Secondly, the phospholipids act as emulsifiers, and their presence can lead to an inefficient phase separation. This is turn leads to lost product in the glycerin phase, and excess soaps and glycerin in the biodiesel phase. For these reasons, a Degumming unit can be installed at the biodiesel plant.
2. Acid Degumming Section
Oil is pumped through a plate and frame heat exchanger where the temperature is heated up In the outlet piping, a mixture of 85% phosphoric acid/15% water is added to the oil. The acid concentration is to be 500 ppm. The acid and oil are mixed in a high shear mixer. Upon exiting the mixer deionized water may be added if necessary to maintain a good water balance. After the water addition, the oil flows to an agitated mix tank. There the residence time allows the acid to react with the NPLs, converting them to HPLs. Citric acid may also be used.
A very small quantity of 20% caustic solution is added to the oil through a static mixer. The caustic will assist in hydrolyzing the phospholipids. A very slight excess, approximately 50 ppm, is added just to be sure enough caustic has been added. This caustic will convert an equivalent amount of FFAs to soap.
This mixture flows to a surge drum for the centrifuge. In the centrifuge the water, gums and HPLs are removed as the heavy phase and the oil comes out as the light phase. The heavy phase flows into a collection tank and is pumped to the wastewater system. The oil undergoes further processing. At this point, the phosphorous level had been reduced from 200 ppm to 50 ppm.
An absorbent is added to the oil. The absorbent contains approximately 65% water. Remaining water must be removed before the absorbent is removed in a filter. Therefore, the oil flows to a vacuum dehydration oil drying system. This will reduce the water level from 2 – 3% to approximately 500 ppm.
The oil and absorbent is pumped to a vertical frame pressure filter. The phosphorous level in the finished oil is below 10 ppm. Solids from the filter are dumped into a hopper and can be land filled.
An alternative to the absorbent is a bleaching clay. However, the absorbent does not retain oil and does not become a flammable solid as with bleaching clays.
4. Safety System Considerations
The elimination of bleaching clays for purifying the oil results in a much safer materials handling operation. There have been numerous landfill fires where autooxidation occurs since the bleaching clays retain oil.