Feedstocks with high Free Fatty Acids cannot go directly to transesterification; first you must go through the esterification process. This is because the transesterification catalyst sodium methylolate reacts with the free fatty acids and forms soap. Instead, esterification of the free fatty acids is accomplished in two catalyst beds in series, with solid catalysts. The catalyst is a proven ion exchange resin. A byproduct water/ methanol stream is produced from this process. Traditional liquid sulfuric acid catalyzed esterification technology uses both acid and then 25% to 50% sodium hydroxide solution to neutralize the spent acid. In addition, emulsions from this sulfuric acid technology are not created by the this process with a solid catalyst.
Esterification 1st Stage
Double degummed oil, yellow grease or any high FFA feed stock flows to a solid catalyzed double bed process that utilizes an ion exchange resin for the catalyst. This process uses a fixed bed configuration with downward flow. A typical design basis is 20% FFAs with the balance being triglycerides with small quantities of water in the ppm range. Double degummed oil, yellow grease or any high FFA feedstock is fed from the esterification feed tank through a pump to a shell and tube heat and raised to the operating temperature. The methanol is also fed from the methanol feed tank (not shown) through pump P-202 to the same shell and tube heat exchanger, and combined with the oil and on to the guard bed (not shown) and the first catalyst bed in a proprietary molar ratio of methanol to FFAs. The guard bed contains the same catalyst and essentially extends the life of the first bed by removing impurities. This means that every feed tank in bulk storage must be tested for FFA content prior to adjusting the methanol feed ratio in the human machine interface in the control room. For feeds higher than 20%, FFAs can be blended with low FFA feedstocks to get below the maximum 20% FFA content. An example of this would be that Palm Fatty Acid Distillate (PFAD) at 90% FFAs can be mixed with RBD soybean oil with essentially no FFAs in a ratio of 100 lbs PFAD to 350 lbs of RBD soybean oil. The second option is to reduce the feed rate to the esterification unit. After the passing through the first bed 90% conversion is achieved leaving 2% FFAs in the reactor effluent. This material is cooled through a heat exchanger and sent to a settling chamber or decanter, to remove the excess methanol and the water of reaction. The water methanol mixture is pumped from the decanter through a pump to the Methanol distillation column feed drum and the methanol distillation column for further processing and recovery of the methanol for recycling. This system will be shown in part 5.
Esterification 2nd Stage
Oil, biodiesel, yellow grease or any high FFA feedstock with remaining FFAs is fed from the settling chamber or decanter, through a pump to another shell and tube heat exchanger and raised to the appropriate operating temperature. Additional methanol is also fed from the methanol feed tank through a pump to the same shell and tube heat exchanger, and combined with the oil and raised to the appropriate operating temperature and on to the second catalyst bed. After the passing through the second bed 90% conversion is achieved again leaving 0.2% FFAs in the reactor effluent. This material is cooled through exit heat exchanger and sent to a second settling chamber or decanter to remove the excess methanol and the water of reaction. The water methanol mixture is pumped from the decanter to the methanol distillation column feed drum (not shown) and the methanol distillation column for further processing and recovery of the methanol for recycling. The FFA free oil now contains biodiesel and triglycerides and is pumped by to a feed drum in the transesterification section of the plant for further processing.