Soapstock acidulation

Vegetable oils are refined through pressing/extraction, degumming, alkali deacidification, decolorization, and deodorization. Alkali deacidification byproduces soapstock. Acidulation of soapstock prepares acid oil which contains FFAs, acylglycerols, and other lipophilic compounds. It is reproduced currently as FFAs, which are used as raw materials for production of soaps, lubricants, and paints. But the demand for FFAs is almost in saturation. Hence, conversion of acid oil to BDF is expected to avoid an oversupply of the industrial FFAs and subsequent price decrease. 

Sulfates are present from the use of sulfuric acid in the soapstock acidulation process. Concerns for sulfates are generated from their contribution to dissolved solids as well as their potential to form odor-causing compounds under anaerobic conditions. Sulfate levels in the model plant presented herein would be on the order of 2000 mg/L. Where controlled, regulatory limits are in the range of 200-300 mg/L. 

Hydrolyzed vegetable oil (HVO) or acid oil is a by-product of alkali refining of crude corn oil, and is obtained by acidulating alkaline soapstock. HVO must contain at least 92% total fatty acids. It is used to control dust and as an energy source in beef and poultry rations. 

Most manufacturers use soapstock to spray on meal for animal feed, or ship the material to acidulators. Some seed oil producers treat soapstock on site with sulfuric acid at a temperature of 90-95 °C to produce acidulated soapstock (Dijkstra and Segers, 2007). Acidulated soapstock is very dark in color with a strong, rancid, burned odor from the free fatty acids and neutral oils. Free fatty acid content varies and can be in excess of 90%. Moisture content as well as unsaponifiables can be substantial and the pH (based on samples provided to Stepan Company) may vary from 3 to 4.5. An example of a typical analysis of an acid oil sample is listed below (Table 6.2). 

Stirred, and a measured amount of sodium hydroxide solution is introduced by spraying on the surface of the oil. An excess of 5-10% sodium hydroxide over the stoichiometric requirement is added to ensure appropriate neutralization of free fatty acids. In this stage, hydrated gums migrate to the water phase. Heating and stirring are stopped when soap breaks are formed. A break forms as soap coagulates with some occluded neutral oil, excess sodium hydroxide, and other impurities. The aqueous soapstock is allowed to settle and subsequently drawn off for acidulation with sulfuric acid to recover a mixture of fatty acids, occluded neutral oil, and other impurities. The mixture is called acid oil. 

Soapstock contains fatty acid soaps and, for oil that is caustically refined, ory-zanol (5-10%). The soaps may be acidulated for feed use and the oryzanol isolated (16). Diethyl ether, alumina chromatography, and crystallization are used for purification of the oryzanol. 

Fluidization with phosphoric acid is not recommended because darkening of the product and hydrolysis may occur. Degumming with acetic anhydride results in fluidized lecithins possibly because PE is acetylated by the reagent. Nonedible lecithins may be fluidized by the addition of acidulated and dried soapstock. 

Acidulated soybean soapstock is the product obtained from the complete acidulation and thorough setting of soapstock, which itself is the by-product obtained from the alkali refining of soybean oil. It is sold on a basis of 95% total fatty acid content. If it falls below 85% total fatty acid content, it may be rejected. Typical analyses are TFAs, 90% moisture, 1% and iodine value 125. In practice, soybean soapstock may be found in combinations with other vegetable oil soapstocks. The buyer should determine if cottonseed soapstock is present, as it may contain gossypol, which is detrimental in nonruminant feeds. Physical properties are medium brown color odor somewhat typical of soybeans, slightly nutty solid when cool and liquid and pumpable at 38 14°C (100-110°F). It qualifies under AAFCO 33.3, IFN 4-17-893 (31). 

Acidulation of soapstock and washwater with 90% to 95% TFA recovery efficiency. 

Method 4 is directed at reducing water content of soapstock sold for acidulation elsewhere. The water phase generated in the decant can have as much or more waste load as that generated from a well-run acidulation plant. Figure 2 shows a relationship developed from one such operation. Note that some gain is reahzed by a moderate pH reduction however, in later phases, the operation essentially becomes an inefficient acidulation process. 

If the plant acidulates soapstock and/or washwater, the resulting acidulation waste will contain sufficient residual heat and acidity to affect the entire waste stream pH and temperature, which obviously assumes an alkali refining process rather than a physical refinery. This process will produce an acidulation effect on residual oils. These oils can then be readily removed in a gravity separation process. 

The process can be used at plants that do not acidulate soapstock however, site-specific smdies should be conducted to determine pollutant removals to be expected. In the overall scheme of things, acidulation of washwater as part of the treatment process will greatly improve effectiveness. 

Acidulation is one of the least desirable processes in the integrated facility. Not only is the process rather difficult to perform effectively, but it generally represents a cost without significant return. Figure 16 depicts a typical acidulation system based on gravity separation. The separations can be performed in either a continuous or batch operation. In operation, soapstock, discharge streams from tank farm collection systems, and other waste streams enter a equalization or holding tank. The facility may or may not add additional caustic at this point to saponify the... 

One problem with acidulation in the integrated facility concerns the emulsion tendencies of certain products, especially phosphatides removed as gums. As indicated earlier, if the gums are not removed from the soapstock stream, but are allowed to enter the acidulation system, a third phase may be evident in the settling tanks. This phase is extremely difficult to split into separate oil and aqueous phases and may require several passes through the system until the emulsion is broken. 

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