Engineering R&D: Simplified food-oil refining
Sodium silicate commands a 50 to 100 percent price premium over sodium hydroxide, and processors need the equivalent to 0.5 to 1 percent of it to the volume of crude oil as a neutralizing agent, depending on the acidity of the vegetable oil. But greater yields, more marketable byproducts and lower maintenance and capital costs can more than offset sodium silicate’s price differential. The technology also opens the market to smaller operators who effectively are closed out by the high cost of commercial-scale centrifuges.
The technology was developed by Ernesto Hernandez, head of the fats and oils processing program at Texas A&M’s Food Protein Research and Development Center, and his former assistant, Steve J. Rathbone. Hernandez received his undergraduate degree in chemical engineering from the University of Guanajuato in Mexico, a Master’s in food technology from the University of Oregon and a Ph.D from University of Massachusetts-Amherst in food engineering. He has been affiliated with Texas A&M for 10 years. Food Engineering recently spoke with Dr. Hernandez about the process and the current state of the oil processing business.
FE: What is sodium hydroxide’s role in oil refining?
Hernandez: One of the first steps in the process is to remove impurities such as free fatty acids through an alkali process involving a solution of sodium hydroxide. Gums that are present also hydrate, and a soapstock is formed. The soapstock has to be removed before bleaching, dewaxing and deodorizing occurs, and this typically is done in a commercial centrifuge, which relies on differences in density and solubility to separate the soapstock. These centrifuges may process 100,000 pounds of oil an hour or more. The price and maintenance cost of this equipment is very high. Suppliers can make smaller centrifuges, but the price doesn’t go down in proportion to the size, so smaller-scale operators can’t compete economically.
FE: How does sodium silicate perform?
Hernandez: When sodium silicate is used, the fatty acids solidify and form a silicate gel, which has useful applications. Sodium silicate has been investigated before in oil processing, along with potassium hydroxide and other chemicals. What we have invented is the filtering technology to remove the gel. A leaf filter is the most efficient and automated way to accomplish this, though the process will work with a plate and frame filter, a rotary filter or any other kind of filter. Both separation and bleaching is done at once with filtration, which was a little bit of a surprise to us. Bleaching typically is an additional process. The silicate gel helps perform this step.
Another surprising thing is the scalability of the process. The filter we used in our pilot plant has an output of 100 pounds per hour. We’ve tested the process at a plant with a 100,000-pound per hour filter, and it worked very well.
FE: You recommend a processing temperature of 175° to 200°F. How does that compare to current processes?
Hernandez: Conventional processes do not go above 160º because there is a danger of deteriorating the oil with sodium hydroxide because it’s such an aggressive agent. Some inactivation of vitamin E occurs, even though it’s less susceptible to destruction than other nutrients. Polyunsaturated oils such as soy are very susceptible to deterioration from thermal treatments. With sodium silicate, that’s not an issue, even with the higher temperatures. That’s important: it’s estimated that vegetable oils deliver the majority of vitamin E intake for most people.
FE: How does the cost of this process compare to conventional methods?
Hernandez: We did a cost-benefit analysis, taking into account the elimination of the centrifuge and its maintenance and also factoring in the higher yields of saleable oil that are realized. About 2 percent of oil typically becomes entrained in the soap. This process can reduce those losses by 15 to 20 percent. The combination of these cost savings more than offsets the added cost of the sodium silicate.
FE: Besides this technology, what other options exist?
Hernandez: Physical refining is a technology that has been available for years and is beginning to be applied. It relies on distillation to remove acids. Europeans have no choice in finding an alternative to sodium hydroxide, and they are applying physical refining. But physical refining is more costly, and it requires very high temperatures that no oils can tolerate without nutritional degradation. Vitamin E is removed with physical refining.
FE: How do the byproducts of centrifugation and filtration compare?
Hernandez: Sodium hydroxide waste is very unfriendly to the environment and can cause problems in a landfill. Dealing with the waste is one of the biggest issues for oil companies. They’re going crazy trying to find outlets for these free fatty acids. The soap has energy value, and some of it is recycled as livestock feed. But the sodium hydroxide portion is harmful to the animals, and the big poultry producers are insisting that it be removed from their meal. The animals do better without it.
Sodium silicate is much less chemically active, and instead of a soap, the byproduct is a neutral gel. It is a very good pelletizing material, and some feed companies are using it for that purpose.
As an agglomeration agent, silicate reduces soap content to less than 80 parts per million, which is within many oil companies’ spec of less than 100 ppm before bleaching. Centrifuges leave considerably more, so to reduce levels, companies use water or silica gel, which of course is a byproduct of this technology.
FE: Filtration makes more sense for a new oil plant than one that already is committed to centrifuges. Are new plants being built?
Hernandez: Currently there is over-production of vegetable oil, so that discourages additional capacity. On the other hand, there are new opportunities for specialty oils and niche products like organic oils and non-GMO oils, products that didn’t exist until recently. People are diversifying, new products are appearing on shelves. That represents an opportunity for new companies. Sodium silicate and filtration can help them fill niches at a much lower capital cost.
FE: Are you optimistic that sodium silicate and filtration will be applied commercially in the next couple of years?
Hernandez: It’s already happening in small plants in El Salvador and Barbados, where the U.S. doesn’t have patent agreements. We train professionals from oil refining companies in three weeklong short courses each year. We discuss conventional processes as well as this technology and spend about 30 percent of our time applying them in the pilot plant.