The separation of many classes of compounds by selective adsorption on molecular sieves or zeolites as well as other adsorbents is well known. Also, various separations based on the degree of unsaturation are known, e.g., esters of saturated fatty acids from unsaturated fatty acids with X or Y zeolites exchanged with a selected cation from U.S. Pat. No. 4,048,205, monoethanoid fatty acids from diethanoid fatty acids with cross-linked polystyrenes, e.g., "Amberlite" from U.S. Pat. No. 4,353,838. A process for separating a mixture of triglycerides, based on the iodine values, is shown in U.S. Pat. Nos. 4,277,412 and 4,284,580 in which permutite and aluminated silica gel adsorbents, respectively, can be used. Similarly, diglycerides have been separated from triglycerides with omega zeolites or silica as the adsorbents, as disclosed in Zinnen U.S. Pat. No. 4,770,819. The refining of oils by admixing them with magnesium silicate to adsorb coloring matter and free fatty acids from glyceride oils is disclosed in U.S. Pat. No. 2,639,289.
U.S. Pat. No. 4,056,468 discloses a combination process of adsorption of aqueous solutions on a silica gel concentration agent and subsequent liquid-liquid extraction of lipophilic-soluble components of the adsorbed species with a lipophilic solvent. Triglycerides and fatty acids are among the lipophilic-soluble materials disclosed that can be isolated from aqueous solutions; however, it is not apparent from the disclosure that fatty acids can be separated from triglycerides by the process. Furthermore, the disclosure relates to analytical separations not suited for continuous bulk separations.
In U.S. Pat. No. 4,877,765, acid-treated amorphous silica was used to remove phospholipids and chlorophyll from glyceride oils as a method of purifying glycerides. There is no teaching of the separation of fatty acids from triglycerides with silica gel.
The use of silica gel in analytical chromatographic separations with various solvent systems is known. Particle sizes of silica gels used in analytical separations ranges from 5 to 50 microns. Also, the removal of various impurities from mixtures including triglycerides is known. However, the usefulness of silica gel as an adsorbent for a bulk separation of fatty acids from triglycerides has not been disclosed or demonstrated.
Illustrative of the analytical separations is Duthic et al, J. Chromatog., 51(2) (1970) pages 319-21 in which fatty acids are isolated from triglycerides with a solvent system of hexane/ethyl acetate/formic acid on plates of silica gel G and developed with sulfuric acid followed by charring in an oven at 120.degree. C. Also, silica gel was utilized in separating polar compounds from non-polar compounds to analyze frying fats according to a report by Wessels, Pure and Applied Chemistry, 55(8) (1983), pages 1381-85. See also Tanaka et al, Lipids 15(10) (1980) pages 872-875.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Patent 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,642,397 is incorporated herein.
I have found an adsorbent, which, in combination with certain desorbent liquids, will selectively adsorb all the fatty acids, mono- and diglycerides and impurities contained in various triglyceride feed material; the triglycerides are relatively non-adsorbed and elute as a class near the void. Thus, the largest component of the feed, the triglycerides are eluted as raffinate and the minor components are adsorbed and eluted as extract by desorption with the desorbent. This so-called rejective separation of the major component is desirable since utilities are lower and adsorbent capacity for the adsorbed components, is lower per unit of output product.
I have discovered a method for separating fatty acids, including mixtures of unsaturated and saturated fatty acids, as a class, from triglycerides. The triglycerides also may be a mixture of triglycerides, including saturated, monounsaturated and polyunsaturated.