NO300550B1 - Process for separating compounds of multi-unsaturation from compounds of minor unsaturation - Google Patents

Description

The invention relates to a method for separating compounds with 2,3 or more ethylenic unsaturations from compounds with fewer or no such unsaturations.

Separation of unsaturated compounds from others, or higher unsaturated compounds from less unsaturated ones, is important in many areas, especially for natural products containing fatty acids. Here, it is often difficult to achieve separation, since compounds with different important nutritional properties or general biological properties have only marginally different physical properties. We have searched for new methods both with regard to solvent systems and with regard to the total separation processes.

The method according to the invention is characterized in that a mixture of the aforementioned polyunsaturated compounds is brought into contact with a liquid capture medium which is selective for the polyunsaturated compounds, the capture medium being sulfolane or other sulfone, which in solution contains silver or other metal ions in pi complex with the polyunsaturated compounds, the capture medium containing the polyunsaturated compounds is contacted with an immiscible release medium which takes up the polyunsaturated compounds, and, usually, the polyunsaturated compounds are recovered from the release medium, and then the capture medium and usually also the release medium are recycled.

The invention is directed to the fractionation of unsaturated compounds within a diverse range. Examples are eicosanoids, tocopherols, tocotrienols, etc. In particular, doubly, triply or even higher ethylenically unsaturated compounds, such as unsaturated fatty acids, fatty alcohols and the like (polyunsaturated compounds), are separated from those that have less ethylenic unsaturation or are completely saturated, using a selective, liquid capture medium. The unsaturated fraction is extracted from the capture medium by bringing it into contact with an immiscible release medium, and is usually recovered from the release medium, although the process can in principle have two steps if the unsaturated material, for example, is not the primarily desired product. The capture medium and usually also the release medium are recycled.

A particular embodiment of the invention is to fractionate mixtures of polyunsaturated fatty acids and related compounds. The method can be successfully carried out with the fatty acids themselves and with derivatives thereof, such as salts, alkyl esters, monoglycerides, diglycerides, triglycerides, phospholipids and amides, and also other compounds containing fatty acid carbon chains with unconjugated double bonds, such as fatty acid alcohols. Mixed glycerides can be fractionated, for example particularly in the recovery of the valuable triglyceride-dilinoleoyl-monogamma-linolenoyl-glycerol (DLMG) from triglyceride mixtures.

With regard to capture media, the sulfones, especially sulfolane, have an unusual combination of properties. Sulfolane, also called tetramethylene sulfone or tetrahydrothiophene-1,1-dioxide, is

the most common, but other available sulfones are 3-sulfolene, also called 2,5-dihydrothiophene-1,1-dioxide and acyclic compounds such as dimethyl sulfone, diiodmethyl-p-tolyl sulfone, and di-(4-hydroxyphenyl) sulfone, also called bisphenol S

Of these, sulfolane itself is widely known for use in enriching the level of unsaturation in fatty oils (Kirk Othmer, section on "Sulpholanes and Sulphones", p. 964, with references to U.S. Patent No. 2,360,860 (1944 ) and Wisniak Br. Chem. Eng. 15(1) 76 (1970)). This is the preferred capture medium for use in the method according to the present invention.

The invention combines for the first time the selective dissolving ability of sulfones for unsaturated fatty acids with the known ability of silver salts and other •type-b' cations to form reversible pi-complexes with the double bonds in unsaturated compounds. This two-component capture medium is the basis for a particularly valuable aspect of the present invention which provides a continuous, efficient, flexible method for the production of products of high quality and purity which are suitable for the food industry and the pharmaceutical industry.

Silver salts, such as the nitrate, readily dissolve in sulfolane in the presence of a proportion of water, for example in commercial grade sulfolane containing 3% by weight of deionized water. Of metals that form pi-complexes with unsaturated compounds, silver is the best known, but the use of salts of copper, gold or other metals with an incomplete electron shell capable of providing the necessary pi-complex formation is not excluded. Silver nitrate has low solubility in anhydrous sulfolane, but a solution of 10 g of silver nitrate in 100 ml of sulfolane water 95:5, based on volume, can be obtained, for example, and similarly a solution of 20 g of silver nitrate in 100 ml of sulfolane water 88:12.

The capture medium is thus particularly suitably constituted by sulfolane, water and silver nitrate. Sulfolane is a viscous, high-boiling, non-toxic, dipolar aprotic solvent that has selectivity for fatty acids and fatty acid esters depending on the molecular weight and degree of unsaturation. Silver has been used in the chromatographic separation of unsaturated compounds. However, so far no free solvent system has been found which dissolves silver salts and at the same time has the ability to dissolve saturated compounds, and the technical and commercial needs associated with the recovery, reuse and recycling of silver are still not satisfied.

A successful performance and degree of selectivity of the method depends on the partition coefficients of the target substance between the feed mixture and the capture medium and between the capture medium and the release medium. Beneficial partitioning ratios for different target extracts can be achieved by adjusting the capture medium, for example, to the amount of water and silver salt used to form the preferred silver-containing sulfolane medium, and by selecting a suitable release medium with the required solubility and polarity properties.

The silver/sulfolane capture medium is very satisfactory in that it enables the separation of fatty acids containing two, three or more double bonds from such compounds which are dienes, monoenes or saturated. Furthermore, a similar level of selectivity is achieved when the fatty acids are present in the much more complex and heterogeneous triglyceride form, from which natural plant seed oils, marine fish oils and fungal biomass oils are formed. In evening primrose, for example, oil-mixed triglycerides, such as DIiMG, which contain one or more triply unsaturated gamma-linolenic acid moieties, are selectively concentrated at the expense of triglyceride types containing various permutations of saturated, monoenic and dienic acyl groups.

An integral part of the present separation method is the use of the release medium to extract the polyunsaturated target substances from the liquid capture medium. The release medium must be largely immiscible with the capture medium and must of course dissolve the polyunsaturated target fraction. It should also preferably have a low boiling point so that it can then be easily separated from the target fraction. Hydrocarbon solvents, such as hexane, or petroleum-hydrocarbon mixtures, or olefins such as cyclohexene, are suitable for many applications of the process, and can be chosen to provide increased extraction of a particular extract by manipulating the partitioning behavior. Extraction of polyunsaturated types of fatty acids with hexane, for example, is essentially quantitative, and the hexane is easily removed and recovered in order to be used again. The sulfolane/silver medium is also easily freed of excess hexane, and is suitable for direct recycling.

Although sulfolane is listed as a solvent component in the capture medium because it is commercially available and inexpensive, other sulfolane derivatives may also be used. Mixtures of sulfones with organic standard reagents such as acetone, ethanol and ethyl acetate can also be successfully used as capture medium in the method.

The salt component in the capture medium need not necessarily be a nitrate. Other soluble salts, for example silver tetrafluoroborate and silver trifluoroacetate, can be used. The capture medium may also include other cations that form pi complexes with unsaturated compounds.

The method according to this invention may conveniently be stated in preferred form to comprise the following steps: A. contacting a mixture to be separated with a capture medium in a multiphase system and allowing a polyunsaturated target product from the mixtures to migrate into the capture medium to form reversible pi complexes therein, B. separate from each other a raffinate phase (containing non-polyunsaturated compounds) and an extract phase containing the capture medium and the target polyunsaturated product, C. bring the extract phase containing the capture medium and the target polyunsaturated product into contact with a release medium that is immiscible with the extract phase, but which takes up the polyunsaturated target product, D. separate the release medium containing the polyunsaturated

target product from the capture medium, which is recycled, and

E. recover the release medium, which is recycled, leaving the polyunsaturated product.

The unit operations or steps of this process are common to many standard solvent extraction processes and may be performed in any convenient manner, but the accompanying general flow diagram provides an illustrative example of the process.

The mixture to be separated is introduced through tube 1 and mixed vigorously with the capture medium, introduced through a separate tube 2, into contact zone 3. The ratio of the medium to the mixture is conveniently between 1:2 and 20:1, and the contact time is between 20 seconds and 100 minutes at temperatures between -80°C and 90°C. Suitable equipment for carrying out this liquid-liquid extraction may include an impact-jet mixer, an agitator vessel, a centrifugal extractor, etc. and it may be used co-currently or counter-currently systems.

In the contact zone, the mixture and the capture medium are brought to form an upper floating raffinate phase and an extract phase which is drawn out separately through the respective pipes 4 and 5. The separation of these two phases can be done by sedimentation, decantation or centrifugation. The raffinate phase can be fed back to the contact zone for re-separation to separate remaining polyunsaturated compounds from non-polyunsaturated species.

The extract phase containing the polyunsaturated fraction complexed with the capture medium is transferred through pipe 6 and distributed towards the release medium (supplied through pipe 7) in the contact zone 8. The same type of equipment as indicated above can be used in this second extraction step and also in the accompanying phase separation, although the operating conditions can be varied. The ratio of the release medium to the capture medium/polyunsaturated complex is conveniently between 20:1 and 1:5, and the contact time is between 20 seconds and 60 minutes at ambient temperature.

The extract phase and the raffinate phase are withdrawn separately through the respective tubes 9 and 10. The raffinate from this second extraction step is the capture medium, and it is fed directly back to the process without further treatment being necessary, due to the small amounts of release medium required for saturating the capture medium can be tolerated by the process.

The extract phase containing the low-boiling release medium and the product is transferred through pipe 1 to the solvent recovery system where the release medium is removed, condensed and collected. The product can be treated by short path distillation to remove all traces of the release medium. It may be necessary occasionally to treat the release medium with saline to remove silver ions, which are readily recovered as silver chloride by filtration followed by removal of water by centrifugation.

The following examples of special separations illustrate the invention. Examples 1 and 2 are the production of capture media, and the rest are separation processes of various types.

EXAMPLE 1

Silver nitrate (10 g) was dissolved under heating to 70°C in water (5 ml) and sulfolane (95 ml) was added with stirring to form a clear solution from which no silver nitrate crystallized at room temperature. This stock solution is suitable for most separations and can be reused more than 20 times in the following examples without degeneration or cross-contamination. It is referred to as 95 SAg.

EXAMPLE 2

Silver nitrate (20 g) was dissolved in 12 ml of water at 40°C and sulfolane (88 ml) was added with stirring. This solution is referred to as 88 SAg, and it is very stable. It is capable of being recycled many times with little loss in selectivity and separation performance.

EXAMPLE 3

Purified evening primrose oil containing 8.2% gamma-linolenic acid expressed by its fatty acid composition and 13.8% of its triglyceride in the form of the isomers of DLMG was used, and 10 g was added to 95 SAg (100 ml) and it was shaken vigorously for a total contact time of 1 minute. It was allowed to separate into two distinct layers over a period of 5 minutes. The upper-floating raffinate phase consisting of saturated and monoenic fatty acid containing triglycerides was decanted. The bottom phase consisting of triglycerides containing polyunsaturated fatty acids in the capture medium was then brought into contact with 20 ml of the release medium hexane and was shaken for 2 minutes. The upper floating extract phase containing oil was decanted. A second addition of 20 mL of hexane was added to the 95 SAg phase and the mixture was shaken for 2 minutes before being set aside for phase separation. The supernatant hexane extract was combined with the first extract and transferred to a solvent recovery vessel where the hexane was removed. The remaining evening primrose oil (2.05 g) was enriched in the intended polyunsaturated fatty acid gamma-linolenic acid by a factor of 2.9, i.e. that the product contained 2 3.8% gamma-linolenic acid and 50% of its triglycerides as DLMG.

EXAMPLE 4

Unrefined sardine oil was converted to its fatty acid ethyl esters by transesterification with sodium ethoxide in ethanol at 60°C. The crude ethyl esters were purified by thin film evaporation at 130°C and a pressure of 0.03 mm to give a colorless mixture of ethyl esters. A sample of this (10 g) was contacted with 88 SAg (80 ml) and shaken vigorously for 5 minutes, then allowed to settle for 20 minutes. The raffinate was decanted and contacted a second time with the capture medium (40 mL), shaken vigorously for 5 minutes and allowed to settle for 20 minutes. Once the overhead raffinate was decanted and discarded, the bottom layer was combined with the bottom layer from the first split and separation. The cyclohexene (30 mL) was contacted with these combined capture media and shaken for 8 minutes. After a subsequent period of 15 minutes, the two phases were completely separated and the upper floating layer of release medium was decanted. The remaining bottom layer was then again extracted twice with 2 x 30 ml aliquots of the cyclohexene. The three cyclohexene extracts were transferred to the solvent recovery vessel where the solvent was removed under vacuum to give a mixture of ethyl esters (2.42 g) enriched in the intended polyunsaturated fatty acids. Eicosa-pentaenoic acid was concentrated from 14.8 to 33.4% and docosahexaenoic acid from 7.6 to 13.8%. The total amount of omega-3 polyunsaturated fatty acids was increased from 24.8 to 52.9%.

EXAMPLE 5

An oil (125 g) extracted from cultivated biomass of the mushroom Mortierella alpina containing arachidonic acid (15%) in both

phospholipid bg triglyceride form was vigorously mixed with 1 liter of 95 SAg containing 5% ethyl acetate. After shaking for 10 minutes, the two phases were allowed to separate for 30 minutes, and the overhead raffinate oil was decanted and discarded. The lower phase was contacted with 50 ml of light petrol as a release medium, mixed thoroughly for 20 minutes and allowed to settle for 60 minutes. The overhead extract phase containing the polyunsaturated enriched oil was decanted and the light gasoline removed by vacuum evaporation. The fungal oil product was enriched in the intended quadruple unsaturated anachidiconic acid by a factor of 2.5. That is, the oil contained 37.5% with a yield of 28% by weight.

EXAMPLE 6

Palm oil residues, i.e. material removed during the deodorization step of the purification operation was treated with acetone at low temperature to remove the bulk of the fatty acids and triglycerides as a crystal fraction by filtration. The filtrate contained mostly sterols, tocopherols and tocotrienols, and 10 g of this was brought into contact with 95 SAg (120 g) and shaken vigorously for 5 minutes. After allowing the two phases to form for 15 minutes, the upper raffinate phase was decanted. The lower phase was contacted with 250 ml of hexane, shaken for 10 minutes and allowed to settle for 20 minutes before decanting the upper extract layer. This was then transferred to a rotary vacuum evaporator where the hexane, the release medium, was removed. The product (2.2 g) contained 60% tocotrienols, compounds containing three double bonds on the phytyl side chain, of which the gamma species were the predominant ones. Furthermore, the product recovered from the above raffinate contained 50% tocopherols, which are similar compounds, but without the content of double bonds on the phytyl side chain.

Claims (3)

1. Process for separating compounds with two, three or more ethylenic unsaturations (polyunsaturated compounds) from compounds with fewer or no such unsaturations, characterized in that a mixture of said compounds is brought into contact with a liquid capture medium which is selective for the polyunsaturated compounds , the capture medium being sulfolane or another sulfone, which in solution contains silver or other metal ions in pi-complex with the polyunsaturated compounds, the capture medium containing the polyunsaturated compounds is brought into contact with an immiscible release medium which takes up the polyunsaturated compounds, and, usually, the polyunsaturated compounds are recovered from the release medium, and then the capture medium and usually also the release medium are recycled. 2. Method according to claim 1, characterized in that the polyunsaturated compounds are present in the naturally occurring triglyceride form such as plant seed oils, marine fish oils or oils from fungal biomass, or in derived forms such as fatty acids or fatty alcohols. 3. Method according to claim 1, characterized in that the sulfone contains a proportion of water which increases the solubility of the metal ion.