Separation of unsaturates

Abstract

 Compounds with two, three or more ethylenic unsaturations (polyunsaturates) are separated from compounds with fewer or no such unsaturations, by contacting a mixture of said compounds with a liquid entrapment medium selective for the polyunsaturates. The entrapment medium containing the polyunsaturates is contacted with an immiscible release medium taking up the polyunsaturates, and normally the polyunsaturates are recovered from the release medium. The entrapment medium and normally also the release medium may then be recycled.

Description

Field of the Invention

[0001] The invention relates to the separation of unsaturated organic compounds from other such compounds or from saturated compounds.

Background

[0002] The separation of unsaturated compounds from others, or more highly unsaturated compounds from less is important in many fields, particularly for natural products containing fatty acids. Here the separation is often difficult to achieve, compounds of importantly different nutritional or general biological properties having only marginally different physical properties. We have sought new approaches in terms of both solvent systems and of overall separation processes.

The Invention

[0003] The invention in one aspect lies in a process for fractionating a diverse range of unsaturated compounds. Examples are eicosanoids, tocopherols, tocotrienols etc. In particular doubly, triply or more highly ethylenically unsaturated compounds such as unsaturated fatty acids, fatty alcohols and the like (polyunsaturates) are separated from those with fewer ethylenic unsaturations or completely saturated, using a selective, liquid entrapment medium. The unsaturated fraction is extracted from the entrapment medium by contacting it with an immiscible release medium, and is normally then recovered from the release medium, though in principle the process can be two stage if for example the unsaturate is not the primarily desired product. The entrapment medium and normally also the release medium are recycled.

[0004] In another aspect the invention lies in the entrapment medium itself and its method of preparation. It also lies in the separated products obtained by the process, of which the unsaturates will normally be the more valuable product and indeed may be novel products in the sense of not having been prepared before in purified form. The fraction not selected by the entrapment medium is however equally a product of the process.

[0005] A particular application of the invention is to fractionate mixtures of polyunsaturated fatty acids and related compounds. The process can be applied successfully to the fatty acids themselves and to their derivatives such as salts, alkyl esters, mono-glycerides, di-glycerides, tri-glycerides, phospholipids and amides as well as to other compounds containing fatty acid carbon chains with unconjugated double bonds, such as the fatty acid alcohols. Mixed glycerides can be fractionated, in particular for example in recovery of the valuable triglyceride dilinoleoyl monogamma linolenoyl glycerol (DLMG) from triglyceride mixtures.

[0006] Considering entrapment media, the sulphones, particularly sulpholane, have an unusual combination of properties. Sulpholane, otherwise tetramethylene sulphone or tetrahydrothiophene-1,1-dioxide, is


is the most common but other available sulphones are 3-sulpholene, otherwise 2,5-dihydrothiophene-1, 1-dioxide


and acyclic compounds such as dimethyl sulphone


diiodomethyl p-tolyl sulphone


and di-(4-hydroxyphenyl) sulphone, otherwise Bisphenol S

[0007] Of these, sulpholane itself is known broadly for use in enriching the unsaturation level in fatty oils (Kirk Othmer, section on "Sulpholanes and Sulphones" p. 964, with references to U.S. Patent 2 360 860 (1944) and Wisniak Br. Chem. Eng. 15(1) 76 (1970).) It is the preferred entrapment medium for use in the process of the present invention.

[0008] The invention for the first time combines the selective solvency of sulphones 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 of unsaturated compounds. This two-component entrapment medium is the basis of a particularly valuable form of the current invention which provides a continuous, efficient, flexible process for the preparation of products of high quality and purity suitable for the nutritional and pharmaceutical industries.

[0009] Silver salts such as the nitrate dissolve readily in sulpholane in the presence of a proportion of water, for example in commercial grade sulpholane containing 3 wt% dionised water. Of metals forming pi-complexes with unsaturated compounds silver is however only the best known, and the use of salts of copper, gold or other metals with incomplete electron shells capable of the required pi-complexing is not excluded. Silver nitrate has a low solubility in anhydrous sulfolane, but a solution of 10g silver nitrate in 100 ml sulfolane water 95:5 by volume can for example be obtained, and similarly a solution of 20g silver nitrate in 100 ml sulfolane water 88:12.

[0010] The entrapment medium is thus particularly suitably made up of sulpholane, water and silver nitrate. Sulpholane is a viscous, high-boiling, non-toxic, dipolar, aprotic solvent which has selectivity for fatty acids and fatty acid esters depending upon the molecular weight and degree of unsaturation. Silver has been used in the chromatographic separation of unsaturated compounds. However no free solvent system which dissolves silver salts whilst having low solubility for saturated compounds has hitherto been found, and the technical and commercial demands associated with recovery, re-use and re-circulation of silver have remained unsatisfied.

[0011] The successful application and degree of selectivity of the process depends upon the partition coefficients of the target substance between the feed mixture and the entrapment medium and between the entrapment medium and the release medium. Favourable partition ratios for different target extractives can be obtained by adjusting the entrapment medium, for example as to the amount of water and silver salt used to make up the preferred silver containing sulpholane medium, and by selecting an appropriate release medium with the required solubilising and polarity properties.

[0012] The silver/sulpholane entrapment medium is very satisfactory in that it allows the separation of fatty acids containing two, three or more double bonds from those that are dienes, monoenes or saturates. Furthermore, a similar level of selectivity is obtained when the fatty acids are present in the much more complex and heterogeneous triglyceride form, of which natural vegetable-seed oils, marine fish oils and fungal biomass oils are composed. For example, in evening primrose oil mixed triglycerides such as DLMG containing one or more triply unsaturated gamma-linolenic acid moeities are selectively concentrated at the expense of triglyceride species containing various permutations of saturated, monoenoic and dienoic acyl groups.

[0013] An integral part of the preferred separation process is the use of the release medium to extract the target polyunsaturated substances from the liquid entrapment medium. The release medium has to be largely immiscible with the entrapment medium and has of course to dissolve the target polyunsaturated fraction; it should also desirably have a low boiling point for ready subsequent separation from the target fraction. Hydrocarbon solvents such as hexane, or petroleum hydrocarbon mixtures, or olefins such as cyclohexene, are suited to many applications of the process and can be selected to give enhanced extraction for a particular extractive by manipulation of partitioning behaviour. Extraction of polyunsaturated fatty acid species with hexane for example is essentially quantitative and the hexane is readily removed and recovered for re-use. The sulpholane/silver medium is also easily freed from excess hexane and is suited to be re-cycled directly.

[0014] Although sulpholane has been quoted as the solution component of the entrapment medium because it is commercially available and inexpensive, other sulpholane derivatives can also be used. Mixtures of sulphones with standard organic reagents such as acetone, ethanol and ethyl acetate can also be used successfully as the entrapment medium in the process.

[0015] The salt component of the entrapment medium need not necessarily be a nitrate. Other soluble salts, for example silver tetrafluoroborate and silver trifluoroacetate, can be used. The entrapment medium can also include other cations which form pi-complexes with unsaturated compounds.

[0016] Conveniently the process of this invention in preferred form may be set out as comprising the following stages:

A. Contacting a mixture to be separated with an entrapment medium in a multiphase system and allowing a target polyunsaturate from the mixture to migrate into the entrapment medium to form reversible pi-complexes therein;

B. Separating from each other a raffinate phase (containing non-polyunsaturates) and an extract phase containing the entrapment medium and target polyunsaturate;

C. Contacting the extract phase containing the entrapment medium and the target polyunsaturate with a release medium immiscible with the extract phase but taking up the target polyunsaturate;

D. Separating the release medium containing the target polyunsaturate from the entrapment medium, which is re-cycled; and

E. Recovering the release medium, which is re-cycled, leaving the polyunsaturate product.

[0017] The unit operations or stages in this process are common to many standard solvent extraction processes and may be carried out in any convenient manner, but the accompanying general flow diagram provides an illustrative example of the process.

[0018] The mixture to be separated is introduced through feed line 1 and admixed vigorously with the entrapment medium, introduced through a separate line 2, in the contact zone 3. Conveniently the ratio of medium to mixture is 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 comprise an impinging jet mixer, an agitation vessel, a centrifugal extractor, etc. and co-current and counter-current systems can be used.

[0019] In the contracting zone the mixture and the entrapment medium are caused to produce a supernatant raffinate phase and an extract phase which are separately withdrawn respectively through lines 4 and 5. The separation of these two phases may be effected by settling, decantation or centrifugation. The raffinate phase can be fed back to the contacting zone for re-partitioning to separate residual polyunsaturates from non-polyunsaturate species.

[0020] The extract phase containing the polyunsaturate fraction complexed with the entrapment medium is transferred through line 6 and partitioned against release medium (supplied via line 7) in the contact zone 8. The same sort of equipment as is indicated above can be used in this second extraction stage and also in the associated phase separation, although the operating conditions may have to be varied. Conveniently the ratio of releasing medium to entrapment medium/polyunsaturates complex is between 20:1 and 1:5 and contact time is between 20 seconds and 60 minutes at ambient temperature.

[0021] The extract phase and the raffinate phase are separately withdrawn respectively through lines 9 and 10. The raffinate from this second extraction stage is the entrapment medium and is fed directly back to process with no further treatment required, since the process can tolerate the small amount of release medium required to saturate the entrapment medium.

[0022] The extract phase containing the low boiling release medium and the product is transferred through line 1 to 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 to treat the release medium with brine occasionally to remove silver ions, easily recovered as silver chloride by filtration with subsequent removal of the water by centrifugation.

[0023] The following examples of particular separations illustrate the invention, Examples 1 and 2 being to preparation of entrapment media, the rest to separation processes of various kinds.

EXAMPLE 1

[0024] Silver nitrate (10g) was dissolved with heating to 70°C in water (5 ml) and sulpholane (95 ml) added with stirring to form a clear solution from which silver nitrate did not crystallise at room temperature. This stock solution is suitable for most separations and may be re-used more than twenty times in the following examples without degeneration or cross contamination. It is referred to as 95 SAg.

EXAMPLE 2

[0025] Silver nitrate (20g) was dissolved in 12 ml of water at 40°C and sulpholane (88 ml) added with stirring. This solution is referred to as 88 SAg, and is very stable. It is capable of being recycled many times with little fall off in selectivity and separation performance.

EXAMPLE 3

[0026] Refined evening primrose oil was taken, containing 8.2% gamma-linolenic acid in terms of its fatty acid composition and 13.8% of its triglyceride in the form of the isomers of DLMG, and 10g was added to 95 SAg (100 ml) and vigorously shaken for a total contact time of 1 minute. It was then allowed to separate into two distinct layers over a period of 5 minutes. The supernatant raffinate phase consisting of saturated and monoeroic fatty acid containing triglycerides was decanted. The bottom phase consisting of triglycerides containing polyunsaturated fatty acids in the entrapment medium was then contacted with 20 ml of the releasing medium hexane and shaken for 2 minutes. The two phases were allowed to settle for 10 minutes. The supernatant extract phase containing the oil was decanted. A second 20 ml of hexane was added to the 95 SAg phase and the mixture shaken for 2 minutes before allowing to settle 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 evening primrose oil remaining (2.05g) was enriched in the target polyunsaturated fatty acid gamma-linolenic acid by a factor of 2.9 i.e. the product contained 23.8% gamma-linolenic acid and 50% of its triglycerides as DLMG.

EXAMPLE 4

[0027] Unrefined sardine oil was converted into 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 0.03mm pressure to yield a colourless mixture of ethyl esters. A sample of this (10g) was contacted with 88 SAg (80ml) and shaken vigorously for 5 minutes and was then allowed to settle for 20 minutes. The raffinate was decanted and contacted a second time with the entrapment medium (40 ml), vigorously shaken for 5 minutes and allowed to settle for 20 minutes. Once the supernatant raffinate was decanted and discarded, the bottom layer was combined with the bottom layer from the first partitioning and separation. Cyclohexene (30 ml) was contacted with this combined entrapment media and shaken for 8 minutes. After a subsequent period of 15 minutes the two phases had completely separated and the supernatant releasing medium layer was decanted. The remaining bottom layer was then extracted twice again with 2 x 30 ml aliquots of cyclohexene. The three cyclohexene extracts were transferred to the solvent recovery vessel, where the solvent was removed under vacuum to yield a mixture of ethyl esters (2.42g) enriched in the target polyunsaturated fatty acids. Eicosapentaenoic acid had been concentrated from 14.8% to 33.4% and docosahexaenoic acid from 7.6% to 13.8%. The total of omega-3 polyunsaturated fatty acids had been increased from 24.8% to 52.9%.

EXAMPLE 5

[0028] An oil (125g) extracted from the cultured biomass of the fungus Mortierella alpina containing arachidonic acid (15%) in both phospholipid and triglyceride forms was vigorously mixed with one litre of 95 SAg containing 5% ethyl acetate. After 10 minutes shaking the two phases were allowed to separate for 30 minutes and the supernatant raffinate oil was decanted and discarded. The lower phase was contacted with 50 ml petroleum spirit as the releasing medium, mixed thoroughly for 20 minutes and allowed to settle for 60 minutes. The supernatant extract phase containing the polyunsaturated enriched oil was decanted and the petroleum spirit removed by vacuum evaporation. The fungal oil product was enriched in the target quadruply unsaturated arachidonic acid by a factor of 2.5 i.e. the oil contained 37.5% with a yield of 28% w/w.

EXAMPLE 6

[0029] Palm oil residue i.e. the material removed during the deodorisation stage of the refining operation was treated with acetone at low temperature to remove the major proportion of fatty acids and triglycerides as a crystal fraction by filtration. The filtrate contained largely sterols, tocopherols and tocotrienols and 10g of this was contacted with 95 SAg (120g) and shaken vigorously for 5 minutes. After allowing 15 minutes for the two phases to form, the upper raffinate phase was decanted. The lower phase was contacted with 250ml hexane, shaken for 10 minutes and allowed to settle for 20 minutes before decanting off the upper extract layer. This was then transferred to a rotary vacuum evaporator where hexane, the release medium was removed. The product (2.2g) contained 60% tocotrienols, compounds containing three double bonds on the phytyl side chain, of which the gamma species was the predominant. Furthermore, the product recovered from the raffinate above contained 50% tocopherols, similar compounds but containing no double bonds on the phytyl side chain.

Claims

1. A process for separation of compounds with two, three or more ethylenic unsaturations (polyunsaturates) from compounds with fewer or no such unsaturations, wherein a mixture of said compounds is contacted with a liquid entrapment medium selective for the polyunsaturates, the entrapment medium containing the polyunsaturates is contacted with an immiscible release medium taking up the polyunsaturates, and normally the polyunsaturates are recovered from the release medium, the entrapment medium and normally also the release medium then being recycled.

2. A process according to claim 1 wherein the polyunsaturates are in the naturally occurring triglyceride form as plant seed oils, marine fish oils or oils from fungal biomass or in derived forms such as fatty acids or fatty alcohols.

3. A process according to claim 1 or 2 wherein the entrapment medium is sulpholane or other sulphone, containing in solution silver or other metal ion pi-complexing with the polyunsaturate.

4. A process according to claim 3 wherein the sulphone contains a proportion of water enhancing the solubility of the metal ion.

5. As such, the metal-ion containing sulphone medium set out in claim 3 or 4.

6. The medium of claim 5, in use in any process for separation of compounds with two, three or more ethylenic unsaturations from compounds with fewer or no such unsaturations.

7. Polyunsaturates separated from compounds with fewer or no ethylenic unsaturations, or such compounds freed of polyunsaturates, by the process of any of claims 1 - 4 or 6.

Drawing