Patent GB-783310-A (GB)
Title: Process for the separation of polycarboxylic acids from mixtures containing them
IPC: C07C51/487
Filed: 1955-04-27
Abstract: Water-insoluble non-aromatic polycarboxylic acids are separated from mixtures containing them by extracting the polycarboxylic acid by means of a hot aqueous solution of a polycarboxylic acid salt in which the carboxyl groups of the acid from which it is derived are largely, and preferably completely, neutralized, and separating the dissolved polycarboxylic acid or acids from the extract.  This separation may be effected by cooling the solution whereby the free polycarboxylic acids crystallize out, or by precipitation with acids. The preferred polycarboxylic acids are those of the formula R(COOH)n in which R signifies an organic residue, preferably a hydrocarbon residue having 1 to 40, more preferably 5 to 35, carbon atoms, and n signifies an integer which is at least 2, and preferably from 2 to 7. The separation may be applied to mixtures of di- and polycarboxylic acids with monocarboxylic acids, hydrocarbons, alcohols, aldehydes, ketones and esters.  Such mixtures are obtained by oxidative breakdown of unsaturated fatty acids, e.g. by treatment with nitric acid, chromic acid or ozone, or by forming epoxy, dihydroxy or hydroxyhalogen derivatives of unsaturated fatty acids which may then be split, for example, by alkali fusion, into mixtures of mono- and polycarboxylic acids.  Mixtures of acids obtained by oxidation of hydrocarbons, or by catalytic addition of carbon monoxide and hydrogen to polyolefines, or unsaturated alcohols, aldehydes or carboxylic acids, are also suitable.  Tri-, penta- or heptacarboxylic acids may be separated from mixtures obtained by reaction of maleic anhydride with unsaturated fatty acids. Suitable salts for use as extractants are those of sodium and potassium, ammonium salts and salts of amines and alkylolamines such as mono-, di- or trimethylamine, mono, di or triethylamine, ethanolamines, propanolamines, tetramethyl ammonium hydroxide or tetraethylol ammonium hydroxide.  Mixtures of salts of different bases may be employed.  The polycarboxylic acids, the salts of which are used as extractants, may have a different molecular weight and a different number of carbon atoms from the polycarboxylic acids which are to be extracted.  When using as extractants the salts of polycarboxylic acids containing three or more carboxyl groups it is unnecessary to use completely neutralized salts; it is sufficient to neutralize said polycarboxylic acid to such an extent that an adequate solubility in water is attained.  This is usually the case when at least two carboxyl groups are neutralized.  The pH values of the extractant solutions usually lie within the range 7.5-4; excess of free bases should not be present.  The extraction of the polycarboxylic acids may be effected batchwise or continuously by bringing the mixture to be extracted into intimate contact with the extractant at raised temperature and separating the two phases, if desired by the assistance of a centrifuge.  The preferred temperatures are in the range of 50 DEG  to 100 DEG  C., but temperatures up to 250 DEG  C. may be used, and a pressure must be maintained in the extraction apparatus sufficient to keep the contents from boiling.  When separating the desired polycarboxylic acids by cooling the hot extract, the operation conditions, i.e. concentration, temperature change on cooling, and nature of extractant, are preferably chosen so as to prevent the extractant from separating with the polycarboxylic acid.  The process may be utilized for fractional extraction or separation of polycarboxylic acids.  Fractional extraction may be accomplished by repeated extraction of the mixture with fresh extractant or by changing the temperature during extraction. Fractional separation may be accomplished by cooling the hot extract slowly and separating the polycarboxylic acids deposited periodically.  The process may also be used to refine fatty acids containing dicarboxylic acids as impurities, e.g. commercial olein. The amount of polycarboxylic acid salt used in the extractant solution need not be accurately equivalent to the amount of polycarboxylic acid present.  Excesses up to 25 times the theoretical amount may be used, but a range of one to ten times the theoretical amount is suitable.  For fractional extraction it is possible to use as little as 0.2 time the theoretical amount.  In examples: (1) the crystallization of sebacic acid, decanedicarboxylic acid and brassylic acid from hot solutions of their monosodium salts is illustrated; (2) the crystallization of brassylic acid from a hot solution thereof in trisodium citrate is illustrated; (3) the crystallization of brassylic acid from a hot solution of the dimethylammonium salt is illustrated; (4) illustrates the crystallization of sebacic acid by cooling hot aqueous solutions containing sebacic acid with the disodium salts of either oxalic, succinic, adipic, phthalic or sebacic acids; (5) a brassylic acid-fatty acid mixture obtained by chromic acid oxidation of unsaturated fatty acids is treated with sodium hydroxide to form a small quantity of disodium salt, then the mixture is heated to 95 DEG and stirred whereby the disodium salt extracts some of the unchanged acid; the mixture is allowed to settle and the aqueous and oily phases are separated and on cooling the aqueous extract the free dicarboxylic acid crystallizes out and is filtered off; the solution of disodium salt obtained as filtrate is then used for further extractions of dicarboxylic acid; (6) a hot aqueous solution of disodium adipate is used to extract commercial oleic acid containing sebacic acid, the oily and aqueous phases are separated in a centrifuge and sebacic acid is precipitated by cooling the aqueous phase, the mother liquor therefrom being used for further extractions; (7) commercial olein containing dicarboxylic acids is extracted countercurrently with aqueous sodium oxalate solution at 90 DEG  C. in a centrifuge, and the aqueous extract is cooled to precipitate the dicarboxylic acids; the refined fatty acids are pratically free from dicarboxylic acids; (8) olein is similarly treated with aqueous disodium sebacate solution to extract sebacic acid therefrom.