Patent GB-791957-A (GB)
Title: Adipic acid crystallization
Filed: 1954-08-20
Abstract: <PICT:0791957/IV (b)/1> Adipic acid is crystallized from aqueous solutions thereof by including in the solution one or more saturated aliphatic monobasic carboxylic acids (other than adipic acid) said acid or acids containing from 2 to 8 carbon atoms, and conducting the crystallization at temperatures from 40 DEG  to 70 DEG  C., the solution containing at least 0.1 per cent by weight of the monobasic acid or acids or at least 1.0 per cent by weight of the dibasic acid or acids, said proportions being based on the total weight of the solution.  Nitric acid may also be present in the solution and in this event the proportion of monobasic acid should preferably be at least 2 per cent by weight of the total nitric acid present and the proportion of by-product dibasic acid should be at least 20 per cent by weight of the nitric acid present.  The process is particularly applicable to the crystallization of adipic acid from the crude reaction mixture obtained by air and subsequent nitric acid oxidation of cyclic hydrocarbons, particularly cyclohexane, the required monobasic and/or dibasic acids being in this case already present in the solution as by-products of the oxidation steps and such a mixture should preferably contain at least 0.5 per cent of the by-product monobasic acids and at least 8 per cent of the by-product dibasic acids and when nitric acid is present it should preferably be in a concentration between 25 and 45 per cent.  The crystallization may be carried out batchwise or in a continuous manner.  In the Figure a vessel (1) is saturated with adipic acid and the solution forced by pump (2) through filter (3) and rotameter (4) into the supersaturator (5) where the temperature is lowered to give the desired degree of supersaturation.  From vessel (5) the solution flows to the crystallization chamber (6) which contains a porous seed basket (7) in which the crystals are collected.  From the top of the crystallization chamber the mother liquor is returned to the saturator (1) by the pipe line (6a).  The valved pipe line (6b) and the valves shown enable the chamber (6) to be by-passed and the solution sent back to the saturator vessel (1).  The crystals collected in the seed basket can be weighed on scales (8) from time to time to determine the rate of crystallization.  The process may be operated by placing a saturated aqueous or aqueous nitric acid solution of adipic acid in the saturator (1) and a weighed quantity of adipic acid seed crystals in the seed basket (7) and determining the flow rate, liquor temperature in the crystallization chamber, and in the saturator and weighing the seed basket at intervals to determine the crystallization rate. The saturated solution of adipic acid is suitably prepared by saturating with adipic acid the nitric acid mother liquor left after crystallization of the first crop of adipic acid crystals from a product obtained from the air and nitric acid oxidation of cyclohexane as described in Specification 633,354 after stripping off the volatile matter, or by saturating a water mother liquor formed by dissolving in water the said first crop of adipic acid crystals and crystallizing adipic acid from that water solution. Data is tabulated to show the effect of the presence of various amounts of the by-product acids on suppressing the nucleation rate of supersaturated aqueous and aqueous nitric acid solutions of adipic acid.  Specified monobasic carboxylic acids which may be used are acetic, propionic, and the normal and branched chain butyric, valeric, caproic, heptoic and caprylic acids and specified dicarboxylic acids are oxalic, malonic, succinic, glutaric, pimelic, and suberic acids.  Specification 733,968, 745,034 and 745,063 also are referred to.