GB767290A - Production of aliphatic carboxylic acids - Google Patents

Abstract

Lower aliphatic carboxylic acids are obtained by oxidizing at elevated temperature in the liquid phase a paraffin hydrocarbon of 4 to 8 carbon atoms with a gas consisting of or comprising molecular oxygen to produce lower aliphatic acids of 1 to 4 carbon atoms, distilling the <PICT:0767290/IV(b)/1> oxidation product to separate substantially all of the volatile materials from the high boiling residues and then oxidizing said high boiling residues at elevated temperature in the liquid phase with a gas consisting of or comprising molecular oxygen to produce lower aliphatic acids of 1 to 4 carbon atoms. The volatile materials are defined as those materials boiling below 170 DEG C. at atmospheric pressure or its equivalent at other pressures whilst the high boiling residues are defined as those boiling above this temperature. The high boiling residues may be mixed with a paraffin hydrocarbon of 4 to 8 carbon atoms and the mixture oxidized. Thus, they may be recycled to the primary oxidation along with the paraffins charged to the oxidation and the residues from the distillation of the resulting oxidation product may be cooled to permit the separation of succinic acid and the latter separated prior to recycle of the liquid residues to the primary oxidation zone along with the fresh hydrocarbon feed. The distillation of the oxidation products may be carried out in two or more columns, thus the non-acidic volatile materials, e.g. those materials boiling below 99 DEG C. in the presence of water may be removed in a first column and the aqueous aliphatic acids in a second column. In view of the azeotroping action of the water present in the oxidation product the separation of the aqueous aliphatic acids from the high boiling residues which have boiling points above 170 DEG C. need not necessarily be carried out by distilling up to a limit of 170 DEG C. since in practice all the aqueous acids can be separated by the use of distillation temperatures of about 150 DEG C. The hydrocarbon employed is preferably a paraffinic hydrocarbon fraction containing hydrocarbons of 4 to 8 carbon atoms, at least 40 per cent by weight of said fraction consisting of C6-C8 hydrocarbons, wherein at least 40 per cent by weight of the C6-C8 paraffins consists of paraffins having one or more methyl branch chains and wherein said fraction boils at a temperature not exceeding 100 DEG C. Suitable feedstocks are straight run petroleum fractions containing a high proportion of branched molecules, the products from the isomerisation of predominatly straight-chain paraffins of 4 to 8 carbon atoms by aluminium chloride, the products obtained by the alkylation of C2-C4 olefins with isoparaffins, or the products obtained by hydrogenation of the mono-olefines produced by dimerisation of lower olefines such as propylene and isobutene. Part of the molecular oxygen used in the oxidation may, if desired, be in the form of ozone and suitable oxidation temperatures are 150-250 DEG C. The oxidations may be carried out in the presence of an oxidation catalyst, e.g. a heavy metal compound, and the oxidation of the high boiling residues may be carried out batchwise or continuously. In examples: (1) a 15-95 DEG C. paraffinic distillation fraction from crude petroleum is oxidized in the liquid phase with air at 160 DEG C. and the product distilled to yield as successive fractions volatile non-acidic products boiling below 99 DEG C. in the presence of water, lower aliphatic acids and water, and high boiling residues boiling above 170 DEG C. The high boiling residues are then oxidized with air at 160-190 DEG C. and the products distilled to yield formic acid, acetic acid, propionic acid, and butyric acid; (2) fresh hydrocarbon is introduced continuously by line (3) (see Figure) through a preheater 13 and introduced into a reactor (1) having air feed points (2) and operated at 170 DEG C. and 600 pounds per square inch pressure. The gaseous and liquid products are removed through line (4), passed to cooler (5) where they are cooled to below 80 DEG C. and are then passed to a gas-liquid separator (6) from which the liquid products are passed to a liquid-liquid separator (8). The lower aqueous acid layer is withdrawn, at least in part, through line (9) and fed via a preheater (15) to a distillation column (11) having a temperature of about 66 DEG C. at the head, about 82 DEG C. at the feed point and 104-105 DEG C. in the reboiler. The distillate is returned by line (12) for admixture with the feed to the reactor (1). The base product is passed by line (14) to a second distillation column (15A) in which the temperature is about 102-105 DEG C. at the head, about 120 DEG C. at the feed point, and 150 DEG C. in the reboiler. The distillate withdrawn by line (16) comprises the aqueous aliphatic acid product whilst the higher boiling residues are withdrawn by line (17), cooled in cooler (18), filtered in filter (19) to remove the precipitated succinic acid and the filtrate is returned for admixture with the feed to reactor (1). The upper layer from separator (8) comprising hydrocarbon, together with part of the lower layer if desired, is also returned to reactor (1) through the preheater (10). The product recovered through line (16) comprises formic acid, acetic acid, propionic acid, and butyric acid. Specification 743,990 is referred to and Specification 743,989 is referred to in the Provisional Specification.