Patent GB-628034-A (GB)
Title: Improvements in or relating to the oxidation of aldehydes and ketones
Filed: 1944-12-08
Abstract: A continuous process for the oxidation of autoxidizable aldehydes and ketones, in the liquid phase with molecular oxygen under super-atmospheric pressure, comprises the introduction of a gas comprising molecular oxygen into a liquid comprising said aldehyde or ketone, pumped through a reactor, in the form of small bubbles to form an emulsion of said gas in the liquid, wherein the emulsion is passed through the reactor at a rate such that a state of turbulence is created in the reactor so as to maintain the gas in the dispersed condition in the liquid and to avoid the formation of a continuous gas phase in the reactor and such that at the point of gas or emulsion entry into the reactor there is a back pressure which is greater than the hydrostatic pressure at that point and is at least one atmosphere above the pressure at the point of emulsion discharge from the reactor.  The reactor may be a tube whose length is many times its diameter, the dispersed condition of the gas phase being maintained by controlling the p  rate of flow of the liquid through the tube.  In a modification of the process, the state of turbulence is created by means of an agitator mechanism; thus, the reactor may be a cylindrical vessel which is completely filled with the gas-liquid raction mixture, the dispersed condition of the gas-phase therein being maintained by means of agitator-mechanism so that no continuous gas-phase exists within the reactor and the gas-liquid mixture is continuously forced through the reactor and discharged from a restricted opening at such a rate as to provide the required back-pressure at the point of entry of the gas to the reactor. Generally, the oxidation is effected by a continuous recirculating process; the emulsion is discharged into a separator, at least a part of the liquid being withdrawn for recovery of the oxidation products and the remainder recycled. According to a preferred method, the oxidizing gas and liquid containing the aldehyde or ketone are passed continuously and simultaneously through a tube of internal diameter not greater than two inches, which is externally cooled, at such rates that in unit time the volume of oxidizing gas fed to the tube at the prevailing pressure is less than that of the liquid feed, and such that the volume of liquid fed to the tube is not less than that amount which, in the absence of the gas, would have a linear velocity in the tube of 7 feet per second, and continuously discharging the resulting intimate mixture of gas and liquid into a separator vessel, wherein the gas and liquid separate at a pressure below that in the tube, normally at substantially atmospheric pressure, and continuously recycling the reaction liquid whilst withdrawing a part thereof for recovery of the oxidation products.  If desired, the intimate mixture of gas and liquid may be passed down the tube.  The oxidizing gas is, preferably, oxygen containing not more than 20 per cent by volume of inert gas, the volume of gas being such that the gas-space developed constitutes 30 to 50 per cent of the volume of the reactor.  The gas and liquid may be introduced via a T-piece near the entrance to the reactor-tube or introduced on the suction side of the circulating pump.  The amount of oxygen in the initial emulsion is preferably such that the effluent permanent gases contain at least 2 per cent by volume of free oxygen.  Diluents or solvents other than those produced in the oxidation may or may not be present; if desired, an excess of the liquid aldehyde or ketone may be used as solvent.  Oxidation catalysts may be present.  The liquid reaction mixture is generally withdrawn from the separator or at a point between the separator and pump.  All or a part of the recirculated portion of liquid may be passed through a cooler. According to examples: (1) a 2.8 per cent solution of acetaldehyde in acetic acid containing cobalt and copper acetates as catalysts is circulated by means of a centrifugal pump through a coiled stainless steel tubular reactor, 24 feet long and 3/8 inch internal diameter, and a separator whilst air is fed in near the tube entrance and acetaldehyde is introduced at a point between the pump and separator and the oxidation product is continuously withdrawn; the various flow rates are such that the pressure at the coil inlet is 24 lbs./sq. inch at a temperature of 49 DEG  C., and 322 grams of acetaldehyde are oxidized to acetic acid per hour per litre of reaction space.  Using an agitator vessel provided with a highspeed stirrer and oxidizing under the same conditions, lower conversions are obtained; (2) the process of (1) is repeated using a 4.7 per cent solution of acetaldehyde and oxygen, and a mixture of acetic acid and acetic anhydride is obtained; (3) methyl propyl ketone containing cobalt acetate is oxidized with oxygen at 95 DEG  C. to give an oxidation product comprising formic and propionic acids, following the procedure of (1) but using a 48 feet tube; under these conditions 143 grams of the ketone are oxidized per hour per litre of reactor space as compared with 28 grams when an agitator provided with a stirrer is used at normal pressure. Reference is made to the similar treatment of unsaturated aldehydes and ketones such as acrolein yielding acrylic acid or crotonaldehyde yielding crotonic acid, which together with the parent aldehyde may undergo secondary reactions with the percompounds formed during the oxidation, and the hydroxylated compounds, such as acetaldol yielding beta-hydroxy butyric acid, both of which may undergo condensation or eliminate water to form the unsaturated aldehyde or acid.  A table shows the effect of variation in linear velocity of the liquid circulated through a reactor tube, on the rate of oxidation of acetaldehyde in acetic acid. Specifications 443,151, 446,259 and 628,035 are referred to, and Specification 510,959 is referred to in the first Provisional Specification.