Patent GB-628035-A (GB)
Title: Improvements in or relating to the manufacture of acetic anhydride
Filed: 1944-12-08
Abstract: Acetic anhydride is produced by the oxidation of acetaldehyde with oxygen by a continuous process comprising continuously circulating a stream of a liquid mixture comprising essentially acetic acid, acetic anhydride, acetaldehyde and an oxidation catalyst through a recirculating reaction system comprising an externally-cooled reaction tube of an internal diameter not exceeding two inches, a separator, wherein a gas phase is separated from the liquid mixture, connected to the outlet end of the tube and a circulating pump connected in a line between the separator and the inlet end of the tube, the volume of the liquid which is present in the other parts of the recirculating reaction system in which it is at substantially the same temperature as that in the reaction tube, the rate of circulation of the liquid mixture being such that, in the absence of gas in the tube, a linear velocity of at least 7 feet per second would be attained in the tube, introducing molecular oxygen into the liquid mixture being circulated through the reaction tube at such a rate that, in unit time, the volume thereof fed in at the prevailing pressure is less than the volume of liquid feed whereby a continuous gas phase is avoided in the reaction tube, withdrawing from the system a portion of the liquid mixture and recovering acetic anhydride therefrom and recirculating the remainder of the liquid mixture to the reaction tube whilst feeding fresh acetaldehyde thereto. Preferably, such an amount of oxidizing gas is used that the gas separating in the separator contains a substantial amount of oxygen, for example, 2 to 25 per cent by volume and the separator is maintained at substantially atmospheric pressure.  The oxidizing gas normally contains not more than 20 per cent of inert gas.  It is also preferred to introduce such an amount of the oxidizing gas that the gas-space developed under the conditions of the oxidation is about 30-50 per cent of the tube volume.  The liquid flow rate is generally such that there is maintained a pressure difference of at least one atmosphere between the tube inlet and the separator.  If desired, the gas/liquid mixture may be passed down the tube.  There may be used straight or bent tubes; preferably, the volume of the tube is such that it contains at least half of the liquid in the system and is at least equal to the volume of the other parts of the liquid circulating system in which the liquid is at substantially the same temperature as that in the tube.  The oxidizing gas and liquid may be introduced via a T-piece near the tube entrance or fed in on the suction side of the pump.  Liquid may be withdrawn for recovery of the products, for example, from the separator or at a point between the separator and pump.  All or part of the circulated liquid may be passed through a cooler located between the separator and pump.  The reaction conditions are preferably such that, employing a temperature of 50 DEG  C., the product withdrawn from the system comprises water, acetic acid and acetic anhydride in a molar ratio of less than 2 : 1 and less than 10 per cent acetaldehyde.  The use of diluents, other than those produced in the reaction, and the addition of acetic acid is not necessary but increases the yield.  According to a typical example, acetaldehyde is oxidized to acetic anhydride and acetic acid by circulating the reaction liquid containing 4.4 per cent acetaldehyde through a stainless steel coil, of 3/4 inch internal diameter and 60 feet length, at a linear velocity of 7.4 feet per second, whilst feeding in fresh aldehyde and oxygen and withdrawing part of the product comprising acetic anhydride and acetic acid.  The effect on the yield of anhydride caused by lowering the linear velocity is shown.  The catalyst used in these experiments is cobalt acetate; a mixture of cobalt acetate and copper acetate (added as the carbonate, if desired) may also be used as catalyst.  Instead of feeding acetaldehyde, a mixture thereof with acetic acid may be introduced.  Specifications 443,151, 446,259, 510,959 and 628,034 are referred to.