Patent GB-579047-A (GB)
Title: Improvements in manufacture of un-saturated aldehydes from acetaldehyde or its homologues
Filed: 1941-02-21
Abstract: <PICT:0579047/IV/1> A process for the manufacture of unsaturated aldehydes by condensing acetaldehyde or a homologue thereof in aqueous solution by means of an alkali-condensing agent and dehydrating the aldol so formed in the presence of a mineral acid, comprises effecting said condensation within a heat-exchanger of high efficiency, absorbing the heat of reaction by circulation of the reaction mixture within the exchanger in countercurrent to a cooling fluid, acidifying the resulting aldol mixture to a pH between 2 and 6, distilling off unchanged aldehyde and dehydrating the aldol by distillation under superatmospheric pressure, the unsaturated aldehyde thus formed being removed as produced in the form of an azeotropic mixture.  The heat-exchangers employed are of a type known per se, the apparatus known as "baffle-plate exchanger" being particularly suitable.  A sufficient amount of alkali-condensing agent is introduced into the mixture of aldehyde and water circulating in such a heat-exchanger so that the composition of the final mixture corresponds to the aldehydealdol equilibrium.  This addition may be made at several points in the exchanger circuit.  The dehydration of the aldol is preferably carried out under a superatmospheric pressure of 0.5 and 5 kilograms per square centimetre.  The distillation of unreacted aldehyde from the aldol solution may also be carried out under a small super - atmospheric pressure.  The process is applicable, in particular, to the preparation of crotonaldehyde from acetaldehyde and a -ethyl-b -propylacrolein from butyraldehyde.  Alkali-condensing agents specified are caustic soda, potash, lime, baryte, and alkaline carbonates.  Fig. 1 shows diagrammatically a plant operated in a continuous way wherein water and aldehyde from tanks 1 and 2 are mixed before passing into a baffle-plate heat-exchanger E (this comprises five elements each having four plates operating in parallel, but only travel lines are shown, the thick lines representing the water-aldehyde mixture flow and the thin lines the cooling liquid flow).  From a tank 6 sufficient aqueous caustic soda for the aldol condensation is introduced at several points in the exchanger circuit by means of valved lines 7, 8, 9 and 10.  The aldol-aldehyde equilibrium mixture flowing out of the exchanger at 11 is mixed with sufficient acid to lower its pH to a value of 2-6 through the medium of acid tank 12 and pipe 13, and is stirred in a tank 14 from which the acidified mixture passes through a pipe 15 to a distillation column 16.  The unchanged aldehyde is distilled off and is condensed at 19 from whence part is returned to the column 16 and part to the aldehyde supply tank 2.  An aqueous concentrated aldol solution flows from the base of the column and is pumped through a pipe 22 to a heated distillation column 24 operated under superatmospheric pressure.  The unsaturated aldehyde formed is removed as an aqueous azeotropic mixture which after condensation in condenser 26, flows via pipe 27 to a separator 28 in which separation into two layers occurs; the upper layer of wet unsaturated aldehyde is removed via line 29 to a column (not shown) where an anhydrous product is recovered, and the lower aqueous layer is returned to the distillation column 24, from the bottom of which flows an aqueous solution containing only salts of the alkali and acid added.  The vapour outlet of the apparatus communicates with a capacity tank C connected with a compressed inert gas (nitrogen) container B and provided with control valves S1 and S2 for adjusting the pressure in the column to a desired value.  In an example illustrating this process crotonaldehyde is prepared from acetaldehyde via aldol. Fig. 2 shows a plant for discontinuous operation, wherein water and aldehyde from tanks 1 and 2 are fed into a tank 32 which is connected by a pipe 3 with a pump 33 so adapted as to cause the liquid in the tank 32 to circulate through exchanger E, from the outlet end of which the cooled liquid is reintroduced via pipe 11 to the tank 32. The pump 33 is put into motion as soon as charging commences so as to remove heat of mixing.  When charging is completed, soda lye from a tank 6 is introduced into the circulating mixture via a line 35 leading to the suction end of pump 33.  This addition may be effected at any other points in the exchanger circuit.  The mixture is continuously pumped from tank 32 through exchanger E and back again until the aldol-aldehyde equilibrium is attained, whereupon the liquid is pumped via line 36 to a tank 14, by opening valve 36a and closing valve 34, and is acidified in tank 14 by addition of acid from tank 12 via line 13. The acidified mixture is then passed via line 15 to the system of distillation columns described above for conversion to unsaturated aldehydes.