GB857135A - Process for the manufacture of isoprene or butadiene - Google Patents

Abstract

<PICT:0857135/IV(b)/1> Isoprene or butadiene is prepared from respectively isobutylene or propylene preferably in a hydrocarbon or ether as solvent by reaction in countercurrent with sulphuric or phosphoric acid of 5-60% by weight concentration and aqueous formaldehyde, distilling the organic product after neutralisation to separate the 4, 4-dimethyl-1, 3-dioxane or 4-methyl-1, 3-dioxane from diluent, cracking the dioxane in the vapour phase in the presence of steam at an elevated temperature and recovering isoprene or butadiene from the organic product by distillation. Referring to the preparation of isoprene, isobutylene preferably dissolved in an inert solvent is fed via storage tank 14 to the bottom of pulsation column 78. The solvent may be benzene, toluene, a saturated aliphatic hydrocarbon or an ether and should boil below the dioxane formed ; preferably a C4 petroleum fraction is used also containing butanes and m-butenes, the reaction conditions being adjusted to ensure selective reaction of the isobutylene. Sulphuric or phosphoric acid and fresh and recycle aqueous formaldehyde are fed to the top of the column from tanks 3 and 4 and 5 respectively. The aqueous phase should contain initially 5-60% by weight of acid and 0,1-15% of formaldehyde, the organic layer finally 10-40% of the dioxane. Reaction is effected at 20 DEG -100 DEG C. and a pressure up to 20 atmospheres gauge to maintain the liquid phase. Piston 80 subjects the contents of the column to a pulsating movement. Organic products are withdrawn from a settling zone at the top of the column and washed in tower 72 with alkali, e.g. caustic soda solution from tank S7. The washed product is distilled in tower 27 to remove solvent overhead, crude dioxane being obtained as bottoms. This is vaporised in thin film evaporator 33, preferably together with water, or steam may be added to the vaporised dioxane to give a molar ratio of crude dioxane to water of 1 : 1-40. Unvaporised material is discarded by line 36. Cracking is effected in reactor 37, e.g. at 200 DEG C. and atmospheric pressure in the presence of a mixture of kaolin, kieselguhr and pyrophosphoric acid. The hot products pass through heat exchanger 40 wherein concentration of acid is effected, condensed and separated in separator 42 into an aqueous formaldehyde layer which is passed to tank 5 and an organic layer which is distilled to obtain isobutylene by line 48,isoprene by line 53 and uncracked dioxane which is recycled by line 54 to evaporator 33. Aqueous effluent from the reactor 78 is concentrated in heat exchanger 40, concentrated acid being recycled by line 41 to tank 3. The overhead vapours containing water and formaldehyde are washed with alkali in tower 56 and condensed in condenser 61. The remaining gases are washed with some of the condensate in tower 67 to recover further amounts of formaldehyde, the liquid then being recycled by line 71 to evaporator 33. The remaining condensate is removed by line 65 to produce further quantities of aqueous formaldehyde. In an alternative method using mixer-settler units in place of reactor 78, part of the fresh feed to tank 14 is diverted and used to extract in countercurrent any of the dioxane entrained in the acid effluent before this is concentrated. The organic material is then fed to the last (in order of direction of aqueous flow) of the mixing units.