Patent GB-587092-A (GB)
Title: Dehydration of polyhydric alcohols
IPC: C07C11/12
Filed: 1945-01-02
Abstract: In the dehydration of polyhydric alcohols of at least 4 carbon atoms and having non-vicinal carbinol groups, the products are fractionated to distil an overhead comprising poly-olefin, anunsaturated alcohol intermediate, an ether by-product, light ends, and water which is treated to separate the alcohol intermediate for recycling. Azeotropes rich in ether by-products may be formed and azeotropic agents may be added.  When dehydrating diacetone glycol, an ether-rich ternary azeotrope is formed from the C6 unsaturated alcohol intermediate, the C8 cyclic ether, and water from the dehydration. Compositions are given of such azeotropes, and of unsaturated alcohol-water and methyl pentadiene-water azeotropes.ALSO:A polyhydric alcohol of at least 4 carbon atoms and having non-vicinal carbinol groups is dehydrated to a polyolefin by heating in the liquid phase in a catalytic dehydration zone directly communicating with a fractionating zone so as to distil an overhead comprising polyolefin, an unsaturated alcohol intermediate, an ether by-product, light ends, and water, this overhead being treated to separate the alcohol intermediate which is recycled to the dehydration zone.  Water of dehydration or fresh water may be introduced into the top of the fractionating zone and the overhead treated in a second fractionating zone to remove as overhead the diene and light ends which are separated in a further fractionation. Bottoms from this second zone are treated to separate the alcohol intermediate for recycling. Azeotroping agents may be added or the water itself may form an ether-rich ternary azeotrope with the ether and the alcohol intermediate.  The polyhydric alcohols used may be prepared by hydrogenating the condensation products of aldehydes or ketones prepared, for example, as described in Specification 502,450. Acetone may be condensed to diacetone alcohol, hydrogenated, and the glycol dehydrated to hexadiene with iodine or hydrogen chloride as catalyst by the process of Specification 572,602. Cyclic ethers are formed as by-products, that in the case of dehydration of diacetone glycol to 2-methyl pentadiene apparently having the formula <FORM:0587092/IV/1> The products distilling over from the dehydration kettle, provided with a fractionation column into which water is introduced, are fractionated in a second column where light ends and a methyl pentadiene-water azeotrope are taken off from a mixture of the ether, unsaturated alcohols and water.  Hydrogen iodide from the iodine catalyst is stripped by adding alkali.  The bottoms are then fractionated to remove water and a ternary azeotrope of ether, alcohol and water, rich in ether, leaving the unsaturated alcohol as bottoms which is recycled to the dehydration kettle.  Alternatively, the crude distillate from the first column is separated into an upper organic layer, part of which is returned as reflux, and an aqueous layer.  Part of the organic layer is then fractionated to remove light ends, and the bottoms are further fractionated to remove dienes and leave an alcohol-ether mixture which is then treated to remove the ether as by azeotroping, and recover the alcohol for recycling. The results obtained by the first method of working are compared with the normal process wherein both the alcohol and ether are refluxed back, diacetone glycol being dehydrated at about 120-145 DEG  C, using 0.05 per cent of iodine catalyst.