| 摘要 |
Methanol and water are removed from the reaction mixture obtained by esterifying terephthalic acid with methanol, by adding to the reaction-mixture an amount of xylene considerably greater than that required to form an azeotrope with the water present in the mixture, feeding the mixture into the middle section of a distilling column the base of which is maintained at a temperature above the melting-point of dimethyl terephthalate, and operating the column so as to distil over dry methanol from the top of the column and withdraw a sidestream containing substantially all the water from the feed (together with some xylene and some methanol), while a solution of D.M.T. in xylene overflows from the base of the column. In an example, the distillation is carried out in a bubble-cap column having 54 plates; and the feed, which comprises 15,4% D.M.T., 58,5% methanol, 3,2% water and 22,6% xylene, is fed to the 24th plate from the base. The temperatures maintained at the top and base of the column are 63,4 DEG and 167,7 DEG C., respectively; the pressure at the top being atmospheric. The overhead vapour, which is condensed, contains 87% methanol and 10% xylene but only 0,3% water. Part of the condensate is returned to the top of the column as reflux, and the remainder is reused for subsequent esterifications. A liquid side-stream at 75 DEG C. is withdrawn at the 34th plate, and passed to a decanter where it separates into two layers. The lower layer, which comprises 39% water and 58% methanol is withdrawn from the system; while the upper, xylene-rich, layer is returned to the column at the 33rd plate. The product withdrawn from the base of the column comprises 52% D.M.T., 47% xylene and 0,1% water; and is found to have an acidity of 1,2% as terephthalic acid. |
