GB949161A - A process and apparatus for manufacturing high purity p-xylene - Google Patents

Abstract

In the crystallization of p-xylene from a mixture of o-, m- and p-xylene and ethyl benzene containing at least 8% of p-xylene in one stage and the recrystallization of the p-xylene in a second stage, at least two cylindrical crystallization vessels in series are employed in the first stage, each vessel containing <FORM:0949161/C3/1> scrapers attached to the periphery of a rotary cylinder, also containing an agitator in the lower part thereof, and being surrounded by a cooling jacket such that the material in the vessel is slowly cooled and large crystals are formed at the end of the series of vessels; and at least one such crystallization vessel is employed in the second stage. The flow rate over the cooling surfaces is 0.3-5.0 m/sec. The p-xylene crystals from the first stage form a filter cake containing at least 85% of p-xylene in a yield of more than 95%. Recrystallization is effected at -5 to -25 DEG C. The p-xylene product has a purity of more than 98%. As shown in Fig. 1, each crystallization vessel 1 is surrounded by a cooling jacket 11, containing scrapers 3 attached to a rotary cylinder 2 driven by a motor 5, also contains a screw-type stirrer 8 driven by a motor 14, and has an inlet 9 and an outlet 10. Circulation of the feed down through the rotary cylinder 2 and around the annular space between the rotary cylinder and the vessel 1 may be aided by a guide block 17 and a guide cylinder 6. The temperature of the feed may fall by less than 10 DEG C. in each vessel. The residence in each vessel may be at least 1 hour. As illustrated in Fig. 2 (not shown), an initial xylene mixture produced by an isomerization, hydroforming, or platforming reaction may be mixed with a portion of the second stage mother liquor, dehydrated, further cooled to - 11 DEG C. by heat exchange with the first stage mother liquor in a vessel as shown, and then passed for crystallization through a series of six vessels, as shown, wherein cooling is effected to - 60 to - 72 DEG C. The resulting crystals are separated from the mother liquor by centrifugation, melted and passed to storage as the desired product.