| 摘要 |
A two-stage crystallization of p-xylene from a mixed xylenes feed is effected by cooling in the first stage, isothermally removing from the coldest zone crystal-free mother liquor which is discarded, separating the slurry of crystals in mother liquor into mother liquor which is returned to the first stage and crystals which are melted and passed to the second stage where cooling produces a second slurry separated into product crystals and mother liquor at least part of which is recycled to the second stage, any remainder being recycled to the first stage. Crystallization is effected in scraped cylinders with conical bottoms cooled by heat-exchange and slurry is preferably removed and at least partly pumped into the bottom of the crystallizer. The first stage may be effected in a plurality of crystallizers using temperatures of - 30 DEG to - 85 DEG F. in the first zone(s) and - 85 DEG to - 105 DEG F. in the final zone with a total holding time of 4 1/2 -18 hours. Mother liquor is removed from the final zone in an amount which, added to the product p-xylene, equals the feed weight. The liquor is withdrawn through a micro-metallic filter with provision for back flushing to remove crystals or a scraped screen or a mini-cyclone. The second crystallization is effected at 15 DEG to 40 DEG F. and sufficient mother liquor is recycled to maintain a flowing slurry. Separations are effected centrifugally at 600-1200 G. and the crystal cake dried by spinning for 30-60 seconds.
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