Patent GB-744134-A (GB)
Title: Improvements in and relating to the production of -Ð-xylene
Inventor: GODDARD REGINALD EDWIN
IPC: C07C7/14
Filed: 1952-07-11
Abstract: <PICT:0744134/IV(a)/1> p-Xylene is produced from mixtures containing two or more xylenes by a continuous process comprising cooling the mixture in one or more stages to a temperature near to but above its initial crystallization point, then cooling the mixture rapidly by direct admixture with a bulk of already cooled mixture, which contains crystalline p-xylene and is obtained preferably by turbulent passage through a coil immersed in a refrigerant, to a temperature which may be slightly below, but is preferably above, that at which the first eutectic of p-xylene with one or more of the other xylenes would begin to crystallize, withdrawing a part of the mixture at this lowest temperature continuously to a zone where no further cooling is applied and where the residence time of the mixture is sufficiently long for any residual supersaturation to be destroyed and for crystals of p-xylene to be brought into a more suitable condition for separation by filtration, and filtering off said crystals, preferably by centrifuge.  The residence time in the zone where no further cooling takes place is preferably at least 8 hours.  The temperature of the initial cooling stages may be in the range -27 DEG  to -56 DEG C., depending on composition, and the lowest temperature zone may vary between -50 DEG  and -90 DEG  C.  In an example, an isomerizate containing 12 per cent p-xylene, 66.5 per cent o- and m-xylenes, and 20 per cent ethyl benzene, precooled to about -30 DEG  to -40 DEG  C. by heat exchange with cold mother liquor, is fed through line 1 to heat-exchanger 2 in which it is cooled to -40 DEG  to -55 DEG  C., and is then fed to slurry pumping tank 3 which contains cooled mixture having about 8 per cent by weight of solid p-xylene.  A temperature of -75 DEG  to -90 DEG  C. is maintained therein by continuously withdrawing slurry by centrifugal pump 4 and passing turbulently through cooler 5, containing ethylene boiling under pressure.  From tank 3, slurry at about -90 DEG  C. is fed to tank 6 where it remains for 8 to 10 hours.  The feed to the centrifuge 8 is controlled by closing valve 13 when tank 7 is full, then blowing the contents over into the centrifuge by inert gas pressure at a rate controlled by 17. Tank 11 contains pentane for washing the crystals and tank 10 xylene used to remove the crystals from the centrifuge and its basket.ALSO:<PICT:0744134/III/1> p-Xylene is produced from mixtures containing two or more xylenes by a continuous process comprising cooling the mixture in one or more stages to a temperature near to but above its initial crystallization point, then cooling the mixture rapidly by direct admixture, with a bulk of already cooled mixture which contains crystalline p-xylene, and is obtained preferably by turbulent passage through a coil immersed in a refrigerant, to a temperature which may be slightly below but is preferably above, that at which the first eutectic of p-xylene with one or more of the other xylenes would begin to crystallize, withdrawing partof the mixture at this lowest temperature continuously to a zone where no further cooling is applied and where the residence time of the mixture is sufficiently long for any residual supersaturation to be destroyed and for crystals of p-xylenes to be brought into a more suitable condition for separation by filtration, and filtering off said crystals, preferably by centrifuge.  The residence time in the zone where no further cooling takes place is preferably at least 8 hours.  The temperature of the initial cooling stages may be in the range -27 DEG  to -56 DEG C. depending on composition, and the lowest temperature zone may vary between -50 DEG C. and -90 DEG C.  In an example, an isomerizate containing 12 per cent p-xylene, 66.5 per cent o and m-xylenes, and 20 per cent ethyl benzene, precooled to about -30 DEG  to -40 DEG C. by heat-exchange with cold mother liquor, is fed through line 1 to heat-exchanger 2 in which it is cooled to -40 DEG  to -50 DEG C., and is then fed to slurry pumping tank 3 which contains cooled mixture having about 8 per cent by weight of solid p-xylene.  A temperature of -75 DEG  to -90 DEG C. is maintained therein by continuously withdrawing slurry by centrifugal pump 4 and passing turbulently through cooler 5, containing ethylene boiling under pressure.  From tank 3, slurry at about -90 DEG C. is fed to tank 6 where it remains for 8 to 10 hours.  The feed to the centrifuge 8 is controlled by closing valve 13 when tank 7 is full thus blowing the contents over into the centrifuge by inert gas pressure at a rate controlled by 17. Tank 11 contains pentane for washing the crystals and tank 10 xylene used to remove the crystals from the centrifuge and its basket.