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Para-Xylene by Three-Stage Crystallisation from Xylene
Para-xylene melts at 15 °C (Lunge, 1887), so it crystallises out of mixed xylene when this is chilled hard. This is Talbot's three-stage process (US 2,985,694, 1961): chill the dry feed to about −95 °F and spin off the crystals, then warm them in stages in their own recycled liquor and spin again, until they are 98.5 % para-xylene. Para-xylene is the raw material for terephthalic acid and so for PET.
The picture: our diagram of the three stages.
Osezingeni eliphezulu
1 day
Imiyalelo
1
1
Take out the ortho-xylene (preferred)
Take out the ortho-xylene (preferred)
If you can, first distil off ortho-xylene as the bottoms of a tall column: more than 35, better about 60, theoretical plates. The patent prefers this but does not require it.
Izinto zokwakha zalesi sinyathelo:
I-Xylene1 as requiredAmathuluzi adingekayo:
Uhlelo lwekholomu yokucwenga
Ibhayela lesitimu
Ithemomitha Yaselabhorethri2
2
Dry the feed and add a little methanol
Dry the feed and add a little methanol
Remove all water from the xylene (calcium chloride is our choice of drier). Add methanol so ice does not plug the cold equipment: the patent's example uses 49 lb methanol to 13,430 lb feed.
Amathuluzi adingekayo:
I-calcium chloride
I-methanol
Ithange lokwenza amakristalu3
3
Chill to about −95 °F and spin
Chill to about −95 °F and spin
Cool slowly, over about 20 minutes, to between −75 and −120 °F (−59 to −84 °C); the example uses −95 °F (−71 °C). Spin the slurry in a centrifuge: the liquid (about 8 % para) goes out, the crystals are about 74 % para-xylene.
Amathuluzi adingekayo:
I-Crystallizer Epholile
I-Centrifuge YebhasikidiIthemomitha Yaselabhorethri
Amagilavu Amelana Nokushisa
Izibuko Zokuchitheka Kwamakhemikhali4
4
Reslurry, bring to −30 °F, spin
Reslurry, bring to −30 °F, spin
Stir the crystals into the liquid from the next stage and bring the slurry to between 0 and −30 °F (−18 to −34 °C); the example uses −30 °F. Centrifuge again: the crystals are now about 91 % para-xylene.
Amathuluzi adingekayo:
Ithange lokwenza amakristaluI-Crystallizer EpholileI-Centrifuge YebhasikidiIthemomitha YaselabhorethriAmagilavu Amelana Nokushisa5
5
Reslurry warmer, spin and melt
Reslurry warmer, spin and melt
Stir these crystals into the liquid from the last spin and bring to 10–45 °F (−12 to 7 °C). Centrifuge: the crystals are 98.5 % para-xylene. Melt them and bottle. Return the liquids to the earlier stages.
Amathuluzi adingekayo:
Ithange lokwenza amakristaluI-Crystallizer EpholileI-Centrifuge YebhasikidiIthemomitha Yaselabhorethri
Amabhodlela engilazi anezivimboAmathuluzi Adingekayo
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