Method of recovering paraxylene in a crystallization zone
A method of recovering paraxylene in a crystallization zone. The crystallization zone includes at least two crystallization stages and two reslurry stages. The method provides for lower throughput through the crystallization zone, resulting in lower capital costs, reduced electricity in operating separation equipment, as well as reduced refrigeration duty.
1. A process for the recovery of a paraxylene product from a mixture of C8 aromatic hydrocarbons comprising:
feeding the mixture of C8 aromatic hydrocarbons to a first crystallization stage;
separating an effluent of the first crystallization stage in a first solid/liquid separator to form a first paraxylene-lean stream and a first paraxylene cake;
feeding at least a portion of the first paraxylene-lean stream to a second crystallization stage;
separating an effluent of the second crystallization stage in a second solid/liquid separator to form a second paraxylene-lean stream and a second paraxylene cake;
feeding the second paraxylene cake and optionally a portion of the first paraxylene cake to a first reslurry zone, and reslurrying the second paraxylene cake and optionally the portion of the first paraxylene cake with a first reslurry fluid;
separating an effluent of the first reslurry zone in a third solid/liquid separator to form a third paraxylene-lean stream and a third paraxylene cake;
feeding at least a portion of the first paraxylene cake and the third paraxylene cake to the second reslurry zone and reslurrying the at least the portion of the first paraxylene cake and the third paraxylene cake with a second reslurry fluid;
separating an effluent of the second reslurry zone in a fourth solid/liquid separator to form a fourth paraxylene-lean stream and a fourth paraxylene cake; and
recovering at least a portion of the fourth paraxylene cake as a paraxylene product.
2. The process of claim 1 wherein the temperature of the first crystallization stage is greater than the temperature of the second crystallization stage.
3. The process of claim 1 wherein the temperature of the first crystallization stage is greater than −40 ° C.
4. The process of claim 1 wherein the temperature of the second crystallization stage is less than −56.7 ° C.
5. The process of claim 1 wherein the temperature of the second crystallization stage is less than −67.8 ° C.
6. The process of claim 1 wherein at least a portion of the second paraxylene-lean stream is fed to an isomerization reactor.
7. The process of claim 1 wherein at least a portion of the second paraxylene-lean stream is recycled to the second crystallization stage.
8. The process of claim 1 where the mixture of C8 aromatic hydrocarbons comprises greater than 25 wt % paraxylene.
9. The process of claim 1 , wherein at least a portion of the first paraxylene-lean stream is recycled to the first crystallization stage.
10. The process of claim 1 , wherein separating the effluent of the second crystallization stage in the second solid/liquid separator to form the second paraxylene-lean stream and the second paraxylene cake further comprises forming a fifth paraxylene-lean stream richer in paraxylene than the second paraxylene lean stream and recycling the fifth paraxylene-lean stream to the second crystallization stage.
11. The process of claim 1 , wherein separating the second reslurry effluent in the fourth solid/liquid separator to form the fourth paraxylene-lean stream and the fourth paraxylene cake further comprises forming a sixth paraxylene-lean stream richer in paraxylene than the fourth paraxylene-lean stream, and the second reslurry fluid comprises at least a portion of the sixth paraxylene-lean stream.
12. The process of claim 1 , wherein at least a portion of the third paraxylene-lean stream is recycled to the first crystallization stage.
13. The process of claim 1 , wherein the first reslurry fluid comprises at least a portion of the third paraxylene-lean stream.
14. The process of claim 1 , wherein the first reslurry fluid comprises at least a portion of the fourth paraxylene-lean stream.
15. The process of claim 1 , wherein the second reslurry fluid comprises at least a portion of the fourth paraxylene-lean stream.
16. The process of claim 1 , wherein at least one of the first solid-liquid separator, the second solid-liquid separator, the third solid-liquid separator, and the fourth solid-liquid separator comprises a centrifuge.
17. The process of claim 1 , wherein at least one of the first solid-liquid separator, the second solid-liquid separator, and the third solid-liquid separator comprises a filter column.
18. The process of claim 1 , wherein the fourth solid-liquid separator comprises a wash column.
19. The process of claim 1 , wherein the mixture of C8 aromatic hydrocarbons has a paraxylene concentration of less than 75 wt %, and wherein the temperature of the effluent of the second crystallization stage is colder than −70.6 ° C.
20. The process of claim 19 , wherein the mixture of C8 aromatic hydrocarbons is an effluent of a pressure swing adsorption zone adapted to adsorb and desorb paraxylene based on the cycling of partial pressure in the pressure swing adsorption zone.