Control methods and systems for polymer product purge

• US 10,035,864 B2
• Filed: 09/07/2016
• Issued: 07/31/2018
• Est. Priority Date: 11/05/2015
• Status: Active Grant

• Current Assignee: ExxonMobil Chemical Patents Inc. (Exxon Mobil Corporation)
• Original Assignee: ExxonMobil Chemical Patents Inc. (Exxon Mobil Corporation)
• Inventors: Sandell, David J.
• Primary Examiner(s): Lee, Rip A
• Application Number:
  US15/258,226
• Publication Number:
  US 20170129969A1
• Time in Patent Office: 692 Days
• Field of Search: 526 59, 526 60, 526 71
• CPC Class Codes: B01J 2208/00663 Concentration
• CPC Class Codes: B01J 2208/00761 Discharging
• CPC Class Codes: B01J 2208/00769 Details of feeding or discharging
• CPC Class Codes: B01J 2208/00973 Products
• CPC Class Codes: B01J 8/0015 Feeding of the particles in the reactor; Evacuation of the particles out of the reactor
• CPC Class Codes: B01J 8/003 in a downward flow
• CPC Class Codes: B01J 8/24 according to "fluidised-bed" technique B01J8/20 takes precedence
• CPC Class Codes: C08F 2/01 characterised by special features of the polymerisation apparatus used
• CPC Class Codes: C08F 2/34 Polymerisation in gaseous state
• CPC Class Codes: C08F 210/02 Ethene
• CPC Class Codes: C08F 210/14 Monomers containing five or more carbon atoms
• CPC Class Codes: C08F 210/16 Copolymers of ethene with alpha-alkenes, e.g. EP rubbers
• CPC Class Codes: C08F 6/005 from solid polymers
• CPC Class Codes: C08F 6/28 Purification
• CPC Class Codes: C08L 23/0815 Copolymers of ethene with aliphatic 1-olefins

Abstract

Disclosed herein are methods and systems for purging a polymer product of volatiles. The methods and systems are particularly useful in the purging of a polyethylene polymer product produced in a fluidized bed reactor.

Claims

1. A method for purging a polymer product of volatiles, the method comprising:

producing a polymer product comprising volatile species in a reactor;

transferring the polymer product from the reactor to at least one purge bin;

injecting a purge gas into the at least one purge bin to create a purged polymer product and a used purge gas;

discharging the purged polymer product from the at least one purge bin to produce a discharged purged polymer product; and

determining a relationship between purge efficiency, X_o/X_i, and (S*P)/G for one or more species of the volatile species purged from the polymer product, wherein;

X_o is the concentration in ppmw of the volatile species in the discharged purged polymer product;

X_i is the concentration in mol % of the volatile species in a gas phase of the reactor upstream of the at least one purge bin;

S is a production rate in Klb polymer per hour of the reactor;

P is an absolute pressure in psia within the at least one purge bin; and

G is a mass flow rate in lb purge gas per hour in the at least one purge bin.

2. The method of claim 1, wherein the reactor comprises a fluidized bed reactor with a cycle gas loop, and X_i is the concentration in mol % of the volatile species in the cycle gas.

3. The method of claim 1, wherein the reactor comprises a fluidized bed reactor, and X_i is the concentration in mol % of the volatile species in a gas phase at a point within the fluidized bed reactor.

4. The method of claim 1, wherein the at least one purge bin comprises a top section and a bottom section.

5. The method of claim 1, further comprising recovering at least a portion of the used purge gas and recycling it to the at least one purge bin.

6. The method of claim 5, further comprising removing at least a portion of the volatile species comprising four or more carbon atoms from the used purge gas prior to recycling it to the at least one purge bin.

7. The method of claim 5, further comprising removing at least a portion of the volatile species comprising three or less carbon atoms from the used purge gas, wherein the removing occurs in a bottom section of the at least one purge bin.

8. The method of claim 1, further comprising determining a minimum fluidization velocity for the at least one purge bin, and maintaining a purge gas velocity within the at least one purge bin below the minimum fluidization velocity.

9. The method of claim 1, further comprising determining a relationship between purge efficiency, X_o/X_i, and (S*P*M)/G for one or more species of the volatile species purged from the polymer product and M is a molecular weight of the purge gas in lb/lb-mol.

10. The method of claim 9, further comprising adjusting at least one of the parameters S, P, M, or G to improve purge efficiency.

11. The method of claim 9, further comprising maintaining the value of (S*P)/G or (S*P*M)/G at or below a target value.

12. The method of claim 1, wherein the one or more volatile species is selected from the group consisting of 1-hexene, 3-methyl-2-pentene, cis- and trans-2-hexene, cis- and trans-3-hexene, 4-methyl-2-pentene, and 2-ethyl-1-butene.

13. The method of claim 1, further comprising injecting steam into the at least one purge bin.

14. The method of claim 1, wherein the purge gas is nitrogen.

15. The method of claim 1, further comprising heating the polymer product after it is discharged from the reactor and before it is transferred to the at least one purge bin.

16. The method of claim 1, wherein the polymer product comprises polyethylene.