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Improved Granular Polyethylene Stripping During Start-up from Empty Purge Bin

IP.com Disclosure Number: IPCOM000257737D
Publication Date: 2019-Mar-07
Document File: 3 page(s) / 34K

Publishing Venue

The IP.com Prior Art Database

Abstract

Purge bins are used commercially to strip (remove) dissolved hydrocarbons from the polyethylene ("PE") powder in the Gas Phase PE process. The purge bin is a counter-current operation that feeds stripping gas at the bottom and PE powder from the top of the bin. Normally when PE reactors start-up, the purge bin is completely empty and when new reactor production begins, the new powder begins to fill the purge bin from the reactor production and can cause very high hydrocarbon residuals at the outlet due to inadequate time for purging. This occurs since the PE powder that is highly concentrated with hydrocarbons from the reactor immediately falls to the bottom of the bin near the bottom outlet. Initially no powder is discharged from the bin as the bin level increases with ongoing PE production into the top. Eventually the level in the bin gets high and then bottom powder discharging begins and at that time, the powder at the bottom isn't completely purged as it is conveyed downstream to other holding bins or directly to the finishing extruder/mixer. The improved procedure discussed here consists of first filling the purge bin with seedbed powder during or after the time that the reactor is charged with seedbed prior to starting production in the reactor. The purge bin is pre-filled with completely purged powder (called seedbed) at the start-up will then allow the new PE product powder to get purged adequately. This new method will significantly reduce the possibility of getting PE powder at the purge bin outlet that contains high hydrocarbon residuals that might exceed the LEL% (lower explosivity limit) and will also reduce the amount of hydrocarbons emissions that would have occurred without this improved procedure.

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***Improved Granular Polyethylene Stripping During Start-up from Empty Purge Bin***

Abstract:  Purge bins are used commercially to strip (remove) dissolved hydrocarbons from the polyethylene (“PE”) powder in the Gas Phase PE process.  The purge bin is a counter-current operation that feeds stripping gas at the bottom and PE powder from the top of the bin.  Normally when PE reactors start-up, the purge bin is completely empty and when new reactor production begins, the new powder begins to fill the purge bin from the reactor production and can cause very high hydrocarbon residuals at the outlet due to inadequate time for purging. This occurs since the PE powder that is highly concentrated with hydrocarbons from the reactor immediately falls to the bottom of the bin near the bottom outlet. Initially no powder is discharged from the bin as the bin level increases with ongoing PE production into the top. Eventually the level in the bin gets high and then bottom powder discharging begins and at that time, the powder at the bottom isn’t completely purged as it is conveyed downstream to other holding bins or directly to the finishing extruder/mixer.   The improved procedure discussed here consists of first filling the purge bin with seedbed powder during or after the time that the reactor is charged with seedbed prior to starting production in the reactor. The purge bin is pre-filled with completely purged powder (called seedbed) at the start-up will then allow the new PE product powder to get purged adequately.   This new method will significantly reduce the possibility of getting PE powder at the purge bin outlet that contains high hydrocarbon residuals that might exceed the LEL% (lower explosivity limit) and will also reduce the amount of hydrocarbons emissions that would have occurred without this improved procedure.

Background

The normal practice when completely shutting down the reactor and dumping the reactor bed is to also completely empty the purge bin.  The empty bin sets the stage for a more difficult purge bin startup upon which we observe the initial PE powder from the purge bin contains high hydrocarbon residuals.  (Note that this problem may not get noticed without looking specifically for it; lab headspace samples do not get taken until operation is stable).  High hydrocarbon residuals should be reduced to avoid the concentration of the hydrocarbons getting above the lower explosive limit (%LEL).  Additionally, the high hydrocarbons at start-up leads to higher hydrocarbon emissions.  The conventional mitigation methods used during startup are to increase both the purge bin residence time and the specific stripping gas ratio N2/PE (Kg/Kg).  Neither of these mitigations are sufficient. 

Proposed Solution/ Conclusions

During start-up of gas phase PE reactors, the first step is to charge a seedbed of PE powder to the reactor.  Normally the downstream purge bin is completely empty and gets powder feed later after the...

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