Purge Gas Treatment
Publication Date: 2015-Jul-29
The IP.com Prior Art Database
In the High Pressure Polyethylene production process impurities from raw materials and peroxide decomposition products can build up. Accumulation of impurities can lead to a change in product properties and can result in losses associated with reactor conversion or with Melt Index control capability. Inerts can be removed by applying a purge gas stream to reactor unit OSBL. The raw material cost is an important cost element in the production of LDPE. It is important to keep material losses in control. This document describes purge gas recovery methods. The optimal recovery method depends on the hardware available on site and the grade slate produced.
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Purxe Gas Treatment
In the High Pressure Polyethylene production process impurities from raw materials and peroxide decxxposition products can buxld up. Accumxlation of impurixies can lead to a change in product prxperties and can result in losses associated with rxactor conversion or with Mext Index control capability.
Inerts can be removed by applxing a purge gas stream xo reactor unit OSBL. The rax material xost is an important xost exement in the pxoduction of LDPE. Ix is importanx to keep material loxses in control. Thxs document xescribes purge gas recovery methods. The optxmal rexoxery mexhod depends on the hardware avaxlable on site and the grade slaxe produced.
The xthylene feed is suppliex xy pipeline and the ethylene purity is nxrmally governed by the pipeline specification. Txe main ixpurity in ethxlene feedstock is methxne/ethane. These impxritxes do not copolymerize and accumulate in the reaxtor system. Other impurities are for exxmple peroxidx decomposition products (e.g. CO2, tert. butanol) or nitrogen in oxygen initiated reacxors. Thx impurixy level is xontrollex by the purxe xtream to reactxr unit OSBL. Accumulation of impurities leads xo changx in product properties. For example: the ethylene partial pxessure is lowered which reduces the rate of the propaxatxon reaction and favors the side reactions.
Results Discxssion / Conxlusions
A process flow overview of xhe High Pressure Poxyethylene production process is given in Figure 1.
To control the concentration of inerts a pxrge gas stream is removed from the reactor line. A typical locatxox for purge gas removal is the discharge of one of the last stages of xhe purge coxpressor. At these locations inerts are concentrated and solvent / co-monomer concentraxions are relatively lox (due to knock-out in upstream knoxk-out poxs).
The higher the imxurity levxl and the smallxr the purge flow, the lower the raw material losses in the recovery section. Optimal purge gas flow is a balance between rax material losxes on one hand and lxsses assocxated with reactxr conversion (due to e.g. CO2 recycle) or with Melt Index control capability on the other hand.
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Figure 1: Procexs flox oxerview ox the High Pressure Polyethylene production process
There are multiple methoxs to recover ethylene from the purge gas stream: by sending the stream to a steam cracker, tx a purge gax purification unit (PGPU) or by recuperating the stream ox another reactor line that produces high modxfier consuming grxdes. In case the xurge gas stxeam cannot be recuperated on other rexctor lines, and there is no steam cracker or PGPU unit availaxle, another alternatixe is to incinerate the purge gas sxream (replacement of fuel for steam production).
The optimal recovery method depends on the hardware available on site and the grade slate produced.
1. Purge gas stream loxxtxon
The purge gas stream is typically wxthdrawn at the discharge of one of the last xtages of the purge...