(120e) Modeling of the Dynamic Interaction in an LDPE Closed System Purge Bins-Regenerative Thermal Oxidizer (RTO) for Ethylene Emission Mitigation

Common designs of LDPE purge bins involve venting the purge and blend air to atmosphere containing small traces of ethylene.  To mitigate ethylene emission from these bins, a new process design has been proposed to collect vent air-carrying ethylene via a closed ducting system connected to a regenerative thermal oxidizer (RTO) unit. This necessitates a blower at the downstream end to pull the purge air before feeding into the RTO to incinerate the ethylene.  Concerns have been raised regarding the impact of such system on the purge bins, which are not designed to withstand any significant amount of vacuum.  While conceptually steady operation of such system is feasible, operation of purge bins is never steady particularly when the purge bins are not designed to withstand any significant level of vacuum pressure.  With the minimum safe operating pressure at -0.1 psig (-0.68 kPag), and the design RTO inlet pressure being -1"’WC (-0.25 kPag), the bins are forced to operate under slight vacuum conditions with a very narrow allowable margin for error.  The operation is inherently dynamic as LDPE pellets are being conveyed into and out of the bins, purged and mixed in cycle by the interaction of multiple pieces of equipment.   In the event of any equipment failure along the train, pressure in the bins can drop below their minimum vacuum level and cause damage to the shell.  Dynamic interactions between the various components of the system including the blower with its VFD drive, purge and blend air compressors, capacitances and resistive characteristics of the pellets in the bins, as well as the response of the control system all contribute to interesting dynamic interactions that must be attended to. A model capable of simulating various dynamic phenomena has been built using Aspen HYSYS Dynamics TM v7.3 for the entire system, and used to answer what-if operational scenarios.  As an example it was shown that  the addition of a pull blower with its variable frequency drive (VFD), cannot maintain the bin pressure above the vacuum pressure limit of -0.1 psig even with the installation of individual pressure vacuum relief valve (PVRV) on each bin. Any interruption to air flows such as the shut-down of blend blower(s), stop/start conveying or swinging the purge air blower poses a non-negligible risk.  Interaction between the PVRV’s and the blower’s VFD both simultaneously trying to control the bin pressure could lead to pressure fluctuations that exceed the design vacuum pressure of the bins.  This paper will provide an overview of the system, state the design and operational challenges, basis of the dynamic model, and results of several what-if scenarios.