Pre-Start Purge Procedure for Ethylene Cracking Furnaces By Inducing a Draft with Hot Air Flowing through One of the Convection Section Tube Banks

Prior to firing an ethylene cracking furnace, care must be taken to remove any trace combustible materials from the firebox. This is typically done by introducing low pressure steam into the bottom of the firebox to purge stagnant air (with any traces of combustible gases) through to the stack. The steam injection is maintained until at least five furnace volumes have been purged. In cold climates, much of the steam will condense and pool in the bottom of the furnace. This has caused operational issues.

This paper describes an alternative method of performing the purge procedure. Hot air bleed (normally used to decoke the furnace coils) at 185°C from a nearby gas turbine in the plant is directed through the premixed feed tube bank in the convection section and then through the vertical radiant coils. As the preheat tube bank slowly heats up, a natural draft is developed which pulls ambient air through the bottom of the furnace and results in the required purging. A transient model has been developed to assess the feasibility of the new procedure, and a full scale furnace experiment was performed to validate the method.  Data gathered from the full scale experiment along with a technique to be used for real-time monitoring of the purge air flow will be presented and discussed.