Coking Mechanism in Refinery Fractionator Wash Beds

This paper will discuss a novel theory of the coking mechanism in a refinery fractionator wash bed. 

The wash bed of a refinery fractionator is prone to coking/fouling due to the low liquid rates along with high vapor rates and high temperatures.  Generally, the wetting rate at the top of the wash bed is minimized to prevent high-value product loss, but lower wetting rates lead directly to coke formation in the bed.  So the refinery works to balance the economics of product recovery versus unscheduled downtime to replace a coked bed.  Marathon has used Tracerco ThruVision™ technology to help manage the wash bed life while maximizing product recovery during the normal turnaround cycle.  The ThruVision™ technology provides a detailed density map at a specific vertical elevation that can pinpoint specific areas of liquid maldistribution or solids build-up. 

During a turnaround cycle, several towers have experienced heater trips that have led to step changes in the wash bed density seen by the ThruVision™ technology and ΔP.  A review of this data from several refinery fractionators has guided the creation of a new theory on how the coke is formed in a refinery fractionator wash bed.