Correcting design errors can prevent coking in main fractionators

Coking in petroleum refinery main fractionators causes unit shutdowns and significant production losses. High temperatures and long residence times are the causes of coking in these units. Coking, however, is not inevitable. It is a function of design and operating errors. Coke formation in petroleum refinery packed columns is caused by several design errors, including: Adequate vapor distribution--localized dry-out; Ultralow local liquid rate--high residence time; High entrainment--coke fines or visbroken vacuum residue; Inadequate liquid mixing--thermal and composition gradients; and High residence time on collector trays. Each of these design flaws is described, then 5 examples are described and solutions given to the problems.