Counterintuitive revamp results

At the 2011 Spring AIChE meeting (Chicago, Ill.), Sean Hennigan of BP gave a presentation regarding a column troubleshoot that yielded a counterintuitive result. A column bottleneck was ultimately traced to the presence of a vacuum cleaner in a tray downcomer. I wondered out loud, after that presentation, whether all tray downcomers should be equipped with vacuum cleaners to reduce liquid back-ups. At the 2014 Spring AIChE meeting (New Orleans, La.), Hennigan gave another interesting presentation entitled, “A High Capacity Re-Tray that Wasn’t — a Practical Lesson in Counter-Intuitive Thinking.” His story and contentions follow. Generally, in distillation and absorption columns, the replacement of sieve trays with fixed-valve trays yields 5–10% capacity increases. Compared to sieve trays, fixed-valve trays have lower froth heights and lower pressure drops, especially when those trays have larger deck open-areas. Along those lines, Hennigan described an experience where a simple atmospheric-pressure distillation column was revamped by replacing sieve trays with such fixed-valve trays. Upon re-start, the new column’s pressure drop was unstable, and higher, not lower. More importantly, the column’s…