Mega columns, mega issues

New distillation columns are getting much larger. The history of column sizes, and design and construction issues associated with large columns were recently addressed at the AIChE Spring Meeting (New Orleans, La.; March 30 to April 3) by Dan Summers of Sulzer Chemtech (Winterthur, Switzerland; www.sulzer.com), Bob Miller of UOP (Des Plaines,Ill.; www.uop.com) and Henry Kister of Fluor Corp. (Irving Tex., www.fluor.com). They focused on trayed, rather than packed columns. It is entirely possible that the largest-volume distillation column in the U.S. was recently commissioned — a C3 splitter with a diameter of 28 ft and a tangent-to-tangent height of 309 ft. That column included four-pass trays. When high liquid flowrates are encountered at such large diameters, tray designers increasingly consider six- and eight-pass crossflow configurations. Many tray designers avoid odd numbers of passes. As the number of flow passes is increased, the number of column inspection manways should increase accordingly. With very long flow-paths, froth height gradients can cause vapor maldistributions and even vapor crossflow channeling. With large bubbling areas, froth stagnancies can occur. Push devices can correct such maladies, but those devices…