By Henry Z. Kister Fluor Corp. and Milton A. Chaves Hess Corp. |
A survey of tower failures [ 1 ] ranks kettle reboilers as the most troublesome reboiler type in the chemical process industries (CPI). Excessive pressure drop in kettle reboiler circuits is the prominent kettle malfunction, causing liquid to back up in the tower base beyond the reboiler return elevation. This high liquid level leads to premature flooding and capacity loss. In almost all the surveyed cases, the excess pressure drop is due to hydraulic restrictions in the kettle inlet or outlet piping or at the kettle entrance [ 2 ].
What is often unappreciated is that in many of the troublesome cases, entrainment plays a key role in tower base backup. Entrainment from kettle reboilers incurs a static head and more friction, which exacerbates kettle outlet-line pressure drop [ 3 ]. Furthermore, the entrained liquid drops get knocked out in the tower and return to the kettle, raising the flows and pressure drops through the kettle circuit.
Entrainment from kettle reboilers occurs when the disengagement space above the bundle is insufficient to disentrain liquid droplets. Several criteria and rules of thumb for preventing entrainment are available, the most popular of which is keeping the top tube row not higher than 60% of the shell…