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Inadequate attention to level can carry significant consequences well beyond the particular vessel
Removing water or moisture from a liquid stream is a critical operation in many processes. Liquid stream dryers commonly contain either molecular sieves or adsorbents; during the last 20 years, the use of molecular sieves has become much more common. Current large-scale applications include water removal from 95% ethanol, as well as from hydrocarbons.
After being in service for a while, both molecular sieves and adsorbents require replacement or regeneration. Molecular sieve regeneration typically relies on a heated gas stream to desorb the water. As we will discuss, carryover of unexpected materials into the dryer can complicate matters.
A caustic wash was used to pretreat propane (LPG) product from a natural gas plant to remove mercaptans. After the caustic wash and amine treating, the LPG went through a dryer before going to refrigerated storage (Figure). This particular time, the plant rapidly ramped up LPG production. Due to poor control on the caustic wash vessel, the increased LPG rate caused caustic to be carried overhead from the caustic treater. The entrained caustic upset the amine unit and, in turn, led to the carryover of a caustic and amine mixture into the dryer. The upset was not noticed by the plant and normal operations continued for about two days.
Then, the plant initiated the next dryer regeneration. The hot regeneration gas caused the caustic-amine-impregnated molecular sieve to set like concrete. The dryer had to be removed from service and the agglomerated sieve jack-hammered out.
No one had appreciated that it was essential to keep level under control in the wash section. Following the hard lesson from this event, procedures were changed and training was updated to emphasize the importance of maintaining proper upstream control on the caustic wash and avoiding sudden feed changes.
The plant implemented two equipment changes to prevent repetition of this problem:
• A derivative limit was added to the upstream depropanizer overhead accumulation drum that feeds the caustic wash to prevent sudden changes in feed rate without an operator override.
• Both the caustic wash and amine vessels were given an additional, independent alarm to notify the operators of high water-phase liquid levels.
Effective plant operation requires an understanding of the ultimate consequences of unexpected, but possible, feed changes that might result from control failure. Often, a bit of extra instrumentation can provide cheap insurance against extreme consequences.
By Andrew Sloley, contributing editor, and Richard Readshaw, senior staff process engineer for VECO USA Inc. E-mail them at [email protected] and [email protected], respectively.