Extractive Distillation Tower Troubleshooting after Internals Revamp
Bailee Yarbrough, Senior Process Engineer, Process Engineering Separations Member, Dow Inc., Freeport, TX
Chet Davidson, Process Engineering Separations Expertise Area Leader, Dow Inc., Freeport, TX
Tim Cooper, Global Technology Leader, Dow Inc., Plaquemine, TX
E-mail Addresses:
Extractive distillation can make difficult separations feasible with the addition of a solvent, making it an efficient separation method for close boiling components. After a revamp at a Dow plant, an extractive distillation tower was unable to meet the design rates and desired product recovery. The tower had been reconfigured from consisting of all packed internals to including a section of high capacity trays. Immediate flooding after start-up resulted in a shutdown and reinspection of the internals, which showed no damage or fouling. After the second start-up, erratic column behavior continued, temperatures swinging as much as 30 degrees Celsius in one section of the tower, in a matter of minutes. The tower proved difficult to control and still exhibited flooding tendencies at higher rates, even though these rates were almost 15% below the design rate. Improper instrumentation installation led to misleading pressure drop measurements.
Our team was tasked with troubleshooting the tower to provide stable operation and determine how to get the tower towards design rates and target recovery of the product. An assortment of troubleshooting techniques was used to aid in diagnosing the tower including gamma scanning and various lab experiments, which ultimately led to the discovery of the root cause. The gamma scan showed both maldistribution and a rapidly changing density during the scan timeframe, consistent with the temperature fluctuation readings. However, the scan showed no evidence of flooding or severe fouling at the time of the scans. This presentation will focus on the tower’s problems, troubleshooting techniques including the interpretation of the gamma scans, and ultimately the root cause of the premature flooding. It will also highlight the impact of changing internals from packing to trays.
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