(6d) Debottlenecking of a Large Industrial Deethanizer By Gravity Driven Two-Phase Flow Analysis

A large industrial Deethanizer in a NGL fractionation plant located in Mont Belvieu, Texas USA exhibited performance issues in its dehydration section since start-up. This problem forced the operations team to reduce rates and to operate at higher than design pressure to avoid hydrates. Troubleshooting efforts to resolve the problem included mechanical and hydraulic analysis of the internals and performing several column gamma scans. The mechanical integrity of the internals was suspected to be part of the problem. However, several gamma scans on the tower showed flooding conditions in the top of the dehydration section. A fluid dynamics analysis of the hydrocarbon drain pipe bypassing the dehydration section indicated the presence of two-phase flow.

The two-phase flow in the drain pipe explained the flooding conditions at the top of the dehydration section. A gamma scan of the drain pipe was performed and confirmed specific gravity differences at different elevations. The implementation of simple but unique piping modifications addressed the multiphase flow in the hydrocarbon line. The Deethanizer is currently operating at normal production rates.

Today, practicing engineers are still unable to predict with confidence whether a two-phase flow will develop. Many similar piping configurations are found in the chemical process industry, and this paper attempts to provide insights to prevent a recurring design problem.