During an ethylene plant turnaround, a 38-year-old tube bundle of a vertical thermosiphon reboiler was replaced with a new, seemingly identical one. After the turnaround, the reboiler was not able to reach the same duty as with the old tube bundle. The heat duty loss was substantial.
Systematic troubleshooting using detailed column and thermosiphon simulation, neutron backscatter, plant data reconciliation and flow regime analysis ruled out possible reasons such as thermosiphon inlet line plugging, or lowered heat transfer due to steam superheat, or the presence of non-condensables in the steam .
To investigate the reduced heat transfer, light, laser and electron microscopy studies revealed that the gradual degradation of the tube surface, which occurred over the years, produced a much rougher surface than that of the new tubes. This rougher surface is likely to have provided a highly extended microsurface area and large number of nucleation sites that have increased the maximum heat transfer duty of the old reboiler. This theory is supported by a report from the old tube bundle’s first year of operation when similar capacity problems occurred.
This article discusses why replacing old equipment with new may not always result as predicted.