During an ethylene plant turn-around, a 38-year-old tube bundle of a vertical thermosiphon reboiler was replaced with a new, seemingly identical one. However, when the plant returned to full operation after the turn-around, the reboiler was not able to reach the same duty as the old tube bundle.
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, leaving reduced heat transfer on the tube side as the strongest suspect.
To avoid plant shut-down, the issue was temporarily solved by switching from low pressure (1.6 barg) to medium pressure (7.2 barg) steam as heating medium. With the higher steam pressure and increased ?T across the exchanger the original duty was successfully recovered.
To investigate the reduced heat transfer, light, laser and electron microscopy studies revealed that the surfaces of old and new reboiler tubes were very different in roughness and cleanliness, causing differing heat transfer characteristics. The gradual degradation of the tube surface, which occurred over the years, is believed to 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.