(187g) Troubleshooting a Thermosiphon Reboiler - Why New Is Not Always Better Than Old

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 was conducted for the reboiler, using tools including detailed column and thermosiphon simulation, neutron backscatter scanning, plant data reconciliation and flow regime analysis. 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, were ruled out one by one. This left the investigation team with reduced heat transfer on the tube side as the strongest suspect for reboiler malfunction.

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 ÄT over the exchanger the original duty was successfully recovered.

To investigate the reasons for the reduced heat transfer, a metallurgy laboratory was contacted. Results of the 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 between the old and the new tubes. 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.