Improve the Reliability of High Flux Reboilers
The reliably of high heat flux reboilers can be a challenge. The tube life can be at times as low as three months between average mean failure. The proper operation and chemical treatment program can extend this average mean failure to greater than a 400% improvement.
Corrosion is an operating challenge encountered in the DSG reboilers for a typical liquid cracker ethylene unit. Low or high pH and / or oxygen typically cause corrosion. With medium pressure steam as the heat source, DSG reboilers operating at high heat flux or high percentage of water evaporation with respect to circulation flow, can experience departure from nucleate boiling and formation of steam / water interface. These phenomena promote organic / inorganic fouling and can lead to under-deposit localized high pH caustic gouging corrosion, if free caustic is present in the DSG reboilers water or low pH under-deposit corrosion.
Corrosion can also occur in the dilution steam itself if the steam is wet, although this is not a common problem. Tube leaks are the most common cause for shutdown of DSG reboilers. If the reboilers are heated by quench oil, tube leaks could result in massive organic fouling on waterside from quench oil contamination.
A novel operation and chemical treatment approach has proven the present reboilers treatment program at the DSG generation system can be revised to provide a more robust treatment approach. This is due to the fact that the incoming water into the system is pure and hence does not have a buffering capacity. Any change in the feedwater chemistry will lead to pH variations and cause problems.
The benefits of the new approach, while combining with the organic dispersant for hydrocarbon dispersion and amines for neutralization treatment to the DSG can be summarized as follows:
- ·Water in the reboiler has higher buffering capacity which enables it to accept feed water variations and not cause the pH to vary
- Addition of pH booster will neutralize organic acid and prevents under-deposit corrosion through concentration effect.
- Additional polymeric dispersant add another defensive measure which prevents iron deposition.
The correct analysis of a high heat flux duty reboiler combined with proper operation and treatment approach has greatly improve the reliability of an Ethylene Unit's Dilution Steam Reboilers