Kolmetz.Com is a chemical engineering web site that publishes technical articles on distillation, process optimization, operations training, personal improvement, process unit safety and environmental concerns.
CHALLENGES IN A ETHYLENE UNIT DILUTION STEAM GENERATOR SYSTEM
Siang Hua Lee - Titan Petrochemicals (M) Sdn Bhd
Phaik Sim Cheah - Titan Petrochemicals (M) Sdn Bhd
Karl Kolmetz - Westlake Group, Lake Charles, USA
In the thermal cracking of Naphtha, steam is added to reduce the partial pressure of the hydrogen and shift the equilibrium to produce more ethylene. A Dilution Steam Generator (DSG) or a Saturator adds this steam to the feed.
A Dilution Steam Generator receives water from the Quench Tower after pretreatment and vaporizes this water. A Saturator receives water from the Quench Tower after pretreatment and using a trayed or packed column saturates the feed utilizing counter current flow with the re-circulating heated water.
Corrosion, erosion and fouling in the DSG and Saturators are not an uncommon problem. Thus, most systems are designed with a spare for cleaning and repair. Titan has also faced corrosion and erosion challenges in the DSG system in Cracker One particularly at the DSG reboilers, LP water stripper tower, DSG feed pumps and dilution steam condensate lines.
The DSG reboilers in the Cracker Two are beginning to show signs of similar challenges. The type of corrosion observed in Titan is of low pH under-deposit form, normally at the waterline area where two phases exist. So far minimal fouling has been detected. Frequent cleaning and retubing of the reboilers has been required. Water losses via blowdown to the wastewater due to leaking tubes have also increased operating cost.
The LP water stripper has seen acid corrosion and steam erosion resulting in reduction of the overall thickness of the shell and conical section of the tower.
DSG feed pumps casing and impeller have experienced coke erosion and corrosion problems. Sections of the condensate lines have experienced weak acid corrosion and erosion due to water hammering and condensate flashing.