(81g) Caustic Tower Operation – Considerations for Effective Performance

Caustic Tower operation is a key factor to the achievement of on-specification ethylene and propylene production. The tight limit on the allowable contaminant content levels driven by polymer producers necessitates close scrutiny and evaluation of the operation and performance of the Caustic Tower System.

Acid gases are created during the thermal cracking of any given steam-cracker feedstock. The preferred choice for removing these acid gases from the cracked gas is by absorption using dilute caustic soda. The Absorber, or Caustic Tower, is designed and operated to remove these acid gases; namely carbon dioxide (CO2) and hydrogen sulfide (H2S) and to a lesser extent some of the weaker acid gases such as mercaptans (R-SH).

This paper provides an overview of what the Caustic Tower process is, the acid gas and caustic concentrations and reactions of the process, and the key operating parameters. The impact of purchased caustic quality on performance is discussed, along with the handling of the fresh caustic solutions. Caustic Tower fouling and factors involved with its presence are described. Fundamentals of Caustic Tower controls and monitoring are provided to aid in the improvement of performance.