(32c) CSTR Study of Aldol Polymerization Inhibition in Ethylene Plant Caustic Towers

The caustic tower in an ethylene plant removes acid gases H₂S and CO₂ from cracked gas. The cracked gas flows upward while a caustic solution flows downward in a liquid/gas absorption tower that operates at 110 ± 10 °F. The acidic species are absorbed into the caustic where they react to form water and sodium salts. If not removed, these components will cause process equipment corrosion, catalyst poisoning, and ethylene product contamination. The primary concern in caustic towers is fouling and the most common mechanism of polymer formation is the aldol condensation of acetaldehyde. Aldol polymer is formed by the base catalyzed polymerization of acetaldehyde which continually grows in length if left unchecked. As the polymer increases in molecular weight it precipitates from solution leading to fouling. Chemistries exist that react with acetaldehyde and prevent polymer formation and are typically aqueous solutions of amines or hydrides from various sources including amino acids.

Historically aldol polymerization and its inhibition is studied in the lab in 8 oz bottles. Interestingly, the results can vary depending on order of addition, testing temperature, sampling time and the acetaldehyde source. This variation has been used by various suppliers to show their chemistries superiority vs their competition, causing confusion among the ethylene producers. As part of a larger project, Nalco Champion undertook an in-depth study of the variations of testing.

This paper will focus on the test conditions and variations in results as well as a new testing scheme that appears to better mimic the caustic tower conditions. When the lab evaluation method was moved from batch to semi-batch and then to continuous mode, greater insights on the mechanism of inhibition that reflects field performance was obtained. The use of Evaporative Light Scattering Detector proved to be a powerful method in quantifying aldol polymer as a function of time.