(95c) Fouling in the Caustic Tower – Mechanism and Mitigation

In the ethylene plant, by products of the cracking process include acid gases such as CO2, CO, and H2S and carbonyls. The acid gases and carbonyls are contaminants for the olefine products, as well as a corrosion risk in processing equipment. To remove these contaminants, ethylene plants employ an acid gas removal treatment operation. This operation is typically performed via scrubbing the process gas with caustic to neutralize the acid gases via contact in a Caustic Tower. The spent caustic is then treated or disposed of in a wet air oxidation unit, as wastewater or other means.

Certain of the contaminants can undergo polymerization to create polymer that can then lay down on trays or packing in the Caustic Tower, causing a variety of operational issues. These issues include increased differential pressure, reduced scrubbing efficiency, foaming, and ultimately can result in a throughput limitation and even premature shutdown if the Caustic Tower fouling is severe enough. The foulant in the caustic tower has several various names in the olefines production vernacular, such as “red oil” or “red tide”, as well as others that are more descriptive about the physical description of the foulant material.

The purpose of this tutorial is to delineate the various mechanisms for fouling in the Caustic Tower, their impacts on plant operation, and then the various methods that can be employed to mitigate this fouling. Mechanical, operational, and chemical methods will be included.