Management of Caustic Tower Fouling

Ethylene plants utilize caustic towers for the removal of acid gas (H2S/CO2) from the cracked gas downstream of the furnaces. Typically, caustic towers experience base-catalyzed polymeric fouling due to the inherent presence of condensed hydrocarbon in contact with sodium hydroxide solution. This fouling can become severe if left unchecked, so the summary herein outlines strategies for the management of caustic tower fouling. At the root, caustic fouling mechanisms include primarily aldol condensation but also free-radical and Diels-Alder reactions. Fouling progression is a function of many parameters, including the plant feedstock being cracked and associated impurities, the tower operating temperature, the location of the tower within the process, fouling precursors from recycle streams, and any chemistries added to the tower. Additionally, impurities in the fresh caustic solution can also play a role in the overall fouling propensity. All these factors offer levers to control system performance and reduce foaming or flooding impacts. To aid in the discussion, analytical methods for foulant characterization are outlined. Ultimately, a comprehensive understanding of the fouling within caustic systems leads to enhanced plant efficiency and product on-spec for H2S/CO2 concentration.