Red Oil Fouling

A caustic tower is one of the key units in an ethylene plant. Steam cracking furnaces produce ppm quantities of H2S and CO2, both of which pose problems for downstream processes: H2S is a poison for the acetylene converter catalyst, and CO2 is a product contaminant. Caustic (sodium hydroxide, or NaOH) is a strong base that can react with acid gases such as H2S and CO2 to remove them from the cracked gas. Ethylene plants have a caustic tower, and sometimes an amine tower (another acid gas removal unit) upstream when the plant was designed to process very sour feeds. Caustic towers are often divided into sections of different “strengths” of caustic, or amount of residual NaOH remaining. The bottom section where the sour gas is fed is the weak section, then the intermediate section is above that, and the strong section is at the top. Spent caustic is drawn off from the weak section at the bottom, and fresh makeup caustic is added to the strong section at the top.

H2S and CO2 aren’t the only undesirables in the sour gas feed to the caustic tower. Acetaldehyde is also formed in steam cracking furnaces at ppm levels, especially for heavy feed crackers. One thing that is interesting about acetaldehyde is that it has three alpha hydrogens that are somewhat acidic:

When acetaldehyde reacts with caustic, it deprotonates and attacks a second acetaldehyde via the Aldol reaction:

Elimination completes the base-catalyzed Aldol condensation reaction and forms crotonaldehyde:

Crotonaldehyde is actually easier to deprotonate than acetaldehyde. Once formed, it deprotonates and continues to oligomerize:

This oligomerization forms highly conjugated aldehydes that can react with other oligomers or diolefins in the cracked gas to form foulant deposits. At first, these deposits are not difficult to remove with simple solvent washes; over time, however, cross linking reactions render the foulant insoluble. To ensure optimal caustic tower performance, a caustic fouling mitigation plan needs to be in place.