(145c) Minimizing Fouling in Ethylene Primary Fractionators

Braskem is a Brazilian petrochemical manufacturer that operates six olefins plants at four different locations. The company produces 3,500 Kty of ethylene using naphtha as their feed and gas steam cracking as their method. The main function of their primary fractionator tower is to separate the light fuel oil (LFO) fraction from heavy fuel oil (HFO) and to cool the mixture effluent from transfer line exchangers (TLEs), with the recovered heat used in other parts of the plant.

In 2002, one of Braskem’s plants underwent a major renovation, and the 16 single-pass trays in the top tower section of the primary fractionator were replaced. Prior to the replacement, process troubles and bad feed specification contributed to a continuous and high rate of fouling in the system, mainly in the lower fractionator trays. This resulted in a severe capacity loss due to premature flooding, high tower pressure drop (almost double of normal value), abrupt and severe tower bottom level reductions during furnace feed slate switches, separation efficiency reduction, high concentration of heavy components in the pygas, and difficulties controlling quench oil viscosity.

Braskem performed several studies to understand the causes of the tower malfunctions, focusing on internal design and the influence of process variables. Recently, a sampling system was installed to obtain samples from the lower fractionator tray down comer for analyses. The asphaltenes concentration and soluble polymers found in these samples are conducive to quench oil flooding, resulting in fouling accumulation on the tray. These complex fouling mechanisms are coherent to the reactive precursors’ characteristics and lower section tower flooding, which results in polymerization and have been found on some trays above. Therefore using only an antifoulant chemical treatment is insufficient and requires operational changes. Additionally, we were unable to find the maximum allowable values of this fouling accumulation in existing public literature. We decided to implement internal benchmarking between our olefins plants and to identify the key operational parameters required to reduce these heavy component concentrations in the trays. Our conclusion is that the heavy components present in the primary fractionator trays should be monitored and controlled in order to reduce operational troubles.