(33c) Mitigation of Fouling From Vinyl Acetate Recycle to Ethylene Plant

LyondellBasell’s Ethylene plant located in Clinton, Iowa, receives a purge stream from the adjacent polyethylene unit.  This purge stream, which is returned to the quench tower, contains vinyl acetate (VA).  The VA, which is used as a comonomer in the polyethylene unit, polymerizes in the presence of excess caustic to form aldol polymer that contributes to fouling in the caustic tower.  In addition, there is the energy cost associated with reprocessing the ethylene in the purge stream.  A VA recovery system in the polyethylene unit, which was not performing as expected, and a chemical (antipolymerant) injection system in the ethylene unit were in place to treat the fouling problem.

 In order to reduce the operating cost of the fouling problem, the VA recovery system was modified.  The demisters in the knockout drums in the VA recovery system were replaced with a more efficient material, reducing the amount of VA being returned to the caustic tower by approximately two thirds, and therefore allowing a considerable drop in chemical usage. 

Implemented in April, 2009, this change is realizing a reduction in chemical treatment cost based on the lower amount of VA contained in the recycle stream, which paid for the project after one month.