To ensure the successful chemical treatment of a caustic system, fouling mechanisms must be identified and their respective contributions to the overall system fouling evaluated. A variety of methods were used to investigate samples from the customer’s caustic tower. The tests included soluble polymer and filterable solids for the caustic tower streams, nuclear magnetic resonance (NMR) analysis of the skim oil, deposit analysis and Baker Hughes’ proprietary caustic reflux testing to optimize the treatment program. The test samples were skim oil from the neutralization tank, spent/recirculating caustic streams from both the strong/weak sections, and deposits from the caustic tower.
NMR analysis showed the presence of aldol polymer in the skim oil, indicating aldol condensation reactions were involved in the fouling mechanism. Infra-red (IR) analysis of the deposit indicated the presence of a carbonyl group, which confirmed that aldol condensation reaction also contributed to the fouling reaction. The Gas Chromatograph / Mass Spectrum (GC/MS) and pyrolysis GS/MS indicated the presence of free-radical precursors in the deposits, confirming the presence of free radical polymerization.
Testing indicated that free radical polymerization and aldol condensation were strong contributors to the fouling mechanism in this caustic tower. Caustic reflux testing was used to evaluate and screen inhibitors. Based on the caustic reflux test, the free-radical polymer inhibitors and inhibitors of aldol condensation reactions significantly decreased the foulant formation.
Reflux testing provided additional confirmation that the free radical polymerization mechanism and the aldol condensation mechanism were significant contributors to system fouling. Reflux testing showed that the combination of aldol condensation and free radical polymerization inhibitors reduced ~ 82% of foulant in this caustic system. The combination of inhibitors had a synergistic effect, reducing polymer formation more than the effect of introducing each of the inhibitors separately.