UNDERSTANDING CORROSION IN HOT POTASSIUM CARBONATE (HPC) ACID GAS REMOVAL PLANTS

Harjac, Sandra Josipa (2007). UNDERSTANDING CORROSION IN HOT POTASSIUM CARBONATE (HPC) ACID GAS REMOVAL PLANTS Master's Thesis, School of Engineering, University of Queensland.

       
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Author Harjac, Sandra Josipa
Thesis Title UNDERSTANDING CORROSION IN HOT POTASSIUM CARBONATE (HPC) ACID GAS REMOVAL PLANTS
School, Centre or Institute School of Engineering
Institution University of Queensland
Publication date 2007
Thesis type Master's Thesis
Supervisor Associate Professor Andrejs Atrens
Abstract/Summary A study was undertaken in order to understand corrosion in hot potassium carbonate (HPC) plants. A plant focused approach was also necessary which involved a review of historical plant based information and the development of a HPC plant corrosion risk register. The information fiom this study is useful to HPC plants to ensure adequate controls and defences are in place to mitigate corrosion risks to HPC plant integrity. A review of the operational history of HPC acid gas removal plants has shown that corrosion problems have been long standing. Usually a recalcitrant long-standing problem can be identified as occurring at a specific time and be linked with a change to process 1 plant operating conditions and hence it is beneficial to widely review the operating history of a process. Part of this study involved reviewing plant based reports and findings, years of operating history and inspection fmdings for the Santos Moomba gas plant in the Cooper Basin (South Australia) using certain aspects of the six sigma methodology. Review has shown corrosion may be triggered by a number of events. The possible root causes were identified as (1) inadequate concentration of anodic inhibitor, (2) the presence of hydrocarbons, (3) process conditions of high gas loading and/or fiothinglfoarning, (4) formation of a sulfide layer and (5) galvanic interaction. An overview of the Six Sigma methodology is included in this study together with a timeline of plant changes made during the mid 1980's. Electrochemical studies were also conducted and the work presented in this thesis introduces new findings that can be incorporated in HPC plant integrity management. This research studied the influence of steel surface condition and solution chemistry on the critical inhibitor concentration required for spontaneous passivation of carbon steel in solutions typical of those used in a HPC plant. The passivity breakdown of carbon steel in conditions typical of HPC acid gas processing plants was also studied. HPC solution is anodically inhibited. It was determined that the critical inhibitor concentration depended on solution chemistry and on the steel surface condition. An inhibitor concentration of 30 g/l may be required to ensure spontaneous passivation under all conditions. The spontaneous passivation of clean polished carbon steel surfaces required a critical inhibitor concentration of 0.5 to 1.8 gll. A minimum level of v5+ is required for inhibition, so that monitoring the v5+ concentration may be crucial to successfully managing corrosion protection in plant. The pitting potential, evaluated from polarisation curves, indicated resistance to pitting increased with increasing carbonate concentration. The critical chloride concentration was considerably above the maximum concentration of 0.1 wt% recorded in a typical HPC plant. Consequently chlorides should not usually be of concern to plant integrity, but plant chemistry limits should specify that solution chloride concentration be maintained below 0.5 wt% at all times. The presence of DEA has an inhibiting effect on HPC plant corrosion, lowering the corrosion rate of steel in rich solution (29 eqv. wt% K2CO3) by an order of magnitude.

 
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