ABSTRACT

The paper discusses black powder formation in the corrosion experiments with field TEG and lab mixed TEG. TEG samples were collected at the gas receiving facilities and in the TEG dehydration units that delivered gas to a pipeline network. The objective was to determine the corrosion rates and production rate of solids, to better understand the rate controlling mechanisms and to find means to reduce the black powder problem. The experiments were run at 5, 25 and 40 °C, with the test gas consisting of 100 kPa CO2 INTRODUCTIONBlack powder is a term that describes the particulate materials that collect in gas pipelines.1-3 The export gas is usually dried in glycol contactors to meet the export gas specification (ref NORSOK M5064) before entering the pipeline. The gas will be saturated with the TEG from the glycol contactor and the amount is determined by the operational conditions of the scrubber after the TEG-contactor. TEG will condense as the temperature and pressure of the gas decreases after entering the riser/pipeline, and form a glycol film on the pipe wall. Glycol is hygroscopic and will extract water from the gas. Simulations with PVTsim6 at 5 °C and 15 MPa gives a liquid phase containing 5-10 wt% water when the pipeline is operated within the NORSOK specifications4 for dry gas. During operational disturbances the simulations show that the water in the TEG phase may increase to more than 20 wt% at the same temperature and pressure. The TEG-water mixture is corrosive, although much less corrosive than pure water. Studies reported by DeWaard et al.7-8 log f = 1.6(log(W%)-2) (1) where W% is the mass percentage of water in the water-glycol mixture. The glycol correction factor f is a multiplier that gives the corrosion rate in the X% glycol solution when multiplied by the corrosion rate obtained in a glycol-free solution of the same temperature, pH and CO2 partial pressure:(also confirmed in a number of other studies) showed that the corrosion rate in glycol-water mixtures decreases with increasing glycol content and that the effect of MEG, DEG and TEG can be expressed by the empirical glycol correction factor f: It can be various types of corrosion products in addition to other contaminants as salts, sand and dirt. The corrosion products (FeSX, FeCO3, Fe3O4 + other oxides) are formed when the internal steel surface is wetted with a water containing phase with dissolved CO2, H2S, organic acids and/or O2. The composition and the morphology of the products depend on the composition of the liquid phase and the relative concentration of the corrosive gases. and 40 Pa H2S and the salt content of 100 to 10000 ppm. The test duration varied from 60 to 102 days. Corrosion rates were determined from weight loss measurements and varied in the range 1-30 µm/year. The experiments showed that the corrosion rate increased significantly when solid iron sulfide accumulated on the steel surface and the liquid contained high amounts of dissolved salt.

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