There are several types of CO2 absorption processes. Particularly in the process with hot potassium carbonate solution, corrosion inhibitor is added to prevent corrosion of carbon steel process equipment. However, in some cases, the inhibitor may not work beneficially because of the improper process condition. In this study, the effect of gas-liquid flow and inhibitor concentration on corrosion of carbon steel was evaluated by laboratory experiment. The result of this experiment implied that corrosion had occurred at the area of the specimen where the solution was not enough provided. This corrosion occurred even if there was enough amount of inhibitor in the solution. In addition, the black scale consisting of ferrous carbonate on the surface of specimen has locally removed in the gas-rich and liquid-lean condition. These results accord with the cause of damage observed on actual commercial plant. This study revealed the significance of considering gas-liquid flow dimension when designing a CO2 absorption process with hot potassium carbonate solution.
Many CO2 absorption processes with several types of absorption liquor are working all over the world. One of them is the process with hot potassium carbonate solution. Figure 1 is an example of the plant system of CO2 absorption process. The chemical reaction of CO2 absorption is given in the Equation 1.
Potassium carbonate solution contacts with mixed gas containing CO2 that entered the absorption tower. Then, carbonate ion in the solution changes into bicarbonate ion resulting CO2 gas absorption. Solution containing bicarbonate ion is fed into stripping tower and bicarbonate ion returns into carbonate ion, resulting CO2 gas discharge. The absorption liquor circulates and repeats absorption and discharge. Usually, the concentration of potassium carbonate solution used in the process is 20 to 30 wt%. The solution contains at least 2wt% diethanolamine(DEA) as absorption promoting catalyst and some inhibitor for corrosion protection. Sometimes, formic acid can accumulate in the solution as side product. Formic acid does not affect to corrosion in that system because it had been disacidified by potassium carbonate. Hot potassium carbonate solution is corrosive to carbon steel. But usually carbon steel is used as the material of the plant with corrosion protection by inhibitor because of the economic reason. Commonly V2O5 is used as the Inhibitor for corrosion protection. V2O5 in solution exist as a form of potassium metavanadate. Corrosion protection mechanism of Inhibitor is shown in Figure 2. Pentavalent vanadium ion in solution oxidizes the surface of carbon steel. As a result, the carbon steel surface is covered by the corrosion protective magnetite film. There are several reports1-3 referring about the ideal concentration of the inhibitor. One of them says 0.7-1.5wt%. Present study started from the experience of corrosion damage on the real plant and it researched about Inhibitor corrosion protection affected by gas-liquid flow with experimental method.