Effect of n-methyl-2-pyrrolidone and methanol on the carbon steel corrosion in hybrid monoethanolamine solutions

Abstract

Carbon dioxide (CO2) absorption is a matured technology for CO2 capture and natural gas purification process. However, the presence of the acid gas in the unit operation poses corrosion issues which need to be addressed to ensure integrity of the operation system is uncompromised. The present study explores and provides comprehensive information on the effects of process parameter variations on the corrosion behaviour of carbon steel in CO2 absorption systems using methylethanolamine (MEA) blended with methanol (MeOH) or N-methyl-2- pyrrolidone (NMP) as aqueous and hybrid solutions. The process parameters of interest involved amine solutions type, temperature, and CO2 loading. The gravimetric and electrochemical experiments were carried out to investigate the corrosion behaviour of the carbon steel coupons in these solutions. The results indicated that the corrosion rate of carbon steels immersed in different amine solutions increased as the solution temperature and CO2 loading rose. The gravimetric results showed that the carbon steel coupons had the lowest corrosion rate were the ones immersed in MEA+MeOH. Both MEA aqueous and hybrid solutions were examined using Raman spectroscopy analysis and the surface of the carbon steel coupons subjected to the corrosion were studied using field emission scanning electron microscopy (FESEM). However, the Electrochemical Impedance Spectroscopy (EIS) results showed an opposite trend from the gravimetric results. This was because the carbon steel immersed in MEA+H2O had the highest resistance toward corrosion. For the electrochemical measurement method, unlike gravimetric measurement method, the surface of carbon steel coupons after the immersion was not scrubbed and straightaway brought for testing upon lifting from the solution. Hence, the high resistance detected could be due to the initiation of spontaneous passivation on the carbon steel coupons. Multivariable Power Least Squares Method (MPLSM) was adopted to further examine the relationship of the studied parameters to the corrosion behaviour of carbon steels immersed in solutions MEA+H2O and MEA+MeOH. The results showed a positive correlation for MEA +H2O with the immersion time as the most prominent effect affecting polarization resistance. As for the MEA+MeOH, it was found that the lower R2 value of 0.5 indicated that the current measured parameters were not sufficient to represent the response data set. This implies the interaction between parameters could also affect the polarization resistance. The knowledge gained from this project could contribute to industries in reviewing the impact of organic solvents and improving on the corrosion issue in the operation.