Role of aqueous methyldiethanolamine (MDEA) as solvent in natural gas sweetening unit and process contaminants with probable reaction pathway

Aqueous methyldiethanolamine (MDEA) is used as a solvent in natural gas sweetening unit, whose quality deteriorates while absorbing H2S/CO2 causing corrosion and foaming in the process. Total process contaminants of lean MDEA obtained from GASCO Company (Habshan, Abu Dhabi) were determined using different instruments. The major hydrocarbons found in inlet natural gas was analyzed using gas chromatography mass spectrometry (GC-MS). Heat stable salt (HSS) anions and metal ions were detected using ion chromatograph (IC) and inductively coupled plasma optical emission spectrometry (ICP-OES). Liquid chromatography mass spectrometry (LC-MS) and direct sample analysis time of flight (AxION 2TOF) analysis were carried out to determine and major organic degraded products present in lean MDEA solvent. The UV-VIS spectrophotometer also justified the presence of total organic acid anions. Acetate and propionate were found to be present in higher amounts of 2806 and 1614 ppm, respectively. Substantial amount of metal ions were also detected using ICP-OES with maximum level of heavy metal ions of lead and iron of 1.009 and 0.967 ppm, respectively. Large numbers of organic degraded products such as diethanolamine followed by methylethanolamine, N,N-bis(2-hydroxyethyl) glycine (bicine) etc. were identified using LC-MS and AxION 2TOF analyses; which are considered to be major organic thermally degraded products. Thermal degradation experiment was compared with H2S and in presence of both H2S/CO2 to obtain same compounds with varying concentration. On the basis of MDEA degradation as reported by previous researchers and from the analyzed lean MDEA samples, probable reaction pathways have been proposed. (C) 2015 Elsevier B.V. All rights reserved.