UBC Theses and Dissertations
Degradation of diethanolamine solutions by carbonyl sulphide and carbon disulphide Dawodu, Olukayode Fatai
The common industrial practice of using aqueous solutions of diethanolamine (DEA) for the removal of impurities such as carbon dioxide (CO₂), hydrogen sulphide (H₂S), carbonyl sulphide (COS) and carbon disulphide (CS₂) from natural, refinery and manufactured gases often entails irreversible reactions between the solvent and the impurities. This phenomenon is referred to as amine degradation and it not only constitutes a loss of the amine but may contribute to operational problems such as foaming, corrosion and fouling. Degradation of DEA by COS and CS₂ was studied by using a 600 mL stainless steel reactor under the following conditions: DEA concentration 10 - 40 wt%; temperature 120 - 195 °C; COS partial pressure 345 - 1172 kPa; CS₂ volume 2.5 - 10.5 mL (CS₂/DEA mole ratio of 0.055 - 0.233). An analytical procedure consisting of gas chromatography (GC) and gas chromatography/mass spectroscopy (GC/MS) was used to identify over 20 compounds in the partially degraded DEA solutions. The major degradation products are monoethanolamine (MEA), bis hydroxyethyl ethylenediamine (BHEED), bis hydroxyethyl piperazine (BHEP), hydroxyethyl oxazolidone (HEOD), hydroxyethyl imidazolidone (HEI), tris hydroxyethyl ethylenediamine (THEED) and bis hydroxyethyl imidazolidone (BHEI); as well as a dithiocarbamate salt (in the case of the CS₂-DEA systems). In addition, both COS and CS₂ induced degradation formed solid products which were characterized on the basis of solubility, melting point, elemental composition, solid probe GC/MS and infrared analysis. The number of degradation compounds in the COS-DEA and CS₂-DEA systems is large when compared with the three major degradation compounds found in CO₂-DEA systems; this demonstrates that the former systems are distinct and more complicated than the latter system. When COS or CS₂ was contacted with aqueous DEA solution, hydrolysis occurred and H₂S, CO₂, COS and, possibly, CS₂ together with their related ionic species were present in the system. Solubility and hydrolysis experiments were therefore conducted to establish the equilibrium composition of the COS-DEA system prior to the commencement of degradation. A modified Kent-Eisenberg (K/E) model which was developed to correlate the experimental data, showed good agreement between the experimental results and model predictions. Since the K/E and previous models were limited to amine-CO₂ and/or H₂S systems, the present modified K/E model which incorporates COS, is a significant improvement. The rate of degradation of DEA was found to increase with temperature, DEA concentration, COS partial pressure and CS₂ volume. On the basis of the experiments conducted to evaluate the contributions of the various compounds in the partially degraded solutions, reaction schemes were developed for the formation of 18 degradation compounds in the COS-DEA and CS₂-DEA systems. Despite the complexity of the reactions, the overall degradation of DEA was well represented by a first order reaction for the present experimental conditions. A mathematical model based on the major reaction schemes was developed to estimate the concentrations of DEA and the major degradation compounds in the COS-DEA system. Contrary to literature information, experiments conducted with gas mixtures of CO₂ and showed that H₂S enhanced the rate of DEA degradation. A direct result of the combined effects of H₂S and CO₂ on alkanolamines was the production of the corresponding lower order alkanolamines from higher order ones. The resulting mixed amine solution increases the routes for degradation compared to single amine solutions. The study therefore provides an indication of what to expect in terms of degradation when mixtures of alkanolamines are used for gas sweetening.
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