International Journal of the Physical Sciences Vol. 6(25), pp. 5877-5890, 23 October, 2011 Available online at http://www.academicjournals.org/IJPS DOI: 10.5897/IJPS11.237 ISSN 1992 - 1950 ©2011 Academic Journals Review Degradation studies of amines and alkanolamines during sour gas treatment process M. S. Islam 1, R. Yusoff 1, B. S. Ali 1*, M. N. Islam 2 and M. H. Chakrabarti 1 1Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. 2Department of Chemistry, Faculty of Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka-1000, Bangladesh. Accepted 5 August, 2011 Aqueous solutions of alkanolamines are widely used for the removal of acidic gases such as H 2S, CO 2, COS and CS 2 in the sour gas treatment process. In spite of the resistance of alkanolamines to chemical breakdown, plant and laboratory reports indicate that, on prolonged use, alkanolamines may be transformed into undesirable products from which the amines are not easily recovered. This phenomenon, commonly referred to as ‘amine degradation’, not only leads to amine losses, but may also contribute to operational problems such as foaming, corrosion, high solution viscosity and fouling as a result of decreasing the plant life. Amine based gas purification plants having absorption and stripping system, solvents are degraded at these two units mainly by oxidative degradation at absorber condition and thermal degradation at stripper condition. Identification of amine degradation routes, remediation of degradation, types of degradation products and studies on degradation reaction mechanism becomes essential for successful plant operation. Gas chromatography-mass spectrometry (GC-MS), ion chromatograph (IC), high performance liquid chromatography (HPLC), nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infrared absorption spectrometry (FT-IR) are extensively used to identify the various degradation products. A comprehensive review of the latest research activities on alkanolamines degradation, best method for identification, various properties of degradation products and their classification is presented in this paper. This study would help to develop better understanding of amine degradation. Key words: Degradation, alkanolamines, absorption, stripping, sour gas . INTRODUCTION Natural and refinery gases typically contain acid gases in The toxicity of H2S, and the lack of heating value of concentrations ranging from a few parts per million to CO 2, NG or LPG must be purified prior to liquification and tens of volume percent. The major acid gases are use, because of the corrosivity of acidic gases in the hydrogen sulfide (H 2S) and carbon dioxide (CO 2). presence of water. Amines and alkanolamines are widely used in natural gas refining for the sweetening of sour gas by removing CO 2 and H 2S. In the amine based purification plant alkanolamines of primary significance *Corresponding author. E-mail: [email protected]. include monoethanolamine (MEA), diethanolamine [email protected]. Tel: +603-79676896. Fax: +603-7967 (DEA), methyldiethanolamine (MDEA), di-2- 5319. propanolamine (DIPA) and diglycolamine (DGA). For the 5878 Int. J. Phys. Sci. last two decades the sterically hindered amine such as degradation products and methodology of DGP AMP (2-amino-2-methyl-1-propanol) has been suggested identification is very important for a better understanding as an attractive solvent for the removal of acid gases. of amine and alkanolamine based absorption and The advantages of AMP include high equilibrium loading stripping systems. capacity (Davis and Gary Rochelle, 2009). Aqueous The main objectives of this review work are to MDEA solution is used as a solvent for selectively represent the basic degradation study of MEA, DEA, absorbing H 2S (Lepaumiera et al., 2009; Blauwhoff et al., MDEA and PZ with possible degradation pathways. 1984; Closmann, 2009; Klare et al., 2000; Shahi et al., Additionally this work also explains the major cause and 1994; Lepaumier et al., 2009a; Saleem and Essa, 2010). effect of degradation products. However, in removing acid gases with alkanolamines and regenerating them, undesirable compounds are likely to be produced by the irreversible transformation of AMINE BASED ABSORPTION AND STRIPPING alkanolamines, which is called degradation. SYSTEM Degradation causes a loss of alkanolamine, and it may also contribute to operational problems such as foaming, For over seventy years, the alkanolamine process has corrosion and fouling. While some loss is expected in all been considered the best approach in removing H 2S and operations, extreme losses can negatively impact CO 2 for the purpose of purification and separation. It is economics of operating any amine unit. Degraded DEA based on the reaction between weak acid (H 2S and/or solutions usually contain hydroxyethyl oxazolidone CO 2) and weak base (alkanolamines) to give a water (HEOD) bis (hydroxyethy) piperazine (BHEP) and tris- soluble amine acid gas salt. An amine based natural gas (hydroxyethyl) ethtlendiamine (THEED), together with low sweetening plant was configured with absorber and concentrations of various higher boiling compounds regenerator. The temperature range is 40 to 60°C and (Dawodu and Meisen, 1996, 1993; Shahi et al., 1994; 100 to 120°C at the absorber and the stripper, Choy and Meisen, 1980; Dawodu and Meisen, 1991; respectively. The sweet gas obtained from the absorber Dawodu and Axel Meisen, 1994). The degraded solution alternatively exhausts sour gas components at the of MDEA or MDEA/PZ contains EG, DEA, TEA, DMAE, stripper. Figure 1 represents a sweetening process of HMP, BHEP, MP and DMP (Lepaumier et al., 2009a; sour gas. Lepaumier et al., 2009c, b; Oh et al., 2003; Amiri et al., Mainly, amine solvents are used as absorbents in the 2007; Carlson et al., 2001; Haws and Jenkins, 2000). absorber for sweetening natural gas and for prolong use Literature cited sufficient information on MEA degradation of this solvent that is transferred to the stripper by means products including volatile and solid substances. It of regenerating absorbed gases. Absorption and contains more than 15 different degradation products desorption are the common process in these two units identified by many researchers (Strazisar et al., 2003; occurring continuously during the circulation of amine Supap et al., 2006; Lepaumier et al., 2009c, b; Sexton solvent. Natural gas containing several acidic oxides is and Rochelle, 2009; Davis, 2009; Sexton and Rochelle, very harmful for environment and plant operation and for 2010; Strazisar et al., 2010; Haws, 2010). Haws (2010) this reason, these are removed prior to use. Sour gases noted that the degraded solution of DGA contains react with amine solvent during absorption-desorption BHEEU and BHEETU, DIPA contains HMPO. In addition cycles gradually at higher and lower temperature and to these impurities, resulting from acid gas induced amine some of these reactions are given on how the DGP is degradation, industrial amine solutions frequently contain formed as follows (Haws, 2010): other contaminants such as suspended solids, dissolved hydrocarbons, sodium and chloride ions, iron sulfide, HO-C H NH +Cl− ..................................... foam suppressants, corrosion inhibitors and heat stable HO-C2H4NH2 + HCl 2 4 3 .1 salts. The accumulation of contaminants in amine DEA HSS solutions creates major operational problems including + − .................................... degradation (Lepaumier et al., 2009a), foaming HO-C2H4NH2 + H2S HO-C2H4NH3 HS .2 (Thitakamol et al., 2009; Pauley and Reza Hashemi, DEA HSS 1998; Amiri et al., 2007), fouling (Axel Meisen and Abdi, 2010), corrosion (Dawodu and Meisen, 1996), a reduction in active amine content also causes an The degradation reactions 3 to 11 showed formation of increase in solution viscosity. The chemical and physical various degradation products of most common industri- nature of the contaminants makes the development of ally used alkanolamine with acid gas components. The purification schemes inherently difficult. In this regards, degradation reaction 7 also showed that DEA degraded an overview of the degradation process of amines into MEA and on the other hand MDEA was degraded to solvents, degradation reaction mechanism, impact of EG, DEA and TEA (reaction 8). Low molecular weight Islam et al. 5879 Acidic Gases Sweet Gas C1-C4 H2S, CO2 Lean Amine r r o t e a b r r e o n s e b g Sour Gas Components A e R CH4, C2H6,C3H8,C4H10 C5H12, H2O,H2S,C0,C02 N ,H ,O 2 2 2 ∑ Cooler ∑ Heat Exchenger Rich Amine Amines/Alkanolamines + Sour Gas Swwet Gas Natural Gas Components: Absorber Chemicals: CH4, C2H6, C3H8, C4H10, CO Sweet Gas + DGP Amines/Alkanolamines, Water ∆ C5H12, CO2, N2, H2, H2S, O2, H O 2 ∆ DGP= Degradation Products; = Absorber/ Stripper Temperature Figure 1. Sour gas sweetening process. amine MEA is not degraded into amine but it directly articles have been published in the field of degradation in forms DGP like OZD, HEI and HEED (Reaction 4). So, the last three decades. The prevention of amine loss, the higher molecular weight amines are degraded into amine degradation is one of the conditions for successful low molecular weight amines but low molecular weight plant operation and also the key to economic benefits. amines are not. This phenomenon of producing amines Hence, it is very important to identify the route of amine most probably accelerates the absorption process. For degradation. this reason,