Published: November 04, 2011 r2011 American Chemical Society 4720 dx.doi.org/10.1021/je200709h | J. Chem. Eng. Data 2011, 56, 4720–4724 ARTICLE pubs.acs.org/jced Carbon Dioxide Solubility and Monoethylene Glycol (MEG) Degradation at MEG Reclaiming/Regeneration Conditions Maria N. Psarrou,* ,† Leif O. Jøsang, † Kristian Sandengen, ‡ and Terje Østvold † † Department of Materials Science and Engineering, NTNU, N-7491 Trondheim, Norway ‡ STATOIL Research Centre, Rotvoll, N-7005 Trondheim, Norway ABSTRACT: Monoethylene glycol (MEG) is a gas hydrate inhibitor. The large amounts used in gas field industry impose the need for MEG regeneration. During this process the waterÀMEG mixture is heated at low pressure to remove most of the water. It is important to know the carbon dioxide equilibria under MEG regeneration conditions to prevent problems related to mineral precipitation and scale formation. The equilibrium amount of total CO 2 in solution was therefore examined in waterÀMEG solutions at typical MEG regeneration conditions [(50 to 98) % wt MEG, (80 to 140) °C, (50 or 100) mmol 3 kg À1 total alkalinity]. A CO 2 /N 2 mixture of known composition (0.200 % or 0.050 % mol CO 2 ) controlled the CO 2 pressure in the reaction vessel. Discoloration was observed in some experiments, and ion chromatography showed that glycolic and formic acids were the dominating MEG degradation products. Data for the total CO 2 concentration at equilibrium are reported together with a discussion describing why common acid titration is still a valid analysis method although significant amounts of organic acids were present. ’ INTRODUCTION Transporting hydrocarbons and water in long subsea flow lines from satellite fields to a platform or to shore results in new challenges to control hydrates, corrosion, and mineral scale. As the fluids cool down, water will condense, and gas hydrates will form, unless an inhibitor such as monoethylene glycol (MEG) is present. Due to the large amounts of MEG being used for hydrate control, it is usually necessary to recycle it. The main processes for recycling of MEG are the regeneration and reclamation processes. The former implies that the incoming waterÀMEG solution “rich-MEG” is simply heated in a distillation column to remove most of the water. Typical conditions will be ambient pressure and 120 °C. The bottom product is the desired “lean MEG” of around 90 wt % MEG. Salts or other contaminants will consequently follow this “lean MEG” phase. In a reclamation process the waterÀMEG solution (either “lean” or “rich MEG”) is evaporated to remove salts and other contaminants. This is typically achieved under vacuum and at (120 to 150) °C. Sodium bicarbonate is usually added to avoid corrosion in the pipelines. The added bicarbonate will end up in the MEG recycle facility, and to control scale in such a facility it is necessary to have knowledge of the carbonic acid equilibria in eqs 1 and 2. 2HCO 3 À f CO 2 ðgÞþ CO 3 2À þ H 2 O ð1Þ CO 3 2À þ H 2 O f CO 2 ðgÞþ 2OH À ð2Þ In regeneration/reclamation units the CO 2 partial pressure is low, and the reactions, eqs 1 and 2, will be driven to the right releasing CO 2 and increasing pH. According to the definition of alkalinity 1 as the sum of all titratable bases, alkalinity (A T ) for the carbonate system in an aqueous solution (eqs 1 and 2) is given in eq 3 where m denotes concentration (mol 3 kg À1 solvent). The total dissolved CO 2 content (m CO 2 tot ) for this system is described in eq 4 where CO 2 (aq) is negligible at the conditions of interest. A T ¼ m HCO 3 À þ m OH À þ 2m CO 3 2À À m H þ ð3Þ m tot CO 2 ¼ m HCO 3 À þ m CO 3 2À þ m CO 2 ðaqÞ ≈ m HCO 3 À þ m CO 3 2À ð4Þ In reclamation units, the alkalinity can be up to 1000 mmol 3 kg À1 . In this work a lower alkalinity [(50 to 100) mmol 3 kg À1 ] typical for regeneration units was chosen. Carbonate equilibria in MEGÀwater solutions have been studied in literature mainly in connection with salt precipitation. Fosbøl et al., 2 G€ artner et al., 3 and Oosterhof et al. 4 provide elaborate data sets on the solubility of the different species in the NaHCO 3 ÀNa 2 CO 3 ÀMEGÀwater system. Sandengen 5 obtained data for the solubility of sparingly soluble carbonates CaCO 3 , BaCO 3 , SrCO 3 , and 3MgCO 3 3 Mg(OH) 2 3 3H 2 O, while Flaten et al 6 examined the polymorphism of calcium carbonate under MEG reclamation conditions. Thus salt solubility data are available in the literature, but to predict precipitation one obviously also need to know the equilibrium constants of eqs 1 and 2. Hayduk and Malik 7 published a series of data for the whole range of MEG fractions at 25 °C. Later Sandengen 5 extended the range to (25 to 90) °C, with and without NaCl in solution. Recently, Kan and Lu 8 conducted experiments at (3 to 70) °C investigating the salt effect (NaCl) on CO 2 partitioning for this system. However in these studies the partial pressure of CO 2 is a lot higher (P CO 2 ∼10 5 Pa) than in reclaiming and regeneration Special Issue: Kenneth N. Marsh Festschrift Received: July 11, 2011 Accepted: October 28, 2011