Article pubs.acs.org/IECR
Inhibitors of Monoethanolamine Oxidation in CO2 Capture Processes Alexander K. Voice and Gary T. Rochelle* McKetta Department of Chemical Engineering, The University of Texas at Austin 200 East Dean Keeton Street, Stop C0400, Austin, Texas 78712-1589, United States S Supporting Information *
ABSTRACT: Aqueous monoethanolamine (MEA) is a good solvent for postcombustion CO2 capture; however, it is prone to oxidative degradation. The initial test conditions mimicked those found in the absorber of a commercial system: MEA was degraded in the presence of CO2, with high oxygen mass transfer, and with dissolved metals, at absorber temperatures. In later experiments, high temperature cycling was incorporated into the apparatus to better represent a real system. These cycling results suggest that the unadditized 7 m MEA in a real system contacted with 5% oxygen in the absorber and with the stripper operated at 120 °C would experience close to 5% amine loss per week. Inhibitors such as inhibitor A (Inh A), 2,5-dimercapto-1,3,4thiadiazole (DMcT), diethylenetriamine pentaacetic acid (DTPA), hydroxyethylidenediphosphonic acid (HEDP), and methyldiethanolamine (MDEA) reduced MEA oxidation at low temperature by more than 90%. Results of the screening study also confirmed the reliability of ammonia production as an accurate indicator of MEA oxidation under all test conditions, allowing for rapid and accurate screening of new additives. Unfortunately, none of the additives screened at low temperature significantly reduced oxidation with high temperature cycling; thus they are not recommended at this point for use in a commercial CO2 capture system.
1. INTRODUCTION Monoethanolamine (MEA) has many desirable properties for CO2 capture from flue gas, including low cost, high heat of absorption, fast reaction rate with CO2, low viscosity, and solubility at any CO2 loading. MEA also has acceptable cyclic CO2 capacity and moderate volatility, and has been extensively studied for gas treating applications over the past 60 years. These attributes make aqueous MEA (typically 30−40 wt %) an attractive solvent for postcombustion CO2 capture from coal- or gas-fired power plants. The main disadvantage of MEA is that it is susceptible to substantial degradation in the presence of oxygen. Flue gas from coal- or gas-fired boilers can contain up to 5 or 15% oxygen, respectively. Kindrick1 first observed that MEA was especially susceptible to oxidation at absorber conditions and formed ammonia as an oxidation product. Later studies reported the catalytic effect of certain transition metals,2−6 the effect of oxygen mass transfer on the oxidation rate,7 and the presence of formate8 and 1-(2-hydroxyethyl)formamide)9 as major oxidation products. The oxidation mechanism is proposed to be analogous to that for hydrocarbon oxidation.10,11 MEA reacts with oxygen, producing MEA hydroperoxide. MEA hydroperoxide is decomposed by metal ions in solution to produce one new free radical, or by heating to produce two new free radicals12 (Figure 1). Organic peroxides have been detected in previous work, and it is proposed that this species is the key to understanding the oxidation and inhibition mechanisms. Initiation of free radicals by decomposition of hydroperoxides is proposed to control the rate of MEA oxidation in the presence of adequate oxygen mass transfer. Several studies have also reported inhibition of oxidative degradation of MEA by various additives. These additives fall primarily into five broad categories: chelating agents, tertiary © 2014 American Chemical Society
Figure 1. Organic hydroperoxide formation and decomposition by dissolved transition metal ions. Top: Bolland and Gee;10 Robertson and Waters.11 Bottom: Walling.12
amines, free-radical scavengers, oxygen scavengers, and sulfurcontaining antioxidants (Table 1). Other additives have also been proposed, categorized by empirical evidence as stabilizers or color reducers. An ideal oxidation inhibitor would be low cost, potent at low (preferably
