Article pubs.acs.org/IECR
Monoethanolamine Reclamation Using Electrodialysis J. Lim,† A. Aguiar,† C. A. Scholes,† L. F. Dumée,†,‡ G. W. Stevens,† and S. E. Kentish*,† †
Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, Vic. 3010, Australia ‡ Institute for Frontier Materials, Deakin University, Pigdons Road, Waurn Ponds, Vic. 3216, Australia ABSTRACT: Monoethanolamine (MEA) is the benchmark solvent for the capture of carbon dioxide from both natural gas and flue gas streams. Despite its effectiveness in absorbing CO2, this solvent can react with impurities in the gas stream to form heat stable salts and other degradation products. These impurities can cause problems such as an increase in solvent viscosity and corrosion of the operating units. Thus, a number of approaches have been considered to mitigate the occurrence of these problems. In this paper, the use of electrodialysis as an online MEA reclamation process in a postcombustion CO2 capture facility is investigated. The study shows that high heat stable salts removal can be achieved with a high MEA recovery. However, it is necessary to limit the current density, particularly at lower salt concentrations, to reduce water splitting. The stability of the commercial ion-exchange membranes in the highly alkaline solvent is also investigated. The results show that the membranes are stable upon exposure to 30 wt % MEA for at least 4.5 months.
1. INTRODUCTION Postcombustion CO2 capture has been considered as an attempt to reduce global warming effects. The main target for the operation is fossil fuel power plants as they are considered as the major source of greenhouse gas emissions. Among all the techniques under consideration, a solvent based approach using an amine solution is the most feasible and widely accepted technique.1,2 In particular, monoethanolamine (MEA) is often chosen due to its rapid reaction kinetics and high CO2 loading capacity.3−5 The efficiency of the process is related to the reaction between MEA and CO2 to form carbamate anions and protonated amines (Scheme 1). A major drawback from this operation is the formation of heat stable salts (sulfates, formates, acetates, and nitrates), which results from the parasitic reactions between MEA and other impurities (SO2, NO2, and O2) in the feed gas stream. As the reaction is irreversible (Scheme 2), these heat stable salts are difficult to regenerate and cause corrosion problems to the operating units.6−8 It was confirmed in previous studies that the presence of only 500 ppm (0.5 g/L) of heat stable salts may increase corrosion rates by fifty-fold.6,9 Additionally, neutral species and oligomers can also be formed through thermal degradation and polymerization of amine species.10,11 Together with the heat stable salts, these contaminants reduce the amount of active amine available for CO2 capture, hence reducing the efficiency of the CO2 capture process. An increase in the concentration of contaminants will also result in an increase in solution viscosity, which in turn increases the overall energy demand. In order to mitigate the occurrence of these problems, a fraction of degraded MEA needs to be purged and replaced with fresh MEA. As a result, there will be an increase in the operating cost for solvent replacement.8 An alternative solution is to release the trapped amine species by neutralizing the acidic contaminants using a carbonate salt, sodium hydroxide, or potassium hydroxide.12 This step deprotonates the charged amines generated in Scheme 2, freeing them for further reaction © XXXX American Chemical Society
with CO2, but still results in a buildup of total salts. Further, neutralization can lead to precipitation of less soluble salts, causing fouling or foaming in the downstream processes.8 Thermal reclamation can also be implemented, whereby MEA is distilled from the unwanted salts and oligomers, leaving a waste sludge. Nonetheless, all these approaches lead to significant MEA losses and a large volume of waste sludge requiring disposal.13 A membrane-based separation technology, electrodialysis (ED), has also been considered as a method to purify contaminated MEA solvent.14,15 The principle of the operation uses an electrical potential difference as the driving force for separating the ionic species in an aqueous solution. Union Carbide was the first company to develop an electrodialysis process for the purification of alkanolamines used in natural gas processing.16 Known as UCARSEP, this process is now owned by the Dow Chemical Company. Neutralisation is still required to deprotonate the charged amine upstream of the ED unit and this step adds further salt to the process. Further, it is essential that the process operates on solvents of low carbon dioxide loading (lean solvent) to reduce the concentrations of carbamate anions present, which might otherwise lead to amine loss. The process is also best conducted at temperatures of 40 °C or below, and this may necessitate additional cooling.14 However, when operated in this manner, the method has been proven over several decades to remove charged contaminants with high amine recovery in natural gas sweetening operations.14,17 Additional treatments are required to remove the neutral species and oligomers, which are also generated during the MEA scrubbing process. However, there has been little discussion in the scientific literature about the Received: September 4, 2014 Revised: November 3, 2014 Accepted: November 10, 2014
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dx.doi.org/10.1021/ie503506b | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Scheme 1. Reaction of two MEA Molecules with CO2 to Form the Carbamate Anion and a Protonated Amine
within the solvent in a coal-fired power station operation. However, the removal of oxalates and acetates are also considered, as these can still form in postcombustion operations due to oxidative degradation.20 The performance of three different commercial ion exchange membranes using a multicompartment electrodialysis cell is compared.
Scheme 2. Reaction of MEA with Sulfur Dioxide to Form Bisulfite and Sulfite Heat Stable Salts
2. MATERIALS AND METHODS adequacy of this approach for flue gas capture, with the notable exception of papers by Volkov et al.18 and Vitse et al.19 In postcombustion capture from a coal fired power station, concentrations of sulfur and nitrogen oxides will be high, particularly if flue gas desulphurisation is not used. This means that the feed to the electrodialysis unit will be rich in nitrates and sulfates, contrary to the case for natural gas processing, where formates, oxalates, and acetates predominate. It has also been reported that both thermal reclamation and electrodialysis are most effective when the concentration of contaminants is relatively high (>1 wt % or 10 g/liter14,17). Thus, reclamation is often carried out in a batch mode, with a reclamation unit brought on site on a temporary basis to improve solvent quality. However, for large scale postcombustion capture, it would be much more practical if a continuous online reclamation process could be implemented, operating on a slipstream of the circulating solvent at lower impurity concentrations (see Figure 1). In this paper, the potential use of electrodialysis as a reclamation process in postcombustion CO2 capture plant is discussed. The investigation involves testing a series of model solutions with different heat stable salts using an electrodialysis lab-scale rig. The focus is on the removal of sulfates and nitrates, as these are the anions most likely to accumulate
2.1. Materials. Samples were prepared by mixing laboratory-grade potassium oxalate monohydrate [(COOK)2· H2O, 98.5%, Chem-Supply), potassium sulfate (K2SO4, 99%, Ajax FineChem), potassium acetate (CH3CO2K, 99%, ChemSupply), potassium nitrate (KNO3, 99%, Chem-Supply), potassium carbonate (K2 CO3, 99%, Sigma-Aldrich), or potassium chloride (KCl, 99%, Chem-Supply) with purified water (13.2 MΩ cm, Millipore) using a magnetic stirrer. For some samples, the background solution was a 30 wt % mixture of pure monoethanolamine (MEA, 99.5%, Chem-Supply) and purified water. There was no pH adjustment made, and CO2 was not added to any of the solutions. The absorption of CO2 from the atmosphere was minimized during experiments by using storage tanks with small apertures. The ion-exchange membranes were purchased from Membranes International Inc., Astom, and Fuma-Tech GmbH. All membranes were preconditioned to allow for membrane hydration and expansion. This was done by overnight immersion in a 5 wt % NaCl for Membranes International Inc. membranes and in 0.2 wt % KCl solution for Neosepta (Astom) and Fumasep (Fuma-Tech GmbH) membranes. Detailed information on the membrane properties is given in Table 1.
Figure 1. Schematic diagram on the use of electrodialysis to remove charged contaminants in a postcombustion CO2 capture facility using an amine solvent. B
dx.doi.org/10.1021/ie503506b | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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