Article pubs.acs.org/IECR
Enhanced Carbon Capture through Incorporation of Surfactant Additives Jonathan J. Bryant,† Cameron Lippert,† Guojie Qi,†,§ Kun Liu,†,∥ David S. Mannel,† and Kunlei Liu*,†,‡ †
Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, Kentucky 40511, United States College of Engineering, Mechanical Engineering, University of Kentucky, 151 Ralph G. Anderson Building, Lexington, Kentucky 40506-0503, United States
‡
S Supporting Information *
ABSTRACT: The CO2 absorption properties of monoethanolamine (MEA) solutions containing varying amounts of surfactant were examined using different experimental apparatus: a packed column, a stirred reactor, a wetted wall column, and a bubble column. The carbon capture efficiency in the packed column was improved by the addition of surfactant. Surface tension measurements of the different solutions were taken over a range of CO2 loadings, and the addition of surfactant was seen to lower the surface tension of the amine solvent. However, the improvement in carbon capture efficiency could not be fully correlated to the surface tension depression. The addition of surfactant improved CO2 absorption in the packed and bubble columns, while the surfactant inhibited absorption in the stirred reactor and wetted wall column experiments. Visual inspection of solvent flowing down a packed column revealed the gas to be bubbling through the liquid, which increases the gas−liquid interfacial area and thus the overall level of mass transfer of carbon dioxide into solution.
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INTRODUCTION
additives decrease the level of mass transfer of gases (N2, CO2, and O2) into aqueous amine solutions and pure water.10−15 One consideration when evaluating these contradictory results is the surfactant itself. Shorter hydrophobic chain lengths (≤10 carbons) were shown to enhance mass transfer at low concentrations,6−8 whereas surfactants with longer hydrophobic chain lengths (>10 carbons) reduced the level of mass transfer even at low concentrations.6 The decreasing diffusivity of surfactants with longer chain lengths16 may explain the lack of any observable Marangoni effect for surfactants of this type. The surfactant counterion12 and concentration14 also affect the mass transfer. A higher surfactant concentration has a negative effect on the mass transfer; more surfactant at the interface leads to reduced interfacial turbulence (hydrodynamic effect) and increased film resistance (barrier effect). Lower critical micelle concentrations (CMCs) for surfactants with longer hydrophobic alkyl chain lengths17,18 might explain why these surfactants have a negative impact on the mass transfer even at low concentrations. The addition of surfactant (surface-active agent) lowers the surface tension of the solution and creates concentration and surface tension gradients between the gas−liquid interface and the bulk liquid. In this way, we aim to increase the local
There is growing concern about the emission of carbon dioxide into the atmosphere and the associated environmental impact.1 As a result, reducing CO2 emissions is of global interest. Significant progress in this area might be made by reducing carbon dioxide emissions from large, stationary point sources such as power plants.2 For this purpose, carbon capture and storage (CCS) using aqueous amine solvents is well understood and can be implemented at these point sources.3,4 In this report, we discuss some of our work on improving the absorption of CO2 by amines for such applications. Absorption of carbon dioxide by an amine solvent consists of the gaseous CO2 being absorbed into the liquid and the subsequent chemical reaction with the amine. Thus, there are both physical and chemical limitations to this process. The physical aspects of absorption (dissolution of CO2 into the liquid and product and reactants diffusing to and from the gas− liquid reaction interface) are dependent on the CO2, product, and amine concentration gradients and localized mixing.5 In this work, we focus on lowering the physical resistance to CO2 absorption by using a surfactant. Surfactants in aqueous systems have been proposed to increase gas−liquid interfacial turbulence (Marangoni instability) and thereby increase the level of mass transfer,6−8 though it is not clear that this instability is present in systems that feature a rapid chemical reaction such as the CO2−aqueous amine system.9 In fact, there are numerous other reports that say the opposite; i.e., surfactant © XXXX American Chemical Society
Received: December 23, 2015 Revised: May 19, 2016 Accepted: June 14, 2016
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DOI: 10.1021/acs.iecr.5b04906 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research turbulence in the solution and improve the CO2 mass transfer without any detriment to the solvent−CO2 chemistry or added complexity to the process engineering design and operation.
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RESULTS AND DISCUSSION After an initial screening of various surfactants and their effect on the surface tension of 30 wt % aqueous monoethanolamine (MEA), proprietary surfactant S-554 (Chemguard), described as a “short-chain perfluoro-based ethoxylated nonionic fluorosurfactant”, was selected for further study because it satisfies the short-chain criterion and also induces a dramatic reduction in surface tension, which may improve the wetting of the hydrophobic packing material in an absorption column.19,20 We also consider S-554 to possess other advantages: its nonionic nature minimizes nonspecific interactions with other ions in solution, the fluorine substituents may render it more “CO2-philic”,21 and it is nonfoaming, which is important for application in a typical CO2 capture process. The surface tension of a solution containing surfactant is concentration-dependent below the critical micelle concentration (CMC), after which point no further decrease in surface tension is observed. To estimate the CMC of S-554, surface tension measurements of 30 wt % MEA with varying concentrations of S-554 were taken (Figure 1). A capillary
Figure 2. Surface tension of 30 wt % MEA solutions at 40 °C with varying amounts of S-554 additive as a function of carbon loading: (circles) 0, (diamonds) 0.01, (times signs) 0.05, and (squares) 0.3 wt %.
process). This is most likely due to an increase in the CMC of the solution; this property is known to be influenced by ionic strength,22−24 which increases during CO2 absorption (from production of carbamate and protonated amine). Testing a solution with 0.52 C/N, we found the CMC increased to approximately 0.9 wt %. To gauge the efficacy of the surfactant additive in a carbon capture process, CO2 absorption experiments in a packed column were performed. The column was 1.59 in. in diameter. Ceramic Raschig rings (6 mm outside diameter × 3 mm inside diameter × 6 mm height) were used as the random packing material, and the packing height was 21 in. More details about the experimental apparatus are given in the Supporting Information. An S-554 concentration of 0.3 wt % was initially chosen because it provides the maximal decrease in surface tension with minimal addition of surfactant. As seen in Figure 3, the CO2 capture efficiency (moles of CO2 captured per mole of CO2 input × 100%) increases by more than 25% when the solution contains 0.3 wt % S-554. The results are also given in terms of the overall mass transfer coefficients. To correlate this behavior to the decrease in surface tension, solutions of 30 wt % MEA with different concentrations of surfactant were tested. Despite the varying surface tensions of the different solutions, the enhancements in capture efficiency are similar. A plot of the percent increase in capture efficiency as a function of the percent decrease in surface tension shows no correlation between the two, suggesting that the surface tension of the solvent cannot be used as a predictor for the CO2 capture ability (Figure S3). This conclusion is supported by the changing surface tension at different C/N values (Figure 2); the percent enhancement (∼25%) in absorption efficiency is roughly the same at any given carbon loading and consistent over the entire tested range, even though the surface tension depression is greater at lower carbon loadings. To investigate the surfactant’s effect on surface stability, we conducted experiments in a stirred vessel (see Figure S5 for experimental details). Briefly, gas is blown over the surface of the solvent, and absorption occurs only at the surface (the gas− liquid interface). The solvent is stirred by a magnetic stir bar, slowly enough that the surface is undisturbed. The results are shown in Figure 4. A decreased capture rate is observed upon addition of surfactant. Similar to what we observed in the packed column, our results in the stirred reactor are also independent of the surfactant concentration; CO2 capture is inhibited to the same extent at the different concentrations
Figure 1. Surface tension of 30 wt % MEA solutions at 40 °C at varying concentrations of S-554.
tube was used for this purpose. The capillary tube is immersed into the liquid being tested. The height the liquid rises up the tube, along with the liquid density and the tube diameter, is used to calculate the surface tension of the liquid (see the Supporting Information for more details). Because the surface tension is temperature-dependent, the measurements were taken at 40 °C to approximate conditions in a typical CO2 absorber. The surface tension reaches a minimum at 0.3 wt %, so this was taken as an approximate CMC. For applications in CO2 capture, the change in the solvent’s properties during CO2 absorption is also of interest. Figure 2 shows the surface tension of 30 wt % MEA with varying concentrations of S-554 at different carbon loadings (moles of CO2 per mole of N). An increasing trend upon absorption of CO2 is observed. For 30 wt % MEA, the surface tension increases gradually from approximately 60 to 70 dyn/cm. The addition of a small amount of surfactant (0.01 wt %) lowers the surface tension slightly while keeping the same increasing trend upon absorption of carbon dioxide. Higher concentrations of surfactant lower the surface tension noticeably (
