Enhanced Carbon Capture through Incorporation of Surfactant Additives

Abstract

The CO 2 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 CO 2 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 CO 2 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. ■ INTRODUCTIONThere is growing concern about the emission of carbon dioxide into the atmosphere and the associated environmental impact. 1 As a result, reducing CO 2 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 CO 2 by amines for such applications.Absorption of carbon dioxide by an amine solvent consists of the gaseous CO 2 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 CO 2 into the liquid and product and reactants diffusing to and from the gas− liquid reaction interface) are dependent on the CO 2 , product, and amine concentration gradients and localized mixing. 5 In this work, we focus on lowering the physical resistance to CO 2 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 CO 2 −aqueous amine system. 9 In fact, there are numerous other reports that say the opposite; i.e., surfactant additives decrease the level of mass transfer of gases (N 2 , CO 2 , and O 2 ) 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) re...

Enhancements in mass transfer for carbon capture solvents part I: Homogeneous catalyst

“…The surfactant-like activity of 1 in solutions of MEA make determining the source of any enhancement difficult to isolate, although previous studies have shown the addition of surfactants to capture solutions may cause formation of a single layer film on the surface that blocks CO2 gas diffusion into the liquid and artificially depress the local amine concentration, decreasing overall mass transfer. 55 However, the constant surface tension and elasticity in solutions of A2P/AMP and A2P/AMP + 1 indicates there is no surfactant-like activity in this solvent blend and any enhancement is likely a result of the catalyst performing as designed. The lack of correlation between the surface properties and capture efficiency in these solutions seems counterintuitive when compared to traditional models of mass transfer in these systems, however as other recent work has shown, caution must be taken when predicting improvements based on fundamental and idealized laboratory testing methods, as counter-intuitive behavior was observed upon the addition of surfactants to stirred-reactor experiments with a flat gas/liquid interface, 55 much like the wetted-wall experiments here.…”

“…55 However, the constant surface tension and elasticity in solutions of A2P/AMP and A2P/AMP + 1 indicates there is no surfactant-like activity in this solvent blend and any enhancement is likely a result of the catalyst performing as designed. The lack of correlation between the surface properties and capture efficiency in these solutions seems counterintuitive when compared to traditional models of mass transfer in these systems, however as other recent work has shown, caution must be taken when predicting improvements based on fundamental and idealized laboratory testing methods, as counter-intuitive behavior was observed upon the addition of surfactants to stirred-reactor experiments with a flat gas/liquid interface, 55 much like the wetted-wall experiments here. We previously concluded that although the addition of surfactant lowers surface tension of the solution, it does not serve to promote bulk solvent mixing on its own in these flat interface systems.…”

“…MEA has well-documented thermal degradation at temperatures above 120 °C. [51][52][53] The thermal stability of the novel Physical solvent properties such as viscosity 54 and surface tension 55 are known to impact the solution side diffusion and mass transfer resistance for reactants and products, and an increase in these properties have been associated with decreasing mass flux of CO2 into capture solutions.…”

Molecular Modeling of the Physical Properties for Aqueous Amine Solution Containing a CO2 Hydration Catalyst

“…Additionally, these concentrated C3P at the interface may affect CO 2 diffusion from the gas phase to the bulk solution phase. 36 Having a lower MEA concentration at the interface also decreases CO 2 reaction rate at the surface. Note that the slightly smaller MEA concentration and slightly larger water concentration at the vapor−liquid interface are in contrast to their respective concentrations around the C3P catalyst molecule in the bulk solution (see below).…”

“…This aggregation may be expected to hinder the accessibility of some C3P molecules to CO 2 or other reactants and decrease the catalyst performance. This catalyst aggregation, combined with the possibility of surface blocking of CO 2 diffusion, 36 9. As shown in Figure 9b, CO 2 and MEA exhibit maximum peak densities at r = 7.5 Å, which is roughly the size of one C3P molecule (Figure 9a).…”