(590f) Investigating the Impact of Non-Isothermal Behavior and the Role of Water Transfer in Membrane Contactors for CO2 Capture

The capture of carbon dioxide from industrial sources is an attractive option to reduce the atmospheric CO₂emissions. Among the different separation techniques, chemical absorption in amine solvents is expected to be the dominant technology at the early stage of the carbon capture development. The main bottleneck to an industrial deployment lies in the high energy consumption of such processes and subsequently the high cost of capture. Membranes contactors are often foreseen as an efficient way to intensify the gas-liquid absorption process. The idea is to use a porous hydrophobic membrane as a gas-liquid contactor, providing a large and controlled interfacial area between the gas and liquid phases. Such a contactor also allows the use of volatile solvent such as ammonia or DMEA instead of classical amines. Obviously, the addition of a supplementary medium compared to classic gas-liquid contactors such as packing induces an increased mass transfer resistance. Still, the use of membrane contactors provides better overall transfer performances than packing due to the (4 to 20 times) higher interfacial area and allows to operate in a much wider range of phase velocity.

Performances of membrane contactors are widely studied in literature both from experimental and modeling points of view but studies on mass transfer performance almost systematically postulate isothermal conditions and dry flue gases for sake of simplification [1]. However, the water fraction of flue gases is not negligible since flue gases are saturated in water during depollution (e.g. about 10% for flue gases from coal combustion). Moreover, the process is not isothermal in industrial conditions due to the water condensation and evaporation and to the high heat of reaction between CO₂and amine.

Consequently, there is clearly a room for comprehensive analysis of these effects in representative conditions and this work intends to highlight the impact of non-isothermal behavior and the role of water transfer on CO₂capture performance in membrane contactors.

In order to do so, numerical simulations have been performed with a rigorous phenomenological model of chemically enhanced heat and mass transfer in membrane contactors. Thermodynamics of electrolyte solutions (H₂O-CO₂-amines) is modeled with the extended UNIQUAC model, particularly well adapted for the prediction of vapor-liquid and chemical equilibria in amine solution. A rate-based formulation is used for the modeling of heat and mass transfer with consideration of the chemical enhancement due to reaction between CO₂and amines. The adopted model has already proved its accuracy and ability to represent absorption and stripping operations with packing as gas-liquid contactors [2] and has been adapted to deal with the specificity of the membrane.

Before investigating the desired effect, the model has been validated on available experimental data in thermostated conditions with a dry inlet flue gas. Predicted CO₂ capture efficiency are in excellent agreement with experimental measurements for two membrane materials (PDMS and PP) and in a wide range of gas velocities. Thereafter, the model has been used to predict the CO₂ capture efficiencies in non-isothermal conditions with a saturated gas. The temperature profiles of gas and liquid phases are specifically investigated. From the simulation results, it appears that the presence of water in flue gas significantly influences the capture efficiency. For instance with a representative test, the predicted capture efficiency is in the order of magnitude of 90% if the system is considered isothermal whereas the capture efficiency decreases down to 75% if a non-isothermal behavior is considered. This reduction can be interpreted by observing the temperature profiles and direction of transferred water flux along the contactor. Both gas and liquid temperatures reach a maximum corresponding to an elevation around 30°C with respect the phase inlet. As the liquid flows, its temperature increases due to the heat of reaction and the massive condensation of water from flue gas. Although the temperature rise favors the kinetic between CO₂ and solvent, the significant shift of gas-liquid equilibrium (reduction of CO₂solubility) explains the decrease of capture efficiency. The temperature decreases as the liquid gets closer to the gas inlet due to water evaporation and conductive heat transfer with the gas phase.

In this study, insights are given on the impact of water in flue gases and the consideration of non-isothermal effects on CO₂capture efficiency through, using rigorous phenomenological modeling. The consideration of water transfer due to condensation and evaporation as well as heat of reaction leads to a reduction of predicted capture efficiencies. This more representative approach will allow to predict more accurately the intensification factor which can be achieved by the use of membrane contactors instead of classical gas-liquid contactors.

[1] Favre E., Svendsen H.F. (2012). Membrane contactors for intensified post-combustion carbon dioxide capture by gas-liquid absorption processes. Journal of Membrane Science,407-408, 1-7.

[2] Neveux T., Le Moullec Y., Corriou, J.P., Favre, E. (2013). Modeling CO₂ capture in amine solvents: Prediction of performances and insights on limiting phenomena. Industrial & Engineering Chemistry Research, 52(11), 4266-4279.