Enhanced Carbon Dioxide Separation by Amine-Promoted Potassium Carbonate Solution in a Hollow Fiber Membrane Contactor Sara Masoumi, Peyman Keshavarz,* Shahab Ayatollahi, Morteza Mehdipour, and Zahra Rastgoo Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 7134851154, Iran ABSTRACT: An aqueous solution of potassium carbonate is an appropriate absorbent for cost-effective separation of CO 2 from flue gas. Amine-promoted potassium carbonate has the potential to take advantage of both absorbents. In this study, a mathematical model has been developed to simulate the absorption of CO 2 into promoted potassium carbonate solutions in a hollow fiber membrane contactor, where monoethanolamine, diethanolamine, and methyldiethanolamine have been considered as promoters. A numerical scheme was applied to solve the simultaneous partial differential equations in the liquid, membrane, and gas phases, and the results were validated with available experimental data in the literature for all promoters. The effects of the promoter concentration, temperature, gas and liquid flow rates, flow directions, axial diffusion in the gas phase, and possible wetting of the membrane were investigated using the model. The promoted solution with monoethanolamine had much higher flux, about 4 times superior to non-promoted absorbent. Simulation results indicated that the promoted potassium carbonate is only effective in a specific range of operating conditions. The membrane wetting can reduce the flux impressively for all solutions; however, the flux was still much higher than non-promoted solution even at high wetting fractions. 1. INTRODUCTION Carbon dioxide is an important greenhouse gas, and it is naturally found in the earth’s atmosphere as a part of the carbon cycle. Human activities disturb this cycle by adding more carbon dioxide to the atmosphere than can be rejuvenated. Among various techniques for capturing carbon dioxide, membrane processes have observed widespread use in recent decades because of their advantages to traditional processes. Some of these advantages are (1) having a high and constant contact area per unit volume of contactor, (2) easily changing the capacity by adding and reducing the number of modules, (3) no need for a density difference between two fluids, and (4) gas and liquid phases being separated by a membrane; therefore, there is no flooding, loading, weeping, foaming, etc. For the first time, Zhang and Cussler 1,2 used the hollow fiber membrane contactor (HFMC) for CO 2 absorption. They employed a microporous non-wetted polypropylene mem- brane, where aqueous sodium hydroxide solution was used as an absorbent. Since then, HFMCs have been studied to separate some gases, such as CO 2 and SO 2 by Karoor and Sirkar, 3 among others. These investigations indicated that the mass-transfer flux was much higher than those usually found in packed towers. Chemical absorption is one of the most common methods of CO 2 capture. Many solvents are available; however, process selection must be according to economics (solvent cost and energy requirement for solvent regeneration) and cleanup ability. The most widely used chemical solvents are aqueous alkanolamines, such as monoethanolamine (MEA), diethanol- amine (DEA), methyldiethanolamine (MDEA), 2-amino-2- methyl-1-propanol (AMP), and diisopropanolamine (DIPA). 4 Hot potassium carbonate solution and amino alcohol solutions, such as sulfinol, can also be applied for CO 2 separation. Kim and Young 5 used AMP, MEA, and MDEA for separating carbon dioxide from a mixture of CO 2 /N 2 in a polytetrafluoro- ethylene (PTFE) HFMC and noticed that, among the considered absorbents, AMP exhibited a higher absorption capacity and moderate absorption rate. Keshavarz et al. 6-8 presented a mathematical model to simulate the absorption of carbon dioxide and hydrogen sulfide in HFMCs. They also checked the effects of membrane wetting on the separation performance. Dindore et al. 9 explored the absorption of CO 2 and H 2 S using aqueous potassium carbonate as a solvent in cross- flow membrane contactors. Aqueous solution of potassium carbonate was also studied by Mehdipour et al. 10 in a HFMC. They showed that there was an optimum concentration of potassium carbonate at each solution temperature. Golkhar et al. 11 applied nanofluids of nanosilica and carbon nanotube as absorbents in a gas-liquid membrane contactor for CO 2 separation. A mixture of amines was noticed in membrane contactors recently. 12,13 It has been shown that the absorption performance of the activated MDEA was much better than that of the non-activated MDEA. While membrane contactors have several advantages compared to traditional towers for CO 2 separation from flue gas, selection of absorbent is still an important challenge. 14 Amine solutions, especially MEA, have a relatively higher rate of reactions with CO 2 compared to carbonate solutions. Nevertheless, their performance as a solvent is limited because of the high heat of reaction, amine loss, amine degradation, and corrosion. One way to enhance the overall solvent performance is to blend a fast reactant, such as MEA, with a solvent having a low heat of reaction, such as potassium carbonate. The effects Received: June 30, 2013 Revised: August 11, 2013 Published: August 12, 2013 Article pubs.acs.org/EF © 2013 American Chemical Society 5423 dx.doi.org/10.1021/ef401228z | Energy Fuels 2013, 27, 5423-5432