Mass Transfer Kinetics of Carbon Dioxide into Concentrated Aqueous Solutions of Monoethanolamine
Doctoral thesis, Peer reviewed
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Original versionYing, J. Mass Transfer Kinetics of Carbon Dioxide into Concentrated Aqueous Solutions of Monoethanolamine. Doctoral dissertation, Telemark University College, 2013
Global warming arguments have gained more and more attention due to the new regulations of carbon dioxide (CO2) emission in the world. Monoethanolamine (MEA) has been employed as an important industrial absorbent for CO2 capture since the 1930s because of its high reaction rate, relatively low cost, and thermal stability. The concentration of MEA in aqueous solution is generally increased to 30 mass % in the CO2 capture process. The energy consumption is high in the present MEA process because of the high reaction heat of MEA with CO2, and a large number of liquid transportation. To reduce the energy consumption and improve the efficiency of CO2 capture in the present MEA process, further increase in solution concentration of MEA is a potential solution. Basic research on the properties and reaction kinetics with CO2 of concentrated aqueous MEA solution is necessary to perform engineering calculations and important for the dimensioning of pipes, pumps and heat exchangers etc. In this work, a novel solubility apparatus and technique was designed and built for the measurement of physical solubility of a gas in liquid. The technique employs a scaled spiral glass tube with a small drop of mercury inside as a eudiometer as an alternative to a three–branch U–tube setup to keep the system pressure constant, and measure the volume drop of absorbed gas at constant temperature. A “vacuum gas saturation” method is proposed for gas saturation operation in the measurement. The physical solubilities of N2O in pure water over the temperature range from 298.15 to 323.15 K and in aqueous salt MEA solutions at 313.15 K were measured under a constant ambient pressure to validate the new technique. The new solubility apparatus and technique possesses some advantages including easy operation, lower mercury inventory, higher sensitivity and greater accurate. The physical mass transfer coefficients of N2O in aqueous MEA solutions were performed using the new apparatus as well. The physical solubilities of N2O in aqueous MEA solutions over the full concentrations range were measured by the novel solubility apparatus over a temperature range from 298.15 to 323.15 K under a constant ambient pressure. The physical solubilities of CO2 in aqueous MEA solutions were estimated using “N2O analogy” method. The results of the solubility measurements of N2O and CO2 in water and N2O in aqueous MEA solutions agree with literature. A semiempirical model to solubility proposed by Wang et al. was used to correlate the solubilities of N2O and CO2 in aqueous MEA solutions, and the correlation results are in agreement with experiment data. The results show that the solubilities of both N2O and CO2 in aqueous MEA solutions showed negative deviation behaviors from the linear additive principle. The viscosities of aqueous MEA solutions over the full concentration range were measured using a rheometer with a double–gap measuring system at a temperature range from 298.15 to 353.15 K. The measured viscosity data are in good agreement with the literature values. An exponent model proposed by DiGuilio et al. was used to correlate the data and the results are very satisfied for the regression of the viscosities of pure MEA from 298.15 to 353.15 K. The polynomial model proposed by Teng et al. with five parameters is satisfied the aqueous MEA solution. The relationship between the viscosity and mole fraction of MEA shows both positive and negative deviation behavior and the critical mole fraction of MEA was found is 0.2. The molecular diffusivities of N2O in aqueous MEA solutions up to 12 M were studied from 298.15 to 333.15 K using a laminar liquid jet absorber, and the diffusivities of CO2 in aqueous MEA solutions were calculated by the N2O analogy method. A modified construction of the temperature control for the laminar liquid jet was proposed. The relationship between the diffusivity and the viscosity of the solution is roughly in agreement with the modified Stokes–Einstein equations. On the other hand, an exponent mathematical model was used to correlate N2O diffusivities in aqueous MEA solutions satisfactorily for calculation of the diffusivities of CO2 in aqueous MEA solutions. Based on the measured physical properties in this work, the chemical reaction kinetics of CO2 with aqueous MEA solutions over a wide concentration range from 0.5 to 12 M were investigated using a stirred cell absorber with a plane gas–liquid interface over a temperature range from 298.15 to 323.15 K. To satisfy the criterion of pseudo-first-order reaction, low CO2 partial pressure (3 – 4 kPa) was employed. The rates of CO2 absorption in the solutions were determined from the fall in pressure, and the reaction rate constants were determinate by two treatment methods on the same experimental data, viz. a “differential” and an “integral” method, which are derived from the mass balance principle and Henry’s law. The reaction between MEA and CO2 is based on “zwitterion” mechanism in this work. The gas-phase resistance was investigated systematically in the stirred cell. To reduce the gas phase resistances in the measurements of CO2 absorption in the solutions, speeding up the gas phase fans and employing very low inert gas pressures of N2 and solution vapor were suggested. The chemical reaction kinetics of CO2 in aqueous MEA solutions were measured over the concentration range from 0.5 to 12 M by a stirred cell absorber with batchwise operation for both gas and liquid. As same as the dilute solution, the reaction of concentrated aqueous IV MEA solution with CO2 is also first order with respect to MEA and the reaction is in the fast reaction regime. The reaction activation energy (Ea) of aqueous MEA + CO2 is calculated based on the experimental data. The enhanced mass transfer coefficient in liquid phase, kLE, increases with the concentration of MEA solutions but decreases when the molarity of MEA is higher than 8 M. Last, some recommendations are given to the future work. CO2–loaded MEA solution is suggested to focus on in the next–step work, the properties and gas absorption of the system can be measured and discussion by the same experimental method mentioned in this thesis. The gas absorption and desorption from the CO2–loaded aqueous MEA solutions should be performed as well. The issue of heat transfer should be taken into account and investigated when the concentrated aqueous MEA solution is employed in the CO2 capture process. The stirred cell or laminar liquid jet can be employed in these studies under a suitable pressure. However, to obtain more accurate experimental data, some modifications on the construction of both the laminar liquid jet and stirred cell should be made. For example, the absorption cell of the liquid laminar jet can be smaller, and the nozzle or receiver should be adjustable etc. Regarding the modification on the temperature control of these equipments, the main idea is to immerse all the gas and liquid pipes in to the same water bath or its hose. Some suggestions of these modifications are proposed in the appendix of this thesis.