J. Chem. Eng. Data 2001, 46, 1125-1129
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Solubility and Diffusivity of CO2 in Triethanolamine Solutions Mousa K. Abu-Arabi,* A. Tamimi, and Asem M. Al-Jarrah Chemical Engineering Department, Jordan University of Science and Technology, P.O. Box 3030, Irbid, Jordan
The objective of this study was to measure directly the physical solubility and diffusivity of CO2 in aqueous triethanolamine (TEA) solutions via the protonation method. The method is based on the protonation of TEA solutions by HCl prior to contacting the solutions to CO2 to eliminate the solutions reactivity with CO2. The properties were measured for concentrations of 10, 20, and 30 mass% TEA in the solution over the temperature range (20-60) °C. The volumetric and the wetted sphere methods were used to measure the solubility and diffusivity, respectively. The measured data of solubility and diffusivity were well fitted versus temperature by an exponential form. The measured properties by the protonation technique and the available literature values measured by the “N2O analogy” method were compared. Good agreement between the two methods was found.
1. Introduction Natural gas, refinery gas, biogas, and synthetic gas usually contain undesirable compounds, called acid gases, like hydrogen sulfide (H2S) and carbon dioxide (CO2). In most cases these gases must be removed before the gas can be transported and processed. Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diisoropanolamine (DIPA), and methyldiethanolamine (MDEA) are used to remove the acid gases. Knowledge of the solubility and the diffusivity of acid gases in aqueous alkanolamine solutions, which are physical properties, are of primary importance for the prediction of mass-transfer rates in gas-treating processes and in the design of such units. These properties are needed at temperatures above ambient temperature as well as at various solution concentrations. The reaction that occurs between acid gases and alkanolamine solutions upon contacting makes the direct measurements of these properties difficult if not impossible. Because of molecular similarities between CO2 and N2O gases, the latter has been used to estimate CO2 properties in reacting solutions.1-9 This has been referred to as the “N2O analogy “. This later has been extended to dissimilar gases such as COS and H2S due to the lack of a method to estimate these properties.10,11 The equations normally used to calculate the solubility and diffusivity via the N2O analogy are6
HCO2,amine ) k1HN2O,amine
(1)
k1) (HCO2/HN2O)water
(2)
DCO2,amine ) k2DN2O,amine
(3)
k2 ) (DCO2/DN2O)water
(4)
with
with
* To whom correspondence should be addressed. On leave at The Middle East Desalination Research Center, P.O. Box 21, Al-Khuwair, PC 133, Oman. E-mail:
[email protected]. Fax: +968 697 197.
where H and D are the Henry’s constant and the diffusivity, respectively. Initially, k1 and k2 were assumed constants1,2 regardless of the temperature, but later were evaluated as a function of temperature.6-12 The latest correlations for k1 and k2 as a function of temperature was developed by Abu-Arabi et al.12 as follows:
k1 ) 3.347 exp(-272K/T)
(5)
k2 ) 0.688 exp(128K/T)
(6)
The “protonation method” developed by Abu-Arabi et al.,13 which is based on the elimination of the alkanolamine solutions reactivity with CO2, was used in this work. The principle of this method is based on the protonation of alkanolamine solutions by hydrochloric acid (HCl) prior to contacting the solutions with CO2. The solubility and diffusivity of CO2 in diethanolamine solutions were determined by this method.12 Therefore, the main objective of this research was to measure the solubility and diffusivity of CO2 in protonated aqueous solutions of triethanolamine (TEA). 2. Experimental Procedure 2.1. Solution Preparation. The protonation method was followed in this study to prepare protonated aqueous TEA solutions. Hydrochloric acid (12 M) was added to the solutions until the end point was reached. The prepared solutions were 10, 20, and 30 mass % TEA after protonation. Reagent TEA with a purity of 98%, supplied by Across, was used. The purity of the CO2 gas was g99.8%. 2.2. Solubility. The apparatus and the experimental technique used to measure the physical solubility are similar to those used by Haimour and Sandall.7 The principal of the method is to bring a known volume of gas with a known volume of liquid. The amount of CO2 gas absorbed in the protonated TEA solutions is measured volumetrically after equilibrium is reached between the gas phase and the solution at constant temperature and pressure. The volume of the gas absorbed is equal to the volume of liquid minus the measured change in volume. For each solubility measurement, the absorption flask was first purged with CO2 gas saturated with vapors of
10.1021/je000386t CCC: $20.00 © 2001 American Chemical Society Published on Web 08/03/2001
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Journal of Chemical and Engineering Data, Vol. 46, No. 5, 2001
the desired solution at constant temperature. A predetermined volume of degassed solution, which was kept at the same temperature of the experimental run, was injected into the absorption flask. The liquid was agitated with a magnetic stirrer until there was no further change in gas volume. The whole apparatus was kept at constant temperature inside a temperature-controlled bath within (0.5 K. The experimental error in the reported data of solubility is
