Ind. Eng. Chem. Res. 2005, 44, 9239-9243
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Measurement of Carbon Dioxide Solubility in a Solution of Diethanolamine Mixed with Methanol K. N. Habchi Tounsi,† A. Barreau,† E. Le Corre,† P. Mougin,*,† and E. Neau‡ Institut Franc¸ ais du Pe´ trole De´ partement Thermodynamique et Simulation Mole´ culaire, 1 et 4 avenue de Bois Pre´ au, 92850 Rueil-Malmaison Cedex, France, and Faculte´ des Sciences de Luminy, Laboratoire de ChimiesPhysique, 163 avenue de Luminy case 901, 13288 Marseille Cedex 9, France
In this work, we show new phase-equilibrium measurements between carbon dioxide and a mixed solvent composed of water, diethanolamine, and methanol. This work is motivated by the everstricter natural gas specifications and by the resulting need to develop new solutions to respond to this challenge. Original experimental equipment is developed for this purpose. Its working conditions are limited between 323.15 and 423.15 K and up to a pressure of 10 MPa. Gas solubilities are calculated by the synthetic method based on assumptions including the mass balance equations. The equipment is validated with well-known results obtained on the water + diethanolamine + carbon dioxide system. Two solvent compositions have been investigated: 40/40/20 and 40/20/40 wt % of, respectively, water, diethanolamine, and methanol. The temperature range is ∼323.15 to 393.15 K with a pressure up to 3.6 MPa. The measurements are correlated by the Kent and Eisenberg approach. 1. Introduction The removal of acid gases such as CO2 and H2S from gas streams is an important operation in natural gas industries, oil refineries, and petrochemical plants. There are two families of solvents used for this kind of gas treatment: chemical solvents and physical solvents. The most widely used chemical solvents belong to the family of alkanolamines. In the past several decades, aqueous diethanolamine solutions were one of the most widely employed gas-treating solvents for the removal of acid gases from natural gases. Their low vapor pressures make them suitable for gas treatment, since vaporization losses are small. In addition, their moderate enthalpies of vaporization (halfway between those of monoethanolamine (MEA) and methyldiethanolamine (MDEA)) result in a low regeneration cost. On the other hand, methanol is one of the most widely used physical solvents because of its capacity for preferentially absorbing acid gases and its low cost. The use of methanol also facilitates dehydration and, thus, avoids formation of ice and hydrates. The advantages of the two kinds of solvents are combined when a mixed solvent is used. Such a mixture of a physical and a chemical solvent allows increasing the absorption for high-pressure acid gas. In this perspective, a new process of gas sweetening by coupling a chemical solvent (aqueous solution of diethanolamine) and a physical solvent (methanol) is being developed by IFP in partnership with Total. The knowledge of acid gas solubilities in these liquid solvents is essential for the design of the natural gas treatment equipment using this process. In the open literature, there is no reference concerning the solubility of carbon dioxide in such a mixed solvent composed of water, diethanolamine, and methanol. The purpose of this work is to produce such data. * Corresponding author. E-mail:
[email protected]. † Institut Franc ¸ ais du Pe´trole De´partement Thermodynamique et Simulation Mole´culaire. ‡ Faculte ´ des Sciences de Luminy.
We can distinguish two main experimental methods, the analytical and the synthetic methods, as follows: ‚ In the analytical method, both liquid and vapor phases are sampled and analyzed. For example, the vapor-phase composition is determined by chromatography, whereas the liquid phase is analyzed by titration. ‚ In the synthetic method, a mass balance is used for calculating the amount of acid gas absorbed by the solvent. The acid gas quantity introduced in the equilibrium cell is determined based on the knowledge of the PVT (pressure-volume-temperature) conditions. Most data concerning the classical water + alkanolamine systems have been obtained using the analytical method. Many works can be quoted: Lee et al.,1-5 Lawson and Garst,6 Li and Mather,7-9 Lal et al.,10 Kennard and Meisen,11 Maddox and Elizondo,12 and Austgen et al.13 As in our system, where we have added a more-volatile compound, the methanol, the synthetic method should be employed, thus avoiding composition measurements. In this paper, we first describe the apparatus developed for the liquid-vapor equilibrium measurement. This equipment has been tested on a water + diethanolamine and carbon dioxide system before, studying the mixed solvent. In the third part of this work, the data are modeled. 2. Experimental Section 2.1. Apparatus. The apparatus (Figure 1) is divided into two main parts. The first one is the equilibrium cell itself, in which we introduce the solvent and the acid gases. The second part is the tubing connecting the equilibrium cell with several gas reservoirs. The equilibrium cell is made of Hastelloy to avoid corrosion problems, and its volume is 261.4 cm3. The cell is positioned in a LAUDA liquid bath thermostat whose temperature fluctuations do not exceed 0.05 K. It is designed to operate at pressures up to 10 MPa and within a temperature range of 323.15 K to 423.15 K. It is further equipped with stirring rotors to ensure the
10.1021/ie0580250 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/19/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005
point. In this manner, an isothermal curve can be described with 6-10 data points. 3. Exploitation of the Measurements and Results
Figure 1. Experimental equipment for liquid-vapor phase equilibrium measurements. Table 1. Purity of the Different Compounds Used in This Study compounds
origin
purity (%)
CO2 H2O DEA CH3OH
Air Liquide distilled water Aldrich Prolabo
99.998 99.8 99 99.8
homogeneity of the liquid and vapor phases. A Pt 100 Ω probe measures the temperature inside the cell in the liquid phase. The pressure is measured with an HBM pressure sensor limited up to 1 or 10 MPa, depending on the pressure range of the mixture to be studied in the cell. The second part of this apparatus consists of a storage bottle of carbon dioxide, which is immersed in a liquid bath thermostat and equipped with both a platinum probe to measure the temperature and a pressure sensor. Several valves allow the isolation of the equilibrium cell from the storage gas bottle. All the circuit lines are heated to avoid condensation risks. The CO2 storage bottle used in this work has a volume of 161.9 cm3. Other devices are not shown on the figure such as a vacuum system for degassing the solvent before gas injection. The degassing process uses the cryogenic method (the solvent is cooled in order to avoid selective evaporation). Before the first measurements, all the sensors have been calibrated by comparison with reference gauges (DPI 605 Druck). The estimated experimental uncertainties of this apparatus are as follows: σ(T) ) 0.03 K and σ(P) ) 3 kPa for the sensor limited up to 1 MPa, and σ(P) ) 15 kPa for the second one. Table 1 reports the origins and purity of the different compound. 2.2. Experimental Procedure. The experimental procedure is described using carbon dioxide as an example gas. First, the solvent composed of water, diethanolamine, and methanol is prepared and introduced into the equilibrium cell. The degassing of the solvent is carried out by a cryogenic method. The cell is further heated at the desired temperature, and the bubble pressure of the solvent is then measured. Then, the gas injection can begin. A desired CO2 quantity is introduced from the carbon dioxide storage to the equilibrium cell. The equilibrium state in the cell is obtained after ∼1 h. Then a new injection of carbon dioxide is allowed to reach another equilibrium data
Using the static method, we only have direct information on the overall composition in the cell as well as its temperature and pressure. The molar composition in the liquid and the associated vapor phase must be calculated. In this section, we describe the assumptions and calculations to exploit the measurements. The amount of injected carbon dioxide in the equilibrium cell (nCO2) is obtained from the temperature and pressure conditions in the CO2 storage vessel before and after the injection. As the volume of the CO2 storage vessel, and of the tubing connecting this vessel to the equilibrium cell, is known, the number of moles of carbon dioxide injected in the equilibrium cell is then calculated using a specific equation of state. In this work, we have used the IUPAC tables14 as the reference equation of state for carbon dioxide. The amount of solvent (ns) is also exactly known by weighing, and the volume of the equilibrium cell (V) is carefully calibrated. Once equilibrium is reached, we have two mass balances (the solvent is considered here as a single pseudo component), L V nCO2 ) nCO + nCO 2 2
(1)
ns ) nLs + nVs
(2)
and one volume balance:
V ) VL + VV
(3)
In these equations, L and V are, respectively, related to the liquid and vapor phases. The vapor volume is written as V VV ) Z(nVs + nCO ) 2
RT P
(4)
where P and T are the pressure and the temperature, respectively, in the cell in the equilibrium state. Z is the vapor compressibility factor. The liquid volume is given by L v VL ) nLs vs + nCO 2 CO2
(5)
where vs is the solvent partial molar volume and vCO2 is the CO2 partial molar volume in the liquid phase. The following assumptions are made: (i) In eq 4, we assume that the compressibility factor of the vapor phase is equal to that of pure carbon dioxide. (ii) In eq 5, we assume that the solvent partial molar volume and the CO2 partial molar volume depend only on temperature. The solvent partial molar volume is taken equal to its molar volume and measured separately.15 The CO2 partial molar volume is an adjustable parameter. It is obtained from density measurements of the water + alkanolamine solvent16,17 charged with various amounts of carbon dioxide. Note that this volume is assumed to be independent of the solvent composition.
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Figure 2. Solubility of CO2 in a 2 mol/L aqueous solution of DEA at 323.15 K: (b) data of Lee et al.1 and (×) this study.
Figure 3. Loading deviation between this work and the data of Lee et al.1 for the water + diethanolamine (2 mol/L) + CO2 system at 323.15 K.
(iii) We assume that Raoult’s law can be applied on the pseudobinary system (solvent + CO2),
nVs P V nVs + nCO 2
)
nLs Pσs L nLs + nCO 2
(6)
where P is the total pressure and Pσs is the solvent vapor pressure. This value is measured independently. Using these assumptions, we are left with six equations L V , nLs , nCO , nVs , for solving the six unknowns that are nCO 2 2 L V V , and V . We are aware that these assumptions may not be entirely justified. We, therefore, tested the sensibility of the results to each of the assumptions. Concerning the first assumption, we notice that, because of the moderate value of pressure (generally
