Effects of Monoethylene Glycol on Carbon Dioxide Partitioning in Gas

May 6, 2010 - Experimental Study on the Solubility of Carbon Dioxide in Systems Containing Ethane-1,2-diol + Water + Salt (Sodium Chloride or Calcium ...
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Ind. Eng. Chem. Res. 2010, 49, 5884–5890

Effects of Monoethylene Glycol on Carbon Dioxide Partitioning in Gas/ Monoethylene Glycol/Water/Salt Mixed Systems Amy T. Kan,* Haiping Lu, and Mason B. Tomson CiVil and EnVironmental Engineering Department, Rice UniVersity, MS-519, 6100 Main Street, Houston, Texas 77005

This paper extended the experimental results of carbon dioxide partitioning in gas/liquid equilibrium with a NaCl-H2O-monoethylene glycol (MEG) mixed solvent. The experiments were conducted systematically at 3-70 °C, 0-6.0 m NaCl, and 0-99 wt % MEG concentrations. CO2 partitioning to the NaCl-H2O-MEG solution is complicated due to nonideal behavior in the solvent. A self-consistent activity model is proposed to describe the effect of monoethylene glycol (MEG) on CO2 partitioning in gas/MEG/water/salt solutions using pure H2O as the reference state. Pitzer theory was used to calculate the effect of salt, and a semiempirical equation was applied to correlate the effects of MEG at different temperatures and ionic strengths. The equation is applicable to CO2 partitioning at 0-6 mol of NaCl and 0-97% MEG mixed solution at 0-90 °C. This model can be incorporated with Pitzer type activity coefficient models to predict the solubility of carbon dioxide in the presence of MEG and salt solutions. Introduction Monoethylene glycol (MEG), HOCH2CH2OH, is an important raw material with numerous research and industrial applications. It is an amphiprotic solvent used in organic syntheses and electrochemical studies. It is also one of the most commonly used cryoprotectants in internal combustion engines and many other heat transfer applications. In the oil and gas industry, MEG is used as a gas hydrate inhibitor. Under conditions of low temperature and high pressure, gas hydrate may form and cause plugging or blockage of pipelines to impede the transfer of oil and gas and result in shutdown, a loss of production, and the risk of explosion or unintended release of hydrocarbons into the environment either on land or offshore. One way to control the formation of gas hydrate in production is to inject “antifreeze” fluids, such as methanol, ethanol, ethylene glycol, and triethylene glycol, into the wells or pipelines. The system carbon dioxide-water is important to virtually all branches of science. As such, the solubility of carbon dioxide in water is one of the more often studied phenomena in all of physical chemistry. There are a few research studies focused on the vapor-liquid equilibrium (VLE) of CO2 in mixed MEG and H2O. Kobe and Mason,1 Won et al.,2 and Hayduck and Malik3 reported CO2 solubility in a MEG-H2O mixture at equilibrium with ∼1 atm of partial pressure of CO2. The conditional Henry’s law constants of Hayduck and Malik’s data can be related to the MEG mole fraction concentration by a fourth-order polynomial (See Figure S1, Supporting Information). Sandengen4 reported CO2 solubility in MEG (0-99.9 wt %)/NaCl (0-0.7 mol/kg solvent)/H2O solution at 25-90 °C. Regular solution theory is commonly used to model the nonideal thermodynamics of cosolvents. Camper et al.5 used it to model the CO2 solubilities in ionic liquids. Fosbel et al.6 studied the CO2-NaHCO3Na2CO3-MEG-H2O system and concluded that CO2 data are insufficient for model prediction. Kan et al.7 investigated the effect of methanol on CO2,gas partitioning and calcite solubility in 0-3 m (m, mol/kg H2O) NaCl and 0-90 wt % methanol from 4 to 25 °C. In that paper, a model is proposed * To whom correspondence should be addressed. Tel.: 713-348-5224. Fax: 713-348-6360. E-mail: [email protected].

that the activity coefficient of CO2 is described by the specific ion interaction model of Pitzer and a semiempirical equation that represents Born’s equation for the nonelecrolyte effect. The current paper presents experimental result of carbon dioxide partitioning in a gas/liquid system with 0-99% (wt %) MEG, 0-6 m NaCl, and 0.14-0.27 atm of CO2 partial pressure at 3-50 °C. Kan’s model was used to develop the semiempirical equation to represent the MEG effect on the CO2 partition. This equation can be directly used in conjunction with any activity coefficient-based geochemical models, such as the Pitzer ion interaction model, to determine the effect of MEG on CO2 solubility in a mixed salt-MEG medium. Materials and Methods All chemicals used in this study are reagent grade (Fisher Scientific). The purity of MEG used in this study is 99.5% (Acros Organics). All solubility experiments were conducted in a reaction bottle (Figure S2, Supporting Information). The reaction bottle is approximately 304 cm3 in volume and is made of heavy-wall borosilicate glass with three PTFE valves (Ace Glassware, Inc., Little Rock, AR). One valve is connected to a digital pressure gauge ((15 psig full range, 0.05% accuracy, Omega Engineering, Stamford, CT). Two syringes are attached to two other valves. One syringe is used to inject strong acid into the reaction bottle, and the other syringe is designed to remove samples from the reaction bottle. The reaction bottle was submerged in a water bath, and the bath was attached to a heat/refrigeration circulating bath (Neslab). Reaction solutions were vigorously stirred with magnetic stirrers during the entire experiment. For carbon dioxide solubility experiments, 0, 10, 30, 60, 85, and 95% by weight (wt %) of MEG and 0, 1, 3, and 6 m NaCl were used to study the salt and cosolvent effects on carbon dioxide partitioning at 3, 23, 50, and 70 °C from a gas phase containing CO2 at a CO2 partial pressure of 0.13-0.27 atm. One additional experiment was conducted with 99.2 wt % MEG in the absence of NaCl at 23 °C. In a typical experiment, approximately 0.16 g of sodium bicarbonate (∼2 mmol) was added to the reaction bottle. The

10.1021/ie901274v  2010 American Chemical Society Published on Web 05/06/2010

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solution was continuously stirred for over 30 min to allow water vapor to saturate the air in the reaction bottle, which eliminates the effects of water vapor pressure changes on the measurements of carbon dioxide pressure. After the pressure reading of the reaction bottle stabilized to a constant reading, excess sulfuric acid (∼3 mmol) was injected into the reaction bottle to produce carbon dioxide at a final solution pH of less than 2. The pressure change in the reaction bottle, due to the evolution of carbon dioxide gas, was measured using a pressure gauge. The amount of CO2 in the gas phase was calculated using the ideal gas law. Note that the CO2 gas activity coefficient is calculated to be 0.994 at 3 °C and 0.997 at 70 °C, as discussed below, and was assumed to be 1.00, which is within experimental error. The amount of dissolved carbon dioxide in the solution was calculated by mass balance. For example, at 23.7 °C, 0.1688 g of NaHCO3 powder (0.002 mols) and 106.84 g of solution containing 11.3 wt % of MEG and 88.7 wt % of H2O and NaCl at a concentration of 0.993 m were put into the reaction bottle. The cap of the reaction bottle was tightened, and the solution was stirred with a magnetic bar for over 30 min to saturate the air with water vapor in the reaction bottle. The water vapor pressure was measured with a pressure gauge and then was recorded when water vapor was saturated in the reaction bottle. Then, 0.391 mL of 6 M sulfuric acid was added into the reaction bottle to a final pH of 1.58. At this point, NaHCO3 in the reaction bottle was acidified by sulfuric acid to produce carbon dioxide to yield a partial pressure of 2.63 psia, which corresponds to 0.00149 mols of CO2 in the gas phase. By mass balance, the liquid phase CO2 concentration was calculated to be 0.00648 mol/kg H2O. Data Interpretation Equilibrium in the CO2-H2O system can be described using the following two equations: + HCO3 + H ) CO2,aq + H2O

(1)

CO2,aq ) CO2,g

(2)

where CO2,aq is defined as all dissolved CO2, hydrated or not. At the final solution pH of less than 2, the concentrations of bicarbonate and carbonate species are negligibly small, the predominant equilibrium of concern is reaction 2, the equilibrium constant for reaction 2 is typically defined by Henry’s law (eq 3). KH )

aCO2,aq fCO2,gas

(3)

where aCO2,aq and fCO2,gas are the activity and fugacity of CO2. In this study, the general thermodynamic framework of Kan et al.7,8 was used to represent the effect of cosolvent on CO2 partitioning in gas/liquid phases, where the liquid phase is composed of salt, water, and MEG. In the presence of salt and MEG in solution, the relationship of the excess free energy change, ∆Gexcess, due to solution composition change and the overall activity coefficient, γoverall, can be expressed as follows: ∆Gexcess ) RT ln γoverall ) ∆Gsalt + ∆GMEG ) RT ln γS + RT ln γN

(4)

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where ∆Gsalt and ∆GMEG represent the free energy change due to adding salt and MEG to solutions, and γS and γN are the activity coefficients due to “salt effects” and “cosolvent effects”, or the MEG effect in this case. γS is calculated by the Pitzer model, and γN can be calculated via the Born type equation for ions,9-11 and a similar function is used herein to model the effect of MEG on the solubility of carbon dioxide. The concentration of aqueous CO2 is expressed in units of aqueous molality (m). It is important to emphasize that this is as required for conventional activity coefficient calculations, e.g., Pitzer theory, which uses pure water as the reference state for all conditions. Interpretation of CO2 Solubility Data. At a specific experimental condition, the conditional Henry’s law constant (K H′ ) can be determined from experimental data. KH′ )

[CO2,aq] PCO2

pKH′ ) -log10 KH′

(5) (6)

where PCO2 is the partial pressure of CO2 (atm), [ ] represents concentrations of particular chemical species enclosed in the unit of molal (mol/kg H2O), and K H′ (m · atm-1) is dependent on a specific temperature, ionic strength, and cosolvent concentration. The thermodynamic Henry’s law constant KH can be represented by the following expressions: KH )

aCO2,aq fCO2,gas

)

S N × γCO [CO2,aq] × γCO 2,aq 2,aq

PCO2 × γCO2,gas

pKH ) -log10(KH)

(7) (8)

where γCO2,g, γSCO2,aq, and γN CO2,aq are the CO2 activity coefficients in the gas and liquid phases. The values of γCO2,g in eq 7 are calculated on the basis of a Peng-Robinson equation of state. The activity changes of carbon dioxide due to the salt effect have been well studied.12-18 We used Pitzer equations to obtain the activity coefficients of carbon dioxide due to the salt effect, γSCO2,aq.19,20 The Pitzer activity coefficient is obtained from a virial expansion of the excess Gibbs energy,19 and specifically, eq 9 S is used to calculate γCO 2,aq. S ln γCO ) 2mNa+(λNa+-CO2 + λCl--CO2) + 2,aq mNa+ × mCl- × ζNa+-Cl--CO2 (9)

where the λ’s and ζ are second- and third-order interaction parameters, respectively, and these parameters are temperature and ionic strength dependent. In this study, the specific temperature and ionic strength dependent equation of λ’s and ζ are obtained from He and Morse.15 The temperature dependence of pKH (m · atm-1) of Plummer and Busenberg17 (eq 10) was adopted. Note that the complete Pitzer activity coefficient model and Peng-Robinson equations are part of the software (ScaleSoftPitzer) that was used in this study to calculate the parameters.21 pKH ) 108.3865 + 0.01985 × T - 6919.5/T 40.452 × log(T) + 669365/T2 (10) In this study, we conducted experiments to determine PCO2 and [CO2,aq] at equilibrium with solutions of 0-6 m salt and 0-99 wt % MEG. Then, the activity coefficients of carbon

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Table 1. Comparison of the Experimentally Determined pKH versus the Thermodynamic pKH Defined by Plummer et al.17 T (°C)

NaCla (m)

NaHCO3a (m)

H2SO4a (m)

PCO2 (psia)

CO2,aqa (m)

S b γCO 2,aq

γCO2,gasb

pKH,obsc

pKH,calcd

∆pKHe

3.6 3.2 3.2 23.5 23.0 23.4 22.2 23.8 50.0 50.1 50.2 50.1 49.8 50.0 50.0 50.0 50.2 50.0 50.0 50.1 49.6 50.6 49.6 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0

1.00 2.99 5.97 0.00 0.99 1.00 2.99 5.97 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 2.99 2.99 5.98 5.98 1.00 1.00 1.00 1.00 1.00 1.00 1.00 2.99 5.98 5.98 5.98 5.98 5.98

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.02 0.02 0.01 0.02 0.02

0.03 0.03 0.04 0.07 0.08 0.08 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

1.983 2.290 2.499 2.445 2.568 2.578 2.756 3.003 3.197 2.994 2.964 3.080 3.178 3.238 2.256 2.579 3.051 3.099 3.118 3.376 3.311 3.524 3.548 3.390 3.203 2.650 3.421 2.621 3.462 2.516 2.778 2.653 2.723 2.497 2.627 2.726

7.48 × 10-3 6.26 × 10-3 5.26 × 10-3 5.96 × 10-3 5.52 × 10-3 5.46 × 10-3 4.24 × 10-3 3.70 × 10-3 3.70 × 10-3 3.69 × 10-3 3.43 × 10-3 3.42 × 10-3 3.63 × 10-3 3.74 × 10-3 3.06 × 10-3 3.18 × 10-3 3.52 × 10-3 3.75 × 10-3 3.52 × 10-3 2.88 × 10-3 2.92 × 10-3 2.01 × 10-3 2.20 × 10-3 3.22 × 10-3 3.07 × 10-3 2.52 × 10-3 3.46 × 10-3 2.50 × 10-3 3.53 × 10-3 2.23 × 10-3 2.15 × 10-3 1.55 × 10-3 1.50 × 10-3 1.59 × 10-3 1.47 × 10-3 1.49 × 10-3

1.162 1.543 2.360 1.016 1.173 1.173 1.542 2.360 1.162 1.162 1.162 1.162 1.161 1.161 1.161 1.161 1.161 1.161 1.160 1.542 1.542 2.357 2.357 1.161 1.161 1.160 1.161 1.160 1.161 1.159 1.539 2.353 2.350 2.353 2.353 2.353

0.994 0.994 0.994 0.995 0.995 0.995 0.995 0.995 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.996 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997 0.997

1.188 1.205 1.134 1.437 1.429 1.435 1.455 1.367 1.702 1.676 1.703 1.720 1.708 1.704 1.634 1.676 1.704 1.684 1.713 1.711 1.698 1.702 1.666 1.788 1.785 1.790 1.761 1.787 1.758 1.819 1.756 1.694 1.721 1.657 1.712 1.723

1.170 1.163 1.163 1.451 1.445 1.449 1.435 1.454 1.711 1.712 1.713 1.712 1.710 1.711 1.711 1.711 1.713 1.711 1.711 1.712 1.708 1.716 1.708 1.847 1.847 1.847 1.847 1.847 1.847 1.847 1.847 1.847 1.847 1.847 1.847 1.847

0.019 0.042 -0.029 -0.014 -0.015 -0.014 0.021 -0.087 -0.009 -0.037 -0.010 0.008 -0.002 -0.007 -0.078 -0.035 -0.008 -0.028 0.002 -0.001 -0.010 -0.014 -0.042 -0.058 -0.061 -0.057 -0.085 -0.060 -0.089 -0.028 -0.090 -0.153 -0.126 -0.190 -0.135 -0.124

a The calculations of concentrations; activity coefficients are based on the unit of molality, mol/kg H2O. b Both activity coefficients are calculated with ScaleSoftPitzerTM. c pKH,obs is the experimentally determined pKH from eq 7. d pKH,calc is the thermodynamic pKH calculated from eq 10. e ∆pKH ) pKH,obs - pKH,calc.

dioxide due to the MEG effect, γN CO2,aq, could be calculated from eq 11. N γCO 2,aq

)

KH × PCO2 × γCO2,gas S [CO2,aq] × γCO 2,aq

)

S KH × γCO2,gas /γCO 2,aq

KH′

(11)

Results and Discussion CO2 Vapor Liquid Equilibrium in the Absence of MEG. Since CO2 partitioning in the salt solutions is very well understood,22,23 a set of experiments was conducted at every temperature and ionic strength condition in the absence of MEG in order to establish the quality of the experimental data and the Pitzer activity coefficient calculation over a temperature range of 3-70 °C and an ionic strength of 0-6 m. The experimentally determined Henry’s law constants (pKH,Obs) were calculated from eq 7 with the γSCO2,aq calculated from eq 9, γCO2,gas calculated by the Peng-Robinson equation N of state, and assuming γCO 2,aq ) 1.00 (by definition in the absence of MEG, Table 1). These values compared reasonably well to the reported pKH values, e.g., pKH’s of 1.197, 1.47, and 1.71 at 5, 25, and 50 °C24 and the thermodynamic pKH (eq 10) defined by Plummer et al.17 (pKH,Calc). In Table 1, ∆pKH represents the difference between pKH,Obs and the thermodynamic pKH,Calc, i.e., ∆pKH ) pKH,Obs pKH,Calc. The 70 °C data shows a larger deviation from the theoretical pKH, especially at 5.98 m NaCl, indicating that the quality

of the 70 °C data was not as reliable as that of the lower temperature data, and therefore, the data were not included in activity coefficient model development. If we exclude the 70 °C data, the mean and standard deviation of ∆pKH are -0.01 ( 0.02 for 22 observations at 3-50 °C for 0-6 m NaCl solutions. Including the 70 °C data, the mean and standard deviation of ∆pKH are -0.042 ( 0.053 for a total of 35 observations, indicating that the experimental data are internally consistent with the thermodynamic pKH values and Pitzer activity coefficients model. Similarly, Spycher and Pruess22 also observed good results when they used the Pitzer activity coefficient to model the salt effect. CO2 Vapor Liquid Equilibrium in NaCl-H2O-MEG Solvent. In Table 2 are listed the experimental conditions and results of CO2 partitioning at various concentrations of MEG and NaCl and various temperatures. Selective data of Table 2 are plotted in Figure 1. In Figure 1a, the pK H′ of the 1 m NaCl data at 3-50 °C versus the MEG mole fraction concentration are plotted. In Figure 1b, the pK H′ of the 23 °C data at 0-6 m ionic strength versus the MEG mole fraction concentration are plotted. Also included in Figure 1b are the CO2-H2O-MEG data of Hayduk and Malik.3 As discussed earlier, the differences in pK H′ at 0% MEG reflect the expected temperature and ionic strength dependence of Pitzer activity coefficients and the temperature dependence of pKH. If the lines in Figure 1a and b were parallel to each other, the effect of MEG on CO2 partitioning would be independent of the temperature and ionic strength. As shown

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Table 2. CO2 Solubility in 0-6 m NaCl and 0-100 wt % MEG Solutions at 3.2-70 °C NaHCO3a PCO2 T xMEG (28 > C) MEG (wt%) (mole fr) NaCla (m) (m) H2SO4a (m) I (m) (psia) 4.4 3.5 4.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 23.4 22.4 23.7 23.8 23.8 22.5 23.8 23.4 23.2 22.0 23.9 23.1 23.0 22.2 23.0 22.8 23.1 23.0 23.9 23.8 23.8 24.1 23.8 50.1 50.1 50.1 50.1 50.1 49.6 49.6 50.6 50.6 50.6 50.6 50.9 50.9 50.7 50.7 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0

11.1 33.3 63.4 85.7 94.0 11.3 33.0 63.6 86.9 94.9 12.6 34.4 64.2 84.8 94.8 99.2 11.0 11.3 21.8 31.2 42.2 43.0 50.4 60.1 70.9 72.0 80.9 90.7 11.1 32.6 61.9 74.8 88.8 13.2 34.6 64.6 86.1 94.6 12.0 32.8 61.6 85.5 94.6 13.4 33.5 62.1 73.5 88.6 12.6 35.7 63.7 75.7 89.4 12.0 32.8 61.7 85.6 94.6 11.3 32.6 60.0 73.2 86.8 13.7 35.3 64.4 76.3 89.6

0.035 0.127 0.335 0.636 0.820 0.036 0.125 0.336 0.657 0.843 0.040 0.132 0.342 0.619 0.840 0.973 0.035 0.036 0.075 0.116 0.175 0.179 0.228 0.304 0.414 0.428 0.552 0.738 0.035 0.123 0.320 0.463 0.698 0.042 0.133 0.346 0.643 0.835 0.038 0.124 0.318 0.631 0.836 0.043 0.127 0.322 0.447 0.693 0.040 0.139 0.337 0.475 0.711 0.038 0.124 0.318 0.633 0.836 0.036 0.123 0.303 0.442 0.657 0.044 0.137 0.344 0.483 0.714

1.00 0.99 0.99 0.98 0.94 2.99 2.98 2.97 2.92 2.80 5.97 5.96 5.93 5.85 5.59 0.00 1.00 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.99 0.98 0.96 2.99 2.98 2.97 2.96 2.91 5.97 5.96 5.94 5.86 5.63 1.00 1.00 0.99 0.98 0.95 2.99 2.99 2.98 2.97 2.93 5.97 5.97 5.95 5.93 5.86 1.00 1.00 0.99 0.99 0.95 2.99 2.99 2.98 2.97 2.95 5.98 5.97 5.96 5.95 5.89

0.022 0.028 0.050 0.127 0.296 0.023 0.028 0.051 0.136 0.334 0.024 0.031 0.052 0.120 0.337 5.393 0.023 0.023 0.025 0.028 0.034 0.033 0.038 0.045 0.065 0.065 0.094 0.194 0.024 0.030 0.051 0.074 0.165 0.025 0.031 0.054 0.129 0.326 0.023 0.029 0.048 0.122 0.335 0.024 0.030 0.050 0.071 0.160 0.024 0.032 0.053 0.078 0.170 0.017 0.024 0.042 0.095 0.250 0.017 0.020 0.036 0.048 0.100 0.018 0.023 0.035 0.051 0.110

0.036 0.041 0.077 0.193 0.487 0.037 0.043 0.070 0.217 0.521 0.040 0.051 0.093 0.194 0.546 8.017 0.034 0.026 0.034 0.038 0.048 0.049 0.056 0.074 0.103 0.089 0.154 0.290 0.037 0.045 0.077 0.112 0.245 0.041 0.047 0.079 0.194 0.521 0.031 0.042 0.067 0.161 0.427 0.031 0.038 0.065 0.090 0.197 0.032 0.040 0.065 0.098 0.196 0.027 0.037 0.052 0.116 0.407 0.022 0.027 0.056 0.074 0.138 0.026 0.030 0.049 0.072 0.156

1.05 1.06 1.12 1.30 1.72 3.05 3.05 3.10 3.27 3.66 6.03 6.04 6.08 6.17 6.47 13.41 1.05 1.04 1.05 1.06 1.08 1.07 1.08 1.11 1.16 1.14 1.22 1.44 3.05 3.06 3.10 3.14 3.32 6.03 6.04 6.07 6.18 6.48 1.05 1.07 1.11 1.26 1.71 3.04 3.05 3.09 3.13 3.29 6.03 6.04 6.07 6.10 6.23 1.04 1.06 1.09 1.20 1.61 3.03 3.04 3.07 3.10 3.19 6.02 6.03 6.05 6.07 6.15

2.03 2.06 2.21 2.05 1.91 2.19 2.24 2.31 2.11 1.87 2.49 2.51 2.45 2.24 2.02 2.35 2.60 2.63 2.64 2.68 2.71 2.63 2.68 2.61 2.62 2.58 2.58 2.45 2.81 2.79 2.75 2.70 2.49 3.03 3.01 2.82 2.59 2.44 3.13 3.28 3.09 3.10 2.97 3.42 3.38 3.22 3.33 3.12 3.57 3.45 3.46 3.29 3.17 2.66 2.78 2.63 2.42 2.30 2.57 2.40 2.68 2.49 2.49 2.71 2.69 2.50 2.45 2.33

[CO2,aq]a (m) 8.11 × 10-1 1.02 × 10-2 1.66 × 10-2 4.70 × 10-2 1.25 × 10-1 7.72 × 10-3 9.14 × 10-3 1.48 × 10-2 4.76 × 10-2 1.39 × 10-1 6.05 × 10-3 7.15 × 10-3 1.27 × 10-2 3.80 × 10-2 1.29 × 10-2 1.851 5.91 × 10-3 5.81 × 10-3 6.48 × 10-3 6.91 × 10-3 8.02 × 10-3 8.13 × 10-3 8.96 × 10-3 1.06 × 10-2 1.59 × 10-2 1.65 × 10-2 2.53 × 10-2 5.65 × 10-2 4.65 × 10-3 5.70 × 10-3 1.03 × 10-3 1.71 × 10-2 4.70 × 10-2 3.60 × 10-3 4.68 × 10-3 1.09 × 10-2 3.39 × 10-2 9.90 × 10-2 4.19 × 10-3 4.70 × 10-3 9.20 × 10-3 2.54 × 10-2 7.87 × 10-2 2.89 × 10-3 4.12 × 10-3 8.48 × 10-3 1.11 × 10-2 3.27 × 10-2 2.32 × 10-3 3.49 × 10-3 6.65 × 10-3 1.20 × 10-2 3.35 × 10-2 2.49 × 10-3 3.09 × 10-3 8.28 × 10-3 1.53 × 10-2 5.07 × 10-2 2.12 × 10-3 2.57 × 10-3 5.10 × 10-3 7.00 × 10-3 1.66 × 10-2 1.73 × 10-3 2.26 × 10-3 4.85 × 10-3 7.72 × 10-3 2.19 × 10-2

pKH′b S c (m · atm-1) γCO 2,aq 1.232 1.137 0.957 0.473 0.016 1.286 1.223 1.026 0.480 -0.038 1.447 1.378 1.119 0.604 0.029 -1.063 1.477 1.488 1.442 1.421 1.361 1.343 1.309 1.224 1.050 1.027 0.841 0.471 1.614 1.523 1.258 1.032 0.557 1.758 1.642 1.245 0.717 0.225 1.707 1.677 1.359 0.919 0.410 1.905 1.746 1.412 1.308 0.812 2.021 1.828 1.549 1.271 0.810 1.861 1.786 1.334 1.032 0.490 1.916 1.802 1.554 1.385 1.009 2.027 1.907 1.545 1.334 0.859

1.164 1.166 1.177 1.216 1.314 1.544 1.547 1.558 1.612 1.733 2.361 2.366 2.383 2.429 2.588 10.752 1.163 1.161 1.163 1.165 1.168 1.168 1.170 1.175 1.185 1.182 1.201 1.249 1.545 1.548 1.560 1.573 1.625 2.361 2.365 2.380 2.434 2.587 1.163 1.166 1.175 1.208 1.308 1.544 1.547 1.558 1.568 1.614 2.359 2.363 2.377 2.393 2.450 1.161 1.164 1.171 1.194 1.287 1.540 1.542 1.552 1.559 1.586 2.355 2.358 2.367 2.377 2.418

N d γCO 2,aq

calcd N e γCO 2,aq

0.955 0.794 0.506 0.167 0.054 0.853 0.737 0.465 0.128 0.036 0.810 0.689 0.377 0.113 0.028 2.84 × 10-4 0.936 0.929 0.832 0.791 0.712 0.660 0.616 0.506 0.348 0.314 0.206 0.085 0.973 0.770 0.417 0.244 0.076 0.844 0.647 0.258 0.074 0.023 0.845 0.787 0.376 0.133 0.038 1.014 0.703 0.318 0.248 0.077 0.850 0.541 0.284 0.149 0.050 0.886 0.745 0.262 0.128 0.034 0.759 0.583 0.327 0.221 0.091 0.641 0.486 0.210 0.129 0.042

0.946 0.806 0.487 0.145 0.042 0.917 0.725 0.364 0.073 0.017 0.860 0.598 0.227 0.042 0.006 5.02 × 10-5 0.936 0.933 0.861 0.787 0.687 0.677 0.598 0.479 0.331 0.311 0.182 0.062 0.907 0.696 0.345 0.181 0.044 0.839 0.565 0.196 0.028 0.005 0.913 0.733 0.402 0.095 0.021 0.869 0.650 0.299 0.164 0.035 0.832 0.519 0.179 0.076 0.012 0.903 0.709 0.370 0.082 0.018 0.881 0.639 0.300 0.151 0.040 0.807 0.507 0.158 0.064 0.010

a S The calculations of concentrations are based on the unit of molality, m (mol/kg H2O). b pKH′ is calculated by eqs 4 and 5. c γCO 2,aq is calculated by N e N eq 9 in ScaleSoftPitzer. d γCO Calcd γCO 2,aq is calculated by eq 11. 2,aq is based on eq 12.

in Figure 1a and b, the data indicate that the MEG effects are slightly dependent on both the temperature and ionic strength.

The data of this study were compared with CO2 partitioning in a H2O-methanol solution in Figure 2a and b at 0 and 1 m NaCl concentrations. In Figure 2c is a comparison of

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the dielectric constants of methanol and MEG. The differences in pK ′H for CO2 in a H2O-NaCl-cosolvent are similar to that in the binary system of H2O-cosolvent. Also, the differences in pK ′H in H2O-cosolvent are consistent with the difference in their dielectric constants. N The value, γCO 2,aq, shown in Table 2 was calculated from the experimental results via eq 11. The γN CO2,aq values at 3-50 °C in the CO2-NaCl (1 m)-H2O-MEG and CO2-NaCl (6 m)-H2O-MEG systems were plotted in Figure 3a and b. The lines are the predictive curves that were calculated on the basis of a semiempirical equation (eq 12) that will be discussed below. In general, γN CO2,aq decreases when the MEG N concentration increases. The 3-50 °C γCO 2,aq data in Table 2 N and the γCO2,aq values calculated from the CO2-H2O-MEG data of Hayduk and Malik3 were correlated to temperature, ionic strength, and MEG concentration via a semiempirical function (eq 12) as suggested by Kan et al.7 and Hayduk and Malik’s results (Figure S1, Supporting Information) using statistics software (SigmaPlot, Systat Software, Inc., San Jose, CA).

(

N log10(γCO ) ) -2.954 + 2,aq

)

691.6 0.382I xMEGT 1 + I0.5 151.9 2 4 x - 0.670xMEG (12) T MEG

where T is temperature (K), I is ionic strength (m), and xMEG is the MEG concentration in mole fraction. The correlation coefficient of the data to eq 12 is 0.998. The equation shows a small temperature and ionic strength dependence of γN CO2,aq. This is reasonable since the predominant ionic strength and temperature dependence of the reaction is corrected by the Pitzer activity coefficient term and the temperature dependence of the Henry’s law constant. The reasonableness of eq 12 was determined by comparing the calculated versus N observed log10(γCO 2,aq) values, as shown in Figure 4. The N calculated log10(γCO 2,aq) values not only agree to our 3-50 °C and 0-97% MEG and 0-6 m NaCl data and Hayduk and Malik’s data as well, they also agree reasonably well with our 70 °C CO2-NaCl (1-6 m)-H2O-MEG data, as well as Sandangen’s data,4 which covered the range of 25-90 °C CO2-NaCl (0.1-0.7 mol/kg solvent)-H2O-MEG (0-95

Figure 1. Conditional Henry’s law constants for CO2 (pK H′ ) vs MEG mole fraction concentration at (a) three temperatures and the solution phase containing 1 m NaCl and 0-95 wt % MEG and (b) 23 °C and the solution phase containing 0-6 m NaCl and 0-95 wt % MEG.

Figure 2. Comparison of the conditional Henry’s law constants for CO2 (pKH′) versus MEG and methanol concentrations (a) in the absence and (b) in the presence of 1 m NaCl at 23-25 °C. (c) The dielectric constants of the H2O-methanol and H2O-MEG binary systems. The CO2-H2O-methanol pKH′ data are from Sada et al.;25 the CO2-H2O-MEG pKH′ data are from Hayduk and Malik;3 the CO2-H2O-NaCl-methanol data are from Kan et al.;7 the dielectric constants were from Timmerman.26

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N N Figure 3. Plots of γCO 2,aq versus MEG concentration at 3-50 °C and (a) 1 m NaCl-MEG and (b) 6 m NaCl-MEG mixed solutions. The lines are γCO2,aq calculated from eq 12.

N Figure 4. Plots of calculated versus observed log10(γCO 2,aq) for all data of this study and Sandangen’s4 and Hayduk and Malik’s3 data.

wt %). The only data point that was significantly deviated from the 1:1 quality line is Hayduk and Malik’s data at 99.5 wt % MEG concentration. Therefore, eq 12 can be used to model the reaction of CO2 partitioning in the H2O-NaClMEG system at 0-6 m NaCl, 0-97 wt % MEG, and 0-90 °C. It is important to model the CO2 partitioning into the MEGbrine liquid phase at various temperatures, pressures, and salt concentrations in order to properly account for the corrosion and scaling tendency of the brine in the gas pipeline, MEG regeneration, and other surface equipment. The complicated

nonideal relationship of CO2-NaCl-H2O-MEG can be simulated using the results of this study. For example, the effect of adding MEG to two different brines of 58 440 and 306 810 mg/L TDS (1-6 m NaCl) at equilibrium with a gas phase of 1 atm of CO2 partial pressure and 25 °C is illustrated in Figure 5, where the aqueous phase CO2 concentration versus MEG/brine volumetric ratios were compared. Note that CO2 solubility (in units of mol/kg solvent) in a NaClH2O-MEG solution does not change significantly if the brine contains 1 m salt (Figure 5a). On the other hand, the CO2 solubility (in unit of mol/kg solvent) changes considerably in a 6 m salt-MEG solution. The addition of ionic compounds reduces the solubility of carbon dioxide in the solution. At high ionic strength, e.g., in 6 m salt, the solubility of carbon dioxide is much smaller than that in a 1 m ionic strength solution; the addition of MEG dilutes the ionic strength directly. Therefore, the solubility of carbon dioxide increases with the increasing MEG concentration. However, the salt effect is not shown when the aqueous CO2 concentration is expressed on a per kilogram of H2O basis (Figure 5b). In typical industrial applications, e.g., in natural gas production, the impact of the MEG application on scale and corrosion needs to be assessed by geochemical modeling, which typically uses a reference state of H2O. In this paper, the observed results and equations are all developed with a reference state of H2O, which can be used directly in geochemical modeling, using for example ScaleSoftPitzer, to model the impact of MEG on CO2 partitioning in natural gas production tubing.

Figure 5. Comparison of CO2 partitioning to 1 and 6 m brine at equilibrium with 1 atm pressure of CO2 and various volumetric ratios of MEG to brine. The CO2,aq concentrations on the y axis are CO2,aq (mol/kg solvent) in a and mol/kg H2O in b.

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Acknowledgment This research was financially supported by the Brine Chemistry consortium companies of Rice University, including BakerPetrolite, BJ chemical, BP, Champion, Chevron, ConocoPhilips, Halliburtan, Kemira, Marathon, M-I SWACO, Nalco, Petrobras, Saudi Aramco, Shell, StatoilHydro, Total, and Advanced Energy Consortium. Supporting Information Available: The apparatus and literature data on CO2 partitioning in NaCl-H2O-MEG solution. This information is available free of charge via the Internet at http://pubs.acs.org/. Literature Cited (1) Kobe, K. A.; Mason, G. E. Aqueous solutions of alcohols as confining liquids for gas analysis. Ind. Eng. Chem., Anal. Ed. 1946, 18, 78–79. (2) Won, Y. S.; Chung, D. K.; Mills, A. F. Density, viscosity, surface tension, and carbon dioxide solubility and diffusivity of methanol, ethanol, aqueous propanol, and aqueous ethylene glycol at 25C. J. Chem. Eng. Data 1981, 26 (2), 140–141. (3) Hayduk, W.; Malik, V. K. Density, Viscosity, and Carbon Dioxide Solubility and Diffusivity in Aqueous Ethylene Glycol Solutions. J. Chem. Eng. Data 1971, 16 (2), 143–146. (4) Sandengen, K. Prediction of mineral scale formation in wet gas condensate pipelines and in meg (mono ethylene glycol) regeneration plants; Norwegian University of Science and Technology: Trondheim, Norway, 2006. (5) Camper, D.; Scovazzo, P.; Koval, C.; Noble, R. Gas Solubilities in Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2004, 43, 6855– 6860. (6) Fosbol, P. L.; Thomsen, K.; Stenby, E. H. Solubility Measurements in the Mixed Solvent Electrolyte System Na2CO3-NaHCO3-Monoethylene Glycol-Water. Ind. Eng. Chem. Res. 2009, 48 (4), 2218–2228. (7) Kan, A. T.; Fu, G.; Tomson, M. B. Effect of methanol on carbonate equilibrium and calcite solubility in a gas/methanol/water/ salt mixed system. Langmuir 2002, 18 (25), 9713–9725. (8) Kan, A. T.; Fu, G.; Tomson, M. B. Effect of methanol and ethylene glycol on sulfates and halite scale formation. Ind. Eng. Chem. Res. 2003, 42, 2399–2408. (9) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions: The Measurement and Interpretation of Conductance, Chemical Potential and Diffusion in Solutions of Simple Electrolytes, 2nd ed.; Butterworth & Co.: London, 1970; p 571. (10) Bates, R. G. Determination of pH - Theory and Practice; WileyInterscience: New York, 1973. (11) Sen, B., Computational techniques of ionic processes in waterorganic mixed solvents. In Thermodynamic behaVior of electrolytes in mixed

solVents; Furter, W. F., Ed.; American Chemical Society: Washington, DC, 1978; Vol. II, pp 215-248. (12) Harned, H. S.; Davis, R., Jr. The Ionization Constant of Carbonic Acid in Water and the Solubility of Carbon Dioxide in Water and Aqueous Salt Solutions from 0 to 50 °C. J. Am. Chem. Soc. 1943, 65, 2030–2037. (13) Carroll, J. J.; Slupsky, J. D.; Mather, A. E. The solubility of carbon dioxide in water at low pressure. J. Phys. Chem. Ref. Data 1991, 20 (6), 1201–1219. (14) Butler, J. N. Carbon Dioxide Equilibria and Their Applications; Addison-Wesley Publ.: Reading, MA, 1982; p 246. (15) He, S.; Morse, J. W. The carbonic acid system and calcite solubility in aqueous Na-K-Ca-Mg-Cl-SO4 solutions from 0 to 90 °C. Geochim. Cosmochim. Acta 1993, 57 (15), 3533–3554. (16) Duan, Z.; Sun, R. An improved model calculating CO2 solubility in pure water and aqueous NaCl solutions from 273 to 533 K and from 0 to 2000 bar. Chem. Geol. 2003, 193 (3-4), 257–271. (17) Plummer, L. N.; Busenberg, E. The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90 °C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O. Geochim. Cosmochim. Acta 1982, 46 (6), 1011–1040. (18) Weiss, R. F. Carbon dioxide in wwater and seawater: The solubility of a non-ideal gas. Mar. Chem. 1974, 2, 203–215. (19) Pitzer, K. S. Thermodynamics, 3rd ed.; McGraw-Hill: New York, 1995. (20) Harvie, C. E.; Møller, N.; Weare, J. H. The prediction of mineral solubilities in natural waters: The Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3CO2-H2O system to high ionic strengths at 25 °C. Geochim. Cosmochim. Acta 1984, 48 (4), 723–751. (21) Tomson, M. B.; Kan, A. T. ScaleSoftPitzer V. 13; Rice University Brine Chemistry Consortium: Houston, TX, 2009. (22) Spycher, N.; Pruess, K. CO2-H2O mixtures in the geologcial sequestration of CO2. II. Partitioning in chloride brines at 12-100 °C and up to 600 bar. Geochim. Cosmochim. Acta 2005, 69, 3309–3320. (23) Spycher, N.; Pruess, K.; Ennis-King, J. CO2-H2O mixtures in the geological sequestration of CO2. I. Assessment and calculation of mutual solubilities from 12 to 100 °C and up to 600 bar. Geochim. Cosmochim. Acta 2003, 67, 3015–3031. (24) Langmuir, D. Aqueous EnVironmental Geochemistry; Prentice Hall: Upper Saddle River, NJ, 1997. (25) Sada, E.; Kito, S.; Ito, Y., Salt Effect on Carbon Dioxide Solubility in Mixture of Methanol and Water. In Thermodynamic BehaVior of Electrolytes in Mixed SolVents; Gould, R. F., Ed.; The Maple Press Co.: York, PA, 1976. (26) Timmermans, J. The Physico-Chemical Constants of Binary Systems in Concentrated Solutions; Two Organic Compounds Without Hydroxyl Derivatives. Interscience Pubs.: New York, 1959; Vol. I, p 1273.

ReceiVed for reView August 12, 2009 ReVised manuscript receiVed April 20, 2010 Accepted April 23, 2010 IE901274V