Measurements of CO2–H2O–NaCl Solution Densities over a Wide

Nov 20, 2013 - Measurements of CO2–H2O–NaCl Solution Densities over a Wide Range of Temperatures, Pressures, and NaCl Concentrations. Yongchen Son...
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Measurements of CO2−H2O−NaCl Solution Densities over a Wide Range of Temperatures, Pressures, and NaCl Concentrations Yongchen Song, Yangchun Zhan, Yi Zhang,* Shuyang Liu, Weiwei Jian, Yu Liu, and Dayong Wang Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian, Liaoning 116024, People’s Republic of China

ABSTRACT: The density of carbon dioxide + brine solution under supercritical conditions is a significant parameter for CO2 sequestration into deep saline formations. This paper has extended our previous study on density measurements of CO2 + Tianjin brine to the CO2−H2O−NaCl solution by using a magnetic suspension balance (MSB). The measurements were performed in the pressure range (10 MPa to 18 MPa) at a range of temperatures (60 °C to 140 °C) with different concentrations of NaCl (CNaCl = 1 mol·kg−1, 2 mol·kg−1, 3 mol·kg−1, 4 mol·kg−1) and different CO2 mass fractions (w = 0, 0.01, 0.02, 0.03). The influences of pressure, temperature, CO2 mass fractions and NaCl concentration on the CO2−H2O−NaCl solution density were analyzed. The CO2−H2O−NaCl solution density increased almost linearly with an increase in the CO2 mass fraction when the NaCl concentration was less than 4 mol·kg−1 and the temperature was lower than 120 °C. However, at a high concentration of NaCl (CNaCl = 4 mol·kg−1), the density decreased with increasing mass fraction of CO2 when the temperature was over 120 °C. The density of the CO2−H2O−NaCl solution with a high NaCl concentration decreased after dissolving CO2 at high temperatures, which caused the solution to float over the saline layer and increased the risk of CO2 leakage. An empirical model was established to predict the solution density with high accuracy.

1. INTRODUCTION With the development of the economy and an increase in energy consumption, the CO2 concentration in the atmosphere has increased rapidly.1 It is regarded as one of the greatest factors in global warming.2−4 The sequestration of CO2 into deep saline formations, as one form of carbon capture and storage (CCS) technology, has been considered as a promising technology for reducing the amount of CO2 emissions into the atmosphere.5−8 The physical motion of CO2 depends on the brine density and CO2 + brine solution density in the deep saline formations. Under normal conditions, when CO2 dissolves into the brine, the dissolved CO2 increases the density of the solution. Therefore, due to the increase in density, the CO2 will collect at the bottom of the saline formation. On the basis of this phenomenon, Haugan et al.9 proposed a method to sequestrate CO2. Although the density change is very small, even a 0.1 kg·m−3 difference plays an © 2013 American Chemical Society

important role in determining if the solution will sink or buoy. Therefore, the density of the CO2 aqueous solution has an important effect on the safety of CO2 sequestration into deep saline aquifers, and this is also essential for chemical engineering and environmental engineering. Research into the density of aqueous CO2 solution has been published in the past few decades. Parkinson10 and Nighswander et al.11 reported the densities of CO2 + water solution in a temperature range from 80 °C to 200 °C and pressure up to 10 MPa. Haugan et al.9 used the heat of dissolution method to calculate the density change of a CO2 aqueous solution. Ohsumi et al.12 obtained the densities of CO2 aqueous solutions at low CO2 concentrations using a vibrating Received: May 15, 2013 Accepted: November 13, 2013 Published: November 20, 2013 3342

dx.doi.org/10.1021/je400459y | J. Chem. Eng. Data 2013, 58, 3342−3350

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densimeter. Aya et al.13 measured the density of a CO2 aqueous solution based on the volume change of a high pressure vessel. Song et al.14 measured the density change of CO2 + artificial brine solution under high pressures (5 to 12.5) MPa and low temperatures (0.1 to 11) °C. Li et al.15 reported the densities of CO2 + brine solution from Weyburn reservoir at 59 °C and pressures up to 29 MPa. Zhang et al.16 measured the density of CO2 + brine extracted from the Beitang depression located at Bohai Bay Basin in Tianjin city using a magnetic suspension balance (MSB). They discussed the influence of the temperature (40 °C to 80 °C), the pressure (10 MPa to 18 MPa) and CO2 mass fraction (w = 0 to 0.03) on the density of the CO2 saline solution. Yan et al.17 measured CO2 solution density at 50 °C, 100 °C, and 140 °C in 0 M, 1 M, and 5 M NaCl brines at pressures from 5 MPa to 40 MPa. Lu et al.18 obtained the density of a CO2−H2O−NaCl solution with high NaCl concentration by the simulation method and found that the solution density decreased with an increase in the CO2 mass fraction at high temperatures and high NaCl concentrations. However, there is no experimental data to verify the simulation result so far. The temperature and salinity of brine is different under different formation conditions, and the densities of CO2 brine solutions is diverse. The studies above have not clarified the relationship between solution densities and salinities. According to the analysis of Hu et al.,19 the solution densities from different studies are inconsistent with each other, and the accuracy cannot satisfy the requirements of CO2 sequestration assessment. This means that density measurements need to be taken for CO2 brine solutions with different salinities. Thus, more experiments should be performed to complement the existing density data. In this study, based on the MSB, density data of CO2−H2O−NaCl with different concentrations of NaCl (CNaCl = 1 mol·kg−1, 2 mol·kg−1, 3 mol·kg−1, 4 mol·kg−1) and different mass fractions of CO2 (w = 0, 0.01, 0.02, 0.03) were obtained in the pressure range of 10 MPa to 18 MPa and at temperatures of 60 °C to 140 °C. The relationship between solution densities and salinities was analyzed specifically.

by Rubotherm Präzisionsmesstechnik GmbH. The temperature in the measuring cell was controlled with a JULABO FP 50-ME Refrigerated/Heating Circulator. As shown in Table 1, the CO2 and N2 were supplied by Dalian Da-te Gas Ltd. According to the gas chromatographic Table 1. Analysis of Experimental Materials experimental material

purity

impurity

CO2

0.9999

N2

0.99999

NaCl

0.995

O2 < 10.0 ppm, H2 < 1.0 ppm, CO < 1.0 ppm, N2 < 30 ppm, THC < 25.0 ppm, H2O < 5.0 ppm CO < 1.0 ppm, CO2 < 1.0 ppm, THC < 1.0 ppm, H2O < 3.0 ppm F ≤ 0.0002 % Ba ≤ 0.001 % As ≤ 0.00005 % N ≤ 0.001 % Br ≤ 0.01 % Ca ≤ 0.005 % pH(50g·L−1, 25 °C)5.0−8.0 SO4 ≤ 0.002 % PO4 ≤ 0.001 % Mg ≤ 0.002 % K ≤ 0.02 % deionized water

H2O

analysis and dew point analysis, the purity of the CO2 and N2 were 0.9999 and 0.99999, respectively. The NaCl used in this study was produced by the Chinese Medicine Group Chemical Reagent Co., Ltd., with the batch number f20110517. Deionized water was used for the preparation of the NaCl solution. The materials mentioned above were used for density measurement without further purification. The sample temperature was measured with a Pt100 temperature probe (measurement accuracy ± 0.01 °C). The pressure was measured with a pressure sensor (20 MPa, reproducibility 0.08 %). The method previously published by Zhang et al.16 details the experimental setup, procedure, and data analysis. This study used a high precision electronic balance (produced by ShangHai MinQiao Scientific Instrument Corporation, JA10003N, μ = ± 1 mg) to weigh the deionized water and NaCl. First, 1 kg of deionized water was weighed, then different weights of NaCl were weighed based on the different NaCl concentrations (1 mol·kg−1, 2 mol·kg−1, 3 mol· kg−1, 4 mol·kg−1) to be produced. Then, by mixing the NaCl and deionized water, the NaCl solutions were obtained.

2. EXPERIMENTAL SECTION The experimental apparatus is shown schematically in Figure 1. The main apparatus is the MSB, including a microbalance, measuring cell, sinker, magnetic suspension coupling and control system. The MSB used in this work was manufactured

3. RESULTS AND DISCUSSION This study presented density data of CO2−H2O−NaCl system at different pressures, temperatures, NaCl concentrations, and CO2 mass fractions. The density values at different NaCl concentrations are shown in Table 2. The uncertainties caused by the temperature, pressure, CO2 mass fraction, and NaCl concentration can be estimated by the partial differential fitting formula of density. The formula is U (ρ) = {[Ud(ρ)]2 + [UCO2(ρ)]2 + [UNaCl(ρ)]2 }1/2

Figure 1. Scheme of experimental apparatus. 3343

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Table 2. Densities of CO2−H2O−NaCl Solution at different concentrations of NaCl (CNaCl = 1, 2, 3, 4 mol·kg−1) and different CO2 mass fractions (w = 0, 0.01, 0.02, 0.03)a T/°C

P/MPa −1

59.68 59.73 59.96 59.62 59.81 80.05 79.50 80.05 80.17 80.13 100.01 99.88 99.88 99.81 99.85 119.45 119.50 119.50 119.50 119.50 140.01 140.02 140.03 139.97 140.00 59.78 59.73 59.86 59.82 59.74 79.87 79.87 79.78 79.79 79.85 99.87 100.05 100.03 100.07 99.97 119.95 120.00 120.20 120.28 120.14 140.03 140.05 140.18 140.05 140.02 59.74 59.83 59.97 60.05 60.08 80.08 80.04

1 mol·kg NaCl 10.001 12.003 14.007 16.012 18.010 9.995 12.002 13.999 16.005 17.998 10.001 12.005 14.001 16.007 18.001 10.002 12.008 14.005 16.001 18.001 10.006 12.009 13.998 16.004 18.001 1 mol·kg−1 NaCl 10.007 12.004 13.997 16.002 18.006 10.003 12.006 14.009 15.997 17.999 9.996 12.001 13.997 16.002 17.996 9.999 12.008 14.002 16.008 18.000 10.001 11.997 14.004 16.007 17.998 1 mol·kg−1 NaCl 9.997 12.004 13.997 16.002 18.006 10.003 12.008

ρ/g·cm−3

T/°C

P/MPa −1

w = 0 CO2 1.02596 1.02678 1.02752 1.02834 1.02912 1.01463 1.01547 1.01606 1.01679 1.01765 1.00131 1.00225 1.00321 1.00393 1.00474 0.98690 0.98750 0.98882 0.98954 0.99045 0.97045 0.97123 0.97224 0.97321 0.97415 w = 0.0104 CO2 1.02803 1.02882 1.02953 1.03034 1.03115 1.01616 1.01701 1.01787 1.01865 1.01941 1.00233 1.00318 1.00396 1.00485 1.00572 0.98755 0.98858 0.98983 0.99065 0.99167 0.97082 0.97161 0.97268 0.97374 0.97456 w = 0.0201 CO2 1.02933 1.03001 1.03083 1.03156 1.03233 1.01720 1.01804

59.98 60.04 60.05 60.02 60.03 80.08 79.99 80.01 80.04 79.98 99.99 99.95 100.05 100.03 99.97 120.03 120.05 120.07 120.08 120.02 139.99 140.03 139.97 139.99 139.96 60.08 60.11 60.1 60.11 60.13 80.08 79.94 80.05 80.07 80.04 99.97 99.921 100.02 100.01 100.12 119.98 119.97 119.85 119.92 119.92 139.92 139.91 139.94 139.94 139.86 59.98 60.03 60.03 59.99 59.93 80.05 80.06 3344

2 mol·kg NaCl 10.014 12.008 14.003 15.999 18.011 9.996 12.005 14.003 16.007 18.026 9.994 11.999 14.002 15.997 18.003 10.003 11.995 13.999 16.003 18.005 10.008 12.003 14.007 16.003 17.998 2 mol·kg−1 NaCl 9.997 12.004 14.007 16.012 18.005 10.005 12.017 14.011 16.007 18.012 9.998 12.013 14.007 16.014 18.008 10.005 11.997 13.995 16.001 18.011 10.002 12.008 14.003 16.001 18.005 2 mol·kg−1 NaCl 10.009 12.004 14.006 16.003 18.007 10.003 11.996

ρ/g·cm−3 w = 0 CO2 1.06045 1.06117 1.0619 1.06263 1.06339 1.04938 1.05019 1.05095 1.05171 1.05255 1.03724 1.03808 1.03884 1.03964 1.04050 1.02383 1.02464 1.02554 1.02634 1.02722 1.00953 1.01061 1.01133 1.01221 1.01296 w = 0.0102 CO2 1.06350 1.06400 1.06455 1.06541 1.06616 1.05209 1.05282 1.05342 1.05421 1.05501 1.03901 1.03987 1.04057 1.04151 1.04218 1.02522 1.02582 1.02663 1.02730 1.02801 1.01001 1.01104 1.01165 1.01300 1.01383 w = 0.0203 CO2 1.06518 1.06586 1.06655 1.06728 1.06804 1.05363 1.05427

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Table 2. continued T/°C

P/MPa −1

80.12 80.06 80.08 99.94 99.98 100.03 100.07 100.02 120.10 119.99 120.11 120.04 120.10 139.93 140.19 140.09 140.13 140.09 60.04 60.02 60.05 59.96 59.96 80.05 79.89 79.85 80.13 79.96 100.03 100.31 99.81 99.88 100.01 120.05 120.08 120.08 119.98 120.06 140.02 140.05 140.07 140.04 139.95 60.04 59.98 59.98 60.06 60.01 80.08 80.12 80.01 80.08 80.13 99.96 100.02 100.01 100.07 99.96

1 mol·kg NaCl 14.006 15.997 18.003 9.999 12.007 14.003 15.999 17.992 9.997 12.006 14.003 16.001 18.003 9.999 12.007 14.005 16.003 18.004 1 mol·kg−1 NaCl 10.003 12.001 13.995 16.002 17.996 10.001 12.004 14.002 16.008 17.997 10.003 12.001 14.001 16.001 18.003 9.996 12.002 14.001 16.003 17.998 10.006 12.005 14.006 16.004 18.001 3 mol·kg−1 NaCl 10.002 12.007 14.002 16.006 18.002 10.007 12.003 14.002 16.008 18.002 10.005 12.008 14.004 16.005 17.997

ρ/g·cm−3

T/°C

P/MPa −1

w = 0.0201 CO2 1.01882 1.01974 1.02048 1.00370 1.00454 1.00536 1.00622 1.00708 0.98897 0.99002 0.99085 0.99184 0.99265 0.97110 0.97212 0.97317 0.97403 0.97511 w = 0.0298 CO2 1.02999 1.03075 1.03180 1.03251 1.03320 1.01856 1.01949 1.02023 1.02098 1.02185 1.00534 1.00635 1.00741 1.00824 1.00904 0.99094 0.99180 0.99276 0.99374 0.99461 0.97180 0.97270 0.97380 0.97490 0.97595 w = 0 CO2 1.08997 1.09092 1.09164 1.09236 1.09307 1.07828 1.07902 1.07982 1.08053 1.08124 1.06626 1.06701 1.06777 1.06852 1.06936

80.09 80.14 80.11 100.04 100.04 100.11 100.07 100.09 120.08 119.96 119.92 120.06 119.99 140.05 140.05 140.03 140.05 140.04 60.26 60.29 60.27 60.25 60.27 79.93 79.94 79.95 80.05 80.09 100.03 100.01 100.04 100.01 100.03 119.95 119.98 119.99 119.92 120.01 140.02 139.74 139.75 139.88 139.83 60.44 60.39 60.41 60.37 60.40 78.96 79.01 79.02 79.04 78.99 99.57 99.61 99.56 99.60 99.55 3345

2 mol·kg NaCl 13.999 16.006 18.014 10.006 12.001 14.006 16.012 18.006 10.004 11.999 14.006 16.008 18.011 10.006 12.009 14.005 16.001 18.005 2 mol·kg−1 NaCl 9.997 12.002 14.001 16.004 18.001 9.995 12.001 14.005 16.007 18.004 9.9965 12.005 14.005 15.999 18.002 10.004 12.002 14.003 15.995 18.004 10.004 12.002 14.002 16.004 18.001 4 mol·kg−1 NaCl 10.007 12.004 14.008 15.999 18.008 10.000 12.008 14.050 16.005 18.001 10.019 12.007 14.005 16.043 18.017

ρ/g·cm−3 w = 0.0203 CO2 1.05495 1.05564 1.05627 1.04021 1.04095 1.04147 1.04205 1.04278 1.02573 1.02612 1.02714 1.02772 1.02843 1.01051 1.01141 1.01220 1.01325 1.01420 w = 0.0299 CO2 1.06702 1.06760 1.06818 1.06877 1.06935 1.05473 1.05533 1.05596 1.05655 1.05712 1.04057 1.04121 1.04189 1.04252 1.04321 1.02602 1.02659 1.02751 1.02822 1.02902 1.01100 1.01190 1.01248 1.01350 1.01423 w = 0 CO2 1.12038 1.12122 1.12236 1.12312 1.12430 1.10950 1.10980 1.11110 1.11153 1.11267 1.09720 1.09850 1.09900 1.10012 1.10070

dx.doi.org/10.1021/je400459y | J. Chem. Eng. Data 2013, 58, 3342−3350

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Table 2. continued T/°C

P/MPa −1

120.02 120.09 119.94 120.02 120.11 140.08 140.05 140.01 140.02 139.99 60.04 59.91 59.98 60.06 60.01 80.08 80.08 80.02 80.03 80.11 99.99 99.99 100.01 99.96 99.97 120.08 120.01 119.94 119.94 119.98 140.05 139.97 140.08 140.05 140.02 60.22 60.20 60.03 60.06 60.24 80.21 80.22 80.25 80.24 80.27 100.07 100.03 100.12 100.16 100.17 120.02 120.09 120.21 120.25 120.26 140.08 140.05 140.01

3 mol·kg NaCl 10.001 12.008 14.006 16.002 17.997 10.007 12.005 14.001 16.001 18.004 3 mol·kg−1 NaCl 10.002 12.004 14.002 16.006 18.002 10.002 12.012 14.005 16.001 17.997 10.007 12.003 14.004 16.008 18.002 10.009 12.007 14.006 16.005 18.010 10.002 11.999 14.008 16.007 18.004 3 mol·kg−1 NaCl 10.002 12.004 14.020 16.005 17.999 10.005 12.006 14.002 16.008 18.002 10.007 12.004 14.001 16.007 18.001 10.001 12.008 14.005 16.002 17.997 10.007 12.005 14.001

ρ/g·cm−3

T/°C

P/MPa −1

w = 0 CO2 1.05322 1.05385 1.05475 1.05554 1.05625 1.03823 1.03872 1.03971 1.04063 1.04153 w = 0.0105 CO2 1.09331 1.09403 1.09508 1.09613 1.09671 1.07988 1.08082 1.08156 1.08233 1.08306 1.06821 1.06896 1.06970 1.07045 1.07132 1.05387 1.05470 1.05553 1.05633 1.05711 1.03885 1.03963 1.04058 1.04146 1.04202 w = 0.0201 CO2 1.09530 1.09611 1.09728 1.09808 1.09869 1.08135 1.08210 1.08294 1.08362 1.08431 1.06871 1.06957 1.07033 1.07113 1.07194 1.05487 1.05569 1.05665 1.05732 1.05803 1.03959 1.04031 1.04108

119.64 119.66 119.62 119.63 119.67 139.71 139.77 139.79 139.86 139.91 59.75 59.75 59.79 59.74 59.76 80.05 80.06 80.09 80.14 80.11 99.97 99.92 100.02 100.01 100.12 119.24 119.39 119.31 119.33 119.36 139.74 139.79 139.78 139.81 139.69 60.60 60.39 60.48 60.43 60.42 80.08 80.01 80.14 80.13 80.07 100.02 99.93 100.02 100.05 100.05 120.45 120.55 120.55 120.48 120.52 140.88 141.21 141.01 3346

4 mol·kg NaCl 10.006 12.002 14.002 15.999 18.008 10.007 12.005 14.004 16.003 18.002 4 mol·kg−1 NaCl 9.999 12.005 14.005 16.002 18.010 10.007 12.004 14.008 15.999 18.008 10.007 12.005 14.004 16.003 18.002 10.004 12.004 14.001 16.005 18.005 10.006 12.004 13.997 16.005 18.004 4 mol·kg−1 NaCl 10.002 12.008 14.006 16.003 18.008 9.995 11.998 14.007 16.004 18.009 10.004 11.998 14.005 16.007 18.004 9.945 11.998 14.007 16.004 18.004 10.004 12.001 14.001

ρ/g·cm−3 w = 0 CO2 1.08696 1.08771 1.08852 1.08930 1.09043 1.07235 1.07315 1.07397 1.07476 1.07553 w = 0.0102 CO2 1.12257 1.12329 1.12426 1.12492 1.12581 1.10955 1.11026 1.11141 1.11245 1.11321 1.09846 1.09917 1.09944 1.10060 1.10071 1.08683 1.08765 1.08847 1.08920 1.09031 1.07171 1.07249 1.07337 1.07416 1.07504 w = 0.0204 CO2 1.12407 1.12504 1.12568 1.12638 1.12705 1.11327 1.11388 1.11445 1.11513 1.11591 1.09959 1.10031 1.10094 1.10163 1.10238 1.08671 1.08751 1.08838 1.08910 1.09020 1.07114 1.07205 1.07305

dx.doi.org/10.1021/je400459y | J. Chem. Eng. Data 2013, 58, 3342−3350

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Table 2. continued T/°C

P/MPa −1

3 mol·kg NaCl 16.001 18.004 3 mol·kg−1 NaCl 10.001 12.010 14.006 16.001 17.993 10.003 12.007 14.011 15.998 18.002 9.993 12.003 14.002 15.994 18.005 9.996 11.996 13.998 16.005 18.006 10.004 12.011 14.013 16.021 17.997

140.02 139.99 59.20 59.21 59.31 59.25 59.36 78.83 79.78 79.73 79.88 79.83 100.14 100.06 100.05 100.02 100.03 120.22 120.22 120.19 120.16 120.28 140.02 140.06 140.08 139.97 140.03

ρ/g·cm−3

T/°C

P/MPa −1

w = 0.0201 CO2 1.04217 1.04354 w = 0.0305 CO2 1.09874 1.09981 1.10056 1.10102 1.10178 1.08353 1.08433 1.08500 1.08621 1.08784 1.06932 1.07025 1.07127 1.07254 1.07280 1.05584 1.05678 1.05740 1.05800 1.05898 1.04124 1.04173 1.04233 1.04321 1.04431

4 mol·kg NaCl 16.001 18.002 4 mol·kg−1 NaCl 9.999 12.003 14.001 16.005 18.010 10.006 12.002 14.001 16.012 18.009 10.002 12.007 14.001 16.01 18.011 10.002 12.005 14.006 16.008 18.001 10.009 12.009 14.009 16.008 18.005

140.94 141.04 59.15 59.17 59.12 59.11 59.13 79.75 79.29 79.48 79.37 79.35 99.34 99.16 99.38 99.45 99.38 119.65 119.84 119.58 119.71 119.61 139.65 139.80 140.02 139.80 139.88

ρ/g·cm−3 w = 0.0204 CO2 1.07386 1.07457 w = 0.0302 CO2 1.12541 1.12625 1.12697 1.12767 1.12855 1.11356 1.11443 1.11533 1.11611 1.11703 1.10031 1.10067 1.10127 1.10201 1.10292 1.08669 1.08745 1.08825 1.08897 1.09017 1.07055 1.07164 1.07263 1.07353 1.07428

ρ/g·cm−3 is the experimental density in the measurement. Standard uncertainties u are u(T) = 0.01 °C, u(P) = 0.005 MPa, UCO2(ρ) = 0.00035 and UNaCL(ρ) = 0.00068 mol·kg−1, and the combined expand uncertainty of measured densities are u(ρ) = 2·0.22 kg·m−3 (level of confidence = 0.95, k = 2). a

⎤ ⎡ 1 v(T , P) = v(T0 , P0)⎢1 + αT(T − T0) − (P − P0)⎥ KT ⎦ ⎣

Ud(ρ) ⎧⎡⎛ ∂ρ ⎞ ⎤2 ⎡⎛ ∂ρ ⎞ ⎤2 ⎡⎛ ∂ρ ⎞ ⎤2 ⎫1/2 ⎟U ⎥ + ⎢⎜ ⎟U ⎥ ⎬ =⎨ ⎢⎜ ⎟Um ⎥ + ⎢⎜⎝ ⎪ ⎣⎝ ∂v ⎠ v ⎦ ⎪ ⎣ ∂m∗ ⎠ m ∗⎦ ⎭ ⎩⎣⎝ ∂m ⎠ ⎦ ⎪

⎪

where T0 is the reference temperature (20 °C); P0 is the reference pressure (atmosphere); αT is the isobaric thermal expansion of the sinker; KT is the isothermal compressibility of the sinker; and v(T0,P0) is the volume of the sinker under the reference temperature and reference pressure. At the range of −10 °C to 250 °C, αT represents

(2)

UCO2(ρ) =

⎛ ∂ρ ⎞ ⎜ ⎟U ⎝ ∂w ⎠ w

(3)

UNaCl(ρ) =

⎛ ∂ρ ⎞ ⎜ ⎟U ⎝ ∂C ⎠ C

(4)

(5)

αT =

ΔL 1 . L0 T − T0

(6)

where L0 is the initial length of the sinker, ΔL is the length change of the sinker, and L0/ΔL is related to the temperature. The isothermal compressibility change with Young modulus E and Poisson’s ratio γ is

U(ρ) is the uncertainty of density. Ud(ρ), UCO2(ρ) and UNaCl(ρ) represent the uncertainty of the measured density, CO2 mass fractions, and NaCl concentration, respectively. Um, Um* and Uv represent the uncertainty of the measured density caused by m, m*, and v, respectively. m* is the apparent mass of the sinker, m is the true mass of the sinker in a vacuum, and v is the volume of the sinker. During the calculation of eq 2, the following uncertainties should be stated: at the experimental condition, the uncertainty of the MSB includes the uncertainty caused by m*, m, and v, which can be calibrated accurately from their known value as specified in Zhang et al.16 and the technical specification. Uv is an important term from the sinker volume that affects the overall uncertainty, as shown in uncertainty eq 2. The volume of the sinker changes with temperature and pressure, and it can be corrected as

KT =

E 3 − 6γ

(7)

where E and γ are related to the temperature, as E = −3·10−13T 6 + 3·10−10T 5 − 9·10−8T 4 + 1·10−5T 3 − 1·10−3T 2 + 0.194T + 92.71

(8)

γ = −6·10−14T 5 + 3·10−11T 4 − 5·10−9T 3 + 1·10−7T 2 − 1·10−4T + 0.339 3347

(9)

dx.doi.org/10.1021/je400459y | J. Chem. Eng. Data 2013, 58, 3342−3350

Journal of Chemical & Engineering Data

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the CO2−H2O−NaCl solution increased by 0.04 g·cm−3 when the NaCl concentration increased 1 mol·kg−1. The greatest influence on the solution density comes from the NaCl concentration, followed by the temperature, and then the pressure, as seen in Figures 3 and 4. 3.4. Relationship of CO2−H2O−NaCl Solution Density and Mass Fraction of CO2. As shown in Figure 3, the density of CO2−H2O−NaCl increased slightly as the mass fraction of CO2 increased for the same temperature. When the temperature was below 120 °C (60 °C, 80 °C, 100 °C), the density increased with increasing CO2 mass fraction at different NaCl concentrations. The density change was not obvious when the CO2 mass fraction increased from 0 to 0.03. Therefore, the influence of the CO2 mass fraction on the density of the CO2− H2O−NaCl solution was less than that of temperature and pressure. As shown in Figure 3, for the high NaCl concentration of 4 mol·kg−1, the density decreased as the CO2 mass fraction increased at 120 °C and 140 °C. Therefore, the effects of the CO2 mass fraction on the density were different between high temperatures (120 °C and 140 °C) and low temperatures (less than 120 °C), which was also proved by Lu et al.18 with a numerical simulation. This is possibly due to the sodium and chloride ions from the NaCl dissolving into the water molecules. The CO2 molecules also dissolved into water when the CO2 was injected. With an increase in the temperature, the sodium and chloride ions swelled and occupied the space of the water, and then the CO2 molecules were squeezed out of the water molecules, which resulted in the increase in volume and decrease in CO2 solution density. On the basis of the analysis of the experimental results, it can be conjectured that if the NaCl concentration was smaller, this phenomenon (when the CO2 concentration of the solution is higher, the density is smaller) will appear at higher temperatures. Therefore, the simulation results of Lu et al.18 were verified in that there is a decrease of solution density at high NaCl concentration (4 mol·kg−1) and high temperature (> 120 °C); the experiment results were very similar to the simulation. At high temperatures and high brine salinities, the brine with dissolved CO2 will move upward due to the buoyancy force and increase the risk of CO2 leakage. This indicates that if the saline formation has a high temperature, it will not be suitable for use as a CO2 storage site. 3.5. Density Modeling of CO2−H2O−NaCl Solution. Based on the experimental data, an empirical model was built, as shown in eq 10

From eq 2, the uncertainties of pressure and temperature are estimated as u(T) = 0.01 °C and u(P) = 0.005 MPa, respectively; from eq 3, UCO2(ρ) and UNaCl(ρ) are assessed to be UCO2(ρ) = 0.00035 mol·kg−1 and UNaCl(ρ) = 0.00068 mol· kg−1, respectively. U(ρ) is the combined expanded uncertainty of the solution densities that are 2·0.22 kg·m−3. (level of confidence = 0.95, k = 2). The densities of NaCl−H2O solution were compared with the previous literature data.20−22 We plot a linear fitting curve of NaCl−H2O solution density with different NaCl concentration in Figure 2, where the solid line is for 100 °C and

Figure 2. Comparison between our experimental densities of NaCl− H2O solution and previous literature data. Legend: ■, our experiment at 100 °C and 10 MPa; ▲, our experiment at 120 °C and 18 MPa; □, Khaibullin at 100 °C and 10 MPa; △, Khaibullin at 120 °C and 18 MPa; ○, Sharygin at 100 °C and 10 MPa; solid line, linear fitting of density vs NaCl concentration at 100 °C and 10 MPa; dashed line, linear fitting at 120 °C and 18 MPa.

dashed line is for 120 °C. As Figure 2 shows, our experimental densities have excellent linearity with NaCl salinity at 100 and 120 °C and match well with Khaibullin’s20,21 data when NaCl concentration is less than 3 mol·kg−1. The data of Sharygin22 at 3 mol·kg−1 agree well with our experimental data, and the absolute deviation is less than 0.09 %. However, the deviation between our data and Khaibullin’s20,21 data is large when NaCl concentration is more than 3 mol·kg−1. 3.1. Relationship of CO2−H2O−NaCl Solution Density and Pressure. As shown in Figure 3a, for the same temperature and NaCl concentration, the solution density increased with increasing pressure. The density increased by 0.003 g·cm−3 with a pressure increase from 10 MPa to 18 MPa under the same CO2 mass fraction. 3.2. Relationship of CO2−H2O−NaCl Solution Density and Temperature. The temperature has a significant effect on the solution density. As shown in Figure 4, the solution density decreased with an increase in the temperature. When the temperature increased, the molecular thermal motion increased, which weakened the interaction between molecules, so the space occupied by the molecules relatively increased, leading to a decrease in the solution density. Under the same NaCl concentration CNaCl = 2 mol·kg−1, the solution density decreased by 0.06 g·cm−3 with a temperature increase from 60 °C to 140 °C. 3.3. Relationship of CO2−H2O−NaCl Solution Density and NaCl Concentration. As shown in Figure 4, for the same temperature and mass fraction of CO2, the solution density increased as the NaCl concentration increased. The density of

2

ρ=

∑ (s1i + s2iP + s3iC + s4iC1/3 + s5iw1/2 + s6iw1/3)T i i=0

(10) −3

−1

where ρ (g·cm ), P (MPa), T (°C), C (mol·kg ), and w are the density of the solution, pressure, temperature, NaCl concentration, and CO2 mass fraction respectively. s1i through s6i are the fitting parameters as listed in Table 3. Figure 5 shows the relative deviation between the experimental density and the value calculated from eq 10 as a function of pressure. The largest error in the model is no more than 0.262 %, and the average absolute deviation of the model is 0.080 %. The density of CO2−H2O−NaCl under different conditions can be obtained through eq 10, which could provide data for CO2 storage in deep saline formations. 3348

dx.doi.org/10.1021/je400459y | J. Chem. Eng. Data 2013, 58, 3342−3350

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Figure 3. Density of CO2−H2O−NaCl solution versus CO2 mass fraction. (a) 3 mol·kg−1 NaCl at 120 °C, (b) 4 mol·kg−1 NaCl at 120 °C, (c) 3 mol·kg−1 NaCl at 140 °C, and (d) 4 mol·kg−1 NaCl at 140 °C. Legend: ■, 10 MPa; ○, 12 MPa; ▲, 14 MPa; ▽, 16 MPa; ◀, 18 MPa.

Figure 4. Density of CO2−H2O−NaCl solution versus temperature at different NaCl concentrations (10 MPa, w = 0.02). Legend: □, CNaCl = 1 mol·kg−1 ; ●, CNaCl = 2 mol·kg−1; △, CNaCl = 3 mol·kg−1 ; ▼, CNaCl = 4 mol·kg−1.

Figure 5. Deviations between experimental densities of CO2−H2O− NaCl and the correlation of eq10 versus pressure at different temperatures. Legend: ■, 60 °C; □, 80 °C; ●, 100 °C; ○, 120 °C; ▲, 140 °C.

4. CONCLUSIONS In this paper, brine was simulated by NaCl solution with different concentrations. Density data of CO2−H2O−NaCl system was obtained using an MSB. Measurements were performed in the pressure range (10 to 18) MPa at

temperatures of (60 to 140) °C with different mass fractions of CO2 (w = 0, 0.01, 0.02, 0.03) and different concentrations of NaCl (CNaCl = 1 mol·kg−1, 2 mol·kg−1, 3 mol·kg−1, 4 mol·kg−1). The influence of pressure, temperature, CO2 mass fraction, and NaCl concentrations on the solution density were analyzed. As the pressure and NaCl concentration increased, so did the

Table 3. Coefficients in Eq 10 i

s1i

s2i

s3i

s4i

s5i

s6i

0 1 2

0.8868 1.5789·10−3 −1.1438·10−5

4.6120·10−4 −1.9575·10−6 1.3304·10−8

5.0708·10−3 3.3686·10−4 −1.5925·10−6

0.14891 −2.0919·10−3 1.0504·10−5

1.1711·10−2 −5.3000·10−5 −1.1061·10−7

−7.4335·10−3 1.8936·10−5 1.5172·10−7

3349

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(10) Parkinson, W. J.; De Nevers, N. J. Partial Molal Volume of Carbon Dioxide in Water Solutions. Ind. Eng. Chem. Fundam. 1969, 8, 709−713. (11) Nighswander, J. A.; Kalogerakis, N.; Mehrotra, A. K. Solubilities of carbon dioxide in water and 1 wt % NaCl solution at pressure up to 10 MPa and temperature from 80 to 200°C. J. Chem. Eng. Data 1989, 34, 355−359. (12) Ohsumi, T.; Nakashiki, N.; Shitashima, K.; Hirama, K. Density change of water due to dissolution of carbon-dioxide and near-field behavior of CO2 from a source on deep-sea floor. Energy Convers. Manage. 1992, 33, 685−690. (13) Aya, I.; Absolute measurement on density of CO2 solution in hydrate region. Proc. Japanese Chem. Eng. Symposium, Miyazaki 2000. (14) Song, Y.; Nishio, M.; Chen, B. Measurement of the density of CO2 solution by Mach-Zehnder interferometry. Ann. N. Y. Acad. Sci. 2002, 972, 206−212. (15) Li, Z. W.; Dong, M. Z.; Li, S. L.; Dai, L.M.. Densities and solubilities for binary systems of carbon dioxide +water and carbon dioxide + brine at 59°C and pressures to 29 MPa. J. Chem. Eng. Data 2004, 49, 1026−1031. (16) Zhang, Y.; Chang, F.; Song, Y.; Zhao, J.; Zhan, Y.; Jian, W. Density of carbon dioxide + brine solution from Tianjin reservoir under sequestration conditions. J. Chem. Eng. Data 2011, 56, 565−573. (17) Yan, W.; Huang, S.; Stenby, E. H. Measurement and modeling of CO2 solubility in NaCl brine and CO2-saturated NaCl brine density. Int. J. Greenhouse Gas Control 2011, 5, 1460−1477. (18) Lu, C.; Han, W. S.; Lee, S. Y.; McPherson, B. J.; Peter, C. Effects of density and mutual solubility of a CO2-brine system on CO2 storage in geological formations: “Warm” vs. “cold” formations. Adv. Water Resour. 2009, 32, 1685−1702. (19) Hu, J.; Duan, Z.; Zhu, C.; Chou, I.-M. PVTx properties of the CO2−H2O and CO2−H2O−NaCl systems below 647 K: Assessment of experimental data and thermodynamic models. Chem. Geol. 2007, 238 (3−4), 249. (20) Khaibullin, I. H.; Borisov, N. M. Experimental investigation of thermal properties of H2O + NaCl and H2O + KCl solutions under phases equilibrium. Teploenergetika. 1966, 4, 518−523. (21) Khaibullin, I. K.; Borisov, I. M. Study of the density of liquid phase of the systems at high parameters of state by gamma-ray technique. Teploenergetika 1963, 10, 78−82. (22) Sharygin, A. V.; Wood, R. H. Volumes and heat capacities of aqueous solutions of ammonium chloride from the temperatures 298.15K to 623 K and pressures to 28 MPa. J. Chem. Thermodyn. 1996, 28, 851−872.

density of the solution, but the density decreased as the temperature increased. The influence of the CO2 mass fraction was less than that of the pressure, temperature, and NaCl concentration on the density of the CO2−brine solution. This study showed that the density of the CO2−H2O−NaCl solution increased almost linearly with increasing CO2 mass fraction when the NaCl concentration was less than 4 mol·kg−1 and the temperature was lower than 120 °C. However, the solution density with the highest concentration of NaCl (4 mol· kg−1) decreased with an increasing mass fraction of CO2 at high temperatures (> 120 °C). The density of brine decreased after dissolving CO2; therefore, the brine will float over the saline layer, close to the cap rocks, instead of depositing below it, which means that aquifers with high temperatures are not suitable for the safe storage of CO2.

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AUTHOR INFORMATION

Corresponding Author

*Y. Zhang. E-mail: [email protected]. Phone/Fax: (86)-411-84708015. Funding

The financial support from National Natural Science Foundation of China (51006016 and 51106019), National Program on Key Basic Research Project (2011CB707304), Liaoning Provincial Natural Science Foundation of China (201202028), Research Fund for the Doctoral Program of Higher Education of China (20100041120040), and the Fundamental Research Funds for the Central Universities is gratefully acknowledged. The authors send thanks for the editor’s kind work and the reviewers’ constructive comments. Notes

The authors declare no competing financial interest.

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