PressureâDensityâTemperature Measurements of Binary Mixtures

Article pubs.acs.org/jced

Pressure−Density−Temperature Measurements of Binary Mixtures Rich in CO2 for Pipeline Transportation in the CCS Process Michela Mazzoccoli,* Barbara Bosio, and Elisabetta Arato Department of Civil, Chemical and Environmental Engineering, University of Genoa, via Opera Pia 15, 16145 Genoa, Italy ABSTRACT: Often, power plants and industrial production plants large point sources of CO2are situated at long distances from storage locations; thus the transportation of the resulting CO2-rich streams, containing small amounts of impurities, is required from the point of capture to the storage site, making it very important to understand the thermodynamic properties of CO2-rich mixtures. The range of conditions studied in this work are of great interest for carbon capture and storage applications, in particular for pipeline transportation of CO2-mixtures. Pressure−density−temperature measurements were taken for binary mixtures of carbon dioxide with nitrogen, oxygen, or argon. Density was measured for all of the binary systems for temperatures between 273.15 K and 293.15 K and pressures between 1 MPa and 20 MPa, using a vibrating tube densimeter, Anton Paar DMA 512-HPM. The molar CO2 concentration was greater than 85 % for all mixtures tested.

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INTRODUCTION Today, reducing the emissions of greenhouse gases has become one of the most important challenges to be faced. CO2 emissions must be reduced by 50 % to 80 % by 2050 (in comparison with 1990 levels).1 Increasing energy efficiency and renewable energy levels may offer the most sustainable solutions for guaranteeing energy supplies; however the benefits will most probably be obtained only in the long term. Therefore, for the short to medium term, fossil fuels will continue to be a primary energy resource, and the issue has become how these fossil fuels can be used in a more environmentally friendly way. An answer could be the carbon capture and storage (CCS) processa good option in a portfolio of technologies with the potential to cut the CO2 emissions that consents the continued use of fossil fuels.1 Transportation is a crucial issue in CCS technology, as power and industrial production plants where CO2 is captured are usually located at long distances from the storage locations, requiring the transportation of CO2 from the point of capture to the storage site. For the purposes of transportation, the CO2-mixture must have high density and purity levels. Furthermore, for final safe storage, a CO2 molar concentration of over 95 % is required.2 Therefore the requirement for transportation may also influence the type of capture technology chosen. One of the main transport solutions is via pipeline. CO2-rich streams will most likely be transported in the “dense” stage (above critical pressure, Pc, and under critical temperature, Tc, in Figure 1), so that the volumes to be transported are not large, in contrast with the gas phase when the density is very low. It is extremely important to know the thermodynamic properties of the different CO2-mixtures to be transported under these conditions. The type and quantity of impurities contained in CO2-mixtures depend on the fuels and on the type of capture technology used. If the CO2 is captured from power plants using © 2012 American Chemical Society

Figure 1. PT diagram for pure CO2.

postcombustion capture, N2, O2, H2O, NH3, SOx, and NOx may be present. In the case of oxy-combustion capture, normally N2, O2, SO2, H2S, and Ar are present, while for precombustion capture, H2, CO2, N2, H2S, and CH4 may be found.3 Even so, volumetric experimental data for the CO2-mixtures of interest in the CCS process to date are limited. To our knowledge there are no published data on the volume of CO2− O2, CO2−CO, and CO2−NH3. The volumetric experimental data of mixtures containing Ar, H2, and H2S cover a small range of temperature and pressure,3 while a higher number of experimental data on volume have been published for CO2−N2 and CO2−CH4 mixtures.3−7 A preliminary comparison between p−ρ−T experimental data found in literature of CO2−N2 and CO2−CH4 binary mixtures Received: June 14, 2012 Accepted: September 6, 2012 Published: September 19, 2012 2774

dx.doi.org/10.1021/je300590v | J. Chem. Eng. Data 2012, 57, 2774−2783

Journal of Chemical & Engineering Data

Article

Figure 2. Schematic diagram of experimental apparatus. Reprinted with permission from Paolo Chiesa of LEAP.

and different equations of state have been made by authors.8 However more volumetric experimental data on CO2-mixtures in pipeline transport conditions are needed; thus an experimental campaign was realized at LEAP (Energy and Environmental Laboratory of Piacenza). Volumetric experimental data of CO2−N2, CO2−O2, and CO−Ar systems in supercritical conditions obtained at LEAP using a vibrating tube densimeter have been published by Mantovani et al.9 This paper reports new experimental results for p−ρ−T measurements of CO2−N2, CO2−O2 and CO2−Ar mixtures in the liquid and vapor stages at different temperatures (273.15 K, 283.15 K, and 293.15 K) and for the pressure range 1 MPa to 20 MPa.

The measurement circuit is composed of the measurement devices: the vibrating tube densimeter (VTD) and pressure and temperature meters. The vibrating tube densimeter is a DMA 512-HPM model produced by Anton Paar. The uncertainty associated with the vibrating period10 is ± 10−8 s. Temperature is controlled by means of a thermostatic bath (T1) supplied by Huber, model CC3-K12, with a temperature range from 253 K to 473 K and a stability of ± 0.02 K. The temperature is maintained by an electric resistance (EH2). Temperature measurements are supplied by two Pt100 platinum probes (PP1 and PP2), calibrated in the Milan Polytechnic laboratory. The temperature measurement accuracy is ± 0.05 K. Pressure measurements are taken by different pressure transmitters supplied by GE Druck of the series PTX611. Three sensors (VPT group) are high temperature transmitters and one sensor (LPT) a lower temperature transmitter. The VPT group is maintained at 288 K by an electric resistance (EH1). The pressure limits of the three sensors are 0.6 MPa, 6 MPa, and 25 MPa, respectively. The LPT transmitter is maintained at 258 K by a thermostatic bath (T2) produced by Huber, model CC1-K6, with a temperature range from 253 K to 423 K with a stability of ± 0.02 K; its pressure limit is 25 MPa. The pressure sensors are calibrated using a hydraulic balance produced by Scandura which, in turn, was calibrated at the Scandura Calibration center for the pressure ranges 1 MPa to 6 MPa and 6 MPa to 20 MPa. Corrections for the actual temperature, local gravity, and transmitter positions have all been applied. The estimated accuracy achieved for pressure measurements is 0.03 %. The temperature is maintained at the desired value inside the measurement circuit, where valves V6, V6R, V7, V8, V9, and V9R are located, by a thermostatic bath (TB) containing a heat exchanger (HE). The thermostatic bath was produced by Huber, model CC1 Variostat CC, with a temperature range from 253 K to 423 K and a stability of ± 0.02 K.

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EXPERIMENTAL SECTION Experimental Apparatus. A schematic diagram of the experimental apparatus is provided in Figure 2. The apparatus provides simultaneous density, pressure, and temperature measurements for a single phase fluid of known composition during an isothermal transformation. The allowed temperature range is from 263 K to 388 K, and the allowed pressure range from vacuum conditions to 20 MPa. The apparatus consists of a pressurization circuit, a mixture vessel, a measurement circuit, a data acquisition system, and a vacuum circuit. The vessel containing the mixture is a cylindrical titanium vessel, 20 mm in diameter and 100 mm in length. A sliding piston is housed inside the vessel, separating the two chambers: the low chamber (LC) contains the sample fluid; the pressure chamber (PC) contains the pressurization gas (normally nitrogen). The pressure level of the pressurization circuit can be controlled by means of the hand pump (HP), and it cannot exceed 30 MPa: when this value is reached, the rupture disk (RD) breaks and depressurizes the whole circuit to maintain its structural integrity. To prevent the disk rupture, a differential manometer (MA) is introduced in the pressurization line. It can work from 0.1 MPa up to 40 MPa, and it is produced by Nuova Fima. 2775



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Experimental Procedure. The vibrating tube densimeter is able to measure the density of only a single phase fluid; therefore different testing procedures are used in the case of a mixture that is a subcooled liquid or a superheated vapor. In the vapor phase tests, as suggested by refs 11 to 13, the pressure transducers (VPT group) and the thermostatic bath TB must be set at a temperature higher than that of the chosen isothermal line, to avoid condensation of the fluid and formation of two phases in a device other than the vibrating tube densimeter. In this case, in fact, the two fluid phases have a different composition to the overall mixture, and the vibrating tube densimeter measures the density of a fluid with an unknown composition. The temperature of the vibrating tube densimeter must be set at the temperature of the chosen isothermal line. Before starting the experiment, the measurement circuit must be made to work under vacuum. The vacuum pump must be started with the V4 valve closed to conserve the sample mixture, and the V10B valve closed to insulate the measurement circuit from the external environment. All of the other valves must be left open to allow the fluid contained inside the measurement circuit to be completely extracted. When a pressure level under 10 Pa is reached inside the measurement circuit, the V10A valve must be closed and the vacuum pump stopped. This operation ensures that the effects of contaminant substances on the tested mixture are negligible. To obtain the p−ρ−T measurements for the vapor phase, the V8 and V9 valves must be closed and the V7 valve opened. The V4 valve must be opened fully to allow the mixture to flow inside the measurement circuit, and the V6/V6R valves regulated to obtain the filling speed required. The latter must be lower than 0.005 MPa·s−1 as suggested by refs 11−13. Pressure, temperature, and density are measured every second starting from 1 MPa. During the test, the three different pressure transducers composing the VPT group must be used in sequence to obtain more precise pressure measurements. Furthermore, as the transducers approach their upper pressure limit, they must be isolated from the measurement circuit to guarantee that their integrity is preserved. When the dew point is reached, a two-phase fluid forms inside the vibrating tube densimeter, and the density values cannot be effectively measured, thus ending the test. In the case of the liquid phase test, as suggested by Bouchot and Richon,11 the pressure transducers (LPT) and the thermostatic bath TB must be set at a temperature lower than that of the chosen isothermal line such as the first vapor bubble forms inside the vibrating tube densimeter.

The data acquisition system consists of an Aglient datalogger, model 34970A (DL1), for sampling the pressure and temperature signals, and of an Anton Paar device, model mPDS 2000 V3 (DL2), for sampling density/period signals. The two instruments are connected to a personal computer that, using an acquisition system developed in Labview, processes and stores the values measured. The vacuum circuit is connected to the measurement circuit and is needed to prevent gas residues inside from contaminating the sample mixture. It is possible to use two vacuum circuits: one is composed of the vacuum pump (VP), Pirani gauge 1 (PG1), and valve V5, the other one is composed of the same vacuum pump, Pirani gauge 2 (PG2), and valve V11. Using the vacuum pump, a pressure under 10 Pa can be obtained inside the measurement circuit. The values of the pressure are registered by a data logger for the vacuum sensors (DL3). For the tests, high purity carbon dioxide (99.998 %) and high purity nitrogen, argon, and oxygen (99.999 %) were supplied by Rivoira S.p.A. Table 1. Molar Fraction x of the Binary Mixtures Investigateda N2a

N2b

O2a

O2b

Ara

Arb

xCO2

0.9873

0.8785

0.9558

0.8512

0.9605

0.9276

xN2

0.0127

0.1215 0.0442

0.1488 0.0395

0.0724

xO2 xAr a

Standard uncertainty u(x) = 0.0002.

Figure 3. Vessel loading circuit. Reprinted with permission from Paolo Chiesa of LEAP.

Table 2. FPCM Model Parameters: the Young’s Modulus E, the Standard Temperature T1, the Linear Dilatation Coefficient α, the Poisson’s Ratio ν, the Internal Radius, ri00, and the External Radius, re00, of the Vibrating Tube under Vacuum parameter

value

E(T)/MPa

⎞ ⎛ 1 1 ⎟⎟ E(T ) = E(T1) + 1.174·10−5⎜⎜ 1298.51/ T − 1298.51/ T 1 exp −1 −1 ⎠ ⎝ exp

E(T1)/MPa T1/K α/K−1 ν ri00/mm re00/mm

2.07·105 298.15 1.14·10−5 0.307 1.073 1.588 2776



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Table 3. Experimental Values of Density ρ with Standard Uncertainty u(ρ) at Temperature T and Pressure p for Pure CO2a

a

p/MPa

ρ/kg·m−3

1.001 2.001 3.001 3.300 3.500 4.001 5.001 6.000 7.000 8.003 9.001 10.002 11.003 12.000 13.004 14.002 15.001 16.002 17.000 18.001 19.001 20.000

T = 273.15 K 21.2 45.7 77.3 89.9 928.1 932.4 940.7 948.2 955.3 961.9 968.1 974.1 979.7 985.1 990.1 995.0 999.7 1004.3 1008.6 1012.8 1017.0 1020.8

u(ρ)/kg·m−3

p/MPa

ρ/kg·m−3

0.1 0.1 0.1 0.1 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

1.000 2.001 3.001 4.000 4.400 5.001 6.003 7.000 8.001 9.001 10.001 11.001 12.000 13.001 14.001 15.000 16.001 17.001 18.002 19.001 20.000

T = 283.15 K 20.4 43.1 70.9 108.4 130.1 869.1 882.2 893.3 903.0 912.2 920.4 928.2 935.5 942.2 948.3 954.3 960.1 965.5 970.8 975.7 980.4

u(ρ)/kg·m−3

p/MPa

ρ/kg·m−3

0.1 0.1 0.1 0.1 0.1 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

1.011 2.001 3.001 4.000 5.000 5.600 5.800 6.001 7.003 8.001 9.002 10.001 11.001 12.000 13.000 14.000 15.003 16.003 17.001 18.002 19.002 20.000

T = 293.15 K 19.8 41.0 66.2 97.3 140.4 182.2 774.1 783.3 809.1 827.5 843.2 856.1 867.7 877.9 887.3 895.9 904.0 911.5 918.6 925.0 931.2 937.1

u(ρ)/kg·m−3 0.1 0.1 0.1 0.1 0.1 0.1 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

Standard uncertainty u(T) = 0.05 K and relative standard uncertainty ur(ρ) = 0.03 %.

Table 4. FPCM Model Optimized Parameters: The Mass, M0, and the Length, L00, of the Vibrating Tube under Vacuum and the Isothermal Linear Expansion Coefficient γT at Different Temperatures T vapor phase T/K 273.15 283.15 293.15

M0/L00/kg·m−1 −2

5.67·10 5.60·10−2 5.60·10−2

The fluid can then be introduced into the vessel. If possible, it should be introduced in the liquid phase to maximize the mass supplied. The pure carbon dioxide can be introduced as a liquid as its critical temperature is sufficiently high and a simple cooling circuit is sufficient. During the filling operation, valves V11 and V14 must be closed, while valves V4 and V15 must be opened. The amount of carbon dioxide introduced is calculated as the difference between the current weight and the one measured during the previous step. If a further chemical species is to be introduced, the procedure is the same as that described above for introducing carbon dioxide. The only difference is that, in this case, no cooling circuit is needed because the other components (N2, O2, and Ar) are introduced in the gaseous phase, due to their complex condensation conditions. This procedure does not allow a mixture of a definite composition to be produced so, in general, a number of filling attempts are necessary to achieve a satisfactory composition that is close enough to the one desired. For each contaminant species, two different CO2-rich mixtures are investigated, their molar composition reported in Table 1. The uncertainty factor for the molar fraction9 calculated is ± 2·10−4. Densimeter Calibration. A vibrating tube densimeter, generally speaking, consists of a hollow U-shaped tube. The two ends of the tube are locked onto an isolating block. Two magnets are mounted on the free section of the tube, and two coils are wound around the magnets. The drive coil is connected to a current source, while the frequency counter induces a current with a frequency equal to that of the vibrating tube. The resonant frequency of the tube is measured by analyzing the signal from the pick up coil. When the density of the fluid contained inside the tube changes, the resonant frequency of the tube changes as a consequence of the variation in the tube mass. This is the reason why the density value can be calculated from the vibrating period when a suitable model is used.

liquid phase

γT/MPa−1 −5

2.12·10 8.84·10−6 8.65·10−6

M0/L00/kg·m−1 −2

5.71·10 5.72·10−2 5.73·10−2

γT/MPa−1 7.38·10−6 8.01·10−6 9.17·10−6

The fluid pressure is raised to 20 MPa by means of the pressurization circuit. Once the pressure required is reached, the valves V4 and V6 are closed. The p−ρ−T values for the liquid phase are obtained by an emptying transitory. In this case, the LPT pressure transducer is used as it can work at low temperatures; thus valve V8 must be opened, whereas valve V7 must be closed. Valves V9/V9R are regulated to obtain the required emptying speed ( 10 %), the isothermal line at 293.15 K shows a trend similar to that of a mixture in a supercritical phase (Figures 7 and 9); thus the critical temperature (Tcm) of the mixture may be considered to decrease as the concentration of these contaminant species increases. In fact, the Tc of pure CO2 is ≈303.15 K, while for CO2−N2 mixtures (xCO2 ≈ 87 %) and for CO2−O2 mixtures (xCO2 ≈ 85 %), the Tcm is probably nearer to 293.15 K. To prove the reliability of the results showed in this work, a comparison with other literature experimental data, where possible, has been done. As already said, no volumetric experimental data of CO2−O2 mixtures in subcritical conditions have been published, and very few data are available for CO2−Ar mixtures; therefore only a comparison for CO2−N2 binary mixture was possible, even if the volumetric data in the same conditions of temperature, pressure, and composition of our experimental results are few. The results of this work resulted in consistence with the available experimental data reported by Brugge et al.,6 con-

sidering volumetric data at the same temperature (≈ 283 K) and different pressures and compositions, and reported by Ely et al.,7 considering experimental data at the same pressure values (9 MPa, 12 MPa, and 16 MPa) and different temperatures and compositions. Some differences were noticed in the comparison with the data reported by Arai et al.;4 however some of latter results are also not consistent with other experimental works.6

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CONCLUSIONS New p−ρ−T experimental data are presented for six binary mixtures rich in carbon dioxide under working conditions similar to those applying for CO2 pipeline transportation. The mixtures contain carbon dioxide and gases, such as oxygen, argon, and nitrogen, that are noncondensable under such conditions and are among the main contaminants in flue gases treated by CCS processes. In scientific literature where, to date, so little volumetric data on carbon dioxide-based mixtures are available, this study could be seen as providing an important contribution in promoting the further study of pipeline transportation of CO2-rich streams from the capture plant through to the storage site. 2782



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l′Hydrogène Sulfure, Mesures par Densimétrie à Tube Vibrant et Modélisation. Ph.D. thesis, Paris School of Mines, France, 2005. (13) Khalil, W. Développement d′un Appareil Automatise de Mesure Simultanée d′Equilibres de Phases et de Proprietà Volumetriques. Exploitation des Données Volumetriques pour le Calcul Prédictif de Grandeurs Thermodynamiques Dérivées. Ph.D. thesis, Paris School of Mines, France, 2006. (14) Sanmamed, Y.; Dopazo-Paz, A.; Gonzalez-Salgado, D.; Troncoso, J.; Romani, L. An accurate calibration method for high pressure vibrating tube densimeters in the density interval (700 to 1600) kg·m−3. J. Chem. Thermodyn. 2009, 41, 1060−1068. (15) Span, R.; Wagner, W. A New Equation of State for Carbon Dioxide covering the fluid region from the triple −point temperature to 1100 K at pressures up to 800 MPa. J. Phys. Chem. Ref. Data 1996, 25, 1509−1596. (16) Mazzoccoli, M. Carbon Capture, Storage and Transportation (CCS&T) process: general aspects and focus on CO2 pipeline modeling. Ph.D. thesis, University of Genoa, Italy, 2011.

Moreover the comparison made, where possible, between the results and the experimental data found in literature proved the reliability of the volumetric data showed in this work. As the knowledge of the thermodynamic properties of CO2rich mixtures containing small amounts of impurities becomes very important in the CCS process and a suitable equation of state (EOS) under the appropriate conditions for pipeline transport, in particular with a high CO2 concentration, has not been clearly defined yet, a comparison between the new experimental data here presented and the results of different EOS could be fundamental. A further development of this work, regarding a comparison with both cubic and nonanalytical EOS to evaluate the accuracy of them for density prediction, is going to be published by the authors.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +39 010 353 2588, fax: +39 010 353 2589, e-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experimental campaign presented in this work was performed at LEAP laboratory. The authors would like to thank sincerely all of the LEAP staff.

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REFERENCES

(1) Special report on carbon dioxide capture and storage. IPCC: Geneva, 2005. (2) De Visser, E.; Hendriks, C.; Barrio, M.; Mølnvik, M. J.; De Koeijer, G.; Liljemark, S.; Le Gallo, Y. Dynamis CO2 quality recommendations. Int. J. Greenhouse Gas Control 2008, 2, 478−484. (3) Li, H.; Jacobsen, J. P.; Wilhelmsen, Ø.; Yan, J. PVTxy properties of CO2 mixtures relevant for CO2 capture, transport and storage: Review of available experimental data and theoretical models. Appl. Energy 2011, 88, 3567−3579. (4) Arai, Y.; Kaminishi, G.; Saito, S. The experimental determination of the P-V-T-x relations for the carbon dioxide-nitrogen and the carbon dioxide-methane systems. J. Chem. Eng. Jpn. 1971, 4, 113−112. (5) Magee, J. W.; Ely, J. F. Isochoric (p,v,T) measurements on CO2 and (0.98 CO2 + 0.02 CH4) from 225 to 400 K and pressures to 35 MPa. Int. J. Thermophys. 1988, 9, 547−557. (6) Brugge, H. B.; Holste, J. C.; Hall, K. R.; Gammon, B. E.; Marsh, K. N. Densities of Carbon Dioxide + Nitrogen from 225 to 450 K at pressures up to 70 MPa. J. Chem. Eng. Data 1997, 42, 903−907. (7) Ely, J. F.; Haines, W. M.; Bain, B. C. Isochoric (p, Vm, T) measurements on CO2 and on (0.0982 CO2 + 0.018 N2) from 250 to 330 K at pressures to 35 MPa. J. Chem. Thermodyn. 1989, 21, 879−894. (8) Mazzoccoli, M.; Bosio, B.; Arato, E. Analysis and comparison of Equations-of-State with p−ρ−T experimental data for CO2 and CO2mixture pipeline transport. Energy Procedia, 2012, 23, 274−283. (9) Mantovani, M.; Chiesa, P.; Valenti, G.; Gatti, M.; Consonni, S. Supercritical pressure-density-temperature measurements on CO2-N2, CO2-O2 and CO2-Ar binary mixtures. J. Supercrit. Fluids 2012, 61, 34−43. (10) Bouchot, C.; Richon, D. An enhanced method to calibrate vibrating tube densimeters. Fluid Phase Equilib. 2001, 191, 189−208. (11) Bouchot, C.; Richon, D. Direct Pressure-Volume-Temperature and Vapor-Liquid Equilibrium Measurements with a Single Equipment Using a Vibrating Tube Densimeter up to 393 K and 40 MPa: Description of the Original Apparatus and New Data. Ind. Eng. Chem. Res. 1998, 37, 3295−3304. (12) Rivollet, F. Etude des Propriétés Volumétriques (PVT) d′Hydrocarbures Légers (C1-C4), du Dioxyde de Carbone et de 2783


PressureâDensityâTemperature Measurements of Binary Mixtures

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