Validation of a New Apparatus Using the Dynamic Method for

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Validation of a New Apparatus Using the Dynamic Method for Determining the Critical Properties of Binary Gas/Gas Mixtures Niramol Juntarachat, Salma Bello, Romain Privat, and Jean-Noel̈ Jaubert* Université de Lorraine, Ecole Nationale Supérieure des Industries Chimiques, Laboratoire Réactions et Génie des Procédés, 1 rue Grandville, 54000 Nancy, France ABSTRACT: In this paper, the performance and the reliability of a new apparatus to determine the critical properties of pure components and gas/gas binary mixtures (i.e., mixtures for which the two components are gaseous at ambient conditions) with a known overall composition are studied. Critical temperatures (Tc) and critical pressures (Pc) of binary mixtures composed of carbon dioxide + a light nalkane (CO2 + n-propane, CO2 + n-butane) or of two light nalkanes (n-propane + n-butane) are measured. For the first time, the ability of our apparatus to determineusing the dynamic modethe critical loci of gas/gas binary systems is tested. The critical points are visually determined by observing the critical opalescence and the simultaneous disappearance and reappearance of the meniscus in the middle of the view cell which withstands operations up to 673 K and 20 MPa. The experimental critical data in this work are compared with previously measured literature data and with values predicted by the PPR78 thermodynamic model.

1. INTRODUCTION The critical point specifies the conditions (temperature, pressure, composition) at which at least two phases in equilibrium become strictly identical. Although multiple types of critical points exist, this paper only deals with vapor−liquid critical points. Knowledge of mixture critical data is important for both practical and theoretical reasons as well as for operation conditions of supercritical fluid extraction and for the development of thermodynamic models. As discussed by Privat and Jaubert,1 to improve the accuracy of prediction and extend the range of applicability of predictive equations of state, mixture critical data are absolutely necessary. This is the reason why it was decided to acquire a new apparatus, to calibrate it, and to perform critical point measurements for binary mixtures. In this study, the critical properties are determined by observing the critical opalescence and the simultaneous disappearance of the meniscus from the middle of the cell on increasing of the temperature and the reappearance of the meniscus on slow decreasing of the temperature. This paper is the third one of a series aimed at validating our apparatus. In our first paper,2 the critical loci of three binary systems (n-pentane + n-heptane, n-pentane + n-decane, and nheptane + n-decane) for which the two components were liquid under standard conditions were measured. The second paper3 was devoted to the measurement of the critical properties of three binary systems (CO2 + n-pentane, CO2 + n-heptane, and CO2 + n-decane) for which one component is liquid and the second is gaseous at ambient conditions. This work focuses on the validation of the performance and the reliability of the new apparatus by measuring the critical points of pure components and binary mixtures for which the two components are gaseous © XXXX American Chemical Society

in standard conditions. The critical points for pure components (CO2, n-propane and n-butane) and the critical loci for binary systems (CO2 + n-propane, CO2 + n-butane, and n-propane + n-butane) were experimentally determined using the dynamic mode. Such systems, which are of the highest interest in enhanced oil recovery (EoR) processes,4,5 were selected to compare our data with those available in the open literature. Additionally, the well-established PPR78 thermodynamic model6−15 was used for predicting the critical loci of the above-mentioned binary mixtures. This model was preferred to the PR2SRK16 predictive equation of state (EoS) since the Peng−Robinson EoS is more widely used than the Soave− Redlich−Kwong EoS to simulate EoR processes.

2. EXPERIMENTAL PROCEDURE 2.1. Materials. All compounds were purchased from commercial sources and used without any further purification. The purities and the suppliers of the compounds are listed in Table 1. 2.2. Apparatus and Procedure. A schematic diagram and photos of the apparatus used in this work are presented in Figures 1 and 2, respectively. It has been developed by the ARMINES company hosted by the Ecole Nationale Supérieure des Mines de Paris for the measurement of the critical properties (critical temperature and critical pressure) of pure substances and multicomponent mixtures with known overall Received: November 4, 2012 Accepted: January 16, 2013

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dx.doi.org/10.1021/je301209u | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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stored at high pressure inside the gas cylinder in which they are marketed (it is the case for pure CO2) can be directly transferred from this cylinder to the volumetric press. Practically, the heaviest gas compound (n-propane or nbutane)contained at low pressure in the gas cylinderis directly filled into the syringe pump and then pumped to the volumetric press that has been evacuated using a vacuum pump (Oerlikon Leybold, model Trivac D2.5) and weighed using a microbalance (Sartorius AG, model MSE) having a precision of 1 mg. The filling and pumping flow rates are regulated by the pump controller. At the end of the filling, the volumetric press and its contents are weighed. This procedure can be repeated several times so that the required amount of the first component is acquired in press. The syringe pump and the transfer circuits are evacuated before each filling and pumping procedure. To obtain maximum amount of gas transferable in each procedure, the syringe pump and the press were cooled using a cryostat bath (Fischer Scientific model Poly stat 37+R5) and an ice water bath, respectively. The same procedure is applied to the lighter gas compound if it is available in a lowpressure gas cylinder (this is the case for n-propane). It is important to ensure that pressure in the syringe pump is higher than that in press before opening the valve of the press. After each transfer, this valve is immediately shut to avoid equilibrium between the pump and the press, causing the first component to enter the pump thus changing its initial mass. The accurate composition of mixture can thus be calculated using the known mass of the two compounds. The volumetric press was then coupled with flanges for pressurization in the next step. The loading part involves the transfer of the mixture contained in the volumetric press to a 260 cm3 syringe pump (Teledyne Isco, model 260D) without a change in the initial overall composition of the mixture. Under standard conditions, the mixture usually exists in two phases (liquid and vapor) in

Table 1. Purity and Supplier of the Compounds Used in This Study compound

purity

supplier

CO2 n-propane n-butane

99.998 vol. % > 99.990 vol. % 100 %

Messer Messer Messer

composition. This apparatus can work using two different modes: dynamic or static.2 The temperature and pressure upper limits of application are 673 K and 20 MPa for the dynamic method and 493 K and 20 MPa for the static method. When the dynamic mode is used, the magnetic stirrer is removed from the oven since the circulation of the fluid ensures an efficient stirring. On the other hand, such a magnetic stirrer is absolutely necessary in the static mode explaining why the working temperature is limited at 493 K (it is the highest temperature that the stirrer can endure). In this study, only the dynamic mode is used. A critical point can be determined by visually observing the critical opalescence and the simultaneous disappearance and reappearance of the meniscus, that is, of the liquid−vapor interface from the middle of the view cell. The experimental setup can be divided into three parts: (1) preparation of sample, (2) loading, and (3) measurement. The first part of the experimental setup involves the preparation of a mixture with a known overall composition. It includes a 150 cm 3 volumetric press, in which the homogeneous mixture is prepared and stored and a 260 cm3 syringe pump that is used to increase the pressure of the pure gaswhich is stored at low pressure inside the gas cylinder in which it is commercializedand to transfer it into the volumetric press (see Figure 3). Such a procedure which allows increasing the quantity of matter contained in press was used for pure n-propane and pure n-butane, which are sold in cylinders, the pressure of which is lower than 6 bar. Gases

Figure 1. Schematic diagram of the apparatus. PS: pressurized source; VPr: volumetric press; SP: syringe pump; VP: vacuum pump; HE: heat exchanger; EM: electric motor; MS: magnetic stirrer; VC: view cell; AT: air thermostat bath (oven); TP: platinum resistance temperature probe; PT: pressure transducer; TR: temperature regulator; BD: bursting disc; FV: flow regulation valve; DAS: data acquisition system; CPU: central processor unit; V: valve. B



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Figure 2. Photos of the apparatus used in this study.

balance (Desgrange and Huot 5202S, CP (0.3 to 40) MPa). Pressure calibration errors are estimated to be ± 0.4 kPa in the (0.1 to 7.1) MPa range and ± 0.8 kPa in the (7.1 to 18.1) MPa range. A video acquisition system (Panasonic, model WVCP290/G) that consists of an endoscope and a video camera is placed behind a window of the air thermostat bath. It is connected to a monitor for observing the view cell and the critical phenomena. Before starting a measurement, the view cell and the circuits were cleaned, evacuated, and closed. The air thermostat was set below the expected critical temperature of the studied sample. The measurement started by opening the outlet valve V4 of the pump for filling a half of the view cell with liquid that passed through a short heating section before entering into the cell from the bottom. Then, the regulation valve V8 was opened, and a continuous flow was achieved through the view cell at a rate higher than 2.5 mL·min−1. The pumping flow rate (for fine adjustment) and the regulation valve (for coarse adjustment) were regulated to maintain the half way liquid level in the cell. The temperature of the air thermostat was increased regularly at a rate of 0.5 K·min−1. Doing so, the pressure regularly increased in the cell until the vapor−liquid interface became cloudy, thick, and then occupied the entire cell at the critical point. At the same time, the cell became orange, thus characterizing the critical opalescence. After reaching one uncoloured supercritical phase, the temperature of the cell was gradually decreased by opening the ventilator of the air thermostat bath, and the pressure of the cell was decreased by slightly opening the regulation valve or decreasing the pumping flow rate. At a little above the critical temperature and pressure, the fluid color changed from colorless to yellow, then from yellow to yellow−red, and finally became dark orange. Then the vapor−liquid interface reappeared in the middle of the view cell. The critical temperature and pressure were recorded at the point where complete darkness was observed in the cell. For each measurement, the temperature-increasing and decreasing processes were repeated at least eight times, and the average of the recorded temperatures and pressures was taken as the critical values reported here.

Figure 3. Schematic diagram of the transfer processfor a gas contained at low pressure inside the gas cylinderto the volumetric press.

the press. The mole fraction of the most volatile component is higher in the vapor phase and lower in the liquid phase compared to the less volatile component and vice versa. Hence, it is important to pressurize the press to obtain a monophasic liquid (i.e., an homogeneous mixture) before transfer to the syringe pump, avoiding only the liquid phase of the mixture going into the pump and changing the overall composition. Pressure used in this process thus needs to be higher than the bubble pressure of the mixture studied at the chosen temperature. The volumetric press coupled with flanges was connected to a pressurized nitrogen gas cylinder for pressurizing and to the syringe pump for transferring. Later on, valve V1for the pressurization of the mixture in presswas opened, and then valves V2 and V3 were opened for the transfer of the mixture to the pump. The measuring part is similar to the previous works of Guilbot et al.,17 Horstmann et al.,18−21 Gil et al.,22 and Soo et al.23 A 4 cm3 titanium-flanged sapphire cell is placed in an oven. The bottom flange holds a magnetic stirrer that is coupled with an electric motor (IKA RW 14 basic). The cell operates up to 20 MPa and 673 K. The cell temperature is measured using a Pt100 probe centered in the cell and connected to an electronic display. The experimental setup for calibration of the Pt100 probe is identical to that carried out by Soo et al.,23 and temperature calibration errors are ± 0.02 K and ± 0.1 K for the (323 to 443) K range and the (443 to 573) K range, respectively. The cell pressure is measured with a pressure transducer (Druck, model PTX611) which is kept at constant temperature of 373.15 K by means of a heating cartridge regulated via a PID regulator (West, model 6100). The pressure gauge was calibrated against a dead weight pressure

3. RESULTS AND DISCUSSION In this work, the validation of the performance and the reliability of the new apparatus have been studied by measuring the critical points of pure components and binary mixtures for which literature data are available. The experimental process for the measurement of the critical point using the dynamic method was investigated. The critical points of pure C



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components (CO2, n-propane and n-butane) and of binary mixtures containing two gases were experimentally determined throughout the entire range of mole composition. The experimental critical points with different mole fraction and their uncertainties (classically determined by following the statistical method recommended by NIST) for the binary systems CO2 (1) + n-propane (2), CO2 (1) + n-butane (2), and n-propane (1) + n-butane (2) were listed in the Tables 2, 3, and Table 2. Experimental Critical Temperatures (Tc) and Pressures (Pc) for the System CO2 (1) + n-Propane (2) at Different Mole Fractions (x1) and Their Standard Uncertainties u x1 0 0.11569 0.21289 0.36536 0.49283 0.67726 0.82536 0.92515 1

u(x1)

Tc/K

u(Tc)/K

Pc/MPa

u(Pc)/MPa

0.00006 0.00009 0.00007 0.00008 0.00008 0.00008 0.00004

369.84 362.84 356.93 344.98 333.78 319.00 307.12 303.46 304.30

0.03 0.06 0.07 0.09 0.04 0.13 0.06 0.06 0.03

4.2426 4.7906 5.1928 5.8991 6.2759 6.5287 6.6897 6.8938 7.3798

0.0027 0.0049 0.0095 0.0030 0.0092 0.0096 0.0090 0.0014 0.0029

Figure 4. P−T projection of the critical loci for the systems CO2 (1) + n-propane (2), CO2 (1) + n-butane (2), and n-propane (1) + n-butane (2). Solid lines: predicted curves with the PPR78 model. Dashed lines: vaporization curves of pure compounds. ●: experimental critical points (this work). Other symbols: experimental critical points found in the open literature (CO2 + n-propane: red □,18 red ◇,28 red ∗,29 red ×,30 red +,31 red ○;32 CO2 + n-butane: □,18 ◇,28 ■,32 +,33 ×,34 ∗,35 ○;36 n-propane + n-butane: blue □,18 blue ◇,23 blue +,25 blue ×,26 blue ∗27).

Table 3. Experimental Critical Temperatures (Tc) and Pressures (Pc) for the System CO2 (1) + n-Butane (2) at Different Mole Fractions (x1) and Their Standard Uncertainties u x1 0 0.10522 0.17362 0.26514 0.33653 0.43027 0.51597 0.63551 0.73943 0.87324 0.94027 1

u(x1)

Tc/K

u(Tc)/K

Pc/MPa

u(Pc)/MPa

0.00003 0.00003 0.00003 0.00003 0.00003 0.00003 0.00002 0.00003 0.00003 0.00002

424.82 418.38 412.48 405.58 398.71 388.66 377.29 359.42 342.29 318.81 309.41 304.30

0.03 0.04 0.06 0.09 0.11 0.08 0.07 0.04 0.08 0.12 0.03 0.03

3.7983 4.4940 4.9997 5.6419 6.2036 6.8676 7.4263 7.9441 8.0122 7.4485 7.2242 7.3798

0.0023 0.0034 0.0032 0.0058 0.0095 0.0026 0.0037 0.0045 0.0107 0.0114 0.0058 0.0029

4, respectively. Additionally, the PPR78 thermodynamic model was used for predicting the critical curves of the studied systems. The comparison between the experimental critical data in this work, literature data, and the predictions by the PPR78 model is shown in Figures 4, 5, and 6, which correspond to pressure−temperature (P−T), temperature−composition (T− x1), and pressure−composition (P−x1) diagrams, respectively.

Figure 5. T−x projection of the critical loci for the systems CO2 (1) + n-propane (2), CO2 (1) + n-butane (2), and n-propane (1) + n-butane (2). Solid lines: predicted curves with the PPR78 model. Dashed lines: vaporization curves of pure compounds. ●: experimental critical points (this work). Other symbols: experimental critical points found in the open literature (CO2 + n-propane: red □,18 red ◇,28 red ∗,29 red ×,30 red +,31 red ○;32 CO2 + n-butane: □,18 ◇,28 ■,32 +,33 ×,34 ∗,35 ○;36 n-propane + n-butane: blue □,18 blue ◇,23 blue +,25 blue ×,26 blue ∗27).

Table 4. Experimental Critical Temperatures (Tc) and Pressures (Pc) for the System n-Propane (1) + n-Butane (2) at Different Mole Fractions (x1) and Their Standard Uncertainties u x1 0 0.25401 0.49409 0.75213 1

u(x1)

Tc/K

u(Tc)/K

Pc/MPa

u(Pc)/MPa

0.00004 0.00003 0.00003

424.82 413.84 402.02 386.48 369.84

0.03 0.04 0.03 0.03 0.03

3.7983 4.0140 4.1837 4.3091 4.2426

0.0023 0.0006 0.0051 0.0011 0.0027

For the system n-propane + n-butane, which belongs to the type I fluid phase behavior according to the classification of binary phase diagrams proposed by Van Konynenburg and Scott,24 a very strong correlation between our experimental results, the literature data,18,22,25−27 and the PPR78 model is obtained in all projections. Only the experimental results from Nysewander et al.25 are higher compared to the rest. For the systems containing CO2, which belong to the type II fluid phase behavior,24,9 Figure 4 shows that the experimental results in this D



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data and with predictions by the PPR78 model. For the system n-propane + n-butane, a very good correlation between the experimental data in this work, literature data, and predictions by the EoS was obtained. For systems containing CO2, the critical data in this work correlate perfectly with the critical loci predicted by the PPR78 thermodynamic model, although a small variation with the literature critical pressures is observed at very high mole fractions of CO2. Due to the scatter of the many experimental data, such a difference is certainly insignificant. Furthermore, the correlation between all of the data in terms of critical temperatures is better than in terms of critical pressures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33 3 83 17 50 81. Fax: +33 3 83 17 51 52.

Figure 6. P−x projection of the critical loci for the systems CO2 (1) + n-propane (2), CO2 (1) + n-butane (2), and n-propane (1) + n-butane (2). Solid lines: predicted curves with the PPR78 model. Dashed lines: vaporization curves of pure compounds. ●: experimental critical points (this work). Other symbols: experimental critical points found in the open literature (CO2 + n-propane:red □,18 red ◇,28 red ∗,29 red ×,30 red +,31 red ○;32 CO2 + n-butane: □,18 ◇,28 ■,32 +,33 ×,34 ∗,35 ○;36 n-propane + n-butane: blue □,18 blue ◇,23 blue +,25 blue ×,26 blue ∗27).

Notes

The authors declare no competing financial interest.

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REFERENCES

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work correlate accurately with the predictions by the PPR78 model. This is not surprising since we know by experience that such a model is able to predict with high accuracy the critical loci of binary mixtures containing a light alkane and CO2, and we thus can be confident in our experimental data. A small variation with literature data for the system CO2 + npropane18,28−32 and for the system CO2 + n-butane18,28,32−36 is observed at high mole fractions of CO2 where the critical pressures in this work are slightly lower than those in literature. However due to the scatter of the experimental data measured by the different authorsespecially in the vicinity of pure CO2we can conclude to a satisfactory agreement between our new data and those previously published. Figures 4, 5, and 6 highlight that the quality of the data depends strongly on the experimental method. As an example, the critical points of the system CO2 + n-propane from Poettmann and Katz28 and Reamer et al.29 are much higher than those measured by other authors. These authors measured the pressure as a function of volume at a given temperature and composition. The bubble and dew pressures were obtained by evaluating the P−V curves, and the critical coordinates could be found by an iterative process. This paper allows concluding that such a technique is not accurate enough. Additionally, the correlation between our experimental data, the literature data, and the predictions by the PPR78 model of the critical temperatures is stronger than that of the critical pressures as can be seen in Figures 5 and 6.

4. CONCLUSION The performance and reliability of the new apparatus were studied by measuring the critical points of pure compounds and mixtures for which literature data were available. The critical properties of pure compounds (CO2, n-propane, and n-butane) and binary mixtures containing two gases (CO2 + n-propane, CO2 + n-butane, and n-propane + n-butane) were determined using the dynamic method. The critical loci of these binary mixtures were also predicted using the PPR78 thermodynamic model. The experimental results were compared with literature E



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Validation of a New Apparatus Using the Dynamic Method for

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