Bubble-Point Measurements of n-Butane + n ... - ACS Publications

Article pubs.acs.org/jced

Bubble-Point Measurements of n‑Butane + n‑Octane and n‑Butane + n-Nonane Binary Mixtures Elisabeth Mansfield* and Stephanie L. Outcalt Applied Chemicals and Materials Division, National Institute of Standards and Technology (NIST), Boulder, Colorado 80305, United States S Supporting Information *

ABSTRACT: Mixtures of small gaseous hydrocarbons with longer chain hydrocarbons are of interest to the natural gas industry as well as other industries in which separations are critical. In particular, binary mixtures of n-nonane are of interest, because n-nonane was recently incorporated into the GERG-2008 equation of state, but there is little experimental vapor−liquid equilibrium (VLE) data available to support the equation. The bubble-point pressures of four compositions of each of the binary mixtures n-butane + n-octane and n-butane + n-nonane were measured over the temperature range of 270 to 370 K. The data and the expanded uncertainty (at a 95 % confidence level, k = 2) of each point are reported. Additionally, the data are compared to existing literature data for the n-butane + n-octane and the GERG-2008 equation for both systems. This is the first report of vapor−liquid equilibrium measurements on n-butane + n-nonane binary mixtures.

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INTRODUCTION Natural gas is mainly composed of methane, but the contributions of heavier hydrocarbons have an impact on the thermodynamic properties of the fuel. For processing, transport, and storage of natural gas, the industry can rely on the thermophysical property data incorporated into on the GERG equation of state. The GERG-2004 equation of state incorporated the thermophysical property data of 18 different components present in natural gas, along with other equations of state in existence to develop a universal equation of state for use by the industry.1 In 2012, the expanded GERG-2008 incorporated n-nonane, n-decane, and hydrogen sulfide.2 The development of the GERG-2008 model exposed a large gap in the understanding of vapor−liquid equilibria (VLE) data for binary mixtures, especially those containing n-nonane. In addition, there is a general lack of VLE data for mixtures of shortchain hydrocarbons with longer-chain hydrocarbons. To begin to address these gaps, this paper provides bubble-point measurements for four different n-butane + n-octane compositions as well as four compositions of n-butane + n-nonane. The n-butane + n-octane data are compared to the GERG-2008 equation of state through the use of REFPROP3 and the only other set of vapor− liquid equilibria data available for n-butane + n-octane mixture.4,5 The bubble-point data for n-butane + n-nonane reported here are the first measurements reported in the literature for n-butane + n-nonane mixtures. Those data are also compared to the GERG2008 equation of state.2

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purification. The stated manufacturer purities were as follows: n-butane 99.9 %; n-nonane 99 %; and n-octane ≥99 %. These purities were confirmed in our laboratory by analysis with gas chromatography−mass spectrometry (GC-MS) or GC with flame ionization detection (GC-FID) (Table 1). Spectral peaks Table 1. Measured and Manufacturer Determined Purity of Mixture Components chemical n-nonane

n-octane n-butane

GC-MS or GC-FIDa

99 % 99 % 99 % ≥99 % 99.90 %

99.46 % 99.53 % 99.49 % 99.14 % 99.97 %

water content 53.7 ± 20 ppm 47.0 ± 20 ppm 27 ± 20 ppm n/a

a

GC-MS was used for n-butane analysis, but GC-FID was used to measure purity of n-nonane and octane because it is more quantitative.

were interpreted with guidance from the NIST/EPA/NIH Mass Spectral Database.6 Water content was verified using coulometric Karl Fischer titrations according to ASTM Standard Test Method E1064-00 (Table 1).7 Mixture Preparation. Mixtures were prepared gravimetrically in sealed 300 mL stainless steel cylinders. n-Octane or n-nonane were added first then degassed by freezing in liquid nitrogen and evacuating the headspace. The closed cylinder was then heated to drive out impurities, and the freeze/pump/thaw

MATERIALS AND METHODS

Received: April 1, 2015 Accepted: June 11, 2015 Published: June 30, 2015

Materials. The pure fluids included in each mixture were obtained from commercial sources and used without further This article not subject to U.S. Copyright. Published 2015 by the American Chemical Society

manufacturer specification

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DOI: 10.1021/acs.jced.5b00308 J. Chem. Eng. Data 2015, 60, 2447−2453

Journal of Chemical & Engineering Data

Article

Figure 1. Schematic of the apparatus used to make the bubble-point measurements.

Table 2. Measured Bubble Point Pressures for the System n-Octane (1) + n-Butane (2) at Temperature T, Pressure P, and Liquid Mole Fraction x1 = 0.306a


x1 = 0.306 ± 4.37 × 10−3

x1 = 0.480 ± 5.29 × 10−3

T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

270.00 275.00 280.00 285.00 290.00 295.00 300.00 305.00 305.00 310.00 310.00 315.00 315.00 320.00 320.00 320.00 325.00 325.00 330.00 330.00 335.00 335.00 340.00 340.00 345.00 345.00 345.00 350.00 350.00 355.00 360.00 365.00 370.00

64.98 77.75 93.27 111.18 131.26 154.32 180.19 207.88 209.26 236.52 239.96 270.22 273.17 307.33 310.60 307.01 341.82 342.79 386.60 386.40 435.59 435.20 489.27 488.38 546.84 547.63 546.47 609.39 608.74 676.99 748.36 825.10 906.98

5.60 5.64 5.69 5.74 5.78 6.26 5.88 5.93 5.93 5.98 5.98 7.03 7.03 7.08 7.08 7.08 7.13 7.13 7.17 7.17 7.22 7.22 7.27 7.27 7.32 7.32 7.32 7.37 7.37 7.42 7.49 7.58 7.67

8.62 7.26 6.10 5.16 4.41 4.06 3.26 2.85 2.84 2.53 2.49 2.60 2.57 2.30 2.28 2.31 2.08 2.08 1.85 1.86 1.66 1.66 1.49 1.49 1.34 1.34 1.34 1.21 1.21 1.10 1.00 0.92 0.85

−0.96 −1.73 −1.45 −1.08 −0.99 −0.02 −0.44 −0.83 −0.16 −2.09 −0.62 −2.40 −1.29 −2.69 −1.60 −2.79 −4.81 −4.51 −4.74 −4.80 −4.64 −4.73 −4.45 −4.64 −4.38 −4.23 −4.45 −4.24 −4.36 −4.08 −4.09 −4.05 −4.01

270.00 275.00 280.00 285.00 290.00 295.00 295.00 300.00 300.00 305.00 305.00 310.00 310.00 310.00 315.00 320.00 325.00 330.00 335.00 340.00 345.00 350.00 350.00 355.00 355.00 360.00 360.00 365.00 365.00 370.00 370.00 375.00 375.00

49.65 59.60 71.02 83.65 98.40 114.84 114.92 132.73 135.61 152.99 151.69 176.73 176.74 174.79 201.93 227.44 255.12 285.47 318.74 363.14 385.54 425.06 426.53 472.38 470.68 521.60 523.26 574.43 572.48 630.81 628.45 690.15 688.06

3.65 3.71 3.77 3.82 3.88 3.94 3.94 4.00 4.00 4.06 4.06 4.12 4.12 4.12 5.62 5.67 5.71 5.76 5.81 5.86 5.90 5.95 5.95 6.00 6.00 6.05 6.05 6.11 6.11 6.16 6.16 6.21 6.21

7.35 6.22 5.30 4.57 3.95 3.43 3.43 3.02 2.95 2.66 2.68 2.33 2.33 2.36 2.78 2.49 2.24 2.02 1.82 1.61 1.53 1.40 1.40 1.27 1.28 1.16 1.16 1.06 1.07 0.98 0.98 0.90 0.90

−4.96 −5.07 −5.17 −5.72 −5.69 −5.83 −5.76 −6.36 −4.10 −6.58 −7.49 −5.99 −5.98 −7.16 −6.02 −7.07 −8.08 −8.90 −9.51 −7.50 −12.83 −13.63 −13.24 −13.15 −13.56 −13.04 −12.68 −12.88 −13.26 −12.71 −13.14 −12.65 −12.99

a

a

Standard uncertainties u are u(T) = 0.03 K. The values of u(x1) and u(P) are given in the table.


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x1 = 0.628 ± 3.12 × 10−3

x1 = 0.746 ± 8.92 × 10−3

T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

270.00 275.00 280.00 285.00 290.00 295.00 300.00 305.00 305.00 310.00 310.00 315.00 315.00 320.00 325.00 330.00 335.00 340.00 345.00 345.00 350.00 355.00 355.00 360.00 365.00 370.00

37.86 44.89 53.08 62.60 73.04 84.81 98.21 111.43 112.75 127.80 129.18 145.84 142.85 165.91 187.50 211.44 237.33 263.80 288.81 291.67 318.91 339.92 336.59 363.30 400.55 436.69

3.84 3.89 3.93 3.98 4.02 4.07 4.12 4.17 4.17 4.21 4.21 5.75 5.75 5.79 5.82 5.86 5.90 5.94 5.97 5.97 6.01 6.05 6.05 6.09 6.13 6.17

10.15 8.66 7.41 6.36 5.51 4.80 4.19 3.74 3.69 3.30 3.26 3.94 4.03 3.49 3.11 2.77 2.49 2.25 2.07 2.05 1.89 1.78 1.80 1.68 1.53 1.41

−5.50 −6.55 −7.13 −7.24 −7.77 −8.16 −8.19 −9.87 −8.57 −9.77 −8.60 −9.69 −11.98 −9.43 −9.41 −9.17 −9.00 −9.49 −11.25 −10.16 −11.70 −15.81 −16.96 −19.38 −18.94 −19.51

270.00 275.00 280.00 285.00 290.00 295.00 295.00 300.00 300.00 300.00 305.00 305.00 305.00 310.00 310.00 310.00 315.00 315.00 320.00 320.00 325.00 330.00 335.00 340.00 345.00 350.00 355.00 360.00 365.00 370.00

29.36 34.27 40.16 46.73 54.04 62.81 60.31 72.00 72.61 69.73 82.32 80.00 80.01 93.82 91.13 91.42 103.52 103.99 117.81 118.07 132.66 149.42 167.90 187.47 209.83 231.06 256.09 280.76 305.56 328.54

4.42 4.45 4.49 4.53 4.56 4.60 4.60 4.64 4.64 4.64 4.67 4.67 4.67 4.71 4.71 4.71 6.17 6.17 6.20 6.20 6.23 6.26 6.29 6.32 6.36 6.39 6.42 6.45 6.49 6.52

15.05 13.00 11.18 9.69 8.44 7.32 7.62 6.44 6.38 6.65 5.68 5.84 5.84 5.02 5.17 5.15 5.96 5.93 5.26 5.25 4.70 4.19 3.75 3.37 3.03 2.76 2.51 2.30 2.12 1.98

0.85 −1.45 −2.65 −3.89 −5.11 −5.14 −9.50 −6.06 −5.16 −9.52 −6.67 −9.77 −9.75 −7.09 −10.25 −9.89 −10.51 −10.01 −10.08 −9.84 −10.33 −10.13 −9.75 −9.69 −8.98 −9.71 −9.40 −9.95 −11.02 −13.17

a


cycles were repeated a minimum of three times. Following this step, the sample cylinder was weighed. A balance with a precision of 0.1 mg was used in the preparation of the mixtures. Utilizing the double-substitution weighing design of Harris and Torres,8 measurement of the mass of each component consisted of weighing four masses: (1) a reference cylinder of approximately the same mass and volume as the empty sample cylinder, (2) the sample cylinder, (3) the sample cylinder plus a 20 g sensitivity weight, and (4) the reference cylinder plus the 20 g sensitivity weight. This weighing sequence was repeated three times for each mass determination. The density of ambient air was calculated on the basis of measurements of temperature, pressure, and relative humidity, and the sample masses were corrected for the effects of air buoyancy.9 n-Butane was then added gravimetrically to the sample cylinder, and the sample was subjected to a minimum of three freeze/pump/thaw cycles again. The mass of n-butane was then determined. Sample cylinders were prepared with the goal of filling the sample cylinder to between 280 mL and the maximum volume of 300 mL at the target composition, at ambient temperature. The standard deviation of the repeat weighings was at most 1.5 mg. The uncertainty of the measured mixture composition will be discussed in detail in a later section. Measurements. A schematic of the instrument used to make the measurements is shown in Figure 1 and has been previously described in detail.10 Briefly, a cylindrical stainless steel cell with an internal volume of 30 mL housed the sample. Sapphire windows at both ends of the cell allowed the sample liquid level

a


to be viewed externally. The cell and all of the system valves were housed inside a temperature-controlled, insulated aluminum block. Prior to loading a sample into the system, the system was evacuated and then cooled to approximately 270 K. Reported pressures have been adjusted to reflect any offset due to the vacuum in the chamber. Sample pressure measurements were recorded in 5 K increments from 270 to 370 K. As the cell temperature was increased, the liquid inside expanded, and it was necessary to periodically release a small amount of liquid from the bottom of the cell to maintain a vapor space. Repeat measurements were conducted at a minimum of two temperatures for each mixture composition to establish the repeatability of the measurements and to determine if the loss of small amounts of the liquid phase affected the sample composition to the extent that duplicate measurements at a given temperature yielded different bubble-point pressures. Under this measurement configuration, attempts were made to ensure that the most accurate bubble points of the sample are measured, but assumptions are made. These assumptions include: (1) the liquid composition in the cell is equal to the bulk composition of the mixture in the sample bottle, and (2) by loading the cell almost full of liquid with only a very small vapor space remaining, the pressure of the vapor phase is the bubblepoint pressure of the liquid composition at a given temperature. 2449



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Table 6. Measured Bubble Point Pressures for the System n-Nonane (1) + n-Butane (2) at Temperature T, Pressure P, and Liquid Mole Fraction x1 = 0.251a


x1 = 0.251 ± 4.05 × 10−3

x1 = 0.498 ± 6.90 × 10−3

T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

270.00 275.00 280.00 280.00 285.00 285.00 285.00 290.00 290.00 295.00 295.00 300.00 300.00 305.00 305.00 310.00 310.00 315.00 315.00 320.00 320.00 325.00 325.00 325.00 325.00 330.00 330.00 335.00 335.00 340.00 340.00 345.00 350.00 355.00 360.00 365.00 370.00

69.74 83.90 100.08 100.13 119.13 119.21 119.71 140.64 141.22 164.84 165.49 191.86 192.52 222.30 222.81 255.51 256.41 290.38 292.53 326.93 329.35 368.93 369.13 368.98 372.40 417.55 419.86 470.31 470.24 527.75 527.77 590.19 656.64 729.81 808.08 889.92 979.01

5.09 5.13 5.18 5.18 5.23 5.23 5.23 5.28 5.28 5.33 5.33 5.38 5.38 5.43 5.43 5.48 5.48 6.60 6.60 6.64 6.64 6.69 6.69 6.69 6.69 6.73 6.73 6.78 6.78 6.83 6.83 6.88 6.93 7.00 7.08 7.17 7.28

7.30 6.12 5.18 5.17 4.39 4.39 4.37 3.75 3.74 3.23 3.22 2.80 2.79 2.44 2.44 2.14 2.14 2.27 2.26 2.03 2.02 1.81 1.81 1.81 1.80 1.61 1.60 1.44 1.44 1.29 1.29 1.17 1.06 0.96 0.88 0.81 0.74

−5.13 −5.28 −5.51 −5.45 −5.20 −5.13 −4.69 −5.04 −4.61 −4.96 −4.55 −4.99 −4.63 −4.89 −4.65 −5.07 −4.70 −5.90 −5.12 −7.22 −6.43 −7.82 −7.76 −7.80 −6.81 −7.62 −7.03 −7.50 −7.52 −7.36 −7.36 −7.19 −7.18 −6.92 −6.71 −6.74 −6.58

270.00 270.00 275.00 275.00 280.00 280.00 285.00 285.00 290.00 290.00 295.00 295.00 300.00 300.00 305.00 305.00 310.00 310.00 315.00 315.00 320.00 320.00 325.00 325.00 330.00 330.00 335.00 335.00 340.00 340.00 345.00 345.00 350.00 350.00 355.00 355.00 360.00 360.00 365.00 365.00 370.00 370.00

47.11 47.17 56.72 56.37 67.46 67.05 79.92 79.45 94.10 93.48 109.93 110.12 127.66 127.96 147.42 146.82 169.36 169.11 193.57 191.87 220.19 220.58 248.85 249.43 277.96 276.53 309.64 308.07 330.61 331.30 364.06 364.48 403.59 403.23 445.81 445.87 491.34 491.66 540.07 540.38 591.98 592.32

3.53 5.43 3.58 3.58 3.64 3.64 3.69 3.69 3.75 3.75 3.80 3.80 3.86 3.86 3.92 3.92 5.51 3.98 4.03 4.03 4.09 4.09 4.15 4.15 4.21 4.21 4.27 4.27 4.33 4.33 4.39 4.39 4.45 4.45 4.51 4.51 4.57 4.57 4.64 4.64 4.70 4.70

7.49 11.51 6.31 6.35 5.39 5.42 4.62 4.65 3.98 4.01 3.46 3.46 3.02 3.02 2.66 2.67 3.26 2.35 2.08 2.10 1.86 1.86 1.67 1.66 1.51 1.52 1.38 1.39 1.31 1.31 1.21 1.20 1.10 1.10 1.01 1.01 0.93 0.93 0.86 0.86 0.79 0.79

−36.42 −32.01 −35.13 −35.98 −34.50 −35.34 −33.48 −34.28 −32.42 −33.30 −31.58 −31.35 −30.77 −30.46 −29.98 −30.50 −29.18 −29.38 −28.41 −29.55 −27.66 −27.43 −27.18 −26.88 −27.65 −28.31 −27.95 −28.61 −33.30 −33.02 −34.17 −34.02 −33.68 −33.80 −33.22 −33.20 −32.65 −32.56 −32.04 −31.96 −31.42 −31.34

a


Uncertainty Analysis. The expanded uncertainty for our bubble-point measurements was previously reported. Briefly, the uncertainty is calculated by the root-sum-of-squares method,11 taking into account five principle sources of uncertainty: temperature, pressure, sample composition, measurement repeatability, and head pressure correction. The standard platinum resistance thermometer (SPRT) and the pressure transducer used for our measurements were calibrated immediately prior to beginning measurements. A difference of pressure at 0.03 K from the measured temperature was factored into the calculation to account for uncertainty in the SPRT. The manufacturer’s stated uncertainty of the pressure transducer is 0.01 % of full range, or 0.7 kPa. As a conservative estimate of the pressure uncertainty, the greater of 0.7 kPa or 0.1 % has been used in the calculation of the overall combined uncertainty of the bubble-point pressures reported here. The uncertainty in the composition of the mixture is by far the most difficult to estimate accurately. Sample purity,

a


uncertainty in the determined masses during sample preparation, and the transfer of the mixture sample into the measuring system will affect the composition of the fluid mixture. To account for the possibility that the degassing of the samples was not complete, a calculation was done assuming that air represented a 0.001 mole fraction impurity in each of the mixtures. Nitrogen was used to represent air in the calculations. The repeatability of our bubble-point measurements was determined by repeating measurements at a minimum of two temperatures for each sample studied. The standard deviation was then taken as the repeatability. To be conservative in our uncertainty estimates, the largest of the standard deviation values 2450



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x1 = 0.738 ± 5.63 × 10−3

x1 = 0.786 ± 5.37 × 10−3

T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

270.00 275.00 280.00 285.00 290.00 295.00 300.00 300.00 305.00 310.00 315.00 320.00 325.00 325.00 330.00 335.00 340.00 345.00 350.00 350.00 355.00 360.00 365.00 370.00

24.75 29.36 34.89 41.31 48.45 56.60 65.53 65.54 75.28 86.21 98.19 111.31 125.24 125.68 140.73 158.07 176.81 196.21 216.54 216.58 240.46 266.24 289.19 307.23

3.21 3.26 3.31 3.36 3.41 3.46 3.50 3.50 3.55 3.60 4.62 4.66 4.71 4.71 4.75 4.79 4.83 4.87 4.91 4.91 4.96 5.00 5.04 5.08

12.96 11.10 9.48 8.13 7.03 6.10 5.35 5.35 4.72 4.18 4.71 4.19 3.76 3.74 3.37 3.03 2.73 2.48 2.27 2.27 2.06 1.88 1.74 1.65

−123.98 −121.41 −116.96 −112.03 −107.93 −103.57 −100.02 −99.99 −97.09 −93.89 −90.90 −88.05 −85.88 −85.22 −83.26 −80.11 −77.13 −75.02 −73.35 −73.32 −70.14 −67.01 −66.67 −69.63

270.00 275.00 280.00 280.00 285.00 290.00 295.00 295.00 300.00 300.00 300.00 305.00 305.00 310.00 310.00 310.00 315.00 315.00 315.00 320.00 320.00 320.00 320.00 325.00 330.00 335.00 335.00 340.00 345.00 345.00 350.00 350.00 355.00 355.00 360.00 365.00 370.00

22.30 26.36 30.85 30.95 36.29 42.16 47.74 48.76 55.17 54.91 56.35 63.30 64.43 73.09 72.37 73.55 82.24 82.50 83.61 94.74 92.90 93.20 93.41 105.16 118.56 132.58 132.71 147.71 164.00 164.27 181.76 181.85 201.58 200.66 221.64 243.88 268.56

3.29 3.33 3.37 3.37 3.42 3.46 3.50 3.50 3.55 3.55 3.55 3.59 3.59 3.63 3.63 3.63 5.14 5.14 5.14 5.18 5.18 5.18 5.18 5.21 5.24 5.27 5.27 5.31 5.34 5.34 5.38 5.38 5.41 5.41 5.45 5.48 5.52

14.75 12.63 10.93 10.90 9.41 8.20 7.34 7.18 6.43 6.46 6.29 5.67 5.57 4.97 5.02 4.94 6.26 6.24 6.15 5.46 5.57 5.55 5.54 4.95 4.42 3.98 3.97 3.59 3.26 3.25 2.96 2.96 2.68 2.70 2.46 2.25 2.05

−131.84 −128.86 −126.73 −125.98 −122.10 −119.07 −120.41 −115.80 −116.16 −117.21 −111.65 −112.51 −108.76 −106.60 −108.66 −105.31 −105.24 −104.58 −101.87 −98.31 −102.24 −101.59 −101.14 −98.09 −94.09 −91.05 −90.87 −88.14 −85.33 −85.02 −82.34 −82.25 −78.77 −79.59 −76.33 −73.35 −69.88

a


for each mixture was used as the repeatability value in the calculation of overall combined uncertainty for each point in that mixture. The pressure transducer was maintained at 313 K during measurements. For temperatures of 320 K and above, the head pressure was calculated for each point and treated as an uncertainty in the calculation of the overall uncertainty in the reported bubble-point pressures. The reported overall combined uncertainty for each point was calculated by taking the root sum of squares of the pressure equivalents of the temperature and composition uncertainties, the uncertainty in pressure, the measurement repeatability, and head pressure corrections. This number was multiplied by two (coverage factor, k = 2) and is reported as an uncertainty in pressure as well as a percent uncertainty for each bubble point.

a


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measurements previously presented for this instrument in Outcalt and Lee.10 Figure 3 shows the percent deviations between the measured n-butane + n-octane bubble-point data presented here and the GERG-2008 equation of state2 represented in REFPROP.3 Overall, the data fall within 10 % of the predicted values of the equation. There seems to be a trend toward greater deviations from the predicted values with increasing percentage of n-octane in the mixture. The split in the deviations seen around 315 K may be due to the constant temperature of the pressure transducer (313 K) resulting in condensation of the vapor in the line between the pressure transducer and the cell above that temperature. This was corrected for as discussed in the uncertainty section. Figure 4 shows the percent deviations between the measured n-butane + n-nonane bubble-point data and the GERG-20082 equation of state represented in REFPROP.3 The data here vary up to 132 % from the prediction

RESULTS AND DISCUSSION Bubble point pressures for eight compositions of n-butane binary mixtures were measured from 270 to 370 K (Tables 2−9). For each table, the temperature and a corrected pressure are given. The uncertainty in the pressure was calculated and reported for each point and is given as an absolute value, as well as a percentage. The deviation from the GERG-2008 equation of state as implemented in REFPROP3 is given in the final column. To validate the performance of the vapor−liquid equilibrium apparatus, a n-butane + propane binary mixture was measured and is reported in Table 10. The n-butane + propane system was chosen as a good representative mixture to validate the system, because it has a significant VLE data available in the literature.10,12 A deviation plot between literature measurements and this work is presented in Figure 2 and is comparable to the 2451



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Table 10. Measured Bubble Point Pressures for the System n-Butane (1) + Propane (2) at Temperature T, Pressure P, and Liquid Mole Fraction x1 = 0.783a x1 = 0.783 ± 5.27 × 10−3 T/K

P/kPa

u(P)/kPa

(u(P)/P) × 100

(1 − PEOS/Pexp) × 100

270.00 275.00 280.00 285.00 290.00 295.00 300.00 300.00 305.00 310.00 310.00 315.00 315.00 320.00 320.00 325.00 330.00 335.00 340.00 340.00 345.00 345.00 350.00 350.00 355.00 360.00 365.00

161.73 190.96 224.35 263.64 306.26 352.67 405.55 405.44 464.18 529.19 526.37 600.27 599.84 678.24 677.69 762.89 855.71 956.39 1065.90 1060.94 1179.04 1178.98 1306.47 1306.63 1443.53 1591.47 1747.66

3.83 4.02 4.25 4.50 4.79 5.10 5.46 5.46 5.86 6.29 6.29 6.77 6.77 7.27 7.27 7.84 8.47 9.13 9.89 9.88 10.49 10.49 11.31 11.31 12.12 12.99 14.00

2.37 2.11 1.89 1.71 1.56 1.44 1.35 1.35 1.26 1.19 1.20 1.13 1.13 1.07 1.07 1.03 0.99 0.95 0.93 0.93 0.89 0.89 0.87 0.87 0.84 0.82 0.80

0.50 0.31 0.26 0.82 0.79 0.42 0.41 0.39 0.41 0.46 −0.07 0.43 0.36 0.41 0.33 0.32 0.31 0.28 0.29 −0.18 −0.14 −0.15 −0.11 −0.10 −0.09 −0.02 −0.11

Figure 3. Deviations from the GERG-2008 equation of state for mixtures of n-octane and n-butane as a function of temperature.

a


Figure 4. Deviations from the GERG-2008 equation of state for mixtures of n-nonane and n-butane as a function of temperature.

Figure 2. Deviations from the GERG-2008 equation of state for mixtures of propane and n-butane as a function of temperature.

in the equation. In contrast to the experimental data for the n-butane + n-octane mixtures, the experimental data for the n-butane + n-nonane systems exhibit a very clear trend of deviations from the predicted GERG-2008 values with mixture composition. For higher mole % of n-nonane, the deviations are greater. To support the observation of this large difference, two similar compositions of n-butane + n-nonane mixtures were made (73.8 mol % n-nonane and 78.6 mol % n-nonane).

Figure 5. Experimental deviations from the GERG-2008 equation of state for mixtures of n-octane and n-butane as a function of temperature (filled symbols) as compared to data from literature5 (open symbol). 2452



Article

(8) Harris, G. L.; Torres, J. A. NIST IR 6969 Selected laboratory and measurement practices and procedures to support basic mass calibrations; National Institute of Standards and Technology: Gaithersburg, MD, 2003. (9) Picard, A.; Davis, R. S.; Glaser, M.; Fujii, K. Revised formula for the density of moist air (CIPM-2007). Metrologia 2008, 45, 149−155. (10) Outcalt, S. L.; Lee, B.-C. A Small-Volume Apparatus for the Measurement of Phase Equilibria. J. Res. Natl. Inst. Stand. Technol. 2004, 109 (6), 525−531. (11) Outcalt, S. L.; Lemmon, E. W. Bubble-Point Measurements of Eight Binary Mixtures for Organic Rankine Cycle Applications. J. Chem. Eng. Data 2013, 56, 1853−1860. (12) Kayukawa, Y.; Fujii, K.; Higashi, Y. Vapor-Liquid Equilibrium (VLE) Properties for the Binary Systems Propane (1) + n-Butane (2) and Propane (1) + Isobutane (3). J. Chem. Eng. Data 2005, 50, 579− 582. (13) Goldberg, R. N.; Weir, R. D. Conversion of Temperatures and Thermodynamic Properties to the Basis of the International Temperature Scale of 1990. Pure Appl. Chem. 1992, 64 (10), 1545−1562.

For these mixtures, the deviations from the predicted values are approximately 80−100 % and both sets have decreasing deviation from the predicted values with increasing temperature. Only one previous VLE data set exists in the literature4,5 for n-butane + n-octane binary mixtures. The temperatures for the bubble-point measurements were converted with the International Temperature Scale of 1990 (ITS-1990).13 Deviations from the GERG-20082 equation of state in REFPROP3 were calculated for each data point and plotted in Figure 5. These data4,5 begin at 335 K, so there is a temperature range of approximately 35 K of overlap with the data presented here. The literature data deviates 3.3 % or less from the equation of state.

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CONCLUSIONS The first bubble-point measurements on n-butane + n-nonane binary mixtures are reported, along with a series of n-butane + n-octane binary bubble-point measurements. The experimental measurements for the n-butane + n-octane binary systems deviate up to 10 % from the values predicted by GERG 2008, while the n-butane + n-nonane experimental measurements were observed to deviate up to 140 %. These deviations suggest that the GERG2008 equation could be improved in the temperature range of 270−370 K.

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ASSOCIATED CONTENT

S Supporting Information *

Supplemental figure depicting pressure and temperature relationship. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jced.5b00308.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: elisabeth.mansfi[email protected]. Phone: 303-497-6405. Fax: 303-497-5030. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The purity analysis of the pure fluids was provided by Dr. Jason Widegren and Dr. Tara Lovestead of NIST. REFERENCES

(1) Kunz, O.; Klimeck, R.; Wagner, W.; Jaeschke, M. The GERG-2004 Wide-Range Equation of State for Natural Gases and Other Mixtures; VDI Verlag GmbH: Düsseldorf, Germany, 2007. (2) Kunz, O.; Wagner, W. The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion of GERG2004. J. Chem. Eng. Data 2012, 57 (11), 3032−2091. (3) Lemmon, E. W.; Huber, M. L.; McLinden, M. O. REFPROP: Reference Fluid Thermodynamic and Transport Properties v9.1; National Institute of Standards and Technology: Gaithersburg, MD, 2013. (4) Fichtner, D. A. Phase Relations of Binary Hydrocarbon Series nButane-n-Octane. Master’s Thesis, The Ohio State University, 1962. (5) Kay, W. B.; Genco, J.; Fichtner, D. A. Vapor-Liquid Equilibrium Relationships of Binary Systems Propane-n-Octane and n-Butane-nOctane. J. Chem. Eng. Data 1974, 19 (3), 275−280. (6) Stein, S. E. NIST/EPA/NIH Mass Spectral Database Standard Reference Data; National Institute of Standards and Technology: Gaithersburg, MD, 2005. (7) Developed by Subcommittee E15.01. ASTM E1064-00 Standard Test Method for Water in Organic Liquids by Coulometric Karl Fischer Titration; ASTM International: West Conshohocken, PA, 2000. 2453


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