Article pubs.acs.org/jced
Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Experimental Phase Equilibrium for the Binary System of n‑Pentane +2-Propanol Using a New Equilibrium Cell and the Static Total Pressure Method Wayne Michael Nelson,*,† Sivanna Naicker,†,‡ Paramespri Naidoo,† Suresh Ramsuroop,‡ and Deresh Ramjugernath† †
Thermodynamics Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College Campus, Durban, South Africa ‡ Department of Chemical Engineering, Durban University of Technology, Steve Biko Campus, Durban, South Africa ABSTRACT: A new apparatus based on the static total-pressure (staticsynthetic) method was designed and commissioned. The novelty of the apparatus involves the placement of the loading valves, allowing metering of components directly into the liquid phase of the equilibrium cell. The new apparatus was tested and the experimental procedure developed by measuring phase equilibrium data (T−P−z) for a number of different systems available in the literature. Three of these test systems are presented in this paper, namely, the binary systems of n-hexane + 2-butanol, n-pentane + 2-butanol, and n-pentane + ethanol at temperatures of 329.21, 317.17, and 303.11 K, respectively. The modeled data (T−P−x) compare well to data available in the literature. New T−P−z data were measured for the binary system of n-pentane + 2-propanol at temperatures of 313.11, 323.11, and 333.12 K. The T−P−x−y data were modeled using both the combined and direct method.
1. INTRODUCTION Multistage separation units used for the purification and recovery of fluids account for a significant portion of capital and operating costs of chemical plants. Consequently, accurate and reliable phase equilibrium data are imperative for the design and optimization of industrial separation schemes. Experimental techniques used to acquire phase equilibrium data can be widely grouped, based on the method of determining composition, into two main categories, viz., analytic and synthetic methods. In most instances, analytic methods involve sample withdrawal followed by composition analysis. Sample removal and analysis can be very difficult in certain cases, even with modern capillary samplers. In contrast, synthesizing mixtures of known composition is in most cases much simpler; as the calibration and operation of equipment used to synthetically prepare mixtures is trivial in comparison to equipment necessary to remove and analyze samples. Generally, phase equilibrium data generated via synthetic techniques, due to their simplicity, are often more reliable than analytical-type methods. However, the data obtained from synthetic-type methods are often more limited, often the vapor phase is not experimentally determined, compared to that of analytic methods. A typical experimental setup of an apparatus following the “static-synthetic” or static total pressure method for measuring phase equilibria at low to moderate pressure involve a temperature-regulated equilibrium cell of known volume into which known amounts of degassed components can be injected.1 From this setup the temperature, pressure, and overall (total) composition can be obtained. The corresponding liquid and © XXXX American Chemical Society
vapor phase compositions must be calculated through a thermodynamic model and material balance. Within our research group, an apparatus (equilibrium cell volume of ∼190 cm3) following the “static-synthetic” was developed and then later automated.2−4 One of the aims of this study was to successfully design and commission a new “static-synthetic” apparatus of comparatively low volume (60 cm3) to complement the aforementioned apparatus. Accordingly, the equipment of Uusi-Kyyny et. al5 and Rarey and Gmehling1 were also used as guidelines for the development of this apparatus. The intention of this publication is to introduce the new experimental apparatus developed by our laboratories. The experimental apparatus was extensively tested and the experimental technique fine-tuned by comparing the measured phase equilibrium data with data available in the literature, viz., n-hexane + 2-butanol at 329.21 K, n-pentane + 2butanol at 303.17 K, and n-pentane + ethanol at 303.11 K. Thereafter, new T−P−z data for the n-pentane + 2-propanol binary system at temperatures of 313.11, 323.11, and 333.12 K were measured and modeled to give T−P−x−y data using the combined and the direct method.
2. MATERIALS AND METHODS 2.1. Materials. The specifications and physical properties of the chemicals used during the experimental measurements are Received: October 11, 2017 Accepted: January 19, 2018
A
DOI: 10.1021/acs.jced.7b00892 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 1. Pure Component Parameters, Purities, and Properties, as Well as the Expanded Uncertainty (k = 2) density at 313.15 K (g·cm−3) component n-pentane n-hexane ethanol 2-propanol 2-butanol
supplier Sigma-Aldrich Merck Merck Sigma-Aldrich Sigma-Aldrich
experimentala 0.627 0.642 0.772 0.768 0.789
c
literatureb 0.6261 0.6411 0.7722 0.7679 0.7895
refractive index at 293.15 K experimentald
literaturee
supplier purity wt %
GC analysis % peak area
1.357 1.375 1.361 1.377 1.395
1.358 1.375 1.361 1.377 1.395
≥99 ≥99 ≥99.5 99.5 ≥99
>99.9 >99.2 99.9 >99.9 >99.2
c
U(T) = 0.05 K; U(ρ) = 0.001 g·cm−3; data recorded at 101 kPa, U(P) = 1 kPa. bData for the liquid density (ρ) from NIST TDE.16 cData at 293.15 K. U(T) = 0.05 K; U(n) = 0.001; data recorded at 101 kPa and a standard wavelength of 589 nm, U(P) = 1 kPa.7 eData for the refractive index (n) from the literature.20 a
d
Figure 1. Schematic of the “static-synthetic” apparatus. IC, immersion circulator; LB, liquid bath; LV, liquid vent line; OS, overhead stirrer motor; P, syringe pump; PP, platinum resistance temperature probe; PT, pressure transducer; RF, Round-bottom flask; TR, temperature regulation; VL, Vacuum line; VP, Vacuum Pump.
impellors were located near the bottom (see Figure 1). The internal stirrer was magnetically coupled to an overhead Neodymium magnet, driven by an overhead stirrer (Heidolph; RZR-2020). This arrangement allowed for thorough agitation of the cell contents. The total working volume of the cell was determined to be 60.01 cm3. The equilibrium cell contained five inlet ports, three of which were sealed by 1/8″ Valco fittings (pressure transmitter, vacuum, and liquid venting lines), the other two ports were connected directly to valves linked using 1/8″ SS tubing to two high accuracy syringe pumps (Isco Teledyne; 100 DX; capacity ∼100 cm3). The syringe pumps contained degassed liquid and were used to accurately dispense known amounts of component into the cell. The temperature of the syringe pumps and subsequent loading lines were temperature regulated to within 0.1 K. A ball valve was used to meter the liquid contents of each syringe pump into the equilibrium cell. These ball valves were sealed directly onto the wall of the equilibrium cell via a copper gasket (see Figure 1). The pure components contained within each syringe pump were dispensed directly into the liquid phase of the mixture within the equilibrium cell, since the valves were sealed
displayed in Table 1. The gas chromatograph (GC) peak area percentages, densities, and refractive indices for the pure components, including the expanded uncertainties, are also listed in Table 1. The densities and refractive indices were measured in-house using a densimeter (Anton Paar; DSA 5000 M) and refractometer (Atago; RX-7000), respectively. The GC peak area percentages were determined via a GC (Shimadzu; 2014) equipped with a thermal conductivity detector and Porapak Q packed column. 2.2. Apparatus. A schematic of the experimental setup is displayed in Figure 1. The equilibrium cell was fabricated from stainless steel (SS) 316L. The cell was machined from a SS billet and consists of a single flanged end (top flange) that was sealed via a single O-ring compressed by six M4 bolts. The equilibrium cell was submerged into a 30 dm3 fluid-filled bath; the temperature of the fluid within the bath was controlled by an immersion circulator (Grant; TX 150). A magnetic stirrer ensured sufficient mixing of the components within the cell. The internal stirring device consisted of an 8 mm SS rod supported between two SS ball bearings, a Neodymium magnet (OD 28 mm; grade N40H; Au−Cu-plated) was attached near the top of the rod, and two B
DOI: 10.1021/acs.jced.7b00892 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
enable the liquids to be injected into the liquid phase under pressure and to prevent backwash. The temperature and pressure of the equilibrium cell were recorded by two 100 Ω platinum resistance thermometer (Pt100) probes (WIKA; 1/10 DIN) and a single pressure transmitter (WIKA P-10; 0−5 bar absolute; accuracy 0.05% of span), respectively. The pressure transmitter and its associated SS line connected to the equilibrium cell were temperature-regulated. The signals of these instruments were logged by a data acquisition unit (Agilent 34972A) connected to a desktop computer. The entire setup also uses an auxiliary degassing apparatus. Reliable degassing was a necessity for accurate phase equilibrium measurements involving two liquids components. The vacuum distillation method, similar to that of Van Ness and Abott, was implemented in this work.6 The liquid component to be degassed was contained in a 250 mL roundbottom flask and was distilled under vacuum in a Vigreux fractionating column under total reflux. A two stage vacuum pump (Edwards; RV3) was used to create the vacuum for the degassing apparatus and the equilibrium apparatus. 2.3. Calibrations and Experimental Uncertainty. The Pt100 probes and pressure transmitter were calibrated using inhouse standards (uncertainties listed in Table 2), a WIKA CTH 6500 and Mensor CPC 6000 (300 kPa), respectively. The syringe pumps have a supplier stated flow accuracy of 0.3% of the set point. The accuracy and reliability of the syringe pumps were tested via calibration. Distilled water was loaded into the syringe pumps and the pumps were temperature-regulated. The pumps were then controlled at constant pressure. Thereafter known amounts of water were dispensed into a beaker located on a mass balance (Mettler Toledo; AB204-S; resolution 10−4 g). The amount of liquid dispensed from the pump (determined via volume readings given by the pump; at a resolution of 10−4 ml) were compared to the gravimetric readings from the mass balance. Following
directly onto the wall of the equilibrium cell. This arrangement eliminated dead volumes associated with the loading lines and in theory thus allow for a lower total capacity of the equilibrium cell. The syringe pumps were operated in constant pressure mode, to Table 2. Standard Uncertainty Estimates and Influences of the Variables in this Work source of uncertainty
estimate
distribution
Pressure (P) P reference (kPa): Mensor CPC 3000 u(P) = 0.0001P (300 kPa) correlation for P (kPa): (500 kPa) 0.2 accuracy of vapor pressures (kPa) 0.7 Temperature (T) T reference (K): CTH 6500 0.02 correlation for T (K) 0.06 Total Compositiona (zi) L b liquid molar volume (Vm) u(VLm) = 0.01VLm (T and P affect) syringe pump flow accuracy (Q) u(Q) = 0.005Q Liquid phase compositionc (xi) volume of cell (cm3) 0.6 vapor phase composition u(yi) = 0.05yi liquid molar volumed u(VLm) = 0.005VLm V compressibility mixture vapor phase (Z ) u(ZV) = 0.02ZV pressure (kPa) 0.6 temperature (K) 0.06
normal rectangular triangular rectangular rectangular rectangular rectangular rectangular rectangular rectangular rectangular rectangular rectangular
a
Standard uncertainties used to estimate the uncertainty in the total composition synthesized within the equilibrium cell. bLiquid molar volume at 313.15 K and 20 bar (conditions for the liquid contained within the syringe pumps). cStandard uncertainties used to estimate the uncertainty in the modeled liquid phase composition. dLiquid molar volume at equilibrium temperature and pressure.
Table 3. Pure Component Vapor Pressures for the Experimental (Exp), Reference (Ref), and Modeled (Mod) Data (PR EoS + MC Alpha Function), Including the Measured Temperature (T), Pressure (P), and the Expanded Uncertainty (k = 2)a referenceb
experimental Texp/K
Pexp/kPa
(Pexp − Pref)/kPa
Pref/kPa
modeled Pmod/kPa
(Pexp − Pmod)/kPa
82.0 82.2 115.6 158.9 213.7
0.0 −0.1 0.0 0.0 0.0
30.3 37.1 45.0 54.1 67.1 76.5
0.1 −0.1 −0.2 0.4 −0.2 0.0
13.9 18.1 23.4 30.0 38.1
−0.1 0.0 0.0 −0.1 0.0
3.3 10.6 14.7
0.1 −0.2 0.2
n-Pentane
a
303.11 303.17 313.12 323.10 333.12
82.0 82.1 115.6 158.9 213.7
81.9 82.0 115.6 159.0 214.4
308.11 313.11 318.10 323.11 329.21 333.13
30.4 37.0 44.8 54.6 66.9 76.5
30.6 37.2 45.0 54.0 66.9 76.4
313.11 318.11 323.11 328.11 333.13
13.8 18.1 23.4 29.9 38.1
13.9 18.2 23.6 30.3 38.6
303.16 323.16 329.17
3.4 10.4 14.9
3.3 10.8 14.8
0.1 0.1 0.0 −0.1 −0.7 n-Hexane −0.2 −0.2 −0.2 0.6 0.0 0.1 2-Propanol −0.1 −0.1 −0.2 −0.4 −0.5 2-Butanol 0.1 −0.4 0.1
U(T) = 0.06 K; U(P) = 0.6 kPa. bReference data from NIST TDE.16 C
DOI: 10.1021/acs.jced.7b00892 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
components were degassed via vacuum distillation. The degassed liquids, sealed in two 250 mL round-bottom flasks, where transferred into their respective evacuated syringe pumps by gravity. After loading, the syringe pumps were immediately pressurized to 2 MPa to avoid leaks under vacuum, and subsequently stored for vapor pressure and phase equilibrium measurements. The vapor pressures of both components were then tested by loading the components individually (∼15 cm3) into the evacuated equilibrium cell. The temperature of the equilibrium cell was then regulated and the saturated vapor pressure recorded. This method was used to check the degassing and pump loading procedures; it was also necessary to fill (with liquid) the internal “ball” within the Swagelok ball valve that was attached directly to the equilibrium cell. Following the vapor pressure validation, the equilibrium cell was then vented and evacuated, and subsequently Component 1 was volumetrically metered into the cell to slightly less than half its volume (∼25 cm3; depending on the molar masses of the components). The equilibrium cell was maintained at constant temperature and the vapor pressure remeasured. A low quantity (∼2 cm3) of Component 2 was then metered into the equilibrium cell and the mixer activated. Under rapid mixing (∼600 rpm) equilibrium was quickly obtained (
