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J. of Supercritical Fluids 28 (2004) 1–9 Vapor–liquid equilibrium of carbon dioxide with ethyl caproate, ethyl caprylate and ethyl caprate at elevated pressures Weng-Hong Hwu, Jaw-Shin Cheng, Kong-Wei Cheng, Yan-Ping Chen ∗ Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC Received 16 July 2002; accepted 30 January 2003 Abstract Vapor–liquid equilibrium (VLE) data were measured for CO 2 with ethyl caproate, ethyl caprylate, and ethyl caprate using a semi-flow t
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  J. of Supercritical Fluids 28 (2004) 1–9 Vapor–liquid equilibrium of carbon dioxide with ethyl caproate,ethyl caprylate and ethyl caprate at elevated pressures Weng-Hong Hwu, Jaw-Shin Cheng, Kong-Wei Cheng, Yan-Ping Chen ∗  Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC  Received 16 July 2002; accepted 30 January 2003 Abstract Vapor–liquid equilibrium (VLE) data were measured for CO 2  with ethyl caproate, ethyl caprylate, and ethyl caprate using asemi-flow type apparatus at 308.2, 318.2 and 328.2 K over the pressure range from 1.6 to 9.2 MPa. In this paper, VLE data arereported. The VLE data were also correlated using the Soave–Redlich–Kwong and the Peng–Robinson equations of state withvarious mixing rules. It is shown that both equations of state agreed well with the experimental data.© 2003 Elsevier B.V. All rights reserved. Keywords:  Vapor–liquid equilibrium; Carbon dioxide; Esters; High pressure; Data 1. Introduction Vapor–liquid equilibrium (VLE) data at elevatedpressures are becoming important owing to the in-creasing applications of the dense gases or supercriti-cal fluids. Carbon dioxide is the most commonly usedsupercritical fluid for extraction and materials pro-cessing because of its non-toxicity and low criticaltemperature and pressure. VLE data for carbon diox-ide with esters at high pressure are not adequate [1].Recently, some experimental data for carbon dioxidewith esters are presented in literature [2,3], which are useful for thermodynamic modeling and process de-sign. However, solubility data for carbon dioxide inesters at high pressures are limited. In this study, asemi-flowapparatuswasusedtomeasuretheVLEdata ∗ Corresponding author. Fax:  + 886-2-2362-3040.  E-mail address:  ypchen@ccms.ntu.edu.tw (Y.-P. Chen). at high pressures for three binary systems of CO 2  withethylcaproate,ethylcaprylate,andethylcaprate.Ethylcaproate (C 8 H 16 O 2 ) and ethyl caprate (C 12 H 24 O 2 ) areused in either organic synthesis or the production of essential oil. Ethyl caprylate (C 10 H 20 O 2 ) is used in themanufacturing of cosmetics or in food industry. Theexperimental measurements were carried out at tem-peratures of 308.2, 318.2, and 328.2 K. The pressureranged from 1.6 to 9.2 MPa. The experimental datawere also correlated using the Soave–Redlich–Kwong(SRK) [4] and the Peng–Robinson (PR) [5] equations of state with various mixing rules. 2. Experimental 2.1. Chemicals Liquefied carbon dioxide was available with pu-rity more than 99.8 mass percentage from San-Fu 0896-8446/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0896-8446(03)00028-7  2  W.-H. Hwu et al. / J. of Supercritical Fluids 28 (2004) 1–9 Nomenclature a ,  b  parameters in the equation of state K   equilibrium ratio,  K  =  y  /   xk  ,  m  binary interaction parameters in the mixing rules P  pressure P vp vapor pressure of ester compound  R  gas constant T   temperature V   volume of CO 2 v  molar volume  x  mole fraction of liquid phase  y  mole fraction of vapor phaseSubscriptsc critical point i ,  j  component  i  or  j Superscriptscal calculatedexp experimentalChemical Co (Taiwan). Ethyl caproate, ethyl capry-late, and ethyl caprate were purchased from Acros Co.The purity of these chemicals was more than 99%. Allchemicals were used without further purification. Re-fractive indices and density are shown in Table 1. Therefractive indices of the pure compounds were mea-sured at 293.2 ± 0.1 K using an Abbe refractometer,Atago 3T, with an accuracy of   ± 0.0001. The densi-ties of pure chemicals were measured at 293.2 ± 0.1K using the Anton Paar DMA 60/602 density meterwith an accuracy of  ± 1.0 × 10 − 2 kg m − 3 . 2.2. Apparatus A semi-flow phase equilibrium apparatus used inthis study is shown in Fig. 1. The apparatus was com- Table 1Comparison of the measured refractive indices and densities of pure fluids with literature dataComponent  n D (293.2 K)  ρ  (293.2 K, kg m − 3 ) Purity (mass%)Experimental Literature [13] Experimental Literature [13] Ethyl caproate 1.4070 1.4073 871.2 873.0 >99.0Ethyl caprylate 1.4178 1.4178 868.2 a 866.0 a >99.0Ethyl caprate 1.4252 1.4256 864.0 865.0 >99.0 a Measured at  T  = 291.2 K. posed of three sections as the sample loader, equilib-rium cell, and composition analyzer. Similar experi-mental apparatus and procedures have been reportedin the previous literature [6,7].Pure CO 2  from a high pressure container was liq-uefied through a cooler with the temperature between268 and 263 K. It was then compressed by a meteringpump (ConstaMetric 3200 P/F, LCD Analytical Inc),and was heated through a pre-heating coil immersedin a water bath before charging into the pre-saturationand phase equilibrium cells. Each head of the pumpwas equipped with a cooling jacket, and aqueous al-cohol at a temperature between 268 and 263 K wascirculated to improve the fluid compression. A de-sired pressure was set in the experiment and a back pressure regulator (Tescom) was used to maintain  W.-H. Hwu et al. / J. of Supercritical Fluids 28 (2004) 1–9  3Fig. 1. Schematic diagram of the experimental apparatus. a constant pressure during the phase equilibriummeasurements.One pre-saturation cell and an equilibrium cell,each with a volume of 300 cm 3 , were used in thisstudy. The cells (Whitey) were made by stainlesssteel and were immersed in a water bath at the ex-perimental temperature. The experimental tempera-ture and pressure were measured using a calibratedquartz thermometer (INS), and a calibrated pressuregauge (Heise). The accuracy for temperature andpressure measurements is  ± 0.1 K and  ± 0.02 MPa,respectively. The metering valves (Autoclave) andneedle valves (Whitey) were also maintained at theexperimental temperature to ensure the equilibriumcondition.After the equilibrium cells, pressure was reduced toambient value and the solvent (ester) component fromthe liquid or vapor phase sample was collected in aflask and cooled by an ice bath. A wet tester meter(Ritter, TG50) was employed to measure the volumeof the solute (CO 2 ) in the vapor phase. The volumeof the liquid phase was determined by measuring thevolume displaced in a column filled with water. Theestimated accuracy for these measurements is betterthan ± 0.25%. 3. Experimental procedures Pure liquid solvent of ester compound was firstlyfed into the pre-saturation and equilibrium cells us-ing a mini pump. Air in these cells was displaced bythe flowing CO 2 . The pre-equilibrium and equilibriumcells were maintained at a constant temperature in awater bath, and CO 2  was charged into the cells undera desired pressure. VLE was usually reached within 2h, and either the vapor or liquid sample was then ex-panded to atmospheric pressure through the meteringvalves. These samples were analyzed using the gravi-metric method in this study. The number of moles of the minor ester compound that might vaporize into thegas phase was corrected using the ideal gas equationof state  n = P vp V   /   RT  , where,  P vp was the vapor pres-sure of the ester compound and  V   was the volume of CO 2  measured by the wet tester meter. The reportedequilibrium compositions are the average values of atleast three repeated measurements. The flow rate of CO 2  was maintained about 20 l h − 1 during the experi-ments. It had also been varied from 10 to 40 l h − 1 , andno change had been observed for the measured com-positions. With these procedures, it is ensured that thereported compositions are the equilibrium values. The  4  W.-H. Hwu et al. / J. of Supercritical Fluids 28 (2004) 1–9 Table 2Comparison of the VLE measurement results for the binary mixture of CO 2  (1) + 1-octanol (2) at 328.2 K P  (MPa) Literature data [8]  P  (MPa) This work   x 1  y 1  x 1  y 1 4.00 0.2406 0.9996 3.00 0.1818 0.99966.00 0.3533 0.9997 5.00 0.2925 0.99978.00 0.4785 0.9993 7.00 0.4139 0.999510.00 0.5856 0.9977 10.10 0.5931 0.997912.00 0.6674 0.9876 11.90 0.6607 0.990915.00 0.7772 0.9435 13.30 0.7243 0.9755 estimatedreproducibilityofthemeasuredcompositionis better than  ± 2% for the minor component in theliquid phase, while that for the minor component inthe vapor phase is estimated to be ± 1.0 × 10 − 4 molefraction. 4. Results and discussion To ensure the reliability of the experimental data,VLE for carbon dioxide + 1-octanol system was mea-sured at 328.2 K. The experimental results are shownin Table 2, and the graphical presentation is demon-strated in Fig. 2. It is observed that the experimentalresults using the present apparatus are in satisfactoryagreement with those in the previous literature [8].VLE for three binary mixtures of CO 2  with ethylcaproate, ethyl caprylate and ethyl caprate are re-ported in Tables 3–5, respectively. Graphical presen- tations of these experimental results are depicted inFigs. 3–5. It is observed that the solubility of carbondioxide in the liquid phase increases consistently withthe decrease of the molecular weight of ester. Carbondioxide has the largest solubility in ethyl caproateowing to its smallest molecular weight.In this study, the VLE data were correlated usingthe equation of state method. The PR equation [5]: P   =  RT  v − b − av(v  +  b)  +  b(v − b) (1)and the SRK [4]: P   =  RT  v − b − av(v  +  b) (2)were employed to correlate the experimental results.Critical properties and acentric factor employed arelisted in Table 6. Table 3Experimental VLE results for the binary mixture of CO 2  (1) + ethylcaproate (2)Pressure (MPa) Composition Equilibriumratio  K  1  x 1  y 1 T  = 308.2 K  1.699 0.2823 0.9994 3.5402.414 0.3834 0.9994 2.6073.060 0.4750 0.9994 2.1043.706 0.5558 0.9994 1.7984.404 0.6282 0.9994 1.5915.117 0.7050 0.9994 1.4185.748 0.7832 0.9991 1.2766.462 0.8480 0.9990 1.178 T  = 318.2 K  1.699 0.2301 0.9992 4.3422.380 0.3163 0.9992 3.1593.026 0.4023 0.9992 2.4843.740 0.4719 0.9992 2.1174.387 0.5341 0.9991 1.8715.101 0.6036 0.9991 1.6555.816 0.6671 0.9987 1.4976.462 0.7346 0.9985 1.3597.177 0.7965 0.9984 1.2537.823 0.8541 0.9976 1.168 T  = 328.2 K  1.733 0.2090 0.9987 4.7782.414 0.2920 0.9987 3.4203.094 0.3635 0.9986 2.7473.758 0.4318 0.9986 2.3134.438 0.4882 0.9986 2.0455.135 0.5504 0.9985 1.8145.799 0.6054 0.9982 1.6496.496 0.6540 0.9982 1.5267.177 0.7039 0.9982 1.4187.823 0.7571 0.9981 1.3188.538 0.8093 0.9964 1.2319.218 0.8463 0.9963 1.177
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