Determination of vapour–liquid and vapour–liquid–liquid equilibrium of the chloroform–water and trichloroethylene–water binary mixtures

Fluid Phase Equilibria 289 (2010) 80–89

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Determination of vapour–liquid and vapour–liquid–liquid equilibrium of the chloroform–water and trichloroethylene–water binary mixtures Hu-Sheng Hu ∗ Institute of Nuclear Energy and New Energy Technology, Tsinghua University, Beijing, 102201, China

a r t i c l e

i n f o

Article history: Received 9 August 2009 Received in revised form 3 November 2009 Accepted 5 November 2009 Available online 13 November 2009 Keywords: Determination Vapour–liquid equilibrium Vapour–liquid–liquid equilibrium Chloroform–water Trichloroethylene–water

a b s t r a c t The vapour–liquid equilibrium (VLE) and vapour–liquid–liquid equilibrium (VLLE) data are the basis for the design of distillation columns for the recovery or removal of residual extractant such as chloroform from wastewater. In this study, a new dynamic condensate-circulation still was designed and used for determination of VLE and VLLE of chloroform–water and trichloroethylene–water binary systems at 101.3 kPa. The phase diagrams of equilibrated vapour-phase composition versus overall liquid-phase composition y1 –x1 (where subscript 1 references organic component in the binary system) and equilibrated temperature versus vapour-phase or overall liquid-phase composition T–x1 (y1 ) were plotted by using the VLE and VLLE data and the data on mutual solubility. The experimental results showed that the two binary systems used have highly non-ideality and form heteroazeotropes, and their heteroazeotrope points are (t = 55.8 ◦ C, x1 = y1 = 0.856) for chloroform–water and (t = 73.9 ◦ C, x1 = y1 = 0.716) for trichloroethylene–water system. The activity coefficients of the component acted as solvent are in general, while the activity coefficients of the component acted as solute are very high in the equilibrated liquid phase. Correlation of the VLE and VLLE data respectively for the two binary systems with the NRTL (non-random two liquids) activity coefficient model gave satisfactory results. © 2009 Elsevier B.V. All rights reserved.

1. Introduction N,N-Dimethylfomamide (DMF) is widely used as a solvent in tanneries and as a vesicant in the manufacture of polyurethane. Both of these processes produce large quantities of wastewater containing DMF. It is now commercially important to treat these waste streams to remove and recover the DMF with the joint benefits of reducing the chemical oxygen demand (COD) of the waste stream and reuse of the recovered DMF. Historically the waste stream has been treated by direct distillation. This process has many disadvantages, it consumes large amounts of energy due to the high latent heat of vapourisation of the water, it is difficult to treat the wastewaters containing low concentrations of DMF, and the DMF can be hydrolysed to form dimethylamine and methyl acid during the distillation that cause secondary pollution of the environment. The author of this paper has recently developed a new alternative technique [1] that involves the extraction of DMF firstly from the wastewater using chloroform as the extractant, followed by distillation of HCCl3 from the extracted solvent for separation and recovery of DMF and HCCl3 . This new process has many significant merits, such as a lower energy consumption due to the

∗ Tel.: +86 10 89796088; fax: +86 10 89791464. E-mail address: [email protected]. 0378-3812/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2009.11.006

low latent heat of vapourisation of chloroform (one-ninth that of water), and little or no hydrolysis of DMF during distillation since hardly any water is present in the extracted solvent, further, this method is suitable for the treatment of wastewaters containing low concentrations of DMF. However, since chloroform is slightly soluble in water (approximately 0.8 wt%), its recovery from the extraction raffinate is important. Since there is a large difference between the vapour pressure of chloroform and water vapour, they are likely to form a binary azeotrope when heating the raffinate, hence, distillation is the preferred method for the recovery of chloroform from wastewater. However, in the literature the VLE (vapour–liquid equilibrium) data for the HCCl3 –H2 O binary system, which form the basis for the design of a distillation column that is used for the recovery of chloroform from wastewater, are incomplete. Gmehling and Onken compiled a large amount of VLE data for many aqueous-organic systems, but no data are available for the CHCl3 –H2 O binary system. Leighton and Calo [2] reported the distribution coefficients of chloroform in an air–water system over the temperature range of 0–30 ◦ C; Turner et al. [3] comprehensively reviewed and measured the vapour–liquid partition coefficients and activity coefficients of several organic compounds, including chloroform and trichloroethylene in water over the temperature range of 15–60 and 15–47 ◦ C, respectively, but there are no extant VLE data available over the temperature range of 60–100 ◦ C for chloroform–water system and 47–100 ◦ C for trichloroethylene–water system.

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In this study, the VLE and VLLE (vapour–liquid–liquid equilibrium) data at 55–100 ◦ C for a CHCl3 –H2 O binary system and mutual-solubility data are investigated in detail. In addition, the VLE and VLLE data at 74–100 ◦ C for the trichloroethylene–water system are also investigated in detail, because the trichloroethylene is probably as candidate of extractant for extracting DMF from waste water and recovery of the extractant from its extraction raffinate is required. Moreover, the trichloroethylene is widely used for rinsing metal parts, thus recovery or removal of it by distillation from rinsing wastewater is again required. The composition of the equilibrated liquid phases and vapour phase and boiling points of the liquid at 101.3 kPa for the two binary systems were measured, further, phase diagrams of equilibrated vapour-phase composition versus liquid-phase composition (y–x) and the equilibrated temperature versus composition of the vapour or liquid phase (T–x(y)) were plotted. In addition, the isobaric VLE and VLLE data were correlated with the NRTL activity coefficient model. 2. Experimental The VLE data are often determined using a best-known Othmer still or Othmer-type still (e.g., Gilmont and Conti still) [4], as well as a modified Scatchard still [5] or a Raal still [6]. These stills have Cottrell pump feature that can ensure a intimate contact between vapour and liquid phase, however, they are not suitable for determination of the VLLE data because there is no a electromagnetic stirring equipped with in the still where there are two liquid layers since the system determined is partial miscible, thus no a good mixing and intimate contact between two liquid phases is ensured. The Ellis equilibrium still, similar to a condensate-circulating Kortümtype still [4] is also used for obtaining the VLE data, but they are again not suitable for determination of the VLLE data, because they have no Cottrell pump although they may have electromagnetic stirring in the still, moreover, their condensate-samplers are so deep that they can deposit and accumulate a certain amounts of the heavier liquid phase there (the condensate generally forms two liquid phases if the system measured is partial miscible) during the condensate reflows back to the still, hence, the samples from those concave sampler do not represent the true compositions of vapour phase being in equilibrium with liquids. In order to measuring the VLLE data, Haddad and Edmister [5] used a specifically designed Hands and Norman equilibrium still for the systems whose condensate forms two liquid phases. However, the Hands and Norman still lacked a Cottrell pump that guaranteed an intense vapour–liquid-phase exchange. In this study, based on the consideration of overcoming or eliminating the above still’s drawbacks, a new condensate-circulating still is designed, as shown in Fig. 1. This still is an all glass one. The heating tube(2), whose outer surface is winded with the electrical resistance wire, acts as a Cottrell pump, that ensures an intimate contact between vapour and liquid phases. When heating the tube(2), an equilibrium mixture of liquid and vapour are discharged out from the top outlet of the tube, and the vapour separates from liquid and enters the condenser(8), and the liquid is spurted to the opposite wall inside the separation-equilibrium chamber(3) and quickly spreads into the liquid membrane along the inside wall and flows downward reaching the bottom of the equilibrium chamber(3), in where there is electromagnetic stirring for strongly mixing the two liquid phases thus the equilibrium between the two liquid phases is quickly reached. The outer surface of the equilibrium chamber(3) is lagged with insulating material so as to prevent the mixture from fractionation. The condensate in the form of two-liquid phases from condenser(8) and liquid from separation-equilibrium chamber(3) are adequately mixed and uniform dispersed even emulsified in the mixing chamber(5) before they enter the heating tube(2). The bottom of chamber(5) is specif-

Fig. 1. apparatus for determining the vapour–liquid–liquid equilibrium. 1: Voltage regulator; 2: electric heater (Cotrell pump); 3: separation–equilibrium chamber for vapour phase and liquid phase; 4 and 7: electromagnetic stirrer; 5: mixing chamber; 6: thermostatic water bath; 8 and 11: condenser; 9: precision thermometer; 10: three-way valve for sampling condensate from vapour phase; 12: separation chamber for two-liquid-phase samples; 13: pressure transducer; 14: valve for discharge of liquid; 15: buffer tank; 16: two-stage cylinder pressure regulator; 17: high pure (grade #5) nitrogen cylinder; 18: valve to vacuum system; 19: three-way valve for sampling liquids.

ically designed to be an plane inclined by a slope of 20◦ , this structure is of great advantage to emulsification of the two liquid phases so as to ensure the stability of the temperature in the equilibrium chamber(3). The sampler(10) equips with a 3-port valve, through which the condensate (one or two liquid phases) flows via a reflowing pipe back to the still without accumulating any heavier liquid phase (organic) in the sampler. When sampling vapour is needed, the 3-port valve is conducted to the right-hand outlet thus the condensate is diverted into a cone-shaped tube with graduations of 0.01 ml, then the tube is placed in a centrifuger for phase-separation. Similarly, when sampling liquid is needed, the liquid in the separation-equilibrium chamber(3) is diverted in a small amount by the 3-port valve(19) into the sample-separation chamber(12), where the temperature is controlled by the water bath at the same temperature as that of separation-equilibrium chamber(3) for separation of two-liquid-phase samples, then the liquid samples are taken from the sample-separation chamber(12) using a syringe. During experiment, the reflow rate of the condensate from vapour phase is controlled by manual operation of the voltage regulator(1) for adjusting electric-heating power applied. The absolute pressure of the system is controlled at 101.3 kPa by manual operating carefully the two-stage cylinder pressure regulator(16) installed on the top of the cylinder(17) for controlling the flow rate of high pure (grade #5) nitrogen applied into the buffer tank (10-l) (15), and the pressure is measured by a pressure transducer (KLP800-AK, Guang Zhou Kun Lun automatic control equipment Co.)(13) with a digital-display unit (CH6/A-H(s)RTB1). The pressure measurement device was checked by measuring the normal boiling point of water. The system temperature is measured by a precision glass thermometer(9) with scale of 0–50 or 50–100 ◦ C. The uncertainties of pressure and temperature measured are p{+0.1 kPa and −0.1 ◦ C, respectively. t{+0.05 −0.15 The concentrations of chloroform in the aqueous phase and trichloroethylene in aqueous phase are determined using a colori-

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metric method [7] with a visible light spectrophotometer (model 722, Shang Hai Heng Ping Scientific Instrument Co.). The uncertainty of measurement of composition (x1 ) with this kind of analysis is x1 {+0.000008 mol fraction. −0.000012 The concentrations of water in the organic phase are determined using a volumetric Karl Fischer titration method [8] with a KF-II automated titration system (Shang Hai Bao Shan Jing Gong Electronic Instrument Co.). The uncertainty of measurement of composition (x2 ) is x2 {+0.00010 mol fraction. The uncertainty −0.00016 of measurement of vapour composition (y1 ) is estimated to be y1 {+0.0015 mol fraction. −0.0025 All of the chemical reagents used such as Carl-Fischer reagent, NaOH and pyridine are of analytic purity (Bei Jing Chemical Regent Co.). The fresh chloroform (analytic purity, purity ≥99.0 wt%, H2 O 0.03 wt%, stabilizer-ethyl alcohol 0.3–1.0 wt%) is washed two times before being used with deionised water (1000 ml water per 100 ml chloroform) for removal of the stabilizer-ethyl alcohol. After washing, the concentration of ethyl alcohol is less than 0.01 wt%. The trichloroethylene (purity ≥99.9 wt%, H2 O 0.004 wt%) is also of analytic purity. The deionised water (electrolytic conductivity 0.01 mS/m) is used throughout the experiment.

3. Results and discussion 3.1. Vapour–liquid and vapour–liquid–liquid equilibrium data The density data of pure liquid components used in this paper are taken from the literature [9,10] and regressed against each other as functions of temperature expressed as the following Eqs. (1)–(3) with regression coefficient R2 = 1, 0.999 and 0.9984, respectively.

The relative errors between reproduced density data and original ones are very small, e.g., less than 0.2% for Eq. (1), and less than 0.06% for Eq. (2). The reproduced density data from these equations are also consistent with (for Eqs. (1) and (3)) or considerably better than (for Eq. (2)) those calculated using the modified Rackett equation [11]. 1 = −0.0019T 2 − 0.7139T + 1860.7

(for chloroform)

(1)

2 = −0.0035T 2 + 1.8659T + 755.43

(for water)

(2)

2

1 = −0.005T + 1.6504T + 1403

(for trichloroethylene)

(3)

where the subscripts 1 and 2 refer to organic component and water, respectively, the i denotes the density of pure liquid i (unit: kg/m3 ), and T is the system temperature (unit: K). The density of each single-liquid phase (aqueous or organic) can be calculated by the following Eqs. (4) and (5) [12]: aq

aq

aq = ϕ1 1 + (1 − ϕ1 )2 org

(4)

org

org = ϕ1 1 + (1 − ϕ1 )2 aq ϕ1

(5)

org ϕ1

and are the volume fractions of organic component where in aqueous and in organic phase, respectively. The relative errors of density of each single-liquid phase between calculated and experimental measured by a Li volumetric flask (250 ml,whose top has an outlet tube with graduation 0.1 ml) are less than 0.07% for the two binary systems used in the experiment. The data on the isobaric VLE for chloroform–water and trichloroethylene–water systems in the entire concentration range of x1 are given in Tables 1 and 2, respectively, where t is the boiling point (◦ C) of the liquid mixture; and y1 is the mole fraction of

Table 1 Comparison of the results correlated with experimental measured (for chloroform(1)–water(2) system). Equilibrium temperature, t (◦ C)

Experimental values x1

61.2 61.0 59.0 57.0 56.0 55.8 55.8 55.8 55.8 55.8 55.8 55.8 55.8 56.0 56.5 57.0 62.0 63.0 69.0 71.0 75.0 76.0 78.0 82.0 84.0 90.0 93.0 94.0 97.0 98.0 99.0 100

a

x1 is the overall liquid-phase composition.

a

NRTL mode y1

1.00000 0.99859 0.99625 0.99209 0.98796 0.95701 0.81035 0.56031 0.34415 0.18360 0.08791 0.02438 0.00691 0.00136 0.00122 0.00117 0.00076 0.00056 0.00046 0.00038 0.00031 0.00025 0.00018 0.00015 0.00012 0.00011 0.00008 0.00006 0.00005 0.00003 0.00002

1.000 0.982 0.951 0.900 0.871 0.855 0.860 0.860 0.849 0.860 0.849 0.860 0.855 0.854 0.842 0.838 0.820 0.805 0.754 0.736 0.710 0.688 0.647 0.619 0.577 0.472 0.439 0.338 0.165 0.095 0.012

0.00000

0.000

1 1.0 1.0 1.0 1.0 1.0 1.1 1.3 1.8 3.0 5.6 11.6 42.2 147.9 745.2 804.2 823.2 1043.5 1354.0 1278.8 1407.8 1502.2 1772.6 2088.2 2133.6 2347.9 1854.1 2225.9 2275.2 1250.1 1057.9 262.7

2

t

Region y1

62.3 69.5 74.1 65.7 21.0 4.6 2.0 1.4 1.1 1.0 0.9 0.9 0.9 0.9 1.0 0.8 0.9 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.8 0.9 1.0 1.0

0.0 −12.3 −13.3 −13.9 −13.9 −11.7 −12.1 −13.2 −17.3 −21.8 −22.2 −4.9 20.9 24.3 22.9 22.5 16.6 7.7 7.3 3.8 1.7 −2.6 −5.9 −4.6 −5.3 −0.6 −0.3 −1.1 1.0 0.7 0.3

0 0.004 0.003 −0.006 −0.006 0.053 0.040 0.078 0.176 0.303 0.305 0.064 −0.091 −0.100 −0.107 −0.109 −0.084 −0.037 −0.034 0.009 0.065 0.130 0.193 0.226 0.251 0.180 0.223 0.176 0.030 0.004 −0.034

1.0

0.0

0.000

Standard deviation SDR (y1 )

0.138

VLE

VLLE

VLE

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83

Table 2 Comparison of the results correlated with experimental measured (for trichloroethylene(1)–water(2) system). Equilibrium temperature, t (◦ C)

Experimental values y1

1

1.00000 0.99919 0.99785 0.99680 0.99548 0.99287 0.94610 0.77109 0.61474 0.29263 0.15059 0.07062 0.01932 0.00924 0.00019 0.00018 0.00017 0.00017 0.00016 0.00016 0.00015 0.00013 0.00011 0.00009 0.00008 0.00007 0.00006 0.00004 0.00002 0.00001 0.00001

1.000 0.976 0.935 0.879 0.830 0.727 0.706 0.706 0.714 0.722 0.737 0.710 0.718 0.718 0.718 0.715 0.713 0.690 0.677 0.670 0.640 0.586 0.530 0.466 0.375 0.306 0.252 0.167 0.090 0.035 0.025

1.0 1.1 1.1 1.1 1.1 1.1 1.1 1.4 1.8 3.7 7.4 15.2 56.8 118.7 5728.7 5988.7 5981.8 5668.5 5480.5 5304.3 5162.4 5450.3 5208.7 5745.4 4706.8 4110.2 3525.1 3119.9 3163.2 3654.3 3026.1

100.0

0.00000

0.000

x1

a

NRTL mode

87.1 84.5 82.0 79.5 77.0 73.9 74.0 74.0 74.0 74.0 73.8 74.0 73.6 73.6 73.6 74.0 75.0 77.0 78.0 79.0 80.0 81.8 83.8 85.0 88.0 89.4 92.5 95.0 97.0 98.0 99.5

a

Region

2

t

y1

53.9 60.1 82.7 91.0 106.5 15.0 3.5 2.0 1.1 0.9 0.9 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.8 0.8 0.9 0.9 1.0 1.0 1.0 1.0 1.0 1.0 1.0

0.0 −1.0 −1.4 −2.5 −3.5 −4.6 −0.5 −3.0 −3.4 −7.9 −11.1 −10.8 6.1 23.9 7.2 5.6 5.6 6.3 6.9 7.6 7.2 4.0 3.5 0.1 1.5 1.4 3.0 2.3 0.6 −0.8 0.5

0.000 0.016 0.030 0.010 −0.002 −0.051 0.036 −0.052 −0.011 0.200 0.338 0.309 −0.009 −0.165 −0.017 0.003 0.014 0.008 0.002 −0.001 −0.011 0.015 0.005 0.038 −0.017 −0.051 −0.067 −0.064 −0.030 −0.006 −0.008

1.0

0.0

0.000

Standard deviation SDR (y1 )

0.101

VLE

VLLE

VLE

x1 is the overall liquid-phase composition.

organic component in the vapour phase being in equilibrium with one or two liquids; and x1 is total mole fraction of organic component in the equilibrated liquid phase with one or two liquids, aq org x1 = (1 − ˇ)x1 + ˇx1 , where the ˇ is phase split, is the fraction aq org of the organic phase in the total liquids; x1 and x1 are mole fractions of organic component in aqueous and in organic phase, respectively. It can be seen from Table 1 for chloroform–water sys-

tem that, in the range of x1 from 0.0013 to 0.9880, the boiling point t maintains a constant (55.8 ◦ C), and the y1 also maintains a constant (0.856), so this x1 range is a invariant region with respect to temperature and vapour composition. In fact, this x1 range corresponds to a two-liquid-phase region, i.e., a three-phase (one vapour phase and two liquid phases, denoted by VLL)-coexistence region, the other ranges correspond to single-liquid-phase regions, i.e., within

Table 3 Correlation of VLLE data with NRTL equations (for chloroform(1)–water(2) system). x1 a

texp (◦ C) aq x1,exp = org x1,exp = y1,exp = aq 1,exp = aq 2,exp = org 1,exp = org 2,exp = aq 1,calc = aq 2,calc = org 1,calc = org 2,calc = aq x1 = org x1 = t aq = torg aq y1 = org y1 = a

Averaged

0.95701

0.81035

0.56031

0.34415

0.18360

0.08791

0.02438

0.00691

55.80 0.00130 0.9930 0.855 785.72 0.90 1.03 128.76 844.80 1.00 1.00 116.61

55.80 0.00132 0.9927 0.860 779.14 0.87 1.04 119.81 845.42 1.00 1.00 114.28

55.80 0.00133 0.9927 0.860 770.27 0.87 1.04 119.81 845.84 1.00 1.00 114.28

55.80 0.00136 0.9927 0.849 744.14 0.93 1.02 128.52 846.48 1.00 1.00 114.28

55.80 0.00117 0.9908 0.860 880.82 0.87 1.04 94.30 837.10 1.00 1.00 98.27

55.80 0.00106 0.9908 0.849 957.75 0.93 1.02 101.15 826.59 1.00 1.00 98.27

55.80 0.00130 0.9895 0.860 788.27 0.87 1.04 82.60 844.93 1.00 1.00 89.17

55.80 0.00135 0.9920 0.855 756.62 0.90 1.03 112.67 846.21 1.00 1.00 107.88

x1 is the overall liquid-phase composition.

55.80 0.00127 0.9918 0.856 807.84 0.89 1.03 110.95 842.17 1.00 1.00 106.63 4.71E−06 −0.0002 1.33 −0.79 0.00065602 −0.0012239

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H.-S. Hu / Fluid Phase Equilibria 289 (2010) 80–89

Fig. 3. (a–c) y1 –x1 phase diagrams for trichloroethylene(1)–water(2) system.

Fig. 2. (a–c) y1 –x1 phase diagrams for chloroform(1)–water(2) system.

x1 from 0.0 to 0.0013 is an aqueous phase; within x1 from 0.988 to 1.0 is an organic phase. The similar results are observed in Table 2, and the invariant region is the range of x1 from 0.00019 to 0.9929 for trichloroethylene–water system. In order to further clearly describe the changing laws of the variables, such as equilibrium temperature, equilibrated liquid and vapour compositions varying with the overall liquid composition x1 , the equilibrium phase diagrams are plotted in this study. At a constant total pressure of 101.3 kPa, the equilibrated vapour-phase compositions (y1 ) and the corresponding overall liquid composition (x1 ) form the y1 –x1 phase diagram, shown in Fig. 2. Fig. 2(b) and (c) are close-up views of specific regions of the phase diagram shown in Fig. 2(a). It can be seen from the plot for chloroform–water system that, the y1 increases sharply as x1 increases from 0 to 0.0013, after which it maintains almost a constant 0.856 when x1 varies in a wide range from 0.0013 to 0.988. This implies the formation of the azeotrope when x1 is in the range of 0.0013–0.988, with the azeotropic composition x1 = y1 = 0.856. The x1 values 0.0–0.0013 and 0.988–1.0 correspond

to single-liquid-phase regions, i.e., the aqueous phase and organic phase, respectively. The similar result is observed in Fig. 3 for the trichloroethylene–water system, i.e., the values of y1 remain a constant of 0.716 as x1 varies from 0.00019 to 0.9929. Fig. 4(a) shows the phase diagram of equilibrated temperature versus compositions, T–x1 (y1 ), at a constant total pressure of 101.3 kPa. Fig. 4(b) and (c) are close-up views of specific regions of the phase diagram shown in Fig. 4(a). It can be seen from Fig. 4 that there exists a lowest-temperature point C on the dew-point curve at y1 = 0.856 (t = 55.8 ◦ C), which corresponds to equilibrated liquid-phase compositions point A (x1 = 0.0013, t = 55.8 ◦ C) on the left-branch of boiling-point curve and point B (x1 = 0.988, t = 55.8 ◦ C) on the right-branch of boiling-point curve. This implies that the CHCl3 –H2 O binary system shows strong positive deviations from Raoult’s law. The point C is just the azeotropic point. The points A and B on the boiling-point curves are called turning points, the values of which are depended on the solubilities of chloroform in aqueous phase and water in organic phase at azeotropic temperature of 55.8 ◦ C, respectively. The range of x1 0.0013–0.988 corresponds to a three-phase (VLL)-coexistence region and the other ranges correspond to regions where a single liquid and vapour coexist. In the VLL-coexistence region, with x1 increasing from 0.0013 to 0.988, the boiling points keep at 55.8 ◦ C, so the so-called azeotropic curve is a perfectly horizontal straight line; in addi-

H.-S. Hu / Fluid Phase Equilibria 289 (2010) 80–89

85

Fig. 4. (a–c) T–x1 (y1 ) phase diagrams at 101.3 kPa for chloroform(1)–water(2) binary system.

Fig. 5. (a–c) T–x1 (y1 ) phase diagrams at 101.3 kPa for trichloroethylene(1)–water(2) binary system.

tion, y1 maintains almost a constant 0.856 (also see Table 3) in the three-phase-coexistence region. According to the Gibbs rule, in the three-phase (VLL)-coexistence region the numbers of freedom degrees is 0 (invariant system), i.e., the azeotropic curve must be a horizontal straight line, and y1 must be a constant. Therefore, these phase diagrams for binary system are in accord with the Gibbs rule. The similar situations are observed in Fig. 5 for the trichloroethylene–water system. The azeotropic curve is also a horizontal straight line with the turning point A (x1 = 0.00019, t = 73.9 ◦ C), B (x1 = 0.9929, t = 73.9 ◦ C) and azeotropic point C (y1 = 0.716, t = 73.9 ◦ C). In each phase diagram, Figs. 4 and 5, except for a dew-point curve, boiling-point curves and azeotropic-point curves, there are two mutual-solubility curves (binodal curves). The solubility of either organic component in aqueous phase or water in organic phase slightly increases as the temperature increases. For further inspecting the properties of temperature, liquid and vapour compositions in the three-phase (VLL)-coexistence region, the data on the isobaric VLLE for the two binary systems are presented in Tables 3 and 4, respectively. It can be seen from these

tables that the equilibrium temperature texp , equilibrated two aq org liquid-phase compositions x1,exp and x1,exp , and vapour compositions y1,exp all maintain almost constant when x1 varies in the three-phase (VLL)-coexistence region. This further proves these VLL-coexistence regions are “invariant regions”. 3.2. Activity coefficient of liquid-phase components Under the normal or negative pressure, the condition of vapour–liquid equilibrium can be expressed as Eq. (6) below [13]: pyi = psi xi i

(6)

where p is the total pressure (unit: kPa) of system, psi is the saturated vapour pressure (unit: kPa) of pure liquid component i; x1 is the overall composition of component i in the liquid mixture;  1 is the activity coefficient of component i in liquid mixture. Eq. (6) can be written as, i =

pyi psi xi

(7)

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H.-S. Hu / Fluid Phase Equilibria 289 (2010) 80–89

Table 4 Correlation of VLLE data with NRTL equations (for trichloroethylene(1)–water(2) system). x1 a

texp (◦ C) aq x1,exp = org x1,exp = y1,exp = aq 1,exp = aq 2,exp = org 1,exp = org 2,exp = aq 1,calc = aq 2,calc = org 1,calc = org 2,calc = aq x1 = org x1 = t aq = torg aq y1 = org y1 = a

Averaged

0.94610

0.77109

0.61474

0.29263

0.15059

0.07062

0.01932

0.00924

74.00 0.00018 0.9927 0.706 6055.03 0.81 1.07 110.04 7062.89 1.00 1.00 96.56

74.00 0.00018 0.9933 0.706 6035.22 0.81 1.07 119.98 7075.80 1.00 1.00 99.37

74.00 0.00018 0.9930 0.714 6084.67 0.78 1.09 111.51 7090.35 1.00 1.00 97.96

74.00 0.00022 0.9927 0.722 4974.55 0.76 1.10 103.92 8055.57 1.00 1.00 96.56

73.80 0.00018 0.9933 0.737 6383.92 0.73 1.13 108.26 7049.50 1.00 1.00 99.37

74.00 0.00016 0.9933 0.710 6780.17 0.79 1.08 118.30 6669.44 1.00 1.00 99.37

73.60 0.00022 0.9929 0.718 4953.96 0.78 1.11 109.93 8128.84 1.00 1.00 97.60

73.60 0.00017 0.9927 0.718 6395.70 0.78 1.11 106.54 6978.31 1.00 1.00 96.57

73.88 0.00018 0.9930 0.716 5957.90 0.78 1.09 111.06 7263.84 1.00 1.00 97.92 −1.50E−06 −0.0010 6.16 −3.00 0.0070 0.0072

x1 is the overall liquid-phase composition.

The saturated vapour pressures of pure organic component and water were calculated from the Antoine equations (8)–(10) (psi : kPa), respectively [14], log ps1 = 5.9629 −

1106.94 t + 218.55

log ps2 = 7.07396 − log ps3 = 3.153 −

1657.46 t + 227.02

1315.3 t + 230.0

(for chloroform)

(8)

(for water)

(for trichloroethylene)

(9) (10)

The activity coefficients of organic component and water in liquid phase, denoted by  1 and  2 , respectively, calculated using Eq. (7). It can be seen from Fig. 6 (Fig. 6(b) and (c) are close-up views of specific regions of the phase diagram shown in Fig. 6(a)) as well as from Table 1 (for chloroform–water system) that in the range of x1 = 0.0013–0.988,  1 and  2 are ordinary values, but when x1 falls in the single-liquid-phase region the activity coefficients of the component acted as solute are very high, e.g.,  1 = 750–2300 when x1 is near the endpoint (x1 < 0.0013) of horizontal coordinated axis. Since all the activity coefficients are obtained by calculated using Eq. (7) from the overall liquid composition x1 , in Fig. 6 the values

of  1 and  1 corresponding to the range of x1 = 0.0013–0.988 (twoliquid-phase region) are only apparent ones, i.e., the dashed lines correspond to two-liquid phases, so the plotted values by dashed lines have no physical significance there. The similar situations are observed for the trichloroethylene–water system. In Table 2, the activity coefficients of the component acted as solute are very high, e.g.,  1 = 3000–5980 when x1 is less than 0.00019; and the values of  1 and  2 corresponding to the range of x1 from 0.00019 to 0.9929 have no physical significance because this range corresponds to two-liquid phase. The true values of activity coefficients of component i in aqueaq ous phase and in organic phase in two-liquid-phase region, i,exp org

and i,exp , are calculated using Eqs. (11) and (12) after experimentally measuring the compositions of two liquid and vapour phases, respectively. These values calculated for the two systems are tabulated in Tables 3 and 4, respectively. aq

i,exp =

pyi,exp

(11)

aq

psi xi,exp

Table 5 Comparison the data from this work with the literatures (for system A: chloroform(1)–water(2); B: trichloroethylene(1)–water(2)). System

Temp. (◦ C)

A A A A A A B B

25 25 35 35 60 60 35 35

System

Temp. (◦ C)

A A B B

Activity coefficients,  1

Reference

857 880 837 805 1120 1040 7600 7410

Turner et al. [3] This work Turner et al. [3] This work Svetlanov et al. [15] This work Schoene et al. [17] This work

Solubility of organic compound in water (mg/l)

Activity coefficients,  1

Reference

20 20 25 25

8200 8400 1000 1140

810 890 7300 8010

Turner et al. [3] This work Neely [18] This work

System

Temp. (◦ C)

Solubility of water in organic compound (wt%)

Activity coefficients,  2

Reference

A A B B

25 25 25 25

Partition coefficient, K1 = y1 /x1 202 200 280 296 960 950 990 940

0.093 0.097 0.033 0.032

162 170 233 240

Staverman [16] This work Kirk and Othmer [19] This work

H.-S. Hu / Fluid Phase Equilibria 289 (2010) 80–89

87

3.3. Correlation of VLE and VLLE data At first, the VLE data in entire concentration range of x1 from Tables 1 and 2 were correlated by Van Laar activity coefficient model [20] and Margules model [21], respectively. The results showed that the plots of x1 /(x1 ln  1 + x2 ln  2 ) versus x1 /x2 and x2 /(x1 ln  1 + x2 ln 2 ) versus x2 /x1 were nonlinear, and the plots of ln( 1 /x2 ) versus x1 and of ln( 2 /x1 ) versus x2 were also nonlinear. This implied that neither the Van Laar nor the Margules model were suitable for correlating the VLE data obtained for the two binary systems in the entire concentration range of x1 . Further, it has been pointed out in the literature that the NRTL model [22] but not the Wilson activity coefficient model [13] is suitable for correlation of the VLLE data. Therefore, the NRTL model Eqs. (13) and (14) are used to correlate the VLE data from Table 1 for HCCl3 –H2 O binary system in entire concentration range of x1 :



ln 1 =

x22

 ln 2 = x12

2 21 G21

(x1 + x2 G21 )2 2 12 G12

(x2 + x1 G12 )2 g

+ +

−g

12 G12 (x2 + x1 G21 )2 21 G21 (x1 + x2 G21 )2 g



(13)

 (14)

−g

where 21 = 12RT 11 , 21 = 21RT 22 , G12 = exp(−˛12 ), G21 = exp(−˛21 ). The model interaction energy parameters g12 − g11 and g21 − were obtained by minimizing the objective function g22 m F= [(y1,exp − y1,cal )2 + (y2,exp − y2,cal )2 ] using the nonlinear k=1 least-square method. During the iterative calculation of g12 − g11 and g21 − g22, the secant formula (Eq. (15)) was used for the iterative calculation of the boiling point t, and take the experimental values texp and (texp +0.5) as the initial values of tn−1 and tn−2 . For finding the other model parameter ˛, an optimization method: dimensionality-reduction method [23] were used. The results correlated including optimized parameters ˛, g12 − g11 and g21 − g22, and corresponding differences between experimental and correlated (t), (y1 ) were shown in Tables 1 and 6. tn = tn−1 −

tn−1 − tn−2 H(tn−1 ) H(tn−1 ) − H(tn−2 )

(15)

2

Fig. 6. (a–c) Experimentally obtained activity coefficients of liquid-phase components for chloroform(1)–water(2) system. The dashed and doted lines correspond to the two-liquid-phase region, so the plotted values by dashed and doted lines have no physical significance there.

org

i,exp =

pyi,exp org

psi xi,exp

(12)

It can be seen from Table 3 for chloroform–water system that in three-phase (VLL)-equilibrium region the averaged activity coeffiaq org cients of the component acted as solvent, 2,exp and 1,exp , are in general, to be 0.87 and 1.04, respectively; while the averaged activaq org ity coefficients of the component acted as solute, 1,exp and 2,exp , are very high, to be 807 and 110, respectively. Both equilibrium temperature texp and vapour compositions y1,exp keep at almost constant. As for the trichloroethylene–water system, the situations are similar to that of chloroform–water system, as shown in Table 4. A comparison between the data collected in this study and those given in the literatures is shown in Table 5. It can be seen that the partition coefficients, activity coefficients of organic compound in liquid phase and data on solubility of organic compound in water or water in organic compound are close to those published in the literature [3,15–19].

where H(tn ) = y − 1, H(tn ) is a normalization function; n i=1 i refers the number of times of iteration. It can be seen from Tables 1 and 6 that the use of the NRTL model for correlating the VLE data for water–chloroform binary system obtained from Table 1 over the entire range of overall liquid composition x1 yields satisfactory results; the precise values of ˛, g12 − g11, and g21 − g22 for NRTL model were found to be 0.42, 9890.03, and 19974.4, respectively; the standard deviation SDR (y1 ) was found to be 0.138. The predicted y1 is plotted in Fig. 7. Fig. 7(b) and (c) are close-up views of specific regions of the phase diagram shown in Fig. 7(a). It can be seen that in the single-phase region the predicted values of y1 are close to the experimental ones. However, dashed line corresponds to two-liquid-phase region, so the plotted values of y1 by the dashed line have no physical significance there. The true values of y1 , as we known from Table 1, keep at almost a constant 0.856. It is necessary for us to separate correlation of the VLLE data for the two-liquid-phase region from correlation of the VLE data for the single-liquid-phase region. For correlating the VLLE data from Table 3 for chloroform–water system in the three-phase (VLL)-coexistence region, we use NRTL activity coefficient model and take the same objective function and optimization method as above correlation of VLE data. The conditions of equilibrium among three phases (VLL) differ from above and are expressed as [13], aq

aq

pyi = psi xi i

org

= psi xi

org

i

(16)

88

H.-S. Hu / Fluid Phase Equilibria 289 (2010) 80–89

Table 6 Parameters of the activity coefficient models. Binary systems

Chloroform(1)–water(2) Chloroform(1)–water(2) Trichloroethylene(1)–water(2) Trichloroethylene(1)–water(2)

NRTL equations

Remarks

˛

g12 − g22

g21 − g11

0.42 0.45 0.40 0.40

9890.03 13774.51 12323.62 13557.8

19974.4 17748.99 23240.78 23412.9

In the entire range of overall x1 In VLLE equilibrium region In the entire range of overall x1 In VLLE equilibrium region

The results correlated are tabulated in Tables 3 and 6. It can be seen from Table 6 that the precise values of interaction energy parameters ˛, g12 − g11, and g21 − g22 for NRTL model were found to be 0.45, 13774.51, and 17748.99, respectively. The correlation of the VLLE data using NRTL model can give satisfactory results with SDR (y1 ) of 0.0007 and 0.001, SDR (t) of 1.3 and 0.8 ◦ C corresponding to vapour-aqueous phase equilibrium and vapourorganic phase equilibrium, respectively. It can be seen from Table 3 that, the correlated activity coefficients of the component acted as org aq solvent, 1,cal and 2,cal , are in general, to be almost 1.0, respectively; while the correlated activity coefficients of the component aq org acted as solute, 1,cal and 2,cal , are very high, to be 842 and 106,

Fig. 8. (a–c) Predicted and experimentally obtained vapour-phase compositions for trichloroethylene(1)–water(2) system. The dashed line corresponds to two-liquidphase region, so the predicted values by the dashed line have no physical significance there.

respectively. That is, these correlated activity coefficients are very close to those of experimental ones. The method for correlation on VLE and VLLE data from Tables 2 and 4 for trichloroethylene–water system are similar to that of above chloroform–water system, and similar results are obtained, as shown in Tables 2, 4 and 6, respectively, and the predicted y1 is plotted in Fig. 8. 4. Conclusions Fig. 7. (a–c) Predicted and experimentally obtained vapour-phase compositions for chloroform(1)–water(2) system. The dashed line corresponds to two-liquid-phase region, so the predicted values by the dashed line have no physical significance there.

It is feasible to use the new dynamic condensate-circulation still designed for determination of the data on isobaric vapour–liquid

H.-S. Hu / Fluid Phase Equilibria 289 (2010) 80–89

equilibrium (VLE) and vapour–liquid–liquid equilibrium (VLLE) for chloroform–water and trichloroethylene–water binary partial miscible systems. For the two binary systems, on each T–x1 (y1 ) phase diagram there exists a lowest-temperature point (azeotropic point) in the dew-point curve, this implies these systems all exhibit marked non-ideal property and strongly immiscibility. Moreover, on each T–x1 (y1 ) phase diagram the azeotropic-point curve is a perfectly horizontal straight line, this implies the boiling point texp is constant in the three-phase (VLL)-coexistence region, in addition, the VLLE data for each binary system showed that the vapour comaq org positions y1,exp and the two liquid-phase composition, x1 and x1 , also remain almost constant although the overall liquid composition x1 may change. The activity coefficients of the component acted as solvent are in general, while the activity coefficients of the component acted as solute are very high. Both correlation of the VLE data in the entire range of x1 and correlation of the VLLE data with the NRTL activity coefficient model gave satisfactory results. List of symbols F objective function H normalization function Gij expression in NRTL equation gij energy parameter (J mol−1 ) interaction between component i and j in NRTL equation k number of data m total number of the data P pressure (kPa) R universal gas constant (=8.314 J mol−1 K−1 ) T temperature in Kelvin (K) t temperature in degrees Celsius (◦ C) total mole fraction of liquid phases component i x1 aq xi mole fraction of liquid phase component i in aqueous phase org mole fraction of liquid phase component i in organic xi phase yi mole fraction of vapour-phase component i SDR standard deviation Subscripts calc calculated data from NRTL equation exp experimental data i component i n number of times of iteration Superscripts s saturated property aq aqueous liquid phase org organic liquid phase Greek letters ˛ non-randomness parameter in NRTL equation i activity coefficient of liquid-phase component i aq i activity coefficient of liquid-phase component i in aqueous phase org activity coefficient of liquid-phase component i in organic i phase

 ij i aq ϕi org ϕi aq  org 

89

parameter in NRTL equation density of pure liquid component i volume fraction of component i in aqueous phase volume fraction of component i in organic phase density of aqueous phase density of organic phase difference value

Acknowledgments The author is indebted to the reviewers and professor Peter T. Cummings editor for good advices in revision of this manuscript. References [1] H.-S. Hu, M.-D. Yang, J. Dang, Huan Jing Ke Xue Yan Jiu 17 (4) (2004) 40–43. [2] D.T. Leighton, J.M. Calo, J. Chem. Eng. Data 26 (4) (1981) 382–385. [3] L.H. Turner, Y.C. Chiew, R.C. Ahlert, D.S. Kosson, AIChE J. 42 (6) (1996) 1772–1788. [4] D. Zudkevitch, Chem. Eng. Commun. 116 (1992) 41–65. [5] P.O. Haddad, W.C. Edmister, J. Chem. Eng. Data 17 (3) (1972) 275–278. [6] J.D. Raal, R.K. Code, D.A. Best, J. Chem. Eng. Data 17 (2) (1972) 211–216. [7] H.-W. Yang, X.-E. Hu, Shi You Hua Gong 32 (1) (2003) 60–61. [8] S. Scaccia, Talanta 67 (4) (2005) 676–681. [9] G.-Q. Liu, L.-X. Ma, J. Liu, Hua Xue Hua Gong Wu Xing Shu Ju Shou Ce You Ji Jian (Handbook of Chemistry and Chemical Engineering Physical Property Data Organics Volume), Chemical Industry Press, 2002, pp. 117. [10] G.-Q. Liu, L.-X. Ma, J. Liu, Hua Xue Hua Gong Wu Xing Shu Ju Shou Ce Wu Ji Jian (Handbook of Chemistry and Chemical Engineering Physical Property Data Inorganics Volume), Chemical Industry Press, 2002, pp. 3. [11] C.F. Spencer, R.P. Danner, J. Chem. Eng. Data 17 (2) (1972) 236–241. [12] Y.-C. Wang, K.-C. Ling, J. Shen, Mei Zhuan Hua 30 (2) (2007) 77–80. [13] S.M. Walas, Phase Equilibria in Chemical and Engineering, Butterworth Publisher, 1985, pp. 380–385. [14] Y. Yuan, Hua Xue Gong Cheng Shi Shou Ce (Handbook of Chemical Engineers), Chinese Mechanical Press, Beijing, 2000, pp. 39–63. [15] E.B. Svetlanov, S.M. Velichko, M.I. Levinskii, Y.A. Treger, R.M. Flid, Russ. J. Phys. Chem. 45 (4) (1971) 488. [16] A.J. Staverman, Rec. Trav. Chim. Pays-Bas, 60 (1941) 836 (as cited in L.H. Turner, et al., AIChE J. 42 (6) (1996) 1772–1788). [17] K. Schoene, J. Steinhauses, Fresen. Z. Anal. Chem. 321 (1985) 538 (as cited in L.H. Turner, et al., AIChE J. 42 (6) (1996) 1772–1788). [18] W.B. Neely, National Conference on Control of Hazardous Material Spills, New Orleans, 1976 (as cited in L.H. Turner, et al., AIChE J. 42 (6) (1996) 1772–1788). [19] R.E. Kirk, D.F. Othmer, Encyclopedia of Chemical Technology, 2nd ed., vol. 5, Wiley, pp. 183. [20] J.J. Van Laar, Z. Phys. Chem. 72 (1910) 723 (as cited in J. Gmehling, U. Onken, Vapour–Liquid Equilibrium Data Collection Aqueous-Organic Systems(S1) Chemistry Data Series, vol. 1, Patr1a Published by DECHEMA, 1977, pp. XXVI). [21] M. Margules, S.-B. Akad, Wiss. Wien, Math. –Maturwiss. KI.II. 104 (1895) 1234 (as cited in J. Gmehling, U. Onken, Vapour–Liquid Equilibrium Data Collection Aqueous-Organic Systems(S1) Chemistry Data Series, vol. 1, Patr1a Published by DECHEMA, 1977, pp. XXVI). [22] H. Renon, J.M. Prausnitz, AIChE J. 14 (1968) 135. [23] C.-G. Zhou, Y. Liao, You Hua Fang Fa Ji Qi Cheng Xv She Ji (Optimization Method and Procedures Design), Chinese Railroad Press, 1989, pp. 168–172. Hu-Sheng Hu, senior engineer, was born in 1965 in China. He held bachelor’s degree from the Science and Engineering University of Kunming in 1988, and held master’s degree from the Institute of Process and Engineering of Chinese Academy of Science in 1997. In the same year, he entered Tsinghua University and began to research the treatment of waste water and volatile organic compound, explored the extraction, distillation and adsorption new separation technology. So far, he has published about 20 research papers on the national and international journals.