water azeotropic mixture by extractive distillation

water azeotropic mixture by extractive distillation

Separation and Purification Technology 122 (2014) 73–77 Contents lists available at ScienceDirect Separation and Purification Technology journal homep...

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Separation and Purification Technology 122 (2014) 73–77

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Entrainer selection for separating tetrahydrofuran/water azeotropic mixture by extractive distillation Zhi-gang Zhang, Dong-hao Huang, Ming Lv, Peng Jia, De-zhang Sun, Wen-xiu Li ⇑ Liaoning Provincial Key Laboratory of Chemical Separation Technology, Shenyang University of Chemical Technology, Shenyang 110142, China

a r t i c l e

i n f o

Article history: Received 18 May 2013 Received in revised form 30 October 2013 Accepted 31 October 2013 Available online 8 November 2013 Keywords: Extractive distillation Entrainer selection Hydrogen bond VLE

a b s t r a c t The mixture of tetrahydrofuran (THF) and water is a minimum boiling azeotrope due to the strong hydrogen bond effect. Some entrainers with different molecular structures which can potentially disrupt hydrogen bonds have been used to improve the relative volatility of THF–water system. After cautiously screening, we find that dimethyl sulfoxide (DMSO) as a hydrogen bond breaker is more effective to break the azeotrope than 1, 2-propanediol proposed by Songlin Xu etc. To validate our viewpoint, the vapor–liquid equilibrium (VLE) data of the two ternary systems: THF (1) + water (2) + 1, 2-propanediol (3) and THF (1) + water (2) + DMSO (3) were measured at 101.32 kPa. And these data are also valuable for the design of extractive distillation process. The nonrandom two-liquid (NRTL) model was applied to correlate the binary and ternary VLE data. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction THF is extensively used as reagent or solvent in organic chemistry. It can mix well with the vast majority of organic solvents. Unfortunately, THF and water can form a minimum boiling azeotrope at atmospheric pressure, 63.5 °C, containing 83.69% THF in mole fraction [1]. This may be caused by the strong hydrogen bond effects between THF and water. So it is difficult to obtain high purity THF from the mixture by conventional distillation or rectification. To date, many methods have been used to break the azeotrope. For instance, M. Seiler broke the azeotrope successfully by applying hyperbranched polyesters and hyperbranched polyesteramide [2], while Lu Jie etc. did a lot of investigation on various membranes in pervaporation [3,4]. However, these processes have not been put into use widely, and extractive distillation is still a common method to separate this azeotropic mixture [5–8]. A suitable entrainer can reduce the apparatus investment and energy consumption apparently. Thus, selecting an appropriate entrainer is a vital point for the extractive distillation of THF. In the process of research, we find that for this strong polar system, hydrogen bonds play a key role in forming the azeotrope. So the azeotrope can be effectively broken if an entrainer can disrupt hydrogen bonds between THF and water. Therefore, the relative volatility between the components will increase obviously. According to this perspective, a series of entrainers are selected including ⇑ Corresponding author. Tel.: +86 02489383736. E-mail address: [email protected] (W.-x. Li). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.10.051

glycols, amides, sulfocompound etc. which potentially disrupt hydrogen bonds and increase the relative volatility of the mixture. Their selectivities are investigated respectively. Then the effects of the entrainers with different molecular structures on the hydrogen bonds are explored in detail. After cautiously screening, more effective entrainer dimethyl sulfoxide (DMSO) is selected. Hydrogen bond effect widely exists in strong polar azeotropic system, so our analysis will give some enlightenment on entrainer selection of other strong polar azeotropic system similar with THF–water system. 1, 2-propanediol had been proposed as an entrainer for separating the mixture by Xu and Wang [5]. Their results were obtained via a HYSYS simulation but the relevant VLE data did not provided. In this paper, we explored the separation effects of 1, 2-propanediol and DMSO seriously by investigating the phase equilibrium behaviors of the two ternary systems: THF (1) + water (2) + 1, 2propanediol (3); THF (1) + water (2) + DMSO (3) and measuring the VLE data at 101.32 kPa since these data are also pivotal for design of extractive distillation process. 2. Materials and methods 2.1. Chemicals THF, DMSO, 1, 2-propanediol, dimethylacetamide (DMAC) and dimethyl formamide (DMF) are supplied by Sinopharm Group CO. Ltd. The purities of all reagents are confirmed to be analytical grade by chromatography. The water used in experiments is deionized water which is prepared by our laboratory.

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2.2. Apparatus and procedure We employed equilibrium still (manufactured by NGW, Wertheim, Germany) described by Hunsmann [9] to implement vapor–liquid equilibrium tests. The capacity of the still is about 100 mL and it is equipped with a reflux condenser. A proper amount of ternary mixture was injected in the still, and heated until the whole system reached equilibrium. This process usually lasted about 1 h. Then samples from the top and bottom of the still were taken to analyze the contents. We applied a quartz thermometer to measure the equilibrium temperature and a manostat to regulate the system pressure. The contents of the samples were analyzed by an Agilent 7890A gas chromatograph (GC). We adopted the thermal conductivity cell detector, a 30 m, 0.320 mm i.d., 0.25 lm, capillary column and programmed heating-up to obtain perfect peak of each component. We set oven start temperature as 372.15 K, maintained 3.5 min, then increased by 50 K/min to 473.15 K and stay 7 min. Moreover, we set injector and detector temperatures as 493.15 K and 523.15 K respectively.

Fig.1. Selectivities of different entrainers varying entrainer mole fraction d, DMF; j, DMAC; N, 1, 2-propanediol; ., DMSO.

3. Results and discussion

bond effect between THF and water will be decreased when following conditions emerge: (1) The entrainer can only form hydrogen bonds with one of the azeotropic components; (2) The entrainer forms much stronger hydrogen bonds with one component than the other. By this way the relative volatility between THF and water can be sufficiently increased thanks to the addition of entrainer and the separation becomes feasible. Based on this view, a series of potential entrainers were selected to implement vapor–liquid equilibrium tests and the basic information of the entrainers was listed in Table 1. Each experiment was repeated three times to eliminate contingency. The selectivity of each entrainer with different entrainer contents was figured out and shown in Fig. 1. Obviously, sorting the entrainers by selectivity

3.1. Entrainer selection Entrainer selection is the key point for the extractive distillation technique. Selectivity is the main evaluation index of separation effect, which is defined as

Si;j ¼ ai;js =ai;j

ð1Þ

where ai,j, and ai,js are the relative volatilities of the two key components before and after the entrainer added. We can suppose the vapor as an ideal behavior at atmospheric pressure. So, VLE equation [10] can be simply rewritten as

pyi ¼ ci xi psat i

ð2Þ

where xi and yi are the mole fractions of component i in the vapor and liquid phase respectively containing entrainer; ci represents the activity coefficient of component i; p is the total pressure of the equilibrium system; psat is the saturated vapor pressure of pure i component i at system temperature. We can approximate ai,j at 1 sat for the system is binary azeotrope. In addition, psat is a constant, i =pj therefore

Si;js ¼ ai;js ¼

Y i =X i ci psat i ¼ Y j =X j cj psat j

Table 2 VLE data for THF (1) + water (2) system at 101.3 kPa.

ð3Þ

The mole fractions Xi, Yi are on entrainer-free basis, which can be obtained through phase equilibrium experiments. Activity coefficient ci can be correlated using the thermodynamic model [11]. In this paper, we use NRTL to correlate VLE data since it gives better agreement with the experimental results. As mentioned above, strong hydrogen bond effect between THF and water causes the azeotrope. But we find that the hydrogen

T/K

x1

Y exp 1

Y cal 1

REy1

366.76 357.14 347.65 343.15 340.65 339.45 338.95 338.70 338.19 338.14 338.16 338.15 338.05 338.15 339.26

0.003 0.008 0.024 0.050 0.066 0.090 0.160 0.237 0.341 0.464 0.680 0.790 0.851 0.913 0.958

0.257 0.480 0.660 0.733 0.750 0.787 0.800 0.809 0.827 0.857 0.857 0.869 0.891 0.911 0.941

0.245 0.467 0.654 0.726 0.755 0.781 0.802 0.810 0.820 0.824 0.844 0.861 0.879 0.915 0.946

4.707 2.726 0.937 0.989 0.704 0.781 0.235 0.074 0.841 3.860 1.564 0.938 1.349 0.393 0.475

exp The relative error of y1 : REY 1 ¼ jY exp  Y cal  100. 1 j=Y 1 1

Table 1 Basic information of selected entrainers [12]. Entrainers

DMSO

1,2-Propanediol

DMAC

DMF

Molecular structure Molecular weight Boiling point (101.3 kPa) T/K Densities(298.15 K) q/(g cm3) Dipole moment (303.15 K) l/(103Cm) Evaporation heat Hm/(kJ mol1)

(CH3)2S = O 78.13 462.15 1.0958

HOCH2CH2CH2OH 76.09 460.45 1.0381

CH3CON(CH3)2 87.12 439.25 0.9366

HCON(CH3)2 73.10 426.15 0.9487

13.34

7.51

12.41

12.88

52.92

538.1

52.3

47.545

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Z.-g. Zhang et al. / Separation and Purification Technology 122 (2014) 73–77 Table 3 VLE data for THF (1) + water (2) + 1, 2-propanediol (3) system at 101.3 kPa. x3 = 0.05

x3 = 0.10

T/K

X1

Y exp 1

368.65 361.85 352.65 346.65 343.65 341.99 340.65 340.65 339.33 339.65 338.77 338.60 339.35 339.65 340.65

0.003 0.007 0.024 0.039 0.060 0.078 0.131 0.183 0.331 0.406 0.554 0.666 0.725 0.849 0.946

0.258 0.440 0.651 0.758 0.799 0.810 0.827 0.830 0.856 0.849 0.859 0.876 0.882 0.898 0.950

Y cal 1

REY1

T/K

0.246 0.455 0.646 0.751 0.801 0.808 0.836 0.835 0.850 0.854 0.867 0.874 0.877 0.901 0.951

4.654 3.407 0.768 0.924 0.250 0.222 1.094 0.605 0.701 0.589 0.931 0.228 0.567 0.324 0.131

368.65 361.85 352.65 346.65 343.65 341.99 340.65 340.65 339.33 339.65 338.77 338.60 339.35 339.65 340.65

x3 = 0.20 X1

Y exp 1

Y cal 1

REY1

T/K

X1

Y exp 1

Y cal 1

REY1

0.005 0.007 0.024 0.039 0.060 0.078 0.131 0.183 0.331 0.406 0.554 0.666 0.725 0.849 0.946

0.258 0.440 0.651 0.758 0.799 0.810 0.827 0.830 0.856 0.849 0.859 0.876 0.882 0.898 0.950

0.246 0.455 0.646 0.751 0.801 0.808 0.836 0.835 0.850 0.854 0.867 0.874 0.877 0.901 0.951

4.654 3.407 0.768 0.924 0.250 0.222 1.094 0.605 0.701 0.589 0.931 0.228 0.567 0.324 0.131

373.44 367.18 358.10 346.35 343.15 342.68 342.00 342.15 341.95 341.70 341.65 341.65 341.45 342.15 343.05

0.003 0.005 0.010 0.040 0.112 0.182 0.253 0.366 0.455 0.566 0.630 0.707 0.802 0.891 0.941

0.130 0.355 0.602 0.819 0.852 0.868 0.870 0.879 0.877 0.878 0.884 0.895 0.899 0.939 0.962

0.130 0.350 0.589 0.826 0.858 0.870 0.870 0.873 0.876 0.879 0.885 0.892 0.909 0.941 0.966

0.460 1.409 2.054 0.788 0.677 0.172 0.023 0.640 0.090 0.188 0.079 0.342 1.112 0.187 0.478

Mole fraction on entrainer-free basis X1, Y1: X1 = x1/(x1 + x2); Y1 = y1/(y1 + y2). exp  Y cal  100. The relative deviation of Y 1 : REY 1 ¼ jY exp 1 j=Y 1 1

(descending order) we obtained: DMSO, 1, 2-propanediol, DMAC and DMF. DMSO is a typical solvent of proton receptor. In its particular molecular structure, S atom and O atom are connected by dpp bond which is different from general carbon oxygen double bond essentially. The electronegativity of O atom is so strong that the shared pairs of electrons are deviated to O atom seriously. Therefore, DMSO can strongly associate with water through firm hydrogen bonds, but negatively combine with THF since THF is also a proton receptor. Whereas 1, 2-propanediol possesses the capacity of accepting and providing protons simultaneously due to its two hydroxyl. So 1, 2-propanediol can associate both water and THF with hydrogen bonds, which weakens its selectivity. DMF and DMAC belong to amides. They both contain CH3 which can provide electron cloud to O atom. In addition, the lone pair electron of N atom can conjugate with O atom, so the O atom shows a

stronger electronegativity than one of ordinary carbonyl. As mentioned above, DMF and DMAC also perform positively on the separation. 3.2. Vapor–liquid equilibrium In this section, three series of VLE experiments varying mole fractions of entrainers were carried out to explore the effects of 1, 2-propanediol and DMSO on the phase equilibrium of THF– water system. By contrast, we also provided VLE data of THF (1) + water (2) system in Table 2. And the VLE data of ternary systems THF (1) + water (2) + 1, 2-propanediol (3), THF (1) + water (2)+DMSO (3) were listed in Tables 3 and 4, respectively. We applied NRTL model to correlate the data of the systems. The relative error between experimental and calculated value was defined and listed in each corresponding table. Binary interaction parameters

Table 4 VLE data for THF (1) + water (2)+DMSO (3) system at 101.3 kPa. x3 = 0.05

x3 = 0.10

x3 = 0.20

T/K

X1

Y exp 1

Y cal 1

REY1

T/K

X1

Y exp 1

Y cal 1

REY1

T/K

X1

Y exp 1

Y cal 1

REY1

366.12 353.08 345.48 341.76 338.03 337.73 337.65 337.67 337.69 337.71 337.72 337.93 338.15 338.67 339.15

0.004 0.011 0.026 0.057 0.127 0.198 0.268 0.360 0.513 0.598 0.710 0.802 0.872 0.914 0.954

0.242 0.562 0.673 0.750 0.821 0.832 0.835 0.839 0.841 0.853 0.856 0.869 0.890 0.915 0.942

0.243 0.561 0.685 0.748 0.826 0.831 0.836 0.841 0.845 0.851 0.855 0.870 0.891 0.915 0.943

0.248 0.181 1.783 0.272 0.720 0.076 0.081 0.238 0.486 0.234 0.133 0.115 0.067 0.013 0.034

369.65 363.32 353.58 346.02 342.05 339.02 339.18 339.25 339.33 339.39 339.43 339.44 339.65 339.91 341.11

0.001 0.011 0.022 0.049 0.065 0.120 0.205 0.291 0.371 0.490 0.599 0.679 0.727 0.832 0.924

0.151 0.333 0.590 0.706 0.768 0.850 0.871 0.871 0.871 0.875 0.886 0.887 0.886 0.910 0.954

0.161 0.344 0.576 0.697 0.770 0.851 0.870 0.870 0.873 0.876 0.876 0.880 0.883 0.900 0.946

6.490 3.401 2.479 1.278 0.221 0.139 0.080 0.106 0.260 0.105 1.034 0.756 0.327 1.021 0.840

368.96 351.28 342.85 341.07 340.88 340.74 340.76 340.66 340.62 340.88 340.96 341.65 342.03 342.65 343.74

0.002 0.015 0.045 0.087 0.129 0.215 0.327 0.443 0.557 0.653 0.740 0.782 0.833 0.888 0.936

0.190 0.643 0.786 0.853 0.876 0.897 0.904 0.906 0.908 0.911 0.915 0.934 0.939 0.960 0.981

0.190 0.644 0.785 0.864 0.877 0.897 0.904 0.906 0.907 0.911 0.926 0.934 0.946 0.964 0.980

0.105 0.202 0.117 1.304 0.106 0.033 0.011 0.055 0.022 0.033 1.184 0.012 0.746 0.349 0.112

Mole fraction on entrainer-free basis X1, Y1: X1 = x1/(x1 + x2); Y1 = y1/(y1 + y2). exp The relative deviation of Y 1 : REY 1 ¼ jY exp  Y cal  100. 1 j=Y 1 1

Table 5 Regressed binary interaction parameters using NRTL model. Component i

Component j

Aij

Aji

Bij/K

Bji/K

aij

THF (1) THF (1) Water (2) THF (1) Water (2)

Water (2) DMSO (3) DMSO (3) 1, 2-propanediol (3) 1, 2-propanediol (3)

1.274 3.8117E5 1.2449 2.454 0

4.919 7.6568E6 1.7524 3.817 0

157.781 347.549 586.801 648.458 300.624

733.402 73.937 1130.215 1457.245 467.900

0.473 0.300 0.300 0.300 0.300

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listed in Table 5 were regressed by the famous simulation program, Aspen Plus. According to Tables 2–4, we obtained X–Y diagrams of the two ternary systems shown in Figs. 2 and 3. DMSO and 1, 2-propanediol are both able to break the azeotrope at a specific entrainer content. The azeotropic point disappears when the system just contains 10% DMSO. Contrastively, to achieve similar effect, about 20% 1, 2-propanediol must be added. It suggests that as an extractive distillation entrainer DMSO is more effective than 1, 2-propanediol for the THF–water system. The influence of the 1, 2-propanediol and DMSO on relative volatility of THF to water was illustrated by Figs. 4 and 5. The relative volatilities were calculated with Eq. (3). As shown in Figs. 4 and 5, both of the two entrainers increase the relative volatility. The more entrainer is added, the higher relative volatility is got. Eventually, this makes the relative volatility between THF and water far surpass 1, especially close to the azeotropic point and then the expected separation process turns into possible. This indicates that DMSO presents a more remarkable solvent effect than 1, 2-propanediol under identical condition.

Fig. 4. Relative volatility of THF (1) to water (2) with different contents of 1, 2propanediol: s, without entrainer; j, x3 = 0.05; N, x3 = 0.10; ., x3 = 0.20; solid lines, correlated using NRTL model.

Fig. 2. X–Y diagram for THF (1) + water (2) + 1, 2-propanediol (3) system: s, without entrainer; j, x3 = 0.05; N, x3 = 0.10; ., x3 = 0.20; solid lines, correlated using NRTL model.

Fig. 5. Relative volatility of THF (1) to water (2) with different contents of DMSO: s, without entrainer; j, x3 = 0.05; N, x3 = 0.10; ., x3 = 0.20; solid lines, correlated using NRTL model.

Fig. 3. XY diagram for THF (1) + water (2) + DMSO (3) system: s, without entrainer; j, x3 = 0.05; N, x3 = 0.10; ., x3 = 0.20; solid lines, correlated using NRTL model.

Fig. 6. TX,Y diagram for THF(1) + water (2) + 1,2-propanediol (3) system with different contents of 1,2-propanediol: d, X1 (without entrainer); s,Y1(without entrainer); j, X1 (x3 = 0.05); h, Y1 (x3 = 0.05); N, X1 (x3 = 0.10); 4, Y1 (x3 = 0.10); ., X1 (x3 = 0.20); r, Y1 (x3 = 0.20); solid lines, correlated using NRTL model.

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systems similar with THF–water which form azeotropes due to strong hydrogen bond association. We can infer that for these mixtures, hydrogen bond breakers can be good choices to serve as entrainers in extractive distillation process. This conclusion will be beneficial for entrainer selection of strong polar system. VLE data of the binary system THF (1) + water (2) and ternary systems THF (1) + water (2)+1, 2-propanediol (3), THF (1) + water (2) + DMSO (3) measured at 101.32 kPa can be applied in designing extractive distillation process. Acknowledgements This work is financially supported by the National Science Foundation of China (Project No. 21076126) and Program for Liaoning Excellent Talents in University (LR2012013). References Fig. 7. T-X,Y diagram for THF(1) + water (2) + DMSO (3) system with different contents of DMSO: d, X1 (without entrainer); s, Y1(without entrainer); j, X1 (x3 = 0.05); h, Y1 (x3 = 0.05); N, X1 (x3 = 0.10); 4, Y1 (x3 = 0.10); ., X1 (x3 = 0.20); r, Y1 (x3 = 0.20); solid lines, correlated using NRTL model.

T–X, Y diagrams for ternary systems with different contents of entrainers were presented in Figs. 6 and 7. From the figures, it can be seen that the addition of entrainer increases phase equilibrium temperature and higher fraction of entrainer produces higher equilibrium temperature. That means, the addition of entrainer increases the heating load of reboiler and increases the energy consumption. The equilibrium temperatures are very nearly equal under same entrainer contents because the boiling point of DMSO is close to 1, 2-propanediol. However, achieving approximate solvent effect, less DMSO is needed. So if DMSO is applied in the extractive distillation process, a large amount of energy can be saved. 4. Conclusions After this investigation, we confirm that DMSO as a hydrogen bond breaker is more capable than 1, 2-propanediol in breaking THF–water binary azeotrope. Thus DMSO can be served as a more effective entrainer in extractive distillation process to separate THF–water mixture. In addition, we often meet some other

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