Isothermal liquid-liquid equilibrium data at 313.15 K and isobaric vapor-liquid-liquid equilibrium data at 101.3 kPafor the ternary system water –1-butanol –p-xylene.
Vicente Gomis[*], Alicia Font, María Dolores Saquete, Jorge García-Cano
University of Alicante. PO Box 99 E-03080 Alicante (Spain)
Tel. +34 965903400.
Water, 1-butanol, p-xylene, liquid-liquid equilibrium, vapor-liquid-liquid equilibrium, experimental data.
Abstract
The vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibria of the ternary system water – 1-butanol –p-xylene have beendetermined.Water – 1-butanol – p-xylene is atype 2 heterogeneous ternary system withpartially miscible water – 1-butanol and water – p-xylene pairs. By contrast, 1-butanol –p-xylene is totally miscible under atmospheric conditions. This paper examines the vapor-liquid equilibrium in both heterogeneous and homogeneous regions at 101.3kPa of pressure. Liquid-liquid equilibrium data at 313.15Khave also been determined, and for comparison, the obtained experimental datahave been calculated by means of several thermodynamic models: UNIQUAC, UNIFAC and NRTL. Some discrepancies were found between the vapor-liquid-liquid correlations;however, the models reproduced the liquid-liquid equilibrium data well. The obtained data reveal a ternary heterogeneous azeotropewithmole fraction composition:0.686 water, 0.1461-butanol and 0.168 p-xylene.
1.Introduction
Ever since the economic crisis of 1973, due to a sharp rise in the price of oil, and especially during the late 80s, concerns regarding the lack of energy supplies and the effects of man’s use thereofon the environment have taken on greater importance. Various factors, including a relianceon oil producing regions, the inexorable drop in world oilreserves, the rise in carbon dioxide (CO2) air concentration – resulting in an increased global surface temperature (greenhouse effect) – have pushed developed countries toward the goal of reducingtheir fossil fuel consumption.
To achieve this goal, and because economic development is energy intensive, the only viable alternative appears to be to substitute part of the fossil fuel consumed by other energy sources. In several sectors, such as electricity production, fossil fuels have been substituted by nuclear energy on the one hand, but also by renewable energies such as solar farms or wind generators on the other. However,the transport sector is one of the most dependent on fossil fuelsand it is important to note that approximately 33 % of total CO2 emitted to the environment by human activities in 2012 [1]came from this sector.
Thus,initiatives to reduce polluting emissions in the transport sectorareof key importance ifgovernmentsare to fulfill their promises[2] regarding the control of CO2 emissions.
Over the short term, substitution of oil by a renewable fuel seems the only viable alternative to achieving the emissions rate goal.
One of the most widely used renewable fuelsover the last few years has been ethanol, produced from crop fermentation.Employing ethanol as a fuelis associated with several problems: it has corrosive properties, making it difficult to transport through pipelines;it can damage engines; it is easily hydratedby moisture in the air, which means it has to be handled with care in order to avoid hydration;andit has a lower energy content than gasoline. Finally,thecost of producing it(including agricultural, fermentation and subsequent purification processes) makes it difficult to compete with petrol as a fuel.
An alternative to bioethanol in recent years has been biobutanol. This is because butanol has more desirable properties as a fuel than ethanol (its energy content is 86% of gasoline’s,versus only 67% in the case ofthe latter) and, in addition,it is not associated withthe same problems (as highlighted in the previous paragraph).
To use biobutanol as a fuel, it must be separated from the other substances present during its production, especially water. Several processes, which are more or less energy consuming,can be used for the purpose of purifying biobutanol. If it isintended to be usedas a gasoline component, a possible technique to accomplish this is azeotropic distillation, using gasoline components as entrainers– to obtain a blend of butanol and gasoline that is very low in water content.
To follow previous studies [3,4] on the viability of using hydrocarbons as entrainers in the dehydratation of butanol, it would be useful to obtain experimental data on the vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibria for another hydrocarbon, such as p-xylene.The purpose of this would be to determine the ternary system water – 1-butanol – p-xylene, if p-xylene werethe hydrocarbon to be used as entrainer in a separation step.
Water – 1-butanol – p-xylene is atype 2 heterogeneous ternary systemwith partially miscible water –1-butanol and water – p-xylene pairs. However, the pair 1-butanol –p-xylene is totally miscible under atmospheric conditions. Thepresent paper is concerned withthe determination of the vapor-liquid equilibriain both heterogeneous and homogeneous regions at 101.3kPa,and the liquid-liquid equilibrium data at 313.15K.
2.Experimental
2.1. Chemicals
Ultrapure water, prepared using a MiliQPlus system, was employed inexperiments The rest of the chemicals were used as supplied.. 1-Butanol was provided by Merck at a chemical purity higher than 99.5%. p-Xylene was provided by Merck at a chemical purity higher than 99%. The internal standard 2-propanol was provided by Merck at a purity of more than 99.8 %. The moisture content of all compounds was measured by the Karl Fisher titration technique and was found tobe 640 ppm, 290 ppm and530 ppm for 1-butanol, p-xylene and 2-propanol respectively.
2.2. Experimental procedure
2.2.1. Liquid-liquid equilibrium determination
The experimental procedure that was followed to obtain the liquid-liquid equilibrium data can be found in a previous paper [3]. However, for the sake of convenience, the most important aspects of it are reproduced here:
Mixtures of water, 1-butanol and p-xyleneof known mass were put inside glass tubesand sealed with septum caps. These tubes were then introduced in a thermostatic bath to maintain their temperature constant at 313.15K. Aftershaking them, their contents were allowed to settle until two phases separated out. Samples from both phases were extracted from the tubes and placed in vials along with 2-propanol, which served as internal standard.
These vials were analyzed in a gas chromatograph equipped with a thermal conductivity detector (TCD).
The chromatograph was a Shimadzu GC14B equipped with a 2m x 3mm Porapack Q 80/100 packed column, running the Shimadzu CLASSVP Chromatography Data System. The temperature of the column was493.15K,while it was513.15K in the injector and the TCD.The helium flow rate was 50 ml/min.
The organic-phase water content was verified by a Karl Fisher titration.The aqueous-phase organics content was also determined by gas chromatography,but this time usinga Flame Ionization Detector (FID). The chromatograph in question was a Thermo Trace gas chromatograph by Thermo Fischer equipped with a DB624 column. The helium flow was set to 50 mL/min at a split-less ratio of 1:50. The temperatures of analysis were 523.15K in the injector and 573.15K in the detector, while the column temperature was rampedfrom 313.15K up to 473.15Kin increments of 40 K /min and held constant for one minuteafter ramping.
2.2.2. Vapor-liquid-liquid determination.
This procedure is explained in detail in a previous publication [3]. In a modified Fisher instrument for vapor-liquid equilibrium determinations, coupled to an ultrasound homogenizer that permits thorough mixing between heterogeneous phases, ternary mixtures were introduced and heated up to the boiling point while the pressure was measured and held fixed at 101.3kPa. The vapor was returned to the boiling chamber after condensing,whilea fraction thereof waspumped offby means of a six-port valve to a chromatographfor analysis. The liquid phases were separated from the vapor and returned to the boiling chamber by a differentconduit than the vapor. Liquid phase samples could be collected from the liquid returning to the boiling chamberby means of a solenoid valve.
After equilibrium had been reached, samples were taken from both the vapor and the liquid. The vapor wasanalyzed by gas chromatography in a TCD detector. The liquid sample was put in a hermetic tube, sealed with a septum cap. This sample was subjected to the same procedure as used during the liquid-liquid equilibrium determination; however,this time the thermostatic bath was maintained at the boiling temperature of each sample. The organic and aqueous phaseswere placed in separate vials together with the 2-propanol internal standard, and analyzed in the same manner as in the liquid-liquid determination. The conditions of the chromatographic analysis were also the same as in the case of the liquid-liquid equilibrium determination.
In the homogeneous region, there is one liquid and one vapor phase. The liquid phase sample, once it was taken out from the system, was put inside a vial with a known amount of 2-propanol as internal standard and was analyzed by gas chromatography with the TCD in the same conditions as the heterogeneous phases.
3. Results
3.1 Liquid-Liquid Equilibrium Results
The results obtained from the liquid-liquid equilibrium experiments at 313.15K are presented in Table 1, andplotted as a ternary phasediagram in Figure 1. For comparison, Figure 1 also includes the liquid-liquid equilibrium data obtained by Letcher et al.[5] at 298.15K. As can be seen, the size of the heterogeneous regionof this system does not vary with rising temperature in the range of temperatures studied.
The obtained data has been correlated by means oftwo thermodynamic models: UNIQUAC and NRTL.The software CHEMCAD was used to perform the correlations [6]. In the case of the NRTL model, the alpha parameter was fixed at 0.2. The binary interaction parameters calculated by the two models are recorded in Table 2. This table also shows the composition deviationof each of the models. For the purpose of assessing the accuracy of the models in relation to the system in question, the experimental data and those calculatedby the models have been plotted in Figure 2. Figure 2 also shows the equilibrium data predicted by the group contribution model original UNIFAC.
Inspection of Figure 2 shows that NRTL and UNIQUAC accurately reproduce the experimental data. However,the UNIFAC model predicts a heterogeneous region for this system that is not consistent with either experiment or the other two models, since it is too large.
3.2 Vapor-Liquid-Liquid Equilibrium Results
Table 3 showsthe vapor-liquid equilibrium data from the homogeneous region. The vapor-liquid-liquid equilibrium data obtained for the heterogeneous region are recorded in Table 4.The compositions are expressed in mole fractions and the boiling temperature (in Kelvins)of eachmixtureisalso shown. All the data presented so far have been subjected to the Wisniak thermodynamic consistency test [7] and are thermodynamically consistent. The Antoine parameters for this test are shown in Table 5[8,9].
In order to visually inspect the obtained data,several figures have been plotted.
First among these is Figure 3, which shows the liquid mixtures pertaining to the homogeneous region and their corresponding equilibrium vapor phases. It is interesting to note that although the liquid mixtures covermost of the homogeneous region, their respective vapors occur within only a small region of the ternary diagram. Many liquid phases in the homogeneous region have an associatedequilibrium vapor phase with a butanol composition rangingbetween 0.18 and 0.56 mole fractions, and between 0.1 and 0.4 mole fractionsin p-xylene. As a consequence, if ahomogeneous liquid mixturewere to undergo a distillation process the condensed vapor would probably be a two phase mixture.
Additionally, Figure 4shows a plot of the vapor–organic and –aqueous equilibria listed in Table 4. The straight lines connect several equilibrium organic and aqueous phase pairs. The dashedlines, in turn, connect these liquid phases with their associated vapor phase. Figure4 has several notable features. As was seen in the case of the liquid-liquid equilibrium, this type 2 system likewisepossesses an aqueous phase whose composition does not vary very much relative to its respective organic phase. As for the vapor region for heterogeneous liquids – in this case the vapor line lies within a narrow composition range. In fact, themaximum concentration of p-xylene that can be read off the vapor line is 0.25,and in the case of 1-butanol rangesbetween 0 and 0.25. The shape of the equilibrium triangles,as well as the variation in boiling temperature, arouses the suspicion that this system possesses a ternary heterogeneous azeotrope. It has not been possible to find any bibliographic reference regarding the existence of the ternary water – 1-butanol – p-xylene. While it has also not been possible to determine the azeotrope’s composition experimentally, it has been possible to predict it by means of interpolation of the experimental data. Thiscalculated ternary azeotrope has the following composition in mole fraction: 0.686, 0.146 and 0.168 for water, 1-butanol and p-xylene respectively. The corresponding liquid phases in equilibrium with the azeotrope have also been calculated by interpolation of the experimental data, with 0.152, 0.385 and 0.463, and 0.989, 0.011 and 0.0001 mole fractions of water, 1-butanol and p-xylene in the organic and aqueous phases respectively.
The present system can be compared with another comprised of similar compounds: water -1-butanol – hydrocarbon. Figure 5makes this comparison by showing the vapor-liquid-liquid equilibrium of the system water -1-butanol – cyclohexane, obtained previously [4].It is evident thatthe vapor line in the cyclohexane system is longer than in the system containing p-xylene. The ternary azeotrope also has a much lower 1-butanol concentration in the case of cyclohexane.
As was done for the liquid-liquid equilibrium, several correlations were performed to test the accuracy ofthermodynamic models in predicting the vapor-liquid-liquid equilibrium of this system. The same models were again used here. The data employedto carry out these correlations were the homogeneous and heterogeneous experimental data presented in this paper, the binary water – 1-butanol vapor-liquid equilibrium data from DECHEMA[9]and the binary azeotropes of the pairs water – 1-butanol, water – p-xylene and 1-butanol – p-xylene from reference[10]. The parameters and the standard deviations in composition and boiling temperature obtained from the correlationsusing UNIQUAC and NRTLare presented in Table 6.
To assess the adequacy of these models, the experimental vapor and organic phases and those obtained through interpolation by UNIQUAC and NRTL,and prediction by UNIFAC,are plotted in Figure 6. Since it is very little changed, the aqueous phase has notbeen plotted. The vapor line predictedby all these models resembles the experimental one. However,when the organic phases are compared, greater discrepancies become apparent:none of the models accurately reproduce the behavior of the organic phase, nor the azeotropic compositions.Concretelythe discrepancies can be considered noteworthy for increasing ratios of 1-butanol and water. The bigger differences between experimental and calculated data appear near the binary water – 1-butanol pair.
4. Conclusions
The liquid-liquid equilibrium of the water – 1-butanol – p-xylene systemhas been determined at 313.15K. Contrary to the data and findings of other authors, the present authors have found this equilibrium to be little affected by temperature. In addition, the vapor-liquid equilibriaof both the homogeneous and heterogeneous regions have beendetermined experimentally.The observed behavior of these equilibria implies that the vapor in equilibrium with the liquid phases in this system tends to be located in the heterogeneous region. This fact would make heterogeneous azeotropic distillation a suitable candidate for separatingsuch mixtures. The system exhibits a ternary heterogeneous azeotrope.
On the other hand, several thermodynamic models have been tested against the obtained experimental data and while the liquid-liquid equilibrium at 313.15K was rightly predicted by some of those models (NRTL and UNIQUAC), the vapor-liquid-liquid equilibrium exhibited discrepancies between predictions and experimental data. It would be, in fact, necessary to improve the thermodynamic models if the simulations that use those models as bases to simulate industrial processes want to be more accurate in their predictions and thus more useful in the design step.
Nevertheless the experimental data presented in this paper would fill the gap in the literature regarding this system and might help in the testing of new or modified thermodynamic models.
5. Acknowledgment
The authors thank the DGICYT of Spain for the financial support of project CTQ2009-13770.
References
(1) European Environment Agency (2012).Retrieved March 13, 2013, from
(2)Kyoto Protocol to the United Nations Framework Conventionon Climate Change, United Nations 1998
(3)V. Gomis, A. Font, M.D. Saquete, J. García-Cano.LLE, VLE and VLLE data for the water–n-butanol– n-hexane system at atmospheric pressure.Fluid Phase Equilibria 316 (2012) 135– 140
(4) V. Gomis, A. Font, M.D. Saquete, J. García-Cano.Liquid-liquid, vapor-liquid and vapor-liquid-liquid equilibrium data for the water–n-butanol–cyclohexane system at atmospheric pressure: experimental determination and correlation. J. Chem. Eng. Data, 58, (2013) 3320–3326