Solvent Effects on Vapor−Liquid Equilibria of the Binary System1‑Hexene + n‑HexaneBeatriz Marrufo,† Ben Rigby,‡ Jordi Pla-Franco,‡ and Sonia Loras*,‡
†Departamento de Ingeniería Química Basica, Universidad del Zulia, 4011, Maracaibo, Venezuela‡Departamento de Ingeniería Química, Escuela Tecnica Superior de Ingeniería, Universitat de Valencia, 46100 Burjassot, Valencia,Spain
ABSTRACT: In order to study the separation of 1-hexene and n-hexane, two sol-vents, 2-pentanol and ethyl-butyrate, are tested as possible entrainers for an extractivedistillation. In this way, isobaric vapor−liquid equilibrium (VLE) data at 100 kPahave been measured for the two ternary systems formed by the initial mixture andone of the mentioned solvents: 1-hexene + n-hexane + ethyl butyrate and 1-hexene +n-hexane + 2-pentanol. VLE data for the four constituent binary systems have alsobeen measured. These systems are 1-hexene + ethyl butyrate, n-hexane + ethyl butyrate,1-hexene + 2-pentanol, and finally n-hexane + 2-pentanol. All binary systems showmoderate positive deviations from the ideal behavior and do not form an azeotrope.The well-known local composition models Wilson, UNIQUAC, and NRTL have beenused for correlating VLE data. Prediction with the UNIFAC method has been alsoobtained.
■ INTRODUCTION
Olefins and paraffins have very similar boiling points and are onlyseparated by a few degrees in temperature. When this occurs,conventional distillation is not a recommended technique due itsexpensive cost, as a consequence of the high reflux ratios andlarge number of stages required for carrying out the separa-tion. In these cases, one alternative is extractive distillation, whichuses a third component called an entrainer to alter the relativevolatility of the components to be separated. However, only afew specific solvents allow an efficient and economical extractivedistillation process; thus, entrainer selection is a crucial step dur-ing the design stage. The choice of entrainer can be carried outcorrectly with complete and accurate vapor−liquid equilibrium(VLE) data of the systems formed by the components to beseparated and the possible solvent.The present work tackles a part of thermodynamic research on
the separation of paraffin and olefin mixtures using differentsolvents. In this work, the initial mixture is formed by 1-hexeneand n-hexane, and ethyl butyrate and 2-pentanol are the entrainercandidates due to certain alcohols and esters being recommendedas entrainers for separation of hydrocarbons.1 An ideal entrainerwill increase the relative volatility considerably between the 1-hexaneand hexane and it will be very selective. Another important char-acteristic that makes an entrainer more suitable is that it is one of theso-called “ecofriendly” solvents. In the present paper, both proposedcompounds can be considered green solvents.In this work, we measured isobaric VLE data for the two ternary
systems 1-hexene (1) + n-hexane (2) + ethyl butyrate (3) and 1-hexene (1) + n-hexane (2) + 2-pentanol (3) and four constituentbinary systems 1-hexene (1) + ethyl butyrate (3), n-hexane (2) +ethyl butyrate (3), 1-hexene (1) + 2-pentanol (3), and n-hexane(2) + 2-pentanol (3) at 100 kPa. In a previous paper,2 we reported
VLE data for the binary system 1-hexene (1) + n-hexane (2).Isobaric VLE data of binary hexane + 2-pentanol system wasobtained by Linek et al.3 at different pressures but none wasrecorded at 100 kPa. However, for the other binary systems and forthe ternary systems, no VLE data have been previously published.
■ EXPERIMENTAL SECTIONChemicals. The chemicals 1-hexene (99+ % mass), ethyl
butyrate (100 w≥ 99.5, puriss. p.a.) and 2-pentanol (100w≥ 99)were supplied by Sigma-Aldrich and the chemical n-hexane(99.5+ % mass) was supplied by Fluka. Purity grade of 2-pentanolwas increased to 99.99 % after being treated in a batch multistagerectification process in a Fischer SPALTROHRHMS-500 column.The other reagents were used without further purification sinceimpurities are smaller than the detection limit of the analyticalmethod used. The specifications of the used chemicals are sum-marized in Table 1.
Received: September 3, 2012Accepted: October 25, 2012Published: November 12, 2012
Apparatus and Procedure. The VLE data and the vaporpressure of the pure compounds were determined using adynamic-recirculating still (Pilodist VLE 100 D) equipped with aCottrell circulation pump. This still is capable of handling pres-sures from 0.25 to 400 kPa, and temperatures up to 523 K.The equilibrium temperature was measured with a digital HartScientific thermometer, model 1502A, and a Hart Scientific Pt 100probe,model AlB0888, calibrated at the Spanish InstitutoNacionalde Tecnica Aeroespacial. The uncertainty is estimated to be 0.02 K.A Pilodist M101 pressure control system was used to measure andcontrol the pressure and the heating power. The pressure is keptconstant by means of a vacuum pump with and an electrovalve.The manometer was calibrated using the vapor pressure ofultrapure water. The uncertainty is estimated to be 0.05 kPa.In each VLE experiment, first step was to fix and hold the
pressure using a vacuum pump, and then turn on the heating andstirring systems of the liquid mixture. The experiment was run-ning at constant pressure until equilibrium was reached. Equi-librium conditions were assumed when no changes in temper-ature and pressure were recorded. Then, samples of liquid andcondensate vapor were taken for analysis. To verify equilibriumconditions, the vapor and liquid were continuously analyzed untila value less than 0.001 of the variation of the mole fraction ofboth liquid and vapor phase was reached. The sample extractionswere carried out with special syringes which allowed the with-drawal of small volume samples.Analysis. Compositions of the liquid and condensed phases
were determined using a Varian CP-3800 gas chromatograph(GC), after calibration with gravimetrically prepared standard solu-tions. A flame ionization detector was used together with a 30 m,0.25 mm i.d. capillary column CP-Wax 52 CB. The GC responsepeaks were treated with Varian Star No. 1 for MS Windows. Thecolumn, injector, and detector temperatures were (353.15, 473.15,and 493.15) K, respectively, for all systems. Very good peakseparation was achieved under these conditions, and calibrationanalyses were carried out to convert the peak area ratio to themass composition of the sample. The average absolute devia-tion in the mole fraction was usually less than 0.001.
■ RESULTS AND DISCUSSION
Pure Component Vapor Pressures. The pure componentvapor pressures for ethyl butyrate and the pure component vaporpressures for 2-pentanol, Pi
0, were determined experimentallyusing the same equipment as that used to obtain the VLE data.The pertinent results appear in Table 2 for ethyl butyrate andin Table 3 for 2-pentanol. The measured vapor pressures werecorrelated using the Antoine equation:
= −+
P AB
T Cln /kPa
/Ki ii
i
0
(1)
whose parameters Ai, Bi, and Ci are reported in Table 4 togetherwith the Antoine parameters for n-hexane and 1-hexene obtainedin a previous work2 and were fitted by a nonlinear optimizationmethod to minimize the average relative deviation in pressure(ARDP). The vapor pressures of ethyl butyrate were correlatedwith an ARDP of 0.07 %. A value for the ARDP equal to 0.18 %was obtained for the case of 2-pentanol. For both compoundsexperimental data are in good agreement to the equation reportedby Reid et al.4 since this equation gives a correlation of theexperimental vapor pressures reported in this work with aARDP = 0.39 % for the ethyl butyrate and a ARDP = 1.01 % forthe 2-pentanol.
Binary Systems. The temperature T and the liquid-phase xi,and vapor-phase yi mole fractions at 100.0 kPa for the systems1-hexene (1) + ethyl butyrate (3), n-hexane (2) + ethyl butyrate (3),1-hexene (1) + 2-pentanol (3), and n-hexane (2) + 2-pentanol(3) are reported in Tables 5−8 and plotted in Figures 1−4.The activity coefficients (γi) were calculated from the follow-ing equation5 assuming nonideality of both liquid and vaporphases:
∑ ∑γ δ δ= +− −
+ −yP
x PB V P P
RTPRT
yyln ln( )( )
2(2 )i
i
i i
ii i ii k ji jk0
L 0
(2)
Table 2. Experimental Vapor Pressure (Pi0) of Ethyl Butyratea
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where T and P are the equilibrium temperature and pressure,Vi
L is the molar liquid volume of component i, Bii and Bjj are thesecond virial coefficients of the pure gases, Pi
o is the pure-component vapor pressure, Bij the cross second virial coefficientand
δ = − −B B B2ij ij jj ii (3)
The standard state for the calculation of activity coefficientsis the pure component at the pressure and temperature of thesolution. Equation 2 is valid at low and moderate pressureswhen the virial equation of state truncated after the second
coefficient is adequate to describe the vapor phase of the purecomponents and their mixtures, and liquid volumes of thepure components are incompressible over the pressure rangeunder consideration. The molar virial coefficients Bii and Bijwere estimated by the method of Hayden and O’Connell6
using the molecular parameters suggested by Prausnitz et al.7
The critical properties of all components were taken fromDIPPR.8
According to the results, the binary systems show positivedeviations from ideal behavior and do not form any azeotrope.
Table 5. Experimental Vapor−Liquid Equilibrium Datafor the Binary System 1-Hexene (1) + Ethyl Butyrate (3)at 100.0 kPaa
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The thermodynamic consistency of the VLE data, for eachbinary system, has been verified with the Fredenslund test.9
Pertinent consistency details and statistics are presented in Table 9,and it can be seen that the consistency criteria (AADy < 0.01) wasachieved using a two parameter Legendre polynomial for thebinary systems with ethyl butyrate and three parameters for thebinary sytems with 2-pentanol.The VLE data for each binary systemhave been correlated using
local composition models (Wilson, NRTL, and UNIQUAC) andpredicted by the UNIFAC contribution method.9,10 For theWilson model, molar liquid volumes of pure components havebeen estimated with the Rackett equation,11 and for UNIQUACmodel, volumeand surfaceparameterswere taken fromDECHEMA.12
The parameters of these models have been determined mini-mizing the following objective function (OF)
Figure 3. Experimental VLE data for the system 1-hexene (1) +2-pentanol (3) at 100.0 kPa:●, experimental data; solid line, smootheddata using the NRTL model with the parameters given in Table 10,dotted−dashed line, predicted by UNIFAC method.
Figure 4. Experimental VLE data for the system n-hexane (2) +2-pentanol (3) at 100.0 kPa:●, experimental data; solid line, smootheddata using the NRTL model with the parameters given in Table 10,dotted−dashed line, predicted by UNIFAC method.
Table 9. Consistency Test Statistics for the Binary Systems1-Hexene (1) + Solvent (3) and n-Hexane (2) + Solvent (3)
Figure 1. Experimental VLE data for the system 1-hexene (1) + ethylbutyrate (3) at 100.0 kPa: ●, experimental data; solid line, smootheddata using the NRTL model with the parameters given in Table 10;dotted−dashed line, predicted by UNIFAC method.
Figure 2. Experimental VLE data for the system n-hexane (2) +ethyl butyrate (3) at 100.0 kPa: ●, experimental data; solid line,smoothed data using the NRTL model with the parameters givenin Table 10; dotted−dashed line, predicted by UNIFAC method.
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∑= ×−
+ | − |=
⎛⎝⎜⎜
⎞⎠⎟⎟
T TT
y yOF 100i
Ni i
ii i
1
expt calc
exptexpt calc
(4)
and are reported in Table 10, together with the obtainedaverage deviations of the correlation. An inspection of theresults given in that table shows that the three compositionmodels are adequate for the description of the VLE of thebinary systems, without any significant difference betweenthem. However, it must be pointed out that the deviationsobtained for the prediction with UNIFAC are larger.Ternary Systems. VLE data for the ternary system 1-hexene
(1) + n-hexane (2) + ethyl butyrate (3) are reported in Table 11 andFigure 5 and VLE data for the ternary system 1-hexene (1) +n-hexane (2) + 2-pentanol (3) are reported inTable 12 and Figure 6.The activity coefficients (γi)were calculated fromeq 2, and themolarvirial coefficients were estimated as well as for the binary systems.The ternary data were found to be thermodynamically consistent bythe Wisniak and Tamir13 modification of the McDermott−Ellistest.14 The test requires that D < Dmax for every experimental point,where the local deviation D is given by
∑ γ γ= + −=
D x x( )(ln ln )i
N
ia ib ia ib1 (5)
and N is the number of components. The maximum deviationDmax is given by
Table 10. Parameters and Correlation Statistics for Different GE Models for the Ternary Systems 1-Hexene (1) + n-Hexane (2) +Solvent (3)
Aij Aji bubble point
model system i + j J·mol−1 J·mol−1 αij ARDTa/% 100·AADy1b 100·AADy2
aAverage relative deviation in temperature. bAverage absolute in vapor phase composition. cMolar liquid volumes of pure components have beenestimated with the Rackett equation.11 dReference 2. eTernary estimation from binary parameters. fVolume and surface parameters from DECHEMA.12gCalculations based on UNIFAC.9,10
Figure 5. Diagram of VLE for the ternary system 1-hexene (1) +n-hexane (2) + ethyl butyrate (3) at 100.0 kPa: ●, liquid-phase molefractions; Δ, vapor-phase mole fractions.
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∑
∑ ∑
∑
γ γ
= + + + + Δ
+ + Δ + | − |Δ
+ + + + + Δ
=
= =
=
− −
⎛⎝⎜⎜
⎞⎠⎟⎟D x x
x y x yx
x xP
px
x x B T C T C T
( )1 1 1 1
( ) 2 ln ln
( ) {( ) ( ) }
i
N
ia ibia ia ib ib
i
N
ia ibi
N
b ia
i
N
ia ib j a j b j
max1
1 1
1
2 2
(6)
The errors in the measurements Δx, ΔP, and ΔT were aspreviously indicated. The first and fourth are the dominant terms
in eq 6. For each experimental point reported in this work thevalue of D was always smaller than the value of Dmax.VLE data for both ternary systems have been estimated by
using the Wilson, NRTL, and UNIQUAC models with thebinary interaction parameters obtained from the regression ofbinary data and also are predicted by the UNIFAC method.Table 10 lists the average deviations between experimental andcalculated temperatures and vapor phase mole fractions of thecomponents. The three models represent the data successfully.Thus, these models can be used to calculate boiling pointsfrom liquid phase compositions at the system pressure. As anexample, boiling isotherms calculated for both ternary systemswith the aid of the NRTL model are presented in Figure 7when ethyl butyrate is the solvent and Figure 8 for the case of2-pentanol.
Table 11. Experimental Vapor−Liquid Equilibrium Data forthe System 1-Hexene (1) + n-Hexane (2) + Ehtyl Butyrate (3)at 100.0 kPaa
Figure 6. Diagram of VLE for the ternary system 1-hexene (1) +n-hexane (2) + 2-pentanol (3) at 100.0 kPa: ●, liquid-phase mole frac-tions; Δ, vapor-phase mole fractions.
Figure 7. Boiling isotherms (K) for the ternary system 1-hexene (1) +n-hexane (2) + ethyl butyrate (3) at 100.0 kPa calculated with NRTLmodel with the parameters given in Table 10.
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Solvent Effects. The solvent effects in the VLE of the mix-ture 1-hexene + n-hexane will be discussed taking into accounttwo aspects: analysis of the residue curve map,15 and changes inrelative volatility. Furthermore, both solvents will be comparedwith each other in order to recommend the most suitableentrainer for the separation of the mixture 1-hexene + n-hexaneby extractive distillation.In Figure 9 and Figure 10, residue curves simulated by Aspen
properties v7.3 using the NRTL model with the experimental
parameters reported in Table 10 are shown. As can be seenin these figures, in both ternary systems there is only a onedistillation region with three singular points (nodes and saddles)which correspond with the three pure component vertices.1-Hexene is an unstable node; n-hexane is a saddle, and ethylbutyrate in Figure 5 and 2-pentanol in Figure 6 are both a stable
node. Thus, 1-hexene could be obtained as an overhead productand any of the two solvents will be the bottom product. Thisbehavior is consistent with the boiling points of the componentsemployed here. In this case the residue curve maps give littlevaluable information.A more useful tool to evaluate the influence of adding a solvent
in a binary mixture is to calculate the change in the relativevolatility of the initial mixture. In this occasion, the value of therelative volatility of 1-hexene to n-hexane is very close to unity(α12 = 1.177).2 In this sense, Table 13 provides values of meanrelative volatilities in the presence of the solvent (α12
S ) for twodifferent solvents compositions (for x3 = 0.7 and 0.8). NRTLmodel has been used in order to calculate the relative volatilities.
Table 12. Experimental Vapor−Liquid Equilibrium Data forthe System 1-Hexene (1) + n-Hexane (2) + 2-Pentanol (3)at 100.0 kPaa
Figure 8. Boiling isotherms (K) for the ternary system 1-hexene (1) +n-hexane (2) + 2-pentanol (3) at 100.0 kPa calculated with NRTLmodel with the parameters given in Table 10.
Figure 9. Residue curve map for the ternary system 1-hexene (1) +n-hexane (2) + ethyl butyrate (3) at 100.0 kPa simulated by Aspen splitusing the NRTL model with the parameters given in Table 10.
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Moreover, liquid molar fractions of each ternary system versusthe differences between molar fractions of the vapor phaseand the liquid phase of above systems are plotted in Figure 11.These representations are made on a solvent-free basis, so the
effect of adding solvent can be more appreciated. Data from boththe Table 13 and the Figure 11 imply that the two proposedsolvents show a poor behavior as extractive distillation entrainersbecause the two values of mean relative volatility in the presenceof the solvents are closer to unity than the original solvent-freevalue. Additionally, a reversal in the initial mixture volatility isproduced when ethyl butyrate is employed as entrainer.This behavior implies the refusal of the candidate entrainers
for extractive distillation to separate 1-hexene from n-hexane.
■ CONCLUSIONSIn order to study the 1-hexene + n-hexane separation process,two solvents, ethyl butyrate and 2-pentanol have been proposedas possible entrainers in an extractive distillation process. Hence,consistent VLE data at 100.0 kPa have been determined for thebinary systems formed by 1-hexene (1) or n-hexane (2) with eachproposed solvent. Also the two ternary systems constituted bythe initial binary mixture and the solvent have been measured.Wilson, NRTL, and UNIQUACmodels correlated well the binarysystems and yielded good estimation for the ternary system,without any remarkable difference.From the results, it is observed that both solvents decrease the
relative volatility of 1-hexene to n-hexane, ethyl butyrate eveninverts this relative volatility for high compositions of this solvent.So, these solvents are not good entrainers for extractive distillationto separate 1-hexene from n-hexane.
FundingFinancial support from theMinisterio de Ciencia e Innovacion ofSpain, through Project No. CTQ2010-18848, is gratefullyacknowledged. B.M. and J.P.-F. have been funded by a grantfrom La Universidad del Zulia of Venezuela and from theMinisterio de Economia y Competitividad of Spain (BES-2011-04636 6), respectively.
NotesThe authors declare no competing financial interest.
■ REFERENCES(1) Brix-Berg. http://www.brix-berg.com/ruleofthumbgraph.htm (ac-cessed July 27, 2012).(2) Marrufo, B.; Aucejo, A.; Loras, S.; Sanchotello, M. Isobaric Vapor-Liquid Equilibrium for Binary Mixtures of 1-Hexene + n-Hexane andCyclohexane + Cyclohexene at 30, 60 and 101.3 kPa. Fluid Phase Equilib.2009, 279, 11−16.(3) Linek, J.; Wichterle, I. ELDATA. Int. Electron. J. Phys-Chem. Data1995, 1 (4), 265−274.(4) Reid, R. C., Prausnitz, J. M., Sherwood, T. K. The Properties of Gasesand Liquids; McGraw-Hill: New York, 1977.(5) Van Ness, H. C.; Abbott, M. M. Classical Thermodynamics ofNonelectrolyte Solutions; McGraw-Hill: New York, 1982.(6) Hayden, J.; O’Connell, J. A Generalized Method for PredictingSecond Virial Coefficients. Ind. Eng. Chem. Process Des. Dev. 1975, 14,209−216.(7) Prausnitz, J.; Anderson, T.; Grens, E.; Eckert, C.; Hsieh, R.;O’Connell, J.Computer Calculation for Multicomponent Vapor-Liquid andLiquid-Liquid Equilibria; Prentice Hall: Englewood Cliffs, NJ, 1980.(8) Daubert, T. E.; Danner, R. P. Physical and ThermodynamicProperties of Pure Chemicals. Data Compilation; Taylor & Francis:Bristol, PA, 1989.
Figure 10. Residue curve map for the ternary system 1-hexene (1) +n-hexane (2) + ethyl butyrate (3) at 100.0 kPa simulated by Aspen splitusing the NRTL model with the parameters given in Table 10.
Table 13. Mean Relative Volatility in the Presence ofthe Solvent (α12
S ) Calculated Using the NRTL Model forthe System 1-Hexene (1) + n-Hexane (2) + Solvent (3)at 100.0 kPa
Figure 11. VLE data plotted on a solvent-free basis for the system1-hexene (1) + n-hexane (2) + solvent (3) at 100.0 kPa. Continuousline4 for x3 = 0.00: Dashed line for x3 = 0.70, calculated using the NRTLmodel with the parameters given in Table 10: ···, ethyl butyrate; --,2-pentanol.
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(9) Fredenslund, A.; Gmehling, J.; Rasmussen, P. Vapor-LiquidEquilibria Using UNIFAC. A Group Contribution Method; Elsevier:Amsterdam, 1977.(10) Hansen, H. K.; Rasmussen, P.; Fredenslund, A.; Schiller, M.;Gmehling, J. Vapor-Liquid Equilibria by UNIFAC Group Contribution.5. Revision and Extension. Ind. Eng. Chem. Res. 1991, 30, 2352−2355.(11) Rackett, H. G. Equation of State for Saturated Liquids. J. Chem.Eng. Data 1970, 15, 514−517.(12) Gmehling, J.; Onken, U. Vapor-Liquid Equilibrium DataCollection; DECHEMA: Frankfurt, 1977.(13) Wisniak, J.; Tamir, A. Vapor-Liquid Equilibria in the TernarySystem Water-Formic Acid-Acetic Acid and Water-Acetic Acid-Propionic Acid. J. Chem. Eng. Data 1977, 22, 253−260.(14) McDermott, C; Ellis, S. R. M. A Multicomponent ConsistencyTest. Chem. Eng. Sci. 1965, 20, 293−296.(15) Doherty, M. F.; Malone, M. F. Conceptual Design of DistillationSystems; McGraw-Hill: New York, 2001.
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