Vapor-Liquid Equilibria at (33.33, 66.66, and 101.33) kPa and Densities at 298.15 K for the System Methanol + Methyl Lactate Maria Teresa Sanz, Beatriz Calvo, Sagrario Beltra ´ n,* and Jose Luis Cabezas Departamento de Ingenierı ´a Quı ´mica, Universidad de Burgos, 09001 Burgos, Spain Isobaric vapor-liquid equilibrium data have been experimentally determined for the binary system methanol + methyl lactate at (33.33, 66.66, and 101.3) kPa. All the experimental data reported were thermodynamically consistent according to the point-to-point method of Fredenslund. The activity coefficients were correlated with the NRTL and UNIQUAC equations for the liquid-phase activity coefficients. The densities and derived excess volumes for the same mixture are also reported at 298.15 K. Introduction Catalytic distillation can be an alternative process for recovering purified lactic acid from the aqueous solutions obtained in the fermentation processes where it is gener- ated. In the catalytic distillation process, esterification of lactic acid is carried out to obtain an ester that can be purified by distillation and then hydrolyzed into a purified lactic acid. For the design of a catalytic distillation process, some basic information is needed such as pure component properties, reaction kinetics, chemical equilibrium, mass transfer characteristics, and vapor-liquid equilibrium of the mixtures involved in the process. In a previous publica- tion, 1 the reaction kinetics of autocatalyzed and heteroge- neously catalyzed lactic acid esterification with methanol were measured. The activity coefficients needed to take into account the real behavior of the liquid phase in the correlation of the experimental reaction rates by the different models were predicted by UNIFAC. Accounting for the nonideality of the liquid phase results in a smaller residual error in the fitting process because the models describe more closely the actual reaction kinetics. 2 The use of experimentally determined activity coefficients, instead of predicted values, would further improve the correlation results. This work is one of a series directed to the experimental determination of the vapor-liquid equilibrium (VLE) of different mixtures that may be formed with the components involved in the esterification of lactic acid with methanol. In this manuscript, a mixture of two nonreacting compo- nents is considered, that is, methanol and methyl lactate. Experimental Section Chemicals. Methyl (S)-(-)-lactate, purchased from Acros (Belgium) with a reported purity of 97%, was purified by vacuum distillation. The final purity was 99.99 wt %, as determined by gas chromatography (GC). Methanol (Lab- Scan, 99.9%) was stored over activated 3-Å molecular sieves to keep it dry. The water content of methanol and methyl lactate was determined with a Karl-Fisher apparatus (Mitsubishi Kasei CA-20) and was found to be below 0.003 wt % in both cases. As an additional purity check, some physical properties of the pure components were measured and compared with values reported in the literature. The results are presented in Table 1. The experimental boiling point of methyl lactate is not included in this table (see Results and Discussion Section, subsection on Saturation Pressures). Apparatus and Procedure. Densities, F, were mea- sured with an Anton Paar (DMA-5000) densimeter with an accuracy of (0.005 kg‚m -3 and refractive indexes, n(D), with an Abbe-type refractometer with an accuracy of (0.0002. An all-glass still of the Gillespie type with circulation of both the liquid and vapor phases was used for experimental determination of VLE and vapor pressure data. This apparatus has been previously described and used in our laboratory to obtain experimental vapor pres- sures and VLE data. 7,8 The still was operated under a nitrogen atmosphere. The total pressure of the system was monitored with a digital manometer and controlled to the desired value (within 0.05 kPa) with a pressure controller (Normastat 75) that allowed dry nitrogen to be injected into or released from the still to achieve an inert atmosphere until thermodynamic equilibrium was reached. Atmo- spheric pressure was measured with a Lambrecht-type barometer. The boiling point temperature ((0.05 K) in the equilibrium still was measured with a digital thermometer (Ertco-Hart, Model 850). Sample Analysis. The liquid and vapor phases were analyzed using a Hewlett-Packard GC (Model 6890) equipped with a flame ionization detector (FID). The GC column was a 25 m × 0.5 μm bonded phase fused silica, FFAP, capillary column. The injector and detector were at 453 K and 523 K, respectively. The oven was operated at variable, programmed temperature, from 428 K (where it was kept for 2 min) to 553 K at a rate of 30 K‚min -1 . Helium (38 mL‚min -1 ), 99.999% pure, was used as the carrier gas. Concentration measurements were accurate to (0.0005 mole fraction. Results and Discussion Saturation Pressures. Some vapor pressure data for methanol were obtained with the still described in the Experimental Section. A comparison of the experimental vapor pressures, and those obtained from the Antoine constants taken from the literature, 10 gives an average * To whom correspondence should be addressed. E-mail: beltran@ubu.es. 1003 J. Chem. Eng. Data 2002, 47, 1003-1006 10.1021/je025513v CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002