Measurement of vapor-liquid equilibria for diethylamine + methanol system represented double azeotropy on elevated pressure

Accurate phase equilibrium data can be used for the design and operation for separation processes based on phase behavior. An azeotrope is the mixture consists of two or more substances, the equilibrium vapor composition is equivalent to the liquid composition, and the equilibrium temperature (or pressure) indicates extreme value. If the mixture to be separated is an azeotropic system, no separation into pure components can be achieved by simple distillation. Knowledge of the acurate azeotropic data is necessity for chemical process synthesis. According to the Dortmund Data Bank [1], approximately 47% of the stored vapor-liquid equilibrium (VLE) data show azeotropic behavior. There are two types of azeotrope for binary systems, which are positive and negative azeotropes, generally. The binary system of diethylamine + methanol is known for the polyazeotropy at given temperature or pressure. The system forms a homogeneous maximum boiling azeotrope at 101.3 kPa. The second minimum boiling azeotropic point appears in the VLE at the condition of elevated pressure, around 300 kPa. Aucejo et al. [2] has measured the VLE at 101.3 and 300 kPa and discussed the detail of these phenomena thermodynamically. The purpose of this work is describing the trajectory of two different of azeotropic points on elevated pressure from experimental data.


Introduction
Accurate phase equilibrium data can be used for the design and operation for separation processes based on phase behavior.An azeotrope is the mixture consists of two or more substances, the equilibrium vapor composition is equivalent to the liquid composition, and the equilibrium temperature (or pressure) indicates extreme value.If the mixture to be separated is an azeotropic system, no separation into pure components can be achieved by simple distillation.Knowledge of the acurate azeotropic data is necessity for chemical process synthesis.
According to the Dortmund Data Bank [1], approximately 47% of the stored vapor-liquid equilibrium (VLE) data show azeotropic behavior.There are two types of azeotrope for binary systems, which are positive and negative azeotropes, generally.The binary system of diethylamine + methanol is known for the polyazeotropy at given temperature or pressure.The system forms a homogeneous maximum boiling azeotrope at 101.3 kPa.The second minimum boiling azeotropic point appears in the VLE at the condition of elevated pressure, around 300 kPa.Aucejo et al. [2] has measured the VLE at 101.3 and 300 kPa and discussed the detail of these phenomena thermodynamically.The purpose of this work is describing the trajectory of two different of azeotropic points on elevated pressure from experimental data.

Experimental
Experimental apparatus and techniques, mainly developed in our laboratory, for the determination of VLE.The measurements were made in two deferent original equilibrium stills with circulation of both the vapor and liquid phases, equipped with a Cottrell pump.An all-grass VLE still [3], as shown in Figure 1, was used for the measurements at 101.3 kPa.The overall charge of the apparatus was about 90 cm 3 of the solution.A stainless-steel VLE still with three pressure-tight grass windows (Hiaki et al., unpublished paper) as shown in Figure 2, was used at 405.3 and 506.6 kPa.The overall charge of the apparatus was about 120 cm 3 of the solution.
Diethylamine and methanol, supplied by the Wako Pure Chemical Co. Ltd. were special grade reagents.The equilibrium composition of the samples was determined using a gas chromatograph (Shinadzu model GC-17A) equipped with a thermal conductivity detector and auto sampler.The column packing was HP-5.The accuracy of liquid, xi, and vapor, yi, mole fractions is estimated to be 0.002 mole fraction.The temperature was measured with a calibrated platinum resistance thermometer (Automatic System Laboratories model F250) with an accuracy of 0.03 K.The Pressure controller (Druck model DPI520 and RUI 100) was used with an accuracy of + 0.025% for the range of 1 to 6 bar absolute.

Results and discussion
Isobaric VLE were measured for the binary system of diethylamine (1) + methanol (2) at 101.3, 405.3 and 506.6 kPa.The activity coefficients  i were calculated using the following equation: The vapor pressures of the pure components, P i S , were obtained using the Antoine equation constants.The experimental data were tested for thermodynamic consistency by the Van Ness method [4].The results of consistency test indicate that the VLE data for systems are thermodynamically consistent as shown in Table 1.• This work -Margules eq.
01021-p.3 Figures 3-5.The system forms a homogeneous maximum boiling azeotrope at 101.3 kPa.The second minimum boiling azeotropic point appears in the VLE at the condition of elevated pressure, around 300 kPa [1].Two azeotropic compositions of both maximum and minimum boiling points change with presented pressure.
The activity coefficients were correlated with the the nonrandom two-liquid (NRTL) [5] and Margules [6] equations.The sum of the squares of relative deviations in activity coefficients was minimized during optimization of the parameters.For the experimental isobaric system of diethylamine (1) + methanol (2) at all pressure condition, the Margules equation yielded the lowest mean deviations between the experimental and calculated activity coefficients.The parameter values and average absolute deviations using the Margules equation are shown in Table 2.
Margules equation:  3-5.The azeotropic data, which were determined on the basis of the experimental data, are shown in Table 3 and Figure 6.Acording to the Arrenius prots of ln P vs. 1/T az , the azeotropic points of this system will be estimated to disappear at the pressure of around 0.7 MPa.

Table 3 .
Azeotropic data for diethylamine (1) + methanol (2) at each experimental pressure: T az1 and x 1az1 , maximum boiling azeotropic data; T az2 and x 1az1 , minimum boiling azeotropic data.Two azeotropic compositions of both maximum and minimum boiling points change with presented pressure.