High pressure vapor–liquid equilibria measurements and modeling of butane/ethanol system and isobutane/ethanol system
Introduction
Bioethanol is considered to be one of the most promising alternatives to fossil fuels in the short and medium term. Several processes have been developed to produce bioethanol from corn or lignocellulosic biomass. Bioethanol production processes typically comprise four parts: pretreatment, scarification, fermentation and dehydration [1], [2], [3], [4]. The development of energy-saving technology for each process is desired.
Many energy-saving dehydration processes for bioethanol production have been developed, such as membrane separation, zeolite adsorption and azeotropic distillation [5], [6], [7], [8], [9]. Horizoe et al. [10], [11], [12] developed an ethanol dehydration system with an extractive distillation using a light hydrocarbon solvent. To effectively develop this process, vapor–liquid equilibria at a wide range of conditions were needed.
Vapor–liquid equilibrium data of butane/ethanol system has been previously reported in the literature by Deak et al. [13], who measured a bubble point with the Cailletet apparatus at 323–523 K, and Soo et al. [14] who measured the composition of the vapor and liquid phases with ROLSI™ capillary samplers, analyzed by gas chromatography at 323–423 K. Kretschmer et al. [15], Holderbaum et al. [16] and Dahlhoff et al. [17] also reported the VLE of butane/ethanol system. Vapor–liquid equilibrium data of isobutane/ethanol system has been reported by Ouni et al. [18], who measured VLE data at 313 K using the total pressure method. Furthermore, the Barker method was used to convert the measured PTZ data into PTXY data. In our dehydration process, operation temperature range is from 310 to 410 K, so VLE data of isobutane is still limited.
In this study, the high-pressure vapor–liquid equilibria of butane/ethanol system and isobutane/ethanol system were measured by the minimum continuous flow method using process gas chromatography at 313–403 K. Experimental PXY data were compared with reference data. The modified RK (Redlich-Kwong) EOS which is developed by Twu et al. [19], [20] parameters were regressed and PXY data were also compared with PSRK (predictive Soave–Redlich–Kwong) EOS [21].
Section snippets
Materials
Table 1 lists the purity and supplier used in this study.
Apparatus and measuring procedure
A diagram of the vapor–liquid equilibrium apparatus is shown in Fig. 1. The apparatus comprised an equilibrium cell with a view window (950 cc), a liquid phase circulation magnetic pump, a vapor phase circulation magnetic pump and a process GC. Two gas buffers (both 950 cc) were placed before and after the vapor phase circulation magnetic pump to prevent pressure fluctuation. The equilibrium cell and circulation pump were immersed in a
Modeling
The experimental data were correlated with the modified RK EOS which was proposed by Twu et al. [19], [20] and allows the accurate representation of polar/non-polar systems. The experimental data were also compared with PSRK EOS, which is group contribution equation of state based on a combination of the Soave–Redlich–Kwong equation of state and a mixing rule whose parameters are determined by the UNIFAC method [21].
Modified RK EOS
Measurement results
The PXY data from the butane/ethanol system is shown in Table 2 and plotted in Fig. 2. Our data relating to the liquid phase correlates well with the data obtained by Deak et al. [13]. With regard to the vapor phase, our data shows slightly larger values than those reported by Soo et al. [14]. The PXY data relating to the isobutane/ethanol system shown in Table 3 and plotted in Fig. 3 appears to agree well with the data of Ouni et al. [18].
Correlation results
Fig. 4, Fig. 5 show a comparison between the
Conclusion
The isothermal vapor–liquid equilibria for butane/ethanol and isobutane/ethanol systems were obtained at 313–403 K. Experimental data is in good agreement with reference data. For the modified RK EOS, calculated concentration shows good agreement with experimental one, though a slight deviation was observed in vapor phase concentration. For PSRK EOS, vapor phase concentration shows good agreement with experimental data, but liquid phase concentration shows a systematic deviation especially on
Acknowledgement
This work was performed as “Development of alcohol dehydration technology” funded by JX Engineering Corporation (Japan).
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