Elsevier

Fluid Phase Equilibria

Volumes 322–323, 25 May 2012, Pages 135-141
Fluid Phase Equilibria

Phase behavior for carbon dioxide/tetraalkoxysilane systems

https://doi.org/10.1016/j.fluid.2012.02.018Get rights and content

Abstract

Phase diagrams of carbon dioxide (CO2)/tetramethoxysilane (TMOS) and CO2/tetraethoxysilane (TEOS) binary systems were measured. Liquid phase measurements were performed by observing the bubble point pressure with a synthetic method. Vapor phase measurements were conducted with a flow type apparatus. For the CO2/TMOS system, liquid phase lines were measured at temperatures from 313.2 to 393.2 K, and CO2 molar fractions from 0.15 to 0.85, and vapor phase lines were obtained at temperatures 313.2, 353.2 and 393.2 K and pressures up to 12 MPa. For the CO2/TEOS system, measurements were conducted only for liquid phase lines at temperatures from 313.2 to 373.2 K and CO2 molar fractions from 0.25 to 0.75. Phase behavior of these two systems was almost the same with each other. Experimental data were correlated with the Peng–Robinson equation of state (PR EoS), and the molecular interaction parameter kij and lij for both systems were determined. For large difference in the molecular size of CO2 and tetraalkoxysilanes, it was found that the lij could be negligible.

Highlights

► We first measured the phase diagrams of CO2/tetramethoxysilane (TMOS) and CO2/tetraethoxysilane (TEOS) binary systems. ► With using the Peng–Robinson EOS, the phase behaviors were successfully correlated. ► Binary interaction parameters of these two systems were determined. ► These systems were found to be almost the ideal mixtures.

Introduction

Supercritical carbon dioxide (scCO2) has many advantages such as low cost, non-toxicity, non-flammability, chemical inertness, easily accessible supercritical conditions, and tunability of physicochemical properties by varying pressure and/or temperature [1], and has been widely utilized in extraction process [2], drying [3], particle formation [4], chromatography [5], formation of polymeric foams [6], and so forth. Especially, scCO2 has been regarded as an alternative for blowing agent of polymers instead of chlorofluorocarbons that deplete ozone layer, and chemical blowing agents that stay behind in the products [7].

In recent years, for the effective energy usage, development of high performance heat insulator is the subject of research for engineers. A polymeric foam inside the foam filled with silica aerogel [8], [9] is regarded as one of the promising candidates for the high performance heat insulator. This composite material will be commercially produced with a scCO2 assisted extruding of the mixture of the polymer and tetraalkoxysilane, the main material of silica aerogel [10]. The foaming, or the nucleation and growth of the gas bubble inside the polymer, is essentially the separation of gaseous CO2 from the thermodynamically stable mixture of polymer/tetraalkoxysilane/CO2, which is caused by the thermodynamic disturbance such as pressure and/or temperature change. Thus, the forming, especially the nucleation, depends on the phase behavior of the systems. Therefore, to design the new process and to find the mechanism of the polymeric forming, phase behavior of the CO2/tetraalkoxysilane binary system and the CO2/tetraalkoxysilane/polymer ternary system is very important. Data for high CO2 fraction range of CO2/TEOS binary system is already reported at low temperature range [11], [12] though, data for low CO2 fraction range at high temperature is necessary for the process design. However, for our knowledge, such data have never been reported in the literature. Further, limited numbers of the phase equilibrium data for the systems including silane compounds were reported.

In this study, vapor–liquid equilibrium of CO2/tetramethoxysilane (CO2/TMOS) and CO2/tetraethoxysilane (CO2/TEOS) systems was measured at wide ranges of temperatures and pressures with using two different types of experimental apparatus, synthetic and flow type. Further, experimental results were correlated with the Peng–Robinson equation of state (PR EoS) [13] with the van der Waals one fluid mixing rule, and molecular interaction parameters kij and lij for CO2/TMOS and CO2/TEOS systems were determined. This work will contribute not only to the understanding of forming but to the accumulation of the phase equilibrium data of siliceous systems.

Section snippets

Materials

Carbon dioxide (purity 99.99%) was purchased from Showa Yozai Co., and used without further treatment. Tetramethoxysilane (>98.0%) and tetraethoxysilane (>96.0%), both GC grade, were purchased from Tokyo Kasei Co., and used as received. Table 1 lists the properties of materials used in this work. Methanol (>99.8%) was purchased from Kanto Chemical Co., Inc., and used after the dehydration with using molecular sieve 3 A.

Apparatus and procedures

In this work, phase diagram of binary systems was measured with two methods.

Theory

The experimental data are correlated with Peng–Robinson equation of state (PR EoS) shown below;p=RTνba(T)ν(ν+b)+b(νb)where p, T, ν and R are the pressure, temperature, molar volume and the gas constant respectively, and a(T) and b are the PR EoS parameters [13]. For pure component, the parameter a(T), a function of temperature, and the parameter b are given bya(T)=0.45724RTc2Pcα(T)α(T)=[1+κ(1Tr)]2b=0.07780RTcPcwhere Tc, Pc and Tr represent the critical temperature, the critical pressure and

CO2/methanol system

Ensuring the reliability of the experimental data, bubble point pressure and dew point pressure were measured for CO2/methanol system at 323.2 K with a synthetic method and a flow type apparatus. Experimental results are also compared with the literature. Experimental results for the CO2/methanol system are shown in Fig. 3 and Table 2. Brunner et al. [19] and Leu et al. [20] reported the vapor–liquid equilibrium for CO2/methanol with using an equilibrium cell and a gas chromatograph at 323.2 K.

Conclusions

Phase diagrams of the CO2/TMOS and the CO2/TEOS binary systems were obtained at wide range of temperature and pressure. These two systems had almost the same phase diagram from each other. These systems do not have the VLLE and the LLE regions presumably due to the nonpolarity of the TMOS and the TEOS.

Experimental data were successfully correlated by the PR EoS with van der Waals one fluid mixing rule. For large difference in the molecular size of CO2 and tetraalkoxysilanes, it was found that

Acknowledgment

The authors thank for the financial support from NEDO.

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