Solubility of carbon dioxide in methanol from 213.15 K to 273.15 K: Measurement and modeling
Graphical abstract
Experimental data and calculated results by SRK-vdW-Wilson for carbon dioxide(1) + methanol(2) from 213.15 K to 273.15 K: ●, measured data; solid line (─), calculated results by SRK-vdW-Wilson.
Introduction
The Rectisol process, which was jointly developed by the German Lurgi and Linde in the 1950s and then independently developed, is a physical acid gas removal process using methanol as the organic solvent at subzero temperatures [[1], [2], [3]]. It can purify acid gas to obtain very small quantities of acid contaminants in the treated gas and produce a concentrated H2S stream as Claus Plant feed for sulfur production and CO2 off-gas essentially free of H2S [4]. Although the Rectisol Process has been successfully used for the removal of CO2 and H2S from gas mixtures produced by partial oxidation of oil or gasification of coal, there is still a need for accurate vapor-liquid equilibrium (VLE) data [5]. The VLE data of the carbon dioxide + methanol system at temperatures below 273.15 K are an important part of the Rectisol technology. The lowest operating temperatures of the absorption unit in the Rectisol process is generally about 213.15 K [6]. Therefore, the study of phase equilibrium for the system at subzero temperatures is extremely important in the Rectisol process.
The research on the VLE of the system at temperatures above 273.15 K has been reported in many pieces of literature. The earliest data were published by Krichevskii et al. [7], Katayama et al. [8], Ohgaki et al. [9], Rousseau et al. [10], Weber et al. [5], Chang et al. [11], Brunner et al. [12], and Hong et al. [13]. Subsequently, a large number of investigators [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]] constantly examined and improved the VLE data of the system at temperatures above 273.15 K.
However, less attention has been paid to VLE at temperatures below 273.15 K in literature. The solubility and VLE data that can be retrieved for the binary mixtures of the carbon dioxide + methanol system have been measured by Bezdel et al. [39], Shenderei et al. [40], Yorizane et al. [41], Ferrell et al. [42], Weber et al. [5], Chang et al. [11], Hong et al. [13], Schroedter et al. [43] and Naidoo et al. [44] at temperatures below 273.15 K. However, the solubility data of carbon dioxide in methanol at temperatures below 273.15 K are still scarce. Hong et al. [13] pointed out that there was a shortage of VLE data for the carbon dioxide + methanol system, particularly vapor phase compositions to go with liquid phase compositions at low temperatures. Till now, the published VLE data [5,11,13,[39], [40], [41], [42], [43], [44]] at temperatures below 273.15 K can't fully meet the needs of the Rectisol process calculation. Based on these solubility data, the binary interaction parameters can be calculated and corrected using chemical composition databases and thermodynamic methods of the process flow simulation software such as PRO/II and Aspen Plus, so that the calculation model of the Rectisol process can be built. The calculation model can not only analyze the existing production process, provide suggestions for improvement of the process, and increase the economic efficiency, but also meet the needs of the engineering design for new projects. Yang et al. [45] obtained reliable simulation results by revised the binary interaction parameters referring to the DECHEMA database in the study of the full CO2 capture process based on the Rectisol technology. Therefore, the determination and improvement of the solubility database of carbon dioxide in methanol at temperatures below 273.15 K have important industrial value.
The thermodynamic methods are also the key to studying the binary and multicomponent VLE in the Rectisol process. This is based on the fact that the complex multicomponent system in the Rectisol process contains associating fluids such as methanol, and its vapor and liquid phases are highly non-ideal. Recently, the PC-SAFT [46,47] equation of state (EOS) can be considered as an appropriate thermodynamic method for the Rectisol process simulation [4,45,[48], [49], [50]]. However, these desired simulation results were obtained by modifying interaction parameters, meaning that the interaction parameters provided by existing VLE databases such as DECHEMA and NIST used in Aspen Plus software are also not satisfactory for predicting phase behavior in the Rectisol process. The literature [34,51] shows that the statistical associating fluid theory (SAFT) [52]-type EOSs have the ability to provide satisfactory calculations for VLE data of the carbon dioxide + methanol system at temperatures from 273.15 K to 352.60 K. Similarly, the cubic-plus-association (CPA) [53,54] EOS can correlate well with VLE data for the binary system at 273.15 K [55]. Whereas, phase behaviors of the binary system below 273.15 K using CPA or SAFT-type EOS have been seldom investigated. Early studies [5,11,42] of phase behavior for the system below 273.15 K have shown that the Soave-Redlich-Kwong (SRK) [56]-type and Peng-Robinson (PR) [57]-type EOSs also do not exhibit sufficient accuracy for correlating with VLE data of the system. Later, Naidoo et al. [44] found that the PR-WS-NRTL model provided an excellent correlation to VLE data at 263.15 and 273.15 K. Interestingly, the cubic EOS may be able to accurately predict the phase behavior of the highly non-ideal system by combining activity coefficient models from 213.15 K to 273.15 K. Furthermore, activity coefficient model parameters are generally not published for this system at subzero temperatures. In this work, two “” approaches were tried to describe the phase behavior of the system from 213.15 K to 273.15 K.
Overall, in order to supplement reliable and essential data for design, operation, and optimization of the Rectisol process, the solubility data of carbon dioxide in methanol were measured at temperatures from 213.15 K to 273.15 K and pressures from vacuum up to 3.3450 MPa in this paper. The SRK EOS with the van der Waals (vdW) [58] one-fluid mixing rule applying the non-random two-liquid (NRTL) [59] activity coefficient model, and the SRK EOS with the vdW mixing rule applying the Wilson [60] model, were separately used to correlate the experimental data and predict VLE data of the system.
Section snippets
Chemicals
The chemicals used in the experiment were carbon dioxide, hydrogen, methanol, and ethanol. Hydrogen was used as carrier gas for sample transfer and gas chromatographic (GC) analysis. Ethanol was used as a medium for heat transfer. The source, initial mass fraction purity, purification method, final mass fraction purity and analysis method are shown in Table 1.
Apparatus and procedure
As shown in Fig. 1, the experimental apparatus included an equilibrium cell, a refrigerating coil, a constant temperature alcohol bath,
Thermodynamic correlation
The relationship of VLE system can usually be expressed as follows [62]:where is the temperature of the system, is the absolute pressure of the system, is the saturation (vapor) pressure of pure liquid at temperature , and are the mole fractions of component in the liquid and vapor phase, respectively, is the activity coefficient of component in the liquid phase, is the universal gas constant, is the fugacity coefficient for
Reliability of the experiment
For testing the reliability of the apparatus and the experimental methods, the experimental data were compared with the available literature data. As shown in Fig. 3, the measured solubility data of carbon dioxide in methanol at 273.15 K are in good agreement with the literature [5,11,41]. Weber et al. [5] obtained solubility data in two different ways at this temperature. It can be confirmed that the performance of the apparatus is excellent.
Correlated results of models
In this part, the isothermal solubility data of
Conclusions
In this paper, the isothermal solubility data of carbon dioxide in methanol were measured at temperatures from 213.15 K to 273.15 K and pressures from vacuum up to 3.3450 MPa. The consistency of measured data and literature values at 273.15 K proves the reliability of the experiment. The calculated results of the SRK-vdW-NRTL model show false liquid-liquid splitting. The model is not entirely suitable for the correlation of the experimental data. The SRK-vdW-Wilson model provides satisfactory
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