Thermodynamic Modeling of Water-Hydrogen Chloride-Methanol System

Thermodynamic Modeling of Water-Hydrogen Chloride-Methanol System

Sina Hassanjani Saravi and Chau-Chyun Chen

Department of Chemical Engineering, Texas Tech University

 

Abstract

Hydrogen chloride as a strong inorganic acid is vastly used in industry e.g. refining metals.  The binary system of HCl+water forms a multiphase VLLE mixed solvent which needs to be investigated. Numbers of experiments have been previously reported for data for HCl+water binary system and they show the binary system forming azeotrope and two liquid phases. Fritz and Fuget [1], Vrevskii et al. [2], Storonkin and Susarev [3] and Haase et al. [4] reported various experimental VLE data. Also Haase et al. [4] reported experimental LLE data. In terms of modeling, Wang et al. [5] modeled VLE of the system based on OLI-MSE model. Another study done by Thomsen [6] showed that with use of electrolyte UNIQUAC model, the HCl+water binary can be characterized at concentrations up to 6 molal. However, he only reported the mixed solvent results at three different temperatures (0, 25 and 80 °C). Also Nichols and Tylor [7] studied many aqueous electrolyte solutions including HCl+water system and parameterize the Pitzer model for 0 to 110 °C and with maximum concentration of 16 molal. In this study, a comprehensive thermodynamic model is developed based on symmetric electrolyte NRTL (eNRTL) activity coefficient model. The model includes all critical thermodynamic properties in three categories: 1) phase equilibrium properties such as vapor-liquid equilibrium and liquid-liquid equilibrium, 2) calorimetric properties such as liquid enthalpy and heat capacity, and 3) speciation properties such as molecular and ionic species concentrations. With use of binary interaction parameters accounting for molecule-molecule binary interactions and molecule-electrolyte interactions, the model accurately calculate all thermodynamic properties including mean ionic activity coefficient, osmotic coefficient, vapor pressure, boiling point, dew point, excess enthalpy and heat capacity. The model is validated with data covering temperature up to 400 K. Furthermore, the HCl+water model is extended to cover the ternary system of HCl+water+methanol which often is used in industry and chemical processes such as preparing methyl esters of volatile fatty acids. Moreover methanol is often used as a co-solvent, for example it may be combined with other components such as hydrogen chloride [8]. Prior studies on this HCl+water+methanol ternary system are rare. However Lu et al. [9] reported experimental VLLE data (TPxy) for this system in atmospheric pressure and temperature range from 70-95 °C. Furthermore they calculated the mean ionic activity coefficient based on Pitzer model up to 8 molal. Also Deyhimi et al. [10] determined mean molal activity coefficients for HCl in the mixture based on potentiometric data of a galvanic cell and modeled the system by use of Pitzer and Pitzer–Simonson–Clegg ion-interaction equation with HCl concentration limited to 5.75 molality. In contrast, the eNRTL model provides a comprehensive thermodynamic model for the ternary system. The model results are shown in a triangle phase diagram which covers the entire concentration range from 0 up to 100% for methanol, water, and hydrogen chloride.

References:

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