Vapour liquid equilibrium measurements for process design
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Doctoral thesis (article-based)
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Chemical engineering report series, Kemian laitetekniikan raporttisarja, 45
AbstractIn recent years it has become increasingly important to develop new oxygenate and isooctane technologies and processes that meet the continuously stricter environmental requirements. Some of the new process schemes use renewable raw materials in order to meet the European Union biofuel requirements. One of the most important requirements for the design of such separation processes includes the knowledge of vapour liquid equilibrium (VLE) behaviour. There are methods to estimate VLE but for the final design and with new systems VLE needs to be determined experimentally. Unfortunately, the existing equipment used for the VLE measurements suffer from labour intensiveness. The application of automation to VLE measurement apparatuses provides increasing accuracy and speed, thus reducing the cost for VLE measurements. In the present work three different apparatuses were developed. Using the constructed apparatuses, VLE was measured for relevant systems in the modelling and design of oxygenate and isooctane technologies. Firstly, a static apparatus for VLE measurements was built, which allowed the analysis of samples from the liquid and vapour phases, by means of an automated sampling system. Measurements were made on ethanenitrile + 2-methylpropane and ethanenitrile + 2-methylpropene systems. No VLE measurements were not found in the current literature for the systems measured with the static apparatus in this work. The systems measured disclosed a positive deviation from Raoult's law. In addition, azeotropic behaviour was observed for the ethanenitrile + 2-methylpropane system. Secondly, a circulation still was made. The still was used to obtain isobaric and isothermal VLE data for nine alkane + alcohol and alkene + alcohol binary systems. An on-line system with circulation of the samples was tested with two analysis methods, mass spectrometry and gas chromatography. The on-line system was then applied to the ethanol + 2,4,4-trimethyl-1-pentene and 2-propanol + 2,4,4-trimethyl-1-pentene systems at atmospheric pressure and vapour pressure was determined for 2-methoxy-2,4,4-trimethylpentane. VLE measurements were made for the methanol + 2-methoxy-2,4,4-trimethylpentane system. Again, no VLE measurements were found in existing for most of the systems measured with the recirculation still. The results exhibited positive deviation from Raoult's law. All the systems measured exhibited azeotropic behaviour, with the exception of the methanol + 2-methoxy-2,4,4-trimethylpentane system. Thirdly, a static total pressure apparatus was constructed. With the manual version of the apparatus 12 binary systems consisting of alkanes + 2-butanol and alkenes + alcohols were measured. The static total pressure apparatus was upgraded to one of a computer-controlled level, which requires substantially less labour than the manual version of the apparatus. Using the computer-controlled version, measurements were made for five binary systems consisting of 2-methylpropene + alcohols. Most of the measurements made with the static total pressure apparatus were for systems for which measurements have not been available earlier. The systems measured exhibited positive deviation from Raoult's law and some of the systems exhibited azeotropic behaviour. The gamma-phi approach was used for modelling the systems measured. The vapour phase was calculated with the Soave modification of the Redlich-Kwong-equation and the Wilson activity coefficient model was used for modelling the liquid phase behaviour. Legendre-polynomials were used in the Barker's method for the data reduction of the static total pressure measurements. In addition to the Wilson equation parameters, NRTL and UNIQUAC activity coefficient model parameters were also determined for the C4-alkene + alcohol systems measured with the static total pressure apparatus. The Wilson equation provided the best fit of the measurements, compared to NRTL and UNIQUAC models. The Antoine-equation was used for describing the vapour pressures of the pure components with the exception of the static total pressure measurements, for which the actual measured vapour pressure values were used.
vapor, liquid, equilibrium, measurement, apparatus, alcohol, hydrocarbon, mixtures
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