Improving Thermodynamic Consistency Among Vapor Pressure, Heat of Vaporization, and Liquid and Ideal Gas Heat Capacities

• Main Author: Hogge, Joseph Wallace
• Format: Others
• Published: BYU ScholarsArchive 2017
• Subjects: multi-property optimization; vapor pressure; heat capacity; heat of vaporization; Chemical Engineering
• Online Access: https://scholarsarchive.byu.edu/etd/6634; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=7634&context=etd

Vapor pressure (Pvap), heat of vaporization (ΔHvap), liquid heat capacity (Cpl), and ideal gas heat capacity (Cpig) are important properties for process design and optimization. This work focuses on improving the thermodynamic consistency and accuracy of the aforementioned properties since these can drastically affect the reliability, safety, and profitability of chemical processes. They can be measured for pure organic compounds from the triple point, through the normal boiling point, and up to the critical point. Additionally, ΔHvap is proportional to the derivative of vapor pressure with respect to temperature through the Clapeyron equation, and the difference between Cpl and Cpig is proportional to the derivative of heat of vaporization with respect to temperature. In order to improve temperature-dependent correlations, all the properties were analyzed simultaneously. First, a temperature-dependent error model was developed using several versions of the Riedel and Wagner Pvap correlations. The ability of each correlation to match Cpl data was determined for 5 well-known compounds. The Riedel equation performed better than the Wagner equation when the best form was used. Second, the Riedel equation form was further modified, and the best correlation form was found for about 50 compounds over 7 families. This led to the development of a new vapor pressure prediction method using different Riedel equation forms to fit Pvap, ΔHvap, and Cpl data simultaneously. Seventy compounds were tested, and the error compared to liquid heat capacity data dropped from 10% with previous methods to 3% with this new prediction method. Additionally a differential scanning calorimeter (DSC) was purchased, and melting points (Tm), enthalpies of fusion (ΔHfus), and liquid heat capacities (Cpl) were measured for over twenty compounds. For many of these compounds, the vapor pressure data and critical constants were re-evaluated, and new vapor pressure correlations were recommended that were thermodynamically consistent with measured liquid heat capacity data. The Design Institute for Physical Properties (DIPPR) recommends best constants and temperature-dependent values for pure compounds. These improvements were added to DIPPR procedures, and over 200 compounds were re-analyzed so that the temperature-dependent correlations for Pvap, ΔHvap, Cpig, and Cpl became more internally consistent. Recommendations were made for the calculation procedures of these properties for the DIPPR database.