The first-order phase transition of melting for molecular crystals by Frost–Kalkwarf vapor- and sublimation-pressure equations

Thermodynamic quantities in the coexistence of the liquid and the solid phases for Frost–Kalkwarf vapor- and sublimation-pressure equations are investigated at an isobaric process. Gibbs free energy changes in the gaseous and the liquid phases, ΔG GL, has been derived from the Frost–Kalkwarf vapor-pressure equation. Similarly, Gibbs free energy changes in the gaseous and the solid phases, ΔG GS, may be estimated by the Frost–Kalkwarf sublimation-pressure equations which are determined by data of sublimation pressures and temperatures for 24 substances. In coexistence between the liquid and the solid phases, Gibbs free energy changes in the liquid and the solid phases, ΔG LS, may be expressed as the difference of ΔG GL and ΔG GS. The melting temperatures and enthalpy changes of melting are evaluated by numerical calculations for 24 substances. The behaviors of H2O for the neighborhood at the melting and the boiling points are investigated. The Gibbs free energy indicates two polygonal lines. Entropy, volume and enthalpy jump from the solid to the liquid phase at the melting point and from the liquid to the gaseous phase at the boiling point. The heat capacity does not diverge to infinity but shows a finite discrepancy at the melting and the boiling points. This suggests that first-order phase transitions at the melting and the boiling points may occur.

The Development of SF6 Green Substitute Gas

Due to its high greenhouse effect, the use of SF6 as the main insulating gas is restricted in the electric power field. Along with the aim of environmental protection, the search for new alternative gases with a lower greenhouse effect and higher insulation strength has received a lot of attention. The properties of alternative gases have a vital impact on the performance of medium-voltage power distribution equipment. Firstly, based on the existing liquefaction temperatures of SF6/N2, SF6/CO2, and SF6/CF4, the calculated liquefaction temperatures were expanded to 0.7 MPa. Combining the Antoine vapor-pressure equation and the basic law of vapor–liquid balance, the vapor pressures of SF6/N2, CF3I/N2, c-C4F8/N2, C4-PFN/N2, C4-PFN/CO2, and C5-PFK/CO2 were obtained. Secondly, the critical breakdown field strength was analyzed for C4-PFN/CO2, C5-PFK/CO2, SF6, CF3I/N2, C5-PFK/Air, and c-C4F8/N2. Finally, the GWPs of SF6/N2, C4-PFN/N2, C4-PFN/CO2, C5-PFK/CO2, and C5-PFK/N2 were discussed. The results show that the liquefaction temperature gradually decreases as the pressure rises; SF6/N2 has the highest vapor pressure at −5 °C; the critical breakdown field strengths of several mixtures are higher than that of SF6.

Property Data Estimation for Hemiformals, Methylene Glycols and Polyoxymethylene Dimethyl Ethers and Process Optimization in Formaldehyde Synthesis

Polyoxymethylene dimethyl ethers (OMEn) are frequently discussed as alternative diesel fuels, with various synthesis routes considered. OME3–5 syntheses demand significant amounts of thermal energy due to the complex separation processes that they entail. Therefore, innovative process designs are needed. An important tool for the development of new processes is process simulation software. To ensure sound process simulations, reliable physico-chemical models and component property data are necessary. Herein we present the implementation of a state-of-the-art thermodynamic model to describe the component systems of formaldehyde-water and formaldehyde-methanol using Microsoft® Excel (2010, Microsoft Corp, Redmond, WA, USA) and Aspen Plus®, (V8.8, Aspen Tech, Bedford, MA, USA) determine the deviation between the calculated results and experimental literature data, and minimize the deviation by means of parameter fitting. To improve the accuracy of the estimation of the missing property data of hemiformals and methylene glycols formed from formaldehyde using group contribution methods, the normal boiling points were estimated based on molecular analogies. The boiling points of OME6-10 are determined through parameter regression in accordance with the vapor pressure equation. As an application example, an optimization of the product separation of the state-of-the-art formaldehyde synthesis is presented that helps decrease the losses of methanol and formaldehyde in flue gas and wastewater.

Dew Points of C4F8/N2 Gas Mixtures

C4F8/N2 gas mixtures are possible substitutes for SF6, because of its good insulation characteristics and low influence to environment. The thrust of this paper is to verify the dew point of C4F8 and C4F8/N2 gas mixtures, the dew points of C4F8/N2 mixtures are achieved using experimentally measuring and theory calculating method. The experimental data obtained is compared to that predicted from the Clausius-Clapeyron equation, Trouton’s rule and vapor pressure equation. The results show that the experimental data verify well with the predictions. So the prediction method can be used to predict the dew point of C4F8/N2 gas mixtures for different charging pressures.