Prediction of the ideal maximum operational temperature of hydrocarbon and HFCs as working fluids of organic Rankine cycle power plants based on transition state theory

Abstract

The use of organic working fluids in organic Rankine cycle (ORC) power plants may lead to thermal decomposition, which can negatively impact system performance and safety. Therefore, it is essential to determine the safe operating temperature range of the working fluids. In this paper, a simplified theoretical method based on the transition state theory is proposed to predict the ideal maximum operational temperature of working fluids. This method involves considering the initial decomposition reaction of the working fluids and calculating the total reaction rate of the initial decomposition using the transition state theory. By setting an acceptable decomposition rate, the ideal maximum operational temperature of the working fluid can be determined. Using this method, the total reaction rate constants of several typical hydrocarbon and hydrofluorocarbon working fluids, including n-butane, isobutane, n-pentane, isopentane, neopentane, n-hexane, HFC-245fa, HFC-134a, HFC-125, HFC-236fa, HFC- 227ea, HFC-143a, and HFC-152a, were obtained in the temperature range of 300~700K. The predicted maximum operational temperatures were in good agreement with most experimental results. Furthermore, by analyzing the branch ratio of each initial decomposition path of these working fluids, it was found that C-C bond cleavage contributed mainly to the decomposition of hydrocarbons, while the removal of HF and C-C bond cleavage contributed mainly to the decomposition of HFCs. As the CC bond breakage paths and HF removal reaction paths have high branching ratios (>99%), only considering these reactions in the reaction rate calculation can greatly reduce the calculation cost without significantly impacting the prediction accuracy.