Transcritical CO2+Fluorobenzene High-Temperature Heat Pumps Thermally Integrated with Air Separation Unit Systems: Mixture Bubble Points Measurements and Cycle Analysis

Carbon dioxide (CO₂) is a commercially viable working fluid for transcritical high-temperature heat pumps (HTHPs), but its low critical temperature (31°C) limits efficiency with cold sources above 40°C. Using a CO₂-based mixture with a higher critical temperature dopant can mitigate this issue by reducing temperature differences at the evaporator and enhancing heat transfer. This study focuses on fluorobenzene (C6H5F) as a dopant due to its high critical temperature, low cost, negligible global warming potential, and regulatory compliance in Europe. To address the lack of vapor-liquid equilibrium (VLE) data for CO₂-fluorobenzene mixtures, an experimental campaign is conducted at the University of Brescia to measure bubble points using an isochoric apparatus. The data are used to select the most suitable equation of state for cycle calculations. The heat pump’s analysis – both at design and off-design- is evaluated in a case study of integration with an Air Separation Unit (ASU). The authors selected this specific case study because it represents one of the most continuous processes in the industry, whereas the integration of heat pumps is often challenged by the intermittency of heat sources in industrial applications. Waste heat from the ASU’s main air compressor is exploited through an intermediate closed-loop water cooling circuit operating at around 90°C. The heat pump upgrades this thermal energy to supply a local district heating network with a design supply temperature of 110°C. At this design point, the system achieves a coefficient of performance (COP) of 7.7, decreasing to 6.2 for higher off-design supply temperatures (e.g., 127°C).