Thermodynamic modeling of formic acid with SAFT-VR Mie DBD for energy applications: Heat pumps, Rankine cycles, and CO₂ electrochemical reduction

Formic acid (HCOOH) has attracted renewed interest as a sustainable chemical intermediate, liquid hydrogen carrier, and potential working fluid for thermodynamic cycles. This work presents a comprehensive thermodynamic study of pure formic acid and its mixtures using the SAFT-VR Mie doubly bonded dimer (DBD) equation of state. The pure component parameters of formic acid were optimized against vapor pressures, liquid densities, and vaporization enthalpies, achieving an excellent description of the experimental data. Binary interaction parameters were adjusted for formic acid mixtures with water, CO₂, and acetic acid, and an accurate description of vapor-liquid equilibria and excess enthalpies is obtained. The model was applied to simulate high-temperature heat pumps and Rankine cycles using carboxylic acids as working fluids. It is found that formic and acetic acids exhibit higher coefficients of performance than conventional refrigerants at elevated temperatures, but face significant practical constraints including corrosion issues, limited temperature range, low operating pressures, and large volumetric flow rates. An integrated process for high-grade formic acid production via CO₂ electrochemical reduction is simulated, combining three-compartment electrolysis cells with pressure-swing distillation for product purification. Energy integration using a formic acid heat pump to supply the distillation reboiler duty reduces the overall electricity consumption of the process making it viable in terms of operating cost. This study provides insights into the thermodynamic feasibility and practical limitations of formic acid in both energy conversion systems and sustainable chemical production processes.