AIChE Journal Highlight: Comparing Exergy and Heat Pump Work in Distillation Systems

With the growing detrimental consequences of greenhouse gases, the chemical process industries are under intense scrutiny to reduce emissions. A significant share of these emissions stems from the combustion of fossil fuels used to meet process heating demands in the sector. Notably, distillation annually accounts for roughly 2.5 Quad (1 Quad = 1.055×10¹⁵ kJ) out of about 100 Quad of the total energy consumption in the U.S. Given the high thermal duty associated with reboiler operation, distillation systems represent a critical opportunity for reducing process heating-related emissions.

Upon closer examination, it becomes evident that the distillation process does not inherently “consume” heat; rather, in most cases, the thermal energy supplied to the reboiler is largely conserved and ultimately discharged at the condenser as low-grade “waste heat” that must be rejected from the system. Importantly, separation in distillation is not achieved directly through the reboiler heat itself, but through the thermodynamic work realized by degrading the reboiler heat to the condenser temperature. Heat pumps capitalize on this fundamental characteristic of distillation by upgrading the heat rejected at the condenser for reuse in vaporizing the liquid at the reboiler. The electrical work required for this heat upgrading can be significantly lower than the original reboiler duty in...