Exploration on two-stage latent thermal energy storage for heat recovery in cryogenic air separation purification system

Abstract

Massive heat in the exhaust air of cryogenic air separation purification system is wasted in conventional systems. Its intermittent and dynamic characteristics are major constraints to achieve heat recovery. Cascaded latent thermal energy storage (LTES) is considered as a promising solution, but actual application is rarely reported. This paper initially investigates a two-stage LTES tank which is incorporated with a pilot plant of cryogenic air separation purification system, and a test under a real operation condition is performed. The tank contains five heat storage units in high-temperature stage and five units in low-temperature stage, which are loaded with commercial paraffin of melting temperatures 70 °C and 48 °C, respectively. Moreover, heat pipes combined with vertical fins are assembled to increase the heat transfer rate. Results show that 51369.5 kJ heat is saved in a charging-discharging cycle by introducing LTES tank to air separation system. 64.7% of waste heat in the exhaust air could be recovered based on waste heat criterion of 40 °C. Also worth noting that payback period of LTES tank is 2.3 years, and 25784.3 kg CO2 emission is reduced per year. The LTES demonstrates good performance when compared with other reported LTES for industrial application.