Dynamic modeling of a mechanically coupled organic Rankine-vapor compression system for compression heat recovery based on an improved lumped parameter model

Abstract

Compression heat recovery is getting attention in cryogenic air separation units for its high energy saving potential, while the organic Rankine-vapor compression system (ORVC) is considered as an attractive technology for both compression heat recovery and air pretreatment. As the states of the compression heat inevitably change with the ambient environment, dynamic simulation of the ORVC-air compression heat recovery system (ORVC-ACS) is carried out in this paper. An improved lumped parameter model that enables to solve the dynamic model with complex structure and on large-time scale is proposed and validated. The working fluid density in the heat exchanger is treated as phase-based average in this improved model. The average relative deviations between the simulation results and experimental data are less than 6.8%. The annual dynamic characteristics and varying-duty operation performance of the ORVC-ACS are further studied. The results show that the ORVC-ACS could basically work under the design condition, with the maximum temperature fluctuation amplitude of about 6.9 °C, response time of about 4–6 min without varying-duty conditions and 8 min with duty changing. Besides, for a 60,000 Nm3/h scale ORVC-ACS, the annual energy saving reaches about 13.75 GWh, equivalent to CO2 emission reduction of about 9.9 Mt, and heat recovery ratio of about 76.4%, illustrating great energy saving and environmental protection performance. This research may benefit subsequent control scheme design and actual operation.