Performance limit of gas compression processes enhanced by self-utilization of compression waste heat

Abstract

Self-utilization of compression waste heat has been proposed for the energy saving of gas compression process, in which the low-grade compression waste heat is recovered to generate cooling capacity in situ to cool the inlet gases of each stage compressor through thermally-driven refrigeration cycles. Efficiency enhancement of the gas compression process is achieved, since the actual compression process in this manner gets closer to an ideal isothermal compression process. However, the performance limits remain unclear due to the lack of consensus on internal coupling mechanism in the literature. Based on the theory of gas compression and thermo-power conversion, a thermodynamic model under ideal conditions is established by sequential Carnot cycles to analyze the performance limit of enhanced gas compression processes for a 60,000-Nm3/h scale air separation unit. The effects of key factors including cooling capacity distribution, number of compression stages, and ambient temperature on the energy conservation are further analyzed. The results show that when the ambient temperature is 25 ℃, the ideal energy saving rate of the three-stage self-enhancement system can reach 7.4%. The boundary maps of the performance improvement of the gas compression systems are given for reference in practice. In addition, a parametric analysis is carried out extending to the general compression systems. The results show that the single-stage pressure ratio, adiabatic index and isentropic efficiency of compressors, which directly affect the grade of waste heat, are the most critical factors affecting the performance of the self-enhancement system, while the distribution of cooling capacity has little effect. This study proves the superiority of this emerging improvement measure over traditional exothermic compression and multistage compression with effective interstage cooling, along with guidance for estimating energy saving potential under different conditions and in different applications.