Performance optimization of turboexpander-compressors for energy recovery in small air-separation plants

In this paper, a reverse-bootstrap turboexpander-compressor (TEC) scheme is proposed for the direct recovery of expansion work into the cooling capacity of small air-separation plants. A mathematical model is established for the codesign and off-design analysis of TEC. The off-design performance is predicted based on the mean streamline method and is validated by the experimental data. The isentropic coefficient is proposed to evaluate the overall performance improvement of the reverse-bootstrap TEC. The results indicate that the use of a reverse-bootstrap compressor in the original turboexpander can increase the isentropic coefficient by 12% compared with that of the traditional braking turboexpander under design condition. The codesign of the turboexpander and the coaxial compressor can improve the coupling characteristics and further increase the isentropic coefficient by 7.1% under design condition, which shows cooling power improvement by 8.1% and reduction of required heating power by 1.7%. The results and analysis demonstrate that the proposed reverse-bootstrap TEC with the codesign method is an effective way for the energy recovery in small refrigeration systems.