Thermodynamic, economic, and environmental performance evaluation of boil-off gas condensate recovery by liquid nitrogen with power generation as byproduct

Abstract

As stricter national standards for volatile organic compounds (VOCs) emissions are implemented, the emission of VOCs is a growing concern. There is an urgent necessary to optimize the existing condensate recovery system structure to meet national emission standards for boil-off gas. In existing VOCs recovery methods, the drawbacks include the frequent regeneration of adsorbent materials, limitations in the condensation recovery of isomers, and the lack of self-sufficiency in power generation. Therefore, a novel boil-off gas condensate recovery system by liquid nitrogen (LN) was proposed in this study. Based on the first and second laws of thermodynamics, a numerical optimization model of the system was established and the system performance of thermodynamic, economic and environmental was analyzed. The results show that the system achieves condensation and recovery of boil-off gas. The LN pump has an optimal outlet pressure. When the pressure is 1.95 MPa, the system achieves maximum net power output, and when the pressure is 0.85 MPa, the system achieves maximum LN savings. Additionally, the higher the ambient temperature, the greater the net power output. At lower ambient temperatures, the system supplies a greater cooling capacity. The system demonstrates good economic and environmental performance under higher ambient temperatures and outlet pressure of LN pump. When the pressure is 1.95 MPa, the system achieves the maximum net present value, standard coal savings and carbon dioxide emission reduction. The system has a maximum payback period of only 0.35 years. The system could be used in engineering applications to improve the overall performance of the liquefied natural gas plant.