Energy, exergy and economic analyses of an optimal use of cryogenic liquid turbine expander in air separation units

Abstract

Cryogenic liquid turbine expanders have emerged quite recently as a replacement for traditional Joule-Thomson valves in cryogenic processes for energy saving purpose. However, introducing a liquid turbine expander into the cryogenic air separation unit (ASU) can influence the operating points of ASU components, and the maximum benefits arising from the liquid turbine expander have not yet been reported. To fill such a gap, thermodynamic models are first established for ASU components, including the main air compressor, booster air compressor, cryogenic gas and liquid turbine expanders, main heat exchanger, cryogenic pump, and distillation column. To achieve the best possible use of the liquid turbine expander, an optimization method is developed by incorporating the thermodynamic models with the particle swarm optimization algorithm, in which the total minimization power consumption of ASU is the optimization goal while the flow rates and pressures of the high pressure air stream and the air stream drawn from the booster air compressor middle section together with the temperature of the medium pressure air stream exiting the main heat exchanger midway act as the optimization variables. Then, energy, exergy and economic analyses are conducted and the corresponding indexes are chosen to quantitatively evaluate the influence of the liquid turbine expander on the air separation process. For the case study, ASUs of 35000 Nm3/h oxygen production capacity with and without a cryogenic liquid turbine expander are optimized, respectively. The results show that the ASU total power saving through the use of liquid turbine expander is 4.2-fold its output shaft power; thus, the energy consumption of unit oxygen decreases by 2.12% and the overall exergy efficiency increases by 1%. In addition, the economic analysis shows that the period of return of the liquid turbine expander in the ASU is estimated to be 1.75 years.