Analysis of Real Gas Effects in Natural Gas Turbo-Expanders

Abstract

During transport and storage of natural gas, natural gas turbo-expanders are used to throttle the high pressure gas and to increase the system efficiency by extracting additional work. This study reports how real gas effect influence prediction of the flow field in the turbo-expander focusing on mass flow rate and efficiency. The tested thermodynamic gas models include the polytropic ideal gas law (PIG), Peng-Robinson equation of state (PR), and Redlich-Kwong equation of state (RK). Simulations have been performed with Ansys CFX 17.0. and methane has been selected as the working fluid. Real gas equation of state is validated by comparing the calculated compressibility factor with that of measured REFPROP. Simulation with the RK EOS has been selected as the reference. PIG model shows a 13% and 20% difference the mass flow rate and efficiency, respectively. PR and RK models show similar results, showing less than 2.1% difference in mass flow rate and efficiency. Predicted density determines mass flow rate. With the blocking effect due to separation bubble, the effective area is the smallest in the PIG law and the mass flow rate is also the smallest. The smaller efficiency arises from larger loss. The losses are analyzed at the nozzle, nozzle-impeller interface, and impeller, respectively by entropy generation. With the large expansion ratio, nozzle wake and separation bubble on the impeller suction surface, PIG law shows 3 times larger loss than that of reference. Mass flow rate and efficiency are important design factors, so real gas effects need to be considered for turbo-expander design and analysis.