Two-Phase Cryogenic Expander for Nitrogen Rejection and Cooling Process in Natural Gas Applications

Abstract

Abstract A typical refinery process of natural gas requires cyclic cooling (liquefaction) and vaporization of the fluid in stages to separate and remove certain impurities, primarily nitrogen, from the ‘raw’ gas. Natural gas from underground sources often has nitrogen contamination due either to its natural occurrence or from previous nitrogen injection to enhance recovery of the natural gas. Nitrogen content must be limited or reduced by a nitrogen rejection process in order to meet the heating value requirement for the natural gas. This process is fundamentally a refrigeration cycle, within which the process pressure must be reduced; traditionally via a throttling process with a Joule-Thomson valve. Replacing the Joule-Thomson valve with a two-phase flashing expander can improve the overall efficiency of this refrigeration process with enhanced cooling, and electrical power co-generation. This paper considers prior art, and reviews the current design considerations and technical challenges for a two-phase expander to be used in the Nitrogen Rejection process. Hydraulic and aerodynamic components of such two phase expanders are reviewed, and their operational characteristics are studied in this paper. Thermodynamic analyses of cooling and liquefaction processes are conducted to quantify the benefits of two phase expanders in terms of cooling, energy recovery and production rate. Site operational data and performance characteristics of an actual two-phase expander are reviewed to better understand two phase expansion within the expander.