On small-scale liquefaction of natural gas with supersonic separator: Energy and second law analyses

Abstract

Under certain conditions supersonic separators promote superior natural gas conditioning over traditional routes. An instigating possibility consists in using supersonic separators to obtain liquefied natural gas that exhibits global growth. This work analyses natural gas liquefaction through a low-scale process using realistic non-isentropic supersonic separator whose design was proposed in the literature. Results are compared with other compact adiabatic expansion routes like the Joule-Thomson and turbo-expander, all starting from natural gas at (298.15 K, 80 bar) and reaching saturated liquefied natural gas at (115.4 K, 1.25 bar). The objective was to demystify the notion that 100% liquefaction per pass is possible with supersonic separator by obtaining the maximum liquefaction fraction per pass for the three compact routes and unveiling whether supersonic separators lead to a less power intense, technically, thermodynamically and economically superior short-scale liquefaction process. To this end, a HYSYS Unit Operation Extension was created to simulate supersonic separators with rigid-geometry, following expected physical behavior and satisfying the 2nd Law of Thermodynamics. Results include a 2nd Law analysis of processes leading to thermodynamic efficiency and liquefaction lost work. Supersonic separator route reached the highest Net Present Value of all routes but was somewhat outperformed by turbo-expander route in terms of power consumption, thermodynamic efficiency and lost power.