On the sustainability of small-scale expansion-based natural gas liquefaction: Supersonic separator, Joule-Thomson, and turbo-expander

Expansion-based liquefaction technologies are prime candidates for small scale offshore natural gas liquefaction, aiding in reducing NG chain logistical costs, due to their compactness. This study assesses the sustainability and profitability of three expansion-based small-capacity natural gas liquefaction technologies: the rigid-geometry supersonic separator, the Joule-Thomson valve and the turbo-expander. The rigid-geometry supersonic separator was simulated for liquefaction of nearly pure methane via a thermodynamically rigorous steady-state modeling. Supersonic separator inlet conditions were optimized via a response-surface predictor of its critical flowrate in terms of temperature and pressure. Such optimum inlet conditions were found to be T = -15.79 degrees C, P = 80 bar for a maximum 2.5 Mach. With these conditions a multi-criteria analysis was conducted combining sustainability metrics to heuristic criteria into a single one-dimensional index, the Sustainability Degree. Turboexpander was unveiled as the most sustainable process as it has the highest SD = 11.39 due to its lowest power intensity (0.78 kWh/kgLNG), highest net present value (15.89 MMUSD) and minimal carbon emissions. Single unit supersonic separator has a medium performance (SD = 3.5), weighed down by the large natural gas recycle needed, while Joule-Thomson is the worst process due to the largest power consumption (1.66 kWh/kgLNG) and the lowest net present value (4.4 MMUSD).