ENERGY SAVING IN LNG PRODUCTION
Natural gas (generally, pressure regulated pipeline or wellhead gas) conversion into LNG takes place at gas temperature of -260F (-162 C) and pressure of approximately 4 psig. Per UVI’s arrangement a separate gas flow cooled by a combined Joule-Thomson and vortex phenomenon is applied to precool LNG feed. The core of this UVI technology is the non-freeze (self-heating) Vortex Pressure Reducer configured as Dual Stream Vortex Tube (VT) that operates as a primary pressure regulator of non preheated pipeline or wellhead gas.
A flow diagram and an example of the VT-enhanced LNG production at pipeline pressure regulation station (PRS) is shown below. The specified PRS gas inlet and outlet pressures here are 800 psi and 100 psi, the pipeline high pressure gas temperature is 40F. The VT is set to split its inlet flow in the 60% (cold outlet) to 40% (hot outlet) proportion; this value produces the greatest possible cooling duty (BTU/HR)
The high pressure pipeline gas at 800 psi and 40F undergoes pressure reduction and energy separation in the Self Heating Vortex Tube forming two outlets: cold at -71F and hot at 93F. The VT cold outlet at -71F is heat exchanged (HEX) with the taken upstream of the PR and previously dried LNG feed (LF). Assuming the use of a HEX with a conventional 10F thermal approach the LF comes out of the HEX at -61F. After cooling the LNG feed flow the VT cold and hot outlets are then combined and the combined flow at some 45F is dumped into the PRS downstream pipe. Since the VT flow is substantially smaller of the pipeline gas flowrate, the VT discharge pressure will adapt to the downstream pipe’s pressure.
The energy saved with the ‘vortex’ precool of LNG feed is expressed as follows:
E saved = ∆ T vortex/∆T, where ∆T vortex is the ‘vortex’ LNG feed precool value and ∆T is the gas temperature difference between pipeline gas temperature and -260F
Energy saved: -61F – (-260F)/40F – (-260F) = 0.639. Savings 38%
The energy saving value is variable and is proportional to the available VT’s ‘in/out’ gas pressure ratio at a Pressure Letdown facility. At large pressure ratios where the VT can operate in series, the energy saving nearly doubles.
It is understood that VT flow is always just a portion of the main flow passing through a Letdown facility. It is also understood that no single cu. Ft. of VT gas flow is lost in the course of vortex LNG feed precool operations.
Similarly to other UVI’s non-freeze (self- heating) Vortex Tubes applications in pressure regulation of natural gas, the Vortex Tubes used for LNG feed precool have no practical limits on gas flow rate and pressure. In case a given gas flow requires a very large VT, multiple VT units operating in parallel, or a custom designed/engineered oversized VT are used.
The Vortex LNG feed precool at letdown facility additionally benefits the pipeline / wellhead operator since the non freeze vortex tube provides for pressure regulation of the high pressure gas which, otherwise, would be warmed up in the PRS upstream Line Heater