ENERGY SAVING IN LNG PRODUCTION
Natural gas (generally pressure regulated pipeline or wellhead gas) conversion into LNG takes place at gas temperature of -260°F (-162°C) and pressure of approximately 4 psig. Large energy consumption required to achieve cryogenic temperatures can be substantially reduced by utilizing the available at letdown facilities pressure differential to precool LNG feed. Per UVI’s arrangement a separate pressure regulated gas flow cooled by a combined Joule-Thomson and vortex phenomenon is applied to precool LNG feed gas. The core of this UVI technology is the non-freeze (self-heating) Vortex Pressure Reducer (VPR). The VPR configured as Dual Stream Vortex Tube (VT) operates as a primary pressure regulator of non-preheated pipeline or wellhead gas.The ‘vortex’ chilling generated in the Vortex Tube’s assisted pressure regulation of natural gas is an energy saving and environmentally friendly addition to a conventional process of LNG production. The ‘vortex’ solution offers a substantial precool of LNG feed gas without consumption of any external, manmade source of cryogenic
A flow diagram and example of the VT-enhanced LNG production at a typical pipeline pressure regulation station (PRS) is shown below. The specified PRS gas inlet and outlet pressures here are 800 psi (54.5 bar) and 100 psi (7 bar), the pipeline high pressure gas temperature is 40°F (4.5C). 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
The high pressure pipeline gas at 800 psi (54.5 bar) and 40°F (5C) undergoes non-freeze pressure reduction and the subsequent energy separation in the Self Heating Vortex Tube forming two outlets: cold at negative 71°F (-57°C) and hot at 93°F (34°C). The VT cold outlet at -71°F is heat exchanged (HEX) with the, taken upstream of the PR and previously dried, LNG feed. Assuming the use of a HEX with a conventional 10°F (5°C) thermal approach the LNG feed comes out of the HEX at -61°F (-52°C). After cooling the LNG feed flow, the VT cold and hot outlets are then combined and the combined flow at some 45°F (7°C) 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 -260°F
Energy saved: -61°F – (-260°F)/40°F – (-260°F) = 0.66 Saving 34% of Energy Required to Produce LNG!
'Vortex' 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 vortex tube can operate in series, the energy saving nearly doubles (well head applications) or triples (CNG powered ‘vortex’ LNG gas precool)
In the course of vortex LNG feed precool, VT flow is always just a portion of gas main flow passing through a Letdown facility; the technology also provides for no single cu. ft. of VT gas flow lost in operations.
There is no practical limit on vortex tube gas flow rate and pressure in the vortex LNG feed gas precool.