1. Thermodynamic Enthalpy of Cryogenic Liquefaction
Transforming high-pressure feed gas into subcooled atmospheric liquid LNG requires extracting three sequential thermal duties:
- Sensible cooling of vapor from feed temperature (\(+25^\circ\text{C}\)) to dew point (\(\approx -50^\circ\text{C}\)).
- Latent heat of condensation across the hydrocarbon boiling envelope (\(-50^\circ\text{C}\) to \(-100^\circ\text{C}\)).
- Liquid subcooling down to \(-162^\circ\text{C}\) to prevent boil-off gas (BOG) flashes during low-pressure storage transfer.
The total cryogenic thermal cooling duty \(Q_{\text{cryo}}\) across the Main Cryogenic Heat Exchanger (MCHE) is:
\[ Q_{\text{cryo}} = \dot{m}_{\text{LNG}} \cdot \Delta h_{\text{liq}} \quad [\text{MW}_{\text{thermal}}] \]
where \(\Delta h_{\text{liq}} \approx 850\text{--}920\ \text{kJ/kg LNG}\) depending on feed pressure and heavy hydrocarbon content.
2. Specific Power Consumption (\(W_{\text{spec}}\)) by Liquefaction Architecture
The thermal efficiency of a liquefaction process is quantified by its specific power consumption, defined as mechanical shaft work required per unit mass of produced LNG:
\[ W_{\text{spec}} = \frac{P_{\text{shaft}}}{\dot{m}_{\text{LNG}}} \quad [\text{kWh/tonne LNG}] \]
Commercial thermodynamic benchmarks under ISO conditions (\(25^\circ\text{C}\) ambient):
- Propane Pre-Cooled Mixed Refrigerant (C3MR): \(280\text{--}320\ \text{kWh/t}\) (Thermodynamic state-of-the-art for baseload plants).
- Dual Mixed Refrigerant (DMR): \(320\text{--}360\ \text{kWh/t}\) (Eliminates pure propane for offshore FLNG).
- Single Mixed Refrigerant (SMR / PRICO): \(380\text{--}440\ \text{kWh/t}\) (Low equipment count for small-scale plants).
- Nitrogen Dual Expander: \(490\text{--}560\ \text{kWh/t}\) (Zero hydrocarbon refrigerant inventory).
3. Main Cryogenic Heat Exchanger (MCHE) Spool-Wound Dynamics
In mixed refrigerant cycles, the MCHE utilizes spool-wound aluminum tube bundles inside a vertical shell. High thermal efficiency requires matching the refrigerant warming curve to the natural gas cooling curve, maintaining a narrow temperature approach (\(\Delta T_{\text{app}} = 2.0\text{--}4.0^\circ\text{C}\)). The Coefficient of Performance (COP) is:
\[ \text{COP} = \frac{Q_{\text{cryo}}}{P_{\text{shaft}}} \]
4. Driver Machinery Sizing & Scope 1 Decarbonization
Compressor shaft power \(P_{\text{shaft}}\) determines driver sizing. When driven by industrial gas turbines (Frame 7/9) or aeroderivative gas turbines (LM6000), fuel gas consumption and direct Scope 1 \(\text{CO}_2\) emissions are evaluated as:
\[ \dot{m}_{\text{fuel}} = \frac{P_{\text{shaft}}}{\eta_{\text{driver}} \cdot \text{LHV}_{\text{fuel}}} \qquad \dot{m}_{\text{CO}_2} = \dot{m}_{\text{fuel}} \cdot \left( \frac{44}{16} \right) \quad [\text{t/h}] \]
Electrification via variable-speed electric motors (E-Drive) eliminates on-site combustion emissions, reducing plant carbon footprint to zero Scope 1.