1. The thermodynamic minimum
For a gas at ambient T_amb and pressure being cooled and condensed to LNG at atmospheric pressure and its boiling point (-161 °C for pure methane), the reversible (exergy) work is set by the gas-side enthalpy and entropy change:
Using a methane proxy with |ΔH| ≈ 920 kJ/kg and an effective |ΔS| ≈ 5.4 kJ/(kg·K) calibrated to the published ideal minimum, at ~308 K ambient Wmin ≈ 743 kJ/kg ≈ 0.20 kWh/kg LNG — consistent with the literature ideal minimum work of ≈ 0.18–0.20 kWh/kg to liquefy natural gas from near-ambient (Mokhatab et al., 2014). The best modern AP-X trains achieve ≈ 0.27 kWh/kg, i.e. an exergy efficiency of roughly 70–75 % relative to this ideal minimum. Real cycles approach the limit by matching the cooling-curve shape with a mixed refrigerant that vaporizes progressively across the temperature range, minimizing log-mean temperature-difference (exergy) losses.
2. Cycle comparison
| Cycle | SP (kWh/kg) | Typical train size | Use case |
|---|---|---|---|
| AP-X (Air Products) | 0.27 | 5–8 MTPA | Mega-trains (Qatar, Sabine Pass) |
| C3MR (Air Products) | 0.295 | 4–5 MTPA | Workhorse; ~60 % of global capacity |
| DMR (Shell) | 0.31 | 4–5 MTPA | Arctic (Sakhalin) & warm-climate variants |
| Cascade (ConocoPhillips) | 0.33 | 3.5–5 MTPA | ConocoPhillips Optimized Cascade ® |
| SMR (Black & Veatch PRICO) | 0.345 | 0.5–2 MTPA | Mid-scale, lower CAPEX |
| N₂ expander | 0.45 | < 1 MTPA | FLNG / FPSO / peak shaver (Brayton cycle) |
The pattern is clear: cycles whose refrigerant is a mixed-component blend (MR) follow the gas-cooling curve more closely than pure-component cascades — fewer log-mean temperature-difference losses → lower exergy destruction → lower SP. Pure N₂ is the simplest and lowest-CAPEX but pays an exergy penalty because it never changes phase (Brayton refrigeration).
3. Ambient T impact
SP rises roughly 0.5 % per °F above 95 °F. Mechanism: warmer cooling-medium temperature raises the hot-side condensing pressure of the refrigerant, increasing compressor head and shaft work for the same cold-side duty. Hot-climate plants (Qatar, Trinidad, Australia) lose ~ 15 % of nameplate capacity in summer if not de-bottlenecked.
Liquid-water cooling (sea or river) gains 2–4 % over air cooling at the same dry-bulb because the wet-bulb sink temperature is lower and stays cooler in daytime.
4. Refrigerant inventory
| Cycle | Inventory (tonnes / MTPA) | Composition |
|---|---|---|
| C3MR | 5,000 | ~2,000 t C3 + ~3,000 t MR (N₂/C1/C2/C3) |
| DMR | 4,500 | Warm MR + cold MR (two mixed-refrigerant loops) |
| SMR | 3,500 | Single MR with N₂/C1/C2/C3/iC5 |
| Cascade | 6,000 | Discrete C1, C2, C3 loops; high inventory total |
| AP-X | 7,000 | C3MR + nitrogen sub-cooling loop |
| N₂ | 500 | Compressed gaseous N₂ only — small inventory, no flammable |
Inventory matters for HSE (jet-fire vapor cloud after a major release) and for the operational cost of importing and adjusting refrigerant composition during commissioning. The N₂ cycle is the only fully non-flammable refrigerant — a significant advantage for FLNG where the deck space and tank inventory are tightly constrained by Class society rules.
5. References
- GPSA Engineering Data Book §16 — Hydrocarbon Recovery (incl. LNG); cycle specific-power benchmarks.
- Lim, W.; Choi, K.; Moon, I. (2013). "Current status and perspectives of LNG plant design." Ind. Eng. Chem. Res. 52, 3065–3088.
- Roberts, M.J.; Petrowski, J.M.; et al. (1997). "The reduction of LNG capital cost." Air Products technical paper.
- Mokhatab, S.; Mak, J.; Valappil, J.; Wood, D. (2014). Handbook of Liquefied Natural Gas. Gulf Publishing.
- Pillarella, M.; Liu, Y.-N.; Petrowski, J.; Bower, R. (2007). "The C3MR liquefaction cycle: versatility for a fast growing, ever changing LNG industry." 15th International Conference on LNG.
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