(5b) Non-Ideal Carnot Model Provides Insights on LNG Liquefaction Processes

The thermodynamic Carnot engine was originally derived to describe the conversion of heat to work. Due to its reversibility, a Carnot engine in reverse operations also acts as a model for a heat pump (refrigerator). Interestingly, the derivation of the Carnot refrigeration model was complete long before the invention of the first working machine. Nowadays, the cryogenic industry has evolved into very large-scale applications, such as in air separations, ethylene, and LNG plants. To describe such systems, sophisticated simulators are required. The primitive Carnot model appears to be out of place in modern applications.

This work will use non-ideal Carnot models to represent typical heat pumps of two-phase (e.g. cascade) or single-phase (e.g. gas-cycle) configurations. It will be demonstrated that a model using 50% Carnot efficiency is adequate for representing the performance envelope of existing refrigeration systems. Following this, the non-ideal Carnot model is extended to represent single and dual refrigerant circuits. The implications of the crossover temperature from high-temperature to low-temperature in a dual refrigeration circuit will be addressed.

LNG liquefaction processes cover a wide spectrum of designs, for example, refrigerants of pure- versus multi-components, circuits of single-, dual-, versus multiple-loops, configurations of single- versus double-string provisions. Collectively they form an ideal showcase to highlight the points demonstrated by the model. Strict discretion will be applied that only the trend and selected statistics be discussed, but not the identities of individual examples. It is not the purpose of this presentation to differentiate licensed LNG liquefaction processes.