(337b) Challenges in Low Temperature Process Design

The aim of this work is to show the challenges in Low Temperature Process Design and how thermodynamic tools make it possible to develop an efficient design. Low temperature processes need large quantities of energy to operate, hence any improvement in technology or design methodology have significant relevance in this industrial field. Large amounts of energy are needed to drive the compressors for the refrigeration cycles and energy consumption increases at lower temperature levels. Therefore, such processes must be designed as efficient as possible.

Pinch Analysis (PA), has become the primary thermodynamic tool for designing heat transfer networks [1]. PA concepts have been successfully used and extended over the years to fit various types of processes and design constraints i.e. [2-4]. However, PA has its limitations when it is applied for designing cryogenic processes since the only design variable used is temperature.

In low temperature process design, temperature is not the only important variable. The pressure level has an enormous influence on energy consumption, thus it must also be considered as a design variable. Often when temperature and pressure are design variables, stream phase may have a significant importance in the processes. In fact, many low temperature processes deal with the phase change as their main objective, i.e. LNG processes and air separation. Consequently, temperature, pressure and stream phase are the main design variables for low temperature processes [5]. For this reason, basic PA is not sufficient to deal with all the requirements for this type of designs.

Generally, process engineering uses the second law of thermodynamics to evaluate process performance since it indicates how efficient energy is used in the process [6, 7]. Exergy Analysis (EA) is not commonly used as a process design tool, more often to measure the quality of the resulting process, although it has a high potential to aid the design of efficient processes. The combined use of PA and EA as design tools for low temperature processes has given encouraging results in our research group. Aspelund et al. [8] proposed a design methodology (ExPAnD) combaining PA and EA and they used it in a Liquefied Energy Chain (LEC) [9] for utilizing stranded natural gas for power production with offshore liquefaction, ship transport, and onshore re-gasification.

The exergy content in a process stream can be divided into chemical exergy and thermo-mechanical exergy. The thermo-mechanical exergy can be further divided into temperature based exergy and pressure based exergy. Therefore, EA can handle the low temperature process design variables and it can also be used as the main process efficiency driver. EA definitions however, are still not homogeneous in the literature and within the scientific community. In particular, there are different definitions of exergy efficiency available, and this work suggests a general and standardized definition that is also suitable for sub-ambient processes.

References

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