(7b) Thermodynamic Study on Refrigeration System and Application Analysis

A refrigeration system generally works by indirectly transferring the thermal energy from low-temperature sources to high-temperature sinks at the expense of electricity or mechanical work, such that the source temperature can be lowered or maintained at a certain value. In many industrial processes, the refrigeration system is employed for chilling and freezing the final liquid/solid products or liquefying the gas intermediates/products for the downstream cryogenic separation. It would be impossible for chemical/petrochemical process industries with the absence of the refrigeration system, such as the production of ethylene, propylene, oxygen, nitrogen, and ammonia. The performance of a refrigeration system is closely related to product quality and energy use efficiency, and thus the plant profitability. Meanwhile, because of the low-temperature operational characteristic, the refrigeration system inherently runs at a potential safety risk. Thus, in chemical/petrochemical industry practice, the performances of a refrigeration system under both normal and turnaround conditions are always the major concerns for the plant owners, engineers, and operators. Therefore, the operational optimality of a refrigeration system should be studied at the very beginning of the conceptual design stage. The design of a chemical plant refrigeration system is a very challenging task. The complexity mainly manifests in the following two aspects: i) An industrial refrigeration system usually needs to satisfy the cooling demands from many different units working at different temperatures. Thus, a refrigeration system should simultaneously work in different recycling loops at multiple temperature and pressure levels. Meanwhile, different refrigeration loops are highly interacted, and thus should be well integrated in terms of minimizing electricity or mechanical work consumptions. ii) A complex refrigeration system sometimes needs to accomplish different refrigeration tasks covering a wide temperature range. For instance, the refrigeration system at ethylene plants needs cooling different process streams changing from 23 to -100.8 Celsius degree. Thermodynamically, it is impossible by using only one refrigerant as the sole working media. Thus, multiple refrigerants have to be employed under this situation, such that each refrigerant will have its own working loops and cover the refrigeration tasks within a specific temperature range. Meanwhile, different recycling loops from different refrigerants may be optimally and cascadely integrated to jointly accomplish some refrigeration tasks. Note that there is still lacking of the systematical study on the optimal design of chemical industrial refrigeration system with the consideration of integrated multi-refrigerants and multi-working loops simultaneously. A typical example with such a complexity is the refrigeration system of an ethylene plant, where the refrigerants of ethylene and propylene are cascaded and coordinated to work in multiple loops for the cryogenic separation of many process streams. To address the conceptual design of such a cascade refrigeration system under various industrial constraints, a systematic optimization model containing multi-refrigerant and multi-temperature loops have to be developed and solved. In this paper, a general MINLP (mixed-integer linear programming) model for the optimal synthesis of the cascade refrigeration system in ethylene plants has been developed, where multiple refrigerants with multiple recycling loops are simultaneously addressed. Meanwhile, the exergy-temperature (B-T) chart combined with the exergy analysis, for the first time, is presented to comprehensively analyze the thermodynamic nature of a refrigeration system, which provides a solid foundation for the conceptual design of a complex refrigeration system. The efficacy of the developed methodology is demonstrated through a case study with industrial data.