Frank Del Nogal, Jin-Kuk Kim, Simon Perry and Robin Smith
Centre for Process Integration, The University of Manchester, PO Box 88, Manchester, M60 1QD,United Kingdom
The use of cold energy should be systematically integrated with process streams in low-temperature systems for energy savings and sustainable low-carbon society. In this paper, newdesign methodology for cascaded mixed refrigerant systems with multi-stage heat exchanges has been proposed, which systematically screens, evaluates and optimizes key decision variables in the design of refrigeration cycles (i.e. economic trade-off, partition temperature, refrigerant compositions, operating conditions, refrigerant flowrate). The integrated design and optimization for overallcryogenic energy systems is also addressed to reflect system interactions between driver selections and design of refrigeration systems. Two case studies are illustrated to demonstrate the advantage using developed design methods.
Keywords: Low temperature energy systems, Mixed refrigerants, Power systems, Synthesis, Optimization
1. Provision of low-temperature cooling
The provision of coldenergy to process industries has gained far less attentions from process engineering community, compared to energy systems which is based on high-temperature energy carrier (e.g. steam), although sub-ambient cooling has a significant potential for energy saving in practice. Effective use of cold energy is vital to ensure the cost-effectiveness of low-temperature processes, as significant powerrequirement for compression is one of major energy consumptions in the provision of cryogenic cooling to process streams.
One of widely-used techniques to save energy requirement in the cryogenic energy systems is to apply a heat integration technique, such that most appropriate levels and duties for refrigeration are determined to match them against GCC (grand composite curve), as shown in Figure1 (Linnhoff et al., 1982; Linnhoff and Dhole, 1989; Smith 2005). The GCC represents overall characteristics of energy systems, and this provides better understanding how to design the refrigeration cycles. Figure 1 illustrates the cycle in which pure refrigerant is employed as a working fluid, and two levels of cooling for process streams are facilitated by using multiple expansion. If one levelof refrigeration is provided, all the cooling duty is provided at Level 2, which results in large compressor shaftpower requirements.
The thermodynamic efficiency of the simple cycle can be improved by introducing economizer, vapor cooler and inter-cooler with multi-level expansion (Wu, 2000, Smith 2005). The cascading two simple cycles, in which different refrigerant is used, is a useful wayto reduce shaftpower requirements for compressor when large temperature range is to be covered by refrigeration. Another important degree of freedom for energy saving in refrigeration system is the decision for how to reject heat to or remove heat from process stream(s). These considerations often lead to have a complex cycle with multi-levels and/or cascaded arrangement, which consists of largenumber of unit.
Figure 1. Refrigeration with pure refrigerant
The use of mixed refrigerants in the cycle can simplify the structure of refrigeration cycle as well as reduce compression duty significantly. As illustrated Figure 2a, the close match between hot (process) stream and cold (refrigeration) stream can be achieved by using mixed refrigerant, while pure refrigerant cannot avoidthermodynamic inefficiency due to large gap existed between two streams. The shape of refrigeration stream in Figure 2a depends on the composition of refrigerants and its operating conditions. When large temperature range is to be cooled by mixed refrigerant systems, cascade arrangement is also possible (Figure 2b). Other structural variations to obtain a better match between hot and cold stream...