(397e) Preliminary Synthesis of Work Exchange Networks
Energy production, usage, costs, and associated CO2 emissions are global concerns. Many process industries including gas processing, refineries, petrochemicals, air, ammonia, urea, etc. are major consumers of energy. Little doubt exists that our first action to make such plants more sustainable should be to increase energy efficiency and integration. However, the major focus so far has been heat integration (Hasan et al., 2009; Furman & Sahinidis, 2002). Some recent work has begun to address various other energy-related networks such as steam-turbine (Han et al., 2006), utility (Bruno & Grossmann et al., 1998), compressor (Razib et al., 2009), etc. Del Nogal et al. (2010) studied the synthesis of mechanical driver and power generation to find a feasible network to meet the demand of different compressor stages. Aspelund et al. (2007) studied the integration of pressure and heat energy in the context of subambient cooling process using mostly thermodynamic and heuristic based approach. However, to our knowledge, the idea and synthesis of work exchange networks that we propose in this work has not been formalized or studied.
Compression is a major consumer of energy in many chemical plants. Invariably, some process streams may need energy in the form of mechanical work for compression, while others may lose energy through expansion. The common approach is to use utility-driven compressors for compressing low-pressure streams and expansion through a valve or turbine for high pressure streams. This clearly offers an opportunity for exchange of work between high-pressure and low-pressure streams using single-shaft-turbine-cum-compressors (SSTC) and major savings in energy are possible. Some plants such as natural gas and air liquefaction have begun to exploit this on an ad hoc basis. However, to the best of our knowledge, no systematic integration methodology exists in the literature for Work Exchange Network Synthesis (WENS).
While the basic idea of a WENS is analogous to HENS (Heat Exchanger Network Synthesis) or MENS (Mass Exchanger Network Synthesis), it poses several major challenges mainly due to the complex operating characteristics (e.g. head vs. flow) of compressors and turbines. Moreover, heat and power being alternate forms of energy, both heat and pressure energy must be considered in WENS, as opposed to only heat in HENS. Razib et al. (FOCAPD, 2009) proposed a Mixed Integer Nonlinear Programming (MINLP) formulation for Compressor Network Synthesis (CNS) to integrate pressure energy among multiple streams. They used a stage-wise superstructure to consider various plausible network configurations. However, their preliminary work did not consider the real operational constraints related to surging, choking, and shaft speed. They also assumed linear cost functions for compressors and other equipment. In this work, we propose and formalize the novel concept of WENS, where the energy supply and demands of process streams at different pressures are integrated to minimize the total fixed and operating cost of the process. The network involves equipment such as centrifugal compressors, gas turbines, electric motors, generators, valves, heaters, coolers, and single-shaft-turbine-compressors (SSTC) that exchange work among multiple low-pressure and high-pressure streams. We develop a multi-stage compression / expansion supersturcture for each stream and develop a novel and complex mixed-integer nonlinear programming formulation for WENS. We simplify and incorporate the complex operating curves of turbines and compressors to the etxent possible to make the problem solvable. Finally, we demonstrate the benefits of our novel approach through a meaningful and realistic case study.
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