An extended method for work and heat integration considering practical operating constraints

Abstract

The development of methodologies for the simultaneous work and heat integration has increasingly been the focus of recent research. Approaches may vary among optimization and heuristic-based methods considering direct and indirect work exchange, in addition to, or with the development of new strategies for heat recovery. This work presents a strategy for the synthesis of work and heat exchanger networks (WHEN) considering the use of single-shaft-turbinecompressor (SSTC) units. The method is based on a meta-heuristic approach and aims fundamentally at synthesizing WHEN that may operate within industrial-like conditions, which are often narrower than those considered in the literature due to simplification assumptions. Therefore, in the present work, practical temperature upper/lower-bound constraints are considered for pressure manipulation units, and the number of coupled units per shaft is limited. Evidently, these constraints yield additional difficulties for the optimization method. Therefore, new calculation blocks are included in a previous block-based model. The method considers inlet and outlet temperatures as decision variables, making the maintenance of solutions within feasible range more efficient during the optimization runs. Moreover, a new Simulated Annealing (SA) based strategy is developed for deciding optimal compressor/turbine couplings in a model that considers a preset number of "slots" per shaft. The method aims at minimizing total annual costs (TAC) and is tested over four case studies. The first two are used both as benchmark for TAC comparison to those reported in the literature as well as for testing the new constraints. The other two cases are investigated for TAC and energy-wise improvements to original designs. Considerable economic improvements and better use of energy are attained in all cases. The method also proved efficient in maintaining solutions within practical operating ranges.

Multiperiod work and heat integration

“…The model is presented in detail in our previous study [32]. Decision variables are presence/absence of heat exchangers, their heat loads and stream split fractions, presence/absence of pressure manipulation units and their inlet/discharge temperatures.…”

“…The model developed in this work consists in a complete revamp of the simultaneous WHEN synthesis model presented in our previous study [32]. That WHEN model is based on the idea of streams passing multiple times through a heat exchange area.…”

“…The derivation of the model is similar regarding energy balances, pressure/temperature-related, shaft-work rate and heat exchange area calculations. The detailed derivation of these equations is presented in the referred work [32]. Figure 1 illustrates the master superstructure concept and the blocks used in the model.…”

“…Note that, for simplicity, some blocks such as individual compressors/turbines that were present in Ref. [32] were removed, since equivalent structures can be obtained with the hybrid blocks presented in Figure 1. Figure 1.…”

“…Pavão et al [30] presented the block-based superstructure, where calculation blocks for each unit type were placed in the pressure manipulation region, and also included the concept of each stream passing multiple times through a heat exchange region, previously used by Wechsung et al [25] and Onishi et al [26]. The method was later extended to evaluate multiple electricity-related scenarios [31] and practical operating constraints such as maximum number of coupled units and more realistic temperature limits for compressors and turbines [32]. A pinch-based approach was developed by Pavão et al [33] for efficient determination of pressure manipulation routes in WHEN.…”