*6.1. Case Description*

A case is studied in this section to illustrate the proposed synthesis method. The most cost-saving network solution is expected. The case comprises four hot and four cold process streams, for which the characteristics, including heat capacity flowrates, supply temperatures and target temperatures are given in Table 1. Supply and target temperatures of cooling water utility are 25 ◦C and 35 ◦C. Three pressure levels of steam in low, medium and high pressure levels (LPS, MPS, HPS) are defined as 170.4 ◦C, 198.3 ◦C and 263.9 ◦C with the corresponding pressure, so the production and utilization of the steam should be optimized according the performance the entire system. Turbine exhaust steam condensate is 45.8 ◦C [18]. Overall heat transfer coefficients are fixed at 1.0 and 0.5 kW·m−2· ◦C−<sup>1</sup> for steam heating and non-steam heating, respectively. The prices of boiler fuel natural gas and generated power are 0.227 \$·kg−<sup>1</sup> and 0.076 \$·kWh−<sup>1</sup> [18], respectively. Efficiencies of boiler and turbine are set to be 0.9 and 0.7 [17]. Annual operation time is 8000 h·y<sup>−</sup>1. Minimum heat transfer temperature difference is set to be 10 ◦C. Coefficients for turbine cost *atur* and *btur* are 81,594 \$·y−<sup>1</sup> and 18.52 \$·kWh−1·y−<sup>1</sup> [18]. Coefficients for boiler cost *aboil* and *bboil* are 101,840 \$·y−<sup>1</sup> and 3.441 \$ h·kg−1·y−<sup>1</sup> [15].

**Table 1.** Process data of the case study.


It is believed that the structure of HEN and the operation of utility system will impact the entire system greatly. Thus, two design schemes with two scenarios of each are investigated within the synthesis. The schemes are defined according to different condensate recovery manners. In the first scheme, condensates of exhaust steam and multi-level steam (HPS, MPS, LPS) are collected in the deaerator which works at a certain operating temperature [18], and then sent to the boiler. In the other scheme, condensates of multi-level steam are recovered without flowing through the deaerator, and exhaust steam at the turbine end is no longer needed to maintain the operating temperature of the deaerator. For each scheme, two scenarios are studied according to different HEN superstructures. In the first scenario, steam utilities are only used at stream ends. While in the second scenario, the proposed structure is adopted, as using the steam not only within inner-stage but also within inter-stage. Therefore, four situations for the whole system are studied and compared to demonstrate the method.
