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## Part 6

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**Part 6**Synthesis of Heat Exchanger Networks**6.1 Sequential Synthesis**Minimum Utility Cost**Example 1**Steam: 500 C Cooling water: 20 – 30 C Minimum recovery approach temperature (HRAT): 20 C**Remarks**• LP for minimum utility consumption leads to the same results as the Problem Table in Pinch method. • The transshipment model can be generalized to consider multiple utilities to minimize total utility cost. • This model can be expanded so as to handle constraints on matches. • This model can also be expanded so as to predict the matches for minimizing the number of units. • We can embed the equations of the transshipment model within an optimization model for synthesizing a process system where the flows of the process streams are unknown.**Example 2**HP Steam: 500 K, $80/kW-yr LP Steam: 380 K, $50/kW-yr Cooling Water: 300 K, $20/kW-yr HRAT: 10K**Sequential Synthesis**Minimum Utility Cost with Constrained Matches**Heat Exchange Options**• Hot stream i and cold stream j are present in interval k (see figure in the previous page). • Cold stream j is present in interval k, but hot stream i is only present at higher temperature interval (see figure in the next page).**Example 1**Steam: 500 C, $80/kW-yr Cooling water: 20 – 30 C, $20/kW-yr Minimum recovery approach temperature (HRAT): 20 C The match between H1 and C1 is forbidden.**Condensed Transshipment Model**The annual utility cost: $9,300,000.**Expanded Transshipment Model**Annual Utility Cost: $15,300,000 Heating Utility Load: 120 MW Cooling Utility Load: 285 MW**Sequential Synthesis**Prediction of matches for minimizing the unit number**Heat Balances**The constraints in the expanded transshipment model can be modified for the present model: • The heat contents of the utility streams are given. • The common index i can be used for hot process and utility streams; The common index j can be used for cold process and utility streams.**Example 1**Steam: 500 C Cooling water: 20 – 30 C Minimum recovery approach temperature (HRAT): 20 C**Sequential Synthesis**Automatic Generation of Network Structures**Basic Ideas**• Each exchanger in the superstructure corresponds to a match predicted by the MILP model (with or without pinch partition). Each exchanger will also have as heat load the one predicted by MILP. • The superstructure will contain those stream interconnections among the units that can potentially define all configurations. The stream interconnections will be treated as unknowns that must be determined.**Embedded Alternative Configurations**• H1-C1 and H1-C2 in series • H1-C2 and H1-C1 in series • H1-C1 and H1-C2 in parallel • H1-C1 and H1-C2 in parallel with bypass to H1-C2 • H1-C1 and H1-C2 in parallel with bypass to H1-C1