Heat Exchanger Networks - Stream Data

Consider first the issue of changing the process conditions and how those changes might be directed to improve the heat recovery. 

If a process change, such as a change in distillation column pressure, allows shifting a hot stream from below the pinch to above, it has the effect of increasing the overall hot stream duty above the pinch and therefore decreasing 

Another way to relate these principles is to remember that heat integration will always benefit by keeping hot streams hot and keeping cold streams cold5. 

2 THE TRADE-OFFS BETWEEN PROCESS CHANGES, UTILITY SELECTION, ENERGY COST AND CAPITAL COST 

Having to readjust the capital-energy trade-off after every process change would be a real problem if it were not for the existence of total cost targeting procedures discussed in Chapter 17. 

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HEAT EXCHANGER NETWORK STREAM DATA - SUMMARY... 

Example 16.1 The process stream data for a heat recovery network problem are given in Table 16.1. A problem table analysis on these data reveals that the minimum hot utility requirement for the process is 15 MW and the minimum cold utility requirement is 26 MW for a minimum allowable temperature diflFerence of 20°C. The analysis also reveals that the pinch is located at a temperature of 120°C for hot streams and 100°C for cold streams. Design a heat exchanger network for maximum energy recovery in the minimum number of units. 

Effective temperatures. When extracting stream data to represent the heat sources and heat sinks for the heat exchanger network problem, care must be exercised so as to represent the availability of heat at its effective temperature. For example, consider the part of the process represented in Figure 19.8. The feed stream to a reactor is preheated from 20°C to 95°C before entering the reactor. The effluent from the reactor is at 120°C and enters a quench that cools the reactor effluent from 120°C to 100°C. The vapor leaving the quench is at 100°C and needs to be cooled to 40°C. The quenched liquid also leaves at 100°C but needs to be cooled to 30°C. How should the data be extracted ... 

Given the information provided by the stream data (see also section 8.2), determine the minimum total annualized cost heat exchanger network by providing ... 

Heat exchanger network Simultaneous minimization of both the total annual cost and the composite environmental index. Analytic hierarchy process Chen et al. (2002) smdied only one case, whereas Wen and Shonnard (2003) smdied three cases with different stream data. Chen et al (2002) Wen and Shonnard (2003)... 

Composite Curves enable to determine directly the Minimum Energy Requirements (MER) from stream data without ever calculate heat exchangers. These are the minimum hot d minimum cold utility required for driving the heat exchanger network, with a minimum driving force of AT at Pinch. 

For example, accurate analysis of a heat exchanger network performance requires values of flow rate, inlet and outlet temperatures of hot and cold streams for each heat exchanger from test runs as well as exchanger areas. Simulated duties of hot and cold streams of the heat exchanger are compared with test run data. If there is mismatch, simulation input data should be compared with the test run data, and suitable adjustments of stream flow rates and/or temperatures are required to minimize the mismatch. 

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