Process streams heat exchanger networks

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. 

Fig. 4. (a) Process streams ia heat-exchange network where A and B represent hot streams, C and D cold streams, (b) The soHd lines represent superstreams, composites constmcted from the process streams of (a). The dashed line represents the hot stream horizontally repositioned to generate a... 

Combinatorial. Combinatorial methods express the synthesis problem as a traditional optimization problem which can only be solved using powerful techniques that have been known for some time. These may use total network cost direcdy as an objective function but do not exploit the special characteristics of heat-exchange networks in obtaining a solution. Much of the early work in heat-exchange network synthesis was based on exhaustive search or combinatorial development of networks. This work has not proven useful because for only a typical ten-process-stream example problem the alternative sets of feasible matches are cal.55 x 10 without stream spHtting. 

It is convenient to represent a heat exchanger network as a grid see Figure 3.24. The process streams are drawn as horizontal lines, with the stream numbers shown in square boxes. Hot streams are drawn at the top of the grid, and flow from left to right. The cold streams are drawn at the bottom, and flow from right to left. The stream heat capacities CP are shown in a column at the end of the stream lines. 

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 ... 

Early work in process synthesis focused on the solution of specific problems, such as the best sequence of distillation columns to perform separation of components in feedstreams into product streams. Another early problem was the synthesis of heat-exchanger networks. 

Heat exchangers are a prime means of conserving energy in process plants by exchanging heat between process streams that need to be cooled and those that need to be heated. Much attention is directed toward optimization of heat conservation and recovery by selecting the proper heat exchanger network. 

One might classify several of these heat exchanger network synthesis algorithms into two broad classes. There are several algorithms which view the synthesis problem as one which selects the next hot process stream/cold process stream match to make. 

A process has four streams with the characteristics given below. Devise a heat-exchange network to maximize the annual savings as compared to no heat exchange. Use a minimum approach temperature ATmin = WC. 

More sophisticated techniques can solve problems with multiphase shell-and-tube exchangers, phase changes of process streams, and varying overall heat transfer coefficients. We may analyze a heat exchanger network as a single heat... 

Do not transfer heat across the pinch point. For example, any process stream heat that is transferred from one side of the pinch point to the other side only increases the requirements for both utilities. The optimal network uses the least number of heat exchangers. 

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