Heat integration recovery

It must be emphasi2ed that any energy costs for the separation in the tradeoffs shown in Fig. 10.7 must be taken within the context of the overall heat integration problem. The separation might after all be driven by heat recovery. 

The appropriate placement of reactors, as far as heat integration is concerned, is that exothermic reactors should be integrated above the pinch and endothermic reactors below the pinch. Care should be taken when reactor feeds are preheated by heat of reaction within the reactor for exothermic reactions. This can constitute cross-pinch heat transfer. The feeds should be preheated to pinch temperature by heat recovery before being fed to the reactor. 

Establish the heat integration potential of simple columns. Introduce heat recovery between reboilers, intermediate reboilers, condensers, intermediate condensers, and other process streams. Shift the distillation column pressures to allow integration, where possible, using the grand composite curve to assess the heat integration potential. 

Consider now the consequences of placing simple distillation columns (i.e. one feed, two products, one reboiler and one condenser) in different locations relative to the heat recovery pinch. The separator takes heat Qreb into the reboiler at temperature Treb and rejects heat Qcond at a lower temperature Tcond There are two possible ways in which the column can be heat integrated with the rest of the process. The reboiler and condenser can be integrated either across, or not across, the heat recovery pinch. 

Consider now a few examples of the use of this simple representation. A grand composite curve is shown in Figure 21.2a. The distillation column reboiler and condenser duties are shown separately and are matched against it. The reboiler and condenser duties are on opposite sides of the heat recovery pinch and the column does not fit. In Figure 21.2b, although the reboiler and condenser duties are both above the pinch, the heat duties prevent a fit. Part of the duties can be accommodated, and if heat integrated,... 

Constraints might be applied for the sake of reducing the capital costs (e.g. to avoid long pipe runs). In addition, constraints might be applied to avoid complex heat integration arrangements for the sake of operability and control (e.g. to have heat recovery to a reboiler from a single source of heat, rather than two or three sources of heat). 

Example 21.1 Two distillation columns have been sequenced to be in the direct sequence (see Figure 21.8). Opportunities for heat integration between the two columns are to be explored. The operating pressures of the two columns need to be chosen to allow heat recovery. Data for Column 1 and Column 2 at various pressures are given in Tables 21.1 and 21.2. 

The appropriate placement of distillation columns when heat integrated is not across the heat recovery pinch. The grand composite curve can be used as a quantitative tool to assess integration opportunities. The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the... 

Aggarwal and Floudas (1992) extended the synthesis approach of nonsharp separations so as to allow for heat integration alternatives. The pressure of each column and the key component recoveries are treated explicitly as optimization variables and a two-level decomposition approach was proposed for the solution of the resulting MINLP model. 

The first alternative simplifies the heat integration of the chemical process, and the second simplifies the interface between the chemical process and the secondary loop. CEA (Leybros, 2009) has designed their S-I process for the first alternative, as the requirements for the reactor expected to be used are most suitable for it. GA has developed flow sheets for both alternatives. Sandia National Laboratories (SNL) is a partner of both CEA and GA in the operation of a demonstration loop for the S-I cycle. SNL is charged with design and operation of the sulphuric acid decomposition section. They have developed a bayonet-heater design for the decomposer which incorporates internal heat recovery. As a result, the outlet temperature of the bayonet heat modules is too low to use in the HI decomposition section. Thus, helium is utilised in the HI decomposition section, as in the CEA flow sheets. 

Fig. 1.2. Schematic flow configurations of heat-integrated processes for coupling endothermic and exothermic reactions, (a) Countercurrent flow of process streams, (b) Cocurrent flow of the process streams in the reactor stages and heat recovery in separate circuits.

In order to compare the economics as well as the overall C02 emissions from each schematic studied in this joint venture, a reference case was analyzed. The reference case included only the process steps associated with coal gasification, shift, and hydrogen purification, but none of the steps associated with C02 sequestration or coalbed methane recovery. Three other process schemes were examined in this study and compared to the reference case. Figure 1 depicts simplified process flow diagrams for the reference case and the other three schemes (note the overall heat integration for each scheme is not shown). The top portion of the figure shows the process steps that are the same for each scheme up to hydrogen purification, while the operations inside the dashed boxes represent the steps that differ between the four cases. 

Consider possible heat recovery and heat integration strategies for all the processes analyzed. 

ARS/HRS (advanced recovery system with heat-integrated rectifier simplification)—Cold fractionation. 

Another solution is given in Fig. 7.3 (Dimian, 1996). This time the heat integration considers a more global viewpoint based on site integration . Excess heat available at high temperature is exported to the utility system. The heat needed to drive the distillation columns is imported from the steam network, at a temperature level compatible with the site policy. Exported energy as high-pressure steam is more valuable, and can be used to produce electricity in a combined heat and power cycle. Therefore, heat recovery is more efficient if treated as a plantwide problem. 

Several hot or cold utilities may be used in a heat recovery project. GCC is the appropriate conceptual tool. Let s review the heat integration of the streams presented in Table 10.1. The same hot load as in Fig. 10.14 can be shared between two steam levels high-pressure (HP) and low-pressure (LP) steam. In the GCC the solution is simple represented by horizontal segments placed at temperatures corresponding to their pressures, as in Fig. 10.23 for three steam levels. This feature makes possible to specify exactly the amount required by each utility. Simple targeting can be applied to optimise their amount if the prices are significantly different. 

In the second step we introduce cost elements sensitive to recycles. For gas recycle we should account for the cost of compression plus compressor depreciation, both on annual basis. For liquid recycle we should consider the operating costs of the distillation column plus the recovery of investment. Preliminary heat integration around the chemical reactor has a feed-effluent heat exchanger (FEHE), as well as a furnace, necessary for start-up and control (Figure 17.3). A rigorous analysis will be present in the next section. 

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