Integrated heat recovery

Polley GT, Reyes Athie CM and Gough M (1992) Use of Heat Transfer Enhancement in Process Integration, Heat Recovery Syst CHP, 12 191. 

Following the above mentioned indications, two example lines with integrated heat recovery and complete flue gas cleaning systems are presented. 

A major feature of the Kellogg process is the integrated heat recovery system to utilize the otherwise waste heat from various process streams and from the reformer flue gas, which is used to raise 100-bar superheated steam. The steam is let down in a turbine, which drives the synthesis gas compressor. Passout steam at 40 bar is used for process steam and for other machine drives. Passout steam from some of the smaller turbines at 4.5 bar is used for minor process heating duties. This total energy concept is largely responsible for the efficiency improvement over earlier processes. 

Smith, R., and Jones, P. S., The Optimal Design of Integrated Evaporation Systems, Heat Recovery Systems and CHP, 10 341, 1990. 

TABLE 6.6 Stream Data for Heat Recovery Between Two Areas of Integrity... 

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. 

Fig. 1. Schematic of nitric acid from ammonia showing integration of reactor heat recovery, power recovery from tailgas, and air compression (3).

The cold recovery device consists of an exhaust air humidifier with an integrated heat exchanger and the supply air heat exchanger, which are connected by a fluid circuit. The first can be described as an indirect evaporative cooler. The cold recovery device is able to transport 83% of the maximum possible enthalpy difference from the exhaust air to the supply air. 

Table 16.6 Stream data for heat recovery between two areas of integrity.

Ahmad S and Hui DCW (1991) Heat Recovery Between Areas of Integrity, Comp Chem Eng, 15 809. 

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). 

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