Cross-pinch heat transfer

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. 

The algorithm may calculate an increase in Qnmin and Qcmin- This means that the match is transferring heat across the pinch or that there is some feature of the design that will cause cross-pinch heat transfer if the design was completed. If the match is not transferring heat across the pinch directly, then the increase in utility will result from the match being too big as a result of the tick-off heuristic. 

Figure 16.8 Three forms of cross pinch heat transfer.

The net heat fiow across Pinch is zero. Consequently, the system can be split into two stand-alone subsystems, above and below the Pinch. Above the Pinch there is need only for hot utility, while below the Pinch only cold utility is necessary. For given AT the hot and cold utility consumption identified so far becomes Minimum Energy Requirements (MER). No design can achieve MER if there is a cross-pinch heat transfer. 

Below the pinch, the system rejects heat and so is a net heat source. When a heat recovery system design does not have cross-pinch heat transfer, that is, from above to below the pinch, the design achieves the minimum hot and cold utility requirement under a given ATmin-... 

On the other hand, if cross-pinch heat transfer is allowed, what could happen In Figure 9.10b, cross-pinch (XP) amount of heat is transferred from above to below the pinch. The system above the pineh now loses XP units of heat to the system below the pinch. To restore the heat balance, the hot utility must be increased by the same amount, that is, XP units. Below the pinch, XP units of heat are added to the system therefore, the cold utility requirement also increases by XP units. The consequence of a cross-pinch heat transfer (XP) is that both the hot and cold utilities will increase by the XP amount, the cross-pinch duty. 

A helical baffle bundle built in this w produces two distinct flow regions. The area outside of the adjacent baffle contact diameter tends to produce a stable helical cross flow. However, inside the diameter where adjacent baffles touch is a second region where vortical flow is induced but in which the intensity of the rotational component tends to decrease as one approaches the center of the bundle. For a fixed flow rate and helix angle, this tendency may be minimized by the proper selection of the baffle contact diameter. With the correct selection, stream temperatures may be made to be close to uniform across the bundle cross section through the shell. However, below a critical velocity (for the baffle configuration and fluid state), the tendency for nonuniformity of temperatures increases as velocity decreases until ever-increasing portions of the central core surface area pinch out with respect to temperature and become ineffective for further heat transfer. 

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