Heat recovery schemes

The flue gas exits the cyclones to a plenum chamber in the top of the regenerator. The hot flue gas holds an appreciable amount of energy. Various heat recovery schemes are used to recover this energy. In some units, the flue gas is sent to a CO boiler where both the sensible and combustible heat are used to generate high-pressure steam. In other units, the flue gas is exchanged with boiler feed water to produce steam via the use of a shell/tube or box heat exchanger. 

The distillation train first separates the benzene/toluene byproduct from main crude styrene stream (8). Unconverted EB is separated from styrene (9) and recycled to the reaction section. Various heat recovery schemes are used to conserve energy from the EB/SM column system. In the final purification step (10), trace C9 components and heavies are separated from the finished SM. To minimize polymerization in distillation equipment, a dinitrophenolic type inhibitor is co-fed with the crude feed from the reaction section. Typical SM purity ranges between 99.90% and 99.95%. 

Warning In all heat recovery schemes, it is very important to minimize transport losses keep ducts and pipes (for hot flue gas, hot air, and steam) short and very well insulated. Similarly, when preheating loads, if they must be transported hot, keep the distances short and cover them with insulation while being transported. 

Suppose, however, that the customer insists on minimum application of feedback controls with no feedforward compensation or overrides. V at column design philosophy should be followed Ha Tng had considerable adverse experience with columns with primitive controls, particularly sidestream drawoff columns, and columns with heat recovery schemes, we suggest the following ... 

Provide 100 percent reserv e capacity in heat-recovery schemes or avoid them altogether. 

Cblumn to be fed is involved in a heat-recovery scheme where the vapor from one column furnishes part or all of the reboil heat for another column or columns. 

Interactions. Elaborate heat-recovery schemes are often highly interactive how is this to be dealt with ... 

Inerts (low boiler) balance. With elabcarate heat-recovery schemes, this is sometimes a problem. Too high a concentration of inerts or low boilers will blanket process-to-process heat exchangers too low a concentration will result in product losses through the vents. 

In view of the above, it is apparent that control of columns with heat-recovery schemes is more difficult than control of conventirmal columns. 

Heat recovery scheme—single source, multiple loads... 

For tight pressure contol, we should use these models with caution. Most of the tight column pressure controls we have studied have closed-loop resonant frequencies in the range of 0.5-2 cpm. For the upper value one should make at least a rov h check of condenser and reboiler dynamics. It may be of interest that the only applications of tight pressure control we have found are in heat-recovery schemes where the vapor from one column serves as the heating medium for the reboiler of another column, and perhaps furnishes heat to other loads. If the vapor flow must be throttled to each load, constant up- and downstream pressures help good flow control. 

Integration of control systems of multiple plant imits. This is additionally significant in many power development projects where modules or cogeneration or waste-heat recovery schemes are commissioned after the main imit has run for a while (add-ons). 

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