Excess Subcooling or Superheat

Excess feed subcooling steps up condensation and liquid traffic in the column. This is countered by the control system or the operator, who will cut back reflux and condenser duty. Overall, column vapor and liquid loads will change only slightly, and excess subcooling will not be troublesome. 

If reflux cannot be cut back (e.g., in an unrefluxed stripper, in azeotropic distillation, or when the packed section above the feed is close to its minimum wetting limit), boilup will need to be raised to compensate for the excess subcooling. Vapor and liquid traffic below the feed and reboiler duty will rise and effectively lower the column feed capacity. Premature flooding may result. If the lower capacity or higher reboiler duty cannot be tolerated, feed preheating (Fig. 12.5a) 

In another case (150a), a subcooled feed entered the bottom sump of a packed rectifier. Preheating this feed consumed 40 percent of the boilup the balance entered the rectifier s packed section. When the feed was shut off, the heat sink was eliminated, causing excessive vapor flow and consequent flooding in the packing. The problem was solved by installing an override controller that would cut back boilup upon excessive packing pressure drop. 

The above discussion pertains to subcooling and superheating effects that tend to persist. Additional considerations apply to handhng subcooled or superheated feeds during ahnormtil operation, such as startup, shutdown, or utility failure. Some of these are  

When a highly suhcooled feed enters a hot column at startup, fast feed introduction or a sudden stepup of feed rate can induce rapid vapor condensation. The ill effects and preventive measures are siiiiilar to those discussed in guidelines 1 to 4 in Sec. 11.7. 

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