Distillation-Effect of Feed Preheat

           Increasing feed preheat to a distillation tower typically makes fractionation worse, assuming that the reflux rate is fixed. That means, that if the concentration of the heavy component in the overhead product is held constant, then the concentration of the light product in the tower’s bottom product will increase.

            But why?

            The reason is the reboiler must decrease. If the reflux rate is held constant, and the concentration of the heavy component in the overhead is held constant, then the condenser duty (i.e., heat removal) is also constant. Thus, if I increase the heat input with the feed, it follows that the reboiler duty must go down to maintain the tower heat balance.

            A decrease in the reboiler duty will also decrease the vapor flow through the trays between the feed tray and the bottom tray. As this vapor flow goes down, the stripping section trays don’t work as hard. Or, we can say that the stripping factor is reduced. Or, perhaps, the rate of tray deck leakage increases. Either way, the amount of the lighter component slipping into the bottom of the distillation tower will increase.

            What then are the benefits of increased feed preheat? There must be such benefits, otherwise most of our towers would not have feed preheaters:

 

• Saves Energy – The feed preheater normally uses waste heat or low pressure steam, with little value. This saves more valuable reboiler steam.
• Supplements Reboiler Capacity – If the tower is not condenser limited, but limited by the capacity of the reboiler, then more feed preheat will permit higher reflux rates, and hence better fractionation efficiency.
• Stops Flooding in Bottom Section Trays – If the tower is limited by flooding or entrainment in the bottom stripping trays, then more feed preheat, which reduces the reboiler duty, will improve fractionation. This happens because the vapor velocities will go down, as the feed preheat goes up.

 

So, depending on the hydraulic capacity of the stripping trays, feed preheat can make fractionation better or worse.