Feed preheater heat duty, increase

Up to this point, we have suggested that the weight flow of vapor up the tower is a function of the reboiler duty only. Certainly, this cannot be completely true. If we look at Fig. 4.2, it certainly seems that increasing the heat duty on the feed preheater will reduce the reboiler duty. 

Let us assume that both the reflux rate and the overhead propane product rate are constant. This means that the total heat flow into the tower is constant. Or, the sum of the reboiler duty, plus the feed preheater duty, is constant. If the steam flow to the feed preheater is increased, then it follows that the reboiler duty will fall. How does this increase in feed preheat affect the flow of vapor through the trays and the fractionation efficiency of the trays ... 

The feed heater is used to increase the feed temperature and control the reactor inlet temperarnre. Although the heater efficiency depends on how it is designed and operated, the heater duty is determined by feed preheating requirement. The feed heater outlet temperature can also be a function of the reaction temperature profile—that is, an ascending temperature profile will lower the heater outlet temperature. For a given feed preheat, the heater duty is mainly a function of the heat of reaction and heat recovery. A process engineer can determine the ways to maintain process heat recovery, heater efficiency, and heat flux. 

For fired heaters, the limitations could be heat flux or TWT. The former is applied to heaters with pressure drop larger than 20 psi, which is the most common. The latter is for low-pressure drop heaters. When the heater duty must be increased to handle duty much larger than design, the existing heater may be insufficient in meeting the limits. Installing a new heater could be very expensive. The most effective way to avoid this is to increase feed preheating via process heat recovery. 

One of the most effective reformer modifications is to use heat from the convection section to preheat radiant section feed. This will reduce radiant section duty and firing rate. The effectiveness of this option is limited only by the risk of coking in the preheat coil, the metallurgy of the preheat coil and the metallurgy of the radiant inlet system. This option has been used to increase capacity by 10% without increasing the arch temperature in the radiant section86. 

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