Subcooled feed

The last design variable which needs to be fixed before the design can proceed, but which is of lesser importance, is feed condition. Subcooled feed (i.e., below bubble point)... 

For instance, for subcooled feed q> 1, for saturated liquid q = 1, and for saturated vapor q— 0. Upon introducing also the reflux ratio... 

For the case of a boiling-point liquid or subcooled feed, vFi = 0, lFi — FX(, and hence the moles of vapor entering plate / — 1 is equal to the moles of vapor leaving plate j. Thus, b, / /, may be computed from the number values found for Vfi/di and vfi/bi as follows... 

When the appropriate values for lFi and vFi are employed, Eq. (2-48) may be used to calculate b-Jdi for a feed of any thermal condition. For bubble-point liquid and subcooled feeds, = FXt and vFi = 0. For feeds that enter the column as dew-point and superheated vapors, vFi = FX and lFi = 0. 

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. 

Figure 17.7e shows column pressure control by adjusting column boilup. This method is complex, but it has worked smoothly in some instances (234). Either a flooded or a nonflooded reflux drum can be used in the latter case, reflux drum level can regulate the rate of condenstaion. Bottom flow is regulated by the bottom sump level. This method may be beneficial in some stripping columns receiving subcooled feeds, where feed temperature variations can affect column pressure to a larger extent than overhead condenser action. 

Midtioomponent sqnration, two-feed column Following replacement of trays ly structured packing, bottom product could not meet specifications. HETP below the feed was half that expected. Raising reflux ratio did not improve se>aration. Problem was solved by preheating the hi y subcooled feed to its bubble point. Hi y subcooled feed can cause a premature coltunn bottleneck. 

The caloric factor / is obtained from a heat balance over the feed cross section. Since for the preheating of a subcooled feed to boiling conditions the condensation of vapor AG, is necessary. 

Perhaps someone has retrofitted the stripper feed to preheat crude before it flows to the stripper. This is a good energy-saving scheme for the crude unit however, a steam stripper will not work on subcooled feed. The feed must be at its bubble point (i.e., the tower drawoff temperature). Otherwise, the steam will not be an effective stripping agent. 

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