Many problems have been encountered in making head measurements in and around distillation columns, that is, column AP, liquid level, and specific gravity. Of these column-base level is probably the one where difficulties are most often encountered. It is the subject of this section. These difficulties are at least partially process oriented in the sense that a column base with associated reboiler is often very complex. Heat must be transferred, liquid must be partially vaporized, and liquid and vapor must be properly separated. In the discussion that follows, we will assume that process design is adequate. We will be concerned only with problems associated with determining level by means of head measurements.
Regardless of the type of transmitter used—AP or displacer—the basic equation relating liquid level, density, and head is (see Figure 11.5):
APH = head, lb force/ft2, between two nozzles gL = local gravity acceleration, ft/sec2
gc = mass-force conversion factor
= 32.2 ft lb mass/sec2 lb force pL = liquid phase density, lb mass/ft3
pT = vapor phase density, lb mass/ft3
H = liquid—vapor interface elevation above bottom nozzle, feet
AHn = nozzle-to-nozzle spacing, feet
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