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is subcooled, its enthalpy is negative, causing a decrease in vapor leaving the tower by a factor L~L. The stream L (1 - a~) is called internal retlux. Summing the effect of subcooled reflux with that of variable feed enthalpy yields their combined effects on distillate flow: D = V + yd - (1 - qL)L (11.12)
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If distillate flow is made the dependent variable, it then depends not only on reflux and vapor rates, but also on feed and reflux enthalpies. Composition is so difficult to control under these conditions, that specialpurpose computers are often used to compensate for QF and q~.~ But if distihate flow is made independent of the heat balance, no such measures are necessary. Manipulation of heat input to control column temperature is not recommended for most applications. Because the temperature of a boiling pure liquid is constant, the sensing element is usually moved up the tower to a t.ray where the temperature of the saturated liquid is a measure of its composition. Temperature is meant to infer composition-yet because it is temperature, many engineers feel that it is a function of heat input. It has been shown, however, that D/F is 100 times more effective in controlling composition than V/F. The reason that manipulation of heat input affects temperature is beause reflux is on flow control, such that D is dependent on V. If the temperature is too low, additional heat is sent to the reboiler, which ultimately increases distillate flow. With this arrangement, the heat balance is deliberately upset in order to alter the material balance.
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Control over product quality can best be achieved by manipulat ing the material balance, free from disturbances in the heat balance. This means that the composition co&roller should set the flow of one of the product streams. Whether distillate or bottoms flow is chosen to be manipulated depends on their relative flow rates. The greater absolute flow accuracy will be obtained by manipulat~ing the smaller flow. So before a decision is made, a complete material balance should be drawn and the flows of the various streams compared. Whenever distillate flow is set to hold the material balance, the bottoms-level controller must manipulate bottoms flow. This allows the heat input to be set independently to establish the separation capability of the tower. For this reason, whenever comparison of the product flow rate does not overwhelmingly favor manipulat8ion of bottoms flow for composition control, distillate flow should be selected. A temperature element located part way up the tower is no assurance of absolute quality control. Changes in separation can alter t he composi-
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tion profile to the extent that product quality could vary somewhat without a noticeable deviation in temperature. The variability is relative to the dist ance between the product and the measuring element. Consequently, an analyzer in t,he product stream provides a much more reliable measure of quality. Icurthermore, some separations are so difficult that the temperature profile is too gradual to hold any meaning.1
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If bottoms flow is under level control, heat input to the column may be fixed at any desired value. Or it may be manipulated to control bott,oms composition, if distillate flow is manipulated to control its own composition. But in any ease, a cascade loop should be used. Steam is the most common source of heat to reboilers. Assuming that steam quality is reasonably constantj, heat flow is regulated simply by a steam flow loop. Occasionally hot, oil is used t,o supply heat to the reboiler. In t his event , heat flow is not linear with oil flow, as Fig. 9.3 verifies, and another indication must be used. Vapor loading in a tower is sensed as t he differential pressure across the trays. A differential-pressure measurement from top to bott,om of the tower can then serve as an index of vapor flow, with the trays acting like an orifice.5 Differential-pressure control by manipulation of heat input is a very fast loop, almost as f&t as an ordinary flow loop. Hence it is very responsive to variations in tower loading such as would be encountered where feed or producbt streams are in t,he vapor phase. In fact it is recommended in these situations even when steam is the heat,ing medium. If bottoms flow is chosen to be manipulated for composition control, bottoms level must be cont,rolled from steam flow. This is not as s,imple a process as in most liquid-level loops, because of the reboilcr between the manipulated and the cont,rolled variables. Since smooth control over boilup is mandatory, this loop must, be heavily damped. Consequent ly, a wide proportional band is necessary, and reset is relied upon t o niaint,ain level. JIost of the vapor is eventually rekned t o the reboiler as liquid some time later, again affcc%ing liquid level. This results in a natural period of several minut.es. But because bottoms flow in this instance is small, relative to the rate of boiling, lit,tle coupling exists, and upsets are few.
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