Partially Flooded Reboilers

The partially flocxled reboilcr is similar in many ways to the partially flooded condenser. Usually, although not always, it is a verticad thermosyphon reboiler. As discussed in Chapter 4, Section 2, it is controlled by throttling the stsam condensate, which in turn varies the condensate level in the shell and thereby the heat-transfer area for condensation. That area covered by liquid permits only sensible heat transfer fixMn the condensate this is a small heat load compared with that of the condensing steam and is treated as negligible. 

Hnal signal flow diagram for base level control cascaded to steam flow control 

Preliminaiy signal flow diagram for flooded reboiler 

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Use of lowest pressure steam available. In many plants excess low-pressure steam is availhigh-pressure steam. Where reboiler AT might be too small if the steam were throttled, c ie may use a partially flooded reboiler (see Chapters 4 and 15) and throttle ccmdensate. Since low-pressure steam is seldom available at constant pressure or steam quality, pressure and temperature compensation of flow measurements is highly desirable if steam is throtded instead of condensate. 

Figure 15.10 (Continued) Techniques for preventing heating-side problems in reboilers. (/) an arrangement which restricted vapor flow passages and caused hammering and baffle erosion when surface was partially flooded (g) a vapor belt to minimize entry impingement problems. (Part f, from N. H. Wild excerpted by special permission from Chemical Engineering, April 21,1969. Copyright by McGraw-Hill, Inc., New York, NY 10020.)...

With condensate tubes partially flooded, any further variations in steam flow to the reboiler will affect both the AT in the reboiler and the fraction of tube surface covered by condensate. These two often interact, giving rise to a sluggish, sometimes erratic response. The steam trap will offer little to assist with the control of the condensate level. Further, if the reboiler load changes are sudden, the above-mentioned equilibrium will be difficult to establish or sustain. Instead, cycling may develop, with the control valve... 

The flow of steam to a process heater or reboiler may also be manipu lated by a valve in the condensate line. The rate of heat transfer is actually changed by partially flooding the exchanger with condensate. Because a change in condensate level is necessary to affect steam flow, this system may respond more slowly than direct manipulation of steam flow, hut it has the distinct advantage of requiring a much smaller valve. 

A fourth degree of freedom is consumed to control column pressure. The valves available are condenser cooling (by far the most commonly7 used), reboiler heat input, and feed (if the feed is partially vapor). If a flooded condenser is used, the cooling water valve is wide open and an additional valve, typically located between the condenser and the reflux drum, is used to cover or expose heat-transfer area in the condenser. 

The column has 30 sieve trays, with a total condenser and a partial reboiler. The solvent enters tray 5 and the feed enters tray 15, from the top. The pressure in the condenser is 1.1 atm the pressure at the top tray is 1.2 atm, and the pressure at the bottom is 1.4 atm. The reflux ratio is 5 and the bottoms rate is 960 kmol/h. Use the nonequilibrium model of the ChemSep program to estimate the separation achieved. Assume that the vapor and the liquid are both well mixed and that the trays operate at 75% of flooding. In addition, determine from the tray-by-tray results the average Murphree tray efficiency for each component. 

G3. A distillation column is being designed to process a feed that is 10 mol% ethanol and 90 mol% water. The feed rate is 100 kmol/h, and the feed is a saturated liquid at a pressure of 5.0 atra The column has a partial reboiler, a total condenser, N = 10 in Aspen notation, D = 10, and L/D = 2. Do tray sizing for a sieve try with one pass, 0.4572 m tray spacing, 85% approach to flood (use Jim Fair s flooding calculation method), and default values for other design variables. 

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