Intermediate reboilers

Another design option that can be considered if a column will not fit is use of an intermediate reboiler or condenser. An intermediate condenser is illustrated in Fig. 14.5. The shape of the box is now altered because the intermediate condenser changes the heat flow through the column. The particular design shown in Fig. 14.5 would require that at least part of the heat rejected from the intermediate condenser be passed to the process. An analogous approach can be used to evaluate the possibilities for use of intermediate reboilers. Flower and Jackson," Kayihan, and Dhole and Linnhofl have presented procedures for the location of intermediate reboilers and condensers. 

Establish the heat integration potential of simple columns. Introduce heat recovery between reboilers, intermediate reboilers, condensers, intermediate condensers, and other process streams. Shift the distillation column pressures to allow integration, where possible, using the grand composite curve to assess the heat integration potential. 

Intermediate Reboiler. Inclusion of an intermediate reboder moves the heat-input location up the column to a slightly colder point. It can permit the use of waste heat for rebod when the bottoms temperature is too hot for the waste heat. 

Miiltiple feeds, sidestreams, and/or intermediate condensers and/or intermediate reboilers are required or desirable. 

Low-pressure steam flow rate to the intermediate reboiler is ratioed to feed rate, (e) Vapor sidestream flow rate is set by a temperature controlier holding tray 10 temperature. 

However we choose to look at it, a basic distillation column has two control degrees of freedom. When we turn to more complex column configurations with sidestreams, side strippers, side rectifiers, intermediate reboilers and condensers, and the like, we add additional control degrees of freedom. These more complex systems are discussed in Sec. 6.8. 

More complex columns (with sidestreams, multiple feeds, intermediate reboiler or condenser, etc.) have more degrees of freedom and require... 

FIG. 13-29 McCabe-Thiele diagram for columns with and without an intermediate reboiler and an intermediate condenser. 

FTocesses for the production of tertiary amyl methyl ether (TAME) Brockwell et ah, Hyd. Proc., 70(9), 133 (1991)]. Highly endothermic reactions may require intermediate reboilers. None of these heat management issues preclude the use of reactive distillation, but must be taken into account during the design phase. Comparison of heat of reaction and average heat of vaporization data for a system, as in Fig. 13-97, gives some indication of potential heat imbalances [Sundmacher, Rihko, and Hoffmann, Chem. Eng. Comm., 127, 151 (1994)]. The heat-neutral systems [-AH (avg)]... 

Intermediate Reboilers and Condensers A distillation column of the type shown in Fig. 13-2a, operating with an interreboiler and an intercondenser in addition to a reboiler and a condenser, is diagramed with the solid lines in Fig. 13-38. The dashed lines correspond to simple distillation with only a bottoms reboiler and an overhead condenser. Total boiling and condensing heat loads are the same for both columns. As shown by K ihan [Am. Inst. Chem. Eng. J. Symp. Ser. 76, 192, 1 (1980)], the adclition of interreboilers and intercondensers increases thermodynamic efficiency but requires additional stages, as is clear from the positions of the operating lines in Fig. 13-38. 

The distillation column sketched above has an intermediate reboiler and a vapor side-stream. Sketch a control concept diagram showing the following control objectives ... 

Low-pressure steam flow rate to the intermediate reboiler is ratioed to feed rate. 

Low base level overrides the low-pressure steam flow to the intermediate reboiler. 

Figure 3-3. Countercurrent cascade of flash chambers with intermediate reboilers and condensers, p...

A variety of other configurations and modifications of the basic design shown in Figures 3-6 and 3zZ are possible. Valve trays (see Chapter lOi are popular. Downcomers can be chords of a circle as shown or circular pipes. Both partial and total condensers and a variety of reboilers are used. The column may have multiple feeds, sidestream withdrawals, intermediate reboilers or condensers, and so forth. The column also usually has a host of temperature, pressure, flow rate, and level measurement and control devices. Despite this variety, the operating principles are the same as for the sinple distillation column shown in Figure 3-6. 

Another modification that is used occasionally is to have an intermediate reboiler or an intermediate condenser. The intermediate reboiler removes a liquid side stream from the column, vaporizes it, and reinjects the vapor into the column. An intermediate condenser removes a vapor side stream, condenses it, and reinjects it into the column. Figure 4-23A illustrates an intermediate reboiler. 

Figure 4-23. Intermediate reboiler (A) balance envelopes, (B) McCabe-Thiele diagram...

An energy balance around the column will show that without an intermediate reboiler is equal to Qr + Qi with the intermediate reboiler (F, z, q, x, Xg, p, L7D constant). Thus the amount of energy required is unchanged what changes is the tenperature at which it is required. Since Xg > Xg, the tenperature of the intermediate reboiler is lower than that of the reboiler, and a cheaper heat source can be used. (Check this out with equilibrium data.)... 

Intermediate condensers are useful since the coolant can be at a higher tenperature. See Problem 4.C6. Intermediate reboilers and condensers are fairly common because they allow for more optimum use of energy resources, and they can help balance column diameters (see Section 10.4). During normal operation, they should cause no problems however, startup maybe difficult (Sloley. 1996). 

B2. Several ways of adapting existing columns to new uses were listed. Generate new methods that might allow existing systems to meet product specifications that could not be met without modification. Note that you can postulate a coiiplex existing column such as one with an intermediate reboiler. 

C5. Derive the operating equations for the two middle operating sections when an intermediate reboiler is used (see Figures 4-23A and 4-23BF Show that the operating line with slope of L7V goes through the point y = x = x. ... 

E3. A distillation column with at total condenser and a total reboiler is separating ethanol from water. Reflux is returned as a saturated liquid, and boilup is returned as a saturated vapor. CMO can be assumed. Assume that the stages are equilibrium stages. Column pressure is 1 atm. A saturated liquid feed that is 32 mol% ethanol is fed to the column at 1000 kmol/h. The feed is to be input on the optimum feed stage. We desire a distillate conposition of 80 mol% ethanol and a bottoms conposition that is 2 mol% ethanol. A liquid side stream is removed on the eighth stage from the top of the column at a flow rate of S = 457.3 kmol/h. This liquid is sent to an intermediate reboiler and vaporized to a saturated vapor, which is returned to the column at its optimum feed location. The external reflux ratio is Lq/D = 1.86. Find the optimum feed locations of the feed and... 

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