Reboiling with Sensible Heat

The above considerations generally apply regardless of the control scheme used. A discussion of the common control schemes used for reboiling with sensible heat follows. 

A common problem with sensible heat reboilers is variation in heating medium inlet temperature. A heat input (Btu) controller (Fig. 7.2e) can alleviate this problem. Heat input control can be implemented either with a direct (Fig. 7.2e) or a bypass control system. The Btu controller compensates for changes in heating medium temperature. Heat input controllers are psuticularly useful in services where the temperature difference across the exchanger is small (AT effects dominate), and small variations in heating medium temperature significantly affect boilup. 

The dominant heating and cooling duties associated with a distillation column are the reboiler and condenser duties. In general, however, there will be other duties associated with heating and cooling of feed and product streams. These sensible heat duties usually will be small in comparison with the latent heat changes in reboilers and condensers. 

Rich/lean amine exchangers are usually shell-and-tube exchangers with the corrosive rich amine flowing through the tubes. The purpose of these exchangers is to reduce the reboiler duty by recovering some of the sensible heat from the lean amine. 

For vertical tubes, the superficial vapor velocity (based on the total heat-transfer surface) can be obtained by multiplying the value calculated from the preceding equation by 0.22. This assumes that there is adequate liquid circulating past the surface to satisfy the mass balance. For thermosiphon reboilers, a detailed analysis must be made to establish circulation rate, boiling pressure, sensible heat-transfer zone, boiling heat-transfer zone, and mean temperature difference. If hquid circulation rates are not adequate, ah hquid will be vaporized and superheating of the vapor wih occur with a resultant decrease in heat-transfer rates. 

Rgure 17.2 Reboiler sensible heating medium control, (a) Reboiler bjrpass, poor control (b) reboiler bypass, good control (c) reboiler bypass with PDC control (d) direct flow control (e) Btu control. 

Additional separation can be obtained by operating without a liquid water phase in the column. Reducing the number of phases increases the degrees of freedom by one. Operation must be at a temperature higher than that predicted by Eq. f8-15L or a liquid water layer will form in the column. Thus, the column must be heated with a conventional reboiler and/or the sensible heat available in superheated steam. The latent heat available in the steam cannot be used, because it would produce a layer of liquid water. Operation without liquid water in the column reduces the energy requirements but makes the system more complex. 

To reduce the CO2 content of the absorber exit gas stream, the majority of the lean solvent can be fed to the absorber to irrigate the second packed bed down from the top. A portion of the lean solvent will be cooled to about 190 F and fed to the top of the absorber as shown in Figure 3-5. The cooler solvent in contact with the exit gas stream at the top of the absorber has a lower equilibrium vapor pressure of CO2. However, this modification does add a sensible heat load to the system that somewhat increases the regenerator reboiler duty. 

The stripper is heated by a reboiler that typically requires 0.9 to 1.1 lb of heating steam per gallon of solvent to be regenerated. Lean solvent leaves the stripper bottom at a temperature of 235° to 245°F and contains 0.12 to 0.15 mol CO2 per mol MEA. Reboiler energy must be sufficient to provide 820 Btu/lb of CO2, representing the heat of reaction with MEA, the sensible heat load to raise the solvent temperature, and the steam present in the overhead vapor. 

We wish to make an analysis of column bases with associated reboilers where there is significant liquid holdup. The analysis should take into account the temperature of the entering liquid and the sensible heat effect of the liquid mass. Such an analysis also applies to vaporizers with associated separators or knockout drums. 

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. 

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