Reboilers and Vaporizers

The design methods given in this section can be used for reboilers and vaporizers. Reboilers are used with distillation columns to vaporize a fraction of the bottom product, whereas in a vaporizer essentially all the feed is vaporized. 

The choice of the best type of reboiler or vaporizer for a given duty will depend on the following factors  

The nature of the process fluid, particularly its viscosity and propensity to fouling  

Forced-circulation reboilers are especially suitable for handling viscous and heavily fouling process fluids see Chantry and Church (1958). The circulation rate is predictable, and high velocities can be used. They are also suitable for low vacuum operations and for low rates of vaporization. The major disadvantage of this type is 

Thermosiphon reboilers are the most economical type for most applications but are not suitable for high viscosity fluids or high vacuum operation. They would not normally be specified for pressures below 0.3 bar. A disadvantage of this type is that the column base must be elevated to provide the hydrostatic head required for the thermosiphon effect. This will increase the cost of the column supporting structure. Horizontal reboilers require less headroom than vertical but have more complex pipework. Horizontal exchangers are more easily maintained than vertical, as tube bundles can be more easily withdrawn. 

The amount of liquid vaporized in the reboiler should not be more than 80%, otherwise this will tend to lead to excessive fouling of the reboiler. For kettle reboilers, there is no recirculation. But for thermosyphon reboilers, a recirculation ratio can be defined as  

This usually lies between 0.25 and 6. The greater the value of recirculation ratio, the less fouling there is in the reboiler. Lower values tend to be used in horizontal thermosyphons and higher values (greater than 4) used in vertical thermosyphons. The recirculation ratio is a degree of freedom at the discretion of the designer. This should be fixed later when the detailed design is carried out. 

Given these arguments, it is not surprising that the most common design of reboiler is the vertical thermosyphon. 

Tw = wall temperature of the heating surface (°C) I sat = saturation temperature of the boiling liquid (°C). 

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B. Gilmour Equation reportedly was used successfully in many reboilers and vaporizers ... 

A simple model for side-rectifiers suitable for shortcut calculation is shown in Figure 11.12. The side-rectifier can be modeled as two columns in the thermally coupled direct sequence. The first column is a conventional column with a condenser and partial reboiler. The second column is modeled as a sidestream column, with a vapor sidestream one stage below the feed stage4. The liquid entering the reboiler and vapor leaving can be calculated from vapor-liquid equilibrium (see Chapter 4). The vapor and liquid streams at the bottom of the first column can then be matched with the feed and sidestream of the second column to allow the calculations for the second column to be carried out. 

Reboilers and Vaporizers Piate Heat Exchangers Direct-Contact Heat Exchangers Finned Tubes... 

It is important to check that the design and operating heat flux is well below the critical flux. Several correlations are available for predicting the critical flux. That given by Zuber et al. (1961) has been found to give satisfactory predictions for use in reboiler and vaporizer design. In SI units, Zuber s equation can be written as... 

Dry wall region Ultimately, if a large fraction of the feed is vaporized, the wall dries out and any remaining liquid is present as a mist. Heat transfer in this region is by convection and radiation to the vapor. This condition is unlikely to occur in commercial reboilers and vaporizers. 

Saturated, bulk boiling is the principal mechanism of interest in the design of reboilers and vaporizers. 

Figure 12.8 A typical vapor gap problem, (a) Lower part of column flooded (full of liquid). (b) Reboiler started. Liquid "drains into reboiler and vaporizes. Vapor travels up. Liquid travels too slowly into sump. A vapor gap forms under Iwttom tray, (c) Bottom tray fails when attempting to support liquid column above it. When it fails, vapor gap shifts upward. Second tray now attempts to support liquid column, id) End result.

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... 

Step 1 Start-up. In this step, the acetone and methanol mixture is placed in the bottom of the column together with some entrainer. Heat is added in the reboiler and vapor boil-up moves up the column. At the same time, entrainer water is fed continuously into the middle of the column at a flowrate (to be specified). The column is run under total reflux conditions until the acetone of the top product reaches its purity specification of 95 mol%. 

A batch distillation task is started up by boiling the liquid in the reboiler and vaporizing it at a rate determined by the heat transfer rate. The vapor generated is sent through the colunm and... 

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