Reboilers and vaporisers

The design methods given in this section can be used for reboilers and vaporisers. Reboilers are used with distillation columns to vaporise a fraction of the bottom product whereas in a vaporiser essentially all the feed is vaporised. 

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

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

Thermosyphon 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 thermosyphon 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 Horizontal exchangers are more easily maintained than vertical, as tube bundle can be more easily withdrawn. 

Kettle reboilers have lower heat-transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials, and have a high residence time. They will generally be more expensive than an equivalent thermosyphon type as a larger shell is needed, but if the duty is such that the bundle can be installed in the column base, the cost will be competitive with the other types. They are often used as vaporisers, as a separate vapour-liquid disengagement vessel is not needed. They are suitable for vacuum operation, and for high rates of vaporisation, up to 80 per cent of the feed. 

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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 vaporiser design. In SI units, Zuber s equation can be written as ... 

Dry wall region Ultimately, if a large fraction of the feed is vaporised, 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 vapour. This condition is unlikely to occur in commercial reboilers and vaporisers. 

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

The complex phenomena involved in heat transfer to a boiling liquid are discussed in Volume 1, Chapter 9. A more detailed account is given by Collier and Thome (1994), Westwater (1956, 1958), Rohsenow (1973) and Hsu and Graham (1976). Only a brief discussion of the subject will be given in this section sufficient for the understanding of the design methods given for reboilers and vaporisers. 

The separation of liquid mixtures by distillation depends on differences in volatility between the components. The greater the relative volatilities, the easier the separation. The basic equipment required for continuous distillation is shown in Figure 11.1. Vapour flows up the column and liquid counter-currently down the column. The vapour and liquid are brought into contact on plates, or packing. Part of the condensate from the condenser is returned to the top of the column to provide liquid flow above the feed point (reflux), and part of the liquid from the base of the column is vaporised in the reboiler and returned to provide the vapour flow. 

A centrifugal pump is to be used to circulate liquid (density 800 kg/m3 and viscosity 0.5 mN s/nr) from the reboiler of a distillation column through a vaporiser at the rate of 400 em /s, and (o introduce the superheated liquid above the vapour space in the reboiler which contains liquid to a depth of 0.7 in. Suggest a suitable layout if a smooth bore 2.5 mm pipe is to be used. The pressure of the vapour in the reboiler is i tcN/nr and the NPSH required by the pump is 2 in of liquid. 

A fluid whose properties are essentially those of o-dichlorobenzene is vaporised in the tubes of a forced convection reboiler. Estimate the local heat-transfer coefficient at a point where 5 per cent of the liquid has been vaporised. The liquid velocity at the tube inlet is 2 m/s and the operating pressure is 0.3 bar. The tube inside diameter is 16 mm and the local wall temperature is estimated to be 120°C. 

The normal practice in the design of forced-convection reboilers is to calculate the heat-transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporisation in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapour-liquid separation vessel. 

Depending on the reboiler duty, a part of the material is vaporised and the vapour travels upward both through the plate holes and downcomers and almost instantly reaches the condenser. 

Calculated the vapour flow rate leaving the reboiler for the duty and liquid heat of vaporisation. 

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