Reboiler pool boiling

Kettle reboilers, and other submerged bundle equipment, are essentially pool boiling devices, and their design is based on data for nucleate boiling. 

The boiling film coefficient for a kettle reboiler can be estimated from the correlation for pool boiling. Equation 15.96 gives one such method due to Palen15. However, the correlation requires the heat flux to be known, and therefore the heat transfer area to be known. Hence the calculation will need to be iterative. An initial estimate of the overall heat transfer coefficient of 2000 W-m 2-K 1 gives ... 

Kettle reboilers generate vapor for process purposes by boiling the liquid outside a bundle of horizontal tubes. This process was historically considered to be an example of pool boiling, with the coefficient calculated by a typical nucleate boiling correlation and a maximum heat flux either fixed by experience [48] or by some modification of Equation (6.94) reflecting the size of the tube bundle [49, 50]. 

With kettle reboilers, the liquid level at the coltunn base is set by the reboiler liquid level plus the head for overcoming reboiler circuit friction. The boiling mechanism is pool boiling with some convective effects. Kettle reboilers normally operate with high ( s 80 percent) fractional vaporization and are therefore prone to fouling. 

Electric resistance vaporizers Classic reboilers or submerged resistance heating elements in normal shells for pool boiling duty. Useful for low to medium capacity loads and more common metals of construction. Can have combined superheat coils in the same containment vessel. (See Fig. V-11.)... 

The mechanism of boiling is essentially nucleate pool hoiling. In hoth styles of rehoiler the liquid velocity is relatively low compared to thermosiphon units. Jacobs provides an extensive comparison of advantages and disadvantages of essentially all the reboiler types used in industrial plants. Palen and Taborek conducted extensive studies of available data and proposed nucleate boiling equations to correlate various data from the available 14 equations down to a selected 6 for detailed study. The study was limited to various hydrocarbons and hydrocarbon mixtures. Their conclusions after computer correlations of the results from several equations were as follows. 

Kettle type, Figure 12.52 in which boiling takes place on tubes immersed in a pool of liquid there is no circulation of liquid through the exchanger. This type is also, more correctly, called a submerged bundle reboiler. 

KETTLE-TYPE BOILERS. A kettle-type boiler, or reboiler as it is called when connected to a distillation column, is shown in Fig. 15.11. A horizontal shell contains a relatively small tube bundle, two-pass on the tube side, with a floating head and tube sheet. The tube bundle is submerged in a pool of boiling liquid. 

A total condenser is used to produce liquid reflux and distillate product. The reboiler is a partial reboiler (vapor is boiled off a liquid pool). The composition of this liquid pool is the same as the bottom product ODmposition. Thermosyphon, ketde, internal, and forced-circulation reboilers ate aU usually partial reboilers. 

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