Reboiler flow regimes

Figure 6.2 Typical flow regime boundaries for a small-scale thermosiphon reboiler. (From Chexal and Bergles, 1972. Copyright 1972 by American Institute of Chemical Engineers, New York. Reprinted with permission.)...

When the liquid arrives at the reboiler base, it is usually subcooled because of the effect of static pressure and heat losses from the line. When the liquid enters the tubes, heat is applied to the liquid. Initially, the subcooled liquid is heated to its boiling point by sensible heat transfer only. After the boiling point is reached, vaporization begins and two-phase flow regimes are established. 

If the liquid in contact with the wall is not sufficiently superheated to sustain bubble nucleation, then heat removal is by convection through the liquid film, with evaporation occurring at the liquid-vapor interface of the vapor core. This is ofl n the dominant mechanism in thermosiphon reboilers, particularly when they operate in the annular flow regime (discussed below). 

Flow regimes. Five main flow regimes are important in reboiler operation (Fig. 15.2). In order of increased vaporization these are... 

Quantitative criteria for determining the flow regimes existing in a specific reboiler are presented elsewhere (e.g., 113,114). An extensive mechanistic review of boiling is also available (282). 

Heat transfer drops off> rate than expected and Ap increases faster than expected fouling because of oversized kettle reboiler on distillation column or change in pH or flow regime laminar when design was turbulent or higher level of contamination in fluids or crud carry over from upstream equipment (e.g. silica from catalyst in upstream reactor) or compensation for oversize by reduced coolant flow-rate. 

For the above reasons, mist flow should be avoided. Mist flow tends to occur when fractional vaporization across the reboiler is high. The transition to mist flow is termed "dryout or "burnout (81, 113, 282, 312). A detailed mechanistic description of this regime is available elsewhere (29). 

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