Kettle reboiler fouling

Fouling. The downflow movement of liquid makes fouling a less severe problem with conventional internal circulation reboilers (Fig. 15. le) than with kettle reboilers. Fouling, however, can be severe if tubes are unflooded. The bathtub arrangement (Fig. 15.96) does not share the liquid downflow benefit, and fouling can be as much as a problem as with kettle reboilers. The isolating chamber forms a dirt trap, and must be blown down (e.g., by perforating the chamber floor). 

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. 

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 ... 

Reboilers are sometimes inserted into the bottom of a tower. These are called stab-in reboilers. It is not a terribly good idea, because it makes it more difficult to fix a leaking or fouled reboiler, without opening the tower itself. However, the kettle reboiler, shown in Fig. 5.7, has essentially the same process performance characteristics as the stab-in reboiler, but is entirely external to the tower. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

Kettle reboilers. Kettle reboilers have low heat transfer rates, a high fouling tendency, a high plot space consumption, and are expensive. They are therefore not very popular in distillation practice. Situations in which they are preferred are ... 

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. 

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. 

I have seen towers equipped with kettle reboilers flood due to high liquid levels a dozen times in my career. The story is always the same. The elevation difference between the reboiler vapor return nozzle and the overflow baffle inside the kettle is only 2 or 3 ft. The designer has forgotten about shell-side fouling. 

Of course, if the steam from the kettle will be used to reboil some nearby column, entrainment is not a major problem. If the steam is going into superheater tubes, however, serious entrainment will lead to solid deposits inside the superheater tubes, and this may cause these tubes to overheat and fail. Fouling of turbine blades would also result. 

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