Forced circulation reboiler

Advantages and disadvantages will in general correspond to the type of reboiler to which forced circulation is applied. The advantages and disadvantages shown are in addition. 

In vacuum service, the large fraction of the tube length used for sensible heating leaves little density difference for thermal circulation. This fact, plus the frequent need for circulating viscous materials, points towards forced-circulation reboilers for vacuum service. 

Because the circulation rate is set by the designer, forced-circulation reboilers can be designed with more certainty than natural circulation units. 

CHANTRY, W. A. and Church, D. M. (1958) Chem. Eng. Prog. 54 (Oct.) 64. Design of high velocity forced circulation reboilers for fouling service. 

Forced circulation reboilers should be designed for velocities of 10-15 ft/sec (3-5 m/sec).53... 

Figure 5.6 shows a once-through forced-circulation reboiler. Such a reboiler differs from a thermosyphon reboiler in that it has a pump to force circulation, rather than rely on natural or thermosyphon circulation. This seems rather wasteful—and it is. 

The great advantage of forced circulation is that careful calculation of the pressure drop through the reboiler and associated piping is not critical. But, as we can see in Fig. 5.6, the operator now has two tower-bottom levels to control. Further, if the hot-side liquid level rises above... 

Figure 5.6 Forced-circulation once-through reboiler.

Most often, forced circulation is used with fired reboilers. If flow is lost to such a reboiler, furnace tube damage is likely to result. Hopefully, this is less likely to occur with a forced-circulation reboiler. Also, the higher pressure drop of a furnace may force the designer to use a pump. Sometimes, we also see a forced-circulation reboiler system, if the reboiler heat is to be recovered from a number of dispersed heat sources that are far away from the tower, and hence a lot of pressure drop has to be overcome. 

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.

Reboilers are heat exchangers that are used primarily to provide boilup for distillation and similar towers. All types perform partial vaporization of a stream flowing under natural or forced circulation... 

Forced circulation reboilers may be either horizontal or vertical. Since the feed liquid is at its bubblepoint, adequate NPSH must be assured for the pump if it is a centrifugal type. Linear velocities in the tubes of 15-20 ft/sec usually are adequate. The main disadvantages are the costs of pump and power, and possibly severe maintenance. This mode of operation is a last resort with viscous or fouling materials, or when the fraction vaporized must be kept low. 

Example 8.12 summarizes the results of such calculations made on the basis of data in Heat Exchanger Design Handbook (1983). Procedures for the design of kettle, thermosiphon and forced circulation reboilers also are outlined by Polley (in Chisholm, 1980, Chap. 3). 

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