Forced Circulation Reboilers

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. 

The design procedure must start with a specific geometry and heat transfer surface and a specific percentage vaporization. Then the heat transfer coefficient is found, and finally the required area is calculated. When the agreement between the assumed and calculated surfaces is not close enough, the procedure is repeated with another assumed design. The calculations are long and tedious and nowadays are done by computer. 

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). 

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. 8.6, the operator now has two tower-bottom levels to control. Further, if the hot-side liquid level rises above the reboiler return nozzle, the force of the vapor and liquid rushing back into the column will cause the trays to flood, but the reboiler heat input will not be affected. 

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. 

Reboilers are sometimes inserted into the bottom of a tower. These are called stab-in reboilers. It is not a terribly good idea, because 

Note that in a kettle reboiler the bottoms product level control valve does not control the level in the tower it controls the level on the product side of the reboiler only. The liquid level on the boiling or heat-exchanger side of the kettle is controlled by the internal overflow baffle. But what controls the tower-bottom liquid level  

To answer this, let us see how such a gravity-fed or kettle reboiler works  

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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. 

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.

The still for this system is a simple flash still with a forced circulation reboiler. The saponified, crude glycerine stream is fed into the recirculation pump on the suction side. The recirculation rate is in the range of 40-50 times of the crude glycerine feed rate. The top of the stiU vessel is equipped with a pad style entrainment separator to eliminate carryover. 

Forced-circulation reboilers are especially suitable for handling viscous and heavily fouling process fluids see Chantry and Church (1958). The circulation rate is predictable, and high velocities can be used. They are also suitable for low vacuum operations and for low rates of vaporization. The major disadvantage of this type is... 

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