Heat transfer reboilers

The final restriction of simple columns stated earlier was that they should have a reboiler and a total condenser. It is possible to use materials fiow to provide some of the necessary heat transfer by direct contact. This transfer of heat via direct contact is known as thermal coupling. 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

The modified Palen-SmaU method can be employed for reboiler design using finned tubes, but the maximum flux is calculated from A, the total outside heat-transfer area including fins. The resulting value of refers to A. ... 

Baffles are provided for heat-transfer purposes. When shell-side baffles are not required for heat-transfer purposes, as may be the case in condensers or reboilers, tube supports are installed. 

Compute condenser and reboiler heat-transfer rates from Eqs. (13-159) and (13-160). 

The preceding discussion on reflux assumes that the condenser is not limiting when the reflux is raised. For a severely limited condenser, an evaluation must first be made of the condenser heat transfer before analyzing the effect of a reflux increase with Smith-Brinkley. Likewise, a limiting reboiler or trays close to flood would have to be evaluated prior to Smith-Brinkley calculations. 

For thermosyphon reboilers, the hydraulic aspects are as important as the heat transfer aspects. The design of thermosyphon reboiler piping is too broad a subject for this handbook. Some good articles on the subject can be found in References 2-14. Reference 3 is particularly good for horizontal thermosyphon reboilers. Table 1 has typical vertical thermosyphon design standards. 

An alternate means of reboiler control is to remove the control valve from the steam line and provide a condensate level controller for the chest cascaded from the tray temperature. The alternate method uses A tube surface for control, with the condensate covering more or less tube surface to vary the area exposed to condensing stream. Condensing area is many times more effective for heat transfer than area covered by relatively stagnant condensate. The reboiler must have extra surface to allow part of its surface to be derated for control purposes. 

In another ineident of the same nature, the oil caught fire. A furnace supplied heat transfer oil to four reboilers. One was isolated for repair and then pressure-tested. The water was drained out of the shell, but the drain valve was 8 in. above the bottom tube plate, and so a layer of water was left in the reboiler (Figure 12-2). 

When the reboiler was brought back on line, the water was swept into the heat transfer oil lines and immediately vaporized. This set up a liquid hammer, which burst the surge tank. It was estimated that this required a gauge pressure of 450 psi (30 bar). The top of the vessel was blown off in one piece, and the rest of the vessel was split into 20 pieces. The hot oil formed a cloud of fine mist, which ignited immediately, forming a fireball 35 m in diameter. (Mists can explode at temperatures below the flash point of the bulk liquid see Section 19.5.)... 

Figure 10-97A. Heat transfer data of reboilers boiling a pure hydrocarbon at low pressure in horizontal and vertical reboilers. (Used by permission Yilmaz, S. B. Chemical Engineering Progress, V. 83, No. 11, p. 64, 1987. American Institute of Chemical Engineers. All rights reserved.)...

Kem deserves a lot of credit for developing design methods for many heat transfer situations and in particular the natural circulation phenomena as used for thermosiphon reboilers and shown in part in Figures 10-96A-D. 

This method for vertical thermosiphon reboilers is based on semi-empirical correlations of experimental data and is stated to predict heat transfer coefficients 30 percent, which is about the same range of accuracy for most boiling coefficient data. The advantage of this method is that it has had significant design experience in the industry to support it. It is also adaptable to other types of reboilers used in the industry. See Figures 10-110 and 10-111. 

After the proper pressure halance-heat transfer relationships are established, the calculations are summarized in connection with other heat flow resistances in the reboiler. 

Johnson, A. 1., Circulation Rates and Over-All Temperature Driving Forces in a Vertical Thermosiphon Reboiler, presented at AlChE Heat Transfer Sym., Louisville, KY, Mar. 20 (1955). 

Chapter 10, Heat Transfer, has been updated and now includes several important design techniques for difficult condensing situations and for the application of thermosiphon reboilers. 

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