Reboilers kettle

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. 

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  

Note that it is the elevation, or the static head pressure, in the tower that drives the kettle reboiler. That is why we call it a gravity-fed reboiler. Also, the pressure in the kettle will always be higher than the pressure in the tower. This means that an increase in the reboiler heat duty results in an increase of liquid level in the bottom of the tower. 

Should the liquid level in the bottom of the tower rise to the reboiler vapor return nozzle, the tower will certainly flood, but the reboiler heat duty will continue. Unfortunately, reboiler shell-side fouling may also lead to tray flooding. This happens because the fouling can cause a pressure-drop buildup on the shell side of the reboiler. 

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 

Liquid flows out of the tower into the bottom of the reboiler s shell. 

The domed top section of the reboiler separates the vapor and the liquid. 

Kettle reboilers consist of a bundle of tubes in an oversize shell. Submergence of the tubes is assured by an overflow weir, typically 5-15 cm higher than the topmost tubes. An open tube bundle is preferred, with pitch to diameter ratios in the range of 1.5-2. Temperature in the kettle is substantially uniform. Residence time is high so that kettles are not favored for thermally sensitive materials. The large shell diameters make kettles uneconomic for high pressure operation. Deentraining mesh pads often are incorporated. Tube bundles installed directly in the tower bottom are inexpensive but the amount of surface that can be installed is limited. 

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The column inventory also can be reducdd by the use of low-holdup column internals, including the holdup in the column base. As the design progresses, other features can be included to reduce the inventory. Thermosyphon reboilers have a lower inventory than kettle reboilers. Peripheral equipment such as reboilers can be located inside the column. ... 

A general method for analyzing kettle reboiler performance is by Fair and Klip, Chem. Eng. Prog. 79(3), 86 (1983). It is effectively limited to computer application. 

Kettle reboilers are generally assumed to require neghgible pressure drop. It is important to provide good longitudinal liqmd flow paths within the shell so that the hquid is uniformly distributed along the entire length of the tubes and excessive local vaporization and vapor binding are avoided. 

The kettle reboiler is shown in Fig. ll-3.5ishell-side, this common design provides adequate dome space for separation of vapor and hquid above the tube bundle and surge capacity beyond the weir near the shell cover. 

Proportional Slow to moderate Small Moderate Pressure, temperature, and level where offset is not objectionable. Kettle reboiler level, drying-oven temperature, pressure-reducing stations... 

In addition to the type description code there is also a shorthand that is used for classifying heat exchangers. The first element of the shorthand is the nominal diameter, which is the inside diameter of the shell in inches, rounded off to the nearest integer. For kettle reboilers and chillers ii emember the kettle has a narrow end and a fat end), the nominal diame-tci is the port diameter (the narrow end) followed by the shell diameter, each rounded off to the nearest integer. 

The reboiler provides the heat input to an amine stripper, which reverses the chemical reactions and drives off the acid gases. Amine reboilers may be either a kettle reboiler (see Chapter 3) or an indirect fired heater (see Chapter 5). 

Figure 10-1F. Kettle reboiler. ( 1988 by Tubular Exchanger Manufacturers Association, Inc.)...

Kettle Reboiler U-Bundle or Floating Tube Sheet Single Shell Poss... 

These units are superior in thermal performance to kettle reboilers. 

Figure 10-97A compares horizontal and vertical units in the same hydrocarbon boiling service at low pressures and shows that the horizontal units are more favorable in the same service than vertical units and even more so when the mean temperature difference is low. Figure 10-97B compares horizontal and vertical thermosiphon units with kettle reboilers when boiling the same hydrocarbon mixture also see Fair, Jacobs, and Rubin. ... 

Figure 10-97B. Heat transfer data of reboilers boiling a hydrocarbon mixture in horizontal and vertical thermosiphon reboilers compared to a kettle reboiler. (Used by permission Yilmaz, S. B. Chemical Engineering Progress, V. 83, No. 11, p. 64, 1987. American Institute of Chemical Engineers. All rights reserved.)...

Figure 10-104. Kettle reboiler—estimate of shell diameter. Example If the vapor rate is 50,000 Ib/hr and the bundle is 25 ft, then V/L is 2,000 lb/(hr)(ft). Entering the curve at this V/L ratio, with an operating pressure of 50 psia and a bundle diameter of 24 in. gives an estimate for shell l.D. of 40 in. (Used by permission Palen, J. W., and Small, W. M. Hydrocarbon Processing, V. 43, No. 11, 1964. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

The article of Fair and Klip presents a detailed analysis of the necessary design features and equations for horizontal kettle reboilers, horizontal thermosiphon reboilers, and vertical thermosiphon reboilers. Other useful references on reboilers are 185,186,188,190,192,194,195,196,197, and 201. 

Figure 9.59. Kettle reboiler with hairpin tubes...

A kettle reboiler heat transfer area 25 m2, carbon steel shell and tubes, operating pressure 10 bar. 

Figure 12.8. Kettle reboiler with U-tube bundle (based on figures from BS 3274 1960)...

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. 

Kettle reboilers, and other submerged bundle equipment, are essentially pool boiling devices, and their design is based on data for nucleate boiling. 

In a tube bundle the vapour rising from the lower rows of tubes passes over the upper rows. This has two opposing effects there will be a tendency for the rising vapour to blanket the upper tubes, particularly if the tube spacing is close, which will reduce the heat-transfer rate but this is offset by the increased turbulence caused by the rising vapour bubbles. Palen and Small (1964) give a detailed procedure for kettle reboiler design in... 

Palen, J. W. and Taborek, J. (1962) Chem. Eng. Prog. 58 (July) 39. Refinery kettle reboilers. 

The plant has a spare kettle reboiler available with the following specification. U-tube bundle, 50 tubes, mean length 4.8 m, end to end. 

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 ... 

The boiling film coefficient for a kettle reboiler can be estimated from the correlation for pool boiling. Equation 15.96 gives one such method due to Palen15. However, the correlation requires the heat flux to be known, and therefore the heat transfer area to be known. Hence the calculation will need to be iterative. An initial estimate of the overall heat transfer coefficient of 2000 W-m 2-K 1 gives ... 

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