Kettle reboiler

Product column reboiler kettle type, heat transfer area 4 m2, design pressure 2 bar, materials stainless steel. 

As an exercise, the reader is invited to demonstrate that both for condenser and reboiler, the degrees of freedom are (A +4), identical with a flash. Typically, the specifications are input stream N. + 2) variables plus two others. Outlet pressure is usually imposed. The remaining variable may be liquid or vapour fraction, including bubble-point liquid (1=1), dew-point vapour (1=0), or sub-cooled liquid or superheated vapour (unusual). The above specifications enable to compute the duty Q, but this may be given also as specification. Note also that in steady state flowsheeting the reflux drum is included in the simulation of condenser. The type of condenser (partial, total, or sub-cooled liquid), as well as the type of reboiler (kettle or thermosyphon) does not change the analysis. 

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 ... 

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. ... 

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. 

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. 

Kettle-type reboilers, evaporators, etc., are often U-tube exchangers with enlarged shell sec tions for vapor-liquid separation. The U-tube bundle replaces the floating-heat bundle of Fig. 11-36. ... 

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. 

Kettle-type-reboiler costs are 15 to 25 percent greater than for equivalent internal-floatiug-head or U-tube exchangers. The higher extra is applicable with relatively large kettle-to-port-diameter ratios... 

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

The function of reboilers has already been discussed. These components are essentially heat exchangers that are used to transfer heat to bring the liquid at the bottom of the column to its boiling point (refer also to discussions in Chapter 1). The principle types employed are jacketted kettles, simple kettle type reboilers, internal reboilers, and thermo-syphon reboilers. Examples of each type are illustrated in Figure 7. 

G is a split flow. The fluid comes in and goes both way.s around the longitudinal baffle and then exits. H is very rare a double split flow. J is a divided flow. K is a kettle type reboiler, which is a special type and is best explained by looking at the example AKT in Figure 3-9. Kettle types are common where there is a boiling liquid or where gas is liberated from shell fluid as it is heated. The weir controls the liquid, making sure the tubes are always immersed in liquid. Gas that flashes from the liquid can exit the top nozzle. 

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. 

A pull-through, floating head, kettle-type reboiler having stationary head integral with the tubesheet, 23-in. port diameter and 37-in. inside shell diameter with tubes 16 ft long is denoted as SIZE 23/37-192 TYPE CKT,... 

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). 

When MEA is used in the presence of COS and CS2, they react to form heat-stable salts. Therefore, MEA systems usually include a reclaimer, The reclaimer is a kettle-type reboiler operating on a small side stream of lean solution. The temperature in the reclaimer is maintained such that the water and MEA boil to the overhead and are piped back to the stripper. The heat-stable salts remain in the reclaimer until the reclaimer is full. Then the reclaimer is shut-in and dumped to a waste disposal. Thus, the impurities are removed but the MEA bonded to the salts is also lost. 

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

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

For horizontal thermosiphon/natural units the boiling fluid is almost always on the shell side, with the heating medium in the tubes. In the vertical units the reboiling of the fluid is in the tubes. For kettle units, the boiling is in the shell. ColUns suggests a rule of thumb that if the viscosity of the reboUer is less than 0.5 centipoise (cp), the vertical thermosiphon should be considered, but when the viscosity is more than 0.5 cp, the horizontal reboUer is probably more economical. 

Figure 10-96B. Horizontal thermosiphon reboiler on distillation column shell and tube design, not kettle. Boiling in shell.

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.)...

The horizontal natural circulation systems do not use a kettle design exchanger, but rather a 1-2 (1 shell side, 2 tube-side passes) unit, with the vaporized liquid plus liquid not vaporized circulating back to a distillation column bottoms vapor space or, for example, to a separate drum where the vapor separates and flows back to the process system and where liquid recirculates back along with make-up feed to the inlet of the horizontal shell and tube reboiler. See Figures 10-96A-C. 

The kettle unit used in the reboiling service usually has an internal weir to maintain a fixed liquid level and tube coverage. The bottoms draw-off is from the weir section. The reboiling handled in horizontal thermosiphon units omits the disengaging space because the liquid-vapor mixture should enter the distillation tower where disengaging takes place. The chiller often keeps the kettle design but does not use the weir because no liquid bottoms draw off when a refrigerant is vaporized. 

Figure 10-103A. Maximum heat flux boiling outside horizontal tubes kettle and internal reboilers. When using the estimate from this curve, a safety factor of 0.7 also should be used. (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. 

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