Types of Reboilers

The common types of reboilers (Fig. 15.1) are vertical thermosiphon, horizontal thermosiphon, forced-circulation, kettle, and internal. The advantages and disadvantages of each type are compared in Table 

This table and the following discussion are intended to serve as a general guide. In a specific application, reboiler selection may also be influenced by other factors. The application of the common reboiler t5rpes is described below. 

Rgure 15.1 Reboiler tjrpes. (a) Vertical thermosiphon (b) horizontal thermosiphon (c) forced circulation id) kettle (e) internal. 

Boiling side Tube Shell Shell Tube Shell 

Heat transfer rate High Moderately high Low to moderate High Low to Moderate 

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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. 

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. 

Select flow loop geometry, i.e., type of reboiler arrangement. 

The choice of the best type of reboiler or vaporiser for a given duty will depend on the following factors ... 

We would like to compare the closedloop dynamic performance of two types of reboilers. 

Four types of reboilers are discussed in this chapter ... 

There are dozens of other types of reboilers, but these four represent the majority of applications. Regardless of the type of reboiler used, the following statement is correct—almost as many towers flood because of reboiler problems as because of tray problems. 

Regardless of the type of reboiler used, the tower bottom product temperature has to be the same, so as to make product specifications. This is shown in Fig. 5.5. However, the reboiler outlet temperature must always be higher in the circulating reboiler than in the once-through reboiler. This means that it is more difficult to transfer heat in the former than in the latter. 

Figure 8.2 shows a common type of reboiler failure. The steam trap on the condensate drain line has stuck open. A steam trap is a device intended to open when its float is lifted by water. The steam trap remains open until all the water drains out of the trap. Then, when there is no more water to keep the trap open, it shuts. But, if the float sticks open, steam can blow through the steam trap. This is called a blown condensate seal. The average vapor velocity through the tubes... 

The next column parameters were imposed as follows feed specification liquid at bubble point feed flow rate 0.0053 kmol s" side stream flow rate 0.001455 kmol s" type of condenser total condenser pressure 760 mm Hg reflux ratio 3 type of reboiler total reboiler heat duty 417 kW bottom liquid volume 0.146 m tray surface (for top and bottom sections) 0.451 m tray surface (for right and left side of the dividing wall) 0.2255 m weir height 0.025 m hole diameter 0.002 m. 

On the technical scale, different types of reboilers have been developed [38], Depending on the distillation requirements, various reboiler constructions can be used. The simplest ones are heat exchangers or double jackets evaporators constitute more elaborate technical constructions. The advantage of the latter systems is generally the short residence time and the handling of products with difficult physical properties, such as those with high viscosity or a tendency to crystallisation. These evaporators have very short direct contact times and, therefore, allow distillation of heat-sensitive products. 

The other type of reboiler commonly used is termed the down ow reboiler due to the fact that LOX enters at the top of the core and flows down. Vapor and liquid exits at the base of the core. The exit stream should have suf cient liquid flow to wash contaminants from the core and prevent dry boiling. A constant purge is still needed, however, from the column sump. 

If x, is set equal to Xg in Eq. (18.23), is also equal to so the operating line for the stripping section crosses the diagonal at point (xg, Xg). This is true no matter what type of reboiler is used, as long as there is only one bottom product. The lower operating line could then be constructed using the point (xg, Xg) and the slope L/(L — B), but a more convenient method is described in the discussion on feed plates in the next section. 

In the common type of reboiler shown in Figs. 18.7 and 18.11, the vapor leaving the reboiler is in equilibrium with the liquid leaving as bottom product. Then and are coordinates of a point on the equilibrium curve, and the reboiler acts as an ideal plate. In Fig. 18.12 is shown the graphical construction for the reboUer (triangle cde) and the bottom plate (triangle abc). Such a reboiler is called a partial reboiler. Its construction is shown in detail in Fig. 15.11. 

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. 

FIGURE S-11 This type of reboiled absorber can lose efficiency as absorption is improved in the fractionator reflux drum. 

The parameters for the distillation column must be specified next. Clicking on Blocks, Cl, and Setup on the left-hand side of the Data Browser opens the window shown in Figure 3.15. There are several page tabs. The first is Configuration, on which the total number of stages, the type of condenser, the type of reboiler, the numerical convergence method, and two other variables need to be specified. We consider each of these below. 

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