Internal reboilers

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. 

Internal reboilers are similar in concept and designed accordingly to the ketde units, but this style does not have a... 

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.)...

Can equipment sets be combined (e.g., replacing reactive distillation with a separate reactor and multi-column fractionation train installing internal reboilers or heat exchangers) to reduce overall system volume ... 

Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

Figure 15.9 Variations on kettle and internal reboiler arrangements, (a) Kettle reboiler with liquid drawn from column sump (6) internal reboiler with an isolating chamber.

Internal reboilers are similar in principle to kettle reboilers. The main operating difficulties experienced with internal reboilers are ... 

Liquid level. Measurement and control of liquid level is a major problem in columns equipped with internal reboilers. Because of vapor bubbling through the liquid, froth rather than pure liquid exists at and above the bundle, as was demonstrated by field tests (156). The froth aeration tends to increase with reboil rate, and is difficult to predict. This makes it difficult to relate apparent to actual liquid level. 

The consequences of unflooding the top of the tube bundle are similar to those described for kettle reboiler. On the other hand, a liquid level too high above the bundle may initiate column flooding. The following can be helpful for minimizing problems of level measurement and control in columns equipped with internal reboilers ... 

Distribution. With tray towers, liquid distribution to the reboiler is uneven. With wide-boiling mixtures, this may prompt composition pinches. Further, light components can be depleted near the inlet or the top of the bundle, causing temperature pinches. In most cases, however, distribution is not a major problem because the bundle is small and internal reboiler AT is large. 

This method is particularly helpful to find fuUy or partially optimized solutions for RD design variables (Malone and Doherty, 2000). The objective function for the RD problem is commonly composed of two basic terms annual operating cost (e.g. consumption of raw materials, steam and cooling water) and the annualized investment i.e. column, internals, reboiler and condenser). The constraints are formed from the MESH equations on each tray, material balances at the top and bottom of the column, kinetic and thermodynamic relationships and logical relationships between process variables and the number of trays. 

Fig. 3 Effect of composition of charge on (a) duration of the process, (b) internal reflux and inverse of the internal reboil ratios.

If the feed is fed into the colnmn without intermediate storage, it will not be possible to manipulate the vapor flows in both colnmn sections independently. This is also tme for the liqnid flows. The number of correcting variables is then two H or C and R (Fig. 34.1). On the vapor side, however, an extra degree of freedom can be used (C or H respectively), if it is used for pressure control. In the vapor line to the condenser and in the internal reboiler, no valves are permitted These flows are determined by C andFf. 

Internal reboilers, usually in the form of horizontal, cylindrical bundles of U-tubes with the heating medium in the tubes, have been used to a modest extent. In comparison with external reboilers, pumping and pressure-drop considerations are eliminated, as are piping and reboiJer shell. Offsetting these in part, however, is the fact that the column base must be longer (taller) to provide... 

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