Natural circulation reboiler

External kettle reboiler, natural circulation viscosities <200 mPa-s. 

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. 

Figure 10-96A. Horizontal thermosiphon reboiler, a. Recirculating feed system, b. Once-through feed system. Both are natural circulation. (Used by permission Yilmaz, S. B. Chemical Engineering Progress, V. 83, No. 11, 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. 

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. 

Natural circulation reboilers are effective and convenient units for process systems operating under pressure. They are usable in vacuum applications but must be applied with care, because the effect of pressure head (liquid leg) on the boiling point of the fluid must be considered. The temperature difference between the heating medium and boiling point of the fluid may be so small as to be impractical, regardless of the tube length in a vertical unit. 

For once-through natural circulation reboilers, the liquid backup height is calculated from the pressure balance equation. If this height, plus an allowance for froth, reaches the bottom tray level, flooding of the tower will occur. 

Because the circulation rate is set by the designer, forced-circulation reboilers can be designed with more certainty than natural circulation units. 

The three reboilers in Figure 15.13 are shown under natural circulation. The flow of liquid from the column to the reboiler is created by the difference in hydrostatic head between the column of liquid feeding the reboiler and the vapor-liquid mixture created by the reboiler. 

Blumenkrantz and Taborek (1971) applied the density effect model of Boure to predict instability in natural-circulation systems in thermosiphon reboil-ers used in the petrochemical industry. An important conclusion of their work was that similarity analysis in terms of the model s dimensionless groups can be used to extrapolate threshold stability data from one fluid to another. 

In a thermosyphon or natural-circulation reboiler, there is, of course, no source of air. The aerated liquid is a froth or foam, produced by the vaporization of the reboiler feed. Without a source of heat, there can be no vaporization. And without vaporization, there will be no circulation. So we can say that the source of energy that drives the circulation in a thermosyphon reboiler is the heating medium to the reboiler. 

When considering the steam side of steam heated reboilers, it is best to think about the reboiler as a steam condenser. The steam, at least for a conventional horizontal reboiler, is usually on the tube side of the exchanger, as shown in Fig. 8.1. The steam is on the tube side, because the shell side was selected for the process fluid. If the reboiler is a thermosyphon, or natural-circulation reboiler, then low-process-side pressure drop is important. For a horizontal reboiler, it is easiest to obtain a low pressure drop for the fluid being vaporized by placing it on the shell side. 

Vertical Thermosiphon Reboilers Vertical thermosiphon reboilers operate by natural circulation of the hquid from the still through the downcomer to the reboiler and of the two-phase mixture... 

CALANDRIAS. Vertical shell-and-tube units, known as calandrias, natural-circulation or thermosiphon reboilers, are generally the tnost economical vaporizers for distillation and evaporation operations. A typical arrangement is shown in Fig. 

Horizontal reboilers, with natural circulation, have a simple circulation system. Liquid flow from an elevated drum, tower bottom or tower trapout boot through a downcomer pipe to the bottom of the exchanger shell. The liquid is heated and leaves the reboiler in the return piping as a vapor or vapor-liquid mixture and flows back tc the tower or drum. There is no pressure difference between the inlet and outlet nozzles. The circulation is forced by the static head difference between the two liquid columns (see Figure 7-72 Use the exchanger centerline as a reference line. 

Circulation rate through the reboiler is fixed by the driving force and the resistance to flow. In a fixed piping system, it is a function of the liquid level in the reboiler sump in the case of natural circulation, and of pump design and operation in the case of forced circulation. 

The regions of commercial interest are the nucleate and film-boiling regions (P3). Nucleate boiling occurs in kettle-type and natural-circulation reboilers. 

Natural circulation calandrias (or thermosiphon reboilers) depend upon density differences to produce required flow rates. Vaporization creates an aerated liquid with a density less than that of the liquid in the system. The hydraulic head resulting from this density difference causes the fluid in the system to circulate. Circulation rates are high with liquid-to-vapor ratios ranging from 1 to 50. 

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