REBOILERS AND VAPORIZERS

Introduction Reboilers are used with distillation columns to vaporize a fraction of the bottom product In a vaporizer essentially all the feed is vaporized. The design methods given in this section can be used for reboilers and vaporizers.

Principal types of reboilers 1. Forced circulation: the fluid is pumped through the exchanger, and the vapor formed is separated in the base of the column.

2. Thermosiphon: vertical exchangers with vaporization in the tubes or horizontal exchangers with vaporization in the shell.

3. Kettle type: in which boiling takes place on tubes immersed in a pool of liquid; there is no circulation of liquid through the exchanger.

Choice of Type The choice of the best type of reboiler or vaporizer for a given duty will depend on: 1. The nature of the process fluid, particularly its viscosity and propensity to fouling. 2. The operating pressure: vacuum or pressure 3. The equipment layout, particularly the headroom available.

Forced-circulation reboilers are especially suitable for handling viscous and heavily fouling process fluids. They are also suitable for low vacuum operations and for low rates of vaporization. Thermosiphon reboilers are the most economical type for most applications but are not suitable for high viscosity fluids or high vacuum operation.

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 thermosiphon type. They are suitable for vacuum operation and for high rates of vaporization

Boiling Heat Transfer Fundamentals The mechanism of heat transfer from a submerged surface to a pool of liquid depends on the temperature difference. At low temperature differences, when the liquid is below its boiling point, heat is transferred by natural convection. As the surface temperature is raised, incipient boiling occurs, vapor bubbles forming and breaking loose from the surface.

The agitation caused by the rising bubbles and other effects caused by bubble generation at the surface result in a large increase in the rate of heat transfer. This phenomenon is known as nucleate boiling. As the temperature is raised further, the rate of heat transfer increases until the heat flux reaches a critical value. At this point, the rate of vapor generation is such that dry patches occur spontaneously over the surface, and the rate of heat transfer falls off rapidly.

Typical pool boiling curve (water at 1 bar).

Estimation of Boiling Heat Transfer Coefficients In the design of vaporizers and reboilers, the designer will be concerned with two types of boiling: pool boiling and convective boiling. Pool boiling is the name given to nucleate boiling in a pool of liquid, such as in a kettle-type reboiler or a jacketed vessel. Convective boiling occurs where the vaporizing fluid is flowing over the heated surface.

Pool Boiling In the nucleate boiling region, the heat transfer coefficient is dependent on the nature and condition of the heat transfer surface. It is not possible to present a universal correlation that will give accurate predictions for all systems. The correlation given by Forster and Zuber can be used to estimate pool boiling coefficients:

The reduced pressure correlation given by Mostinski is simple to use:

Critical Heat Flux It is important to check that the design and operating heat flux is well below the critical flux. In SI units, Zuber’s equation can be written as: A reduced pressure equation for critical heat flux:

Film Boiling Heat transfer in the film-boiling region will be controlled by conduction through the film of vapor.

Example Estimate the heat transfer coefficient for the pool boiling of water at 2.1 bar, from a surface at 125 ˚C. Check that the critical flux is not exceeded.