Heat exchanger boiling liquids

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. 

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 reflux is induced by means of externally cooled liquid pumparounds. A pumparound simply removes hot liquid from the tower, pumps it through a heat exchanger and then introduces this cooled liquid into the tower a few trays above. Use of pumparounds allows a better distribution of tower loadings than if all the heat were removed from the VPS using an overhead condenser. Four to six trays between sidestreams and two pumparounds are normally specified for a lube VPS. The three liquid sidestream products to be used as lube plant feed stocks are steam stripped to remove lighter boiling components which condense with tire sidestreams. 

The factors that affect the evaporation process are concentration in the liquid, solubility, pressure, temperature, scaling, and materials of construction. An evaporator is a type of heat exchanger designed to induce boiling and evaporation of a liquid. The major types of evaporator are... 

Condensed mode operation. To avoid accumulation of liquid in the reactor the dew point of the gas must be kept above reactor temperature. When operating in condensed mode, higher-boiling components in the recirculating gas are allowed to condense in the heat exchanger, and this liquid is fed back to the reactor where it evaporates. 

Convective heat exchange, natural or forced Radiant heat transfer, e.g. furnaces Evaporation, e.g. in evaporators Condensation, e.g. in shell and tube heat exchanges Heat transfer to boiling liquids, e.g. in vaporizers, boilers, re-boilers ... 

Heat transfer operations. Heat transfer fluids other than steam and cooling water utilities are sometimes introduced into the design of the heat exchange system. These heat transfer media are sometimes liquid hydrocarbons used at high pressure. When possible, higher boiling liquids should be used. Better still, the flammable material should be substituted with a nonflammable medium such as water or molten salt. 

Robertson, J. M., 1983, The Boiling Characteristics of Perforated Plate-Fin Channels with Liquid Nitrogen in Upflow, in Heat Exchangers for Two-Phase Flow Applications, ASME HTD-Vol. 27, pp. 35-40. (4)... 

For certain applications multi-pass arrangements are used. When a plate heat exchanger is used instead of the tubular unit, boiling on the heating surfaces is avoided by increasing the static head using a line restriction between the plate pack and the separator. Compared with tubular units, lower circulation rates and reduced liquid retention times are important advantages. Plate-type units are discussed further in Section 14.7.7. 

Note that in a kettle reboiler, the bottoms product level control valve does not control the level in the tower it controls the level on the product side of the reboiler only. The liquid level on the boiling or heat-exchanger side of the kettle is controlled by the internal overflow baffle. But what controls the tower-bottom liquid level ... 

There are many reactors in industry that use evaporative cooling. The liquid in the reactor boils to remove reaction heat. The vapor leaving the reactor is condensed in an external heat exchanger, and the liquid is returned to the reactor. Clearly the vapor phase is important in these autorefrigerated reactors. In addition to chemical kinetics, the vapor-liquid equilibrium properties influence the design of the reactor-condenser system. 

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