EVAPORATION

EVAPORATION

1.         INTRODUCTION

            In evaporation the vapor from a boiling liquid solution is removed and a more concentrated solution remains. In the majority of case the evaporation process called evaporation refers to the removal of water from an aqueous solution. Typical examples: evaporation solution of sugar, sodium chloride, glue, milk, and orange juice. In these cases the concentrated solution is desired product and the evaporated water is normally discarded.

2.         TYPES OF EVAPORATION EQUIPMENT AND OPERATION METHODS

2.A      General types of evaporators

In evaporation, heat is added to a solution to vaporize the solvent, which is usually water. The heat is generally used a vapor such as steam on one side of a metal surface, with the evaporating liquid on the other side. The general types of equipment are discussed below.

1.Open kettle or pan. The simplest evaporator consists of an open pan or kettle in which the liquid is boiled. The heat is supplied by condensation of steam in a jacket or in coils immersed in the liquid. In some cases the kettle is direct-fired. These evaporators are inexpensive and simple to operate, but the heat economy is poor. In some cases, paddles or scrapers are used for agitation.

2.Horizontal-tube natural circulation evaporator.

The horizontal-tube natural circulation evaporator is shown in Fig. 8.2-1a. The horizontal bundle of heating tube is similar to the bundle of tubes in heat exchanger. The steam enters the tubes, where it condenses. The steam condensate leaves at the other end of the tubes. This type of evaporator is relatively cheap and is used for non- viscous liquids with high heat transfer coefficients and liquids that do not deposit scale. Since liquid circulation is poor, they are unsuitable for viscous liquids.

3.Vertical-type natural circulation evaporator. In this type of evaporator, vertical rather than horizontal tubes are used; the liquids is inside the tubes and the steam condenses outside the tubes. Because of boiling and decreases in density, the liquid rises in the tubes by natural circulation, as shown in Fig. 8.2-1b, and flows downward through a large, central open space or downcomer. The natural circulation increase the heat-transfer coefficient. This type is not used with viscous liquids. It is often called the short-tube evaporator. The basket type is widely used in the sugar, salt, and caustic-soda industries.

4.Long-tube vertical-type evaporator. Since the heat-transfer coefficient on the steam side is very high compared to that on the evaporating-liquid side, high liquid velocities are desirable. In a long-tube vertical-type evaporator, shown Fig. 8.2-1c, the liquid is inside the tubes. The tubes are 3 to 10 m long and the formation of vapor bubbles inside the tubes causes a pumping action, which gives quite high liquid velocities. Generally, the liquid passes through the tubes only once and is not recirculated. This is widely used for producing condensed milk.

5.Falling-film-type evaporator. A variation on the long-tube-type evaporator is the falling film evaporator, wherein the liquid is fed to the top of the tubes and flows down the walls as a thin film. Vapor-liquid separation usually takes place at the bottom. This type is widely used for concentrating heat-sensitive materials such as orange juice and other fruit juices, because the holdup time is very small (5 to 10 s or more) and the heat-transfer coefficient are high.

6.Forced-circulation-type evaporator. The liquid-film heat transfer coefficient can be increased by pumping to cause forced circulation of the liquid inside the tubes. This could be done in the long-tube vertical type shown in Fig. 8.2-1c by adding a pipe connection shown with a pump between the outlet concentrate line and the feed line. In the forced-circulation type, however, the vertical tubes are usually shorter than in the long-tube type, as shown in Fig.8.2-1d. This type useful for viscous liquids.

7.Agitated-film evaporator. The main resistance to heat transfer in an evaporator is on the liquid side. One way to increase turbulence in this film, this is done in a modified falling-film evaporator with only a single, large, jacketed tube containing an internal agitator. Liquid enters at the top of the tube and as it flows downward, it is spread out into a turbulent film by the vertical agitator blades. The concentrated solution leaves at the bottom and vapor leaves through a separator and out the top. It is used with heat-sensitive viscous material such as rubber latex, gelatin, antibiotics, and fruit juices. However, it has a high cost and small capacity.

8.Open-pan solar evaporator. A very old yet still-used process is solar evaporation in open pans. Saltwater is put in shallow open pans or troughs and allowed to evaporate slowly in the sun to crystallize the salt.