8.2. TYPES OF EVAPORATION EQUIPMENT AND OPERATION METHODS

8.2A. General Types of Evaporators

In evaporation, heat is added to a solution to vaporize the solvent, which is usually water. The heat is generally provided by the condensation of a vapor such as steam on one side of a metal surface, with the evaporating liquid on the other side. The type of equipment used depends primarily on the configuration of the heat-transfer surface and on the means employed to provide agitation or circulation of the liquid. 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 tubes is similar to the bundle of tubes in a heat exchanger. The steam enters the tubes, where it condenses. The steam condensate leaves at the other end of the tubes. The boiling liquid solution covers the tubes. The vapor leaves the liquid surface, often goes through some de-entraining device such as a baffle to prevent carryover of liquid droplets, and leaves out the top. This type of evaporator is relatively cheap and is used for nonviscous liquids with high heat-transfer coefficients and liquids that do not deposit scale. Since liquid circulation is poor, they are unsuitable for viscous liquids. In almost all cases, this evaporator and the types discussed below are operated continuously, that is, the feed enters at a constant rate and the concentrate leaves at a constant rate.

3. Vertical-type natural circulation evaporator

In this type of evaporator, vertical rather than horizontal tubes are used; the liquid 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. This natural circulation increases the heat-transfer coefficient. This type of evaporator is not used with viscous liquids. It is often called the short-tube evaporator. A variation is the basket type, where vertical tubes are used but the heating element is held suspended in the body so there is an annular open space as the downcomer. In this way, it differs from the vertical natural circulation evaporator, which has a central instead of annular open space as the downcomer. 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 in 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. Contact times can be quite low in this type. In some cases, as when the ratio of feed to evaporation rate is low, natural recirculation of the product through the evaporator is effected by adding a large pipe connection between the outlet concentrate line and the feed line. 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 coefficients 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. Additionally, in some cases a separate and external horizontal heat exchanger is used. This type of evaporator is very 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, and hence the heat-transfer coefficient, is by actual mechanical agitation of this liquid 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. This type of evaporator is very useful with highly viscous materials, since the heat-transfer coefficient is greater than in forced-circulation evaporators. It is used with heat-sensitive viscous materials such as rubber latex, gelatin, antibiotics, and fruit juices. However, it has a high cost and small capacity. For interested readers, Perry and Green (P1) give more-detailed discussions and descriptions of evaporation equipment.

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.

8.2B. Methods of Operation of Evaporators

1. Single-effect evaporators

A simplified diagram of a single-stage or single-effect evaporator is given in Fig. 8.2-2. The feed enters at T_F K and saturated steam at T_s enters the heat-exchange section. Condensed steam leaves as condensate or drips. Since the solution in the evaporator is assumed to be completely mixed, the concentrated product and the solution in the evaporator have the same composition and temperature T₁, which is the boiling point of the solution. The temperature of the vapor is also T₁, since it is in equilibrium with the boiling solution. The pressure is P₁, which is the vapor pressure of the solution at T₁.

If the solution to be evaporated is assumed to be dilute and like water, then 1 kg of steam condensing will evaporate approximately 1 kg of vapor. This will hold if the feed entering has a temperature T_F near the boiling point.

The concept of an overall heat-transfer coefficient is used in the calculation of the rate of heat transfer in an evaporator. The general equation can be written

Equation 8.2-1

where q is the rate of heat transfer in W (btu/h), U is the overall heat-transfer coefficient in W/m² · K (btu/h · ft² · °F), A is the heat-transfer area in m² (ft²), T_s is the temperature of the condensing steam in K (°F), and T₁ is the boiling point of the liquid in K (°F).

Single-effect evaporators are often used when the required capacity of operation is relatively small and/or the cost of steam is relatively cheap compared to the evaporator cost. However, for large-capacity operation, using more than one effect will markedly reduce steam costs.

2. Forward-feed multiple-effect evaporators

A single-effect evaporator as shown in Fig. 8.2-2 is wasteful of energy, since the latent heat of the vapor leaving is not used but is discarded. Much of this latent heat, however, can be recovered and reused by employing a multiple-effect evaporator. A simplified diagram of a forward-feed triple-effect evaporation system is shown in Fig. 8.2-3.

If the feed to the first effect is near the boiling point at the pressure in the first effect, 1 kg of steam will evaporate almost 1 kg of water. The first effect operates at a temperature that is high enough that the evaporated water serves as the heating medium to the second effect. Here, again, almost another kg of water is evaporated, which can then be used as the heating medium to the third effect. As a very rough approximation, almost 3 kg of water will be evaporated for 1 kg of steam in a three-effect evaporator. Hence, the steam economy, which is kg vapor evaporated/kg steam used, is increased. This also holds approximately for more than three effects. However, the increased steam economy of a multiple-effect evaporator is gained at the expense of the original first cost of these evaporators.

In forward-feed operation as shown in Fig. 8.2-3, the fresh feed is added to the first effect and flows to the next in the same direction as the vapor flow. This method of operation is used when the feed is hot or when the final concentrated product might be damaged at high temperatures. The boiling temperatures decrease from effect to effect. This means that if the first effect is at P₁ = 1 atm abs pressure, the last effect will be under vacuum at a pressure P₃.

3. Backward-feed multiple-effect evaporators

In the backward-feed operation shown in Fig. 8.2-4 for a triple-effect evaporator, the fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect. This method of reverse feed is advantageous when the fresh feed is cold, since a smaller amount of liquid must be heated to the higher temperatures in the second and first effects. However, liquid pumps must be used in each effect, since the flow is from low to high pressure. This reverse-feed method is also used when the concentrated product is highly viscous. The high temperatures in the early effects reduce the viscosity and give reasonable heat-transfer coefficients.

4. Parallel-feed multiple-effect evaporators

Parallel feed in multiple-effect evaporators involves the adding of fresh feed and withdrawal of concentrated product from each effect. The vapor from each effect is still used to heat the next effect. This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt.