Evaporators

EVAPORATORS

It is convenient to classify evaporators into the following: natural circulation evaporators, forced circulation evaporators, and film evaporators.

Natural Circulation Evaporators

Forced Circulation Evaporators

Film Evaporators

The Efficiency of Evaporators

Vapor Removal and Liquid Entrainment

Evaporation without Boiling

Natural Circulation Evaporators

Small-scale evaporators consist of a simple pan heated by jacket, coil, or by both. Admission of the heating fluid to the jacket induces a pool boiling regime in the vessel. 

FIGURE 10.3 (A) Evaporator with calandria and (B) climbing film evaporator.

Very small evaporators may be open, the vapor escaping to the atmo-sphere or into a vented hood. Larger pan evaporators are closed, the vapor being led away by pipe. Small jacketed pans are efficient and easy to clean and may be fitted for the vacuum evaporation of thermolabile materials. However, because the ratio of heating area to volume decreases as the capacity increases, their size is limited, and larger vessels must employ a heating coil. This improves evaporating capacity but makes cleaning more difficult.

The large heating area of a tube bundle is utilized widely in large-scale evaporators. Horizontal mounting, with the heating fluid inside the tube, is limited by poor circulation to the evaporation of nonviscous liquids in which the bundle is immersed. Normally, the tube bundle is mounted vertically and is known as a calandria. The boiling of liquids in a vertical tube and the earlier regimes of this process operate in a calandria. The length of tubes and the liquid level are such that boiling occurs in the tubes and the mixture of vapor and liquid rises until the entire calandria is just submerged. A typical evaporator is shown in Figure 10.3A. The tubes are from 1.2 to 1.8 m in length and 5.1 to 7.6 cm in diameter. The low density of the boiling liquid and vapor creates an upward movement in the tubes. Vapor and liquid separate in the space above the calandria, and the liquid is returned to the pool at the base of the tubes by a large central downcomer or through an annular space between the heating element and the evaporator shell. Feed is added and concentrate is withdrawn from the pool, as shown in the figure. As long as the viscosity of the liquid is low, good circulation and high heat transfer coefficients are obtained.

In some evaporators, the calandria is inclined and the tubes are lengthened.

Forced Circulation Evaporators

On the smallest scale, forced circulation evaporators are similar to the pan evaporators described above, modified only by the inclusion of an agitator.

Vigorous agitation increases the boiling film coefficient, the degree depending on the type and speed of the agitator. An agitator should be used for the evaporation of viscous materials to prevent degradation of material at the heated surfaces.

Some large-scale continuous units are similar to the natural circulation evaporators already described. The natural circulation induced by boiling in a vertical tube may be supplemented by an axial impeller mounted in the downcomer of the calandria. This modification is used when viscous liquids or liquids containing suspended solids are evaporated. Such units are employed in evaporative crystallization. In other forced circulation evaporators, the tube bundle becomes, in effect, a simple heat exchanger through the tubes of which the liquid is pumped. Commonly, the opposing head suppresses boiling in the tubes. Superheating occurs, and the liquid flashes into a mixture of liquid and vapor as it enters the body of the evaporator.

Film Evaporators

In the short tubes of the calandria, an intimate mixture of vapor and liquid is discharged at the top. If the length of the tube is greatly increased, progressive phase separation occurs until a high-velocity core of vapor is formed, which propels an annular film of liquid along the tube. This phenomenon, which is a stage of flow when a liquid and a gas pass in the same direction along a tube, is employed in film evaporators. The turbulence of the film gives very high heat transfer coefficients, and the bubbles and vapor evolved are rapidly swept into the vapor stream. Although recirculation may be adopted, it is possible, with the high evaporation rates found in long tubes, to concentrate the liquid sufficiently in a single pass. Since a very short residence time is obtained, very thermolabile materials may be concentrated at relatively high temperatures. Film evaporators are also suitable for materials that foam badly. Various types have been developed, but all are essentially continuous in operation, their capacity ranging from a few gallons per hour upward.

The climbing film evaporator, which is the most common film evaporator, consists of tubes 4.6 to 9.1 m in length and 2.5 to 5.1 cm in diameter mounted in a steam chest. This arrangement is described in Figure 10.3B. The feed liquid enters the bottom of the tubes and flows upward for a short distance before boiling begins. The length of this section, which is characterized by low heat transfer coefficients, may be minimized by preheating the feed to its boiling point. The pattern of boiling and phase separation follows, and a mixture of liquid and vapor emerges from the top of the tube to be separated by baffles or by a cyclone separator. Climbing film evaporators are not suitable for the evaporation of viscous liquids.

In the falling film evaporator, the liquid is fed to the top of a number of long heated tubes. Since gravity assists flow down the tube, this arrangement is better suited to the evaporation of moderately viscous liquids. The vapor evolved is usually carried downward, and the mixture of liquid and vapor emerges from the bottom for separation. Even distribution of liquid must be secured during feeding. A tendency to channel in some tubes will lead to drying in others.

The rising-falling film evaporator concentrates a liquid in a climbing film section and then leads the emerging liquid and vapor into a second tube section, which forms a falling film evaporator. Good distribution in the falling film section is claimed, and the evaporator is particularly suitable for liquids that increase greatly in viscosity during evaporation.

In mechanically aided film evaporators, a thin film of material is main-tained on the heat transfer surface irrespective of the viscosity. This is usually achieved by means of a rotor, concentric with the tube, which carries blades that either scrape the tube or ride with low clearance in the film. Mechanical agita-tion permits the evaporation of materials that are highly viscous or that have a low thermal conductivity. Since temperature variations in the film are reduced and residence times are shortened, the vacuum evaporation of viscous ther-molabile materials becomes possible.

The Efficiency of Evaporators

In the pharmaceutical industry, economic use of steam may not be of overriding importance because the small scale of the operation and the high value of the product will not justify the additional capital costs of improved heating effi-ciency. In other industries, heating costs impose more efficient use of heat. This is secured by utilizing the heat content of the vapor emerging from the evapo-rator, assumed, until now, to be lost in a following condensation. Two methods commonly used are multiple effect evaporation and vapor recompression.

In multiple effect evaporation, the vapor from one evaporator is led as the heating medium to the calandria of a second evaporator, which, therefore, must operate at a lower temperature than the first. This principle can be extended to a number of evaporators, some stages working under vacuum. The limit is set by the relation of the cost of the plant and the vacuum services with the cost of the steam that is saved.

In evaporators employing vapor recompression, the vapor emerging is compressed by mechanical pumps or steam jet ejectors to increase its temper-ature. The compressed vapor is returned to the steam chest.

Vapor Removal and Liquid Entrainment

Vapor must be removed from the evaporator with as little entrained liquid as possible. The two determining factors are the vapor velocity at the surface of the liquid and the velocity of the vapor leaving the evaporator. On a small scale, surface vapor velocities will be low, but with increase in scale, the adverse ratio of surface area to volume creates higher velocities. Droplets formed by the bursting of bubbles at the boiling surface may then be projected from the surface. In addition, foam may form. Various devices may be used to control entrainment at or near the surface. A high vapor space is provided above the boiling liquid to allow large droplets to fall and foam to collapse. Baffles may be used in the vapor space to arrest entrained droplets. Where allowable, antifoaming agents, such as silicone oils, can be used to depress foaming.

Stokes’ law shows that vapor of particular characteristics will carry droplets upward against the force of gravity. Any entrained liquid not inter-cepted in the body of the evaporator will, therefore, be carried forward in the higher-velocity stream of the vapor uptake. Some droplets will be caught here, the quantity depending on the geometry of the duct and the velocity of the vapor. At atmospheric pressure, the latter might be 17 m/sec. In vacuum evaporation, much higher velocities may be used. When the quantity of entrained liquid is high, the vapor is commonly led to a cyclone separator. This is employed with frothing materials and the vapor-liquid mixture leaving a climbing film evaporator. In the separator, the entrained liquid is flung out to the walls by centrifugal force and may be collected or returned to the evapo-rator. The vapor is led to a condenser.

Evaporation without Boiling

During heating, some evaporation takes place at the surface of a batch of liquid before boiling begins. Similarly, liquids that are very viscous or that froth excessively may be concentrated without boiling. The diffusion of vapor from the surface is then described by equation (4.5) as:

where N_A is the number of moles evaporating from unit area in unit time, kg is the mass transfer coefficient across the boundary layer, R is the gas constant, T is the absolute temperature, P_Ai is the vapor pressure of the liquid, and P_Ag is the partiale pressure of the vapor in the gas stream. kg is proportional to the gas velocity.