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What is an Evaporator?

Evaporation is a process that is used to concentrate on the solution of a solvent. The solvent is usually water. The solvent is further vaporized to produce a concentration solution. Evaporation is very much different from distillation because evaporation doesn’t involve the separation of vapors into components. An evaporator consists of a heat exchanger, valves, manifolds, controls, pumps, and condenser.

Evaporators are used in various processes including pharmaceuticals, foods and beverages, pulp and paper, chemicals, polymers and resins, inorganic salts, acids, bases, and many other things. In this article, we will discuss the various types of evaporators, its advantages and disadvantages, and its applications.

How Evaporator Work-:

Evaporators are used in industries to concentrate the product through the process of evaporate water . Evaporators can be in range of single effect to seven effects which depends upon the volume of product to be process through the evaporators.

The technology is based on basic principle of reduction of quantity by concentrating the Product and subsequently separation of water. Product effluent is fed to the evaporator to concentrate up to concentrate up to 30% to 90% solids concentration. The total process is under vacuum and the vapors generated in the system are re-used to economize steam consumption in multiple effect evaporation system with the help of thermal vapor recompression system. The thermal vapor recompression use vapors generated in the evaporator and compressed it by steam and the compressed vapors are used as heating medium in the evaporator, reuse of vapors as a heating media reduces the steam consumption makes evaporators much economical than other traditional methods of evaporation .

Types of Evaporators

• Falling Film Evaporators
• Force Circulation Evaporators
• Batch Evaporators
• Wiped Film Evaporators
• Scraped Surface Evaporator
• Thin Film Evaporators

Falling Film Evaporator

In Falling Film Evaporators the liquid flows downwards in the form of a film along with the vapors at the core in a parallel flow. The film mechanism and low residence time makes this evaporator ideally suited to heat sensitive non-salting and non-crystallizing solutions. The liquid to be evaporated is initially pre heated to the boiling temperature and enters the heating tubes via distribution plates in the top bonnet of the evaporator. In these evaporators sufficient wetting of the heating surface is very important for the heat economy and trouble free operation. The wetting rates can be increased by extending or splitting the individual evaporator effects. Falling Film Evaporators can be particularly useful when the temperature difference between the liquid to be evaporated and the heating media is low.

Applications of Falling Film Evaporator :

The concentration of dairy products, sugar solutions, phosphoric acid, and black liquor.

 

Force Circulation Evaporators

Forced Circulation Evaporators are short tube evaporators and are ideally used when the boiling of feed liquid on the heating surfaces is to be avoided due to the salting and fouling characteristics of the liquid. Hence in these evaporators the flow velocity in the tubes is kept very high.

In these evaporators the liquid is circulated at high velocities in the tubes of the heat exchanger while steam is provided in the chest. This liquid is partially evaporated when the pressure is reduced in the vapor-liquid separator to the corresponding boiling temperature. High volume,  low head axial flow circulation pumps are used for recirculating the crystal laden slurry in the evaporator and hence the operation of this evaporator is independent of the temperature difference. In the crystallizing operation crystals can be separated from the crystallizing slurry by adjusting the velocity of the circulating stream. These evaporators are ideal for handling liquors with high viscosity or which have a tendency to salt and scale during evaporation.

Applications of Force Circulation Evaporator :

Applications in which forced circulation is used includes urea, sodium sulfate, sodium chloride, magnesium chloride, caustic potash, citric acid, and ammonium sulfate.

 

Batch Evaporators

Batch evaporators are filled with the dilute material, heat is applied, and the material is gradually concentrated to the desired point by the application of heat and removal of vapors.

Additional dilute material might be added as the concentration occurs to replenish the evaporated liquid to a given level. Since each gallon of dilute material brings some solids with it into the evaporator, the evaporator eventually achieves the final solids concentration.

 

Scraped Surface Evaporators

This Evaporator Usually selected for products which are highly viscous or in paste / syrup forms products having a tendency to scale or salt out during evaporation can also be handled in this evaporator.

The rotating blades of these evaporators ensure that no material stays on the heat transfer surface for long duration thus preventing thermal degradation.

The liquid flow can be bottom to top or top to bottom. This flow pattern depends on the fluid properties, chances of scale or crystal formation. This system can be used along with the falling film or forced circulation evaporators as last finisher stage.

 

Wiped Film Evaporators

These WFE evaporators have found a niche in difficult evaporation operations. Though this style evaporator can be used in almost any application, the WFE excels in distillation steps where products are heat sensitive, viscous, tend to foul heated surfaces, or are high temperature boiling.

Wiped Film Evaporators are operating in plants world wide. Wiped Film Evaporators can be installed in a variety of configurations. WFEs with internal condensers are called short path evaporators and can operate under high vacuum. Wiped Film Evaporators without internal condensers are called top vapor outlet (TVO) and employed as column reboilers or in very dirty applications.

 

Thin Film Evaporators

With Thin Film Distillation, a substantial decrease of boiling temperature is obtained by reducing the operating pressure. This allows thermal separation of products that would be destroyed by conventional vacuum distillation (pot still or distillation column) because of the necessary high temperatures and long residence time.

In a Thin Film Evaporator the raw material is heated up on the internal surface of a heated tube until the lower boiling component starts to evaporate. These vapors are then liquefied on the cold tubes of a condenser. In simple thin film evaporator design the condenser is located outside but as close to the evaporator as possible.

Applications of Thin Film Evaporators :

Widely used to purify organic chemicals including natural oils, fatty acids, herbicides, insecticides
Food concentration and pharmaceuticals including, vegetable and fruit purees, plant and vegetable extracts, biological solutions, etc.