Thermal energy in the vapour evolved from a boiling solution and can be utilized to vaporize more water if there is a temperature drop for heat transfer in the desired direction. In the case of multiple-effect evaporation systems, this temperature drop is created by progressively lowering the boiling point of the solution in a series of evaporators by operating them under lower absolute pressures.
The desired driving force (temperature drop) can also be created by increasing pressure (and, therefore, the condensing temperature) of the vapour evolved by (a) Mechanical recompression or (b) Thermal recompression.
The compressed vapour having a higher condensing temperature is then fed to the steam chest of the evaporator from which it came. So the economy of the evaporator is also increased by recompressing the vapour from the evaporator and condensing it in the steam chest of the same evaporator. In this method, the vapour from the evaporator is compressed to a saturation pressure of steam to upgrade the vapours to the conditioning of the original steam to permit the use of heating media. The cost of supplying the required amount of compression is usually smaller than the value of latent heat in the vapour. By this, we can obtain multiple effect economies in a single effect.
Mechanical Recompression :
In this method, the vapour evolved from the evaporator is compressed to a somewhat higher pressure by a positive displacement (or) centrifugal compressor and fed to a heater as steam. As the saturated temperature of the compressed vapour is higher than the boiling point of the solution, heat flows from vapour to solution and more vapours are generated. The principle of mechanical vapour recompression is depicted in the below figure.
In this method, the vapour is compressed by means of a steam jet ejector. Here the high-pressure steam is used to draw and compress the major part of the vapours from the evaporator while the remaining part of the vapour is separately condensed for compensating motive steam added.
Thermal recompression is better suited than mechanical recompression to vacuum operation. Jets are cheaper and easier to maintain than compressors. Disadvantages of thermal recompression include low mechanical efficiency of jets and lack of flexibility to meet changed operating conditions.
By this vapour recompression, we can obtain multiple effect economy in the single effect. The question that arises is why not the compressor the vapour from the evaporator and expand a small amount of heat to get a kg of steam instead of spending a large amount of heat energy in a boiler to obtain the same kg of steam? The major reasons are
(1) The vapour compression evaporator and small temperature drop make compression economical. The optimum temperature drop is 10 oF whereas in multiple effect evaporator may work on a temperature drop of 100 oF. Thus the single effect must have as much surface in one effect as the sum of all the heating surfaces in multiple effects.
(2) The compression equipment is expensive.
(3) They must be a standby steam capacity for supplying heat for starting.
(4) If the boiling point elevation of the solution is appreciable then the energy needed for compression increases very rapidly.
Forced circulation evaporators
In natural circulation evaporators, the liquid enters with a velocity of 0.3 to 1 m/s and generally, the heat transfer coefficients are very low and particularly with viscous liquids. So, whenever we are dealing with a concentration of highly viscous and scale-forming solutions there is no alternative but to use forced circulation evaporators. By increasing the velocity of the liquid flow (generally 2 to 6 m/s) through the tubes heat transfer coefficients increase enormously and the high liquid velocity which is resulting from the pumps prevents scale formation on heating surfaces.
Two types of forced circulation evaporators.
(i) horizontal heating type
(ii) vertical heating type
They consist of a centrifugal pump, a 1-2 shell & tube heat exchanger in the case of the horizontal heating element and a 1-1 shell & tube heat exchanger in the case of the vertical heating element, an evaporator body with vapour outlet at the top, deflector plate, an outlet for product discharge at the bottom.
Centrifugal pump forces the liquid through the tubes at high velocity and gets heated by condensing steam on the shell side. The solution becomes superheated and flashes into a mixture of vapour and liquid in the evaporator body. The deflector plate separates the vapour and liquid in the vapour space, and vapours are removed at the top, and concentrated liquid comes out at the bottom.
1. High transfer coefficients obtained even with viscous solutions.
2. Whenever we are dealing with a concentration of highly viscous and scale-forming solutions forced circulation evaporators to prevent scale formation on heating surfaces
3. Residence times are low so that heat-sensitive viscous materials also can be used.
The main disadvantage of forced circulation evaporators is the high pumping cost.
To handle high viscous solution in continuous operations external pumping system is provided by forced circulation evaporators. Salt, plastic, polymer, and pharmaceutical industries use it for crystallizing, concentrating and thickening products.
Long vertical tube evaporator
These are two types.
(i) Long vertical tube falling film evaporator
(ii) Long vertical tube rising film evaporator
(i)Long tube vertical (climbing film) rising film evaporator:
This consists of a long tubular heating element incorporating tubes 25 mm to 50 mm and 3 to 10 meters in length. And also it contains at the top a separator or vapour space for removing entrained liquid from the vapour. The deflector is provided just above the tube sheet.
The feed at near boiling is fed to the bottom of the evaporator. It is then pumped inside the tubes. Steam is provided on the shell side. Liquid and vapour are separated in the vapour separator at the top. The deflector acts as both a primary separator and foam breaker. Multiple effects are used to achieve a higher steam economy.
Its counter heat exchange design gives the ability to handle foam and forth forming-type materials.
(ii)Long tube vertical falling film evaporator:
Construction: This consists of a long tubular heating element incorporating tubes 50 mm to 250 mm and 3 to 10 meters in length.
The feed to the evaporator is fed to the top of the evaporator through efficient liquid distributors. Steam is given on the shell side. The concentrate is collected at the bottom. The same operation is repeated in multiple effects to achieve steam economy.
The main problem in this type of evaporator is that of distributing the liquid uniformly as a film inside the tubes. This is done by a set of perforated metal plates above a carefully levelled tube sheet.
- Used in desalination industries to separate water and salts in seawater. Optimized models of the multi-effect system having smooth or double-fluted tubes are used to obtain a better performance ratio. Submerged vertical tube evaporators developed from the classic falling film evaporators are used to provide fresh water with better thermal efficiency.
- It is employed in a high-capacity refrigeration system that uses propane as the refrigerant.
- Used widely in liquid food evaporation.
Evaporation and Types of Evaporation Equipment
The objective of Evaporation: To concentrate a dilute solution consisting of non-volatile solute and volatile solvent.
In this unit operation, the solvent to be evaporated is generally water and the concentrated solution is a product. The vapour generated usually has no value, it is condensed and discarded.
Types of evaporation:
1. Single effect evaporation: The solution to be concentrated flows inside the tubes. The heating medium is steam condensing on metal tubes. Usually, the steam enters at 3 atm abs. and boiling liquid is under moderate vacuum. This increases the temperature difference between the steam and boiling liquid. When a single evaporator is used, the vapour from the boiling liquid is condensed and discarded. This is called single-effect evaporation. It is simple but utilizes steam ineffectively. To evaporate 1 kg of water from the solution we require 1-1.3 kg of steam.
2. Multiple effect evaporation: Increasing the evaporation per kg of steam by using a series of evaporators between the steam supply and condenser is called multiple effect evaporation
Properties of evaporating liquids that influence the process of evaporation:
- Concentration: As the concentration increases, the viscosity and density increase thereby the boiling point of the solution increases. The increase in boiling point and viscosity reduces the rates of heat transfer in evaporators.
- Foaming: Solutions like organic compounds tend to foam during vaporization.The foam is carried away along with vapour leading to heavy entrainment.
- Scale: Solutions deposit scales on the heating surface .overall heat transfer coefficient drastically decreases and leads to the shutting down of the evaporators for cleaning of the tubes.
- Temperature sensitivity: Pharmaceutical products, fine chemicals and foods are damaged when heated to moderate temperatures for relatively short times. So special techniques are employed to reduce the temperature of the liquid and the time of heating.
- The material of construction: Evaporators are made of some kind of steel. however many sols attack ferrous metals and are contaminated by them. Copper, nickel, and stainless steels can also be used.
Classification of evaporators:
Evaporation is a process of vaporization of a volatile solvent from a solution in order to increase the concentration of the solute. The types of evaporation equipment are:
(1) Natural circulation evaporators
(a) Long tube vertical falling film evaporator
(b) Long tube vertical rising film evaporator
These are used generally for simple evaporation operations either as a single effect or multiple effects.
(2) Forced circulation evaporators
(a) Forced circulation evaporator with a horizontal heating element
(b) Forced circulation evaporator with a vertical heating element
These are used generally for salting, viscous and scale-forming solutions.
(3) Agitated film evaporator
The main advantage of this type is to give high heat transfer coefficients even with very high viscous liquids. This is a modified falling film evaporator. Mechanical agitation is used to reduce the viscosity of very high viscous solutions.
Industry applications of various evaporators:
Long tube vertical rising film evaporator:
These types of evaporators are widely used in industries for the handling of foaming, frothy liquors, and used for the production of condensed milk
Long tube vertical falling film evaporator:
These types of evaporators are widely used in industries for concentrating highly heat-sensitive materials such as orange juice, food materials etc. which require short residence times. And used for the production of condensed milk.
Forced circulation evaporators:
These types of evaporators are widely used in industries for salting, viscous and scale forming solutions, and used in crystallizing operations in which it is necessary for the solids to be in suspension.
Agitated film evaporator:
The main advantage of this type is it gives high heat transfer coefficients even with very high viscous and heat-sensitive liquids such as gelatin, rubber latex, antibiotics and fruit juices. But these are very costly and will be having smaller capacities.