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  2. 2. EVAPORATORSEvaporators are used for reducing product volume, remove water prior to drying, and toimprove product storage life.  Evaporation is a phase transition from liquid phase to vapor phase below the boiling point at a given temperature.  Evaporation usually occurs on the surface.Importance of Evaporators:-This equipment is used to get the following actions:  Improve product storage life  Reduce transportation costs  Prepare the products for next operations like drying and crystallization.  Recovery of solvent  Minimize the chances of chemical reactions which may deteriorate the product  Decrease the impact of microbial growth.Factors Affecting Evaporation:-Factors which are affecting the rate of evaporation are in following: 1) Temperature:- The rate of evaporation is directly proportional to the rate of temperature. Higher the temperature higher will be rate of evaporation but evaporation is maximum at boiling point of the liquid. Thermo stable products can be evaporated at high temperatures but thermo labile products will have to be evaporated at low temperatures. Many substances like glycosides, alkaloids, hormones, enzymes and antibiotics may decompose even at below 100°C hence they should be evaporates at a careful temperature. Extremely heat sensitive products may be evaporated under reduce pressure. But the time required for evaporation is also very critical. 2
  3. 3. 2) Surface Area:- The rate of evaporation is directly proportional to the surface area of the vessel exposed to evaporation. Greater the surface exposed to evaporation higher will be rate of evaporation.3) Agitation:- During evaporation the upper layer of liquid to be evaporated has a tendency to form a scum or layer which lowers the rate of evaporation. Therefore it becomes necessary to agitate or stirrer the solution under evaporation. This will also prevent the decomposition of product at the bottom due to excessive heat and will also prevent the settling of solids at the bottom of container. SELECTION OF EVAPORATOR The choice made on the selection of evaporator equipment is based on following points: Type of Product required:- The selection of method and apparatus to be used for evaporation dependsupon the type of product required and Product characteristics, including heat sensitivity,viscosity and flow properties, foaming tendency, fouling and precipitation, boiling behavior,etc. Evaporating pans or stills may be used to produce liquid or dry products butfilm evaporators may be used to get liquid products. Economic Factors:- While selecting the method and apparatus employed for evaporation, due consideration must be given to economy of labour, fuel, floor area and materials. The recovery of solvents and the utilization of ‘waste’ heat are also important factors which contribute significantly in the reduction of overall costs. 3
  4. 4.  Capacity and operational data, including quantities, concentrations, temperatures, annual operating hours, change of product etc.  Required operating media, such as steam, cooling water, electric power, cleaning agents, spare parts, etc.  Standards and conditions for manufacture delivery, acceptance, etc.  Site conditions, such as available space, climate (for outdoor sites), connections for energy and product, service platforms, etc. TYPES OF EVAPORATORS There are different types of evaporators that are differing from each otheraccording to the method and capacity of their work. Briefly evaporators are classified as:  Small Scale Methods  Large Scale Methods Small Scale Methods:- Small quantities of liquids may be evaporated in a porcelain or glass dish.Generally direct heating by Bunsen burner or electric hot plates used for evaporation, but itshould not be employed, as direct heating leads to decomposition of the substances’ towardsthe end of evaporation. A fixed maximum temperature can easily be attained by employingdifferent types of baths as a source of indirect heating. A water bath is most suitable when theliquids are to be heated up to 100°C.They are simple and inexpensive. Sand bath or oil bathcontaining liquid paraffin or soft paraffin may be used when higher temperatures up to 300 °Care required. To prevent decomposition, whole of the liquid should not be evaporated todryness. The last traces of solvent from the concentrated liquid may be removed undercontrolled temperatures. 4
  5. 5. Applications of Small Scale Methods:  Used commonly in laboratory procedures and preparations of products.Advantages of small scale methods:  Simple and inexpensive  Limited resources are required  Quick process Large Scale Methods:- Different evaporating equipments are used in large scale manufacturing ofproducts.Which are described briefly in following: 1) Evaporatoring Pans: On a manufacturing scale, solutions containing water in large quantity are evaporated through open pans Called evaporating pans. They consist of hemispherical or shallow pans made of copper, stainless steel, aluminum or other metal surrounded by a steam jacket. 5
  6. 6. The hemispherical shape is most suitable because it affords the best surface/volume ratio forheating. These pans may be fixed or made to tilt to remove the product.For convenience in emptying the fixed pans, a tube passes from the lowest point of inner pan,through the jacket, which fitted with the tap to withdraw the evaporated liquid from the panwhen desired.Advantages:  They are simple, easy and cheap to construct.  They are easy to use and clean.  Stirring of the evaporating liquids can be done easily.Disadvantages:  The whole of liquid is heated all the time which may lead to decomposition of the components. 6
  7. 7.  On the evaporating surface, scum is rapidly formed which decreases evaporation. Solids may be deposited at the bottom which makes stirring necessary.  These pans can only be used for evaporating aqueous and thermo-stable liquids extracts. They cannot be used for evaporating extracts containing organic solvents like alcohols etc.  The room in which evaporating pans are used for evaporating the liquids, must be fitted with efficient exhaust fans otherwise the room will be filled with dense fog of condensed vapours and water will start falling from the room and along the sides of walls. 2) Evaporating stills: Evaporating stills are quite similar to evaporating pans. It consists of a vessel made up of copper or stainless steel, with a cover which is fitted with a water condenser so that the solvent is condensed and collected in a receiver. At the lowest point of the pan, a tube fitted with a tap is connected for the removal of product from the pan. The pan is surrounded by a steam jacket. For cleaning the pan or easy removal of the product the cover of the vessel may be fitted with quick-release system of clamps.Advantages:  They are easy to construct and quite easy to clean and maintain.  As the vapours are condensed. It increases the rate of evaporation.  A receiver a vacuum pump can be fitted to the apparatus by which evaporation under reduce pressure can be carried out at low temperature.  The still can be used for evaporation of both aqueous and organic solvents. 7
  8. 8. Disadvantages:  The whole of the liquid is o be heated all the times which may lead to deterioration of the product.  The heating surface is limited. Note: This method of evaporation is used widely in pharmaceutical industries when small batches of liquids are to be evaporated. 3) Long tube Evaporators: Long tube evaporators have wide range. Most frequently used long tube evaporators are discuss in following. a) Calandria type evaporator: Commonly it is known as short tube or single effect evaporator. Generally tubes are employed in order to get sufficient heating surfaces with steam on the outside and liquid inside. In same order still further to increase the capacity the tubes are multiplied forming a Calandria. Features:  Vertical type of evaporator.  Tubes sheets extending across the body and central down take.  Material which has to be evaporated is introduced in tubes.  The tube may be about 1.2m long and 5 cm in diameter, but the size varies with the nature of the substances.  There is space below containing a steam coil to give extra heating capacity and large enough to afford circulation of liquid. 8
  9. 9. Advantages:  High heat transfer rates at high temperature differences  Ease of cleaning  Relatively inexpensiveDisadvantages:  Large floor space and weight  Poor heat transfer at low temperature differences  Not use for thermolabile products 9
  10. 10. b) Climbing film Evaporators:- It is also known as long tube or rising film evaporator.The theory of climbing film evaporator’s working is that the ascending force of thissteam produced during the boiling causes liquid and vapors to flow upwards in parallelflow. At the same time the production of vapor increases and the product is pressed as athin film on the walls of the tubes, and the liquid rises upwards. This co-current upwardmovement against gravity has the beneficial effect of creating a high degree ofturbulence in the liquid. This is advantageous during evaporation of highly viscousproducts and products that have a tendency to foul the heating surface. Usually there must be a rather high temperature difference between the heatingand boiling sides of this type of evaporator. Otherwise the energy of the vapor flow is notsufficient to convey the liquid and to produce the rising film. The length of the boilingtubes will typically not exceed 23 ft (7m). 10
  11. 11. Advantages:  Reduced floor space requirement  Relatively high heat transfer cofficient  Ability to handle foamy liquidsDisadvantages:  High head room requirement  Hydrostatic head at the bottom may increase product temperature and cause temperature sensitivity problems. c) Horizontal Film Evaporator:- Also known as falling film evaporator. This type of evaporator is generally made of long tubes (4-8 meters in length) which are surrounded by steam jackets. The uniform distribution of the solution is important when using this type of evaporator. The solution enters and gains velocity as it flows downward. This gain in velocity is attributed to the vapor being evolved against the heating medium, which flows downward as well. This evaporator is applicable to highly viscous solutions so it is frequently used in the chemical, food, and fermentation industry. 11