evaporators.pptx

Evaporation 1  EVAPORATION Definition:- Theoretically, “Evaporation means simply vaporization from the surface of a liquid. Vaporization of a liquid below its boiling point is called evaporation.” Thus, no boiling occurs and the rate of vaporization depends on the dif...

Evaporation 1  EVAPORATION Definition:- Theoretically, “Evaporation means simply vaporization from the surface of a liquid. Vaporization of a liquid below its boiling point is called evaporation.” Thus, no boiling occurs and the rate of vaporization depends on the diffusion of vapour through the boundary layers above the liquid. 2 Thus a practical definition of evaporation is: “The removal of liquid from a solution by boiling the solution in a suitable vessel and withdrawing the vapour, leaving a concentrated liquid residue.” This means that heat will be necessary to provide the latent heat of vaporization and, in general the rate of evaporation is controlled by the rate of heat transfer. Evaporators are designed, therefore, to give maximum heat transfer to the liquid, with the largest possible area, a suitable temperature gradient. 3  Mechanism When heat applied in solution the motion of molecules increase and molecules present in the surface overcome the surface tension of the liquid and it evaporates because surface molecules have less cohesive force than others. 4  Evaporation Introduction Vaporization of a solvent from a solution to make it concentrated.  Solvent volatile  Solute non-volatile in nature. We are left with thick liquor It is different from drying only, as in drying the solvent is vaporized to have a solid end product 5 APPLICATION OF EVAPORATION Evaporation is one of the most important processes in the manufacture of pharmaceuticals. It is used in the preparation of 1. Liquid extracts, soft extracts & dry extracts. 2. In the concentration of blood plasma & serum. 3. It is also used in the manufacture of drugs containing, antibiotics, enzymes, hormones & many other substances. Hajvery University (Faculty of Pharmacy) 6 4) Used in purification of vitamins. 5) Concentration of proteins. 6) Concentration of biological products. 7) Stripping of solvents from vegetable & plant or herbal extracts. 8) Removal of water & solvents from fermentation broths. 9) Concentration of penicillin & related products Hajvery University (Faculty of Pharmacy) 7  Solution properties Knowledge of solution properties is important for the design of the equipment for evaporation. Some of the important properties of the solution are given below, Concentration Initially, the solution may be quite dilute and the properties of the solution may be taken as the properties of solvent. As the concentration increases, the solution becomes viscous and heat transfer resistance increases. The crystal may grow on the heating coil or on the heating surface. The boiling points of the solution also rise considerably. Solid or solute contact increases and the boiling temperature of the concentrated solution became higher than that of the solvent as the same pressure (i.e. elevation in boiling point). Foaming Many of the materials like organic substance may foam during vaporization. If the foam is stable, it may come out along the vapor known as entrainment. Heat transfer coefficient changes abruptly for such systems. Degradation due to high temperature The products of many chemical, food, pharmaceutical industries etc. are very temperature sensitive and they may get damaged during evaporation. Thus special case or technique is required for concentrating such solution. Scaling Many solution have tendency to deposit the scale on the heating surface, which may increase the heat transfer resistance. These scales produce extra thermal resistance of significant value. Therefore, scaling in the equipment should not be ignored thus de-scaling becomes an important and routine matter. Equipment material The material of the equipment must be chosen considering the solution properties so that the solution should neither be contaminated nor react with the equipment material. 8  Factors affecting Evaporation There are seven factors to affect evaporation. 1. Temperature 2. Surface area 3. Agitation 4. Atmospheric aqueous vapour pressure 5. Atmospheric pressure on the liquid under evaporation 6. Type of product required 7. Economic factors 9 1) Temperature: The rate of evaporation is directly proportional to the temperature. 2) Surface area: The rate of evaporation is directly proportional to the surface area of the vessel exposed to evaporation. 3) Agitation: is necessary for evaporation. 4) Atmospheric aqueous vapour pressure: The rate of evaporation is inversely proportional to the atmospheric aqueous vapour pressure. 10 5) Atmospheric pressure on the liquid under evaporation: The rate of evaporation is inversely proportional to the atmospheric pressure on the liquid under evaporation. 6) Type of product required: The selection of the method and apparatus to be used for evaporation depends upon type of product required. 7) Economic factors: When selecting the method and apparatus the economic factors are important 11 EVAPORATION UNDER At reduced pressure evaporation increases. Advantage: REDUCEDE PRESSURE 1. Evaporation occurs at low temperature, with less risk of damage to heat sensitive materials. 2. A lower operating temperature gives higher temperature gradients, without the need of excessive steam pressures. 3. The lower the operating, temperature the lower the steam pressure that can be used. 12  EQUIPMENT USED FOR EVAPORATION Equipments which are used for evaporation are called Evaporators. Types of Evaporator: Evaporators are divided mainly into three groups. 1. Natural circulation evaporator  Types: i) Evaporating pans ii) Evaporating stills iii) Short tube evaporator. 13 2) Forced circulation evaporator. 3) Film evaporator Types: i) Wiped Film evaporator ii)Long Tube Evaporator a) Climbing film evaporator b) Falling film evaporator 14  1) NATURAL CIRCULATION EVAPORATORS Working principle: The movement of the liquid results from convection currents set up by the heating process. Convection currents: The process in which heat moves through a gas or liquid as the hotter parts rises and the cooler part sinks. 15 Salt production 16 Lithium production 17  Evaporating Pans “On a manufacturing scale, liquid extracts containing water are evaporated in open pans called evaporating pans.” Construction: The evaporating pan consists of Hemispherical shallow made of Copper Stainless steel Alluminium Enameled iron Steam jacket The hemispherical shape gives the best surface\volume ratio for heating and the largest area for the disengagement of vapour. http://www.pharmainfo.net/book/pharmaceutical-machines/ 18 steam-jacketed-kettle-or-evaporating-pan Working: 19 20  Advantages Evaporating pan or steam jacketed kettle is constructed both for small scale and large scale batch operations. Evaporating pan or steam jacketed kettle is a simple in construction and easy to operates, clean and maintain. Its cost of installation and maintenance is low. For the construction of the evaporating pan materials such as copper, stainless steel and aluminum so on wide variety of materials can be used. Stirring of the contents or materials and removal of 21 the products is easy.  Disadvantage 1) The whole of the liquid is heated all the time which may lead to decomposition of the components. 2) On the evaporating surface foam is rapidly formed which decreases evaporation. Solids may be deposited at the bottom which make the stirring necessary. 3) This pan can only be used for evaporating aqueous and thermostable liquid extracts. 4) They can not be used for evaporating extracts containing organic solvents like alcohol etc. 22  ii) Evaporating still Construction It consists of a jacketed-evaporating pan with a cylindrical cover that connects it to a condenser. The over all assembly is called still. The cover is clamped with the evaporating pan. 23 Working : 24 Advantages: Easy to clean and maintain. Allow the equipment to be used for slovents other than water. E.g. ethanol. Disadvantages: All the liquor is heated all the time. The heating surface is limited. 25 Horizontal tube evaporation 26 iii) Short tube evaporator Construction: The evaporator is a cylindrical vessel. The lower portion of the vessel consists of a nest of tubes with the liquor inside and steam outside– this assembly is called calendra. The specifications of calendria are as follows: Tube length: 1–2m Tube diameter: 40 – 80 mm Diameter of evaporator: 2.5 m Number of tubes: 1000 The feed inlet is at the top of the calendra. The product outlet is placed at the bottom of the evaporator. Steam inlet and outlet is placed from the side of the calendria. 27 Working: 28 Advantages 1. Use of tubular calendria increases the heating area, possibly by a factor of 10 to 15 compared to that of an external jacket. 2. Increasing the rate of heat transfer. 3. Condenser and receiver can be attached to run the evaporation under vacuum with nonaqueous solvents. Disadvantages 1. Since the evaporator is filled to a point above the level of the calendria, a considerable amount of liquid is heated for a long time. The effect of this continual heating can be reduced to some extent by removing concentrated liquor slowly from the outlet at the bottom of the vessel. 29 2. Complicated design, difficult for cleaning and maintenance.  2) Forced circulation evaporators Construction: The evaporator consists of a short tube calendria and a large cylindrical vessel (body of the evaporator) for separation of vapor and liquid takes place. The liquor inlet is provided at the side of the cylindrical vessel. A pump is fitted in between the calendria and the body of the evaporator. A tangential inlet for liquid under high pressure is placed at neck of the body of the evaporator. The vapor outlet is placed at the top of the body and it may be passed through a condenser to collect the condensed liquid. 30 Working principle:. 31  Advantages 1) These have an great advantage over natural circulation evaporators in that the rapid liquid movement improves heat transfer, especially with viscous liquids or materials that deposit solids or foam readily. 2) The equipment is suitable for operation under reduced pressure due to the capacity of the process to overcome the effect of greater viscosity of liquids. 3) These evaporators possess rapid evaporation rate. 32 4) These evaporators are mainly used for thermolabile materials. 5) These can also be used in practice for the concentration of insulin and liver extracts. Disadvantage: Corrosion-erosion can occur, due to high circulation velocities. Salt deposits detach and accumulate at the bottom. 33  3) FILM EVAPORATORS Film evaporators spread the material as a film over the heated surface, and the vapor escapes the film. Following are the types of film evaporators. i) Wiped Film evaporator ii)Long Tube Evaporator a) Climbing film evaporator b) Falling film evaporator 34 35  i) Wiped film Construction: evaporators A form of film evaporator coming into increasing use is the wiped film evaporator or rotary film evaporator, which contains of a single, short tube of wide diameter, better described as a narrow cylindrical vessel,1 or 2 meters in length. A section across the evaporator is shown here where it will be seen that the vessel is surrounded by a heated jacked. Through the vessel is a bladed rotor, with a clearance of the order of 1mm between the tips of the rotor blades and the wall of the vessel. 36 Working: 37  ii) Long tube evaporators (Climbing film evaporators) Construction: The heating unit consists of steam-jacketed tubes, having a length to diameter ratio of about 140 to 1, so that a large evaporator may have tubes 50 mm in diameter and about 7 m in length. The liquor to be evaporated is introduced into the bottom of the tube, a film of liquid forms on the walls and rises up the tubes, hence it is called climbing film evaporator. 38 Working: 39 ii) Long tube evaporators (Falling film evaporators) Construction: The heating unit consists of steam-jacketed tubes, having a length to diameter ratio of about 140 to 1, so that a large evaporator may have tubes 50 mm in diameter and about 7 m in length. 40 Working principle 41 Advantages: Because of obtaining good heat transfer the method being especially useful with liquids that are too viscous to be processed in units in which the film is formed naturally. Disadvantages: A major disadvantage of falling film evaporators is the potential instability of the falling film Expense to manufacture and install the instrument is high. Difficult to clean and maintain. Hajvery University (Faculty of Pharmacy) 42  Evaporation Material of Construction Mostly made of steel For highly corrosive fluids, special materials are used like, Cu, Nickle, Stainless steel, aluminium etc 43  Evaporation Method of feeding Variations in the Multiple effect has come from the mode of feed supply There are four possible feeding arrangements Forward feed Backward feed Mixed feed Parallel feed 44  Figure: (a) Forward feed (b) Backward feed flow patterns in four effect evaporator. 45  Figure: (c) Mixed feed (d) Parallel feed flow patterns in four effect evaporator. 46  Evaporation Types of Evaporators I. Evaporators with heating medium in jacket II. Vapor heated evaporators with tubular heating surfaces  Horizontal tube evaporators  Evaporators with tubes placed vertically  Evaporators with short tubes  Single effect evaporators47  Multiple effect evaporators  Evaporation Steam jacketed kettles/Batch type pan evaporators Simplest and oldest type Steam supplied in the jacket gives it’s heat content and condensate leaves through the outlet. 48  Evaporation Advantages Used for both small scale and large scale operations Simple in construction and easy to operate Low maintenance and installation Disadvantages Heat economy is less 49 Not suitable for heat sensitive materials  Evaporation Horizontal Tube evaporators Steam is passed through the tube which are immersed in the pool of liquid to be evaporated. Feed is introduced into the evaporator until the steam compartment is immersed. The feed absorbs heat and solvent is evaporated. Use: Best suited for non-viscous liquids because of poor circulation 50  Evaporation Vertical Tube evaporators Liquid is passed through the vertical tubes and steam is supplied from outside the tubes It consists of short vertical tubes, typically 1-2 m long and 50 – 100 mm in diameter are arranged inside the steam chest. 51 The tube bundle is located in the bottom  Evaporation Climbing Film evaporators Liquid is passed through the vertical tubes and steam is supplied from outside the tubes It consists of short vertical tubes, typically 1-2 m long and 50 – 100 mm in diameter are arranged inside the steam chest. 52 The tube bundle is located in the bottom  Evaporation Heat transfer coefficient The heat flux and the evaporator capacity are affected by the overall heat transfer coefficient. The heat transfer coefficient is influenced by the design and method of operation of the evaporator. Coefficient is a reciprocal of five 53 individual resistances: the steam film  Evaporation Steam film coefficient Steam will have high heat transfer coefficient. Depends upon the type of condensation. No scaling as there is no solute The presence of noncondensable gas seriously reduces the steam film coefficient. 54 Tube side high possibility of scaling, thus  Evaporation Liquid-side coefficient Liquid side coefficient depends upon the velocity of liquid. In case of viscous materials, the resistance of the liquid side controls the overall rate of heat transfer to the boiling liquid. Forced circulation gives higher heat 55 transfer coefficient.  Because of the difficulty of measuring individual coefficients in an evaporator, experimental results are usually expressed in terms of overall coefficients. 56  Evaporation Industrial applications In sugar industry In dehydrating milk, which is then used in many food products In fertilizer plants Evaporation is not only removing water In the production of refined petroleum 57 products, more volatile compounds are