evaporation

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Presentation Transcript

Introduction : 

Introduction Many pharmaceutical processes involve heating of materials, such as- Evaporation Distillation Drying Sterilization Extraction Crystallization evaporation

Sources of heat : 

Sources of heat

Sources of heat : 

Sources of heat

Steam heat is used for transfer of heat for various processes. : 

Steam heat is used for transfer of heat for various processes. Reasons for the widespread use of steam are: Steam has a very high heat content Heat is given up at constant temperature The raw-material-water-is cheap and plentiful Steam is clean, odorless and tasteless, so no chance of contamination of a product. It can be used at high pressure to generate electric power and low-pressure exhaust steam used for process heating. Steam is easy to generate, distribute and control. Steam is generated by heating water in a boiler.

Evaporation is a process of vaporizing large quantities of volatile liquid to get a concentrated product. : 

Evaporation is a process of vaporizing large quantities of volatile liquid to get a concentrated product. evaporation

Evaporation is a surface phenomenon, i.e., mass transfer takes place from the surface. Thus no boiling occurs. : 

Evaporation is a surface phenomenon, i.e., mass transfer takes place from the surface. Thus no boiling occurs. evaporation

Practical definition is-evaporation is the removal of solvent from the solution by boiling the liquor in a suitable vessel & withdrawing the vapor, leaving a concentrated liquid residue in the vessel : 

Practical definition is-evaporation is the removal of solvent from the solution by boiling the liquor in a suitable vessel & withdrawing the vapor, leaving a concentrated liquid residue in the vessel evaporation

Slide 8: 

Either solutions or suspensions. Liquid must be volatile, while the solute must be non-volatile Material should be thermo-stable Liquid is water with low solid content Liquid may be less viscous than water or so viscous that it will hardly flow. Properties of material to be evaporated

Slide 9: 

Evaporation is the escape of fast-moving particles from the surface of a liquid.

Distinguishing features of evaporation with other heat processes. : 

Distinguishing features of evaporation with other heat processes.

applications : 

applications Manufacture of bulk drugs :- evaporation process is used in pharmacy practice, pharmaceutical industries, chemical industries etc. Manufacture of biological products :- such as insulin, biochemical products (penicillin) & plant products. Preparation of blood products (blood plasma & serum) involves evaporation. Enzymes & antibiotics are prepared. Miscellaneous :- demineralised water after condensation (Distillation)

Equipment used for the evaporation are known as evaporators.Steam is generally used as a source of heat. : 

Equipment used for the evaporation are known as evaporators.Steam is generally used as a source of heat. Heat transfer in evaporators Theory

Evaporator capacity : 

Evaporator capacity The total amount of heat required can be obtained by considering a simple diagram of an evaporator. Theory Feed vapor Steam Condensate Product

Theory : 

Theory Let F kg be the feed/hr to the evaporator, whose solid content is XF (weight fraction) & enthalpy is hF J/kg. Let L kg be the product collected/hr from the evaporator, whose solute composition is XL (weight fraction) & enthalpy is hL J/kg. Let V kg be the vapor liberated/hr from the evaporator, whose solute composition is y (weight fraction) & enthalpy is hV J/kg. In most evaporators, the vapor is pure water as there is no entrainment & therefore y is zero.

Material balancing : 

Material balancing The material balance can be obtained from total material entering & total material leaving. It can be represented by an equation for the evaporation process. Feed (kg)=product collected(kg)+vapor liberated(kg) F=L+V Written for material balancing in terms of mass x weight fraction for the solute as: FXF=LXL+Vy

Energy balancing : 

Energy balancing Steam is supplied for evaporation. Let S kg be the steam supplied/hr with an enthalpy of hS J/kg. Let C kg be the condensate removed having an enthalpy of hC J/kg. The heat balance can be obtained from total heat entering & total heat leaving. It can be represented by Heat entering (J)=heat leaving (J) Also written as Heat in feed +heat in steam=heat in thick liquor(product)+heat in vapor+heat in condensate+heat lost by radiation Loss of heat by radiation is less and can be neglected. So the equation written in terms of enthalpy as: FhF+ShS=LhL+VhV+ChC

Energy balancing : 

Energy balancing Temperature difference is the difference between the saturation temperature of the steam & the boiling point of liquid. Generally, a temperature difference of 20-300 C is sufficient for rapid evaporation of solution. But in practice, the feed may have the temperature less than boiling point of the liquid. The steam may be superheated & the condensate may get cooled. All these factors influence evaporator calculation with respect to mass balance.

Improving heat transfer coefficient : 

Improving heat transfer coefficient Evaporator is considered as a heat exchanger. Like any exchanger, heat is transferred from steam to the product. The general equation for heat transfer can be expressed as : Q=UAΔt Where Q=rate of heat transfer, W U=overall heat transfer coefficient, W/m2K A=heating surface area, m2 Δt=temperature difference, K

Improving heat transfer coefficient : 

Improving heat transfer coefficient Heating surface (A) must be large. For this purpose more number of tubes are mounted in parallel to form a unit known as calandria. The overall heat transfer coefficient, U, can be regarded as follows.

Improving heat transfer coefficient : 

Improving heat transfer coefficient The overall coefficient is approx. nearer to the lowest surface coefficient of the film. Any factor that improves the coefficient on the boiling side increases overall coefficient almost proportionally. Some of the factors affecting these conditions are:

Improving heat transfer coefficient : 

Improving heat transfer coefficient Pumping liquid at high velocities (forced circulation) through the tubes has beneficial effect. Such high velocities tend to decrease the thickness of the viscous film & the buffer layer. Overall coefficient may be seriously lowered because of- Corrosion of the surfaces. Deposition of solid material from the evaporating liquid. The presence of noncondensable gases in the heating steam (air). Efforts should be made to prevent the above effects. Forced circulation is an important measure, i.e. increase the velocity of liquid flow to increase the capacity of the evaporator.

Factors influencing/affecting rate of evaporation : 

Factors influencing/affecting rate of evaporation M α A(b-b’)/p Where, M=rate of evaporation A=the surface area of liquid P=the atmospheric pressure b=maximum aqueous vapor pressure b’=pressure due to vapor of the liquid present in the air.

Factors influencing/affecting rate of evaporation : 

Factors influencing/affecting rate of evaporation Surface area Temperature Vapor pressure Atmospheric pressure Agitation Moisture content of the feed Type of the product require Time of evaporation Film & deposits Economic factors

Surface area : 

Surface area M α A In a shallow vessel rate of evaporation is fast. Greater the surface area, greater will be the rate of evaporation

temperature : 

temperature M α temperature Higher temperature→greater value of b →greater the evaporation Evaporation is maximum at the boiling point of liquid. At given temperature →some molecules have higher kinetic energy & some molecules have lower KE. Fast moving molecules escape from surface of the liquid in to vapor while slow moving ones remain behind. When temp. of the liquid raised →more molecules acquire sufficient kinetic energy & escape from the surface (below the B.P.) Below B.P. →vapor formed only from the surface. At the B.P. →vapor formed throughout the body of liquid. Glycoside & alkaloids →decompose at high temp. Hormones, enzymes & antibiotics →heat sensitive. So, special techniques require for evaporation. e.g. malt extract by under reduced pressure , antibiotics are prepared concentrated by freeze drying.

Vapor pressure : 

Vapor pressure M α vapor pressure of liquid Evaporation can be accelerated by increasing the difference (b-b’) this may be achieved in two ways: By increasing the max. vapor pressure (b). Vapor pressure increases with increase in temp. By decreasing the actual pressure due to vapor of evaporating liquid.

Atmospheric pressure : 

Atmospheric pressure M α 1/p Lower the p value →greater the evaporation Lower the external pressure →lower the B.P. of liquid →greater the evaporation Liquid with low B.P. →quickly evaporate b’coz higher vapor pressure at lower temp. Outer atmospheric pressure is dry →value of b will be low →greater the evaporation

agitation : 

agitation During evaporation liquid form a layer→that lower the rate of evaporation If agitation →it will break layer →so rate of evaporation increases →& also prevent decomposition of preparation at the bottom.

Moisture content of the feed : 

Moisture content of the feed If moisture →drug constituents undergo hydrolysis at high temp. To prevent it →first go for low temp. exposure & →then go for higher temp. for final concentration. For e.g. dry extract of belladonna

Type of product required : 

Type of product required To decide the apparatus for evaporation process. OPEN PAN →for liquid or dry concentrate FILM EVAPORATOR →for liquid concentrate SPRAY DRYER →for dry products with good solubility VACCUM EVAPORATOR →for porous product suitable for granules e.g. granular extract of cascara for tablet preparation

Time of evaporation : 

Time of evaporation Longer exposure →greater evaporation →especially for thermostable constituents Exposure for short time →at higher temp. →less destructive Exposure for long time →at lower temp. →more destructive So, FILM EVAPORATOR are more useful.

Film & deposits : 

Film & deposits Vegetable extract evaporation in steam pan→film formation on the surface→or precipitated matter may deposit on heating surface FILM→reduces evaporating surface & PRECIPITATED MATTER→hinders the transfer of heat To avoid this →efficient stirring is required.

Economical factors : 

Economical factors Labor, fuel, floor space & material are primary consideration. Recovery of solvents & utilization of waste heat involve considerable reduction of costs. For evaporation, heat should be latent heat of vaporization, so evaporator is designed to give max. heat transfer to the liquid

Slide 35: 

Classification of evaporators Small scale (laboratory scale) Porcelain dish Glass equipments large scale (industrial) Natural circulation Forced circulation Film evaporators Improved economical Evaporating pan Short tube evaporator Long tube Climbing film Falling film Horizontal film evaporator Wiped film evaporator Rotary film Multiple Vapor compression

Evaporating pan : 

Evaporating pan Principle :- natural circulation evaporator Construction :- Hemispherical structure or shallow an inner pan called kettle It is enveloped with an outer pan called jacket. Hemispherical shape affords the best surface/volume ratio for heating. Space between kettle & jacket, in which steam is passed. Pan may be fixed or tilt to remove the product. (tilted pan should have capacity NMT 90 liters.) For smaller quantities→kettle is made up of single sheet of metal. For larger quantities →kettle is made up of several sheets that are welded.

Evaporating pan : 

Evaporating pan Material for construction :- Should be good conductor of heat. Copper →as a kettle, excellent material because of good conductivity, but if acidic materials, some quantity of copper would dissolved. So tinned copper is used. Iron →as a jacket, because minimum conductivity, to prevent rusting of jacket, tinned or enameled iron is used. Inlet of steam & outlet for non-condensed gas at the top of kettle. Outlet for the condensate at the bottom of the jacket Outlet for the concentrated product at the bottom of the kettle.

Evaporating pan : 

Evaporating pan Working :- Aqueous extract is placed into a evaporating pan Steam is supplied through the inlet. Steam gives out heat to the contents & the condensate leaves through the outlet. The contents must be stirred manually for small volumes & mechanically for large volumes. Rate of evaporation is fast at initial stage & decreases gradually as the liquid gets concentrated. Room where evaporation is carried must be fully ventilated , otherwise the room is quickly filled with fog of condensed vapor & water falls from the roof & runs down the walls. Fans fitted over pan remove vapor, prevent condensation in room, & accelerate the rate of evaporation by quickly removing saturated air from the surface of the liquid.

Evaporating pan : 

Evaporating pan Uses :- Concentration of aqueous & thermostable liquors. For liquid extracts containing water as a menstruum. E.g. liquid extract of liquorice. Advantages :- For both large & small scale Simple in construction & easy to operate, clean & maintain. Cost of installation & maintenance is low. Construction with wide variety of materials like copper, aluminum, stainless steel. Stirring & removal of product is easy.

Evaporating pan : 

Evaporating pan Disadvantages :- Heat economy is less, so, cost per unit material production is more. Not suitable for heat sensitive material due to long time of exposure. Heating area decreases as the product gets more concentrated. It is open type, so vapor passes into the atmosphere, which can lead to saturation of the atmosphere, slowing evaporation as well as causing discomfort. B.P, of water can’t be reduced, so reduced pressure can’t be created in open type evaporator.

Slide 41: 

Classification of evaporators Small scale (laboratory scale) Porcelain dish Glass equipments large scale (industrial) Natural circulation Forced circulation Film evaporators Improved economical Evaporating pan Short tube evaporator Long tube Climbing film Falling film Horizontal film evaporator Wiped film evaporator Rotary film Multiple Vapor compression

Horizontal film evaporator : 

Horizontal film evaporator Principle :- Steam is passed through the horizontal tubes, which are immersed in a pool of liquid to be evaporated. Heat transfer takes place through the tubes & the liquid outside the tubes gets heated. The solvent evaporates & escapes from the top of the evaporator. The concentrated liquid is collected from the bottom.

Horizontal film evaporator : 

Horizontal film evaporator Construction :- Large cylindrical body with conical or dome-shaped top & bottom. Made up of cast iron or plate steel. Body range from 1.8 to 2.4 meters diameter & from 2.4 to 3.6 meters height. Lower part of body consists of a steam compartment with an inlet for steam at one end & a vent for noncodensed gas on the other end. Condensate outlet from the bottom of the steam compartment. In the steam compartment, 6 to 8 stainless steel horizontal tubes are placed. The tubes are cut long enough so that they project about 25 mm beyond the tube sheet on the both ends. The width of steam compartment is usually half the diameter of the body. At one convenient point inlet for feed is provided. One outlet for vapor is placed at the top of the dome. Another outlet for thick liquid is placed at the centre of the conical bottom of the body.

Horizontal film evaporator : 

Horizontal film evaporator Working :- The feed is introduced into the evaporator. Steam is introduced into the steam compartment. The horizontal tubes receive heat from the steam & conduct it to the liquid due to temp. gradient. Steam condensate passes through the corresponding outlet. The feed absorbs heat & solvent gets evaporated. The vapor then escapes through the outlet placed at the top. This process is continued until a thick liquid is formed, which can be collected from the bottom outlet.

Horizontal tube evaporator : 

Horizontal tube evaporator Uses :- Best suited for non viscous solutions that do not deposit scales or crystals on evaporation. E.g. cascara extract. Advantages :- Cost per square meter of heating surface is usually less.

Slide 46: 

Classification of evaporators Small scale (laboratory scale) Porcelain dish Glass equipments large scale (industrial) Natural circulation Forced circulation Film evaporators Improved economical Evaporating pan Short tube evaporator Long tube Climbing film Falling film Horizontal film evaporator Wiped film evaporator Rotary film Multiple Vapor compression

Short tube evaporator : 

Short tube evaporator Principle :- Liquid is passed through the vertical tubes & the steam is supplied from outside the tubes. Heat transfer takes place through the tubes & the liquid inside the tubes gets heated. The solvent evaporates & escapes from the top of the evaporator. The concentrated liquid is collected from the bottom. vertical tube evaporator

Short tube evaporator : 

Short tube evaporator Construction :- Large cylindrical body made up of cast iron with dome shaped top and bottom. Inside the body, calandria is fitted at the bottom. Calandria consists of a number of vertical tubes, whose diameter ranges from 0.05 to 0.075 meters & length of 1-2 meters. About 100 such tubes are fitted in a body measuring 2.5 meters or more diameter. Inlet are provided for steam & feed. Outlet are provided for vapor, concentrated product, non-condensed gas and condensate.

Short tube evaporator : 

Short tube evaporator Working :- Steam is introduced outside the tubes. The condensate is passed through the corresponding outlet & non-condensed gas escapes through the vent. The feed is introduced in such a way as to maintain the liquid level slightly above the top of the tubes. The liquid inside the tubes is heated by the steam & begins to boil. As the liquid boils, it spouts up through the tubes & returns through the central down-take. It sets up a circulation of hot liquid, which enhances the rate of heat transfer. The vapor escapes through the top outlet. The steam is supplied until required concentration of the product is obtained. Finally the product can be withdrawn from the bottom outlet.

short tube evaporator : 

short tube evaporator Uses :- In the manufacture of cascara extract, sugar, salt, caustic soda. Advantages :- Tubes increases the heating surface nearly 10-15 times when compared with steam jacketed kettle. Vigorous circulation enhances the rate of heat transfer. It can be connected to a condenser & receiver, which further increases rate of evaporation. Also used for volatile solvents. No. of units can be joined to obtain more efficient effect. (multiple effect evaporator)

short tube evaporator : 

short tube evaporator Disadvantages :- The liquid is maintained above the level of the calandria. So the upper layers of the liquid need a long time for getting heated. This problem can be minimized by removing concentrated liquid slowly from the bottom. Complicated construction, so installation cost increases. Cleaning & maintenance is difficult. During operation, the pressure inside the evaporator increases. In large evaporators, where the liquid depth may be of the order of 2 meters, the pressure increases to 25 kilopascals, leading to elevation of the boiling point by 5 to 60 C. this reduces the effective temp. gradient & may affect heat sensitive materials.

short tube evaporator : 

short tube evaporator In short tube evaporator, circulation depends on completely on boiling. Steam is supplied into the calandria to induce boiling. When steam supply is stopped, automatically boiling stops. As a result the particles settle down. These particles act as a nuclei which grow as crystals. Therefore, this evaporator is sometimes used as a crystallizing evaporator. If such crystallization is undesirable, the problem can be avoided by installing a propeller in the central portion close to the bottom. By increasing the RPM of the propeller, the capacity of the evaporator can be doubled. Short tube vertical evaporator with propeller

short tube evaporator : 

short tube evaporator Uses :- mild steel or cast iron is used, so suits well for clear liquid & crystallizing solutions. Non-corrosive liquids & mild scaling solutions can also be handled. Advantages :- heat transfer coefficient are high due to high temp. gradient values. It require low head-room. Cleaning & maintenance is easy. It is relatively inexpensive. Disadvantages :- high floor space, more in weight. Relatively more liquid is retained. Rate of heat transfer decreases due to high viscosity liquids. Short tube vertical evaporator with propeller

short tube evaporator : 

short tube evaporator Conical bottom & sometimes a flat bottom. Boiling is quiet violent. Spouting of liquid leads to entrainment. This problem can be avoided by placing a deflector over the tubes. In this case, down-take is annular instead of being central. Advantage :- entire heating element is a single unit. Complete unit can be removed for repairs. Deflector prevents entrainment losses completely Basket type evaporator

Slide 55: 

Classification of evaporators Small scale (laboratory scale) Porcelain dish Glass equipments large scale (industrial) Natural circulation Forced circulation Film evaporators Improved economical Evaporating pan Short tube evaporator Long tube Climbing film Falling film Horizontal film evaporator Wiped film evaporator Rotary film Multiple Vapor compression

climbing film evaporator : 

climbing film evaporator Principle :- The tubes are heated externally by steam. The preheated feed enters from the bottom & flows up through the heated tubes. The liquid gets heated, the liquid near the wall becomes vapor & forms small bubbles. These tend to fuse to larger bubbles, which travel up in the tubes along with entrapped slug. The liquid films are blown up from the top of the tubes & strikes entrainment separator (deflector) kept above. This throws the liquid concentrate down in to the lower part from where it is drawn. Rising tube evaporator

Climbing film evaporator : 

Climbing film evaporator Construction :- The heating unit consists of steam jacketed tubes. Tubes (long & narrow) are held between two plates. A deflector is placed to the top of the vapor head. The evaporator carries steam inlet, vent outlet & condensate outlet. The feed inlet is from the bottom of the steam compartment.

Climbing film evaporator : 

Climbing film evaporator Working :- The preheated liquid feed is introduced from the bottom of the unit. The height of the liquid column is maintained low, i.e. 0.6-1.2 meters above the bottom tube sheet. Steam enters in to the spaces outside the tubes through the inlet. Heat is transferred to the liquor through the walls of the tubes. The liquid becomes vapor & forms smaller bubbles, which tend to fuse to larger bubbles. They travel up in the tubes along with slug. As more vapor is formed, the slug of liquid is blown up in the tubes facilitating the liquid to spread as a film over the walls. Finally the mixture of liquid concentrate & vapor eject at high velocity from the top of the tubes.

Climbing film evaporator : 

Climbing film evaporator Working :- The deflector prevents entrainment, and acts as a foam breaker. Vapor leaves from the top, while concentrate is collected from the bottom. Uses :- Thermolabile sub. Can be concentrated. E.g. insulin, liver extract, vitamins. Clear liquids, foaming liquids & corrosive solutions in large quantities can be evaporated. Deposited film can be removed quickly by increasing feed rate or reducing steam rate.

Climbing film evaporator : 

Climbing film evaporator Advantages :- Large area for heat transfer. Liquid flows at a high velocity, so resistance for heat transfer at the boundary layers is reduced. So heat transfer enhanced. Time of contact between the liquor & heating surface is very short. (1 second for heater & 20 seconds in evaporator). So suitable for heat sensitive materials. Unlike short tube evaporator, the tubes are not submerged, so there is no elevation of B.P. due to hydrostatic head. Suitable for foam-forming liquids, because of deflector. Low hold up & small floor space.

Climbing film evaporator : 

Climbing film evaporator Disadvantages :- Expensive Complicated construction Difficult to clean & maintain Large head space is required Not for viscous liquids, salting liquids and scaling liquids If high feed rate, liquor may be insufficiently concentrated. If low feed rate, film can’t maintained. Dry patches may form on the tube walls.

Slide 62: 

Classification of evaporators Small scale (laboratory scale) Porcelain dish Glass equipments large scale (industrial) Natural circulation Forced circulation Film evaporators Improved economical Evaporating pan Short tube evaporator Long tube Climbing film Falling film Horizontal film evaporator Wiped film evaporator Rotary film Multiple Vapor compression

falling film evaporator : 

falling film evaporator Principle :- Feed enters from the top and flows down the walls of the tubes. The liquid gets heated rapidly due to heat transfer from steam. The liquid boils & becomes vapor, which forms small bubbles. They tend to fuse to form larger bubbles, which travel down the tubes. Concentration takes place during this downward journey. Vapor and liquid are separated at the bottom.

falling film evaporator : 

falling film evaporator Construction :- It resembles climbing film evaporator, but it is inverted. The heating unit consists of steam jacketed tubes. The feed inlet is from the top of the steam compartment. The other provisions are steam inlet, vent & condensate outlet remain same. The outlet for the product is provided at the bottom & is connected to a cyclone separator.

falling film evaporator : 

falling film evaporator Working :- Steam is supplied into the steam compartment. Feed enters from the top of the tubes. The temp. of the boiling liquid is same as that of the vapor head. The feed flows down the walls of the tubes. The liquid gets heated rapidly. The liquid boils & become vapor, which forms small bubbles. These tend to fuse to form large bubbles, which travel down the tubes. Concentration takes place during this downward journey. Vapor & liquid are separated in the cyclone separator.

falling film evaporator : 

falling film evaporator Uses :- To separate volatile & non-volatile material, when the feed is of low viscosity. For the concentration of yeast extract, manufacture of gelatin, extract of tea & coffee. Used for heat sensitive materials such as fruit juices. Advantages :- Suitable for high viscous liquids, because the flow of vapor film is assisted by gravity. Liquid hold up is less & hold up time is very small. Not over heated during passage, heat transfer coefficient is high even at low boiling temp. Highly acidic & corrosive feeds can be concentrated using impervious graphite tubes & rubber lined vapor heads.

falling film evaporator : 

falling film evaporator Disadvantages :- Easy distribution of feed is by using the perforated plate or by spray nozzles, so not useful for the suspension as the solids clog the perforated plate. Not suitable for salting or scaling liquids. Feed distribution is poor. For continuous supply, liquid may recirculated or rate of feed is kept high.

Slide 68: 

Classification of evaporators Small scale (laboratory scale) Porcelain dish Glass equipments large scale (industrial) Natural circulation Forced circulation Film evaporators Improved economical Evaporating pan Short tube evaporator Long tube Climbing film Falling film Horizontal film evaporator Wiped film evaporator Rotary film Multiple Vapor compression

Forced circulation evaporator : 

Forced circulation evaporator Principle :- Liquid is circulated through the tubes at high pressure by means of a pump. Boiling does not takes place because boiling is elevated. Forced circulation of liquid creates form of agitation. When the liquid leaves the tubes & enters the vapor head, pressure falls suddenly This leads to flashing of super heated liquor. Thus evaporation is effected.

Forced circulation evaporator : 

Forced circulation evaporator Construction :- The steam jacketed tubes are held between two tube sheets. Tubes are 0.1 m inside diameter & 2.5 m long. The part of tubes projects into the vapor head. Which consists of a deflector. Vapor head is connected to a return pipe, which runs downward and enters into the inlet of a pump.

Forced circulation evaporator : 

Forced circulation evaporator Working :- Steam is introduced into the calandria. Pump sends the liquid to the tubes with a positive velocity. As the liquid moves up through the tubes, it gets heated & begins to boil. As a result the vapor & liquid mixture rushes out of the tubes at a high velocity. This mixture strike the deflector which throws the liquid downward. This result in an effective separation of liquid & vapor. The vapor enters the cyclone separator & leaves the equipment. The concentrated liquid returns to the pump for further evaporation. Finally the concentrated product is collected.

Forced circulation evaporator : 

Forced circulation evaporator Uses :- It is operated under reduced pressure, so suitable for thermolabile substance. For concentration of insulin & liver extract. Suitable for crystallizing operations where crystals are to be suspended at all times. Advantages :- High heat transfer coefficient due to rapid liquid movement. Salting, scaling & fouling are not possible due to forced circulation. Suitable for thermolabile substance, high viscous preparations because of pumping mechanisms. Disadvantages :- Hold-up of liquid is high. Expensive because pump is required.

Slide 73: 

Classification of evaporators Small scale (laboratory scale) Porcelain dish Glass equipments large scale (industrial) Natural circulation Forced circulation Film evaporators Improved economical Evaporating pan Short tube evaporator Long tube Climbing film Falling film Horizontal film evaporator Wiped film evaporator Rotary film Multiple effect evaporator Vapor compression

Multiple effect evaporator : 

Multiple effect evaporator Vertical tube evaporator discussed earlier is a single effect evaporator. Such evaporator is connected in a several ways so as to achieve large scale evaporation as well as greater economy. They are not used in pharmaceutical industry, but the principle are of interest and should be understood. This is illustrated using an example of triple effect evaporator with a parallel feed mechanism.

Multiple effect evaporator : 

Multiple effect evaporator Advantages :- Suitable for large scale & for continuous operation. Highly economical when compared to single effect. About 5 evaporators can be attached. Construction :- 3 single effect evaporators are attached i.e. triple effect evaporator. The other aspects of construction remain same as single effect. The vapour from 1st evaporator serves as a heating medium for 2nd evaporator. Same as for 2nd and 3rd. Last evaporator is connected to a vacuum pump.

Multiple effect evaporator : 

Multiple effect evaporator Working :- Parallel feed :-a hot saturated solution of the feed is directly fed into each of the three effects in parallel without transferring the material from one to another. This is commonly used in the concentration of the salt solution, where the solute crystallizes on concentration without increasing the viscosity. Operations :-the equipment is at room temp. & at atm. Pressure at the beginning. The liquid feed is introduced into all the 3 evaporators up to the level of upper tube sheets. And the following operations are attempted.

Multiple effect evaporator : 

Multiple effect evaporator The vent valves V1,V2&V3 are kept open & all other valves are closed. Now high vacuum is created in the liquid chambers of evaporators. The steam valve S1 & condensate valve C1 are opened. Steam is supplied. Steam first replaces cold air in the steam space of 1st evaporator. The supply of steam is continued until the desired pressure P0 is created in the steam space of 1st evaporator. At this pressure, the temperature of the steam is t0. Steam gives its temperature to the liquid feed in the 1st evaporator and gets condensed. Condensate is removed through the valve C1.

Multiple effect evaporator : 

Multiple effect evaporator Due to heat transfer, the liquid temperature increases & reaches the B.P. during this process, vapour well be generated from the liquid feed. So, formed vapour displaces air in the upper part of 1st evaporator. Moreover, the vapour also displaces the air in the steam space of the 2nd evaporator. After complete displacement of air by vapour in the steam compartment of 2nd evaporator, the valve V2 is closed. The vapour of 1st evaporator transmits its heat to the liquid of 2nd evaporator & gets codensed. Condensate is removed through the valve C2. these steps continue in the 3rd evaporator also.

Multiple effect evaporator : 

Multiple effect evaporator As the liquid in 1st evaporator gains temp., the difference in temp. between the liquid & steam decreases, hence, the rate of condensation decreases. As a result, the pressure in the vapour space of 1st evaporator gradually increases to P1 by increasing temp. to t1, which is the B.P. of the liquid in 1st evaporator, & decreasing the temp. difference (t0-t1). A similar change takes place in the 2nd evaporator & the liquid reaches the B.P. similarly, the process will be repeated in 3rd evaporator. Finally 3 evaporators come to a steady state with the liquid boiling in all the 3 bodies.

Multiple effect evaporator : 

Multiple effect evaporator As boiling proceed, liquid level in 1st evaporator comes down. Feed is introduced through the feed valve to maintain the liquid level constant. Similarly evaporation of liquid takes place in 2nd & 3rd evaporators. To maintain the liquid levels constant, feed valves F2 & F3 are used for 2nd & 3rd evaporator respectively. This process is continued until the liquid in all the evaporators reaches the desired viscosity. Now the product valves are opened to collect the thick liquid. Thus in this evaporators, there is continuous supply of feed, continuous supply of steam & continuous withdrawal of liquid from all 3 evaporators. Hence, evaporators work continuously.

Multiple effect evaporator : 

Multiple effect evaporator The evaporators can also be fed by Forward feed method:-- mother liquor is introduced into 1st then transferred to 2nd & then to 3rd. Backward feed method:-- mother liquor is introduced into 3rd then transferred to 2nd & then to 1st. Mixed feed method:-- mother liquor is introduced into 2nd then transferred to 3rd & then to 1st .

Multiple effect evaporator : 

Multiple effect evaporator Economy of multiple effect evaporator It is the quantity of vapour produced per unit steam admitted. Feed is admitted at its B.P. so it does not require any more heat to raise its temp. Hence, the supplied steam is condensed to give heat of condensation. This heat will then transferred to the liquid. The heat transferred now serves as latent heat of vaporization, i.e. liquid undergoes vaporization by receiving heat. Loss of heat by means is negligible.

Multiple effect evaporator : 

Multiple effect evaporator Economy of multiple effect evaporator So economy of evaporator may be expressed as: Economy of evaporator=total mass of vapour produced/total mass of steam supplied. In single-effect evaporator, steam produces vapour only once. So Economy of single effect evaporator=N units of vapour produced/N units of steam supplied=1 In multiple effect evaporator, one unit of steam produces vapour many times, depending on the no. of evaporators connected. So Economy of multiple effect evaporator=N units of vapour produced/1 unit steam supplied=N Therefore, economy of multiple effect evaporator is N times the economy of the single effect evaporator.

Slide 84: 

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