Evaporation presentation

Hajvery University (Faculty of Pharmacy) 1

Evaporation

Dr.Zulcaif Ahmad

2

TABLE OF CONTENTS• EVAPORATION

DefinationMechanismFactors affecting the Evaporation

• EVAPORATION UNDER REDUCEDE PRESSURE• EQUIPMENT USED FOR EVAPORATION • APPLICATION OF EVAPORATION• REFERENCES

Hajvery University (Faculty of Pharmacy)


EVAPORATIONDefination:-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.


• 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.


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.


Factors affecting the Evaporation • There are seven factors to affect evaporation.1. Temperature2. Surface area3. Agitation4. Atmospheric aqueous vapour pressure5. Atmospheric pressure on the liquid under

evaporation6. Type of product required7. Economic factors


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.


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


EVAPORATION UNDER REDUCEDE PRESSURE

• At reduced pressure evaporation increases. Advantage:

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.


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.


2) Forced circulation evaporator.3) Film evaporator

Types: i) Wiped Film evaporatorii)Long Tube Evaporator

a) Climbing film evaporatorb) Falling film evaporator


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.


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.


• Working:• The dilute solution is taken

in the pan. Steam is introduced through the steam inlet into the jacket to heat the pan. In these evaporators the movement of the liquid results from convection currents set up by the heating process. The concentrated liquid is collected through the outlet placed at the bottom of the pan.



Advantages1) They are simple, easy and cheap to construct.2) They are easy to use and clean.3) Stirring of the evaporating liquids can be

done easily.


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.


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.


Working :• The dilute liquid is fed

into the still, the cover is clamped. Steam is introduced into the jacket. The liquid is evaporated and condensed in the condenser and collected. The product (i.e. concentrated liquid) is collected through the product outlet.


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.


iii) Short tube evaporatorConstruction:• 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 – 2 m• 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.


Working:• The liquor in the tubes is

heated by the steam and begins to boil, when the mixture of liquid and vapor will shoot up the tubes (in a similar manner to that of a liquid that is allowed to boil to vigorously in a test-tube).

• The product is collected through the product outlet.


• 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.

• 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.


Working principle:• Feed is introduced through the

liquor inlet. Pump will force the liquid through the calendria. Steam heats the liquid inside the calendria. As it is under pressure in the tubes the boiling point is elevated and no boiling takes place. As the liquor leaves the tubes and enters the body of the evaporator through the tangential inlet there is a drop in pressure and vapor flashes off from the superheated liquor. The concentrated liquid is pumped out through the product outlet and the vapor is collected through the vapor outlet.


Advantages1) 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.


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.


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 evaporatorii)Long Tube Evaporator

a) Climbing film evaporatorb) Falling film evaporator



i) Wiped film evaporators• Construction:• 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.


Working:• The liquor is introduced at the

top of the vessel and spread as a film over the heated wall by the action of the rotor.

• Evaporation occurs as the liquor passes down the wall, vapour is taken to a condenser and the concentrated liquor withdrawn at the bottom of the vessel.

• The evaporator is therefore a form of single tube, falling film evaporator in which the film is formed and agitated mechanically.


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.


Working:• At the upper end, the mixture of vapor and

concentrated liquor enters a separator, the vapor passes to a condenser, and the concentrated liquid to a receiver.

• Cold or pre heated liquor is introduced into the tube.

• Heat is transferred to the liquor from the walls and boiling begins.

• Ultimately sufficient vapor has been formed for the smaller bubbles to unite to a large bubble,

• filling the width of the tube and trapping a ‘slug’ of liquid above the bubble.

• As more vapor is formed, the slug of liquid is blown up the tube, the tube is filled with vapor, while the liquid continues to vaporize rapidly, the vapor escaping up the tube and, because of friction between the vapor and liquid, the film also is dragged up the tube upto a distance of 5 to 6 metres.


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.


Working principle• The liquor to be evaporated

is introduced at the top of the evaporator tubes and the liquor comes down due to gravity.

• The concentrate and vapor leaves the bottom. They are separated in a chamber where the concentrate is taken out through product outlet and vapor from vapor outlet.


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.


APPLICATION OF EVAPORATION• Evaporation is one of the most important

processes in the manufacture of pharmaceuticals.

• It is used in the preparation of1. 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.


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


Refrences

• Cooper and Gunn’s, Tutorial Pharmacy Edited by S.J.Carter, Sixth Edition, Part two, Pharmaceutical Technology, Unit # 14,Page number, 164-172.

• http://www.sspindia.com/prd/forced-eve-img1.jpg

• http://www.avta-us.com/images/lab_horiz.jpg• http://www.sws.uiuc.edu/atmos/statecli/instr

uments/panevap.jpg

http://www.sspindia.com/prd/forced-eve-img1.jpg

http://www.sspindia.com/prd/forced-eve-img1.jpg

http://www.avta-us.com/images/lab_horiz.jpg

http://www.sws.uiuc.edu/atmos/statecli/instruments/panevap.jpg

http://www.sws.uiuc.edu/atmos/statecli/instruments/panevap.jpg


THANK YOU

Evaporation presentation

Health & Medicine