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# Drying

introduction, theory of drying, applications of drying, construction & working about fluidised bed dryer,use of tray dryer,construction about vacuum dryer, construction & working about drum dryer, construction about spray dryer

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### Drying

1. 1. INTRODUCTION TO DRYING PROCESSES Mr. R. R. Patil Dr. Shivajirao Kadam College of Pharmacy, Kasabe digraj, Sangli
2. 2. Introduction  “ Drying is defined as the final removal of liquid from solids by vaporisation with the aid of heat.”  The equipment used for drying is called dryer.
3. 3. Theory of drying  Drying involves both heat & mass transfer operations.  To dry material, two steps are needed viz.. i. Heat must be supplied to provide latent heat of vaporisation. ii. The liberated vapour must be removed by moving an air steam.
4. 4. Cont..  Consider that, a film of liquid is formed at the surface of the material being dried. The rate of evaporation of this film is related to the rate of heat transfer, as per equation..  dw/dØ = q / λ ……. eq.1  Where, dw/dØ = the rate of evaporation pounds of water per hour q = overall rate of heat transfer (BTU per hr) λ = latent heat of vaporisation of water (BTU Per
5. 5. Cont..  The rate of diffusion of moisture into the air stream is expressed by the rate equations, similar to those for heat transfer. The driving force is a humidity differential, whereas, heat transfer is a temperature differential. Thus eq.1 can be written as.. dw/dØ = K’ A (Hs – Hg)……..eq.2 Where, dw/dØ = the rate of diffusion as pounds of water per hr. K’ = the coefficient of mass transfer. A = area of the evaporating surface in square feet. Hs = the absolute humidity at the evaporating surface. Hg = the absolute humidity in the passing air stream
6. 6. Cont…  K’ is not a constant but it varies with the velocity of the air stream passing over the evaporating surface thus.. K’ = cGn where, c = a proportionality constant G = the rate of flow of air n = a fractional exponent usually about 0.82 After certain period, the rate of evaporation is equal to the rate of diffusion of vapour. The rate of heat transfer can be equated with the rate of mass transfer i.e eq.1 can be equated with eq.2 i.e
7. 7. Applications of drying  Used in manufacturing of granules.  Used to reduce the bulk & weight of material.  It helps in the preservation of crude drugs or plant from mould growth.  It helps in the size reduction process.
8. 8. Equipments…  Tray dryer  Tunnel dryer  Rotary dryer  Fluidised-bed dryer  Vacuum dryer  Freeze dryer
9. 9.  It is the simplest form of dryer, in this arrangement of heater at the bottom in cabinet e.g. laboratory oven.  In that the best type of tray dryer is directed circulation form, in which air is heated & is directed across the material in a controlled flow.  In modern tray dryers, a uniform temperature & air flow is maintained by the use of a well cabinet with strategically placed fans & heating coil.  In that heater is fixed in a such way that the air is reheated before passing over each shelf.  When the air passes over each shelf a certain amount of heat is given up to provide latent heat of vaporisation. TRAY DRYER
10. 10. Cont..  USE: for crude drugs, powders, granules. etc.
11. 11. TUNNEL DRYER  It is the same like tray dryer but only modification in which oven is replaced by a tunnel.  In which, receive the damp material at one end & discharge the dried product at the other end.  In that conveyor belt system can also be used to pass the material through the tunnel.
12. 12. Cont..
13. 13. ROTARY DRYER  It is a modified form of the tunnel dryer.  In which the material pass through a rotating cylinder, counter current to the stream of heated air.  It is a cylindrical shell which is mounted with slight slope, so that the material fed in at one end will move through it on slow rotation.  Rotation speed near about 10 rpm.  USE: powders, granules for continuous drying purpose.
14. 14. Cont..
15. 15. FLUDISED BED DRYER  It is a best dryer because, in that rapid drying occurs.  Theory: If a gas is allowed to flow upward through a bed of solid particles at a velocity greater than the velocity of the particles, the particles are partially suspended in the gas stream.  The resultant mixture behaves like a liquid, so solids is said to be fluidised.
16. 16. Cont..
17. 17. Cont..  It have two types: Vertical & Horizontal fluidised bed dryer.  In that fan is mounted in the upper part of the dryer.  Air is heated by heaters & passed through the wet material contained in a drying chamber.  This chamber is fitted with a wire mesh support at the bottom.  The air flow rate is adjusted by re-circulation control.  Fabric filter bags are provided to prevent the passage of fine particles.  It is used for batch type dryer & the drying chamber is removed from the unit for charging
18. 18. Cont..
19. 19. VACUUM DRYER  This equipment is a good example of conduction drier. The vacuum oven consists of a jacketed vessel to withstand vacuum within the oven.  There are supports for the shelves giving a larger area for conduction heat transfer. The oven can be closed by a door.  The oven is connected through a condenser and liquid receiver to a vacuum pump.  Operating pressure can be as low as 0.03-0.03
20. 20. Cont.. Condenser Connection to Vacuum Pump Condensat e Receiver Steam or Water Jacket
21. 21. FREEZE DRYER or LYOPHILIZATION  It is also called as sublimation drying process  In that process water is sublimed & removed from the product after it is frozen at the reduced pressure.  Theory:  Material is frozen in a container connected to a high vacuum system.  Vapour pressure of material is reduced below the vapour pressure at which it is dried i.e below the triple point.  Under these conditions, heat transferred is used as latent heat of sublimation to the product by means of radiant heat.  Ice sublimes to vapours.  Water vapours are removed by condensation.
22. 22. Cont..  Construction:  Chamber for vacuum drying: having shelves for keeping the material.  Vacuum source: vacuum pump is connected to the chamber.  A heat source: heat is provided by conduction or radiation or both.  A vapour removal system: the condenser consists of large cooled surface by solid carbon dioxide mixed with acetone or ethanol.
23. 23. Cont..  Working:  Pre-treatment : first solution is concentrated under normal vacuum dryer before introducing in the chamber.  Pre-freezing : ampoules, vials & bottles having aq.solution is packed & frozen in cold shelves at a temperature below -500C.  Primary drying :  The material is spread.  Temperature & pressure are kept below the triple point of water.  Heat is supplied.  Ice sublimes into vapours.  Vapours are removed from the chamber.  98-99% moisture removes during primary drying.
24. 24. Cont..  Secondary drying : remaining moisture is removed by vacuum drying at 50-600C. Secondary drying takes about 10-20 hrs.  Packing : biological products are dried packed in aseptic condition immediately after drying.  Use: thermolabile subst, microbiological cultures, biological products.
25. 25. Drum Dryer or Film Drying  It consists of a drum of about 0.75-1.5 m in diameter and 2-4 m in length, heated internally, usually by steam and rotated on its longitudinal axis.  Operation:  The liquid is applied to the surface and spread to a film, this may be done in various ways, but the simplest method is that where the drum dips into a feed pan. Drying rate is controlled by using a suitable speed of rotation and the drum temperature. The product is scraped from the surface of the drum by means of a doctor
26. 26. Cont..
27. 27. SPRAY DRYER  The spray dryer provides a large surface area for heat and mass transfer by atomizing the liquid to small droplets.  These are sprayed into a stream of hot air, so that each droplet dries to a solid particle.  The drying chamber resembles the cyclone ensuring good circulation of air, to facilitate heat and mass transfer, and that dried particles are separated by the centrifugal action.  Spray dryer can be operated efficiently at various feed rates.
28. 28. Cont..