Drying (Pharmaceutics)

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Drying V V S Narayana Reddy Karri Lecturer Dept of Pharmaceutics JSS College of Pharmacy, Ootacamund ( JSS University, Mysore ) Rocklands Udhagamandalam.


Definition Drying is defined as the removal of small amounts of water or other liquid from a material by the application of heat .


Applications In the pharmaceutical industry it is used as a unit process in the manufacture of granules which can be dispensed in bulk or converted into tablets or capsules. Drying can also he used to reduce the bulk and weight of the material , thereby lowering the cost of transportation and storage. It helps in the preservation of crude drugs of plant from mould growth , which occurs due to presence of moisture. It helps in the size reduction of crude drugs . The presence of moisture in the crude drug does not allow it to get powdered easily.


Equipment Tray drying Tunnel dryers Rotary dryers Vacuum drying Fluidized-bed drying Freeze dryers Drum dryer Spray dryer Dryers used for Solutions and Suspensions Dryers used for Pharmaceutical granulation


Example: Laboratory Oven A fan is fitted to the oven, the forced hot air is circulated which helps in increasing the heat transfer . The best type of a tray dryer is that of the air is heated and is directed across the material in a controlled flow. The material to be dried is spread on the trays . The trays used have solid, perforated, or wire mesh bottoms . In tray dryer, hot air is continuously circulated. Forced convection heating takes place to remove moisture from the solids placed in trays. 1. TRAY DRIER


Example: Laboratory Oven In modem tray dryers , a uniform temperature and air flow is maintained by the use of a well-insulated cabinet with strategically placed fans and heating coils. As shown in Fig. there is an alternate arrangement of the shelves so that air can flow uniformly without any obstruction (as indicated by the arrows). Heater is fixed in such a way that the air is re-heated 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 . Uses Tray dryers are used for drying of crude drugs, chemicals, Powders and granules used in tablet manufacturing. 1. TRAY DRIER..

2. Tunnel Dryer:

2. Tunnel Dryer A tunnel dryer is a modification of the tray dryer in which the oven is replaced by a tunnel which receives damp material at one end and discharges the dried product at the other end. It may be operated batch wise or continuously . The tunnel dryers can be of any size and the movement of solid material can be effected by placing it on trays on rails and at suitable intervals a tray may be pushed into the inlet which displaces the tray from the dry end. Inlet Outlet Tunnel

3. Rotary Dryer:

3. Rotary Dryer Rotary Dryer is a modified form of the tunnel dryer in which the material is passed through a rotating cylinder , counter current to the stream of heated air . It consists of a cylindrical shell (about 10 m in length) which is mounted with a slight slope , so that the material fed in at one end will move through it on its slow rotation at about 10 revolutions per minute . Heating of the cylindrical shell is done directly by passing hot air current or indirectly by steam tubes or heated gases circulated through an outer shell. Due to the rotation of the cylinder, material is turned over and drying takes place from individual particles. Uses The rotary dryer is used for the purpose of drying any powdered or granular solid. On a large scale operation, rotary dryers are used for continuous drying.

4. Fluidized bed dryer:

Theory In a fluidized bed dryer, good contact between hot air and particles to be dried is obtained which causes rapid drying. If a gas is allowed to flow upward through a bed of solid particles at a velocity greater than the settling velocity of the particles, the particles are partially suspended in the gas stream. Each individual solid particle is surrounded by the drying gas with the result that drying takes place in a much shorter period. 4. Fluidized bed dryer

4. Fluidized bed dryer…:

Two types of fluidized dryers are used in the pharmaceutical industry. Vertical fluidised bed dryers Horizontal fluidised bed dryers A typical vertical fluidised bed dryer is shown in Fig. The fluidising air stream is induced by a fan which is mounted in the upper part of the dryer. The air is heated to the required temperature in air eaters and passed through the wet material contained in a drying chamber fitted with a wire mesh support at the bottom. The fabric filter bags are provided to prevent the passage of fine particles . This type of fluidised bed dryer is a batch type dryer and the drying chamber is removed from the unit for charging and dumping. The fluidised bed dryers are available in different capacities ranging from 5 kg to 200 kg with an average drying time of about 20-40 minutes. 4. Fluidized bed dryer…

4. Fluidized bed dryer…:

Advantages of Fluidised Bed Dryer. It gives a high drying rate . The fluidised bed dryer method is 15 times faster than the routine method of a tray dryer. The method is suitable for drying of thermo labile materials because heating time is minimized. In a fluidised bed dryer, drying takes place from individual particles and not from the entire bed. The fluidised bed dryer has a high output and occupies a small floor space. Disadvantages of Fluidised Bed Dryers The turbulence produced when hot air is passed through the material may cause attrition of some material, which leads to the production of fines. The vigorous movement of solid particles in hot air can lead the generation of an electric charge . Hence suitable precautions must be taken in this regard. 4. Fluidized bed dryer…

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It consists of a jacketed vessel made of a material which can withstand vacuum within the oven and steam pressure in the jacket. The oven is connected to a vacuum pump through a condenser and a receiver . Generally, vacuum oven is operated at the pressure about 0.03 to 0.06 bar . At this pressure water boils at 25-35°C. In the pharmaceutical industry an oven of the size of about 1.5 m cubes (1cubic meter=1000 lit) having 20 shelves is commonly used. Nowadays, the vacuum ovens with a number of small compartments and doors are available rather than one big compartment with a heavy door. 5. Vacuum Dryer

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Advantages of Vacuum Dryer The dryer is useful for drying of heat sensitive materials as drying takes place at a low temperature. A porous and soft dry extract is produced when a liquid extract is dried in a vacuum dryer. The valuable solvent can be recovered from the extract, when it i is dried in a vacuum dryer as solvent vapors are condensed in the condenser. Disadvantages of Vacuum Dryers The labor and running costs are rather high. The vacuum dryer is of limited capacity. 5. Vacuum Dryer

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Freeze drying is also known as lyophilization . Principle : In freeze drying, water is removed from the frozen state by sublimation, i.e., direct change of water from solid into vapour without conversion to a liquid phase . Theory: In this process, the material is frozen in a suitable container connected to a high vacuum system, so that the vapor pressure of water vapor is reduced to less than that of the material being dried. Thus it reduces the temp and pressure to values below the triple point of water . In these conditions, any heat transferred is used as latent heat and ice sublimes directly to vapor . 6. Freeze Dryer

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Construction: A chamber for vacuum drying A Vacuum source A heat source A vacuum removal system as vacuum pump 6. Freeze Dryer…

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Working: 1. Pretreatment: This step is done to reduce the volume of solution to be dried. The solution is pre-concentrated under normal vacuum drying. This reduces the actual drying by 8-10 times . 2. Pre-freezing: This done to solidify water . The vials and bottles in which aqueous solution is packed are frozen at a temperature below -50 o C 3. Primary drying: The temperature and pressure is kept below triple point of water for the sublimation of water. Heat is supplied which transfers as latent heat and ice sublimes directly into vapor state . Primary drying removes about 98-99% moisture 6. Freeze Dryer…

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Working: 4. Secondary drying: The moisture left in the primary drying is removed by an ordinary vacuum drying . The vacuum drying is done at a temperature of 50 o -60 o C 5. Packing 6. Freeze Dryer…

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Advantages: The product obtained is light and porous having excellent solubility Drying takes place under vacuum ; hence, oxidation is minimized as there is no contact with air. The heat sensitive material can be dried The sterility of the product can be maintained Disadvantages: The process is very expensive because a complicated plant is used The period of drying is long , it usually not less than 10 hours 6. Freeze Dryer…

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Dryers used for Solutions and Suspensions

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Sometimes solutions and suspensions are required to be converted into solids. This is done by using special types of dryers. The main purpose of these dryers is to spread the liquid to a large surface area so that drying can occur rapidly. Two types of dryers are used for this purpose: The dryers in which there is spreading of the liquid in the form of a thin film e.g. Drum Dryer. The dryers in which there is dispersing of the liquid as a spray of small droplets e.g. Spray Dryer.

7. Drum Dryer:

The drum dryer consists of a hollow roller with a smoothly polished external surface heated internally by steam. It rotates on its longitudinal axis. The liquid to be dried is placed in a trough known as a "feed pan". The liquid is picked up by the roller as it rotates, covering the surface in a thin film which gets dry during the revolution of the roller. On the completion of one revolution, the dried film is removed mechanically by a scraper known as a "doctor knife". Advantages of Drum Dryer The dryer is compact. It occupies much less space as compared to a spray dryer. The product obtained in the drum dryer is in the form of flakes which are used conveniently for many purposes. The liquid is exposed to heat for a few seconds In order to reduce the temperature of drying, the drum can be enclosed in a vacuum chamber. There is rapid drying because the liquid gets spread in the form of thin film over a large area resulting in rapid heat and mass transfer. 7 . Drum Dryer

7. Drum Dryer…:

Disadvantages of Drum Dryer Technical skill is required to operate a drum dryer. A careful control on feed rate, film thickness, speed of rotation and temperature is necessary. 7 . Drum Dryer… Applications A drum dryer is used for drying of solution or suspension e.g. drying of milk products, starch products, ferrous salts and suspension of Zinc oxide or Kaolin.

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8 . Spray Dryer Spray Dryer In a spray dryer the liquid to be dried is sprayed in the form of mist. The minute droplets are readily evaporated and get converted into solid particles, which all to the bottom of the chamber. The vapours are transferred into a separator where the fine dry particles which are carried along with vapours are separated and collected. Spray dryers are available in many forms and designs. A typical spray dryer is shown Fig. 12-7 The drying chamber is just like the cyclone separator, so as to ensure good circulation heat and mass transfer and also to ensure that the dried particles are separated by the centrifugal action.

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8 . Spray Dryer… The character of the particles depends on in the form of droplets. As such it is import atomiser . Two types of atomisers are used: 1. Jet atomiser 2. Rotary atomiser Jet atomisers are easily blocked resulting in variation of the droplet size. Rotary atomisers are preferred to avoid this problem. Advantages of Spray Dryer The liquid when sprayed through an atomiser gets converted into Due to rapid evaporation, the droplets do not attain a small droplets and it also provides a large surface area for heat and mass transfer so that evaporation is rapid. of latent temperature so that most of the heat used is in the form heat of vaporisation . The dry product obtained has a high density, hence it has an excellent solubility. Labour costs are low. The product obtained is free flowing having almost spherical particles which is quite helpful in the tablet manufacturing. Disadvantages of Spray Dryer 1. The spray dryer is very bulky and expensive. It occupies a large space. 2. The thermal efficiency is quite low. Applications Spray dryer can be used for drying of any substance both in solution or in suspension. Spray dryers are very useful for drying of thermolabile substances. Citric acid, borax, sodium phosphate, hexamine, gelatin and extracts are dried by a spray dryer. The suspensions of starch, barium sulphate and calcium phosphate are also dried by a spray dryer. Milk, soap and detergents, too are dried by a spray dryer.

9. Microwave Drying:

9. Microwave Drying It is the method of drying in which electromagnetic radiation (radio frequency from 3-150 MHz or microwave frequency at 915 or 2450 MHz) is applied to the material to be dried. If polar solvent molecules are present, the electromagnetic field induce orientation of the dipoles in molecules. As the field oscillates, the polar solvent molecules oscillate with the field, resulting in increased kinetic energy from dipolar molecules and their collisions with others This manifested in thermal or heat energy Applications: Drying of thermoliable pharmaceutical materials such as vitamins, enzymes, proteins and flavors.

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