Drying and Pharmaceutical Dryer

 

DRYING

·      Drying involves removal of water or another solvent by evaporation from a solid, semi-solid or liquid by application of heat and finally a liquid free solid product is obtained.

·         Drying is accomplished by thermal techniques but non-thermal drying processes such as squeezing wetted sponge, adsorption by desiccant (desiccation) and extraction are also used.

·      In bio products like food, grains, and pharmaceuticals like vaccines, the solvent to be removed is almost invariably water.

Difference between Drying and Evaporation

Drying

Evaporation

Done to get a stable dry product.

Final product is either concentrated

Suspension or wet slurry.

Removal of less amount of moisture.

Removal of large amount of liquid.

Drying occurs below boiling point.

Evaporation occurs more at boiling point.

Emphasize on solid product.

Emphasize on reducing the volume.

 

·         The main objectives of drying include to preserve foods and increase their shelf life by reducing the water content and water activity; avoid the need for use of refrigeration systems for transport and storage (expensive); reduce space requirements for storage and transport.

·         In pharmaceutical technology, drying is carried out for one or more of the fallowing reasons:

1. To avoid or eliminate moisture which may lead to corrosion and decrease the product or drug stability.

2.       To improve or keep the good properties of a material like granules, e.g. Flowability, compressibility.

APPLICATIONS OF DRYING

·         Preparation of bulk drugs: In the preparation of bulk drugs, drying is the final stage of processing. A few examples are dried aluminum hydroxide, spray dried lactose and powdered extracts.

·         Preservation of drug products: Drying is necessary in order to avoid deterioration. For examples protection of blood products, skin, tissues and crude drugs from microbial growth.

·         Improved characteristics: Drying produces materials of spherical shape, uniform size, free flowing and enhanced solubility.

·         Improved handling: To reduce the cost of transportation of large volume materials.

·         To make the materials easy or more stable for handling. Drying reduces moisture content.

·         Drying as final step: Drying is the final step in evaporation, filtration, and crystallization

Mechanism of drying

• Involves both heat and mass transfer

• Rate of evaporation is related to rate of heat transfer as

             dW/d=q/ƛ

• The rate equation for mass transfer

            dW/d=KA (Hs-H g )

Moisture content

• Bound moisture and free moisture

• % moisture content=   weight of water in samplex100/ weight of dry sample

• If exactly 5 g of moist sample is brought to a constant dry weight of 4g, calculate loss on drying and moisture content

• Percent MC is more realistic value in determination of dryer load capacity

• Equilibrium moisture content depends on

– Temperature and humidity of air

– Properties of the material

Rate of drying & Time of drying

Classification and types of dryers

·         Static Bed Dryers (Ex. Tray Dryers, Freeze Dryer)

·         Moving Bed Dryer (Ex. Drum Dryer)

·         Fluidised Bed Dryer

·         Vacuum Dryer

·         Pneumatic Dryer (Ex. Spray Dryer)

TRAY DRYER

Principle

·         The basic working principle of this incredible machine is the continuous circulation of hot air.

·   In the tray dryer, moisture is removed from the solids that are placed in the tray by a forced convectional heating.

·         The moist air is removed is partially but in a simultaneous fashion.


Construction

·    Tray Dryer is used for the best drying results in conventional process.

·         It is a double walled cabinet with Single or Two doors.

·         The gap between two walls is filled with high density fibre glass wool insulation material to avoid heat transfer.

·         Doors are provided with gaskets.

·         Stainless steel trays are placed on the movable trolleys.

·         Tray Dryer is provided with control panel board, process timer, Digital temperature controller cum indicator etc.

·         Tray Dryer is available in capacities ranging from 6, 12, 24, 48, 96, 192 trays.

Working

·         In tray dryer hot air is continuously circulated. Forced convection heating takes place to remove moister from the solids placed in trays.

·         Simultaneously the moist air is removed partially.

·         Wet solid is loaded in to the trays. Trays are placed in the chamber.

·         Fresh air is introduced through in let, which passes through the heaters and gets heated up.

·         The hot air is circulated by means of fans at 2 to 5 meter per second.

·         Turbulent flow lowers the partial vapour pressure in the atmosphere and also reduces the thickness of the air boundary layer.

·         The water is picked up by the air. As the water evaporates from the surface, the water diffuses from the interior of the solids by the capillary action.

·         These events occur in a single pass of air. The time of contact is short and amount of water picked up in a single pass is small.

·         Therefore, the discharged air to the tune of 80 to 90 % is circulated back through the fans. Only 10 to 20% of fresh air is introduced.

·         Moist air is discharged through outlet. Thus, constant temperature and uniform air flow over the materials can be maintained for achieving uniform drying.

·         In case of the wet granules as in tablets and capsules drying is continued until the desired moister content is obtained.

·         At the end of the drying trays or trucks are pulled out of the chamber and taken to a tray dumping station.

Advantages

·         Each batch is handled as a separate entity.

·         It is more efficient in fuel consumption.

·         It is operated batch-wise.

·         It is simple to use.

·         It provides tendency to over-dry the lower trays.

·         It requires little labor costs – merely load and then unload.

Disadvantages

·         The process is time-consuming.

·         It requires extra cost.

·         Not suitable for oxidizable and thermolabile substances.

Uses

·         Tray dryer is used in the drying of the sticky materials.

·         It is used in the drying of the granular mass or crystalline materials.

·         Plastic substances can be dried by the tray dryers.

·         Wet mass preparations and pastes can be dried in a tray dryer.

·         In the tray dryers the crude drugs, chemicals, powders and tablet granules are also dried to obtain free flowing materials.

·         Some types of equipments can be dried in the tray dryers.

Variants:

·      Tray dryer may be operated under vacuum called vacuum tray dryers.

·    Another is Tunnel dryer. In this type, trucks are loaded with wet materials at one end of the tunnel.

·         The tunnel comprised of a number of units, each of which is electro-statically controlled.

·         The solids get dried and the product is discharged at the other end of the tunnel.

 

DRUM DRYER OR ROLL DRYER

Principle

·         In drum drying, the heated surface is the envelope of a rotating horizontal metal cylinder.

·         The cylinder is heated by steam condensing inside, at a pressure in the range of 200 to 500 kPa bringing the temperature of the cylinder wall to 120–155°C.


Construction

·         A drum dryer consists of one or two horizontally mounted hollow cylinder(s) or drums of about 0.75-1.5 m in diameter and 2-4 m in length, made of high-grade cast iron or stainless steel, a supporting frame, a product feeding system, a scraper, and auxiliaries.

·         The drum is heated internally by steam, and rotated on its longitudinal axis.

·         The external surface of the drum is polished.

·         Liquid or slurry is placed as feed in a pan. The drum is partially dipped in pan.

·         The spreader is used to spread liquid film evenly on roller.

·    The rotation of the drum adjusted so that all of the liquid is fully vaporized.

·    The drum is rotated continuously.

·         The dried deposits can be scrapped off with the help of doctor knife.

Working

·         As the drum rotates, the liquid material gets adhere to external surface of drum.

·      The liquid is spread as film on to the surface. The drying of the material is done by process of steam when passed in to the drum.

·         By the mechanism of conduction, the heat gets transferred in to drum and drying process takes place.

·         The material is completely dried during whole process during its revolution.

·         The dried material is scrapped by the knife and that fall in to the bin.

Advantages

·         Drying takes place in less time.

·         It is suitable for thermo sensitive drugs.

·         It occupies less space.

·         In order to reduce the temperature of drying the drum can be enclosed in a vacuum chamber.

·         Rapid drying takes place due to rapid heat and mass transfer.

Disadvantages

·         Maintenance cost is high.

·         Skilled operations are essential to control thickness of film.

·         It is not suitable for less solubility products.

·         The operating conditions are critical.

·    It is necessary to introduce careful control on feed rate, film thickness, and speed of drum rotation and drum temperature.

Pharmaceutical Uses of Drum Dryer or Roll Dryer:

·         Drum dryer takes viscous liquids, slurries, suspensions, and pastes as input material to be dried and produces the output as powders or flakes.

·         Drum  dryers  find  application  majorly  in  Food  &  Dairy  industry  and  Chemical & Pharmaceutical Industries for drying various type of pasts and slurries.

Variants:

·         A vacuum drum dryer encloses both drum and feed line in a vacuum chamber to facilitate drying of heat sensitive materials.

·         In large scale, instead of one drum, two drums are set in parallel, rotating in opposite direction with a common feed inlet.

SPRAY DRYER

Principle

Spray drying is an industrial process for dehydration of a liquid feed containing dissolved and/or  dispersed  solids,  by transforming  that  liquid  into  a  spray  of  small  droplets  and exposing these droplets to a flow of hot air.


Construction

 

·         A spray dryer is composed of a feed pump, atomizer, air heating unit, air dispenser, drying chamber (diameter of the drying chamber ranges between 2.5 to 9.0 m and height is 25 m or more) and also systems for exhaust air cleansing and also powder recovery/separator. 

·         The spray disk atomizer is about 300 millimeters in diameter and rotates at a speed of 3,000 to 50,000 revolutions per minutes.

·         In the spray dryer the liquid to be dried is atomized into the good droplets that are tossed radially into a relocating stream of warm gas.

Working

·         A spray dryer takes a liquid stream and separates the solute or suspension as a solid and the solvent into a vapor. The solid is usually collected in a drum or cyclone. The liquid input stream is sprayed through a nozzle into a hot vapor stream and vaporized. Solids form as moisture quickly leaves the droplets.

·         The three stages that occur in a spray dryer before drying is accomplished include:

§  Atomization

§  Spray-air mixing and moisture evaporation.

§  Dry product separation from the exit air.

·         The nature of the final product obtained after drying in a spray dryer depends on;

§  The design and operation of the spray dryer.

§  The physicochemical properties of the feed.

Advantages       

·         Product quality and properties can be effectively controlled and maintained through the entire drying operation.

·         Thermolabile products/ pharmaceuticals can be dried at atmospheric pressure and low temperature.

·         Spray dryer permits high- tonnage production in continuous operation adaptable to conventional PLC control (Programmable Logic Controller) and it is relatively simple to operate.

·         Feedstock  in  solution,  slurry,  emulsion,  paste,  and  melt  form  can  be  dried  if pumpable.

·         Corrosion problem is minimal and the selection of materials of construction of spray dryer is simplified since the dried material comes in contact with the equipment surfaces in an anhydrous condition.

·         Spray dryer produces dry powder particles of controllable particle size, shape, form, moisture content, and other specific properties irrespective of dryer capacity and heat sensitivity.

·         Spray  dryer  handles  a  wide  range  of  production  rates  and  provides  extensive flexibility in  its  design  that  is  product  specification  are  readily met  through  the selection of appropriate spray dryer design and its operation from a wide range of available design.

·         It is energy-intensive equipment because specific heat of evaporation can be supplied in a short time. The temperature difference across the drying chamber is relatively small and an appreciable amount of heat is lost with exhaust air.

Disadvantages

·         Spray dryer is bulky and also expensive to install.

·         It is difficult to clean after use.

·         It has a low thermal efficiency that is a lot of heat is wasted during operation.

·         Solid materials cannot be dried using spray dryers.

·         Product degradation or fire hazard may result from product deposit on the drying chamber.

Uses

·         Spray  dryer  is  used  in  drying  pharmaceuticals  like  penicillin,  blood  products, enzymes, vaccines, etc.

·         It is used in the production of excipients and co-processed excipients with increased flowability, compatibility, and tablet disintegration.

·         To improve drug compressibility and reduce capping tendencies in crystals.

·         It  is  equally  used  in  the  preparation  of  matrix  microcapsule  containing  drug substances and a biodegradable polymer in order to obtain controlled drug release formulation.

·         It is employed in enhancing solubility and dissolution rates of poorly soluble drugs by formation of pharmaceutical complexes or via the development of solid dispersion thus increasing bioavailability.

·         It  is  used  in  the  production  of  dry  powder  formulation/dry  powder  aerosol  and thermolabile materials.

·         Apart from its applications in the pharmaceutical industries, spray dryers also find use in; Chemical industries, Ceramic industries, Food industries, etc.

·         Biochemical industries e.g. algae, fodder antibiotics, yeast extracts, enzymes, etc.

·         Environmental pollution control e.g. flue gas desulfurization, black liquor from paper- making etc.

Variants

·         Spray dryer can be constructed in such a way as to suit sterile products.

·         It can be operated under closed conditions to recover solvents.

·         It can be operated under oxygen free environment.

·         The same equipment can be used for spray congealing.

·         It is useful for encapsulation (coating) of solid and liquid particles.

FLUIDIZED BED DRYER

Principle 

·         The equipment works on a principle of fluidization of the feed materials.

·         In fluidization process, hot air is introduced at high pressure through a perforated bed of moist solid particulate.

·         The wet solids are lifted from the bottom and suspended in a stream of air (fluidized state).

Construction

·        
The dryer is made up of stainless steel or plastic.

·         A detachable bowel is placed at the bottom of the dryer, which is used for charging and discharging.

·         The bowel has a perforated bottom with a wire mesh support for placing materials to be dried.

·         A fan is mounted in the upper part for circulating hot air.

·         Fresh air inlet, prefilter and heat exchanger are connected serially to heat the air to the required temperature.

·         The temperature of hot air and exit air are monitored.

·         Bag filters are placed above the drying bowl for the recovery of fines.

Working

·         The wet granules to be dried are placed in a detachable bowl. The bowl is inserted in the drier.

·         Fresh air can pass through a pre-filter, which is then heated when passing through a heat exchanger.

·         Hot air flows through the bottom of the bowl.

·         At the same time, fan start to rotate.

·         The air speed increases gradually.

·         When the velocity of air is greater than the sedimentation rate of the granules, the granules remain suspended in the gas stream.

·         After specific time, a pressure point is reached in which the friction drag on a particle is equal to the force of gravity.

·         The granules rise in the container due to high gas velocity of 1.5 to 7.5 meter per minute and then fall back. This state is known as fluidized state.

·         The gas surround to each granule do dry them completely.

·         The air comes out of the dryer passing through the filters in the bag.

·         The entrained particles remain adhered to the interior surface of bags.

·         Periodically bags are shaken to remove entrained particles.

Advantages

·         It takes less time to complete drying as compared to other dryer.

·         Drying is achieved at constant rate.

·         Handling time is also short.

·         It is available at different sizes with different drying capacity.

·         The equipment is simple and less labor cost required.

·         More thermal efficiency.

·         Drying capacity is more than other dryer.

·         It facilitates the drying of thermo labile substances since the contact time of drying is short.

·         It is batch type or continuous type process.

Disadvantages

·         Many organic powders develop electrostatic charge during drying.  To avoid this efficient electrical grounding of the dryer is essential.

·         Chances of attrition of some materials resulting in production of fines.

Pharmaceutical

·         It is used for drying of granules in the production of tablets.

·         It is used for coating of granules.

·         It can be used for three operations such as mixing, granulation and drying.

Variants:

·         Plug flow dryer: It is a rectangular fluid bed dryer having different compartments for fluidisation.

·         The material is made to move from inlet through different compartments to outlet.

·         Different drying condition can be maintained in the compartments.

·         Often the last compartment is fluidised with cold gas to cool the solids before discharge.

VACUUM DRYER

Principle

Vacuum drying is generally used for the drying of substances which are hygroscopic and heat sensitive, and is based on the principle of creating a vacuum to decrease the chamber pressure below the vapour pressure of the water, causing it to boil. Hence, water evaporates faster. The heat transfer becomes, i.e., rate of drying enhances substantially.


Construction

·         The oven is divided into hollow trays which increases the surface area for heat conduction. The oven door is locked air tight and is connected to vacuum pump to reduce the pressure. The materials to be dried are kept on the trays inside the vacuum dryer and pressure is reduced by means of vacuum pump.

·         The enclosed space (approximately 1.5 meter cube) is divided in to a number of portions by means of 20 hollow shelves, which are part of the jacket. These shelves provide larger surface area (about 45 to 50 meter square) for conduction of heat. Over the shelves, metal trays are placed for keeping the material. The oven door can be locked tightly to give an air tight seal. The oven is connected to a vacuum pump by placing condenser in between.

Working

·         The tray that are present in the dryer are used to dry the material that are placed in the shelves and the pressure is reduced to 30 to 60 Kps by vacuum pump.

·         The door closes firmly and steam passes through the jacket space and the shelves.

·         So the heat transfer is carried out by the conduction mechanism.

·         When evaporating under vacuum, the water is evaporated from the material at 25  300C.

·         The vapour goes to the condenser.

·         After drying vacuum line is disconnected.

·         Then the materials are collected from the tray.

Advantages

·         Material handling is easy.

·         Hollow shelves which are electrically heated can be used.

·         It provides large surface area. So the heat can be easily transfer through the body of the dryer and last drying action takes place.

·         Hot water can be supplied through the dryer, which help in drying process at the desired temperature.

Disadvantages

·         Dryer is a batch type process.

·         It has low efficiency.

·         It is more expensive.

·         Labour cost is too high.

·         Needs high maintenance.

·         There is a danger of overheating due to vacuum.

Uses

·         Vacuum dryer can be used for drying of following:

·         Heat sensitive materials, which undergo decomposition.

·         Dusty and hygroscopic material.

·         Drugs containing toxic solvents. These can be separated in to closed containers.

·         Feed containing valuable solvents. These are recovered by condensation.

·         Drugs which are required as porous end products.

·         Friable dry extracts.

FREEZE DRYER

Principle

Freeze drying or lyophilisation is a drying process used to convert solutions or suspensions of labile materials into solids of sufficient stability for distribution and storage. The fundamental principle in freeze-drying is sublimation, the shift from a solid directly into a gas. Just like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it. Drying is achieved by subjecting the materials to temperature and pressures below the triple point.


Construction

·         It consists of drying chamber in which trays are loaded.

·         Heat supply in radiation source, heating coils.

·         Vapour condensing or adsorption system.

·         Vacuum pump or stream ejector or both.

·         The chamber for vacuum drying is generally designed for batch operation.

·         It consists of shelves for keeping the material.

·         The distance between subliming and condenser must be less than the mean path of molecules. This increases the rate of drying.

·         The condenser consists of relatively large surface cooled by solid carbon dioxide slurred with acetone or ethanol.

·         The temperature of condenser must be much lower than evaporated surface of frozen substance.

·         In order to maintain this condition, the condenser surface is cleaned repeatedly.

Working

The following steps are involved in the working of freeze dryer

1.       Preparation and pretreatment

2.       Prefreezing for solidifying water

3.       Primary Drying

4.       Secondary Drying

5.       Packing

·         Primary drying is a top-down process with a well-defined sublimation front moving through the product as it dries. Above the ice surface interface is dried product, or “cake; below the interface is product with ice crystals still remaining to be sublimed.

·         After primary freeze-drying is complete and all ice has sublimed, in the area where ice has been removed, desorption of water from the cake occurs. This process is called secondary drying and has started in the primary drying phase.

·         The product appears to be dry but the residual moisture content may be as high as 7–8%.

·         Packing is done by replacing vacuum with inert gas, bottles and vials are closed.

Advantages

·         It is suitable for drying heat sensitive products

·         Freeze dried products is porous and easy to dehydrated and instantly dissolved.

·         Drying takes place at very low temperature, so that enzyme action is inhibited and chemical decomposition, particularly hydrolysis, is minimized.

·         Denaturation of protein does not occur.

·         Loss of volatile material is less.

·         Sterility can be maintained.

Disadvantages

·         The process is very slow.

·         Expensive process.

·         It is not a general method of drying, but it is limited to certain type of valuable products that cannot be dried by any other means.

·         The period of drying is high.

·         The product is prone to oxidation, due to the high porosity and large surface area.

·         Therefore, the product must be vacuum packed or with an inert gas or in container.

Uses

·         It is used in production of injection, solutions, and suspension.

·         It is also used for production of blood plasma and its fractionated products, bacterial and viral cultures, antibiotics and plant extracts, steroids, vitamins and enzymes.

·         Food product like mushroom, meat products can be dried by this method.

·         Coffee and tea concentrates and citrus fruit juices are also dried by this method.

·         Pharmaceutical  companies  often  use  freeze-drying  to  increase  the  shelf  life  of products, such as vaccines and other injectable.

·         By removing the water from the material and sealing the material in a vial, the material can be easily stored, shipped and later reconstituted to its original form for injection.

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  1. Best notes about pharmaceutical dryer

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