Drying

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Phamaceutical Engineering - Dryng lecture note

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DRYING 1 Girish.B KLECOP Belgaum Mr.Girish.B M.Pharm girishanbhog@gmail.com

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CONTENT Introduction of Drying Applications of Drying Mechanism of Drying Theory of Drying Classification & Types of Dryers 2 Girish.B KLECOP Belgaum

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Drying It is defined as “ the removal of small amounts of water or other liquid from a material by the application of heat.” It also includes removal of volatile liquids or water from another liquid or gas or a Suspension. Drying is possible when the environment is unsaturated with the water vapour. Hence humidity is an important determinant for drying of solids. 3 Girish.B KLECOP Belgaum

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The product obtained from an evaporator is either concentrated solutions or suspensions or a wet slurry, whereas that from a dryer is substantially dry solid. The process of evaporation involves removal of much more quantity of liquid per hour than in drying. Method of drying Thermal drying – application of heat Non thermal drying (freeze drying) 4 Girish.B KLECOP Belgaum

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Non thermal drying Expression The expression of solid to remove liquid. e.g. Squeezing of wetted sponge Extraction The extraction of liquid from solid by use of solvent. Adsorption By use of desiccants such as anhydrous calcium chloride 5 Girish.B KLECOP Belgaum

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Absorption The absorption of moisture from gases by passage through a sulfuric acid column. Desiccation Desiccation of moisture from a solid by placing it in a sealed container with a moisture-removing material e.g. silica gel in bottle. 6 Girish.B KLECOP Belgaum

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Applications of Drying Preparation of bulk drugs Ex. Dried aluminium hydroxide Spray dried lactose Powdered extracts Preservation of drug product Drying is necessary to avoid deterioration. Ex.1. Crude drugs of animal & vegetable origin, synthetic & semi synthetic drugs, aspirin & penicillin's tablets are undergo chemical decomposition process. 2. Blood products, skin & tissue undergo microbial decomposition. 7 Girish.B KLECOP Belgaum

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Improved characteristics Drying produces material of spherical shape, uniform Size, free flowing & enhanced solubility. Ex. Granules are dried to improve the fluidity & Compression characteristics. Improved handling Removal of moisture makes the material light in weight & reduces the bulk thus the cost of transportation will be less & storage will be efficient. Stability enhancement By removal of moisture significantly reduces rate of chemical reactions, chances of microbial attack or enzymatic actions and thus improves stability. 8 Girish.B KLECOP Belgaum

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Theory of Drying In a wet solid mass, moisture is present in two forms, Bound moisture It is the minimum water (moisture) held by the material that exerts an equilibrium vapor pressure less than the pure water at the same temperature. Substances containing bound water are often called hygroscopic substances. b) Unbound moisture It is the amount of water held by the material that exerts an equilibrium vapor pressure equal to that of pure water at the same temperature. 9 Girish.B KLECOP Belgaum

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Unbound water exists mostly in the voids of the solids. Thus, in a non hygroscopic material, all the liquid is unbound water. In a hygroscopic material, the unbound moisture is the liquid in excess of the equilibrium moisture content. Measurement of the moisture in a wet solid is referred as moisture content , or MC: X 100 weight of water in sample weight of dry sample % MC = 10 Girish.B KLECOP Belgaum

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Hygroscopic materials Water in : Fine capillaries Cell & fibre wall Physical interaction Vapour pressure of wet solids is less than the vapour pressure of pure water Bound Water Contains Condition Vapour pressure of wet solids is equal to vapour pressure of water Non – hygroscopic materials Water in void spaces Unbound Water Contains Condition 11 Girish.B KLECOP Belgaum

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Mechanism of Drying Drying involves two process – Heat Transfer It takes place from the heating medium to the solid ; except in dielectric or high frequency electric drying, where heat is generated within the solid & flows to exterior surface. Mass Transfer It involves movement of the moisture to the surface of the solid & its subsequent evaporation from the surface. The transfer of vapors from the surface to the surrounding is affected by external conditions like temperature, humidity, air flow rate, pressure & evaporating surface exposed. 12 Girish.B KLECOP Belgaum

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Equilibrium Moisture Content (EMC) It is the amount of water present in the solid which exerts a vapor pressure equal to the vapor pressure of the atmosphere surrounding it. EMC in a wet mass is shown below. 13 Girish.B KLECOP Belgaum

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Depending upon temperature and humidity conditions, solids may absorbs or lose moisture. Desorption When air is continuously passed over the solid containing moisture more than EMC, then solid loses water continuously till EMC is reached. This phenomenon is known as desorption. 14 Girish.B KLECOP Belgaum

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Sorption When air is continuously passed over the solid containing moisture less than EMC, then solid absorbs water continuously till EMC is reached. This phenomenon is known as sorption. 15 Girish.B KLECOP Belgaum

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Measurement of EMC The solid samples are placed in a series of closed chambers such as desiccators. Each chamber consist of solutions which maintain a fixed relative humidity in the enclosed air spaces. Solid samples are exposed to several humidity conditions, the exposure is continued until the material attains a constant weight. The difference in the final & initial weights gives the moisture content. 16 Girish.B KLECOP Belgaum

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Factors Affecting EMC Nature of material Nonporous insoluble solids have an EMC zero EX. Talc For fibrous or colloidal organic substances, EMC values are high. For porous solids, EMC values are much higher & variable. Nature of Air For air of zero humidity, EMC of all materials is zero. As the temperature of air increases, the EMC of solid decreases. 17 Girish.B KLECOP Belgaum

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Free Moisture Content (FMC) FMC is the amount of water that is free to evaporate from the solid surface. Free moisture content (FMC) = total water content – EMC The moisture present in the solid can be expressed on a wet weight or dry weight basis. % loss on Drying (LOD) = Mass of water in sample (kg) Total mass of wet sample (kg) × 100 % moisture content (MC) = Mass of water in sample (kg) Mass of the dry sample (kg) × 100 18 Girish.B KLECOP Belgaum

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The rate of drying of a sample can be determined by suspending the weight material on a balance in a drying cabinet and measuring the weight of sample as it dries as a function of time. Behavior Of Solids During Drying or Rate Of Drying Drying Rate = Weight of water in sample(kg) Time(h) × weight of dry solid (kg) 19 Girish.B KLECOP Belgaum

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Drying rate curve It is obtained by plotting a graph of FMC on X- axis & drying rate On Y- axis. Depending on external conditions & internal mechanism of fluid flow, solids show different drying patterns. 20 Girish.B KLECOP Belgaum

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A typical drying cycle of a solid can be divided into three distinct zones, 1. Initial Adjustment Period 2. Constant Rate Period 3. Falling Rate period 21 Girish.B KLECOP Belgaum

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Drying Rate Curve 22 Girish.B KLECOP Belgaum

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1. Initial Adjustment Period It is time corresponding to AB curve, which is also called as ‘Heating Up Period’ . During this period, solids absorbs heat & temperature is increase i.e. An wetted substance when kept for drying it absorbs heat from surrounding & vaporisation of moisture takes place which cools the surface. Heat flows to the cooled surface at higher rates, leads to rise in temperature & evaporation again. 23 Girish.B KLECOP Belgaum

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This continues & after some time heating & cooling rate becomes equal. This temperature is equal to wet bulb temp. of drying air & is referred by the point B on graph. 24 Girish.B KLECOP Belgaum

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2. Constant rate period It is time corresponding to BC curve in graph. The temperature remains constant & rate of drying is constant. During this period, there is a continuous liquid film over the surface of solid. Moisture evaporating from the surface is replace by the water diffusing from the interior of the solid. Also the drying rate remains constant as show in curve by region BC. Rate of diffusion = Rate of evaporation 25 Girish.B KLECOP Belgaum

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As drying proceeds, the coarse capillaries are completely depleted of water & solid fails to maintain uniform film. The area over which moisture film is not present is known as ‘ dry spot ’. Such dry spot start appearing & drying rate start falling & point (point C) at which decrease in drying rate start is referred to as the critical moisture content (CMC) 26 Girish.B KLECOP Belgaum

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3. First falling Rate period Also known as Period of Unsaturated surface drying. It is time corresponding to CD curve. During this period, surface water is no longer replaced at a rate fast enough to maintain a continuous film on the surface. Dry spots begin to appear & rate of drying begins to fall off. The point D is referred to as second critical point. 27 Girish.B KLECOP Belgaum

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Second falling rate period It is time corresponding to DE in graph. During this Period, rate of drying falls even more rapidly than the first falling rate & no film is present on surface. At the end, the drying rate becomes zero & moisture content of solids at this point (Point E) referred to as Equilibrium Moisture Content(EMC) . 28 Girish.B KLECOP Belgaum

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EMC is defined as, the mass of water per unit mass of dry solid when drying limit has been attained by use of air at any given temperature and humidity. 29 Girish.B KLECOP Belgaum

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Type of dryer Mechanism Examples Static bed dryer Systems in which there is no relative movement among the solid particles being dried, although there may be bulk motion of the entire drying mass. Tray dryer Freeze dryer Moving bed dryer System in which the drying particles are partially separated so that they flow each other. Drum dryer Fluidized bed dryer Systems in which the solid particles are partially suspended in an upward moving heated gas system. Fluidized bed dryer Pneumatic dryer System in which drying particles are entrained & conveyed at a high velocity gas stream. Spray dryer CLASSIFICATION OF DRYERS 30 Girish.B KLECOP Belgaum

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Tray drier Principle In the tray dryer, hot air is continuously circulated. Forced convection takes place to remove moisture from the solids placed in trays. Simultaneously, the moist air is removed partially. This is also called as shelf, cabinet or compartment dryer. Construction It consist of rectangular chamber whose walls are insulated. Trays are placed inside the heating chamber. 31 Girish.B KLECOP Belgaum

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Laboratory dryers contains 3 trays minimum & in industry 20 tray. Each tray is rectangular or square & about 1.2 to 2.4 meters square in area. Trays are usually loaded from10 to 100 mm deep. Distance between bottom of upper tray & surface of the substance loaded in the subsequent tray must be 40mm. 32 Girish.B KLECOP Belgaum

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Dryer is fitted with a fan for circulating air over the trays in the corner of chamber, direction vanes are placed to direct air in the expected path. Fig :- Tray Dryer 33 Girish.B KLECOP Belgaum

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Working :- Wet solids is loaded into trays. Fresh air is introduced through inlet, which passes through the heaters & gets heated up. 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 & also reduces the thickness of air boundary layer. 35 Girish.B KLECOP Belgaum

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As water evaporates from the surface, the water diffuses from the interior of solid by capillary action. These occur in single pass of air. The time of contact is short & amount of water is picked up in a single pass is small. Therefore discharged air to the tune of 80 – 90% is circulated back through fans. only 10-20% of fresh air is introduced. 36 Girish.B KLECOP Belgaum

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Advantages Handling of materials can be done without losses It can be operated batch wise, for following reasons: a) each batch can be handled as separate entity. b) equipment is readily adjusted for use in drying of variety of materials. c) valuable products can be handled efficiently. Disadvantage It requires more labour to load & unload, Time consuming 37 Girish.B KLECOP Belgaum

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Uses Following substances can be dried, Sticky materials Plastic substances Precipitates Chemicals Tablet granules & powders 38 Girish.B KLECOP Belgaum

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Drum drier Principle A heated hallow metal drum rotates on its longitudinal axis, which is partially dipped in the solution to be dried. The solution is carried as a film on the surface of the dryer & Dried to form a layer. A suitable knife scraps the dried material, while drum is rotating Construction It consist of a horizontally mounted hollow steel drum of 0.6 – 3.0 meters diameter & 0.6 – 4 meters in length, whose external surface is polished. Below the drum, feed pan is placed in a such way that the drum dips partially into the feed. 39 Girish.B KLECOP Belgaum

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On one side of the drum a spreader is placed & on other side a doctor’s knife is placed to scrap the dried material. A storage bin is placed connecting the knife to collect the material. Fig: Drum Dryer 40 Girish.B KLECOP Belgaum

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Working Steam is passed inside the drum. Drying capacity is directly proportional to the surface area of the drum. Heat is transferred by conduction to the material. Drum is rotated at a rate of 1- 10 revolutions per minute. the liquid material present in the feed pan adheres as a thin layer to the external surface of drum during its rotation. Material is completely dried during its journey in slightly less than one rotation. the dried material is scrapped by knife, which then falls into a storage bin. The time of contact of the material with the hot metal is 6-15 seconds only 43 Girish.B KLECOP Belgaum

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Advantages Drying time is less only few seconds. Hence , heat sensitive material can be dried. Less space as compared to spray dryer. Rates of heat & mass transfer are high. Product obtained is completely dried & is in the final form Disadvantages Maintenance cost is higher than spray dryer. Skilled operators required . It is not suitable for solutions of salts with less solubility. 44 Girish.B KLECOP Belgaum

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Uses It is used for drying solutions, slurries, suspensions etc. & also drying of products like, Milk product Starch product Ferrous salt Suspensions of zinc oxide & kaolin Antibiotics Yeast & pigments Malt & glandular extracts Insecticides. Calcium & barium carbonates. 45 Girish.B KLECOP Belgaum

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Spray Dryer Principle The fluid to be dried is atomized into fine droplets, which are thrown radially into a moving stream of hot gas. Temperature of droplets is immediately increased & fine droplets get dried in the form of spherical particles. This process completes in few seconds before the droplets reach the wall of dryer. Construction It consist of a large cylindrical drying chamber with a short conical bottom, made up of stainless steel. Diameter of 2.5- 9.0 meters & height 25.0 meters or more. 46 Girish.B KLECOP Belgaum

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An inlet for hot air is placed in the roof of the chamber & another inlet carrying spray disk atomizer is set in the roof. spray disk atomizer is about 300 mm in dia. & rotates at a speed of 3000 to 50,000 rpm. Bottom of the dryer is connected to a cyclone separator. Fig :- Spray dryer 47 Girish.B KLECOP Belgaum

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Working Drying of material in spray dryer involves three stages. Atomization of liquid The feed is introduced through the atomizer either by gravity or by using suitable pump to form fine droplets. The properties of final product depends upon the droplet form. Atomizer of any type: 1. pneumatic atomizer 2. pressure nozzle type 3. spinning disc atomizer Rate of feed adjusted in a such a way that droplets should be completely dried before reaching walls of drying chamber. 50 Girish.B KLECOP Belgaum

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b) Drying of liquid droplets Surface of a liquid drop is dried immediately to form a tough shell. Liquid inside must escape by diffusing through the shell at a particular rate. Heat transfer from outside to inside takes place at a rate greater than liquid diffusion rate. As a result, heat inside mounts up which allow the liquid to evaporate. This leads to increase in internal pressure, which causes droplets to swell. The shells thickness decreases where as permeability for vapour increases. If the shell is neither elastic nor permeable it ruptures & internal pressure escape. 51 Girish.B KLECOP Belgaum

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c) Recovery of dried product Centrifugal force of atomizer drives the droplet to follow helical path. Particles are dried during their journey & finally fall at the conical bottom. All these processes are completed in a few seconds. Particle size of the final product ranges from 2 to 500 mm. Particle size depends upon solid content of the feed, liquid viscosity, feed rate & disc speed. Capacity of spray dryer – 2000 kg/hr 52 Girish.B KLECOP Belgaum

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Advantages It is continuous process & drying completes within 3 to 300 sec. Labour cost is low. Product of uniform & controllable size can be obtained. Fine droplets form provide large surface area for heat & mass transfer. Product shows excellent solubility. 5. Either solution or suspensions or thin paste can be dried in one step to get final product ready for package. 53 Girish.B KLECOP Belgaum

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6. Drying of sterile product & Reconstituted product. 7. Globules of an emulsion can be dried with the dispersed phase inside & layer of continuous phase outside. Disadvantages It is very bulky & expensive. Such huge equipment is not always easy to operate. The thermal efficiency is low, as much heat is lost in the discharged gases. 54 Girish.B KLECOP Belgaum

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Uses Spray dryers are used compulsorily, if : The product is a better form than that obtained by any other dryer. The quantity to be dried is large. The product is thermolabile, hygroscopic or undergo chemical decomposition. 55 Girish.B KLECOP Belgaum

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A few product that are dried using spray dryer are: Citric acid Gelatin Barium sulphate Detergents Methyl cellulose Ferrous sulphate Pepsin Vaccines Penicillin Calcium sulphate Acacia Borax Fruit juices Sulphur Blood Pancreatin Hormones Chloramphenicol Sodium phosphate Extracts Milk Lactose Plasma Vitamins Serum Starch 56 Girish.B KLECOP Belgaum

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Fluidised Bed Dryer Principle Hot air is passed at high pressure through a perforated bottom of container containing granules to be dried. The granules are lifted from the bottom & suspended in the stream of air. This condition is called Fluidized state. The hot gas Surrounding every granule to completely dry them. Thus, materials or granules are uniformly dried. 57 Girish.B KLECOP Belgaum

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Construction Two types of bed dryers are available 1. Vertical fluid bed dryer 2. Horizontal fluid bed dryer The dryer is made up of stainless steel or plastic. A detachable bowl is placed at the bottom of the dryer, which is used for charging & discharging. 58 Girish.B KLECOP Belgaum

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The bowl has perforated bottom with a wire mesh support for placing materials to be dried. A fan is mounted in the upper part of circulating hot air. Fresh air inlet, prefilter & heat exchanger are connected serially to heat the air to the required temperature. The temp. of hot air & exit air are monitored. Bag filters are placed above the drying bowl for the recovery of fines. 59 Girish.B KLECOP Belgaum

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Fig : Fluidized bed Dryer 60 Girish.B KLECOP Belgaum

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Working The wet granules to be dried are placed in the detachable bowl & it is pushed into dryer. Fresh air is allowed to pass through a prefilter, which subsequently gets heated by passing through a heat exchanger. The hot air flows through the bottom of the bowl. The air velocity is gradually increased. When the velocity of the air is greater than settling velocity of granules,the granules remain partially suspended in the gas stream. A point of pressure is reached at which frictional drag on the particle is equal to the force of gravity. 62 Girish.B KLECOP Belgaum

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The granules rise in the container because of high velocity gas & later fall back in a random boiling motion. This condition said to be Fluidised State. The gas surrounds every granule to completely dry them. The air leaves the dryer by passing through bag filters. The entrained particles remain adhered to the inside of the bags. Periodically the bags are shaken to remove the entrained particles. 63 Girish.B KLECOP Belgaum

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Advantages It requires less time to complete drying i.e. 20 to 40 min. compared to 24hr of tray dryer. Different sizes with different drying capacity from 5 to 200 kg/hr. Handling simple & low labour cost. Thermal efficiency is 2 to 6 times than tray dryer. Mixing efficiency is also high as compared to other dryers. 64 Girish.B KLECOP Belgaum

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6. Hot spots are not observed in the dryer. 7. It facilitates the drying of thermolabile substances, as contact time is short. 8. It can be used for batch type or continuous type. 9. It has high output from a small floor space. 10. The free movement of individual particles eliminates the risk of soluble material migrating as may occur in static bed. 65 Girish.B KLECOP Belgaum

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Disadvantages Many organic powders develop electrostatic charge during drying. To avoid this, efficient earthing of dryer is essential. The turbulence of the fluidized state of granules may cause attrition of some materials resulting in the production of fines. but using a suitable binding agent this problem can be solved. 3. Fine particles may become entrained & must be collected by bag filters. Uses It is popularly used for drying of granules in the production of tablets. It can be used in three operations like mixing, granulation & drying. It is modified for coating of granules. 66 Girish.B KLECOP Belgaum

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Vacuum dryer Principle In vacuum dryer, material is dried by the application of vacuum. When vacuum is created, the pressure is lowered so that water boils at a lower temperature. Hence, water evaporates faster. The heat transfer becomes efficient i.e. rate of drying enhances substantially. Construction It is made up of a cast iron heavy jacketed vessel. It is so strong that it can withstand high vacuum within the oven & steam pressure in the jacket. The enclosed space is divided into a number of portions by means of 20 hollow shelves, which are part of jacket. These shelves provide large area for conduction of heat. 67 Girish.B KLECOP Belgaum

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Over the shelves, metal trays are placed for keeping the material. The oven door can be locked tightly to give an air tight seal. oven is connected to a vacuum pump by placing condenser in between. Fig : Vacuum Dryer 68 Girish.B KLECOP Belgaum

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Working Material to be dried is spread on trays. Trays are placed on the shelves. Pressure is decreased up to 30 to 60 kilopascals by means of vacuum pump. door is closed firmly. Steam or hot air is supplied into the hollow space of jacket & shelves. heat transfer by conduction takes place. At this vacuum, evaporation of water from the material takes place at 25-30°C, on account of lowering of boiling point. Water vapour passes into the condenser where condensation takes place. 70 Girish.B KLECOP Belgaum

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Advantages Large surface area for heat transfer. Handling of material, trays & equipment is easy. Easy to switching over to next material. Hot water of desired temperature can be supplied. Electrically heated hollow shelves can be used. 71 Girish.B KLECOP Belgaum

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Disadvantages Heat transfer coefficient are low. Limited capacity & used for batch process. More expensive than tray dryer. labour & running cost is also high. There is danger of over heating as the material is in contact with steam heated surface for longer period. 72 Girish.B KLECOP Belgaum

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Uses 1. Heat sensitive materials, which undergo decomposition. 2. Dusty & hygroscopic material. 3. Drugs containing toxic solvents. these can be separated into closed containers. 4. Feed containing valuable solvents. These are recovered by condensation. 5. Drugs which required as porous end products. 6. Friable dry extracts. 73 Girish.B KLECOP Belgaum

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Freeze Dryer It is also known as lyophilization i.e. system is made solvent loving for removing the same. 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 liquid phase. Solid-liquid- vapour equilibrium phase diagram of water is useful to decide the experimental conditions. The drying is achieved by subjecting material to temperature & pressure below the triple point. Under this conditions, any heat transferred is used as latent heat & ice sublimes directly into vapour state. The water vapour is removed from the system by condensation in a cold trap maintained at a temperature lower than frozen material. 74 Girish.B KLECOP Belgaum

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Construction Freeze dryer consist of , Drying chamber in which trays are locked. Heat supply in the form of radiation source, heating coils. Vapour condensing or adsorption system. Vacuum pump or steam ejector or both. Fig : Freeze Dryer 75 Girish.B KLECOP Belgaum

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Working The working of freeze dryer consist of following steps. Preparation & pretreatment The volume of solution introduced into the container is limited by its capacity. Therefore pretreatment is essential. The solutions are preconcentrated under the normal vacuum tray drying. This reduces the actual drying by 8 to 10 times. Prefreezing to solidify water Vials, ampoules or bottles in which the aqueous solution is packed are frozen in cold shelves (- 50ᵒC). The normal cooling rate is about 1 to 3 Kelvin/ minute so that large ice crystals with relatively large holes are formed on sublimation of ice. This is also responsible for giving a porous product. 78 Girish.B KLECOP Belgaum

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3. Primary Drying It means sublimation of ice under vacuum. The temp. & pressure should be below the triple point of water i.e. 0.0098ᵒC & 4.58 mmHg for sublimation, when water is alone present. When a Solution of a solid is dried, the depression of freezing point of water occurs. Hence, it is essential that the temperature be brought below the eutectic point. The pressure & temp. at which the frozen solid vaporizes without conversion to liquid is referred to as the eutectic point. Depending on the drug substances dissolved in water, the eutectic point is determined. The usual range is from -10ᵒC to -30ᵒC. The conditions of 1 to 8 K below eutectic point is sufficient. 79 Girish.B KLECOP Belgaum

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Vacuum is applied to the tune of about 3 mmHg on the frozen sample and the temperature is linearly increased about 30 ᵒC in a span of 2 hrs. Heat (About 2900 kilojoules/ Kg) is supplied which transfer as latent heat & ice sublimes directly into vapour state. As the drying proceeds, thickness of dried solids increases. Primary drying stage removes easily removable water, about 98% to 99%. 80 Girish.B KLECOP Belgaum

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Secondary Drying It is removable of residual moisture under high Vacuum. The temp. of solid is raised to as high as 50 to 60ᵒC but vacuum is lowered below that is used in primary drying. The rate of drying is very low & it takes about 10 to 20 hrs. 5. Packing After vacuum is replaced by inert gas, the bottles & vials are closed. 81 Girish.B KLECOP Belgaum

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Uses It is used for drying of number of product, Blood plasma & its fractionated product. Bacterial & viral culture. Antibiotics & plant extracts. Steroids, vitamins & enzymes. 82 Girish.B KLECOP Belgaum

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Advantages Thermo labile substances can be dried. Denaturation does not occur. Migration of salts & other solutes does not take place. Moisture level can be kept as low as possible. Product is porous & uniform. Sterility can be maintained. Material can be dried in its final container such as single dose & multiple dose 83 Girish.B KLECOP Belgaum

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Disadvantages The product is prone to oxidation, due to high porosity & large surface area. Therefore, the product should be packed in vacuum or using inert gas. Equipment & running cost is very high. The period of drying is very high. Time can not be shortened. It is difficult to adopt the method for solutions containing non-aqueous solvents. 84 Girish.B KLECOP Belgaum

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