drying

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DRYING DEEPIKA BAIRAGEE ASSISTANT PROFESSOR M PHARMA (QUALITY ASSURANCE) Dr. APJ ABDUL KALAM UNIVERSITY, INDORE

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DRYING It is defined as “the removal of small amount of water or other liquid from a martial 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 vaour . Hence humidity is an important determinant for drying of solids.

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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 hours than in drying. METHOD OF DRYING Thermal drying-application of heat Non thermal drying (freeze drying)

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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 Absorption: The absorption of moisture from gases by passage through a sulphuric 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.

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APPLICATION OF DRYING Preparation of bulk-drugs: Ex. Dried aluminium hydroxide, spray dried lactose, powder extracts Preservation of drug product: Drying is necessary to avoid deterioration Ex. 1. Crude drugs of animal and vegetable origin, synthetic and semi synthetic drugs, aspirin and penicillin’s tablets are undergo chemical decomposition process. 2. Blood products, skin and tissue undergo microbial decomposition. Improved characteristics: Drying produces material of spherical shape, uniform size, free flowing and enhanced solubility. Ex. Granules are dried to improve the fluidity and compression characteristics.

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Improved handling: Removal of moisture makes the material light in weight and reduce the bulk thus the cost of transportation will be less and 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 improve stability.

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THEORY OF DRYING In a wet solid mass, moisture is present in two forms. Bond moisture It is the minimum water held by the material that exerts an equilibrium vapour pressure less than the pre water at the same temperature. Substances containing bond water are often called Hygroscopic substances. b. Unbound moisture It is the amount of water held by the material that exerts an equilibrium vapour pressure equal to that of pure water at the same temperature.

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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: %Moisture content = weight of the dry sample/ weight of water in sample X 100

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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 and flows to exterior surface. Mass transfer It involves movement of the moisture to the surface of the solid and its subsequent evaporation from the surface. The transfer of vapours from the surface to the surrounding is affected by external conditions like temperature, humidity, air flow rate, pressure and evaporating surface exposed.

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EQUILIBIRUM MOISTURE CONTENT (EMC ) It is the amount of water present in the solid which exerts a vapour pressure of the atmosphere surrounding it. EMC in a wet mass is shown below. Depending upon temperature and humidity conditions, solids may absorbs or lose moisture.

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DESORPTION When air is continuously passed over the solid containing moisture more than EMC, then solid losses water continuously till EMC is reached. This phenomenon is known as desorption.

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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.

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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 and initial weight gives weights gives the moisture content.

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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 and 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.

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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 content-EMC The moisture present in the solid can be expressed on a wet weight or dry weight basis % loss on dryig (LOD) = mass of water in sample (kg)/total mass of wet sample (kg) X 100 % moisture content (MC) = mass of water in sample (kg)/ mass of the dry sample (kg) X 100

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BEHAVIOR OF SOLIDS DURING DRYING OR RAE OF DRYING 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 s function of time. Drying rate = weight of water in sample/ time X weight of dry solid

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DRYING RATE CURVE If is obtained by plotting a graph of FMC on x-axis and drying rate on y-axis. Depending on external conditions and internal mechanism of fluid flow, solids show different drying patterns. A typical drying cycle of a solid can be divided into three distinct zones. Initial adjustment period Constant rate period Falling rate period

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INITIAL ADJUSTMENT PEROID It is time corresponding to AB curve, which is also called as ‘Heating up period’. During this period, solids absorbs heat and temperature is increase i.e.. An wetted substance when kept for drying it absorbs heat from surrounding and vaporisation of moisture takes place which cools the surface. Heat flows to the cooled surface at higher rates, leads to rise in temperature and evaporation again. This continues and after some item heating and cooling rate becomes equal. This temperature is equal to wet bulb temperature of drying air and is referred by the point B on graph.

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CONSTANT RATE PERIOD It is time corresponding to BC curve in graph. The temperature remains constant and 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 As drying proceeds, the coarse capillaries are completely depleted of water and 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 and dring rate start falling and point at which decrease in drying rate start is referred to as the critical moisture content.

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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 and rate of drying begins to fall off. The point D is referred to as second critical point. 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 and no film is present on surface. At the end, the drying rate becomes zero and moisture content of solids at this point referred to as equilibrium moisture content.

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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. ‘

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Types 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 and conveyed at a high velocity gas stream. Spray dryer

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TRAY DRYER PRINCIPLE In the tray dryer, hot air is continuously circulated. Forced convection takes place to remove moisture form 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 who's walls are insulated. Trays are placed inside the heating chamber. Laboratory dryers contains 3 trays minimum and in industry 20 tray.

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Each tray is rectangular or square and about 1.2 to 2.4 meters square in area. Trays are usually loaded from 10 to 100 mm deep. Distance between bottom of upper tray and surface of the substance loaded in the subsequent tray must be 40mm. Dryer is fitted with a fan for circulating air over the trace in the corner of chamber, direction vanes are placed to direct air in the expected path.

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WORKING Wet solids are loaded into trays. Fresh air is introduced through inlet, which passes through the heater and gets heated up. Hot air is circulated by means of fans at 2 to 5 m/s. turbulent flow lowers the partial vapour pressure in the atmosphere and also reduces the thickness of air boundary layer. As water evaporates from the surface, the water diffuses from the interior of solid by capillary action. This occurs in single pass of air. The time of contact is short and amount of water is picked up in a single pass is small. Therefor this charged air to the tune of 80-90% is circulated back through fans. Only 10-20% of fresh air is introduced.

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ADVANTAGES Handling of materials can be done without losses It can be operated batch wise, for following reasons: Each batch can be handled as separate entity Equipment is readily adjusted for use in drying of variety of materials Valuable products can be handled efficiently DISADVANTAGE It requires more labour to load and unload, time consuming. USES Sticky materials Plastic substances Precipitates Chemicals Tablet granules and powders

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DRUM DRYER 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 and 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 m diameter and 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. On one side of the drum a spreader is placed and 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.

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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 b 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.

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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 and mass transfer are high Product obtained is completely dried and 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

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UESE It is used for drying solutions, slurries, suspensions etc. and also drying of products like Milk product Starch product Ferrous salt Suspensions of zinc oxide and kaolin Antibiotics Yeast and pigments Malt and glandular extracts insecticides Calcium and barium carbonates.

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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 and 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 large cylindrical drying chamber with a short conical bottom, made up of stainless steel. Diameter of 2.5-9 meters and height 25 meters or more. An inlet for hot air is placed in the roof of the chamber and another inlet carrying spray disk atomizer is set in the roof. Spray disk atomizer is about 300mm in diameter and rotates at a speed of 3000 to 50000 rpm. Bottom of the dryer is connected to a cyclone separator.

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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 products depends upon the droplet form atomizer of any type: Pneumatic atomizer Pressure nozzle type Spinning disc atomizer Rate of feed adjusted in a such a way that droplets should be completely dried before reaching walls of drying chamber

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b. Drying of liquid droplet 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 well. The shells thickness decrease where as permeability for vapour increases. If the shells is neither elastic not permeable it rupture and internal pressure escape.

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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 and finally fall at the conical bottom. All these processes are completed in a few second. Particle size of the final product ranges from 2 to 500mm. Particle size depends upon solid content of the feed, liquid viscosity, feed rate and disc speed Capacity of spray dryer-2000kg/hr

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ADVANTAGES It is continuous process and drying completes within 3 to 300 sec. labour cost is low Product of uniform and controllable size can be obtained Fine droplets form provides large surface area for head and mass transfer. Product shows excellent solubility Either solutions or suspension or thin plate can be dried in one step to get final product ready for package. Drying of sterile product and reconstituted product Globules of an emulsion can be dried with the dispersed phase inside and layer of continuous phase outside. DISADVANTAGES It is very bulky and expensive Such huge equipment is not always easy to operate The thermal efficiency is low, as much heat is lost in the discharged gases.

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USES 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

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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 vaccine penicillin Calcium sulphate Acacia Borax Fruit juices Sulphur Blood Pancreatin Hormones chloramphenicol Sodium phosphate Extracts Milk Lactose Plasma Vitamins Serum starch

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FLUIDIZED 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 and suspended in the stream of air. This condition is called Fluidized state. The hot gas surrounding every granule to completely dry them. Thus, material or granules are uniformly dried.

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CONSTRUCTION Two types of bed dryers are available Vertical fluid bed dryer 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 and discharging. 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 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.

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WORKING The wet granules to be dried are placed in the detachable bowl and 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 particles is equal to the force of gravity. The granules rise in the container because of high velocity gas and later fall back in a random boiling motion. This condition said to be Fluidized State. The gas surrounds every granules 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.

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ADVANTAGES It requires less time to complete drying i.e. 20 to 40min. Compared to 24hr of tray dryer. Different sizes with different drying capacity from 5 to 200 kg/hr Handling simple and low labour cost. Thermal efficiency is 2 to 6 times than tray dryer. Mixing efficiency is also high as compared to other dryers. Hot spots are not observed in the dryer. It facilitates the drying of thermolaible substances, as contact time is short. It can be used for batch type or continuous type. It has high output from a small floor space. The free movement of individual particles eliminates the risk of soluble material migrating as may occurs in static bed.

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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. Fine particles may become entrained and 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 and drying. It is modified for coating of granules.

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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 and 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. 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.

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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 and 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 vapor passes into the condenser where condensation takes place.

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ADVANTAGES Large surface area for heat transfer. Handling of material, trays and 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. DISADVANTAGES Heat transfer coefficient are low Limited capacity and used for batch process. More expensive than tray dryer labour and running cost is also high. There is danger of over heating as the material is in contact with steam heated surface for longer period.

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USES Heat sensitive materials, which undergo decomposition. Dusty and hygroscopic material. Drugs containing toxic solvents, these can be separated into closed containers. Feed containing valuable solvents. These are recovered by condensation. Drugs which required as porous end products. Friable dry extracts.

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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- lidquid-vapour equilibrium phase diagram of water is useful to decide the experimental conditions. The drying is achieved by subjecting material to temperature and pressure below the triple point. Under this conditions, any heat transferred is used as latent heat and 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..

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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.

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WORKING The working of freeze dryer consist of following steps Preparation and 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. 2. Prefreezing to solidify water Vials, ampoules or bottles in which the aqueous solution is packed are frozen in cold shelves. 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.

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3. Primary Drying It means sublimation of ice under vacuum. The temperature and pressure should be below the triple point of water i.e. 0.0098˚C and 4.58mmHg 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 and temperature 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 -10˚C to 30 ˚C. The conditions of 1 to 8k below eutectic point is sufficient. Vacuum is applied to the tune of about 3mmHg on the frozen sample and the temperature is linearly increased about 30˚C in a span of 2hrs. Heat is supplied which transfer as latent heat and 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%.

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4. Secondary drying It is removable of residual moisture under high vacuum. The temperature of solid is raised to as high as 50 to 60 ˚C but vacuum is lowered below that is used in primary drying. The rate of drying is very low and it takes about 10 to 20hrs. 5. Packing After vacuum is replaced by inert gas, the bottles and vials are closed. USES It is used for drying of number of product Blood plasma and its fractionated product Bacterial and viral culture Antibiotics and plant extracts Steroids, vitamins and enzymes

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ADVANTAGES 1.Thermo labile substances can be dried. 2. Denaturation does not occur. 3. Migration of salts and other solutes does not take place. 4. Moisture level can be kept as low as possible. 5. Product is porous and uniform 6. Sterility can be maintained 7. Material can be dried in its final container such as single dose and multiple dose. DISADVANTAGES The product is prone to oxidation, due to high porosity and large surface area. Therefore, the product should be packed in vacuum or using inert gas. Equipment and 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.

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