UNIT 3 DRYING.pptx

Drying
Introduction
Mechanism
Theory of drying
Equipment's

1. PHARMACEUTICAL ENGINEERING (BP304T) Unit- IiI Drying Name: Mrs. Pooja Eknathji Waware Assistant Professor G H RAISONI UNIVERSITY SCHOOL OF PHARMACY

2. CONTENTS: OBJECTIVES OF DRYING APPLICATIONS OF DRYING MECHANISM OF DRYING PROCESS RATE OF DRYING CURVE CLASSIFICATION OF DRYERS TRAY DRYER DRUM DRYER OR ROLL DRYER SPRAY DRYER FLUIDIZED BED DRYER (FBD) VACUUM DRYER FREEZE DRYER

3. Drying Drying is define as the removal of small amounts of water or other liquid from a material by application of heat. In general drying process involves two steps. 1) Heat transfer 2) Mass transfer Drying and Evaporation are relatively same term are distinguish in terms of removal of water. Hear are some differences between drying and evaporation

4. Difference between Drying and Evaporation Drying Evaporation In drying processes the main operation usually carried out on solid materials, e.g. powders or products In Evaporation processes the main operation usually carried out on liquid materials, e.g. solution or products Drying in the most of the cases means the removal of relatively small amounts of water from solids. Evaporation includes the removal of large amounts of water from solutions. Drying involves the removal of water at temperatures below its boiling point. Evaporation involves the removal of water by boiling solutions. In drying, water is usually removed by circulating air over the material in order to carry away the water vapour . While in evaporation, water is removed from the material as pure water vapour mixed with other gases.

5. OBJECTIVES OF DRYING The main O bjectives of drying include to preserve foods and increase their shelf life by reducing the water content and water activity; A void the need for use of refrigeration systems for transport and storage (expensive); R educe space requirements for storage and transport . In pharmaceutical technology, drying is carried out for one or more of the following 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. Flow ability, compressibility

6. APPLICATIONS OF DRYING Preservation of drug product : Drying is necessary in order to avoid deterioration. For examples protection of blood products, skin, tissues and crude drugs from microbial growth . Effervesant tablets, synthetic and semisynthetic drugs undergo chemical decomposition. Preparation of bulk drugs: In the preparation of bulk drugs, drying is the final stage of processing. examples – dried aluminium hydroxide , spray dried lactose and powdered extracts.

7. Improved characteristics: Drying produces materials of spherical shape, uniform size, free flowing and enhanced solubility . Granules are dried to improve the fluidity & compression characteristics these are essential for production of tablets and capsules.

8. Improved handling Removal of moisture makes the material light in weight and reduces bulk. T hus cost of transportation will be less & storage will be efficient. To make the materials easy or more stable for handling. Drying reduces moisture content

9. Theory of drying In a wet solid mass, water may be present as bound water and unbound water. Bound Water: bound water is the minimum water held by the material that exerts an equilibrium vapour pressure less than the pure water at the same temperature. Substances containing bound water are often called H ygroscopic substances. Unbound water : 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. Unbound water exist mostly in the voids of solids. Thus, in a non hygroscopic material, all the liquid is unbound water.

10. In 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 𝒘𝒂𝒕𝒆𝒓 𝒊𝒏 𝒔𝒂𝒎𝒑𝒍𝒆 𝑾𝒆𝒊𝒈𝒉𝒕 𝒐𝒇 𝒅𝒓𝒚 𝒔𝒂𝒎𝒑𝒍𝒆 × 𝟏𝟎𝟎

11. Hygroscopic materials Water in: - fine capillaries - Cell and fiber walls - Physical interaction Vapour pressure of wet solids is less than the v.p of pure water Non-Hygroscopic materials Water in void spaces Vapour pressure of wet solids is equal to v.p of pure water Bound water UnBound water

12. Mechanism of Drying Process Drying involve two steps- Heat Transfer It takes place from the heating medium to the solid material. Mass Transfer It involves the transfer of moisture to the surface of solids and subsequently vapourisation from the surface into surrounding.

13. Diffusion theory Diffusion theory the rate of flow of water is proportional to moisture gradient. According to this theory, moisture movement may be as follows: Water diffuses through the solid to the surface and subsequently evaporates in to surroundings. Evaporation of water occurs at an intermediate zone, much below the solid surface, then vapours diffuse through the solid into air.

14. Capillarity Theory Capillary theory is applicable to porous granular solids with network of inter-connected pores and channels. As the drying starts, a meniscus is formed in the capillary and exerts a force. This is the driving force is greater in small pores compared to the large pores. Therefore, small pores pull more water from the larger pores and thus large pores get emptied first. This theory is applicable to hygroscopic material.

15. Equilibrium Moisture Content (EMC) : It is the amount of water present in the solid which exerts a vapour pressure equal to the vapour pressure of the atmosphere surrounding it. Depending upon temperature and humidity conditions solids may absorbed or loss 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. Sorption: When air is continuously passed over the solid containing moisture less than EMC, then solid absorbs water continuously till EMC reached. This phenomenon is known as sorption.

16. Measurement of EMC : The solid samples are placed in a series of closed chamber such as desiccators. Each chamber consist of solutions which maintain a fixed relative humidity in the enclosed air spaces. Solids 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.

17. Factor 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 zero humidity, EMC of all materials is zero. As the temperature of air increases, the EMC of solid decreases.

18. Free Moisture Content (FMC): It 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)/ total mass of dry sample (kg) ×100

19. Behavior of solids During Drying or Rate 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 as a function of time. Drying rate = Weight of water in sample (kg)/ Time(h) ×Wt. of dry solid (kg)

20. 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.

21. 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 increases i.e. an wetted substance when kept for drying it absorbed heat from surrounding & vaporization of moisture takes place which cools the surface. Heat flows to the cooled surface at higher rates, leads to rise in temperature & evaporation again.

22. 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

23. 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 .

24. 4. 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 ( E) referred to as Equilibrium Moisture Content(EMC ) .

25. Classification of Dryers Static Bed Dryers ( Ex. Tray Dryers, Freeze Dryer ) Moving Bed Dryer (Ex. Drum Dryer ) Fluidized Bed Dryer Vacuum Dryer Pneumatic Dryer (ex. Spray Dryer )

26. TRAY DRIER Principle of Tray Dryer: In tray dryer hot air is continuously passed over wet mass . Heat transfer takes place by forced convection to remove moisture from solids placed in trays . Construction : It consists of a rectangular chamber whose walls are insulated. Trays are placed inside the heating chamber. The number of trays may vary with the size of the dryer.

27. Each tray is rectangular or square and about 1.2 to 2.4 meters square in area . Trays are usually loaded from 10.0 to 100 millimeters deep. Dryer is fitted with a fan for circulating air over the trays . Electrically heated elements are provided inside (rather than outside) to heat the air. In the corner of the chamber, direction vanes are placed to direct air in the expected path .

28. WORKING  Wet solid is loaded into trays. Trays are placed in the chamber .  Fresh air is introduced through inlet which passes through the heaters and heated up .  The hot air is circulated by means of fans at 2 to 5 meter per second .  The water is picked up by air and moist air is removed from outlet .  During the cycle of drying only 10 to 20 % of fresh air is introduced and 80 to 90% air is circulated back . https://www.youtube.com/watch?v=dSTb3e80400

29. MERITS  The method is operated batch wise.  Attrition is not observed.  Loading and unloading can be done without losses. DEMERITS  Only a fraction of the solid particles is directly exposed .  The method is costly and time consuming. USE  Sticky materials, plastic substances, granular mass or crystalline materials, precipitates and paste can be dried in a tray dryer.

30. DRUM DRYER OR ROLLER DRYER Principle In drum dryer heated hollow cylindrical drum is rotate on longitudinal axis, which is dipped into the solution to be dried. The solution is carried as a film on the surface of the dryer and dried to form a layer. Dried material is removed with the help of the knife . Construction The drum dryer consists of a horizontally mounted hollow steel drum of 0.6 to 3.0 meters diameter and 0.6 to 4.0 meters length, whose external surface is smoothly polished. Below the drum, feed pan is placed in such a way that the drum dips partially into the feed.

31. On one side of the drum a spreader is placed and on the other side a doctor’s knife is placed to scrap the dried material . A storage bin (or a conveyor ) is placed connecting the knife to collect the material .

32. WORKING Steam is passed inside the drum. Heat transfer coefficient of the drum metal is high. Heat is transferred by conduction to the material. Simultaneously drum is rotated at the 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 the drum during its rotation. The materials are completely dried during its journey in slightly less than one rotation (from one side to another side of the drum). The dried materials are scrapped by the doctor’s knife, which than falls into a storage bin.

33. The dried materials are scrapped by the doctor’s knife, which than falls into a storage bin. The time of contact of the material with hot metal is 6 to 15 seconds only . Therefore processing conditions such as film thickness, steam temperature are closely controlled .

34. MERITS: The method gives rapid heat drying and mass transfer are higher The entire material is continuously exposed to heat source . The equipment is compact. Heating time is short being only a few seconds. The product obtained is completely dried and is in the final form. DEMERITS: Operating conditions are critical . Attrition is not possible. Skilled operators are essential to control feed rate ,film thickness, speed of rotation and temperature. Maintenance cost of a drum dryer is higher than spray dryer . It is not suitable for solutions of salts with less solubility.

35. Use Drum dryer is used for drying solution, slurries, suspensions etc . The products dried are milk products, starch products, ferrous salts, suspensions of zinc oxide, suspension of kaolin, yeasts, pigments, malt extracts, antibiotics, DDT, calcium, insecticides and barium carbonates

36. SPRAY DRYING Principle: In spray drier the fluid to be dried is atomized in to 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.

37. Construction It consist of large cylindrical drying chamber with a short conical bottom made up of stainless steel.(diameter 2.5 to 9.0m and height 25m or more) An inlet for hot air placed in the roof and another inlet carrying a spray disk atomizer is also set in the roof. The spray disk atomizer is about 300 mm in diameter and rotates at a speed of 3000 to 50,000 rpm. Bottom of the dryer is connected to cyclone separator.

38. Working of Spray Dryer Drying of material in spray dryer involves 3 stages: Atomization of liquids: The feed is introduced through the atomizer either by gravity or using suitable pump. Drying of the liquid droplets: Fine droplets are dried in the drying chamber by supplying hot air through the inlet. Recovery of the dried product: Centrifugal force of atomizer drives the droplets to follow helical path. Particles are dried and collected at the conical bottom. All these processes are completed in few seconds. Particle size obtained is ranging from 2 to 500 mm. maximum size of spray dryer has capacity of 2000 kg/hr. https://www.youtube.com/watch?v=0o4ZCjHnaRw

39. Uses It is used in the production of dry powder formulation/dry powder aerosol and thermolabile materials. Product quantity to be dried is large. Spray dryer is used in drying pharmaceuticals like penicillin, blood products, enzymes, vaccines, etc Apart from its applications in the pharmaceutical industries, spray dryers also find use in; chemical industries, ceramic industries, food industries, etc.

40. Advantages It is continuous process & dried completely 3 to 30 sec.  Labor cost is low  Product of uniform & controllable size is obtained  Products shows excellent solubility. Disadvantages It is bulky and expensive  Huge equipment not always easy to operate  Thermal efficiency is low  Suitable for drying of sterile products

41. Fluidized Bed Dryer ‘ Fluidized’ means something that behaves like liquid. In the fluidized bed dryer, the mixture of solids and gas behave like a liquid and solid are called fluidized . It provides good contact between hot air and particles to obtain efficient drying . The hot air is passed through a mesh, which supports the conical vessel with a porous base. This vessel is filled with powder to be dried.

42. Principle : In fluidized bed dryer hot air is passed at high pressure through a perforated bottom of the container. The granules are lifted from the bottom and suspended in the stream of air, this condition is called as fluidized state. The hot gas surrounding every granules to completely dry them.

43. 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.

44. Working The wet granules to be dried are placed in a detachable bowl. The bowl is inserted in the drier. Fresh air can pass trough a prefilter , which is then heated when passing trough 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.

45.  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. Uses:  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

46. Advantages Require less time for drying i.e., 20 to 40 min. It is available in different sizes with the drying capacity 5 to 200 kg Labour cost are low Also used for mixing the ingredients Hot spot are not observed in the dryer Higher drying temp. can be used

47. Disadvantages Some powder are organic which develop electrostatic charges to avoid this, efficient electrical earthing of the dryer is essential Chances of attrition of some materials resulting in production of fines.

48. 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

49. Construction : It is made up of cast and 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. 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.

50. W orking 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 30℃. The vapour goes to the condenser. After drying vacuum line is disconnected. Then the materials are collected from the tray. https://www.youtube.com/watch?v=HqP45slStpw

51. 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

52. 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

53. Uses 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.

54. 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. Sold-liquid- vapour equilibrium phase diagram of water Is usefull to decide the experimental conditions.

55. The drying is achieved by subjecting material to temperature & pressure below the triple point. under this conditions, any heat transferred is used as a 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.

56. construction It consists of Drying chamber in which trays are loaded . Heat supply in radiation Vacuum pump or stream ejector or both. source, heating coils . Vapour condensing or adsorption system. Vacuum pump or ejector o r both.

57. Working The following Preparation and pretreatment Preparation and pretreatment Prefreezing for solidifying water Secondary Drying Primary Drying Packing

58. 1.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 . 2. Prefreezing for solidifying water: Vials, ampoules or bottles in which the aq. Solution is packed are frozen in cold shelves (-50ºc) The normal cooling rate is about 1 to 3 Kevin/minute so tghat large ice crystals with relatively large holes are formed on sublimation of ice. This is also responsible for giving a porous product.

59. 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 for mmHg, sublimation, when water is alone present. when a solution a solid is dried, the depression of freezing pont 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. This usual range is from -10ºc to 30ºc. The condition of 1 to 8 K below eutectic point is sufficient.

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

61. 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. Packing: After vacuum is replaced by inert gas, the bottles & vials are closed.

62. Uses It is used in production of injection, solutions, and suspension used for production of blood plasma and its fractionated products bacterial and viral cultures antibiotics and plant extracts steroids, vitamins and enzymes .

63. Advantages Denaturation of protein does not occur. Loss of volatile material is less . Sterility can be maintained Thermo labile substances can be dried. migration level can be kept as low as possible. Material can be dried in its final container such as single done & multiple dose.

64. 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.

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