Industrial Pharmacy I Drying I Lecture Notes PDF

Industrial Pharmacy I Aram I. Ibrahim Assistant Lec. MSc Pharmaceutics Principles of Pharmaceutical Processing Drying I 1 Outline Drying Applications of drying Theory of dryi...

Industrial Pharmacy I Aram I. Ibrahim Assistant Lec. MSc Pharmaceutics Principles of Pharmaceutical Processing Drying I 1 Outline Drying Applications of drying Theory of drying Behavior of solids during drying 2  Drying Cant · 85 , be free. completely Poss ↑ A drying is defined as the removal of a liquid from a material by the application of heat, and is accomplished by the transfer of a liquid from a surface into an unsaturated vapour phase. Drying is typically intended for adjustment of moisture levels in solid materials. In practice this liquid refers to the water, but at times certain volatile substances may need to be removed = Drying and evaporation are distinguishable by the relative quantities of liquid removed from the solid. ↓ Same process BUT Drying removed small of 3 : amount moisture. will be evaporation : Large amount removed Non-thermal drying There are many non-thermal methods of drying, for example: ▪ The expression of a solid to remove liquid (the squeezing of a wetted sponge). ▪ The extraction of liquid from a solid by the use of a solvent, adsorption of water from a solvent by the use of desiccants (anhydrous calcium chloride). - ▪ Absorption of moisture from gases by passage through a sulfuric hygroscopic substance. acid column. ▪ Desiccation of moisture from a solid by placing it in a sealed container with a moisture-removing material (silica gel). 4 Applications of drying for heat sensitive materials s ⑨ Drying is most commonly used in pharmaceutical manufacturing as a unit process in the preparation of granules. Another application is found in the processing of materials, e.g. the preparation of dried aluminum hydroxide,& spray drying of lactose. Drying also can be used to reduce bulk and weight, thereby lowering the cost of transportation and storage. 5  Purpose of drying ⑤ For recovery of drug during synthesis D For adjustment of moisture content in powder and granules Impart special properties to material, e.g., spray-dried lactose for better & flow Reduce weight of material Improve stability by reducing hydrolysis of drug Improve shelf life by reducing microbial growth 6 Types of moisture Moisture held in the microstructure of solid that cannot be easily removed called Bound water, has a vapor pressure lower than that of pure water. Moisture other than the bound water is called Unbound water. Removal of bound water requires more energy as compared to the unbound water. 7  Latent heat Latent Heat of Fusion: The energy required to change a substance from solid to liquid (or vice versa) without changing its temperature. For instance, when ice melts, it absorbs heat from the surroundings but remains at 0°C until fully melted. Latent Heat of Vaporisation: The energy required to change a substance from liquid to gas (or vice versa) without a temperature change. An example is boiling water: the water remains at 100°C until it has completely turned into steam. - Equal temperatures > -· 8  THEORY① OF DRYING Drying involves both heat and mass transfer operations. Heat must be transferred to the material to be dried in order to supply the latent heat required for vaporization of the moisture. The rate of evaporation of this film is related to the rate of heat transfer by the equation: dW/dθ = q/λ ✔ dW/dθ is the rate of evaporation ✔ q is the overall rate of heat transfer ✔ λ is the latent heat of vaporization of water 9 THEORY OF DRYING Secondly, mass transfer is involved in the diffusion of water through the material to the evaporating surface. The driving force is a humidity differential, whereas for heat transfer, it is a temperature differential. Du The rate equation for mass transfer: dW/dθ = k’ A(Hs -Hg ) ✔ dW/dθ is the rate of diffusion ✔ k’ is the coefficient of mass transfer ✔ A is the area of the evaporating surface ✔ Hs is the absolute humidity at the evaporating surface ✔ Hg is the absolute humidity of the passing air stream 10 THEORY OF DRYING After an initial period of adjustment, the rate of evaporation is equal to the rate of diffusion - of vapor, and the rate of heat transfer can be equated with the rate of mass transfer, a = k’ A(Hs -Hg ) dW/dθ = q/dW/dλ - If the overall rate of heat transfer, q, is expressed as the sum of the rates of heat transfer by convection, radiation, and conduction, is expanded to the form: dW/dθ = (qc + qr + qk )/λ = k’ A(Hs−Hg ) 11 THEORY OF DRYING The rate of drying may be accelerated by increasing any of the individual terms..555) Ens The rate of convective heat transfer, qc, can be increased by: Increasing the air flow rate -o Raising the inlet air temperature. The rate of radiation heat transfer, qr , can be stepped up by: Introducing a high-temperature radiating heat source into the drying chamber. & The rate of conduction heat transfer, qk, can be stepped up by: Reducing the thickness of the material being dried Allowing it to come in contact with raised-temperature surfaces Increasing the air velocity increasing the coefficient of mass transfer 12 Dehumidifying the inlet air, thus increasing the humidity differential, (Hs -Hg ). Dry bub is greater than wet bulb Temperature , except when humidity is equal , then they become equal. THEORY OF DRYING Dry-Bulb Temperature It is simply the ambient condition, i.e., the temperature of dry air, and is typically measured with a conventional thermometer. Wet-Bulb temperature It gets its name because a wet permeable membrane, such as wet gauze, is used in conjunction with a regular thermometer for temperature measurement. The air temperature is measured while a wet gauze is wrapped around the thermometer bulb. Since water is evaporated from the gauze, it leaves evaporative cooling on the thermometer. Consequently wet-bulb temperature is lower than dry-bulb temperature. However, at 100% relative humidity dry-bulb temperature is equal to wet-bulb temperature, since no evaporation of water takes place from wet gauze. 13  Behavior of Solids during Drying je of material in a certain dryer? 3 How would one know if 8 or 12 hours are required to dry a batch weight - - gijs 9. S E Sanswer. The rate of drying of a sample can be determined by suspending the wet material on a scale or balance in a drying cabinet and measuring the weight of the sample as it dries as a function of time. 14 Segment AB-Initial Adjustment When a wet solid is first placed in a drying oven, it begins to absorb heat and increase in temperature. At the same time, the moisture begins to evaporate and this tends to cool the drying solid. After a period of initial adjustment, the rates of heating and cooling become equal and the temperature of the drying material stabilizes. As long as the amount of heat transfer by radiation is relatively small, the temperature reached equals the wet-bulb temperature of the drying air. i O Oro latet 15 Segment AB-Initial Adjustment This period of initial adjustment is shown as segment AB. If the wet solid is initially at a higher temperature than the wet-bulb temperature, it cools down following segment AB. wet bulk - Temperature -- - 16 Segment BC-Constant Rate Period At point B, the temperature is stabilized and remains constant as long as there is a film of moisture remaining at the surface of the drying solid. Between points B and C, the moisture evaporating from the surface is replaced by water diffusing from the interior of the solid at a rate equal to the rate of evaporation. The rate of drying is constant, and the time BC is the constant rate period. The drying rate during this period depends on the air temperature, humidity, and speed, which in turn determine the temperature of the saturated surface. 17 Point C-Critical Moisture Content At point C, the surface water is no longer replaced at a rate fast enough to maintain a continuous film. Dry spots begin to appear, and the rate of drying begins to fall off. The moisture content at which this occurs is referred to as the critical moisture content. 2 Critical Moistrlue Content 18 Segment CD-First Falling Rate Period At and below the critical moisture content the movement of the moisture from the interior is no longer sufficient to saturate the surface. Between points C and D, the number and area of the dry spots continue to grow, and the rate of drying falls steadily. The time CD is referred to as the first falling rate period or the period of unsaturated surface drying. Since the surface is no longer saturated, it tends to rise above the wet-bulb temperature. 19 Segment DE-Second Falling Rate Period At point D, the film of surface water is completely evaporated, and the rate of drying depends on the rate of diffusion of moisture to the surface of the solid. Point D is referred to as the second critical point. Between points D and E the rate of drying falls even more rapidly than the first falling rate, and time DE is called the second falling rate period. This section is controlled by vapour diffusion, a factor largely dependent on particle size due to its influence on the dimensions of the pores and channels. During this period, the surface temperature approaches the temperature of the drying air. 20  Point E-Equilibrium Moisture Content When the drying rate is equal to zero, starting at point E, the equilibrium moisture period begins, and the solid is in equilibrium with its surroundings, i.e. its temperature and moisture content remain constant. Continued drying after this point is a waste of time and energy. The condition in which a material is in equilibrium with its surroundings, neither gaining nor losing moisture, may be expressed in terms of its equilibrium moisture content, equilibrium relative humidity, or water activity. These values may differ greatly for various materials, and in addition to affecting drying, they affect physical and chemical stability, susceptibility to microbial growth, and packaging requirements. 21  Any Thanks Questions? 22