Drying Prepared by: Mohammad Mahareeq Definitions Definitions • Equilibrium moisture X*. This is the moisture content of a substance when at equilibrium with a given partial pressure of the vapor. • Bound moisture. This refers to the moisture contained by a substance which exerts an equilibrium vapor pressure less than that of the pure liquid at the same temperature. • Unbound moisture. This refers to the moisture contained by a substance which exerts an equilibrium vapor pressure equal to that of the pure liquid at the same temperature. • Free moisture. Free moisture is that moisture contained by a substance in excess of the equilibrium moisture: X – X*. Only free moisture can be evaporated, and the free-moisture content of a solid depends upon the vapor concentration in the gas. • These relations are shown graphically in the following figure for a solid of moisture content X exposed to a gas of relative humidity A. Types of Moisture Definitions Example Drying Definition: The removal of a liquid from a material by the application of heat. It is accomplished by the transfer of the liquid from the surface of the material into the vapor phase. There are other methods to achieve drying than with the application of heat such as: Desiccation in a sealed container with silica gel. Freeze drying by application of vacuum. Drying Purpose: It is most commonly used in preparation of granules. Other uses include: Spray drying of lactose. Increase stability of moisture sensitive drugs (aspirin) Preservation of animal and vegetable drugs. Psychrometry Psychrometry: is the determination of the vapor concentration and carrying capacity of the gas. The most common drying in pharmaceutics involves removal of water from the material by air (air-water system). The carrying capacity determines: Rate of drying. Extent of drying (equilibrium content). Psychrometry Psychrometric charts (humidity charts) are helpful in understanding the relationship between temperature and humidity of the air-water vapor system at constant pressure. Psychrometry Humidity: is the concentration of water vapor in a gas. Absolute humidity: weight of water vapor per unit weight of dry air. Saturation humidity: The absolute humidity at which the partial pressure of water vapor in the air is equal to the vapor pressure of free water at the same temperature (100% relative humidity), curve CDE. Relative humidity: the ratio of the partial pressure of water vapor in the air to the vapor pressure of free water at the same temperature. Which is also the ratio of the absolute humidity to the saturated humidity at that temperature. Psychrometry Dew point: is the temperature to which a water vapor-air system must be cooled to become saturated. The air holds the maximum amount of water without condensation taking place. When the vapor-air mixture is cooled below its dew point water droplet form (2-phase system). Psychrometry Raising the temperature of a given air leads to reducing its relative humidity and thus can accept more water during the drying process. Psychrometry Wet bulb temperature: the equilibrium temperature reached by an evaporating surface when the rate of heat transferred to the surface by convection is equal to the rate of heat lost by evaporation. it can be measured by a thermometer whose bulb is covered by a wick saturated with water. The actual temperature of the air measured by an ordinary thermometer is the dry bulb temperatures. Psychrometry Psychrometry The wet bulb temperature is a function of the temperature and the humidity of the air thus it can be used to determine the humidity. The constant wet bulb temperature line (AD) can be used to determine the relative humidity: If the dry bulb temperature is 60 F and the wet bulb temperature is 54 F. The intersect of 60 F and the 54 F constant wet bulb temperature corresponds to 53 G/p absolute humidity. The same vapor-air mixture at 60 F is saturated at point C corresponding to 78 G/p. So the relative humidity is equal to (53/78)*100% ~70% Humidity measurement Gravimetric methods: A known amount of air is passed on a pre-weighed amount of phosphorous pentoxide. The increase of weight of phosphorous pentoxide represents the amount of water in that sample. Wet-bulb and dry bulb thermometers. Hygrometer: the instrument utilizes a material that has different properties at different humidities. Example hair or wood fiber that expands or shrinks with changes in humidity. Digital hygrometer. Dew point hygrometer: by observing the temperature at which moisture begins to form on a polished surface in contact with air. Theory of drying Drying involves both heat and mass transfer. Heat must be transferred to the surface of the material in order to provide the latent heat of evaporation for the moisture. Mass transfer involves the diffusion of water through the material to the evaporating surface, diffusion of the resultant vapor into the passing air stream. Drying Tests • Rate of drying can be determined for a sample of a substance by suspending it in a cabinet or duct in a stream of air. • The weight of the sample can be measured as a function of time. • The following conditions should be as closely as possible to the large-scale operation: Drying Tests The sample should not be too small. The sample should be similarly supported in a tray or frame. It should have the same ratio of drying to nondrying surface. It should be subject to similar conditions of radiantheat transfer. The air should have the same temperature, humidity and velocity (both speed and direction). • If possible several tests should be made on samples of different thicknesses. Rate-of-drying Curve • From the data obtained during the test, a curve of moisture content as a function of time can be plotted (fig a). • This is useful in determining the time required for drying larger batches under the same drying conditions. • The data is then converted into rates of drying, expressed as N = mass/(area) (time) and plotted against moisture content (fig. b) a b Rate-of-drying Curve • This can be done by: Measuring the slopes of tangents drawn to the curve of Fig a) or by, Determining from the curve; small changes in moisture content (∆X) for corresponding small changes in time (∆θ). • Calculating the rate as: (N=Ss ∆X/A ∆θ) where; Ss: the mass of dry solid. A: the wet surface over which the gas blows and through which evaporation takes place. • There are two major parts to the rate curve of Fig(b) A period of constant rate (B-C). A period of falling rate (C-E). Rate-of-drying Curve • If a solid is initially very wet, the surface will be covered with a thin film of liquid. The rate at which moisture evaporates can be described in terms of a gas mass-transfer coefficient ky: Nc = Ky (Ys – Y) Ys : humidity of the gas at liquid surface which is the saturated humidity at temperature ts. Y: humidity of the main gas stream. • Evaporation of moisture absorbs latent heat of evaporation, the liquid surface will come to, and remain at, an equilibrium temp. such that the rate of heat flow from the surroundings to the surface exactly equals the rate of heat absorption. • When the average moisture content of the solid has reached a value Xc (the critical moisture content), so the surface film of moisture has been reduced by evaporation that further drying causes dry spots to appear. Rate-of-drying Curve • The value of N must fall and this gives rise to the first part of the falling—rate (the period of unsaturated surface drying). From points C to D. • On further drying (points D to E) the rate of drying falls more rapidly than before. At point E the moisture content of the equilibrium value X* for the prevailing air temp. and humidity, and drying stops. Time of Drying Time of drying The Mechanisms of Batch Drying • Consider the section of a material drying in a stream of gas as shown in the (Fig. 12.11) Zs: solid thickness. Zm: tray thickness. TG: temp. of drying gas. Y: humidity of the gas (mass moisture/mass dry gas). G: mass velocity of the gas (mass/(time)(area)). A: evaporation surface area (upper). Ts: temp. of the surface. The Mechanisms of Batch Drying • The drying surface receives heat from several sources: - qc by convection from the gas stream. - qk by conduction through the solid. - qr by direct radiation from a hot surface at temp. TR. • • • q = qc + qk + qr The flux of evaporation is equal to Ns. N: the flux of drying (mass of moisture evaporated/(area) (time) s: the latent heat of evaporation. N s = q The Mechanisms of Batch Drying Acceleration of Drying Rate Classification of Dryers • Two useful classifications: Based on the method of heat transfer. On the method of solid handling. • Based on the method of solids handling, dryers are divided into: Static-bed dryers. Moving bed dryers. Fluidized-bed dryers. Pneumatic dryers. Static-Bed Systems Tray and Truck Dryers: • • • • • Most commonly used in pharm. operations. Tray dryers are called shelf, cabinet or compartment dryers. Truck dryer is one in which the trays are loaded on trucks (racks equipped with wheels). Trays are loaded from 0.5-4.0 inches deep with at least 1.5 inches clearance between the surface and the bottom of the tray above. Batch type dryers. Static-Bed Systems • Widely used in pharm. Industry for: Each batch can be handled as separate entity. Batch size of pharm. Industry is relatively small compared to chem. Ind. The same equipment can be used for drying a wide variety of materials. • Classified as direct or indirect: Most are direct, in which heating is accomplished by the forced circulation of heated air. Indirect type utilize heated shelves or radiant heat sources inside the drying chamber. Static-Bed Systems • • • • Trays used have solid, perforated, or wire mesh bottoms. To achieve uniform drying, there must be a constant temp. and uniform airflow over the material being dried. Energy sources for heating the drying air are electricity for small units and steam for larger units. Controlled air temp. between 40 – 120oC permit heat sensitive materials to be dried. Tunnel and Conveyor Dryers • • • Truck dryers for continuous drying. Semi continuous operation for tunnel dryers type. Conveyor dryers are truly continuous. Turbo-Tray Dryers Moving-Bed Systems Turbo-Tray Dryers: • A continuous shelf moving-bed dryer. • Consists of a series of rotating annular trays arranged in a vertical stack, rotating slowly at about 0.1 -1.9rpm. • Does low temperature drying as low as 60°F without vacuum or up to 1200°F • Heated air is circulated over the trays by turbo-type fans. • Each tray is provided with a slot cut into the tray, as well as a leaving rate for spreading the solid. • Solid fed in the top is spread upon the top tray to a uniform thickness. • As the tray revolves, the solid is pushed through the slot by a separate wiper rake, to fall upon the tray beneath. • This type is considered faster than tunnel dryers for the turbotray dryer continuously exposes new surfaces to the air. Tumbling Dryers Tumbling Dryers • • • • • • • Batch type dryer. Double cone shape is the most common. Operates under vacuum. Provides controlled low-temp. drying. Increased rate of drying. Heat is supplied to the tumbling charge by contact with the heated shell and by heat transfer through the vapor. Optimum conditions are established by varying the vacuum, temp. and rotation speed. Tumbling Dryers • • • • The normal charge would be about 60% of the total volume. Drying times of 2-12 hrs are expected. For drying tablet granules, 2-4 hrs periods are expected instead of the 18-24hrs obtained with hot air ovens. Mixing and granulation can be done in the same equipment and could precede drying. Fluidized-Bed Systems • • • Fluidization is applied to processes in which a loose, porous bed of solids is converted to a fluid system having the properties of surface leveling flow and pressure-depth relationships, by passing the fluid up through the bed. Gas velocity is greater than the setting velocity of the particles and less than the velocity for pneumatic conveying. Fluidization technique is efficient for the drying of granular solids. Fluidized-Bed Systems • Advantages over conventional tray drying: Granules have the proper particle size, for good fluidization. (i.e no need for further grinding). It has a two fold to six fold advantage in the thermal efficiency. Faster in both drying and handling time than the tray dryer, because of the high interfacial contact between drying air and solids. No hot spots are produced and higher drying temp. can be employed. Type of Fluidized-Bed Systems 1. Vertical. 2. Horizental. Type of Fluidized-Bed Systems 1. Vertical FBD: • The unit is of batch type. • The fluidizing air stream is induced by a fan in the upper part of the equipment. • Air is heated in an air heater and flows upward through the wet material. • Air flow rate is adjusted by means of damper. • The bag filter at the top of the drying chamber is to prevent carrying over of fine particles. Type of Fluidized-Bed Systems • Capacity varies from 5kg to 200kg. • Average drying time from 20-40 min. • Designed for the direct preparation of tablet granulations as well as for the drying of conventionally produced wet granulations. • The unit can be used as granulator, the dry ingredients are placed in the chamber and fluidized while the granulating liquid is sprayed into the bed. Granules are dried by heating the fluidizing air. Freeze Drying • Used for pharm. products: Which lose their viability in the liquid state. Which deteriorate if dried in air at normal atmospheric pressures. Which are heat-sensitive. Which may react with oxygen. • Materials or products need such type of drying as blood serum, plasma, antibiotics, hormones, bacterial cultures, vaccines and many food stuffs are dehydrate by freeze drying. Freeze Drying • • The process is also refered as lyophilization, gelsication or drying by sublimation. The material to be dried is: First cooled and frozen. Then subjected under a high vacuum to heat (supplied by conduction or radiation or both). • • The frozen liquid sublimes leaving only the solid. The process depends on the phenomenon of sublimation, whereby water passes directly from the solid state (ice) to the vapor state. Freeze Drying • • • Sublimation can take place at pressures and temperatures below the triple point. (4.579) mm Hg absolute (4579 microns) and 0.0099oC) The pressure and temperature at which the frozen solid vaporizes without conversion to a liquid is referred to as the eutectic point. Freeze drying is carried out at temperatures and pressures well below this point, to prevent the frozen water from melting. Freeze Drying • • In practice freeze drying of pharmaceuticals is carried out at temperatures of 10oC to 40oC and pressures of 2000 to 100 microns. Basic requirements for freeze drying: The vapor pressure of the water on the surface of the material must be higher than the partial pressure of the enveloping atmosphere. Latent heat of vaporization must be introduced to the drying solid at such a rate as to maintain desirable temp. levels at both the surface and interior. Evaporated moisture must be removed. Freeze Drying • Freeze dryers are composed of four basic components: • • Chamber for vacuum drying. Vacuum source. Heat source. Vapor-removal system. Vacuum is achieved by pumps, steam ejectors, or a combination of the two. Heat is provided by conduction or radiation or by both. Freeze Drying • Water vapor is removed by: Condensers: water vapor removed from the chamber condenses in a from of thin layer of ice, the ice is removed by melting it with a heated fluid or by means of scraper blades in the continous operation. Desiccants: liquid or solid desiccants are employed in the initial vapor removal to enhance the efficiency of the pumps removing the water vapor. Pumps.