Industrial Pharmacy Drying PDF

Industrial Pharmacy  Drying Definition : Drying is the final removal of small amount of water from solids or relatively solid material (usually by heat) Drying is commonly the last stage in a manufacture process Purposes of drying 1. To reduce the cost of tr...

Industrial Pharmacy  Drying Definition : Drying is the final removal of small amount of water from solids or relatively solid material (usually by heat) Drying is commonly the last stage in a manufacture process Purposes of drying 1. To reduce the cost of transport 2. To make a material suitable for handling. 3. To provide definite properties (flowability). 4. To avoid the presence of moisture (prevent drug degradation and equipment corrosion).  Drying Evaporation Removal of less amount of Removal of large amount of moisture liquid Drying occurs below boiling Evaporation occurs more at point boiling point Done to get a stable dry Generally done for product concentration Emphasize on solid product Emphasize on reducing the volume Bound moisture This water retained in such a way that it exerts a vapor pressure less than that of free water at the same temperature. Such water may be retained in small capillaries, adsorbed on surface in or solution in cell walls. Unbound moisture : Unbound moisture in a hygroscopic material is that moisture in excess of the equilibrium moisture content corresponding to saturation humidity. Free moisture :It is the moisture in a sample above the equilibrium moisture content that can be removed by drying process. Hygroscopic material: It is a material that may contain bound moisture. Non-Hygroscopic material: It is a material that may contain no bound moisture. Equilibrium moisture content EMQ: It is the limiting moisture to which a given material can be dried under specific conditions of air temperature and humidity. Critical Moisture Content CMC  It is the average moisture content when the constant rate period ends. Nature of material 1-Crystalline solids : Water is held in open surface and interstitial spaces between particles; EMC for crystalline solids is almost zero. E.g sand 2-Amorphous solids: Water is an integral part of the molecular structure and entrapped in fine capillaries and interior pores. So amorphous solids are more difficult to be dried. Example : Lactose.  Drying Curve Consider drying of porous insoluble substance e.g sand Period Io: Initial period during which drying rate may increase or decrease from its initial value. Period I:(Constant drying rate period) Early stage of drying, during which drying CMC Drying rate rate remain constant until free moisture is removed I Period II:(First falling rate period) II IO The drying rate decreases with the continuous decrease in water content. FMC Period III:( second falling rate period) III It is the stage of drying immediately following period II during which the drying rate decreases with decreasing water content but EMC Moisture content in different rate than during period II The limit to which the water content can be reduced by drying for an infinite length of time is known as the Equilibrium Moisture Content (EMC). Drying Curve Period I :(The constant drying rate period) Begins at the free moisture content and ends at the critical moisture content. It is characterized by CMC Drying rate uniform rate of drying and a I constant surface and IO interior temp of the II material. It is the steady state FMC period reached at the end of III period I0. The period continues as EMC Moisture content long as water is supplied to the surface of the material In the drying of porous, insoluble solids, as rapidly as evaporation the mechanism of liquid water transfer takes place. from the interior to the surface appears to be by capillary action.  Effect of Shrinkage Shrinkage is an important factor in controlling the drying rate as a result of decreasing the moisture content. a) Rigid, porous, or nonporous solids do not shrink appreciably during drying. b) Colloidal and fibrous materials undergo severe shrinkage as the moisture is removed from them. This has three effects: 1) The first is that it alters the surface of the material per unit weight as in case of vegetables and foodstuffs. 2) A hardened layer on the surface is developed , impervious to the flow of moisture either as liquid on vapor, that occurs in the drying of such materials as clay and soap (it is the more serious effect is the there ). 3) Cause the material to warp or check or otherwise change its gross structure, This often happens in drying wood. For remedy :For materials that tend to warp or check, or for materials that tend to develop a hard surface layer, it is sometimes desirable to dry with moist air. to decrease the humidity difference between the air and the surface of the solid in order to slow down the drying and so decrease the effect of shrinkage.  Drying Equipment Dryers Direct dryer Batch dryer Indirect Continuous dryer dryer Advantages of batch operation: Ease of construction of a small batch dryer compared with the continuous dryers which needs expert and effort. It can be used for different materials Control humidity during batch operation is possible. Advantages of continuous dryers: 1-Easy in operation. 2-Lower unit cost compared with batch drying 3-Less time consuming.  Advantages of direct dryer 1- It is less costly. 2- Control temperature of air - so used with heat - sensitive materials. Disadvantage of direct dryers: The overall thermal efficiency is low due to the loss of energy in the exhaust air and the saturation of this air by moisture which cannot accept any further moisture. Losses occur in case of powdery materials in the circulating air. The product or the solvent may react with oxygen in air leading to several problems In indirect dryers heat is transferred to wet material by conduction through a solid metal surface. Disadvantage of indirectly heated dryers: is the danger of overheating the product through the heating surface.  1-Compartment dryer 2-Vacuum tray dryer 3-Tumbling dryers: Batch 4- Freeze dryers: DRYERS 1- Turbo-shelf dryer Single 2-Drum dryer Double Continuous Vacuum 3-Spray Dryers: 4-Rotary Dryers 5- Tunnel Dryers  Batch Dryers 1- Vacuum tray dryer: (Vacuum shelf dryer) Structure A rectangular iron shell which contains a number of hollow shelves (D) which filled with steam or hot water. during operation Vertical manifolds (A and B) at either side. A short connection C extends from either manifold to each shelf. One of these manifolds (A) is for the introduction of steam and the other (B) is for the removal of condensate and non-Condensed gases. It desirable when a material on trays have to be dried more rapidly than can be done by passing a stream of air over them and Vacuum shelf dryer; A, yet maintain a temperature lower than would correspond to the steam-inlet manifold, B, evaporation of water at atmospheric pressure. condensate-outlet manifold, C, connections to manifolds from shelves; D, shelves.  How does it work The material is spread on the shelves of trays The door is closed and the interior of the dryer is placed under vacuum by means of a vacuum pump. The steam in the shelves gradually heats the material to a temperature such that the water will evaporate under vacuum in the dryer. This water vapor is collected and condensed in a condenser placed between the dryer and the vacuum pump. Advantages: 1. Suitable for thermo labile pharmaceuticals where drying is carried out at low temperature. 2. Suitable for materials which must be kept from contact with air or other oxidizing gases. 3. If the liquid to be vaporized is a valuable solvent, this can be easily collected in the condenser. 4. Warm water at any desired temperature can be used instead of steam. Disadvantage: 1. Only the surface of material undergo drying (so one uses tumbling dryers). 2. Each unit has small drying capacity.  2- Tumbling dryers:  The drying process takes 2-3 hours.  The capacity of equipment is only 60% ,i.e., 40% space).  The shell is heated by jacket with hot water or by direct heating.  The water vapor is removed by air or filter through a vacuum. Advantages 1- Efficient drying, because of tumbling Changes the surface of material continuously so give complete drying. 2- Less time consuming.  3- Freeze drying : (lyophilic drying" or "lyophilization". Principle Freeze drying is the removal of ice or other frozen solvents from a material through the process of sublimation. Low temperatures (freezing) and high vacuum are used 1. As it is the most expensive method with low drying rates so used on a large scale when other methods are inadequate. 2. used when high rates of decomposition of the drug materials occur during normal drying e.g. Fruit juices, loss of flavor or odor and proteins are partly denatured. 3. Drying of blood plasma and some antibiotics are important large- scale. On a smaller scale, used for the dehydration of bacteria, vaccines, blood fractions and tissues. 4. Used for drying serum, plasma, antibiotics, hormones and other heat sensitive materials.  Principle of Freeze drying technique (lyophilic drying)or (lyophilization). The phase diagram for water The diagram consists of three separate areas representing the phases of water, solid, liquid, and vapor. The point O is the only point where all the three phases can coexist, and is known as the triple point. On heating at constant atmospheric pressure ice will melt when the temperature rises to 0 C. At this constant temperature and pressure it will then change to water. Continued heating will raise the temperature of the water to 100 C where, if heat addition is continued, the liquid water Temperature will be converted into water vapour at 100 C.  The phase diagram for the water  If , however, solid ice is maintained at a pressure below the triple point then on heating the ice will sublime and pass directly to water vapour without passing through the liquid phase. This sublimation, and therefore drying, can occur at a temperature below 0 C.  This will only happen if the pressure is prevented from rising above the triple point pressure.  It may be thought that as the process takes place at a low temperature the heat required to sublime the ice will be small. The phase diagram for water  Precautions 1-Under conditions of high vacuum, water vapor must be trapped or eliminated continuously. To maintain drying 2-Heat must be supplied to the frozen solid to balance the latent heat of sublimation without melting the frozen solid The phase diagram for water  Basic components of freeze dryers It composed of five basic components 1) A chamber for vacuum drying. 2) A vacuum source. 3) A heat source (latent heat of sublimation) 4) A vapor-removal system. 5) Compressor for freezing (frozen cycle) Freeze Dryer Stages of the freeze drying process 1- Freezing stage:  The liquid material is frozen before the application of vacuum to avoid frothing.  The bottles are rotated slowly and almost horizontally in a refrigerated bath. The liquid freezes in a thin shell around the inner surface of the bottle.  Freezing is slow and large ice crystals form, which is a drawback of this method. 2 - Vacuum application stage: The containers and the frozen material must be connected to a vacuum source sufficient to drop the pressure below the triple point and remove the larger volumes of low – pressure vapor formed during drying. 3 - Sublimation stage (primary drying ):  Heat of sublimation must be supplied. Under these conditions the ice slowly sublimes, leaving a porous solid which still contains about 0.5% moisture after primary drying 4- Secondary drying: The removal of residual moisture at the end of primary drying is performed by raising the temperature of the solid to as high as 50 or 60 C. 5- Packaging: Attention must be paid to packaging freeze-dried products to ensure protection from moisture. Containers should be closed without contacting the atmosphere.  Disadvantages (limitation ) of freeze drying 1-The porosity, ready solubility and complete dryness yield a very hygroscopic product. Unless products are dried in their final container and sealed in situ, packing require special conditions. 2-The process is very slow and uses complicated plant, which is very expensive 3- It is not a general method of drying but limited to certain types of valuable products. 4- Increase time and handling required for processing (more time con- suming).