Basic of Fluid Bed Dryer: 1 Basic of Fluid Bed Dryer Index: 2 Index Components in Fluid Bed Dryer (FBD) What is FBD Different Types Effect of Air Flow in FBD Wruster Process Typical Components of a Fluid Bed System: 3 Typical Components of a Fluid Bed System Air preparation unit Product container Exhaust filter Exhaust blower Control panel Air distribution plate Product Spray nozzle Solution deliver What is Fluidised Bed: 4 What is Fluidised Bed A Fluidised Bed is a bed of solid particles with a stream of air or gas passing upward through the particles at a rate great enough to set them in motion Different types of Fluidized Bed: 5 Different types of Fluidized Bed Slugging bed – A slugging bed is a fluid bed in which air bubbles occupy entire cross sections of the vessel and divide the bed into layers Boiling bed – A boiling bed is a fluid bed in which the air or gas bubbles are approximately the same size as the solid particles Channeling bed – A channeling bed is a fluid bed in which the air (or gas) forms channels in the bed through which most of the air passes Spouting bed – A spouting bed is a fluid bed in which the air forms a single opening through which some particles flow and fall to the outside What happens when the Air Flow is High: 6 What happens when the Air Flow is High At higher airflow rates , Agitation becomes more violent The movement of solids becomes more vigorous The bed does not expand much beyond its volume at minimum fluidization. Such a bed is called an aggregative or bubbling fluidized bed A bubbling bed Effect of airflow through the bed: 7 Effect of airflow through the bed As the air travels through the particle bed, it imparts unique properties to the bed For example the bed behaves like a liquid. It is possible to propagate wave motion, which creates the potential for improved mixing. The surface area of fluidized particles is large, Larger Area Improves heat transfer, reduces process time and imparts reproducible operating parameters. In a bubbling fluidized bed, no temperature gradient exists within the mass of the fluidized particles. This isothermal property which results from the intense particle activity in the system Thus, the fluid bed can be used to agglomerate particles, improve flow properties, produce coated particles, pellets, or tablets, taste-mask bitter products or effect uniform chemical reactions in a controlled fashion Spouted bed and draft tube: 8 Spouted bed and draft tube The spouted bed is a combination of a jet-like, upward-moving, dilute, fluidized phase It is surrounded by a slow, downward-moving bed through which gas percolates upward. Whether or not a particle bed can be made to spout depends on gas flow, bed depth, inlet-nozzle diameter, and particle diameter. The spouted-bed principle was successfully implemented for coating in Wurster's 1959 invention. Wruster Process: 9 Wruster Process In the Wurster process, the particles or tablets to be coated are fluidized in an upward-moving air stream A high-velocity air stream is introduced into the fluidized bed, causing a spout A draft-tube partition is placed around the spout to prevent the particles in the spout from colliding with the particles descending in the fluidized bed. A cyclical flow of particles is thus created Wurster Process A standard Wurster chamber with a flat bottom plate and a container with a dished air distributor plate: 10 Wurster Process A standard Wurster chamber with a flat bottom plate and a container with a dished air distributor plate Wruster Process: 11 Particles, beads, or tablets enter the high-velocity spout, they are uniformly accelerated and physically separated from each other High-velocity air and the particles move up The coating is applied by a spray nozzle mounted at the base of the spout The process air that moves the particles also serves to dry the coating. Due to Large amount of air used, excellent drying is achieved by this process Air in the spout spreads out to fill the expansion chamber when the air stream and particles clear the top of the partition The particles settle on the top of the bed of fluidized particles Because the bed of particles is fluidized by air additional drying occurs as the particles descend to the bottom of the bed and reenter the partition they are accelerated again by the high-velocity air stream and receive additional coating Wruster Process Distributor Plate: 12 Distributor Plate The tapered and dished bottom plate in the Wurster Chamber is designed to help particles, beads, or tablets enter the coating zone, thus facilitating uniform cyclical flow of the particles and subsequent uniform coating Conditioned air is introduced form the bottom of batch fluid bed The velocity of a given volume of air determines how fluidization will be achieved Air-distributor plates covered with a 60mesh or finer screen provide an appropriate means of air supply to the bed Also regulates the air Plates are identified by Percentage of open area Interchangeable Plates with 4%, 6%, 8%, 12%, 16%, and 30% openings are available to provide a range of loading capacities Distributor Plate (contd.): 13 Various size batches can be produced efficiently and with uniform quality Low bulk density product requires low incoming air velocity a large open area plate best suits for uniform fluidization Restricts product to enter into the filter housing Prevents filter plugging More dense product requires high incoming air velocity a smaller open area plate provide more velocity is suitable A dished distributor plate is preferred because it facilitates the flow of product through the coating zone when a bottom-coating technique is used Distributor Plate (contd.) Conditioning of Air: 14 Conditioning of Air Because heated air is used to dry the product during the drying, agglomerating, and coating processes, the drying capacity of the air must be carefully monitored Conditioning of Air: 15 During fluid bed drying, the product passes through three distinct temperature phases Conditioning of Air Conditioning of Air (contd.): 16 Phase 1 - material is heated from ambient temperature to approximately the wet-bulb temperature of the air in the dryer. Phase 2 - this temperature is maintained until the material's moisture content is reduced to the critical level. Material holds no free surface water at this point and the temperature starts to rise further Phase 3 In some cases, the temperature continues to rise until it equals the temperature of the air in the dryer In most processes the drying is stopped before the material reaches this terminal temperature. Conditioning of Air (contd.) Dry & Wet Bulb Temperature: 17 Dry & Wet Bulb Temperature The Dry Bulb temperature, usually referred to as air temperature, is the air property that is most common used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air. Effect of Air: 18 Effect of Air When low-temperature fluidizing air is used, climatic conditions can play a significant role in the fluid bed process. In geographic locations where the absolute humidity varies during the year, its effect on the relative humidity of the heated, fluidized air becomes pronounced. Table – I shows the effect of outside conditions on process conditions One approach, raising the inlet-air temperature, is limited by the product's heat sensitivity Each degree of increase in the inlet temperature is less beneficial than is a corresponding decrease in the outlet-air temperature Effect of outside conditions on process conditions : 19 Table I Typical Outside Climatic Conditions Process Air Temp. Typical Exhaust Air Conditions Temperature and Humidity Moisture Content (g, H 2 O/kg dry air) Temperature and Humidity Moisture Content (g, H 2 O/kg dry air) 30 °C, 85% RH 23 50 °C 35 °C, 80% PH 28 4 °C, 30% RH 1.5 50 °C 30 °C, 85% RH 23 90 °C 70 °C, 80% RH 220 4 °C, 30% RH 1.5 90 °C Effect of outside conditions on process conditions Effect of Air: 20 In a typical drying process, Suspended particles in a concurrent air stream are kept relatively cool by evaporation Inlet-air temperature can be much higher than the product degradation point If powder clings to equipment surfaces, it may scorch Effect of Air Effect of Air: 21 Second criteria is to reduce the outlet air temperature A unit volume (Outlet Stream of cool air by weight is more compared to inlet warmer air stream) Outlet temperature has greater influence on energy use and productivity per degree of change Lowest practical setting of outlet temperature benefits energy and productivity and product quality Cooler outlet air raises humidity significantly, and this may restrict the allowable temperature if product moisture is increased Effect of Air Effect of Air: 22 Certain resins and other heat-sensitive materials longer residence time in the drying zone permits a lower outlet temperature without increased product moisture longer residence time has an effect similar to that of raising the outlet temperature. Uniform particle size has the same effect Non uniform or larger particles need a higher temperature, a longer time to dry or both Effect of Air Air-Handling System: 23 Air-Handling System Air enters the fluid bed unit's air-handling system, it can be heated, cooled, humidified, dehumidified, or filtered A typical layout of an air-conditioning system is illustrated Air-Handling System: 24 Filtration : A coarse dust filter is placed where the air enters the system. High Efficiency Particulate Air (HEPA) filters are used to filtered air after heating Dehumidification : A cooler followed by a droplet catcher dehumidifies the air Heating : Process air is heated by a finned-tube heat exchanger The most widely heating method is face and bypass airflow damper system used in conjunction with a flooded steam coil The face and bypass system offers the advantages of precise temperature control ( + 1 ° C or less) and the ability to change temperatures rapidly during the process with little or no offset Air-Handling System Air-Handling System: 25 Humidification : During cold and dry seasons, air humidity may actually be lower than desirable and re humidification may be necessary. A clean-steam injector can be used for this purpose after the cold and dry air is heated. Humidification is used primarily to maintain constant inlet conditions and process times . when preheating is required a bypass loop can be used for preconditioned air. Preheating allows to attain the required process temperature & humidity within the vessel before fluidization Air-Handling System Heat and mass Transfer: 26 Heat and mass Transfer Conditions for heat transfer in Fluidized bed are extremely favorable A dry packed bed of particles is a good thermal insulator (because there is very little contact between adjacent particles) Little heat flows by conduction, even when the thermal conductivity of the solid is high Heat transfer by convection is also small Equilibrium is achieved because the particles at the wall are frequently replaced by particles from the interior of the bed The high rate of mixing is caused by passing bubbles, which are essential to maintain heat transfer Heat and mass Transfer: 27 Heat and mass Transfer The equilibrium temperature for any surface from which water is evaporating adiabatically (i.e., without heat transfer from its surroundings) is the wet-bulb temperature As water evaporates in the air, the dry-bulb temperature of the air falls and the humidity increases, but the heat content remains the same. In an adiabatic dryer, the wet-bulb temperature is the true temperature of the solid surface during this constant-rate period Heat and mass Transfer: 28 Heat and mass Transfer In heat transfer, the maximum rate of mass transfer that occurs during drying is proportional to surface area turbulence of the drying air the driving force between the solid and the air the drying rate Pressure Drop: 29 Pressure Drop To move air in a fluid bed blowers or exhaust fans mounted outside impart motion and pressure to the air using a paddle - wheel action Different type of Pressure that act in Air Handling System Velocity Pressure Static Pressure ( Operating Duct System) Negative Static Pressure (Exhaust System) Different Pressure: 30 Different Pressure The moving air acquires a force or pressure component in its direction of motion because of its weight and inertia is called velocity pressure and measured in millimeters of water column (wc) Pressure that is independent of air velocity or movement is always present known as static pressure, it acts equally in all directions. Negative static pressure will exist on the inlet side of the fan Total pressure is the combination of static and velocity pressures Blower Pressure: 31 Blower Pressure Blower size is determined by calculating the pressure drops (^P) created by all the components of the completed system. For example A 28 in. blower creates a pressure differential between the exhaust and inlet blower that is equal to the pressure at the bottom of a 28-in high water column The blower in a fluid bed system discharges directly into the atmosphere, which is designated as having a pressure of zero (gauge) Therefore, a 28-in. blower creates a ^P of 28 in. between the exhaust and inlet Because the exhaust has a zero pressure, the blower has a negative pressure of 28 in. of water. The ^P created by the fan is dissipated by the equipment located in the system The exhaust flap dissipates excess ^P not needed to fluidize the material This flap is controlled by a pneumatic positioner that allows an infinite number of settings Blower Pressure: 32 Blower Pressure A blower with a suitable ^P will fluidize the process material properly However, a blower without enough ^P will not allow proper fluidization of the material, resulting in a longer drying time If the blower develops too much ^P, the control of fluidization will be very difficult A properly sized blower should develop ^P so that the exhaust flap will be used in the 50 80% open position Pressure experienced by various pieces of equipment in a hypothetical situation: 33 Pressure experienced by various pieces of equipment in a hypothetical situation Sr No. Equipment ^P (in. wc) 1. Inlet rough filter 0.5 2. Heater housing 0.5 3. HEPA filter 2 4. Inlet plenum & bottom plate 5 5. Processing material 4 6. Filter bags 7 7. Exhaust explosion protective valve 2 8. Air duct 0.5 9. Capacity for processing & exhaust air flap 6.5 Total 28 Exhaust Pressure Control: 34 Exhaust Pressure Control The exhaust flap dissipates excess ^P not needed to fluidize the material This flap is controlled by a pneumatic positioner that allows an infinite number of settings A blower with a suitable ^P will fluidize the process material properly and without enough ^P will not allow proper fluidization of the material, resulting in a longer drying time. If blower develops too much ^P, the control of fluidization will be very difficult. A properly sized blower should develop ^P so that the exhaust flap will be used in the 50 80% open position. When equipment such as scrubbers and duct filters is added, the ^P of the blower can be increased to compensate for the additional resistance created in the system. Exhaust filter systems: 35 Exhaust filter systems Containing all the product inside the fluid bed system by using an exhaust air filter is one of the most important aspects of fluid bed processing. The ideal filter material should retain all of the product particles in the container while allowing process air to pass through. Cotton, polyester, polypro-pylene, nylon, and expanded poly-tetra-fluoro-ethylene are the most commonly used materials. These filters can be obtained in 1- to 25-p.m sizes. The particle size of the product being processed and the type of unit operation (coating, agglomerating, or drying) will dictate the level of porosity of the filter material that should be used. The filter can cause a significant ^P, many process failures result from the selection of filter media that have openings of the wrong size. Process failure can also occur when the filter clogs because of excessive fluidization of fine powder or when filters are improperly cleaned during the process. Too fine filter will impede fluidization, causing excessive ^P, and a too-coarse filter will cause loss of valuable product carried by process air. Process considerations: 36 Process considerations Airflow in drying Airflow in granulation Airflow in coating Airflow in drying: 37 Airflow in drying One constraint in using a fluid bed dryer is the inability to achieve uniform fluidization. An indication of good fluidization is a free downward flow of the granulation at the sight glass of the drying bowl, but such limited observation could be misleading. This situation can be detected by monitoring the outlet-air temperature. Every product has a unique constant rate of drying period in which the bed temperature remains relatively constant for a significant length of time. if outlet-air temperature rises more rapidly than anticipated, it is an indication that fluidization is incomplete. In such cases, the process must be stopped, the granulation stirred to distribute the moist material and the process restarted. Airflow in granulation : 38 Airflow in granulation While the granulating solution is being sprayed, the exhaust flap is controlled to achieve proper fluidization. The objective of this process is to fluidize the powder for maximum exposure to the spray nozzle. Whereas over fluidization may produce uneven or lumpy agglomerates and may also cause filter plugging, under fluidization may stall the bed and ultimately lead to bed collapse. Filters can also become plugged Minimum fluidization velocity depends on particle size and particle humidity, as both changes during the granulation process As the product dries, its density changes and less airflow is required Airflow in coating: 39 Airflow in coating Tablet, Pellet and Particle Coating are all performed in fluid bed equipment using a top spray, a bottom spray with a Wurster column, or a rotary coater The coating process involves the deposition of droplets on the substrate material, followed by spreading and coalescing of the droplets, which form a continuous layer as they adhere to the matrix. Throughout the process, solvent is evaporating. It is advisable to control ambient air dew points in organic solvent processes and in aqueous coating operation. Conclusion: 40 Conclusion The fluidized-bed unit is one of the most versatile pieces of production equipment in the process industries. A fundamental understanding of the mechanism of fluidization, normal pressure drops across various elements, the effects of variable climatic conditions, and heat and mass transfer during various unit operations will minimize the number of process problems encountered.