Fluidization

The processing technique employing a suspension or fluidization of small solid particles in a vertically rising stream of fluid usually gas so that fluid and solid come into intimate contact. This is a tool with many applications in the petroleum and chemical process industries. Suspensions of solid particles by vertically rising liquid streams are of lesser interest in modern processing, but have been shown to be of use, particularly in liquid contacting of ion-exchange resins. However, they come in this same classification and their use involves techniques of liquid settling, both free and hindered (sedimentation), classification, and density flotation.

1. Fluidization Mr. Sagar Kishor savale [Department of Pharmacy (Pharmaceutics)] [email protected] 1 Department of Pharmacy (Pharmaceutics) | Sagar savale

2. Fluidization The processing technique employing a suspension or fluidization of small solid particles in  a vertically  rising stream of  fluid usually   gas so that  fluid and solid come into intimate contact. This is a tool with many applications in the petroleum and chemical process industries. Suspensions of solid particles by vertically rising liquid streams are of lesser interest in modern processing,  but have  been shown to be of use, particularly in liquid contacting of ion-exchange resins. However,  they come  in this same classification and their use involves techniques of liquid settling, both free  and hindered  (sedimentation), classification, and density flotation . Many important industrial processes rely upon intimate contact between a fluid (liquid or gas) and a granular material. In earlier applications the fluid flows through a static bed of granules supported on a grid. Provided the material is suitable, great improvement in mixing and contact is achieved if the granule size is properly matched to the upward velocity of the fluid. The particles of material will be supported by the drag force and the bed is said to be “fluidized”. The fluidized beds show following liquid or fluid like properties - Lighter float on top of the bed (i.e. object less dense than the bulk density of the bed) The surface stays horizontal even in tilted bed. The solids can flow through an opening in the vessel just like a liquid. The beds have a “static” pressure head due to gravity. Levels between two similar fluidized beds equalize their static pressure heads. It has a zero angle of repose. Assumes the shape of vessel that contains it 2

3. A gas fluidized bed may have the appearance of boiling liquid. It has bubbles, which rise and appears to the burst. The bubbles result in vigorous mixing and generally horizontal free surface. The motion of the bed varies with the fluid flow rate. At high velocity, particles may become entertained and transported by the fluid. History   Fluid-bed processing technology was first introduced into the pharmaceutical industry over 39 years ago as a means of rapidly drying powdered and granulated materials. Subsequently, in the late 1960’s the addition of spray nozzles allowed fluid bed drying technology to be extended to cover this granulation process. Wurster fluid bed coating process   Wurster in 1950 designed Wurster fluid-bed and this process is called as “ Wurster process” The salient features of the Wurster design consist of: 3

4. Coating chamber Inner partition, the diameter of which is approximately 50% that of the coating chamber, Air-distribution plate drilled with larger diameter holes in the central portion than those on the periphery, Spray nozzle located at the center of the air distribution plate, Geometric limitation on Wurster design normally prevents the diameter of the insert exceeding 9”. This limitation is imposed by the need to ensure that all products accelerating through the spray zone is uniformly coated. However, most pharmaceutical industry used custom –built equipment , and simply paid appropriate license free to the Wisconsin alumni research foundation , to whom the patent right were assigned (WARF ). Different Approaches In Fluid-bed Replacement of coarse filter screens with a filter bag assembly, Extension of the expansion chamber to permit deceleration of finer particles and minimize in the filter system. Additionally , functional coatings are applied for a multiplicity of purposes, including taste masking, enteric coating, and controlled-release coating. To meet these needs, three basic processing approaches have evolved   Top spray Bottom spray Tangential spray 4

5. Huttlin-Kugel coater   It does not consist of a central processor. The product container of Kugel coater is spherical. Fluidized air introduced by tube that passes down the central of the product container. A series of spray nozzle are also located at the bottom of the product container in such a way that fluidizing air create a balloon effect to keep the product being coated away from the spray nozzles. Addition of multiple spray nozzles intended to maximize uniformity of distribution of coating. 5 Fig – figure for the three basic types of fluid-bed coating processes

6. Principle The principle of operation of fluidized system are based on the fact that if a gas is allowed to flow through abed of particulates solid at velocity greater than the settling velocity of the particle and less than the terminal velocity for pneumatic conveying an equal to the minimum velocity of fluidization (V mf) , the solids get partially suspended in the steam of upward moving gas. The gas steam neglect the gravitational pull due to the weight of particle enable the suspended state of solid. The resulting mixture of solid and gas behave like liquid and thus rigidly solid are called fluidized. The solid particles are continually caught up in eddies and fall back in a random boiling motion so that each fluidized particle is surrounded by the gas steam for effective drying or granulation or coating purpose. In the process of fluidization their occurs an intense mixing between the solid or gas resulting in uniform condition of temperature, composition and particle size distribution throughout the bed . Classification of fluidized system   Fluidization in coating Fluidization in granulation Fluidization in drying 6

7. Theory of Fluidized bed system – Concept Fluidization is a condition in which the solid particles are supported by drag forces caused by the gas phase passing through the interstices among the particles at some critical velocity. It is an unstable condition in that the superficial gas velocity upward is less than the terminal settling velocity of the solid particles. The gas velocity is not sufficient enough to entrain and convey continuously all the solids. When a group of particles is described as being fluidized, it is said that they are suspended through the drag caused by the upward flow of a fluid. As the upward flow of fluid in a packed bed of solids is increased, the pressure drop increases proportionally. At certain velocity, the force of drag on the particles is sufficient to counteract the force of gravity. Beyond this velocity, resistance to the flow is at a maximum and the bed pressure drop becomes constant with an increasing flow. This velocity is denoted as the minimum fluidization velocity and is a fundamental parameter used to characterize fluidization behavior. The use of a fluidized bed dryer for drying farm products is widely known and accepted, and literally thousands of fluidized bed dryers are operating throughout the food and chemical processing industries. In contrast with this industrial development, the fundamental research on fluidized bed dryer has not made similar progress and the design of an industrial fluid bed dryer is still very much an art based on empirical knowledge . Stages of fluidization Gas flow through the column from the blower to the drying chamber is characterized with increased gas flow as fixed bed, delayed bubbling, bubbling fluidization, slugging fluidization, turbulent fluidization, and fast fluidization and dilutes pneumatic conveying regimes as shown in fig 2. The transition from fixed bed to fluidization is delineated by the minimum fluidization velocity Umf, which correspond to the lowest gas velocity at which all bed particles are suspended by the gas. 7

8. The value of Umf is obtained experimentally by determining the level of pressure drop across the bed as the superficial gas velocity in the fixed bed regimes increases. Several relationships for the evaluation of the minimum fluidization velocity have been developed. Examples include the relationship developed by Linoya. This equation is shown below in equation 1as: Where h gas is the gas velocity r is the gas density, dp the diameter of particles and βE1 and βE2 are the Ergun parameters depending on the particles sphericity and bed void age at incipient fluidization. Ar is the Archimedes number defined as: 8 Fig 2 flow pattern in gas –solids fluidized beds

9. Where the particle density and g is the gravity. According to Teunou and Poncelet, the Ergun expression can be approximated when the particle’s diameter exceeds 100μm. In that case, Umf becomes Umf = dp > 100µm ………………(3) The onset of bubbling is indicated by the minimum bubbling velocity, Umb . The minimum bubbling velocity strongly depends on the particles properties being dried. This velocity is usually greater than the Umf for fine particles. In this case, a bubble free fluidization regime between Umf and Umb thus exist for particles with fine aggregates where due to their size, an inter-particle forces. plays a significant role. For a fine particle solid, Geldart and Abrahamsen gave a relationship for Umbas For particles with larger diameter, experimental results have shown that Umb is usually less than Umf. When the superficial gas increases further, gas bubbles become larger and slugging is said to occur when the bubbles grow larger to size comparable to the column diameter. The minimum slugging velocity Ums can be estimated by an equation given by Stewart and Davidson as However this equation is applicable for beds with dimension H/D The turbulent and fast fluidization regimes are considered to be high velocity fluidization regimes. 9

10. Two different definitions are used to distinguish between the bubbling regime and the turbulent regime. The first defines Uc, the superficial gas velocity at which the standard deviation of the pressure fluctuations reaches a maximum. At the onset of turbulent regime , Uc is believed to reflect the condition at which bubbles coalescences and break up reach a dynamic balance. The second definition indicates a transition from bubbling to turbulent fluidization based on Uk, a superficial velocity at which the root mean square standard deviation of pressure fluctuation begins to level off with increasing U. The implication of the above is that the bubbles coalescence and breaking up is stabilized. The transition from the turbulent to fast fluidization is said to occur at the transport or settling velocity Ut , where a significant number of particles are carried out from the top of the column. The settling velocity is the air velocity over which transportation by dragging or pneumatic conveying occurs. At this stage, a sudden change of pressure drop with increasing solids flow rate disappears when the superficial gas velocity exceeds Utr . The transition from fast fluidization to pneumatic conveying is characterized by the disappearance of a dense –phase region and large amplitude pressure fluctuation in the bottom sector of the riser. The settling velocity Ut for non-spherical particles can be evaluated using the equ6 as suggested by Kunii and Levens pei as: Ø - shape factor, which accounts for non-spherical particle shape, it is also known as the particle sphericity and dp is the dimensionless particle diameter .it is pertinent to note that besides the gas particle ,it is the size and density of the particles that determines the fluidization velocity needed to obtain a homogeneous fluidized bed. The larger and denser the particle are, the higher the fluidization velocity must be to keep the particle fluidized. 10

11. Fluidized bed system Fluid bed granulation with subsequent fluid bed drying and coating, are the three most important methods to produce granules for pharmaceutical manufacturing. Fluid bed granulating, drying and coating operations use the same mechanical principle by which air is drawn through the machine. The airflow generally is developed by turbine fan suction located at the exhaust end. First air enters through the air handler, where it is conditioned for the process and then it is drawn into the product processing area. The design of gas distributor plate is essential for the optimum behavior of the material in a fluidizing process. Filter bags separate the product from the process air before the air exits into the atmosphere through an outlet air duct. The ideal filter material should retain all of the product particles in the container while allowing process air to pass through. A high-shear mixer can be placed in-line with a fluid bed processor. After the mixture is granulated in a high-shear mixer, the dense material is transported to the fluid-bed dryer to dry and if any type of coating is required then it further passes to the fluid-bed coating. These three unit operations and the transfer between them can be controlled by a single controller. 11 fig 3. Diagram of a traditional fluid bed processing system.

12. Principle of fluidized bed drying This concept is based on the similarity between traditional spray-drying and the top-spray fluid-bed process. Both methods atomize liquid droplets into a chamber of hot air to facilitate drying. The process air volume can be adjusted in the top-spray process. By operating with a large air volume, liquid can be spray-dried in the chamber at an inlet air temperature much lower than that used for traditional spray drying. In the beginning of the fluid-bed spray-drying process, the liquid flow rate must be low to allow for a high evaporation rate. Once spray-dried particles are formed, the rate of liquid addition can be relatively high because of the combined effect of particle agglomeration and the layering/spray-drying of the liquid. In some cases, inert carriers or particles of the raw material can be used to seed the mixture, thereby inducing the formation of spray-dried particles. Finished products obtained from fluid-bed spray-drying generally are larger, have better dispensability and flow ability, and exhibit a narrow particle-size distribution compared with products obtained via traditional spray drying. Principle of fluidized bed granulation Powders are fluidized and a binder solution or suspension is sprayed onto the fluidized particles, creating liquid bridges which form agglomerates from the powder. As soon as the desired size of the agglomerates is achieved, spraying is stopped and the liquid evaporated. The structures created by the liquid bridges are then maintained by solid binder bonds. Whatever has been liquid inside the agglomerates is now void, as such permitting modified size and porosity of the agglomerates for their intended function, e.g. for compression into tables or fast dissolving instant drink applications. The lack of kinetic energy in the agglomeration zone results in light structures with plenty of internal capillaries. 12

13. Principle of fluidized bed coating Fluidization of granules is achieved in a cylindrical chamber by the upward flow of drying air. The airflow is controlled so that the more air enters through the source, causing the granules to rise in the chamber. The granules fall on the wall of chamber and again enter in the airstream. The coating solution is continuously applied from the spray nozzle located either at bottom or tangential to the chamber. The efficient coating is mainly depends upon some of factors - coating material addition rate, nature of coating material. 13

14. Processing Fluidized bed drying The fluidized bed dryer system used is shown in the fig 2 below. It comprises the blower with air discharge capability between 0 – 130m3/mm of air flow, two heating band, a drying chamber and a vent at the top of the chamber. The properties of the air flowing around the drying product are a major factor in determining the rate of removal of moisture. The capacity of air to remove moisture is principally dependent on its initial temperature and humidity, the greater the temperature and lower the humidity, the greater the moisture removal capacity of the air. The air from the blower passes through the heater which in conjunction forms hot air. The changes in air conditions when air is heated and passed through a bed of moist products, produces a drying process. As air moves through the grain bed, it absorbs moisture under adiabatic drying; sensible heat in the air is converted to latent heat. The absorption of moisture by air would be the difference between the absolute humidity’s at each point. When a gas is passed upwards through the material as shown in the fig 2 , the gas will at low flow rates merely enters through the fixed bed of the particles. As the gas velocity increases, the pressure drop across the particle layer will increase in proportion to the gas velocity until the pressure drop reaches the equivalent of the weight of the particles in the bed divided by the area of the bed. At this point, all particles are suspended in the upward flowing gas and the frictional force between particles and gas counter balances the weight of the particles. The layer of particles is now said to be incipiently fluidized, although the homogenous particle layer behaves like a liquid, only moderate particle mixing takes place. When the gas velocity is increased further above Umf , the gas velocity for incipient fluidization, any additional fluidization gas will pass through the particle layer as bubbles. The gas bubbles will be small at the gas distributor, however they coalesce rapidly and rise through the particle layer, causing vigorous mixing of the fluidized particles. At still higher gas velocities, a point is reached at which the drag forces are increased to a degree that the particles becomes entrained within the gas stream and are carried from the fluid beds.     14

15. Fluidized bed granulation As fluidized bed granulator have a same design as that of fluidized bed dryer the powder particles are fluidized in a stream of air, but in addition granulation fluid to be sprayed from nozzle on the bed of powders. Heated and filtered air is blown or sucked thought the bed of unmixed powder to fluidized particles and mixes the powder; fluidisation is actually a very efficient mixing process. Granulating fluid is pumped from reservoir through a spray nozzle position over the bed of particle. The fluid cause the primary powder particle to adhere when the droplet and powder colloid. Escape of material from the granulation chamber is prevented by exhaust filters, which are periodically reintroducing the collected material into the fluidized bed. Sufficient liquid is spray to produce granules of required size, at which point spray is turnoff but the fluidized air is continue. The wet granules are then dried into the heated air stream. Fluidized bed coating Firstly the granules are introduced as fluid bed in the fluidized bed coater; the granules kept flowing in the coating chamber by blowing of airflow through the air inlet. Then from the coating material reservoir the material is travelled through the means of pipes or capillaries. The coating material is then sprayed through the nozzle with a specific velocity. Due to the continuous application of coating material the granules are get coated, then by adjusting the temperature of the inlet air the granules are get dried. 15 Fig 2 Fluidized Bed Dryer system used

16. Equipments In the fluidized bed system the basic equipment used is same in drying , granulation and coating but depending upon the type of operation some modifications are done in the basic design of the fluidized bed reactor. the basic fluidized bed reactor is that of follows: 16 Solids Solids Gas Gas Gas bubbles Solid particles Distributor

17. Depending upon the input direction of the airflow nozzle the fluidized bed is divided into 4 main types Types of fluid beds Fluid beds used drying, granulation and coating are classified according to the nozzle position (top, bottom and tangential) and the operating conditions (continuous or batch).Continuous fluid bed is widely used in the food industry but rarely in the production of enzymes. Batch fluid bed reactors having a cylindrical or conical shape . Air are distributed through a bottom grid with an adequate partition and size of holes. Reactors are equipped with one or more nozzles and sometimes with a mechanical stirrer as well. In general basically four types of fluid beds which are shown in diagram : T op spray bottom spray wurster type rotor with side spray 17 (a) = top spray , (b )=bottom spray , (c )=wurster type , (d )=rotor with side spray

18. Top spraying is the oldest and simplest type of the fluid bed with spray nozzle placed at the top of the chamber and air blowing from bottom. It is still used for wet granulation but the efficiency and quality regarding coating is generally poor and it is now often replaced by bottom spray or the wurster type. Generally, granules prepared by top spray have a looser structure and are more porous than granules prepared by other type of fluid beds. Bottom spraying gives shorter distance between nozzle and bed thereby reducing the premature drying of binder/coater liquid before impact and leads to greater coating efficiency. This type of fluid bed is very important for coating but the risk of unwanted agglomeration during coating is higher than in a top spray due to higher concentration of wet particles. Wurster type fluid bed is the modification of the bottom spray fluid bed. By inserting a fixed cylinder into the chamber the circulation of the particle is changed and the drying rate is increased, reduces the risk of agglomeration. This fluid bed system is particularly suited for coating.Rotor with side spray consists of a disc rotating in the fluidizing chamber and the liquid solutions are added tangentially from the top of the reactor. The combination of rotation and bottom-up air flow provides specific properties such as higher spherical shape and density to resulting particles. This type is mainly used for coating but the quality remain same as that of wurster type of fluid bed. Some of fluidized bed Equipments are given below 18

19. 19 Fig 4: fluidized bed coater

20. 20 Fig 5: fluidized bed granulator

21. Advantages of the fluidized bed system Liquid like bahaviour, easy to control, rapid mixing, uniform temperature and concentration. Resists the rapid temperature change, hence responses slowly to change in operating condition and avoids temperature runaway with exothermic reaction. Applicable for large and small scale operations Heat and mass transfer rate are high, requiring smaller surface. Continuous operation. Easy of process due to stable conditions. Disadvantages of fluidized bed system Bubbling beds of fine particles are difficult to predict and are less deficient. Particle break-up is common. Pipe and vessel walls error due to the collision of the particles. Non uniform flow patterns. Size and type of particles, which can handle by this technique, are limited. Due to the complexity of fluidized bed behavior, there are often difficulties in attempting scale up from smaller scale to industrial grades. 21

22. Applications Application of fluidized bed system: Fluidized beds are used as a technical process which has ability to promote high levels of contact between gases and solids. In fluidized bed a characteristics set of basic properties can be utilized ,indispensible to modern process and chemical engineering .these properties include: Extremely high surface area contact between fluid and solid per unit bed volume. High relative velocities between the fluid and the dispersed solid phase. High level of inter mixing of the particulate phase, frequent particle-particle and particle-wall collision. Taking an example of food processing industry: fluidized beds are used to accelerate fringing in some IQF (individually quick frozen). These fluidized bed tunnels are typically used on small food products like peas, sliced vegetables. The fluid used in the fluidized bed may also contain a fluid of catalytic type; that’s why it is also used to catalyses the chemical reaction and improve the rate of reaction. Fluidized beds are also used for efficient bulk drying of material. Fluidized bed technology in dryers increase the efficiency by allowing for the entire surface of the subject of the drying to the suspended and therefore expose to air. This process can also be combined with heating and cooling as the application, if necessary. Fluidized bed dryers used in drying of various materials such as powder, tablets, granules, coals, fertilizers, plastic material. This process is being used in a granulation of the pharmaceutical powders. Fluidized bed coater are used widely for coating of powders,granules,tablets,pallets,beds held in suspension of column of air. Fluidized bed coating is used for test masking, enteric release and barrier films on tablets and powders. Bottom spray coating is used for sustained release and enteric release products. Fluidized bed system is also employed in non-pharmaceutical fields in comparison to their use in pharmaceutical fields as there are numerous apparatus, process and products parameters that affect the quality of final pharmaceutical products. 22

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