INTRODUCTION:
Fluid bed processing technology was first introduced into the pharmaceutical industry over 39 years ago as a means of rapidly drying powder and granulated materials. Subsequently, in the late 1960’s the addition of spray nozzle allowed fluid bed drying technology to be extended to cover this granulation process.
Fluidized bed processor is widely use equipment is granulation, palletization, drying of pharmaceutical manufacturing. Fluidized bed processor is made up of stainless steel and it contains exhaust air duct, coating chamber, funnel like modification, support screen, air atomization nozzle, liquid feed for nozzle, atomizing air supply, fluidizing air supply. The principle of fluidized bed processor involved in such a technique may be either, top spray, bottom spray, and tangential spray process. It is depending upon position of spray gun in the equipment. The top spray is popular for active granulation and palletization technique.1 The bottom spray (Wurster) fluid bed method is very popular in pharmaceutical industry for active layering and for coating to modify or control drug release because it produces superior film compared with other coating techniques. The tangential spray (Rotary) method has been used for granulating and pelletizing with subsequent coating. Fluidized bed processor is kind of equipment used extensively in pharmaceutical industries to reduce the moisture content of pharmaceutical powders and granules warm air is blown through a perforated distributor to rapidly and gently dry materials until the required residual moisture content is reached. Moisture evaporated from the product is exhausted with the drying air, providing very short processing times dryer capacities is range from 5-kg to 200 kg and the average drying time is about 20-40 min.2 The first equipment reported in the pharmaceutical literature to provide continuous batch wet granulation was fluid bed drying equipment. Which was modified by the addition of spry nozzle or fluid injectors to provide addition of liquid binding and adhesive agents to dry powder materials for powder agglomeration followed by drying in same equipment. Commercial apparatus ranges from laboratory models that, by changing the bowl have a volume between 0.2 and 2 L, giving a capacity of few grams up to about 1 kg, up to production machines. Fluid bed coaters are used in coating material by the fluid bed technology With fluid bed coating, particles are fluidized and coating fluid sprayed on the dried. Small droplets and low viscosity of the spray medium ensure an even product coating. The coating formulation is invariably added as a solution or suspension via spray gun.3
Theory of Fluidization:
The processing technique employing a suspension 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 tool with many applications in the pharmaceutical industries. solid particles by vertically rising liquid stream are lesser interest in modern processing, but have been shown to be use, particularly in liquid contacting of ion exchange resins.
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 granule supported on a grid. provide the material is properly matched to the upward velocity is 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 properties4.
· Lighter float on the top of the bed4
· The surface stays horizontal stays horizontal even in tilted bed
· The solid can flow through an opening in the vessel just like a liquid.
· The beds have a “static” pressure head due to gravity.
· It has a zero angle of repose
· Assume the shape of vessel that contain it
Principle of Fluidization:
The principle of fluidization is based upon the fact that if a gas is allowed to flow through a bed of particulates solid at velocity greater than the settling velocity of the particle and less than a terminal velocity of pneumatic conveying an equal to the minimum velocity of fluidization (Vmf), the solids get partially suspended in the steam of upward moving gas. The gas steam neglects the gravitational pull due to the weight of particles enable the suspended state of solid. The resulting mixture of solid and gas behave like liquid and thus rigidly solids are called fluidized. The solid particles are continually caught up in eddies and fall back in 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.5
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 fluidization, slugging fluidization, turbulent fluidization and fast fluidization and dilute pneumatic conveying regimen 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 particle is suspended by the gas.
The value of Umf is obtained experimentally by determine the level of pressure drop across the bed as the superficial gas velocity in the fixed bed regimen increase.6 Several relationships for the evaluation of the minimum fluidization velocity have been developed. Example include the relationship developed by Linoya. This equation is shown below in the equation 1 as:
When h gas is the gas velocity r is the gas density,
dp the diameter of particles and 
are Ergum parameters depending on the
particle’s sphericity and bed void age at incipient fluidization. Ar is the
Archimeds number defined as:
Where the particle density and g is the gravity. According to Teunou and Poncelet, the Ergun expression can be approximated when the particle diameter exceeds 100µm. in that case, Umf becomes
Umf= dp> 100µm……
The onset of bubbling is indicated by the minimum bubbling velocity, Umb. The minimum bubbling velocity strongly depends on the particles properties being dried. The velocity is usually greater than the Umf for fine particles. in this case, a bubble free fluidization regimen between Umf and Umb thus exist for particles with fine aggregates where due to their size, an inter-particle forces plays a important role.7 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 regimens are considered to be high velocity fluidization regimens. Two different definitions are used to distinguish between the bubbling regime and turbulent regime. The first defines Uc, the superficial gas velocity at which the standard deviations of the pressure fluctuations reach maximum. At the onset of turbulent regime, Uc is believed to reflect the condition at which bubbles coalescences and breakup 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 of 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.8 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 regimen 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 equation 6 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 homogeneous fluidized bed. The larger and denser the particle are, the higher the fluidization velocity must be to keep the particle fluidized.9
Fig:1 Flow pattern in gas- solid fluidized beds a) Fixed bed, b) Bubbling regimen, c) slugging regimen d) turbulent regimen e) fast fluidization f) pneumatic conveying
Conditions for fluidization:
· The minimum velocity at which a bed of particles fluidizes is a crucial parameter needed for the design of any fluidization operation.
· The details of minimum velocity depend upon a number of factors, including the shape, size, density and poly-dispersibility of the particles.
· The density, for example, directly alter the net gravitational force acting on the article, and hence the minimum drag force, or velocity, needed to lift a particle.
· The shape alters not only the relationship between the drag force and velocity, but also the packing properties of the fixed bed and the associated void spaces and velocity of fluid through them.
· To find the minimum fluidizing velocity, Umf, experimental and theoretical approaches can be used.
Advantages of the fluidized bed system
· Liquid like behavior, easy to control, rapid mixing, uniform, temperature and concentration.
· Resist the rapid temperature change, hence responses slowly to change in operation condition and avoids temperature runway with exothermic reaction.
· Applicable for large and large- and small-scale operations.9
· Heat and mass transfer rate high, requiring smaller surface.
· Easy of process due to stable conditions.
· Continuous operation.
Disadvantages of fluidized bed system
· Bubbling beds of fines particles are difficult to predict and are less deficient.
· Particle break-up is common.
· Non-uniform flow patterns.
· Due to the complexity of fluidized bed behavior, there are often difficulties.
· Pipe and vessel walls error due to the collision of the particles.
· Size and type of particles, which can handle by this technique, are limited.
Spraying Techniques of Fluidized Bed Processor:
Depending on the position/ location of the spray nozzle fluid bed processes are characterized by,
1. Top spray
2. Bottom spray
3. Tangential spray
Top Spray:
The top spraying process is commonly used for wet granulation, coating, drying. Spray nozzle is placed above the expansion chamber. The process is employed in the formation of granules and fine powder. The fluidized powder is wetted with fluid/binder/starch paste by using top spraying system.10 The top spraying system contain spraying nozzle precise dosing peristaltic pump. The fluidized powder is wetted until liquid bridge are formed between particles. Segregation of powder doesn’t take place as different powder is glued together on a small scale. Since there is no impinging force in in fluid bed equipment. The granules are weak and having good solubility. Once the granules are made, they are dried in the hot air. This technology used for formed granules for capsule, tableting, free flow properties, dust free process, good dispensability and excellent solubility sue to porous structure. This design of coater is ideal for coating large quantities of powders, pellets, microencapsulation and crystals.11
Bottom spray:
The wurster process is commonly used for to coat particles, sphere, granules and tablets. System have been developed use with a different coating of formulation including aqueous/organic solvents, hot saturated solutions and hot melts.12 The discrete particle in a fluidized bed using differential air flow to produce a cyclic movement of material. The location of spray nozzle at the bottom of fluidized bed of particles sets our wurster process apart from other coating methods.13 with the help of high air pressure the bed of particle moves upward in a cyclic motion inside a chamber as it is coated with an atomized material to create a core-shell structure.14 This process shows that the coating material can be applied efficiently to individual particles while controlling for agglomeration. The process can be continued until the uniform film thickness is achieved.15
Tangential spray:
The nozzle is introduced at the side of product container/expansion chamber during processing. Three mechanical forces cause particle movement, mixing and granulation.16 First, the spinning of disk generates a centrifugal force. Second, a lifting force is generated by the hot air pass to the adjustable disk gap. Third, gravitational force cause material to fall down on to the disk, this force provides proper mixing and result in granules, drying, coating with good content uniformity. The tangential spray is commonly use in granulation process, spheronization layering and coating of pellets.17
Fig: 2 Spraying Techniques a) Top spray b) bottom spray c) Tangential spray