Fluid bed dryer
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In a fluid bed dryer, good contact between hot air and particles to be dried so obtained which causes rapid drying.
If a gas is allowed to flow up wards through a bed of solids particles at a velocity greater than the setting velocity of the particles, the particles are partially suspended in the gas stream.
The resultant mixture of solids and gas gehave like a liquid and the solids are said to be fluidised. Each individual solid partilces is surrounded by the drying gas with the result that the drying takes palce in a much shorter period.
Mover over the intense mixing between the solids and hot provides a uniform conditions of temperature, composition and particle size distribution.
Two types of fluidized bed dryers are used in the pharmaceutical industry. There are
1. Vertical fluidized bed dryers
2. Horizontal fluidized bed dryers
In the fluidized bed dryer, hot air or gas is passed at high pressure through a perforated bottom of the container containing granules to be dried.
The granules are suspended in the stream of air and are lifted from the bottom. This condition is called fluidized state.
The hot air is surrounded every granules to completely dry them. Thus materials or granules are uniformly dried.
Two types of fluidized bed dryers are available in the pharmaceutical industry. There are vertical fluidized bed dryers and horizontal fluidized bed dryers.
The construction of the vertical fluidized bed dryer is made up of the stain less steel or plastic. A detachable bowl is placed at the bottom of the dryer, which is used for charging and discharging of the materials.
The bowl has a perforated bottom with a wire mesh support for placing the materials to be dried. A fan is mounted in the upper part for circulating hot air. Fresh air inlet, prefilter and heat exchanger are connected serially to heat the air to the required temperatures.
The temperature of the hot air and exit air are monitered. Bag filters are placed above the drying bowl for the recovery of the fines.
The air flow is adjusted by means of recirculation control and fabric bags are provided to prevent the passage of the fine particles. This type of the fluidised bed dryer is a batch type dryer and the drying chamber is removed from the unit for the charging and dumping.
The different capacities ranging from 5 kg to 200 kg with an average drying time of about 20 - 40 min of the fluidised bed dryers are available
Horizontal vibrating conveyer fluidised bed dryers are used for continuous drying of a large volume of the materials.
The wet granules to be dried are placed in a detachable bowl. The bowl is pushed in to the dryer. Fresh air is allowed to pass through a prefilter, which subsequently gets heated by passing through a heat exchanger.
The hot air flows through the bottom of the bowl. Simultaneously fan is allowed to rotate. The air velocity is gradually increased. When the velocity of the air is greater than the settling velocity of granules, the granules remains partially suspended in the gas stream.
After some times a point of pressure is reached at which the frictional drag on the particles is equal to the force of the gravity. The granules rise in the container because of high velocity gas 1.5 to 7.5 m per min and later fall back in a random boiling motion.
This condition is said to be fluidised state. The gas surrounds every granules to completely dry them. The air leaves the dryer by passing through the bag filters. The entrained particles remain adhered to the inside the surface of the bags. Periogically the bags are shaken to remove the entrained particles.
Intense mixing between the granules and hot gas is provided uniform conditions of the temperature, composition and particle size distribution. Drying is achieved at constant rate and falling period is very short. Any attempt to increase the air velocity may result in entrainment.
The residence time for the drying is about the 40 min. the materials is left for the some times in the dryer for reaching ambient temperature. The bowl is taken out for the discharging. The end product is free flowing.
u It is used for the popularly drying of the granules in the production of the tablets.
u It is used for the three operation such as mixing, granulation and drying.
u It is modified for coating of granules.
1) It takes less time to complete drying that is 20 to 40 mins. Compared to 24 hours of tray dryer.
2) Handling time is also short. It is 15 times faster than the tray dryer.
3) It is available in different sizes with the drying capacity ranging from 5 to 200 kg per hour.
4) The drying containers are mobile, making handling simple and reducing labour costs.
5) The thermal efficiency is 2 to 6 times greater than the tray dryer.
6) It is also used for the mixing the ingredients and its maixing efficiency is also high.
7) Hot spots are not observed in the dryer because of its excellent mixing and drying capacities.
8) Higher drying temperatures can be used that are not posible in tray dryer and drying capacities.
9) It facilitates the drying of thermolabile substances since the contact time for the drying is short.
10) It can be used either as batch type or continuous type.
11) It has a high out put from a small floor space.
12) The free movement of the individuals particles eliminates the risk of soluble materials migrating as may occurs in static beds
1) Many organic powders develops electrostatic charges during drying. To avoid this efficient electrical earthing of the dryer is essential.
2) The turbelence of the fluidsed state of granules may causes attrition of some materials resulting in the production of fines.
3) But using a suitable binding agents this problem can be solved. Fine particles may become entrained and must be collected by bag filters.
Plug flow dryer:
It is rectangular fluid bed dryer having different compartments for fluidisation. The materials is made to move from inlets through different compartments to outlet. Different drying conditions can be maintained in the compartments. Often the last compartments is fluidised with cold gas to cool the solids before discharge.
1. Mehta RM, Pharmaceutics-I: introduction to the drying process (1996), Page no: 188 - 190.
2. C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices - filtration, 2001, page no. 397 - 400.