FBD Working Principle
Functions Before Fluid Bed Drying Process
The precondition for an air heating unit is to heat the main tower. Compression air inspection is done to assure that joints formed by inflation gaskets are leakproof.
The main function is done by the following processes
- Loading
- Air Inlet
- Fluidization
- Drying
- Filter Bags Shaking
Loading
The initial startup step in a fluid bed dryer is to add a fresh batch of wet granules to the product chamber. The Feeding material can be sucked from the high shear mixer chamber through a feeding tube under negative pressure.
Air Inlet
The Blower is switched ON from the control panel. As the blower becomes operational so it draws the air continuously from Air Heating Unit and into the tower through the lower plenum.
Fluidization
Inlet air is blown through a static powder bed. As the velocity of the air increases so does the space between the powder particles until and unless the powder particles become suspended in a bed.
Drying
The drying process is taking place in further three stages until and unless the endpoint is achieved. At the endpoint, the solid particles moisture level is equal to or less than 1%.
- Preheating
Wet particles hang in dry and hot air currents. By conventional heating of the body of the particle, the surface moisture evaporates as heat flows through the body. The speed of drying increases as the particles absorb more heat. The moisture lost during preheating is less but the temperature of the bed rises steadily.
- Constant Rate
It is called a constant rate because the rate of drying remains constant as a function of time. By plotting a graph, the amount of moisture lost versus time gives a linear graph. In the pre-heating stage, the bed’s temperature was rising but in this stage, particles remain in the same temperature. It is the case because any energy supplied is taken up by the evaporating molecule as the transition of the liquid to the gaseous state.
- Falling Rate
In this stage usually particle losses, most of their moisture and are near the drying endpoint. The surface moisture is reduced any remaining moisture is drawn from the porous core through capillary action. As the temperature of the vapor decreases, so does the temperature of the bed. Drying stops when the moisture level drops from less than 1%.
Fluidized Bed Dryer is working on the principle of fluidization of the feed material. Fluidization is the introduction of hot air at high pressure through a perforated bed of moist solid particles.
In the fluidized bed dryer, the particles are particularly suspended in a mobile gas stream (Hot Air), the air serves as a boiling liquid in the sense that the particles are first lifted upward and then they fall back. This method provides better heat transfer and better mass transfer. This mixture of solid particles and air behave like a liquid, and the solid is said to be fluidized. This method is more efficient for the drying of granular solids. In this method each granule is surrounded by hot air, so the material is effectively dried.
Filter Bags Shaking
When the blower is functional it draws and increases airspeed from the fluidized bed dryer. The airflow has very small elements called fines. If these particles pass the filter bag so they will be expelled to the outside surrounding environment. The fine will be capture in the filter bag hole. These fines cause the formation of a dust layer that accumulates the filter bag and causes a dropping in pressure. The dust layer is removed by mechanical shaking. This is done by pneumatic cylinders at regular intervals.
Exhausting
It is also called secondary filtering unit. Air exhausted from the FBD may go through the second filtering stage. It captures all those fine particles that are passed from the filter bag.
Discharge of The Dried Material
Discharge is the removal of dried material from the FBD. Discharging can be done manually by opening the lock and wheeling the product container on its trolley to the next process equipment.
Parameters to Be Controlled in Fluidized Bed Dryers System
There are three main parameters to be controlled during Drying Process
Process Parameters
Position of The Heaters
The position of Heaters should be adjusted to gain desired flow of air with the desired temperature. It should be adjusted as that neither should be very closed nor away. If it is placed too much closed to the outlet of the blower, then the desired temperature will not be achieved.
Temperature
Drying time is inversely proportional to the inlet air temperature. The drying speed depends upon the internal temperature of the solid bed. Air temperature also has an impact on moisture diffusion in drying. At high temperature, the particle absorbs more heat for evaporation and increasing evaporation rate and in low temperature, less heat is absorbed and the evaporation rate is decreased.
Humidity
The drying process is inversely proportional to the air humidity. For faster drying, the humidity should be at a minimum. Humidity can be reduced by making the particle size small due to less internal diameter.
Air Flow Speed
Airflow speed should be controlled for efficient fluidization to occur. For an efficient drying process, airflow speed should be optimized. If the speed is too high, then the gas will not be heated to our target temperature. And if it is too slow then fluidization will not occur.
Air Velocity
The exhaust of air from the blower should be achievable. At high air velocity, the moisture level of the particle rapidly decreases. At high air velocity, the mass and heat transfer in the bed can be easily carried and as a result, easy fluidization occurs. An increase in the bubble size of the bed occurs with an increase in the air velocity. The functionality of the fluidized bed dryer dominates by the velocity of the gas than the temperature.
Apparatus Parameters
- This type of parameter is related to the structure of the apparatus
- Air distribution inside the apparatus is influenced by the position of the plate
- The Shape of the apparatus in the conical base type gives better productivity and fluidity
- In the case of Coating, Nozzle height plays a vital
- For the operation process, both negative and positive pressure plays an important role
Product Parameters
- Drying time increases in case of the high initial moisture content of a material
- The batch should be small and optimized for better results