Air Separator
Principle
Similar to cyclone separators, the air separator also works on the same principle. Powders containing very fine particles are sometimes difficult to separate by cyclone separators. The combination of air current and centrifugal force is made in such cases. Due to centrifugal force, resulting in separation of fines from air and coarse particles, which are then discarded. These fines are collected at the bottom.Construction
This device consists of a conical bottom mounted on a vertical cylindrical metal vessel, Figure. An upper part of the vessel has a tangentially mounted feed inlet. A central outlet for fluids is located at the top portion of the conical portion, while a base outlet is located at the base. To prevent air short-circuiting directly from the inlet into the outlet, the fluid outlet pipe extends down below the inlet section. In the center of the vessel, rotating discs and rotating blades are attached to the shaft. Fine and coarse/heavy particles are discharged from separate bottom outlets.
Working
In the upper part of the vessel, the feed enters tangentially through the inlet. The feed falls on the blades as they rotate. An air jet is produced by the rotating blades, as indicated in Figure above. Fine particles are blown away on the walls by the centrifugal force generated by the jet, to the bottom, where they are collected. A greater number of coarser particles requires a shorter distance from the center of the separator. As a result, they fall into a sediment collection zone at the discharge of the separator.Uses
- Process streams are often collected by dust collectors to extract valuable granular solids or powders.
- By using it, submicron size particles can be separated that are too small to be handled by sieving.
- Often used as an air pollution control device, it is used to maintain or improve air quality in pharmaceutical production areas.
- A mist collector removes fine liquid droplets of particulate matter from the air to improve the quality of the workplace air.
- In order to vent exhaust gases to the atmosphere, granular solid pollutants need to be removed from them.
- Coolant and oil mists, as well as metal working fluids, can be collected using this device.
Merits
- Installing it is simple.
- It has a variable rotor speed and adjustable airflow.
- The product output is high.
- Cleaning and maintenance are simple.
- As far as output and product quality are concerned, it outperforms sieving.
- Batch and continuous modes of operation are available.
Demerits
- The efficiency decreases if the particles are too fine (<5 µm).
- Wet and sticky materials are not suitable for it.
Bag Filter
Principle
Two stages are involved in the separation process. Through suction, the gas-containing dust is sucked through a cloth bag (i.e., a bag that contains dust) and separation results. Pressure is then applied to the bags to allow the powder adhering to them to fall, collecting at the bottom.Construction
In a metal container are suspended a series of cotton and wool bags. Feed is received in the bottom portion of the device in the form of a hopper. On top of the vessel, there is a bell-crank lever arrangement that changes the filtering action into shaking.
Working
The gas containing dust enters the hopper during the filtering period. After that, it is inserted inside the bags and atop the instrument. In this apparatus, a vacuum fan creates a pressure lower than atmospheric pressure. When the bag holds particles, they are trapped.Shaking period - The bell-crank lever rotates during this time and changes the damper's position. Outside air enters the metal casing through the top, thereby breaking the vacuum. Furthermore, the bags shake violently when shaken or jerked. Dust or fine particles are dislodged from the bags as a result of these shaking movements. In order to collect dust to the maximum extent possible, the hopper is further withdrawn from the conical base.
Uses
- Dust particles are separated from dusty gases by bag filters in large industrial units. Likewise, fabric dust collectors are bag filters.
- Fabric dust collectors are the most cost-effective and efficient types of industrial dust collectors.
- Since bag filters can reach 99% efficiency, they are regarded as the most efficient dust collectors.
- Cyclone separators are often used with bag filters.
- Together, they clear the air within a room.
Merits
- It is possible to achieve very high collection efficiencies (99.9%+) by varying the grain loading and particle size at the inlet. In a certain range of particle size and concentration, fabric collectors maintain a constant static pressure and efficiency, unlike other types of single dust collectors.
- The combustion fuel sulfur content does not affect the efficiency of the collector as it does with ESPs.
- Sensitivity to particle size distributions has been reduced.
- It does not require high voltage.
- It is possible to collect flammable dust.
- It is possible to remove smoke and fumes at sub-micron levels by using special fibers or filter aids.
- There are a number of collector configurations, sizes, and inlet and outlet locations to choose from.
Demerits
- A high acid or alkaline atmosphere may significantly shorten fabric life, especially at high temperatures.
- Unless special materials are used, materials can only withstand a maximum of 550 degrees Fahrenheit.
- The condensation of moisture or the collection of hygroscopic materials can result in fabric plugging, low cleaning efficiency, or large pressure losses.
- To minimize cake leakage or assist in the removal of certain dusts, special fabric treatments may be required.
- Fabric bags are prone to melting or burning at extreme temperatures.
Elutriation
Principle
In the process of elutriation, particles fall into a rising fluid and are categorized into two sizes. Sorting columns move fluid at a certain speed to separate particles. The particles with the highest terminal velocities sink to the bottom, and the particles with the lowest terminal velocities rise to the top and are lifted away. The terminal velocity of particles in fluids can be calculated using Stokes' law for Reynolds number.
In this equation, ρ is the specific gravity of the particle, ρo is the specific gravity of the fluid medium, r is the radius of the particle, η is the viscosity of the medium, Vm is the terminal velocity of the particle, and g is the gravitational acceleration.
In the presence of constant volumetric flow rates of rising fluids during elutriation, the diameter of the columns determines the velocity of rising fluids. If the diameter of the column is narrow, the fluid velocity will be high while if the diameter is wide, the fluid velocity will be low. A rising medium with higher velocities enables coarser particles to settle, whereas one with lower velocities allows finer particles to settle. Various classes of particles can be obtained by separating the sample into columns of increasing diameter and connecting them in series. According to Stokes's Law, the particles cannot exceed a certain size.
Construction
A gravity system basically consists of a vertical column with an intake for suspensions near the bottom, an outlet for coarse particles at the base, and an overflow for fluids near the top, as shown in Figure. Despite having only one column for separation, a velocity gradient extends the length of the tube, resulting in the particles being separated based on their sizes. The procedure described in Figure can be repeated if more than one fraction is required.
As the area of the cross-section increases in successive tubes with the same flow rate, the velocity will decrease, resulting in fractions.
Working
Separating particles requires first levigating the material and then transferring the paste to an elution tank. Particles are settled independently in a large tank filled with water. The contents of the tank are stirred to ensure uniform distribution of particles. Unless disturbed, unsettled coarse particles will sink to the bottom whereas unsettled small particles will remain suspended in the liquid. As such, these fines can be moved to the next elutriator, where the same separation process can be carried out to obtain more fractions of fines.Uses
- Either a simple or multiple elutriators can be used for size reduction purposes following a size reduction process, with the goal of separating large particles, which may then be returned to be ground again, used for another purpose, or discarded.
- Solids like kaolin or chalk can be separated using this method with liquids, which is often followed by wet grinding and sedimentation or elution.
- Gases may be used to separate finer solids that are too slow to separate in liquids, to dissolve water-soluble substances, or to improve the drying process.
Merits
- A continuous process is involved.
- Sedimentation produces a slower separation.
- It is possible to add several columns based on the fractions needed.
- No skills are required.
- Sedimentation is a slower process.
Demerits
- Sometimes, dilution of the suspension may not be desirable.
- Rather than separating particles according to their features (for example, surface or shape), it divides them according to their sedimentation properties.
- This method cannot separate particles of the same type with the same sedimentation property.
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