Solid Gas separation (cyclone - filtration)
The separation of gas and the solids that it carries are of prime importance in process industries. Solids can be in the gas stream on purpose, as it is for example for the pneumatic conveying line, in case the recuperation of 100% of the solid at the end of the transport phase is required to have an economical process. But the solids can also be present as pollution, as a dust, and growing environmental concerns will require to have a dust collection system to clean the gas prior to its emission to the atmosphere.
Different technologies of separation exist and fit different purposes. Cyclones, Scrubbers and filters are presented in this webpage.
Such collection phenomena is well represented by the Stokes number which can be calculated for a particle in a stream of fluid
Equation 1 : Stokes Number
Sk = Inertial force / viscosity force
ρD=specific gravity of dispersed phase (solids) (kg/m3)
uCD=uC-uD=Difference of velocity between dispersed phase and continuous phase (m/s)
d=diameter of particles (m) - if not all same dimension, take d50
Cm=correction of Cunningham - Millikan of the drag coefficient
μc=viscosity of the continuous phase (Pa.s)
Dc=characteristic dimension of the separation equipment
If the Stokes number is low, it means that the particle has a tendency to follow the fluid when it is changing direction : it will be difficult to separate them in a separator built under an inertia principle.
What happens is the following : when the flow of particles + fluid is exposed to a certain target (for example, a plate), particles with a too large diameter (= having a high Stokes number) will not be able to follow the fluid lines around the obstacle and will be collected by the obstable by inertial impact. The 1st diameter at which particles will not be collected is the "cut" diameter of the separator.
For very small particles (diameter < 1 micron), the physical phenomena that dominates is the brownian movement. Diffusional transfer, that can lead to an interception of the small particles, will be in competition with convective transfer.
If the convective transport is high (high fluid velocity), the interception will not be possible. As a consequence, catching very small particle will require to operate at low fluid velocity in order to let the opportunity to particles to travel by brownian movement to a target that will collect them.
It is a special collection mode used typically in separator having a disengagement volume, especially for large particles. The particles will sediment due to a higher terminal settling velocity compared to the gas velocity.
The table below is listing the key Solid - Gas separation process equipment that can be found in process industries
Table 1 : Industrial Solid - Gas separators
|Equipment||Detailed in this page|
All these equipment can be used as industrial dust collectors. Some of them, especially cyclones coupled to filters can be positionned at the end of pneumatic conveying lines for recovering the product being transferred. Scrubbers or electrostatic precipitators are more used on very diluted stream, as dust removal systems in order to ensure the dust control for air leaving the process.
Cyclones are very simple equipment based on an inertial separation principle. Gas and particles enter in the cyclone (usually constituted of a cylindrical body, a conical outlet for solids and a top axial pipe outlet for gas) tangentially.
A vortex is created due to the centrifugal force that applies and the gas circles down to the conical bottom. Particles with a too large diameter are then pushed against the wall and separated from the gas. In the conical part, the gas flow reverses its direction to go up via the central part of the cyclone, and exits on the top by the gas outlet pipe. The solids are travelling down the wall and are collected at the bottom of the conical part.
It should be noted that the gas, when reversing its direction, can pick-up again some solids (re-entrainment) which would reduce the separation efficiency. In order to avoid such phenomena, the design of the cyclone and its operation (make sure there is never solids accumulated in the cone) are of prime importance.
Figure 1 : Cyclone design and principle of separation
Scrubbers are very efficient Solid - Gas separators which are working on the principle of contacting the particles with a liquid (mostly water). Where cyclones are often limited to particle sizes > 1 micron, scrubbers can separate particles well below 1 micron. Their efficiency is mainly due to the fact that they are combining different interception mechanisms
- Intertial (particles hit the wetted internal - packing sometimes - of the scrubber)
- Interception (particles are collected by the water droplets or wetted surfaces if they pass close enough)
- Diffusion (for very small particles < 0.3 micron
In some cases, those mechanisms are reinforced by others due to the presence of humidity (with condensation on a particle for example, or agglomeration of particles).
Figure 1 : Scrubber design and principle of separation
Scrubbers present however a very big disadvantage : they generate large amount of slurries which then needs to be further treated. Nowadays, considering the environmental requirement, the use of scrubbers must be reserved to applications where an easy use of the effluents is possible or where other separator techniques can definitively not be used.
Gas and Solid can also be separated on a solid media called filter. There are different mechanism playing in the separatino of gas and solids, and the physics is actually changing when the filter is new or has been used for some time.
For a new filter, the particles will be able to penetrate deep in the filter structure and be collected (or not, filters, as other techniques of separation, have a certain efficiency) by inertia and diffusion.
During use, particles will accumulate in and on the surface of the filter, and will finally form a cake. Subsequent filtration is therefore the effect of the filtration inside the filter but also through the cake that has been formed. The more the quantity of particles, the more the pressure drop will be high. It will then be necessary to replace or clean the filter.
Cleaning is sometimes performed automatically by applying a reverse flow of gas. It is particularly useful in bag filters located at the reception of pneumatic conveying lines. For pneumatic conveying lines, the high loads of solids and the process purpose make filtration the only choice for collecting the product ; it is sometimes combined with a cyclone in order to avoid to saturate the filter too quickly.
A key sizing parameter for filters is the filtration velocity (also called gas to cloth ratio). It is usually pretty low around 1 - 5 cm/s depending on the technology ; with some exceptions like the pre-filters of HVAC that can go up to 1.5 m/s.
Principles of Powder Technology
Operations Unitaires Mecaniques Depoussierage Devesiculage, N. Midoux, Cours ENSIC
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