Question 8.12: Cyclone Separator The waste gas from a combustion process c......
Cyclone Separator
The waste gas from a combustion process contains particles that are separated in a cyclone separator prior to discharge into the atmosphere. The waste gas with a density of 0.9 kgm^{–3} contains a range of particle sizes as shown in Table 8.2. For the particle size distribution and grade efficiency of the cyclone, determine the overall efficiency of the separator.
TABLE 8.2
| Particle Distribution Size and Cyclone Grade Efficiency | ||||||
| Size range (μm) | 0–5 | 5-10 | 10-20 | 20-40 | 40-80 | 80-160 |
| Mass (g) per 100 g | 8 | 14 | 33 | 24 | 12 | 9 |
| Grade efficiency (%) | 25 | 45 | 75 | 90 | 95 | 100 |
A cyclone separator is a device used to separate particles from air or a gas stream and is commonly used in spray drying to collect the dried product. It consists of a vertical cylindrical section with a tapering conical section beneath as shown in Figure 8.9 and is used to separate particle sizes typically ranging from 5 to 200 μm, which enter the separator tangentially with a high velocity of up to 30 ms^{–1} . A vortex flow is therefore created inside the cyclone. The direction of the flow of the outer vortex is downward and close to the cyclone walls, but the flow reverses in the lower part of the cyclone to rise up the centre in a vortex rotating about a turbulent central core, with the gas flow leaving via the outlet in the top. The separated particle-free air or gas leaves the top of the separator. The particles leave via the bottom for collection and recovery.
Taking a basis of 100 g of particles retained, the cumulative mass retained is calculated as shown in Table 8.3, for which the overall efficiency is therefore 77%.
TABLE 8.3
| Particle Size Distribution | ||||||
| Size range (μm) | 0-5 | 5-10 | 10-20 | 20-40 | 40-80 | 80-160 |
| Retained per 100 g | 1.7 | 5.2 | 27.2 | 23.4 | 10.4 | 9.0 |
| Cumulative retained per 100 g | 1.7 | 6.9 | 34.1 | 57.5 | 68.0 | 77.0 |
| Emitted per 100 g | 5.25 | 13.0 | 19.8 | 22.1 | 23.0 | 23.0 |
