Inertial Separation Using Cyclone
A cyclone is a hot gas cleaning device used in all types of gasifiers [1]. The inertial separation devices like cyclones operate using mass and acceleration principles for separation of heavier solids from lighter gases. A cyclone can be operated at temperatures in excess of 1000°C, and is one of the oldest and most commonly employed devices for solids separation in syngas. They utilize centripetal acceleration to reduce the long times otherwise required for small particles to settle by gravity. A basic design of a cyclone used in the syngas cleaning is shown in Figure 11.5 [1]. As shown, the gas stream enters a "double vortex" that first forces particulate outward and downward in an outer vortex. This outer swirling motion separates particulate matter from the vapors by inertial forces. The gas stream is then redirected into an inner and upward moving vortex before exiting the device through a "vortex finder." Several approaches to cyclone design are based on the characteristics of particles and the gas stream [24]. In general, a "cut point" is
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Figure 11.5 Cyclone used in biomass syngas cleaning. (Reprinted with permission from reference [1]; copyright 2013 Elsevier). |
established where a certain size particle obtains a balance between centrifugal and drag forces. Even though cyclones are a mature technology, process advancements are still occurring in this field.
There are some new designs in cyclones, and one new design operates as a reverse flow gas cyclone using partial recirculation, and it has shown a separation efficiency that is superior to the classical Stairmand high efficiency (HE) designs [24]. Simple design and lack of moving parts are important features in cyclones. Cyclones can be operated at high temperature, which is typically limited only by mechanical strength and stability of the construction materials. They are often operated hot to prevent condensation of water, tar, and other contaminants that might otherwise foul or corrode the cyclone. With their robust nature and efficient removal of particulate matter larger than 5 mm, cyclones are typically the first cleanup device applied to a gas stream. However, many processes require more stringent particulate matter removal down from sizes below 1 mm. Therefore, gas cleaned in the cyclone is typically sent to a filtration-type gas cleaning system. These second-stage cleanup systems can remove much smaller particles in the gas stream [1].