Authors: Ray A. Cocco a, Ulrich Muschelknautz b, Ben Freireich a, S. B. Reddy Karri a, and Ted Knowlton a
a Particulate Solid Research, Inc. 4201 W. 36th Street, Building A, Chicago, IL 60632 USA
b MK Engineering, Heinrich-Fuchs-Str. 101, 69126 Heidelberg, Germany
Source: This paper was published in the 13th International Conference on Fluidized Bed Technology in 2021.
Abstract: With fluidized bed, circulating fluidized bed (CFB) and even pneumatic conveying operations, reverse-flow cyclones are a critical component in a successful process. Although the designing principle for cyclones is straightforward, the hydrodynamics in these units are not. This poor understanding of the hydrodynamics has limited improved cyclone operations which is why cyclone designs has not changed significantly in 150 years.
This is especially true with the symmetrical design of the cyclone. With the exception of the inlet, everything else on a cyclone is symmetric. For example, the gas outlet tube (also called the vortex finder) immersed into the cyclone, also called the vortex finder, is located concentrically, i.e. its axis coincides with the axis of the cyclone. However, it has been shown in numerous recirculating cyclones of CFB power plants that eccentric positioning of the vortex finder axis improves the cyclone efficiency without affecting the pressure drop [Muschelknautz and Muschelknautz, 1999; Ispen, et al., 2014]. By modifying the vortex finder, the circulating material became finer and its quantity increased resulting in a considerable improvement of the boiler performance, the heat transfer in the combustion chamber, improved the optimum combustion temperature to ~900°C, and reduced the amount of unburnt filter ash. In addition, this optimization led to cost savings of operational costs.
Trefz  gave a qualitative explanation of the positive effect of eccentrically shifting the vortex finder axis on the cyclone efficiency based on flow measurements in cyclones. At the top of the cyclone barrel region, there is a secondary flow through the boundary layer of about 10% of the total flow carrying particles towards the center. The vortex finder forces this secondary flow around the tube down to the opening allowing to remove particles from that flow by centrifugal force. In cyclones with a tangential inlet and a concentric vortex finder, however, this separation is not optimal. The axis of the vortex flow is pushed away from the geometrical center of the cyclone by the incoming gas flow and as a consequence the tangential velocity of the flow around the vortex finder is not symmetrical. In the section of the vortex finder surface which is closer to the cyclone axis, the tangential velocity stagnates and the secondary flow streams vertically downwards with practically no circumferential velocity. In this section, the particle separation is poor. By shifting the vortex finder axis into the “eye” of the vortex a more uniform flow around the vortex finder is achieved and its separation efficiency improves considerably.
Recently, numerical granular-fluid simulations of high loaded cyclones with a concentric and with an eccentric vortex finder have been performed by applying the CFD code Barracuda Virtual Reactor® of CPFD, LLC. The CFD results confirmed that eccentrically shifting the vortex finder results in a better collection efficiency without compromising the pressure drop. The calculations were compared with measurement data from commercial high loaded cyclones in CFB boilers and the origin of the improvement due to the eccentric shifting the vortex finder was studied.