(5c) Design Criteria for Multi Cyclones in a Limited Space

Multi cyclones are generally applied with the intention to separate a gas-solids feed with a higher efficiency than what may be achieved by applying a single cyclone consuming the same energy. Principally, increasing the number of parallel cyclone cells and decreasing at the same time their size improves the efficiency of a multi cyclone without changing its pressure drop, provided a uniform distribution of the gas and solids feed into each single cyclone cell can be achieved and bypass flows of gas through the solids discharge openings from one cyclone cell to the other within the common housing can be avoided.

Multi cyclones are made up either from standard cyclone cells or from uniflow cyclone cells, mostly equipped with axial inlets for vortex generation, Fig. 1. The latter devices are appropriate in space limited applications, e.g. as third stage separators in fluid catalytic cracking (FCC) processes [1]. In standard cyclones the gas flow reverses its direction. Pure gas and collected particles are leaving the device at opposite sides. In contrast, uniflow cyclones have gas and particles passing through them in only one direction making them much more compact than standard cyclones and easily to integrate into pipe lines. However, their efficiency for separating a specified gas-solids flow at a given pressure drop is generally slightly lower than that of standard cyclones [2]. Consequently, a multicyclone consisting of uniflow cyclone cells has usually a somewhat lower efficiency than its standard cyclone counterpart, but needs also less space, if both devices have the same number of cells.

Fig. 1: Multi cyclones consisting of 16 standard cyclone cells (left) and of 36 uniflow cyclone cells (right) using approximately the same cross sectional area of the housing.

Often space available for an industrial dedusting device is limited. Under the restriction of limited space a multi cyclone made from uniflow cyclone cells can have decisive advantages over a standard cyclone solution. One of those advantages is that under certain conditions a uniflow cyclone solution can separate a specified gas-solids feed with a similar or even higher efficiency than a multi cyclone based on standard cyclone cells while having the same pressure drop and the advantage of easy integration into pipe lines at the same time. This may occur when uniflow cyclone cells are allowed to occupy the same volume as their standard cyclone counterpart. It arises from the fact that due to their compact design more uniflow cyclone cells can be installed into a given volume than standard cyclone cells would have place in it. The conditions regarding geometry and operation data leading to this result are identified by a systematic analysis applying well proven calculation models for standard cyclones according to [3] and uniflow cyclones as described in [2]. Constraints arising from installation costs as well as from operation issues e.g. regarding maldistribution of the gas-solids flow onto the single cyclone cells and potential erosion are taken into consideration.

References

[1] Krishnamurthy, S., Koves, W.,2013: Apparatuses and methods for gas-sold separations using cyclones. Uop Llc. Patent US-8419835 B2.

[2] Muschelknautz, U., 2017: Performance comparison of uniflow cyclones and standard cyclones. Proceedings of 12^th International conference on fluidized bed technology, Krakow, 421-428.

[3] VDI Heat Atlas, 2010: Cyclones for the precipitation of solid particles. Springer-Verlag, 2^nd Edition, Chapter L3.4, 1226-1237.