(652d) Cyclone Design and Performance at Restriced Space

Cyclone separators are widely used in industry for separating solid particles from gases. The standard cyclone has gas and particles exiting the device at opposite sides and is either equipped with a tangential inlet or – mostly for the usage in multiclones - with an axial inlet. A competitive but less explored alternative to the standard cyclone is the uniflow cyclone. Like a pipe, a uniflow cyclone has gas and particles passing through it in only one direction. Like in the standard cyclone the vortex flow is generated either by swirl vane inserts or by a tangential inlet at the entrance of the device. In contrast to standard cyclones clean gas and separated particles leave the uniflow cyclone at the same side. Compared to standard cyclones, uniflow cyclones are much more compact. This design reduces their investment costs and makes them particularly interesting for applications with limited space. It also allows a simple and cost-effective implementation in piping systems. Even though the principle of uniflow cyclones has long been known, not much literature has been published on how to design and to calculate them. In recent years comprehensive experimental and theoretical studies of uniflow cyclones have strongly improved their understanding and led to approved design criteria and calculation methods similar to those which have already been achieved for standard reverse flow cyclones.

Often space for a dedusting unit in an industrial plant is limited. If saving of space is of high priority or if there is even only a limited space available for purifying a given gas volume flow the question arises whether under the restriction of restricted space uniflow cyclones may be preferable over standard cyclones. This question is addressed by applying well proven calculation programs for standard cyclones according to [1,2] and for uniflow cyclones as described in [3-5]. The results indicate in agreement with experiments:

For separating a specified gas solids mixture at a specified pressure drop
- a uniflow cyclone requires significantly less space (diameter and length) than a standard reverse flow cyclone but is generally also slightly less efficient, at least in its present state design.
- the efficiencies of the standard cyclone and of the uniflow cyclone approach each other with increasing gas volume flow, i.e. decreasing cyclone size, and furthermore with increasing pressure drop, particle size and/or particle density.
- a uniflow cyclone can be more efficient than a standard cyclone if space for dedusting the gas solids flow is limited and the available pressure drop is low. In other words, uniflow cyclones can achieve higher separation efficiencies per volume than standard cyclones for low pressure drops.

A deeper understanding of those results is obtained by analyzing the principal differences between standard cyclones and uniflow cyclones regarding their performance data. This analysis includes amongst others the quantitative relationship between the gas and particle velocities and the performance data of both cyclone types. Its implication on erosion potentials in both cyclone types is discussed as well.

The results described above are transferable to multicyclone systems, i.e. systems of many parallel cyclone cells within a common housing having a common solids hopper for solids discharge. Principally increasing the number of parallel cyclone cells and decreasing at the same time their size improves the efficiency of a multiclone without changing the base area and without affecting the pressure drop, provided a uniform distribution of the gas and the solids feed into each single cyclone cell can be achieved and bypass flows through the solids discharge openings from one cyclone cell to the other can be avoided. In many cases multicyclones use swirl tube cells. Analogously, multicyclones can be made from uniflow cyclone cells. Often these devices are used in space-limited applications, e.g. as third-stage separators in fluid catalytic cracking (FCC) processes [6-8]. Multiclones using uniflow cyclone cells at ambient conditions are applied e.g. in combustion engines of agricultural machines, locomotives, construction machines and others for pre-cleaning the suction air of the engine before the final filter [9,10]. Industrial mobile cleaners are a further area of application.

[1] E. Muschelknautz, V. Greif, Cyclones and other gas-solids separators, in Circulating Fluidized Beds (Eds: J. R. Grace, A. A. Avidan, T. M. Knowlton), Blackie Acad. & Profess., London (1997) 181-213.
[2] VDI-Wärmeatlas Atlas, Zyklone zum Abscheiden fester Partikel aus Gasen. 12th German ed., 2018.
[3] U. Muschelknautz, Analytical approach for calculating the separation efficiency of uniflow cyclones, Chem. Eng. Technol. 35 (12) (2012) 2099-2106.
[4] U. Muschelknautz, Performance comparison of uniflow cyclones and standard cyclones, Proceedings of 12th International conference on fluidized bed technology, Krakow, 421-428, 2017.
[5] U. Muschelknautz, Design criteria of multi cyclones in a limited space. Proceedings of 8th World Congress on Particle Technology, Orlando, USA, 2018.
[6] W. Peng, A. C. Hoffmann, H. Dries, Separation Characteristics of Swirl-Tube Dust Separators, AIChE J. 50 (1) (2004) 87-96.
[7] M. Kraxner, U. Muschelknautz, S. B. R. Karri, R. Cocco, T. M. Knowlton, Applicability of a Uniflow Cyclone as a Third Stage Separator in the FCC-Process. AIChE – American Institute of Chemical Engineers – Annual Meeting, Minneapolis / Minnesota, USA, 2011.
[8] S. Krishnamurthy, W. Koves, Apparatuses and methods for gas-solid separations using cyclones. Uop Llc, Patent US-8419835 B2, 2013.
[9] V. Greif, Multizyklon-Blöcke zur Staubabscheidung vor Verbrennungsmotoren – Auslegung und Betriebserfahrungen. VDI Berichte 1511 (1999), p. 215-226.
[10] M. Kraxner, Empirical evaluation of design criteria for uniflow cyclones in multicyclone boxes, PhD Thesis, TU Munich, Germany, 2013.