(5a) An Empirical Comparison of Two Different Cyclone Designs in the Usage of a Third Stage Separator

Actually the Third Stage Separator (TSS) in a FCC-process is mainly constructed as a standard reverse flow cyclone which can be designed by approved calculation models, as for example by using the model of Barth/Muschelknautz [1]. The FCC-process efficiency mainly depends on the cyclone performance i.e. on its separation efficiency and its pressure loss. The conical shape of the lower separation body of a reverse flow cyclone leads to high tangential velocities and therefore to high erosion which strongly affects its life time. A second cyclone type, which is not yet well investigated, is the uniflow cyclone. Its essential advantages compared to the reverse flow cyclone are its compact construction and its easy integration into pipes. A further advantage is its low pressure loss and therefore its low energy consumption. Postma et al. investigated the usage of uniflow cyclones (swirl tubes) in the FCC-Process, which requires high process reliability [2]. A study on the applicability of uniflow cyclones as a TSS was carried out in 2010 at PSRI laboratories and showed acceptable performance data for the usage within FCC-process engineering [3].

Standard Reverse Flow Cyclones vs. Uniflow Cyclones

The simplicity of design and the lack of moving parts in cyclone separators are key to their ubiquitous presence in process industry. In some applications, where high gas flow rates must be processed, the physical size of a conventional reverse flow cyclonic unit can become very large. The uniflow cyclone or axial cyclone, is a interesting alternate design, which is able to deliver comparable efficiencies in a very compact geometry. The name is derived from the flow field pattern generated inside the cyclone. Unlike reverse flow cyclone, the gas enters and leaves the cyclone in the same direction (see Figure 1).

Fig. 1: Depiction of a uniflow cyclone and its flow field pattern.

Compared to a reverse flow cyclone design, the uniflow cyclones can be built approximately two-third smaller in volume at comparable efficiencies. Wide spread adoption of this design has been limited by the absence of a universally accepted and validated design approach. The existing literature mainly focuses on special applications, as shown in the research work of Gauthier et al. or Zhang et al. [4,5]. The highly turbulent particle flow patterns offer a complex parameter system which makes the dimensional analysis rather complicated. For instance, numerous design variations of the inlet swirl generator (Figure 1) are possible, and each approach will result in remarkably different flow field within the cyclone. Therefore, an empirical approach is most apt for studying uniflow cyclones.

To facilitate the understanding of its operation, the design structure of a uniflow cyclone can be divided into four major sections in the direction of the flow, namely Swirl Vane Inlet, Separation Chamber, Vortex Finder and Particle Outlet.

State of the art

Even though the working principle of this cyclone type is well-known, there is hardly any literature which investigates the design, the estimated efficiency or pressure drop. To close this gap the MCI is driving systematic research activities since 2008 to improve the separation efficiency of uniflow cyclones by optimizing the geometric design [6,7,8,9]. As outcome of these activities, there are several applications established in process and automotive industry, as for example in air-pre-cleaning systems for internal combustion engines used in off-road vehicles (e.g. building and agricultural engines).

Experimental

To compare the two cyclone systems on a fair basis, the executed test series show data for efficiency and pressure drop and the erosion potential at comparable operating conditions. The experimental testing is performed on a standard reverse flow cyclone and a uniflow cyclone with various operation configurations differing from each other regarding the vortex finder diameter, the volume flow rate and pressure recovering systems. A test stand which allows comparing effectiveness and optical accessibility for phenomenological observations is set up at MCI laboratories, by using selected FCC-Powder which typically reaches the Third Stage Separator.

References:

[1] VDI Heat Atlas, 11th Edition. VDI-Book. Berlin [u.a.]: Springer Vieweg, 2013.

[2] Postma, R.S., Hofmann, A.C., Dries, H.W.A., Williams, C.P., The Use of Swirl Tubes for Dedusting, World Congress on Particle Technology 3, Brighton, UK, 1998.

[3] Kraxner, M., Muschelknautz, U., Karri, S.B.R., Cocco, R., Knowlton, T.M., Applicability of a Uniflow Cyclone as a Third Stage Separator in the FCC-Process, AIChE - American Institute of Chemical Engineers - Annual Meeting, Minneapolis / MN, USA, 2011.

[4] Gauthier, T.A., Briens, C.I., Bergougnou, .A., Galtier, P., Uniflow cyclone efficiency study, Powder Technology, Nr.62: 217-225, 1990.

[5] Zhang, Y., Wang, X., Riskowski, G.L., Christianson, L.L., Ford, S.E. Particle separation efficiency of a uniflow deduster with different types of dust, Transactions of ASHRAE, 1999.

[6] Muschelknautz, U., Pattis, P., Reinalter, M., Kraxner, M., Design Criteria of Uniflow Cyclones for the Separation of Solid Particles from Gases, CFB10 – 10th International Conference on Circulating Fluidized Beds and Fluidized Bed Technology, Sunriver / OR, USA, 2011.

[7] Kraxner, M., U. Muschelknautz, S. Wechner, S. Ackermann, V. Greif, J. Bolda, Influence of the Inlet Vane Geometry on the Uniflow Cyclones Performance, AIChE-Annual Meeting, Pittsburgh / PA, USA, 2012.

[8] Kraxner, M., Skarke, B., Kofler, T., Pillei, M., Pressure Drop in Uniflow Cyclones: Investigation on an Empirical Calculation Model, CFB-11, Beijing, China, 2014.

[9] Kraxner, M., Empirical Evaluation of Design Criteria’s for Uniflow Cyclones in Multicycloneboxes, Doctor Thesis, TU München, Germany, 2013.