Design criteria for multicyclones in a limited space
Graphical abstract
Introduction
Cyclone separators are widely used in industry for separating solid particles from gases. They are applied preferably for reasons of high operational safety due to their simple and robust construction, usability for product recovery, stable and generally low pressure drops and in many applications comparable low invest and operation costs. In addition, they can be operated under extreme conditions including large gas volume flow rates as well as high solids concentrations, temperatures and pressures, representing a comparative advantage over other gas-solids separation technologies. The minimum particle size that can be collected by a cyclone generally decreases with decreasing cyclone size. This effect is used in multicyclones, i.e. systems of many parallel cyclone cells within a common housing having a common solids hopper for solids discharge. Multicyclones are generally used to increase the separation efficiency beyond the level achievable with a single larger cyclone. Principally increasing the number of parallel cyclone cells and decreasing at the same time their size improves the efficiency of a multicyclone without changing the flow cross-section and without affecting its pressure drop, provided that 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 standard reverse flow cyclone cells with an axial inlet, also called swirl tubes, which are shown in Fig. 1 left (In the strict sense of the word swirl tubes denote that type of a cyclone with a purely cylindrical shape [1]). Often those devices are used for separating fine particles, e.g. as a third stage separator in the fluid catalytic cracking (FCC) process operating at high temperatures and pressures [2,3]. An application at ambient conditions is e.g. the purification of dryer flue gases in the food industry. Analogously multicyclones can be made from uniflow cyclone cells, Fig. 1 right. Those devices are especially used in space-limited applications, e.g. as compact alternatives to the swirl tube multiclone as a tertiary separator in the FCC process [4]. Since decades uniflow multiclones are also applied in combustion engines of agricultural machines, locomotives, construction machines and others where they pre-clean the suction air of the engine before the final filter [5]. Industrial mobile cleaners are a further area of application.
Both cyclone types, the swirl tube and the uniflow cyclone, have axial inlets with swirl vane inserts for vortex generation. Their main difference is that in swirl tubes the gas flow reverses its direction and pure gas and collected particles are leaving the device at opposite sides, whereas uniflow cyclones have gas and particles passing through them in only one direction exiting at the same end of the device. Due to those design differences, uniflow cyclones can be built much more compact and can easily be integrated into pipelines.
Even though the principle of uniflow cyclones has been known for a long time, not much literature has been published on how to design and to calculate them. So far research on uniflow cyclones has been restricted to special applications such as a short-contact time reactor for high solids loadings [6]. To fill this gap systematic experimental studies have been performed recently [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]]. A significant challenge to make uniflow cyclones accessible to general industrial practice is an accurate, reliable and fast design method. For calculating the performance data of conventional reverse flow cyclones, approved analytical models with high precision exist which are often preferred over methods based on numerical simulation with long computing times. With this background, a new analytical approach for calculating the separation efficiency and the pressure drop of uniflow cyclones for practical design work has been developed [26,27]. This approach is based on an equilibrium orbit concept similar to that applied in the Barth-Muschelknautz model for reverse flow cyclones, which has been proven to work successfully in a wide range of industrial applications. More detailed information about this model can be found e.g. in [1,28,29].
Applying the calculation models for standard cyclones and for uniflow cyclones which both are based on the same physical concepts indicated that uniflow cyclones require significantly less space for separating a specified solids-gas mixture than standard reverse flow cyclones but are generally also slightly less efficient, at least in their present state design [30,31]. Experiments comparing the performances of various standard and uniflow cyclone geometries confirm those differences [22]. The model for uniflow cyclones has been validated by extensive experimental data covering various cyclone sizes (diameters between 110 mm and 292 mm), geometries, as well as operation data including various solids feeds and types of solids [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]].
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 limited space available for purifying a given gas volume flow the question arises whether under the restriction of limited space a multicyclone made from uniflow cyclone cells may be preferable over its swirl tube counterpart. This question is addressed by applying well-proven calculation programs for standard cyclones according to [28,29] and for uniflow cyclones as described in [26,27].
Section snippets
Outline of the calculation method for uniflow cyclones
The model for standard cyclones according to [28,29] assumes a limited loading capacity of the gas stream splitting the separation process into two steps. At any solids loading μ in excess of this critical loading μlim, the solids are immediately separated from the gas at the inlet to the cyclone. The solids remaining in the gas are separated in the cyclone barrel and in the inner vortex below the gas outlet tube with a second, inner separation efficiency ηi. The total separation efficiency of
Results and discussion
To compare both multiclone types, a typical industrial application has been considered and investigated in a systematic way: 30 kg/h fine solid particles shall be removed from 14,000 m3/h exhaust gas of a reactor with a gas density of ρg = 1.2 kg/m3 and a gas viscosity of ηg = 2∙10−5 Pa s at a specified cyclone pressure drop. Regarding the pressure drop two cases are studied, 1000 Pa (0.145 Psi) and 3000 Pa (0.290 Psi). A typical application under those conditions is the purification of dryer
Conclusions
Recent studies show that state-of-the-art uniflow cyclones are slightly less efficient than standard reverse flow cyclones regarding purification of a specified gas-solids feed at a given pressure drop. Nevertheless, their more compact construction in comparison to reverse flow cyclones and their applicability as inline separator can make them preferable in applications with limited space. The present investigation compares multiclones using swirl tube cells with those using uniflow cyclone
Nomenclature
- A
Parameter in drag law (−)
- Af
Total friction area in the cyclone (m2)
- Aw
Sedimentation area for wall separation (m2)
- B
Parameter in drag law (−)
- C
Parameter in drag law (−)
- D
Parameter in fractional efficiency curve ηF,i(d,z) (−)
- Dc
Cyclone diameter (m)
- Dcore
Core diameter of swirl vane inserts (m)
- DVF
Vortex finder diameter (m)
- d
Particle size (m)
- dc
Cut-off size (m)
- dc,1
Cut-off size of separation in the inner vortex flow (m)
- dc,2
Cut-off size of separation in the main flow part through the ring chamber (m)
- dc,e
U. Muschelknautz earned his Diploma in Physics from University of Bonn, Germany in 1989, Doctoral Degree in Physics from Max Planck Institute for Solid State Research, Stuttgart, Germany in 1993 and Post-Doc in Physics from Centre National de la Recherche Scientifique (CNRS), Grenoble, France 1993–1994. He has been a Professor of Mechanical Process Engineering and Fluid Dynamics and Head of R&D Particle Removal Technology in MCI Management Center Innsbruck from 2005 to 2014. He is currently
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U. Muschelknautz earned his Diploma in Physics from University of Bonn, Germany in 1989, Doctoral Degree in Physics from Max Planck Institute for Solid State Research, Stuttgart, Germany in 1993 and Post-Doc in Physics from Centre National de la Recherche Scientifique (CNRS), Grenoble, France 1993–1994. He has been a Professor of Mechanical Process Engineering and Fluid Dynamics and Head of R&D Particle Removal Technology in MCI Management Center Innsbruck from 2005 to 2014. He is currently working as a Managing Director of MK Engineering, Heidelberg, Germany since 1994. His research interests are Cyclone Separators, Pneumatic Conveying, Scrubbers, Filters, Fluidization, Multi-phase flow modelling.