Separation and Purification Technology 179 (2017) 236–247

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Separation and Purification Technology

journal homepage: www.elsevier .com/locate /seppur

Analysis of the effect of counter-cone location on cyclone separatorefficiency

http://dx.doi.org/10.1016/j.seppur.2017.02.0121383-5866/� 2017 Elsevier B.V. All rights reserved.

E-mail address: m.wasilewski@po.opole.pl

Marek WasilewskiFaculty of Production Engineering and Logistics, Opole University of Technology, 76 Proszkowska St., 45-758 Opole, Poland

a r t i c l e i n f o

Article history:Received 13 September 2016Received in revised form 3 February 2017Accepted 5 February 2017Available online 8 February 2017

Keywords:Cyclone separatorModeling flow phenomenaCounter-coneKiln gas bypass systemsClinker burningCFD

a b s t r a c t

This paper analyzes the possibility of optimizing the structure of cyclone separators through the applica-tion of an additional compartment in the form of a counter-cone. Analysis of studies published so far indi-cates that few of them concern such improvements for cyclone separators, and those that do focus solelyon determining the effect of a counter-cone on the efficiency of solid particle separation. This paper alsoassesses the effect of a counter-cone on pressure drop. Furthermore, the risk of excessive agglomerationof particles in the lower area of the conical part of the cyclone due to the proposed structural modifica-tions was assessed. Based on the obtained study results, was proposed a method for the determination ofthe optimum counter-cone location.Fifteen variants of geometric configurations of the counter-cone were tested using two study methods -

Computational Fluid Dynamics (CFD) and experimental research. The Reynolds-averaged Navier–Stokesequations with the Reynolds stress turbulence model (RSM) were used in the analysis. Cyclone separatorsfrom a real industrial installation, i.e. a kiln gas bypass system for cement clinker burning, were used forthe research. This allowed for additional validation of the numerical models used and parametrization ofcalculation and boundary conditions.The application of a counter-cone was found to benefit the basic parameter characterizing cyclone sep-

arators, i.e. separation efficiency.� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Number of studies have addressed the development andimprovement of cyclone separators. Initially, such studies involvedexperimental and analytical research. Many methods for calculat-ing pressure drop and the efficiency of solid particle separationin cyclone separators (i.e. two parameters characterizing the effi-ciency of these devices) have been developed. The most commonlyused models that allow determining the first of these parametersinclude the models developed by Stairmand [1], Barth [2], Shep-herd and Lapple [3] or Casal and Martinez [4]. On the other hand,for example, Barth [2], Rietema [5], Mothes and Löffler [6] andMuschelknautz [7,8] proposed the models that help to determinethe separation efficiency – with a small share of the solid phase.A separate set of data required to design cyclone separators arerelationships that describe the characteristic geometrical dimen-sions (e.g., a, b, De, S, h, Hc, H or B). In this case, the results obtainedin studies by Lapple, Stairmand and Swift may prove useful. Wanget al. [9] discuss the geometrical relationships described by theseresearch studies. The precise geometric dimensioning is very

important, since it is directly reflected in the efficiency of cyclone– often also in the operating costs. Chiefly, there should be a searchfor a compromise between high efficiency of separation and lowpressure drop. Svarovsky [10] proposed the relationship betweenthese parameters, which allow designing a ‘reasonable’ cyclonedesigns.

With the rapid development of computer technology andimproved access to increasingly efficient computational hardware,a new research method has emerged that helps solve the problemof multiphase flows inside cyclones, termed computational fluiddynamics (CFD). Boysan et al. [11] were the first to use CFDmethodcalculations for the analysis of flow in a cyclone and to calculateseparation efficiency. A review of studies on the subject allowsthe conclusion that the researchers used different numerical mod-els. Some authors used the k-e RNGmodel [12–15]. The RSMmodelis the most common model for the study of cyclone separators. Anexample of this type of research are studies [16–21]. Its mainadvantage rests in the possibility to map turbulent stress with ahighly anisotropic character. The RSM model enables more precisetest results to be obtained in comparison to the k-e RNG model.Azadi et al. [22] compared these two models. Their study resultsshow greater precision of the RSM model in comparison to

Nomenclature

a height of the gas inletb width of the gas inletB diameter of the cyclone lower (dust) outletBh distance from the base of the counter-cone to the lower

outletCD drag coefficientCFD computational fluid dynamicsdp diameter of a particleD cyclone body diameterDe diameter of the cyclone gas outletDs counter-cone diameterDPM discrete phase modelDij the stress diffusion termFk momentum transport coefficientg acceleration of gravityGi inlet particle mass flow rateh height of the cyclone cylindrical sectionH total height of the cycloneHc cyclone cone part heightk turbulence kinetic energyP pressurePij the shear production termPRESTO Pressure Staggering Option

p0 dispersion pressureQi inlet gas volumetric flow rateRANS Reynolds average Navier–StokesRSM Reynolds stress models the source termsd distance between the counter-cone and the conical part

of the cycloneS height of the outlet duct in the interior of the cycloneSIMPLE semi-implicit method pressure-linked equationst timeui(j, k) gas velocity to direction i (j, k)up particle velocityu0

i(j, k) fluctuating velocity to direction i (j, k)a apex angle of the counter-coneDP pressure drop in a cyclone separatord Kronecker factoreij the dissipation termm dynamic viscosity of gasPij the pressure-strain correlation termq density of gasqp density o a particlesij the Reynolds stress tensor

Fig. 1. Location of the counter-cone and a method for determining its dimensions,according to Muschelknautz [28].

M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247 237

experimental research. Moreover, the authors of some studiescombine these two models. The work of Houben and Pirker [23]is an example here. At the initial stage of simulation, they usedthe k-e RNG model to stabilize the flow, and then switched tothe RSM model.

In recent years, a third model – Large Eddy Simulation (LES) – isgaining in popularity in the study of cyclones. There are two vari-ants of this model – Finite Volume Large Eddy Simulation (FV-LES)and Lattice-Boltzmann Large Eddy Simulation (LB-LES). Gronaldand Derksen [24] made a comparison of these two variants withRANS-RSM model. They pointed some limitations of RANS model.In turn, Pirker et al. [25] proposed a hybrid approach – they useda LB-LES and RANS-RSM model in their study. Other studies thatused FV-LES model are [26,27].

One of the methods for optimizing the structure of cyclone sep-arators in cases where the objective function involves maximizingseparation effectiveness is to equip the traditional structures withan additional element in the form of a counter-cone in the lowerarea of the separator. This solution can be applied in most typesof cyclones. The goal of the additional element (also called a Chi-nese hat, a vortex stabilizer or an apex cone) is to limit secondary cir-culation of the separated particles. Hoffmann & Stein [28]concluded that secondary circulation results from the vortex tail(the place where the axial direction of movement changes) movingback from the lower particle tank and adhering to the wall of theconical part of the separator. Due to rotational movement, the solidparticles that drop to the tank are moved by the rising gas. Analysisof studies on the issue [29–37] (presented below) enables theobservation that, while using such additional design solutions,the counter-cone diameter, apex angle and correct location shouldbe precisely selected. Incorrect selection of these parameters mayhamper the drop of the material to the tank, reduce separation effi-ciency, and increase the risk of blockage in the lower outlet of thecyclone.

Analysis of studies on the subject leads to the conclusion thatfew researchers have focused on methods for selecting counter-cone dimensions or a scientific description of the effects of applica-tion of the additional compartment. Muschelknautz, the author ofmany research papers on the design of cyclone separators, defined

[29] the location of the counter-cone (above the lower outlet) and amethod for determining its parameters (Fig. 1).

In contrast, Kabsch [30] suggests that the counter-cone belocated below the conical part of the separator. Krambrock [31]proposes a similar location to Kabsch, adding that the diameterof the counter-cone should be marginally greater than thediameter of the outlet tube (De), and the apex cone should be equalto 90�.

Kepa conducted studies [32,33] in which he compared the twosuggested locations of the counter-cone (below the cyclone outletand in the conical part of the cyclone) using CFD. He concluded thatplacing the counter-cone below the conical part is more beneficial,as it helps increase separation efficiency.

In turn, Yoshida et al. [34–36] studied the apex angle within the40–80� range. They found that the optimal value of the angle foroptimizing separation efficiency is 70�. Furthermore, they observedthat increasing the gap between the counter-cone and the wall ofthe conical part of the cyclone increased the amount of small par-ticles that can be extracted into the particle tank.

238 M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247

Obermair et al. [37] analyzed two structural variants of acounter-cone. In the first variant, the apex angle was 90�, and thedevice itself was located under the lower outlet of the cyclone. Inthe second variant, the apex angle was 120�, and the counter-cone was located over the outlet of the cyclone. The latter solutionled to an increase in separation efficiency (by 2%) and an increasein pressure drop by about 200 Pa (the value of about 1200 Pa up to1400 Pa).

The present study analyzed and described multiphase flowinside a cyclone separator equipped with a counter-cone. Imple-menting this sort of improvement is one of the simplest methodsof increasing separation efficiency - it may be used both withnew cyclone separators which are being designed and improvingexisting. The cost of this improvement also is not high and it doesnot require for a cyclone separator to be offline for a long period oftime in case of improving existing and operating installations. Themain goal of the study was the elaboration of a universal methodenabling determination of the optimum location of the counter-cone. To this end, analysis of the effect of the geometric dimensionsand location of an additional barrier on the efficiency of cycloneseparators was necessary.

2. Materials and methods

Since exploitation data from real industrial installations fre-quently show considerable discrepancies with research results, itwas decided that this study would be based on the actual opera-tional conditions of a particular installation. In order to accomplishthis task, was selected the cyclone separator design used for thekiln gas bypass system in one of the cement plants in Poland(Fig. 2). The task of gas bypass systems used in the systems ofdry clinker burning is the extraction of a part of the process gassesout of the rotary kiln. This allows a reduction in the formation of

Fig. 2. Flow sheet of the bypass system in the

growths in cyclones of individual stages of the cyclone suspensionpreheater. This negative phenomenon stems from the formation ofclosed alkali circuits (condensed volatile compounds from theexchange column are returned to the rotary kiln, where, due tothe high temperature, they become volatilized). Now, in mostinstallations around the world the amount of gasses removed tobypass ranges from 5% to 10%. Particle-laden gasses enter thebypass system, where they undergo intense cooling with cool air.This allows the volatile components to condense on the surfaceof the raw material grains present in the kiln off-gasses. Next,the particles are separated in cyclone separators and directed out-side the system. The extraction of gasses into the bypass systeminvolves heat loss. One percent of the extracted gasses correspondsto a loss of about 15–25 kJ/kg � cl. Therefore, the percentage shareof the bypassed gasses should be precisely adjusted to the condi-tions inside the installation, and the individual devices that makeup the bypass system should be optimized.

Key elements of the bypass system are cyclone separators, thetask of which is to extract volatile components deposited on solidparticles and vent them outside the system. Analysis of the struc-ture of these devices has showed that the system used cycloneswith a helical-roof inlet. Such cyclone designs are common in avariety of industry branches. Moreover, this constitutes an advan-tage of the tested design – the usefulness of the obtained resultsmay not be restricted to the construction of cyclones used inbypass systems. Fig. 3 shows the geometry of the tested cyclonewith particular emphasis on the key element of the analyzeddevice. In the following stage, installation balance was performedand the granulometric distribution of dust supplied with gassesto the bypass system was determined. For the analyzed installa-tion, the separation efficiency was 89% and the value of pressuredrop 1250 Pa. As the cyclones, which are operating in a dual sys-tem, had an identical geometry, the study was conducted on only

installation for cement clinker burning.

Fig. 3. Base variant of the cyclone (mm).

Table 1Flow parameters included in the CFD study.

Gaseous phase Solid phase

Qi 5.23 m3/s Gi 0.18 kg/sq 1.225 kg/m3 qp 2800 kg/m3

M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247 239

one such device. The gaseous and solid phases were divided pro-portionally. Based on collected data, flow parameters included inthe study were defined (Table 1).

Fig. 4. Schematic diagram of the experimental setup. 1. Inlet pipe; 2. Flow meter; 3. Feed8. Air exhaust fan; 9. Bag filter.

2.1. Experimental research

In order to conduct experimental research, an experimentalinstallation was designed and constructed (Fig. 4). The test standwas equipped with two installations which supplied the testmedia. The gas phase was supplied to the system via a fanequipped with a frequency inverter, which allowed for regula-tion of the fan’s rpm. On the other hand, the solid phase wassupplied using a feeder. In order to eliminate the phenomenonof non-separated particles leaving the system, the installationwas equipped with a fabric filter. The main element of the teststand was a cyclone separator constructed of acrylic glass, themain component of which was PMMA. Its geometrical design(relations of characteristic dimensions) corresponded to the realstructure used in a bypass system. The model used in the exper-imental study was performed in the scale of 1:3. Like in the caseof model scale, a flow similarity (1:3) was applied and thishelped to maintain the ratio of gas phase and solid phase share(Gi/Qi). The inlet velocity of cyclone in both cases was 15 m/s.Additionally, equal ratio between density of solid phase andthe gas phase was used.

The measurements started, when the flow was established andlasted 30 s. The value of the pressure drop was determined viameasurement of the pressure difference at the inlet and outlet ofthe cyclone. The separation efficiency was defined via the determi-nation of the difference between the solid phase mass supplied tothe system and the weight of separated solids. In addition, aftereach measurement series, solid phase separated in the lower tankwas analyzed for particle size, which allowed determining the effi-ciency of separation efficiency of particular solid fractions. In orderto limit measurement errors, tests for each variant were conductedin three repetitions, and mean values were assumed for theanalysis.

Fifteen modifications to the structure of the separator weredesigned and analyzed. The modifications involved the installationof a counter-cone with different geometrical parameters inside thecyclone and these were proposed based on an analysis of theresults of other studies on the subject. As other researchers suggest[28–36] that the location of the additional element plays the key

ing system; 4. Pressure sensors; 5. Cyclone separator 6. Dust hopper; 7. Outlet pipe;

Table 2Description of the proposed variants of the location and geometry of the counter-cone.

Oznaczenie Bh Ds [mm] a [�]

a1 0.15B 368 85a2 0.15B 436 95a3 0.15B 520 105b1 0.1B 368 85b2 0.1B 436 95b3 0.1B 520 105c1 �0.1B 368 85c2 �0.1B 436 95c3 �0.1B 520 105d1 �0.15B 368 85d2 �0.15B 436 95d3 �0.15B 520 105e1 �0.35B 368 85e2 �0.35B 436 95e3 �0.35B 520 105

240 M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247

role, five counter-cone locations were proposed (denoted in thepaper as a,b,c,d,e). Its location was marked Bh. In order to apply auniversal character to the conducted study, the value of the param-eter was made dependent on the diameter of the lower outlet ofcyclone B. Depending on the location of the counter-cone, the Bh

value was from �0.35 B (variant e) to 0.15 B (variant a). Thisparameter constituted the basis for the proposal of the optimumcounter-cone location. Moreover, for each of the five counter-cone locations three apex angles were proposed. They weremarked with the symbols 1 (85�), 2 (95�) and 3 (105�). This further

Fig. 5. Variants of the applied modific

enabled a testing of the effect of the cone apex on the efficiency ofcyclone separators. A full summary of the performed changes ispresented in Table 2 and in Fig. 5.

2.2. CFD studies

The study was conducted based on CFD using the finite volumemethod with the Ansys Fluent 14 software package. The pressure-based segregated solver method was applied. Differential equa-tions were solved using the Semi-Implicit Method for PressureLinked Equations (SIMPLE) algorithm to accurately determine thecoupling of the pressure and velocity fields, in order to satisfythe continuity equation for momentum. The second-order upwindinterpolation method was used to determine the representativesamples of the constituent values on the surface of the control vol-umes. Turbulent flow was modeled using the RANS model andaccording to the closing hypothesis (detailed model) for the Rey-nolds Stress Model (RSM). The unsteady solver was used with atime step of 0.001 s. The standard wall function was used to solveturbulent flow problems in the wall regions. The reflect boundaryconditions were used for the walls. Collisions between particlesand the walls of the cyclone were assumed to be perfectly elastic(coefficient of restitution is equal to 1) [16,38].

The presence of the solid phase was modeled using the Euler-Lagrange method, applied through Ansys Fluent 14 as the discretephase model (DPM). The DPM with one-way coupling was used forthe description of the dispersed phase. Hoekstra method [39] wasadopted as a criterion, which allows determining the effectivenessof separation of particles of various diameters. The particles, which

ations (internal dimensions, mm).

Table 3Details of CFD settings.

Inlet Inlet velocity 15 m/sTurbulence intensity 5%Hydraulic diameter 0.57 m

Outlet Outflow boundary condition hasbeen used for the outlet.

The walls of the cyclone The standard walls functionDiscrete phase model One-way coupling

8 fractions distributed over theinlet surface 15 lm–200 lmStochastic trackingMax. number of steps 50,000Step length factor 5

Pressure-Velocity Coupling SIMPLETerms of convergence: Continuity 10�6

Terms of convergence: Other 10�3

Spatial discretization: Pressure PRESTOSpatial discretization: Momentum,

Turbulent kinetic energy, Turbulentdissipation rate, Reynolds stresses

Second order upwind

Turbulence model: RANS

@ui

@xi¼ 0 ð1Þ

quj@ui

@xj¼ � @P

@xiþ @

@xjþ l @ui

@xjþ @uj

@xi

� �� �þ @sij

@xjð2Þ

sij ¼ �qu0iu

0j ð3Þ

[40–42]Detailed model: RSM, Adopted standard numerical coefficients rk, Cl, re, Ce1,

Ce2, k

@

@tðqu0

iu0jÞ þ

@

@xkðquku0

iu0jÞ ¼ Dij þ Pij þ pij þ eij þ s ð4Þ

Dij ¼ � @

@xkqu0

iu0ju

0k þ ðp0u0

jÞdik þ ðp0u0iÞdjk � l @

@xku0iu

0j

� �� �ð5Þ

Pij ¼ �q u0iu

0k

@uj

@xkþ u0

ju0k

@ui

@xk

� �ð6Þ

pij ¼ p0 @ui

@xjþ @uj

@xi

� �ð7Þ

eij ¼ �2l@u0

i

@xk

@u0i

@xkð8Þ

[41–43]Two-phase flow: Discrete phase model

Fk ¼18l qpd

2p

qpd2p

� CDRep24

ð9Þ

CD ¼

24Re for Rep 6 124ð1þ0:15Re0:687p Þ for 16Rep61000

Rep

0:44 for Rep > 1000

8>><>>:

ð10Þ

Rep ¼dpq up � uj��

l ð11Þ

[41,42,44,45]

M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247 241

reached the lower tank, were considered as separated, and thosewhich left the cyclone through vortex finder were counted asunseparated.

Table 3 provides detailed information about the establishedconditions for the CFD modeling.

During initial research, three densities (cell numbers of about330,000, about 570,000 and about 720,000) of the hexagonalmeshes were analyzed for the base structure of the cyclone. A dif-ferent variable mesh density was applied depending by the cycloneto take into account areas with the highest flow disturbances. Itwas found that the value of about 570,000 ensured high compli-ance, compared to the results from an industrial installation andexperimental studies, with the possible low load of the computingunit. A summary of the obtained values is presented in Table 4. Inthe next step of the research, hexagonal meshes were generated forall analyzed geometrical variants. The density of these meshes ran-ged from a cell number of about 558,000 to about 579,000. The dif-ferences resulted from the proposed structural modifications. Fig. 6shows the computational mesh and the cell number according toelement quality for the base variant.

3. Results and discussion

The first step involved validation of the obtained results usingCFD tests and experimental research was also conducted. The anal-ysis concerned the value of pressure drop and separation effi-ciency. A comparative procedure was carried out for all variants.For the base variant, an additional comparison of the obtainedresults was possible with the use of two test methods, with thedata originating from the industrial installation. The conductedvalidation was aimed at quantitative estimation of the errors stem-ming from the assumptions in the numerical model andparametrization of the calculation and boundary conditions. Theobtained values of pressure drop and separation efficiency for thebase variant show a broad convergence. Mean error for the pres-sure drop was 7%; maximum 10% and minimum 5%. On the otherhand, these values for separation efficiency were 3.5%, 3.9%, and3.1%, respectively. The comparison of CFD test results and valuesfrom industrial installations for the base variant revealed that errorfor the pressure drop was approximately 7% and for the separationefficiency approximately 6%. Detailed values of the parameters dis-cussed in the validation process are presented in Tables 4 and 5.The consistent values obtained for pressure drop and separationefficiency for the base variant show the correct selection of the cal-culation process parameters. Slight differences may stem fromimperfections in the calculation models, which require the intro-duction of certain simplifications and empirical coefficients withhigh levels of uncertainty.

The basic indicators used to assess the proposed modificationsand their effect on the efficiency of cyclone separators were asfollows:

� change in separation efficiency as a result of the introducedmodifications;

� change in pressure drop as a result of the introducedmodifications;

� concentration of solid particles in the area of the counter-cone.

The third of these parameters formed the basis for determiningto what extent the proposed modifications may have affected therisk of excessive agglomeration of particles in the lower area ofthe conical part of the cyclone (due to the introduction of thecounter-cone).

Table 5 shows the values obtained for separation efficiency andpressure drop for the proposed variants.

Table 4Analysis of computational mesh density sensitivity.

Mesh Cell numbers Total separation efficiency [%] Pressure drop [Pa]

CFD results Exp. results Industrial installation result CFD results Exp. results Industrial installation result

1 330,000 83.15 87 89 1055 1210 12502 570,000 83.65 13443 720,000 83.55 1377

Fig. 6. Discretization of the computational area for the base variant according to element quality.

Table 5Values for separation efficiency and pressure drop for each variant.

Variant of modification Total separation efficiency [%] Pressure drop [Pa]

CFD results Exp. Results Results compatibility [%] CFD results Exp. Results Results compatibility [%]

Base 83.65 87 96.1 1344 1210 90.0a1 86.63 89.9 96.4 1367 1257 92.0a2 86.13 89.2 96.6 1372 1272 92.7a3 85.87 89 96.5 1390 1295 93.2b1 86.52 89.7 96.5 1374 1280 93.2b2 86.04 89.2 96.5 1384 1284 92.8b3 85.34 88.5 96.4 1389 1290 92.9c1 85.21 88.3 96.5 1372 1275 92.9c2 84.97 88 96.6 1375 1280 93.1c3 84.53 87.5 96.6 1378 1282 93.0d1 84.92 87,9 96.6 1377 1280 93.0d2 84.75 87.8 96.5 1391 1302 93.6d3 84.55 87.5 96.6 1400 1330 95.0e1 84.66 87.6 96.6 1379 1285 93.2e2 84.57 87.5 96.7 1397 1315 94.1e3 84.93 87.8 96.7 1371 1271 92.7

242 M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247

M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247 243

Analysis of results began with the key parameter, i.e. separationefficiency. All proposed variants were found to provide higher sep-aration efficiency than the base variant (in relation to the valueobtained through CFD). Variant a1 was most favorable, whereasthe lowest values were obtained for variant c3. Considering thevalues (presented in Table 5) obtained for the proposed geometricvariants, it can be observed that the counter-cone location was themost significant. The highest values were obtained for modification(Bh = 0.15B). Separation efficiency decreased as the counter-conewas lowered. Placing the entirety of the counter-cone below thecyclone outlet (modification e) was found to be the least beneficial.The apex angle of the counter-cone also had a significant effect.Analyzing the proposed apex angle for particular positions ofcounter-cone, it can be concluded that the angle of 85� (denotedby 1) ensured the highest values of separation efficiency. For vari-ants a and b (for which the highest values of separation efficiencywere obtained), this angle ensured more efficient solid particles

Fig. 7. Separation efficiency of each variant ac

Fig. 8. Separation efficiency of each variant acc

removal by about 0.5% (for the results of CFD) compared to anangle of 95�. Increasing the angle to 105� caused a further deterio-ration in the value of this parameter (by 0.26% relative to the angle95� for variant a, and 0.7% relative to angle 95� for variant b). Thedeterioration of the values obtained with an increasing apex angleis probably due to the reduction in the gap between the counter-cone and the wall of the conical part of cyclone. This may adverselyaffect the discharge of the fine particle to the lower tank. Theexception is the value obtained when the counter-cone waslocated entirely in the lower tank (variant e). In this case, the high-est value was obtained for the angle of 105�.

An additional analysis of separation efficiency for the individualdiameters of solid particles was necessary to fully describe theeffect of geometrical modifications on separation efficiency. Fig. 7shows values of the parameter for individual fractions of the solidobtained using CFD modeling, whereas Fig. 8 demonstrates exper-imental research results. The figure includes particle diameters of

cording to particle diameter (CFD results).

ording to particle diameter (Exp. results).

244 M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247

�60 lm for better clarity (above this value, separation efficiencyequalled 100% for all variants).

The obtained values show that the separation efficiency of thesmallest particles (dp = 15 mm) was of key importance, as theseparticles led to the highest differences resulting from the intro-duced modifications. The highest separation efficiency values wererecorded for the a1 variant (59% of the CFD tests and 71% of exper-imental research) and lowest for the c3 variant (51% for CFD and62% for experimental). These values are considerably higher thanin the base variant (50% for CFD tests and 60% for experimentalresearch). Fig. 9 presents partial separation efficiency for varianta1 and base variant. Larger fractions of solid particles caused minorchanges as a result of the introduced modifications. Changes in theseparation efficiency values for both test methods (CFD and exper-imental) have similar trends. However, values obtained with the

Fig. 9. Partial separation efficiency

Fig. 10. Pressure drop va

experimental method are higher. This may stem from the effectof the formation of fine particle agglomerations of the solid bodyand collisions between the particles. Especially the phenomenonof fine particles agglomeration on wall surface can play a large rolein the case. Analyzing the studies [46–48], one might encounterthe term of ‘fishhook’– describing the phenomenon of fine solidparticles agglomeration on cyclone walls – explaining the causeof the differences in the results of the two research methods. Gro-nald [48] detailed in his study the three types of particles agglom-eration: one large cluster to which the small particles adhere to, achain of small particles, and the many small particles that adhereto each other. The mapping of these phenomena in the case ofCFD study is difficult. It requires the use of additional models,which contributes to a significant increase in the load of calcula-tion units. Some researchers undertook the study in this area. An

for variant a1 and base variant.

lues for each variant.

Fig. 11. Distribution of static pressure for variants a1 and d3.

Fig. 12. Velocity vectors in the lower area

M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247 245

example can be the study [47] in which Houben focused on the lastof the abovementioned types of particles agglomeration. It is worthto note that this phenomenon can be very important, especially fora large share of solid phase in a diphase mixture. Therefore, it isreasonable to undertake further studies on the phenomenon ofparticles agglomeration using CFD method.

All variants were found to have a negative effect on pressuredrop. This was a natural result of introducing an additional com-partment along the upward vortex flow of the gas. Fig. 10 presentspressure drop values obtained for two test methods for all ana-lyzed geometries. Changes in the location and dimensions of thecounter-cone have a minimal effect on the obtained values forthe analysis of results obtained using the CFD method (valuesremain in the range from 1367 to 1400 Pa). On the other hand,for experimental research the obtained range was larger – from1257 Pa to 1330 Pa. Independent of the research method, similareffects were observed in individual variants as a result of the intro-duction of structure changes. In both cases, the lowest value wasobtained for variant a1 and the highest for variant d3. The distribu-tion of pressure fields for these variants is presented in Fig. 11. Theconducted observations suggest that the low pressure zone islocated on the axis of the cyclone separator. The closer to the wallsof the device, the higher the pressure. The lowest pressure valueswere recorded just behind the top outlet from the cyclone. Whenthe cone is located at a lower position (variant d3) it causes thegas vortex to lengthen. This may translate into a lower pressuredrop value.

Fig. 12 shows vectors of the gas phase velocity near the counter-cone. This allows the representation of the phenomenon of flowdisturbance in the region. An analysis of the figure allows anobservation of the effect of the vortex end ‘clinging’ to the upperfragment of the counter-cone. This enables a limitation of theabduction of solid particles settling on the cyclone walls near thebottom outlet. For conventional designs, the end of the gas phasevortex may, to a greater extent, approach the walls of the device,

of the cyclone (variants a1 and d3).

Fig. 13. Concentration of solid particles in the area of the cyclone outlet (variants a1, a2 and a3).

246 M. Wasilewski / Separation and Purification Technology 179 (2017) 236–247

which contributes to a decrease in separation efficiency. A similartendency can be observed when comparing the two design variantspresented in Fig. 12. When the counter-cone is located at a lowerposition (variant d3), the end of the vortex is placed at a lowerposition than for variant a1. Therefore, the vortex area, to a greaterextent, approached the walls of the cyclone separator in the area ofthe bottom outlet.

The conducted research was complemented with an assess-ment of the risk of excessive agglomeration of particles in thelower conical part of the cyclone due to the proposed structuralmodifications. As already mentioned in the study, agglomerationphenomenon usually leads to an improved efficiency of dustremoval. However, in special cases, an excessive agglomerationmay hinder ‘free sliding’ down of a set of particles that adhereto each other. Such a situation may occur when the distancebetween the surface of the counter-cone and the wall of thecyclone is too small. In this case, analysis of particle concentrationin the area of counter-cones conducted using CFD tests provednecessary. The point of reference were the maximum values ofthe parameter for all design variants. The obtained valuesremained in the range from 0.43 kg/m3 to 1.50 kg/m3. The lowestrisk (lowest values of the analyzed parameter) occur when thecounter-cone is entirely in the lower tank (e) – independent ofthe apex angle. On the other hand, the highest risk (highest valuesof the analyzed parameter) occurs for the variants in which theapex angle is 105� (with the exception for the above-mentionedvariant e). This particularly applies to the counter-cone locatedat the highest position. This may lead to impeded sliding of theseparated particles to the lower tank. Fig. 13 presents exampleconcentrations of particles for cases when the counter-cone islocated at the highest level (variants a1, a2 and a3) to show theeffect of the apex angle on particle concentration. To maintain fig-ure clarity, only the lower region of the cyclone is presented, i.e.the region which underwent design changes. The highest concen-tration of the solid phase for the a3 variant occurs in the bottomarea of the counter-cone. For these variants, the analyzed param-eter values - maximum particle concentration, varied in the rangefrom 0.64 kg/m3 to 1.50 kg/m3.

4. Conclusions

The following conclusions can be drawn from this study into theanalysis of flow inside cyclone separators equipped with an addi-tional structural element in the form of a counter-cone:

� The application of a counter-cone in each of the studied geo-metrical variants led to an increase in solid particle separation.

� The location of the counter-cone was found to be the key geo-metrical parameter. The results show that the counter-coneshould be located above the lower outlet of the cyclone. The

optimum location of the counter-cone (assuming maximizationof the separation efficiency) can be obtained when the distanceof the counter-cone base from the bottom outlet (Bh) is 0.15 B(Bh = 0.15 B).

� The apex angle of the counter-cone also plays a significant role.From the obtained results, it was determined that the value ofthe parameter should be 85�.

� Improvement in the separation efficiency was observed for par-ticles smaller than 60 mm. This results from the fact that thephenomenon of re-entrainment of the particles particularlyrelates to fine particles and in particular particles with diameterup to 15 mm.

� The application of an additional structural element with a highapex angle (>100�) may entail the risk of excessive agglomera-tion of solid particles in the area of the counter-cone. In extremecases, this may negatively affect separation efficiency.

� A negative effect of the introduction of an additional compart-ment is an increased pressure drop inside the cyclone separator.Therefore, the application of a counter-cone is especially viableif the optimization of separation efficiency is desired and aslong as the installation can withstand an increased pressuredrop.

� Validation of the results (based on the values of separation effi-ciency and pressure drop) obtained via CFD tests and experi-mental research allowed us to determine that numericalmodels were correctly selected and the calculation and bound-ary conditions were correctly parametrized.

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