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Brazilian Journal of Chemical EngineeringPrint version ISSN 0104-6632

Braz. J. Chem. Eng. vol.17 n.4-7 So Paulo Dec. 2000

http://dx.doi.org/10.1590/S0104-66322000000400010

PERFORMANCE ANALYSIS ANDDESIGN OF SMALL DIAMETER

CYCLONES

M.R.T. Halasz 1 and G. Massarani 2*

1PEQ/COPPE/UFRJ, 2PEQ/COPPE/UFRJC.P.68502, CEP 21945-970, Rio de Janeiro - RJ, Brazil

Phone: +55 21 590-2241, Fax: +55 21 590-7135E-mail: [email protected]

(Received: September 20, 1999 ; Accepted: April 6, 2000)

Abstract - In this work, the effect of the configuration on thecollection efficiency and pressure drop in small diameter cyclones isevaluated based on neural networks (Functional Link Networks). Theexperiences were conducted at LSP/COPPE Laboratory in a Stairmand high efficiency prototype (Dc= 5 cm) with variable overflow diameter. Three different configurations were tested, and it ispossible to observe a significant increase in the collection efficiency with the reduction of theoverflow diameter.Keywords: small diameter cyclone, neural networks, radioactive wastes.

INTRODUCTION

This work stands for one of the development steps of an efficient gas treatment system, involving highperformance small diameter cyclones with a high separation efficiency, for treatment of gases from radioactivewaste incineration.

Annually a great amount of radioactive waste is generated, originated in nuclear power plants, radioisotopeapplications in industry, teaching and research institutions, hospitals, etc. [Raduan, 1993]. A traditional way oftreating those wastes is using incineration, which is able to reduce the amount of waste for the safe disposal infillings or storage facilities. The incineration process generates gases, ashes and vapors that must be treatedbefore disposing. In general, the first step of such treatment is accomplished using cyclones, responsible for theremoval of part of the particle solids.

The cyclones are solid-gas separators that use the same principle as the centrifuges, i.e., sedimentation bycentrifugal field, and have been used in industry since the beginning of this century [Dirgo, 1985]. The cyclonespresent as main features the extreme versatility, low operational and maintenance costs, easy construction andinstallation [Leith and Licht, 1972].

Brazilian Journal of Chemical Engineering - Performance analysis and d... http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0104-66322...

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The cyclone collection efficiency is directly affected by the cylindrical section diameter. The decrease in thisparameter causes an immediate increase in the centrifugal field intensity formed inside the cyclone, increasingthe separation efficiency. Small diameter cyclones are defined as the cyclones with diameter smaller than 5.0cm, that can be used in the separation of small particles (smaller than 10mm) with high separation efficiency.

Small diameter cyclones can be used for treating radioactive wastes, due to their better ability to separatesmaller particles than the traditional cyclones [Bttner, 1986 and Kim and Lee, 1990].

In this work a FLN (Functional Link Network) neural network was used for obtaining a model capable ofdescribing the particle collection efficiency. The ortogonal estimator procedue of Billings et al. (1988) was used

as implemented by Henrique and Lima (1996). The Matlab package was used for carrying out the computations.

In the literature it is possible to find a multitude of experimental data for cyclones with body diameter in therange of 20 to 100 cm. The experimental data used in this work, however, are related to small diametercyclones with body diameter in the range of 1 to 5 cm. These data were obtained from Bttner (1986), Kim andLee (1990) and Griffiths and Boysan (1992).

It is possible to build a model, using neural networks, that is able to predict the separation efficiency of smalldiameter cyclones with a good degree of confidence. In this work a neural network (Functional Link Network)was used, trained with literature data and validated using experimental results obtained for small diametercyclones with the high efficiency Stairmand configuration. The equation obtained is presented below. In adetailed analysis (Figure 1) it can be noted that the separation efficiency tends to increase with a decrease inthe overflow diameter (keeping the other parameters constant).


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(1)

where

(2)

It is worth to note that the equation is a function of operational conditions [flow (Q), diameter of the particle tobe separated (dp) and ratio between fluid viscosity and particle density (m/rs)], the equipment characteristic

relations [(a/D), (b/D), (De/D), (H/D), (h/D), (B/D), (S/D)] and cylindrical section diameter, D. Thelimit values for the parameters are reported in Table 1.

The pressure drop is an essential parameter in the design of cyclone systems that allows to evaluate the blowerspecifications. Equation 3 represents one expression that can be used to calculate this parameter.

(3)

where NH is a dimensionless parameter and depends on the cyclone proportions, that by Shephered and Lapple

(1939) is equivalent to:


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(4)

According to Figure 2, that represents Equation 3, it can be observed that the decrease in the overflow diameter(keeping other parameters constant) leads to an increase in the pressure drop in the cyclone.

Usually the methodology to determine the equations and design parameters for cyclones is semi-empirical andwell known for some configurations. The equations for the design of a high efficiency Stairmand cyclone can bewritten according to the methodology proposed by Massarani (1991).

where K = 0.041(5)

where (6)

(7)

(8)

D* is the cut diameter of the equipment, i.e., the diameter of the particle that is separated with 50% efficiency,K is the dimensionless constant that considers the cyclone geometry, DP is the pressure drop between the feedand the overflow, v is the velocity at the cylindrical section of the cyclone, r is the fluid density b is adimensionless constant, which depends on the cyclone configuration, is the global collection efficiency andh(dp) is the grade collection efficiency.

In this work the collection efficiency is evaluated for three different cyclone configurations, resulting from thevariation of the diameter of the high efficiency Stairmand cyclone.

MATERIALS AND METHODS


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The experimental system presented in Figure 3 was assembled using a high efficiency Stairmand cyclone with 5cm diameter. Three different outlet diameters were adapted to this cyclone (De = 2.50, 2.35 and 2.25 cm). The

feed inlet diameter could not be altered, because this section was fixed to the cylindrical section. A dust bunkerwas adapted to equipment underflow outlet, in order to collect the particle material. One-inch diameter pipingwas connected to the cyclone overflow outlet and to the blower suction inlet. An expansion tank was connectedto the piping, between the blower and the cyclone, in order to avoid the retention of small particles in therotameter.

The feed system is composed of two parts: one feed bin of 30 cm height and 8 cm of cylindrical sectiondiameter, and a rotating disk of 20 cm diameter. The feed bin outlet is attached to a vibrator, which facilitatesthe particle flow from the feed bin. The rotating disk is used to guide the particles to the cyclone inlet.

In this work a gas-solid system was investigated. The solid materials studies are described in Table 2. Thematerial present physical properties similar to the ashes resulting from the radioactive waste incineration (rs =

3.0g/cm3 and dpmean = 20mm). It could be observed that 40 wt% of the sand and 30 wt% of the neodymium

carbonate particles have diameter smaller than 10 mm. It was possible to adjust the particle distribution of

every condition studied using the Rosin-Rammler-Bennet (RRB) model, described by the following equation.

(9)

where X is the mass fraction of particles with diameter smaller than D; D and m are model parameters, whichrepresent the diameter correspondent to X=0.632 and to the dispersion, respectively.

The particle distribution was determined using the Malvern Masterizer X equipment and the solid densitypresented in Table 2 was determined using classic picnometry.

RESULTS AND DISCUSSION

Experiments using three different configurations of the small diameter cyclone were carried out. The firstconfiguration (1) was the high efficiency Stairmand cyclone with De/Dc = 0.50, the second (2) and the third


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(3) were variations of the high efficiency Stairmand cyclone with De/Dc = 0.47 and De/Dc = 0.45, respectively.

When the collection efficiency is high, it could not be observed almost any differences between the particledistribution curves of the feed and the underflow. This leads to imprecision in the cut diameter determinationand consequently in the determination of the design parameters, following the methodology described byMassarani (1991) (Eq. 5, 7 and 8), which considers the grade collection efficiency. In this case, it is possible touse the strategy proposed by Yuan (1996), where the design is based only on the global collection efficiency.

The global efficiency obtained with the three different configurations studied can be observed in Figures 4 and5. The experimental results correspond to the predicted by the neural network, showing that when the overflowdiameter is decreased, the collection efficiency increases. However, it could be noted by the experimentalresults that this increase is more significant when configurations (1) and (2) rather than when configurations(2) and (3) are compared.

It is possible to note from Figure 6 that the pressure drop increases with the flow rate increase, as expected. Inthis case the equation 3 is very good to describe the pressure drop.


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CONCLUSIONS

It was confirmed through the Neural Network prediction and by the equation proposed by Shephered and Lapple(1939) that the decrease in the overflow diameter of small cyclones leads to an increase in the pressure dropand in the collection efficiency. The experimental results show that below a certain ratio (De/Dc), the increase in

the collection efficiency is small when compared to the increase in the pressure drop.

NOMENCLATURE

a cyclone inlet height [L]

b cyclone inlet width [L]

B cyclone lower (dust) outlet diameter [L]

dp diameter of the particle to be separated [L]

D cylindrical section diameter [L]

Dc cylindrical section diameter [L]

De cyclone outlet diameter [L]

D* cut diameter of the equipment [L]

D diameter correspondent to X=0.632 [L]

h cyclone cylinder height [L]

H cyclone height [L]

K dimensionless constant [-]

m model RRB dispersion [-]

NH dimensionless parameter [-]

Q flow rate [L3/q ]S cyclone gas outlet pipe length [L]

v velocity at the cylindrical section of the cyclone [L/q ]vi inlet velocity [L/q ]X mass fraction of particles with diameter smaller than D [-]

b dimensionless constant [-]DP pressure drop between the feed and the overflow [M/Lq 2]

global collection efficiency [-]

h(dp) grade collection efficiency [-]m fluid viscosity [M/Lq ]r fluid density [M/L3]

rs particle density [M/L3]

ACKNOWLEDGEMENTS


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This work is part of the project entitled "Modelagem, Simulao e Controle de Processos Qumicos", FAPERJnumber E-26/150.970/99. Halasz would also like to acknowledge CNEN for the scholarship.

REFERENCES

Billings, S.A., Chen, S., Korenberg, M.J., Identification of MIMO non-linear systems using a forward-regressionorthogonal estimator, I.J. Control, V. 49, p. 2157 2189, (1989). [ Links ]

Bttner, H., Investigation on Particle Collection in Small Cyclones, J. Aerosol Science, V. 17, No. 3, p.537-541,Great Britain, (1986). [ Links ]

Dirgo, J. and Leith, D., Cyclone Collection Efficiency: Comparison of Experimental Results with TheoreticalPredictions, Aerosol Science and Technology, V. 4, Elsevier Science Publishing Co, (1985). [ Links ]

Griffiths, W.D. and Boysan, F. , An Assessment of the Application of Computational Fluid Dynamics (CFD) toModel the Performance of a Range of Small Sampling Cyclones, J. Aerossol Science, V. 23, Suppl. 1, p.S587-S590, Pergamon Press Ltd., (1992). [ Links ]

Henrique, H.M. and Lima, E.L., Model structure determination in neural network models, accept for publicationin Chemical Engineering Science, RR/1999/000858, (1996). [ Links ]

Kim, J.C. and Lee, K.W., Experimental Study of Particle Collection by Small Cyclones, Aerosol Science andTechnology, V. 12, Elsevier Science Publishing Co, p.1003-1015, (1990). [ Links ]

Leith, D. and Licht, W., The Collection Efficiency of Cyclone Type Particle Collectors a New TheoreticalApproach, A. I.Ch. E. Symposium Series 68, p.196 - 206, (1972). [ Links ]

Massarani, G., "Projeto e Anlise de Desempenho de Ciclones e Hidrociclones II" Revista Brasileira deEngenharia, V. 8, No. 2, p.86, (1991). [ Links ]

Raduan, R.N. and RzyskI, B.M., Instalao de Repositrios de Subsuperfcie para Rejeitos Radioativos Requisitospara Avaliao de Impacto Ambiental, Anais do Simpsio Sobre Engenharia Nuclear e Meio Ambiente, SeoLatino Americana da American Nuclear Society, Rio de Janeiro, p. IV - 35 IV 43, (1993). [ Links ]

Shepherd, C.B. and Lapple, C.E., Flow Pattern and Pressure Drop in Cyclone Dust Collectors, Ind. Eng. Chem., V.31, No. 8, p. 972-984, (1939). [ Links ]

Yuan, H., Thew, M.T. and Rickwood, D., Separations of Yeast with Hydrocyclones, Hydrocyclones96, p.135-149,London, (1996). [ Links ]

*To whom correspondence should be addressed

Associao Brasileira de Engenharia Qumica

Rua Lbero Badar, 152 , 11. and.01008-903 So Paulo SP Brazil

Tel.: +55 11 3107-8747Fax.: +55 11 3104-4649Fax: +55 11 3104-4649

[email protected]


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