Cyclone Modelling Results at High t and High p

7/24/2019 Cyclone Modelling Results at High t and High p

1/15

A numerical and experimental study on a high efficiency

cyclone dust separator for high temperature

and pressurized environments

Mi-Soo Shin a, Hey-Suk Kim a, Dong-Soon Jang a,*,Jin-Do Chung b, Matthias Bohnet c

a Chungnam National University, 220 Gung-dong, Taejon 305-764, Republic of Koreab Hoseo University, Baebang-Myun, Asan city Chung-Nam 336-795, Republic of Korea

c Institute of Chemical Engineering Technical university of Braunschweig, Langer Kamp 7, 38106 Braunschweig, Germany

Received 23 January 2004; accepted 5 November 2004

Available online 20 December 2004

Abstract

A numerical and experimental study has been made for the development of high efficiency cyclone dust

separator applicable to the extreme environments of high pressure of 6 bar and temperature up to 400 C.

The main objective of this study is to develop a handy and reliable computer program and thereby to figure

out the physical mechanism of dust collection for high temperature and pressure condition.

The program is developed using Patankars SIMPLE method for the application of 2-D axi-symmetric

flow field. The two-equation turbulence kemodel is employed for the resolution of Reynolds stresses. Fur-

ther the particle trajectory calculation is made by the incorporation of drag, centrifugal and coriolis force in

a Lagrangian frame.

The calculated results predict well the general trend and its magnitude of the experimentally measured

pressure drop with the condition of increased pressure and temperature as a function of flow rate. Further,

experiment shows that the increase of pressure and temperature generally affect significantly the collectionefficiency of fine particle less than 10 lm but the effect of pressure and temperature appears contrary each

other. That is, the increase of pressure increases the collection efficiency, while the increase of the temper-

ature results in the decrease of the efficiency over a certain range of flow rate. This is explained well by the

1359-4311/$ - see front matter 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.applthermaleng.2004.11.002

* Corresponding author. Tel.: +82 42 821 6677; fax : +82 42 823 8362.

E-mail address: [email protected] (D.-S. Jang).

www.elsevier.com/locate/apthermeng

Applied Thermal Engineering 25 (2005) 18211835
mailto:[email protected]:[email protected]


2/15

variation of the gaseous density and viscosity effect on the drag force and also confirmed successfully by the

result of numerical calculation. Therefore the decrease of fractional collection efficiency caused by the high

operating temperature can be remedied by the increase of operating pressure.

In order to investigate in more detail the effect of extreme condition on the physics of collection effi-ciency, a series of parametric numerical investigations are performed in terms of major cyclone design or

operational parameters such as tangential velocity and vortex finder length, etc. As expected, tangential

velocity plays the most important effect on the particle collection even for the elevated temperature and

pressure condition. But there is no remarkable difference noted between reference and extreme condition.

And the length of vortex finder has relatively insignificant effect on collection characteristics but the dia-

meter of vortex finder plays an important role for the enhancement of collection efficiency.

The incorporation of a proper turbulence model of the nonlinear term appeared by relative velocity

between gas and particle phase for drag calculation of particle trajectory is considered as one of the impor-

tant tasks for the more accurate resolution of physical feature for elevated temperature and pressure con-

dition in near future.

2004 Elsevier Ltd. All rights reserved.

Keywords: Cyclone separator; Pressure drop; Collection efficiency; High temperature and pressure

1. Introduction

The development of new energy saving technologies in power generation causes a strong impe-

tus to develop new devices for hot flue gas cleaning at pressurized condition. A cyclone is consid-ered as one of advantageous tools for high temperature gas cleaning due to their simplicity and

low maintenance requirements. By using suitable materials and methods of construction cyclonesmay be adapted for use in extreme operating conditions such as high temperature, high pressure,and further corrosive gases environments. Therefore, the objective of this study is to investigate

the behavior of dust particle at an elevated temperature and pressure condition.The schematic diagram of cyclone is shown as inFig. 1. The dirty gas enters the cyclone tan-

gentially, describes a descending outer vortex, inverts the direction of motion due to the action ofincrease of static pressure and ascends by an inner vortex exiting at the cyclone top through thevortex finder.

The larger particles are swept into the cyclone wall by a centrifugal force, which is locally op-posed by aerodynamic drag of radial direction, and are carried towards the bottom of the cyclone

by the descending outer vortex. The finer particles exit at the top with the carrier gas, togetherwith coarser particles that may have been re-entrained and swept by the ascending inner vortex.Since cyclones have been used extensively in various industries, a considerable number of experi-

mental and theoretical investigations have been performed on cyclone separators to the present.

Among these, Stairmand[1]presented one of the most popular design guides which suggested thatthe cylinder height and exit tube length be, respectively, 1.5 and 0.5 times of the cyclone body

diameter for the design of a high efficiency cyclone. Bryant et al. [2]observed if the vortex touchedthe cone wall, particle re-entrainment occurred and Leith[3]and Bhatia and Cheremisinoff[4]dis-

cussed the effects of the cone opening size. Rongbiao et al. [5] were suggested that flow ratestrongly influenced the efficiency and the reduction in cone size results in higher collection

1822 M.-S. Shin et al. / Applied Thermal Engineering 25 (2005) 18211835


3/15

h

H

B

De

a

b

Dust tube

Dust

Cleaned gas

Dirty gas

Dirty gas

Dc

S

H : Cyclone heighth : Cylinder height

S : Exit tube lengthDc : Cyclone body diameterDe: Gas exit diametera : Inlet height

b : Inlet widthB : Dust outlet diameter

Fig. 1. Schematic diagram of cyclone illustrating geometrical dimensions.

Nomenclature

u axial velocity (m s1

)v radial velocity (m s1)w tangential velocity (m s1)CD drag coefficient (dimensionless)Re Reynolds numberdp particle diameter (m)

Greek letters

/ general specific dependent variablesq density (kg m3)C/ diffusion coefficient of/S/ source term of/

l dynamic viscosity (kg m1 s1)k turbulent kinetic energy (J kg1)e turbulent kinetic dissipation rate (J kg1 s1)

Subscripts

g gas

p particle

M.-S. Shin et al. / Applied Thermal Engineering 25 (2005) 18211835 1823


4/15

efficiency without significantly increasing the pressure drop. Bohnet et al. [68] investigated the

effects of the pressure drop and grade efficiency at high temperature and high pressure.Although our knowledge of what goes on inside a cyclone has increased over the years, the

exact mechanisms of removing particles and effects of each cyclone dimension as shown inFig. 1 are still not fully understood due to the complexity of the turbulence itself and the interaction

between particle and carrier gas. In addition, a radical change of operating conditions by high tem-perature and pressure add even more difficulties to the complicated problem. Therefore, most exist-ing cyclone theories are based on simplified models or are heavily dependent upon empiricism. The

theoretical models need experimentally defined factors to fit theory on experimental data [6].Recently, research efforts by computational fluid dynamics are frequently carried out for the res-

olution of flow field and dust particle behavior with different degree of numerical and modeling accu-

racy in order to assist in the time consuming experimental works. In conjunction with the complexflow structure, numerical simulation is momentarily not able to completely substitute experiments

but can reduce, to a certain degree, experimental costs for design and optimization. Therefore, thepurpose of this study is to help better understand the particle removing characteristics of cycloneseparator especially by exploring the effects of extreme operating conditions caused by high temper-

ature and pressure. To this end a relatively well-established numerical method for the simple 2-D axi-symmetric approximation of cyclone geometry is adopted in this study. The computer program

developed is validated by the comparison of the experimental data performed in the laboratoryof Prof. Bohnet in the dept of Chemical Engineering of University of Braunschweig.

2. Experimental set-up

To receive reliable data for extreme operating conditions, the set-up shown in Fig. 2has beeninstalled. It allowed measurements of pressure drop and grade efficient in a temperature range 20600C at a maximum pressure of 6 bar. A three stage compressor delivers the flow rate, which is

adjusted by two control valves in the bypass. The pressure is adjusted by one control valve at theend of the set-up. The air is heated electrically with two heaters. A rotating brush dosing deviceinstalled within a pressure vessel is used to feed solids continuously to the gas. A valve in the main

stream controls the flow rate through the dosing devices. The collection efficiency curve is deter-mined by measuring the particle size distribution and the particle concentration within the inlet

and outlet gas stream of the cyclone. These data are measured with two light scattering aerosolcounters in-line and simultaneously. The cyclones are fixed in a pressure vessel. This simplifies

the variation of the cyclone geometry, because the cyclone itself has to be designed temperatureresistant only[911].

3. Numerical modeling

3.1. Governing equation

The basic gas-phase conservation equations for mass, momentum, energy, turbulence quantities

and species concentration can be expressed, in Eulerian cylindrical framework, as



5/15


6/15

3.2. Numerical algorithm and solution procedure

The solution of the Eulerian gas phase equations are done by a control-volume based finite-dif-ference procedure. A detailed description of this method is given by Patankar [12]. The methodrequires the division of the computational domain into a number of control volumes, each asso-

ciated with a grid point. The governing differential equations in each control volume profile areapproximated in each coordinate direction. In this study, power-law scheme is employed forthe discretization of the convection term appeared in the governing equation (1). A system of dis-

cretized linear equations as Eq.(6)is solved iteratively due to the nonlinear feature of the equationimplicitly imbedded in the coefficient the discretized equation [12].

aP/P aE/EaW/W aN/N aS/S b 6

Fig. 3. Particle trajectories in a control volume.

Table 1

/ and C/ expression for 2-D cylindrical coordinate

Variables / C/ S/

Axial momentum u leff o

ox leff

ou

ox

1

r

o

or leffr

ov

ox

op

ox

Radial momentum v leffo

ox leff

ou

or

1

r

o

or leffr

ov

or

2leff

v

r2

qw2

x

op

or

Tangential momentum w leff leffr2

qv

r

1

r

oleffor

w

Kinetic energy k leff

rkGk1qe

Kinetic energy dissipation rate eleff

rs

e

j C1Ck1C2qe

Temperature T k

Cp

Gj1 2leffou

ox

2

ov

or

2

v

r

2" #leff

ow

ox

2 r

o

or

w

r

2

ou

or

ov

ox

2" #

C1= 1.44, C2= 1.92, Cl= 0.92, rj= 0.9, re= 1.22



7/15

where, aE, aW, aN, aS, and aP are coefficients of east, west, north, south and main grid nodes,

respectively.Fig. 4represents the boundary condition of cyclone employed in this study.

4. Results and discussion

As mentioned above, numerical calculation for the cyclone is performed using the computerprogram developed in this laboratory and compared with experimental data performed at the

Institute of Chemical Engineering of University of Braunschweig. In order to investigate the ef-fects of pressure drop and particle separation efficiency, the trajectories of particles are calculatedwith various geometrical variables (vortex finder length, vortex finder diameter) and operating

condition (tangential velocity, flow rate). The detailed dimensions and operating conditions aresummarized inTable 2.

First of all,Fig. 5shows the comparison of calculated and measured pressure drop curves as

function of flow rate for various temperature and pressure conditions of temperature up to400C and pressure up to 6 bar. The calculation of pressure drop is made by the difference be-

tween exit and inlet area-weighted static pressure. As shown in theFig. 5, the comparison showsfairly good agreement over the entire temperature, and pressure range except the case of high flow

rate with 6 bar, over which there are no experimental data available for direct comparison. Butthere are some visible difference observed for the case of reference condition, that is, atmosphericoperating pressure, 1.1 bar and room temperature and the difference becomes more visible as the

increase of flow rate. The explanation of this difference is not clear at this stage but is consideredpossibly due to the combined effect of the numerical and experimental error by the increased flowrate. As might be expected, for a given volumetric flow rate, the pressure drop generally increases

Inlet

condition

Right wall

B.C

Bottom wall

B.C

Outflow

B.C

Symmetry

B.C

Top

wall

R1

R2

RF

Fig. 4. Boundary conditions of cyclone used in this study.



8/15

with the increase of gaseous density, that is, high pressure and low temperature condition. This isattributed to the effect of increased dynamic pressure and thereby large pressure drop by the

increase of density for the same flow rate.

0

0.2

0.4

0.6

0.8

1

0.1 1 10

particle size(m)

efficiency,

18 ,1.1bar,60 /h

400 ,1.1bar,60 /h

m3

m3

Fig. 6. The effect of temperature on the measured fraction collection efficiencies for 18 and 400 C temperature.

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140 160 180

Flow rate(m3/h)

P(mbar)

Calculated

15 , 6 bar

200 , 6bar

400 , 6bar

15 , 1.1bar

Fig. 5. Measured and calculated pressure drops as a function of flow rate for the condition of elevated temperature and

pressure.

Table 2

The specification of cyclone geometry and operating condition employed in this study

Variables Geometry (mm)

a* Dc* De* H* h* S* B*

Standard 13 100 32 248 55 71 31

Flow rate (m3/h) 10160 (reference flow rate condition: 60)

De/Dc 0.2, 0.4, 0.6

S/Dc 0.2, 0.5, 0.72, 1.0

Tangential velocity (vane angle) 1.01 m/s (30), 1.75 m/s (45), 3.03 m/s (60)

dp[lm] 110

* ReferFig. 1.



9/15


10/15

0

0.2

0.4

0.6

0.8

1

0.1 1 10

particle size(m)

efficiency,

18 ,1.1bar,60 /h

400 ,6.0bar,60 /h

Fig. 8. The combined effect of increased pressure and temperature on the measured collection efficiency as a function of

particle size for the condition of 1.1 bar, 18 C and 6 bar, 400 C.

0 0.05 0.1 0.15 0.2X

0

Y

(a) T= 15o

C, P=1.1 bar, 60m3/hr

0 0.05 0.1 0.15 0.2X

0

Y

(b) T= 400o

C, P=1.1 bar, 60m3/hr

0 0.05 0.1 0.15 0.2X

0

Y

(c) T= 15o

C, P=6.0 bar, 60m3/hr

0 0.05 0.1 0.15 0.2X

0

0.01

0.02

0.03

0.04

0.05

0.01

0.02

0.03

0.04

0.05

0.01

0.02

0.03

0.04

0.05

0.01

0.02

0.03

0.04

0.05

Y

(d) T= 400o

C, P=6.0 bar, 60m3/hr

x

r

Inlet

Outlet

Fig. 9. Particle trajectory of various particles (110lm) with high temperature and pressure (collection efficiency:

(a) 90%, (b) 85%, (c) 93%, (d) 87%).



11/15

shows a little bit better efficiency and vice versa for the particle size more than 1 lm. Based on

the result of thisFig. 8, it is clear that the decrease of collection efficiency by the increase of tem-perature can be alleviated by the proper increase of operating pressure of cyclone.

A number of calculated particle trajectories have been performed with variation of the pressureand temperature for the flow rate of 20, 40, and 60 m3/h, in which representative particle trajec-

tories are classified with particle size and initial starting location at inlet port. For the case of60 m3/h, the particle trajectories are presented in Fig. 9 together with the overall collection effi-ciency based on the result of the representative particle trajectories. As shown in the figure, the

collection efficiency generally increases with the increase of pressure and decrease of temperature.The highest collection efficiency is 93% for the 6 bar and room temperature and the lowest is 85%for the 1.1 bar and 400 C. For the case of the flow rate 40 m3/h, the overall collection efficiency

shows a little lower or so.In general, the calculated result is fairly consistent with the qualitative trend of the experimental

observation but the direct comparison with calculation and experiment is not made at this stagesince the finite grouping of representative classification with particle size and initial location is notsufficient to give fairly accurate quantitative evaluation.

0 0.05 0.1 0.15 0.2

X

0

0.01

0.02

0.03

0.04

0.05

Y

(a) De/D

C= 0.2

0 0.05 0.1 0.15 0.2X

0

0.01

0.02

0.03

0.04

0.05

Y

(b) De/D

C= 0.36

(Standard)

0 0.05 0.1 0.15 0.2X

0

0.01

0.02

0.03

0.04

0.05

Y

(c) De/D

C= 0.6

x

r

Fig. 10. Trajectory of particles (110lm) with the increase of diameter of vortex finder (collection efficiency: (a) 94%,

(b) 87%, (c) 84%).



12/15

In order to figure out further the effect of increased temperature and pressure on the cyclone

efficiency a series of parametric investigation have been performed in terms of important cycloneoperating and design variables. Since the pressure and temperature cause opposite effect on cy-

clone collection efficiency, the combined effect of pressure and temperature does not show anyconsistent and noticeable difference compared to the result of reference condition. Figs. 1012rep-

resent the particle trajectories calculated with 6.0 bar and 400 C for the flow rate of 60 m3/h.

0 0.05 0.1 0.15 0.2X

0

0.01

0.02

0.03

0.04

0.05

Y

(a) S/DC

= 0.2

0 0.05 0.1 0.15 0.2X

0

0.01

0.02

0.03

0.04

0.05

Y

(b) S/DC

= 0.5

0 0.05 0.1 0.15 0.2X

0

0.01

0.02

0.03

0.04

0.05

Y

(d) S/DC

= 1.0

0 0.05 0.1 0.15 0.2X

0

0.01

0.02

0.03

0.04

0.05

Y

(c) S/DC

= 0.71(standard)

x

r

Fig. 11. Particle trajectory of various particles (110lm) with increase of vortex finder length (collection efficiency:

(a) 78%, (b) 87%, (c) 87%, (d) 86%).



13/15


14/15

sufficient ability to collect particles whose diameters are much smaller than 1 lm at high tangen-

tial velocity.

5. Conclusion

Based on a series of numerical investigation and experiment, a number of useful conclusions

can be drawn for the case of extreme operating condition caused by high temperature and pres-sure condition.

The calculated results predict well the general trend and its magnitude of the experimentally

measured pressure drop with the condition of increased pressure and temperature as a functionof flow rate. Further, experiment shows that the increase of pressure and temperature generally

affect significantly the collection efficiency of fine particle less than 10 lm but the effect of pressure

and temperature appears contrary each other. That is, the increase of pressure increases the col-lection efficiency, while the increase of the temperature results in the decrease of the efficiency overa certain range of flow rate.

In order to investigate in more detail the effect of extreme condition on the physics of collection

efficiency, a series of parametric numerical investigations are performed in terms of major cyclonedesign or operational parameters such as tangential velocity, vortex finder length and diameter.Tangential velocity plays the most important effect on the particle collection even for the elevatedtemperature and pressure condition. But in general there is no remarkable difference noted between

reference and extreme condition. Therefore, the decrease of fractional collection efficiency by theoperating condition of high temperature can be remedied by the increase of operating pressure.

The incorporation of a proper turbulence model of the nonlinear term appeared by relativevelocity between gas and particle phase for drag calculation of particle trajectory is considered

as one of the important tasks for the more accurate resolution of physical feature for elevated tem-perature and pressure condition in near future.

References

[1] C.J. Stairmand, The design and performance of cyclone separators, Trans. Inst. Chem. Eng. 29 (1951) 356383.

[2] H.S. Bryant, R.W. Silverman, F.A. Zenz, How dust in gas affects cyclone pressure drop, Hydrocarbon Process. 62

(1983) 8790.

[3] J. Dirgo, D. Leith, Performance of theoretically optimized cyclones, Filterat. Separat. 22 (1985) 199225.

[4] M.U. Bhatia, P.N. Cheremisinoff, Cyclones, in: P.N. Cheremisinoff (Ed.), Air Pollution Control and Design for

Industry, Marcel Dekker, New York, 1993.

[5] S.H. Rongbiao, K.W. Xiang, K.W. Park, K.W. Lee, Effects of cone dimension on cyclone performance, Aerosol

Sci. 32 (2001) 549561.

[6] C.H. Kim, J.W. Lee, A new collection efficiency model for small cyclones considering the boundary layer effect,

J. Aerosol Sci. 32 (2000).

[7] M. Michael, W. Martin, Pressure loss and separation characteristics calculation of a uniflow cyclone with a CFD

method, Chem. Eng. Technol. 23 (2000) 753758.

[8] M.A. Silva, S.A. Nebra, Numerical simulation of drying in a cyclone, Drying Technol. 15 (6-8) (1997) 17311741.

[9] M. Bohnet, Cyclone separation for fine particles and difficult operating conditions, KONA Powder Particles (12)

(1994) 6976.



15/15

[10] M. Morweiser, M. Bohnet, Design calculation of aerocyclones for extreme operating conditions, in: 12th

International Congress of Chemical and Process Engineering, 1996, pp. 5566.

[11] M. Morweiser, M. Bohnet, Influence of operating conditions on grade efficiency and pressure drop of aerocyclone,

in: 3rd International Conference on Multiphase Flow, ICMF98, 1998, pp. 18.[12] S.V. Patankar, Numerical Heat Transfer and Fluid Flow, Hemisphere, Washington, DC, 1980.

[13] D.S. Jang, Single and Two-Phase Reacting Flow Predictions Modeling of Nonequilibrium Effects, Turbulent

Particle Dispersion and Nitrogen Oxide Formation in Pulverized Coal Combustor, Doctor Thesis, Louisiana State

University, USA, 1987.


Cyclone Modelling Results at High t and High p

Documents

Transcript of Cyclone Modelling Results at High t and High p