Week # 8

MR Chapters 9

• Tutorial • MR #9.1

MARTIN RHODES (2008)

Introduction to Particle

Technology, 2nd Edition.

Publisher John Wiley & Son,

Chichester, West Sussex,

England.

Gas Cyclones

• During processing and handling of particulate solids, separation of particles from suspension in a gas may be required

• Generally, particles larger than about 100 mm can be separated by gravity settling

• For particles less than 100 mm, more energy intensive methods such as filtration, wet scrubbing and electrostatic precipitation must be used

• Gas cyclones are best suited as primary separation devices and for relatively coarse particles

• Electrostatic precipitator or fabric filter may be used downstream to remove very fine particles

• Gas cyclones are generally not suitable for separation involving suspensions with a large proportion of particles less than 10 mm

• Most common type of cyclone is known as reverse flow cyclone separator

• Inlet gas is brought tangentially into the cylindrical section

• A strong vortex is created inside the cyclone body

• Particles in the gas are subjected to centrifugal forces which move them radially outwards, against the inward flow of gas and towards the inside surface of the cyclone

• Direction of vortex flow reverses near the bottom of the cylindrical section

• Gas leaves the cyclone via the outlet in the top

• Solids at the wall are pushed downwards by the outer vortex and out of the solids exit

• Gravity has been shown to have little effect on the operation of the cyclone

Flow Characteristics

• Rotational flow in the forced vortex within the cyclone body gives rise to a radial pressure gradient

• This pressure gradient, combined with the frictional pressure losses at the gas inlet and outlet and losses due to changes in flow direction, make up the total pressure drop

• Pressure drop, measured between the inlet and gas outlet, is usually proportional to the square of gas flow rate through the cyclone

• A resistance coefficient, the Euler number Eu, relates the cyclone pressure drop Dp to a characteristic velocity:

• Where rf is the gas density

• The characteristic velocity v can be defined based on the cross-section of the cylindrical body of the cyclone

• Where q is the gas flow rate and D is the cyclone inside diameter

• The Euler number represents the ratio of pressure forces to the inertial forces acting on a fluid element

• Value is practically constant for a given cyclone geometry, independent of the cyclone body diameter

Efficiency of Separation

• Consider a cyclone to which solids mass flow rate is M, mass flow discharged from the solids exit orifice is Mc

(known as the coarse product) and solids mass flow rate leaving with the gas is Mf (known as the fine product)

• Total material balance on the solids may be written:

• Component material balance for each particle size x (assuming no breakage or growth of particles within the cyclone):

• Where dF/dx, dFf/dx and dFc/dx are the differential frequency size distributions by mass (mass fraction of size x) for the feed, fine product and coarse product respectively

• Total efficiency of separation of particles from gas, ET, is defined as the fraction of the total feed which appears in the coarse product collected

• The efficiency with which the cyclone collects particles of a certain size is described by the grade efficiency, G(x):

• Using the notation for size distribution described above:

• Combine to find an expression linking grade efficiency with total efficiency of separation

• Above equation relates size distribution of feed, coarse product and fine product

• In cumulative form, this becomes

• Consider a reverse flow cyclone with a cylindrical section of radius R

• Particles entering the cyclone with the gas stream are forced into circular motion

• The net flow of gas is radially inwards towards the central gas outlet

• The forces acting on a particle following a circular path are drag, buoyancy and centrifugal force

• The balance between these forces determines the equilibrium orbit adopted by the particle

• Drag force is caused by the inward flow of gas part the particle and acts radially inwards

• Consider a particle of diameter x and density rp following an orbit of radius r in a gas of density rf and viscosity m

• Let the tangential velocity of the particle be U and the radial inward velocity of the gas be Ur

• If we assume that Stokes’ law applies under these conditions then the drag force is given by:

• The centrifugal and buoyancy forces acting on the particle moving with a tangential velocity component U

at radius r are,

• Under the action of these forces, the particle moves inwards or outwards until the forces are balanced and the particle assumes its equilibrium orbit

• A relationship between U and the radius r for the vortex in a cyclone is needed

• For a rotating solid body, U = rw, where w is the angular velocity and for a free vortex Ur = constant

• For the confined vortex inside the cyclone body, it has been found experimentally that the following holds approximately:

• If we also assume uniform flow of gas towards the central outlet,

• Combining,

• Where r is the radius of the equilibrium orbit for a particle of diameter x

• If we assume that all particles with an equilibrium orbit radius greater

than or equal to the cyclone body radius will be collected,

• Then substituting r = R, we derive the expression below for the

critical particle diameter for separation, xcrit:

• Values of the radial and tangential velocity components at the

cyclone wall, UR and UR, may be found from a knowledge of

cyclone geometry and gas flow rate

• This analysis predicts an ideal grade efficiency curve

• All particles of diameter xcrit and greater are collected

• All particles of size less than xcrit are not collected

• In practice, gas velocity fluctuations and particle-particle interactions result in some particles larger than xcrit being lost and some particles smaller than xcrit being collected

• Consequently, the cyclone does not achieve such a sharp cut-off as predicted by the theoretical analysis

• Grade efficiency curve for gas cyclones is usually S-shaped

• Particle size for which the grade efficiency is 50%, x50, is often used as a single number measurement of the efficiency of the cyclone

• x50 is also known as the equiprobable size since it is that size of particle which as a 50% probability of appearing in the coarse product

• In a large population, 50% of the particles of this size will appear in the coarse product

• x50 is sometimes simply referred to as the cut size of the cyclone

Scale-Up of Cyclones

• Scale-up of cyclones is based on a dimensionless group, the Stokes number

• Characterizes the separation performance of a family of geometrically similar cyclones

• Stokes number Stk50 is defined as:

• Where m is gas viscosity, rp is solids density, v is the characteristic velocity and D is the diameter of the cyclone body

• Physical significance of the Stokes number is that it is a ratio of the centrifugal force (less buoyancy) to the drag force, both acting on a particle of size x50

• For large industrial cyclones the Stokes number, like the Euler number, is independent of Reynolds number

Range of Operation

• For a particular cyclone and inlet particle concentration, total efficiency of separation and pressure drop vary with gas flow rate as follows:

• Theory predicts that efficiency increases with increasing gas flow rate

• In practice, total efficiency curve falls away at high flow rates because re-entrainment of separated solids increases with increased turbulence at high velocities

• Optimum operation is achieved somewhere between points A and B, where maximum total separation efficiency is achieved with reasonable pressure loss

• Position of point B changes only slightly for different dusts

• Correctly designed and operated cyclones should operate at pressure drops within a recommended range

• For most cyclone designs operated at ambient conditions, this is between 500 to 1500 Pa

• Within this range, total separation efficiency ET increases with applied pressure drop

• Above the top limit the total efficiency no longer increases with increasing pressure drop and it may actually decline due to re-entrainment of dust from the dust outlet orifice

• It is therefore wasteful of energy to operate cyclones above the limit

Dynamics of Particulate systems (Part 3)

Chi-Hwa WangDepartment of Chemical and Biomolecular Engineering, National University of

Singapore, 4 Engineering Drive 4, Singapore, 117585, Singapore. E-mail: chewch@nus.edu.sg.

Investigation on Electrostatics in Fluidized BedYongpan Cheng, Eldin Wee Chuan Lim, Chi-Hwa Wang

Department of Chemical and Biomolecular Engineering

National University of Singapore, 4 Engineering Drive 4, Singapore

Jun Yao, School of Energy Research, Xiamen University, Xiamen, China

Guoqing Guan, North Japan Research Institute for Sustainable Energy, Hirosaki University,

Matsubara, Aomori, Japan

Chihiro Fushimi, Atsushi Tsutsumi, Collaborative Research Center for Energy Engineering,

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo

153- 8505, Japan

Y. Cheng, E.W.C. Lim, C.H. Wang, G. Guan, C. Fushimi, M. Ishizuka, A. Tsutsumi" Electrostatic

Characteristics in a Large-Scale Triple-Bed Circulating Fluidized Bed System for Coal

Gasification", Chem. Eng. Sci., 75, 435-444 (2012).

21

Outlines

Background for study of electrostatics.

Hydrodynamics in triple-bed combined circulating

fluidized bed.

Equivalent current in electrostatics in the riser and

downer.

Induced current in electrostatics below the cyclone.

Conclusions.

22

Background

Hazards

Alter hydrodynamics.

Cause agglomeration.

Interfere with instrumentation.

Generate nuisance discharge.

Create the danger of explosions.

………

Reduction of electrostatics.

Adding fine particles.

Adding anti-static agents.

Being well-grounded.

Increasing humidity.

Improving wall smoothness.

Altering pipe wall materials.

……..

23

Electrostatics are generated through tribocharging due to the

collisions and frictions between particle-particle or particle-wall

Ruins after explosions in Mill City Museum

in Minneapolis

Experimental set-up

24

Riser: Ф 0.1m ×15.7m

Downer: Ф 0.1m× 6m

Moving bed: Ф 0.15 m× 5 m

BFB: 0.75m × 0.27m × 3.4 m

Triple-Bed combined Circulating Fluidized Bed (TBCFB)

MPCT

MPCT

gsbUgrU

gdU

gdU : Air velocity in downer

grU : Air velocity in riser

gsbU : Air velocity in gas-sealing bed

Silica sand particles with mean particle

size are circulating in the

Circulating fluidized bed.

25

Particle size distribution

Sand particles are adhered to acrylic wall due to strong electrostatics.

Strong electrostatics can be observed.

128μmpd

Modular Parametric Current Transformer (MPCT)

When particles with charges pass through the tube, the equivalent

current will be formed due to the electromagnetic waves. This current

can be detected by MPCT, with the resolution of

Two locations for measurements: Top of riser, Middle of downer.

The flow is fully developed there.

26

Laptop computer

Air

Riser

MPCT

Electronics

card

Data acquisition

system

Particles with

charge

Current

-610 A

Variation along riser at air velocity in gas-sealing bed

Pressures decrease with the increasing height along the riser.

Solids holdups at the bottom are the highest, and almost approach

constant at the top region of riser.

27

4000 8000 12000 16000

0

2

4

6

8

10

12

14

16

He

igh

t H

,m

Pressure P,Pa

Ugr=6m/s

Ugr=8m/s

Ugr=9m/s

Ugr=10m/s

0.00 0.02 0.04 0.06 0.08

0

2

4

6

8

10

12

14

He

ight H

,m

Solids holdup

Ugr=6 m/s

Ugr=8 m/s

Ugr=9 m/s

Ugr=10 m/s

s s/( )P g H r D D

Solids holdup is calculated with pressure gradient method

gsb 0U

Solid mass flux variations under different

air superficial velocities in riser at

There is little variation on the solids mass flux due to the constant

driving force between bottoms of riser and gas-sealing bed.

28

5 6 7 8 9 10 1160

80

100

120

140

160

So

lid m

ass flu

x G

s, kg

/(m

2s)

Air superficial velocity in riser Ugr,m/s

gsb 0U

grU

Variations of average current and solids holdup in downer

The solids holdup and average current increase rapidly

first, then increase mildly, consistent with the trend of

solids mass flux.

29

0.00 0.02 0.04 0.06 0.080.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

Ave

rag

e c

urr

en

t I m

, A

Ugsb

, m/s

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Ave

rag

e s

olid

s h

old

up

,es

Induced current measurements below cyclone

30

Copper sheet

Polymer film

Aluminum film

Electrometer

1. Acrylic tube wall; 2. Copper sheet. 3. Polymer film. 4. Aluminum film

Induced current

Charge is transferred from particles to the wall,

then to the ground, thus forms the induced current

Instantaneous fluctuations of induced current at

With increasing solids mass flux, the

induced current is increased greatly.

At high solids mass flux, there is a

peak, which means the sparks

happen at this point.

31

0 10 20 30 40 50 600.0

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

3.0x10-6

Gs=92 kg/(m2s)

Ind

uce

d c

urr

en

t I,

A

Time t,s 0 10 20 30 40 50 600.0

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

3.0x10-6

Gs=228 kg/(m2s)

induced c

urr

ent

I,A

Time t,s

0 10 20 30 40 50 600.0

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

3.0x10-6

Gs=248 kg/(m2s)

Induced c

urr

ent

I,A

Time t,s

gr =6 m/sU

gsb 0U

gsb 0.068m/sU

gsb 0.034m/sU

Accumulated charge on the wall

Accumulated charges on the wall is significant at high

solids mass fluxes.

32

0 10 20 30 40 50 600.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

Charg

e v

ariation Q

,C

Time t,s

Ugsb=0

Ugsb=0.034 m/s

Ugsb=0.068 m/s

Q I dt

Summary

With increasing air velocity in the gas-sealing bed,

solids mass flux increased first, then approached a

constant. The solids holdups and average currents in

both riser and downer have the same trend.

The flow pattern in the riser has great influences on the

instantaneous equivalent currents.

The dominant frequencies in the riser are almost fixed,

the dominant frequencies in the downer are focused on

the low frequency regime.

Induced current on the wall increases with increasing

solids mass flux.

33

Studies of Solid-Solid Mixing Behaviors

in a Downer RectorYongpan Cheng, Eldin Wee Chuan Lim, Chi-Hwa Wang

Department of Chemical and Biomolecular Engineering

National University of Singapore, 4 Engineering Drive 4, Singapore

Guoqing Guan

North Japan Research Institute for Sustainable Energy, Hirosaki University,

Matsubara, Aomori, Japan

Chihiro Fushimi, Atsushi Tsutsumi

Collaborative Research Center for Energy Engineering, Institute of Industrial

Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-

8505, Japan

C. Fushimi, G. Guan, Y. Nakamura , M. Ishizuka, A. Tsutsumi, Y. Suzuki , Y. Cheng, E.W.C

Lim, C.H. Wang “Mixing behaviors of cold–hot particles in the downer of a triple-bed

combinedcirculating fluidized bed”, Powder Technology, 221, 70-79 (2012). 34

35

Adapted from C. Fushimi, A. Tsutsumi, “Advanced-Integrated Gasification Combined Cycle with Exergy

Recuperation,“, The University of Tokyo-Imperial College London Joint Symposium on Innovation in Energy Systems

at Imperial College London,, 1 February 2008.

Co-production flowchart for gasification

36

Heat carried particlesCOCO2

CO､H2

Pyrolysisgases

CharChar+particles

Steam

Pyrolyzer

CoalCombustor

Heat carried particles

Gasifier

Purification

Power generator

Steam

Gas turbine

air

Cooling water

Steam turbine

O2

Waste heat recovery boiler

Next generation high-efficiency A-IGCC (advanced Integrated coal

Gasification Combined Cycle) system

Power generator

Power efficiency can be increased by 9%

Outlines

Numerical simulation on mixing of sand and coal with

Discrete Element Method in downer.

Experimental study on mixing of cold and hot sand in

downer.

Conclusions.

37

38

Lab-scale triple-bed circulating fluidized bed

Riser: Ф40mm ×3,600mm

Downer: Ф 40mm× 2150mm

BFB: 300mm × 300mm × 500mm

Sand particles

Coal

Good mixing between sand and coal

particles is needed to guarantee

significant heat transfer between them

Normal Tangential

Governing equations

Continuity

39

0

f

f ut

rr

f D

f f e f

u Fuu p u g

t V

rr m m r

Momentum

Drag force1

D

V v vF

Drag coefficient

p

p

0.687

p p

p

24Re 0.5

Re

24 1.0 0.15Re / Re 0.5 Re 1000

0.44 Re 1000

D pC

p

pRerd vr

m

2

2

1.65

1.75(1 )150(1 )0.8

3(1 ) 0.8

4D

vn

L L

C v

r

r

Numerical method and parameters

40

Discrete Element Method is used with coupling between EDEM and FLUENT

Ergun, Wen & Yu model is used to calculate the drag force exerted on the particles by air

Sand density 2600 kg/m3

Coal density 1250 kg/m3

Sand particle diameter 1 mm

Coal particle diameter 1 mm

Diameter of nozzle 20 mm

Coefficient of restitution 0.9

Coefficient of friction 0.5

Coefficient of rolling friction 0.01

air velocity 2.5~10 m/s

Sand solids mass flux 87.5 kg/(m2s)

Coal solids mass flux 8.75 kg/(m2s)

Mixing of coal and sand particles at

41

Normal arrangement Tangential arrangement

f 2.5 m/sU

Mixing of coal and sand particle at

42

Normal arrangement Tangential arrangement

f 10 m/sU

Solids holdup distribution along downer at

43

Normal

Coal Sand

Z=0.1

Z=0.2

Z=0.5

Tangential

Coal Sand

f 2.5 m/sU

0

0.022

0

0.042

High holdupLow holdup

(Continued)

44

Z=1.0m

Z=1.5m

Z=1.9m

Normal

Coal CoalSand Sand

Tangential

Calculation of mixing index

Divide a certain section (length is 0.1m,

located at z=0.2, 0.5, 1.0, 1.5, 1.9 in a

downer into equal-sized 100 bins.

To minimize the influence of transient

fluctuations, the numbers of sand and coal

particle passing through each bin are

calculated within 1 s.

z

45

z=0.2

z=1.0

z=1.9

In each bin, mass ratio of coal over sand

3

coal coalcoal coal coal

i3sand sand

sand

4

3=4

3

coal

sandsand sand

N rm N

Xm NN r

r r

rr

Mixing index

Average mass ratio

46

22

p

1

N

i i

i

w X X

iw Weighting factor:

mass fraction of particle in a certain sample over the all the samples.

2

0 (1 )X X 0 Mixing index when it is completely segregated.

p

m

0

1I

Dimensionless mixing index

0 : Completely segregated 1: Completely mixing

1

1 N

i

i

X XN

Variation of mixing indices along downer

Along downer, mixing indices increase first, then approach constant

Tangential arrangement has higher mixing indices than normal

arrangement.

Higher air velocity corresponds to higher mixing indices.

47

0.0 0.5 1.0 1.5 2.00.6

0.7

0.8

0.9

1.0

Mix

ing

ind

ex I

M

z, m

Normal Tangential

Uf=2.5m/s

Uf=10 m/s

Experimental set-up

48

Riser: Ф0.1m ×15.7m

Downer: Ф 0.1m× 6m

Moving bed: Ф 0.15m× 5m

BFB: 0.75m × 0.27m × 3.4m

Experimental study on mixing in pilot plant

Numerical Study on the Hydrodynamics Behaviour in Biomass and Sewage

Sludge Co-Gasification

Coal Gasification

Coal Gasification System

• Heterogeneous reactions between char

and gases:

• Homogeneous gas phase reactions:

Objective:

To study the coal gasification in the downer reactor of a triple-bed combined circulating

fluidized bed.

Reaction in Gasifier

* Cheng, Yongpan, and Chi-Hwa Wang. "Numerical study on coal gasification in the downer reactor of a triple-bed combined

circulating fluidized bed." Industrial & Engineering Chemistry Research 53.16 (2014): 6624-6635.

Distribution of species volume fraction in the upper quarter section of the downer reactor.

* Cheng, Yongpan,

and Chi-Hwa Wang.

"Numerical study on

coal gasification in

the downer reactor of

a triple-bed combined

circulating fluidized

bed." Industrial &

Engineering

Chemistry Research

53.16 (2014): 6624-6635.

Coal Gasification

Main Findings:

• With increasing particle size beyond

500 μm, coal particles could not be

gasified completely due to shorter

residence time, leading to

decreasing production of CO and

H2.

• With increasing coal feeding rates,

the volume fraction of CO increased

monotonically, while that of H2

increased first and then approached

a constant value due to the limited

moisture availability in the coal

samples.

• With increasing air feeding rates,

more char and volatiles could be

decomposed into light gases.

* Cheng, Yongpan, and Chi-Hwa Wang. "Numerical study on coal gasification in the downer reactor of a triple-bed combined

circulating fluidized bed." Industrial & Engineering Chemistry Research 53.16 (2014): 6624-6635.

Effect of stream mass flow rate Effect of coal mass flow rate

Effect of particle size on produced gas composition

Results:

Co-gasification of biomass and solid waste

Comparisons of produced gas composition

and temperature profiles with experiments

Mole fractions of gas species

Results:

Y.P Cheng, Z. Thow, C.H. Wang, “Biomass Gasification with CO2 in a Fluidized Bed”, Powder Technology, 296, 87-101 (2016).

0 5 10 15 20 250.0

0.1

0.2

0.3

0.4

Mole

fra

ctio

n

Moisture contents, %

CO

CO2

H2

CH4

Effect of Wood

Moisture

Content

Effect of Wood

Particle Size

0 2 4 6 8 100.0

0.1

0.2

0.3

0.4

0.5

0.6

Mo

le fra

ctio

n

Wood particle diameter,mm

CO

CO2

H2

CH4

1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.60.00

0.05

0.10

0.15

0.20

CO

mo

le fra

ctio

n

CO2/biomass mass ratio

20% CO2

40% CO2

60% CO2

80% CO2

1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.60.20

0.25

0.30

0.35

0.40

0.45

0.50

CO

2 m

ole

fra

ctio

n

CO2/biomass mass ratio

20% CO2

40% CO2

60% CO2

80% CO2

Effect of CO2/Biomass Mass Ratio

Co-gasification of biomass and solid waste

Main Finding:

• Produced gas quality and cold gas

efficiency are enhanced with increasing

CO2-to-biomass mass ratio.

• 60% of CO2 in gasifying agent could be

defined as the optimal case due to higher

quality of produced gas and higher cold

gas efficiency.

• CO and CH4 fractions decrease while H2

and CO2 fractions increase with increasing

moisture content from 5 to 20%. In

contrast, CO, H2 and CH4 fractions

decrease while CO2 fractions increases

with increasing particle size of 2 ~ 8 mm.

• With higher moisture content and larger

particle size of wood, cold gas efficiency

and CO2 conversion rate are decreased.

Results:

Y.P Cheng, Z. Thow, C.H. Wang, “Biomass Gasification with CO2 in a Fluidized Bed”, Powder Technology, 296, 87-101 (2016).