week #7 dynamics of particulate systems (part 2) 7.pdfon the electrostatics of pneumatic conveying...

Week #7Dynamics of Particulate systems (Part 2)

Chi-Hwa WangDepartment of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4,

Singapore, 117585, Singapore. E-mail: [email protected].

On the Electrostatics of Pneumatic Conveying of Granular Solids Using Electrical Capacitance Tomography (ECT)

Kewu Zhu1, Chi-Hwa Wang1,2,

Shuji Matsusaka3, Hiroaki Masuda3

1 Singapore-MIT Alliance, 4 Engineering Drive 3, Singapore 117576

2 Department of Chemical & Environmental Engineering,

National University of Singapore, 4 Engineering Drive 4, Singapore, 117576

3 Department of Chemical Engineering,

Kyoto University, Kyoto 615-8510, Japan

CV1

Recycle

Feed Hopper

Solid Feed

Hopper

Air from

compressor

mains

CV2

CV3

Rotary air

lock feeder

Rotameter

Weight

Indicator

PC

0.71 m

0.85 mECT

Experimental

P

Physical properties of particles

used in this work

Present study considered coarse PP particles compared to 500m glass powder, and anti-static powders

Material Mean particle diameterx103

(m)

Particle density (kg/m3)

Glass powder 0.5 2980

Polypropylene2.8 1123

Larostat-519 0.02 520kg/m3

Pneumatic Conveying Electrostatic Analysis

– Characterize wall charge accumulation.

– Investigate concentration measurement under static charge effect.

– Examine the effects of operation conditions on the wall charge accumulation.

– Study the mechanism for special pattern formation.

Schematic diagram of voltmeter measurement

h

U

h = 43 mm

h1 = 4 mm

= 450

1

2

3

4

5

6

1 – sensor 4 – voltmeter

2 – pipe segment 5 – support

3 – copper plane 6 – clamp

h1

Contact potential measurement

Electrometer

Power voltage supply

Powder

Vibrating

upper electrode

Lower electrode

Electromagnetic

shield

Thermohygrostat

Computer

' 20 / 2

AV V dSample Au a

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 500 1000 1500

Time (Min)

Vo

lta

ge

(V

)

Glass Powder

PVC

PP

Larostat 519

Contact potential measurement for particles and inner wall of PVC pipe

Variation of time averaged solids concentration distribution –influence of electrostatic charges

horizontal pipe1.2 m from downstream bend

Gs = 0.08 kg/s

Charge accumulation at the wall during pneumatic conveying system – effect of conveying velocity

Vertical pipeZ =2.05 m

Gs = 0.08 kg/s

Charge accumulation at the wall during pneumatic conveying system – by ECT & electrostatic voltmeter

t, sec

0 1000 2000 3000 4000 5000 6000 7000 8000

vo

lta

ge

, k

v

-16

-14

-12

-10

-8

-6

-4

-2

0

0.00

0.02

0.04

0.06

0.08

0.10

0.12

A

B

C

D

Pneumatic conveying of polypropylene granules. Solids mass flow rate Gs

= 0.08 kg/s. A: surface potential, U = 18.9 m/s, horizontal conveying; B:

surface potential, U = 17.3 m/s, vertical conveying; C: solid concentration

, U = 18.9 m/s, horizontal conveying; D: solid concentration, U = 17.3

m/s, vertical conveying.

(a)

(b)

t

(d)

(c )

N

S

EW

Slugging flow

Ug = 13.0 m/s

Gs = 7.0 kg/(m2.s)

Z = 2.05 m

Distribution of polypropylene particles in a vertical riser flow –annular capsule flow

K. Zhu, S.M. Rao , C.H. Wang, and S.

Sundaresan “Electrical Capacitance

Tomography Measurements on the

Vertical and Inclined Pneumatic

Conveying of Granular Solids“, Chem.

Eng. Sci. 58(18) 4225-4245 (2003).

Induced current measurement

Test station B

Polymer film

electrometer

Pipe wallSections A & C

Aluminum foil

t, sec

0 5 10 15 20

i, m

icro

A

-40

-20

0

20

40

60

80

(a)

time, sec

0 5 10 15 20

s

0.0

0.2

0.4

0.6

0.8

1.0

plane1

plane 2(b)

(a) MPCT measurement

(b) ECT Measurement

U = 14.3 m/s, Gs = 0.08 kg/s

Moving capsule flow

Influence of Larostat-519 antistatic powder

U = 23.4 m/s, Gs = 0.08 kg/s(a) w/o larostat(b) with larostat

K. Zhu, S.M. Rao , Q.H. Huang, C.H. Wang, S Matsusaka, and H. Masuda, “On the Electrostatics of Pneumatic Conveying of Granular Materials Using Electrical Capacitance Tomography“, Chem. Eng. Sci., 59(15) 3201-3213 (2004).

Scanning electron micrographs of polypropylene particles – after conveying

(a) (b)

(a) – with addition of larostat 519 powder

(b) – without larostat 519 powder

Glass Powder Tests

Experiments are repeated for glass powder (meandiameter = 500 mm, density = 2980 kg/m3). Similartrends in the induced current are found withcomplicated polarity changes.

It is noted that the induced currents detected for afresh pipe segment (as high as several nA) aresignificantly larger than that for a used pipe (less than0.1nA).

The surface potential measured using the electrostaticvoltmeter increases slowly from -0.2kV only to around~3 kV for a used pipe, while the surface potentialincreases drastically to a value higher than 20 kV for afresh pipe.

This observation confirms the induced currentmeasurement results and suggests that attrition ofconveying pipe may considerably alter the pipeproperties for static electricity.

Summary (5)

ECT measurement drifting is due to electrostatic charge

Reliable solid volume fraction can be obtained if the electrostatic charge influence is considered

Wall charge accumulation are characterized by ECT measurement and this is consistent with electrostatic voltmeter observation

Wall electrostatic charges are enhanced with lower conveying velocities at the same solids flow rate

Current due to charged particles detected by MPCT were in good agreement with ECT measurement

Addition of antistatic powder (larostat 519) can reduce the charge generation

Discrete Element Method Simulation for Pneumatic Conveying Systems

i

i

k

ii i c,ij d,ij f ,i

j 1

k

ii ij

j 1

dvm m g f f f

dt

dI T

dt

Cundall, P. A. and O. D. L. Strack. A discrete numerical model for granular

assemblies. Geotechnique, 29, 47–65. 1979.

•Discrete Element Method (DEM)

•Originally developed for describing the mechanical

behavior of assemblies of discs and spheres

DISCRETE ELEMENT METHOD

ctanff

RR

ttvf

nnvf

tf

nf

nmax,s

jjii

iir

i,tij,dt

iiri,nij,dn

iij,ti,tij,ct

iij,ni,nij,cn

COMPUTATIONAL FLUID DYNAMICS

f

f f

u 0

uuu p g F

•Coupling between CFD and DEM via fluid-particle drag

force model

•Additional source term in momentum equation to

represent reaction force on fluid

FLUID DRAG FORCE

f ,i f 0,i i

2

f 0,i d0,i f i i i i i

2

10 p,i

f f

f 0.5c R u v u v

1.5 log Re3.7 0.65exp

2

Di Felice, R. The voidage function for fluid-particle interaction systems. Int.

Journal Multiphase Flow, 20, 153–159. 1994.

GRANULAR FLOW SIMULATIONShape of particles Spherical Number of particles 5310 Particle diameter, d 1.8 10-3 m

Particle density, p 2980 kg m-3

Particle-particle coefficient of restitution, ep 0.95 Particle-wall coefficient of restitution, ew 0.97

Spring constant in discrete model, 5000 N m-1

Solids volume fraction, 0.15

Aspect ratio, /d 33.3

Simulation time step, t 10-7 s

Wang, C.-H., R. Jackson, S. Sundaresan. Instabilities of fully developed rapid flow of a granular material in a channel.

Journal of Fluid Mechanics, 342, 179–197. 1997.

GRANULAR FLOW SIMULATION

f = 0.0f = 0.1

Kx = 0.1

f = 10.0

Kx = 3.5

GAS FLUIDIZATION

Zhu, K., S. M. Rao, C.-H. Wang, S. Sundaresan. Electrical capacitance tomography measurements on vertical and

inclined pneumatic conveying of granular solids. Chemical Engineering Science, 58(18), 4225–4245. 2003.

Dispersed Flow Plug Flow

PNEUMATIC CONVEYING

Shape of particles Spherical

Type of particles Polypropylene

Number of particles 500, 1000, 1500, 2000

Particle diameter, d 2.8 10-3 m


Spring constant in force model, 5.0 103 N m-1

Viscous contact damping coefficient, 0.35

Coefficient of friction 0.3

Gas density, f 1.205 kg m-3

Gas viscosity, f 1.8 10-5 N s m-2

Pipe diameter 0.04 m

Pipe length 1.0 m

Computational cell size 4 mm 4 mm

Simulation time step, t 10-7 s

Material properties and system parameters

VERTICAL PNEUMATIC CONVEYING

HORIZONTAL PNEUMATIC CONVEYING

PHASE DIAGRAMS

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

12 14 16 18 20 22 24 26

Gas velocity (m s-1

)

So

lid

flo

w r

ate

(k

g s-1

)

2000 particles

1500 particles

1000 particles

500 particles

Plug Flow

Dispersed Flow

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

8 12 16 20 24 28 32

Gas velocity (m s-1

)

So

lid

flo

w r

ate

(k

g s-1

)

2000 particles

1500 particles

1000 particles

500 particles

Slug Flow

Homogeneous Flow

Moving dunes

MD/H

S/H

vertical pneumatic conveying horizontal pneumatic conveying

Transient development of solid flow rates

0.0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10

Time (s)

So

lid

flo

w r

ate

(k

g s-1

)

24 m s-1

(Dispersed Flow)

22 m s-1

(Dispersed Flow)

20 m s-1

(Dispersed Flow)

18 m s-1

(Dispersed Flow)

15 m s-1

(Transition Dispersed/Plug Flow)

14 m s-1

(Plug Flow)0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10

Time (s)

So

lid

flo

w r

ate

(k

g s-1

)

30 m s-1

(Homogeneous Flow)

26 m s-1

(Homogeneous Flow)

22 m s-1

(Homogeneous Flow)

18 m s-1

(Transition Homogeneous/Moving dunes)

14 m s-1

(Moving dunes)

10 m s-1

(Moving dunes)


PRESSURE DROPS

100

200

300

400

500

12 14 16 18 20 22 24 26

Gas velocity (m s-1

)

Pre

ssu

re d

rop

(P

a m

-1)

2000 particles

1500 particles

1000 particles

500 particles

0

200

400

600

800

1000

8 12 16 20 24 28 32

Gas velocity (m s-1

)P

ress

ure

dro

p (

Pa

m-1

)

2000 particles

1500 particles

1000 particles

500 particles


Summary (7)

The Discrete Element Method utilizing a linear spring-dashpot-friction slider force-displacement model was coupled with Computational Fluid Dynamics and used for the simulation of pneumatic conveying of granular material in both vertical and horizontal pipes in this study.

The simulation results obtained were in good agreement with previously reported experimental observations in terms of the types of flow patterns arising at different operating conditions used.

These various flow regimes and their corresponding operating conditions have been represented in the form of phase diagrams. Quantitative data such as solid flow rates and pressure drops of the gas also show good agreement with well established trends for such pneumatic conveying operations.

Electrostatic Characterization

J. Yao, Y. Zhang, C.H. Wang, S. Matsusaka, H. Masuda, “Electrostatics of the Granular Flow in a Pneumatic Conveying System“, Ind. Eng. Chem. Res., 43, 7181-7199 (2004).

Disperse flow – pattern observed in the vertical

pipe

Initial condition

Two hours later

The clusters were located fairly high up in the pipe and traveled along a curved path by the pipewall. These clusters appeared and disappeared intermittently in an unpredictable manner.

J. Yao, Y. Zhang, C.H. Wang, S. Matsusaka, H. Masuda, “Electrostatics of the Granular Flow in a Pneumatic Conveying System“,Ind. Eng. Chem. Res., 43, 7181-7199 (2004).

Ring flow - vertical granular pattern

Initial condition

Fifteen minutes later

Particles were observed totravel in a spiral fashion up the vertical pipe along thepipe wall. This resulted in a ring or annulus structure with high particle concentrations adjacent to the wall and a relatively empty core region

J. Yao, Y. Zhang, C.H. Wang, S. Matsusaka, H. Masuda, “Electrostatics of the Granular Flow in a Pneumatic Conveying System“,Ind. Eng. Chem. Res., 43, 7181-7199 (2004).

Induced current measurement

Test station B

Polymer film

electrometer

Pipe wallSections A & C

Aluminum foil

K. Zhu, S.M. Rao , Q.H. Huang, C.H. Wang, S Matsusaka, and H. Masuda, “On the Electrostatics of Pneumatic

Conveying of Granular Materials Using Electrical Capacitance Tomography“, Chem. Eng. Sci., 59(15) 3201-3213 (2004).

t, sec

0 5 10 15 20

i, m

icro

A

-40

-20

0

20

40

60

80

(a)

time, sec

0 5 10 15 20

s

0.0

0.2

0.4

0.6

0.8

1.0

plane1

plane 2(b)

(a) MPCT measurement

(b) ECT Measurement

U = 14.3 m/s, Gs = 0.08 kg/s

Moving capsule flow

K. Zhu, S.M. Rao , Q.H. Huang, C.H. Wang, S Matsusaka, and H. Masuda, “On the Electrostatics of Pneumatic Conveying of Granular Materials Using Electrical Capacitance Tomography“, Chem. Eng. Sci., 59(15) 3201-3213 (2004).

Time(second)

I(A

)

2000 4000 6000 8000

0

5E-08

1E-07

1.5E-07

2E-07

2.5E-07

Disperse flow

Half-ring flow

Ring flow

Negative

Induced current – vertical pipe

Time(second)

Ch

arg

eQ

(C)

0 2000 4000 6000

0

2E-05

4E-05

6E-05

8E-05

Disperse flow

Half-ring flow

Ring flow

Negative

(a) Comparison of the current value (negative) for the three flows. (b) Comparison of the charge accumulation for the three flows.

J. Yao, Y. Zhang, C.H. Wang, S. Matsusaka, H. Masuda, “Electrostatics of the Granular Flow in a Pneumatic Conveying System“, Ind. Eng. Chem. Res., 43, 7181-7199 (2004).

Summary: Electrostatics in Pneumatic Conveying

Air flow rate is a key factor determining the electrostatic behavior of granularflow. The lower the air flow rate, the higher the induced current and particlecharge density. These in turn lead to particle clustering and the formation ofsuch structures as half-ring and ring in the vertical conveying pipe.

Electrostatic effects increase with time. The charge accumulated at the pipewall increases with time and the rate of increase seems constant for each ofthe three types of flow. Particle charge density also increases with time andthis may account for clustering behavior occurring at the vertical pipe walleven when a high air flow rate is used and the dominant flow regime is that ofdisperse flow. Pipe wall material has an obvious effect on the electrostatics ofthe granular flow.

Electrostatic effects depend on composition for particle mixture. Thecommercially available anti-static agent, Larostat-519 powder, was found toreduce electrostatic effects within the system effectively.

The mechanism of electrostatic charge generation for the granular flow in thepneumatic conveying system mainly depends on tribroelectrification due tostrong force effect on the surface when the particles slide on the pipe wall.

DEM Simulation

• Newton’s Laws of Motion

• Force-displacement Model

N

1j

i,fiij,dij,ci

i mdt

dm fgff

v

N

1j

ij

i

idt

dI T

ω

ij,ni,nij,cn δf

ij,ti,tij,ct δf

iiri,nij,dn nnvf

jjiiiiri,tij,dt RωRωttvf

Reversed flow in pneumatic conveying in an inclined pipe

g

DEM Simulation

• Fluid Drag Force Model 1

ii,0fi,f

ff

iiii

2

i

2

ifi,0di,0f Rc5.0 vuvuf

2

Relog5.1exp65.07.3

2

i,p10

2

5.0

i,p

i,0dRe

8.463.0c

f

iiiif

i,p

R2Re

vu

Di Felice, R. The voidage function for fluid-particle interaction systems. Int. J. Multiph. Flow 1994, 20, 153.

Fluidized bed simulation using DEM

DEM Simulation

• Computational Fluid Dynamics

0t

u

Fguuu

u

fff

f Pt

Pneumatic Conveying simulations using DEM

V2

V1

0 0.25 0.5 0.75 10

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

V2

V1

0 0.25 0.5 0.75 10

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

V2

V1

0 0.25 0.5 0.75 10

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

Simulation Conditions

Material Properties and System ParametersShape of particles Spherical

Type of particles Polypropylene

Number of particles 500, 1000, 1500, 2000

Particle diameter, d 2.8 10-3

m


Spring constant in force model, 5.0 103 N m

-1

Viscous contact damping coefficient, 0.35

Coefficient of friction 0.3

Gas density, f 1.205 kg m-3

Gas viscosity, f 1.8 10-5

N s m-2

Pipe diameter 0.04 m

Pipe length 1.0 m

Computational cell size 4 mm 4 mm

Simulation time step, t 10-7

s

Rao, S. M.; Zhu, K.; Wang, C. H.; Sundaresan, S. Electrical capacitance tomography measurements on the pneumatic

conveying of solids. Ind. Eng. Chem. Res. 2001, 40, 4216.

Simulation Conditions

• Particles first allowed to settle under gravity for 0.5 s

before gas flow was initiated

• Periodic boundary conditions applied to the solid phase

to simulate an open flow system

• Solid concentration, , defined as overall volume

fraction of particles divided by volume fraction of

particles at maximum packing (0.64)

Results and Discussion

Dispersed Flow

= 0.08

Gas velocity 14 m s-1

Plug Flow

= 0.32

Gas velocity 14 m s-1

W.C. Lim, C.H. Wang, and A.B. Yu, “Discrete Element Simulation for Pneumatic Conveying of Granular Material” AIChE Journal, 52, 496-509 (2006).


Stratified Flow

= 0.08, Gas velocity 10 m s-1

Moving dunes


Slug Flow


Homogeneous Flow




• The different flow regimes in vertical

pneumatic conveying are represented

in the form of phase diagrams

• Dashed lines separate regions

representing different flow regimes

while dashed circles enclose regions

where transition between two adjacent

flow regimes might be taking place

• In vertical pneumatic conveying, the

dispersed flow regime is dominant at

high gas velocities and low solid

concentrations while the plug flow

regime is dominant otherwise

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

12 14 16 18 20 22 24 26

Gas velocity (m s-1

)

So

lid

flo

w r

ate

(k

g s-1

)

= 0.32

= 0.24

= 0.16

= 0.08

Plug Flow

Dispersed Flow



• Similarly, the homogeneous flow

regime is dominant at high gas

velocities and low solid concentrations

while the slug flow regime is dominant

otherwise in horizontal conveying

•At low gas velocities and solid

concentrations, effects of gravitational

settling result in the formation of the

moving dunes and stratified flow

regimes

• Intermediate values of gas velocities

involve transitions between moving

dunes and homogeneous flow and

between stratified and homogeneous

flow

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

8 12 16 20 24 28 32

Gas velocity (m s-1

)

So

lid

flo

w r

ate

(k

g s-1

)

= 0.32

= 0.24

= 0.16

= 0.08

Slug Flow

Homogeneous Flow

Moving dunes

MD/H

S/H



• The solid concentration profile for

dispersed flow in vertical pneumatic

conveying shows that solid

concentrations are higher near the

walls than in the center of the pipe

• This trend is similar for all gas

velocities simulated

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.00 0.01 0.02 0.03 0.04

Radial position (m)

So

lid

co

nce

ntr

ati

on

Gas velocity

14 m s-1

16 m s-1

18 m s-1

20 m s-1

24 m s-1



• The solid concentration profiles in

horizontal pneumatic conveying show

quantitatively the effects of

gravitational settling which results in

higher solid concentrations along the

bottom wall of the pipe

•As before, the solid concentration

profiles are quantitatively similar for

different gas velocities used

0.00

0.01

0.02

0.03

0.04

0.00 0.10 0.20 0.30 0.40 0.50

Solid concentration

Ra

dia

l p

osi

tio

n (

m)

Gas velocity

14 m s-1

18 m s-1

22 m s-1

26 m s-1

30 m s-1


week #7 dynamics of particulate systems (part 2) 7.pdfon the electrostatics of pneumatic conveying...

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