fluidisation pneumatic/hydraulic conveying · fluid & particulate systems fluid &...

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Fluid and Particulate Systems 424521 /2018

FLUIDISATION

PNEUMATIC/HYDRAULIC CONVEYING

Ron ZevenhovenÅA Thermal and Flow Engineering

[email protected]

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6.1 Fluidised beds (FBs) :basic features

Åbo Akademi University - Värme- och Strömningsteknik Biskopsgatan 8, FI-20500 Åbo / Turku Finland

RoNz 2april 2018

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april 2018 Åbo Akademi University - Värme- och Strömningsteknik Biskopsgatan 8, FI-20500 Åbo / Turku Finland

3

Fluidised beds: basics

Bubbling fluidised

bedBR98

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FluidisationPhenomenon when solid particles are exposed to an up-going gas (or liquid) flow. When the gas velocity in a packed bed is increased to a level at which the pressure loss corresponds to the bed gravity, the bed expands and the particles will draw away from each other. The bed has then turned from a packed bed into a fluidised bed. The increased void fraction enables the gas to easier pass through, and a kind of equilibrium is attained.

A fluidised bed combustorapril 2018 RoNz 4Åbo Akademi University - Värme- och Strömningsteknik

Biskopsgatan 8, FI-20500 Åbo / Turku Finland

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8Fluidisation

The individual particles in a fluidised bed are in constant motion, colliding witheach other and with the walls of the vessel. Different flow types occur in fluidised beds and spouting beds.

spoutingbed

increasing gas velocity

april 2018 RoNz 5Åbo Akademi University - Värme- och Strömningsteknik Biskopsgatan 8, FI-20500 Åbo / Turku Finland

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Liquid-like behaviour

For visual purposes.


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8Fluidisation

Advantages:- good mixing of the particles → fairly uniform concentration and temperature- the vessel of well-mixed solids represents a large thermal flywheel that resists temperature changes- large contact surface between particles and gas → efficient heat and mass transfer, resulting in fast chemical reactions- constantly fresh particle surface due to abrasion- in certain cases, easy to handle due to the liquid like behavior of the gas-particle suspension

Disadvantages:- may need high fan power- particles may sometimes crumble too fast, sometimes get lumped together to an agglomerate, which can be difficult to fluidize- difficult to realize the principle of counter flow- erosion of the vessel and pipes can be big- expensive to regain particles (and powder)- inefficient contacting between gas and particles in bubbling beds of fine particles- rapid mixing cause non-uniform residence times of solids


Simplified model:Solid phase = perfectly mixedFluid phase = in plug flow

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Fluidised beds: applicationIndustrial examples:- drying- synthesis reactors- cracking- metallurgical processes- gasification of hydrocarbon and coke- FBC: combustion of solid fuels

(with SO2-capturing limestone)- polymerisation


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8Design considerations

1. Can the fluidised bed process be realized?

2. Pressure loss in order to evaluate the needed fan power.

3. Porosity (voidage) from the measurement of the vertical pressure profile.

4. Minimum fluidisation velocity required to transform the packed bed into a fluidised bed.

5. Terminal velocity of the particles in order to clarify when significant entrainment occurs.

6. Dimensional analysis for evaluating experimental and industrial fluidisationConditions: scale-up


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Geldart’s FB classificationThe first step in designing a fluidisation process is to clarify if it can be realizedwith the particles and gas in question. The particle diameter and density, as wellas the gas density, will tell what kind of fluidizing behavior can be expected.

Cohesive: difficult to fluidize due to cohesion

example: cement

Aeratable: bubble free velocity range exists

example: cracking catalyst

Sand-like: bubbles occur almostimmediately

example: construction sand

Spoutable: can be fluidised in aspouting bed, poormixing

example: coffee beans

Geldart’s classification of powders at room temperature and atmospheric pressure (derived for ambient air….)


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11

Fluidisation regimes

and Geldart’s

classification

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12

Vertical particle concentration

(“density”) profiles for various

fluidisation regimes

KL91

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6.2 FB Pressure drop


RoNz 13april 2018

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Pressure drop

distributor (perforatedsupport plate)

bed

How much fan power will be needed? The amount can be calculated from the press loss of the air distributor and the bed: power ≈ flow * pressure drop.

air inlet distributoror ”windbox”

The pressure drop over the air distributor that is required for uniform fluidisation is of the order of 0.2-1 times the pressure drop over the bed. Usually lowest pressure drop in circulating fluidised beds; highest in bubbling beds.


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8Pressure loss in the distributor

2

2holeg

distrloss,

wp d

g

gholedRe

dw

d

The pressure loss in an distributor can be calculated with a similar theory as used for connections in parallel: one opening several / many

d

11.2

Red ζd

100 2.163

300 2.041

500 2.163

1000 2.441

2000 2.687

>3000 2.777


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Pressure loss across the bed

The pressure loss in a packed bed is proportional to w1…2, but when the velocity is so high that the bed becomes fluidised, the pressure loss is more or less constant. The well aerated gas-solid suspension can then easily deform without appreciable resistance, like a liquid. The pressure required for injection of a gas at the bottom is roughly the static pressure of the gas-solid suspension, and is independent of the gas flow rate.

w is the superficialgas velocity

Pressure drop in a fluidised bed.


Useful for determining umf

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17

Pressure drop vs. velocity:

fixed fluidised bed

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Pressure drop across the bed

buoyancygravitydrag FFF

gVgmF pgpdrag

gVgVF pgppdrag

The theoretical constant pressure loss in a fluidised bed can be derived whenbalancing the forces that act on a non-accelerating particle in equilibrium state.

Fdrag

Fbuoyancy

Fgravity

gVgVF )1()(1)( gpdrag

ghghA

F )1(1 gp

cs

drag

ghAghAF )1()(1)( csgcspdrag

gploss 1 hgpapril 2018 RoNz 18Åbo Akademi University - Värme- och Strömningsteknik


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8Non-spherical particles

dpV

V

The bed is usually made up of non-spherical particles and for comparison it is most convenient to derive an equivalent spherical diameter dp, which is defined as the diameter of a equivalent sphere, which have the same volume as the non-spherical particle that the bed is made up of.

In empirical equations (e.g. Ergun equation) it is, however, the particle surface area that is important for evaluating the frictional resistance to gas flow and the heterogeneous chemical reactions. The form factor or shape factor 0 ≤ Ψ ≤ 1 is then used, and it is defined as the surface area of the equivalent sphere divided by the surface area of the actual particle that the bed is made up of.


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Pressure loss across the bedThe buoyancy force Fbuoyancy can in practical cases be neglected and the pressure loss of the fluidised bed can be estimated if the weight of the particles is known.

The packed bed porosity ε is then needed and can be derived by an Experimental approximate formula or from a diagram,

376.0

42.0

where the shape factor, or sphericity Ψ is the surface area of an equivalent sphere (equal volume as the particle) divided by the surface of the actual particle that the bed is made up of (≤1).

Fluidisation Engineering, KL91


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8Pressure drop across the bed

Be aware of difference between pressure drop (or loss) and (static) pressure difference.


For gas fluidised beds: the gas flow in excess of what is needed for fluidisation forms bubbles

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Pressure distribution (vertical)

loss1g10g0 pzgpzgp

The pressure balance for the fluid can be written (neglecting changes in the kinetic energy),

and combining it with the expression for the pressure loss in fluidised beds

gploss 1 hgp

gives the particle concentration (or gas concentration, i.e. porosity ε) at different heights in the fluidised bed when the pressure is measured.

11.3

Note that the fluid can in certain cases be a liquid, with a high density that affects the pressure balance


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8Minimum fluidisation velocity

p

2bedpackedloss,

2 d

wK

h

p

5.31

Re

3001

p3

K

wmf is the velocity when a packed bed is turned into a fluidised bed. It can be derived from the expression for pressure loss in packed beds,

and the expression for pressure loss in fluidised beds,

gpbedfluidizedloss, 1 hgp

These expressions are set equal and w is solved for. Both expressions canadditionally be utilized for evaluating the bed expansion (uniform porosity ε)at w > wmf.

11.4april 2018 RoNz 23Åbo Akademi University - Värme- och Strömningsteknik


Ergun for packed bed:General:

Interesting:independentof particle size!

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Terminal velocity

dt

dwmFFF buoyancydraggravity

02 pg

2Tg

Dprj

gVw

CAgm

Fdrag

Fbuoyancy

Fgravity

It can be derived from

If the particle is not accelerating, the equation can be written

Solving for wT results in

gDprj

pgT

)(2

CA

gVgmw

11.5

wT is the difference in velocity between the gas and the particle in fully developed flow conditions (no acceleration); the velocity of the gas when the particle is kept in place by the gas flow, or the falling velocity of a particle in a stagnant fluid. If the gas velocity around a particle exceeds the terminal velocity, the bed will loose the particle (entrainment).


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8Particles of different sizes

The bed has seldom uniform particles of the exact same size. A mean value ofthe diameter (Sauter mean diameter, SMD) can be derived from, (see course material #5)

i

i

d

X

d

1

from a mixture of particles having the mass fractions Xi with the diameters di.

This gives a mean diameter of the particles corresponding to “the rightparticle surface”, i.e. the particle diameter that has the same specific surface as that of the full distribution. The area can then be estimated from the weight.

11.1april 2018 RoNz 25Åbo Akademi University - Värme- och Strömningsteknik


Taking into account a size distribution p(x): p(x)dx is the chance of particle size x being in size range x x+dx

p

2bedpackedloss,

2 d

wK

h

p

Δ , ·

ρ ·

2 ·Example:

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Particle size distributions - general Particle size measurements give information on how

fractions of sizes are distributed according to number(d0), length (d), surface (d2) or volume ~ mass (d3).

In general:

or discrete:

For example Dsauter = D3,2

Volume mean diameter DVM = D4.3


RoNz 26

See

e.g.

http

://w

ww

.ther

mo

pedi

a.co

m/c

onte

nt/1

108/

Literature: A97!

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6.3 Fluidisation velocity, particle terminal velocity


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Gas velocities

The superficial gas velocity is defined for the whole cross sectional area.

The real gas velocity between the particles is higher and can be estimated if the porosity ε is known.


In a bubbling bedthe gas volume

that exceeds the gas volume for

minimum fluidisationwill form bubbles

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29

Minimum fluidisation velocity umf /1

gH

pFSmf

mf

fb ))(1(

Pressure drop across a fluidised bed (at minimum fluidisation conditons):

Pressure drop across a packed bed (Ergun):

pressure drop p, bed height H,

porosity ,gravity g,

fluid density F, dynamic viscosity F, particle diameter dp, particle density

S,flow velocity u,

particle sphericity

p

F

p

Fpb

d

u

d

u

H

p

2

323

2 )1(75.1

)(

)1(150

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30

Limiting cases: Remf small (“fine”), Remf large (“coarse”)

Minimum fluidisation velocity umf /2

Dimensionless groups: Remf, Ar

0 for large Remf 0 for small Remf

2

3p

2323

)(dAr Re

Re75.1

Re)1(150

F

FSF

F

Fmfpmf

mfmf

gud

Armf

mfmf

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31

Terminal particle (settling) velocity ut31

34

24

122

1

)1(

D

F

S

pt

ptFDFpp

C

g

du

duCgVgm

gravity - lift force (buoyancy) = drag force

mass mp, gravity g, volume Vp

fluid density F, dynamic

viscosity F, drag coefficient cD,

particle diameter dp, particle

density p, terminal velocity ut,

Reynolds number Re

)Re15.01(Re

241000Re2

Re

242Re if ;Re

678.0t

tDt

tDt

F

ptFt

Cif

Cdu

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32/74

Terminal velocity Dimensionless particle size, d* and velocity, u*

Determining terminal velocity, ut:

calculate Ar = dp* Figure u* calculate ut

F

Fpp

p

FSF

2F*

2F

FSFp

*p

udRe

Ar

Re

g)(uu

g)(dArd

31

31

31

31

with

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33/74

Chart for the determination of particle terminal settling velocitythrough a fluid

KL91

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34/74

Geldart’s classification and FB reactor types

F

Fpp

p

FSF

2F*

2F

FSFp

*p

udRe

Ar

Re

g)(uu

g)(d

Ard

31

31

31

31

with

KL91

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6.4 FB air distributors, non-mechanical valves


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Air (gas) distributor


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8Air (gas) distributor

The pressure drop over the air distributor that is required for uniform fluidisation is in the order of 0.2-1 times the pressure drop over the bed.


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38/74

Fluidisation: effect of gas distributor type

Ref: BR98

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39/74

Non-mechanical valvesF

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6.5 Fluidised bed modelling


RoNz 40april 2018

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41/74

The Kunii-Levenspiel bubbling bed model /1

Gas flow = gas flow via emulsion + gas flow via bubbles

i.e., with bed area A, and superficial velocity uo :

flow (uo-umf)*A via bubbles

flow umf *A via emulsion

mfb

b

mfb

mf

bb

uu

uuδ

uu

uuδ

ε

)gd(.u

-1 :emulsion in bed of Fraction

:bubbles in bed of Fraction

0 u u u :solids of velocity Rise

u :gas emulsion of velocity rise lSuperficia

u u :phase emulsion of velocity Rise

:bubbles of velocity Rise

down s,up s,s

mf

mf

mfe

KL91

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42/74

The Kunii-Levenspiel bubbling bed model /2

KL91

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43/74

Bed height and bubble sizeBed height vs. velocity :

Bubble diameter :(Ao ~ bottomdistributor plate area)

Bubble rise velocity:(Davidson & Harrison)

21

bmf0b

2.0

8.00

4.0mf0

b

b

mf0mf

)gd(711.0)uu(u

g

)A4h()uu(54.0d

u

uu

H

HH

http

://w

ww

.cd-

adap

co.c

om/p

ress

_roo

m/d

ynam

ics/

25/im

ages

/uav

_6.jp

g

KL91

Or: let a CFD calculationdetermine all thisbased on your input dataand your model selection

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6.6 FB heat and mass transfer


RoNz 44april 2018

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45/74

Particle fragmentation, attrition, abrasion, ...

BR98

attrition abrasion

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46/74

Emulsion-to-wall heat transfer

a. large particles, short contact time

b. small particles,long contact time

GAK97

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Emulsion-to-wall heat transfer /2

radiationconductionconvectiongasconvectionparticle

radiationconductionconvection

hhfhf

hhh

/,,

/

)(1

Heat transfer coefficient, h (W/m2K) :

where ƒ = fraction of wall covered by particles

problem:particle-to-wall distance, δ ??particle/wall contact time,τ ??

wall coverage, ƒ ??

TKK (Aalto)

98/99

Refs: GAK97,ZKTLB99

particlepparticleparticlegasconvectionparticle ch ,,

1

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Heat transfer in CFB combustion reactorsh ~ 100 … 1000 W/m2K

Ref: GAK97

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Single particle mass transfer in a CFBC riserNumin = 2 2

Compare with standard Ranz- Marshall equation (‘52):

Nu = 2 + 0.6 Re0.5Pr0.33

Imporant aspect consideringheat / mass transfer analogy :

inert, bed material particles areimportant from a heat transferpoint of view, not from a mass

transfer point of view.

Inert particles contribute to heat transfer, not to mass transfer !!

Ref: P98

Nusselt number

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6.7 Exercises 11


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8Exercises 11

11.1 Calculate the mean diameter of a material with following distribution,

11.2 A laboratory scale fluidised bed is fed with air (20ºC) with the volumetric flow of 125 l/min. The distributor (1 mm thick) has 170 small holes, each with a diameter of 0.3 mm. Calculate the pressure loss in the distributor.

11.3 A fluidised bed reactor (h = 10 m, Acs = 2,5 m2) is fed with 12.5 m3/s air (g = 0,310 kg/m3 and g = 44,4·10-6 kg/ms). The bed is made up of particles with a diameter dp of 0,320 mm with a density ρp of 2600 kg/m3. The static pressure has been measured at different heights,

Calculate the porosity, which is supposed to be constant, in the area between 0 m 0.2 m and 7 m 10 m.


RoNz 51

Cumulative weight of a 360 g sample [g]

dp smaller than [μm]

0 50 60 75 150 100 270 125 330 150 360 175

h (m) p (kPa) 0 120 0.2 118 7 112

10 111.8

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Exercises 11 11.4 Calculate the minimum fluidisation velocity wmf for a bed of sharp sand particles (

=160 μm, ψ=0.67, ρp=2600 kg/m3) when the fluidizing gas is ambient air (ρg=1.2 kg/m3, ηg=18·10-6 kg/(m s). The porosity of the packed bed is 0.55.

11.5 Can the particlesa) be retained in the bed although the gas velocity is higher that the terminal velocity?b) be lost from the bed although the gas velocity is less than terminal velocity?

11.6 An industrial fluidised bed reactor is going to use particles with dp=1.50 mm and ρp=2600 kg/m3. The fluidizing gas (ρg=0.45 kg/m3, ηg=4.3·10-5 kg/ms) has a planned superficial velocity of 5 m/s. The porosity need to be known before the actual process can be started, but because the particles and the gas are very expensive, air (ρg=1.2 kg/m3, ηg=1.9·10-5 kg/ms) and particles of a cheaper test material are used instead. Calculate by means of dimensional analysis the values for the test material, dp and ρp, as well as the superficial velocity of the air in the porosity evaluation experiment. Can the industrial fluidised bed reactor be utilized in the test?


RoNz 52

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6.8 Pneumatic conveying


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Solids transport methodsSuspended particles

Pneumatic (hydraulic) conveying Gravity chutes Air slides

Supported particles Belt conveyors Screw conveyors Bucket elevators Vibratory conveyors


RoNz 54

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8Pneumatic conveying

Negative (a) (i.e. ”vacuum” ~ 0.4 atm) and positive (b) pressure conveying, also combined (pushand pull) exists

(a) Dust free feeding, no leakage, toxic / hazardous solids

(b) Large distances, large loadings

Separation by cyclone, usually


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Pneumatic conveying

Physical properties of typical solids for pneumatic conveying


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8Pneumatic conveying: classification


RoNz 57

C06

kg solids / kg gas

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Pneumatic conveying: regimes

Increasingparticleloading

Often dense transport is associatedwith large pressure fluctuations


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8Pneumatic conveying: pressure drop Important for

power consumptioncalculation

Acceleration of particlesrequiressignificantpower

”Acceleration length”

Increasedsolid loading


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Pneumatic conveying

Flow regimediagrams (log-log)for fine particlepneumaticconveying

Jp = 0: only gas


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8Pneumatic conveying: drag reduction

Left:

Right:

Typically at 0.5 – 2 kg/ kg solids loading, turbulent, dp < 0.2 mm


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Minimum transport velocity /1

(taken from Fan and Zhu, 1998 chapter 11)

U = mean gas stream velocity; Dd = pipe diameter;µ, ρ = gas viscosity, density; αp= particle volume fraction;ρp = particle density;


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8Minimum transport velocity /2

(taken from Fan and Zhu, 1998 chapter 11)


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Pneumatic conveying Pressure drop for flow

of solids in pneumaticconveying (comparedto air velocity) for particle velocity us, solid feed rate F,particle terminal velocity uo.

Electrostatic chargingchanges us and Δp !!

Example experimentalresult for a given system at up to 35 m/s air velocity


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8Pneumatic conveying: air movers


RoNz 65

Rotary lobe (”roots”) blower

C06

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6.9 Hydraulic conveying


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8Hydraulic conveying Liquid (water)

instead of gas (air) used for transport

Much lowervelocities

Solids blockageeasier avoided

Three types:– Homogeneous– Settling, vertical– Settling, horizontal

horizontal


PRESSURE DROP ESTIMATION: SEE C06 section 4.2

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Transitional velocities (horizontal)


RoNz 68

C06

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8Critical velocity: Wilson’s diagram (1979)


RoNz 69

C06

For a 0.4 m pipe,0.2 mm particlesstart settling at Ucm ~ 3 m/s ifS = ρS/ρL = 2.65

Corrects to Ucm ~ 2.3 m/s ifS = ρS/ρL = 2.0

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Critical velocity: Wani’s correlations (1982)


RoNz 70

C06

αs = volumetric solids concentration m3/m3.

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8Hydraulic conveying Example: coal mining tailings

transport to a storage ”lagoon” at 1,1 km, rate 15 kg/s


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6.10 Exercises 12


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8Exercises 12

12.1

12.2


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74

Sources / further readingBR98: G L Bormand KW Ragland “Combustion engineering” McGraw-Hill (1998) Chapter 17CRBH83: Coulson, J.M., Richardson, J.F., Backhurst, J.R., Harker, J.H. “Chemical Engineering, Vol. 2 : Unit

Operations” Pergamon Press, Oxford (1983) FZ98: L-S Fan, C Zhu “Principles of gas-solid flows” Cambridge Univ. Press (1998)GAK97: Grace, J.R., Avidan, A.A., Knowlton, T.M. (Eds.) "Circulating fluidised beds", Chapman & Hall, London

(1997)IGH91: Iinoya, K., Gotoh, K., Higashitani, K. “Powder technology handbook”, Marcel Dekker, New York

(1991) KL91: D Kunii, O Levenspiel “Fluidisation engineering” 2nd ed, Butterworth-Heinemann (1991)P98: Palchonok, G.I. “Heat and mass transfer to a single particle in a fluidised bed” Chalmers Univ. of

Technol., Sweden, Ph. D. thesis (1998)PL98: Peirano, E., Leckner, B. “Fundamentals of turbulent gas-.solid flows applied to circulating fluidised bed

combustion” Progr. Energy Combust. Sci. 24(1998) 259-296ZH00: R. Zevenhoven, K. Heiskanen ”Particle technology for thermal power engineers” part 1 & part 2, post-

graduate course ene-47.200, TKK, Espoo, Sept./Oct. 2000ZKLTB99: Zevenhoven, R., Kohlmann, J., Laukkanen, T., Tuominen, M., Blomster, A.-M. “Suspension-to-wall

heat transfer in CFB combustion: near-wall particle velocity and concentration measurements at low and high temperatures” Proc. 6th Int. Conf. on CFB, Würzburg, Germany, August 1999 (J. Werther, Ed.), Frankfurt/Main (1999) 959-965

C06: Crowe, C.T., ed., ”Multiphase Flow Handbook” CRC Press, Taylor & Francis Group (2006) Chapters 4-5A11: Ahlskog, M. ”Dimensioning of Polymer Pipelines for Slurries” MSc thesis ÅA KT VST 2011A97: Allen, T. Particle size measurement. Vol. 1, Powder sampling and particle size measurement, and Vol. 2,

Surface area and pore size determination. Chapman & Hall (1997)

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