S1_10_50_Phillip_Fawell_rev.pdf

8/9/2019 S1_10_50_Phillip_Fawell_rev.pdf

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How flocculant selection can influencesolids dilution requirements inthickener feedwells

M Tanguay1, P Fawell1, A Grabsch1 and S Adkins2

1 2


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Flocculation is a sensitive process

• Flocculated aggregates are

usually fragile.

• Need to achieve the right

balance of applied shear,reaction time, dosage and

solids concentration.

Solids concentration

S e t t l i n

g

r a t e

•

Optimising these variablesin a thickener feedwell can

be quite difficult.


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Where do we come in?

• AMIRA P266 has applied

computational fluid

dynamics (CFD) to full-scale

feedwell optimisation.• Adding a flocculation model

(PB-CFD) allows prediction

of aggregate size.

• Can readily capture solidsconcentration effects on

flocculation performance.


4/18

How to deal with high solids feeds?

• Some feedwells can provide a degree of natural

dilution prior to flocculation, but difficult to control.

• Direct feed dilution elevates settling rate requirements.

• External solids dilution devices (E-duc, Turbo-dil) utiliseclarification zone liquor.

• Flocculant choice can reduce the need for dilution:

–

Limited examples of this having a large influence. – Never previously studied by CFD.

– Focused on one product (Rheomax® DR 1050 from BASF).


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Flocculation kinetics in pipe flow

Profiles obtained for both flocculants across a range of dosages, solids

concentrations and flow rates, then a population balance (PB) applied.

Magnafloc® 336 Rheomax® DR 1050


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Modelling outcomes

3 f D

agg

eff s

p

d

d

Fractal dimension (D f )→ 2.40 with Magnafloc® 336

→ 2.55 with Rheomax® DR 1050

Magnafloc® 336Rheomax® DR 1050

®®®®


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Fractal aggregate structures

Fractal dimension 2.51Fractal dimension 2.05 Fractal dimension 2.40


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Computational domain

Inlet flow rate

Inlet velocity

Feed solid concentration

Flocculant dosage

Flocculant concentration

Overflow rate

1000 m3 h-1

1.5 m s-1

5,10,15,20% w/w

20 g t-1

0.01%

150 m3 h-1

Deliberately set low to keep the bed low

and reduce its impact on feedwell flows


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Example PB-CFD output

• A snapshot in time for

just one condition.

• Shows that there is a

wide variety of particle/aggregate paths.

• 50000 paths considered

for each case.


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Predicted settling velocities

vs. time in thickener30

25

20

15

10

5

00 50 100 150 200

Time (s)

S e t t l i n g v e l o c i t y ( m

h - 1 )

Magnafloc®

336

Rheomax®

DR 1050

5% w/w

10% w/w

15% w/w

20% w/w

Shelf height

Feedwell exit


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Predicted solids throughput

Injected slurry

concentration

Maximum throughput of solids (t h-1)

Rheomax® DR 1050 Magnafloc® 336

5% w/w 168 141

10% w/w 323 18215% w/w 359 157

20% w/w 348 159

• First time PB-CFD has been used to predict throughput.

• First demonstration of the potential impact of

flocculant selection.


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Mean path analysis

• Particle paths processed with a 1D flocculation model

taking conditions (flocculant concentration, solid

fraction, shear rate, etc.) from the CFD solution.

• Aiming to depict process from the point of view of afinite set of particle flowing in the thickener.

• Two approaches attempted:

– to average the processed output of each particle path.

– to process the average of all particle paths.

• Averaging performed on 500 particle paths.


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1D flocculation model vs. full PB-CFD


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1D flocculation model vs. full CFD

• Processing individual particle paths and averaging results is okay.

• Processing the averaged particle paths is not okay.


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But what does it mean?

• Variability in particle paths may play a significant role

in the overall output of the thickener.

• Full scale thickener cannot be treated as a reactor

producing a homogeneous output: – Our CFD modellers get to keep their jobs.

– Still may be scope to refine/speed-up the analysis.

• PB-CFD does show that achieving a denser aggregate

from feedwell flocculation should produce a higher

flux under the right conditions.


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But what does it mean in practice?

• Just because you can get a higher flux, doesn’t mean

you will, or even need to.

• Flocculant selection may have reduced impacts:

– When rise velocity of the thickener is low.

– When applied dosages are low or solids dilution is high.

• In particular, benefits from higher aggregate density

may not be realised in sub-optimal feedwells:

– Shear is too high, leading to excessive breakage.

– Shear is too low, leading to poor mixing/short-circuiting.


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Acknowledgements

• This work has been part of ongoing collaboration

between CSIRO and BASF.

• It also builds upon techniques developed within the

AMIRA P266 “Improving Thickener Technology” series ofprojects (see www.p266project.com), of which BASF has

been a long-term sponsor.
http://www.p266project.com/http://www.p266project.com/

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