Computational Fluid Dynamics

Chemical Engineering Practice VI

Carlos Teixeira – Paulo Ramos - Seyedali Emami

DEQ – FEUP 2011/2012

What is CFD?

The physical aspects of any fluid flow are governed by the

following three fundamental principles [1]:

Mass is conserved;

F = m·a (Newton’s second law);

Energy is conserved.

Continuity equation

Momentum equation

Energy equation

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Evolution of CFD applications:

Figure 1 - Mapping of the Evolution of Patent

Applications filing [2] Figure 2 - Mapping of the Evolution of

Scientifics Articles over time [2]

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CFD Applications:

CFD can be used in [3]:

• Aerospace & Defense

• Automotive

• Construction

• Health care

• Energy

• Chemical Processing

• Etc.

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Application of CFD in water waste treatment

Activated sludge process (ASP) plants includes the following stages

[3]:

Inlet works

Primary settlement

Activated sludge treatment

Secondary settlement

Tertiary treatment

CFD can be used to;

• Find capital cost savings,

• Achieve performance improvements,

• Energy savings.

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Application of CFD in water waste treatment [3]

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Company: Cambados WWTP

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Company : Cambados WWTP

• Working since 1985, this station was the first depuration station at

Oporto city. Despite being a potential user of CDF technology,

actually it isn’t.

• This WWTP receives the wastewater produced in several parishes

of Maia. The station have a capacity of 7040 m3 per day for a total

of 10000 habitants but it only use 1882 m3.

• This station treats wastewater from households and then they are

drained to river with an acceptable level of pollution through an

emissary.

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Company: Treatments

The wastewater treatment process is characterized by two

phases

Treatment of Liquid

phase

Preliminary treatment

Biologic treatment

Treatment of solid phase

Thickening Mechanical dehydration

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Company: Treatments

• The decanted sludge is accumulated and then are returned

to the activated sludge tank. The decanted activated sludge,

when in excess, are send to a thickener tank to reduce the

moisture content. [4]

• After thickening, the sludge is dried, whose concentration

reaches 20% solids, and are destined for composing to the

Parada WWTP. [4]

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Current State of Design

Wastewater treatment vessels are sized according to required

pollutant removal: [5]

- Requires knowledge/assumptions of the flow regime (i.e.

hydrodynamics)

- Vessel configuration design to achieve flow type relies on

empirical correlations and heuristic techniques

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Hydrodynamics: Importance for Wastewater Treatment

The hydraulic behavior in an activated sludge tank, is of

fundamental importance for the efficiency of the process[6].

Hydraulic phenomena with negative effect on performance[6]:

-Short circuiting streams

-Dead volumes

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CFD: Alternatives

Models developed by the International Water Association are

useful tools for the process control. However, they are

unsuitable to model the influence of the reactor geometry; [7]

- Length/width ratio

- Presence of baffles

- Effluent inlet device

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CFD: Alternatives

Full-scale tracer tests are very informative when investigating

hydraulic situations in activated sludge tanks however [6];

-It claims a lot of personal

-And mean residence times are in order of days

So it becomes impracticable

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Why Design Using CFD in a WWTP?

A CFD model can simulate the hydrodynamics of a design before

implementation [8]:

- Reducing lead-up times and costs

- Ultimately lead to optimization of reactor configuration

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Why Design Using CFD in a WWTP?

Empty tanks in ETAR de Cambados

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Conclusions

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• CFD modeling of wastewater processes is developing

• CFD design can lessen reliance on potentially

expensive trial-and-error methods

Acknowledgments

We would like to thank to Eng. Atilia Silva for welcome us

at ETAR de Cambados.

ETAR de Cambados

Address:

Rua de Cambados

Vila Nova da Telha

Maia

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Question?

Thank

you

for

your

attention.

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References

[1] Anderson, Jr., John D.; Computational Fluid Dynamics. ISBN: 0-

07-113210-4

[2] Islabão, G. et al (2010). Technological Trends in CFD Applications.

Journal of Technologyl, Managment and Innovation, 76-83.

[3] http://www.ansys.com/

[4] http://www.ambiente.maiadigital.pt

[5] L. Benedetti et al (2006). Benchmarking of WWTP design by

assessing costs,. Water Science & Technology, 95–102.

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References

[6] Kjellstrand R. et al (2005) Short Circuiting in a Denitryfying

Activated Sludge Tank. Water Science & Technology, Vol. 52, No. 10-

11, pp 79-87, IWA

[7] Moullec, Y. L. (2010). CFD simulation of the hydrodynamics and

reactions in an activated sludge channel. Chemical Engineering

Science, 492 - 498.

[8] Brannock, Matthew (2003). Computational fluid dynamics tools

for the design of mixed anoxic wastewater treatment vessels PhD

Thesis, School of Engineering, The University of Queensland.

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