nunhez.pdf

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Multiphase CFD: the use of Optimization models and the

importance of full scale in Multiphase Modelling

JosRoberto Nunhez and Collab orato rsFaculty of Chemical Engineering, UNICAMP

State University of Campinas

E-mail: [email protected]


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JosRoberto Nunh ezFaculty of Chemical Engineering,

UNICAMP State University of Campinas

E-mail: [email protected]

Nicolas SpogisESSS South America ANSYS representative

Email: [email protected]

Design of a high efficiently hydrofoil through the use of

computational fluid dynamics and multi-objective optimization


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3

Solid SuspensionsGet a Lift

In a project completed at UNICAMP in

Brazil, an optimization procedure was

applied to the design of an impeller to

illustrate how this approach can lead tomore efficient mixing processes (and

why not other processes?) in general.

The suspension of solid particles in a

stirred tank was used to illustrate the

methodology.


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4

ANSYS Advantage

ANSYS Advantageuses real examples.

How engineers,analysts and designerscan use CFD moreeffectively.

Articles detail howSimulation DrivenProduct Developmentcan reducedevelopment time,prototype testing and

time-to-market; andultimately helpcompanies win the racein product innovation.


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5

AIChE Journal

AIChE J, V55, issue 7,pp17231735, 2009


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6

Optimization

Microdigital1983

29 years

PGQE + L-CFD alone

have in their labs 520

cores with aproximatly 30

TFLOPs capacity;


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7

Optimization

1950 A6000

LIGHTNIN Mixers - 1994

44 years


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8

Design of a high efficiently hydrofoil:

Pure Axial Flow

Lack of Radial Flow

Torque Reduction Saving Costs

Through the use of: Computational Fluid Dynamics

Multi-Objective Design Optimization

Study Objectives


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Still provide good bottom

mixing when located

2-3 diameters off bottom.

Allows the use of shorter

and smaller diameter shafts

Customer Benefits

Pure Axial Flow

Results in less side force

Lower bending moments

and hence gives

longer life for gear

reducers and shaft seals.

Lack of Radial Flow

Up to 50%

resulting in increase

of the gear reducer life

use of a smaller

gear reducer.

Torque Reduction

Savings in operating costs

combined with first

cost savings

giving the customer

a two-fold benefit.

Saving Costs

Hydrofoil impellers advantages


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10

CAD and Mesh

Generation

Post Process

CFD Solver

Impeller

Multidisciplinary

Optimization

Impeller optimization steps

Icem Scripts

Batch Scripts

CFX Scripts
http://www.perl.com/


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Modeling considerations

Unsteady Navier stokes equations in theirconservation form;

Mass conservation equation;

This work used the Alternate Rotation Model thatsolves numerically the absolute frame velocity

instead of the relative velocity;

The Frozen Rotor model was used in a 120o sector

of the reactor;


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Modeling considerations

The Shear Stress Transport (SST) turbulence modelwas used since it is more accurate and robust for

the prediction of problems involving flow separation;

In order to predict turbulence correctly, it isimportant to have at least a layer of control volumes

capturing the effect of the boundary layer, which is

assessed by the dimensionless variable y+. The

average y+

is about 1 for the SST turbulence model.


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Impeller Process Types

Energy Level Process Type P/V Relation (kW/m)

Low level Weak solid suspensionLow viscosity fluids

0.2 kW/m

Moderately Heat transfer

Gas dispersion

0.6 kW/m

High Level Heavy solid suspension

High gas dispersion

Emulsification

2 kW/m

Higher Level High viscosity fluids

Pasta 4 kW/m


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CalculateNew rotationto keep P/V

-

Solve Npsteady state

-

Solve P/Vsteady state

Solid Suspensiontransient

Geometry Generation

Mesh Generation

,

Post ProcessNp Nq, Variance, etc

Impeller optimization steps

Input Variables

Output Variables

53.. DN

PNp

35.. NpD

PN


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Input Variables

Parameters Parameter Value

Tank diameter T = 1m

Height of the liquid H= T = 1m

Bottom clearance C=H/4 = 0.25m

H

C


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Input Variables

D/T

R_HD T_HD


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Variable Minimum Value Maximum Value Discrete / Continuum

Impeller diameter 0.4 0.5 Continuum

Root chord 0.1 0.2 Continuum

Tip chord 0.1 0.2 Continuum

Root chord angle 20 degrees(related to rotation axis)

70 degrees(related to rotation axis)

Continuum

Tip chord angle 30 degrees(related to rotation axis)

95 degrees(related to rotation axis)

Continuum

Root profile DAE11, S1223, E387, FX 63-13717 different profiles

Discrete

Tip profile DAE11, S1223, E387, FX 63-137

17 different profiles

Discrete

Input Variables


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Blade ProfilesLow Reynolds Hydrofoil


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Vessel solid concentration

variance

Heavy solid suspension

Pure Axial Flow

Lack of Radial Flow

Pumping effectiveness

Torque Reduction Saving Costs

n

i

i CC

ns

1

22

1

1

p

q

N

N

Pefect

Objective Functions


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A Sobol's algorithm was chosen

to generate an initial population

of 3000 individuals, than 50

individuals was filtered by a D-

Optimal algorithm in order to

reduce the preliminary

investigation computational time.

DOE - Preliminary investigations


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RSM - Response SurfacesSolid Concentration Variance


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RSM - Response SurfacesPumping Effectiveness


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DOEScatter Chart - Preliminary investigations


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Optimization Algorithm

Multi-Objective GeneticAlgorithm (MOGA-II)

MOGAs search method alsohas very interesting aspects:

it allows global solutions to befound

it especially guarantees anactual multi-objectiveoptimization, where the Paretofrontier is defined in the end.

Reproduction operators:

Classical crossover;

Directional crossover;

Mutation;

Selection.


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Pareto Frontier

Dominated Points

Non Dominated Points


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Problems:

Average axial Flow

Average radial flow

High NpTorque

High power consumption

Low solid suspension

Initial Impeller DesignPBT 45


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Solutions:

Pure axial Flow

Lack of radial flow

Low NpTorque

Low power consumption

High solid suspension

Optimized Impeller


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Solutions:Low Hub Separation

Low Tip Vortex

Low Shear

Problems:High Hub Separation

High Tip Vortex

High Shear

Hub and Tip Vortex


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Experimental tests

In the accomplishment of the

experimental tests an agitationsystem was projected that consists:

Vertical Vessel with torospherical

10% bottom;

Sustentation Structure in stainless

steel 316L; Baffles, made use in the interior of

the tank;

SEW Electric Motor - 1,5 kW;

Control panel;

MAGTROL torquimeter TM 300series, accuracy


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Power Number evaluation

6% error


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Solid Suspension

N=290 RPM ->P = 6.09 W N=172 RPM ->P = 6.06 W

N=390 RPM ->P = 10.54 W N=232 RPM ->P = 11.23 W

O


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Optimized Blade Design

S lid S i


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Solid Suspension

O ti i d Bl d D i


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Solid concentration variance wasreduced by 48.5%

Power consumption reduced by84.4%

Pumping effectiveness increasedby 410.2%

When compared to the

performance of a standardpitched blade impeller (45degrees constant Tip ChordAnglePBT45).

Optimized Blade Design

O ti i d Bl d D i P t t 2


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Industrial hydrofoil impellersrequire a higher Power number,compared to the previousimpeller. It should be ideallybetween 0.1 and 0.3

The model was run again using

now another restriction on theoptimization model. We set therestriction Np > 0.2

The model arrived at acompletely different impeller

Optimized Blade DesignPrototype 2Previous impeller

Prototype 2 impeller



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Optimized Blade DesignPrototype 2

VELOCITY VELOCITYVECTOR PLOT



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Optimized Blade DesignPrototype 2

SOLIDSCONCENTRATION

STREAMLINES


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The Importance of Using Three-Phase 3-D Models in

a Real Scale Simulation of Industrial FCC Risers.

University of Campinas

School of Chemical Engineering

Gabriela Cantarelli Lopes


Leonardo Machado da Rosa

Milton Mori


Waldir Pedro Martignoni

Jos Roberto Nunhez


P bli h d


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Introduction

Published papers

FCC P


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IntroductionWhat is the FCC process and how does it work?

part of the petroleum refining:

FCC Process

Crude oil Distillationresiduum

FCC

converts the heavy fractions into more valuablelighter fractions

FCC P


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Introduction

it is necessary because of the increase on the oil

demand and prices along with the decline of the oil

fields;

large profits result from any further improvement

either in the process, or in the equipments.

Why is the FCC process so important to the refineries?

FCC Process

FCC P


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Introduction

FCC Process

Riser reactor

The need for improvement in the FCC

process calls for advances in:

feed injection technology;

highly active catalysts;

geometric configurations.

FCC Process


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Introduction

Experiments Numerical studies

CFD

Multiphase flows

better understanding of the phenomena;

allows the investigation of ways to

improve the performance of the FCC

process.

FCC Process

FCC Process


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FCC Process

Literature review

models present

varying degrees of

simplifications and

assumptions.

Complete physical model of the FCC riser

reactor (Source: Gao et al., 2001)

Research Objective


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Research Objective

3-Dimensional:test of different geometries

2-Phase 3-Phase

CFD simulations using a sophisticated modeling.

Modeling and Simulation


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Eulerian - Eulerian - Lagrangian approach;

solidgas liquid



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EulerianEulerian:

continuous

Conservation Equations: momentum, mass and energy

3-Phase Model

Momentum transfer: Guidaspow drag model;

Heat transfer: Ranz-Marshall correlation;Turbulence: Reynolds stress model (RSM);

Particle fluctuations: KTGF.

M d li d Si l ti


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3-Phase Model

Lagrangian:

discrete

droplet is calculated individually

Trajectory: Gravity + Drag force (Morsi-Alexander);

Heat transfer (gas and droplet) : Ranz-Marshall;

Mass transfer (gas and droplet): difference of concentration.



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Reactions

4-Lump Model:

The set of cracking reactions are simplified



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Setup

Solver: ANSYS FLUENT 12.0;

User-defined functions for heterogeneous

kinetics;

Hexaedrical meshes: 1 million volumes;

Transient simulation: 15 sec. of flow;

16 partitions solved in parallel.



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Geometries

L-shape T-shape

Previous studies in cold

setup:

Smooth exits:less

turbulence and solidbackmixing. High wear

by friction.

Abrupt exits:less

maintenance.



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Geometries



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Cases

gas

+

solid

+

liquid

gas

+

solid

Case 2

gas

+

solid

+

liquid

Case 1 Case 3




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Operating conditions

Feed (oil mass flux) 20 kg/s

Catalyst-to-oil ratio 7 kg/kg

Steam 7 wt%

Feed oil temperature 500 K

Catalyst inlet

temperature

900 K (2-

Phase)

960 K (3-Phase)

Droplet diameter 0.1 mm

Results


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Flow characteristics

Average cross-section values

Results

Results


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Results

Vaporization

Max. res. time: 0.2 seconds(Gao et al., 2001)

Results


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Results

Exit shape effects

Case 1 (L-shape exit) Case 3 (T-shape exit)

Solid velocity streamlines

Results


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Results

Case 1 (L-shape exit) Case 3 (T-shape exit)

Exit effects

Height = 32m

Results


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Results

Height = 32m

Exit effects

Results


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Results

Exit effects

Results


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(Ali et al.,

1997)

9%

Results


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Acknowledgements


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Acknowledgements

The students and colleagues mentioned in the

presentation of both works;

Petrobras for the funding of the FCC project;

ESSS for the support on the Optimizationproject;

The organizers of the workshop Trends in

Physical and Numerical Modeling for IndustrialMultiphase Flow for the invitation.

Brain storm


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Brain storm

Do you believe the use of optimization models withCFD is a promising way to improve equipments

performance?;

Since turbulence in multiphase flow is hardly

scalable, and both area/volume ratio and shearstress change both in scaling up/down, do you agree

we should aim at simulating equipments at their real

size?
http://www.cenpra.gov.br/


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Thank you!
http://www.cenpra.gov.br/

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