Your Success is Our Goal OPTIMIZATION OF HEAT TRANSFER ZONES IN DISTILLATION COLUMNS X International PHOENICS.

Your Success is Our Goalwww.siemens.com/itps1 www.chemtech.com.br

OPTIMIZATION OF HEAT TRANSFER ZONES IN DISTILLATION COLUMNS

X International PHOENICS Users Conference

Melbourne, May 2004

Your Success is Our Goal

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Chemtech Solutions


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Chemtech - A Siemens Company, Rio de Janeiro / RJ – BrazilChemtech - A Siemens Company, Rio de Janeiro / RJ – Brazil

Petrobras / CENPES, Rio de Janeiro / RJ – BrazilPetrobras / CENPES, Rio de Janeiro / RJ – Brazil

Bruno de Almeida BarbabelaBruno de Almeida Barbabela

Flávio Martins de Queiroz GuimarãesFlávio Martins de Queiroz Guimarães

Silvia WaintraubSilvia Waintraub

Glaucia TorresGlaucia Torres

AUTHORS


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INTRODUCTION

The use of empty spray sections (no packing) in heat transfer regions of vacuum towers allows a high deep cut operation, due to the reduction of the pressure drop along the column.

Simulation studies show that a reduction of 1 mmHg in pressure results in an increase of approximately 0.3% in the gas oils yield. This represents a profit of about US$ 13,000,000 per year for a typical unit with a feed rate of 30,000 m3/day. Other benefits are the decrease in investment and maintenance cost.

Petrobras has four vacuum towers in Rio de Janeiro’s refinery designed by Badger Limited in 1975 without any device in the top pump-around section. In the present work we will present the CFD model developed to analyze this kind of tower.


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INTRODUCTION

Spray Nozzles

Chimneys


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• PHOENICS VERSIONPHOENICS VERSION:: 3.4

• MULTI-PHASE MODELMULTI-PHASE MODEL:: IPSA FULL

• INTERPHASE PROPERTIESINTERPHASE PROPERTIES:: By GROUND Coding.

Customization based on PETROX (Petrobras’ Process Simulation Tool) routines for calculation of petroleum fractions properties;

• SPRAY FORMATION MODELSPRAY FORMATION MODEL:: By GROUND Coding.

Spray formation sources based on nozzle characteristics;

GENERAL SETTINGS


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VALIDATION PHASE

OPTIMIZATION PHASE

TUNNING PHASE

Two-dimensional model (simplified)

Three-dimensional model

Two-dimensional model (simplified)

Three-dimensional model

SIMULATION STRATEGY


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GEOMETRY – THE EMPTY SPRAY SECTION


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GEOMETRY – THE SPRAY NOZZLES


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GEOMETRY – THE CHIMNEYS


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ANALYSIS – THE CHIMNEYS

VELOCITY ISO-SURFASE


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• Spray generated droplets have a constant and uniform diameter (though it can change along the tower);

• No interaction between droplets;

• Since the droplets are very small, they behave as a film, without temperature gradient from the bulk of the droplet to the interface;

• The vapor phase has an ideal gas mixture behavior;

• Only the main petroleum fractions were considered for the properties calculation;

• Diffusive transport of the petroleum fractions into the same phase was not considered.

CONSTRAINTS


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ENERGY TRANSFER

The heat transfer between phases is presented below (without convective and diffusive terms) and was based on the heat transfer of small spherical droplets in a continuous vapor phase:

LVp

jjj

jjj Ht

mTTAph

t

TCpV ...... int

Heat Transfer Coefficient (h)

p

D

D

kNuh

.

31

21

Pr.Re.6,00,2 DDNu Nusselt Number


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MASS TRANSFER

The mass transfer follows the model below and was based on the petroleum fractions properties calculated by the PETROX routines:

ipip MANt

m..

,,.. iSipci CCAKN

Mass Transf. Coef.p

Dmic D

NuDK

.,

Molar Flux (Ni)


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ATOMIZATION DROPLET SIZE

Sauter Diameter (D32)

...,,,32 pcpPfD o

Correlations for the Sauter diameter are presented at Mugele [10] and Lefebvre [12].

The Sauter diameter is employed in atomization efficiency studies where mass transfer and chemical reactions are presented.


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VALIDATION

Temperature Profile (phase 5)

40

50

60

70

80

90

100

110

120

24900 25900 26900 27900 28900

Elevation (mm)

Tem

per

atu

re (

Cel

siu

s)

PHOENICS

Experimental


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VALIDATION

Height of Heat Transfer Zone

0

500

1000

1500

2000

2500

3000

3500

0,60 0,80 1,00 1,20 1,40 1,60

Reflux (normalized)

He

igh

t (m

m)

PHOENICS

Exp.

Exp.

PHOENICS


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VALIDATION

40

50

60

70

80

90

100

110

120

24900 25900 26900 27900 28900

Height above ground (mm)

Te

mp

era

ture

(C

els

ius

)

PHOENICS 2D

Field Data

PHOENICS 3D


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VALIDATION – CONCLUSIONS

• The computational model fits well the experimental data for the 2D model, although it must be enhanced for the 3D model.

• No significant drag of the oil droplets was notified at the current operational conditions.

• From the analysis of the results, the current 60o spray cone seems to be not suitable. A wide range nozzle is recommended.


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Vacuum TowersCAPACITY FACTOR ANALYSISRESULTS – CS = 0.05 ft/s

Temperature Velocity


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Temperate Velocity


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RESULTS – SPRAY STABILITY

Heat Transfer Height vs. CsHeat Transfer Height vs. Cs

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Fator de Capacidade (CS)

Alt

ura

de

Tro

ca

Té

rmic

a (

m)

Capacity factor (Cs)

Hei

gh

t (m

)


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RESULTS – DROPLET SIZE

Dragged Liquid vs. Droplet SizeDragged Liquid vs. Droplet Size

Droplet diameter (micron)

Dra

gg

ed L

iqu

id (

%)


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Vacuum TowersWIDE RANGE NOZZLE (90o CONE)RESULTS – CS = 0.14 ft/s



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VelocityTemperature


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Vacuum TowersTWO SETS OF NOZZLES (90o CONE)RESULTS – CS = 0.14 ft/s



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VelocityTemperature


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CONCLUSIONS

• The maximum capacity factor allowed for spray cone stabilization simulated (between 0.325 and 0.375 ft/s) was similar to the values reported by experimental observations and experts opinions;

• Since the simulations showed that the effective height of the heat transfer zone has low dependence on the mean droplet size, it is recommended that the spray nozzles were adjusted to be biased toward the generation of greater droplets in order to minimize the liquid drag;

• The use of wider angles of sprays reduces the effective height of heat transfer zone despite the loss of spray stability;

• The use of two levels of distributors is recommended for improve the spray cones stability and reduce the effective height of the heat transfer zone.


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CONCLUSIONS

• Although some improvements have to be made, CFD seems to be an useful tool to analyze the performance of heat transfer and spray formation in vacuum towers. The use of this technology on the optimization of current towers and on the project of new ones is recommended to improve their performance.


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NEXT STEPS

• Petrobras will use the model to perform other studies:

• Variation of the liquid reflux temperature

• Variation of the height of the spray nozzles

• Calculation of the global heat transfer coefficient

• To enhance the model of the petroleum mixture, considering more fractions in it.


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Kontaktadresse:

Peter MusterI&S GCSchuhstraße 6091052 ErlangenTel: 09131-7-24607

Mail: [email protected]


www.siemens.com/itps1 www.chemtech.com.br

Contact:

Flávio GuimarãesSenior Manager

Tel: +55 (21) 3233-5100

Mail: [email protected]

THANK YOU

Your Success is Our Goal OPTIMIZATION OF HEAT TRANSFER ZONES IN DISTILLATION COLUMNS X International PHOENICS.

Documents

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