Prof Barry Crittenden, Dr Mengyan Yang

FOULING THRESHOLDS IN BARE TUBES AND TUBES FITTED WITH INSERTS

University of Bath

Department of Chemical Engineering

Sustainable Thermal Energy Management Conference

SusTEM2010

Newcastle 2-3 November 2010

OUTLINE

1. The crude oil fouling problem

2. Experimental results from bare tube and

tube fitted with a hiTRAN insert

3. Computational fluid dynamics3. Computational fluid dynamics

4. Equivalent linear velocity for tubes fitted

with inserts

5. Fouling model & threshold modelling

6. Conclusions

PARTNERSHIP

Centre for Process IntegrationSchool of Chem Eng & Analytical Science

University of Manchester

PO Box 88, Sackville Street

Manchester M60 1QD

Prof Robin Smith

Dr Jin-Kuk Kim

Cal Gavin Ltd

Minerva Mill Innovation CentreStation Road

Alcester B49 5ET

Dr Jin-Kuk Kim

Dr Igor Bulatov Martin Gough

Department of Chem Eng

University of Bath

Bath BA2 7AY

Prof Barry Crittenden

Dr Mengyan Yang

Embaffle B.V

Coengebouw

Kabelweg 37, 1014 BA

Amsterdam

Stuart Oakley

Eric van der Zijden

CRUDE OIL PREHEAT EXCHANGER TRAIN

Between 16 and 60 shell & tube exchangers in the pre-heat train

DEPOSITS IN CRUDE PREHEAT EXHANGER TRAINS

DOWNTIME, BUNDLE PULLING AND CLEANING

01E102 rf

0.02

0.025

0.03

TYPICAL GROWTH IN HEAT EXCHANGER FOULING RESISTANCE

Thermal resistance

0

0.005

0.01

0.015

01/0

8/20

0401

/09/

2004

01/1

0/20

0401

/11/

2004

01/1

2/20

0401

/01/

2005

01/0

2/20

0501

/03/

2005

01/0

4/20

0501

/05/

2005

01/0

6/20

0501

/07/

2005

01/0

8/20

0501

/09/

2005

01/1

0/20

0501

/11/

2005

01/1

2/20

0501

/01/

2006

01/0

2/20

0601

/03/

2006

01/0

4/20

06

01E102 rf

Date

SCHEMATIC OF THE PILOT-SCALE PARALLEL TUBE APPARATUS

0

0.0002

0.0004

0.0006

0.0008

0.001

0 1 2 3 4 5

Linear velocity (m/s)

Fo

ulin

g r

ate

(K

m2W

01h

-1)

523K 538K 543K 553K

0

0.0001

0.0002

0.0003

0.0004

0 0.5 1 1.5 2 2.5

Liner velocity (m/s)

Fo

ulin

g r

ate

(K

m2W

-1h

-1)

523K 538K

Bare MDI insert

EXPERIMENTAL CONDITIONS

Velocity Re Initial (clean) surface temperature

(m/s) 250°C 265°C 270°C 280°C

0.5 3600 Bare & MDI Bare & MDI N/A Bare

0.8 5800 Bare N/A N/A Bare

1.0 7300 Bare & MDI Bare & MDI Bare Bare

1.5 11000 Bare & MDI Bare & MDI Bare Bare

1.8 14500 Bare & MDI Bare Bare Bare

2.0 21800 Bare Bare Bare Bare

3.0 26200 Bare Bare Bare Bare

4.0 29000 Bare Bare Bare Bare

hiTRAN TUBE INSERTS

Reproduced from www.calgavin.com

FOULING THRESHOLD CONDITIONS

Temperature

(K)

Linear velocity

bare tube

(m/s)

Linear velocity

tube with insert

(m/s)

523 4.01 2.23

538 4.16 2.43

543 4.48 No data

553 4.58 No data

Over the surface of heat exchanger tubes, in particular with tubes fitted with inserts, it is desirable to know:

The velocity distribution

The shear stress distribution

The temperature distribution

NEED TO KNOW

The temperature distribution

The heat flux distribution

CFD simulation can provide answers for both bare tubes and tubes

with inserts

k-ε model – basic equations

Equation of continuity

Equation of momentum

Equations of turbulent kinetic energy (k) and dissipation rate of turbulent energy (ε):

CFD SIMULATION USING COMSOL SOFTWARE

( ) ( )[ ] ρεησ

ηηρ

ρ−∇+∇+

∇

+⋅∇=∇⋅+

∂

∂ 2T

T

k

T uukkut

k

k

( ) ( )[ ]k

CuukCCut

TT

2

2

2

1

ερρε

σ

ηηερ

ερεµε

ε

−∇+∇+

∇

+⋅∇=∇⋅+

∂

∂

For the heat transfer equations, the turbulence results in an effective

thermal conductivity keff :

keff = ko + kT

kT = CpηT/PrT

CFD SIMULATION FOR FLUID FLOW IN BARE TUBE

Velocity field for flow in bare tube of 19 mm ID

Axial symmetric geometry, the upper boundary

represents the central axial.

Left end: inlet; Right end: outlet

Inlet velocity: 1m/s, bulk temperature: 423K

Shear stress can be calculated from the turbulent viscosity and velocity gradient obtained by CFD.

COMPARISON OF THE SHEAR STRESS OBTAINED

BY CFD AND FRICTION FACTOR METHODS

Velocity (m/s) Shear stress

CFD

(Pa)

Shear stress

Friction factor

(Pa)

Re Friction factor

0.5 0.8 0.8 6909 0.0087

1 2.9 2.8 13818 0.00731 2.9 2.8 13818 0.0073

2 9.9 9.3 27636 0.0061

3 19.2 18.9 41455 0.0053

4 32.2 31.3 55273 0.0052

CFD SIMULATION FOR FLUID FLOW IN TUBE WITH INSERTS

Velocity field

Z (flow direction)

0 0.013

Tube (19mm ID) with medium density inserts (hiTRAN)

Linear flow rate: 1m/s, bulk temperature: 423K

Vertical slice

WALL SHEAR STRESS DISTRIBUTION

20.00

30.00

40.00

50.00

60.00

Sh

ea

r s

tre

ss

(P

a)

0.00

10.00

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

Position in z direction

Sh

ea

r s

tre

ss

(P

a)

0.5m/s 0.7m/s 1m/s 1.5m/s

Shear stress data are obtained from the velocity gradient

and the turbulent viscosity by CFD simulation

COMPARISON OF THE SHEAR STRESS VALUES BETWEEN BARE TUBE AND TUBE WITH INSERTS OBTAINED BY CFD

Velocity (m/s) Wall shear stress

Bare tube (Pa)

Lowest wall shear stress

Tube with insert (Pa)

0.5 0.8 1.3

1.0 2.9 6.4

2.0 9.9 38

PRESSURE DROPS OBTAINED BY CFD SIMULATIONAND MEASURED BY CAL GAVIN

Linear velocity

(m/s)

Pressure drop (kPa/m)

hiTRAN® data

Pressure drop (kPa/m)

CFD simulation

0.5 2.53 4.43

0.8 5.94 6.880.8 5.94 6.88

1.0 9.00 13.12

1.6 21.91 25.33

2.0 33.76 38.46

EQUIVALENT VELOCITY OF THE TUBE FLOW WITH MEDIUM DENSITY INSERTS

Equivalent velocity = 0.2461x2 + 1.1369x

R2 = 0.999

2

2.5

3

3.5

4

4.5

5

Eq

uiv

ale

nt

bare

tu

be

velo

cit

y (

m/s

)

15

20

25

30

35

40

45

Sh

ear

str

ess (

Pa)

■: Velocity; ¤: Shear stress

0

0.5

1

1.5

2

0 0.5 1 1.5 2 2.5 3

Velocity - Tube with inserts (m/s)

Eq

uiv

ale

nt

bare

tu

be

velo

cit

y (

m/s

)

0

5

10

15

Sh

ear

str

ess (

Pa)

APPLICATION OF THE MODEL DEVELOPED FOR FOULING

IN BARE TUBE TO TUBE WITH INSERTS

• Modify Yeap’s model – replace the velocity in the fouling suppression term

with wall shear stress. This model is capable of modelling the effect of

velocity more accurately including the velocity maximum behaviour seen

for Maya crude.

• Use the equivalent linear velocity in the fouling growth term:

• Adopt the concept of equivalent linear velocity which will allow the fouling

data obtained from experiments with a bare tube to be used for prediction

of the fouling inside a tube fitted with an insert.

wm

ssfm

sfmfC

RTETCuB

TuCA

dt

dRτ

µρ

µρ−

+=

−−

−

)/exp(13/23/13/123

3/43/23/2

COMPARISON OF EXPERIMENTAL DATA AND MODEL FITTING

Bare tube

0.00010

0.00020

0.00030

0.00040

Fo

uli

ng

rate

(K

m2/W

h)

0.00000

0 1 2 3 4 5

Linear velocity (m/s)

Fo

uli

ng

rate

(K

m2/W

h)

Experimental data: fouling rate of Maya crude at constant

wall temperature (523K) for the bare tube (Don Philips 1999).

Model parameter E: 50.2 kJ/mol

COMPARISON OF EXPERIMENTAL DATA AND MODEL FITTING

Tube with medium density hiTRAN insert

0.0001

0.0002

0.0003F

ou

lin

g r

ate

(K

m2W

-1h

-1)

0.0000

0.0001

0 0.5 1 1.5 2 2.5 3 3.5

Equivalent Linear velocity (m/s)Fo

ulin

g r

ate

(K

m

Experimental Model fitting

MODEL APPLICATION

0.0001

0.001

Pre

dic

ted

fo

uli

ng

rate

(K

m2/w

h)

0.00001

0.00001 0.0001 0.001

Actual fouling rate (Km2/wh)

Pre

dic

ted

fo

uli

ng

rate

(K

m

Experimental data: Fouling test results of Maya crude in bare tube and tube fitted with medium density insert (Crittenden et al. 2009).

Model parameter E: 50.2 kJ/mol by curve fitting

THRESHOLD CONDITIONS

Bare tube and tube with insert

520

530

540

550

560

Th

resh

old

tem

pera

ture

(K

)

500

510

3.00 3.50 4.00 4.50 5.00

Velocity/Equivalent velocity (m/s)

Th

resh

old

tem

pera

ture

(K

)

Experimental Model predicted

Experimental data: Fouling test results of Maya crude in bare tube

and tube fitted with medium density insert (Don Phillips 1999).

CONCLUSIONS & FUTURE

Parallel tube apparatus has provided crude oil experimental data as a function of linear velocity and surface temperatures with and without

hiTRAN inserts fitted.

CFD modelling allows prediction of surface shear stresses for both bare tubes and tubes fitted with inserts.

The concept of equivalent velocity can be used in Yeap’s fouling model

to draw together the fouling experimental results for Maya crude oil in to draw together the fouling experimental results for Maya crude oil in both a bare tube and a tube fitted with a hiTRAN insert.

The modified Yeap model can also be used to predict he fouling

threshold conditions for both bare tubes and tubes fitted with inserts.

Further CFD simulation will be carried out for the temperature

distributions in bare tubes and tubes fitted with hiTRAN inserts.

The research will be repeated with the Embaffle system, although validation of the CFD predictions will be much more difficult.