CFD MODELING OF TRICKLE BED REACTORS: Hydro-processing …

CFD MODELING OF TRICKLE BED REACTORS: Hydro-processing Reactor

Prashant R. Gunjal, Amit Arora and Vivek V. Ranade

Industrial Flow Modeling GroupNational Chemical Laboratory

Pune 411008Email: [email protected]

mailto:[email protected]

CFD Modeling of Trickle Bed Reactors

OUTLINE

• Trickle Bed Reactors– Applications/ flow regimes– Experiments

• CFD Modeling of Trickle Bed Reactors– Inter-phase momentum exchange/ capillary terms– Estimation of fraction of liquid suspended in gas phase

• Simulating Hydro-processing Reactor– Reaction kinetics/ other sub-models– Influence of reactor scale

• Concluding Remarks


TRICKLE BED REACTORS

• Wide Applications– Hydro-de-sulfurization– Hydro-cracking/ hydro-treating– Hydrogenation/ oxidation– Waste water treatment

• Key Characteristics– Close to plug flow/ Low liquid hold-up– Suitable for slow reactions

– Poor heat transfer/ possibility of mal-distribution– Difficult scale-up


TRICKLE BED REACTORS

Gas & Liquid Flow through Void Spaceε = 26-48 %

GasLiquid


FLUID DYNAMICS OF TBR

Characterization of Packed Bed

Wetting of Solid Surface by

Liquid

Flow Regimes & Global Flow

Characteristics

Mal-distribution, Channeling &

Mixing

ε, ∆P, αL, η, RTD


FLOW REGIMES


EXPERIMENTS AT NCL

• Hydrodynamics– Pressure Drop– Liquid Hold-up

• Conductivity Probes– Residence Time Distribution

• Liquid Distribution at Inlet– Uniform– Non-uniform

• Experimental Parameters– Bed Diameter: 0.1 & 0.2 m– Particle Diameter: 3 & 6 mm– Liquid Velocity: < 24 mm/ s – Gas Velocity: < 0.50 m/ s– Tracer : NaCl

Data Acquisition

From Compressor Liquid Tank

Column with 3mm or 6mm Glass Beads

SeparatorConductivity

Probe

101

Pressure Indicator

Pump


TYPICAL PRESSURE DROP DATA

0

5

10

15

20

25

30

35

0 10 20 30Liquid Flow Rate, Kg/m2s

Pres

sure

gra

dien

t, K

Pa/m

Trickle f low regime

Pulse f low regime


PUBLISHED EXPERIMENTAL DATA

00.20.40.60.8

1

1.21.41.61.8

2

0 5 10 15 20 25

Liquid Velocity VL, (Kg/m2s)

Supe

rfic

ial G

as V

eloc

ity V

g, (m

/s)

φL= ?

Trickle Flow1

3

1. Szady and Sundersan (1991)2. Rao et. al (1983) 3. Specchia and Baldi (1977)

Pulse Flow

Spray Flow

Bubbly Flow

φL=βL

φL=?

φL βL

2


MODELING OF MACROSCOPIC FLOW

• Bed Porosity: Scale of Scrutiny

– Bed diameter/ length: overall ε– Intermediate scale: Gaussian distribution– Smaller than particle diameter: Bi-modal distribution

• Approach Used:

– Experimentally measured or estimated radial variation of axially averaged bed porosity

– Specify porosity by drawing a sample assuming Gaussian distribution with specified variance


CHARACTERIZATION OF PACKED BED

• Radial Variation of Porosity: Mueller (1991)

( )

( )

( )

Function Besselorder zero is J and r/Dr

D/d0.724-0.304b

D/d13.0for 9.864D/d

2.932-7.383a

13.0D/d2.61for 3.156D/d

12.98-8.243a

)e(ar)Jε(1εrε

tho

*P

PP

PP

br*oBB

=

=

≤−

=

≤≤−

=

−+= −


CHARACTERIZATION OF PACKED BED

• Mueller’s CorrelationThree data sets from literature Our experiments

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1Dimensionless Radius

Por

osity

D=0.194m, dp=3mm

D=0.194m, dp=6mm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.2 0.4 0.6 0.8 1

Dimensionless Radius

Poro

sity

Szady and Sundersan (1991)

Rao et.al. (1983)

Spacchia and Baldi (1977)


VARIATION OF BED POROSITY

– Selection of appropriate value is not straight forward: RTD may help

r/R

0.70

0.56 With std dev=0

With std dev=5%

With std dev=10%


CFD MODEL

• Multi-fluid Model (Eulerian-Eulerian)Continuity Equation

0=⋅∇+∂

∂KKK

KK Ut

ρερε

Momentum Balance Equation

( ) ( )RkRKKKK

KKKKKKKK

UUFgU

PUUtU

−++∇⋅∇

+∇−=⋅∇+∂

∂

,

)()(

ρεµε

ερερε


CFD MODEL

• Inter-phase Coupling Terms (Attou & Ferschneider, 2000)

( )⎟⎟

⎠

⎞

⎜⎜

⎝

⎛⎥⎦

⎤⎢⎣

⎡−

−−+⎥

⎦

⎤⎢⎣

⎡−

−=

333.0

2

667.0

22

21

)1()1(

)1()1(

G

L

pG

GLGP

G

L

pG

GGGL d

UUEd

EFε

εε

ερε

εε

εµε

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛⎥⎦

⎤⎢⎣

⎡−

−+⎥

⎦

⎤⎢⎣

⎡−

−=

333.0

2

667.0

22

21

)1()1(

)1()1(

G

S

pG

GGP

G

S

pG

GGGS d

UEd

EFε

εε

ερε

εε

εµε

⎟⎟⎠

⎞⎜⎜⎝

⎛+=

pL

SGP

pL

sLLS d

UEd

EFε

ερεµεε 2

22

21


CFD MODEL

• Capillary Terms

• Attou and Ferschneider (2000)

⎟⎟⎠

⎞⎜⎜⎝

⎛−=−

21

112dd

PP LG σ

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−−

=−L

G

PGLG F

dPP

ρρ

εεσ 5416.0

112

333.0

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

∂∂

⎟⎟⎠

⎞⎜⎜⎝

⎛

−+

∂∂

⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛−

=∂∂

−∂∂

−

L

GG

G

ss

G

/

G

s

p

LG

ρρ

Fzε

εε

zε

εεε

d.σ

zP

zP

2

32

111

14165

32


CFD MODEL

• Capillary Terms

– Extent of Wetting, f (Jiang et al., 2002)

• Scalar Transport

iimiKKiKKKiKK SCDCU

tC

+∇⋅−∇=⋅∇+∂

∂ )( .ρερερε

cLG PfPP )1( −=−

025.01.881 <+=⎟⎟⎠

⎞⎜⎜⎝

⎛

L

G

L

G

L

G forFρρ

ρρ

ρρ


SIMULATED RESULTS

0

5

10

15

20

25

30

35

40

45

0 0.02 0.04 0.06 0.08 0.1

Velo city M agnitude, m/ sec

Non-prewetted Bed

Prewetted Bed

0

5

10

15

20

25

30

35

0 0.1 0.2 0.3Liquid Holdup

Non-prewetted BedPrewetted Bed

Pre-wetted Un-wetted

VL= 6 mm/ s, VG=0.22 m/ s, D=0.114 m, dP= 3 mm, σ=5%

Con

tour

s of

Liq

uid

Hol

d-up

Distributions within the bed


SIMULATED RESULTS

Column Diameter = 0.114 m Particle Diameter = 3 mm, VG=0.22 m/ s


SIMULATED RESULTS

VG= 0.22 m /s, D = 0.114 m VG= 0.22 m /s, D = 0.194 m


GAS-LIQUID FLOW IN TBR

• Estimation of Frictional Pressure Drop & Fraction of Liquid Supported by Gas

– Simulate at two values of ‘g’ (9.7, 9.9 m/s2)

21

12

21

ggLPg

LPg

LPf

−

⎟⎠⎞

⎜⎝⎛ ∆−⎟

⎠⎞

⎜⎝⎛ ∆

=⎟⎟⎠

⎞⎜⎜⎝

⎛ ∆

( )21L

12L gg

LP

LP

−ρ

⎟⎠⎞

⎜⎝⎛ ∆−⎟

⎠⎞

⎜⎝⎛ ∆

=φ

Fraction of Liquid Hold-up Supported by Gas Phase < 1

Solid

Liquid

Gas

( )

LL

Lf ρ

βφ

φ

≤

−⎟⎟⎠

⎞⎜⎜⎝

⎛ ∆=⎟

⎠⎞

⎜⎝⎛ ∆

Q

gLP

LP

LGLGL


SIMULATED RESULTS

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.00 0.20 0.40 0.60 0.80 1.00Liquid Flow Rate x10-1, kg/m2s

Liqu

id S

atur

atio

n a

ndL

Holub et. al. (1993) -Experimental dataSzady and Sundersan (1991)-Exp. DataCFD ResultsSupported Liquid Saturation

Calibrate Model Parameters from ∆P

Can Predict Total and Supported Liquid Volume Fraction

0

2

4

6

8

10

12

14

0.00 0.20 0.40 0.60 0.80 1.00Liquid Flow Rate x10-1, Kg/m2s

Pres

sure

Gra

dien

t KPa

/m

Saez and Corbonell (1985)-Exp. dataSzady and Sundersan (1991)-Increase in liquidSzady and Sundersan (1991)-decrease in liquidCFD Simulations w ithout Capillary ForceCFD Simulations

Operating conditions:VG=0.22m/s, D/dp =55, dp=3mm, Std. Dev.=5%, E1=215, E2=1.75


SIMULATED RESULTS

0

10

20

30

40

50

0 0.2 0.4 0.6 0.8 1Gas Flow Rate, (Kg/m2s)

Pres

sure

Dro

p, (K

Pa/m

) CFD ResultsExperimental Data

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1Gas Mass Velocity, kg/m2s

Liqu

id S

atur

atio

n an

d L

Liquid Saturation- CFD ResultsSupported Liquid SaturationLiquid Saturation- Exp. Data

Data of Spachio & Baldi (1977)

As gas velocity increases, more & more liquid is

supported by gas

Operating conditions: VL=2.8x10-3 m/s, dp=3mm, D/dp=30, Std. Dev.=5%, E1=500, E2=3


SIMULATED RESULTS

0

0.04

0.08

0.12

0.16

0.2

0 2 4 6 8Gas mass velocity, Kg/m2s

Liqu

id S

atur

atio

n an

dL

Liquid Saturation- CFD ResultsSupported Liquid SaturationLiquid Saturation Exp. Data

0

40

80

120

160

200

0 2 4 6 8Gas Mass Velocity, (kg/m2s)

Pres

sure

Dro

p, (K

Pa/m

) Experimental DataCFD Results Data of Rao et al. (1983)

As gas velocity increases, more & more liquid is

supported by gas

Operating conditions:VL=1x10-3m/s, D/dp =15.4, dp=3mm, Std. Dev.=5%, E1=215, E2=3.4


RESIDENCE TIME DISTRIBUTION

0

0.02

0.04

0.06

0.08

0.1

0.12

0 20 40 60 80 100 120Time, (sec)

E(t),

sec

-1

Pre-wetted Bed

Non Pre-wetted Bed

Simulation Results

1

1

1

Experimental Results2

2

2

Operating conditions: VL=2.06x10-3m/s VG=0.22m/s, D/dp =18, dp=6mm, Std. Dev.=5%, E1=180, E2=1.75


MODELING OF HYDRO-PROCESSING REACTOR

• Reaction and Kinetics (Chowdhury et al. 2002)Hydro-desulfurisation (HDS)

SH,LAd

6.1S,L

56.0H,L

HDS2

2

cK1cck

r+

−=SHArH2SAr 22 +→+−

De-aromatisation of Mono-, Di- and Poly- Aromatics

NaphMono_MonoMonoMono CkCkr +−=NapthenesH3ArMono 2 ⇔+−

MonoDi_DiDiDi CkCkr +−=ArMonoH2ArDi 2 −⇔+−

DiPoly_PolyPolyPoly CkCkr +−=ArDiHArTri 2 −⇔+−


SIMPLIFICATIONS/ ASSUMPTIONS

• Pressure drop is insignificant compared to the operating

pressure

• Trickle bed reactor is operated isothermally (efficient heat

transfer)

• Ideal gas law is applicable

• Liquid phase reactants are non-volatile (negligible vapor

pressure)

• Gas-liquid mass transfer is the limiting resistance.

• The catalyst particles are completely wetted


MODEL EQUATIONS

• Mass Balance of Species i

• Source Terms for Gas Phase

• Source Terms for Liquid Phase

( ) ( ) kikkikmikkikkkkikkk SCDCU

tC

,,,.,, ρερερε

ρε+∇−=⋅∇+

∂∂

⎥⎦

⎤⎢⎣

⎡−−= Li

i

GiGLGLii C

HC

aKS

∑=

=

+⎥⎦

⎤⎢⎣

⎡−=

nrj

1jijB rηρLi

i

GiGLGLii C

HC

aKS

∑=

=

=nrj

1jijB rηρiS


MODEL EQUATIONS

• Mass Transfer Coefficient (From Goto & Smith, 1975)

• Solubility of Hydrogen/ H2S (Korsten et al. 1996)

2/1

1

2

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛= L

iL

L

L

LLi

LLi

DG

Dak

ρµ

µα

α

Li

NiH ρλ

υ.

=

220

42

320

210H1.aTaTaTaa

2 ρ++

ρ++=λ

).008470.03670.3exp(2

TSH −=λ835783.0a

1094593.1a

1007539.3a

1042947.0a

559729.0a

4

63

32

31

0

=×=

×=

×−=

−=

−

−

−

: molar gas volumeNυ


CASE STUDIES

• Gas and Liquid Superficial Velocities Increase with Scale

• Wetting gets Better with Scale

Parameters Laboratory Scale Reactor(Chowdhury et al. 2002)

Commercial Scale Reactor(Bhaskar et al. 2004)

Reactor Diameter, mBed Length

Particle Diameter, mBed Porosity

LHSV, h-1Operating Pressure, MpaOperating Temperature, K

Initial H2S Conc., v/v %

0.0190.5 m0.00240.50

1-520-28

573-693 0.5-8

3.816 m

0.00150.36

1-520-80

573-693 0.5-8


KINETICS/ OIL COMPOSITION

• From Chowdhury et al. (2002)

AdK

Kinetic Constants Values

, m3/kmol 50000

K, Dimensionless 2.5 X 1012 exp(-19384/T)

k*mono, m3/kg.s 6.04 X 102 exp(-12414/T)

k*Di, m3/kg.s 8.5 X 102 exp(-12140/T)

k*poly, m3/kg.s 2.66 X 105 exp(-15170/T)

Component Percentage

Ar-S % 1.67

Poly-Ar % 2.59

Di-Ar % 8.77

Mono-Ar % 17.96

Naphthenes % 19.25


SIMULATED RESULTS-1

Temperature Initial Concentration of H2S

0

25

50

75

100

573 583 593 603 613 623 633 643 653

Temp, K

Con

vers

ion,

Ar-

S %

Varying Porosity

Uniform Porosity

Ar-S Exp

0

30

60

90

0 2 4 6 8

Initial H2S Conc Gas Phase ( %, V/V)

Conv

ersi

on, x

(P=4 MPa, LHSV=2.0 h-1, QGNTP/QL=200 m3/m3, TR=320oC, yH2S=1.4% )Symbols denote experimental data of Chowdhury et al. (2002)


SIMULATED RESULTS-2

(P=4 MPa, LHSV=2.0 h-1, QGNTP/QL=200 m3/m3, yH2S=1.4%)Symbols denote experimental data of Chowdhury et al. (2002)

0

10

20

30

40

50

60

70

80

90

573 593 613 633 653Temperature, K

Con

vers

ion,

%

Ar-Ptotaltotalpoly

0

10

20

30

40

50

60

573 593 613 633 653Temperature, K

Con

vers

ion,

%

Ar-D

Ar-M

Ar-Di-Expt

Ar-Mono-Exp


SIMULATED RESULTS-3

0

10

20

30

40

50

60

2 2.4 2.8 3.2 3.6 4

LHSV, h-1

Con

vers

ion,

%

Total-ArAr-Poly

Ar-DiAr-Mono

(P=4 MPa, TR=320 oC, QGNTP/QL=200 m3/m3, yH2S=1.4%, filled symbols are for experimental data of Chowdhury et al., 2002)


INFLUENCE OF REACTOR SCALE

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8 1

Dimensionless Radius

Dim

ensi

onle

ss S

olid

Hol

d-up

0

5

10

15

20

25

30

35

Dim

ensi

onle

ss L

ocal

Axi

al V

eloc

ity

Solid hold-upLocal Axial Velocity

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.2 0.4 0.6 0.8 1

Dimensionless RadiusD

imen

sion

less

Sol

id H

old-

up

0

1

2

3

4

5

6

7

Dim

ensi

onle

ss L

ocal

Axi

al V

eloc

ity

Solid hold-upLocal Axial Velocity

Industrial (LHSV=2)Laboratory (LHSV=3)

(P=4 MPa, TR=320 oC, QGNTP/QL=300 m3/m3)



25

50

75

100

573 593 613 633 653

Temperature, K

Con

vers

ion,

% Lab-scale ReactorCommercial Reactor

0

2000

4000

6000

8000

10000

12000

573 593 613 633 653Temperature, K

Out

let C

onc.

Ar-

S, p

pm

Lab Scale Reactor

Commercial Reactor

(Plab & com=4 & 4.4 MPa, LHSVlab=2.0 h-1, QGNTP/QL=200 m3/m3, yH2S=1.4%)



-20

0

20

40

60

80

100

573 593 613 633 653Temperature, K

Con

vers

ion,

%

Poly-ArTotalPoly-Ar-CommercialTotal-Commercial

505560

6570758085

9095

100

2 2.5 3 3.5 4LHSV, h-1

Con

vers

ion,

%

Ar-S-Lab Scale

Ar-S-Commercial

(Plab & com=4 & 4.4 MPa, LHSVlab & com=2.0 & 3.0 h-1, (QGNTP/QL)lab & com= 200 & 300 m3/m3, yH2S=1.4%)

(Plab & com=4 & 4.4 MPa, Temperature= 593 K,(QGNTP/QL)lab & com= 200 & 300 m3/m3, yH2S=1.4%)



0

20

40

60

80

100

120

2 3 4 5 6 7 8

Pressure, MPa

Con

vers

ion,

%

Poly-Ar

Di-Ar

Mono-Ar

Total

0

10

20

30

40

50

60

70

80

90

100

2 2.5 3 3.5 4LHSV, h-1

Con

vers

ion,

%

Poly-ArDi-ArMono-ArTotal

(QGNTP/QL)lab & com= 200 & 300 m3/m3 T=593 KLHSVlab & com=2.0 & 3.0 h-1 ,yH2S=1.4%

Plab & com=4 & 4.4 Mpa, T=593 K yH2S=1.4%(QGNTP/QL)lab & com= 200 & 300 m3/m3

Continuous lines: commercial; Dotted lines: laboratory scale


CONCLUDING REMARKS

• Macroscopic CFD Model Reasonably Simulates Gas-liquid Flow in Trickle Beds– Liquid hold-up– Residence time distribution– May be used to estimate fraction of suspended liquid

• Further Work is Needed for Understanding Wetting/ Hysteresis to Make Further Progress

• Despite Limitations, CFD Models can be Used to Understand Key Issues in Reactor Engineering Including Scale-up & Scale-down



MODELING OF MESO-SCALE FLOWS

• Single Phase Flow through Packed Bed– Unit cell approach– Simple cubic, FCC, rhombohedral ..– Inertial flow structures, pressure drop, heat transfer

• Interaction of Liquid Drop/ Film with Solid Surface– Regimes of interaction– Dynamic contact angle/ surface characteristics– VOF simulations– Insight into capillary forces???


SINGLE PHASE FLOW

• Single Phase Flow through Packed BedPeriodic Flow

Wall

Wall

Periodic Flow

Wall

Operating Parameters• Cell Length 28mm, 3mm• Fluid Water• Particle Reynolds Number 12-6000


SINGLE PHASE FLOW

Experiments:Suekane et al. AIChE J., 49, 1

Simulations

0

5

0

4.5


SINGLE PHASE FLOW

0

1

2

3

-1 -0.5 0 0.5 1x/r

Vz/V

mea

n

Experimental

Simulat ion-28mm

Simulat ion-3mm

-1

0

1

2

3

4

-1 -0.5 0 0.5 1x/r

Vz/V

mea

n

Experimental

Simulat ion-28mm

Simulat ion-3mm

-1

0

1

2

3

4

5

-1 -0.5 0 0.5 1x/r

Vz/V

mea

n

Experimental

Simulat ion-28mm

Simulat ion-3mm

-1

0

1

2

3

4

5

-1 -0.5 0 0.5 1x/r

Vz/V

mea

n

Experimental

Simulat ion-28mm

Simulat ion-3mm


SINGLE PHASE FLOW

E

S

Vz=8.66mm/s

Max arrow length :A=0.26 mm/sB=0.29 mm/sC=0.44 mm/s

A: z/r=0.14 (experimental) B: z/r=0.28 (experimental) C: z/r=0.42 (experimental)A: z/r=0.14 (experimental) B: z/r=0.28 (experimental) C: z/r=0.42 (experimental)A: z/r=0.14 (experimental)A: z/r=0.14 (experimental) B: z/r=0.28 (experimental)B: z/r=0.28 (experimental) C: z/r=0.42 (experimental)C: z/r=0.42 (experimental)


SINGLE PHASE FLOW

• Inertial Flow Structures were Captured Accurately by CFD Models

• Velocity Distribution in Unit Cells Resembles that in a Packed Bed

• Ergun Parameters may be Obtained through Unit Cell Approach for Semi-structured Packed Bed

• Unit Cell Approach may be Extended to Simulate Two Phase Flows to Understand Wetting


LIQUID SPREADING ON SOLID SURFACES

Computer

CCD Camera

Monitor

Light Diffuser

Light

Drop Flow Over Pellet

Drop Rest on Flat Plate

8.79mm

1mm

φ10.4mm


DROP IMPACT ON SOLID SURFACE


DROP IMPACT ON SOLID SURFACE


FLAT SURFACE

(f)(c)

(d)

(e)(b)

(a)

t=0ms

t=16ms

t=20ms

t=36ms

t=48ms

t=240ms

t=0ms

t=10ms

t=210mst=25ms

t=45ms

t=45ms


DYNAMICS OF DROP IMPACT


MODELS FOR INTERPHASE COUPLING?

Wall Shear Stress on Solid Surface, red~100 Pa

Drop Shape at 12 ms

Shear Strain on Gas-liquid Interface

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CFD MODELING OF TRICKLE BED REACTORS: Hydro-processing …

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