Unsteady State Operation in Trickle Bed Reactors “Modulation of input variables or parameters to create unsteady state conditions to achieve performance

Unsteady State Operation in Trickle Bed ReactorsUnsteady State Operation in Trickle Bed Reactors

“Modulation of input variables or parameters to create unsteady “Modulation of input variables or parameters to create unsteady state conditions to achieve performance better than that attainable state conditions to achieve performance better than that attainable with steady state operation”with steady state operation”

Motivation and ObjectivesMotivation and Objectives Performance enhancement in existing reactorsPerformance enhancement in existing reactors Design and operation of new reactorsDesign and operation of new reactors Lack of systematic experimental or rigorous modeling studies in Lack of systematic experimental or rigorous modeling studies in

lab reactors necessary for industrial applicationlab reactors necessary for industrial application Experimentally investigate unsteady state flow modulation Experimentally investigate unsteady state flow modulation

(periodic operation) for a test hydrogenation system(periodic operation) for a test hydrogenation system Develop model equations incorporating multiphase, Develop model equations incorporating multiphase,

multicomponent transport that can simulate unsteady state multicomponent transport that can simulate unsteady state operation operation

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Strategies for Unsteady State OperationStrategies for Unsteady State Operation

Flow Modulation (Gupta, 1985; Haure, 1990; Lee and Silveston, 1995)

– Liquid or gas flow– Liquid/gas ON-OFF or HIGH-LOW flow– Isothermal/non-isothermal/adiabatic conditions

Composition Modulation (Lange, 1993)

– Periodic switching between pure or diluted liquid/gas– Quenching by inert or product (adiabatic)

Activity Modulation (Chanchlani, 1994; Haure, 1994)

– Enhance activity due to pulsed component– Removal of product from catalyst site– Catalyst regeneration due to pulse

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Gas Limited ReactionsGas Limited Reactions

Partial Wetting of Catalyst Pellets -DesirablePartial Wetting of Catalyst Pellets -Desirable– Internal wetting of catalystInternal wetting of catalyst– Externally dry pellets for direct access of gasExternally dry pellets for direct access of gas– Replenishment of reactant and periodic product removalReplenishment of reactant and periodic product removal

– Catalyst reactivationCatalyst reactivation

Liquid Limited ReactionsLiquid Limited Reactions

Partial Wetting of Catalyst Pellets-UndesirablePartial Wetting of Catalyst Pellets-Undesirable– Achievement of complete catalyst wettingAchievement of complete catalyst wetting– Controlled temperature rise and hotspot removalControlled temperature rise and hotspot removal

Possible Advantages of Unsteady State OperationPossible Advantages of Unsteady State Operation

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Test Reaction and Operating ConditionsTest Reaction and Operating Conditions

C CH

CH

2

3

HC CH

CH

3

3

H+2

Pd/Alumina

Operating ConditionsOperating Conditions

• Superficial Liquid Mass Velocity : 0.05-2.5 kg/m2s• Superficial Gas Mass Velocity : 3.3x10-3-15x10-3 kg/m2s

• Operating Pressure : 30 -200 psig (3-15 atm)• Feed Concentration : 2.5 - 30 % (200-2400 mol/m3)• Feed Temperature : 20-25 oC

• Cycle time, (Total Period) : 5-500 s• Cycle split, (ON Flow Fraction) : 0.1-0.6• Max. Allowed Temperature Rise : 25 oC

Alpha-methylstyrene hydrogenation to isopropyl benzene (cumene)

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, (sec)

s (1-)L(peak)

L (base)

L(mean)

Liquid Limited Conditions (0.4 < )High Pressure,

Low Liquid Feed Concentration

Gas Limited Conditions ( ~ 20)Low Pressure,

High Liquid Feed Concentration

D C

D CeB Bi

eA A*

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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 500 1000 1500 2000

Space time (s)

Con

vers

ion

(X)

Flow Mod. (Cycle=60s, S=0.5)

Steady State

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 500 1000 1500 2000 2500 3000

Space time (s)

Con

vers

ion(

X)

Steady State

Unsteady State (Cycle time= 60 s, Split=0.5)

Comparison of Performance under Gas and Comparison of Performance under Gas and Liquid Limited ConditionsLiquid Limited Conditions

Effect of Cycle Split and Total Cycle Period on Effect of Cycle Split and Total Cycle Period on Performance EnhancementPerformance Enhancement

Gas Limited Conditions ( ~ 20)Operating Conditions : Pressure=30 psig

Cycle Split ()= Liquid ON Period/Total Cycle Period()

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0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 50 100 150Total Cycle Period, s

X(U

S)/

X(S

S)

P=30 psig, C(AMS) feed = 1627 mol/m3Cycle Split = 0.33, L (mean) 0.24 kg/m2s

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.2 0.4 0.6 0.8 1Cycle Split (ON time/Total Cycle Time)

Con

vers

ion

(X)

Effect of Liquid Mass Velocity and Total Cycle Period Effect of Liquid Mass Velocity and Total Cycle Period on Unsteady State Performanceon Unsteady State Performance

0

0.5

1

1.5

2

2.5

0 100 200 300

Total Cycle Period, s

X(U

S)/

X(S

S)

L (mean)=0.137 kg/m2s

L (mean)=0.068 kg/m2s

P=30 psig, C(AMS) feed = 1582 mol/m3Cycle Split = 0.25

Enlargement of enhancement zone at lower mass velocity

, (sec)

s(1-)

L (ON)

s(1-)

L (ON)=L(mean) /

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L (mean)

Effect of Liquid Reactant Concentration on Effect of Liquid Reactant Concentration on Performance EnhancementPerformance Enhancement

Lower conversion at higher feed concentration reduces enhancement even at lower liquid mass velocity

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.2 0.4 0.6 0.8 1Cycle Split

Co

nv

ers

ion

, X(u

s)/

X(s

s)

C(AMS) feed = 2082 mol/m3, L = 0.10 kg/m2s

C(AMS) feed = 1484 mol/m3, L = 0.24 kg/m2s

P = 30 psig, Cycle Period = 60 s

, (sec)

s(1-)

L (ON)

s(1-)

L (ON)=L(mean) /

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L (mean)

Effect of Pressure on Steady and Unsteady Effect of Pressure on Steady and Unsteady State PerformanceState Performance

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200Pressure, psig

Co

nv

ers

ion

, X(u

s),

X(s

s)

Steady State Unsteady State

C(AMS) feed = 1437 mol/m3, L = 0.085 kg/m2sCycle Period = 60 s, Split = 0.33

At low mean liquid mass velocity, unsteady state performance is higher than steady state even as liquid limitation is reached

~ 24 ~ 3

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, (sec)

s(1-)

L (ON)

s(1-)

L (ON)=L(mean) /

L (mean)

Effect of Cycling Frequency on PerformanceEffect of Cycling Frequency on Performance

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

0 0.05 0.1 0.15 0.2Cycling Frequency (Hz)

X(u

s)/

X(s

s)

Cycle Split = 0.5 Cycle Split = 0.2

P= 60 psig, C(AMS) feed = 2084 mol/m3 L(mean) = 0.1 kg/m2s

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 0.05 0.1 0.15 0.2Cycling Frequency, Hz

Co

nv

ers

ion

, X(u

s)

P= 30 psig, C(AMS) feed= 1494 mol/m3, Cycle Split =0.5, L(mean) = 0.1 kg/m2s

Optimum cycling frequency depends upon feed concentration, pressure and cycle split

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Effect of Base-Peak Flow Modulation on Performance Effect of Base-Peak Flow Modulation on Performance Enhancement under Liquid Limited ConditionsEnhancement under Liquid Limited Conditions

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14

1.16

0 50 100 150 200 250Cycle period, s

X(u

s)/

X(s

s)

P = 150 psig, C(AMS) feed = 784 mol/m3Cycle Split = 0.1, L (mean) = 0.14 kg/m2s

L (peak)/L(base) = 3.63

, (sec)

s (1-)L(peak)

L (base)

At high peak to base flow ratio, unsteady state operation gives better performance even under (near) liquid limited conditions (0.4< < 2)

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L(mean) = *L (peak)+(1-)*L (base)

L (mean)

Phenomena occurring under unsteady state operation Phenomena occurring under unsteady state operation with flow modulation in a trickle-bed reactorwith flow modulation in a trickle-bed reactor

time,t

Catalyst (Internally and Externally wetted)

Liquid Full (Holdup=Bed voidage)

Catalyst (Internally wet, externally partially wet)

Liquid films (Holdup = dynamic +static)

Gas accesing liquid and dry catalyst

Catalyst (Internally wet, externally dry)

Liquid films (Holdup = only static)

Gas Accesing dry catalyst

LIQUID PULSE ON

LIQUID PULSE TRANSITON ZONE

LIQUID PULSE OFF

Temperature, Low (=Feed Temperature)

Temperature, Rise (>Feed Temperature)

Temperature, High (>Feed Temperature)

(a)

(b)

(c)

(Only Scenario II)

(Only Scenario II)

GOALGOAL: : To Predict Velocity, Holdup, Concentration and Temperature ProfilesTo Predict Velocity, Holdup, Concentration and Temperature Profiles

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The Model StructureThe Model Structure

z=L

z=0GAS LIQUID

SOLID

C1G

C2G

.

.CnG

C1L

C2L

.

.CnL

NiGS

NiGS

NiLS

NiLS

NiGL

EGS

EGS

ELS

ELS

EGL

EGL

NiGL

t

Cz

u C N a N aG iG IG G iG iGL

GL iGS

GS( )

t

Cz

u C N a N aL iL IL L iL iGL

GL iLS

LS( )

Bulk Phase EquationsBulk Phase Equations

SpeciesSpecies

EnergyEnergy

c B CPB e

CP LSLS

GSGS

E

tk

T

zE a E a

( )( )

11

2

2

( ) ( )L L L L IL L L GLGL

LSSL

E

t

u H

zE a E a

( ) ( )G G G G IG G G GLGL

GSGS

E

t

u H

zE a E a

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Advantages of Maxwell-Stefan Multi-component Advantages of Maxwell-Stefan Multi-component Transport Equations over Conventional ModelsTransport Equations over Conventional Models

Multicomponent effects are considered for individual component transport [k]’s are matrices

Bulk transport across the interface is considered

Nt coupled to energy balance (non zero) Transport coefficients are corrected for high fluxes

[k] corrected to [ko] = [k][[exp([])-[I]]-1

Concentration effects and individual pair binary mass transfer coefficients considered

Thermodynamic non-idealities are considered by activity correction of transport coefficients

Holdups and velocities are affected by interphase mass transport and

corrected while solving continuity and momentum equations

jiij DD

[ ]ln

ij ij ii

j

xx

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Flow Model EquationsFlow Model Equations

uiL,uiG

L,G,P

Staggered 1-D Grid

Z

z

pc

zt

z

u

z

uN IGGILL

LGi

)()()(

)11

(**

MomentumMomentum

ContinuityContinuity

PressurePressure

LL

L

IL L

iGL

GL i iLS

LS it

u

zN a M N a M

G G G IG G

iGL

GL i iGS

GS it

u

zN a M N a M

L L

ILL L IL

ILL L L

LD Liq IG IL IL i

GLGL i i

LSLS i

u

tu

u

zg

P

zF K u u u N a M N a M , ( ) ( )

G G

IGG G IG

IGG G G

GD Gas IL IG IG i

GLGL i i

GSGS i

u

tu

u

zg

P

zF K u u u N a M N a M , ( ) ( )

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Stefan-Maxwell Flux Equations for Interphase Stefan-Maxwell Flux Equations for Interphase Mass and Energy TransportMass and Energy Transport

N J x J xq

iL

iL

i k kL

k

n

ix

1

1

Gas-Liquid FluxesGas-Liquid Fluxes

Liquid-Solid and Gas-Solid FluxesLiquid-Solid and Gas-Solid Fluxes

N J y J yq

iV

iV

i k kV

k

n

iy

1

1

E h T T N H TLL I L i

LiL

Li

n

. ( ) ( )

1

E h T T N H TVV G I i

ViV

Gi

n

. ( ) ( )

1

E h T T N H TLSLS L ILS i

LSiL

Li

n

. ( ) ( )

1 N c k xLSt LS LS [ ][ ].

N c k xGSt GS GS [ ][ ]. E h T T N H TGS

GS G IGS iGS

iG

Gi

n

. ( ) ( )

1

Bootstrap Condition for Multicomponent TransportBootstrap Condition for Multicomponent Transport• Interphase Energy Flux for the Gas-Liquid Transport and Bulk to Catalyst Interphase Energy Flux for the Gas-Liquid Transport and Bulk to Catalyst Interface TransportInterface Transport

• Net Zero Volumetric Flux for Liquid-Solid and Gas-Liquid Interface for Net Zero Volumetric Flux for Liquid-Solid and Gas-Liquid Interface for Intracatalyst FluxIntracatalyst Flux

[ ], i k G ik i ky , ik k nc y ( )/

, y i i

i

y:i i i

VG i

LLy H T H T ( (@ ) (@ ))

[ ], ikCP ik ci kx:k

k

k

nc

ncmx

MM( )/

and

mx cii

ii

xM

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Catalyst Level EquationsCatalyst Level Equations

Approach I: Rigorous Single Pellet Solution of Intrapellet Profiles along with Liquid-Solid and Gas-Solid Equations

Approach II: Apparent Rate Multipellet Model Solution of Liquid-Solid and Gas-Solid Equations

G CiCP L

xc

CiCP L G CiCP L G CiCPL

cx x

dtB

x x x

xcRtCP

i ncnt

i ncnt

j

j ncnt

j ncnt

j ncnt

jncnt, ,

,

, , ,{[ ][ ] [ ]}

( )

( )

1

11

11 1

11

2 11

20

cx x

dtN a N a RtCP

int

int

iLS

jLS

j a biGS

jGS

j a bAppnt

1

1 1 1 11

1 0, ,

G

Type I: Both Sides Externally Wetted

Type II: Half Wetted Type III: Both Sides Externally Dry

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Liquid Holdup and Velocity ProfilesLiquid Holdup and Velocity Profiles

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 10 20 30 40time (s)

Liq

uid

Hol

dup

...

z=0.0

z=0.25

z=0.45

z=0.65

z=0.85

z=1.0

0

0.01

0.02

0.03

0.04

0.05

0.06

0 10 20 30 40

time (s)

Liq

uid

Vel

ocity

(m/s

) …..

z=0.0

z=0.25

z=0.45

z=0.65

z=0.85

z=1.0

Operating Conditions: Liquid ON time= 15 s, OFF time=65 s Liquid ON Mass Velocity : 1.4 kg/m2s Liquid OFF Mass Velocity : 0.067 kg/m2s Gas Mass Velocity : 0.0192 kg/m2s

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Transient Simulation ResultsTransient Simulation Results Alpha-methylstyrene Concentration ProfilesAlpha-methylstyrene Concentration Profiles

Alpha-methylstyrene Concentration during ON cycle of flow modulationFeed Concentration : 1484 mol/m3

Pressure : 1 atm. Reaction Conditions : Gas Limited ( ~ 25)

(Intrinsic Rate Zero order w.r.t. Alpha-MS)

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0

200

400

600

800

1000

1200

1400

1600

0 0.2 0.4 0.6 0.8 1

Axial Location,(z/L)

Con

cent

ratio

n, m

ol/m

3

0.005

1.815

5.815

11.853

19.805

39.833

74.82

99.93

124.817

149.811

174.822

199.818

225.011

250.007

275.01

299.922

Transient Cumene and Hydrogen Concentration ProfilesTransient Cumene and Hydrogen Concentration Profiles

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Profiles show build up of cumene and hydrogen concentration during the liquid ON part of the cycle

0

100

200

300

400

500

600

700

800

900

0 0.2 0.4 0.6 0.8 1

Axial Location,(z/L)

Con

cent

ratio

n, m

ol/m

3

0.005

1.815

5.815

11.853

19.805

39.833

74.82

99.93

124.817

149.811

174.822

199.818

225.011

250.007

275.01

299.922

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25time (s)

Liq.

Pha

se H

2 C

once

ntra

tion,

mol

/m3

z=0

z=0.1

z=0.2

z=0.3

z=0.4

z=0.5

z=0.6

z=0.7

z=0.8

z=1

Alpha-methylstyrene and Cumene Concentration Alpha-methylstyrene and Cumene Concentration Profiles During Flow ModulationProfiles During Flow Modulation

Supply and Consumption of AMS and Corresponding Rise in Cumene Concentration

Operating Conditions: Cycle period=40 sec, Split=0.5 (Liquid ON=20 s) Liquid ON Mass Velocity : 1.01 kg/m2s Liquid OFF Mass Velocity : 0.05 kg/m2s Gas Mass Velocity : 0.0172 kg/m2s

0.341

10.479

20.644

29.291

39.455

0.1

0.2

0.3

0.4

0.5

0

10

20

30

40

50

Cum

ene

Con

c., m

ol/m

3

time, s Axial Location, m

0.1

0.3

0.5

0.7

0.9

0.2275.353

10.15615.852

20.43325.313

29.83134.725

37.225

0

40

80

120

160

200

Alp

ha-M

S co

nc.,

mol

/m3

time, s

Axial Location, m

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Catalyst Level Hydrogen and Alpha-methylstyrene Catalyst Level Hydrogen and Alpha-methylstyrene Concentration Profiles During Flow ModulationConcentration Profiles During Flow Modulation

0.03

5

5.07

81

10.3

263

15.1

737

21.0

159

24.9

087

30.1

751

35.0

302

39.7

204

0.10.2

0.30.4

0.50.6

0.70.8

0.91

0

20

40

60

80

100

120

140

160

Alp

ha-M

S co

ncen

trat

ion,

m

ol/m

3

time,s

Axial Location, m

0.1

0.3 0.

5 0.7 0.

9

0.035

5.0781

15.1737

20.0057

25.459730.1751

35.0302

02468

1012

14

Hyd

roge

n C

once

ntra

tion

, m

ol/m

3

time,s

Axial Position, m

Concentration of Hydrogen during Liquid ON (1:20s, Wetted Catalyst ) and Liquid OFF(20:40 s,

Dry catalyst) for negligible reaction test case

Concentration of Alpha-MS in previously dry pellets during Liquid ON

(1:20s, Wetted Catalyst ) and Liquid OFF(20:40 s, Dry catalyst)

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Simulated Cycle Time and Cycle Split Effects on Simulated Cycle Time and Cycle Split Effects on Unsteady State PerformanceUnsteady State Performance

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3

0 0.1 0.2 0.3 0.4 0.5 0.6

Cycle Split ()

Enh

ance

men

t (ra

te(u

s)/r

ate(

ss)) P=30 psig, Cfeed=1484 mol/m3,

L=0.22 kg/m2s, Cycle time=60 s

2

2.2

2.4

2.6

2.8

3

3.2

3.4

0 20 40 60 80 100 120

Cycle Period, s

Enh

ance

men

t (ra

te(u

s)/r

ate(

ss)) P=30 psig, Cfeed=1484 mol/m3,

L=0.22 kg/m2s, split=0.5

Cycle Split and Cycle Period Effects Agree Qualitatively with Experimental Results

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Transient Fluid Dynamic SimulationTransient Fluid Dynamic Simulationusing CFDLIB (Los Alamos)using CFDLIB (Los Alamos)

2D-Test bedDimensions: 29.7x7.2 cm33x8 (264 cells with preset porosity)Cycle period= 60 sCycle split = 0.25Liquid Velocity = 0.1 cm/s (central point source)Gas Velocity= 10 cm/s (uniform feed)

Gas-Solid and Liquid-Solid Drag Closure: Two-Phase Ergun Equation

Title:

Creator:TECPLOTPreview:This EPS picture was not savedwith a preview included in it.Comment:This EPS picture will print to aPostScript printer, but not toother types of printers.

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Bed Porosity (lighter areas: higher porosity)

Liquid Holdup Comparison between Steady and Unsteady OperationLiquid Holdup Comparison between Steady and Unsteady Operation

Title:


Title:


Title:


Title:


t= 15 s t= 25 s t=40 sSteady State Unsteady State

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SummarySummary

Performance enhancement was seen to be a strong function of the extent of reactant limitation Performance enhancement under gas limited conditions was found to be significantly dependent upon the cycle split, cycle period, liquid mass velocity and cycling frequency Performance enhancement under liquid limited conditions was observed only with BASE-PEAK flow modulation (to a lesser extent than under gas limited conditions)

Rigorous modeling of mass and energy transport by Stefan-Maxwell equations and solution of momentum equations needed to simulate unsteady state flow, transport and reaction has been accomplished. Qualitative comparison with the experimental observations has been successfully demonstrated. The developed code can be used as a generalized simulator for any multicomponent, multi-reaction system and can be converted to a multidimensional code for large scale industrial reactors Fluid dynamic codes (CFDLIB) have been used to demonstrate better flow distribution under unsteady state operation. These codes would help achieve quantitative predictions when used in conjunction with the reaction transport simulator developed in this study

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Recommendations for Future WorkRecommendations for Future Work

Downflow and Upflow ComparisonDownflow and Upflow Comparison

Generalization of the conclusions obtained for complex reactionsGeneralization of the conclusions obtained for complex reactions Steady and Unsteady State ModelsSteady and Unsteady State Models

Implementation for multi-reaction problems and conversion to a flow Implementation for multi-reaction problems and conversion to a flow

sheet based package (ASPEN user model)sheet based package (ASPEN user model)

Implementation for multi-dimensional test cases in the framework of Implementation for multi-dimensional test cases in the framework of

CFD codes (CFDLIB or FLUENT)CFD codes (CFDLIB or FLUENT) Unsteady State ExperimentsUnsteady State Experiments

Testing of reaction networks for possible enhancement in selectivity viaTesting of reaction networks for possible enhancement in selectivity via

flow or composition modulation flow or composition modulation

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Advisors: Prof. M. P. Dudukovic and Prof. M. Al-DahhanAdvisors: Prof. M. P. Dudukovic and Prof. M. Al-Dahhan

Committee members: Prof. B. Joseph, Prof. R. A. GardnerCommittee members: Prof. B. Joseph, Prof. R. A. Gardner

Dr. M. Colakyan (Union Carbide)Dr. M. Colakyan (Union Carbide)

Dr. R. Gupta (Exxon Research)Dr. R. Gupta (Exxon Research)

CREL Industrial SponsorsCREL Industrial Sponsors

Dr. Kahney, Dr. Chou, G. Ahmed (Monsanto)Dr. Kahney, Dr. Chou, G. Ahmed (Monsanto)

Dr. Patrick Mills (Du Pont)Dr. Patrick Mills (Du Pont)

Engelhard, Eastmann ChemicalsEngelhard, Eastmann Chemicals

CREL Students and Research Associates CREL Students and Research Associates

Y. Wu, Y. JiangY. Wu, Y. Jiang

Computer and Laboratory Support Computer and Laboratory Support

Dr. Y. Yamashita, Dr. S. Kumar, Dr. Y. Yamashita, Dr. S. Kumar,

S. Picker, J. KrietlerS. Picker, J. Krietler

Parents, Roommates, and FriendsParents, Roommates, and Friends

AcknowledgementsAcknowledgements

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Documents

Unsteady State Operation in Trickle Bed Reactors “Modulation of input variables or parameters to create unsteady state conditions to achieve performance