CFD MODELING OF TRICKLE BED REACTORS: Hydro-processing Reactor

Transcription

1 CFD MODEIN OF TRICKE BED REACTORS: Hydro-processing Reactor Prashant R. unjal, Amit Arora and Vivek V. Ranade Industrial Flow Modeling roup National Chemical aboratory Pune 48

2 OUTINE Trickle Bed Reactors Applications/ flow regimes Experiments 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

3 TRICKE BED REACTORS Wide Applications Hydro-de-sulfurization Hydro-cracking/ hydro-treating Hydrogenation/ oxidation Waste water treatment Key Characteristics Close to plug flow/ ow liquid hold-up Suitable for slow reactions Poor heat transfer/ possibility of mal-distribution Difficult scale-up

4 TRICKE BED REACTORS iquid as as & iquid Flow through Void Space = 6-48 %

5 FUID DYNAMICS OF TBR Characterization of Packed Bed Wetting of Solid Surface by iquid, P, α, η, RTD Mal-distribution, Channeling & Mixing Flow Regimes & lobal Flow Characteristics

6 FOW REIMES

7 EXPERIMENTS AT NC Pressure Indicator Data Acquisition Column with 3mm or 6mm lass Beads Hydrodynamics Pressure Drop iquid Hold-up Conductivity Probes Residence Time Distribution iquid Distribution at Inlet Uniform Non-uniform From Compressor Separator Conductivity Probe Pump iquid Tank Experimental Parameters Bed Diameter:. &. m Particle Diameter: 3 & 6 mm iquid Velocity: < 4 mm/ s as Velocity: <.5 m/ s Tracer : NaCl

8 TYPICA PRESSURE DROP DATA 35 Pressure gradient, KPa/m Trickle flow regime Pulse flow regime 3 iquid Flow Rate, Kg/m s

9 PUBISHED EXPERIMENTA DATA Superficial as Velocity Vg, (m/s) Spray Flow φ β φ =? Trickle Flow Pulse Flow φ =?. Szady and Sundersan (99). Rao et. al (983) 3. Specchia and Baldi (977) φ =β Bubbly Flow iquid Velocity V, (Kg/m s)

10 MODEIN OF MACROSCOPIC FOW Bed Porosity: Scale of Scrutiny Bed diameter/ length: overall Intermediate scale: aussian 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 aussian distribution with specified variance

11 CHARACTERIZATION OF PACKED BED Radial Variation of Porosity: Mueller (99) a a b r () r * = B = = ( = r/d and ( D/d 3.56) ( D/d 9.864) J )J.98 P.93 P P o B.74 =.34 - D/d is o (ar zero * )e th br for.6 D/d for 3. D/d 3. order Bessel Function P P

12 CHARACTERIZATION OF PACKED BED Mueller s Correlation Three data sets from literature Our experiments Szady and Sundersan (99) Rao et.al. (983) Spacchia and Baldi (977).6.5 D=.94m, dp=3mm D=.94m, dp=6mm Porosity Porosity Dimensionless Radius Dimensionless Radius

13 VARIATION OF BED POROSITY.56 With std dev= With std dev=5% With std dev=% r/r.7 Selection of appropriate value is not straight forward: RTD may help

14 CFD MODE Multi-fluid Model (Eulerian-Eulerian) Continuity Equation = + K K K K K U t Momentum Balance Equation ( ) ( ) R k R K K K K K K K K K K K K U U F g U P U U t U = +, ) ( ) ( µ

15 CFD MODE Inter-phase Coupling Terms (Attou & Ferschneider, ) ( ) + = ) ( ) ( ) ( ) ( p P p d U U E d E F µ + = ) ( ) ( ) ( ) ( S p P S p S d U E d E F µ + = p S P p s S d U E d E F µ

16 CFD MODE Capillary Terms Attou and Ferschneider () = d d P P σ = P F d P P σ ( ) + = s s / s p F z z d. σ z P z P

17 CFD MODE Capillary Terms Extent of Wetting, f (Jiang et al., ) Scalar Transport i i m i K K i K K K i K K S C D C U t C + = + ) (. c P f P P ) = ( < + = for F

18 SIMUATED RESUTS Distributions within the bed Contours of iquid Hold-up Non-prewetted Bed Prewetted Bed Velocity M agnitude, m/sec 35 Non-prewetted Bed 3 Prewetted Bed 5 Pre-wetted Un-wetted V = 6 mm/ s, V =. m/ s, D=.4 m, d P = 3 mm, σ=5% iquid Holdup

19 SIMUATED RESUTS Column Diameter =.4 m Particle Diameter = 3 mm, V =. m/ s

20 SIMUATED RESUTS V =. m /s, D =.4 m V =. m /s, D =.94 m

21 AS-IQUID FOW IN TBR Estimation of Frictional Pressure Drop & Fraction of iquid Supported by as Simulate at two values of g (9.7, 9.9 m/s ) g g P g P g P f = ( ) g g P P = φ Fraction of iquid Hold-up Supported by as Phase < Solid iquid as ( ) f β φ φ = Q g P P

22 SIMUATED RESUTS Pressure radient KPa/m Saez and Corbonell (985)-Exp. data Szady and Sundersan (99)-Increase in liquid Szady and Sundersan (99)-decrease in liquid CFD Simulations w ithout Capillary Force CFD Simulations iquid Flow Rate x -, Kg/m s Can Predict Total and Supported iquid Volume Fraction iquid Saturation and Calibrate Model Parameters from P Holub et. al. (993) -Experimental data Szady and Sundersan (99)-Exp. Data CFD Results Supported iquid Saturation iquid Flow Rate x -, kg/m s Operating conditions:v =.m/s, D/dp =55, dp=3mm, Std. Dev.=5%, E =5, E =.75

23 SIMUATED RESUTS 5 Pressure Drop, (KPa/m) 4 3 CFD Results Experimental Data as Flow Rate, (Kg/ms) As gas velocity increases, more & more liquid is supported by gas Data of Spachio & Baldi (977) iquid Saturation and iquid Saturation- CFD Results Supported iquid Saturation iquid Saturation- Exp. Data as Mass Velocity, kg/ms Operating conditions: V =.8x -3 m/s, dp=3mm, D/dp=3, Std. Dev.=5%, E =5, E =3

24 SIMUATED RESUTS Pressure Drop, (KPa/m) Experimental Data CFD Results Data of Rao et al. (983) as Mass Velocity, (kg/ms) As gas velocity increases, more & more liquid is supported by gas iquid Saturation and iquid Saturation- CFD Results Supported iquid Saturation iquid Saturation Exp. Data as mass velocity, Kg/ms Operating conditions:v =x -3 m/s, D/dp =5.4, dp=3mm, Std. Dev.=5%, E =5, E =3.4

25 RESIDENCE TIME DISTRIBUTION.. Pre-wetted Bed Non Pre-wetted Bed E(t), sec Simulation Results Experimental Results Time, (sec) Operating conditions: V =.6x -3 m/s V =.m/s, D/dp =8, dp=6mm, Std. Dev.=5%, E =8, E =.75

26 MODEIN OF HYDRO-PROCESSIN REACTOR Reaction and Kinetics (Chowdhury et al. ) Hydro-desulfurisation (HDS) Ar S + H Ar + HS r HDS = k + c K.56,H Ad c c.6,s,h S De-aromatisation of Mono-, Di- and Poly- Aromatics Mono Ar + 3H Napthenes r = k C + k Mono Mono Mono _Mono C Naph Di Ar + H Mono Ar r = k C + Di Di Di k _ Di C Mono Tri Ar + H Di Ar r = k C + Poly Poly Poly k _ Poly C Di

27 SIMPIFICATIONS/ ASSUMPTIONS Pressure drop is insignificant compared to the operating pressure Trickle bed reactor is operated isothermally (efficient heat transfer) Ideal gas law is applicable iquid phase reactants are non-volatile (negligible vapor pressure) as-liquid mass transfer is the limiting resistance. The catalyst particles are completely wetted

28 MODE EQUATIONS Mass Balance of Species i C k k t k, i + ( k ku kck., i ) = ( k k Di, m Ck, i ) + k ksi, k Source Terms for as Phase S i Ci = K ia C i H i Source Terms for iquid Phase S i = K i a C H i i C i + j = nr Bη r ij j= S i = j = nr Bη r ij j=

29 MODE EQUATIONS Mass Transfer Coefficient (From oto & Smith, 975) Solubility of Hydrogen/ H S (Korsten et al. 996) / = i i i D D a k µ µ α α i N H i λ υ. = 4 3 H. a T a T a T a a = λ ).847. exp(3.367 T S H = λ a a a.4947 a a = = = = = : molar gas volume υ N

30 CASE STUDIES as and iquid Superficial Velocities Increase with Scale Wetting gets Better with Scale Parameters Reactor Diameter, m Bed ength Particle Diameter, m Bed Porosity HSV, h- Operating Pressure, Mpa Operating Temperature, K Initial H S Conc., v/v % aboratory Scale Reactor (Chowdhury et al. ).9.5 m Commercial Scale Reactor (Bhaskar et al. 4) m

31 KINETICS/ OI COMPOSITION From Chowdhury et al. () Kinetic Constants Values K Ad, m 3 /kmol 5 K, Dimensionless.5 X exp(-9384/t) k*mono, m 3 /kg.s k*di, m 3 /kg.s k*poly, m 3 /kg.s 6.4 X exp(-44/t) 8.5 X exp(-4/t).66 X 5 exp(-57/t) Component Percentage Ar-S %.67 Poly-Ar %.59 Di-Ar % 8.77 Mono-Ar % 7.96 Naphthenes % 9.5

32 SIMUATED RESUTS Varying Porosity Conversion, Ar-S % 5 5 Uniform Porosity Ar-S Exp Conversion, x Temp, K Temperature Initial H S Conc as Phase ( %, V/V) Initial Concentration of H S (P=4 MPa, HSV=. h -, Q NTP /Q = m 3 /m 3, T R =3 o C, y HS =.4% ) Symbols denote experimental data of Chowdhury et al. ()

33 SIMUATED RESUTS- Conversion, % Ar-P total total poly Conversion, % Ar-D Ar-M Ar-Di-Expt Ar-Mono-Exp Temperature, K Temperature, K (P=4 MPa, HSV=. h -, Q NTP /Q = m 3 /m 3, y HS =.4%) Symbols denote experimental data of Chowdhury et al. ()

34 SIMUATED RESUTS-3 Conversion, % Total-Ar Ar-Poly Ar-Di Ar-Mono HSV, h - (P=4 MPa, TR=3 o C, Q NTP /Q = m 3 /m 3, y HS =.4%, filled symbols are for experimental data of Chowdhury et al., )

35 INFUENCE OF REACTOR SCAE Dimensionless Solid Hold-up Solid hold-up ocal Axial Velocity 5 5 Dimensionless ocal Axial Velocity Dimensionless Solid Hold-up Solid hold-up ocal Axial Velocity Dimensionless ocal Axial Velocity Dimensionless Radius Dimensionless Radius aboratory (HSV=3) Industrial (HSV=) (P=4 MPa, T R =3 o C, Q NTP /Q =3 m 3 /m 3 )

36 INFUENCE OF REACTOR SCAE Conversion, % 75 5 ab-scale Reactor Commercial Reactor Outlet Conc. Ar-S, ppm ab Scale Reactor Commercial Reactor Temperature, K Temperature, K (P lab & com =4 & 4.4 MPa, HSV lab =. h -, Q NTP /Q = m 3 /m 3, y HS =.4%)

37 INFUENCE OF REACTOR SCAE Conversion, % 6 4 Poly-Ar Total Poly-Ar-Commercial Total-Commercial Conversion, % Ar-S-ab Scale Ar-S-Commercial Temperature, K HSV, h- (P lab & com =4 & 4.4 MPa, HSV lab & com =. & 3. h -, (Q NTP /Q ) lab & com = & 3 m 3 /m 3, y HS =.4%) (P lab & com =4 & 4.4 MPa, Temperature= 593 K, (Q NTP /Q ) lab & com = & 3 m 3 /m 3, y HS =.4%)

38 INFUENCE OF REACTOR SCAE Conversion, % Poly-Ar Di-Ar Mono-Ar Total Conversion, % Poly-Ar Di-Ar Mono-Ar Total Pressure, MPa HSV, h - (Q NTP /Q ) lab & com = & 3 m 3 /m 3 T=593 K HSV lab & com =. & 3. h -,y HS =.4% P lab & com =4 & 4.4 Mpa, T=593 K y HS =.4% (Q NTP /Q ) lab & com = & 3 m 3 /m 3 Continuous lines: commercial; Dotted lines: laboratory scale

39 CONCUDIN REMARKS Macroscopic CFD Model Reasonably Simulates asliquid Flow in Trickle Beds iquid 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 imitations, CFD Models can be Used to Understand Key Issues in Reactor Engineering Including Scale-up & Scale-down

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41 MODEIN OF MESO-SCAE FOWS Single Phase Flow through Packed Bed Unit cell approach Simple cubic, FCC, rhombohedral.. Inertial flow structures, pressure drop, heat transfer Interaction of iquid Drop/ Film with Solid Surface Regimes of interaction Dynamic contact angle/ surface characteristics VOF simulations Insight into capillary forces???

42 SINE PHASE FOW Single Phase Flow through Packed Bed Periodic Flow Periodic Flow Wall Wall Wall Operating Parameters Cell ength 8mm, 3mm Fluid Water Particle Reynolds Number -6

43 SINE PHASE FOW 5 Experiments: Suekane et al. AIChE J., 49, 4.5 Simulations

44 SINE PHASE FOW 3 Experiment al 4 Experiment al Simulat ion-8mm Simulat ion-3mm 3 Simulation-8mm Simulat ion-3mm Vz/Vmean Vz/Vmean Vz/Vmean x/r Experimental Simulation-8mm Simulat ion-3mm x/r.5 Vz/Vmean x/r Experimental Simulation-8mm Simulat ion-3mm x/r

45 SINE PHASE FOW E Vz=8.66mm /s S Max arrow length : A=.6 mm/s B=.9 mm/s C=.44 mm/s A: z/r=.4 (experimental) B: z/r=.8 (experimental) C: z/r=.4 (experimental)

46 SINE PHASE FOW 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

47 IQUID SPREADIN ON SOID SURFACES Monitor Computer ight Diffuser Drop Rest on Flat Plate ight 8.79mm mm φ.4mm Drop Flow Over Pellet CCD Camera

48 DROP IMPACT ON SOID SURFACE

49 DROP IMPACT ON SOID SURFACE

50 FAT SURFACE t=ms t=45ms t=ms t=36ms (a) (d) t=ms t=45ms t=6ms t=48ms (b) (e) t=5ms t=ms t=ms t=4ms (c) (f)

51 DYNAMICS OF DROP IMPACT

52 MODES FOR INTERPHASE COUPIN? Drop Shape at ms Wall Shear Stress on Solid Surface, red~ Pa Shear Strain on as-liquid Interface