Modeling Flowand Residence Time Distribution (RTD) in an

Modeling Flow andResidence Time Distribution (RTD)

in an Industrial-scale Segmented Reactorby Coupling CFD and Monte Carlo Simulations

Team: Hua Bai, Adriana Vazquez, Amber Stephenson, Jorge Jimenez and Dennis Jewell

Acknowledgement: Paul Gillis

CFD in Chemical Reaction Engineering V (Whistler 2008)

The Dow Chemical CompanyThe Dow Chemical Company

• Motivation• Quantifying flow and RTD in a reactor is critical

for defining appropriate reactor model to predict performance (e.g., yield and selectivity) – What reactor model to use (CSTR, PFR or

combination)– Process optimization– Evaluation of different operating conditions

• This talk: RTD


The Reactor (CFD modeling domain)

Overflow direction

Baffle 1Baffle 2Baffle 3

Baffle 4Baffle 5

Tank volume: >31K gals (120m3)


Liquid surfaces:

Elevation viewA7A1A2A3A4A5A6

B1B2B3B4B5B6

Cell ACell BCell CCell DCell ECell F

From upstream

To downstream

ERL back toCell F via A6

Drain notch

External Recirculation Loop(same for each cell) excluded from CFD modeling domain

Freshfeed

ERL

RTD Strategy

• Measuring RTD in the reactor during operation is expensive and difficult– Lab experiment scale-up is difficult

• Modeling: “2-step” approach– Step 1: obtain the flow field in the reactor (frozen in Step 2)– Step 2: obtain RTD (2 methods)

• Method 1: Predict the outlet response to a pulse or step input at the inlet (transient simulation of passive scalar)

• Method 2: Stochastic particle tracking to track trajectories of tracers (statistically calculate RTD)

– Tracers released at inlets of each cell– Random Walk Model for dispersion due to turbulent eddies– Residence time recorded @ outlets as each tracer exits the cell

• Method 2 chosen in this work – Advantage in dealing with multiple inlets/outlets– Advantage in coupling with Monte Carlo method


Outflow via nozzle BInflow via nozzle ARTDAB

Overflow to next cellInflow via nozzle ARTDAO

Outflow via nozzle BOverflow from previous cellRTDOB

Overflow to next cell

Tracers exiting cell @: (where RT recorded)

Overflow from previous cellRTDOO

Tracers released @:Cell RTD

Unique ChallengesCFD in Chemical Reaction Engineering V (Whistler 2008)

• Multiple inlets/outlets• External Recirculation Loop (ERL), which is excluded from

the CFD modeling domain

Cell RTD components without ERL

OverflowOverflow

ERL

Back to cell via A

(a) Tracers released at Nozzle A1-A7

(b) Tracers released at baffle overflows

(c) Tracers released at drain notches

Iso-surfaces of 5 ft/s

Flow PatternsCFD in Chemical Reaction Engineering V (Whistler 2008)

Iso-surfaces of 3 ft/s

Velocity magnitude contours Flow pathlines:

FLUENT 6.3


RTDAB for Cell B

Mean RT: 74.9 sStd dev.: 79.3 sMin RT: 0.6 sMax RT: 828.9 s

RTD components w/o ERL

Key elements for accurate stochastic tracer tracking

“How to” in FLUENTTedious “Inlet subdivisions” approach- Divide inlet into multiple sub-inlets based on velocity range

- Define “Injection” for each sub-inlet# tracers ~ local mass flow rate

Average 35,000 tracers tracked in each stochastic tracking

•-- Tracer release distribution @ inlets based on local mass flow rate •-- Sufficient number tracers tracked to produce statistically significant results ••-- Proper integration time step

0

20

40

60

80

100

120

140

160

180

200

Cell A Cell B Cell C Cell D Cell E Cell F

Cell turnover time Average Mean RT

Tim

e (s

ec)

Validation of RTD w/o ERL


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Cell A Cell B Cell C Cell D Cell E

Flow split Tracers split

Average deviation to tC= 3.2%

Average deviation to Flow split = 3.4%

tCT

C Q

Vt

OORTD Mean RT for RTDOO

OO Tracer-split ratio in tracking

T

O

Q

QRatio of weir overflow in inflows

Flow split (C): ratio of flow being recycled and total inflow

Tracers split: ratio of tracers exiting Nozzle B and total injected tracers

RTD RTD w/o ERL

OBOBOOOO RTDRTDRTD

(Ave. weighted by tracer split and inflows)

ABABAOAO RTDRTD1

• Key information – Single-pass RTDs for each cell: RTDOO, RTDOB, RTDAO, and RTDAB

– C – (flow recycled back to the cell) / (total inflow entering the cell)– t – lag time through ERL

• Cycles % tracers RTD with effect ofexperienced being tracked flow recycling

0 (1-C) RTDOO

1 (1-C)C RTDOB + t + RTDAO

2 (1-C)C2 RTDOB + RTDAB + 2 t + RTDAO

3 (1-C)C3 RTDOB + 2RTDAB + 3 t + RTDAO

… ……….. ………………n (1-C)Cn RTDOB + (n-1)RTDAB + n t + RTDAO

Number of cycles (n) needed to track all tracers(1-C) + (1-C)C + (1-C)C2 + (1-C)C3 + …+ (1-C)Cn = 1

for example, if C=0.9, 99% tracers would be tracked after 43 cycles (n =43) • “+” operations between the single-pass RTDs (raw data) superposition through

Monte Carlo simulation

Include ERL in RTD


OverflowOverflow

Monte Carlo Schematic


Monte Carlo simulation using the commercial software ARENA by Rockwell Automation Technologies Inc.

p – random number, uniformly distributed between 0 and 1 C – (flow recycled back to the cell) / (total inflow entering the cell)

p<C

p>C

Time=RTDOO

Time=RTDOB+ t

Time= Time + RTDAO

RTDRTDCellCell=Time=Time

Time= Time + RTDAB + t

p>C

p<C

Histograms of RTD w/ERL


0

200

400

600

800

1000

1200

1400

1600

1800

2000

Cell A Cell B Cell C Cell D Cell E Cell F

Cell turnover time (s) Mean RT by Monte Carlo (s)

Validation of RTD w/ERL


'

'

TC Q

VtCell turnover time:

Average deviation to tC= 2.9%

RTD w/ERL ERL: External Recirculation LoopCt

= the liquid volume in the cell + in the ERL

= (the total volumetric flow rate entering the cell)- (the flow being recycled back to the cell)

'TQ

'V

• A model was developed to predict RTD in a reactor with multiple inlets and outlets as well as external recirculation loop (ERL)– by coupling CFD stochastic particle tracking with Monte Carlo

simulation (implemented in ARENA by Rockwell Automation Technologies Inc.)

– validated by matching the reactor/cell turnover times• also by matching “flow split” with “tracers split”

• Critical for accurate RTD predictions during stochastic particle tracking – Tracer release distribution @ inlets proportional to local mass

flow rate – Sufficient number of tracers tracked to produce statistically

significant results– Proper integration time step

Summary CFD in Chemical Reaction Engineering V (Whistler 2008)

Thank YouThank You


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Modeling Flowand Residence Time Distribution (RTD) in an