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
CFD in Chemical Reaction Engineering V (Whistler 2008)
The Reactor (CFD modeling domain)
Overflow direction
Baffle 1Baffle 2Baffle 3
Baffle 4Baffle 5
Tank volume: >31K gals (120m3)
CFD in Chemical Reaction Engineering V (Whistler 2008)
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
CFD in Chemical Reaction Engineering V (Whistler 2008)
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
CFD in Chemical Reaction Engineering V (Whistler 2008)
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
CFD in Chemical Reaction Engineering V (Whistler 2008)
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
CFD in Chemical Reaction Engineering V (Whistler 2008)
OverflowOverflow
Monte Carlo Schematic
CFD in Chemical Reaction Engineering V (Whistler 2008)
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
CFD in Chemical Reaction Engineering V (Whistler 2008)
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
CFD in Chemical Reaction Engineering V (Whistler 2008)
'
'
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
CFD in Chemical Reaction Engineering V (Whistler 2008)
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