Copyright ©, 2018, Sasol
SASOL GROUP TECHNOLOGY
CFD ANALYSIS OF INDUSTRIAL MIXERS AND SEPARATORS
Dr Robin Jordi
ESC 3 September 2018
African Pride Irene Country Lodge
2Copyright ©, 2018, Sasol
Case 1: Tertiary FCC Separator Vessel
Altona Cyclofines™ Third Stage Separator Arrangement: Source 2003 NPRA Annual Meeting
in San Antonio, TX
• Tertiary gas-solid separation vessels are a feature of FCC units, separating catalyst fines and attrition products from regenerator flue gases. The bimodal PSD with significant ultrafine material is a separation challenge.
• Typically a refractory lined high temperature pressure vessel installation with dividing partitions supporting the high-efficiency swirl tubes. The number of swirl tubes is scaled to refinery FCC unit capacity.
• Two variants of swirl tubes, reverse flow and axial flow units are used in this application.
• Feed line is typically via the central axis, with uncoupled inlet ports in the middle chamber.
• Underflow rate is maintained at a small fraction of the inlet volumetric flow, controlled by a downstream flow resistance.
• Overflow is via vortex finder piping into upper chamber and out via the discharge piping to a turbo-expander, slide valve, waste heat boiler and stack etc.
• A Sasol unit experience poor separation efficiency. Blockages of the swirl tube vortex finders are often observed during shutdown inspections.
• Extensive CFD simulation was used in the RCA process troubleshooting.
3Copyright ©, 2018, Sasol
Isokinetic Sampling Data and SEM Image
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1
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7
PD
Fv
Particle Diameter (µm)
SP1 Dataset: PDF Tests 7, 8, 12, 14, 18
Test 7 Test 8 Test 12 Test 14 Test 18
Particle size distribution function at SP1 SEM Image of a Sample at SP2
4Copyright ©, 2018, Sasol
Flow pathlines (m.s-1) Static pressure (Pa)Flow pathlines (m.s-1)Static pressure (Pa)
INITIAL CFD ANALYSIS of SWIRL TUBE
High Fidelity CFD Simulation of the Swirl Tube
● CFD simulation shows unfavourable precession of the inner vortex with attachment to the tube internal wall.
● Inner and outer vortices are strongly impacted by the pressure difference between the middle chamber and the underflow chamber. Large ΔP results in improved performance – but this is a dynamic coupled process.
● Taylor eddies at the base internal wall of the vortex finder tube are noted.
5Copyright ©, 2018, Sasol
CFD Mesh and Solver Settings: Case 1.1
Solver Settings
PV Coupling Phase coupled SIMPLE
Discretization
Gradient LSQ Cell Based
Density QUICK
Momentum QUICK
Volume Fraction QUICK
Turbulence Eqns 2nd Order upwind
Reynolds Stresses 1st Order upwind
Energy QUICK
Time 1st Order Implicit
Models
Multiphase Implicit 2 Phase Eulerian
Turbulence model Linear pressure-strain RS model
Scalable wall fcns
Energy On / No radiation
Gravity On
EOS Ideal gas
Solution
Style Transient / 0.5 ms Timesteps
Other
UDFs Initialisation and solution monitoring
Scheme Online post-processing
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80%
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cen
tage
Inverse Orthogonal Quality (-)
Mesh quality metric
Meshing
Method Ansys Meshing
Mesh type Conformal hexahedral
Mesh cell count 3.52 Million
6Copyright ©, 2018, Sasol
Summary of CFD Findings: As-Built Unit
Unblocked Condition:
• Good outer quasi-free and inner quasi-forced vortex structure development is observed with the swirl tubes.
• Gas flow distribution in the central manifold to each swirl tube is ±1.5 %.
• Particulate separation efficiency simulated with a single with a expected particulate cutsize is consistent with vendor claims of 90% efficiency.
• Precessional instability of high velocity vortices are noted.
Blocked Performance: 25 % of vortex tubes occluded.
• Higher total pressure drop as anticipated by scaling in velocities in the open vortex finder tubes.
• Poorly developed vortex structures observed with the swirl tubes, helical features completely absent with high wall bounded tangential velocities.
• Reversed flow into the discharge ports of the unblocked swirl tubes noted. Inlet distributions skewed by ± 6.0 %
Altona Cyclofines™ Third Stage Separator Arrangement: Source 2003 NPRA Annual Meeting
in San Antonio, TX
7Copyright ©, 2018, Sasol
2015 Shutdown 25 % of Swirl Tubes Blocked
Blocked Performance Summary
● Reverse flow in open swirl tubes outlets in blocked case. Superficial velocity 3.86 m.s-1.
● This upflow velocity would be sufficient to entrain and elutriate 900 µm particles.
● Vortex finder tube frictional resistances control the inlet flow distribution.
0.583 MFU 0.618 MFU
0.583 MFU -0.181 MFU
0.798 MFU
3.86 m.s-1
8Copyright ©, 2018, Sasol
Cyclone Cluster Retrofit Analysis
Velocity magnitude (m.s-1)
Cyclone Cluster Design Methodology
● Proposed solution is to replace the existing swirl tubes with a smaller number of cyclones not prone to blockage.
● Particulate Solids Research Inc. (PSRI), Chicago Cyclone design
procedure was implemented in high quality Fortran 95 coding.
● Optimise a conventional reverse flow cyclone geometry to maximize
separation efficiency – subject to constraints.
● With the cyclone geometry fixed apply the same design against 5
statistically consistent isokinetic sample datasets.
● Major constraints are:
● PSRI Correlation inherent constraints.
● Vessel geometric constraints:
• Adequate barrel size for flow clearance with a suitable
refractory lined vortex finder. Utilizing 135° degree offset
volute inlet with 25 mm of refractory lining.
• Cyclone height < 5 m – rigging access.
• Passage through the equipment manhole.
● Varying the number of cyclones from 5-8 in the cluster.
9Copyright ©, 2018, Sasol
CFD Mesh and Solver Settings: Case 1.2
Solver Settings
PV Coupling SIMPLE
Discretization
Gradient LSQ Cell Based
Pressure PRESTO!
Momentum QUICK
Turbulence Eqns QUICK
Reynolds Stresses QUICK
Time Bounded 2nd Order Implicit
Models
Multiphase Off
Turbulence model Linear pressure-strain RS model
Standard wall fcns
Energy Off
Gravity On
EOS Constant density
Solution
Style Transient / 0.1 ms Timesteps
Other
UDFs Initialisation and solution monitoring
Scheme Online post-processing
Meshing
Method Ansys Meshing
Mesh type Polyhedral Cutcell
Mesh cell count 103.4 Million
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60%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1P
erce
nta
geInverse Orthogonal Quality (-)
Mesh quality metric
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Cyclone Cluster Retrofit Option
Static Pressure (Pa) Gas velocity magnitude (m.s-1) Gas tangential velocity (m.s-1)
Optimised 8 Cyclone Cluster
● Stable vortex structures in 8 cyclone cluster noted. Median efficiency 75.3 ± 4.4 %.
11Copyright ©, 2018, Sasol
CFD Mesh and Solver Settings: Case 1.3
Solver Settings
PV Coupling Coupled
Discretization
Gradient LSQ Cell Based
Pressure PRESTO!
Momentum QUICK
Volume Fraction QUICK
Turbulence Eqns QUICK
Reynolds Stresses QUICK
Time Bounded 2nd Order Implicit
Models
Multiphase Mixture
Dispersed interfaces
Implict body forces
Turbulence model Linear pressure-strain RS model
Standard wall fcns
Energy Off
Gravity On
EOS Constant density
Solution
Style Transient / 1 ms Timesteps
Other
UDFs Solution control
Scheme Online post-processing
Meshing
Method Ansys Meshing
Mesh type Mosaic Poly-Hexcore
Mesh cell count 12.3 Million
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45%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1P
erce
nta
geInverse Orthogonal Quality (-)
Mesh quality metric
12Copyright ©, 2018, Sasol
Case 2: Centrifugal Separator Vessel
Separator Vessel CAD Centrifugal Separator Vessel in Secunda
• Not the initial focus of the CFD study, but a process design inadequacy was identified.
• Vessel has some typical features expected of cyclone separators in gas liquid service:
• Drip ring, downward angled tangential feed nozzle, base baffle to eliminate vapor lock in liquid underflow line, erosion resistant impingement pad on upper barrel etc.
• Similar cyclone separators are used in multiple process plant applications:
• Gas plant (LTX Separator)
• Refinery applications (Thermal Gasoil Units (TGU), Visbreaker Units etc.)
• Chemical plants (Thermoplastic Rubber Plants).
• One of multiple installed units.
• No vortex stabilizer is noted.
• Inlet flow is a two / three phase flow in thermal equilibrium.
• Overhead transfer line to downstream fin-fan heat exchangers – so excessive liquid entrainment is very undesirable.
• Some solid particulates can be entrained during upstream process instability.
13Copyright ©, 2018, Sasol
Case 2: CDF Analysis Cases: As-Is and Proposed Retrofit
As-Is Proposal
Two CFD cases
• Proposed case includes a vortex stabilizer plate
assembly with support strakes to reduce liquid
recirculation.
• Vortex stabilizer design is in accordance with a
Process Engineering design manual.
• Most of the existing vessel and nozzle
dimensions accord well with this design manual.
• Additional conical element on the vortex finder as
utilized in gas-liquid cyclone separators to entrain
liquid film into the gas phase as droplets. Typical
installations have serrated conical elements to
enhance droplet formation.
• Since this study was not a focus of the original
CFD scope, a quick analysis was required.
Hence an assembly meshing analysis with VoF
style phase tracking.
14Copyright ©, 2018, Sasol
CFD Mesh and Solver Settings: Case 2.1 As-Is
Solver Settings
PV Coupling SIMPLE
Discretization
Gradient LSQ Cell Based
Pressure PRESTO!
Density 2nd Order upwind
Momentum 2nd Order upwind
VoF Compressive
Turbulence Eqns 2nd Order upwind
Reynolds Stresses 2nd Order upwind
Energy 2nd Order upwind
Time Bounded 2nd Order Implicit
Models
Multiphase Implicit 2 Phase VoF
Sharp / Dispersed / 10-7 VoF cutoff
Implicit Body Forces
Turbulence model Linear pressure-strain RS model
Standard wall fcns
Energy On / No radiation
Gravity On
EOS Ideal gas
Solution
Style Transient / 1 ms Timesteps
Other
UDFs Initialisation and solution monitoring
Transport Properties
Scheme Online post-processing
Meshing
Method Ansys Fluent Meshing
Mesh type Mosaic Poly-Hexcore
Mesh cell count 4.83 Million
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30%
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60%
70%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1P
erce
nta
ge
Inverse Orthogonal Quality (-)
Mesh quality metric
15Copyright ©, 2018, Sasol
CFD Mesh and Solver Settings: Case 2.2 Retrofit Option
Meshing
Method Ansys Fluent Meshing
Mesh type Mosaic Poly-Hexcore
Mesh cell count 5.93 Million
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10%
20%
30%
40%
50%
60%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Per
cen
tage
Inverse Orthogonal Quality (-)
Mesh quality metric
Solver Settings
PV Coupling SIMPLE
Discretization
Gradient LSQ Cell Based
Pressure PRESTO!
Density 2nd Order upwind
Momentum 2nd Order upwind
VoF Compressive
Turbulence Eqns 2nd Order upwind
Reynolds Stresses 2nd Order upwind
Energy 2nd Order upwind
Time Bounded 2nd Order Implicit
Models
Multiphase Implicit 2 Phase VoF
Sharp / Dispersed / 10-7 VoF cutoff
Implicit Body Forces
Turbulence model Linear pressure-strain RS model
Standard wall fcns
Energy On / No radiation
Gravity On
EOS Ideal gas
Solution
Style Transient / 1 ms Timesteps
Other
UDFs Initialisation and solution monitoring
Transport Properties
Scheme Online post-processing
16Copyright ©, 2018, Sasol
Centrifugal Separator: CFD ResultsStatic pressure (Pa)
Velocity magnitude (m.s-1)
17Copyright ©, 2018, Sasol
Centrifugal Separator: CFD ResultsTangential velocity (m.s-1)
Liquid phase fraction (-)
18Copyright ©, 2018, Sasol
Case 3: Non-Newtonian Inline Static Mixers
Inline Static Mixers CAD Application involves dispersion of a Newtonian fluid into a non-Newtonian fluid.
• Newtonian fluid has a small volume fraction (< 3%) of the overall flow. Fluid density is 1300 kg.m-3 with a viscosity of 32 cP.
• Non-Newtonian fluid is a shear thinning fluid. Fluid rheology was measured over a strain rate range 5 – 1000 s-1. Some evidence of flow memory effects were noted but not analysedhere. Fluid density is ~1200 kg.m-3.
• A Herschel-Bulkley model matched the temperature dependent apparent viscosity / strain rate data well.
• An existing inline static mixer produced poor mixing results. This is a turbulent flow mixer with 11 mixing elements. Calculation showed an apparent Reynolds number of ~10.
• Alternative commercial mixers like the SMX style inline static mixer designed for Laminar non-Newtonian mixing applications were therefore considered.
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Sh
ea
r str
ess (
Pa
)
Shear rate (s-1)
Shear stress vs Shear rate
[Pa] Model [Pa]
19Copyright ©, 2018, Sasol
Inline Static Mixer Element CAD Detail
SMX 238 90° mixer elementsSMX 113 120° mixer elements
SMX (n,Np,Nx) Θ
n: Parameterization
Np : Number of parallel plates, Nx : Number of plate sections, Θ : Plate angle
20Copyright ©, 2018, Sasol
CFD Mesh and Solver Settings: Case 3.1 SMX113
Solver Settings
PV Coupling Coupled
Discretization
Gradient LSQ Cell Based
Pressure PRESTO!
Momentum QUICK
Volume Fraction Modified HRIC
Time Steady
Models
Multiphase Implicit 2 Phase VoF
Sharp
Turbulence Laminar
Energy Off
Gravity On
EOS Constant phase density
Rheology Herschel-Bulkley Model
Bingham pseudoplastic
Solution
Style Pseudo-Transient
Other
UDFs None
Scheme Online post-processing
Meshing
Method Ansys Fluent Meshing
Mesh type Polyhedral Hexcore
Mesh cell count 59.6 Million
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50%
60%
70%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Per
cen
tage
Inverse Orthogonal Quality (-)
Mesh quality metric
21Copyright ©, 2018, Sasol
CFD Mesh and Solver Settings: Case 3.2 SMX238
Meshing
Method Ansys Fluent Meshing
Mesh type Polyhedral Hexcore
Mesh cell count 124.8 Million
0%
10%
20%
30%
40%
50%
60%
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Per
cen
tage
Inverse Orthogonal Quality (-)
Mesh quality metric
Solver Settings
PV Coupling Coupled
Discretization
Gradient LSQ Cell Based
Pressure PRESTO!
Momentum QUICK
Volume Fraction Modified HRIC
Time Steady
Models
Multiphase Implicit 2 Phase VoF
Sharp
Turbulence Laminar
Energy Off
Gravity On
EOS Constant phase density
Rheology Herschel-Bulkley Model
Bingham pseudoplastic
Solution
Style Pseudo-Transient
Other
UDFs None
Scheme Online post-processing
22Copyright ©, 2018, Sasol
Static Mixer CFD Simulation Results: Volume Fraction
SMX 113 SMX 238
23Copyright ©, 2018, Sasol
SMX 113
SMX 238 (Standard mixer design)
Static Mixer CFD Simulation Results: Volume Fraction
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Static Mixer CFD Simulation Results: Velocity Contours
SMX 113 SMX 238
25Copyright ©, 2018, Sasol
SMX 113
SMX 238 (Standard mixer design)
Static Mixer CFD Simulation Results: Static Pressure
26Copyright ©, 2018, Sasol
Static Mixer CFD Simulation Results: Flow Pathlines
SMX238
non-Newtonian phase Newtonian phase
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