SASOL GROUP TECHNOLOGY CFD ANALYSIS OF INDUSTRIAL …

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.


Isokinetic Sampling Data and SEM Image

0

1

2

3

4

5

6

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


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.


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

0%

10%

20%

30%

40%

50%

60%

70%

80%

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

Meshing

Method Ansys Meshing

Mesh type Conformal hexahedral

Mesh cell count 3.52 Million


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


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


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.



Solver Settings

PV Coupling SIMPLE

Discretization


Pressure PRESTO!

Momentum QUICK

Turbulence Eqns QUICK

Reynolds Stresses QUICK

Time Bounded 2nd Order Implicit

Models

Multiphase Off


Standard wall fcns

Energy Off

Gravity On

EOS Constant density

Solution

Style Transient / 0.1 ms Timesteps

Other



Meshing


Mesh type Polyhedral Cutcell


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 1P

erce

nta

geInverse Orthogonal Quality (-)

Mesh quality metric


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 %.



Solver Settings

PV Coupling Coupled

Discretization


Pressure PRESTO!

Momentum QUICK

Volume Fraction QUICK

Turbulence Eqns QUICK

Reynolds Stresses QUICK


Models

Multiphase Mixture

Dispersed interfaces

Implict body forces


Standard wall fcns

Energy Off

Gravity On

EOS Constant density

Solution

Style Transient / 1 ms Timesteps

Other

UDFs Solution control


Meshing


Mesh type Mosaic Poly-Hexcore


0%

5%

10%

15%

20%

25%

30%

35%

40%

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


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.


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.


CFD Mesh and Solver Settings: Case 2.1 As-Is

Solver Settings

PV Coupling SIMPLE

Discretization


Pressure PRESTO!

Density 2nd Order upwind

Momentum 2nd Order upwind

VoF Compressive


Reynolds Stresses 2nd Order upwind

Energy 2nd Order upwind


Models

Multiphase Implicit 2 Phase VoF

Sharp / Dispersed / 10-7 VoF cutoff

Implicit Body Forces


Standard wall fcns


Gravity On

EOS Ideal gas

Solution


Other


Transport Properties


Meshing

Method Ansys Fluent Meshing



0%

10%

20%

30%

40%

50%

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


Mesh quality metric


CFD Mesh and Solver Settings: Case 2.2 Retrofit Option

Meshing




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


Mesh quality metric

Solver Settings

PV Coupling SIMPLE

Discretization


Pressure PRESTO!

Density 2nd Order upwind

Momentum 2nd Order upwind

VoF Compressive


Reynolds Stresses 2nd Order upwind

Energy 2nd Order upwind


Models


Sharp / Dispersed / 10-7 VoF cutoff

Implicit Body Forces


Standard wall fcns


Gravity On

EOS Ideal gas

Solution


Other


Transport Properties



Centrifugal Separator: CFD ResultsStatic pressure (Pa)

Velocity magnitude (m.s-1)


Centrifugal Separator: CFD ResultsTangential velocity (m.s-1)

Liquid phase fraction (-)


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.

0

200

400

600

800

1000

1200

1400

1600

1 10 100 1000

Sh

ea

r str

ess (

Pa

)

Shear rate (s-1)

Shear stress vs Shear rate

[Pa] Model [Pa]


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


CFD Mesh and Solver Settings: Case 3.1 SMX113

Solver Settings

PV Coupling Coupled

Discretization


Pressure PRESTO!

Momentum QUICK

Volume Fraction Modified HRIC

Time Steady

Models


Sharp

Turbulence Laminar

Energy Off

Gravity On

EOS Constant phase density

Rheology Herschel-Bulkley Model

Bingham pseudoplastic

Solution

Style Pseudo-Transient

Other

UDFs None


Meshing


Mesh type Polyhedral Hexcore


0%

10%

20%

30%

40%

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


Mesh quality metric


CFD Mesh and Solver Settings: Case 3.2 SMX238

Meshing


Mesh type Polyhedral Hexcore


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


Mesh quality metric

Solver Settings

PV Coupling Coupled

Discretization


Pressure PRESTO!

Momentum QUICK

Volume Fraction Modified HRIC

Time Steady

Models


Sharp

Turbulence Laminar

Energy Off

Gravity On

EOS Constant phase density

Rheology Herschel-Bulkley Model

Bingham pseudoplastic

Solution

Style Pseudo-Transient

Other

UDFs None



Static Mixer CFD Simulation Results: Volume Fraction

SMX 113 SMX 238


SMX 113

SMX 238 (Standard mixer design)

Static Mixer CFD Simulation Results: Volume Fraction


Static Mixer CFD Simulation Results: Velocity Contours

SMX 113 SMX 238


SMX 113

SMX 238 (Standard mixer design)

Static Mixer CFD Simulation Results: Static Pressure


Static Mixer CFD Simulation Results: Flow Pathlines

SMX238

non-Newtonian phase Newtonian phase

SASOL GROUP TECHNOLOGY CFD ANALYSIS OF INDUSTRIAL …

Documents

Transcript of SASOL GROUP TECHNOLOGY CFD ANALYSIS OF INDUSTRIAL …