Transcript S&C Thermofluids Ltd

S&C Thermofluids Ltd
CFD modelling of adsorption in
carbon filters
E Neininger*, MW Smith** & K Taylor*
* S&C Thermofluids Ltd
** Dstl, Porton Down
Overview
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Background to filter model development
Physics of adsorption modelling
Validation
Implementation in PHOENICS
Future developments
Typical filter application
Impregnated granular
activated carbon
Glass Fibre
Filter
Air Flow
Canister filter for respirator
Modelling Requirements
• Pressure drop
• Contaminant breakthrough time
Other filter geometries
Small scale filter test bed
- 2 cm diameter carbon bed
Carbon monolith filter
- Courtesy of MAST
Flow through filter bed
Flow through packed bed
• Pressure drop
- local voidage distribution coupled to Ergun equation
for pressure loss through bed:
Dp/L = 5 So2(1-e)2mU/e3 + 0.29 So(1-e)rU2/e3
viscous loss
turbulent loss
- earlier work using this equation given good
agreement with experimental data for pressure drop.
• Voidage distribution
- Mueller model good for uniform spherical particles
- uniform voidage gives better comparison with
measured breakthrough times for granular carbon
Adsorption rate
• Two scalar equations solved
- one for transport of contaminant vapour
- one for rate of ‘uptake’ of adsorbed phase
• A linear driving force approach is used for the
adsorption rate, whereby this is proportional to the
amount of remaining capacity
-C/t = 1/e So km (C - Ci)
• Equilibrium uptake determined by adsorption
isotherm = f(C,T)
Adsorption isotherm
• Pentane adsorption isotherm on BPL carbon at
295K
0.35
Uptake (g/g)
0.30
0.25
0.20
0.15
X – experimental data
__ - Dual Dubinin-Astakhov equation
0.10
0.05
0.00
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
p/p0
Validation
Breakthrough of pentane (3lpm flow, 295K, various bed depths)
Validation
Breakthrough of pentane (3lpm flow, 1cm bed depth, 295K)
Saturation of filter bed
Variable inlet concentration
Outflow concentration from pulsed inflow
With no filter
0.5cm filter – experimental
1cm filter – experimental
0.5 filter – CFD
1cm filter - CFD
Adsorption in wet air
-C/t = 1/e So km (C - Ci)
but Ci for pentane limited so that
uptake </= total pore volume - water uptake
Pentane concentration at outlet
4500
4000
Water on Carbon Adsorption Isotherm
3000
2500
0.5
2000
0.45
1500
0.4
1000
0.35
500
0
0.00
20.00
40.00
60.00
80.00
100.00
120.00
Time (mins)
dry air
inlet RH 80%
bed + inlet RH 80%
140.00
Uptake (g/g)
Concentration (mg/m3)
3500
0.3
0.25
DDA
0.2
0.15
0.1
0.05
0
0
0.1
0.2
0.3
0.4
0.5
p/p0
0.6
0.7
0.8
0.9
1
Implementation in PHOENICS
• Pre-processor
• User interface allows rapid input of geometry and
property data
• Writes Q1 file and runs FEMGEN to create mesh
• Run steady-state to establish flowfield then
transient to model adsorption
• Run full transient if inlet flowrate varies with
time
Implementation in PHOENICS
• Pre-processor
• User interface allows rapid input of geometry and
property data
• Writes Q1 file and runs FEMGEN to create mesh
• Run steady-state to establish flowfield then
transient to model adsorption
• Run full transient if inlet flowrate varies with
time
Implementation in PHOENICS
• Pre-processor
• User interface allows rapid input of geometry and
property data
• Writes Q1 file and runs FEMGEN to create mesh
• Run steady-state to establish flowfield then
transient to model adsorption
• Run full transient if inlet flowrate varies with
time
Implementation in PHOENICS
• Customised GROUND Coding
• Pressure drop and adsorption source terms
• Outlet contaminant concentration can be
monitored as run progresses
• Modelling issues
• Cell blockages
Monolith filter model
• Activated carbon
monolith
• Low pressure drop
• Single channel model
• detailed model of one
flow path
• contaminant diffuses
into porous monolith
• can model several
monoliths in series
Monolith – vapour concentration
outlet vapour concentration vs time
Hexane breakthrough
6000
5000
Concentration (mg/m3)
vapour concentration after 6 mins
4000
3000
2000
1000
0
0
20
40
60
Time (min)
80
100
120
Future development of model
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Multiple adsorbents
Non-linear driving force for adsorption
Property database/GUI
Heat of adsorption source terms
Improved solver speed
- optimisation of GROUND coding
- parallel processing
Conclusions
• Requirement for CFD modelling of filters
• CFD model of adsorption process
developed
• Validation of packed bed model
promising
• Monolith model requires validation
• Customised user interface
• Ongoing developments