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

• Background to filter model development• Physics of adsorption modelling• Validation• Implementation in PHOENICS• Future developments

Typical filter application

Air Flow

Impregnated granularactivated carbon

Glass FibreFilter

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:

p/L = 5 So2(1-)2U/3 + 0.29 So(1-)U2/3

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/ So km (C - Ci)

• Equilibrium uptake determined by adsorption isotherm = f(C,T)

Adsorption isotherm

• Pentane adsorption isotherm on BPL carbon at 295K

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

p/p0

Up

take

(g

/g)

X – experimental data

__ - Dual Dubinin-Astakhov equation

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

Pentane concentration at outlet

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00

Time (mins)

Co

nc

entr

ati

on

(m

g/m

3)

dry air inlet RH 80% bed + inlet RH 80%

Adsorption in wet air

-C/t = 1/ So km (C - Ci)

but Ci for pentane limited so that

uptake </= total pore volume - water uptake

Water on Carbon Adsorption Isotherm

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

p/p0

Up

take

(g

/g)

DDA

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

Hexane breakthrough

0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100 120

Time (min)

Co

nce

ntr

atio

n (m

g/m

3)

vapour concentration after 6 mins

outlet vapour concentration vs time

Future development of model

• 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