Adsorption (Porous Catalyst)
The adsorption process can be regarded as being under equilibrium if the adsorption rate is very fast and mass transfer resistance is negligible. In such case, the adsorption depends only on the amount of adsorbent, concentration (or pressure) of adsorbate and temperature. This kind of isotherm adsorption process is implemented in the Adsorption sub-feature of the Porous Medium and Unsaturated Porous Medium features.
In general, the adsorption process deviates to some extent from its equilibrium state. This is dealt with in the adsorption kinetics. With the assumption that the species adsorption processes are independent of each other, and using the Linear Driving Force (LDF) model to represent the adsorbed species-solid (catalyst) mass transfer rate, the overall adsorption rate (SI unit: mol/(m3·s)) is:
(3-121),
where
.
Here, ci is the concentration of bulk species i; cequ, i (SI unit: mol/m3) is the average equilibrium adsorption concentration of bulk species i; hi is the lumped mass transfer coefficient, which can be estimated from empirical equations based on the mass transfer mechanism; ρ (SI unit: kg/m3) is the adsorbate density; and cP,e,i (SI unit: mol/kg) is the equilibrium concentration of the adsorbed species, derived from the adsorption isotherm. The adsorption rate for bulk species i defined by Equation 3-121 is thus equivalent to
(3-122).
For an adsorbed species the corresponding adsorption rate (SI unit: mol/(m2·s)) is expressed as
.
Domain Selection
The domain selection inherits all the domain selections from its parent feature. Several adsorption features can be added to the same domain.
adsorption
Adsorption parameters can be set only for the species that are selected under Adsorbed Species section in the Porous Catalyst feature.
Select the Adsorption isotherm for individual species check box to set different adsorption isotherm models for each adsorbed species. The equations for the different models are shown in the Equation section. Select the adsorption isotherm from the Adsorption isotherm drop-down list. Choose among Langmuir, Freundlich, Toth, BET, or User defined.
With Adsorption isotherm for individual species unchecked, all species use the same adsorption isotherm model. In this case, Adsorption isotherm is available directly below the Adsorption isotherm for individual species check box.
Use the Species check-box to choose the species that are adsorbed. For each selected adsorbate, there are three parameters to fill in; Adsorption isotherm (available when check-box Adsorption isotherm for individual species is checked), adsorption model specific parameters and Mass transfer coefficient.
Select an adsorption isotherm for each adsorbate (available with Adsorption isotherm for individual species being checked).
The adsorption model specific parameters for species i are:
•
Langmuir:
1
Langmuir constant kL,i (SI unit: m3/mol)
2
Adsorption maximum cp,max,i (SI unit: mol/kg).
•
Freundlich:
1
Freundlich constant kF,i (SI unit: mol/kg)
2
Freundlich exponent NF,i (dimensionless)
3
Reference concentration cref,i (SI unit: mol/m3).
•
Toth:
1
Adsorption maximum cp,max,i (SI unit: mol/kg)
2
Toth constant bT,i (SI unit: m3/mol)
3
Toth exponent NT,i (dimensionless).
•
BET (Brunauer-Emmett-Teller):
1
Monolayer adsorption capacity c0,i (SI unit: mol/kg)
2
Saturation concentration cS,i (SI unit: mol/m3)
3
BET constant KB,i (dimensionless).
•
User defined:
1
Adsorption isotherm cP,i (SI unit: mol/kg):
The last parameter to enter for each adsorbate is the Mass transfer coefficient hi (SI unit: m/s).