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 subfeature of the Porous Medium and Unsaturated Porous Medium features.
In general, the adsorption process deviates to some extent from its equilibrium state. With the assumption that the species adsorption processes are independent of each other, the Adsorption node uses the Linear Driving Force (LDF) model to define the adsorbed species-solid (catalyst) mass transfer rate with regard to an adsorption isotherm. The overall adsorption rate (mol/(m3·s)) is expressed as:
(3-1),
Here the adsorption isotherm, cP,e, i (mol/kg), represents the equilibrium value of bulk species i adsorbed per dry mass of adsorbent. cads, i is the local value of the adsorbed species concentration (mol/m2). The available specific surface, and the adsorbent bulk density are denoted by Sreac (1/m) and ρb (kg/m3) respectively. hLFD,i is a lumped mass transfer coefficient, which can be estimated from empirical equations based on the mass transfer mechanism. The mass transfer rate is added as a source or sink term in the equation for the bulk species concentration. The adsorbed species concentration is solved for in the manner of:
(3-2)
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 checkbox 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 list. Choose among Langmuir, Freundlich, Toth, BET, or User defined.
With Adsorption isotherm for individual species cleared, all species use the same adsorption isotherm model. In this case, Adsorption isotherm is available directly below the Adsorption isotherm for individual species checkbox.
Use the Species checkbox to choose the species that are adsorbed. For each selected adsorbate, there are three parameters to fill in; Adsorption isotherm (available when the checkbox Adsorption isotherm for individual species is selected), adsorption model specific parameters and Mass transfer coefficient.
Select an adsorption isotherm for each adsorbate (available with Adsorption isotherm for individual species being selected).
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).