COMSOL 6.2 - Adsorption

Adsorption

The adsorption process can be regarded as being under equilibrium if the adsorption rate is very fast and other mass transfer resistance is negligible. In such cases, the adsorption depends only on the amount of the adsorbent, concentration (or pressure) of adsorbate and temperature. This kind of isotherm adsorption process is implemented in Adsorption subfeature of the Porous Medium and Unsaturated Porous Medium features in the Transport of Diluted Species in Porous Media.

In general, the adsorption process deviates, to some extent, from its equilibrium state. This is dealt with in the adsorption kinetics. With the assumption of species adsorption process being independent of each other and the Linear Driving Force (LDF) model being used to represent the adsorbed species-solid (catalyst) mass transfer rate, the overall adsorption rate is

(3-141)

where

here ci is bulk mixture concentration of species i; cequ, i is the equilibrium concentration of the same species when adsorbed (amount per volume); h i is the lumped mass transfer coefficient, which can be estimated from empirical equations based on the mass transfer mechanism; ρ is the dry bulk density of the material; and cP,e, i is the adsorbed equilibrium concentration (amount per solid mass) defined from an isotherm adsorption model.

Domain Selection

The domain selection inherits the all domain selections from its parent feature. Several Adsorption nodes can be added to the same domain.

adsorption

Adsorption parameters can be set only for these species which are selected under Adsorbed Species section in the Porous Catalyst feature.

Select check box to set different isotherm adsorption models for individual species. The equations for different models are shown under Equation section.

With unchecked, all species use the same isotherm model. In this case, the direct down is available, and specify an adsorption isotherm from one of models, , , , , or .

From the list, select these species which are adsorbate. With the species being selected as adsorbate, there are three kinds of parameters, Adsorption isotherm (available when check box is checked), adsorption model parameters and mass transfer coefficient.

Select an adsorption isotherm for each adsorbate (available with being checked).

Enter adsorption model parameters for species i, parameters are:

•

For :

Enter a kL,i (SI unit: m3/mol) and an cp,max,i (SI unit: mol/kg).

•

For :

Enter a kF,i (SI unit: mol/kg), a NF,i (dimensionless), and a cref,i (SI unit: mol/m3).

•

For :

Enter an cp,max,i (SI unit: mol/kg), a bT,i (SI unit: m3/mol), a NT,i (dimensionless).

•

For :

Enter a c0,i (SI unit: mol/kg), a cS,i (SI unit: mol/m3), and a KB,i (dimensionless).

•

For enter an cP,i (SI unit: mol/kg):

The last parameter for adsorbate is the hi (SI unit: m/s).