Fixed bed adsorption using pore diffusion model
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Hello!
As the title suggests. Almost all of my code is from this thread, specifically given by one Alan Stevens (Freundlich4.m): fixed bed adsorption column model-solving PDE-freundlish isotherm - MATLAB Answers - MATLAB Central (mathworks.com)
I am trying to use the pore diffusion model which is given by:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1450427/image.png)
where q is the amount adsorbed, Ds is the diffusion coefficient of adsorbate in the solid phase, r is the particle radius coordinate. This model is usually applied on the an adsorbent particle.
The initial condition is:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1450432/image.png)
and the boundary conditions are:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1450437/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1450442/image.png)
where, rp is the particle radius, kf is the external mass transfer coefficient, rho_p is the particle density, Ct is the concentration in the bulk fluid at time t, same as "C" in the code in the forum linked above and Cs is the concentration of adsorbate on the surface of the particle.
The dqdt is replaces the dqdt in the following equation:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1450447/image.png)
and has the same boundary and initial conditions as given in the link.
I modified the code but I am confused. Equation 1 and 2 have to be solved simultaneosuly, but this introduces an additional spatial parameter r. So one of the ways this maybe possible is by averaging the values of q at all r at a particular time instant to calculate dqdt and then replacing it in the equations 2 to calculate dCdt. However, this requires equation 1 to be fully solved before starting the calculations for dCdt.
I need help implementing this code. I could really use some.
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Torsten
le 4 Août 2023
Modifié(e) : Torsten
le 4 Août 2023
The "dq/dt" in equation 2 is the change of mass of q in the particle with respect to time. If you set
q_in_particle = integral_{r=0}^{r=r_p} 4*pi*r^2*q dr
and differentiate q_in_particle with respect to time, you arrive at
d(q_in_particle)/dt = 4* pi*r_p^2*D*(dq/dr)_{r=r_p}.
and (dq/dr)_{r=r_p} comes from the second boundary condition in your PDE for q.
18 commentaires
Torsten
le 13 Août 2023
Modifié(e) : Torsten
le 13 Août 2023
As far as I remember, Cs is the equilibrium concentration corresponding to the concentration in the bulk fluid. You must supply Cs as a function of C and maybe of temperature. So I don't know how Csdt comes into play: C (bulk concentration of the adsorbens in the gas phase) and q (adsorbat concentration in the particle) are the variables to be solved for.
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