All the methods are good, the easier method is the optical method, the tracer must be a colorant and the tracer concentration in the reactor output could be followed with a colorimeter. must be careful with the interference of color.

So the measurement, not the calculation, as in the previous question.

Without specifying the kind of tracer, one can only guess.

Regards,

thank you all for the suggestions 

If your goal is to study the concentration distribution and transport inside the porous medium and you can construct the experimental fixed bed,  a good method is transmitted light imaging technique in conjunction with a fluorescent dye tracer (sodium fluorescein). You can adopt cylindrical linear light tubes  so the light transmitted through the transparent reactor reaches a diffusive regime and use a CCD camera. Pay attention to a previous good calibration of "blanck" images with the reactor in saturated conditions and without fluorescein inside!

If your reactor already exists and is not transparent, the best method is to use a ionic tracer and to measure conductivity (and temperature). In this case you can only obtain information on axial dispersion / advection and not about radial dispersion

thank you, Ombretta Paladino, for valuable informations

You can also use a laser (to determine the velocity field) and to add the momentum equations to to the mass balance equation. Only, that's not the point.

Regards,

thank you Mirosław Grzesik

that is a good idea

This was in reference to the previous answer.

Expansive device and a very complicated mathematical description of results. But of course LDV (LDA) method is used in practice. In case of a non-transparent wall, instead of LDV you can use ultrasounds.

Regards,