I have a fixed bed extraction system, where the solvent is moving from top to bottom on a fixed bed. However, the mass transfer stops after some time, even though, the solvent is not reached to maximum solubility
Dear Mr. Ligade,
from your descrition it is not fully clear what the problem exactly is. To my understanding there are mainly two different possibilities:
a) The mass transfer is limited by mass transfer in the liquid
b) the mass transfer is limited in the solid
You can check if a) applied by removing the solute loaded liquid and replacing it by fresh liquid. If the mass transfer happens to be fast again (at least initially), case a) applies. In that case you can increase mass transfer by increasing flow rate of the liquid (leads to elevated mass transfer coefficients), using more liquid (keeps the difference in chemical potential of the solute in the solid and the liquid at a higher level -> increased driving force) or exchange the liquid completely from time to time.
It is also possible that the mass transfer of the solute in the solid is rate limiting at later times (e. g. initally solute close the solid surface is extracted but later the solute from the bulk of the solid phase takes very long to reach the liquid. In that case you could decrease particle size of the solid.
In both cases increasing the process temperature should help as well, because it accelerates all mass transfer processes ...
Try to allow the solvent to flow from the bottom up instead of the top down. This will increase the residence time between the solvent and the fixed bed and will improve the mass transfer.
Hope this helps answer your question.
Professor Yehia Khalil, Yale University, USA
Fellow of the University of Oxford, UK
Also using flow from bottom to top usually ensures complete wetting of the solid substrate with the solvent. In top to bottom percolation this is not certain unless the extraction vessel is saturated and their is some constructive method to keep it that way during operation.
Best regards.
Also see below paper:
Effect of gas sparging on the rate of mass transfer at fixed beds of horizontally stacked screens
Canadian J. Chem. Eng.
Volume 66, Issue 3
June 1988
Pages 497–500
You can increase the contact time by restricting the out flow enough to flood the packed bed full of liquid.
Equilibrium may limit the concentrations in solid and liquid. Usually it is a Log Log relationship, that means the solid doesn't give up the last bit of adsorbed material unless the solvent has none of the desorbed material.
You can refer to isotherms of your system to explain the results.
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