Hi,
I have got some DSC data related to the heat flow in mW (milliWatt I presume?) measured during adsorption/desorption of CO2 onto/from an activated carbon.
I'm getting some inconsistent results so I'd like to make sure I'm not using a wrong calculation (see following steps):
1) Integrate the heat flow curve with respect to the time
I'm using a straight baseline (default option). Is that correct or I should use a different type of baseline?
However, this gives me a value in mJ (milliJoule, mW*s)
2) Convert the value from mJ to kJ (dividing by 10^6)
3) Divide value in kJ by the mass expressed as moles of CO2 adsorbed/desorbed
This finally gives kJ/mol.
Could you please give any feedback about that?
Thanks,
Antonio
1. It depends on the shape of thermogram, if end point is same as first point than default is good choice. Base line corrections are required in most cases by doing blank runs.
2. Conversion and calculation seems ok.
Antonio,
your approach is correct provided the adsorption/ desorption amount of CO2 is negligible with respect to the ab-.initio mass of sample in the DSC pan.
In fact, the DSC output in terms of mW refers to the heat flux needed to equilibrate the original sample. The DSC doesn't know that the mass is changing during the adsorption/desorption process.
In case you want account for this aspect you should perform the same experiment with a TGA (thermo gravimetric analyzer) that allows you to mesure the kinetics of desorption/adsorption (under the same conditions). Then, the heat flux can be modified accounting for the actual mass of the sample in the pan.
amount of CO2 adsorbed/desorbed is within 4% of the initial material weight...
Antonio,
please, account also that during the process the thermal conductivity of the sample may also change. This and the previous answer had the scope to point out that the physics underlying the process you are looking at is rather involved. Thus taking the integral over time of the heat flux includes the approximations described so far.
Hi Antonio,
Mr D'Amore is right, due to in ab processes you can not separate de heat and mass transfer, you have to use a model that relates the heat flux according to mass transfer.
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