Issue in Dubinin calculations about pygaps HOT 7 CLOSED

I have included a rough fix that works for my data (see above commit on my fork). the loadings read in are already in cm3/g stp, so I don't know if this will work on other units/bases. If you look over this and think it makes sense, then let me know and I'll formally submit a pull request etc.

If not, and I just don't understand how this code is meant to work then ignore me!

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Interesting difference. A couple of questions to clarify

• What are the definitions of limiting micropore capacity and limiting micropore volume? Is this total uptake in cm3[gas at stp]/g and cm3[liquid in pores]/g?
• Where is the magic number coming from? Does this essentially convert from the two? If so, can it be confirmed through CoolProp/RefProp?

If the above are correct, the changes made should not have any impact on the final number, so there may be something else at play...

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PS: I don't remember why I did the calcs in base 10, it just makes it more complicated.

from pygaps.

Your definitions are correct.
And yes, this magic number is just the ratio between gas density/liquid density. Unfortunately, CoolProp doesn't have density of nitrogen at 298 K. On running;

adsorbate.gas_density(298)

The following error is produced;

Thermodynamic backend failed with error Temperature to QT_flash [298 K] must be in range [63.051 K, 126.192 K]. Attempting to read parameters dictionary...

The aif file that the isotherm is imported from has relative pressure units and loading units of 'cm3/g stp' - don't know if that reveals anything?

from pygaps.

CoolProp won't work on supercritical gases, true. If it's STP, density should be in 22.4 cm3/mmol / 28k mmol/g (N2 molar mass).

So definitions are right. Note that the volume_gas loading option will give you the isotherm volume of gas at isotherm temperature, so at 77K. In the changes

microp_volume = microp_capacity * gas_density / liquid_density

In terms of units and concepts

[cm3/g, gas at 77k] * [cm3/g, gas at 303k] / [cm3/g, liquid at 77K]

So it's not apples to apples.

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Grand, so I tested this on a file, and it turns out that I'd just been creating the point isotherm, and then the resultant aif file incorrectly.

Essentially;

file = pd.read_csv('isotherm.csv', names=['pressure', 'loading], header=0)
 isotherm = pg.PointIsotherm(
        pressure=file['pressure'],
        loading=file['loading'],
        material='example',
        adsorbate='N2',
        temperature=77,
        temperature_unit='K',
        pressure_unit='bar',
        pressure_mode='relative',
        loading_basis='volume_gas',
        loading_unit='cm3',
        material_basis='mass',
        material_unit='g',
    )
isotherm.to_aif('isotherm.aif')

So the preamble in the resulting aif file looks like;

...
_pygaps_pressure_mode 'relative'
_pygaps_pressure_unit 'None'
_pygaps_material_basis 'mass'
_pygaps_material_unit 'g'
_pygaps_loading_basis 'volume_gas'
_pygaps_loading_unit 'cm3'
_pygaps_temperature_unit 'K'

Changing to

...
_pygaps_loading_basis 'molar'
_pygaps_loading_unit 'cm3(STP)'
...

Fixes this, and running verbose gives much more reasonable pore volume;

isotherm = pgp.isotherm_from_aif('isotherm.aif')
pgc.dr_da_plots.da_plot(isotherm, p_limits=[0,0.1], verbose=True)

Returns

Exponent is: 3
Micropore volume is: 0.773 cm3/g
Effective adsorption potential is 6.35 kJ/mol

I wonder if it's worth warning when the user attempts to create an isotherm with the specific units and bases I have shown here? To me it was the most intuitive way to do so.

Anywho, closing this issue!

from pygaps.

Good stuff!
If it was up to me, I would remove cm3[stp] completely...

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