Abstract
An adsorption isotherm model is deduced based on the Gaussian cumulative frequency distribution principles. This isotherm has three parameters: q max, the 50% adsorbate loading pressure P 50, and the standard deviation σ. The isotherm development hinges on establishing P 50 and σ. These are deduced from a log plot of the isotherm pressure P versus the inverse Gaussian cumulative frequency distribution plot. The slope of this plot is σ and the y-intercept is P 50. The temperature dependency of P 50 is deduced from a plot of log P 50 versus 1/T: this is actually the isosteric plot at 50% loading with slope equal to the heat of adsorption at 50% loading.
The model is tested using the literature methane data for 5A zeolite. Twenty-four isotherms from six different studies are fitted with the model using a q max of 9 g/100 gZ applicable to methane 5A isotherms in the temperature range of the study, as reported in the literature. The standard deviation σ is observed to be 0.98±0.088. The isostere of P 50 indicates a heat of adsorption between 4.62–4.97 kcals/mole consistent with literature values. All 24 isotherms reduce to one characteristic curve when θ is plotted against (P/P 50)1/σ. The θ values approaching zero (<0.05) are shown to be consistent with reported Henry constant data using the heat of adsorption value calculated at 50% loading as is expected for homogeneous adsorption.
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Abbreviations
- a :
-
Constant in (10) and (16)
- k :
-
Boltzmann’s constant
- K :
-
Langmuir equilibrium parameter, kPa−1
- K H :
-
Henry’s constant, g/100 gZ/kPa
- P :
-
Pressure, kPa
- P C :
-
Critical pressure, kPa
- P LM :
-
Log mean pressure for the entire isotherm, kPa
- P 50 :
-
Pressure at 50% loading, same value as P LM, kPa
- P r :
-
Reduced pressure, dimensionless
- P r,50 :
-
Reduced pressure at 50% loading, dimensionless
- R :
-
Gas constant
- q :
-
Zeolite loading, g/100 gZ
- q max :
-
Maximum zeolite loading, g/100 gZ
- T :
-
Temperature, K
- T CAR :
-
Critical adsorbate reduced temperature
- T r :
-
Reduced temperature
- Z :
-
Standard normal probability distribution variable
- u :
-
Multisite Langmuir parameter
- θ :
-
Loading (= q/q max)
- σ :
-
Standard deviation for the isotherm
- −ΔH 0 :
-
Heat of adsorption at 0% loading, kcals/mole
- −ΔH 50 :
-
Heat of adsorption at 50% loading, kcals/mole
- Φ:
-
Cumulative distribution function.
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Abouelnasr, D., Loughlin, K.F. A new Gaussian adsorption isotherm model: test data is methane-5A zeolite. Adsorption 16, 93–101 (2010). https://doi.org/10.1007/s10450-010-9217-2
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DOI: https://doi.org/10.1007/s10450-010-9217-2