The 8th Industrial Fluid Properties Simulation Challenge

Objectives

The 8th Industrial Fluid Properties Simulation Challenge will focus on predicting adsorption isotherms of n-perfluorohexane (n-C₆F₁₄) in the certified reference material BAM-P109 activated carbon.

Timeline

January 2014 - Challenge problem announced
Friday, September 26th, 2014 - Challenge final entries due
November 2014 – Challenge champions announced at the 2014 Fall AIChE meeting

Background

Activated carbons are among the most widely used industrial materials. Due to their low cost of production and high adsorption capacity, they have found use in applications including: gas and electrolyte storage; gas and fluid purification; and catalysts and catalyst supports. Activated carbons are typically synthesized from organic carbonaceous precursors such as woods, coconut shells and coals. By judicious choice of the precursor materials and synthesis conditions, they can be synthesized with a wide range of features such as pore size distribution, specific surface area and chemical composition

With increasing numbers of applications, the ability to predict the performance of activated carbon adsorbents for a wide range of sorbent compounds would be very valuable in pre-optimizing systems and reducing product development time. Molecular simulation techniques in principle could be ideal to predict the adsorption isotherms of activated carbon sorbents for sorbates of varied chemistry as well as for various chemically-modified activated carbon adsorbents.

Although adsorption in porous media has been an area of extensive and vigorous activity in the field of molecular simulation (e.g. in zeolites [1,2], metal-organic frameworks [3,4], nanotubes [5], and other porous carbons [6,7]), it was not the focus of the IFPSC until the recent 7th Simulation Challenge to predict adsorption of C₆F₁₄ in BCR-704 Faujasite zeolite. The 8th Simulation Challenge continues the theme to assess the capability of molecular simulation methods and force fields to accurately predict adsorption in porous media for practically relevant and moderately complex chemical systems in order to benchmark the state-of-the-art capability in this important application area.

Studies of adsorption equilibria by molecular simulation employing both Monte Carlo [8-10] and molecular dynamics [11,12] techniques have become relatively common. However, applying these methods to study adsorption equilibria using force fields (potential energy models) developed for bulk phase conditions remains an open issue. General, transferable force fields that are reasonably accurate over a wide range of state conditions in the bulk are not necessarily transferable to the adsorbed phase. Moreover, active sites on the surface of the adsorbent can dramatically affect the adsorption behavior.

The focus of the current challenge will be to assess the potential to use molecular simulation methods to predict adsorption isotherms for organic compounds. Specifically, the challenge will focus on predicting the adsorption isotherm of n-perfluorohexane on the certified reference material BAM-P109.

Ar, H₂O, N₂, and CO₂ adsorption isotherm studies on the BAM-P109 activated carbon will be carried out by the leading industry expert company Quantachrome and provided to Challenge entrants to aid in validating simulation models. Elemental analysis, XPS studies, and X-ray diffraction studies will also be carried out and provided to Challenge entrants. The Ar, H₂O, N₂, and CO₂ adsorption studies, elemental analysis, XPS and X-ray diffraction studies will be posted on the IFPSC web-site no-later then end of January 2014.

The experimental benchmark adsorption studies for n-perfluorohexane (n-C₆F₁₄) on the BAM-P109 activated carbon will also be carried out by Quantachrome. n-perfluorohexane (n-C₆F₁₄) is a high performance material produced by 3M Company.

Challenge

Predict the adsorption isotherm of n-perfluorohexane (n-C6F14) on the Certified Reference Material BAM-P109 Carbon at a temperature of 273K and at relative pressures of 0.1, 0.3, and 0.6. The relative pressure is defined as that relative to the bulk saturation pressure predicted by the model for the given temperature. At these relative pressures, the micropores of BAM-P109 are expected to be filled with adsorbate, and an accurate simulation model for adsorption in the carbon micropores will be necessary for successful predictions. Evaluating this capability is the primary goal for the 8th Simulation Challenge.

The lowest relative pressure was chosen because it represents the approximate lowest pressure accessible for reliable, reproducible adsorption data for the experimental method and apparatus that is being used to measure the benchmark data. Additional adsorption over the range of 0.1 to 0.6 relative pressure (i.e. in the pseudo-plateau region of the adsorption isotherm) is expected to occur in the mesoporous regions of BAM-P109. Molecular simulation methods used to model adsorption in the micropores may not be directly applicable to predict adsorption in the mesopore regions. However, challenge entrants are encouraged to develop additional approaches to account for mesoporous adsorption (for example, via correlations to the provided experimental benchmark adsorption data) as a secondary aspect of this challenge.

Rules of the Game

Any theory/modeling/simulation method can be used. However, only molecular simulation method-based submissions will be scored and judged for the adsorption at 0.1 relative pressure which is due in large part to adsorption in the microporous region. Submissions employing correlative or alternative methods to predict the adsorption at 0.1 relative pressure are also encouraged, however, but only to demonstrate the capabilities of these approaches. Presentation and publication of papers employing alternative approaches will be considered for acceptance in the IFPSC Challenge Special Session at the Fall 2014 National AIChE meeting and in the subsequent journal special proceedings.
Correlative or alternative methods to predict the adsorption at 0.3 and 0.6 relative pressure which is primarily due to adsorption in the mesoporous region are encouraged and will be scored and judged.
Any force field (or other model parameterization) previously published in the open literature prior to the announcement of this challenge is acceptable.
Force fields (or other models) may be parameterized using any other published physical property data but not via new or previously unpublished experimental studies including n-perfluorohexane adsorption isotherms in activated carbon.
The experimental bulk saturation pressure for n-C₆F₁₄ at 273K used to calculate relative pressures was 0.0855 bar [13]. This data can be used to aid in developing and validating the theory/modeling/simulation method. The relative pressures for reporting challenge results, however, must be based on the bulk saturation pressure predicted by the model.
The bulk saturation pressure predicted by the model must be reported.
Estimates of the uncertainty must be included.

Challenge Scoring

Challenge champions along with 1st, 2nd, and 3rd runners-up will be awarded. Entries will be scored by comparing the predicted adsorption isotherm data to experimentally measured data. For adsorption isotherms, the amount of perfluorohexane experimentally adsorbed at the defined set of pressures will be compared to the amounts predicted at the same pressures. Full credit will be awarded for a prediction that falls within the experimental uncertainty.

The units for comparing between experiment and simulation will be volume [cm³ g^-1] at STP. The scoring will be computed as a mean percent error, M%E, between the computed value and the experimental value at the specified relative pressures. Where the M%E will be computed for n points using

where V_ab,exp and V_ab,sim are the experimental and simulated volumes of absorbed gas, respectively. V_ab,exp is the experimental volume of adsorbed gas ± the experimental uncertainty.

The M%E will be weighted such that the prediction of adsorption at 0.1 relative pressure will account for 60% of the score, while the predictions for adsorption at 0.3 and 0.6 relative pressures will account for 20% of the score each. The weighting reflects the adsorption filling of the microporous region below 0.1 relative pressure followed by further adsorption in the mesoporous region above 0.1 relative pressure.

Other Entry Guidelines

Entries are to be submitted to contest@ifpsc.org on or before the deadline
A submission for this challenge problem is to be in the form of a manuscript suitable for submission to a refereed, archival, scientific journal. The manuscript must contain sufficient detail about the simulation or other method and about the force field (if simulation) so that an experienced simulator could reproduce the results without requiring access to proprietary information. In particular, all potential parameters and molecule geometry parameters must be explicitly specified in the manuscript. The results are to be reported in SI units.
Adsorption isotherm results are to be reported in units of [cm³ g^-1].
An analysis of the uncertainty in the calculated results is required and must be included in the manuscript.
Entries are expected to present results that are statistically significant and to present sufficient supporting evidence to establish this quality. Also, the scientific reasoning behind any new (unpublished) force field parameterizations must be explicitly and precisely expressed in the entry. If there is a consensus among the judges that an entry is of poor quality (uses a method commonly accepted to be fundamentally flawed, presents results that are not statistically significant, fails to provide sufficient supporting data and details, violates the various rules and guidelines established for the competition, or for any other reason that the work would unlikely be accepted in any peer-reviewed scientific journal in the field), that entry will be rejected and will not be considered in the judging.
Entries that represent collaborations between multiple research groups are welcomed.

References

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[2] B. Smit and T. L. M. Maesen, Chem. Rev., 108, 4125 (2008).

[3] S. Keskin, J. Liu, R. B. Rankin, J. K. Johnson, and D. S. Sholl, Ind. Eng. Chem. Res., 48, 2355 (2008).

[4] T. Düren, Y.-S. Bae, and R. Q. Snurr, Chem. Soc. Rev., 38, 1237 (2009).

[5] W. Shi and J. K. Johnson, Phys. Rev. Lett., 91, 015504 (2003).

[6] G. M. Davies and N. A. Seaton, AIChE J., 46, 1753 (2000).

[7] M. B. Sweatman and N. Quirke, Mol. Simul., 31, 667 (2005).

[8] G.E. Norman and V.S. Filinov, High Temp. (USSR), 7, 216 (1969).

[9] S.C. McGrother and K.E. Gubbins, Mol. Phys., 97, 955 (1999).

[10] D. Frenkel and B. Smit, Understanding Molecular Simulation, Academic Press, San Diego (2002).

[11] M. Lupkowski and F. van Swol, J. Chem. Phys., 95, 1995 (1991).