2024 AIChE Annual Meeting

(203a) Thermodynamic Behavior and Activity Coefficient Modelling of Low Pressure Alkane + Alcohol Systems

Authors

Schwarz, C. E. - Presenter, Stellenbosch University
Slabbert, R. M., Stellenbosch University
Buitendach, N., Stellenbosch University
Burger, A., Stellenbosch University
The design of a thermally-driven chemical separation process requires a thorough understanding of the phase behavior of the relevant chemical mixtures. Such understanding not only includes the behavior of one specific system under consideration, but also the phase behavior trends of related chemical series (or ensembles). To model the system as accurately as possible, the most appropriate thermodynamic models need to be selected, requiring a well-informed opinion and understanding of the ability, shortcomings and limitations of common models to correlate practical data. Against this background, this study concerns itself with the modelling of n-alkane + alcohol systems. These systems show significant non-ideality and are often encountered in the petrochemical industry.

The aim of the study was to evaluate the phase behavior of selected n-alkane + alcohol systems, while also comparing experimental data (some newly measured) with corresponding predictions by two prominent activity coefficient models. Mixtures included n-hexane, n-heptane, or n-octane (as alkanes) and ethanol, 1‑propanol, or various butanol isomers (as alcohols). The following objectives had to be fulfilled in pursuit of the aim:

  1. Compare the experimental data to show the effect of molecular mass of the n-alkane and alcohol as well as the effect of branching and the position thereof of the alcohol.
  2. Using the correlative NRTL activity coefficient model [1] as well as the predictive UNIFAC model [2], determine the ability of two prominent thermodynamic models – freely available in process simulators such as Aspen Plus® – to correlate/predict the phase behavior of these systems and to determine if any trends in their correlation/prediction can be observed.

Binary vapor-liquid equilibria data – at a constant pressure of 101 kPa – were collected for (n‑hexane, n-heptane or n-nonane) with (ethanol, 1-propanol or 1-butanol) [3–6], as well as for (n-hexane, n-heptane or n-nonane) with (2‑methyl-1-butanol, 2-methyl-2-butanol, 3-methyl-1-butanol or 3-methyl-2-butanol) [7]. The data were either obtained from reliable sources, or measured in our laboratory [3,7]. As expected, the phase behavior showed significant non-ideality, with many of the systems displaying a positive (temperature minimum, pressure maximum) azeotrope, most probably due to association between the alcohols.

For all the systems considered, the movement of the azeotrope and the size of the two-phase region can qualitatively be related to the difference in the boiling points. The closer the boiling points of the n-alkane and the alcohol are to one another, the smaller the phase envelope and the closer the azeotrope is to the equimolar composition.

An increase in either the n-alkane or the 1-alcohol molecular mass leads to an increase in the boiling points of the components. Therefore, as the n-alkane molecular mass increases, the azeotrope moves from the high n-alkane to the low n-alkane compositions if the alcohol remains the same. Conversely, as the 1-alcohol molecular mass increases, the azeotrope moves from the high 1-alcohol to the low 1-alcohol compositions if the n-alkane remains the same. The movement of the azeotrope is expected and is in agreement with what was seen for higher n‑alkanes + 1‑alcohols [8].

Considering the effect of branching, the same trends are observed; the smaller the difference in boiling point, the closer the azeotropic composition is to equimolar composition and the smaller the phase envelope. The primary alcohols have a higher boiling point than the secondary alcohols and the closer the methyl branch is to the hydroxyl group, the lower the boiling point – following the fact that the molecule is less polar due to the shielding effect of the methyl group. These boiling point observations explain the trends observed for the different methyl branched butanols’ phase behavior with alkanes.

The correlative NRTL model [1] as well as the predictive group contribution UNIFAC model [2] are two commonly used thermodynamic models for low pressure phase behavior where significant non-ideality is present. These models, as implemented in Aspen Plus® were used to correlate/predict the phase behavior of the aforementioned systems.

The thermodynamic modeling was conducted in Aspen Plus® V14 and, where applicable, the built-in data regression system of Aspen Plus® was used to correlate parameters. For both models, the pure component vapor pressures were determined using the extended Antoine parameters with the default values present in Aspen Plus® V14.Further, the miscibility behavior of the predicted/correlated phase behavior of the systems was checked.

The results show that the NRTL model is able to correlate the data well to accuracies within the experimental accuracy of the data. For larger boiling point systems, the UNIFAC model performs surprisingly well considering its predictive nature. However, for systems with closer boiling points, the model shows increasing deviation from the experimental data.

Overarchingly, the phase behavior of n-alkanes + alcohols show positive (temperature minimum, pressure maximum) azeotropes. The location of the azeotropes and the size of the phase envelope can be explained by considering the difference in the normal boiling points of the components. Both the NRTL and the UNIFAC models perform well when correlating/predicting the data, with decreasing accuracy of UNIFAC as the normal boiling points of the components increase.

In the future, a more comprehensive study on the thermodynamic modelling of these systems is needed, including extending the thermodynamic modelling to state of the art models, such as the SAFT-Mie family.

References

[1] H. Renon, J.M. Prausnitz, Local compositions in thermodynamic excess functions for liquid mixtures, AIChE J. 14 (1968) 135–144.

[2] A. Fredenslund, R.L. Jones, J.M. Prausnitz, Group-contribution estimation of activity coefficients in nonideal liquid mixtures, AIChE J. 21 (1975) 1086–1099.

[3] N. Buitendach, Measurement, modelling and uncertainty propagation of low-pressure phase equilibrium data for 1-alcohols and n-alkanes, Masters Thesis in Chemical Engineering, Stellenbosch University, 2024.

[4] J. Ortega, F. Espiau, A New Correlation Method for Vapor−Liquid Equilibria and Excess Enthalpies for Nonideal Solutions Using a Genetic Algorithm. Application to Ethanol + an n-Alkane Mixtures, Ind. Eng. Chem. Res. 42 (2003) 4978–4992. https://doi.org/10.1021/ie030327j.

[5] T. Hiaki, K. Takahashi, T. Tsuji, M. Hongo, K. Kojima, Vapor-Liquid Equilibria of 1-Propanol or 2-Propanol with Octane at 101.3 kPa, J. Chem. Eng. Data 40 (1995) 274–276. https://doi.org/10.1021/je00017a060.

[6] T. Hiaki, A. Taniguchi, T. Tsuji, M. Hongo, K. Kojima, Isobaric Vapor−Liquid Equilibria of Octane + 1-Butanol, +2-Butanol, and +2-Methyl-2-propanol at 101.3 kPa, J. Chem. Eng. Data 41 (1996) 1087–1090. https://doi.org/10.1021/je960112z.

[7] R.M. Slabbert, Evaluation of structural (s)-SAFT-γ Mie using newly measured binary VLE data of alkanes mixed with branched alcohols, Masters Thesis in Chemical Engineering, Stellenbosch University, 2024.

[8] S.H. du Plessis, S.A.M. Smith, C. Latsky-Galloway, C.E. Schwarz, Investigation of the Low-Pressure Phase Behavior and SAFT Modeling of 1-Alcohol and n-Alkane Binary Systems, J. Chem. Eng. Data 69 (2024) 623–638. https://doi.org/10.1021/acs.jced.3c00346.