(246b) Molecular Interactions in Mixtures of High Molecular Mass Alkanes and Alcohols in the Presence and Absence of Supercritical Carbon Dioxide

1. Introduction

High molecular mass alcohols (typically C₁₀ and higher) are produced through the oxygenation of alkanes. As the oxygenation process does not run to completion, the production process of these alcohols results in a mixture of alkanes and alcohols. As the alkane feedstock is usually a range of molecular masses, so too is the alcohol product. As a result, the alkane-alcohol product mixture usually consists of a molecular mass range of both alkanes and alcohols. While the alcohols have a higher boiling point (lower vapour pressure) than the alkanes, cross-over boiling points occur between the higher alkanes and lower alcohols. Further, evidence exists for significant molecular interactions between the alkanes and alcohols resulting in positive deviations from Raoult’s law and in some cases azeotropic behaviour. As a result, separation using traditional distillation is not a viable option and advanced separation processes are required.

One of the possible separation techniques that has shown viability is supercritical CO₂ fractionation [1,2]. To develop and fully understand these technologies detailed phase behaviour of the n-alkanes + 1-alcohols is required both in the absence as well as in the presence of supercritical CO₂. A number of studies have been conducted on the phase behaviour of these systems but to date no systematic study has been presented investigating the complex molecular interactions present.

The aim of this study is to present a systematic investigation of three alkane + alcohol systems where the alkane has two more carbon atoms than the alcohol. The three systems (n-decane + 1-octanol), (n-dodecane + 1-decanol) and (n-tetradecane + 1-dodecanol) will be considered. These systems were considered due to some data already being available in the literature, the general availability of these compounds in pure form allowing easier experimental measurement and the close boiling point temperatures of the alkane and alcohol. Both low pressure vapour-liquid equilibria data in the absence of CO₂ as well as high pressure vapour-liquid equilibria and phase boundary measurements in the presence of CO₂ will be considered. The study will consider the molecular interactions present in the data and compare the molecular interactions in the presence and the absence of CO₂. Further, progression of the molecular interactions with molecular mass will also be considered.

2. Low pressure vapour liquid equilibria

To date only limited low pressure vapour-liquid equilibrium data are available in the literature. As a result a collection of mixtures of n-alkanes + 1-alcohols were recently measured [3]. Data were measured on a modified Gillespie-type still with thermodynamic consistency and verification showing the data to be reliable. Amongst others data were measured for the n-decane + 1-octanol system and n-dodecane + 1-decanol system at 40 kPa. Unfortunately data could not be measured for the n-tetradecane + 1-dodecanol system due to the components having too high boiling points.

The results show that both these systems exhibit positive deviations from ideality with temperature minimum azeotropes at 414.0 K and 0.072 mole fraction 1-octanol for the n-decane + 1-decanol system and 453.8 K and 0.082 mole fraction 1-decanol for the n-dodecane + 1-decanol system. As the molecular masses of the alkanes and alcohols increase the difference in the pure component boiling points decrease and the azeotrope move closer towards an equimolar composition. The positive deviation from Raoult’s law suggest that association is present in the mixture. It can therefore be postulated that the alcohols forms dimers, trimers and multimers and therefore cause significant deviation from ideal behaviour.

3. High pressure phase behaviour

A significant number of studies have been conducted for selected CO₂ + n-alkane + 1-alcohol systems. The majority of these studies have measured the phase boundary or solubility limit resulting in bubble and dew point data. While this data does not provide information on the composition of the co-existing phases, it does provide useful information from which molecular interactions can be inferred. Limited vapour-liquid equilibrium studies have also been conducted where the compositions of the co-existing phases are measured.

High pressure bubble and dew point data have recently been measured for the CO₂ + n-decane + 1-octanol system [4]. Previous measurements of CO₂ + n-dodecane + 1-decanol have also been conducted considering both the high-pressure bubble and dew point measurements [5,6] as well as high pressure phase behaviour measurements [7]. To complement the above data, high pressure bubble and dewpoint measurements were also conducted for the CO₂ + n-tetradecane + 1-dodecanol system. The bubble and dew point measurements were conducted on a previously constructed high-pressure variable volume view cell [8] and the phase equilibria measurements on an analytical variable volume view cell [9,10]. In both cases the setups have repeatedly been shown to produce reliable data.

The data shows that significant molecular interactions are present between the alkane and the alcohol. In all cases co-solvency with a pressure minimum is present. Similar to the low pressure behaviour, it can be postulated that this pressure minimum is the result of association present in the system where the alcohol molecules form dimers, trimers and multimers. These associated molecules then behave differently to that of the monomer alcohols and thus significantly influence the phase behaviour present in the system.

Unfortunately insufficient data exists to clearly investigate the progression with molecular mass. However, indications are that as the molecular mass of the compounds increases, the molecular interactions or their effect decreases. This may be due to the diluted effect of the hydroxyl group that is involved in the molecular association present.

4. Outcome

This study has shown that significant molecular interactions are present in systems involving high molecular mass alkanes and alcohols. This is true for both systems in the absence as well as the presence of CO₂. In the case studied here regions of co-solvency were observed for high n-alkane fractions in the presence of CO₂. These regions of co-solvency correlate with the azeotropes found in the low-pressure phase equilibrium data. Unfortunately, as measurements were conducted at different temperatures, direct qualitative comparison of experimental data is not possible. However, indications are that similar molecular interactions are present both in the absence as well as the presence of CO₂.

5. Future work

While significant data have been measured and have improved the understanding of the molecular interactions present between n-alkanes and 1-alcohols, significant additional work is still required. Further experimental work is required especially for high pressure phase equilibria measurement. Additionally thermodynamic modelling can aid in the understanding of the molecular interactions. Predictive models can be considered to predict the n-tetradecane + 1-dodecanol systems based on the results for lower molecular mass homologs for this series. These predictive models could also be used to extrapolate the data to lower temperatures to allow for comparison with the high-pressure phase behaviour data. Further, various thermodynamic models can be considered to correlate the measured data (both high and low pressure) and provide improved clarity for the co-solvency and azeotropes present with possible linkage between them.

6. References

[1] G.J.K. Bonthuys, C.E. Schwarz, A.J. Burger, J.H. Knoetze, J. Supercrit. Fluids. 57 (2011) 101–111.

[2] C.E. Schwarz, G.J.K. Bonthuys, R.F. van Schalkwyk, D.L. Laubscher, A.J. Burger, J.H. Knoetze, J. Supercrit. Fluids. 58 (2011) 352–359.

[3] S.H. du Plessis, Masters Thesis in Chemical Engineering, Stellenbosch University, 2022.

[4] C.F.O. Momoh, Masters Thesis in Chemical Engineering, Stellenbosch University, 2022.

[5] S.A.M. Smith, C.E. Schwarz, Fluid Phase Equilibria. 406 (2015) 1–9.

[6] M. Zamudio, C.E. Schwarz, J.H. Knoetze, J. Supercrit. Fluids. 84 (2013) 132–145.

[7] F.C. van N. Fourie, C.E. Schwarz, J.H. Knoetze, J. Supercrit. Fluids. 151 (2019) 49–62.

[8] C.E. Schwarz, I. Nieuwoudt, J. Supercrit. Fluids. 27 (2003) 133–144.

[9] F.C. v. N. Fourie, C.E. Schwarz, J.H. Knoetze, Chem. Eng. Technol. 38 (2015) 1165–1172.

[10] F.C. v. N. Fourie, C.E. Schwarz, J.H. Knoetze, Chem. Eng. Technol. 39 (2016) 1475–1482.