(555g) Phase Equilibria in Mixtures Involved in the Synthesis of Isopropyl Myristate Via Esterification of Myristic Acid with Isopropanol

Isopropyl myristate (IPM) is a natural-based fatty acid ester used as emollient in cosmetic applications [1] and as drug delivery solvent in pharmaceuticals for dermal use [2]. This ester is generally produced via esterification of myristic acid (MA) with isopropanol (IPA) to obtain isopropyl myristate (IPM) and water (W) as products. Usually, the process is carried out in a batch or continuous reactor with subsequent separation units that are required to achieve commercial product specifications [3]. The conversion of the esterification reaction is limited by the equilibrium, and it can be shifted by the addition of IPA excess or by removing produced water. Taking this into account, and because of the high relative volatility between the isopropyl myristate and the isopropanol – water mixture, reactive distillation is a potential method to produce isopropyl myristate with a high yield. Additionally, this intensified process would reduce capital and operating costs related to the separation processes.

To enable a correct design and optimization of a reactive distillation column for this system, it is crucial to accurately describe the phase equilibrium behavior of the components of the reactive mixture. Even though the isopropanol – water vapor-liquid equilibrium [4] and some properties of the fatty components (e.g. vapor pressure, density, viscosity) [5, 6] are available in the literature, there are not experimental data for the vapor – liquid and liquid – liquid equilibrium involving mixtures with myristic acid or isopropyl myristate. In previous works, phase equilibria of the reactive mixture have been predicted using group contribution methods (e.g. UNIFAC, UNIFAC-DMD, NRTL combined with UNIFAC), but some predict an unlikely azeotrope between myristic acid and isopropyl myristate [3]. Furthermore, using such models, the predicted equilibrium data are considerably different from each other. For instance, the predicted size of the immiscibility region in the liquid-liquid equilibria are completely different, likewise the tie lines within the dome. Thus, there is need to develop a suitable and accurate thermodynamic model to describe the reactive mixture and to use it in the design of reactive distillation operations.

In this regard, this work was focused on the measurement of the vapor−liquid equilibrium (VLE) for the binary mixtures: IPA−IPM and IPA–MA, and the liquid−liquid equilibrium (LLE) for the ternary mixtures: MA–IPA−W, IPM–IPA–W, MA–IPM–W. The VLE were measured using a glass equilibrium cell under isothermal conditions at three different temperatures. The corresponding equilibrium pressure was measured using a high accuracy gauge (MKS PDR 2000 Baratron - dual capacitance diaphragm). A preliminary calibration was done by measuring vapor pressures of IPA and IPM at different temperatures, ensuring minimal deviations with respect to reported data. In the case of the LLE, they were measured under isothermal conditions at a local pressure of 74.5 kPa, at three different temperatures. The composition of the samples was analyzed by High Performance Liquid Chromatography (HPLC). Additionally, MA and IPM were characterized by Differential Scanning Calorimetry (DSC) to verify the enthalpies involved in phase changes and to assess potential decomposition temperatures. The obtained data passed a thermodynamic consistency test and they were used to fit the binary interaction parameters of NRTL equation. The obtained model can be confidently used for further process modeling and reactive distillation design.

References

[1] M. A. Liebert, «Final Report of the Amended Safety Assessment of Myristic Acid and Its Salts and Esters as Used in Cosmetics,» Journal Of The American College Of Toxicology, vol. 1, nº 4, pp. 55-80, 1982.

[2] P. Klaffenbach y D. Kronenfeld, «Analysis of impurities of isopropyl myristate by gas-liquid chromatography,» Journal of Chromatography A, pp. 330-334, 1997.

[3] d. M. Jong, «Reactive Distillation for Cosmetic. An alternative for the production of isopropyl myristate?,» Technische Universiteit Eindhoven, Enschede, 2010.

[4] P. Marzal, J. B. Montón y M. Rodrigo, «Isobaric Vapor-Liquid Equilibria of the Water + 2-Propanol System at 30, 60, and 100 kPa,» J. Chem. Eng. Data, vol. 41, nº 3, pp. 608-611, 1996.

[5] E. Hammer y A. Lydersen, «The Vapour Pressure of di-n-Butylphthalate, di-n-Butylsebacate, Lauric Acid and Myristic Acid,» Chem. Eng. Sci, vol. 7, nº 1-2, pp. 66-72, 1957.

[6] C. Bonhorst, P. Althouse y H. Triebold, «Esters of Naturally ocurring fatty acids,» Industrial And Engineering Chemistry , vol. 40, nº 12, pp. 2379-2384, 1948.