(169g) Thermodynamic and Transport Data for Design of Processes with Sub-Critical and Super-Critical Fluids

Separation processes and product formulation require the use of solvents or of high temperature in processing steps.

Conventional solvents are potential environmental pollutants, and the application of heat involves high energy consumption; therefore, research is oriented towards the development of new processes with lower environmental impact. High pressure technologies involving sub and super-critical fluids offer the possibility to obtain new products with special characteristics or to design new processes, which are environmentally friendly and sustainable. By using high pressure as a processing tool one can also avoid the legal limitations for solvent residues and restrictions on use of conventional solvents in chemical processes.^1–3

Extraction of substances from plant materials and their “in situ” formulation in products with specific properties is at the moment one of the very promising applications of super-critical fluids.^4,5 Other advanced processes are polymer processing in/with super-critical fluids, use of sub-and super-critical fluids as sustainable reaction media, etc...^6,7

There are several processes using sub- and super-critical fluids which are already developed to the commercial scale, like dry cleaning, high pressure sterilisation, jet cutting, thin-film deposition for microelectronics, separations of value-added products from fermentation broths in biotechnology fields and as the solvent in a broad range of synthesis. All of these applications lead to sustainable manufacturing methods that are not only ecologically preferable but also give the products with very special properties.³

For the design of all high pressure process, data are required on the operating parameters the type and quantity of the solvent, the re-circulation rate and energy consumption. This information can be obtained from phase equilibrium and mass transfer measurements. Therefore, several parameters influencing solubility, mass transfer of target compounds in the SCF, and consequently extraction yield has to be considered prior to choosing the suitable processing method.^8–10 Extract quality depends on pressure and temperature which can seriously influence the composition of the final extracts. In addition, pressure drop effect has to be evaluated and taken into account when optimising parameters to obtain the best ratio between yield and solvent amount and extraction time. There is an additional requirement, namely, highest possible loading of SC solvent should be achieved in extraction step of the processes, while in separation step of the process the solubility of solute in solvent should be the lowest.

Since high solubility of compound of interest in the super-critical solvent is essential for the economy of extraction process the practical analyses shall verify if extraction using super-critical fluid is the suitable technique for the isolation of the target compound.

Special attention will be given to thermodynamic fundamentals of these processes - phase equilibrium data for systems plant extracts or pure substance with different gasses like propane, argon, chlorinated hydrocarbons, sulphur hexafluoride and carbon dioxide.

For evaluation of basic thermodynamic and transport data describing the behaviour of the system at certain conditions phase equilibrium, density, viscosity, dielectric constant, diffusion coefficient and interfacial tension have to be considered.^11–13 However, scientific literature offers a variety of these data, measured at a variety of pressures and temperatures, for several pure compounds¹⁴. Data on behaviour of binary systems at elevated pressures and temperatures are still relatively scarce and comprise methods that are either expensive either time consuming. There are only few contributions suggesting new methods that are quick and inexpensive and above all, less dependent on the nature of the super-critical fluid saturated compound.^15,16

The thermodynamic properties of multi component mixtures and their analysis in terms of interpretative models constitute a very interesting subject, crucial for design and set up of industrial processes which continue to drive research in the study of multi component systems. The characterisation of mixtures through their thermodynamic and transport properties is important from the fundamental viewpoint of understanding the system behaviour which indicates that thorough knowledge of transport properties is essential in many chemical and industrial applications for surface facilities, pipeline systems, and mass transfer operations.¹⁷ Intermolecular interactions between molecules may be understood by defining these properties at different pressures and temperatures both for pure chemicals and their mixtures over the whole composition range. A specific interest should be dedicated to the fact that thermodynamic and transport properties are associated with heat and fluid flow characteristics. Viscosity and interfacial tension are among the most influential parameters on fluid behaviour and consequently in many oil production and processing aspects from porous media to surface facilities.^18,19 In our lecture we will give an extended overview of new developed techniques to determine fundamental thermodynamic data for binary and multicomponent systems of solids with different sub-and super-critical fluids as processing media for production of novel materials with special properties.

In order to determine the hold-up times, transfer studies were performed and for determining mass transport coefficients different mathematical models were tested. Even if mathematical models can be used to predict the phase equilibrium data at unstudied conditions, parameters should be first determined experimentally. Therefore, for design of processes relatively simple new techniques were developed to obtain transport data for systems with different sub-and super-critical fluids.

(1) Brunner, G. Supercritical Process Technology Related to Energy and Future Directions – An Introduction. J. Supercrit. Fluids 2015, 96, 11–20. https://doi.org/10.1016/j.supflu.2014.09.008.

(2) Brunner, G. Supercritical Fluids: Technology and Application to Food Processing. J. Food Eng. 2005, 67 (1), 21–33. https://doi.org/10.1016/j.jfoodeng.2004.05.060.

(3) Knez, Ž.; Markočič, E.; Leitgeb, M.; Primožič, M.; Knez Hrnčič, M.; Škerget, M. Industrial Applications of Supercritical Fluids: A Review. Energy 2014, 77, 235–243. https://doi.org/10.1016/j.energy.2014.07.044.

(4) Škerget, M.; Knez, Ž. Modelling High Pressure Extraction Processes. Comput. Chem. Eng. 2001, 25 (4), 879–886. https://doi.org/10.1016/S0098-1354(01)00662-7.

(5) de Melo, M. M. R.; Silvestre, A. J. D.; Silva, C. M. Supercritical Fluid Extraction of Vegetable Matrices: Applications, Trends and Future Perspectives of a Convincing Green Technology. J. Supercrit. Fluids 2014, 92, 115–176. https://doi.org/10.1016/j.supflu.2014.04.007.

(6) Goodship, V.; Ogur, E. O. Polymer Processing with Supercritical Fluids; iSmithers Rapra Publishing, 2005.

(7) Knez, Ž.; Markočič, E.; Novak, Z.; Hrnčič, M. K. Processing Polymeric Biomaterials Using Supercritical CO2. Chem. Ing. Tech. 2011, 83 (9), 1371–1380. https://doi.org/10.1002/cite.201100052.

(8) Pastore Carbone, M. G.; Di Maio, E.; Iannace, S.; Mensitieri, G. Simultaneous Experimental Evaluation of Solubility, Diffusivity, Interfacial Tension and Specific Volume of Polymer/Gas Solutions. Polym. Test. 2011, 30 (3), 303–309. https://doi.org/10.1016/j.polymertesting.2011.01.004.

(9) Hannay, J. B.; Hogarth, J. On the Solubility of Solids in Gases. Proc R Soc Lond. 1879, 29, 324.

(10) Letcher, T. M. Thermodynamics, Solubility and Environmental Issues; Elsevier, 2007.

(11) Kiran, E.; Gokmenoglu, Z. High-Pressure Viscosity and Density of Polyethylene Solutions in n-Pentane. J. Appl. Polym. Sci. 1995, 58 (12), 2307–2324. https://doi.org/10.1002/app.1995.070581218.

(12) Sengers, J. L.; Kiran, E. Supercritical Fluids: Fundamentals for Application; Kluwer Academic Publishers, 1994.

(13) Škerget, M.; Mandžuka, Z.; Aionicesei, E.; Knez, Ž.; Ješe, R.; Znoj, B.; Venturini, P. Solubility and Diffusivity of CO2 in Carboxylated Polyesters. J. Supercrit. Fluids 2010, 51 (3), 306–311. https://doi.org/10.1016/j.supflu.2009.10.013.

(14) Markočič, E.; Škerget, M.; Knez, Ž. Solubility and Diffusivity of CO2 in Poly(l-Lactide)–Hydroxyapatite and Poly(d,l-Lactide-Co-Glycolide)–Hydroxyapatite Composite Biomaterials. J. Supercrit. Fluids 2011, 55 (3), 1046–1051. https://doi.org/10.1016/j.supflu.2010.10.001.

(15) Tochigi, K.; Okamura, T.; Rattan, V. K. Prediction of High-Pressure Viscosities for Binary Liquid Mixtures Using the EOS-GE Mixing Rule with Low-Pressure Viscosity Data. Fluid Phase Equilibria 2007, 257 (2), 228–232. https://doi.org/10.1016/j.fluid.2007.02.032.

(16) Kravanja, G.; Hrnčič, M. K.; Škerget, M.; Knez, Ž. Interfacial Tension and Gas Solubility of Molten Polymer Polyethylene Glycol in Contact with Supercritical Carbon Dioxide and Argon. J. Supercrit. Fluids 2016, 108, 45–55. https://doi.org/10.1016/j.supflu.2015.10.013.

(17) Kravanja, G.; Knez, Ž.; Knez Hrnčič, M. Density, Interfacial Tension, and Viscosity of Polyethylene Glycol 6000 and Supercritical CO2. J. Supercrit. Fluids 2018, 139, 72–79. https://doi.org/10.1016/j.supflu.2018.05.012.

(18) Hrnčič, M. K.; Kravanja, G.; Škerget, M.; Sadiku, M.; Knez, Ž. Investigation of Interfacial Tension of the Binary System Polyethylene Glycol/CO2 by a Capillary Rise Method. J. Supercrit. Fluids 2015, 102, 9–16. https://doi.org/10.1016/j.supflu.2015.03.015.

(19) Kravanja, G.; Knez, Ž.; Knez Hrnčič, M. The Effect of Argon Contamination on Interfacial Tension, Diffusion Coefficients and Storage Capacity in Carbon Sequestration Processes. Int. J. Greenh. Gas Control 2018, 71, 142–154. https://doi.org/10.1016/j.ijggc.2018.02.016.