Supercritical Fluids - An Overview
Author: Pragya Palak
The interplay of various thermodynamic states has bestowed us with many wonders. Supercritical fluids (SCF) are one of them. They represent a class of substances that exist as a single-phase above their critical point i.e., above their critical temperature and pressure. It is common knowledge that the states of the matter are interchangeable. Vapour on being subjected to low temperature and high pressure can be converted into the liquid state. But along this process there comes a certain temperature, above which the gas cannot be further liquified, no matter the amount of pressure being applied. This temperature is called critical temperature and the pressure corresponding to it is called critical pressure. Thus the condition of temperature and pressure being above their critical state where no distinct liquid and gas phases exist is termed as supercritical. Acetone, carbon dioxide, diethyl ether, propane, nitrous oxide, and water are some of the substances that show supercritical state.
The physicochemical properties of supercritical fluids lie between that of liquid and gaseous state, combining the advantageous properties of both at once. The viscosity of SFCs is quite low and the surface tension is essentially nonexistent. Diffusivity is high, which in combination with low viscosity induces interesting transport phenomena processes.
If one draws a parallel between the fluids(liquids and gases) and supercritical fluids, it can be observed that the latter have a high diffusion coefficient, a low viscosity and a high compressibility factor as compared to the liquid state. And parallel to the properties of the gases, high heat capacity and high thermal conductivity are seen.
What's interesting is that these properties are extremely sensitive to the changes in temperature, pressure, and composition resulting in a myriad of ways in which they can be exploited. For instance, near their critical temperature SCFs feature a liquid-like density. But in contrast to liquids, they are still quite compressible. Thus one manipulates the density of the supercritical fluid by adjusting the pressure. It is known that the solubility of solutes is proportional to the density of the solvent. Therefore whenever the supercritical fluids act as solvents, their solvation power can easily be varied by adjusting the pressure.
SCFs are generally applied with other components to create a binary system. It is because they influence the properties of components with which they are mixed. As an instance let us take the example of substantial solubility of supercritical CO₂ in condensed phases. Supercritical CO₂ drastically reduces the viscosity of the phase along with its surface tension making it possible to handle these materials in numerous process equipment.
ProcessesSupercritical fluids are a flexible tool for processing materials. Following are the few of their many wide range applications.
Mass Transfer Processes
Extraction from solids is one of the most important industrial applications of supercritical fluids. It is used in the extraction of essential oils, the decaffeination of coffee and tea, the defatting of cacao, cleaning of rice, treatment of used rubber tubes, and the production of extracts from fruits, nuts, and other natural materials.
Supercritical drying can advantageously produce aerogels. Aerogels are ultralight porous material derived from a gel, in which the liquid component of the gel has been replaced with a supercritical fluid. This results in solid with extremely low density and thermal conductivity which has many applications. For instance the cosmetic industry.
The low surface tension of supercritical fluids makes it possible to enter the smallest structures. They also reduce the surface tension and viscosity of contaminating oils, which are then removed easily from within the surface.
The low viscosity and high diffusion content of SCFs pave the way to a high and fast penetration into the cores of solid materials. The small supercritical fluid molecules diffuse into porous and natural materials, such as wood, cork, leather, and fabric fibres, carrying molecules such as fungicides, polymers, and dyes.
Other important mass transfer processes are dyeing, tanning, desorption, and greasing.
Reactions with supercritical fluids have attracted wide attention in recent years. They can be used in many reactions as reaction media. It has been observed that supercritical reaction media dissolve catalysts differently than other typical solvents. Data shows that while on one hand, enzymes can catalyze reactions in the supercritical fluids the other hand lays before us how the supercritical CO2 can inhibit biological reactions and be used for sterilization.
Important reactions that can be carried out in supercritical media include Friedel-Crafts alkylation, Hydrogenation, Hydrothermal biomass gasification, and Oxidation in supercritical water.
Supercritical CO₂ makes it ideally suitable to replace organic solvents in polymer processing. Carbon dioxide is an effective dilution agent for polymer melts that significantly increase free volume and thus improve the ability to process materials through reductions in viscosity and interfacial tension. Supercritical fluids also play an important role in polymer recycling.
Emulsions and Microemulsions, Colloids
Many nonvolatile solutes are insoluble in CO₂. Therefore, many recent projects have utilized colloids in CO₂ to include insoluble phases. Whereas the solvent strength of CO₂ is limited,water-in-CO2 (W/C) and CO₂ -in-water (C/W) emulsions and microemulsions solubilize high concentrations of polar, ionic, and nonpolar molecules within the dispersed and continuous phases. These emulsions may be separated easily for product recovery simply by depressurization, unlike conventional emulsions. Thus, these emulsions offer new possibilities for replacing organic solvents in many fields.
We can further agree that the advantages of Supercritical fluids can be narrowed down to the following basic points:
1. Raising the technological hurdles.
2. Enhancing the standard of manufacturing operations.
3. Providing a viable alternative to the traditional ways.
While Supercritical fluids pose a promise of great future ahead there is still a lot to tread on. Due to the lack of knowledge, awareness, and the necessary technological setup, this resource has not been harnessed to its possible extent. But with the changing times, one can hope to see more of them in all the spheres of life.
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