What is Supercritical water?
What is
Supercritical Water?
Subcritical water refers to water that is being kept under conditions of temperature and pressure such that it remains liquid and does not become steam, despite the temperature being above the standard boiling point of water at atmospheric pressure (100 degC).
These conditions typically involve temperatures between 100 degC and 374 degC and pressures high enough to prevent the water from boiling, thus keeping it in the liquid phase. This pressure is lower than the critical pressure of water, which is approximately 221 bar.
Why use
Supercritical Water?
The ionic properties of subcritical water amplify with both temperature and pressure, thereby enhancing its reactivity for processes like hydrolysis. Its lower viscosity and greater diffusivity, as compared to water at room temperature, further endow it with potent solvent capabilities. These characteristics render subcritical water effective for tasks such as the extraction of bioactive compounds or the breakdown of biomass in hydrothermal operations.
Frequently Asked Questions
There are a number of practical and environmental benefits when using water as a process fluid:
1. Tuneable Density
By managing the pressure and temperature, we can adjust the physical attributes of subcritical water to enhance its effectiveness. The density of water in subcritical conditions plays a crucial role, significantly impacting its ability to solvate, its viscosity, and its diffusivity.
At higher pressures and temperatures, the density of water decreases, making it a less polar solvent, thereby enhancing its ability to dissolve nonpolar compounds. Conversely, at lower pressures and temperatures, water maintains its polarity and ability to dissolve polar substances.
This tunability of water's properties under subcritical conditions is what makes subcritical water a versatile and valuable tool in several industrial processes.
2. Tunable Ionic Strength
As temperature and pressure of water increase, its characteristics shift from being predominantly polar to more ionic. This amplifies water's reactivity, transforming it into a powerful solvent for numerous compounds, which may not readily dissolve under cooler, low-pressure conditions. For instance, at 250°C, the pH transitions from 7.0 to 5.5, indicative of a substantial elevation in the ionic strength of hydronium and hydroxide ions compared to their strength at room temperature.
These alterations directly impact the performance of water. The augmented temperatures mainly heighten the likelihood of hydrolysis due to the stronger ionic intensity and diminished pH.
The ionic strength plays a crucial role in determining several key attributes of water, such as its reactivity and power to solvate. This adjustable ionic strength in subcritical water offers a versatile resource in numerous applications, including the extraction of bioactive compounds and the hydrolysis of biomaterials.
3. Environmentally Responsible
Subcritical water extraction and processing is often viewed as environmentally responsible due to several key factors:
- Reduced chemical usage: The process minimizes or eliminates the need for chemical solvents, as water under high temperature and pressure conditions can act as a powerful and versatile solvent itself.
- Energy efficiency: The energy required to heat water to subcritical temperatures is usually less than the energy needed to maintain other solvents at similar temperatures or to reach supercritical conditions.
- Waste minimization: The primary waste product of subcritical water processes is water itself, which is generally non-hazardous and can often be treated and reused, thus reducing waste generation.
- Sustainability: Water is a renewable resource, and the process can be designed to be largely closed-loop, with minimal waste and maximum reuse.
Overall, by minimizing chemical usage, reducing waste, and operating more energy efficiently, subcritical water technologies can provide a more sustainable and environmentally friendly option for a variety of industrial processes.