The Whole Plant Extraction Blog

Exploring Extraction Possibilities with Superheated and Supercritical Water

Fritz Chess

There has been a surge in interest lately in using pressurized water for extracting botanicals. Along with this interest there is confusion and misinformation about how water can be utilized in various kinds of extraction and other industrial processes. So before we look at the ways Eden Labs equipment can be used to do water extractions, it is important to define some of the terms being used.

First we need to define a supercritical fluid in a way that people can relate to instead of a bunch of graphs with pressure and temperature gradients. Think of a pot of water boiling on a stove. The water is boiling at 100C. It is impossible to get the water any hotter than 100C because if you turn up the temperature, it will only boil faster. However, when you put a lid on the pot and secure it down with strong clamps, this dynamic changes. By securing the lid on the boiling pot, steam begins to build pressure quickly in the pot. As pressure builds, the boiling point of the water goes up so at a certain temperature and pressure the boiling will stop unless more heat is applied. If more heat is applied, it will start to boil and pressure will continue to rise. As the temperature and pressure rise, the steam becomes thicker in the vessel and the density of the water drops. At a certain point, the steam and water merge into one state that resembles a very thick fog. This is the supercritical state of water which occurs at 374C and 3,200 psi.

The range of temperature and pressure that occurs between 100C and 374C gives us what is known as Superheated Water. Much of the confusion regarding these two terms probably derives from the common use of the term supercritical as it refers to carbon dioxide. Supercritical CO2 extraction has become a very popular method of extraction for botanicals so people are familiar with the term. Supercritical CO2 occurs at 31C and 1,100 psi.

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What can be done with supercritical water?

Supercritical water has some amazing properties but, as a general rule, extracting compounds from botanicals is not one of them. When the cellulose that binds plant matter together is exposed to supercritical water, it breaks down into simple sugars. Eden Labs has built supercritical water systems for this purpose. Sawdust from the forest products industry is put in a supercritical water state where the wood is converted to sugars which can then be fermented and made into biofuel.

If this process was used to extract a desired constituent from a botanical, the entire batch of plant material would be broken down into a slurry of water, sugar and plant constituents. Many of the desired constituents would be altered or destroyed by the tremendous heat, pressure and chemical reaction unleashed by this technique.

This is where the concept of superheated water comes in. In its natural state, water is the very definition of a polar solvent meaning that it has a molecular structure that has a positive charge on one side and a negative charge on the other. As pressure and temperature of water is raised it becomes increasingly non-polar meaning the electrons are shared equally by the atoms composing the molecule so there is no resulting charge.

Achieving Supercritical Water

By manipulating temperatures and pressures of the water, the solvent properties of the water can be adjusted to the desired polarity. Between 100C and 200C, water has the solvent properties of a water/methanol mixture meaning it has polar and non-polar properties. This range is excellent in providing a very thorough botanical extraction with a wide range of constituents of varying polarities. Traditional Chinese Medicines are an excellent example of this as they often employ a combination of many herbs that need a polar/non-polar solvent. Once superheated water reaches 205C, it becomes a strong non-polar solvent more akin to pure methanol.

Available graphs that chart the polarity changes in water due to temperature and pressure all assume that the water pressure is generated by heating it and creating thermal expansion.  Another way to generate pressure is by pumping. Eden Labs has built high pressure solvent extractors for many years that use this principle. This opens up many more possibilities in manipulating the polarity and extraction properties of water. By using a combination of pumping and heating or cooling, new extraction parameters can be created that have never been graphed. For example, cold water could be pumped up to several thousand psi meaning that it would be at the pressure of superheated water that should be a couple hundred degrees but it is kept cold by a chiller cooling a jacket around the pressure vessel. The temperature could then gradually be raised while under pressure. This is a way that extract fractions could be collected.

A combination of pumping, heating and valving can be employed to take fractions off containing different constituents. For example, at 5C and 2,000 psi, the liquid could be vented off and collected with the resulting compounds that came out of the plant at that combination of pressure and temperature. More water could then be pumped into the vessel and heated to 50C and 2,000 psi. This higher temperature would pull other products that became soluble as the heat was raised. For oils and resins, high temperatures and pressures would generate the non-polar solvent properties needed to put them in solution.

Eden Labs specializes in high pressure extractors that can use a wide variety of solvents and gases. Although we are best known for our supercritical CO2 extractors, we can customize these systems so that they can use a variety of compressed gases as well as pressurized water, alcohol and/or other solvents.

April 07, 2017