Deborah Agboola   |   September 02, 2020

Pressurized Hot Water Extraction

Water as a universal solvent?
Deborah is an inquisitive writer with a forte in technical writing. Her desire to enlighten and inform gives her the extra edge of structuring her writes to an easily understandable form, while retaining its technicality. When she is not…

With cannabis derivatives, particularly concentrates, gaining popularity, experts have tirelessly worked on developing new methods to obtain these extracts, breaking new ground in their mission. Though several advanced techniques have shown considerable efficiency with FDA approval, most of them are not environmentally-friendly due to their use of chemical solvents. Nevertheless, a few of these extraction processes stand out as a result of their eco- and health-safety plus high efficiency, including Ultrasonic Extraction, Microwave Extraction, and Pressurized Hot Water Extraction.

Pressurized Hot Water Extraction (PHWE)

Discovered by Hawthorne and his team in 1994, this technique involves super-heated water with a temperature above its atmospheric boiling point of 100°c /273k, 0.1MPa, but below its supercritical threshold of 374°c/647k, 22.1MPa, as the extractant. The unique features of water as an extractant in this procedure stems from its eco-friendly nature, high suitability for extracting thermal-labile compounds, as well as its ready availability.

Water, a Universal Solvent: Myth or Fact? Water, as a universal solvent, has been a widely disputed theory for a long time, which is most likely due to its properties at room temperature. However, analytical studies have shown that above a specific temperature and pressure, water transmutes into its non-polar form, a state in which it dissolves non-polar compounds like hydrocarbons and organics. Hence, this establishes this theory as a fact, noting it efficiently serves in both the polar and non-polar states.

Understanding the MO in Cannabis Extraction

Over the years, Hawthorne’s invention has evolved from its former exclusive application in environmental analysis to be used in an extensive range of applications, including consumables manufacturing.

Numerous studies to determine the efficiency of PHWE in extracting cannabinoids under varying conditions have shown, at optimal settings of 50°c extraction temperature, 160°c collection vessel, and 45 minutes extraction length time, there is a resultant high yield of the non-psychoactive compounds (CBC, CBG, CBD), collectively CBDT, to the psychoactive compounds (THC and CBN), collectively THCT.

To use the PHWE technique in cannabis extraction, the degassing of the extractant (water) is mandatory to avoid oxidizing the analytes. The process of degassing, which in this case is the removal of oxygen molecules, can be done via sonication or helium purging and should be done for a minimum of 60 minutes if using the latter. To monitor the quality of extracts obtained and the efficiency of the process, chromatographic methods such as the high-performance liquid chromatography (HPLC) and a UV-detector or gas chromatography-mass spectrometer are effective channels.

The PHWE technique offers two ways of conducting cannabis extraction: the dynamic or continuous flow approach and the static or bath system approach. While both are independent procedures, synergy is possible for better efficiency.

Dynamic & Static Pressurized Hot Water Extraction

In dynamic PHWE, there are five primary components required: a pump, an extraction vessel, a heating device, a pressure restrictor, and a collection vial. The pump, optionally an HPLC pump, transports the activated extractant into the extraction vessel containing the sample biomass, through the pressure restrictor, and finally into the collection vessel where vaporization occurs to separate the extractant from the extract.

With the Static PHWE technique, a pump is conventionally an inessential feature, and extraction occurs at the saturated vapor pressure of the vessel. However, if a pump is included, it must be accompanied by two pressure valves for the regulation and maintenance of the fluid’s pressure.

Depending on the technique, a setup must consist of a pressure restrictor efficient enough to maintain the liquid state of the extractant, a heating device strong enough to raise the temperature of the fluid to a high enough level, and generally, a corrosion-resistant extraction vessel to withstand the corroding ability of water attained above the critical point. Regarding their extraction vessel, while dynamic PHWE requires no distinct structure, the process of the static PHWE, which is highly dependent on pressure, is most effective in an autoclave, with a wide enough diameter to allow easy placement of the stirrer, which facilitates mass transfer in the medium.

General Benefits

The green extraction method comes as a relief to industries looking to evolve into eco-friendly and non-toxic operations, most especially the pharmaceutical industry, which most times requires assured high-purity, low toxicity, psychoactive free raw materials.

This method also offers a better entourage effect by preserving the final analyte properties, less toxicity risk by eliminating up to 99.9% of the extractant and other undesired compounds present, through vaporization and a trapping system, as well as better efficiency due to its high selectivity, low surface tension, high kinetics, and easy usage.

Although the cost efficiency of this process is yet to be established, compared to other methods like the supercritical CO2 extraction, this technique is broadly applicable, and consequently, reduces production cost.

Pros and Cons

Evaluating both systems in terms of simplicity and ease of use, the static PHWE takes pre-eminence since it does not require a pump and pressure restrictor in its operation. But considering the efficiency and output quality of both systems, the long residence time of extracts in the static PHWE increases the risk of phytochemicals’ degradation, especially with heat-labile compounds; hence, this method is less preferred.

Looking at the start-up cost and operational cost, the dynamic PHWE is relatively more expensive than the static, due to the additional implements it requires and its high risk of amassed precipitates in its pipes. However, precipitates can be controlled by the addition of an extra pump between the extraction vessel and the pressure restrictor for back-flushing. Or by affixing heating tape around the tube.

Nevertheless, in considering what system to use, the following parameters should be put into consideration: its maximum operating temperature and pressure, the material of the extraction vessel, and general safety precautions.



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