When attempting to determine the optimal size of a CO2 system for research or production purposes, several factors must be taken into account. The prevailing misconception is that individuals often desire a system with future capacity in mind. However, it is important to note that CO2 extractions are most efficient when the extraction vessels are completely filled, allowing the CO2 to thoroughly interact with the material and extract the maximum amount of soluble compounds. Insufficiently filling the extraction vessels creates significant empty spaces where the supercritical fluids can bypass the material, resulting in subpar extraction outcomes. Therefore, the most suitable size is generally determined by the smallest volume that the customer can fill. It is preferable to conduct multiple smaller batches that are completely packed rather than a single large batch that remains underfilled.

Dual vessel systems present a favourable middle ground when considering extractions of varying sizes. For instance, a system equipped with both a 1L and 5L vessel allows for small-scale extractions suitable for research purposes, while also offering the capacity to scale up using the larger 5L vessel.

CO2 is commonly categorized as a non-polar solvent, but this classification only holds partially true. By increasing the density of CO2 through elevated pressure, it becomes capable of dissolving moderately polar compounds. However, highly polar compounds like alcohols and amides do not exhibit solubility in CO2 alone. To facilitate the extraction of such compounds, the introduction of a co-solvent becomes necessary. The co-solvent modifies the polarity of CO2, enabling the extraction of these compounds. Incorporating a co-solvent pump into a Supercritical Fluid Extraction (SFE) system expands the range of extractable compounds, enhancing its versatility.

Furthermore, co-solvent pumps prove valuable in the cleaning process by allowing the addition of a more polar solvent. This addition reduces the cleaning time required for the system, enhancing efficiency.

Subcritical CO2 refers to CO2 operating below its critical point of 31 °C and 74 bar. In this range, CO2 transforms into a high-pressure liquid and becomes suitable for processing materials at lower temperatures. Similar to other fluids, reducing the temperature directly influences the solubility of various compounds. For instance, subcritical CO2 exhibits reduced solubility for plant waxes, but it can still effectively dissolve smaller compounds like alpha and beta acids present in hops. This characteristic allows for a higher level of selectivity in the extraction process.

The primary distinction between a supercritical and a subcritical system lies in the capability to cool below room temperature. In most systems with a volume of 10 L or below, temperature control is achieved through the use of electrical heater bands. However, employing a thermal fluid enables a significantly broader range of conditions, enhancing control over the extraction process. This broader temperature range facilitates better control over extractions, offering more precise and customizable conditions. As we understand that it isn’t always clear which conditions will work best all our systems can be easily retrofitted to achieve subcritical conditions without change to the vessel.

By manipulating the conditions of CO2 we can selectively extract various classes of compounds by adjusting the density of CO2 through changes in pressure and temperature. These principles can also be applied to the collection phase. By varying the pressure and temperature across multiple collectors, we establish a density gradient. This gradient serves to selectively desolubilize different classes of compounds into distinct collectors, enabling a targeted fractionation of your extraction process.

Typically, during the process of fractionation, the collection occurs in a manner where the least soluble compounds are initially gathered, followed by the subsequent collection of more soluble compounds like terpenes in the final collector.

One of the remarkable advantages of using CO2 as a solvent is that when collecting the product from the separator, it returns to its gaseous state, leaving the product uncontaminated.

Nevertheless, we can also harness the reusability of CO2 by recompressing it. While this may not be cost-effective for small-scale systems due to the relatively inexpensive nature of CO2 (depending on the grade), it becomes more sensible as we scale up. The commonly employed approach involves lowering the CO2 pressure to 55 bar (bottle pressure) and recycling it back into a tank.

However, this recycling process can pose challenges, as there can be carryover of materials leading to potential blockages and contamination of the extraction process. By thoroughly understanding the characteristics of the materials and process conditions, these effects can be minimized or eliminated.

The duration of an extraction process depends on the solubility of compounds in CO2. It is crucial to have a clear understanding of the solvent-to-feed ratio, which specifies the required amount of CO2 per gram of material for extraction. Once this CO2 quantity is determined, the flow rate can be utilized to estimate the duration of the extraction process.

However, it should be noted that faster delivery of CO2 does not necessarily lead to quicker extractions. If the velocity of CO2 is excessively high, it can cause channelling within the material, resulting in suboptimal extraction outcomes. Therefore, it is important to strike a balance and ensure an appropriate CO2 velocity that facilitates thorough extraction without compromising the quality.

CO2 exhibits the ability to dissolve a wide array of natural products. This diverse group includes terpenoids, phenolics, sterols, lipids, diterpenes, cannabinoids, alkaloids, carotenoids, waxes, and polyphenols. The solubility of these compounds in CO2 as a supercritical fluid is influenced by factors such as pressure, temperature, and fluid density.