We have discussed how flow meters can aid in troubleshooting the incoming CO2 flow.  They are however, much more powerful than this.  One of the most important metrics in any extraction is the solvent to extract ratio.  This is the grams or kilograms of CO2 per gram or kilogram of biomass / extracted material.  A good example of this is found within the extraction of Hemp or Cannabis.  Most common ratio of CO2 to extracted oil found in the cannabis industry is around 250 g of CO2 per 1 g of crude extract.

Most pumps can operate without a flowmeter, and can function through a calibrated or computed flow

Essentially we know the volume of the piston, the piston stroke, and the speed of the stroke.  Using this we can calculate the flow rate at any given speed.

For crude research this is acceptable and can give you an indication if CO2 extraction is right for your process.  To accurately understand and optimise this you need additional metrics.

Consider the last two case studies (Part 1 and Part 2) on how varying pressure and temperature affects the incoming CO2 density.  If the density alters,  the desired flow rate and the delivered CO2 will also change accordingly

Using accurate temperature control, from the Huber Circulators, we chill the incoming feed thus removing any temperature effects on the CO2 density.  However, if the density begins to reduce during a run this can result in the following delivery:

Density (kg / m3) Computed Flow Rate (g/min) @ density 1000 kg/m3 Actual Flow Rate (g/min) Computed CO2 Delivery after 3 h (kg) Actual CO2 Delivery after 3 h (kg)
950 100 95 18.0 17.1
900 100 90 18.0 16.2
850 100 85 18.0 15.3
800 100 80 18.0 14.4

As the above demonstrates   you can see over a 3 h run without a flow meter based on computed flow (no density management) you can end up under delivering CO2 by up to 20 %.

The assumption in this extraction is you are not recycling the CO2 (to be discussed later).  This can be common in process development when using a small system.

Using a mass flow meter such as the Bronkhorst  we can correct the speed of the pump as the density varies, allowing us to accurately deliver the correct mass of CO2 during an extraction.


Please get in touch if you want to talk to us further about our supercritical or subcritical CO2 extraction systems.