Importance of CO2 Feed from Density to Flow (Part 2 – Temperature) Core Separations

We have considered how pressure affects the incoming CO2 supply.  The bottle is delivered at 55 bar and the pressure drops as the CO2 is consumed.

Temperature effects on the CO2 is equally as important as pressure to CO2 density.

Within an extraction system we can manipulate the temperature at many points within the process, including the that of the incoming CO2 feed.

 We achieve this by controlling the temperature of the condensers (tube in shell heat exchangers) using a liquid recirculating chiller from Huber.

This provides us with control over 2 main factors:

  1. The incoming phase of the CO2. Ensuring that it remains liquid once it reaches the inlet of the pump.
  2. The incoming density of the Liquid CO2.

The CO2 phase is important to ensure the pump remains at its most efficient, however by manipulating the temperature of the CO2 we can control the flow rate.

Most high-pressure CO2 pump manufacturers quote the flow rate in mass either kg/h or g/min because the volumetric flow changes with density. 

There is a misconception that running a pump at its highest flow (high CO2 delivery vs mass of biomass) is the most efficient way to run an extraction.  This is not the case, as it can be very application dependent.  For example, high flow rates are more prone to the effects of extraction channelling. 

Building a pump with a high dynamic range is also a challenge (high and low flow).  In some cases, you may require a pump that can deliver 500g/min of CO2 at a density of 1000 kg/m3, for example.  But the same pump would then struggle to deliver flows below 100 g/min at the same density.

By altering the density of the in-coming CO2 we can influence the flow.  For example, a pump running at a fixed RPM may deliver 100g/min @ a density of 1000 kg/m3.  By increasing the temperature, we can affect the density and thus change the flow rate:

Pressure (bar)

Temperature (°C)

Density (kg/m3)

Flow rate (g/min)

55

8

889

88.9

55

4

916

91.6

55

0

944

94.4

55

-4

966

96.6

55

-8

988

98.8

 

We are now able deliver a lower flow without hardware changes to the pump.  This maybe beneficial on a research system that is connected to both a 5L and 1 L extraction vessel.  A high flow rate of 500g/min maybe required for the 5 L but this is not the case for the smaller vessel.

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