CO2 equilibrium

Controlling CO2 Gas/Liquid Equilibrium

CO2 isn’t a liquid at normal room temperature and pressure. It is a gas.

CO2 equilibriumIn this graph from The Engineering ToolBox, the equilibrium curve is the colored one between the triple point and the critical point.

The Triple Point is the only point in which CO2 exists in a solid, liquid and gaseous phase at the same time, due the specific combination of temperature and pressure. The Critical Point is characterized by the disappearance of the difference between gaseous phase and liquid phase.

When temperature and pressure are increased over the critical point, we have supercritical fluid. The CO2 is at the same time a liquid and a gas at every point along the Equilibrium Curve.

Maintaining the CO2 along this curve will save a lot of energy. Our system is designed to maintain this equilibrium in the reservoir, one of the most important vessels in the system. Without a good control of temperature and pressure at the reservoir’s level, you’ll never be able to have a stable flow in the system.

In our system it’s easy to set the pressure in the reservoir at the desired value. We suggest 48 bar. The system, thanks to our full adaptive automation control, will stabilize the pressure automatically.


The Importance of Cleaning Your System

The cleaning procedure is an important practice: it is just as important as the extraction process.

Any extraction system needs to be cleaned periodically: this period depends on many factors. The following issues all have an impact on the cleaning period:

  • Type of the raw material
  • Quality of the raw material
  • Any pre-treatment of the raw material
  • Number of extractions between cleanings
  • Use of a co-solvent in the extraction process

cleaningThroughout the process the bacteria contamination is very low: during the extraction we have a hostile environment inside the system for bacteria: no oxygen, high pressure and saturation of CO2.  If you use a small amount of ethanol or oil as a co-solvent, the viscosity of the extract is decreased and the deposit of solid parts in the system is reduced dramatically. This condition will change immediately when you stop the system and open the lid of the extractor. Ambient air goes inside the extractor/circuit along with any bacteria in the air. The cleanliness of the ambient air is therefore an important factor.

So it is important to reduce the time that the extractor is open, and the more time the flow of CO2 is arrested, then the higher the probability of precipitation of solids in the piping with negative pressure.

How to Clean the Machine

To clean the piping and exchanger effectively we need a mix of supercritical CO2 and another co-solvent. If apolar or medium polar compounds were extracted, then we use ethanol. If polar compounds were extracted we use water. It is important to remember that ethanol is a flammable fluid, therefore the quantity of ethanol used in the process is very important.

There are two methods you can use to clean the frites:

  • Use the canister during cleaning. Fill the canister with glass marbles, from 8 to 12 mm diameter. If not available, use stainless steel balls, same diameter.
  • Use an ultrasound bath. If the frites are not perfectly cleaned, remove them from the canister and put the discs in the bath.

The small balls will help the ethanol to go from the bottom to the top of the extractor. Use the valve installed on the bottom flange of the extractor to purge ethanol if present at the bottom of the extractor into a safety container. Typical parameters are: pressure at 300 bar and temperature about 60° C/140° F.

The process is fully automatic, there is a sequence written in the program recipe to clean the extraction vessels in the same cleaning process. Separators are only partially cleaned by these procedures. Depending on the quality of the raw material, you may need to open and manually clean the inner wall of the vessels with ethanol and a brush. Check the inner wall of the separators after the automatic cleaning in order to check the status of each separator. If they appear yellow and sticky, they will need to be cleaned manually.

advantages of SC-CO2

Advantages of Supercritical Technology

Traditional technologies in many cases do not offer a satisfactory solution to a specific problem. Furthermore, there is a continuous search to reduce production costs. Pilot scale studies may show that, despite initial high capital costs, operating costs would be lower and the overall feasibility can be proven at certain scale of operation. In addition, supercritical technology allows:

  • Possibility of combining an extraction operation with fractionation column under supercritical conditions to highly concentrate the bioactive components of interest.
  • Possibility of combining a micronization / atomization / dewatering operation with expansion vessel under supercritical/subcritical condition to further make the liquid extract in form of atomized dry powders. This is the way the pharmacological / nutraceutical form will be modified in the future, for the purpose of significantly increasing the bioavailability and action of the active substances.
  • Possibility of using the exhausted material as a secondary product or better as a raw material for other products due to the absence of solvents that may have polluted the raw material.

This is the case of light flours obtained from the extraction of waxes and oils: the protein content of the exhausted product is increased due to the subtraction of fatty compounds, like flours without fats from almonds. It is the case of vegetable waxes separated from the compounds.

Other advantages that SC-CO2 technology has demonstrated in the extraction of special oils include:

  • oxidative stability
  • chemical composition
  • stability of bioactive components maintained during extraction
  • storage and aromatic profile
  • consumer acceptability of such oils.

SC-CO2 is a non-polar FDA-approved solvent (GRASP).

CO2 level

Controlling the CO2 Level

CO2 equilibriumWe say that a liquid is in equilibrium with its gas when the same mass is exchanged from the gas to liquid and from liquid to gas. This happens continuously, and this point is strongly dependent on pressure and temperature. Any change in temperature produces a change in the pressure. When temperature is increased, more liquid CO2 turns to gas, increasing pressure. When temperature is decreased more gaseous CO2 turns in liquid, decreasing the pressure.

The pressure cannot give any information about the liquid CO2 level in the reservoir, but the pressure in a CO2 tank will be the same — in the range of 0.1% to 100% of the liquid level inside! Therefore, a liquid level sensor is necessary to control the level. But the level is strongly dependent on the pressure and temperature inside the tank. So, to have good control of the CO2 liquid level, it is mandatory to have temperature control of the CO2 tank.

There are many points at which we can find equilibrium between gaseous CO2 and liquid CO2. All of these points are situated along the CO2 equilibrium curve. Choosing the best point is part of the system design procedure, because it defines the level of the liquid CO2 in the reservoir. As the CO2 condensing temperature at CO2 bottle pressure is about 14° C (57° F), we chose this value to control the pressure in the reservoir. The control is fully automatic.

efficiency of supercritical CO2

Supercritical CO2’s Extraction Efficiency

Supercritical carbon dioxide isn’t just an efficient solvent for apolar compounds. In fact, if it’s combined with fluid modifiers like water, it becomes a very efficient solvent for medium polar and polar substances (caffeine for coffee decaffeination). The extraction efficiency is better than conventional technologies based on chemical solvents, due to the following features offered by supercritical CO2:

  • Increase of mass’ transport for the zero superficial tension effect, with correlated advantages in the extractive efficiency and in the duration of the extraction (minutes vs. hours).
  • With a minimum modulation of temperature and pressure in the process, the solvent properties are modified in an important way.
  • Totally miscible like gasses.
  • CO2 is a low-viscosity, eco-friendly and green solvent, not flammable, inert and non-toxic.
  • High diffusivity (so it can increase the extractive kinetics).
  • Co-solvents (water, ethanol) can further modify the solvent proprieties.
  • The relatively low extraction temperatures help conserve volatile compounds.
  • This process is not influenced by oxidation: the extraction vessel, full of supercritical CO2, is an inert space for the oxidation process also at high temperature (50-70 °C).
  • Antibacterial and so the final extracts are high quality products, for a microbiology point of view.
  • Cheap: it’s common in the atmosphere (0.04% and rising) and it’s concentrated in a pure solution (99,9% of CO2, gas state,20-25 bar), simply available in safe cylinders or bottles.
  • During the process the CO2 is continually recycled and this reduces the primary cost of extraction. At the same time, the small quantity of solvent lost during the process, and so released in the atmosphere, doesn’t increase global CO2 emissions, because the same CO2 was concentrated from the atmosphere in cylinders.
  • If a polar co-solvent (like water and ethanol) was used for the extraction of polar or medium polar chemical compounds, the extracted substances are easily isolated with the evaporation of the co-solvent.
solubility of supercritical CO2

Controlling Solubility of Supercritical CO2

The Supercritical CO2 extraction process is “geometrically variable.” A supercritical fluid is any compound at a temperature and pressure above its Critical Point.

It can diffuse through solids like a gas, and it can dissolve materials like a liquid. For any pure compound, there is a transition state called “critical” state: for temperatures below the critical temperature (Tc), two phases — liquid and vapor — coexist; for temperatures above Tc, there is only one phase: supercritical fluid. Solubility is a function of pressure and temperature:

  • Solubility increases with increasing pressure at constant temperature.
  • Solubility may increase or decrease when temperatures are raised at constant pressure.

Solubility is related to density. Higher density, higher solubility. This is true from a theoretical point of view, but when applied to a singular compound we may see different results.

The supercritical fluid density always increases with increasing pressure at a fixed temperature, and it always decreases with increasing temperature at a fixed pressure. Solubility depends on this pair of values (pressure/temperature). These values are strictly connected with the solubility of each single compound. For extraction with supercritical fluids, operating conditions are chosen to obtain the selective extraction of compounds of interest, reducing to a minimum the co-extraction of undesired compounds. The selection of the operating conditions depends on the specific compound or compound family to be extracted.


How Separation and Fractionation Works

After the extraction process, the mixture composed of CO2 and solutes leaves the extraction vessel(s) and it is directed to the separation vessels. By varying the pressure, flow and temperature of these vessels, it is possible to induce the selective precipitation of different chemical compounds as a function of their different saturation conditions in the supercritical fluid.

separationFirst of all, the mixture is directed into separator S1 and here, with the use of a lamination valve, the pressure goes down and (due to the Joule-Thomson Effect) the gas that was rapidly expanded during the depressurization process cools because the molecules get the energy using their specific heats, also the temperature of gaseous CO2 drops dramatically, and two important effects are observed:

  • Solvent properties of CO2 changes immediately: Density of the fluid is reduced ten-fold and the expansion of the CO2 changes the speed of the CO2 from centimeters per second to meters per second.
  • All compounds dissolved in the CO2 immediately fall out of solution because the fluid changed its status from supercritical to gaseous and it has become a weak solvent.

S1 is a heated gravimetric separator. This design is particularly effective for the heaviest compounds of the extract. The gravimetric separator works by gravitational force. But this isn’t enough to completely clean up the CO2, in fact other lighter solutes are not separated from the solvent in S1. For this reason there are two additional separators, S2 and S3.

S2 is a heated cyclonic separator. While S1 works using gravity, S2 works using centrifugal force. The fluid moves very quickly creating a vortex inside the vessel. The fluid continues to spin and the particles of extract begin to separate, moving toward the walls of the separator and sliding to the bottom. The temperature of the walls determines which compound will be condensed. Generally, essential oils, like hydrocarbon terpenes, can be found in this separator, as they are lipid-soluble compounds, while oxygenated terpenoids, and sterols travel to separator S3 dissolved in the water micro drops — they are polar compounds and the condensation point (because of heating) is too high to condense them in separator S2.

S3 is a cooled cyclonic separator. Micro droplets of water will condense and collect in this separator along with hydrophilic/oxygenated terpenes and other volatile substances due to their low condensation point.