Frequently Asked Questions

1. How Do Supercritical Fluids Work?

When a gas such as Carbon Dioxide is compressed and heated, its physical properties change and it is referred to as a supercritical fluid. Under these conditions, it has the solvating power of a liquid and the diffusivity of a gas. In short, it has the properties of both a gas and a liquid. This enables supercritical fluids to work extremely well as a processing media for a wide variety of chemical, biological, and polymer extraction.

Near liquid densities increase the probability of interactions between the carbon dioxide and the substrate, similar to a liquid solvent. The gas-like diffusivities of supercritical fluids are typically one to two orders of magnitude greater than liquids, allowing for exceptional mass transfer properties. Moreover, near zero surface tension as well as low viscosities similar to gases, allow supercritical fluids to easily penetrate a microporous matrix material to extract desired compounds. The synergistic combination of density, viscosity, surface tension, diffusivity, and pressure and temperature dependence, allow supercritical fluids to have exceptional extraction capabilities.

Another powerful aspect to supercritical fluid extraction (SFE) is the ability to precisely control which component(s) in a complex matrix are extracted and which ones are left behind. This is accomplished through precise control of several key parameters such a temperature, pressure, flow rates and processing time. Yields from SFE are typically much greater those of extractions performed by traditional techniques. Product purity is high, and decomposition of material almost never occurs due to the relatively mild processing temperatures.

2. Why is Carbon Dioxide Used Most Often in SFE?

Supercritical fluid extraction has emerged as an attractive separation technique for the food and pharmaceutical industries due to a growing demand for “natural” processes that do not introduce any residual organic chemicals. Supercritical carbon dioxide is by far the most commonly used supercritical fluid. The unique solvent properties of supercritical carbon dioxide have made it a desirable compound for separating antioxidants, pigments, flavors, fragrances, fatty acids, and essential oils from plant and animal materials. In the supercritical state, carbon dioxide behaves as a lipophillic solvent and so, is able to extract most nonpolar solutes. Separation of the carbon dioxide from the extract is simple and nearly instantaneous; leaving no solvent residue in the extract, as would be typical with organic solvent extraction. Unlike liquid solvents, the solving power of supercritical carbon dioxide can be easily adjusted by slight changes in the temperature and pressure, making it possible to extract particular compounds of interest. With the addition of small amounts of polar co-solvents, even polar materials can be extracted. Additional advantages of carbon dioxide are that it is inexpensive, available in high purity; FDA approved, and is generally regarded as a safe compound (GRAS). Supercritical carbon dioxide is also desirable for extraction of compounds that are sensitive to extreme conditions because it has a relatively low critical temperature (31°C).

3. Why is Carbon Dioxide Most Often the Media of Choice for SFR?

The properties which make supercritical carbon dioxide an attractive solvent for extraction also apply to its use as a medium for reaction chemistry. A fluid’s most important physical and transport properties that influence the kinetics of a chemical reaction are intermediate between those of a liquid and a gas in the supercritical carbon dioxide. The reactants and the supercritical carbon dioxide frequently form a single supercritical fluid phase. Supercritical fluids share many of the advantages of gas phase reactions including: miscibility with other gases, low viscosities, and high diffusivities, thereby providing enhances heat transfers and the potential for fast reactions. Supercritical fluids are especially attractive as reaction medium for diffusion-controlled reactions involving gaseous reagents such as hydrogen or oxygen.

An example of using supercritical fluids as a reaction medium is the hydrogenation of pharmaceuticals to promote enantio selective hydrogenation to favor a cis or trans version of a molecule during hydrogenation. By performing the reaction in two, instead of three phases, the rate of hydrogenation reactions can be increased over 1,000 times. As a results, the size of the reactor and the associated equipment is less than 1/10th that of conventional autoclave systems. Oils and fatty acid esters, as well as hydrogen are soluble in supercritical carbon dioxide. The reaction rate is increased because excess hydrogen is always available for reaction, and the catalyst pores are not filled with stagnant liquid.

4. How does SFT’s Carbon Dioxide Pump Work?

Various types of pumps can be used for supercritical fluid applications. For medium to large volume processes, a pneumatic booster pump is most often used. A diaphragm pushes against a piston to compress the liquid carbon dioxide to a set pressure point. The air that drives the pump increases the liquid carbon dioxide pressure (boosts) in a ratio of about 100 to 1. So for every 1 psi of air delivered to the pump, the carbon dioxide pressure is boosted by 100 psi. (i.e. Air at 50 psi. will deliver about 5,000 psi of carbon dioxide). The CO2 pressure is controlled by an air regulator which in turn controls the pump operation. Once the desired pressure is selected, the pump pressurizes the overall system to this set point. When the restrictor valve is opened, the pump will continue to actuate to maintain the desired set point.

5. Does the Liquid Carbon Dioxide Pump Shut Off During a Static Extraction?

Yes, the pump will fill/pressurize the extraction vessel up to the set point. If there is no flow of material out of the vessel, the pump will shut off. As soon as the variable restrictor is opened, dissolved materials (analyte) and carbon dioxide begin to flow out of the pressure vessel. The pump will begin to actuate to maintain a pressure set point. Look at the restrictor as a Back Pressure Regulator. As you adjust the restrictor to various flows, the pump will speed up or slowdown accordingly to maintain the overall system set point pressure.

6. Why do the SFT-150 and SFT-250 Require a Chiller / Recirculator?

The chiller is used to transfer heat away from the pump head. Cooling the pump head ensures that only liquid carbon dioxide reaches the pump. This is important because the unit cannot pump gaseous carbon dioxide. The chiller essentially does two things. It counteracts the heat of compression which occurs inside the pump head, and it removes heat caused by friction of the piston moving back and forth. Both of these heat sources need to be kept in check. If the pump head is not cooled, liquid carbon dioxide will enter and immediately flash to gas. The pump will cavitate and will operate inefficiently or not at all.

7. Why Not Use a Helium Head Spaced C02 Tanks Instead of a Chiller?

The Chiller eliminates the need for Helium headspace Carbon Dioxide tanks. The action of pumping heats up the liquid carbon dioxide causing the liquid carbon dioxide to flash in the pumping head to gas. This results in cavitations and low pump efficiency. Cavitation can be eliminated 2 ways: First, by use of a chiller assembly to cool the pump head and/or carbon dioxide fluid to about –5 degrees Celsius, eliminating the cavitation problem. Or, second, by use of a higher delivery pressure of carbon dioxide (as delivered in a helium headspace tank at 1,500 psi). Higher delivery pressure keeps the carbon dioxide from flashing to gas, causing the cavitation problem. However, Helium headspace tanks cost about $145.00/tank. A standard carbon dioxide is on the order of $30/tank. The Chiller assembly pays for itself quickly after about 4-6 months of standard operation. Supercritical Fluid Technologies, Inc. holds a Patent on its “Chill Can” assembly.

8. Why Are Co-solvents Sometimes Used?

A small amount of a co-solvent increases the ability of supercritical carbon dioxide to dissolve polar compounds. Neat supercritical CO2 has dissolving properties similar to hexane. This means that, by itself, carbon dioxide is very good for dissolving relatively non-polar materials. The addition of just a small quantity of co-solvent enhances the solubilizing power of the supercritical carbon dioxide making it possible to extract much more polar molecules. Typical co-solvents include: methanol, ethanol, and water.

9. When and Why is a “Preheater” for the Fluid Recommended?

A liquid CO2 pre-heater is recommended for all extraction work. Regardless of vessel size and despite the use of band heaters, heating efficiency is limited because of the relatively small vessel surface area relative to the total vessel volume. Especially at high flow rates, SFE’s with larger vessels but no preheater will not hold temperature with a high degree of accuracy during dynamic flow. To compensate for the physical limitations of the vessel heaters, a fluid pre-heater is used to regulate the temperature of the carbon dioxide and co-solvent before they reach the main sample vessel. For the most efficient and reproducible extraction work, it is highly recommended that a preheater always be used.

10. Which Extraction / Reaction Vessel is Right for Me?

Supercritical Fluid Technologies, Inc. offers a wide variety of sample vessels and options to meet our clients’ needs. Vessels ranging from 50 mL up to 2000 mL are available for our standard bench scale units (up to 4000 in the SFT-250). 20 liter and larger vessels may be used in our pilot scale processing systems. Many options are available for these vessels from windows to mixing, as required by the application. One issue to keep in mind as you decide on vessel for your application is that these are ASME Design Vessels and they are heavy! For example, a 4000 mL vessel in our bench scale system weighs in at 280 lbs. You will need an engine hoist to move this vessel around the laboratory! Fortunately vessels in both the SFT150 and SFT-250 are mounted on sliding racks. The weight of the vessel becomes an issue only when interchanging vessels. The 50 mL, 100 mL, 500 mL, and 1000 mL vessels, which are ideal for preliminary work, can be handled with little difficulty.

11. What is the Purpose of the SFT-250 “Over Temperature” Logic Controller?

The "over temperature" logic controller keeps the vessel’s outer wall temperature from getting extremely hot and in turn over shooting the internal set temperature of the sample vessel. For example, if you have an internal vessel temperature set to 40 °C, you would set the external wall temperature or "over temperature" controller to 45 °C. In this way you maintain the internal temperature at 40 °C without overshooting the desired temperature. Keep in mind you are heating a very large metal mass in the sample vessel. There is a certain amount of histolysis of heat through the vessel wall. To maintain accurate temperature control a control of both the internal vessel temperature and external wall temperature is the best solution.

12. How Do I Change a Hand-Tight Series Vessel Seal?

Remove the existing O-ring carefully. Be sure that you do not scratch the vessel’s O-ring groove surface with any tools. We recommend using a plastic or wood stick to remove existing O-ring. Clean all surfaces thoroughly with solvent. Clean the inside of the vessel seal. The inside surface is where the O-ring actually seals.

Carefully install the new O-ring in the groove of the cover. New O-rings tend to be stiff and may need to be slowly worked into position. It is sometimes helpful to heat the O-ring in a pot of hot water before installation. This will help relax the O-ring material long enough for installation. Lubricate the O-ring and seal area of the vessel with O-ring grease. A small amount of grease works best. Also lubricate the threads of the nut with process coMPatible thread lubricant. Thread the cover into the body of the vessel until you feel the resistance of the O-ring being forced into the seal area. Do not try to force the O-ring all at once. Work the threads back and forth gently until the O-ring has worked into the seal area. Continue to tighten until the threads bottom out.