Supercritical CO² is very much at the heart of ever-increasing research into green technologies and highly efficient pressure control is pivotal to its’ success. SCO² has fluid properties that are rather unique in that it behaves as neither a gas nor a liquid whilst also having very useful thermal properties. Areas of research therefore include its use as a “green solvent” as it has virtually no toxicity, using supercritical CO² for carbon geo-sequestration and pressurizing CO² to its critical point for converting captured CO² to CO. It also has long-standing uses as for example in the formation of bio-technology scaffolds to support tissue growth.
Equilibar® precision back pressure regulators are globally recognised as one of the key cornerstones for success within research reactor applications using supercritical fluids. Standard solutions are available for pressures up to 6000 psig however as needs increase there are solutions for 10,000 psig and now even 20,000 psig. The BR-Series back pressure valve has also been available for many years and its blockage resistant design has been essential for SCO² applications.
Experience has taught us that the most common configuration for use with supercritical carbon dioxide is a 316 stainless steel body together with a polyimide diaphragm. It is equally clear that the envelope for experimental conditions is ever-increasing so alternative diaphragm materials such as PTFE/Glass, PEEK and metallic are also becoming common, depending of course on those process conditions. Because of its nature as a highly aggressive solvent care should be taken to prolong service life by detailed consideration of the seals and elastomers. Pressure reductions within a system, even when highly controlled, have the potential to damage some O-ring materials as the SCO² permeates into the elastomer and then expands exponentially upon pressure release. Equilibar recommends using an EPDM O-ring for SCO² applications. EPDM (ethylene propylene diene monomer) is a high-density synthetic rubber capable of withstanding high temperatures together with strong acid and alkali resistance. Manufacturers of perfluroelastomers (FFKM) O-rings also recommend certain of their high-durometer (Shore A 90) varieties for resistance to explosive decompression. Kalrez 0090 from Dupont is specifically designed for rapid gas decompression service.
Application Example 1
An Equilibar precision back pressure regulator was used successfully to facilitate supercritical carbon dioxide research at a famous University. Traditional back pressure regulators with poppet type control valves could neither handle the extremely low flow rate nor cope with the plugging problems caused by icing. An Equilibar with a supple PTFE/Glass diaphragm was able to maintain highly stable control whilst also easily handling the phase changes.
Application Example 2
Supercritical CO² has been successfully applied as a solvent and liquid extraction medium as it is both highly efficient and environmentally friendly. Carbon Dioxide itself is readily available in the natural environment and hence non-harmful, is straight forward to capture and recycle and can easily be shifted to its supercritical phase by controlling the temperature and the pressure. Indeed, the ease of this process means that the specific characteristics of the solvent produced can be finely tuned by only small changes in T and P. This capacity for fine tuning is perfect for its effective use within fractionation. Fractionation or Fractional Separation takes place during phase transition within a mixture (gas, solid, liquid) whereby each of these components can be divided.
Research has established that active ingredients can be derived from plants. These essential oils have differing dissolution conditions, so the fine tuning allows for each substance to be separated independently. This selective extraction process can also be used to extract aromatics for the perfume industry or beneficial oils from fish. SCO², and sometimes a co-solvent such as ethanol, is pumped through a heater to reach the pressure and temperature conditions required for the solvency of the plant oils that are present within the batch extractor. Following this comes the fractionation process whereby the mixture is directed through a series of separator vessels in a multistage operation allowing selective and sequential fluid extraction. Each of the separators is set at a very specific pressure, allowing only one component of the mixture to drop out.
In order for each of the extracted components to be most pure, a precise pressure must be maintained in the separators during fractionation. The unique capability of the Equilibar back pressure regulator to maintain precise pressure control makes it ideal for this targeted fractional separation process. In the final stage of the separation, where the SCO² is decompressed for recycling, the Equilibar BR-Series blockage resistant regulator would be required. The expansion of the supercritical fluid during this pressure let down stage is often problematic. The Joules-Thompson effect causes a dramatic temperature drop which can lead to ice formation that may block the internals of traditional back pressure regulators.
In the video seen below an Equilibar BPR resists the build-up of ice in its internal passages and can even overcome potential blockage by frozen oils. After the pressure let down step, the targeted oils are separated and the CO² and co-solvents may then be removed for recycling.
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