SUPERCRITICAL fluids (SCFs) are rapidly replacing organic solvents used in industrial purification, essential oils and fatty acids extraction and re-crystallisation operations because of regulatory and environmental pressures on hydrocarbon and ozone-depleting emissions.

SCF-based processes have helped in eliminating the use of hexane, ether and methylene chloride as solvents which are not environment-friendly.

With increasing scrutiny of solvent residues in pharmaceuticals, medical products, cosmetics, perfumes and neutraceuticals as well as with stricter regulations on VOC and ODC emissions, the use of SCFs is proliferating fast in industrial sectors.

Supercritical fluid extraction (SFE) plants are operating at throughputs of 100,000,000 lbs/yr or more in foods industry. Coffee and tea are decaffeinated via supercritical fluid extraction and major brewers in the US and Europe use flavours that are extracted from hops with supercritical fluids.

SCF processes are being commercialised in polymers, pharmaceuticals, lubricants and fine chemicals industries. This process is advantageously applied to increase product performance to levels that cannot be achieved through traditional processing technologies.

Fatty acids can also be extracted using SCFE. Traditionally, edible oil is being extracted using hexane which could be a health risk. Therefore, use of this technology is need of the time to reduce health hazards and maximise yields of high grades.

Properties of SCF: There are drastic changes in some properties of a pure liquid as its temperature and pressure is increased approaching the thermodynamic critical point. For example, under thermodynamic equilibrium conditions, the visual distinction between liquid and gas phases, as well as the difference between the liquid and gas densities disappear at and above the critical point.

Similar drastic changes exist in properties of a liquid mixture as it approaches the thermodynamic critical loci of the mixture. Other properties of a liquid fuel that change widely near the critical region are thermal conductivity, surface tension, constant-pressure heat capacity and viscosity.

SCF extraction: Supercritical Fluid Extraction (SCFE) is based on the fact that, near the critical point of the solvent, its properties change rapidly with only slight variations of pressure. Supercritical fluids can be used to extract analytes from samples. The main advantage of using supercritical fluids for extraction is that they are inexpensive, extract the analytes faster and are more environment-friendly than organic solvents. For these reasons, supercritical fluid CO2 is the reagent widely used as supercritical solvent.

Advantages of SCFE: SCFs have solvating powers similar to liquid organic solvents, but with higher diffusivities, lower viscosity and lower surface tension. Since the solvating power can be adjusted by changing the pressure or temperature, separation of analytes from solvent is fast and easy. By adding modifiers to a SCF (like methanol to CO2) its polarity can be changed for having more selective separation power. In industrial processes involving food or pharmaceuticals, one does not have to worry about solvent residuals as if a “typical” organic solvent was used.

Candidate SCFs are generally cheap, simple and many are safe. Disposal costs are much less and in industrial processes, the fluids can be simple to recycle. SCF technology requires sensitive process control, which is a challenge. In addition, the phase transitions of the mixture of solutes and solvents have to be measured or predicted quite accurately. Generally the phase transitions in the critical region are rather complex and difficult to measure and predict.

This technology is nascent in Pakistan. The first-ever pilot plant has recently been imported by the University of Agriculture, Faisalabad, with financial support of the ministry of science and technology and installed at the Institute of Horticultural Sciences.

The main objective of setting up of the plant is to extract essential oils from flower petals. Now, experimentation is under way to optimise various critical points for extraction of essential oils from roses, tuberose and chambeli. In the next phase, scope will be broadened and some other potential high value non-traditional horticultural crops as well as medicinal and aromatic plants will be tested for value addition and by products developments. This plant has cost Rs6.5 million. Efforts will also be made to locally fabricate the plant with the help of PCSIR and other engineering departments to lower its cost and make this technology available to farmers at an affordable price.

Crops that will be tested for value addition using SCF technology are rose, tuberose, motia, chambeli, citrus (sour orange), sandal, clove etc. Moreover, through this technology yields are far more than through traditional methods of distillation and solvent extraction. It is beneficial for cosmetics and perfume industry and can help in minimising import of medicines and earn foreign exchange for the country.

Process of operation: In SCF plant, food grade CO2 is used as a solvent which is filled in tank. In chiller, mono ethylene glycol is added to lower the temperature of water to minus 100C. At this temperature, CO2 is pumped into pump which increases the flow rate of CO2 into extractor in which flower petals are placed.

When pressure and temperature of the plant exceeds the critical point and liquid gas is converted into supercritical state, it extracts all volatile compounds from the material (e.g. flower petals) by passing through the material which is collected in the separator. After a batch time, extracted material is collected from drain outlet of the separator. When all extractable compounds have been collected, then plant is de-pressurised to collect sufficient CO2 for recycling which is another benefit of this technology making its operational cost less. Apart from essential oil extraction from rose petals, it is also advantageous that this process does not denature the petals while passing through the process, and these can be used to prepare rose jam (another product of value addition) to increase return.