Soxhlet Extraction Process / Soxhlet Extraction Process notes

 Continuous Hot Extraction (Soxhlet Extraction)

 The apparatus for continuous hot extraction used on a laboratory scale (figure 9.3), consists of a flask, a Soxhlet extractor, and a reflux condenser. The ra w material is placed in a thimble (of filter paper) inserted in the wide central tube of the extractor. The drug after getting moistened with the menstruum is packed into the extractor in a way that the extract outlet present at the bottom is not blocked. The menstruum is placed in the flask and boiled at its boiling point. The resultant vapours rise up the larger right tube in the upper part of the drug and then enter the condenser, where it condenses and drops back on to the drug. During percolation, the menstruum extracts the soluble constituents of the drug. When the extract reaches to the top level of syphon tube, the complete percolate syphons over into the flask. The suction effect of syphoning allows the menstruum to penetrate the drug. Thus, the men struum is required in a limited amount for repeated percolation through the drug. This process is continued till the drug gets completely extracted, and the final extract obtained in the flask is further processed. 

 A simpler design of continuous hot ext raction apparatus, which is also official in I.P. (figure 9.4), has an advantage that the hot vapours which rise up the tube encircle the drug material being extracted. Thus, this apparatus will use a slightly higher extraction temperature than the Soxhlet in which a small fraction of heat from the material bed is lost in the surroundings. However, the condensed menstruum percolates through the drug and drops back into the flask without collecting until syphoning (as in the Soxhlet). Consequently, the macer ation period of the drug with the hot menstruum is lost in this apparatus. This process has the following limitations: 

 1) It is not suitable for drugs having th ermolabile active constituents, e.g., enzymes, alkaloids, anthraquinone derivatives, esters, etc., because this extraction process requires a high temperature and also the extract in the flask is maintained in hot condition during the entire process.

2) It is suitable only for pure solvents , constant boiling mixtures (like alcohol - water), or solvent mixture forming azeotropes. 

3) If the menstruum is an ordinary binary mixture, the vapour composition and the liquid composition will be different. 

 4) It is not used if the drug to be extracted is of such physical nature that it would block the Soxhlet apparatus, e.g., opium, gum, resin, orange peel, etc. 

9.1.2.8. Supercritical Fluid Extraction (SFE) 

The process of SFE involves separating one component (i.e., the extractant) from another (i.e., the matrix) using supercritical fluids ( i.e., the extracting solvent). The extraction is generally from a solid matrix or from liquids. SFE can be used as a sample preparation step for analytical purposes, or it can be used on a larger scale to strip unwanted materials from a product (e.g., decaffeination) or collect a desirable product (e.g., essential oils). Supercritical fluid is a substance at temperature and pressure above its critical point. It can diffuse through solids like a gas and dissolve materials like a liquid. It can be suitably used as a substitute for organic solvents in various industrial and laboratory processes. The most commonly used supercritical fluids are carbon dioxide (CO2) and water, which are used for decaffeination and power generation, respectively. However, modified co -solvents such as ethanol or methanol can also be used. CO2 is used as an extraction solvent for botanicals, it does not leave behind any toxic residues, and its extraction properties can be manipulated by making subtle changes in pressure and temperature. An ideal supercritical fluid has the following properties: 

1) It has highly compressed gases, which combine the properties of gases and liquids. 

2) It can lead to reactions, which are difficult to achieve using conventional solvents. 

3) It has a solvent power similar to light hydrocarbons for most of the solutes. However, the solubility of fluorinated compounds is more in supercritical CO2 than in hydrocarbons; and this increased solubility is important for polymerisation. 

4) It increases solubility with increasi ng density (i.e., with increasing pressure). Rapid expansion of supercritical fluid leads to precipitation of a finely divided solid (this is a key feature of flow reactors). 5) Since it is miscible with permanent gases ( e.g., N2 or H2), this leads to much higher concentrations of dissolved gases than can be achieved using conventional solvents. 

Process 

1) The mixture to be fractionated is passed in the extraction column having a heater along its length. 

2) CO2 is purged through the column. 

3) Once the extraction colu mn is pressurised, the supercritical fluid moving along the column length saturates the drug material. 

4) The operating conditions (i.e., pressure and temperature) are selected. 

5) In the pressure-controlled extraction, the solution is expanded in the separation stage to precipitate the extract and the gas iagain recompressed for recycling. 

6) In the temperature-controlled extraction, the solution is heated which lowers the solvent density and precipitates the extract. The density is then increased by isobaric cooling for recycling. 

7) The working of supercritical fluid extraction system is controlled from a PC, which sets the operating conditions like pressure, temperature, and flow rate. 

8) The PC is programmed to shut down the unit in case of overpressure or over temperature (figure 9.5). 

Advantages 

1) SFE determines the rate at which the extraction can be performed.

 2) The SFE process completes within 20-60 minutes. 

3) Many steps have been eliminated from SFE, thus the accu racy and reproducibility of the extraction is increased.

 4) SFE produces less waste solvents and also offers less exposure of labo ratory personnel to toxic solvents. 

5) SFE yields quantitative recovery of target analytes without loss or degradation during extraction.

 6) SFE offers selective extraction by selecting the fluid polarity and density. 

 7) SFE avoids purification by adsorp tion chromatography and keeps the other active ingredients intact in the matrix. 

8) Supercritical fluids have solute diffusivities in order of magnitude higher and solute viscosities in order of magnitude lower than liquid sol vents, and this increases the extraction efficiency. 

9) The solvent strength of a supercritical fluid can be easily controlled, while the solvent strength of a liquid is constant. 

10) The supercritical fluids are mostly gasses at ambient conditions. 

 Disadvantages 

1) Carbon dioxide (the most commonly used solvent in SFE) cannot be used for extracting polar compounds due to its low polarity

2) Presence of water in SFE process may cause problems. 

 3) In SFE, the matrix effect is unpredictable.

 4) SFE process requires specialised/expensive equipment.

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