Continuous Extraction

Liqiud-Solid Systems

    Extracting a material from a solid generally requirest considerable time, because it is difficult to get the extracting solvent into direct contact with the solute. Because of this, the effective distribution ratios are low and large volumes of solvent are sometimes necessary. 

A. Soxhlet Extractor

    An efficient device for liquid-solid extraction, which eliminates the use of large solvent volumes, is the Soxhlet extractor (Fig. 1). The extracting solvent is placed in the flask at the bottom (A). When the solvent is heated, it goes up the side arm at B. C is closed off, and the siphon return (right side of B) fills with liquid almost immediately and is also closed off. The solvent vapor is condensed in the condenser and drips onto the sample. When the extractor fills up to G, the solvent will siphon back into the reservoir flask and the process will start again. The solvent back into over and over again: therefore, although a large volume of fresh solvent attacks the sample, actually only a small total volume is used.

    A Soxhlet extractor is commonly used in the determination of dieth-ylstilbestrol, a growth enhancer, in poultry meat. 

    

    The Soxhlet extractor works well with solids but not with liquids. Continuous extractors are excellent for use with materials having a low distribution ratio. 

B. Solvent Heavier

    The extractor shown in Fig. 2, is a continuous extractor for use with solvents heavier than water. The solvent is placed in the flask A and heated. The vapor rises to B and the up to C where it is condensed. The liquid cannot get back in the flask because of the seal at B. It runs into the extractor D, drips through the water layer E, and collects at F. The excess solvent containing the extract runs out the bottom at F and back into the flask. This is a continuous process with the extract collecting in the flask. By using chloroform, caffeine can be extracted easily from cola beverages in about 45 min. 

   
C. Solvent Lighter

    The continuous extractor shown in Fig 3. Is used with solvents lighter than water. The solvent is placed in flask A and heated. The vapor rises past B and on to D. It cannot go to F past C, because after an initial few minutes, C is filled with liquid. The vapor condenses in E and drips down into the flared tube F. The liquid will stay in tube F until it builds sufficient pressure to force its way out of the bottom opening: then it bubbles up through the sample solution H and collects at the top G. The excess runs back into the flask by tube C. The ipecac alkaloids can be extracted from expectorants with ether by this technique in about 3 hr.