Immersion extractors

Immersion-type extractors have been made continuous through the inclusion of screw conveyors to transport the soHds. The Hildebrandt immersion extractor (18) employs a sequence of separate screw conveyors to move soHds through three parts of a U-shaped extraction vessel. The helix surface is perforated so that solvent can pass through the unit in the direction countercurrent to the flow of soHds. The screw conveyors rotate at different speeds so that the soHds are compacted as they travel toward the discharge end of the unit. Alternative designs using fewer screws are also available. 

The Hildebrandt total-immersion extractor is shown schematically in Fig. 18-80. The hehx surface is perforated so that solvent can pass through countercurrently. The screws are so designed to compact... 

The American company, French Oil Mill Machinery Company of Piqua, Ohio, developed continuous solvent extraction plants starting in 1939, using a multistage BoUman-type percolation extractor (3). Soon thereafter, the Belgian Company, Extraction De Smet, started manufacturing continuous multistage belt-type extractors in 1945. V.D. Anderson of Cleveland, Ohio, joined the solvent extraction equipment supply market in 1948 with continuous HUdebrandt-type immersion extractors (5). 

Total Immersion Extractors. In a total immersion extractor, the solid material is immersed in and travels through a pool of solvent. The Hildebrand extractor of 1931 (188-190) is an early example. Hildebrand s extractor consisted of two vertical mbes connected at the bottom with a horizontal mbe. All three tubes were filled with solvent. Three screw conveyors moved the solid material down one tube, across the bottom, and up the other tube. Solvent entered near the top of the solids discharge tube and overflowed the solids inlet tube. This flow pattern caused the extracted oil to move away from the solids in a countercurrent fashion. The screws, however, were rough on flakes and produced many fines. 

Batch extractors mix a charge of solids and solvent and then let it drain. Immersion extractors cause the solids to pass through a pool of solvent. Percolation extractors carry the solids through a vapor-light chamber where solvent rains down through the solids, dissolving out the oil, similar to the way a coffee percolator works. There are hve major types of percolation extractors basket, rotary, perforated belt, slid-ing-bed, and rectangular loop (151). 

Two principle types of extractors have been employed—immersion extractors and percolation extractors (Fig. 11.11). An immersion extractor immerses and soaks the material to be extracted in solvent (a laboratory example is the Soxhlet extractor). Immersion extractors require more solvent usage and have material conveyance problems therefore, percolation extractors almost exclusively predominate in the industry today. Early soybean extractors were immersion extractors, but the industry quickly moved to percolation extractors. In a percolation extractor, the solvent percolates by... 

The Hildebrandt extractor operates according to the dipping principle, immersion extractor, body contains two vertical and one horizontal screw, countercurrent flow of solvent and solids. While the solids pass through the unit they are compacted by the screws extract phase is discharged via rod cages. 

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

Batch Extractors. Coarse soHds are leached by percolation in fixed or moving-bed equipment. Both open and closed tanks (qv) having false bottoms are used, into which the soHds are dumped to a uniform depth and then treated with the solvent by percolation, immersion, or intermittent drainage methods. 

A last note about the continuous phase is the fact that it must completely immerse the packing section where the mixing of the two phases takes place. The inner phase between the two liquid phases is therefore to be near the extractor column s dispersed-phase outlet. The extract stream, having gained the transferred solute, exits the column at the opposite end from where the raffinate stream exits. The raffinate stream is the inlet feed stream containing the extracted solute. 

FIGURE 11.12 Crown immersion-type extractor. (Courtesy of Crown Iron Works Company.)... 

In the quenching experiments, the usual coking procedure was initially followed. However, after the required amount of feed was pumped into the drum, the experiment was stopped by quickly removing the drum from the furnace and immersing it in a mixture of ice and water. The quenched contents were carefully removed from the drum, inspected and sectioned. Each section was then extracted in a Soxhlet extractor with toluene, until the toluene extract became colorless. 

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