Screw-Conveyor 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. 

Screw-Conveyor Extractors One type of continuous leaching equipment, employing the screw-conveyor principle, is strictly speaking neither a percolator nor a dispersed-solids extractor. Although it is often classed with percolators, there can be sufficient agitation of the solids during their conveyance by the screw that the action differs from an orthodox percolation. 

Schwartzberg (loc. cit.) reports that screw-conveyor extractors, once widely employed to extract flaked oil seeds, have fallen into disuse for this application because of their destructive action on the fragile seed flakes. 

Vertical-plate extractor. Exemplified by the Bonotto extractor (Fig. 18-79), this consists of a column divided into cylindrical compartments by equispaced horizontal plates. Each plate has a radial opening staggered 180° from the openings of the plates immediately above and below it, and each is wiped by a rotating radial blade. Alternatively, the plates may be mounted on a coaxial shaft and rotated past stationary blades. The solids, fed to the top plate, thus are caused to fall to each lower plate in succession. The solids fall as a curtain into solvent which flows upward through the tower. They are discharged by a screw conveyor and compactor. Like the Bollman extractor, the Bonotto has been virtually displaced by horizontal belt or tray percolators for the extraction of oil seeds. 

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. 

In contrast to the previously described modes of operation, solvent and feed move continuously towards each other in absolute countercurrent extraction. These extractors, e.g. the screw-conveyor extractor, the Bonotto extractor, the Kennedy extractor and extraction batteries with decanter, all move the solid material and are, therefore, mechanically stress objected. For the miscella this requires extended filtration for removing solids. 

In the screw-conveyor extractor, the unit consists of a pressure-resistant cylinder with in-line screws. The solid matrix enters the extractor at the opposite side of the pure solvent. The feed is moved, under the effects of mixing and compaction, to the other end of the screw-conveyor. 

Another type of stationary bed extractor is shown in Figure 5.5. This Hildebrand extractor consists of a U-shaped screw conveyor with a separate auger, or screw, mechanism in each section to transport the solids through the U. Each auger rotates at a different speed to control compaction of the solids. Solids are fed into one end of the U and solvent into the other to provide countercurrent flow. 

The Hildebrandt extractor shown in Fig. 20. consists of a U-shaped screw conveyor with a separate helix in each section. The helices turn at different speeds to give considerable compaction of the solids in the horizontal section. Solids are fed to one leg of the U and fresh solvent to the other to give countercurrent flow. 

The Hildebrandt extractor in Fig. 12.8-2b consists of three screw conveyors arranged in a U shape. The solids are charged at the top right, conveyed downward, across the bottom, and then up the other leg. The solvent flows countercurrently. 

Figure 12.8-2. Equipment for moving-bed leaching [a) Bollman bucket-type extractor, (b) Hildebrandt screw-conveyor extractor.

BoUmann basket conveyor extractor Hildebrandt screw convQfor extractor ... 

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