Continuous-Contact Extractors

The differential extractors are also subject to axial mixing (see Chap. 6) it severely reduces extraction rates because of the deterioration of the concentration differences between phases which is the driving force for mass transfer. This is illustrated in Fig. 10.50, where the real (axial-mixing) concentration profiles show a substantially smaller concentration difference than those for plug flow. If the flow ratio of the liquids is not unity, and it very rarely is, dispersing the liquid flowing at the lower rate will lead to small numbers of dispersed-phase drops, small interfacial areas, and small mass-transfer rates. On the other hand, if the majority liquid is dispersed, the axial-mixing problem is exacerbated. The 

Reviews of calculation methods to account for axial mixing [41] (simplified procedures have been devised [47, 78]) and of axial-mixing data [26] are available. 

Flgiire 1051 Packed extraction tower, light liquid dispersed. 

Correlations for estimating flooding rates are available, and data for mass-transfer coefficients and axial mixing have been summarized (23, 43, 72, 76]. Although axial mixing is less severe than in spray towers, mass-transfer rates are poor. It is recommended instead that sieve-tray towers be used for systems of low interfacial tension and mechanically agitated extractors for those of high interfacial tension. 

The extraction towers previously described are veiy similar to those used for gas-liquid contact, where density differences between liquid and gas are of the 

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Process and Operating Conditions The major parameters that must be fixed or identified are the solvent to be used, the temperature, the terminal stream compositions and quantities, leaching cycle (batch or continuous), contact method, and specific extractor choice. 

After the neutralisation of the alkaline catalyst with an acid, the polyether is diluted with n-hexane. The polyether solution is contacted with water in counter flow using continuous centrifugal extractors. As a consequence of the extraction, two layers appear the upper layer contains a solution of polyether in hexane and the lower layer contains water and the potassium salt. The upper layer is separated and the pure polyether is obtained by solvent vacuum distillation. The disadvantages of this process are a large volume of wastewater and the necessity of solvent recovery and recycling. 

SFE from solids is carried out by continuously contacting the solid substrate with the supercritical solvent. In most cases the solid substrate forms a fixed bed. During extraction, the supercritical solvent flows through a fixed bed of solid particles and dissolves the extractable components of the solid. The solvent is fed to the extractor and evenly distributed to the inlet of the fixed bed. The loaded solvent is removed from the extractor and fed to the precipitator. 

Consider the continuous-contact tower of Fig. 10.57. Althou the raffinate is shown flowing downward as if it were denser, in some instances the solvent-rich, or extract, phase will be denser and will enter at the top. In either case, in what follows subscript 1 will always represent that end of the tower where the raffinate enters and extract leaves, while subscript 2 will indicate where extract enters and raffinate leaves. We are presently unconcerned with which phase is dispersed and which is continuous. If the extractor is fed along the side, Fig. 10.57 and the relationships which follow apply separately to each section above and below the feed inlet. 

Interfacial Mass-Transfer Coefficients. Whereas equiHbrium relationships are important in determining the ultimate degree of extraction attainable, in practice the rate of extraction is of equal importance. EquiHbrium is approached asymptotically with increasing contact time in a batch extraction. In continuous extractors the approach to equiHbrium is determined primarily by the residence time, defined as the volume of the phase contact region divided by the volume flow rate of the phases. 

Coalescence The coalescence of droplets can occur whenever two or more droplets collide and remain in contact long enough for the continuous-phase film to become so thin that a hole develops and allows the liquid to become one body. A clean system with a high interfacial tension will generally coalesce quite rapidly. Particulates and polymeric films tend to accumulate at droplet surfaces and reduce the rate of coalescence. This can lead to the ouildup of a rag layer at the liquid-hquid interface in an extractor. Rapid drop breakup and rapid coalescence can significantly enhance the rate of mass transfer between phases. 

Differential extractors, in which the phases are continuously in contact in the extractor and are only separated at the exits for example, in packed column extractors. 

A solid-liquid extraction is the selective dissolution of a component of a solid material through contact with a liquid solvent. Two methods of accomplishing this are 1) shaking the solid with the solvent in the same container for a specified period of time, and 2) using a Soxhlet extractor, in which fresh solvent is continually cycled through the solid for a specified period of time. 

Solvent extraction. The press cake emerging from a screw press still retains 3 to 15 percent of residual oil. More complete extraction is done by solvent extraction of the residues obtained from mechanical pressing. The greater efficiency obtained in the solvent extraction process encouraged the industry for direct application to oilseeds. In the United States and Europe, continuous extractor units are used in which fresh seed flakes are added continuously and are subjected to a counterflow of solvent by which intimate contact is achieved between the seeds and solvent. The common solvent for edible oil is commercial hexane or heptane, commonly known as petroleum ethers, boiling in the range of 146 to 156°F (63.3 to 68.9°C). After extraction, maximum solvent recovery is necessary for economical operation. The solvent is recovered by distillation and is reused. The extraction oil is mixed with prepress oil for refining. The extracted meals contain less than 1 percent of residual oil. 

Truly continuous is the extraction of liquids if the extractor is replaced by a column. The liquid is pumped continuously onto the head of the pressurised column and flows down by gravity. Supercritical fluid extraction is normally operated in the so-called droplet regime and not in the film regime. This means the liquid in contact with the... 

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