Surface-phase extractor

A method was presented for the immobilization of a thioglycoUc acid moiety on the surface of active sUica gel via a simple and direct synthetic route based on a single-step reaction for the formation of two solid-phase extractors. The selective removal and preconcentration of some heavy metal ions, viz. Cu(II), Zn(ll), and Hg(ll) from natural seawater was successfully accomplished. " ... 

Silica gel-chemically immobilized-Eriochrome black T was synthesized and the surface coverage was found to be 0.38 mmol/g. The stability toward hydrolysis of this silica gel phase in various buffer solutions (pH 1.0-10.0) was studied and evaluated. The applicability of silica gel phase-immobilized-Eriochrome black T as a solid-phase extractor for Zn(II), Mg(II), and Ca(ll) was performed by the batch equilibrium technique and was found to show an order similar to the formation constant values of the three metal ions with the indicator. The separation and selectivity of this modified silica gel for these metal ions, based on a column technique, were found to afford a reasonable performance of the three studied metal ions. The structure of silica gel-chemically immobilized-Eriochrome black T is given in Scheme 20. 

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. 

Fig. 123 shows the typical construction of a mixer-settler extractor. The main parameters usually required for the design of an extractor are maximum output, total holding capacity, organic reagent capacity, mixer capacity, phase contact time, settler surface area and specific settler output. 

Contimious liquid extraction techniques are used when the sample volume is large, the distribution constant is small, or the rate of extraction is slow. The efficiency of extraction depends on many factors including the viscosity of the phases, the magnitude of the distribution constant, the relative phase volumes, the interfacial surface area, and the relative velocity of the phases. Numerous continuous extractors using llghter-than-water and heavier-than-water solvents vee been described [3,2 7,42,73,74]. Generally, either the ligi Pr or heavier density... 

Foster GD, Gates PM, Foreman WT. 1993. Determination of dissolved-phase pesticides in surface water from the Yakima River Basin, Washington, using the goulden large-sample extractor and gas chromatography/mass spectrometry. Environ Sci Tech 27(9) 1911-1917. 

These phenomena of surface energy all are time dependent. The shorter the contact time of the phases, the more difficult it is for them to develop. Therefore, flow and mass transfer in centrifugal extractors are hardly affected. 

The existence of a stagnant laminar fluid film adjacent to the interface is not difficult to visualize, in the case where the interface is stationary, as when the fluid flows along a solid surface. However, this situation seems rather unrealistic with the fluid-fluid interface, as when the surface of the liquid in an agitated vessel is in contact with a gas phase above, or if gas bubbles move upward through a liquid, or when one liquid phase is in contact with another liquid phase in an extractor. 

Liquid holdup in liquid-liquid extractors must be defined for both the heavy and light phases. The light-phase outlet pressure is used to control the relative liquid holdup of the two phases. Higher light-phase outlet pressure increases the light-phase holdup. This pressure has been correlated with the phase density difference, rotor speed, and rotor dimensions (24). In addition, packing characteristics of volumetric surface area and porosity influence liquid holdup and throughout capability (23). 

An extractor column is generally a tall, vertical packed tower that has two or more bed sections. Each packed bed section is typically limited to no more than 8 ft tall, making the overall tower height about 40 to 80 ft. Tower diameter depends fully upon liquid rates, but is usually in the range of 2 to 6 ft. Liquid-liquid extractors may also have tray-type column internals, usually composed of sieve-type trays without downcomers. These tray-type columns are similar to duoflow-type vapor-liquid separation, but here serve as contact surface area for two separate liquid phases. The packed-type internals are more common by far and are the type of extractor medium considered the standard. Any deviation from packed-type columns is compared to packing. 

If you took note of Eq. (7.14), you noted the variable 0 the interfacial surface tension constant. We know how to get surface tension constants for various materials from sources such as Lange [3], but what about interfacial surface tension Here interfacial surface tension refers to the surface between the two liquid phases in the extractor. So how do we obtain or calculate this interfacial surface tension For the answer, please see Fig. 7.12. 

The solute is disseminated in a solid matrix in the most of the supercritical extractions of natural products. If the interactions between solute and solid matrix are not important, the mass transfer models can be developed from the equations of microscope balances to a volume element of the extractor. If the mass transfer resistance is in its solid phases, the mathematical models must consider the solute transport within the solid particles or the surface phenomena. 

The amounts of oil collected in the cold trap and the amount of oil absorbed by the thimble did not always add up to the total amount of oil extracted by the CO2. During extraction and depressurization the oils in the SC phase deposited onto the extractor vessel walls, and onto the inner surfaces of the apparatus. This was confirmed by flushing the system with COj between experiments. Tiny droplets of oil most likely from previous extractions were collected in the cold trap. Therefore, the experimental amount of SC extracted oil was not totally reliable for mass balance purposes. Instead, total amounts of lipid extracted by SC CO, were calculated by subtracting the amount of residual lipids (lipids that were not extracted by CO,) from the amount of lipids present prior to SC COj extraction. Both He initial lipid content and the amount present after SC COj were obtained through similar solvent extraction treatments having small experimental errors. 

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