Zone extraction

HPTLC is a very fast and convenient assay to separate samples components and is often used in Organic Chemistry and in Synthetic approach. Unknown substances, after different display assay, were generally scraped off from the TLC/HPTLC plate, diluted into a tube and transferred into the MS system for structural elucidation and characterization. Now, a TLC-MS interface was developed by CAMAG, which can semi-automatically extract zones of interest and on-line direct them into any brand of a HPLC-MS system. The TLC-MS interface is connected by two fittings to any HPLC instrument coupled with mass spectrometer, without other system configuration adjustments or mass spectrometer modifications. By this way, the unknown substances can be directly extracted from a TLC/HPTLC plate, eluted and resolved by HPLC system and sensitive and selective mass spectrometric signals are obtained within a minute per substance zone [33],... 

Mass transfer in polymeric solutions by molecular diffusion is a comparatively slow process, and in extraction equipment where mass transfer occurs through a thin wiped film, inordinately large equipment surface areas are often required in order to obtain substantial rates of mass transfer. In commercial practice when this situation occurs, the required surface areas may, instead, be obtained by either one of two methods, both of which involve reducing the pressure in the extraction zone. In one approach the extraction pressure is fixed at a value which is less than the equilibrium partial pressure of the monomer or solvent in the polymeric solution fed to the extraction zone. In these circumstances gas bubbles... 

Consider a situation in which a concentrated polymeric solution enters the extraction zone of, say, an extruder in circumstances when the pressure in the extraction zone. Pa, is less than the equilibrium partial pressure of the volatile component in the feed solution. Under these conditions the solution will be supersaturated at the extraction pressure, flashing of the volatile component will occur, gas bubbles of radius Rq will be formed, and the concentration will immediately fall from Wi to wq. If bubble formation occurs by homogeneous nucleation, the rate at which these bubbles will be formed per unit volume of solution should depend on the difference between the equilibrium partial pressure of the volatile component and the devolatilization pressure. Since this pressure difference is greatest when the solution first enters the extraction zone, the rate of formation of bubbles will at first be high but as devolatilization pro-... 

Some useful insight can be developed concerning the influence of material properties and process conditions on devolatilization efficiency by considering the special case when the number of bubbles per unit volume of solution is constant. To fix ideas, assume that all bubbles are formed instantaneously when the solution enters the extraction zone and that no bubbles are ruptured until the very end of the process when all rupture simultaneously. Then the rate of formation can be expressed by... 

On the other hand, if bubble growth is diffusion controlled, then the mass transfer coefficient may be meaningful. However, in this case, the surface area for mass transfer is the surface area of the bubbles entrained in the solution and this depends on the volume of liquid in the extraction zone and not on the surface area of the extraction zone. Clearly, attempts to correlate experimental data for the extraction of a volatile component from a polymeric solution containing entrained bubbles using mass transfer coefficients can be misleading or totally erroneous. 

Instantaneous molar flux of volatile component from nip Pressure in extraction zone Vapor pressure of stripping agent or fugitive component Initial gas pressure in bubble Vapor pressure of volatile component... 

As previously mentioned, cyclohexane was chosen for the fractionation solvent for the second step, since it could transport sufficient quantities of residuum at reasonable temperatures. In the reflux mode, the column of the FDV is operated at a higher temperature than the extraction zone. As the carry-over of residuum decreases, the temperature of the column is reduced to cause the density in this region to increase and consequently more residuum is transported overhead. 

This is repeated until the column is at the same temperature as the extraction zone. In this work., four fractions were brought overhead by operating the column initially at 593 K and then decreasing the temperature to 578, 573, and 563 K as the residuum carry-over approached 1.0 gram per gram-mole of cyclohexane. This concentration value is calculated from the amount of residuum collected from the bottom of the separator after a 30-minute collection period and from the amount of cyclohexane pumped during that period. The column temperatures were selected both from density estimation and from actual experimentation. A more detailed discussion of the development of the operational parameters for the reflux mode will be presented in a future paper. 

If we again consider the example of ICP-MS, the ion beam travels at an approximate velocity of 2.3 X 103 m/sec and fills the 1-inch extraction zone in approximately 11 (isec. If the mass spectral repetition rate is 20 kHz, then 50 psec is required to generate a mass spectrum, translating to a duty factor of 22%. Under similar conditions, we might expect a duty factor of 0.007% from a swept-beam approach. 

In order for further development of high pressure extraction processes, the existing plant will be equipped with varied geometrically shaped extraction zones. The aim is to reach longer residence times. Just as well, the present co-current spraying will be tested in a countercurrent flow with a pressure nozzle only for the liquid phase. 

A conventional SMB can be divided into four zone zone I is between desorbent and extract, zone II is between extract and feed, zone III is between feed and raffinate, zone IV is between raffinate and desorbent as shown in Figure 1. Each column plays the role of different zone during each period between switching time. For example, colunm 3 plays the role of zone in during the first switching period, zone n during the second... 

Ci = inlet volatile concentration (initial volatile concentration in melt at extraction zone) M/1 ... 

With fluids, we think of the pump as the source of pressure as well as the flow rate determining device. However, with supercritical fluids (in contrast to t3q)ical liquids), a pump needs a control point downstream to hmit the passage of molecules per unit time. This restriction then "holds-back" the previously unlimited flow of molecules to a definite, but not always pre-determined level. Ideally then, the restrictor serves to restrict the flow until the density of molecules distributed from the pump through the extraction region right up to the final restriction point in space is such that the operating density desired in the extraction zone is achieved. This is much easier to state in words than it is to achieve in actual experimental practice. This is especially true if you wish to achieve an experimental set of parameters and hold those values over a finite period of time (ranging from minutes to hours) and do it with the statistical precision and accuracy that are necessary to attain the final quantitative analytical results. 

Tuomikoski, S., Virkkala, N., Rovio, S., Hokkanen, A., Siren, H., and FranssUa, S., Design and fabrication of integrated solid-phase extraction-zone electrophoresis microchip. Journal of Chromatography,... 

Separation of two liquid phases after the mass transfer is an important step in the hquid—hquid extraction. Therefore, when designing an extraction unit, a question always arises as howto achieve the separation oftwo phases immediately following phase contact in the extraction zone. In conventional extraction equipment, the operations associated with mixing and separation of two liquids is usually at least partially distinct. 

In an ARD-extractor the asymmetrical stator consists of trays and baffles to divide the column into extraction zones and linked transfer zones. On the rotor mixing discs are mounted. The extraction zone is limited by the stator partition and is separated by the trays into chambers. Phase transport and separation take place in the settling zone behind the partition. Compared to the RDC smaller throughput but better separation efficiency. 

The viability of continuous extraction depends on the availability of lock systems which will enable solid material to be injected into the high pressure extractor and then removed from it without incurring unacceptable loss of solvent. These systems have been discussed by Reimert [24]. (It should be remembered that some solvent will normally be present in voids within the bed of extracted material.) The author finds it useful to classify lock systems into form preserving systems (required for example in the decaffeination of coffee beans), form-changing systems (which could be used, for example for de-oiling seeds or the extraction of the bitter components from hops) and systems in which a solid to be processed is injected into the extraction zone as a suspension in a suitable fluid. This classification applies irrespective of whether the solid is being conveyed into or out of the pressure chamber and is shown in Table 8.5. 

Extruder, Piggy-Back n A system in which the two chief functions of a plasticating extmder - melting and pressure development - are made independently controllable by using two extmders in tandem. The first receives the cold feed, melts it, and delivers the melt to the second extmder, which is essentially a melt pump with possible mixing and/or extraction zones, and which develops the die pressure. Though the piggy-back principle is sound. 

Fig. 3.37 Current-voltage characteristics of an HgCdTe n Tup homojunction extraction diode for different doping levels of the extraction zone...

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