Pulsed extractors

Preselection of a suitable extractor type on the basis of available information and testing at pilot scale Establishment of suitable operating conditions as, for example, the pulse intensity of pulsed extractors or the speed of rotation of centrifugal extractors Determination of the flooding point... 

The apparatus employs a passively mode locked Nd/YAG laser oscillator, a Pockel cell pulse extractor, and Nd/YAG laser amplifier to produce laser pulses at 1064 nm. Non-linear crystals convert 30% of this light to 355 nm, which is used for excitation of the sample. The optical path length of the 355 nm light is varied by a computer-controlled time delay stage. 

Figure 7-17 [15] shows a comparison between the experiment and the calculated flooding point data, Eq. (7-21), with the parameter m= 1.9 for pure binary mixtures and ternary mixtures C D and for the mass transfer direction D C with the parameter m = 1.5. As can be seen from the comparison, the experimental data has been verified by calculation with an accuracy of less than 20 %. It was therefore possible to significantly consolidate and generalise the information available on the loading capacity of non-pulsed extractors, compared to Mersmann s flooding point diagram shown in Fig. 7-14. 

FIG. 23-38 Efficiency and capacity range of small-diameter extractors, 50 to 150 mm diameter. Acetone extracted from water with toluene as the disperse phase, V /V = 1.5. Code AC = agitated cell PPC = pulsed packed column PST = pulsed sieve tray RDC = rotating disk contactor PC = packed column MS = mixer-settler ST = sieve tray. (Stichlmair, Chem. Ing. Tech. 52(3), 253-255 [1980]). 

The apparatus for the PFAM film coating on the slider surface is shown in Fig. 1 (a). The film thickness was measured by the TOF-SIMS as shown in Fig. 1 (b). It used a pulsed primary Ga+ ion beam to impact the surface of the PFAM film with an inset energy of 15 keV, an extractor current of 2 fj,A, beam current of 600 pA, a pulse width of 17.5 ns, and a frequency of 10 kHz, respectively. The positive TOF-SIMS spectra on the slider surface is shown in Fig. 2 where the peaks at m/z 31, 50, 69, 100, and 131 in Fig. 2(a) correspond to the positive secondary ion fragments of CF+, CFj, C2F4, and C3F5, respectively. The peak at m/z 469 apparent in Fig. 2(b) corresponds to the ion C12H7F 15O2H+ which is the characteristic ion of PFAM molecules. Therefore, the positive TOF-SIMS spectra demonstrates the existence of PFAM film [24,25]. The thickness of the PFAM film can be determined... 

Extraction can be performed in stirred tanks if the process proceeds fast and separation of phases is ea.sy, but column extractors are most commonly used. The column can be filled with a particulate material. The liquids flow countercurrently whereby the flow can be uniform or pulsed. Reciprocated and rotary agitators are often used to enhance mass transfer. An example of the latter type is shown in Fig. 7.2-13 (asymmetric rotating disk (ARD) extractor). 

The need to use multiple extraction to achieve efficient extraction required the development of new types of continuously working extractors, especially mixer-settlers and pulsed columns, which were suitable for remotely controlled operations. These new extractors could be built for continuous flow and in multiple stages, allowing very efficient isolation of substances in high yield. A good example is the production of rare earth elements in >99.999% purity in ton amounts by mixer-settler batteries containing hundreds of stages. These topics will be further developed in Chapters 6 and 7. 

Pulsing means that either the whole liquid content of a sieve tray column is continually pushed up and down by a piston that moves to and fro, or the whole plate package is moved up and down [3]. Figure 9.5 illustrates the two extractor constructions schematically. They show about the same efficiency... 

The method of calculation introduced in this chapter not only allows an exact determination of the column diameter for nonpulsed sieve tray columns, but also allows a good estimation of the diameters of pulsed and stirred extractors. For the latter, however, more exact specific equations exist for the flooding point, see for example [1,4]. 

Fig. 9.17 Residence times of (a) continuous and (b) drop phase in a liquid pulsed sieve tray extractor according to measurements (squares and crosses) and according to calculations by the dispersion model (drawn lines). Measured and calculated response curves agree well for the continuous phase, but not for the dispersed phase. (From Ref. 14.)...

The previous sections have pointed out that mathematical models of the processes must be proved by experiments, or adapted to experimental results with the aid of pilot extractors. For economic reasons, pilot extractors are usually much smaller than large-scale industrial apparatus. Pulsed pilot columns, for example, have a diameter between 30 and 250 mm, whereas industrial-size columns are up to 2500 mm and more in size. Thus, the question arises of whether or not the calculations or pilot experiments may be used for the dimensions of large-scale apparatus. This is a general problem for engineers. 

If a rapid, reciprocating motion of relatively short amplitude is applied to the liquid contents of an extraction column, the column is said to be pulsed. The agitation thus provided has been found to give improved extraction rates. Devices of this sort were suggested as early as 1935 (VI), but it was not until the atomic energy programs in the United States and Great Britain developed a need for efficient extractors of low... 

Several methods have been used to produce the pulse action required in these columns. A reciprocating, plunger pump, from which the valves have been removed, may be connected to the extractor as at (a), Fig. 12, with a direct liquid connection between the column and the pump piston. Such an arrangement is perhaps mechanically most reliable, al-... 

The arrangement used is shown in Fig. 12(a). The horizontal plates are pierced with holes of small diameter (approximately J -in. diameter), whose total cross section may be 20 to 25% that of the column. No downspouts are provided, as for ordinary perforated-plate extractors, and both liquids must pass through the same holes. The plate spacing is usually small, 2 in. in large-diameter columns, for example. Columns of this sort may be pulsed at amplitudes of 1 in. and frequencies up to 150 cycles/min., depending upon the circumstances. 

Desirable motion can be imparted to the liquids by reciprocating motion of the plates rather than by pulsing the entire liquid mass. This mode employs much less power and provides equally good extraction efficiency. A 30 in. dia tower 20 ft high is sufficiently agitated with a 1.5 HP motor. Some arrangements of such extractors are shown in Figure 14.13. 

Some novel types and variations of basic types of extractors have been developed, most of which have not found wide acceptance, for instance pulsed rotary towers. The literature of a few of them is listed by Baird (in Lo et al., 1983, pp. 453-457). Here the extractors illustrated in Figure 14.15 will be described. 

System Control The control unit operates all the functions of the extractor. It is composed of a CRT, keyboard, contact closure outputs and inputs and nine control cards each with special functions. This unit requires almost no modification since its functions are highly specialized and the circuits are delicately assembled. The most important function to the user is its ability to interface with other instruments. This is done via the contact closures. Of primary use in our design is the contact closure which controls the event end output pulse. In the Suprex Model 50, this is contact closure B. This event end contact closure is the main interface in our design to other in-... 

In the tests carried out in centrifugal extractors, the extraction and recovery of cesium higher than 99.99% were obtained on simulated effluents, with a very good coherence between calculated flowsheets and experimental results.102 Tests confirmed the feasibility of the implementation of the cesium process in pulsed columns, the latter representing the most adapted contactors for the industrial implementation to overcome the drawback due to the presence of solid matter in waste to be treated. 

Fig. 26. Mechanically agitated industrial contactors, (a) mixer-settler (b) rotating-disk column (c) mixco column (d) asymmetric rotating-disk column (e) pulsed packed column (f) Podbielniak centrifugal extractor. (Reprinted from Doraiswamy, L. K and Sharma, M. M., Heterogeneous Reactions Analysis, Examples and Reactor Design, Vols. I and 2, 1984, John Wiley and Sons.)...

For some applications, instead of pulsed sieve-plate columns, vibrating-plate extractors have been recommended (Rod, 1976). Rod has pointed out some advantages of this type of contactor for the oximation of cyclohexanene. 

Perforated-plate (sieve-plate) towers B. Mechanically agitated extractors 1. Towers with rotating stirrers 2. Pulsed towers... 

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