Sieve-plate pulse column

In the example shown in Fig. 7.6, using an alkylphosphoric acid, about 60 stages are required to give a Co/Ni ratio of about 100. So many stages would be too many for mixer-settler operation, and other types of contactors would have to be considered. In this particular example, a sieve plate pulsed column has been shown to be very effective [3]. Flowever, with the development of the alkylphosphonic and alkylphosphinic acids, the separation of cobalt and nickel can be achieved in very few stages, owing to the high rejection of nickel (see Chapter 11). 

A change in the type of contactor between that used in the pilot plant to that used in the commercial operation can result in problems. This may occur because a particular large-scale contactor was not readily available. For example, in two plants separating chemically similar metals, although sieve plate pulse columns were found to be excellent contactors for the required separation, other columns were chosen for the commercial plant because pulse columns of sufficient size were not readily available. The columns chosen, at least in one case, resulted in very poor physical operations and process efficiency. 

The later methods of uranium refining in the United States similarly involve TBP extraction. The National Lead Company s plant at Femald came into operation during 1954. This uses a 3N nitric acid feed solution prepared from chemical concentrates or high-grade pitchblende with a uranium concentration of 200 g/1. The solvent is 33-5 per cent TBP in a purified kerosene diluent. Phosphoric acid is added in quantities up to 15 per cent on a uranium basis, in order to complex any thorium impurity and prevent its extraction into the solvent. Three separate sieve-plate pulse columns are employed for extraction into solvent, stripping of... 

It is envisaged that this process could be operated with the simple type of mechanically stirred mixer-settler which is satisfactory in the Springfields uranium purification process. The Femald type of sieve-plate pulse column is another alternative. 

L. Burkhart, A. Survey of Simulated Methods for Modeling Pulsed Sieve-Plate Extraction Columns, UCRL-15101, Ames Laboratory, Iowa State University, Ames, Iowa, 1979. 

Fig. 13. (a) Observed and (b) computed formation of a droplet at an orifice in a pulsed sieve-plate extraction column. (Reprinted from Chemical Engineering Science, Volume 50, Ohta M., et al. Numerical analysis of a single drop formation process under pressure pulse condition, pp. 2923-2931, copyright 1995, with permission from Elsevier Science.)... 

Fig, 2,17 Operation range of a pulsed sieve-plate extraction column for a certain free cross-sectional area... 

Pulsed Columns. The efficiency of sieve-plate or packed columns is increased by the appHcation of sinusoidal pulsation to the contents of the column. The weU-distributed turbulence promotes dispersion and mass transfer while tending to reduce axial dispersion in comparison with the unpulsed column. This leads to a substantial reduction in HETS or HTU values. 

The pulsed-plate column is typically fitted with hori2ontal perforated plates or sieve plates which occupy the entire cross section of the column. The total free area of the plate is about 20—25%. The columns ate generally operated at frequencies of 1.5 to 4 H2 with ampHtudes 0.63 to 2.5 cm. The energy dissipated by the pulsations increases both the turbulence and the interfacial areas and greatly improves the mass-transfer efficiency compared to that of an unpulsed column. Pulsed-plate columns in diameters of up to 1.0 m or mote ate widely used in the nuclear industry (139,140). 

Gravity Pulsation Group C Pulsed packed column Pulsating plate column Group D Pulsed sieve plate column Controlled cycling column... 

Pulse-packed columns Pulse sieve-plate columns... 

In the microfluid dynamics approaches the continuity and Navier-Stokes equation coupled with methodologies for tracking the disperse/continuous interface are used to describe the droplet formation in quiescent and crossflow continuous conditions. Ohta et al. [54] used a computational fluid dynamics (CFD) approach to analyze the single-droplet-formation process at an orifice under pressure pulse conditions (pulsed sieve-plate column). Abrahamse et al. [55] simulated the process of the droplet break-up in crossflow membrane emulsification using an equal computational fluid dynamics procedure. They calculated the minimum distance between two membrane pores as a function of crossflow velocity and pore size. This minimum distance is important to optimize the space between two pores on the membrane... 

Pulsed perforated-plate columns (Fig. 12) are fitted with horizontal perforated plates or sieve plates which occupy the entire cross section of the column. The total free area of... 

Pulsed sieve-plate columns have shown good efficiencies for frequencies between 60 and 150 strokes/min and amplitudes under 1 cm. In general, a plate spacing of 10 cm is considered most favorable. However, these columns operate well in only a limited load range and are also susceptible to fouling, particularly with sticky products. To some extent, the reduction in efficiency at underload can be compensated by changing the pulsation characteristics the load can also be artificially increased by recycling the raffinate. 

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. 

In the case of large throughputs, pulse-type sieve plate columns or mixer-settlers are used as extraction apparatuses both for this process step and for the later extraction steps. 

In pulsed sieve-plate towers, the entire column cross-section is occupied with trays, and thus the lighter phase passes through the holes in the upward stroke and the heavy phase in the downward stroke. This will continuously create new interfaces, which improves the mass transfer. By low pulsation intensities the dispersed phase is discon-tinuously moving through the holes (mixer-settler mode). The appropriate relation... 

Mechanically Pulsed Column. The Karr column (Fig. 23) consists of perforated plates ganged on a common shaft which is oscillated by an external drive. The perforated area and hole size are much larger than in typical sieve plate operation. At high amplitude x frequency product in larger columns, the tendency for excessive backmixing can be curtailed by installation of some fixed baffles. 

Pulsed sieve plate column HETS increases exponentially with diameter 0.4 m at 1 m dia. max. diameter 3 m superficial velocity about 5.5 L/s m sieve holes 3 to 8 mm velocities through the holes <0.2 m/s to minimize the formation of small drops. Tray efficiencies about 20 to 30%. 

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