Pulse column

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

In many types of contactors, such as stirred tanks, rotary agitated columns, and pulsed columns, mechanical energy is appHed externally in order to reduce the drop si2e far below the values estimated from equations 36 and 37 and thereby increase the rate of mass transfer. The theory of local isotropic turbulence can be appHed to the breakup of a large drop into smaller ones (66), resulting in an expression of the form... 

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. 

Special safety constraints apply to equipment selection, design, and operation in nuclear reprocessing (269). Equipment should be reHable and capable of remote control and operation for long periods with minimal maintenance. Pulsed columns and remotely operated mixer—settlers are commonly used (270). The control of criticaHty and extensive monitoring of contamination levels must be included in the process design. 

At higher total flow rates, particularly when the Hquid is prone to foaming, the reactor is a pulsed column. This designation arises from the observation that the pressure drop within the catalyst bed cycles at a constant frequency as a result of Hquid temporarily blocking gas or vapor pathways. The pulsed column is not to be confused with the pulse reactor used to obtain kinetic data ia which a pulse of reactant is introduced into a tube containing a small amount of catalyst. 

FIG. IS-S Pulsed columns a) Perforated-plate column with pump pulse generator, (h) Packed column with air pulser. 

FIG. 22-12 Pulsed -column capacity versus column size for 65 percent p-xylene feed. To convert gallons per hour to cubic meters per hour, multiply hy 0.9396 to convert square feet to square meters, multiply hy 0.0929. (McKay et al., prepr., 59th nat. meet. AIChE, East Columbus, Ohio.)... 

Rotating-disk and pulsed columns Continuous, piston flow Dispersed, piston flow... 

Nagel, V. Gilmore, M. Scott, S. Bateman pulsed column pilot-plant campaign to extract cobalt from the nickel electrolyte stream at Anglo Platinum s base metal refinery. International Solvent Extraction Conference, Cape Town, South Africa, Mar. 17-21, 2002, pp 970-975. 

A recent and extremely important development lies in the application of the technique of liquid extraction to metallurgical processes. The successful development of methods for the purification of uranium fuel and for the recovery of spent fuel elements in the nuclear power industry by extraction methods, mainly based on packed, including pulsed, columns as discussed in Section 13.5 has led to their application to other metallurgical processes. Of these, the recovery of copper from acid leach liquors and subsequent electro-winning from these liquors is the most extensive, although further applications to nickel and other metals are being developed. In many of these processes, some form of chemical complex is formed between the solute and the solvent so that the kinetics of the process become important. The extraction operation may be either a physical operation, as discussed previously, or a chemical operation. Chemical operations have been classified by Hanson(1) as follows ... 

Flooding-point data have been correlated by equations analogous to 13.34 and 13.35. This procedure is permissible since, as Thornton 37 and Logsdail and Thornton(38) report, pulsed columns can be operated up to the flooding-point with no droplet coalescence. 

THORNTON, J. D. Trans. Inst. Chem. Eng. 35 (1957) 316. Liquid-liquid extraction. Part XIII. The effect of pulse wave-form and plate geometry on the performance and throughput of a pulsed column. 

Whilst the Schildknecht column is essentially a laboratory-scale unit, a melt-crystalliser of the wash-column type was developed by Phillips Petroleum Company in the 1960s for large-scale production of xylene. The key features of this Phillips pulsed-column crystalliser, as described by McKay et alSss are shown in Figure 15.27. A cold slurry... 

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. 

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. 

Fig. 9.6 Behavior of droplets near the sieve tray in a hquid-pulsed column during downstroke and upstroke of the hquid content.

Irradiated UO2 is dissolved in nitric acid, resulting in a dissolver solution with the approximate composition listed in Table 12.7. This is treated by the Purex process. The main steps in the conventional Purex process are shown schematically in Fig. 12.5. All existing plants listed in Table 12.8 use some variation of the Purex process. Typically, the extractant composition (percentage TBP, diluent) and the extraction equipment (i.e., pulse columns, mixer-settlers, etc.), vary from plant to plant. However, the upper concentration limit is 30% TBP to prevent a phase reversal due to the increased density of the fully loaded solvent phase. 

A pilot plant, containing two pulsed columns, one for extraction and one for stripping, and batchwise evaporation was in operation in Sweden during 1981. Pilot plant operations have also been performed in Holland (Fig. 14.6) and Germany. The experience from these tests shows that the process concept is technically practicable and well proven. The economics of the process, however, are strongly dependent on the cost for disposal of spent pickling liquors. 

Stripping HNO3 and HF from the organie solution with water in a second pulsed column operation and recyeling the aeids back to the pickling bath... 

Fig. 12. Pulse columns (a) liquid-pulsed, perforated-plate column (6) air-pulsed, packed column (T2).

Another property of the iron-defective molybdate is the presence of Mo=0 double bonds on the surface. The hydrogen-abstracting capacity of the catalyst is closely related to Mo6 contained in the Mo=0 as is shown in Sect. 3. There the role of iron is also discussed. It is, however, interesting to note here that pure iron oxides accelerate combustion and that a W03—Fe2(W04)3 catalyst is practically inactive [254], Replacement of iron by chromium is possible but leads to a lower activity [253]. Baussart et al. [46] prepared stoichiometric NiMo04 which showed selective behaviour towards formaldehyde in a pulsed column below 375°C. 

Pulsed columns Low HETS No internal moving parts Many stages possible Nuclear Petrochemical Metallurgical... 

In many lypes of contactors, such as stirred tanks, rotary agitated columns, and pulsed columns, mechanical energy is applied externally in order to reduce the drop size and thereby increase the rate of mass transfer. 

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