Pulsed-Liquid Columns

FIG. 15-52 Pulsed-liquid columns, (a) Sieve tray column with pump-type pulse generator, (b) Packed column with air pulser. 

The droplet velocity is delimited to 2-3 m/s because of the mechanical coti-structiOTi of the droplet generator. At a too low velocity the pulsed liquid column is easily diverted sideways by the rotational movement of the cutter disc. On the other hand, with a too high droplet velocity, the resulting droplet frequency is so high that the friction between the cutter disc and the nozzle plate heats up the liquid considerably. 

Logsdail, D. H. and Thornton, J. D. Trans. Inst. Chem. Eng. 35 (1957) 331. Liquid-liquid extraction. Part XIV. The effect of column diameter upon the performance and throughput of pulsed plate columns. 

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

It is shown, that the performance of a pulsing packed column can be split up into its two component parts, the pulses and the zones in between pulses. The pulses can be described as parts of the bed already in the dispersed bubble flow regime the zones-in between the pulses as parts of the bed still in the gas-continuous regime. The pulse frequency is linearly dependent upon the real liquid velocity. The properties of the pulse, like holdup, velocity and height are quite independent upon all the parameters except gas flow rate. 

Bender et al (1979) have discussed the operating characteristics of pulsed packed columns, wherein the efficiency can be improved by a factor of two through the introduction of pulsations. Raschig rings are not recommended, since the droplets of the dispersed phase are likely to adhere to the packings. Pulsed columns work well in situations where the liquid properties do not change substantially in the column. On the whole, a pulsed packed column is not a very efficient column, and it requires that the density difference between the two phases be more than 0.08 g/cm3. 

The appropriate fraction was collected and rechromatographed on the same column with the same gradient but a different ion-pairing reagent (10 mM triethylamine adjusted to pH 6.5 with trifluoroacetic acid). SchistoFLRFamide was isolated on the basis of cross-reactivity to antibodies raised against the molluscan peptide FMRFamide. Sequence analysis was performed with a pulsed-liquid phase sequencer using Edman chemistry (25). 

Satisfying equation (2) guarantees that no interference between the generated and reflected wave will occur. In the special case that the test liquid is accelerated in a cylinder and stopped abruptly ( Fig. Id ), the length of the pressure pulse equals the length of the liquid column. If such a wave is used for the generation of a tension wave, the wave pattern of the main wave fronts running back and forth in the tube, becomes quiet simple. 

There are two different types of energy input in sieve tray or perforated tray columns (Fig. 6-39) in a pulsed sieve tray extractor (PSE) the liquid column is pulsed in a swing tray extractor (STE) fixed sieve trays are mounted on a swinging axis which is driven by an infinitely variable, directly coupled geared motor, including crank shaft and connecting rod. The top and bottom section are expanded to allow the phases to rest and then separate. 

There are three wo well known types of column that can be used for continuous countercurrent liquid/liquid contacting rotating disc contactors (RDC) and pulsed packed columns (PPC) (these are not to be confused with three-phase packed coplumn reactors, operated under pulsed flow conditions, see section 4.7.2.3). 

When a narrow residence time distribution is desired, good heat transfer will usually require high flow rates, which in combination with a given residence time would result in long or tall reactors. Occasionally very long coaxial tubes are used for single liquid phase processes. For processes wiUi a liquid as the continuous phase, a cascade of stirred tanks (sections 3.3.3 and 7,2,1,3) or a pulsed pack column (section 45,15) may then be interesting alternatives. 

There are many areas of appHcation for liquid/liquid extractors, where non-pulsed packed columns are preferred, e.g. for the separation of corrosive products, which must... 

The following terms are generally used to describe the pulse action Frequency is the rate of application of the pulse action, cycles/time. Amplitude is the linear distance between extreme positions of the liquid in the column (not of the pulser) produced by pulsing. Pulsed volume = amplitude X frequency X column crosssectional area = volumetric rate of movement of hquid, expressed as volume/time or vol-ume/(time-area). 

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