Extraction perforated-plate columns

For stirred tanks with hold-ups up to 0.35 the constant K = 0.024 ( 1For reciprocating-plate extraction columns K = 0.I8, while for pulsed perforated-plate columns AT = 0.18 Kirk-Othmer Encyclopedia of Technology, 1978-1984). 

Perforated-plate columns are semistage in operation. They are reasonably flexible and efficient. A 7-ft-diameter, 80-ft-high perforated-plate column used for extraction of aromatics was reported to have the equivalent of 10 theoretical stages. Because of the simplicity and low cost of packed and perforated-plate columns, they are widely used in industry despite their low efficiency, particularly for processes requiring few theoretical stages and for corrosive systems where absence of mechanical moving parts... 

In 1935, Van Dijck proposed that the extraction efficiency of a perforated-plate column could be improved by either pulsing the liquid contents of the column or by reciprocating the plates. The latter idea was relatively little exploited until the late 1950s, when Karr reported data on a 3-in.-diameter, open-type, perforated reciprocating-plate column. The column was further developed by Karr and Lo by employing baffles. 

The increasing diversity in the applications of liquid extraction has led to a correspondingly diverse proliferation of extraction devices that continue to be developed. This chapter focuses on those fundamental principles of diiiusion, mass transfer, phase equilibrium, and solvent selection that provide a unifying basis for the entire operation. Design procedures for both stagewise and differential contactors alM receive considemtion, including packed and perforated plate columns and mixer-settlers. Some mechanically aided columns are discussed and an attempt is made to conqiare the performance of various equipment tteigns. 

Equation (7.1-31) is used in the deagn extraction equipment sudi as spn colnmns, perforated plate columns, and mixer-settlers. 

Drop Diameter. In extraction equipment, drops are initially formed at distributor no22les in some types of plate column the drops are repeatedly formed at the perforations on each plate. Under such conditions, the diameter is determined primarily by the balance between interfacial forces and buoyancy forces at the orifice or perforation. For an ideal drop detaching as a hemisphere from a circular orifice of diameter and then becoming spherical ... 

Uranyl nitrate was extracted with tributyl phosphate in a 3-in. diameter pulse column with a perforated-plate section height of approximately 9 ft. Plate-free end sections 3-4 in. in diameter and up to 6 ft in length were incorporated in the design to give several minutes of holdup time for phase separation. The pulse was applied by means of reciprocating stainless-steel bellows or a reciprocating piston. 

Countercurrent columns with additional kinetic energy input have found a broad range of industrial applications [42-48]. Examples of extraction towers with energy input are pulsed towers, pulsed packed columns and pulsed perforated-plate towers. A number of units with some form of mechanical agitation are also used (Karr column, Scheibel column, Oldshue-Rushton column, Ktihni column, RZE extractor, RDC and ARD extractor, Graesser contactor). 

A continuous version of-this process is used in Germany (Rg. 15-27). Molten caprolactam, catalyst, and stabilizer are metered into the top of a tower heated to 25Q-260°C by a heat-exchange liquid and maintained at atmospheric pressure. The product slowly passes down through perforated plates in the column as polymerization occurs and is continubusly drawn off at the bottom and metered to spinning machines. This product contains 10 per cent monomer, and the final fiber must be extracted to remove the monomer. 

Consider now the absorption extraction of heavy hydrocarbons under the conditions of counter-current flow in a column absorber presented schematically in Fig. 20.7. Gas of a given composition yo = (yoi, yo2, , yon), where yoi is the molar fraction of i-th component, with the flow rate Qgo enters the bottom part of the column. At the same time, an absorbent with composition xo = (xoi 5 02, , on) and flow rate qo enters the top part of the column. The number of contact stages is equal to N. Each stage is equipped with a perforated plate operating in the ablation regime. This means that the liquid is not collected on the plate, but exists in a dispersed state in the inter-plate space. Each contact stage contains a separation device, for example, a mesh droplet catcher, in which the exhausted absorbent is separated from the gas and directed toward the next plate. 

Perforated plates such as sieve trays used in absorption, distillation or extraction columns. The holes can be covered by caps or valves to avoid weeping in the range of low superficial gas or vapor velocities. The two phases are moving in a crossflow on a tray. 

Liquid distributors are often used to correct liquid flow to a more uniform pattern and/or to add more liquid to the reactor. Liquid distributors are also often used to redistribute the liquid phase as well, but a simpler option, shown in Figure 9.9, would be a perforated plate with directional facing. Liquid collectors are used to channel the liquid into the liquid oudet in order to prevent converted/used liquid to stay in the column bottom. The hardest portion of designing a collector is to not interfere with gas distribution, and the available designs are quite wide and diverse. TBRs, on the other hand, tend to use liquid collectors, which are only used for collecting and extracting liquid. 

Two pailially miscible liquid phases may be contacted for extraction puiposes in a discontinuous or stage-wise manner in columns where the stages take the form of perforated plates, fitted with downcomers to facilitate flow of the continuous phase. Bubble-cap plates have been found ineffective in extraction, because of the lower density difference, lower interfacial tension, and higher viscosity of the disperse phase, compared to gas-liquid systems. 

Skelland and co-woikers have developed a procedure for the design of perforated plate extraction columns. This eliminates the need for experimentally measured stage efficiencies, which are usually costly and troublesome to obtain. Additionally, the validity of such efliciencies in scaled-up application is fir quently uncertain. Currently, the procedure involves use of rate equations for mass transfer during drop formation either at the perforations or at the end of jets issuing from the perforations, during ftee rise or fall of the drops, and during coalescence beneath Mcb i le, to locate a pseudoequilibtium curve. The latter is employed instead of the actual equilibrium curve on the - >a distribution diagram in a stepwise... 

FIGURE 7.4 1 Plates it + 1 and n in a perforated plate liqaid-liqnid extraction column. 

Sieve tray columns have also fotmd an application in staged countercurrent liquid extraction operations. The perforated plates, arranged in much the same way as in gas-liquid contact, act to break up accumulations of the dispersed phase and provide fresh surfaces for renewed mass transfer. Each of the plates, or rather the space between them, is a potential equilibrium stage, but the efficiency E is generally quite low. 

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