AGITATED EXTRACTION COLUMNS

LIQUID-LIQUID EXTRACTION EQUIPMENT

In certain applications, the mass-transfer efficiency of a static extraction column is quite low, especially for systems with moderate to high interfacial tension. In these cases, efficiency may be improved by

mechanically agitating the liquid-liquid dispersion within the column to better control drop size and population density (dispersed-phase holdup). Many different types of mechanically agitated extraction columns have been proposed. The more common types include vari- ous rotary-impeller columns, the reciprocating-plate column, and the rotating-disk contactor (RDC). The following is a brief review. For more detailed discussion, see Liquid-Liquid Extraction Equipment, Godfrey and Slater, eds. (Wiley, 1994); Science and Practice of Liquid- Liquid Extraction, vol. 1, Thornton, ed. (Oxford, 1992); and Hand- book of Solvent Extraction, Lo, Baird, and Hanson, eds. (Wiley, 1983; Krieger, 1991).

Rotating-Impeller Columns A number of different rotating- impeller column extractors have been proposed and built over the years. Only the Scheibel and Kühni designs are reviewed here. For information about the Oldshue-Rushton design, see the previous edi- tion of this handbook. Also see Oldshue, Chap. 13.4 in Handbook of TABLE 15-23 Mass-Transfer Data for Sieve Tray Columns

Column Tray

System diameter, in spacing, in Ref.

Benzene–acetic acid–water 1.97 3.9–6.3 25 1.97 3.2–6.3 24 2.2 2.8–6.3 23 1.6× 3.2 5.9 20 Benzene–acetone–water 3 4, 8 13 Benzene–benzoic acid–water 3 4 13 Benzene–monochloroacetic acid–water 1.97 3.9–6.3 25 Benzene–propionic acid–water 1.97 3.2–6.3 24

Carbon tetrachloride–propionic acid–water 1.97 3.9–6.3 25

Clairsol–water 17.7 13–15 14

Ethyl acetate–acetic acid–water 2 8–24 10

Ethyl ether–acetic acid–water 8.63 4–7.2 15

Gasoline–methyl ethyl ketone–water 3.75 4.5, 6 11

Isopar(M)–water 16.8 12 21

Kerosene–acetone–water 3 4, 8 13

Kerosene–benzoic acid–water 3.63 4.75 1

Kerosene–benzoic acid–water 6 6, 12 9

Isopar-H–benzyl alcohol, methyl benzyl 2 × 12 24 2

alcohol, acetophenone–water

Methylisobutylcarbinol–acetic acid–water 3 6 12

Methyl isobutyl ketone–adipic acid–water 4.18 6 5

Methyl isobutyl ketone–butyric acid–water 4.8 6–23 8

Methyl isobutyl ketone–acetic acid–water 4 6–12 17

9.7 8–24 18, 19 Pegasol–propionic acid–water 4.8 6–11 7 Toluene–benzoic acid–water 8.75 6 16 3.63 4.75 1 3.56 3–9 22 3 6 12 2.72 9 6 2 24 10 Toluene–diethylamine–water 4.18 6 3, 4 Toluene–water 16.8 12 21 9.7 8–24 18 Toluene–acetone–water 16.8 12 21 9.7 8–24 19 4 6–12 17

2,2,4-Trimethylpentane–methyl ethyl ketone–water 3.75 4.5, 6 11

NOTE: To convert inches to centimeters, multiply by 2.54. References:

1. Allerton, Strom, and Treybal, Trans. Am. Inst. Chem. Eng., 39, p. 361 (1943).

2. Angelo and Lightfoot, Am. Inst. Chem. Eng. J., 14, p. 531 (1968). 3. Garner, Ellis, and Fosbury, Trans. Inst. Chem. Eng. (London), 31, p.

348 (1953).

4. Garner, Ellis, and Hill, Am. Inst. Chem. Eng. J., 1, p. 185 (1955). 5. Garner, Ellis, and Hill, Trans. Inst. Chem. Eng. (London), 34, p. 223 (1956). 6. Goldberger and Benenati, Ind. Eng. Chem., 51, p. 641 (1959). 7. Krishnamurty and Rao, Indian J. Technol., 5, p. 205 (1967). 8. Krishnamurty and Rao, Ind. Eng. Chem. Process Des. Dev., 7, p. 166 (1968). 9. Lodh and Rao, Indian J. Technol., 4, p. 163 (1966).

10. Mayfield and Church, Ind. Eng. Chem., 44, p. 2253 (1952). 11. Moulton and Walkey, Trans. Am. Inst. Chem. Eng., 40, p. 695 (1944). 12. Murali and Rao, J. Chem. Eng. Data, 7, p. 468 (1962).

13. Nandi and Ghosh, J. Indian Chem. Soc., Ind. News Ed., 13, pp. 93, 103, 108 (1950).

14. Oloidi and Mumford, ISEC Proc. (Munich, 1986).

15. Pyle, Duffey, and Colburn, Ind. Eng. Chem., 42, p.1042 (1950). 16. Row, Koffolt, and Withrow, Trans. Am. Inst. Chem. Eng., 37, p. 559

(1941).

17. Rocha, Humphrey, and Fair., Ind. Eng. Chem. Process Des., 25, p. 862 (1986).

18. Rocha et al., Ind. Eng. Chem. Res., 28(12), pp. 1873–1878 (1989). 19. Rocha, Cardenas, and Garcia, Ind. Eng. Chem. Res., 28(12), pp.

1879–1883 (1989).

20. Shirotsuka and Murakami, Kagaku Kogaku, 30, p. 727 (1966). 21. Seibert and Fair, Ind. Eng. Chem. Res., 32(10), pp. 2213–2219 (1993). 22. Treybal and Dumoulin, Ind. Eng. Chem., 34, p. 709 (1942). 23. Ueyama and Koboyashi, Bull. Univ. Osaka Prefect., A7, p. 113 (1959). 24. Zheliznyak, Zh. Prikl. Khim., 40, p. 689 (1967).

Solvent Extraction, Lo, Baird, and Hanson, eds. (Wiley, 1983; Krieger, 1991).

Scheibel Extraction Column The original Scheibel column

design consisted of a series of knitted-wire-mesh packed sections placed within a vertical column, with a centrally located impeller between each section and no baffles [Scheibel and Karr, Ind. Eng. Chem., 42(6) pp. 1048–1057 (1950)]. A second-generation Sheibel design [AIChE J., 2(1), pp. 74–78 (1956); U.S. Patent 2,850,362 (1958)] added flat partitions or baffles to the ends of each packed section, and the impellers were surrounded by stationary shroud baffles to direct the flow of droplets discharged from the impeller tips. The new baffling arrangement improved efficiency, allowing design of larger-diameter columns with less power input and decreased height per theoretical stage. A third design by Scheibel [U.S. Patent 3,389,970 (1968)] eliminated the wire-mesh packing and retained the use of baffles and shrouded impellers (Fig. 15-48). The packed sections were replaced by agitated sections. This design was developed because the wire-mesh packed sections were prone to fouling (plug- ging) and difficult to clean. A Scheibel extractor of this type is very well suited to handling mixtures with high interfacial tension and can be designed with a large number of stages. It is not as well suited for systems that tend to emulsify easily owing to the high shear rate generated by a rotating impeller. Because of its internal baffling, which controls the mixing patterns on the stages, the Scheibel column has proved to be one of the more efficient extractors in terms of height of a theoretical stage; this makes it well suited to applications that

require a large number of stages or are located indoors with headroom restrictions. Holmes, Karr, and Cusack [Solvent Extraction and Ion Exchange, 8(3), pp. 515–528 (1990)] have published results compar- ing the efficiency of the Scheibel column to that of other extractors using the system toluene + acetone + water. For additional discussion, see Scheibel, Chap. 13.3 in Handbook of Solvent Extraction, Lo, Baird, and Hansen, eds. (Wiley, 1983; Kreiger, 1991). A related column design called the AP column consists of alternating sections of Scheibel-type agitators and structured packing [Cusack, Glatz, and Holmes, Proc. ESEC’99, Soc. Chem. Ind., p. 427 (2001)]. The high open area of the packing allows for higher capacity while the agitation provides increased efficiency.

FIG. 15-41 Baffle towers. (a) Side-to-side flow at each tray. (b) Center-to- center flow (disk-and-doughnut style). (c) Center-to-side flow. [Reprinted from

Treybal, Liquid Extraction (McGraw-Hill, 1963), with permission. Copyright 1963 McGraw-Hill, Inc.] 0 0.5 1 1.5 0 0.5 1 1.5

V

, cm/s

V

, cm/s

Toluene Dispersed Water Dispersed Butanol Dispersed Toluene/Water Butanol/Water

FIG. 15-42 Capacity characteristics of a baffle tray extractor. Tray overlap = 62 percent. Column diameter = 10.2 cm. [Taken from Seibert, Lewis, and Fair,

0 0.5 1 1.5 0 0.5 1 1.5

V

, cm/s

V

, cm/s

Toluene Dispersed, TS = 30.48 cm Toluene Dispersed, TS = 10.2 cm Water Dispersed, TS = 10.2 cm Toluene Dispersed, TS = 5.1 cm

FIG. 15-43 Effect of tray spacing on baffle tray capacity. [Taken from Seibert,

As with most agitated extractors, the final design of a Scheibel column typically involves scale-up of data generated in a miniplant or pilot-plant test. The column vendor should be consulted for specific information. The key scale-up guidelines are as follows: (1) Dt(2)/Dt(1)

= [Q(2)/Q(1)]0.4_{; (2) Z}

t(2)/Zt(1)= [Dt(2)/Dt(1)]0.70; (3) stage efficiency is

the same for the pilot and full scale; and (4) power per unit volume is the same for each scale [Cusack and Karr, Chem. Eng. Magazine, pp. 112–119 (1991)]. Industrial columns up to 10 ft (3 m) in diameter and containing 90 actual stages have been designed using the following general procedures and a 3-in (75-mm) pilot column:

1. Pilot tests usually are conducted in 3-in (75-mm-) diameter columns. The column should contain a sufficient number of stages to complete the extraction. This may require several iterations on column height.

2. The column is run over a range of throughputs Vd+ Vcand agi-

tation speeds. At each condition, the concentrations of solute in extract and raffinate streams are measured after steady-state opera- tion has been achieved (usually after 3 to 5 turnovers of column volume). At each throughput, the flood point is determined by increasing the agitation until flooding is induced. A minimum of three throughput ranges are examined in this manner. Mass-transfer performance is measured at several agitation speeds up to the flood point.

3. From the above mass-transfer and flooding data, the combina- tion of specific throughput and agitation speed that gives the optimum economic performance for the required separation can be determined. This information is used to specify the specific throughput value [gal(h⋅ft3_{) or m}3_(h⋅m3_{)] and agitation speed (rpm) for the com-} mercial design. However, unlike the RDC and Karr columns, for 0 0.5 1 1.5 2 0 0.5 1 1.5 2

V

, cm/s

V

, cm/s

62% Tray Overlap Zero Tray Overlap Sieve Trays

FIG. 15-44 Effect of tray overlap on baffle tray capacity. System: toluene (d) + acetone + water (c). [Taken from Seibert, Lewis, and Fair, Paper No. 112a, AIChE National Meeting, Indianapolis

FIG. 15-45 Effect of tray overlap on baffle tray capacity. System: n-Butanol (d)+ succinic acid + water (c). [Taken from Seibert, Lewis, and Fair, Paper No. 112a, AIChE National Meeting, Indi-

0 0.2 0.4 0.6 0.8 1 1.2 0 0.1 0.2 0.3 0.4 0.5

V

, cm/s

V

, cm/s

62% Tray Overlap Sieve Trays Zero Tray Overlap

which the specific throughput of the scaled-up version is the same as that of the pilot column, it is a characteristic of the Scheibel column that the throughput of the scaled-up column is on the order of 3 to 5 times greater than that achieved on the 3-in-diameter pilot column. The limited throughput of the 3-in column is due to its restrictive geometry; these restrictions are removed in the scaled-up columns.

4. Once the column diameter is determined, the stage geometry can be fixed. The geometry of a stage is a complex function of the column diameter. In the 3-in pilot column, the stage height-to-diameter ratio is on the order of 1:3. On a 10-ft- (3-m-) diameter column, it is on the order of 1:8. The recommended ratio of height to diameter is Zt(2)/Zt(1)= [Dt(2)/Dt(1)]0.70.

5. The principle of the Scheibel column scale-up procedure is to maintain the same stage efficiency. Therefore, the scaled-up column

will have the same number of actual stages as the pilot column. The only difference is that the stages will be taller, to take into account the effect of axial mixing. With the agitator dimensions determined, the speed is then calculated to give the same power input per unit of throughput. Scheibel found that power input can be correlated by

P= 1.85ρω3_D

5 _(15-166)

where P is the power input per mixing stage, Diis the impeller diam-

eter, ρ is the average liquid density, and ω is the impeller speed (rota- tions per unit time).

Kühni Column Like the Scheibel column, the Kühni column

uses shrouded (closed) turbine impellers as mixing elements on a cen- tral shaft (Fig. 15-49). Perforated partitions or stator plates extend 0 2 4 6 8 10 12 0 0.2 0.4 0.6 0.8 1

In document Perry Hambook (Page 82-85)