12.6. TYPES OF EQUIPMENT AND DESIGN FOR LIQUID–LIQUID EXTRACTION

12.6A. Introduction and Equipment Types

As in the separation processes of absorption and distillation, the two phases in liquid–liquid extraction must be brought into intimate contact with a high degree of turbulence in order to obtain high mass-transfer rates. After this contact of the two phases, they must be separated. In both absorption and distillation, this separation is rapid and easy because of the large difference in density between the gas or vapor phase and the liquid phase. In solvent extraction, the density difference between the two phases is not large and separation is more difficult.

There are two main classes of solvent-extraction equipment, vessels in which mechanical agitation is provided for mixing, and vessels in which the mixing is done by the flow of the fluids themselves. The extraction equipment can be operated batchwise or continuously as in absorption and in distillation.

12.6B. Mixer–Settlers for Extraction

To provide efficient mass transfer, a mechanical mixer is often used to provide intimate contact between the two liquid phases. One phase is usually dispersed into the other in the form of small droplets. Sufficient time of contact should be provided for the extraction to take place. Small droplets produce large interfacial areas and faster extraction. However, the droplets must not be so small that the subsequent settling time in the settler is too large.

The design and power requirements for baffled agitators or mixers have been discussed in detail in Section 3.4. In Fig. 12.6-1a a typical mixer–settler is shown, where the mixer or agitator is entirely separate from the settler. The feed of aqueous phase and organic phase are mixed in the mixer, and then the mixed phases are separated in the settler. In Fig. 12.6-1b a combined mixer–settler is shown, which is sometimes used in the extraction of uranium salts or copper salts from aqueous solutions. Both types of mixer–settlers can be used in series for countercurrent or multiple-stage extraction. Typical stage efficiencies for a mixer–settler are 75–100%.

Figure 12.6-1. Typical mixer–settlers for extraction: (a) separate mixer–settler, (b) combined mixer–settler.


12.6C. Spray Extraction Towers

Packed and spray-tower extractors give differential contact, where mixing and settling proceed continuously and simultaneously (C8). In the plate-type towers or mixer–settler contactors, the extraction and settling proceeds in discrete stages. In Fig. 12.6-2 the heavy liquid enters at the top of the spray tower, fills the tower as the continuous phase, and flows out through the bottom. The light liquid enters through a nozzle distributor at the bottom, which disperses or sprays the droplets upward. The light liquid coalesces at the top and flows out. In some cases the heavy liquid is sprayed downward into a rising, light continuous phase.

Figure 12.6-2. Spray-type extraction tower.


The spray tower has a very large axial dispersion (back-mixing) in the continuous phase. Hence, only one or two stages are usually present in such a tower. Typical performance parameters for a spray tower are given in Table 12.6-1. Despite its very low cost, this type of tower is rarely used. It can be used when a rapid, irreversible chemical reaction occurs, as in neutralization of waste acids (W1).

Table 12.6-1. Typical Performance for Several Types of Commercial Extraction Towers
TypeCapacity of Combined Streams, (VD + VC), m3/m2 ° hApproximate Flooding, (VD + VC), m3/m2 ° hSpacing between Stages, T, cmOverall Height of Transfer Unit, HOL, mPlate Efficiency, EO, %Height of Equilibrium Stage, HETS, mRef.
Spray Tower15–75  3–6 3–6M4, S5
Packed Tower12–30  0.9–1.7 0.4–1.5S4, S5, W1
Structured Packing Tower65–90    0.5–1.6H4
Sieve-Tray Tower27–60 10–25 8–300.8–1.2M4, P4, S4
Pulsed Packed Tower17–2340   0.15–0.3P4, S4, W1
Pulsed Sieve Tray Tower25–35605.1  0.15–0.3S4, W1
Scheibel Tower10–14[*]402.5–20  0.1–0.3P4, S2, S3, W1
Karr Tower30–4080–1005–15  0.2–0.6S2, S4

[*] Throughput for diameter D1 = 7.6 cm. For larger towers of D2 diameter see Eq. (12.6-3).

12.6D. Packed Extraction Towers

A more effective type of tower is made by packing the column with random packing such as Raschig rings, Berl saddles, Pall rings, and so on, which cause the droplets to coalesce and redisperse at frequent intervals throughout the tower. The axial mixing is reduced considerably. A packed tower is more efficient than a spray tower, but back-mixing still occurs and the HETS (height equivalent to a theoretical stage) is generally greater than for the pulsed and mechanically agitated towers discussed later in Sections 12.6F and G.

Packed towers are used where only a few stages are needed (S3) and generally with low-interfacial-tension systems of about 10 dyn/cm or so (W1). When using random packings, it is preferable to choose a material that is preferentially wetted by the continuous phase. For example, stoneware Raschig rings or Berl saddles are used for water as the continuous phase and carbon rings or saddles for toluene as the continuous phase (T2). Packed towers more often use random packing and less often structured packing. Some typical performance values for packed towers are given in Table 12.6-1.

The few data available for structured packing have similar HETS values as packed or sieve-tray towers but give higher capacities (H4). The structured packings used for extraction are similar to those used for distillation.

In packed absorption towers, flooding occurs when the gas velocity is increased until the liquid cannot flow downward and is carried up by the gas out of the tower. In packed liquid-extraction towers, flooding occurs when increasing the dispersed or continuous flow rates causes both phases to leave at the outlet of the continuous phase.

A flooding correlation by Crawford and Wilke (C9) is given in Fig. 12.6-3, where VC and VD are superficial velocities of continuous and dispersed phases in ft/h, ρC and ρD are densities of continuous and dispersed phases in lbm/ft3, Δρis |ρC ¯ρD|, μc is viscosity of continuous phase in lbm/ft · h, a is specific surface area of packing in ft2/ft3 (Table 10.6-1), ε is void fraction of packed section (Table 10.6-1), and σ is interfacial tension between phases in lbm/h2. Note: 1.0 dyn/cm = 28 572 lbm/h2. It is recommended that the design flow rates be set at 50% of flooding due to uncertainties in the correlation.

Figure 12.6-3. Flooding correlation for packed extraction towers. [From J. W. Crawford and C. R. Wilke, Chem. Eng. Prog., 47, 423 (1951). With permission.]


EXAMPLE 12.6.1. Prediction of Flooding and Packed-Tower Diameter

Toluene as the dispersed phase is being used to extract diethylamine from a dilute water solution in a packed tower of 1-in. Pall rings at 26.7°C. The flow rate of toluene V = 84 ft3/h and of water solution L = 56 ft3/ · h. The physical properties of the dilute solutions are: for the aqueous continuous phase (C), ρC = 62.2 lbm/ft3, μC = 0.860 cp = 0.860(2.4191) = 2.080 lbm/fth; for the dispersed phase, ρD = 54.0 lbm/ft3. The interfacial tension σ = 25 dyn/cm (T2). Do as follows:

  1. Predict the flooding velocity.

  2. Using 50% of flooding, determine the tower diameter.

  3. If the separation requires 5.0 theoretical stages, calculate the tower height.

Solution: For part (a), from Table 10.6-1 for 1-in. Pall rings, the surface area a = 63 ft2/ft3 and ε = 0.94 void fraction. Also, σ = (25 dyn/cm)(28 572 lbm/h2)/(dyn/cm) = 714 300 lbm/h2. Then, for the ordinate in Fig. 12.6-3,


From Fig. 12.6-3, the abscissa value is 170. Hence,


Solving, . Also, since VD/VC = V/L = 84/56 = 1.5, the final result gives VD = 108.45 ft/h and VC = 72.30 ft/h for the flooding velocity.

For part (b), using 50% of flooding, VD = 54.2, VC = 36.15 ft/h, and VC + VD = 90.35 ft/h = 90.35/3.2808 = 27.54 m/h. This is still in the typical range given for packed towers in Table 12.6-1 of 12–30 m/h.

The tower cross-sectional area = L/VC = (56 ft3/h)/(36.15 ft/h) = 1.549 ft2. Then, πD2/4 = 1.549 and D = 1.404 ft (0.428 m).

For part (c), use the average HETS for packed towers from Table 12.6-1 of (0.4 + 1.5)/2, or 0.95 m (3.117 ft). Then the tower height is (HETS ft/stage) (number of stages) or 3.117(5.0) = 15.58 ft (4.75 m). Adding about 2 ft to the top and bottom for inlet nozzles and settling zones, the total height = 15.58 + 2 + 2 = 19.58 ft (5.97 m).


To use Fig. 12.6-3, the value of the interfacial tension σ is needed. This is a very important variable for extraction and can vary from about 5 to 50 dyn/cm. The interfacial tension between immiscible phases that must be settled must be sufficiently high for rapid coalescence. Also, high values of σ mean that often extra energy or agitation must be used in the tower for dispersion of one phase in the other. However, too low a value may result in too-slow coalescence or stable emulsions.

To estimate the interfacial tension of a two-phase system when experimental data are not available, Eq. (12.6-1) can be used for type one ternary systems, with an average deviation of about ±15% (T2):

Equation 12.6-1


where σ is the interfacial tension in dyn/cm, xAB is the mole fraction of solvent A in the saturated solvent-rich B layer, xBA is the mole fraction of B in the saturated solvent-rich A layer, xCA is the mole fraction of the distributed solute C in A, and xCB is the mole fraction of solute C in B. This equation holds only for a range of values for [xAB + xBA + (xCA + xCB)/2] between 0.0004 and 0.30. A value of 1.0 indicates complete solution of the phases at the plait point. The range of σ values is 4–52.5 dyn/cm.

EXAMPLE 12.6-2. Estimation of Interfacial Tension

Equilibrium data for the system water (A)–acetic acid (C)–methylisobutyl ketone (MIBK) (B) are as follows in wt %, where the acetic acid concentration is dilute (P4):

Water-rich (A) phaseMIBK-rich (B) phase
Water (A)Acid (B)MIBK (C)Water (A)Acid (C)MIBK (B)
98.4501.552.12097.88
95.462.851.72.801.8795.33

Estimate the interfacial tension for the data point containing some acetic acid.

Solution: The molecular weight for A is 18.02, for B is 100.16, and for C is 60.05. Converting 1.7 wt % (B) to mole fraction xBA in the (A)-rich layer,


Also, 2.85 wt % (C) in the (A)-rich layer becomes xCA = 0.00885. Similarly, 2.80 wt % (C) becomes xAB = 0.1365, and 1.87 wt % (C) becomes xCB = 0.02736.

Substituting into Eq. (12.6-1),


For the binary liquid system, water (A)–MIBK (B), the calculated mole fractions are xBA = 0.002825 and xAB = 0.1074. Then using Eq. (12.6-1), σ = 11.49 dyn/cm.


12.6E. Perforated-Plate (Sieve-Tray) Extraction Towers

Perforated plates or sieve trays are also used for dispersion of liquid drops and coalescence on each tray, as shown in Fig. 12.6-4. This is similar to the sieve trays described in Fig. 10.6-1a for distillation and absorption. The downcomers carry the heavier continuous liquid phase from one tray to the next. The light dispersed phase coalesces below the tray, jets up to the tray above, and then coalesces on the tray above. Overflow weirs are not used on downcomers (S3).

Figure 12.6-4. Perforated-plate or sieve-tray extraction tower.


The holes in the tray are 0.32–0.64 cm in diameter, and the % of open tray area is 15–25% of the column cross-sectional area. Tray spacings of 10–25 cm are used.

The following equation can be used to estimate the fractional tray efficiency Eo in a tray tower (P4):

Equation 12.6-2


where σ is interfacial tension in dyn/cm, T is tray spacing in ft, do is hole diameter in ft, and VD and VC are superficial velocities in ft/s. For systems with high interfacial tensions, heights of transfer units are relatively high and stage efficiencies low.

Some typical operating conditions in tray towers are given in Table 12.6-1. To scale-up towers from small to larger sizes, the same sum of superficial velocities VC + VD should be used (S3). A throughput design value of about 50% of flooding also should be used for all types of extraction towers.

EXAMPLE 12.6-3. Tray Efficiency for Perforated-Plate Tower

Acetic acid is being extracted from water by the solvent methylisobutyl ketone in a perforated-plate tower at 25°C. The flow rate of the continuous aqueous phase is 120 ft3/h and that of the dispersed solvent phase is 240 ft3/h. The interfacial tension is 9.1 dyn/cm. The tray spacing is 1.0 ft and the hole size on the tray is 0.25 in. Estimate the fraction tray efficiency Eo.

Solution: VD/VC = 240/120, σ = 9.1 dyn/cm, T = 1.0 ft, and do = 0.25/12 = 0.02083 ft. Substituting into Eq. (12.6-2),



12.6F. Pulsed Packed and Sieve-Tray Towers

There are many types of towers that are mechanically agitated to increase the mass-transfer efficiency and/or the throughput. An ordinary packed tower or one with special sieve plates can be pulsed by applying a rapid reciprocating motion of relatively short amplitude to the liquid contents. A reciprocating plunger pump, bellows pump, or high-pressure air pulse is externally connected to the space containing the continuous fluid so that the entire contents of the tower move up or down. Continuous inlet flows of continuous and dispersed phases enter and exit the tower.

1. Pulsed packed towers

Pulsing packed towers reduces the HETS considerably, by about a factor of 2 or so. Pulsing is also useful in handling liquids with high interfacial tensions, up to 30–40 dyn/cm (W1). Typical values for HETS of 0.15–0.3 m are given in Table 12.6-1. Since pulsing is uniform across the cross section, scale-up of tower size can be accomplished by using the same value of VD + VC (W1).

2. Pulsed sieve-tray towers

Typical amplitudes used are from 0.6 to 2.5 cm and frequencies from 100 to 250 cycles/min (P4). Typical hole size is 0.32 cm diameter, with 20–25% free space on the tray and 5.1 cm (2 in.) tray spacing. The trays occupy the entire cross section of the tower and there are no downspouts. During upward pulsing, the light liquid is forced through the holes and droplets rise to the tray above. During downward pulsing, the heavy liquid behaves in a similar manner. Typical operating values are given in Table 12.6-1. Pulsing the tower markedly reduces the HETS.

12.6G. Mechanically Agitated Extraction Towers

There are many types of mechanically agitated towers, two of the most important of which are the Scheibel tower shown in Fig. 12.6-5a and the Karr tower shown in Fig. 12.6-5b.

Figure 12.6-5. Mechanically agitated extraction towers: (a) Scheibel rotating-agitator tower, (b) Karr reciprocating-plate tower.


1. Scheibel tower

In the Scheibel tower, a series of rotating turbine agitators form dispersions which coalesce in passing through the knitted mesh. The mixture then passes through the outer settling zone. The tower thus operates as a series of mixer–settler extraction units.

The tower operates with high efficiencies, as shown in Table 12.6-1. In scaling up from a total combined flow rate Q1 for both phases in m3/s to Q2, the diameter D2 is approximately related to D1 by (L4, S2)

Equation 12.6-3


Also, in scaling up from diameter D1 to D2, the HETS varies as follows:

Equation 12.6-4


Equations for scale-up of the column internals are available (L4, S6).

2. Karr reciprocating-plate tower

As shown in Fig. 12.6-5b for the Karr column, the perforated trays are moved up and down to increase agitation and to pulse the liquids. This results in a more uniform drop-size distribution because the shear forces are more uniform over the tower cross section (K4, P4).

Typical parameters are an amplitude of plate movement of 2.5 cm (1 in.), 100–150 strokes/min, and plate spacing of 5–15 cm. Trays contain holes 1.4 cm in diameter and open space of 50–60% (W1). Scale-up procedures for the Karr column are relatively accurate. In scaling up a small tower with diameter D1 to a larger size D2, the total throughput per unit area (VC + VD), plate spacing, and amplitude are kept constant. Then the HETS in m and the strokes per minute (SPM) are scaled up by (K4, P4)

Equation 12.6-5


Equation 12.6-6


Typical performance values are given in Table 12.6-1.

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