Equipment
The liquid-liquid extraction equipment may be classified, according to the construction and operational characteristics into two groups: stagewise contactors and differential contactors. In the former type of contactors there are discrete number of stages in which two phases are equilibrated and then separated either by gravity (mixer-settler and columns) or by disk stack centrifuges. In the differential contactors the composition of phases changes continuously (29), and two types can be considered: column contactors where separation is achieved by gravity, and centrifugal contactors. The continuous countercurrent flow is maintained owing to the difference in the density of the phases (top and bottom), and either the gravity force (vertical columns) or centrifugal force (centrifugal extractors).
2.2.1. Stagewise Contactors
1. Mixers and Mixer-Settlers: A mixer-settler unit consists of two cylindrical tanks, one provided with a centrally located paddle agitator for mixing the phases and the other for settling the mixed phases (see Notes 4 and 5). The operation may be carried out in batch fashion or with continuous flow. For batch operations, the same vessel will serve the purpose for both mixing and settling, whereas for continuous mode, separate vessels are usually used. The mixer-settler units may be arranged in continuous countercurrent mode, either vertically (cascade) or horizontally (batteries) (Fig. 1A-C).
2. Mixer-Disk Stack Centrifuges: When the separation time under gravity is too long, commercially available separators can be used to speed up separation. Diskstack separators are used in a continuous mode for highly efficient separation, and a countercurrent installation of several units can be used to achieve high yields.
The disk-stack separator consists of a rotating bowl with a double-pump discharge for the light and heavy phases. The liquid mixture is fed through the stationary feed pipe into the rotating bowl. It enters the disk stack through rising channels in the disks and is separated between the narrow disks (Fig. 2) (31).
3. Column Contactors a. Perforated Plate Column: Perforated plate (sieve plate) columns are essentially a stack of several small spray columns (usually the diameter of each stage is greater than its height) (Fig. 3). Plates separate the stages with large numbers of small holes either drilled or punched on them.
b. Perforated Rotating Disc Contactor (PRDC): The PRDC consists of a cylindrical vessel containing perforated disks mounted on a central rotating shaft (Fig. 4) (see Note 6).
c. Kuhni Column: This extraction column consists of a cylindrical vessel with a central rotating shaft carrying closed impellers. A perforated plate is located between each stage to control the droplet holdup (Fig. 5) (see Note 6).
d. York-Scheibel Column: This type of extractor consists of a cylindrical vessel with alternate packed and unpacked sections serving as settling and mixing chambers. A centrally mounted rotating shaft carries a number of equally spaced turbine blade impellers rotating within the unpacked sections (Fig. 6). The packed settling chambers are normally filled with York mesh packing having about 90% voids.
2.2.2. Differential Contactors
1. Column Contactors a. Spray Column: Spray columns are the simplest of liquid-liquid extractors involving dispersion of one of the liquids. These vertical columns comprise an empty shell where the light and the heavy liquids are introduced and removed as shown schematically in Fig. 7. The dispersed phase is distributed with the help of a orifice or a nozzle. If the dispersed phase is the light, it is distributed at the bottom, while the heavy phase is distributed at the top of the column.
b. Packed Column: Packed columns are essentially a stack of packings arranged regularly or irregularly in vertical towers over a perforated support, as shown in Fig. 8. These columns are equipped with suitable liquid distributors and
- Fig. 1. Schematic representation of (A) mixer-settler unit, (B) horizontal battery, and (C) vertical cascade.
have grids for packing supports. The packings commonly used are Raschig and Lessing rings, and Intalox and Berl saddles, each possessing specific
- Fig. 2. Schematic representation of a disk stack contactor (31). Continuous recovery of intracellular products by crosscurrent centrifugal extraction using PEG/salt systems has been used (2,5).
Fig. 3. Schematic representation of a Perforated Plate Column (29).
Fig. 3. Schematic representation of a Perforated Plate Column (29).
advantages with respect to cost, surface availability, interface regeneration, pressure drop, weight, and corrosion resistance (30).
- light phase outlet
perforated disks perforated disks e 0
light phase inlet e 0
light phase inlet
heavy phase outlet
Fig. 4. Schematic representation of PRDC. The PRDC was used to extract a recombinant protein, cytochrome b5, from disrupted Escherichia coli cells with ATPS of PEG/salt (38).
c. Horizontal Rotary Contactors: The Graesser contactor consists of a horizontal cylinder in which is mounted a rotor disk assembly on a central shaft along the cylindrical axis. The disk comprises several fixed semicircular cups or buckets around the circle diameter. In operation the interface level is kept along the axis of the shaft (see Note 7).
2. Centrifugal Contactors a. Podbielniak extractor: Consists of an internal cylindrical rotor. Typically, concentric cylinders (perforated or not) are wound inside the rotor, rotating about a horizontal shaft. The centrifugal force created makes the heavy liquid flow countercurrently to the light liquid in the direction of the periphery of the spiral, where it is collected out of the machine. Conversely, the light liquid flows to the center and is removed out (Fig. 9).
b. Centrifugal liquid-liquid chromatography: This system operates in continuous mode with a liquid mobile phase and a liquid stationary phase; the latter remains inside the column by the interaction of the special internal geometry of the apparatus and the centrifugal field.
One type of apparatus design consists of a series of cartridge composed by several fine channels, which are connected in series, and are submitted to a centrifugal field. These devices are called centrifugal partition chromatogra-
rotating shaft light phase heavy phase inlet light phase inlet light phase heavy phase inlet light phase inlet
turbine impeller heavy phase outlet
Fig. 5. Schematic representation of a Kuhni column (39). This type of column has been used by Hustedt et al. (32) for enzyme purification using ATPS.
turbine impeller heavy phase outlet
Fig. 5. Schematic representation of a Kuhni column (39). This type of column has been used by Hustedt et al. (32) for enzyme purification using ATPS.
phy, centrifugal droplet countercurrent chromatography; or Sanki from its fabricate name.
A second type of design is denominated coil planet centrifuge chromatography. It consists of a column made of teflon tube helical coiled in one or several layers and wrapped around a cylinder. The tube is rotating at a certain angular speed around its own axis. The assembly, column plus cylinder, is also rotating at the same angular velocity and in the same direction around the main axis of the system (synchronous movement). In cross-axis coil planet centrifuge, both the rotation axis are positioned in a perpendicular form (Fig. 10A,B).
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