Process Technology: An Introduction - Haan A.B. 2015

7 Extraction and leaching
7.2 Industrial liquid-liquid extractors

Extractors are usually classified according to the methods applied for interdispersing the phases and producing the countercurrent flow pattern. To maximize the dispersion of one phase in the other, and to minimize back mixing, extractors are equipped with trays, packings, or mechanical moving internals. The location of the principal interface depends upon which phase is dispersed. When the light phase is dispersed, the interface is located at the top of the extractor. When the heavy phase is dispersed, the interface is located at the bottom. The solvent can be the heavy or light phase, and dispersed or continuous. Usually the phase that is fed at the highest rate is the dispersed phase.

7.2.1 Mixer-settlers

As shown in Fig. 7.5, mixer-settler systems involve a mixing vessel for phase dispersion that is followed by a settling vessel for phase separation. They are widely used in the chemical process industry because of reliability, flexibility, and high capacity. Although any number of mixer-settler units may be connected together, these extractors are particularly economical for operations requiring high throughput and few stages. Dispersion can be achieved by pump circulation, air agitation, or mechanical stirring. Intense agitation in the dispersion vessel leads to high rates of mass transfer and close approach to equilibrium. However, because the resulting dispersion can be difficult to separate, designs of mixer-settler systems must be carefully balanced between dispersion intensity and time of settling. Scale-up and design of mixer-settlers is relatively reliable, because they are practically free of interstage back mixing and stage efficiencies are high. The main disadvantages of mixer-settlers are high capital cost per stage and large inventory of material in the vessels. In large industrial units, the settlers usually represent at least 75 % of the total volume.

Fig. 7.5: Mixer-settler.

7.2.2 Mechanically agitated columns

Many modern differential contactors employ rotary agitation for phase dispersion. The best known commercial rotary agitated contactors are shown in Fig. 7.6. The Scheibel column is designed to simulate a series of mixer-settler extraction units. An impeller agitates every alternate compartment with selfcontained mesh-type coalescers between each contacting stage. The rotating-disk contactor (RDC) uses the shearing action of a rapidly rotating disk to interdisperse the phases. RDC’s have been used widely throughout the world for propane deasphalting, sulfolane extraction for aromatics/alifatics separation and caprolactam purification. The Oldshue—Rushton column consists essentially of a number of compartments separated by horizontal stator-ring baffles. Kuhni contactors are similar to the Scheibel columns and have gained considerable commericial application. A baffled turbine impeller promotes radial discharge within a compartment. The principal features are the use of a shrouded impeller to promote radial discharge within the compartments.

Fig. 7.6: Mechanically agitated columns: (a) Scheibel column; (b) rotating disk contactor; (c) asymmetric rotating disk contactor; (d) Oldshue—Rushton multiple-mixer column; (e) Kuhni column; (f) Karr reciprocating plate column.

A more energy-efficient way to obtain phase dispersion in a column is reciprocating or vibrating plates in a column. Reciprocation of plates requires less energy and has the same effect in terms of mixing patterns and uniform dispersion. The difference between the different reciprocating-plate columns that have been built for industrial use lies in the plate design. The open-type Karr column is the best known. This type of column has gained increasing industrial application in the petrochemical, pharmaceutical, and hydrometallurgical industries.

7.2.3 Unagitated and pulsed columns

Despite their low efficiency, unagitated columns are widely used in industry because of their simplicity and low cost. They are particularly suited for processes requiring few theoretical stages and for corrosive systems where the absence of mechanical moving parts is advantageous. The three main types of unagitated column extractors are shown in Fig. 7.7. Spray columns are the simplest in construction but suffer from very low efficiency because of poor phase contacting and excessive back mixing in the continuous phase. They generally provide no more than the equivalent of one or two equilibrium stages and are typically used for basic operations, such as washing and neutralization.

Fig. 7.7: Unagitated column extractors: (a) spray column; (b) packed column; (c) perforated plate column.

Packed columns have better efficiency because of improved contacting and reduced back mixing. The used packings in extraction are similar to the ones used in distillation and include random and structured packings. It is important that the packing material should be wetted by the continuous phase to avoid coalescence of the dispersed phase. The main functions of the packing elements are to reduce back mixing in the continuous phase and promote mass transfer due to jostling and breakup of dispersed phase drops. Because the packing elements reduce the cross-sectional area for flow and decrease the velocity of the dispersed phase, the column diameter for a given rate will always be greater than for a spray tower. However, a packed column is commonly preferred over a spray column because the reduced flow capacity is less important than the improved mass transfer.

The sieve tray extractor resembles sieve tray distillation. If the light phase is dispersed, the light liquid flows through the perforations of each plate and is dispersed into drops, which rise through the continuous phase. The continuous phase flows horizontally across each plate and passes to the plate beneath through a downcomer. If the heavy phase is dispersed, the column is reversed and upcomers are used for the continuous phase. Perforated-plate columns are operated semistagewise and are reasonably flexible and efficient.

An increased efficiency of sieve-plate and packed extraction columns is obtained by applying a sinusoidal pulsation to the contents of the column. The well-distributed turbulence promotes dispersion and mass transfer while tending to reduce axial dispersion in comparison with the unpulsed column. A pulsed-plate column is fitted with horizontal perforated plates that occupy the entire cross section of the column. The total free area of the plate is about 20—25 %. Pulsed-packed columns (Fig. 7.8) contain random or structured packing. The light and dense liquids passing counter currently through the columns are acted on by pulsations transmitted hydraulically to form a dispersion of drops. The pulsation device is connected to the side of the column, usually at the base, through a pulse leg. Typical operating conditions are frequencies of 1.5—4 Hz with amplitudes of 0.6—2.5 cm.

Fig. 7.8: Pulsed packed column.

Fig. 7.9: Podbielniak centrifugal extractor. Adapted from [86].

7.2.4 Centrifugal extractors

In centrifugal extractors centrifugal forces are applied to reduce the contact time between the phases and accelerate phase separation. The units are compact, and a relatively high throughput per unit volume can be achieved. Centrifugal extractors are particularly useful for chemically unstable systems such as the extraction of antibiotics or for systems in which the phases are slow to settle. Advantages include short contact time for unstable materials, low space requirement, and easy handling of emulsifed materials or fluids with small density differences. The disadvantages are complexity and high capital and operating costs. Centrifugal extractors have been widely used in the pharmaceutical industry and are increasingly used in other fields.

Fig. 7.9 shows a schematic of the first differential centrifugal extractor used in industry. The Podbielniak extractor can be regarded as a perforated-plate column wrapped around a rotor shaft. This extractor consists of a drum rotating around a shaft equipped with annular passages at each end for feed and raffinate. The light phase is introduced under pressure through the shaft and then routed to the periphery of the drum. The heavy phase is also fed through the shaft but is channeled to the center of the drum. Centrifugal forces acting on the phase-density difference causes dispersion as the phases are forced through the perforations. Other manufacturers of centrifugal extractors are Robatel, Westfalia, Alfa-Laval, and Cinc.

7.2.5 Selection of an extractor

The large number of extractors available and the number of design variables complicate selection of a contactor for a specific application. Some important criteria that should be taken into consideration during contactor selection are:

· — stability and residence time;

· — settling characteristics of the solvent system;

· — number of stages required;

· — capital cost and maintenance;

· — available space and building height;

· — throughput;

· — experience with the type of extractor.

The preliminary choice of an extractor for a specific process is primarily based on consideration of the system properties and number of stages required for the extraction. Choosing a contactor is still both an art and a science. Although cost ought to be a major balancing consideration, in many actual cases experience and practice are the deciding factors. Mechanically agitated devices show some efficiency advantages, but whether these advantages come at the expense of higher cost must be evaluated by designing for the specific system at hand.

Tab. 7.2: Some industrial solid-liquid extraction processes.