A centrifugal extractor multiplies the force of gravity acting on two liq- uid phases. Centrifugal extractors can facilitate a liquid-liquid extrac- tion process by reducing diffusion path lengths and increasing the driving force for liquid-liquid phase separation. They can achieve very high specific throughput with very low liquid residence time. A wide variety of machine types are available, ranging from relatively simple devices used primarily for phase separation or for single-stage liquid- liquid contacting with separation to more complex machines designed to provide the equivalent of multistage liquid-liquid contacting within a single unit. Some machines are designed to handle feeds containing solids such as whole fermentation broth. This section provides a brief overview with a description of several machines for illustration. More detailed descriptions of centrifuge design and performance are avail- able from equipment vendors. For additional discussion, see Janoske and Piesche, Chem. Eng. Technol., 22(3), pp. 213–216 (1999); Leonard, Chamberlain, and Conner, Sep. Sci. Tech., 32(1–4), pp. 193–210 (1997); Blass, Chap. 14 in Liquid-Liquid Extraction Equip- ment, Godfrey and Slater, eds. (Wiley, 1994); Schügerl, Solvent Extrac- tion in Biotechnology (Springer-Verlag, 1994); Otillinger and Blass, “Mass Transfer in Centrifugal Extractors,” Chem. Eng. Technol., 11, pp. 312–320 (1988); and Hafez, Chap. 15 in Handbook of Solvent Extraction, Lo, Baird, and Hanson, eds. (Wiley, 1983; Krieger, 1991).
Centrifugal extractors can be beneficial when the liquid density dif- ference is small, when short contact time is needed to avoid product degradation, when feed and solvent easily emulsify, or in cases where high specific throughput is needed due to limitations in available floor space or ceiling height. Centrifugal extractors also can provide flexi- bility in operation in cases where feed variability is high, by allowing adjustment of feed rate and rotational speed as needed to obtain sat- isfactory performance. Potential disadvantages generally derive from difficulties associated with maintaining high-speed rotating machin- ery, relatively high purchase prices compared to those of some other types of extractors, and limitations as to the number of theoretical stages that can be achieved per machine (generally< 1 or up to 5 or 6
theoretical stages depending upon throughput and the type of machine). Another consideration for some machines with close inter- nal clearances is the potential for plugging if any solids are present in the feed; however, as noted above, some machines are specifically designed to handle and discharge solids.
Commercial-scale centrifuges almost always are continuously fed machines, unless the scale of the operation is very low, as in some low- volume bioprocessing operations where very-high-g operation and long processing times are needed. A continuously fed centrifugal extractor can deliver high multiples of g, but at much lower residence time (given by holdup volume of the feed phase divided by volumetric feed rate) compared to a batch process. The maximum hydraulic capacity (or nominal capacity) of a continuously operated machine often is not realized in commercial applications, because the feed rate needs to be turned down in order to have sufficient residence time for good extraction and phase separation performance.
In evaluating options, it generally is not possible to accurately pre- dict performance because of the complexity of the hydrodynamics within a centrifuge. While high-g operation can promote good perfor- mance, in certain cases the extremely rapid acceleration generated within the machine also can promote backmixing or emulsification. Miniplant tests using small units generally are needed, and vendors often offer testing services.
Single-Stage Centrifugal Extractors The types of centrifuges used in extraction operations are quite varied. Differences include vertical versus horizontal configuration, fluid-filled versus operation with an air core, pressurized or unpressurized operation, generation of low to extremely high multiples of gravitational acceleration (500 up to 20,000× g or higher), as well as differences in the liquid holdup volume, design of internals, internal clearances, and purchase price. The simpler machines, such as the CINC separator from CINC Pro- cessing Equipment, Inc. (Fig. 15-59) and the Rousselet-Robatel model BXP, have relatively large internal clearances. An air core is maintained within the machine, and liquid layers decant over internal weirs. Flow restrictions in the overflow piping need to be minimized to avoid any pressure imbalance between light- and heavy-liquid overflow lines, since this can affect the location of the liquid-liquid interface and the liquid overflow/underflow split. These machines often are used for washing operations and other extraction applica- tions with high K values requiring few theoretical stages. They often serve as the separator in a mixer-settler stage, such that solvent and Untreated Hydrocarbon In Treated Clear Hydrocarbon Out Treating Solution In Treating Solution Out FIBER-FILMTM Contactor
feed are first mixed in a static mixer or a separate vessel before being fed to the centrifuge. Some mixing occurs within the centrifuge itself; so if the extraction is sufficiently fast, solvent and feed might be fed directly to the centrifuge to accomplish both mixing and phase sepa- ration. Multiple units can be connected in a countercurrent mixer- settler cascade if needed. Processes with 5 to 7 units are typical, while processes with as many as 50 units have been reported. Multiple-unit mixer-settler processes utilizing centrifuges at each stage generally involve production of high-value, low-volume products. Stacked-disk types of machines also are available from numerous vendors and may be used in a similar extraction scheme (generally requiring some type of mixer in the feed line). These machines contain an internal stack of conical disks with a small gap between disks on the order of mil- limeters [Janoske and Piesche, Chem. Eng. Technol., 22(3), pp. 213–216 (1999); and Mannweiler and Hoare, Bioproc. Biosystems Eng., 8(1–2), pp. 19–25 (1992)]. Stacked-disk machines can be thought of as inclined-plate or lamella-type decanters operating in a centrifugal field (see “Liquid-Liquid Phase Separation Equip- ment”). They magnify the separation power by greatly reducing the distance the dispersed phase must travel before coalescing at a sur- face, at the expense of somewhat higher complexity and closer inter- nal clearances.
Figure 15-59 shows a cutaway drawing of a CINC separator show- ing an outer annular space where solvent and feed mix before enter- ing the interior of a rotating drum. Although this type of machine is not designed to separate solids from feeds, a clean-in-place option is offered to facilitate periodic removal of solids that accumulate in the internals. In applications in which one or more of the feed liquids is somewhat viscous, special consideration must be given to the design
of the centrifuge internals such that pressure drop through the machine is not excessive. In certain applications, feed with viscosities as high as several hundred centipoise may be handled; however, spe- cial modifications to the internals are needed, and throughput must be reduced compared to that in typical operation. Maximum or nom- inal volumetric flow capacities for CINC machines range from 110 L/h to 136 m3/h (0.5 to 600 gal/min) depending upon the size of the unit. The Rousselet-Robatel design is somewhat similar. These machines range in size from 50 L/h up to 80 m3/h (0.2 to 350 gal/min). They are designed to generate only moderate centrifugal force and are generally limited to applications requiring no more than about 25,000g⋅s (maximum g acceleration times the liquid residence time based on total volumetric flow rate and liquid holdup in the machine). The CENTREK single-stage extractor from MEAB consists of a funnel-shaped centrifugal-bowl centrifuge mounted above a mixing tank containing a submerged stirrer. An internal “hydrolock” is used to control the position of the liquid-liquid interface in the bowl. Accord- ing to the manufacturer, this is especially important for multistage, cascade operation. The unit can tolerate some amount of solids in the feed and is available in nominal capacities of 20 L/h to 20 m3/h (0.1 to 90 gal/min).
Centrifugal Extractors Designed for Multistage Perfor- mance At the other end of the spectrum are the more complex machines designed to provide multistage or differential liquid-liquid contacting and separation within a single unit. Some machines pro- mote formation of very thin films for efficient liquid-liquid contacting and separation. Others provide multiple zones for mixing and separating the phases. All are designed with complex internals and close clear- ances. These machines typically achieve 2 to 5 theoretical stages FIG. 15-59 CINC centrifugal separator. (Courtesy of CINC Processing Equipment, Inc.)
depending upon operating conditions, with some authors claiming as many as 7 or 8 stages.
The classic machine of this type is the Podbielniak extractor avail- able from Baker-Perkins (Fig. 15-60). The body of the extractor is a horizontal cylindrical drum containing concentric perforated cylin- ders. The liquids are introduced through the horizontal rotating shaft with the help of special mechanical seals; the light liquid is fed internally to the drum periphery and the heavy liquid to the axis of the drum. Rapid rotation (up to several thousand revolutions per minute, depending on size) causes radial counterflow of the liquids, which then flow out through the shaft. Materials of construction include steel, stainless steel, Hastelloy, and other corrosion-resistant alloys. The Podbielniak design provides extremely low holdup of liq- uid per stage, and this led to its extensive use in the extraction of antibiotics, such as penicillin and the like, for which multistage extraction and phase separation must be done rapidly to avoid chem- ical destruction of the product under conditions of extraction
[Podbielniak, Kaiser, and Ziegenhorn, Chap. VI in Chemical Engi- neering Progress Symposium Series No. 100, vol. 66, pp. 43–50 (1970)]. Podbielniak extractors have been used in all phases of phar- maceutical manufacturing, in petroleum processing (both solvent refining and acid treating), in extraction of uranium from ore leach liquors, and for clarification and phase separation work. Jacobsen and Beyer [AIChE J., 2(3), pp. 283–289 (1956)] describe operating characteristics and the number of theoretical stages achieved for a specific application.
The Quadronics (Liquid Dynamics) extractor is a horizontally rotated device, a variant of the Podbielniak extractor, in which either fixed or adjustable orifices may be inserted radially as a package. These permit control of the mixing intensity as the liquids pass radially through the extractor. Flow capacities, depending on machine size, range from 0.34 to 340 m3/h (1.5 to 1500 gal/min).
The Luwesta (Centriwesta) extractor is a development from Coutor [Eisenlohr, Ind. Chem., 27, p. 271 (1951)]. This centrifuge revolves about a vertical axis and contains three actual stages. It operates at 3800 rotations per minute and handles approximately 5 m3/h (1300 gal/h) total liquid flow at 12-kW power requirement. Provision is made in the machine for the accumulation of solids separated from the liquids, for periodic removal. It is used, more extensively in Europe than in the United States, for the extraction of acetic acid, pharmaceuticals, and similar products.
The de Laval extractor contains a number of perforated cylinders revolving about a vertical shaft [Palmqvist and Beskow, U.S. Patent 3,108,953 (1959)]. The liquids follow a spiral path about 25 m (82 ft) long, in countercurrent fashion radially, and mix when passing through the perforations. There are no published performance data.
The Rousselet-Robatel LX multistage centrifugal extractor is designed with up to 7 internal mixing/separation stages. Each stage consists of a mixing chamber where the two phases are mixed by means of a stationary agitation disk mounted on a central drum. The high relative speed between the stationary disk and the rotating walls of the mixing chamber creates a liquid-liquid dispersion with high interfacial area to facilitate rapid mass transfer. The agitation disk and the mixing chamber’s inlet and outlet channels form a pump which draws the two phases from the adjacent stages and transfers the dis- persion to a settling chamber, where it is separated by centrifugal force. The manufacturer claims that high stage efficiencies can be achieved. Extract and raffinate phases are removed from the machine by gravity discharge, or an internal centripetal pump can be employed to discharge these streams under pressure. Nominal flow rates range from 25 L/h up to 80 m3/h.
FIG. 15-60 Podbielniak centrifugal extractor. (Courtesy of Baker Perkins, Inc.)