MEMBRANE-BASED PROCESSES
Polymer Membranes Extraction processes employing polymer membranes are sometimes referred to as nondispersive or pertraction operations. The use of membranes in extraction offers a number of potential advantages including (1) constant well-defined mass-transfer area; (2) the ability to operate at very low solvent-to-feed ratios inde- pendent of other operating variables; (3) very low holdup of solvent and product within the extractor, thus providing low residence time similar to a centrifugal extractor; (4) dispersion-free liquid-liquid con- tacting that eliminates the need for liquid-liquid interface control and phase separation; (5) no requirement for a difference in density between liquid phases; and (6) linear scale-up by addition of extra modules, so performance at large scale can be determined directly from small-scale tests using a single module. This last point suggests, however, that the economy of scale may not be as large as it is for extractors that are scaled up as a single larger unit.
The most important advantages that membranes can offer to the process designer are those that overcome an inherent limitation of another type of extractor, as in the ability to handle liquids with close or even equal densities and the ability to operate at extremely low solvent- to-feed ratios. Thus, the types of applications where membranes are likely to be most attractive include applications with close densities and/or a K value greater than 50 or so. In principle, K> 50 would allow operation using a solvent-to-feed ratio of 1 : 25 or less (for an extraction factor of 2), something that can be difficult to accomplish by using con- ventional extractors. To take full advantage, the feed would have to be sufficiently dilute that the loading capacity of the solvent is not exceeded. The primary disadvantages of membrane-based extractors are the added mass-transfer resistance across the membrane, limited fiber-side or tube-side throughput, and concerns about fouling and limited mem- brane life in industrial service. Applications are limited to feeds that are free of solid particles (or can be cost effectively prefiltered); otherwise, the membranes are easily fouled. The useful life of a membrane module also is a critical factor since the frequency with which membrane mod- ules must be replaced has a dramatic impact on overall cost.
The use of nonporous polymer membranes for liquid-liquid extrac- tion suffers from very slow permeation of solute through the mem- brane, although this approach has been developed for a special case
involving reaction-enhanced extraction of an aromatic acid from wastewater through a nonporous silicone membrane into a caustic solution [Ferreira et al., Desalination, 148(1–3), pp. 267–273 (2002)]. For most liquid-liquid extraction applications, however, a porous membrane is used and extraction involves transfer through a liquid- liquid meniscus maintained within the pores. One of the most promis- ing contactors for this type of extraction is the microporous hollow-fiber (MHF) contactor (Fig. 15-69). The MHF contactor resembles a shell-and-tube heat exchanger in which the tube walls are porous and are capable of immobilizing a liquid-liquid interface within the pores. For a hydrophobic polymeric membrane, the aque- ous phase normally is fed to the interior of the fiber (the fiber-bore side), while the organic phase is fed to the shell side. In this configu- ration, the aqueous fluid is maintained at a higher pressure relative to the organic phase, to immobilize the liquid-liquid interface within each pore. Care must be taken to avoid too high an aqueous pressure, or else breakthrough of the aqueous phase can occur. This break- through pressure is a function of the interfacial tension and pore size. Earlier versions of MHF contactors provided a parallel-flow design, but this design suffered from shell-side bypassing [Seibert et al., Sep. Sci. Technol., 28(1–3), p. 343 (1993)]. An improved design that incor- porates a central baffle and uniform fiber spacing is currently available (Fig. 15-69). The dimensions are listed in Table 15-25.
In the baffled design, the shell-side fluid is fed through a central perforated distributor. It flows radially through the fiber bundle, around a baffle located in the middle of the module, and leaves the module through the central distributor. As in conventional extraction, the mass transfer of solute occurs across a liquid-liquid interface. However, unlike in conventional extraction, the interface is main- tained at micrometer-size pores, and three mass-transfer resistances are present: tube-side (kt), shell-side (ks), and pore or membrane-side
(km). The overall mass-transfer coefficient based on the tube-side liq-
uid kotis given by
= + + (15-192)
where mvolis the local slope of the equilibrium line for the solute of interest, with the equilibrium concentration of solute in the tube-side
1 km mvol ks 1 kt 1 kot
FIG. 15-69 Schematic of the Liqui-Cel Membrane Contactor. (Courtesy of Membrana-Charlotte. Liqui-Cel is a registered trademark of Membrana-
liquid plotted on the y axis and the equilibrium concentration of solute in the shell-side liquid plotted on the x axis. Equation (15-192) assumes the tube-side fluid wets the pores.
The mass-transfer efficiencies of various MHF contactors have been studied by many researchers. Dahuron and Cussler [AIChE J., 34(1), pp. 130–136 (1988)] developed a membrane mass-transfer coefficient model (km); Yang and Cussler [AIChE J., 32(11), pp.
1910–1916 (1986)] developed a shell-side mass-transfer coefficient model (ks) for flow directed radially into the fibers; and Prasad and
Sirkar [AIChE J., 34(2), pp. 177–188 (1988)] developed a tube-side mass-transfer coefficient model (kt). Additional studies have been
published by Prasad and Sirkar [“Membrane-Based Solvent Extrac- tion,” in Membrane Handbook, Ho and Sirkar, eds. (Chapman & Hall, 1992)]; by Reed, Semmens, and Cussler [“Membrane Contactors,” Membrane Separations Technology: Principles and Applications, Noble and Stern, eds. (Elsevier, 1995)]; by Qin and Cabral [AIChE J., 43(8), pp. 1975–1988 (1997)]; by Baudot, Floury, and Smorenburg [AIChE J., 47(8), pp. 1780–1793 (2001)]; by González-Muñoz et al. [J. Membane Sci., 213(1–2), pp. 181–193 (2003) and J. Membrane Sci., 255(1–2), pp. 133–140 (2005)]; by Saikia, Dutta, and Dass [J. Membrane Sci., 225(1–2), pp. 1–13 (2003)]; by Bocquet et al. [AIChE J., 51(4), pp. 1067–1079 (2005)]; and by Schlosser, Kertesz, and Mar- tak [Sep. Purif. Technol., 41, p. 237 (2005)]. A review of mass-transfer correlations for hollow-fiber membrane modules is given by Liang and Long [Ind. Eng. Chem. Res., 44(20), pp. 7835–7843 (2005)]. Eksangsri, Habaki, and Kawasaki [Sep. Purif. Technol., 46, pp. 63–71 (2005)] discuss the effect of hydrophobic versus hydrophilic mem- branes for a specific application involving transfer of solute from an aqueous feed to an organic solvent. Karabelas and Asimakopoulou [J. Membrane Sci., 272(1–2), pp. 78–92 (2006)] discuss process and equipment design considerations.
In general, researchers have treated MHF contactors as differen- tial contacting devices. However, Seibert and Fair [Sep. Sci. Tech- nol., 32(1–4), pp. 573–583 (1997)] and Seibert et al. [ISEC ‘96 Proc., 2, p. 1137 (1996)] suggest that the baffled MHF contactor can be treated as a staged countercurrent contactor. Their recommenda- tions are based on studies using a commercial-scale skid-mounted extraction system. Their semi-work-scale study demonstrated the performance advantages of the MHF contactor relative to a column filled with structured packing for a system with a high partition ratio. Seibert et al. [ISEC ‘96 Proc., 2, p. 1137 (1996)] also provide limited economic data for the extraction of n-hexanol from water by using n- octanol. Also see the discussion by Yeh [J. Membrane Sci., 269(1–2), pp. 133–141 (2006)] regarding the use of internal reflux in a cross- flow membrane configuration to boost liquid velocities for enhanced performance.
Liquid Membranes Emulsion liquid-membrane (ELM) extrac- tion involves intentional formation of an emulsion between two immiscible liquid phases followed by suspension of the emulsion in a third liquid that forms an outer continuous phase. The encapsulated liquid and the continuous phase are miscible. The liquid-membrane phase is immiscible with the other phases and normally must be stabi-
lized by using surfactants. If the continuous phase is aqueous, the sus- pended phase is a water-in-oil emulsion. If the continuous phase is organic, the emulsion is the oil-in-water type. This technology differs from traditional liquid-liquid extraction processes in that it allows transfer of solute between miscible liquids by introducing an immisci- ble liquid membrane between them. A typical process involves first forming a stable emulsion and contacting it with the continuous phase to transfer solute between the encapsulated phase and the continuous phase, followed by steps for separating the emulsion and continuous phases and breaking the emulsion. The emulsion must be sufficiently stable to remain intact during processing, but not so stable that it can- not be broken after processing, and this may present a challenge for commercial implementation. The technology is described by Franken- feld and Li [Chap. 19 in Handbook of Separation Process Technology, Rousseau, ed. (Wiley, 1987)].
Potential applications of ELM extraction include separation of aromatic and aliphatic hydrocarbons [Chakraborty and Bart, Chem. Eng. Technol., 28(12), pp. 1518–1524 (2005)], separation and con- centration of amino acids [Thien, Hatton, and Wang, Biotech. and Bioeng., 32(5), pp. 604–615 (1988)], and recovery of penicillin G from fermentation broth [Lee, Lee, and Lee, J. Chem. Technol. Biotechnol., 59(4), pp. 365–370, 371–376 (1994); Lee et al. J. Mem- brane Sci., 124, pp. 43–51 (1997); and Lee and Yeo, J. Ind. Eng. Chem., 8(2), p. 114 (2002)]. The latter application involves transfer of the penicillin G solute (pKa= 2.7) from the continuous phase (consisting of a filtered broth adjusted to a pH of about 3) into the membrane phase (typically n-lauryltrialkymethyl amine extractant dissolved in kerosene) and then into the interior aqueous phase (clean water at a pH of about 8). Lee et al. [J. Membrane Sci., 124, pp. 43–51 (1997)] show that the operation can be carried out in a continuous countercurrent extraction column. The product is later obtained by separating the emulsion droplets from the continuous phase by using filtration, and this is followed by breaking the emul- sion and isolating the interior aqueous phase from the amine extrac- tant phase. A polyamine surfactant is used to stabilize the emulsion during extraction.
Supported liquid-membrane (SLM) processes involve introduction of a microporous solid membrane to serve as a support for the liquid- membrane phase. The microporous membrane provides well-defined interfacial area and eliminates the need for a surfactant. As in the penicillin ELM application described above, SLM applications often employ an extractant solution as the liquid-membrane phase to enable a facilitated transport mechanism. The extractant species interacts with the desired solute at the feed side and then carries the solute across the membrane to the other side, where solute transfers into a stripping solution. Such a process, whether using a surfactant-stabi- lized emulsion or a supported liquid membrane, allows forward and back extraction (or stripping) in a single operation. Ho and Wang [Ind. Eng. Chem. Res., 41(3), pp. 381–388 (2002)] discuss the application of SLM technology to remove radioactive strontium, Sr-90, from conta- minated waters. Other examples involve extraction of metal ions from water [Canet and Seta, Pure Appl. Chem. (IUPAC), 73(12), pp. 2039–2046 (2001)] and recovery of aromatic acids or bases from wastewater [Dastgir et al., Ind. Eng. Chem. Res., 44(20), pp. 7659–7667 (2005)]. One of the challenges encountered in using sup- ported liquid membranes is the difficulty in controlling trans-mem- brane pressure drop and maintaining the liquid membrane on the support; it may become dislodged and entrained into the flowing phases. Various approaches to stabilizing the supported liquid have been proposed. These are discussed by Dastgir et al. [Ind. Eng. Chem. Res., 44(20), pp. 7659–7667 (2005)].
ELECTRICALLY ENHANCED EXTRACTION
An electric field may be used to enhance the performance of an aqueous- organic liquid-liquid contactor, by promoting either drop breakup or drop coalescence, depending upon the operating conditions and how the field is applied. The technology normally involves dispersing an electrically conductive phase (the aqueous phase) within a continuous nonconductive phase, applying a high-voltage electric field (either ac or dc) across the continuous phase, and taking advantage of the effect of the electric field TABLE 15-25 Baffled MHF Contactor
Geometric Characteristics
Baffles per module 1
Module diameter, cm 9.8
Module length, cm 71
Effective fiber length, cm 63.5 Fiber outside diameter, µm 300 Fiber inside diameter, µm 240
Porosity of fiber 0.3
Number of fibers per module 30,000 Contact area per module, cm2 81,830
Interfacial area, cm2/cm3 27
Tortuosity 2.6
Reprinted from Seibert and Fair, Sep. Sci.
Technol., 32(1–4), pp. 573–583 (1997), with
on the shape, size, and motion of the dispersed drops. The potential advantages of this technology include more precise control of drop size and motion for improved control of mass transfer and phase separation within an extractor. Potential disadvantages include the requirement for more complex equipment, difficulties in scaling up the technology to han- dle large production rates, and safety hazards involved in processing flam- mable liquids in high-voltage equipment.
A number of different equipment configurations and operating con- cepts have been proposed. Yamaguchi [Chap. 16 in Liquid-Liquid Extraction Equipment, Godfrey and Slater, eds. (Wiley, 1994)] classifies the proposed equipment into three general types: perforated-plate and spray columns, mixed contactors, and liquid-film contactors. For exam- ple, Yamaguchi and Kanno [AIChE J., 42(9), pp. 2683–2686 (1996)] describe an apparatus in which a dc voltage is applied between two elec- trodes in the presence of a nitrogen gas interface. Aqueous drops form in the presence of the electric field, and they are first attracted to the gas-liquid interface. Once the drops contact the interface, the charge on the drops is reversed, and the drops fall back to coalesce at the bottom of the vessel. Bailes and Stitt [U.S. Patent 4,747,921 (1988)] describe a rotating-impeller extraction column containing alternating zones of high voltage (to promote dispersed drop coalescence) and high-inten- sity mixing (to promote redispersion of drops). In this design, the elec- tric field serves to promote drop coalescence so that dispersed drops experience alternating drop breakup and growth as they move through the agitated column. Scott and Wham [Ind. Eng. Chem. Res., 28(1), pp. 94–97 (1989)] and Scott, DePaoli, and Sisson [Ind. Eng. Chem. Res., 33(5), pp. 1237–1244 (1994)] describe a nonagitated apparatus called an emulsion-phase contactor. This device employs an electric field to induce formation of a stable emulsion or dispersion band, with clear organic and aqueous layers above and below. The aqueous phase is fed to the middle or top of the dispersion band; it flows down through the band and is removed from a clarified aqueous zone maintained at the bottom. The lighter organic phase is fed to the bottom; it moves up through the dispersion band and is removed from the top. The net result is countercurrent contacting with very high interfacial area and significantly improved mass transfer in terms of the number of transfer units achieved for a given contactor height.
Another approach involves electrostatically spraying aqueous solu- tions into a continuous organic phase to create dispersed drops within a spray column contactor [Weatherley et al., J. Chem. Technol. Biotech- nol., 48(4), pp. 427–438 (1990)]. A high voltage is applied between elec- trodes, one connected to a nozzle where dispersed drops are formed and the other placed within the continuous organic phase. Petera et al. [Chem. Eng. Sci., 60, pp. 135–149 (2005)] discuss the modeling of drop size and motion within such a device. For additional discussion, see Tsouris et al. [Ind. Eng. Chem. Res., 34(4), pp. 1394–1403 (1995)], Tsouris et al. [AIChE J., 40(11), pp. 1920–1923 (1994)], Gneist and Bart [Chem. Eng. Technol., 25(2), pp. 129–133 (2002)], Gneist and Bart [Chem. Eng. Technol., 25(9), pp. 899–904 (2002)], and Elperin and Fominykh [Chem. Eng. Technol., 29(4), pp. 507–511 (2006)].
PHASE TRANSITION EXTRACTION AND TUNABLE SOLVENTS
Phase transition extraction (PTE) involves transitioning between sin- gle-liquid-phase and two-liquid-phase states to facilitate a desired separation. Ullmann, Ludmer, and Shinnar [AIChE J., 41(3), pp. 488–500 (1995)] showed that extraction of an antibiotic from fermen- tation broth into an organic solvent could be improved by transition- ing across a UCST phase boundary using heating and cooling. The results showed much higher stage efficiency compared to a standard extraction technique without phase transition and much faster phase separation. The phase transition may be induced by a change in tem- perature or a change in composition through addition and/or removal of organic solvents or antisolvents [Gupta, Mauri, and Shinnar, Ind. Eng. Chem. Res., 35(7), pp. 2360–2368 (1996)]. Alizadeh and Ashtari describe a temperature-induced phase transition process for extracting silver(I) from aqueous solution using dinitrile solvents [Sep. Purification
Technol., 44, pp. 79–84 (2005)]. Another process that exploits a phase transition to facilitate separation and recycle of solvent after extraction utilizes ethylene oxide–propylene oxide copolymers in aqueous two- phase extraction of proteins [Persson et al., J. Chem. Technol. Biotech- nol., 74, pp. 238–243 (1999)]. After extraction, the polymer-rich extract phase is heated above its LCST to form two layers: an aqueous layer containing the majority of protein and a polymer-rich layer that can be decanted and recycled to the extraction.
Another approach utilizes pressurized CO2to control phase splitting and tune partition ratios in organic-water mixtures. Addition of pres- surized CO2yields an organic phase rich in CO2(the gas-expanded phase) and an aqueous phase containing little CO2. Adrian, Freitag, and Maurer [Chem. Eng. Technol., 23(10), pp. 857–860 (2000)] report data demonstrating the ability to induce phase splitting in the com- pletely miscible 1-propanol + water system by pressurization with CO2 at near-critical pressures above 74 bar (about 1100 psia). The authors also show that the partition ratio for transfer of methyl anthranilate from the aqueous phase to the organic phase can be varied between 1 and about 13 by adjusting pressure and temperature. Jie Lu et al. [Ind. Eng. Chem. Res., 43(7), pp. 1586–1590 (2004)] demonstrate a reduc- tion in the lower critical solution temperature for the partially miscible THF+ water system by addition of CO2at more moderate pressures (on the order of 10 bar, or about 145 psia). The authors show that the partition ratio for transfer of a water-soluble dye from the organic phase to the aqueous phase can be increased dramatically by increas- ing CO2pressure. For more detailed discussion of gas-expanded-liquid techniques used to facilitate various reaction and extraction processes, see Eckert et al., J. Phys. Chem. B, 108(47), pp. 18108–18118 (2004).
IONIC LIQUIDS
The potential use of ionic liquids for liquid-liquid extraction is gaining considerable attention [Parkinson, Chem. Eng. Prog, 100(9), pp. 7–9 (2004)]. Ionic liquids are low-melting organic salts that form highly polar liquids at or near ambient temperature [Rogers and Seddon, Sci- ence, 302, p. 792 (2003)]. The potential use of ionic liquids to extract metal ions from aqueous solution is discussed by Visser et al. [Sep. Sci. Technol., 36(5–6), pp. 785–804 (2001)] and by Nakashima et al. [Ind. Eng. Chem. Res., 44(12), pp. 4368–4372 (2005)]. In another example, phenolic impurities are extracted from an organic reaction mixture using an acidic ionic liquid such as methylimidazolium chloride [BASF promotional literature (2005)]. After extraction, the extract phase is separated by evaporation of the phenolic content, and the raffinate containing the desired product is washed with water to remove small amounts of ionic liquid that saturate that phase. Other potential appli- cations are described in Ionic Liquids IIIB: Fundamentals, Challenges, and Opportunities, Rogers and Seddon, eds. (Oxford, 2005). The pos- sibility of switching a solvent system from ionic to nonionic states also is being investigated [Jessop et al., Nature, 436, p. 1102 (2005)]. The authors report that a 50/50 blend of 1-hexanol and 1,8-diazabicyclo- [5.4.0]-undec-7-ene (DBU) becomes ionic when CO2 is bubbled through the solution. The CO2reacts to form a mixture of 1-hexylcar- bonate anion and DBUH+cation, a viscous ionic liquid. The reaction can be reversed by using N2to strip the weakly bound CO2from solu- tion. This returns the solution to its less viscous, nonionic state and pro- vides a basis for a switchable solvent system.
The challenges involved in using ionic liquids for extraction appear similar to those encountered using nonvolatile extractants dissolved in a diluent, including difficulty dealing with buildup of heavy impurities in the solvent phase over time. Additionally, solvent stability and recovery need to be very high for the process to be economical due to the high cost of makeup solvent. Potential advantages include the pos- sibility of obtaining higher K values, allowing use of lower solvent-to- feed ratios, and simplification of extract and raffinate separation requirements. For example, volatile components may easily be removed from the ionic liquid by using evaporation under vacuum instead of multistage distillation; and, in certain cases, the solubility of ionic liquid in the raffinate may be very low.