Supercritical fluid processing Separations and reactions are applications that point to a bright future for this environmental control technology
Charles A. Eckert John G . Van &ten Thomas Stoicos Universiry of Illinois Urbana, Ill. 61801 Separations and detoxification reactions often can be run advantageously using supercritical fluids (SCFs) as processing media. The use of a supercritical solvent frequently permits extraction of very dilute toxic species, either directly or in a two-step process. SCF solvents not only increase reactivity but offer the potential for simultaneous separation and detoxification. Supercritical fluids offer excellent potential as process solvents in chemical and environmental engineering applications. An SCF is a substance that has been heated above and compressed beyond its critical temperature and critical pressure. As such, it exists as a single fluid phase, with some characteristics of gases and liquids, as well as some particular properties of its own. Figure 1 shows that it is possible to move directly from a liquid to a gas without phase separation simply by taking a path through the SCF region of the phase diagram. By operating in the supercritical reeion. it is oossible to take advantage of v&ety i f interesting and useful imperties. In the vicinity of the critical point, density is an extremely SD function of pressure; in fact, at the ( cal point compressibility becomes .L.llnite. Densities also become quite high at relatively moderate pressures-at only 200 bar and 35 OC the density of C a is close to 0.8 gkc. The special properties of fluids near a critical point make them ideal media for mass transfer. Primarily, very high density means that there is a COR-
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0015936W86/09~0319$01.50/0 0 1986 American Chemical Sociely
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spondingly high capacity for solutes. In fact, the solubility varies exponentially with the solvent density. The high compressibility means that within this critical region, these properties undergo large changes with relatively small changes in operating conditions. For example, the solubility of the solute phenanthrene in supercritical ethylene undergoes a l @ change in solubility with only a 150-bar change in operating pressure. This makes solubility programming possible; a desired solute solubility is set by proper selection of system temperature and pressure. Furthermore, the transport properties of SCFs are most favorable. Molecular diffusivity of SCFs is substantially higher-by one to two orders of magnitude-than that for normal liquids. Moreover, the viscosity is almost as low as that for gases, facilitating both pumping and natural convection. Many common substances have critical temperatures near ambient, making them especially attractive as process fluids. These include carbon dioxide (30 O C ) , ethylene (10 "C), ethane (32 OC), fluoroform, (45 OC), and sulfur hexafluoride ( 4 6 O C ) . At higher temperatures, one might consider aliphatic or aromatic hydrocarbons, or polar and protic compounds such as alcohols, ammonia, or even water. Carbon dioxide has been especially attractive for use in the food, pharmaceutical, and pollution control industries because it is relatively hazardless and nontoxic. Other solvents, however, especially tailored mixtures, frequently have superior properties. A significant cost factor for many separation processes is the recovery of the extracting solvent. This is usually an energy-intensive step. In this case, however, the solvent is in the supercritical state and therefore can be recovered by reducing the pressure and changing the temperature. From the process engineering point of view, it is desirable to design a temperature-controlled s e p aration with only minimum pressure drop requirements.
FIGURE 1
Pressure-temperature diagram of a pure materiala
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Liquid region
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a Retention volumn (mL SCF) 'From d8afornaceous so11by supercritical ethylene at 50
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Application of SCF technology Many of the early investigations of SCFs as solvents were reported in the patent literature and have resulted in a number of applications that are either already in commercial use or are under intensive investigation (I, 2). In Kerr McGee's ROSE (residuum oil supercritical extraction) process, supercritical pentane is used to remove valuable petroleum components from still bottoms. Decaffeination of coffee is accomplished by soaking the beans in water and then immersing them in supercritical CO?; the water is essential for mass transfer of the caffeine from
within the bean. Among the processes still in the experimental stage are the removal of active ingredients from hops and spices and the removal of nicotine from tobacco. This latter process holds considerable advantages over conventional liquid extraction because the supercritical solvent leaves the essential aromas in the leaves and maintains them in a physically unaltered state. An area of particular interest to analytical chemists is that of supercritical chromatography. By using an SCF as the mobile phase. capacity ratios (retention times) may be reduced by more than loo0 times, compared with those that result from standard gas chromatography (3). As with high-performance liquid chromatography, SCF chromatographs can be programmed by pressure rather than temperature to speed elution times. This in turn reduces the possibility of destroying thermally labile compounds. Of most interest, however, are the rapidly developing possibilities For using this new technology in various aspects of environmental control. Basically, there are three categories of Drwesses For such amlications OF SCF .. iechnology : One-steo seoararion. The SCF is out into d;reci contact with anotLer phase and is used to remove a contaminant. Ultimate purification and recovery of the material removed may or may not be economical. Twcwtep separation. The contaminated phase is put into contact with a second intermediate phase, not the SCF, such as an adsorbent. The contaminant is first transferred to the intermediate phase, most often at ambient pressure, and removed by an SCF in a separate, second step at an elevated pressure. Again the subseauent omcessinr! of the contaminant is inkaterial. Reactive seoaration. The SCF is out into direct 'contact with the contaminated material. It simultaneously dissolves the material and serves as a reaction medium for a specific chemical change, such as a detoxification reaction. The one-step separation is most a p plicable when the concentration of the material to be removed is relatively high. For example, a number of processes for recovering alcohols from aqueous solutions with supercritical C02 have been proposed (4, 5). Certainly, one can envision comparable extraction of many other organics, such as higher alcohols, aromatics, esters, ketones, and aldehydes, from water or other solvents by similar means. Such an SCF extraction would be especially advantageous For the removal and sub-
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sequent separation of a series of multiple, hard-to-separate species. Ringhard and Kopfler have reported good results For extracting fairly dilute contaminants from water in a one-step process (6). Botanical Resources has patented a process that uses supercritical C 0 2 to remove pyrethrin-a naturally occurring insecticide-from pyrethrum flowers (7). Pyrethrin is highly toxic to insects but almost totally nontoxic to warm-blooded animals. An additional advantage is that it decomposes with time and does not accumulate in the environment. As a consequence, the insects do not develop a resistance to the substance. The capital and operating costs ofthis SFC extraction process are about one-half those of the conventional multistep organic solvent method. Another example of the single-step separation is the decontamination of solids by extraction with an SCF. Knopf and co-workers have removed DDT from soil samples using supercritical COz (8). At the University of Illinois we have studied the removal from soil of chlorinated aromatics, such as trichlorophenol, as model compounds for polychlorinated biphenyls (PCBs) and dioxins. Figure 2 shows the results of a semibatch flow experiment in which supercritical ethylene is used to remove virtually all trichlorophenol from a soil sample. In the vast majority of separation studies using either the one-step or twostep process, pure COz was used as the only SCF. But other solvents, especially tailored solvent mixtures, almost always offer significant technical and economic advantages. The soil decontamination study mentioned above is just one of many examples that demonstrate this point. The two-step process offers significant advantages in the concentration of very dilute contaminants. By the use of a properly chosen intermediate phase, contaminants in part-per-million or even part-per-billion concentrations in a gas or liquid stream can be greatly concentrated in the additional step either for recovery or detoxification. Perhaps the most interesting application proposed for the two-step process is in the augmentation of current methods of wastewater treatment that use either activated carbon or synthetic resinous adsorbents for the purification of industrial waste streams. In these systems, the solvent is used to regenerate beds of adsorbent that have become saturated with contaminant, allowing the adsorbent to be recycled. Current methods, such as steam stripping and thermal regeneration, are energy intensive and therefore expensive to use. Researchers at Arthur D.
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c Little (ADL) have made several studies comparing supercritical and conventional methods and have found that SCF stripping can indeed be economically attractive (9, 10). A typical flow sheet for one such process is shown in Figure 3. In one study, the compounds atrazine and dinitrobutylphenol (contaminants in the waste stream of a pesticide plant) were successfully removed from activated carbon at an estimated cost of 14 to 29 cents per pound of carbon, which compares Favorably with thermal regeneration at 29 to 36 cents per pound. This technology has also been applied to the regeneration of resinous adsorbents. Although these special adsorbents are often Far more efficient and have far greater selectivity than activated carbon, they are temperature sensitive and therefore expensive to recycle because thermal regeneration cannot be used. Qpically, the resin is regenerated using a solvent wash-anddistillation process, which is highly energy intensive. Because SCFs can operate at moderate temperatures and can be Far more readily recycled than liquid solvents can, SCFs appear ideal for this process. ADL found that using a resin followed by supercritical washing saves 7143% of the cost of running a conventional process that uses carbon adsorbent followed by thermal regeneration for such diverse compounds as alachlor, phenol, and acetic acid. Removal and concentration of trace contaminants from gas streams by the two-step SCF process also are possible. Eppig and co-workers report advantageous regeneration efficiencies and economics for activated-carbon regeneration after treating air streams that Envimn. Sci. Technol..Vol. 20. No. 4, 1986 321
contain traces of gasoline, alcohols, or
FIGURE 3
Arthur D. Little process for regenerating an adsorbent from wastewater purification Pressure reduction valve
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FIGURE 4
Modar process tor oxidative detoxification of waste streams
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