2298
Ind. Eng. Chem. Res. 1987, 26, 2298-2306
Soave, G. Chem. Eng. Sci. 1972,27, 1197. Strausz, 0. P. In The Oil Sands of Canada-Venezuela 1977;Redford, D. A., Winestock, A. G., Ed.; CIM Special Volume 17; Wiley: New York, 1977; p 146. Svrcek, W. Y.; Mehrotra, A. K. J. Can. Pet. Technol. 1982,21(4),31. Teja, A. S.; Rice, P. Ind. Eng. Chem. Fundam. 1981, 20(1),77. Ward, S. H.; Clark, K. A. Reprt 57, 1950; Research Council of Alberta, Alberta, Canada.
Winniford, R. S. J. Inst. Pet. 1963, 49(475), 215. Wong, D. S. H.; Sandler, S. I.; Teja, A. S. Fluid Phase Equilibr. 1983, 14, 79. Yen, T. F.; Dickie, J. P. J . Inst. Pet. 1968, 54(530), 50. Received for review November 5, 1985 Revised manuscript received March 17, 1987 Accepted July 22, 1987
Rapid Expansion of Supercritical Fluid Solutions: Solute Formation of Powders, Thin Films, and Fibers Dean W. Matson, John L. Fulton, Robert C. Petersen, and Richard D. Smith* Chemical Methods and Separations Group, Chemical Sciences Department, Battelle, Pacific Northwest Laboratories, Richland, Washington 99352
Supercritical fluids (or dense gases) have been established as suitable solvents for many nonvolatile s) solute condensation occurring during the or thermally labile compounds. The rapid expansion of a supercritical fluid solution through a nozzle results from solute nucleation and particle growth processes which may be impacted by a variety of experimental parameters. The qualitative effects of solute concentration and of the presence of electrolytes on the physical characteristics of material formed during this process have been explored. The technique is shown to be applicable to both inorganic (SiOz) and organic materials (polymers) using a range of supercritical solvents and thus has potential for a variety of applications involving the formation of powders, thin films, and fibers. It is shown that an intimately mixed powder of two inorganic materials (Si02 and KI) or an inorganic and organic combination (KI and poly(viny1 chloride)) can be obtained, suggesting the feasibility for the formation of unique amorphous mixtures.
I. Introduction Supercritical fluids, dense gases above their critical temperatures and pressures, have been shown to have a number of unique solvent characteristics (McHugh and Krukonis, 1986) and can often be used to dissolve solutes having negligible vapor pressures (Smith and Udseth, 1983; Randall, 1983; Paulaitis et al., 1983). Furthermore, the rapid expansions of highly dilute supercritical fluid solutions through a nozzle have been shown to allow transfer of individual molecular species of low volatility to the gas phase, where mass spectrometric techniques may be used for their study (Smith and Udseth, 1983; Randall, 1983). Rapid expansions of more concentrated supercritical fluid solutions containing low vapor pressure solutes can be used to produce powders and films, resulting from homogeneous nucleation of the solute species present in the solutions prior to expansion (Smith, 1986). The production of particles upon decompression of supercritical fluid solutions was first noted over a century ago (Hannay and Hogarth, 1880), and the formation of particles during the expansion of supercritical solutions through a valve has more recently been observed for a number of different systems (Paulaitis et al., 1983; McHugh and Krukonis, 1986; Larson and King, 1986). We are currently investigating the rapid expansion of the supercritical fluid solutions (RESS) process, which involves an expansion of the solution through a well-defined orifice under conditions which allow some degree of control over the solute nucleation and growth phenomena during the expansion. This technology offers the potential to produce thin fiis,fine powders with narrow size distributions, and intimate mixtures of amorphous materials under the nonequilibrium conditions inherent in the RESS expansion (Petersen et al., 1986; Matson et al., 1986a,b). Theoretical models for free jet expansions (e.g., Anderson (1971), Shapiro (19531, and Murphy and Miller (1984)) provide a basis for understanding the details of the RESS expan-
Table I. Critical Properties of Solvents Used i n This Study critical critical critical density, E/mL solvent oressure. bar temo. "C 96.8 0.217 propane 42.5 132.4 0.234 ammonia 114.0 196.6 0.237 n-pentane 33.7 ethanol 63.8 243.1 0.276 374.1 0.325 water 221.2
sion process which affect solute nucleation and growth and, therefore, the physical properties of RESS products. This understanding will allow some degree of control over the RESS process and permit the tailoring of RESS product morphologies to meet specific requirements for any number of possible applications. The RESS technique has potential for use with a wide range of both inorganic and organic materials. This report discusses the results of some of our initial investigations involving a number of ceramic and preceramic materials, as well as a variety of organic polymers. 11. Experimental Section A. The RESS Apparatus. The RESS process utilizes the dramatic change in dissolving power experienced by a solvent as it is rapidly expanded from conditions where it exists as a supercritical fluid to a much lower pressure (and temperature) environment where it exists as a gas. This transition of the solvent from a supercritical fluid having significant dissolving capacity to a gas having negligible dissolving power encourages the rapid nucleation and growth of low vapor pressure solute particles, provided sufficient solute density exists in the expansion jet. The RESS process is therefore considered a general technique, useful for any solvent-solute systems that can be maintained as a solution at supercritical fluid conditions. Consequently, the process has been applied to a number of different solute materials using solvents having a range
0888-588518712626-2298$01.50/0 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 11,1987 2299
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Figure 1. Schematic illustration of the RESS apparatus as used for supercritical water. Inset the RESS expansion.
of critical parameters. The critical parameters of some of the solvents utilized for this investigation are summarized in Table I. Basic components of a RESS apparatus consist of a supply of solution, a pump to pressurize the system, a region in which the solution is heated to above its critical temperature, and an orifice through which the supercritical fluid solution is expanded. For initial studies designed to investigate the RESS process, solute products were collected by impaction on an appropriate substrate placed in the expansion jet. A schematic -am of the experimental apparatus used to study the supercritical Si0,-water system is shown in Figure 1. Hydrostatic pressure of -590 bar was maintained in an externally heated high-temperature, highpressure autoclave (High Pressure Equipment, Inc., Model GC-5) containing chips of high-purity SiO, glass. Pressure was provided to the system by a high-pressure air driven liquid pump (Haskel, Model DSTF-150) and controlled with a back-pressure regulator (Tescom, Model 26-1721). The autoclave was generally kept at subcritical temperatures for the studies reported here and was allowed to equilibrate at temperature and pressure for several hours prior to sample collection. When the valve downstream from the autoclave was opened (Figure l),the solution moved from the autoclave into a length of '/8-in.-o.d. stainless steel tubing (0.5-0.8 m) maintained a t a higher temperature where it was heated to above its critical temperature prior to expansion. Heating of the line was accomplished by using the output from a high-current dc power supply (Hewlett-Packard, Model 6453A) which, in tum, was controlled with a process controller (Research, Inc., Model 640) and a thermocouple feedback. Subsequent to heating, the supercritical fluid solution was allowed to expand through a nozzle, which for the Si0,water system studies typically consisted of -5 mm of 60-pm4.d. stainless steel capillary silver soldered into a
short length of l/lB-in.-o.d. stainless steel tubing. This type of nozzle was easily connected to the end of the supercritical transfer line by using an appropriate high-pressure union. Powder samples were collected inside a partially evacuated chamber (0.1-1.0 bar) to minimize the potential health hazards associated with the presence of airborne silica particles. The experimental apparatus used for other oxide and non-oxide materials were in principle, the same as that used for the Si0,-water system. The arrangement was simplified in many w e s , however, by pumping premixed solutions directly into the heated supercritical transfer line (bypassing the autoclave) or by collecting samples at ambient atmosphericconditions in a fume hood. The use of premixed solutions was appropriate where solutes exhibited sufficient solubility in a liquid solvent at ambient temperatures and pressures and where solubility did not decrease to the point where the solute precipitated as the solution was heated to supercritical temperatures. Fused silica nozzles of 25-pm i.d. were commonly used for the polymer solutions using organic solvents, and no backpressure regulator w a required ~ because of lower flow rates and minimal pressure fluctuations resulting from the use of a chromatographic pump (Altex, Model llOA). B. Product Characterization. Initial characterization of RESS products was accomplished by using optical microscopy, with scanning electron microscopy (SEM) or transmission electron microscopy (TEM) utilized for more detailed morphological analysis. Further characterization of selected samples was performed by using one or more of several techniques. Qualitative X-ray fluorescence was applied for elemental analysis of particles and was also used to probe chemical homogeneity when more than one solute component was expected to be present in the product. Specific surface areas of powder samples were measured by nitrogen gas adsorption (BET method). Phase characteristics of powder samples were probed in the bulk product by using X-ray powder diffraction and at the microstructural level by using dark-field TEM analysis. Infrared analysis was used to determine the extent of solvent inclusion in RESS polymer products. Melting points of polymer products were determined and compared with the melting points of polymers initially loaded into the autoclave to establish the extent of fractionation which might have occurred by selective solvation of lower molecular weight oligomers during the dissolution-extraction process. 111. Results and Discussion A. Rapid Expansion of Supercritical Fluid Solutions. The RESS process, in which the supercritical fluid solvent density rapidly drops and solute nucleation occurs, depenh on the characteristics of the fluid expansion. This expansion depends to a large extent on the flow properties of the,nozzle across which the expansion is allowed to take place. The expansion of a supercritical fluid solution through a capillary nozzle, as it occurs during the RESS process, can be separated into several distinct stages (inset, Figure 1). These include a subsonic expansion which occurs through the length of the nozzle itself, a brief supersonic "free jet" expansion stage which occurs immediately upon the fluid exiting the nozzle, and a final stage during which the jet interacts significantly with background gas present in the expansion region. Expansion of a supercritical fluid through a sufficiently short capillary tube is essentially an adiabatic process, and the flow rate of the fluid through the tube can be approximated by using the methods described by Lapple (Lapple, 1943; Levenspiel, 1977). For the conditions of
2300 Ind. Eng. Chem. Res., Vol. 26,No. 11, 1987
Enthalpy (kcalikgm)
Figure 2. Enthalpy-entropy-pressure diagram for H20(after Perry and Green (1984)).
interest in the RESS process (i.e., a high inlet pressure and short nozzle), the flow through the nozzle is choked and the fluid velocity cannot exceed the speed of sound. For capillary nozzles with aspect ratios (LID) on the order of 100, the fluid expansion occurring within the tube produces a density at the exit which is typically 50% of the inlet density (corresponding to a pressure which is 40% of the initial pressure) (Smith et al., 1986). As the fluid expands adiabatically and approaches the speed of sound at the nozzle exit, its enthalpy is equal to the initial fluid enthalpy, less the value of the kinetic energy gained during the expansion to that point (-&lo% of the preexpansion enthalpy of the fluid). Beyond the nozzle, the RESS expansion jet is characterized by two distinct stages; a brief isentropic "free jet" expansion near the nozzle and a region of significant mixing with background gases further downstream. The boundary between these postnozzle expansion stages is characterized by a set of shock fronts resulting from the deceleration of the jet to subsonic velocities due to jetbackground gas interactions. These shock fronts include a barrel shock wave, reflected shock waves, and a shock front perpendicular to the jet axis (the Mach disk) (Ashkenas and Sherman, 1966; Randall, 1983; Liepmann and Roshko, 1957). The distance from the nozzle tip to the Mach disk can be estimated as 0.67D(Po/P1)1/2,where D is the orifice diamter and Po and PI are the initial preexpansion fluid pressure and the background pressure in the expansion region, respectively (Ashkenas and Sherman, 1966). This corresponds to distances from less than 1to approximately 3 mm for the conditions typically used for the supercritical water-based RESS systems. The bulk of the fluid density drop occurring subsequent to the fluid exiting the nozzle takes place during this brief free jet expansion stage. As the fluid approaches the Mach disk, entropy of the jet begins to increase due to collisional processes with gas in the background region. Beyond the Mach disk, significant mixing between the jet and background gases again contributes to an entropy increase in the jet. Figure 2 is the enthalpy-entropy-pressure diagram for H,O which shows various isotherms and the two-phase region where liquid and vapor exist in equilibrium (after Perry and Green (1984)). Also shown are the lines of constant moisture content within the two-phase region. By use of the above model to describe the different stages of the RESS expansion, a diagram, such as Figure 2, can be utilized to estimate the phase characteristics of the ex-
pansion jet if the initial fluid conditions are defined. From the pressure and temperature of the fluid upstream of the nozzle, the initial fluid enthalpy is known, and the enthalpy and pressure at the nozzle tip can be accuratelydetermined from the predicted flow of a compressible fluid in a tube (Lapple, 1943; Levenspiel, 1977). Another convenient reference point is the Mach disk where the jet reverts from supemonic to subsonic velocity. At the Mach disk the fluid pressure and temperature (and, as a result, the sonic velocity) are lower than at the nozzle tip. For H20, sonic velocities vary from 640 m/sat 550 "C and 600 bar to 436 m/s at 150 "C and 200 bar (Meyer et al., 1979). Because the velocity is slightly lower, the enthalpy of the fluid at the Mach disk will be slightly higher than the enthalpy at the nozzle tip, yet lower than a t the nozzle inlet. The enthalpy of the fluid at the Mach disk will typically be between 90% and 100% of the preexpansion enthalpy. Within the supersonic isentropic region, the expansion process is complex and the jet may enter the two-phase expansion region. The pressure-temperature phase conditions must, however, lie in a region of the phase diagram bounded by the entropy of the jet at the Mach disk and an isenthalpic line equal to the entropy at the nozzle exit. An approximate value of the average fluid density at the Mach disk can be determined from a simple mass balance by measuring the radius of the jet at this point and realizing that the velocity at the Mach disk is slightly less than the velocity at the nozzle tip. From this simple measurement, the density of a single-phase H 2 0 fluid at the Mach disk is estimated to be on the order of 50 times smaller than the density at the nozzle exit. Consequently, it is clear that a significant fraction of the expansion occurs within the isentropic expansion stage. In the region beyond the Mach disk, the turbulent flow of the jet rapidly mixes background gases with the fluid. Here, the tsmperature and pressure of the background gases have a large effect on the conditions within the jet. As shown in Figure 2, a preexpansion temperature of
