4626
Ind. Eng. Chem. Res. 1996, 35, 4626-4634
Application of Rapid Expansion of Supercritical Solutions in the Crystallization Separation Guo-Tang Liu and Kunio Nagahama* Department of Industrial Chemistry, Tokyo Metropolitan University, Minamiohsawa, Hachioji, Tokyo 192-03, Japan
The rapid expansion of supercritical solutions (RESS) has been applied to separate the hydrocarbon mixture. The RESS experiments for the naphthalene and phenanthrene mixture using supercritical carbon dioxide have been carried out to study the influence of various RESS conditions on separation as well as on morphology. High pre- and postexpansion temperatures enhanced the naphthalene composition of the precipitate more than the preexpansion solution composition. By diluting the preexpansion mixture with pure CO2 and then increasing the postexpansion temperature, an increase in the size of the particles precipitated from mixtures was found. Increasing continuously the phenanthrene composition of the preexpansion mixture solution, the shape of the crystals changed from flat-sheet (pure naphthalene) to needlelike (pure phenanthrene). Most of the experiments were carried out at the extraction temperature of 308.2 K and at the pressure of 14.0 MPa. The highest naphthalene composition of particles precipitated from the saturated fluid mixture (0.883 mole fraction of naphthalene) reached a 0.960 mole fraction of naphthalene at a higher postexpansion temperature than that of the eutectic. 1. Introduction The benefits of supercritical fluids have been widely reviewed, and the reports of their application in a variety of processes are continually appearing. Supercritical fluids are widely used as extracting agents in supercritical fluid extractions, transporting media in chromatographic separations, desorbing media in regeneration of adsorbent, reaction media in some material processing, and precipitating fluids in RESS (rapid expansion of supercritical solution) or GAS (gas antisolvent recrystallization) processes. Whatever the purpose of all these applications, the common stage is the dissolution of the selected solute into the selected fluid (except for GAS) and, moreover, the variation of the solubility with operating conditions of pressure and temperature. RESS (Matson et al., 1986, 1987a,b) is a promising new process for the production of small and uniform particles, which has been described in a review paper (Tom and Debenedetti, 1991). In the RESS process, the supercritical fluid is used as a solvent for the solute, and the mechanical perturbation which occurs when the solution is expanded across a fine throttling device such as a capillary or an orifice nozzle can lead to a high supersaturation in a very short period (microsecond) of phase separation. Consequently, the microstructural materials may be precipitated by density reduction of the fluid. A sudden fast increase in supersaturation mostly prompts an outburst of homogeneous nuclei, and the development of very small particles with a narrow size distribution is expected to occur. More recently, most experimental studies with RESS have focused on a systematic investigation of the effects of process variables upon product characteristics (Lele and Shine, 1992, 1994; Reverchon et al., 1993, 1995; Tom et al., 1994; Ksibi et al., 1995; Mawson et al., 1995). Besides, one-dimensional compressible flow models have been developed to study the dynamics as well as the nucleation and growth of crystals for RESS (Lele and Shine, * Author to whom correspondence is addressed. E-mail:
[email protected]. Fax: +81-426-77-2821. Telephone: +81-426-77-2822.
S0888-5885(96)00142-X CCC: $12.00
1992; Kwauk et al., 1993; Debenedetti et al., 1993). In contrast to RESS, the GAS process is a technique for precipitating or crystallizing solutes dissolved in a liquid solvent by using a compressed fluid antisolvent. In recent years, several studies have been aimed at polymers (Yeo et al., 1993a, 1995; Randolph et al., 1993; Dixon et al., 1993) and pharmaceutical compounds (Chang and Randolph, 1989; Yeo et al., 1993b; Schmitt et al., 1995). There have been several reports on the crystallization of pure organic solids from supercritical solution. For example, Tavana and Randolph (1989) developed a technique of batch crystallization to study the crystallization mechanism from supercritical carbon dioxide. In the study of the nucleation of benzoic acid from supercritical carbon dioxide, the effects of different cooling and decompression profiles on crystal size were determined. They confirmed the validity of the conventional concept that crystals are formed by nucleation and subsequent growth. Furthermore, Debenedetti (1990) has studied theoretically the homogeneous nucleation of phenanthrene crystals from the supercritical carbon dioxide solution. Berends et al. (1993) presented some results of nucleation and growth of fine benzoic acid and phenanthrene crystals from supercritical carbon dioxide. The growth phenomena, mechanism, and kinetics of naphthalene seed crystals in a supercritical carbon dioxide solution were recently explored by Tai and Cheng (1995). They reported that, compared with the conventional crystallization precesses such as those from a liquid solution and vapor phase, the crystal growth of naphthalene in supercritical fluid showed characteristics in the growth mechanism and kinetics that are similar to liquid-solution growth but not to vapor growth. Crossover of solubilities in the supercritical fluid region and its potential use for separations (retrograde crystallization) were discussed by Chimowitz and Pennisi (1986). In the last few years, such retrograde crystallizations have been studied experimentally by Johnston et al. (1987) and Kelley and Chimowitz (1989) and theoretically by Chimowitz et al. (1988) and Foster et al. (1991). In particular, Sako and co-workers (1990) © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4627
Figure 1. Experimental setup: C, cooler; E, equilibrium cell; F, filter; G, gas meter; H, heater; J, preheater coil; P, HPLC pump; PCV, backpressure regulator; PI, pressure gauge; S, crystallization cell; TIC, temperature controller; TR, trap; WB, constant-temperature bath.
reported the experimental results for the purification of naphthalene + phenanthrene using retrograde crystallization using supercritical CO2. The highest purity was 96.5 wt % for naphthalene and 97.1 wt % for phenanthrene obtained from a binary solid mixture composed of 50 wt % naphthalene and 50 wt % phenanthrene. The aim of this experimental work is to develop the RESS technique for the fractional crystallization of organic mixtures. In this study, we investigated the effects of key process variables on RESS behavior for the naphthalene + CO2 and naphthalene + phenanthrene + CO2 systems. Particularly, our goal is to observe the morphology of precipitated crystals and to probe into the possibility of RESS process for organic mixture separation. This work may be the first exploratory study on the separation for a two-solute mixture by only RESS. 2. Experimental Section 2.1. Experiments. The experimental RESS apparatus is shown in Figure 1. This is similar to that proposed by Mohamed et al. (1989a) and allows the independent control of all process variables: pre- and postexpansion pressure and temperature and supercritical solution composition before expansion. The apparatus is divided into extraction, dilution, and precipitation units. In the extraction unit, a carbon dioxide solvent was supplied from a gas cylinder and liquefied through a cooler, C (Scinics Co., CH-201), and then it was pressurized to the desired pressure by a HPLC pump (Jasco, PU-980) at a constant flow rate. The cell E1 (stainless steel, 22 cm length, 2.2 cm o.d.) or E2, composed of two columns (stainless steel, 22.5 cm length, 0.9 cm o.d. each), was used. For the pure naphthalene crystallization experiments, E1 was charged with about 7 g of naphthalene. For most of the two-solute mixture crystallization experiments, E1 was charged with about 7 g of an equal weight solid mixture of naphthalene and phenanthrene, and this is because the solubility of each component in the fluid is thermodynamically independent of the mixed solid feed composition. In the case of precipitation run from a fluid mixture of adjusted composition, E1 and E2 were operated in parallel with the first column of naphthalene (typically 3-4 g) and the second column of phenanthrene (3-4 g). The extraction cell was placed in an air bath controlled by a microprocessor PID temperature control unit (Tokyo Seisakusho Co., FC-410). Through all experimental
Figure 2. Expansion nozzle.
runs, the solid solutes were packed with 3 mm glass beads to prevent the solute from caking. As a dilution unit, a second HPLC pump (Jasco, PU980) was used to introduce a diluent solvent (CO2) to adjust the preexpansion composition before entering the crystallizer. In this manner, independent control of the composition of the supercritical solution was obtained prior to expansion. In the precipitation unit, the supercritical fluid mixture, exiting the extraction and dilution units, entered a thermostated injector line, and then it was throttled across a fine-diameter orifice. As shown in Figure 2, a 0.28 mm thick, 35 µm diameter orifice (L/D ) 8) has been used along with an orifice of 75 or 90 µm diameter. The orifice was observed by a microscope before and after each run to ensure that it was not clogged with fine particles. The expansion chamber is a visual observation cell with a see-through glass window. A sample of the precipitated crystals was then collected for analysis either on a small glass plate or in a small glass bottle (24 mm i.d. × 50 mm), which was positioned at the center of the crystallization cell approximately 55 mm away from orifice exit. Two 2 µm frit filters were placed at the entrance of the preheated line and at the exit of the precipitated cell to prevent plugging of the expansion nozzle and the metering valve. The crystallization pressure was controlled by a backpressure regulator. In addition, a line was installed to prepressurize the crystallization cell up to the desired pressure with pure CO2 before each run. The running time of an experiment was varied from 10 to 15 min, and the typical flow rates are between 1.2 and 2 standard L/min. At the end of a run, the
4628 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
Figure 3. Solid-liquid equilibria of the naphthalene + phenanthrene system under atmospheric pressure.
pump feeds are stopped and the system pressure is slowly lowered using the micrometering valve. 2.2. Characterization. The morphology of precipitated particles was observed with respect to their shape and size by an optical microscope (Olympus BH-2), and about 100 target particles were counted for the determination of particle size distribution. The crystallinities of the collected particles were analyzed through an X-ray diffractometer (MC Science, MXP3). The compositions of solid mixtures of naphthalene and phenanthrene coprecipitated from a supercritical CO2 solution were verified by a gas chromatograph (Simazu, GC-8A) with a SE-30 column. 2,6-Dimethylnaphthalene was added to the sample as an internal standard substance for analysis. 2.3. Materials. Reagent-grade naphthalene (+99%) and phenanthrene (+98%) were obtained from Kanto Chemical Co. and Tokyo Kasei Co., respectively. Highpurity carbon dioxide (more than 99.95%, Kayama Sanso Co.) was used as the solvent. 2,6-Dimethylnaphthalene of +99% purity was supplied from Wako Chemical Co. All chemicals were used without further purification. 3. Solubilities of Naphthalene + Phenanthrene Mixtures in CO2 Before measuring the solubilities of naphthalene + phenanthrene solid mixtures in supercritical carbon dioxide, the solid-liquid equilibria for the naphthalene + phenanthrene systems were measured under atmospheric pressure by a differential scanning calorimeter (Seiko Co., DSC 200). As shown in Figure 3, the system is a simple eutectic mixture with a eutectic temperature of 49.5 °C, which is higher than the experimental extraction temperature of this work. The ICT data (Mortimer, 1928) are complementary and shown together in Figure 3. For this system, the phase behavior of solid-liquid equilibria under higher pressure has been studied by Dams and Schlunder (1991). Accurate mixture solubility measurements were performed with two objectives: to ascertain saturation for each solute in the contacting column and to obtain correlation of solubility data for calculating the concentration of solute at different RESS operating conditions. In a previous study (Liu and Nagahama, 1996), the
Figure 4. Solubilities of a naphthalene + phenanthrene + carbon dioxide system.
solubilities of naphthalene + phenanthrene mixtures in supercritical carbon dioxide at 35 °C were reported in detail. The solubilities of phenanthrene for naphthalene + phenanthrene mixtures in carbon dioxide at lowpressure conditions were too small to be determined. So as to predict the solubilities under a wide range of pressures, the experimental data have been correlated successfully using the Yu-Lu-Iwai equation of state (Yu and Lu, 1987; Yu et al., 1987; Iwai et al., 1989). On the basis of the correlated interaction parameters (Liu and Nagahama, 1996), we calculated the solubilities of naphthalene and phenanthrene at low as well as high pressures. The results are shown in Figure 4, and due to the effect of the second solute as a cosolvent on the solubility of the first solute, the solubility enhancements occurred on the average of 27% for naphthalene and 130% for phenanthrene. 4. Results and Discussion of the RESS Experiment 4.1. Morphology of Pure Naphthalene. To examine the effect of main operational parameters on the RESS process, experiments for the naphthalene/CO2 system were first performed as model materials, whose RESS experiment has already been investigated by Mohamed et al. (1989a,b). We summarized the results in Table 1, where the densities of pure CO2 were calculated by IUPAC EOS (Altunin et al., 1976). In addition, X-ray diffraction patterns obtained for the precipitated naphthalene particles indicated the complete retention of the crystalline structure even after the expansion. 4.1.1. Effect of Preexpansion Temperature (Tpre). An increase of the preexpansion temperature, which is the nozzle temperature before expansion through the orifice, resulted in a marked increase of the mean particle size (runs P-1-P-3 in Table 1 and Figure 5) over the range of 87-120 °C. Increasing the preexpansion temperature leads to a decrease in CO2 density and a concurrent increase in the solute’s saturated partial pressure within the preexpansion supercritical solution.
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4629 Table 1. RESS Experimental Conditions and Results on Naphthalene + CO2 run no.
Pext (MPa)
FCO2 (kg/m3)
Tpre (°C)
orifice diameter (µm)
Ppost (MPa)
Tpost (°C)
yna (×104)
particle size (µm)
max particle size (µm)
P-1 P-2 P-3
20.4 20.3 20.4
869.5 868.6 869.5
87 105 125
Increasing Preexpansion Temperature 40 2.50 45 40 2.50 45 40 2.50 45
167 167 167
5-8 8-10 15-25
25 27 42
P-3 P-4 P-5
20.4 20.4 20.3
869.5 869.5 868.6
125 125 125
Decreasing Expansion Chamber Temperature 40 2.50 45 167 40 2.50 30 167 40 2.55 15 167
15-25 8-12 5-10
42 80 20
P-6 P-7 P-8
20.5 15.0 10.9
870.2 815.6 742.0
125 125 125
Decreasing Naphthalene Concentration 90 2.50 45 90 2.50 45 75 2.50 45
167 145 113
8-13 10-17 15-25
40 45 40
P-9 P-1 P-10
20.1 20.4 20.4
867.0 869.5 869.5
86 87 87
Decreasing Expansion Chamber Pressure 75 4.36 45 40 2.50 45 40 0.10 45
167 167 167
3-7 5-8 5-8
15 25 23
P-3 P-11 P-6
20.4 20.5 20.5
869.5 870.2 870.2
125 125 125
167 167 167
15-25 20-25 8-14
42 40 40
Increasing Orifice Diameter 40 2.50 45 75 2.55 45 90 2.50 45
Figure 5. Naphthalene particle size distributions evaluated at different preexpansion temperatures (Tpre).
The decrease of the solvent density is responsible for the decrease of the solvent strength. Inversely, the concurrent increase in the solute’s saturated partial pressure is responsible for the increase in the naphthalene solubility. The total effect of the two competing phenomena always results in an increase in the saturated naphthalene concentration in the supercritical fluid, since extraction pressures in this study were always outside of the retrograde region (Tsekhanskaya et al., 1964). Therefore, the preheated preexpansion solution becomes unsaturated so there would not be any nucleation or growth and hence results in a decrease in the supersaturation ratio (volume unit), which is the ratio of the solute composition to the saturated solubility at expansion conditions. That is, the decrease of the ratio with temperature brings about the increase of the particle size. As shown in Figures 5 and 6, narrow particle size distributions were also observed at every condition. 4.1.2. Effect of Expansion Chamber Temperature (Tpost). Expansion chamber (crystallization cell) temperature is another important variable controlling morphology. Decreasing it from 45 to 15 °C caused
a decrease of the saturated solubility of naphthalene and resulted in an increase in the supersaturation ratio and a concurrent decrease of the mean particle size (Table 1). Particle size distributions at three different expansion chamber temperatures are shown in Figure 7. The standard deviation of the distribution, normalized to the mean of the distribution, was relatively sensitive to the temperature of the expansion chamber. 4.1.3. Effect of Extraction Pressure (yna). Essentially, the change of the extraction pressure brings about a change of preexpansion naphthalene concentration. Some experimental results presented in Table 1 (runs P-6-P-8) and Figure 8 show the particle size distributions at various naphthalene concentrations. Decreasing the extraction pressure leads to a decrease in the solvent density and then results in a decrease in the solvent strength as well as the naphthalene solubility. Therefore, it leads to a decrease of the supersaturation ratio at the expansion chamber and results in an increase in the particle size. The increase of the particle size with a decrease of the solute concentration corresponded to those reported for RESS of organic solutes by Mohamed et al. (1989a) and Reverchon et al. (1993), which may be explained by the classical theory of nucleation (Mohamed et al., 1989b). Mawson et al. (1995), however, clearly observed an increase in the particle size with increasing concentration for RESS of a polymer. 4.1.4. Effect of Changes of Expansion Chamber Pressure (Ppost) and Orifice Diameter. As seen from Table 1 (runs P-9, P-1, and P-10; runs P-3, P-11, and P-6), a decrease of the expansion chamber pressure or an increase of the orifice diameter did not result in a considerable change in the morphology of naphthalene. For instance, it may be due to the fact that the change of the expansion chamber pressure from 4.2 MPa to atmospheric pressure causes a small depletion of the saturated naphthalene concentration and a little change of the supersaturation ratio as shown by our correlated calculation. As pointed out by Smith et al. (1986), at the same pre- and postexpansion pressure conditions, a larger diameter orifice provides only higher total flow rates without bringing a change of axial velocity and thus has no effect on the crystallization process. Our experimental results and those of Ksibi et al. (1995) provide clear evidence to support the argument. For
4630 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
Figure 7. Naphthalene particle size distributions evaluated at different postexpansion temperatures (Tpost).
Figure 8. Naphthalene particle size distributions evaluated at different preexpansion naphthalene concentrations (yna).
is defined as follows:
β)
Figure 6. Optical photomicrographs of naphthalene crystals produced at different preexpansion temperatures (Tpre): (a) 87, (b) 105, (c) 125 °C.
the precipitation of naphthalene from supercritical CO2, our results were almost consistent with the previous studies (Mohamed et al., 1989a,b), but, in general, the diameter effect is significant for the use of capillary as an expansion device (Lele and Shine, 1992; Tom et al., 1994). Furthermore, Mawson et al. (1995) also showed a very clear transition from particles to fibers of a polymer by switching from an orifice to a capillary nozzle. 4.2. Separation of the Naphthalene + Phenanthrene Mixture by RESS. The enrichment factor β
xna/xph yna/yph
(1)
where xna and xph are the mole fractions of naphthalene and phenanthrene in precipitated crystal, and yna and yph are the mole fractions of naphthalene and phenanthrene in supercritical CO2 extracted from the equilibrium cell under the experimental condition. If β is larger than unity, it means that enrichment of naphthalene is carried out by controlling RESS conditions. Fractional crystallization of the naphthalene + phenanthrene mixture by RESS using CO2 was conducted to accomplish the enrichment by exploiting the crystallization as a rate-governed process, because the crystallization is based on the balance between nucleation and growth rates. All experimental data for the naphthalene + phenanthrene + CO2 system are summarized in Table 2. The expansion chamber pressure (Ppost) was always maintained at 2.5 MPa for all of the experiments due to obtained maximum supersaturation ratios.
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4631 Table 2. RESS Experiments on Naphthalene + Phenanthrene + CO2 run no.
Pext (MPa)
Tpre (°C)
Tpost (°C)
M-1 M-2 M-3
13.7 15.8 18.3
135 135 135
M-4 M-5 M-1 M-6 M-7
13.7 14.0 13.7 14.0 14.0
135 135 135 135 135
15 25 35 50 60
M-1 M-8 M-9 M-10
13.7 13.7 14.0 14.0
135 100 80 60
35 35 35 35
M-5 M-11 M-12 M-13 M-14
14.0 14.0 14.0 14.0 14.0
135 135 135 135 135
25 25 25 25 25
dilution ratio (R)
particle size (µm)
max particle size (µm)
precipitated composition (xna)
Increasing Concentrations of Naphthalene + Phenanthrene Mixture 35 0 3-10 70 35 0 3-10 100 35 0 2-8 30
enrichment factor (β)
0.909 0.912 0.907
1.33 1.38 1.30
Increasing Expansion Chamber Temperature 0 5-10 90 0 5-15 75 0 3-10 70 0 5-40 230 0
0.868 0.905 0.909 0.938 0.960
0.87 1.26 1.33 2.01 3.19
Decreasing Preexpansion Temperature 0 3-10 70 0 2-10 25 0 2-10 15 0 2-10 30
0.909 0.879 0.873 0.878
1.33 0.97 0.92 0.96
Increasing Dilution Ratio 5-15 10-30 10-40 10-50 15-50
0.905 0.918 0.945 0.919 0.914
1.26 1.49 2.28 1.51 1.41
0 1/3 1 3 4
In this section, we will discuss the possibility of RESS separation of a binary mixture by focusing on the concentration change of solid mixtures collected in a sampling glass bottle. Each reported datum point is the average value of at least two samples. The sample compositions of the mixture precipitates were generally within 0.005 in the mole fraction of naphthalene. 4.2.1. Effect of Extraction Pressure. The effect of extraction pressure on the enrichment factor was primarily examined. As can be seen from Table 2 (runs M-1-M-3), the compositions of precipitated crystals were found to be rather insensitive to changes in extraction pressures ranging from 13.7 to 18.3 MPa. According to our results of solubility measurement of this mixture in supercritical CO2 (Liu and Nagahama, 1996), increasing the extraction pressure from 13.7 to 18.3 MPa, the preexpansion composition ratio (yna/(yna + yph)) between two solid solutes has not largely changed (from 0.883 to 0.875, Figure 4). Therefore, it leads to the fact that the compositions of precipitated crystals have almost not changed. 4.2.2. Effect of the Expansion Chamber Temperature. In a way similar to that of the precipitation of pure naphthalene, the expansion chamber temperature (Tpost) had a strong influence on the precipitation process of this system. As shown in Table 2, the naphthalene mole fraction of precipitated crystals with the RESS process at 50 and 60 °C, which are higher than the eutectic temperature, became clearly much higher, and the maximum of enrichment ratio at 60 °C attained 3.19. We found that the expansion chamber temperature was the major determining factor in the RESS separation process. 4.2.3. Effect of the Preexpansion Temperature and Dilution Ratio. As shown in Table 2, the change of preexpansion temperature (Tpre) did not result in the enrichment of naphthalene with the exception of that at 135 °C (run M-1). According to the interpretation suggested by Smith et al. (1986) and Reverchon et al. (1993), a preexpansion temperature of lower than 100 °C may result in the expanding fluid (CO2) traversing the two-phase region during an adiabatic expansion. In these cases (runs M-8-M-10), the naphthalene mole fractions of precipitated crystals were almost equal to
75 100 200 150 180
the preexpansion composition (xna ) 0.883), and the change of Tpre has no effect on fractional separation (β Z 1). In order to get a big concentration change of aromatic mixtures in SC CO2 or to reduce the supersaturation ratio during crystallization, we added pure SC CO2 as a diluent solvent to an extract solution stream before entering the crystallizer. The dilution ratio R is defined as the flow rate of extracted solution divided by the flow rate of pure solvent. Among five experiments in Table 2, the highest enrichment factor was observed at R ) 1. 4.3. Morphology of Precipitated Mixture. 4.3.1. Particle Size and Shape. To investigate the morphology change of solid mixtures precipitated by RESS, we made microscopic observations with respect to particle size as well as shape. The composition ratio of naphthalene to phenanthrene in the solid mixture produced by RESS was adjusted by controlling flow rates passing through two equilibrium cells (E1 and E2 in Figure 1) with two HPLC pumps. The experiments were conducted at the following conditions: extraction temperature and pressure of 35 °C and 14.0 MPa; preexpansion temperature of 135 °C; and expansion temperature and pressure of 35 °C and 2.5 MPa. Figure 9 shows the transition of the morphology of solid mixtures precipitated by the expansion of various compositions of solute mixtures dissolved in SC CO2. For pure naphthalene the shape was flat-sheet (Figure 9a), and for pure phenanthrene it was fine needle (Figure 9e). Decreasing the naphthalene mole fraction in the solid mixture (xna), a small number of needlelike particles (phenanthrene crystal) appeared in between the flat-sheet-like naphthalene matrix (Figure 9b). Further decreasing xna, there naphthalene and phenanthrene coexisted in the agglomerated particles and fine needles (see Figure 9c,d); while naphthalene may happen to agglomerate, larger elongated particles are also present due to the existence of phenanthrene. A similar habit modification of the main crystal due to an increase of the impurity concentration for benzoic acid crystallization from a liquid solution of benzoic acid + ochlorobenzoic acid + toluene was reported by Kitamura and Nakai (1982). X-ray diffraction patterns of the naphthalene + phenanthrene solid mixtures produced
4632 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
Figure 9. Optical photomicrographs of pure naphthalene, pure phenanthrene, and naphthalene + phenanthrene mixtures of different compositions (Pext ) 14.0 MPa, Tpre ) 135 °C, Tpost ) 35 °C, Ppost ) 2.5 MPa): (a) pure naphthalene, (b) xna (mole fraction of naphthalene) ) 0.86, (c) xna ) 0.42, (d) xna ) 0.24, (e) pure phenanthrene.
by RESS were obtained. The patterns showed that they were simply superimposed by these two crystalline solids, retaining identical structure and crystallinity of the virgin solids. 4.3.2. Supposed Crystallization Mechanism. According to the recent study on the expansion behaviors during the RESS process by Lele and Shine (1992), the fluid flow is divided into three regions as illustrated in Figure 2. Employing a thin orifice as an expansion
device, a large density reduction (nearly 10%) of supercritical fluids occurred at the entrance of the orifice, and the residence time in the orifice was on the order of 10-7 s, during which time the density decreased further by 7% (Lele and Shine, 1992). Finally the fluid is expanded in the supersonic free jet region where much larger density reduction occurs. Thus, segregation of the dissolved solute may take place in any of these three flow regions.
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4633
In our typical RESS experiment of the solute mixture, the supersaturation ratios for naphthalene and phenanthrene in the expansion chamber were estimated at around 430 and 1.5 × 104, respectively, which were calculated from the solubility data. In addition, the naphthalene and phenanthrene mixture is a simple eutectic system as mentioned previously. As a result of the big supersaturations, nucleation and growth of solid particles may occur to some degree even in the entry region of the orifice due to streamline contraction, which leads to phase separation (solid particle formations from the expansion fluid solution). Particularly, phenanthrene tends to precipitate there more easily due to very large supersaturation ratio than naphthalene does. In fact, our experiments showed that the phenanthrene compositions of the crystals precipitated around the entry region of the orifice were higher than that of the preexpansion mixture. Inside the orifice a large number of crystal nuclei of naphthalene and also phenanthrene were further formed, and subsequently both of them grow to crystals in an expansion fluid. In Figures 9a-e, it is expected that large flat-sheet crystals are naphthalene particles and fine needlelike crystals are phenanthrene particles. Based on the observations with respect to morphology and solid composition, we supposed the crystallization mechanism during the fluid expansion as follows. Due to the much larger supersaturation of phenanthrene, its nucleation and crystal growth may have taken place even inside the nozzle and result in fine particle formation. On the other hand, the large-sized naphthalene crystals may be formed dominantly after passing the orifice due to no larger supersaturation. The solid particles of pure phenanthrene and pure naphthalene may be precipitated separately in the sample bottle, because they are a simple eutectic system. Even after passing through the free jet region in the bottle, the fluid flow rate may still be high enough to blow off the small particles (mainly consisting of phenanthrene) to the outside of the glass bottle and the crystals left inside the bottle become richer in naphthalene than those coprecipitated in the free jet region. Such a suggested mechanism can be supported from the data in Table 2; that is, the larger the particle size is, the higher the enrichment factor is. In other studies mainly on the RESS formation of polymers (Lele and Shine, 1992; Mawson et al., 1995), they discussed the RESS formation mechanism in terms of the time scale available for phase separation, which governs the morphology of the precipitated polymer. In the case a of naphthalene + phenanthrene + CO2 mixture, we believe that such a phase separation does not occur in the preheating zone. 5. Conclusion According to the experimental results for crystallization of pure naphthalene from a SC CO2 solution by RESS, we found that the particle size of pure naphthalene was sensitive to pre- and postexpansion temperatures and also to preexpansion naphthalene concentration in SC CO2. The composition and the morphology of crystals precipitated from SC CO2 solution containing naphthalene + phenanthrene by RESS were influenced strongly by conditions during the expansion process. High preand postexpansion temperatures enhanced richer naphthalene composition of the precipitate than that of the preexpansion mixture solution. The highest enrichment
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Received for review March 12, 1996 Revised manuscript received September 3, 1996 Accepted September 6, 1996X IE960142V
X Abstract published in Advance ACS Abstracts, October 15, 1996.
