Ind. Eng. Chem. Res. 2000, 39, 4853-4857
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Fast Particle Size and Droplet Size Measurements in Supercritical CO2 E. Marioth,* B. Koenig, H. Krause, and S. Loebbecke Fraunhofer Institut fu¨ r Chemische Technologie, Joseph-von-Fraunhofer-Strasse 7, D-76327 Pfinztal, Germany
Polymerization and chemical reactions taking place in supercritical fluids (SCFs) are often combined with the formation and precipitation of insoluble products. The properties and quality of a formed product are significantly influenced by its particle size. To control and regulate the product precipitation during an SCF process measurement, techniques are required to monitor online particle sizes and particle growth. This paper presents the adaptation of the so-called three-wavelength-extinction technique to a dynamic sc-CO2 process containing fine solid particles or droplets of immiscible liquids. This technique allows the simultaneous determination of mean particle sizes and particle concentrations in dispersed systems. The measuring principle is based on the extinction of monochromatic light at three different wavelengths due to absorption and dispersion, as described by the MIE theory. However, for the application of the three-wavelengthextinction technique to a SCF process, the strong influence of pressure and temperature on the optical properties of supercritical fluids must, in particular, be considered. Systematic studies investigating TiO2 particles suspended in sc-CO2 have shown that reliable particle size measurements can be achieved whenever the specific refractive index of sc-CO2 is taken into account. Particle sizes determined by the three-wavelength-extinction technique under sub- and supercritical conditions were in excellent accordance with particle sizes obtained by conventional offline measurement techniques. In addition to the characterization of fine particles, the threewavelength-extinction technique is also suitable for size measurements of droplets in sc-CO2. For example, droplets being formed during the mixing of dimethylformamide (DMF) and scCO2 and dimethyl sulfoxide (DMSO) could be observed and analyzed in real time. In general, one can conclude that the three-wavelength-extinction technique is a suitable tool for monitoring and analyzing the formation of particles and droplets during a supercritical fluid process. 1. Introduction Polymerizations and chemical reactions taking place in supercritical fluids are often combined with the formation and precipitation of insoluble products. For continuous operations accompanied with particle formation, fast measurement techniques for online process control are required. One important parameter is the control of the particle size. To follow the particle size online means that the measurement technique primarily has to be fast and as accurate as necessary. For the special demands arising from of the properties of supercritical fluids (sc-fluids), the measurement technique has to be insensitive to small changes in the fluid conditions, such as small changes in pressure, temperature, etc. The so-called three-wavelength-extinction technique (in the following denoted 3-λ technique) was supposed to be a promising technique with respect to the above-mentioned criteria.1 This technique was arbitrarily developed for the measurement of particles in the submicron range in exhaust streams of engines.2 At the University of Karlsruhe, Dr.-Ing. Tremmel used the 3-λ technique to examine particle formation in firing plants.3 The first applications to supercritical fluid processes were done by Tu¨rk et al. In their work, cholesterol particles formed via the RESS process were measured after the nozzle in a subcritical gaseous fluid.4 * Author to whom correspondence should be addressed. E-mail:
[email protected]. Phone: +49-721-4640-239. Fax: +49-721-4640-111.
The results of the measured particle sizes obtained by the 3-λ technique were compared with results obtained by conventional offline particle size measurement techniques. 2. Theory The 3-λ technique is based on the extinction of monochromatic light (e.g., laser light) due to absorption and dispersion effects. By passing through a collective of particles, the initial intensity I0 of the wavelength λ decreases to the intensity I. This decrease in intensity obeys the Lambert-Beer law if spherical particles possessing a monodisperse particle size distribution are assumed and multiple dispersion is neglected.
(
πx2p I ) I0 exp -NL Qext(xp,λ,m) 4
)
(1)
with I0 the initial intensity; I the reduced intensity; N the particle number concentration; xp the particle size; Qext the extinction coefficient; m the complex refraction index, m ) n + ik, where n is the real part due to disperion and k is the imaginary part due to absorbance; L the optical path length, and λ the wavelength. The unknown particle number concentration can be eliminated by dividing the logarithmic intensity ratios for two different wavelengths. This ratio is called the extinction quotient EQ1,2.
10.1021/ie0001873 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/2000
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Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
() ()
ln EQ1,2 )
I I0
I ln I0
λ1
)
Qext(xp,λ1,m)
(2)
Qext(xp,λ2,m)
λ2
The extinction coefficient Qext can be calculated by the MIE theory.5 The particle size distribution p(xp) with a log-normal distribution, a number-averaged mean diameter xj50,0, and a standard deviation σ can be written as
[
2
[ln(x) - ln( xj50,0)] 1 exp p(xp) ) 2σ2 σx2πx
]
(3)
For the application of the Lambert-Beer law to a particle collective having a particle size distribution p(xp), eq 1 must be modified to
[
I ) I0 exp -NL
∫0
∞
x2p p(xp)π Qext(xp,λ,m) dxp 4
]
(4)
The usage of a second extinction quotient EQ2,3 has several advantages. It avoids systematic errors, which can occur when the shape of the EQ1,2 versus xp curve is not clear, for example, when one value for EQ1,2 can be assigned to two or more particle sizes.6 This is the case whenever the particle material absorbs radiation of one of the wavelengths used. From EQ2,3, the correct particle size can be determined. By using three wavelengths, it is also possible to determine an additional parameter of the particle collective or one that changes during the experiment, such as the standard deviation of the particle size distribution (i.e., during a precipitation process) or the real or imaginary part of the refractive index of the particle material. 3. Experimental Section 3.1. Setup. For particle size measurements under dynamic conditions in supercritical carbon dioxide, a circular-flow apparatus was set up, containing a gear pump and a high-pressure view cell. In bypass, an extraction cell was mounted, which allowed for particle material to be loaded into the high-pressure apparatus. At the entrance of the high-pressure view cell, a connection to a volumetric working HPLC pump was installed in order that liquid solvents could be added to the CO2 stream. In all experiments, the flow rate of the solvent was 0.5 mL/min. A schematic diagram of the experimental setup is shown in Figure 1. The 3-λ apparatus is a commercial product from Wizard-Zahoransky GmbH (D). The system was equipped with three lasers emitting at wavelengths of 674, 814, and 1311 nm, which allow the measurement of particle sizes, according to MIE theory, in a range from about 50 nm to 4.2 µm. The pressure was measured at the entrance of the fluid into the view cell and at the extraction cell. The temperature was measured after the view cell and again at the extraction cell. Every temperature sensor measured directly in the fluid stream. The pressure measurement was realized by digital pressure indicators (Digibar). The temperature was controlled and regulated with PID regulators and heating tapes. The laser beam was situated near the outlet of the view cell at a distance of 15 cm from the inlet of the organic liquid. The inlet of the organic liquid was a
Figure 1. Diagram of the experimental setup. Table 1. Refractive Indices of Different Solvents8 and Mixtures Thereof liquid
refractive index
acetone DMSO DMF DMF/acetone 1:1 acetone/DMSO 1:1 acetone/DMSO 5:1 acetone/DMSO 1:5 acetone/DMSO 1:10
1.35 - 0.1i 1.42 - 0.1i 1.43 - 0i 1.39 - 0.05i 1.38 - 0.1i 1.36 - 0.1i 1.40 - 0.1i 1.41 - 0.1i
T fitting with a normal 1/4-in. tube (inner diameter ) 1 mm). For the ultrasound treatment, the entire extraction cell was placed in a heated water bath with an ultrasound generator at the bottom. The frequency and the amplitude of the ultrasound was not specified. 3.2. Materials. Particle size measurements applying the three-wavelength-extinction technique were performed by using titanium dioxide (TiO2) particles as reference materials. The size of the TiO2 particles was also determined by offline techniques applying a Malvern Mastersizer (Type S). The TiO2 particles used can be described by the following parameters: xp (0.5) ) 0.6 µm (mean particle size), σ ) 0.4 µm (width of particle size distribution), m ) 2.593 - 0.1i (from Malvern Mastersizer database), spherical shape, and no interactions with solvents. The TiO2 was received from Riedel de Ha¨en, Hannover (Germany). The sphericity was determined by SEM. For the investigation of microemulsions in sc-CO2, DMF and different DMSO/acetone mixtures were used. The refractive indices and densities for pure sc-CO2 were taken from literature.7 The values for the refractive indices of the organic solvents are listed in Table 1. The values for the mixtures were estimated from the refractive indices of the pure substances. 4. Results 4.1. Investigation of Suspensions. 4.1.1. TiO2 Particles in Liquid CO2. For statistical consideration, nine particle size measurements of TiO2 in liquid CO2 were carried out (number of runs, n ) 9). Liquid CO2 was used under the following conditions: refractive index, m ) 1.19; pressure, p ) 60 bar; and temperature, T ) 293 K. The average value of all of the measured mean particle sizes amounted to xj50,3 ) 0.73
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4855
Figure 2. 3-λ experiment with TiO2 in liquid CO2.
µm. (xj50,3 quantifies the average geometric mean diameter of all measured particles relative to their volume). The average standard deviation, σ, of the average mean particle sizes was 0.11 µm. That means that the detected mean particle sizes of the TiO2 particles are very close to the value obtained by the Mastersizer. As an example the time-resolved measurement of the particle size in an experimental run using ultrasonic treatment to distribute the particle material is shown in Figure 2. The curve shows the time-resolved measurement with four significant phases. During the first 50 s, the particles were washed out of the extraction vessel. The second phase shows the effect of the ultrasound treatment, indicated by the appearance of a larger mean particle size. The third phase shows a decreasing particle size because of deflocculation and, after the agglomerates are destroyed, a stable mean particle size of about 0.6 µm. 4.1.2. TiO2 Particles in Near-Critical CO2. Because compressed CO2 is subject to remarkable changes of its refractive index in the near-critical region by slight changes of pressure and temperature, the stability of the size measurement results was investigated in this region (n ) 4). The near-critical state of CO2 was achieved at a refractive index, m, of 1.06; a pressure, p of 80 bar; and a temperature, T, of 305 K. Although small temperature differences appeared during four experiments, and therefore the refractive index changed also, the results show no conspicuous deviations. The average mean particle size of the inserted TiO2 amounts to xj50,3 ) 0.54 µm. The measured mean particle sizes are very close to the values obtained by the Mastersizer. The overall average standard deviation of σa ) 0.155 µm indicates small changes of the detected mean particle sizes over all measurements. 4.1.3. TiO2 Particles in Supercritical CO2. One object of the study was to look at the effects of density or refractive index changes on the accuracy of the 3-λ particle size measurement technique. To follow this aim, CO2 was compressed under the six different conditions listed in Table 2.
Figure 3. TiO2 particle sizes measured at different sc-CO2 refractive indices. Table 2. Densities and Refractive Indices of CO2 at Different Supercritical Conditions pressure (bar)
temp (K)
density (g/cm)
refractive index
90 130 120 150 100 100
315 318 318 348 373 393
0.45 0.70 0.67 0.47 0.19 0.17
1.11 1.16 1.17 1.10 1.04 1.04
All values obtained (xj50,3 ) 0.61 µm) are situated near the reference size from the Malvern Mastersizer measurement. The average standard deviation of σa ) 0.16 µm indicates a narrow range of the measured particle size data. Consequently, stable and repeatable size measurements are also obtained by the 3-λ technique when the refractive index of the compressed CO2 changes significantly (Figure 3). 4.2. Investigation of Microemulsions. By applying the 3-λ technique, we investigated whether macroscopic solvent droplets are formed in the supercritical phase, what the size of these droplets is, and how the droplet size is influenced by the pressure, temperature, and composition of the solvent mixture. In this study DMF, DMSO, and mixtures of acetone with DMF and acetone with DMSO were examined. The relevant data are listed in section 3.2. Unfortunately, the droplet sizes measured with the 3-λ technique could not be compared with data obtained
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Table 3. Average Mean Droplet Size of DMF in Compressed CO2
liquid CO2 sc-CO2
Table 5. Measured Droplet Sizes of Different DMSO/ Acetone Mixtures in CO2
pressure (bar)
temp (K)
avg mean droplet size (µm)
avg std deviation (µm)
density (g/cm)
60 150
293 353
2.2 3.05
0.48 0.53
0.79 0.37
Table 4. Measured Mean Droplet Size of DMSO in sc-CO2 CO2 density (g/cm)
temp (K)
pressure (bar)
avg mean droplet size (µm)
avg std deviation (µm)
0.25 0.47 0.51 0.66
318 328 318 328
80 115 100 150
2.62 2.97 2.98 3.29
0.45 0.43 0.30 0.28
by offline analysis, because such techniques are not available. All measurements were interpreted after steady-state conditions were achieved. Saturation of the CO2 with the organic substance, droplet formation, and droplet swelling due to CO2 diffusion into the droplet were not measured. Nevertheless, these procedures have significant influence on the droplet size at steadystate conditions. The refractive indices of the saturated CO2 and of the CO2-saturated droplets could not be measured. For the interpretation, the refractive indices of the pure fluids were taken. For acetone/CO2 mixtures, no droplet formation was observed. This corresponds with the fact that acetone and sc-CO2 are completely miscible. 4.2.1. DMF/CO2 Mixtures. The formation of DMF droplets was investigated in liquid CO2 and sc-CO2. In both cases, droplet formation was observed (Table 3). In comparison to the TiO2 experiments, the standard deviation of the droplet size distribution is greater, corresponding to a broader distribution. The average mean droplet sizes are of one order of magnitude so a significant effect of the different states of CO2 could not be observed. 4.2.2. DMF/Acetone/sc-CO2 Mixtures. Although pure DMF forms droplets in sc-CO2, mixtures with acetone do not. No droplet formation was observed, because acetone works as a modifier that increases the solubility of DMF in sc-CO2. 4.2.3. DMSO/sc-CO2 Mixtures. The other observed solvent is DMSO. For the droplet size measurements, different supercritical conditions were chosen at temperatures between 313 and 328 K and pressures between 80 and 150 bar. The average droplet sizes measured under these conditions are listed in Table 4. It can be observed that the droplet size slightly increased when the density was increased. 4.2.4. DMSO/Acetone/sc-CO2 Mixtures. Investigation of the droplet formation behavior of DMSO/acetone mixtures in sc-CO2 were realized for four different mixtures (Table 5). For the 1:1 mixture, the formation of droplets was not easy to repeat, because acetone works as a modifier. As mentioned above, acetone is completely miscible with sc-CO2 under this conditions. To verify this presumption, the mixture ratio was set to DMSO/acetone ) 1:5. As expected, no droplets were observed or detected. As a consequence, the content of DMSO was increased. The mixture ratio DMSO/acetone ) 5:1 had more consistent droplet formation behavior, but in general the repeatability was not sufficient. With
DMSO/acetone sc-CO2 avg mean avg std ratio density temp pressure droplet size deviation (cm/cm) (g/cm) (K) (bar) (µm) (µm) 1:5 1:1 5:1 10:1 10:1
0.67 0.67 0.67 0.67 0.71
318 318 318 318 323
120 120 120 120 150
no droplets 2.13 3.01 2.08 1.71
0.67 0.67 0.67 0.71
a mixture ratio of DMSO/acetone ) 10:1, it was possible to form droplets in a repeatable way. The average mean droplet sizes measured show no trend depending on the mixture ratio. All results are of the same order of magnitude. The average standard deviation σa ) 0.69 ( 0.02 µm for all experiments with DMSO/acetone mixtures indicates a broad droplet size distribution. 5. Conclusions This study has shown that the three-wavelengthextinction technique is a suitable method for measuring the size of particles and droplets in sub-, near-, and supercritical CO2 with sufficient accuracy. Because particle and droplet sizes are measured within seconds, the 3-λ technique fulfils the requirements for online control during high-pressure processes. TiO2 particles were used as an inert and wellcharacterized reference material to validate the 3-λ technique. Particle sizes measured with conventional offline techniques (Malvern Mastersizer) could be reproduced in compressed CO2 by the fast online method. A significant influence of CO2 density (depending on pressure and temperature) or CO2 refractive index on the size measurement was not observed. Because the 3-λ technique allows also for the detection and size measurement of droplets, it can be applied to the investigation of microemulsions. It was shown that DMF and DMSO form droplets when added to supercritical CO2, which can be measured online. The droplet size measurement is not significantly influenced by the CO2 pressure, temperature, or mixture ratio. Furthermore, it could be shown that the droplet size increases slightly when the sc-CO2 density is increased. Literature Cited (1) Pfeiffer et al. Optisches Multiwellenla¨ngen-Extinktionsverfahren-angewandt zur online Messung der Gro¨βe und Volumenkonzentration von Partikeln im Abgas von Dieselmotoren. VDI Ber. 1995, 285-300. (2) Zahoransky, R. A.; Kuhnt, W.; Laile, E. Transient on-line particle measurements in the undiluted exhaust of internal combustion engines. American Association for Aerosol Research, Sixteenth Annual Meeting, Denver, CO, Oct 13-17, 1997. (3) Tremmel, A. Elektronenstrahlinduzierte Partikelbildung in Abgasen von Feuerungsanlagen: Einsatz eines optischen In-situ Meβverfahrens. Ph.D. Dissertation, Universita¨t Karlsruhe, Karlsruhe, Germany, 1993. (4) Meyer, J.; Katzer, M.; Schmidt, E.; Cihlar, S.; Tu¨rk, M. Comparative particle size measurements in lab-scale nanoparticle production processes. Third World Congress on Particle Technology, Brighton U.K., July 1998. (5) Mie, Beitra¨ge zur Optik tru¨ber Medien, speziell kolloidaler Metallo¨sungen. Ann. Phys. (Berlin) 1908, 25 (3), 377-445. (6) Wittig, S.; Feld, H.-J.; Mu¨ller, A. Das Dispersionsquotientenverfahren zur optischen Partikelgro¨ βenbestimmung. 3. TecflamSeminar, Karlsruhe, Germany, 1987; pp 61-75.
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 4857 (7) Lo¨bbecke, S. UV-Vis Spektroskopische Untersuchungen von reinem und mit Nitroaromaten beladenem komprimierten Kohlendioxid. Diploma theses, Philipps-Universita¨t Marburg, Marburg, Germany, 1994. (8) Lide, D. R. Handbook of Chemistry and Physics, 79th ed.; CRC Press LCC: New York, 1998.
Received for review February 4, 2000 Revised manuscript received September 25, 2000 Accepted October 2, 2000 IE0001873
