Size distributions of particles obtained from a gas cylinder

Size Distributions of Particles Obtained from a Gas Cylinder The presence of stable, submicron particles in ethylene gas obtained from a gas cylinder at pressures higher than about 50 bar has been established by measuring size distributions of the particles by means of an electrostatic analyzer and an extended electric mobility method after Knutson ["Fine Particles" (B. Y. H. Liu, Ed.), Academic Press, New York, 1979]. INTRODUCTION Gases like ethylene, ethane, methyl ether, CO2, CF3C1, N20, C3Hs, CF2C12, CzF4CIz, H2S, and other gases have been and will be used in gas jet recoil transport systems to improve transport efficiencies (2-5). This improvement of transport efficiencies is due to aerosol particles generated in some expanding gases. After Auman et al. (6) two conditions must be fulfilled for the formation of these aerosol particles. First the gas must partly condense during the expansion from the high pressure in the gas tank to the lower pressure in the aerosol transport system leading to the formation of relatively short-lived droplets of condensed gas. Second the gas must contain small amounts of impurities which are present in sufficient amount even in commercially available high-purity gases. The gas and the impurity condense before or during the expansion and the liquid gas droplets contain the impurity. Furthermore, impurity molecules may be frozen out on the cold droplets which have temperatures down to about -100°C in the case of C2H4. After evaporation of the droplets of liquefied gas the impurity is left behind in the form of an aerosol. Because oversaturation of the gas with the impurity is very sensitive to the density of the gas, it could be possible that the reduction of the high density of the gas and not the low temperature of the gas droplets is responsible for the aerosol formation. However, from experiments of Aumann et al. (6) with extremely purified gases it follows that a possible oversaturation of the expanded gas with the impurity vapor is not responsible for the aerosol formation. There exists no critical concentration below which aerosol formation is impossible. A relatively direct method of establishing the presence of these small aerosol particles is provided by measuring their size distribution by an electric mobility method used in this study. For our investigations we used ethylene gas as an aerosol generator. The gas was subjected to a rapid expansion in a gas regulator and the resultant cooling caused homogeneous nucleation leading to short-lived droplets of C2H4. After evaporation of the droplets the size distribution

of the residual particles was measured. The purpose of the present work is to report on the results of such experiments. EXPERIMENTAL TECHNIQUE A schematic representation of the experimental arrangement is displayed in Fig. 1. The cylinders of ethylene gas (purity > 99.6 vol%) were obtained from Messer-Griesheim GmbH/Frankfurt, and the internal pressures were approximately 100 bar. They were operated at room temperature. (The critical pressure and temperature for ethylene gas are 50.7 bar and 9.25°C (7)). The gas from the cylinder flowed through a regulator and a gas flow meter and was then diluted by filtered laboratory air. During the experiments the gas regulator was held at constant temperature by a thermostat. The diluted ethylene gas with the suspended particles entered an electrostatic classifier with a bipolar charger. The charger established an equilibrium charge distribution and the electrostatic classifier extracted mobility fractions selected by the applied voltage. The electric current carried by the extracted mobility fraction was measured by a collecting filter/electrometer arrangement. The mobility analysis used is described in detail by Knutson (1). From the mobility data the resulting size distribution was extracted. RESULTS AND DISCUSSION The expansion of the gas in the reducing valve takes place at constant enthalpy (Joule-Thomson effect) which leads to cooling of the gas. The obtained temperature difference is proportional to the pressure difference of the gas. In our experiments the pressure of the expanded gas was about 1 bar and the pressure in the gas tank ranged from ~120 to ~1 bar, and one can expect a decrease of the gas temperature with increasing pressure in the gas tank. Figure 2 shows three size distributions of the "impurity aerosols" obtained for pressures in the gas tank of 115, 65, and 59 bar and a constant gas ftowrate of 30 liters/hr. There is a decrease of particle size of the aerosol with de-

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0021-9797/80/030297-04502.00/0 Journal of Colloid and Interface Science, Vol. 74, No. 1, March 1980

Copyright © 1980 by Academic Press, Inc. All rights of reproduction in any form reserved.

298

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creasing gas pressure in the tank and below about 50 bar for ethylene no more particles are formed. This means that for p ~< 50 bar cooling o f the gas by rapid expansion is not sufficient to cause h o m o g e n e o u s nucleation leading to short-lived droplets o f ethylene.

For a higher pressure in the gas tank, the gas temperature in the reducing valve is lower, and supersaturation of ethylene is higher. Therefore, the size of the droplets o f liquefied gas increases with increasing gas pressure in the tank, and the same is true for the residual particles. For particles b e l o w about 0.02 tzm there is an increasing influence of diffusion losses b e t w e e n the reducing valve and the measuring device on the size distribution. Figure 3 s h o w s the size

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PARTICLE DIAMETER.pm FIG. 2. Size distributions of the "impurity aerosols": , ,,,, 10 2 (1)p = 115 bar, Q = 30 liters/hr, tt = 22°C, tth = 20°C. Q01 0,I (2) p = 65 bar, Q = 30 liters/hr, tt = 22°C, tth = 20°C. PARTICLE DIAMETER.~m (3) p = 59 bar, Q = 30 liters/br, tt = 22°C, tth = 20°C. FIG. 3. Size distributions of t h e " impurity aerosols": p = pressure in the gas tank; Q = flowrate o f the gas; (4) p = 73 bar, Q = 40 liters/hr, tt = 22°C, tth = 20°Ctth = temperature o f the thermostat; tt = temperature • (5) p = 73 bar, Q = 20 liters/hr, tt = 22°C, tth = 20°C. o f the gas tank. ,

Journal of Colloid and Interface Science, V o l . 74, N o . 1, M a r c h 1980

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299

FIG. 4. Plume of liquefied ethylene droplets.

distributions of the "impurity aerosols" obtained for gas flowrates of 20 and 40 liters/hr and a constant gas pressure of 73 bar. There is an increase of the number concentration of the smaller particles for higher flowrates perhaps due to shorter times for the droplets to grow by coagulation and condensation. In our investigations the reducing valve was held at a constant temperature of 20°C by a thermostat. Nevertheless, the cold gas may cool some parts of the interior of the valve, which may have a temperature differing from the one of the bath. This "self-cooling" of the valve is very intensive without a temperature stabilizing bath, and one gets an increase of particle size up to diameters of ~0.6 /zm (4) for the same fiowrates of ethylene given above. From experimental studies (6) we know that the aerosols obtained from a gas cylinder are formed from the nonvolatile impurities which are contained in the gas and that these impurities consist of heavy hydrocarbons having mass numbers up to about 400. It seems to be inevitable that impurities of sufficient amount are present even in high-purity gases as they can be bought, because "high-purity" gases are made from very impure gases and it is very difficult to purify an extremely impure gas to such an extent that no traces of impurity are present and because the solubility of nonvolatile compounds in compressed gases is drastically increased at high pressures (6). Besides the interest in the size distributions of the aerosols there is the question of the lifetime of droplets of condensed gas and impurity particles. A droplet of condensed gas

evaporates in the course of time because it is colder than the surrounding gas and the gas molecules are transferring their energy of thermal motion to the droplet. For example the lifetime of droplets of liquefied ethylene suspended in ethylene gas (1 bar, 20°C) with diameters of 0.1 /zm was estimated to be smaller than 0.1 sec (4). For very high flowrates of ethylene and without use of a thermostat for the valve, "self-cooling" of the valve is especially powerful, and at the exit of the valve the droplets of liquefied ethylene become visible (Fig. 4). To study the lifetime of the "impurity aerosol," the aerosol was filled in a 2-liter glass bottle and illuminated by a 5-roW H e - N e laser for a short time every hour. The particles could be observed even after 10 hr; this means that the evaporation rate of the particles is very small. Under vacuum conditions of an electron microscope the particles are stable, but evaporate in a short time by the electron beam. For the ethylene gas used in our experiments the impurity probably consisted of pump oil or similar compounds. It should be possible to add different substances of low vapor pressure to the gas in the tank to get ultrafine aerosols of the desired impurity.

REFERENCES 1. Knutson, E. O., "Fine Particles" (B. Y. H. Liu, Ed.), Academic Press, New York, 1979. 2. Wiesehahn, W. J., D'Auria, J. M., Dautet, H., and Pate, B. D., C a n a d . J. P h y s . 51, 2347 (1973).

Journal of Colloid and Interface Science, Vol.74, No. 1, March 1980

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NOTES

3. Aumann, D. C., Presuhn, R., and Weismann, D., G.S.l.-Bericht 71-75, 41 (1975). 4. R6big, G., Thesis, Giessen (1975). 5. Wollnik, H., Wilhelm, H. G., R6big, G., and Jungclas, H., Nucl. Instr. Meth. 127, 539 (1975). 6. Aumann, D. C., Presuhn, R., and Weismann, D., Nucl. Instr. Meth., submitted. 7. Messer Griesheim, Gase hoher Reinheit, Frankfurt.

Journal of Colloid and Interface Science, Vol. 74, No. 1, March 1980

G. ROBIG

H. G. SCHEIBEL J. PORSTENDORFER lnstitut fftr Biophysik Strahlenzentrum der Justus-Liebig-Universit?tt Leihgesterner Weg 217 Federal Republic of Germany

Giessen

Received March 26, 1979; accepted July 17, 1979