Real time measurement of aerosol size distribution using MASTERSIZER

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 44, August 2006, pp Real time measurement of aerosol size distribution using MASTERSIZER R Baskaran*, V Subramanian & T S Selvakumaran Radiological Safety Division, Safety Group, Indira Gandhi Center for Atomic Research, Kalpakkam Received 14 February 2005; revised 20 March 2006; accepted 2 May 2006 An Aerosol Test Facility (ATF) has been set-up at Radiological Safety Division (RSD), Indira Gandhi Center for Atomic Research (IGCAR), Kalpakkam, for the aerosol studies on fast reactor safety. In order to measure the Mass Median Diameter (MMD) of the aerosols in real time, a laser based instrument MASTERSIZER (M/s MALVERN, UK) is used. Mastersizer is provided with a liquid flow cell and a powder spray unit which are useful for the off-line measurement. For the real time measurement, an aerosol flow cell is designed and fabricated. It is integrated with the MASTERSIZER and the aerosol chamber of ATF. Aerosols are drawn through the aerosol flow cell whose optical windows are aligned with the laser path of MASTERSIZER. In this paper, the details and the performance test results of the aerosol flow cell are presented. Performance test of the aerosol flow cell is carried out with polystyrene latex particles of diameter µm and methylene blue aerosols. Sodium oxide aerosols are generated and injected into the aerosol chamber, and the time evolution of MMD of sodium oxide aerosols has been studied. Keywords: Mastersizer, Aerosol size, Aerosol test facility, Fast reactor safety IPC Code: B01J19/00 1 Introduction In the case of most unlikely event of Core Disruptive Accident (CDA) in Liquid Metal Fast Breeder Reactor (LMFBR), the sodium slug may impact the reactor roof slap. This provides a pathway for the escape of radioactive material (fission products and fuel material) and sodium into the containment 1. Fuel and fission product vapours will condense and form aerosols 2. For the aerosol studies of fast reactor safety analysis, an Aerosol Test Facility (ATF) has been designed, fabricated, installed and tested 3,4 at RSD, IG- CAR. ATF mainly consists of an aerosol chamber made of SS-304 and having 1 m 3 volume. The chamber has eight ports through which aerosol source and diagnostic equipments are connected to the chamber. A thermal plasma torch is used as a source for producing aerosols 5,6 of fuel and fission product equivalent materials. A sodium combustion cell is used for producing sodium oxide aerosols 7. Our main objective is to measure the time evolution of mass concentration and size distribution of aerosols generated inside the aerosol chamber. These measurements will provide: (i) input data for HAARM (Heterogeneous Aerosol Agglomeration Revised Model) code 8, which predicts the behaviour of aerosols inside the containment building of LMFBR after CDA, (ii) co- * rb@igcar.ernet.in agglomeration process of fuel equivalent material, fission products and sodium 9. The instantaneous measurement of size distribution of aerosols can be done with light scattering device. Other methods, such as Beta-guard counter 10 and Quartz Crystal Microbalance 11, determine the masssize distribution in a interval of time. For the real time measurement of aerosol particle size distribution in ATF, a laser based aerosol measurement apparatus MASTERSIZER (M/s Malvern, UK) is used. The Mastersizer uses the principle of ensemble dif technique. A laser beam is expanded and then collimated into a beam of several mm diameter (2 mw He-Ne laser, 633 nm, with 18 mm beam diameter), which passes through the particle cloud (14.3 mm length). Particles in the beam scatter light in all directions. A receiving lens is used to focus both the transmitted beam and forward scattered beam onto a detector located at the focal plane of the lens. The transmitted light is focused to a point on the optical axis while the diffracted light forms a series of concentric rings (Faunhofer dif pattern). As the receiving lens performs a Fourier transform on the scattered light, light scattered at a given angle θ by a particle located any where in the illuminated sample volume will be focused at a same radial position in the detector array. The Mastersizer measures the volume-size distribution of particles in the laden volume

2 BHASKARAN et al.: REAL TIME MEASUREMENT OF AEROSOL SIZE DISTRIBUTION USING MASTERSIZER 577 of air from μm. The measurement of vol-d 50 (50 th percentile of volume size distribution) is the same as Mass Median Diameter (MMD), since measurement is carried out only for a single material at any given time for which the density is included. The instrument is provided with a liquid flow cell and a powder spray unit and these units are useful for the off-line measurement. In order to measure the aerosol size distribution in real time in the aerosol chamber of ATF, an aerosol flow cell has been designed fabricated and integrated with the MASTERSIZER. A pump with a flow rate of few l/min drags the aerosols, which pass through the optical window of the aerosol flow cell. The aerosol size range in the chamber is in the order of μm. The performance evaluation of the aerosol flow cell has been studied with the methylene blue aerosols and latex particles. In this paper, the description and the performance evaluation results of the aerosol flow cell are presented. With this setup, the time dependant measurement of MMD of sodium oxide aerosol is carried out in the aerosol chamber of ATF and these results are presented. 2 Aerosol Flow Cell A schematic diagram of the aerosol flow cell is shown in Fig. 1. Aerosol flow cell is a hollow stainless steel pipe with rectangular cross-section of 14.3 mm width and 103 mm breadth and it has a length of 730 mm. Both the ends of the cell are provided with flanges. The aerosol chamber is connected in one side and the other side is connected with an air displacement pump. Using a flow control the pump draws aerosols from the chamber through the flow cell and the aerosols are fed back to the aerosol chamber. The gas flow rate (about 0.5 lpm) is adjusted to have a laminar flow. In order to have an optical path for the MASTERSIZER, suitable openings are made in side plates of the flow cell. In the openings, grooves are made and laser windows are connected. The distance between the windows is kept at 14.3 mm, which is one of the standard settings of the Fig. 1 Schematic diagram of the aerosol flow cell Fig. 2 Experimental set-up for the performance evaluation of the aerosol flow cell MASTERSIZER. Parallelism between both the laser windows is an extremely important factor for the maximum transfer of laser power. Suitable fabrication procedures are adopted to get parallelism within ± 0.1 deg. 3 Performance Test Results of Aerosol Flow Cell The experimental set-up for the performance evaluation of the aerosol flow cell is shown in Fig. 2. The aerosol flow cell is placed such that the optical windows are in line with the laser path. Initially the optical path is aligned with the MASTERSIZER and the background is measured. The aerosol flow cell was tested with the polystyrene latex spheres (M/s Polyscience, USA) of diameter μm and methylene blue aerosols. Polystyrene latex spheres are produced and injected in the flow cell and the aerosol volume-size distribution is measured. Fig. 3 shows the volume-size distribution of polystyrene latex spheres and the mass median diameter is 2.55 μm. Methylene blue aerosols are produced by atomization and the aerosols are injected in the flow cell. The volume-size distribution of methylene blue aerosol is shown in Fig. 4. The Mass Median Diameter (MMD) for methylene blue aerosols is 15.8 μm. Using Quartz Crystal Micro Balance, aerosol mass-size distribution is measured at the outlet of aerosol flow cell. QCM is having many stage cascade impactors that operate with multiple sets of paired piezoelectric crystals. In each pair, one crystal is used as mass sensor on which aerosols deposit and the other one is used as reference crystal. An increase in

3 578 INDIAN J PURE & APPL PHYS, VOL 44, AUGUST 2006 mass on sensitive crystal due to deposition of aerosol causes the natural resonant frequency to decrease in direct proportion to the increased mass. The change in beat frequency of the sensitive and reference crystal is the measure of mass deposit of the aerosol. Model PC-2 of M/s California Measurements INC, USA is used in our experiments. The impactor operates at a flow rate of 240 ml/min and has 10 stages, which segregates particles from 0.05 to 25.0 µm at a particle density of 2g/cm 3. QCM measurements are repeated and the results are presented in Table 1. From the QCM manual 12 (Page No.6a), the size range for various stages is obtained for the particle density of 1 g/cc (aerodynamic diameter) and only this size range is reported in Table 1. The Mass Median Aerodynamic Diameter (MMAD)is determined by plotting cumulative mass percentage versus effective cut-off diameter () in terms of aerodynamic diameter. Fig. 5 shows the cumulative mass percentage versus effective cut-off diameter () in terms of aerodynamic diameter for the measurement-3 and the MMAD is 15.5 μm. The mean MMAD for the three measurements is μm. Using the density of methelene blue (1.26 g/cc), the MMD is determined to be 16.4 μm. The MMD value obtained from MAS- TERSIZER measurements differs nearly by 4% from the QCM measurements. Thus, the aerosol flow cell is tested for the measurement of smaller and larger size particles and it is qualified for the use with the MAS- TERSIZER. Fig. 3 Volume-size distribution of polystyrene latex spheres obtained by MASTERSIZER with the aerosol flow cell Fig. 4 Volume-size distribution of methylene blue aerosols obtained by MASTERSIZER with the aerosol flow cell Size Range (μm) (μm) Table 1 Results obtained from QCM measurements Measurement 1 Measurement 2 Measurement 3 Cumulative % less than Cumulative % less than Cumulative % less than > MMAD (μm) Average MMAD =18.5 μm Average MMD = 16.4μm = effective cut-off diameter; MMAD = mass median aerodynamic diameter; MMD = mass median diameter

4 BHASKARAN et al.: REAL TIME MEASUREMENT OF AEROSOL SIZE DISTRIBUTION USING MASTERSIZER Real Time Measurement in ATF Schematic diagram showing the integration of the MASTERSIZER and the aerosol flow cell with the aerosol chamber is shown in Fig. 6. A constant airflow (~ 0.5 lpm) from the aerosol chamber is maintained by a pump and a laminar flow of aerosol in the flow cell is maintained. The aerosols are injected back into the aerosol chamber. The flow Reynolds number for the diameter of 14.3 mm (optical path) with 0.5 lpm is 5.2 which is very much lesser than gas flow Reynolds number 2000 to exceed turbulent flow condition 13. The initial pressure inside the chamber at the instant of aerosol generation is at 4 kpa in excess. These two factors help in filling the aerosol flow cell with aerosols and the changes in the aerosol parameters in the sample volume reflect the changes in the aerosol parameters inside the chamber at any instant. About 5 g of sodium was loaded in the sodium combustion cell of ATF and it was heated up to 550 o C in an argon environment. Then, argon is let out from the sodium combustion cell and air is let in. By combustion, sodium was burnt and the aerosols were produced. The aerosol volume-size distribution as a Fig. 5 Cumulative mass percentage versus effective cut-off diameter () Fig. 7 Volume-size distribution of sodium oxide aerosols at time t = 0 and t = 1 min from the time of injection of aerosols into the aerosol chamber Fig. 6 Schematic diagram showing the integration MASTERSIZER together with the aerosol flow cell in ATF

5 580 INDIAN J PURE & APPL PHYS, VOL 44, AUGUST 2006 function of time is measured using MASTERSIZER. The volume-size distributions of sodium oxide aerosols at time t = 0 and t = 1 minute are shown in Fig. 7. The growth of aerosol size is seen in these measurements. The measurement can be performed with a minimum time delay as low as 1 second and it takes only few milli-seconds for making measurement, processing and displaying the results. The results can be analyzed at a later stage. The changes in volume-size distribution observed at time t=0 and t=1 minute shows the growth of particle size due to coagulation of aerosols. Such a fast change in the aerosol parameters inside chamber is determined using the combination of flow cell and MASTERSIZER. 5 Conclusion An aerosol flow-cell has been designed, fabricated and tested. The Mastersizer has been integrated with ATF through the aerosol flow cell and the changes in volume-size distribution of aerosols in the aerosol chamber are studied. References 1 Berthoud G, Longest A W, Wright A L & Schutz W P, Nuclear Tech, 81 (1988) Walter A E & Reynolds A B, Fast Breeder Reactors, Pergamon Press (1981). 3 Baskaran R, Selvakumaran T S & Subramanian V, Indian J of Pure & Appl Phys, 42 (2004) Baskaran R, Subramanian V & Selvakumaran T S, Generation of Aerosols Using Thermal Plasma Torch for Fast Reactor Safety Studies, IASTA Bulletin, 16 (2004) Young R M & Pfender E, Plasma Chemistry & Plasma Processing, 5 (1985) 1. 6 Venkatramani N, Current Sci, 83 (2002) Baskaran R V, Subramanian & Selvakumaran T S, Indian J Environ Protection, 24 (2004) Gieseke J A, Lee K W & Reed L D, HAARM-3 User Manual, NRC Report BMI-NUREG-1991 (1978). 9 Gieseke J A et al., Characteristics of agglomerates of sodium oxide aerosol particles, BMI-NUREG Klein F, Ranty C & Sowa L, J Aerosol Sci, 15 (1984) Ward M D & Buttry D A, Science, 249 (1990) User Manual, Air Particle Analyser QCM Cascade Impactor System, California Measurements, INC, California 91024, USA, Willeke K & Baron P A, Aerosol Measurement: Principles, Techniques, & Applications, Van Nostr & Reinhold, New York (1993).