Journal of Scientific and Industrial Metrology

Keywords

PM2.5; Aerosol generator; PGSS; Mixing kettle

Introduction

The standard aerosol generation is important for the PM2.5 monitors calibration. Various techniques are known for aerosol generation such as Atomization method, Fluidized Bed method and Agglutination method [1]. However, the limitations of these traditional methods must be focused that Atomization method could only be applied to powder materials dissolved easily in liquid [2], the Fluidized Bed method is well known for the unevenly distributed aerosol particles [3], and the Agglutination method is only applicable to the particular materials with reaction characteristic [4]. A new approach is therefore needed in the generation of PM2.5 standard aerosol.

The generation of nano-micron particles based on supercritical fluids, especially the PGSS approach, has made a great progress in recent years. PGSS involves first dissolving the supercritical fluid in the molten liquid to form the saturated solution, then spraying saturated solution through the nozzles, causing the nano-micron particles to be generated [5]. With the advantages of the lower operating pressure, less dosage of CO₂ and no organic solvent, etc., compared with other supercritical fluids technologies, such as Rapid Expansion of Supercritical Solutions and Supercritical Anti-Solvent [6], PGSS has been widely applied. PGSS was firstly introduced by Weidner [7] in 2003 who chronically committed to the research of the micronization of polyethylene glycol [8,9]. Tandya et al. [10] had generated cyclosporine particles for 1 μm using PGSS. Wang [11] had generated menthol / wax microcapsule from 2 μm to 30 μm. These above showed that PGSS is a better method for the PM2.5 standard aerosol generation with the advantage of controlling the diameter and distribution of the particles by only adjusting the temperature and pressure of the mixing kettle. Therefore, a PM2.5 standard aerosol generation process based on the PGSS was developed, as well as a PM2.5 standard aerosol generator was designed in this paper.

Process of PM2.5 Standard Aerosol Generation

The PM2.5 standard aerosol generator based on PGSS is shown in Figure 1.

Figure 1: Schematic diagram of PM2.5 standard aerosol generator based on PGSS. 1 - CO₂ cylinder; 2, 6, 8, 10, 11, 12, 14, 15, 16, 17, 19, 20, 21, 27 - Valve; 3 - Purifier; 4 - Flow Meter; 5 - Refrigeration Equipment; 7 - High - Pressure Pump; 9 - Buffer Kettle; 13 - High - Pressure Mixing Kettle; 18 - High - Pressure Mixing KettleII; 22 - Buffer Kettle; 23 - Flow Meter; 24 - Crystal Kettle; 25 - Metering Pump; 26 - Solution Tank.

1. The CO₂ from the CO₂ cylinder is fed through the flow system, the value 27 is kept opening to exhaust, and then the CO₂ cylinder valve should be closed before the experiment.

2. Materials packed in high-pressure mixing kettle 13 or 18, are heated using the thermostat in a temperature higher than the fusion point of them after a predetermined period of time to melt them.

3. Opening the refrigeration equipment to preset to the cooling temperature, then the relevant valves 2, 6, 8, 10, 11 (or 15) are turned on. The CO₂ from the CO₂ cylinder passes through purifier, refrigeration equipment, high-pressure pump in turn resulting in a supercritical state, and then flows into highpressure mixing kettle.

4. The supercritical carbon dioxide in high - pressure mixing kettle is maintained at the preset pressure and temperature for 0.5 h to 5 h so as to mix well with the melted materials.

5. Upon opening the valve 16, the supercritical carbon dioxide is introduced through bypass pipeline into the nozzle, then the valve 14 or 19 is turned on that the mixture from highpressure mixing kettle is sprayed through the nozzle with a decompression and expansion, to avoid the blocking in the generation of PM2.5 aerosol, and the supercritical carbon dioxide inside the nozzle designed this work can further mixes with the melted materials, as well as further disperses small droplets. Then the aerosol generated gets through the valve 27 into subsequent equipment.

The PM2.5 Standard Aerosol Generator Design

The PM2.5 standard aerosol generator based on PGSS consisted of the following main components: CO₂ cylinders, purifier, refrigeration equipment, and high - pressure pump, thermostat, two high-pressure mixing kettle, crystal kettle, nozzles. The last three components were designed as the main task in this work. This design can meet with the requirement of the temperature and pressure according to Chinese Standards. The others were selected from standard component which could also meet with the experiment requirements.

The design of high - pressure mixing kettle

The design parameters of the high - pressure mixing kettle were as follows: maximum working pressure of 15 MPa, maximum design temperature of 100°C, and inner diameter of 30 mm. According to these design parameters, the high-pressure mixing kettle was designed and calculated according to Chinese Standard TSG R0004 - 2012 [12] and Chinese Standard GB150-2011 [13]. The thickness of the shell, the maximum working pressure and the thickness of the head can be calculated using the formula 1 - 3, respectively. The structure of the high - pressure mixing kettle was shown in Figure 2.

Figure 2: Diagram of high - pressure mixing kettle. 1 – Nut; 2 - Bolt; 3 - Cylindrical Shell; 4 - Flange; 5, 12 - Flat Head; 6 - Inlet Connection; 7 - Outlet Nozzle; 8 - Flat Gasket; 9 - Sintered Plate; 10 - Sintered Plate Fixed Ring; 11 - Emptying Tube.

(1)

(2)

(3)

Where,δ is shell thickness; Di is inner diameter; [σ]^t is allowable stress of material; φ is seam coefficient; p_c is design pressure, δ_e is effective thickness, D_G is the center diameter under the pressing force, δ_p is thickness of the head, and K is structure coefficient of the head (Figure 2).

The design of crystal kettle

The design parameters of the crystal kettle were as follows: maximum working pressure of 15 MPa and inner diameter of 55 mm. The crystal kettle structure was also designed up to China's Nation Standard and Profession Standard [13], for calculating the thickness of the shell and the head obtained from the Equation (1) and (2), respectively, it was shown in Figure 3.

Figure 3: Diagram of crystal kettle. 1 - Flat Head; 2 - Flat Gasket; 3 – Nut; 4 - Stud Bolt; 5 - Sealing Gasket; 6 - Kettle Body; 7, 9 - Supercritical Fluid Inlet Connection; 8 - Mixture Entrance; 10 – Nozzle; 11 - Pressure Strip; 12 – Window; 13 - Sintered Plate Fixed Ring; 14 - Sintered Plate; 15 - Emptying Tube

The design of nozzle

The nozzle shown in Figure 4 is a coaxial three - channel internal - mixing size - adjusting nozzle. The main components of it include the lock nut, seal ring, taper sleeve and core. The taper sleeve was designed according to Chinese Standard JB4732 - 2005 [14]. The thickness of the taper sleeve can be calculated according to the formula 6. The thread stress can be calculated according to the formula 7. The spraying of mixture was a critical step in the generation of PM2.5 aerosol, and the size of aerosol particles was affected by adjusting the nozzle outlet for the clearance size which would decrease as the taper sleeve rising owing to the certain angle between the core and taper sleeve. It meant that different diameters were achieved in one nozzle rather than in several different conventional nozzles. This work not only greatly reduced the number of nozzles and the manufacturing cost, but also was convenient to replace the nozzles frequently during the experiment work.

Figure 4: Diagram of coaxial three - channel internal-mixing size - adjusting nozzle. 1 - Outlet; 2 - Central Pipeline; 3 - Bypass Pipeline; 4 - Lock Nut; 5 - Seal Ring; 6 - Taper Sleeve; 7 - Mixing Chamber; 8 - Core

(4)

(4)

Where, D_i is the inner diameter of taper sleeve, D_o is the outer diameter of taper sleeve, P_c is the design pressure, K₁ is load combination factors, S_m is stress intensity, δ₁ is the thickness of taper sleeve, σ_m is the thread stress, and F is axial load.

Manufacturing and Testing of the PM2.5 Standard Aerosol Generator

Manufacturing of the equipment

The PM2.5 standard aerosol generator was manufactured by Nantong Huaan Chaolinjie Co. LTD. High-pressure mixing kettle, crystal kettle and the PM2.5 standard aerosol generator assembled are shown in Figures 5a-5c, respectively.

Figure 5: Pictures about PM2.5 standard aerosol generator. (a) High-pressure mixing kettle (b) Crystal kettle (c) PM2.5 standard aerosol generator.

Testing of the equipment

The experiment was firstly carried out with the high-pressure mixing kettle keeping at 15 MPa and 90°C for 90 min. Then mixture was sprayed through the nozzle into the crystal kettle which was maintained in 5.6 MPa at ambient temperatures keeping the sprayed time for 120 min. In this research, the particles of polyethylene glycol 8000 were generated using PGSS method, collected on the 1000 mesh sieve in Figure 6a, and the Micro-morphology of them was then investigated by the scanning electron micrograph (SEM) shown in Figure 6b.

Figure 6: The experiment result obtained with high-pressure mixing kettle keeping at 15 MPa, 90°C for 90 min (a) Polyethylene glycol particles (b) SEM photograph.

Subsequently experiment was conducted with the high-pressure mixing kettle keeping at 20 MPa and 80°C for 90 min. Then mixture was sprayed through the nozzle into the crystal kettle which was maintained in 5.6 MPa at ambient temperatures keeping the sprayed time for 120 min. The particles of polyethylene glycol 8000 generated using PGSS method were shown in Figures 7a and 7b.

Figure 7: The experiment result obtained with high-pressure mixing kettle keeping at 20 MPa and 80°C for 90 min (a) Polyethylene glycol particles (b) SEM photograph.

As shown in Figures 6b and 7b, the particles were small and uniform for overall distribution with more than 90% smaller than 2.5 μm in diameter, and the others were larger than 2.5 μm because a large number of tiny particles aggregated on the mesh sieve in the colleting process. In order to solve the particle aggregation problem, the crystal kettle would be equipped with electrostatic ion-generator to electrify the particles and produce repulsive force so as to overcome the effects of van der Waals forces to avoid aggregation, and the aerosol was directly sprayed into the aerosol spectrometer to detect the aerodynamic diameter of aerosol particles.

Summary

In this paper, the PM2.5 standard aerosol generator based on the PGSS was firstly designed and manufactured. And the results of preliminary experiments, in which the nano-micron particles were generated with polyethylene glycol 8000 as the raw material, showed that more than 90% of the aerosol particles were smaller than 2.5 μm in diameter, this result is similar to the Atomization method with diameter in the range of 1.3 μm - 5.8 μm [1], and the others were larger than 2.5 μm due to the aggregation of tiny particles on the mesh sieve in the collecting process. And further study on the generation process conditions is still required in subsequent experiments to realize steady generation of standard PM2.5 aerosol and make the device as a steady source of PM2.5 aerosol. PM2.5 standard aerosol generator will have great significance for the realization of PM2.5 monitors testing and calibration.

Acknowledgement

The work reported here was supported by the Shandong Institute of Metrology and the Department of Shandong province. We express our grateful thanks to the Science and Technology Development Plan Project of Shandong Province (2014GSF117026) and Project of Shandong Province Higher Educational Science and Technology Program (J13LM08) for their financial support.

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