- Email: [email protected]
Simple and cost-effective powder disperser for aerosol particle size measurement
Available online at www.sciencedirect.com
Powder Technology 187 (2008) 27 – 36 www.elsevier.com/locate/powtec
Simple and cost-effective powder disperser for aerosol particle size measurement P. Tang a , D.F. Fletcher b , H.-K. Chan a,⁎, J.A. Raper c a
Faculty of Pharmacy, University of Sydney, NSW 2006, Australia School of Chemical and Biomolecular Engineering, University of Sydney, NSW 2006, Australia Department of Chemical and Biological Engineering, University of Missouri-Rolla, MO 65409-1230, USA b
c
Received 20 December 2007; accepted 7 January 2008 Available online 16 January 2008
Abstract Commercial dry powder dispersers needed in conjunction with particle size measurement equipment are usually quite expensive (of the order of thousands of dollars). We have found that a simple vacuum generator can be used as a cost-effective disperser (US$50). Comparison with other commercial dispersers, small scale powder disperser (SSPD) model 3433 (TSI, Shoreview, USA) and Scirocco dry powder disperser (Malvern, Worcs, UK), showed that our disperser worked as efficiently as these expensive dispersers. Crystalline mannitol (less than 1% moisture content) and amorphous BSA (8.5–9.2% moisture content) smooth spherical particles were used to test the dispersion capability of the unit. Smooth spherical particles were chosen because they are more cohesive than corrugated particles due to increased contact points. Therefore, sufficient dispersions of other less cohesive particles should be able to be achieved using the optimum conditions reported here. The effects of air pressure, sample weight, and nozzle size of the disperser were investigated. Comparison of the particle size distributions between wet and dry measurements were used to determine the dispersion efficiency. Quantitative comparisons were made using the values of D(v,0.5) and span. The best dispersion was found using a 1.00 mm nozzle and the maximum percentage differences in D(v,0.5) and span are 23% and 19%, respectively, with more than 200 mg mannitol powder dispersed with pressures of 50, 70, 90 psi. Using BSA powders, the maximum percentage differences of D(v,0.5) and span are 37% and 25%, respectively. As was the case for the commercial devices, the dispersion of BSA particles could not be improved even when the pressure of the compressed air was increased. © 2008 Elsevier B.V. All rights reserved. Keywords: Powder disperser; Size measurement; Aerosol
1. Introduction Dry powder dispersers have been widely used and are required to de-agglomerate powder in order for commercial instruments to measure particle size distributions based on laser diffraction [1] and time-of-flight techniques [2]. It is recognized that complete dispersion of dry particulate solids, especially in the size range below 20 μm, is difficult to achieve due to the strong cohesive forces, namely van der Waals, magnetic, electrostatic, and forces due to solid or liquid bridges. In general, the
⁎ Corresponding author. E-mail address: [email protected] (H.-K. Chan). 0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.01.003
strength of these forces increases with decreasing particle size. It has been reported that the van der Waals force, the main interparticle attraction force, is approximately 100 times stronger than gravity for 10 μm particles [3]. For 1 μm particles, this force increases to 1000 g (g being the gravity force) and for 0.1 μm, it increases to between 104 and 105 g. De-agglomeration in the dry powder feeder associated with the Malvern Mastersizer 2000 (Malvern, Worcs, UK) laser diffraction instruments is achieved by accelerating the particles close to sonic speed along a tapered transport tube. This mechanism provides the shear forces and collisions between the particles and the feeder's walls in order to break up the aggregates. Pressure titration (from 0.1 to 4.0 bar) is recommended to find the optimum pressure that can break up aggregates without fracturing the individual particles. The optimum pressure is found when the
28
P. Tang et al. / Powder Technology 187 (2008) 27–36
particle size analysis from dry and wet dispersions agree. Previous versions of dry powder dispersers associated with time-of-flight equipment, such as the Aerosizer™ LD (Amherst Process Instruments, Hadley, MA) are based on the application of a fluidized pulsed jet, impaction with a disperser pin, and acceleration of the agglomerated powder particles through a sonic nozzle [4,5]. There are four controllable variables available to obtain optimum dispersion for different materials. These variables are: shear force (0.5 to 4.0 psi), feed rate (1000 to 10,000 counts/s), transport velocity from the fluidized bed to the disperser pin (high or low), and pin vibration (on or off). In a small scale powder disperser model 3433 (TSI, Shoreview, USA), abrasive paper is glued to a rotating table to provide a rough surface against which to brush the powder and thus de-agglomeration is encouraged before the powder is sucked through a venturi tube where further de-agglomeration takes place. The shear force in the venturi tube cannot be varied. The newer dry powder disperser associated with Particle Size Distribution Analyser 3603 (TSI, Shoreview, USA) de-agglomerates powder by the action of two pulsating, high (not sonic) velocity air jets with pressure ranging from 2 to 5 psi. A dry powder disperser, called Tornado™, manufactured by Beckman Coulter GmbH (Krefeld, Germany) breaks up the powder aggregates by swirling air, sucked through a ring gap above the sample via rotation of the sample container support, leading to strong impaction with controllable intensities [6]. Pressure titration (0.1 to 4 bar) is also recommended to find the optimum pressure. Sympatec GmbH (Zellerfeld, Germany) claims that their manufactured dry powder disperser, called Rodos, is able to disperse particles down to 0.1 μm [7]. For dosing, a vibratory feed and an inlet funnel connected to the dispersing line is used for less adhesive material, while for extremely fine or adhesive products a rotating table with a compacting roller is recommended. The ultimate de-agglomeration of the powder is achieved by generating a vacuum through expansion of compressed gas applied to an injector in order to accelerate the particles as soon as they arrive in the dispersing line [1]. The pressure of compressed air can be varied from 0.1 to 6 bar to achieve optimum dispersion. Three different sizes of dispersing line (4, 6, and 10 mm) are available to enhance dispersion for more cohesive particles.
The dispersing unit manufactured by Fritsch GmbH (Oberstein, Germany) uses mechanical and pneumatic forces to deagglomerate the powder [8]. An amplitude-controlled vibratory feeder is used for material dosing and the dispersion occurs in a two-phase annular gap nozzle where powders are accelerated at a high flow rate. The default pressure is 3 bar (maximum 4 bar), however it can be reduced to 1 bar for brittle material to avoid milling of particles. Therefore, it can be seen that commercial dispersers are readily available but generally they are quite expensive (of the order of thousands of dollars), and tailor-made equipment requires substantial engineering fabrication. We have found that a simple unit designed originally to create a vacuum using compressed air (Pisco, Illinois, USA), can be used as a suitable alternative. This unit is relatively cheap (approximately US$50) and readily available. Mannitol and bovine serum albumin (BSA) particles produced by spray drying were used to test the unit, representing powders of different cohesiveness. 2. Method 2.1. Particle size measurement Size measurement was done using a Mastersizer S laser diffractometer (Malvern, Worcs, UK) with an aerosol mounting unit (Fig. 1a) to which the disperser's outlet was attached. The other end of the aerosol mounting unit was connected to a vacuum pump to draw out the particles after passing through the measurement zone. The disperser unit (Pisco, Illinois, USA) has two perpendicular inlets for compressed air and powder feed (Fig. 1b). A funnel, having a grid of 1 mm diameter holes to help break up large particle agglomerates, was fitted inside the powder feed inlet. The powder was dispersed by injecting compressed air through nozzles of various diameters (0.25, 0.5, 0.75, and 1.0 mm). Crystalline mannitol with moisture content less than 1% (Fig. 2a) and amorphous bovine serum albumin with moisture content 8.5–9.2% (Fig. 2b) spherical particles of different sizes were used to test the disperser unit. Only smooth spherical particles were tested because it has been shown that they are more cohesive than corrugated spherical particles [9] due to the
Fig. 1. (a) Disperser unit fitted onto the aerosol mounting unit of Mastersizer S for size measurement; (b) Disperser unit to disperse powder to individual particles.
P. Tang et al. / Powder Technology 187 (2008) 27–36
29
(measurement time of 1.0 s) for higher powder loads. These sweep numbers were chosen to ensure that data collection was only done when there were particles passing through the laser beam. Therefore the number of sweeps was increased with increasing sample size. For the wet measurement, the analysis model used was polydisperse and the 300RF lens (size range: 0.05–900 μm) was fitted. Since the sample was circulated continuously between the sample holder and the lens, more sweeps could be chosen to collect enough data. For this, the sweep number was set at 2000 (default value from the equipment). The effects of sample weight, nozzle diameter, and compressed air pressure on powder dispersion were investigated. To study the sample weight effect, samples of 100, 200, and 500 mg were used at each nozzle diameter and pressure. For the 0.25, 0.50, and 0.75 mm nozzles, the compressed air pressure was set at 70 and 90 psi for the 500 mg sample weight. Lighter samples and lower pressures were not used because insufficient dispersed particles could be delivered to the measurement zone (i.e. as indicated by low obscuration). For the 1.0 mm nozzle, the sample weight was varied at 100, 200, 300, 500, 1000 mg and the pressure was varied at 50, 70, and 90 psi. These values were chosen to give enough dispersed particles during size measurement. Each measurement was done in triplicate to ensure reproducibility. 2.2. Computational Fluid Dynamics (CFD) simulation
Fig. 2. Scanning electron micrographs of (a) mannitol and (b) bovine serum albumin spherical particles used to test the disperser. Magnification: 8000×.
presence of more contact points. Therefore the optimum condition found for these particles should be applicable to different shaped particles of the same material. The particle size distributions were represented by the D(v, 0.5), which is the equivalent volume diameter at 50% cumulative volume determined by the Mastersizer S. The width of the size distribution was expressed via the span, calculated as [D(v,0.9) − D (v, 0.1)] / D(v,0.5). For mannitol, particle size analysis was based on the real refractive index (RI) (1.52), imaginary RI (0.1), and RI of air as the dispersion fluid (1.00). For BSA, the real and imaginary RIs are 1.55 and 0.02, respectively. To determine the efficiency of the disperser unit, the reported size of the dry measurement was compared with the wet measurement when the particles were suspended in chloroform (RI of 1.44) in which complete dispersion was achieved. The surfactant polyoxyethylene-sorbitan monooleate (Sigma-Aldrich, St. Louis, USA) was added to the suspension (0.2% v/v) which further sonified for 5 min to help disperse the particles. For dry measurement, the analysis model chosen in the Mastersizer S was the compressed range and the 300F lens (size range: 0.5–900 μm) was fitted along with the aerosol mounting unit. The number of detector scans (sweeps) was set at 100 (measurement time of 0.2 s) for 100 mg powder and 500
Simulations were performed using the commercial CFD software ANSYS CFX11 (http:www.ansys.com/cfx) to study the effect of introducing a swirl flow in the high pressure gas line just upstream of the nozzle exit. Fig. 3 shows the disperser geometry for the swirl flow case. The CFD model solved conservation equations for mass, momentum and energy, together with the Shear Stress Transport (SST) turbulence model. The fluid was treated as an ideal gas with the molecular weight of air. At the inlet the total pressure was set to 70 psi, corresponding to the reservoir conditions, and at the outlet the static pressure was set to be atmospheric. The flow in the nozzle was calculated and then particles were injected as a post process step from the location of the feed tube. The particles were given a density of mannitol (1500 kg m− 3) and were tracked through the gas using the Schiller–Naumann drag law and were subjected to turbulent dispersion by the gas stream. They were given a coefficient of restitution of one at the walls (i.e. they bounced with no loss of energy).
Fig. 3. Disperser geometry created in ANSYS CFX11. The swirler is shown in orange, the inlet from the gas reservoir is shown in blue and the feed location of the particles is shown in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
30
P. Tang et al. / Powder Technology 187 (2008) 27–36
3. Results and discussion 3.1. Comparison between Pisco disperser and other dry powder dispersers The performance of the Pisco disperser was compared with small scale powder disperser (SSPD) model 3433 (TSI, Shoreview, USA) and Scirocco dry powder disperser (Malvern, Worcs, UK) (Fig. 4). The shear force in the Scirocco dry powder disperser was set at the highest value (4 bar) because all lower values were found to be insufficient to achieve complete dispersion. The force in the SSPD could not be varied and was fixed at approximately 0.5 to 0.6 bar. All the measurement was done using a Malvern Mastersizer S except for the Scirocco dry powder disperser which was connected to a Malvern Mastersizer 2000. The sample loaded in the SSPD and Scirocco dispersers was approximately 10 mg, while 200 mg was put in the Pisco disperser. Fig. 4 shows that the Pisco disperser performed as well as the more expensive dispersers. The particle size distribution (PSD) for mannitol powder (Fig. 4a) is very close to that obtained from the wet dispersion. However, none of the dry powder dispersers could reproduce the small mannitol particles peak (particles smaller than 1 μm) shown in the particle size distributions measured from the wet dispersion. As for BSA, the PSD of dry dispersions showed the presence of large aggregates (larger than 20 μm) that could not be broken up. The truncation of the particle size distributions of the dry powders at the fine end (Fig. 4b, PSD obtained using SSPD and Pisco dispersers) was due to the limited range of the 300F optical set up in Malvern Mastersizer S that can only measure particles above 0.5 μm. Unfortunately, with this particular setup in Malvern Mastersizer S, the aerosol mounting unit can only be fixed with the 300F lens instead of the 300RF lens which has a wider size range (0.05–900 μm) normally used for wet dispersion. The truncation was not observed with the PSD obtained from Scirocco disperser because it was connected to Malvern Mastersizer 2000 which has wider size range (0.01 to 10,000 μm).
Table 1 Comparison of D(v,0.5) and span of mannitol and bovine serum albumin particles using small scale powder disperser 3433 (TSI, Shoreview, USA) and Pisco disperser Mannitol D(v, 0.5) μm Wet dispersion Dry dispersion with SSPD 3433 Dry dispersion with Mastersizer 2000 Dry dispersion with Pisco disperser
Bovine serum albumin Span
D(v, 0.5) μm
Span
2.46 (0.02) 1.11 (0.04) 8.27 (0.01) 1.99 (0.01) 2.82 (0.11) 1.48 (0.09) 5.62 (0.75) 2.66 (0.34) 2.58 (0.12) 1.12 (0.05) 8.24 (0.15) 4.87 (0.11) 2.40 (0.10) 1.21 (0.28) 8.99 (0.74) 2.52 (0.86)
The value of D(v, 0.5) and span of the PSDs shown in Fig. 4 are tabulated in Table 1. The Pisco disperser was able to disperse the mannitol and BSA particles efficiently as indicated by D(v,0.5) and span. With the SSPD 3433, many large BSA particles were stuck in the connecting tube between the unit and Mastersizer S, hence the D(v,0.5) measured were smaller than that obtained from the wet dispersion. 3.2. Optimisation of Pisco disperser using different nozzle size The comparison between the Pisco disperser and the other dispersers discussed in Section 3.1 was done using the default 1.00 mm nozzle inside the Pisco disperser. We manufactured different size nozzles, which were 0.25, 0.50, 0.75 mm to investigate the effect of nozzle size on disperser performance. Table 2 shows the sample weight before dispersion and the amount of powder that had gone through the measurement zone. The obscuration, which is the relative amount of light scattered or absorbed by particles, is a good indicator of the particle concentration within the measurement zone. The ideal range of obscuration for dry measurement in the Mastersizer S is 1−10% [10], which guarantees the effective range of signal to noise ratio to avoid multiple scattering.
Fig. 4. Particle size distribution (PSD) obtained from Mastersizer S for (a) mannitol and (b) bovine serum albumin particles.
P. Tang et al. / Powder Technology 187 (2008) 27–36
31
Table 2 Size measurement results for mannitol powder using the disperser unit with 0.25, 0.50, 0.75, and 1.00 mm nozzles Run No
Sample
Pressure
D(v,0.5)
(mg)
(psi)
(μm)
Loaded
Sized
(mg)
Span
Obscuration
(%)
0.25 mm nozzle 1 2 3a 4
96.62 (1.37) 231.22 (22.39) 507.58 (5.65) 497.46 (2.88)
34.98 (18.72) 64.08 (28.44) 124.71 (78.38) 38.40 (6.55)
90 90 90 70
10.46 (11.69) 3.12 (0.41) 2.80 (0.34) 2.97 (0.55)
17.37 (10.70) 19.82 (8.27) 12.36 (10.40) 42.98 (13.43)
1.63 (2.14) 4.10 (3.99) 10.50 (8.58) 1.60 (0.61)
0.50 mm nozzle 5a 6a 7 8
110.61 (6.48) 200.19 (5.16) 499.00 (10.41) 507.68 (1.99)
71.21 (5.34) 80.53 (18.23) 218.82 (97.74) 56.48 (13.93)
90 90 90 70
2.37 (0.01) 2.46 (0.01) 2.56 (0.03) 2.77 (0.17)
1.12 (0.05) 1.59 (0.25) 10.86 (3.68) 33.60 (8.16)
4.00 (0.26) 5.53 (3.90) 7.90 (4.44) 2.43 (2.19)
0.75 mm nozzle 9 10 a 11 12
104.07 (2.31) 201.39 (4.19) 490.31 (11.08) 503.27 (7.55)
84.92 (9.24) 164.63 (9.50) 407.93 (44.98) 418.20 (10.25)
90 90 90 70
2.38 (0.09) 2.32 (0.05) 2.46 (0.16) 2.45 (0.08)
14.97 (27.52) 1.30 (0.01) 27.94 (12.14) 35.63 (14.34)
4.28 (1.59) 7.67 (0.50) 16.15 (3.82) 2.85 (0.81)
1.00 mm nozzle 13 a 14 a 15 a 16 a 17 a 18 a
101.53 (5.44) 205.44 (6.89) 552.23 (8.25) 1035.81 (12.31) 498.3 (7.23) 486.14 (6.74)
92.31 (2.55) 180.25 (9.88) 450.44 (7.82) 827.31 (6.17) 420.45 (9.23) 350.18 (26.55)
90 90 90 90 70 50
3.23 (0.05) 2.55 (0.12) 2.28 (0.06) 2.14 (0.05) 2.48 (0.19) 2.59 (0.26)
2.56 (0.25) 1.52 (0.14) 1.40 (0.09) 1.35 (0.07) 1.50 (0.28) 1.65 (0.32)
2.17 (1.45) 4.45 (2.36) 14.23 (3.22) 18.71 (3.89) 12.43 (4.05) 5.80 (1.38)
Fully dispersed mannitol particles: D(v, 0.5) = 2.11 (0.01) μm and span = 1.38 (0.05). The numbers in brackets indicate the standard deviation. a Indicates optimum conditions for a particular nozzle size.
In the sample dispersion with the 0.25 mm nozzle, there was a wide size distribution (i.e. span) for all sample weights and pressures, indicating that this nozzle failed to completely disperse the powder. There was also a large variation in the amount of powder being dispersed, as shown by the sized sample in Table 2. In all experiments, the powder left behind in the funnel was further re-dispersed following the same method. The values of D(v, 0.5) and span were similar to that obtained from the initially dispersed portion, indicating that this portion was indeed representative of the whole powder sample. Using the lower pressure of 70 psi (run 4 in Table 2) provided less dispersive energy resulting in a larger span than that obtained at 90 psi for similar powder weight. The best dispersion using the 0.25 mm nozzle was obtained at 90 psi with the 500 mg powder sample. For the 0.50 mm nozzle, the optimum pressure and sample weight that produced size and span comparable with the wet measurement values were at 90 psi and 100, 200 mg. For the 0.75 mm nozzle, these optimum values were 90 psi and 200 mg powder. Using 1.00 mm nozzle, increasing the sample weight at 90 psi yielded better dispersion indicated by the smaller difference of D(v,0.5) and span values between wet and dry dispersion. For all nozzles, when lower pressure was used to disperse the same amount of powder the span value increased indicating the existence of non-dispersed aggregates. The results presented in Table 2 can be explained in terms of the dispersion mechanism discussed below. The compressed air
entering from the left port (Fig. 5a), pulled in the powder from the sample port (Fig. 5b), forcing the powder clumps to pass through the disperser orifice resulting in dispersed powder exiting (Fig. 5c). The clumps break up due to the turbulence, the shear force created by the compressed air causing collisions between particles, and collisions between particles and disperser walls as they pass through the device. If the air velocity is too high, too many powder clumps are forced to go through the disperser orifice at the same time, causing the orifice to clog prematurely, thus reducing the amount of powder dispersed. Since only a small amount of powder passes through the orifice, fewer collisions between particles and
Fig. 5. Mechanism of powder dispersion. a: compressed air injected through the nozzle; b: powder entering the disperser orifice; c: dispersed powder comes out.
32
P. Tang et al. / Powder Technology 187 (2008) 27–36
Fig. 6. De-aggregation of powder inside disperser (a) small amount of powder producing fewer collisions between particle–particle and particles–walls (b) large amount of powder producing more collision resulting in better dispersion.
walls occur and hence dispersion is less efficient (Fig. 6a). The fact that this is the case indicates that turbulence is not playing a major role. Better dispersion can be achieved if there are more collisions so that more powder can be dispersed (Fig. 6b). The air velocity through the 0.25 mm nozzle was so high that there was insufficient time for all powder clumps to go through the orifice before it was blocked. As a result, only small amounts of powder were sucked in through the hole: 36% for 100 mg, 28% for 200 mg, 25% for 500 mg at 90 psi, 8% for 500 mg at 70 psi. Consequently, there might not be enough interaction with other particles or the disperser walls to break up the clumps. Hence wide size distributions were obtained. At a low powder loading (100 mg), performance was even worse,
with little break up to individual particles, as evidenced by the high D(v,0.5) values. With the 0.5 mm nozzle, the air velocity was lower so the clumps had enough time to enter the orifice consecutively before it was blocked. Therefore more powder was analysed: 64% for 100 mg, 40% for 200 mg, 44% for 500 mg at 90 psi. As more powder entered the disperser orifice, there were more collisions between particles and particles–walls resulting in better dispersion. As a result, the D(v,0.5) was closer to the actual size and the span was closer to the actual value than those obtained using 0.25 mm nozzle. When 70 psi was used, the air had insufficient energy to pull the powder through the orifice (11% for 500 mg) and did not generate enough shear force to break up the clumps. Similarly for the 0.75 mm nozzle, at constant pressure increasing the powder load from 100 to 200 mg provided better dispersion because there were more collisions to de-aggregate the clumps. Air velocity through the nozzle increases as the nozzle size decreases. With higher velocity, the number of collisions between particles and the disperser wall at the exit path are reduced (Fig. 7a) because all flow is in the axial direction. If swirl flow could be achieved (Fig. 7b), the collision frequency would increase. However, using the current setup, it is impossible to create very much radial flow. In the simulations high gas velocities of up to 600 m s− 1 were observed at the exit to the inner tube and these gave rise to very high levels of turbulent kinetic energy (104 m2 s− 1). When the swirl flow was present the same high velocities and turbulence kinetic energies were maintained but in addition there was a swirl velocity of around 100 m s− 1 in the region of the particle inlet. Particles ranging in size from 10–1000 μm were released from the particle inlet (shown in red in Fig. 3) and tracked through the domain. Obviously as the particles interact with the flow and the walls they de-agglomerate so the size will change. Here we illustrate the effect of swirl flow on the particle behaviour for particles of 100 μm. It is evident from Fig. 8 that adding swirl results in many more wall impacts that would lead to de-agglomeration. It also has the effect of increasing the residence time of the particles in the tube, so that they are subjected to the high levels of fluid shear and turbulence for a much longer period.
Fig. 7. Different patterns of gas flow inside the Pisco Disperser (a) axially; (b) radially (i.e. swirl flow).
P. Tang et al. / Powder Technology 187 (2008) 27–36
33
Fig. 8. Results of CFD simulation showing particle–wall interactions (a) without swirl flow and (b) with swirl flow.
dry particle sizes indicates a good dispersion over the range of conditions studied. In order to further investigate the effectiveness of dispersion with the 1.00 mm nozzle, particle size distributions of smaller mannitol and BSA powders were measured in both wet and dry samples (Fig. 9). The dry measurement was done in triplicate to ensure reproducibility (all shown in Fig. 9). Fig. 9 also shows the presence of some very large particles of approximately 100 μm in diameter. However, these large particles were not seen in the electron micrographs shown in Fig. 2 or the particle size distributions obtained from analysis of the wet samples. These seemingly large particles could be due to either inefficient dispersion or thermal mixing. Thermal mixing is caused by the temperature difference between the compressed and surrounding air, and would cause the laser beam to deflect from its normal path causing an artifact seen as large particles. However, this phenomenon was less likely to take place because there was an inadequate time for heat transfer between the
In general, with the 0.25, 0.50, and 0.75 mm nozzles, there was a large variation in the quantity of powder measured with powder loading. This significant variation was also reflected in span values. The 1.00 mm nozzle achieved the optimum collision frequency between particles and particle–walls indicated by the fact that D(v,0.5) and span values are very close to the wet dispersion values. Since the 1.00 mm nozzle gives the best dispersion compared with other nozzles, mannitol powders of different size were used to further test its dispersion efficiency. Runs 13–18 (Table 2) show that low powder loading (less than 200 mg), resulted in large differences of D(v,0.5) (as high as 53%) between wet and dry samples. Increasing the loading to above 200 mg reduced the difference in D(v,0.5) between the wet and dry samples to less than 23%. For this reason, all further testing (Table 3) used powder loading greater than 200 mg. Table 3 gives values of D(v,0.5) and span measured from wet and dry measurements for larger mannitol particles obtained at various pressures and sample weights. Comparison of wet and
Table 3 Size measurement results for mannitol powder using the disperser unit with 1.00 mm nozzles Run No
Sample Before
1 2 3 4 5 6
Pressure
D(v,0.5)
Span
Obscuration
Sized
(mg)
(mg)
(psi)
(μm)
198.40 (5.65) 369.93 (19.01) 558.52 (6.30) 1032.56 (34.14) 228.99 (5.75) 221.69 (15.81)
128.10 (6.74) 287.91 (22.26) 446.47 (56.02) 719.67 (133.50) 166.03 (7.46) 142.28 (17.89)
90 90 90 90 70 50
2.83 (0.04) 2.94 (0.03) 2.98 (0.15) 2.95 (0.26) 2.99 (0.06) 3.09 (0.13)
(%) 1.17 (0.05) 1.32 (0.13) 1.36 (0.25) 1.46 (0.51) 1.24 (0.03) 1.25 (0.18)
Fully dispersed mannitol particles: D(v, 0.5) = 3.00 (0.03) μm and span = 1.41 (0.01). The numbers in brackets indicate the standard deviation.
2.47 (0.31) 6.33 (2.57) 13.70 (7.64) 18.80 (10.57) 5.23 (0.71) 5.90 (2.26)
34
P. Tang et al. / Powder Technology 187 (2008) 27–36
Fig. 9. Wet and dry particle size distributions (PSD) of (a) mannitol, wet measurement: D(v,0.5) = 2.05 (0.01) μm and span = 1.56 (0.03); (b) BSA, wet measurement: D(v,0.5) = 3.59 (0.02) μm and span = 2.11 (0.01); (c) BSA, wet measurement: D(v,0.5) = 4.58 (0.04) μm and span = 2.15 (0.02); (d) BSA, wet measurement: D(v,0.5) = 3.65 (0.06) μm and span = 2.16 (0.04). FD indicates PSD of fully dispersed particles obtained from wet measurement (dotted line).
surroundings and the gas to take place to any significant extent. What was more likely to happen was that not all submicron particles (indicated by the peak b1 μm in particle size distributions shown in Figs. 4 and 9) could be dispersed completely due to their cohesiveness. Although some of the measurements had high obscuration values outside the ideal range (Table 2 runs 3, 11, 15, 16, 17 and Table 3 runs 3 and 4), multiple scattering was not considered to take place. When the incident light hits more than one particle, the intensity of the scattered light received by the detector is much lower. Had it occurred, multiple scattering would lead to undersizing of the real particle size. This did not occur for any of the cases studied.
Velocity bias is a big concern in dry powder measurement where small and large particles travel through the measurement zone at different speeds [11]. This happens when the air within the measurement zone is not moving and hence particles coming out from the disperser at high speed will decelerate once they are in contact with the still air. Fine particles decelerate more rapidly than the larger ones and therefore stay in the measurement zone for a longer period. This causes the fine particle proportion to be over reported. The vacuum pump attached to the aerosol mounting unit provides a stream of moving air within the measurement zone and therefore the velocity bias effect was eliminated. Additionally, we did not observe shifts to smaller sizes in the particle size distribution.
P. Tang et al. / Powder Technology 187 (2008) 27–36
35
The most efficient dispersion of mannitol and BSA powders was found with the 1.0 mm nozzle. The maximum percentage differences of D(v,0.5) and span between wet and dry dispersion of mannitol powders for sample weight more than 200 mg across all pressure values tested are 23% and 19%, respectively. Using amorphous BSA powders that had higher moisture content than mannitol, the dispersion was not improved when the pressure was increased from 50 to 70 and further to 90 psi. The average differences of D(v,0.5) and span obtained with BSA powders are 29.21 ± 8.36% and 18.10 ± 7.45%, respectively. With the 0.75 mm nozzle, the optimum dispersion was achieved at 90 psi and 200 mg loaded mannitol powder, giving the maximum percentage differences of D(v,0.5) and span 10% and 6%, respectively. 0.50 mm nozzle gives maximum differences of 17% and 19% for D(v,0.5) and span, respectively at 90 psi and 100 mg mannitol powder. The 0.25 mm nozzle was not suitable because of the big differences of D(v,0.5) and span are 33% and 800%, respectively at 90 psi and 500 mg mannitol powder. It appears that the main mechanism taking place inside the disperser unit is particle–particle and particle–wall collision caused by the shear force from the compressed air. Compressed air with higher pressure provides more energy which further enhances the collision frequency. Nozzle size governs the velocity and amount of powder passing through the disperser orifice. More collisions take place when more powder is introduced to the disperser, producing better dispersion. However, for smaller nozzles, the high velocity in the axial direction leads to fewer collisions of the clumps with the walls. The 1.00 mm nozzle shows the best results compared with the smaller nozzles. 4. Conclusion A simple and cost-effective powder disperser has been shown to be suitable to disperse mannitol and BSA powders. The disperser has been shown to disperse the powder as efficiently as the more expensive commercial device. The disperser allows the
Fig. i. Results of Computational Fluid Dynamics simulation showing (a) gas velocity; (b) Mach number; (c) Turbulence Kinetic Energy inside the powder disperser.
Fig. ii. The Mach surface is located at the nozzle outlet.
36
P. Tang et al. / Powder Technology 187 (2008) 27–36
adjustment of the pressure of compressed air to achieve the best dispersion specific to the material in order to avoid milling. For 0.25–0.75 mm nozzles, the best dispersions were obtained at a particular value of powder loading and pressure. The 1.00 mm nozzle, however, is more robust since it gives good dispersion across all pressures tested and all powder loadings above 200 mg. As with the more expensive dispersers, some parameters which include powder loading and pressure need to be optimized to generate complete dispersion. The only disadvantage with this disperser is the larger quantity of loaded sample required to achieve effective dispersion compared with commercial powder dispersers. However this disadvantage is countered by the advantage that it is more likely that the larger sample is representative of the powder to be measured. The CFD simulation results illustrate that the introduction of swirl has a very important effect on the flow in the nozzle and the subsequent behaviour of the particles. The present study was performed to investigate the likely effect of introducing swirl. It is clear that CFD could be used to study this in much greater depth and could be used to optimize the amount of swirl (which can be changed by varying the swirler design) and the optimum particle injection location. Acknowledgement We would like to thank Dr Herbert Chiou for providing technical drawing of the Pisco disperser. One of the authors is currently at the National Science Foundation. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Appendix A Computation Fluid Dynamics simulation on the disperser was performed to obtain gas velocity, Mach number, and level of turbulence. Mach number is a parameter used to describe the velocity of a jet and calculated as a ratio of the diameters of the outlet of nozzle divergent section and the throat of the convergent section [12].
Fig. i(a) shows that the gas speed at the nozzle outlet was supersonic. Normally to achieve supersonic flow, de Laval nozzle (a convergent–divergent nozzle usually used to accelerate gas speed) is used and Mach 1 is achieved at the throat of such nozzle. However, if the outlet profile is correct, it is possible to accelerate to Mach N1, as shown in Fig. i(b). There is no such throat in the disperser nozzle so the Mach 1 surface is located at the outlet of the nozzle, as shown in Fig. ii, and the pressure ratio is correct to generate supersonic flow. The high level of turbulence shown in Fig. i(c) will lead to de-agglomeration of the powder. References [1] K. Leschonski, S. Rothele, U. Menzel, A special feeder for diffraction pattern analysis of dry powders, Part. Charact. 1 (1984) 161–166. [2] J.P. Mitchell, M.W. Nagel, Time-of-flight aerodynamic particle size analysers: their use and limitations for the evaluation of medical aerosols, J. Aerosol. Med. 12 (4) (1999) 217–240. [3] M. Corn, in: C.N. Davis (Ed.), Aerosol Science, Academic Press, London, 1966, pp. 359–390. [4] J.F. Bohan, Dry powder dispersion system for particle size analysis using aerodynamic time-of-flight, Powder Handl. Proc. 8 (1) (1996) 59–61. [5] T.A. Poole, 1995 Dry powder dispersion system for particle size measurement. Amherst Process Instruments, Inc: USA. [6] U. Koenig, A new quality for dispersing powders in particle size analysis, GIT Lab. Fachz. 47 (3) (2003) 224–225. [7] M. Puckhaber, Increasing the quality of metal powder with qualitative particle size analysis, Powder Metall. (2000) 16–17 April ed. [8] G. Bumcke, Particle size analysis in the laboratory technology and equipment. Fritsch GmbH: Oberstein, Germany, 1990. [9] N.Y.K. Chew, P. Tang, H.-K. Chan, J.A. Raper, How much particle surface corrugation is sufficient to improve aerosol performance of powders? Pharm. Res. 22 (1) (2005) 148–152. [10] Malvern Instruments 2001, Transfer of methods from the Mastersizer S and Mastersizer X to the Mastersizer 2000, http://www.malvern.co.uk/ labeng, issue MRK525-01, accessed on 23 February 2006. [11] Malvern Instruments 2001 Controlling sample presentation during dry powder laser diffraction measurement, http://www.malvern.co.uk/labeng, issue MRK643-01, accessed on 23 February 2006. [12] F.A. Holland, R. Bragg, Fluid Flow for Chemical Engineers, 2nd ed. Edward Arnold, London, 1995.
Powder Technology 187 (2008) 27 – 36 www.elsevier.com/locate/powtec
Simple and cost-effective powder disperser for aerosol particle size measurement P. Tang a , D.F. Fletcher b , H.-K. Chan a,⁎, J.A. Raper c a
Faculty of Pharmacy, University of Sydney, NSW 2006, Australia School of Chemical and Biomolecular Engineering, University of Sydney, NSW 2006, Australia Department of Chemical and Biological Engineering, University of Missouri-Rolla, MO 65409-1230, USA b
c
Received 20 December 2007; accepted 7 January 2008 Available online 16 January 2008
Abstract Commercial dry powder dispersers needed in conjunction with particle size measurement equipment are usually quite expensive (of the order of thousands of dollars). We have found that a simple vacuum generator can be used as a cost-effective disperser (US$50). Comparison with other commercial dispersers, small scale powder disperser (SSPD) model 3433 (TSI, Shoreview, USA) and Scirocco dry powder disperser (Malvern, Worcs, UK), showed that our disperser worked as efficiently as these expensive dispersers. Crystalline mannitol (less than 1% moisture content) and amorphous BSA (8.5–9.2% moisture content) smooth spherical particles were used to test the dispersion capability of the unit. Smooth spherical particles were chosen because they are more cohesive than corrugated particles due to increased contact points. Therefore, sufficient dispersions of other less cohesive particles should be able to be achieved using the optimum conditions reported here. The effects of air pressure, sample weight, and nozzle size of the disperser were investigated. Comparison of the particle size distributions between wet and dry measurements were used to determine the dispersion efficiency. Quantitative comparisons were made using the values of D(v,0.5) and span. The best dispersion was found using a 1.00 mm nozzle and the maximum percentage differences in D(v,0.5) and span are 23% and 19%, respectively, with more than 200 mg mannitol powder dispersed with pressures of 50, 70, 90 psi. Using BSA powders, the maximum percentage differences of D(v,0.5) and span are 37% and 25%, respectively. As was the case for the commercial devices, the dispersion of BSA particles could not be improved even when the pressure of the compressed air was increased. © 2008 Elsevier B.V. All rights reserved. Keywords: Powder disperser; Size measurement; Aerosol
1. Introduction Dry powder dispersers have been widely used and are required to de-agglomerate powder in order for commercial instruments to measure particle size distributions based on laser diffraction [1] and time-of-flight techniques [2]. It is recognized that complete dispersion of dry particulate solids, especially in the size range below 20 μm, is difficult to achieve due to the strong cohesive forces, namely van der Waals, magnetic, electrostatic, and forces due to solid or liquid bridges. In general, the
⁎ Corresponding author. E-mail address: [email protected] (H.-K. Chan). 0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.01.003
strength of these forces increases with decreasing particle size. It has been reported that the van der Waals force, the main interparticle attraction force, is approximately 100 times stronger than gravity for 10 μm particles [3]. For 1 μm particles, this force increases to 1000 g (g being the gravity force) and for 0.1 μm, it increases to between 104 and 105 g. De-agglomeration in the dry powder feeder associated with the Malvern Mastersizer 2000 (Malvern, Worcs, UK) laser diffraction instruments is achieved by accelerating the particles close to sonic speed along a tapered transport tube. This mechanism provides the shear forces and collisions between the particles and the feeder's walls in order to break up the aggregates. Pressure titration (from 0.1 to 4.0 bar) is recommended to find the optimum pressure that can break up aggregates without fracturing the individual particles. The optimum pressure is found when the
28
P. Tang et al. / Powder Technology 187 (2008) 27–36
particle size analysis from dry and wet dispersions agree. Previous versions of dry powder dispersers associated with time-of-flight equipment, such as the Aerosizer™ LD (Amherst Process Instruments, Hadley, MA) are based on the application of a fluidized pulsed jet, impaction with a disperser pin, and acceleration of the agglomerated powder particles through a sonic nozzle [4,5]. There are four controllable variables available to obtain optimum dispersion for different materials. These variables are: shear force (0.5 to 4.0 psi), feed rate (1000 to 10,000 counts/s), transport velocity from the fluidized bed to the disperser pin (high or low), and pin vibration (on or off). In a small scale powder disperser model 3433 (TSI, Shoreview, USA), abrasive paper is glued to a rotating table to provide a rough surface against which to brush the powder and thus de-agglomeration is encouraged before the powder is sucked through a venturi tube where further de-agglomeration takes place. The shear force in the venturi tube cannot be varied. The newer dry powder disperser associated with Particle Size Distribution Analyser 3603 (TSI, Shoreview, USA) de-agglomerates powder by the action of two pulsating, high (not sonic) velocity air jets with pressure ranging from 2 to 5 psi. A dry powder disperser, called Tornado™, manufactured by Beckman Coulter GmbH (Krefeld, Germany) breaks up the powder aggregates by swirling air, sucked through a ring gap above the sample via rotation of the sample container support, leading to strong impaction with controllable intensities [6]. Pressure titration (0.1 to 4 bar) is also recommended to find the optimum pressure. Sympatec GmbH (Zellerfeld, Germany) claims that their manufactured dry powder disperser, called Rodos, is able to disperse particles down to 0.1 μm [7]. For dosing, a vibratory feed and an inlet funnel connected to the dispersing line is used for less adhesive material, while for extremely fine or adhesive products a rotating table with a compacting roller is recommended. The ultimate de-agglomeration of the powder is achieved by generating a vacuum through expansion of compressed gas applied to an injector in order to accelerate the particles as soon as they arrive in the dispersing line [1]. The pressure of compressed air can be varied from 0.1 to 6 bar to achieve optimum dispersion. Three different sizes of dispersing line (4, 6, and 10 mm) are available to enhance dispersion for more cohesive particles.
The dispersing unit manufactured by Fritsch GmbH (Oberstein, Germany) uses mechanical and pneumatic forces to deagglomerate the powder [8]. An amplitude-controlled vibratory feeder is used for material dosing and the dispersion occurs in a two-phase annular gap nozzle where powders are accelerated at a high flow rate. The default pressure is 3 bar (maximum 4 bar), however it can be reduced to 1 bar for brittle material to avoid milling of particles. Therefore, it can be seen that commercial dispersers are readily available but generally they are quite expensive (of the order of thousands of dollars), and tailor-made equipment requires substantial engineering fabrication. We have found that a simple unit designed originally to create a vacuum using compressed air (Pisco, Illinois, USA), can be used as a suitable alternative. This unit is relatively cheap (approximately US$50) and readily available. Mannitol and bovine serum albumin (BSA) particles produced by spray drying were used to test the unit, representing powders of different cohesiveness. 2. Method 2.1. Particle size measurement Size measurement was done using a Mastersizer S laser diffractometer (Malvern, Worcs, UK) with an aerosol mounting unit (Fig. 1a) to which the disperser's outlet was attached. The other end of the aerosol mounting unit was connected to a vacuum pump to draw out the particles after passing through the measurement zone. The disperser unit (Pisco, Illinois, USA) has two perpendicular inlets for compressed air and powder feed (Fig. 1b). A funnel, having a grid of 1 mm diameter holes to help break up large particle agglomerates, was fitted inside the powder feed inlet. The powder was dispersed by injecting compressed air through nozzles of various diameters (0.25, 0.5, 0.75, and 1.0 mm). Crystalline mannitol with moisture content less than 1% (Fig. 2a) and amorphous bovine serum albumin with moisture content 8.5–9.2% (Fig. 2b) spherical particles of different sizes were used to test the disperser unit. Only smooth spherical particles were tested because it has been shown that they are more cohesive than corrugated spherical particles [9] due to the
Fig. 1. (a) Disperser unit fitted onto the aerosol mounting unit of Mastersizer S for size measurement; (b) Disperser unit to disperse powder to individual particles.
P. Tang et al. / Powder Technology 187 (2008) 27–36
29
(measurement time of 1.0 s) for higher powder loads. These sweep numbers were chosen to ensure that data collection was only done when there were particles passing through the laser beam. Therefore the number of sweeps was increased with increasing sample size. For the wet measurement, the analysis model used was polydisperse and the 300RF lens (size range: 0.05–900 μm) was fitted. Since the sample was circulated continuously between the sample holder and the lens, more sweeps could be chosen to collect enough data. For this, the sweep number was set at 2000 (default value from the equipment). The effects of sample weight, nozzle diameter, and compressed air pressure on powder dispersion were investigated. To study the sample weight effect, samples of 100, 200, and 500 mg were used at each nozzle diameter and pressure. For the 0.25, 0.50, and 0.75 mm nozzles, the compressed air pressure was set at 70 and 90 psi for the 500 mg sample weight. Lighter samples and lower pressures were not used because insufficient dispersed particles could be delivered to the measurement zone (i.e. as indicated by low obscuration). For the 1.0 mm nozzle, the sample weight was varied at 100, 200, 300, 500, 1000 mg and the pressure was varied at 50, 70, and 90 psi. These values were chosen to give enough dispersed particles during size measurement. Each measurement was done in triplicate to ensure reproducibility. 2.2. Computational Fluid Dynamics (CFD) simulation
Fig. 2. Scanning electron micrographs of (a) mannitol and (b) bovine serum albumin spherical particles used to test the disperser. Magnification: 8000×.
presence of more contact points. Therefore the optimum condition found for these particles should be applicable to different shaped particles of the same material. The particle size distributions were represented by the D(v, 0.5), which is the equivalent volume diameter at 50% cumulative volume determined by the Mastersizer S. The width of the size distribution was expressed via the span, calculated as [D(v,0.9) − D (v, 0.1)] / D(v,0.5). For mannitol, particle size analysis was based on the real refractive index (RI) (1.52), imaginary RI (0.1), and RI of air as the dispersion fluid (1.00). For BSA, the real and imaginary RIs are 1.55 and 0.02, respectively. To determine the efficiency of the disperser unit, the reported size of the dry measurement was compared with the wet measurement when the particles were suspended in chloroform (RI of 1.44) in which complete dispersion was achieved. The surfactant polyoxyethylene-sorbitan monooleate (Sigma-Aldrich, St. Louis, USA) was added to the suspension (0.2% v/v) which further sonified for 5 min to help disperse the particles. For dry measurement, the analysis model chosen in the Mastersizer S was the compressed range and the 300F lens (size range: 0.5–900 μm) was fitted along with the aerosol mounting unit. The number of detector scans (sweeps) was set at 100 (measurement time of 0.2 s) for 100 mg powder and 500
Simulations were performed using the commercial CFD software ANSYS CFX11 (http:www.ansys.com/cfx) to study the effect of introducing a swirl flow in the high pressure gas line just upstream of the nozzle exit. Fig. 3 shows the disperser geometry for the swirl flow case. The CFD model solved conservation equations for mass, momentum and energy, together with the Shear Stress Transport (SST) turbulence model. The fluid was treated as an ideal gas with the molecular weight of air. At the inlet the total pressure was set to 70 psi, corresponding to the reservoir conditions, and at the outlet the static pressure was set to be atmospheric. The flow in the nozzle was calculated and then particles were injected as a post process step from the location of the feed tube. The particles were given a density of mannitol (1500 kg m− 3) and were tracked through the gas using the Schiller–Naumann drag law and were subjected to turbulent dispersion by the gas stream. They were given a coefficient of restitution of one at the walls (i.e. they bounced with no loss of energy).
Fig. 3. Disperser geometry created in ANSYS CFX11. The swirler is shown in orange, the inlet from the gas reservoir is shown in blue and the feed location of the particles is shown in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
30
P. Tang et al. / Powder Technology 187 (2008) 27–36
3. Results and discussion 3.1. Comparison between Pisco disperser and other dry powder dispersers The performance of the Pisco disperser was compared with small scale powder disperser (SSPD) model 3433 (TSI, Shoreview, USA) and Scirocco dry powder disperser (Malvern, Worcs, UK) (Fig. 4). The shear force in the Scirocco dry powder disperser was set at the highest value (4 bar) because all lower values were found to be insufficient to achieve complete dispersion. The force in the SSPD could not be varied and was fixed at approximately 0.5 to 0.6 bar. All the measurement was done using a Malvern Mastersizer S except for the Scirocco dry powder disperser which was connected to a Malvern Mastersizer 2000. The sample loaded in the SSPD and Scirocco dispersers was approximately 10 mg, while 200 mg was put in the Pisco disperser. Fig. 4 shows that the Pisco disperser performed as well as the more expensive dispersers. The particle size distribution (PSD) for mannitol powder (Fig. 4a) is very close to that obtained from the wet dispersion. However, none of the dry powder dispersers could reproduce the small mannitol particles peak (particles smaller than 1 μm) shown in the particle size distributions measured from the wet dispersion. As for BSA, the PSD of dry dispersions showed the presence of large aggregates (larger than 20 μm) that could not be broken up. The truncation of the particle size distributions of the dry powders at the fine end (Fig. 4b, PSD obtained using SSPD and Pisco dispersers) was due to the limited range of the 300F optical set up in Malvern Mastersizer S that can only measure particles above 0.5 μm. Unfortunately, with this particular setup in Malvern Mastersizer S, the aerosol mounting unit can only be fixed with the 300F lens instead of the 300RF lens which has a wider size range (0.05–900 μm) normally used for wet dispersion. The truncation was not observed with the PSD obtained from Scirocco disperser because it was connected to Malvern Mastersizer 2000 which has wider size range (0.01 to 10,000 μm).
Table 1 Comparison of D(v,0.5) and span of mannitol and bovine serum albumin particles using small scale powder disperser 3433 (TSI, Shoreview, USA) and Pisco disperser Mannitol D(v, 0.5) μm Wet dispersion Dry dispersion with SSPD 3433 Dry dispersion with Mastersizer 2000 Dry dispersion with Pisco disperser
Bovine serum albumin Span
D(v, 0.5) μm
Span
2.46 (0.02) 1.11 (0.04) 8.27 (0.01) 1.99 (0.01) 2.82 (0.11) 1.48 (0.09) 5.62 (0.75) 2.66 (0.34) 2.58 (0.12) 1.12 (0.05) 8.24 (0.15) 4.87 (0.11) 2.40 (0.10) 1.21 (0.28) 8.99 (0.74) 2.52 (0.86)
The value of D(v, 0.5) and span of the PSDs shown in Fig. 4 are tabulated in Table 1. The Pisco disperser was able to disperse the mannitol and BSA particles efficiently as indicated by D(v,0.5) and span. With the SSPD 3433, many large BSA particles were stuck in the connecting tube between the unit and Mastersizer S, hence the D(v,0.5) measured were smaller than that obtained from the wet dispersion. 3.2. Optimisation of Pisco disperser using different nozzle size The comparison between the Pisco disperser and the other dispersers discussed in Section 3.1 was done using the default 1.00 mm nozzle inside the Pisco disperser. We manufactured different size nozzles, which were 0.25, 0.50, 0.75 mm to investigate the effect of nozzle size on disperser performance. Table 2 shows the sample weight before dispersion and the amount of powder that had gone through the measurement zone. The obscuration, which is the relative amount of light scattered or absorbed by particles, is a good indicator of the particle concentration within the measurement zone. The ideal range of obscuration for dry measurement in the Mastersizer S is 1−10% [10], which guarantees the effective range of signal to noise ratio to avoid multiple scattering.
Fig. 4. Particle size distribution (PSD) obtained from Mastersizer S for (a) mannitol and (b) bovine serum albumin particles.
P. Tang et al. / Powder Technology 187 (2008) 27–36
31
Table 2 Size measurement results for mannitol powder using the disperser unit with 0.25, 0.50, 0.75, and 1.00 mm nozzles Run No
Sample
Pressure
D(v,0.5)
(mg)
(psi)
(μm)
Loaded
Sized
(mg)
Span
Obscuration
(%)
0.25 mm nozzle 1 2 3a 4
96.62 (1.37) 231.22 (22.39) 507.58 (5.65) 497.46 (2.88)
34.98 (18.72) 64.08 (28.44) 124.71 (78.38) 38.40 (6.55)
90 90 90 70
10.46 (11.69) 3.12 (0.41) 2.80 (0.34) 2.97 (0.55)
17.37 (10.70) 19.82 (8.27) 12.36 (10.40) 42.98 (13.43)
1.63 (2.14) 4.10 (3.99) 10.50 (8.58) 1.60 (0.61)
0.50 mm nozzle 5a 6a 7 8
110.61 (6.48) 200.19 (5.16) 499.00 (10.41) 507.68 (1.99)
71.21 (5.34) 80.53 (18.23) 218.82 (97.74) 56.48 (13.93)
90 90 90 70
2.37 (0.01) 2.46 (0.01) 2.56 (0.03) 2.77 (0.17)
1.12 (0.05) 1.59 (0.25) 10.86 (3.68) 33.60 (8.16)
4.00 (0.26) 5.53 (3.90) 7.90 (4.44) 2.43 (2.19)
0.75 mm nozzle 9 10 a 11 12
104.07 (2.31) 201.39 (4.19) 490.31 (11.08) 503.27 (7.55)
84.92 (9.24) 164.63 (9.50) 407.93 (44.98) 418.20 (10.25)
90 90 90 70
2.38 (0.09) 2.32 (0.05) 2.46 (0.16) 2.45 (0.08)
14.97 (27.52) 1.30 (0.01) 27.94 (12.14) 35.63 (14.34)
4.28 (1.59) 7.67 (0.50) 16.15 (3.82) 2.85 (0.81)
1.00 mm nozzle 13 a 14 a 15 a 16 a 17 a 18 a
101.53 (5.44) 205.44 (6.89) 552.23 (8.25) 1035.81 (12.31) 498.3 (7.23) 486.14 (6.74)
92.31 (2.55) 180.25 (9.88) 450.44 (7.82) 827.31 (6.17) 420.45 (9.23) 350.18 (26.55)
90 90 90 90 70 50
3.23 (0.05) 2.55 (0.12) 2.28 (0.06) 2.14 (0.05) 2.48 (0.19) 2.59 (0.26)
2.56 (0.25) 1.52 (0.14) 1.40 (0.09) 1.35 (0.07) 1.50 (0.28) 1.65 (0.32)
2.17 (1.45) 4.45 (2.36) 14.23 (3.22) 18.71 (3.89) 12.43 (4.05) 5.80 (1.38)
Fully dispersed mannitol particles: D(v, 0.5) = 2.11 (0.01) μm and span = 1.38 (0.05). The numbers in brackets indicate the standard deviation. a Indicates optimum conditions for a particular nozzle size.
In the sample dispersion with the 0.25 mm nozzle, there was a wide size distribution (i.e. span) for all sample weights and pressures, indicating that this nozzle failed to completely disperse the powder. There was also a large variation in the amount of powder being dispersed, as shown by the sized sample in Table 2. In all experiments, the powder left behind in the funnel was further re-dispersed following the same method. The values of D(v, 0.5) and span were similar to that obtained from the initially dispersed portion, indicating that this portion was indeed representative of the whole powder sample. Using the lower pressure of 70 psi (run 4 in Table 2) provided less dispersive energy resulting in a larger span than that obtained at 90 psi for similar powder weight. The best dispersion using the 0.25 mm nozzle was obtained at 90 psi with the 500 mg powder sample. For the 0.50 mm nozzle, the optimum pressure and sample weight that produced size and span comparable with the wet measurement values were at 90 psi and 100, 200 mg. For the 0.75 mm nozzle, these optimum values were 90 psi and 200 mg powder. Using 1.00 mm nozzle, increasing the sample weight at 90 psi yielded better dispersion indicated by the smaller difference of D(v,0.5) and span values between wet and dry dispersion. For all nozzles, when lower pressure was used to disperse the same amount of powder the span value increased indicating the existence of non-dispersed aggregates. The results presented in Table 2 can be explained in terms of the dispersion mechanism discussed below. The compressed air
entering from the left port (Fig. 5a), pulled in the powder from the sample port (Fig. 5b), forcing the powder clumps to pass through the disperser orifice resulting in dispersed powder exiting (Fig. 5c). The clumps break up due to the turbulence, the shear force created by the compressed air causing collisions between particles, and collisions between particles and disperser walls as they pass through the device. If the air velocity is too high, too many powder clumps are forced to go through the disperser orifice at the same time, causing the orifice to clog prematurely, thus reducing the amount of powder dispersed. Since only a small amount of powder passes through the orifice, fewer collisions between particles and
Fig. 5. Mechanism of powder dispersion. a: compressed air injected through the nozzle; b: powder entering the disperser orifice; c: dispersed powder comes out.
32
P. Tang et al. / Powder Technology 187 (2008) 27–36
Fig. 6. De-aggregation of powder inside disperser (a) small amount of powder producing fewer collisions between particle–particle and particles–walls (b) large amount of powder producing more collision resulting in better dispersion.
walls occur and hence dispersion is less efficient (Fig. 6a). The fact that this is the case indicates that turbulence is not playing a major role. Better dispersion can be achieved if there are more collisions so that more powder can be dispersed (Fig. 6b). The air velocity through the 0.25 mm nozzle was so high that there was insufficient time for all powder clumps to go through the orifice before it was blocked. As a result, only small amounts of powder were sucked in through the hole: 36% for 100 mg, 28% for 200 mg, 25% for 500 mg at 90 psi, 8% for 500 mg at 70 psi. Consequently, there might not be enough interaction with other particles or the disperser walls to break up the clumps. Hence wide size distributions were obtained. At a low powder loading (100 mg), performance was even worse,
with little break up to individual particles, as evidenced by the high D(v,0.5) values. With the 0.5 mm nozzle, the air velocity was lower so the clumps had enough time to enter the orifice consecutively before it was blocked. Therefore more powder was analysed: 64% for 100 mg, 40% for 200 mg, 44% for 500 mg at 90 psi. As more powder entered the disperser orifice, there were more collisions between particles and particles–walls resulting in better dispersion. As a result, the D(v,0.5) was closer to the actual size and the span was closer to the actual value than those obtained using 0.25 mm nozzle. When 70 psi was used, the air had insufficient energy to pull the powder through the orifice (11% for 500 mg) and did not generate enough shear force to break up the clumps. Similarly for the 0.75 mm nozzle, at constant pressure increasing the powder load from 100 to 200 mg provided better dispersion because there were more collisions to de-aggregate the clumps. Air velocity through the nozzle increases as the nozzle size decreases. With higher velocity, the number of collisions between particles and the disperser wall at the exit path are reduced (Fig. 7a) because all flow is in the axial direction. If swirl flow could be achieved (Fig. 7b), the collision frequency would increase. However, using the current setup, it is impossible to create very much radial flow. In the simulations high gas velocities of up to 600 m s− 1 were observed at the exit to the inner tube and these gave rise to very high levels of turbulent kinetic energy (104 m2 s− 1). When the swirl flow was present the same high velocities and turbulence kinetic energies were maintained but in addition there was a swirl velocity of around 100 m s− 1 in the region of the particle inlet. Particles ranging in size from 10–1000 μm were released from the particle inlet (shown in red in Fig. 3) and tracked through the domain. Obviously as the particles interact with the flow and the walls they de-agglomerate so the size will change. Here we illustrate the effect of swirl flow on the particle behaviour for particles of 100 μm. It is evident from Fig. 8 that adding swirl results in many more wall impacts that would lead to de-agglomeration. It also has the effect of increasing the residence time of the particles in the tube, so that they are subjected to the high levels of fluid shear and turbulence for a much longer period.
Fig. 7. Different patterns of gas flow inside the Pisco Disperser (a) axially; (b) radially (i.e. swirl flow).
P. Tang et al. / Powder Technology 187 (2008) 27–36
33
Fig. 8. Results of CFD simulation showing particle–wall interactions (a) without swirl flow and (b) with swirl flow.
dry particle sizes indicates a good dispersion over the range of conditions studied. In order to further investigate the effectiveness of dispersion with the 1.00 mm nozzle, particle size distributions of smaller mannitol and BSA powders were measured in both wet and dry samples (Fig. 9). The dry measurement was done in triplicate to ensure reproducibility (all shown in Fig. 9). Fig. 9 also shows the presence of some very large particles of approximately 100 μm in diameter. However, these large particles were not seen in the electron micrographs shown in Fig. 2 or the particle size distributions obtained from analysis of the wet samples. These seemingly large particles could be due to either inefficient dispersion or thermal mixing. Thermal mixing is caused by the temperature difference between the compressed and surrounding air, and would cause the laser beam to deflect from its normal path causing an artifact seen as large particles. However, this phenomenon was less likely to take place because there was an inadequate time for heat transfer between the
In general, with the 0.25, 0.50, and 0.75 mm nozzles, there was a large variation in the quantity of powder measured with powder loading. This significant variation was also reflected in span values. The 1.00 mm nozzle achieved the optimum collision frequency between particles and particle–walls indicated by the fact that D(v,0.5) and span values are very close to the wet dispersion values. Since the 1.00 mm nozzle gives the best dispersion compared with other nozzles, mannitol powders of different size were used to further test its dispersion efficiency. Runs 13–18 (Table 2) show that low powder loading (less than 200 mg), resulted in large differences of D(v,0.5) (as high as 53%) between wet and dry samples. Increasing the loading to above 200 mg reduced the difference in D(v,0.5) between the wet and dry samples to less than 23%. For this reason, all further testing (Table 3) used powder loading greater than 200 mg. Table 3 gives values of D(v,0.5) and span measured from wet and dry measurements for larger mannitol particles obtained at various pressures and sample weights. Comparison of wet and
Table 3 Size measurement results for mannitol powder using the disperser unit with 1.00 mm nozzles Run No
Sample Before
1 2 3 4 5 6
Pressure
D(v,0.5)
Span
Obscuration
Sized
(mg)
(mg)
(psi)
(μm)
198.40 (5.65) 369.93 (19.01) 558.52 (6.30) 1032.56 (34.14) 228.99 (5.75) 221.69 (15.81)
128.10 (6.74) 287.91 (22.26) 446.47 (56.02) 719.67 (133.50) 166.03 (7.46) 142.28 (17.89)
90 90 90 90 70 50
2.83 (0.04) 2.94 (0.03) 2.98 (0.15) 2.95 (0.26) 2.99 (0.06) 3.09 (0.13)
(%) 1.17 (0.05) 1.32 (0.13) 1.36 (0.25) 1.46 (0.51) 1.24 (0.03) 1.25 (0.18)
Fully dispersed mannitol particles: D(v, 0.5) = 3.00 (0.03) μm and span = 1.41 (0.01). The numbers in brackets indicate the standard deviation.
2.47 (0.31) 6.33 (2.57) 13.70 (7.64) 18.80 (10.57) 5.23 (0.71) 5.90 (2.26)
34
P. Tang et al. / Powder Technology 187 (2008) 27–36
Fig. 9. Wet and dry particle size distributions (PSD) of (a) mannitol, wet measurement: D(v,0.5) = 2.05 (0.01) μm and span = 1.56 (0.03); (b) BSA, wet measurement: D(v,0.5) = 3.59 (0.02) μm and span = 2.11 (0.01); (c) BSA, wet measurement: D(v,0.5) = 4.58 (0.04) μm and span = 2.15 (0.02); (d) BSA, wet measurement: D(v,0.5) = 3.65 (0.06) μm and span = 2.16 (0.04). FD indicates PSD of fully dispersed particles obtained from wet measurement (dotted line).
surroundings and the gas to take place to any significant extent. What was more likely to happen was that not all submicron particles (indicated by the peak b1 μm in particle size distributions shown in Figs. 4 and 9) could be dispersed completely due to their cohesiveness. Although some of the measurements had high obscuration values outside the ideal range (Table 2 runs 3, 11, 15, 16, 17 and Table 3 runs 3 and 4), multiple scattering was not considered to take place. When the incident light hits more than one particle, the intensity of the scattered light received by the detector is much lower. Had it occurred, multiple scattering would lead to undersizing of the real particle size. This did not occur for any of the cases studied.
Velocity bias is a big concern in dry powder measurement where small and large particles travel through the measurement zone at different speeds [11]. This happens when the air within the measurement zone is not moving and hence particles coming out from the disperser at high speed will decelerate once they are in contact with the still air. Fine particles decelerate more rapidly than the larger ones and therefore stay in the measurement zone for a longer period. This causes the fine particle proportion to be over reported. The vacuum pump attached to the aerosol mounting unit provides a stream of moving air within the measurement zone and therefore the velocity bias effect was eliminated. Additionally, we did not observe shifts to smaller sizes in the particle size distribution.
P. Tang et al. / Powder Technology 187 (2008) 27–36
35
The most efficient dispersion of mannitol and BSA powders was found with the 1.0 mm nozzle. The maximum percentage differences of D(v,0.5) and span between wet and dry dispersion of mannitol powders for sample weight more than 200 mg across all pressure values tested are 23% and 19%, respectively. Using amorphous BSA powders that had higher moisture content than mannitol, the dispersion was not improved when the pressure was increased from 50 to 70 and further to 90 psi. The average differences of D(v,0.5) and span obtained with BSA powders are 29.21 ± 8.36% and 18.10 ± 7.45%, respectively. With the 0.75 mm nozzle, the optimum dispersion was achieved at 90 psi and 200 mg loaded mannitol powder, giving the maximum percentage differences of D(v,0.5) and span 10% and 6%, respectively. 0.50 mm nozzle gives maximum differences of 17% and 19% for D(v,0.5) and span, respectively at 90 psi and 100 mg mannitol powder. The 0.25 mm nozzle was not suitable because of the big differences of D(v,0.5) and span are 33% and 800%, respectively at 90 psi and 500 mg mannitol powder. It appears that the main mechanism taking place inside the disperser unit is particle–particle and particle–wall collision caused by the shear force from the compressed air. Compressed air with higher pressure provides more energy which further enhances the collision frequency. Nozzle size governs the velocity and amount of powder passing through the disperser orifice. More collisions take place when more powder is introduced to the disperser, producing better dispersion. However, for smaller nozzles, the high velocity in the axial direction leads to fewer collisions of the clumps with the walls. The 1.00 mm nozzle shows the best results compared with the smaller nozzles. 4. Conclusion A simple and cost-effective powder disperser has been shown to be suitable to disperse mannitol and BSA powders. The disperser has been shown to disperse the powder as efficiently as the more expensive commercial device. The disperser allows the
Fig. i. Results of Computational Fluid Dynamics simulation showing (a) gas velocity; (b) Mach number; (c) Turbulence Kinetic Energy inside the powder disperser.
Fig. ii. The Mach surface is located at the nozzle outlet.
36
P. Tang et al. / Powder Technology 187 (2008) 27–36
adjustment of the pressure of compressed air to achieve the best dispersion specific to the material in order to avoid milling. For 0.25–0.75 mm nozzles, the best dispersions were obtained at a particular value of powder loading and pressure. The 1.00 mm nozzle, however, is more robust since it gives good dispersion across all pressures tested and all powder loadings above 200 mg. As with the more expensive dispersers, some parameters which include powder loading and pressure need to be optimized to generate complete dispersion. The only disadvantage with this disperser is the larger quantity of loaded sample required to achieve effective dispersion compared with commercial powder dispersers. However this disadvantage is countered by the advantage that it is more likely that the larger sample is representative of the powder to be measured. The CFD simulation results illustrate that the introduction of swirl has a very important effect on the flow in the nozzle and the subsequent behaviour of the particles. The present study was performed to investigate the likely effect of introducing swirl. It is clear that CFD could be used to study this in much greater depth and could be used to optimize the amount of swirl (which can be changed by varying the swirler design) and the optimum particle injection location. Acknowledgement We would like to thank Dr Herbert Chiou for providing technical drawing of the Pisco disperser. One of the authors is currently at the National Science Foundation. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Appendix A Computation Fluid Dynamics simulation on the disperser was performed to obtain gas velocity, Mach number, and level of turbulence. Mach number is a parameter used to describe the velocity of a jet and calculated as a ratio of the diameters of the outlet of nozzle divergent section and the throat of the convergent section [12].
Fig. i(a) shows that the gas speed at the nozzle outlet was supersonic. Normally to achieve supersonic flow, de Laval nozzle (a convergent–divergent nozzle usually used to accelerate gas speed) is used and Mach 1 is achieved at the throat of such nozzle. However, if the outlet profile is correct, it is possible to accelerate to Mach N1, as shown in Fig. i(b). There is no such throat in the disperser nozzle so the Mach 1 surface is located at the outlet of the nozzle, as shown in Fig. ii, and the pressure ratio is correct to generate supersonic flow. The high level of turbulence shown in Fig. i(c) will lead to de-agglomeration of the powder. References [1] K. Leschonski, S. Rothele, U. Menzel, A special feeder for diffraction pattern analysis of dry powders, Part. Charact. 1 (1984) 161–166. [2] J.P. Mitchell, M.W. Nagel, Time-of-flight aerodynamic particle size analysers: their use and limitations for the evaluation of medical aerosols, J. Aerosol. Med. 12 (4) (1999) 217–240. [3] M. Corn, in: C.N. Davis (Ed.), Aerosol Science, Academic Press, London, 1966, pp. 359–390. [4] J.F. Bohan, Dry powder dispersion system for particle size analysis using aerodynamic time-of-flight, Powder Handl. Proc. 8 (1) (1996) 59–61. [5] T.A. Poole, 1995 Dry powder dispersion system for particle size measurement. Amherst Process Instruments, Inc: USA. [6] U. Koenig, A new quality for dispersing powders in particle size analysis, GIT Lab. Fachz. 47 (3) (2003) 224–225. [7] M. Puckhaber, Increasing the quality of metal powder with qualitative particle size analysis, Powder Metall. (2000) 16–17 April ed. [8] G. Bumcke, Particle size analysis in the laboratory technology and equipment. Fritsch GmbH: Oberstein, Germany, 1990. [9] N.Y.K. Chew, P. Tang, H.-K. Chan, J.A. Raper, How much particle surface corrugation is sufficient to improve aerosol performance of powders? Pharm. Res. 22 (1) (2005) 148–152. [10] Malvern Instruments 2001, Transfer of methods from the Mastersizer S and Mastersizer X to the Mastersizer 2000, http://www.malvern.co.uk/ labeng, issue MRK525-01, accessed on 23 February 2006. [11] Malvern Instruments 2001 Controlling sample presentation during dry powder laser diffraction measurement, http://www.malvern.co.uk/labeng, issue MRK643-01, accessed on 23 February 2006. [12] F.A. Holland, R. Bragg, Fluid Flow for Chemical Engineers, 2nd ed. Edward Arnold, London, 1995.