Discrete Element Method (DEM) has been widely used to simulate powder behavior for the past several decades. DEM has many advantages compared to the continuum model; it is possible to include attraction forces such as liquid bridge, electrostatic and van der Waals forces and it accurately predicts the motion of individual particles in a system. One of the major drawbacks of DEM is the large calculation cost. Many of the particles used in industry are stiff and the contact time is short during collisions, which may require significantly small time step (in the order of micro to nanoseconds) in simulation. This makes it difficult or even impossible to complete simulations within acceptable time scale. Therefore, in order to increase the time step and reduce calculation cost, the particle stiffness is often reduced from the original material property. Although this approach is widely accepted for dry and relatively coarse particles where only contact force is dominant, it is reported that the particle behavior changes drastically when attraction forces are exerted on particles; particles with reduced stiffness become more “cohesive” than original particles. Hence, attraction forces are sometimes reduced to counter-balance this effect. In the present study, the effectiveness and limitations of this method are further discussed.
Dry powder inhalations (DPIs) is a dosage form for delivering powdered medicine to the lungs by an inhaled air flow of a patient, which is mainly used as a treatment for respiratory diseases such as bronchial asthma and chronic obstructive lung disease. The site of the human respiratory system where the powder inhalation formulation reaches depends on the size of the particle. Recent researchers actively conducted the design of DPIs for effective transportation in the human respiratory system, and they generally evaluated the lung reachability using a cascade impactor. Understanding particle motions and deposition behavior in not only a cascade impactor but also a respiratory system is necessary to improve the efficiency of DPI.
In the present study, we proposed the usage of a numerical calculation model combining computational fluid dynamics (CFD) and discrete element method (DEM). We constructed a cascade impactor model and a simple lung model to calculate the behavior of the fluid and particle transportation with CFD-DEM simulations. CFD-DEM simulations using a cascade impactor model demonstrated the experimental data of transportation ratios to each stage by adjusting the adhesion based on JKR theory. Furthermore, we investigated the effect of particle properties (particle sizes and particle densities) and the inhalation patterns (healthy and patient patterns) on the lung reachability of particles. As a result, we revealed that the relationship between the aerodynamic size and the lung reachability depends exponentially. The obtained relationship is essential knowledge for the design guide of DPIs because it enables ones to estimate the lung reachability of particles from the aerodynamic size.
1.Introduction
Jet mil is widely used for the process for dry powder milling. It has advantage of milling the sample without heating up the sample, because of its adiabatic expansion feature. However, in order to maximize the throughput, keeping the particle size small enough, optimizing the milling conditions is necessary. Conditions such as inlet pressure of the air, and feeding rate must be optimized by many trials before running the process for production. Those trials take lots of time and workload. Even after determining the optimized conditions, the size of the milled particles and throughput change with the variation of the lot of the materials to be milled. The flowability of the material also changes with time, which effects to the feeding rate of the feeder, and the particle size after the milling. To solve those problems. we propose real time measurement of the particle size of the milled powder, and discuss about the feed back control of the particle size.
2.Experimental method
We put a laser diffraction particle size distribution analyzer “Insitec” (Malvern Panalytical Ltd.), at the exit of a jet mill “STJ-200” (Seishin Enterprise Co., Ltd.). We used lactose for the milling sample. The inlet pressure of the jet mill was fixed to 0.6 MPa, and changed the feeding rate of the material.
3.Result
We could see the increase of the particle size, when we increased the feeding rate. This is caused by the insufficient milling force due to the too many particles in the mill. Therefore, we could change the particle size and the throughput, by just changing the feeding rate. We tried to optimize the feeding rate, using the real-time measurement data of the particle size analyzer. By using technology, the optimization of the jet mill process is possible, regardless the particle size and hardness of the materials to be milled.
The entry of micrometer-sized outdoor particulate matters into indoor spaces via the re-entrainment of these particles from their sediment layer formed on medium-performance filter media used in air handling units of office buildings might have been of a certain concern. In the present study, we investigated the effect of loading of the medium-performance filter media, where micrometer-sized particles were deposited, with APA solution on the suppression of their re-entrainment by exposing to intermittent fast air-flow blowing.
We selected two types of unwoven and charged filter media as test media, the single-layer media composed of mixed fine fibers and coarse ones and the binary-layer ones composed of upper coarse fiber layer and lower fine one separately. The test media in sheeted or pleated form were loaded with Kanto loam particles at a given loading density, and then 10 ppm of APA solution at its given density. After one day for drying the test media, the media were exposed to intermittent blowings of fast air-flow at a given face velocity to count a number of particles released from the test media. For comparison the number of released particles from test media without APA solution loading or with DI water loading were counted in the same manner.
Fig. 1 compares the percentages of decrease in number of released particles from the binary-layer or single-layer filter media in sheeted or pleated form loaded with APA solution. It was found that the single-layer media had better performance on preventing re-entrainment of particles by APA-solution loading than the binary-layer ones. In addition, the pleated media were less effective in lowering the numbers of released particles from themselves compared to the sheeted ones. It was caused by the discrepancy of loading density between test particles and APA solution in the pleated media.
