(44e) Effect of Particle Properties on Evaluation of Flowability Using a Test of Powder Discharge By Air Pressure

Cohesive powders are widely used in various industries such as chemical reaction, material production, food processing and pharmaceutical development. Recently the cohesive powder used in these processing is increased because it has the advantage, i.e. having superior reaction based on a large specific surface area. However, the cohesive powder handling in the surrounding air is extremely difficult by the influence of the cohesive force which is relatively strong to the gravity force in case of the reducing particle diameter. For example, the bridge and the flooding of cohesive powders are encountered in the practical situation and these troubles need to be avoided. As this issues, it is well known that these phenomena are related to the evaluation of powder flowability. Therefore, it is very important subject to the evaluation of flowability of cohesive powder.

In this study, we experimentally investigated to the flowability and cohesiveness of the cohesive powder using a test of the powder discharge by the air pressure. The powder used was three kinds of Calcium hydroxide having the difference of particle diameter and moisture contest. These powders were belonging to the Geldart group C particle. We were analysis the mass flow rate of cohesive powder and the average air pressure acting on powder bed where these parameters were calculated by the discharge mass of the powder and the air pressure as a function of the elapsed time, respectively. Furthermore, the separation force acting on the powder was estimated by the based on Rumpf equation which the air pressure at the beginning of the powder discharge, the initial void fraction, the particle diameter and the particle density were substituted in. Cohesiveness was expressed as the ratio of separation force and gravity force. Additionally, van der Waals force, electrostatic force and liquid bridge force on the powder were also calculated. Finally, we performed the evaluation of flowability to three kinds of Calcium hydroxide powder and examined the influence of cohesiveness of the powder against flowability of the powder.

Experimental device was consists of the filling part of the cohesive powder and the air supplying section at the top and the bottom of a vessel. The powder filling vessel was made by the cylindrical acrylic pipe which has the inside diameter of 50mm and the height of 150mm. The air distributors to the uniform air supplying were arranged at the top and the bottom of a cylindrical vessel. The supplying air which produced by the compressor was introduced through the three way valve and the air was supplied simultaneously from distributors to the powder bed in a vessel. By using this operation, the powder in a vessel can be discharge from a circular orifice at the central of bed bottom. In this experiment, a circular orifice diameter was fixed 4mm. The flow rate of air to the top and the bottom of the powder bed was adjusted by the mass flow controller. The discharge mass of the powder through an orifice was measured by the load cell which is installed at the bottom of a receiving vessel. The air pressure acting on the top of the powder bed was measured by the pressure sensor which is located on 150mm from the bottom of vessel.

Powder used is as following. Three kinds of Calcium hydroxide which has the different particle diameter, the same density and the different material moisture was used in this study, and these powders called as Ca(OH)₂ -A, Ca(OH)₂ -B and Ca(OH)₂ -C. As for the particle properties, the average particle diameter of Ca(OH)₂ -A and B were 7μm and Ca(OH)₂ -C was 13μm, the density was 2240kg/m³, and the moisture content of Ca(OH)₂ -A, B and C were 0.56, 0.1 and 5.5%, respectively. Furthermore, the Carr’s flowability index of three powders which measured by the powder tester were 25, 28 and 23 with Ca(OH)₂ -A, B and C respectively. According to the evaluation of Carr’s index to these powders were classified as “Bad” flowing of the powder. Additionally, all the powders belongs to the group C particle in the Geldart classification. This means that the powder to handle in this study is very cohesive.

The experimental conditions in the present study is as following. The initial filling mass of powder to a cylindrical vessel was fixed 70g. The flow rate of supplying air at the top and the bottom of the powder bed in a vessel were varied from 0.067×10^-5 to 1.5×10^-5 m³/s to give the difference air pressure acting on the powder bed. Three kinds of powder was filling naturally in a cylindrical vessel and then the initial height of the powder bed was recorded because the initial void fraction was estimated to all the experimental conditions, where the average initial void fraction of Ca(OH)₂ -A, B and C were 0.840, 0.830 and 0.804.

As the result, we obtained the following results.

(1) The cohesive powder flow by using our method was classified by three phase such as “Dischargeable flow”, “Transient flow” and “Non dischargeable flow” as well as previous work. Furthermore, this method could be evaluated to the flowability and the cohesiveness of the powder as following.

(2) Powder flowability to three kinds of Calcium hydroxide was evaluated by the relationship between the mass flow rate and the average air pressure acting on the top of the powder bed. The mass flow rate of Ca(OH)₂ -B was increased gradually with increasing the average air pressure. On the other hand, the mass flow rate of other powders clearly appeared the fluctuating trend, especially, the mass flow rate of Ca(OH)₂ -C against the average air pressure was confirmed a large change. It is therefore concluded with the powder flowability that Ca(OH)₂ -B has a highest flowability and Ca(OH)₂ -C was lowest flowability.

(3) Cohesiveness of three kinds of Calcium hydroxide was estimated as the ratio of the separation force and the gravity force. The results showed that the ratio of the separation force and the gravity force on Ca(OH)₂ -B was lower than that of other particles. Meanwhile, the ratio of the separation force and the gravity force on Ca(OH)₂ -C indicated the highest value. This result suggested that the cohesiveness of Ca(OH)₂ -C was the most of strong.

(4) As the result that the flowability of Ca(OH)₂ -C was low, it could be infer that the moisture content and the cohesiveness were influenced to this phenomenon.