Hossein Ahmadian

Analysis of Tribo-Electric Charging of Spherical Beads Using Distinct Element Method

Tribo‐electrification is an important phenomenon in powder handling, as it could cause hazards and nuisance, such as dust explosion, adhesion to walls and blockage of pipelines leading to powder losses and difficulties in flow control. At present, there are only a few methods available to predict the dynamic charging of bulk powders, and they are not suitable for testing a small quantity of powders or handling highly active powders, for which full isolation would be needed. In this paper, the operation of a simple test device is analysed. Tribo‐electrification of a single spherical bead inside a sealed capsule is induced through a horizontal motion using a shaking device at various frequencies. The single particle contact charge obtained from experiments is incorporated in numerical simulations based on Distinct Element Method (DEM) and is used to predict the transient charge accumulation of an assemblage of beads. In the model, the tribo‐charging of a single bead is related to the contact area of the bea...

Analysis of enzyme dust formation in detergent manufacturing plants

In the detergent industry, enzymes are used to catalyze the breakdown of tough stains such as oils and fats, which cannot be easily removed with surfactants and bleaches. In the 1960s, a number of plant workers developed respiratory allergies from inhaling enzyme dust over long periods. The response from the enzyme manufacturing companies was to encapsulate the enzymes by granulation, fluid bed coating, extrusion and other techniques. The end users of encapsulated enzymes such as the detergent industry concentrated on the installation of engineering containment and dust extraction systems. These were combined with other safe systems of work, including airborne monitoring and health surveillance. The result was a significant decrease in the concentration of airborne enzyme dust in the working environment and virtually eliminated the incidence of respiratory allergy. However, enzyme dust generation has not been eliminated completely and the integrity of the enzyme encapsulates remains a key control point. In 2000, the Enzyme Dust Consortium was formed with the objective to design a validated test method with a protocol (for granule qualification and quality control) to correlate with dust levels in plant. Current test methods for dust formation do not replicate the mechanical stresses in the detergent plant and limit the enzyme manufacturers ability to develop better granules. Our efforts are focused on evaluating the prevailing mechanical stresses in detergent manufacturing plants and their effect on enzyme granule attrition. Dust sampling of the factory ventilation ductwork show a large variation in enzyme dust levels. This may be attributed to the presence of some weak granules. Nevertheless, the likely sources and mechanisms of enzyme dust need to be identified. Once a new testing procedure is established, enzyme manufacturers will be able to design and produce improved products that will result in a further reduction of enzyme exposure in detergent manufacturing facilities.

CFD modelling of a pilot-scale counter-current spray drying tower for the manufacture of detergent powder

ABSTRACT A steady-state, three-dimensional, multiphase computational fluid dynamics (CFD) modeling of a pilot-plant countercurrent spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass, and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian–Lagrangian approach. The continuous-phase turbulence is modeled using the differential Reynolds stress model. The drying kinetics is modeled using a single-droplet drying model, which is incorporated into the CFD code using user-defined functions (UDFs). Heat loss from the insulated tower wall to the surrounding is modeled by considering thermal resistances due to deposits on the inside surface, wall, insulation, and outside convective film. For the particle–wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature, and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content, and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements.

Mechanical Failure of Grains in Sheared Granular Media: Effect of Size Ratio

Recent studies have shown that, when the size ratio of grains (ratio of size of a grain normalised to the average size of surrounding grains) exceed about five, the stress state of the grains is dominantly hydrostatic (fluid‐like) in dense granular packing subjected to shearing. This behaviour explains the reason for the retardation tendency of breakage of large particles observed in sheared dense granular media. However, most of the engineering applications dealing with sheared random particulate media, for example mixing of detergent grains, often involve particles having size ratio less than five and information on the nature of stress experienced by grains in such scenario remains unknown, an aspect addressed in the present work using Distinct Element Method (DEM) simulations. Our stress map of particles precisely defines the stress transition:‐for particles with size ratio greater than ∼3, the stress state is dominantly hydrostatic; and between ∼1.66 and 3, the stress state gradually transit form fro...