Kordnejad, Marzieh (PhD)

Tribo-electric charging and electrostatic effects in fluidized bed dryers

M. Kordnejad, I. Roghair, M. van Sint Annaland

STW 0.52 e-mail: m.kordnejad@tue.nl

Context

Gas−solid fluidized beds are commonly used in many chemical industries for various processes such as coating, drying, and mixing, and extensively for polymerization processes. In these systems electrostatic charges are typically generated due to continuous particle−particle and particle−container wall contacts, resulting from a phenomenon known as tribo-electrification.

These charges can accumulate in dielectric materials, such as polymers. The buildup of electrostatic charges in fluidized beds can interfere with the hydrodynamics in the reactor, resulting in serious operational problems, safety issues and even shutdown. Thus there are pressing safety and economic incentives to prevent over-accumulation of charges in gas–solid fluidized beds. In order to do this, the relevant phenomena and mechanisms of generation, accumulation, dissipation and separation of electrostatic charges need to be well understood.

Many industrial processes have long been troubled by triboelectric charging of powders and particulate systems. In the pharmaceutical industry, many products are produced in granular form and triboelectric charging can cause these particles to aggregate, leading to quality control problems of the final product.

Despite the negative consequences of excess electrostatic charge accumulation, the electrostatic charge generation, dissipation, and mitigation mechanisms and the relationship between the electrostatic charge level and incipient wall sheet formation are poorly understood.

Designing experiments and modeling together in order to calculate the electrostatic charges accumulating in a gas-solid fluidized bed will reveal a better understanding of the mechanism and mitigation strategies of electrostatic charging in this system. Some solutions, especially different antifouling agents, have been proposed and applied in laboratory to dissipate electrostatic charging and wall sheeting. This results can applied in industrial scale.

Research questions

  • What kind of properties does the ideal anti-fouling/anti-static agent have?

  • Can triboelectric charge generation be minimized by changing operation mode?

  • Can electrostatic charges be expelled through reactor design?

Methods

Many articles have been published on experimental work into electrostatic charge generation in fluidized bed system, but there is not such a large emphasis on modeling techniques. We propose to use a Discrete Element Model (DEM), in which individual particles in a fluidized bed can be simulated in a deterministic fashion, and in which the gas-solids interaction, particle-particle and particle-wall collisions are taken into account. We will extend this technique so as to include triboelectric (dis)charging, computation of the electric field (Maxwell’s equations, notably Gauss’s Law), and the interaction of the charged particles with the electric field. Moreover, the model will be able to compute gas phase concentration fields, and can account for the changes in permittivity.

Experiments will be performed to validate this model by doing laboratory experiments alongside to each other.