Drying in Vibrating Fluidized Beds

Sören Lehmann, M.Sc.

Poster

Motivation

The fluidized bed technology is widely spread across a variety of industrial applications, such as drying, coating, agglomeration, combustion, etc. The general principle of fluidization is the formation of a fluid-like state. Therefore, a bed of solid particles is penetrated by a gas stream (fluidization gas). Fluidization is achieved, when the drag forces of the gas flow on the particles are in equilibrium with the gravitational forces on the particles. In this state the mixture of particles and gas show fluid-like properties.

Fluidized bed dryers are commonly utilized in a wide range of industrial applications. In comparison to other drying principles the convective fluidized bed dryer has many advantages. The fluidized state of the product results in good mixing of drying gas and the product. Thus higher rates of energy and mass transfer can be achieved, compared to contact dryers. This results in shorter drying time respectively larger amounts of dried product. Furthermore, the highly homogenous fluidized bed allows drying at lower temperatures and avoids the building of hot spots in the product. All these characteristics make fluidized bed dryers especially interesting for drying of temperature sensitive powders e.g. in the pharmaceutical and food industry.

Milk powder for instance has small particle sizes (diameters < 300 µm). Inter particle forces are particularly dominant at these small diameters. Cohesive behavior and agglomeration are common phenomena that hinder the fluidization. To overcome the cohesive forces and achieve proper fluidization external energy has to be applied to the particle bed. Vibrating/shaking of the fluidized bed apparatus is a commonly applied option in industrial processes.

Experimental Setup

Nearly all published research on vibrating fluidized beds is conducted in very small, lab scale devices. Hence, a new pilot plant scale test rig has been designed and constructed at the institute. Vibration can be applied to the fluidized bed. This fluidized bed dryer is equipped with numerous measurement devices to 1) investigate flow behavior and fluidization characteristics of the fluidized bed and 2) to gather detailed information about the drying process. Air is used as fluidization and drying medium. The process air is conditioned and the flow rate of air as well as the vibration frequency and amplitude can be adjusted. The test plant is equipped with a glass wall which enables visual observations.

Parameter studies are conducted to identify potential improvement and optimization for the fluidization as well as the drying characteristics of the process.

Simulation

The experimental data is furthermore used to develop and validate a vibrating fluidized bed model for the dynamic flow sheet simulation tool DYSSOL. By considering the entire process chain and not only the fluidized bed dryer, a detailed model of the whole drying process is expected to give valuable information for the design of future plants as well as process up-scaling.

Project funding and start date

Project started 2016 in cooperation with Tetra Pak.

Thesis Topics

Topics for Bachelor/Project/Master thesis are available now:

Experimental work as well as simulation based projects are open.

Do not hesitate to contact me, if you are interested.

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