Hydrodynamics of Turbulent Fluidized Beds

Tom Wytrwat, M.Sc.

Poster

Motivation

Fluidized bed technology is one of the major industrial processes for handling and product treatment of particulate materials. This technology is used in food and pharmaceutical industry especially for coating as well as in chemical, petroleum and energy industry to carry out solid-gas and catalytic reactions. Fluid catalytic cracking (FCC) is a process commonly used for cracking of heavy mineral oils in circulating fluidized bed reactors. But also combustion processes for energy generation are operated in fluidized beds to use the advantages of high reaction surface areas and high heat transfer rates due to fluidization of the solid material.

Climate change leads to rethinking of the commonly used processes which emit high amounts of greenhouse gases as carbon dioxide. For this reason, it is the task of research to find ways to minimize emissions. Chemical looping combustion (CLC) is a fluidized bed combustion process which is designed for carbon capturing and storage. Thus, it is one opportunity to contribute to a change in the current emission of greenhouse gases.

Aims of the project

CFD Simulation of CLC carried out in coupled fluidized bed reactors including chemical reaction behavior is complex and leads to high simulation times. Thus, it is of interest to use predictive fluid dynamic models to decrease the computational effort. For this reason, the aim of this project is the generation of a semi-empirical model which describes the fluid dynamics in turbulent fluidized beds using particles of group B according to Geldart’s classification.

Experimental setup

Fluidized beds of different sizes in laboratory and pilot scale are used for the investigation of the fluid dynamics. The plants have diameters in a range between 0.05 m and 1.0 m. Combustion processes are carried out at temperatures around 1000°C. To investigate the influence of temperature on fluidization behavior some plants are heated electrical to operate them at these temperatures. Velocities between 0.3 and 6.0 m/s can be adjusted in the plants. Different plants available at the institute are listed in table 1.

An important parameter for the description of fluidized beds fluid dynamics is the solid holdup. Therefore, knowledge about the solid holdup in a fluidized bed reactor is of high interest for plant operation. A method to measure the solid holdup is the measurement of the pressure drop over the height of the fluidized bed. Each plant has several pressure sensors for solid holdup measurements. Furthermore, pressure measurements are done to determine transitions between different fluidization regimes, whereby they only give a radial averaged value for the solid holdup. For this reason, also local probe measurements are performed inside the fluidized beds to determine local solids concentration profiles. Capacitance probes are based on the principle of the alteration of the permittivity in a measuring volume depending on the solid content. They were introduced in the 1990th at the institute and were well developed for different fields of application. Besides, using a two channel capacitance probe it is possible to determine velocities of bubbles or clusters which are moving in the fluidized bed. In the dilute upper region of the fluidized bed a suction probe is used for solid sampling. The samples can be investigated regarding the composition and particle size distribution. Thus, conclusions on segregation effects can be carried out. 

Table 1: Fluidized bed plants for fluid dynamic investigations
fluidized bed diameter height operating conditions
0.05 m 0.8 m ambient
0.1 m 1 m ambient
0.1 m 15 m up to 1000°C
0.15 m 8 m up to 1000°C
0.4 m 15 m ambient
1.0 m 6 m ambient

Funding

  • TOTAL Research & Technology Gonfreville, France

Contact Details

Tom Wytrwat
Tom Wytrwat
research assistant
phone: +49 40 428 78 21 93
email: tom.wytrwat@tuhh.de