Introductory Theory on Fluidization

2. THEORY

2.1 Packed columns and Fixed Beds

A packed column consists of specifically shaped particles contained within a column. Generally a packed column is used to bring two phases in contact with one another. Normally one fluid will wet the packing and flow as a film over its surface. The second fluid will pass through the remaining volume of the column. This promotes a high interfacial area between the two phases and a high degree of turbulence between the two fluids. The fluid’s can be liquid or gas, and can be entered into the column either at the top or bottom. Figure 1 represents a typically layout used for fluidization.

figure 1: Typical packed column set up used for fluidisation

A packed column is constructed using a metal, glass, ceramic or plastic shell preferably with a corrosion resistant lining. The column should be mounted as vertical as possible to promote even liquid distribution. The packing material or bed rests on a support plate that should be designed to offer minimal resistance to the passage of fluids (75% free area is recommended for the passage of gas ,(Richardson, J. F. & Harker, J.H. with Backhurst, J.R.( 2002)).

It is important to be able to predict the pressure drop over a packed column for the various flow rates of the fluids. These criteria are used extensively in plant design. Packed columns in which the solids are stationary relative to each other are referred to as fixed.

The first experimental work with fixed beds was carried out by Darcy (Darcy, H.P.G. (1856)) in 1830 in Dijon and translated into English in 2004 (Bobeck, P. (2004)). His work resulted in the “Darcy’s Law” relationship, which can be expressed as equation 1.

(1)

Here, B is termed the permeability coefficient and is dependent

only on the physical properties of the bed. is the viscosity of the fluid and is the bed thickness. is the average velocity of the fluid and is the pressure drop.

Alternatively the expression for fluid flow through a fixed bed is given by equation 2, the Carmen-Kozeny equation,(Richardson, J. F. & Harker, J.H. with Backhurst, J.R.( 2002)).

(2)

2.2.1 Fluidised Beds

When a fluid is passed downwards through a bed at any flow, no relative movement occurs between the particles unless the initial layout of the particles was unstable. Where the flow is streamline, the bed will behave as a fixed bed and can be treated with the fixed bed equations. When a fluid is passed upwards through a bed, the bed will behave as a fixed bed only at relatively low flow rates. A simple force balance on a particle, figure 2, shows competing forces.

figure 2. A force balance on a particle in a bed.

Thus when the frictional drag forces are equal to the apparent weight of the particle, the bed will begin to rearrange