Many years ago, I was lucky enough to work in “Launch Pad” in the science museum. There were lots of wonderful activities and the following blog posts are about the activities. Launch Pad has been replaced by the equally wonderful “Wonderlab”
The fluidised bed
The fluidised bed was just a large bowl of table salt with an added extra – an air blower.
When I used to teach science to year 7 (11/12 years of age) I would teach them that there were three states of matter. One of them was a solid, which was defined as having a fixed shape and volume and couldn’t flow.
The fluidised bed seems to contradict the idea that solids can’t flow.
The Salt Bowl. Salt which is packed solid acts as if it were a fluid when air is blown through it.
The video below shows how you can make your own simple fluidised bed
Put some salt into a container and feel it. It is still, dry and obviously solid.
If you had a big enough container full of salt you could stand on the salt because it would support your weight.
If you have made your own fluidised bed blow some air through the salt. You will find that things begin to change. When you put your hand in now you can feel the salt moving around and the bubbles rippling under your fingers. The salt looks, feels and behaves like a liquid at its boiling point even though it is still a “solid”. Liquids and gases (fluids) cannot support your weight, and so your hands disappear in the “fluidised” salt like someone trying to walk on quicksand.
Quicksand is a colloid consisting of fine granular material (such as sand, silt or clay) and water.
Quicksand forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight. Quicksand can form in standing water or in upward flowing water (as from an artesian spring). In the case of upward flowing water, forces oppose the force of gravity and suspend the soil particles.
The saturated sediment may appear quite solid until a sudden change in pressure or shock initiates liquefaction. This causes the sand to form a suspension and lose strength. The cushioning of water gives quicksand, and other liquefied sediments, a spongy, fluid-like texture. Objects in liquefied sand sink to the level at which the weight of the object is equal to the weight of the displaced soil/water mix and the submerged object floats due to its buoyancy.
A colloid is a mixture in which one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance.
So, in the fluidised bed the solid salt has become fluidised.
What is happening?
In the fluidised bed the air blows between the grains of salt and allows them to move about freely. The speed of the air is critical for this to happen. If the air blows too fast the salt will be blown away and if it is blown too slowly the bed of salt will not move at all.
The moving salt particles are interesting because they behave like the particles (molecules) in everyday liquids, such as water in a saucepan. It shows us something about the states of matter.
In a solid, the molecules are packed tightly together and do not move around. In a liquid, however the molecules are freer and move past each other (as in the “fluidised” salt). In a gas there is more room again and the molecules have greater freedom.
By providing energy (such as heat) to a solid we can produce a liquid and by giving more energy to a liquid we can produce a gas. This is the kinetic theory of matter and the fluidised bed is a demonstration of what happens to the molecules in a liquid.
Does the salt behave like a fluid?
You have seen that the particles of salt move like the molecules in a fluid. The fluidised bed shows other properties of fluids. As in real fluids some things float in the bubbling salt and some things sink.
If you have made a fluidised bed find two objects of similar shape but of very different weights. The light one could be made of polystyrene and the heavy one of a convenient piece of metal or stone. Place them where the bubbles appear when the blower has been switched on. The heavy object will sink and the light one will float. If you bury them both, the light one rises to the surface. Flotation is one of the properties of fluids so the salt does behave like a fluid.
The heavy object sinks because it is denser than the light one. It has more weight for its size so it disappears in the bubbles. The light object should have the same shape and size as the heavy object but it has less stuff inside it. Because of this the airstream can keep it afloat. Other objects, such as boats, float on liquids, like the sea, for similar reasons of density.
Using fluidised beds
Fluidised beds were very useful in some types of industry. They are a relatively recent technological innovation (1950’s) and have been used for processes as different as freezing peas and burning coal (the latter is no longer of importance as coal fired power stations are being phased out worldwide).
Fluidised beds have many advantages over more traditional industrial methods. Gases and solids can be mixed very thoroughly so that reactions between them can take place more quickly and efficiently.
Fluidised beds can provide very effective even heating and cooling. For example, vegetables are frozen on fluidised beds with air at -38oC blown through them. The food is very efficiently evenly frozen.
It is important to realise that salt is most definitely a solid. Each individual crystal has a fixed shape, fixed volume and it cannot flow.
Principles of fluidised beds
Solid particles can be moved by a fast stream of air or other gas – for example, sand blown by the wind on the sea shore and dust sucked into vacuum cleaners. The particles will not be moved unless the air flow is fast enough. If the air velocity is high the sand will be carried for long distances, but at a lower velocity eddies will be formed in which the sand moves around but does not travel far. This ability of moving air to entrain small solid particles and keep them in motion enables a fluidised bed to be formed.
Imagine a box containing sand resting on a mesh. If air is blown slowly upwards through the mesh, it percolates between the sand particles without disturbing them. When the velocity of the air stream is gradually increased, a point is reached when individual particles are forced upwards; they become supported by the air stream and begin to move about, although the bed continues to have a fairly well-defined surface.
At still higher velocities an important change occurs; the bed becomes turbulent with rapid mixing of the particles. Bubbles, similar to those in a briskly boiling liquid, pass through the bed and the surface is no longer well defined but diffused. A bed of solid particles in this state is said to be ‘fluidised’, because it has not only the appearance but also some of the properties of a boiling fluid.
There are lower and upper limits of air velocity between which satisfactory fluidisation of sand or any other granular substance will take place. The minimum velocity of the air stream causing fluidisation is termed ‘minimum fluidising velocity’. For a bed of any material, the larger the particles, the greater the velocity of the air or other gas required to fluidise it; for particles of a given size, the heavier they are, the greater the fluidising velocity needs to be.
In practice, a fluidised bed will contain particles of different sizes. The operating limits are set, on the one hand, by the minimum of air velocity needed to keep the particles fluidised and, on the other hand, by the maximum velocity that can be used before an excessive quantity of material is blown out of the bed.
For a given height of containing vessel, there is a maximum depth of fluidised bed which can be maintained at a given fluidising velocity, depending on the size of the particles. If more particles are added, entrainment will occur until the level reduces back to the equilibrium depth. If the equilibrium depth is not adequate for the design requirements, it is possible to obtain a deeper bed by continuously separating the entrained particles and returning them to the bed. The greater the difference between the required depth and the equilibrium depth, the greater will be the rate of recycling of the particles which is necessary.
By using a high rate of recycle, it is even possible to maintain a bed of fine particles of a size which would otherwise be entirely entrained from the containing vessel. However, in this case the appearance of the bed if different. Instead of there being a bubbling bed with a well-defined surface, the fine particles fill the entire vessel. but with a relatively low concentration. The actual concentration depends on the rate at which particles are recycled. This type of bed is a “fast fluidised bed” or a “fast recycling bed”. The designs in which the solid material forms a well-defined bed are said to be “bubbling beds” and such is the rate of development in fluidised bed combustion that they are sometimes referred to as “conventional bubbling beds”.
In some applications the natural solids mixing in a fluidised bed is enhanced by using a non-uniform distribution of air to the bed often coupled with an inclined air distributor. This type of system is referred to as a “circulating bed”.
A fluidised bed of solids behaves in many ways like a liquid and has the following important characteristics:
o A bubbling bed finds its own level. If the vessel containing the fluidised bed of solids is tilted from a horizontal position, the surface of the bed remains level.
o Provided the fluidised state can be maintained; the bed can be transferred from one container to another as though it were a liquid.
o Solid particles in a fluidised bed are violently churned about; rapid mixing occurs and any added particles are quickly distributed through the bed.
o Objects can float or sink in a fluidised bed according to their density, as in a liquid.
o When a fluidised bed is heated, the thorough mixing enables heat to be rapidly transferred from one part to another, ensuring near uniformity of temperature as in a stirred liquid. This is in contrast to conditions in a bed of stationary particles, in which heat is transferred by the much slower process of conduction from one layer of particles to another. Temperature differences in beds of stationary ‘Particles can therefore be very high.
The mixing which occurs in a fluidised bed causes heat to be rapidly transferred to a cooler surface immersed in it (for example, a water tube). The constant movement brings a continuous supply of hot particles to this surface.
The evenness of the fluidised bed temperature enables automatic control to be more precise than in other coal-burning systems.