BC Lecture 1 - Sedimentation Flashcards
Most appropriate methods for separating solids suspended in a fluid?
- For dilute systems, or relatively large particles of fairly dense solids, a gravity settling tank may be appropriate
- For more dilute systems of smaller and/or light particles, a centrifuge may be more appropriate
- For very fine particles, or where a very high separation efficiency is required, a barrier system such as a filter or membrane may be needed
- For highly concentrated systems, a gravity thickener may be adequate or, for more stringent requirements, a filter may be needed
5 factors of hindered settling?
- The interactions of particles-particles and particles-vessel wall is significant
- The large particles are hindered by the small particles, which increase the effective resistance of the suspending medium for the large particles
- The upwards velocity of the displaced fluid flowing in the interstices between the particles is significant, so the apparent settling velocity (relative to a fixed point) is significantly lower
- The velocity gradients in the suspending fluid flowing upwards between the particles are increased (since the area available for flow is now smaller), resulting in greater shear forces
- Because of the high surface area to volume ratio for small particles, surface forces (e.g shear) are important, resulting in flocculation and ‘clumping’ of the smaller particles into larger effective particle groups
Two modes of settling
- Suspension with a range of particle size rations less than 6:1
In this case, all the particles settle at about the same velocity in the constant composition zone (B), leaving a layer of clear liquid (A) above. As the sediment (D) builds up, however, the liquid that is squeezed out of this layer serves to further retard the particles just above it, resulting in a zone of variable composition (C).
- Case (b) is less common and corresponds to a broad particle size range, in which the larger particles settle at a rate significantly greater than that of the smaller ones, and consequently there is no constant composition zone.
What is thickening?
Thickening involves increasing the solids content of a slurry or suspension by gravity settling in order to achieve separation (or partial separation) of the solids and the fluid. Because concentrated suspensions and/or fine particle dispersions are often involved, the result is usually not a complete separation of the solids from the liquid but is instead a separation into a more concentrated (underflow) stream and a diluted (overflow stream).
Difference between thickeners and clarifiers
The clarifier is designed to produce a clean liquid overflow with a specified purity, whereas the thickener is designed to produce a concentrated underflow product with a specified concentration.
Three zones in a thickener/clarifier
Top, or clarifying zone contains relatively clear liquid from which most of the particles have settled. Any particles remaining in this zone will settle by free settling.
The middle zone is a region of varying composition through which the particles move by hindered settling.
The bottom zone is a highly concentrated settled or compressed region containing the settled particles, the particle settling rate in this zone is very slow.
Applications of packed beds
- Adsorption columns, e.g water removal, air separation, purifications
- Catalytic reactions, e.g hydrocracking of crude oils
- Ion exchange, e.g water purification
- Car exhausts with catalytic converters
- Kitchen cooker hoods
Why does velocity of flow increase along the bed when gases are passed through a packed bed?
As gases are compressible, their physical properties will change as they flow through the bed. For an isothermal bed (constant temperature), in steady flow the mass flow rate will remain constant along the bed. As the pressure decreases along the length of the bed, the volume of the gas must increase and hence the velocity of flow increases along the bed
What happens in the bed when the flow rate is first increased, and then subsequently decreased again in a fluidised bed?
Increase
Before the point of minimum fluidisation, the bed acts like a packed bed because the particles are stationary. So, the pressure drop across the bed can be described using the CK or Ergun equations depending on the flow regime. When u reaches umf, the minimum fluidising condition, the upward drag force imposed by the flowing fluid is just sufficient to overcome the static friction of the particles so that the particles no longer rest on each other.
As the velocity is further increased, the pressure drop passing through the bed falls slightly because of the sudden increase in the area of fluid flow.
After this point, the bed is said to be fluidised. As u is increased further, the bed starts to rise (expands), i.e its height h increases and as a consequence, the bed voidage also increases. It is this reason that the pressure drop reaches an approximately constant value that is independent of the fluid velocity. The increase in bed height will continue until the height of the bed reaches that of its container and thereafter particles are swept out of the bed in fluid flow, this is called elutriation.
What happens in the bed when the flow rate is first increased, and then subsequently decreased again in a fluidised bed?
Decrease
If the fluid velocity is reduced, the bed contracts until it reaches the condition where the particles are just resting on one another. The porosity then has the max stable value which can occur for a fixed/packed bed of the particles.
If the velocity is further decreased, the structure of the bed then remains unaffected provided that the bed is not subject to vibration. The pressure drop (EF) across this reformed fixed bed at any fluid velocity is less than that before fluidisation.
What is particulate fluidisation?
With a liquid, the bed continues to expand as the velocity is increased after fluidisation minimum, and it maintains its uniform character, with the degree of agitation of the particles increasing progressively.
What is aggregative fluidisation?
With a gas, however, uniform fluidisation is frequently obtained only at low velocities. At higher velocities two separate phases may form-a continuous phase,
often referred to as the dense or emulsion phase, and a discontinuous phase known as the lean or bubble phase. The fluidisation is then said to be aggregative.
Slugs, channels and spouting
In some cases the bubbles become very large and are of comparable size to the diameter of the bed in which case they are called slugs. On the other hand, beds of fine, cohesive particles may display channels and cracks. When the particles are large enough, the gas may pass through the bed in a central channel, producing a spouted bed.
