Filtration Flashcards
What is filtration?
A solid liquid separation technique where by a solid is harvested from a suspension by means of a porous medium or a screen which retains the solids but allows liquid to pass through.
What are the four sections of a filter set up in order from bottom to top?
Filter support, filter medium, filter cake and suspension/slurry.
Why would you use a filter material that has pores that are bigger than the particles to be filtered?
Once the cake has begun to form, the larger pores reduce the pressure drop and increase the rate of filtering. This is at the expense of a small amount of material being lost initially.
What is deep bed filtration?
When particles penetrate a long way into a filter medium and the medium is thick.
What are the 5 critical factors in filtration?
- Pressure drop across the filter and cake formed.
- Surface area of the filtrate
- The viscosity of the filtrate
- The resistance of the filter cake
- Resistance of the filter medium
How do you calculate the volume of filtrate that has passed?
dV/dt = A^2 (-delta P) / (r Mu v V)
dV/dt = Volume of filtrate that has passed in time t
A = area
delta P = pressure drop
t = specific resistance
Mu = viscosity of filtrate
v = volume of cake deposited per unit of filtrate
V = filtrate volume
How do you calculate the volume of cake deposited per unit of filtrate?
v = lA / V = c / (1-e)
l = cake depth A = Area V = volume of filtrate c = suspension volume concentration e = cake voidage
How does V vary with the change in pressure when there is a constant rate of filtration?
V = -k deltaP
How does Volume filtered relate to rate of filtration at a constant pressure?
dV / dt = k / V
and volume filtered = k (t^1/2)
What is an incompressible cake?
No matter how large the pressure put on the cake, it will not change volume.
What is a compressible cake?
The cake will change volume depending on how much pressure is put on it.
Why is cake compression bad?
Cake compression is no recoverable/inelastic usually and it increases the pressure drop, reducing the rate of filtration.
How does sedimentation effect filtration?
Larger particles settle on top of the filter. This is good because it can lead to a lower density cake, which decreases the pressure drop.
What is delayed cake filtration?
Mechanical scraping is used to keep the cake thickness at a constant and increases the rate at which the suspension is thickened.
What is cross flow filtration?
If the suspension is recirculated with flow across the filter then the suspension can be thickened. Essentially removes a lot of the solvent to thicken the slurry without forming a cake.
