1. Filtration Reading Materials:
M. Rhodes, Introduction to Particle Technology, Wiley. Chapter 6
Perry’s Handbook, Chapter 18
Coulson and Richardson’s Chemical Engineering, Volume 2, Chapter 7

2. What is Filtration?
The process of separating solid from liquid using a porous medium which retains the solid but allow the liquid to pass.

3. Applications Widespread use for recovery of solids from solution
Produce a cake of moist solids
Cake may be concentrated from flotation or other fine separation process
Tailings are also filtered in some plants
Use to clarify Pigment liquid, produce potable water and remove solids from lubrication oils and fuels
May be operated using gravity, vacuum and pressure methods.

4. Filter Medium Most important consideration
Affect the efficiency of separation
Function: to support the filter cake
Characteristic:
Able to retain solids
Mechanically strong. Corrosion resistance
Relative coarse material
Material: cotton, wool, nylon, silk, glass fiber, etc.

5. Types of Filters

7. Filter selection based on
Duty requirements
Nature of solids being filtered

8. Pressure Filters Virtual incompressibility of solids Advantages:
Higher flow rates
Better washing and drying
Disadvantages:
Difficult to continuously remove solids
Usually a batch process

9. Pressure Filters
Plate and Frame Press
Chamber Press

10. Vacuum Filters Batch vacuum filters Continuous vacuum filters
Rotary drum
Rotary disc
Horizontal belt

11. Filtration Theory

12. An Illustration of Filtration Mechanism

13. An Illustration of Filtration Mechanism

14. Factors Affecting the Filtration Process
Properties of fluid (i.e. viscosity, density and corrosive properties)
Nature of solids (i.e. particle size and shape, size distribution and packing characteristics)
Concentration of solids in suspension
Quantity materials to be handled and its value
Valuable products (I.e. solids, liquid or both)
Do we need to wash the filtered solids?
Effect of contaminants to the product
Pretreatment of feed liquor (e.g. heated)

15. Factors Affecting the Rate of Filtration
The drop in pressure from the feed to the far side of the filter medium
The area of filtering surface
The viscosity of the filtrate
The resistance of the filter cake
The resistance of the filter medium and initial layers of cake
Voidage of the Cake
If the voidage of the cake is constant, this case is called incompressible cake
If the voidage of the cake changes with time, it is called compressible cake

17. Incompressible Cake Pressure Drop
Contributors to the pressure drop: the cake and the filter medium
For porous flow under laminar flow conditions for both cake and medium, Kozeny equation can be used:
Where:
and B is the volume of filtrate passed in time t

18. The value of the cake thickness X can be found from mass balance
A Case of Small Cake
The value of the cake thickness X can be found from mass balance
Vs (intake) = Vs (out)
dx*A*(1-eps) = (dB + dx*A)
dx*A*(1-eps) = Vs (intake) …… Unit is m3s of solid
But (dB + dx*A)= Vl (out) has a unit m3L liquid so need to * with , solid volume fraction, c, (m3s / m3L )
m3s = m3L * m3s / m3L ……….. m3s = m3s
Where c is volume fraction of solid in the feed and can be obtained by:
ms is mass per unit volume of feed, and s is the solid density (the density of the actual particles, not the density of a volume of the material (or bulk density)

19. Pressure Drop of Small Cake
, OR
, mass of solids per unit volume

20. Basic Equation of Filtration
Define pressure resistances of cake and medium as:
are cake and medium resistance
Assume that m/s < < 1-, then we get:

21. If we define specific resistances as:
Assume that m/s < < 1-, then we get:

22. 3.1

23. 0.8

24. 2

25. Two Modes of Filtration
Constant Pressure
When we keep the constant pressure, the flow rate will be decreased
Constant Rate
Under the conditions of constant rate, the pressure drop will increase as the cake is built up

26. Constant Rate
Pressure Drop: Or

28. Constant Pressure
Integrated form:
OR :

29. B (m3)

31. Constant Pressure : at the beginning
If the medium resistance is negligible

35. Example
Repeat the previous example using the following correlation:
By drawing
Vs B

38. m

40. m

44. = 4000
= 1000 2

47. Compressible Filter Cake
Caused by the increase of resistance to flow through the cake:
Consolidation
Occurs when there is a variation in size particles
As the cake builds up, the small particles will eventually fill up the pores. So increase the specific resistance of the cake
Compression
As the pressure increases, the particles are forced to pack more closely, resulting a higher resistance
This effect is irreversible, with the cake showing a specific resistance equivalent to the highest pressure achieved
This effect is the main effect occurring in compressible cake

48. Specific resistance at any point in the cake:

53. Filtration Equipment Bed Filters
Suitable for filtering large volumes of liquid with low solid content
Washed by backwashing using clean liquid which flows upwards
through the bed, fluidize it and releasing the trapped particles

54. Plate and Frame Filters
A series of plates and frame arranged alternatively and supported on rail
Slurry enters through the frames, and filtered on the cloth (between plate
& frame)
The plate has a ribbed surface to allow the filtrate to collect and be
discharged
Disadvantage: time to dismantle it!

55. Chamber Press
A modified plate and frame press
Surfaces of the plates are recesses
The slurry is fed through the center of the press and passes through hole
in the center of each cloth

56. Leaf Filters

57. Bag Filters

58. Continuous Rotary Filters

59. Continuous Rotary Filters

61. Operating Cycle
Five basis stages:
Filter
Dewater
Wash
Solids Removal

63. Solution

64. Solution

67. Solution

68. Solution