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

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

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.

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.

Types of Filters

Filter selection based on Duty requirements Nature of solids being filtered

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

Pressure Filters Plate and Frame Press Chamber Press

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

Filtration Theory

An Illustration of Filtration Mechanism

An Illustration of Filtration Mechanism

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)

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

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

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)

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

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:

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

3.1

0.8

2

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

Constant Rate Pressure Drop: Or

Constant Pressure Integrated form: OR :

B (m3)

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

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

m

m

= 4000 = 1000 2

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

Specific resistance at any point in the cake:

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

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!

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

Leaf Filters

Bag Filters

Continuous Rotary Filters

Continuous Rotary Filters

Operating Cycle Five basis stages: Filter Dewater Wash Solids Removal

Solution

Solution

Solution

Solution