FILTRATION

FILTRATION

NUR ISTIANAH,ST.MT.,M.Eng

Introduction

Filtration may be defined as the separation of solids from liquids by passing a suspension through a permeable medium which retains the particles.

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Konsep dasar

Filtrat Residu

Feed

Types of filtration

1. Surface filters

2. Depth filters

1.Surface filters

used for cake filtration in which the solids are deposited in the form of a cake on the up-stream side of a relatively thin filter medium.

Figure2. Mechanism of cake filtration

2.Depth filters

used for deep bed filtration in which particle deposition takes place inside the medium and cake deposition on the surface is undesirable.

Figure 3. Mechanism of deep bed filtration

The fluid passes through the filter medium, which offers resistance to its passage,

under the influence of a force which is the pressure differential across the filter.

rate of filtration = driving force/resistance

•The filter-cake resistance is obtained by

multiplying the specific resistance of the filter

cake, that is its resistance per unit thickness, by the thickness of the cake.

•The resistances of the filter material and pre-

coat are combined into a single resistance called the filter resistance.

•It is convenient to express the filter resistance in terms of a fictitious thickness of filter cake.

•This thickness is multiplied by the specific resistance of the filter cake to give the filter resistance.

Factor affected on filtration

• Pressure drop ( ∆P )

• Area of filtering surface ( A )

• Velocity of filtrate ( v )

• Resistance of filter cake ( α )

• Resistance of filter medium ( R

_m

)

• Properties of slurry ( μ)

rate of filtration = driving force/resistance

-(P) or Pressure

drop

Filter cake ()

Filter medium (R_m) Viscosity ()

Filter medium resistance; R

Normally be constant but may vary with time (as a result of some penetration of

solid into the medium) and sometimes may also change with applied pressure

(because of the compression of fiber in the

medium).

•As the overall pressure drop across an

installed filter include losses not only in the medium but also in the associated piping and in the inlet and outlet ports

•It is convenient in practice to include all

these extra resistances in the value of the

medium resistance R.

Constant pressure filtration

• Equation is useful because it covers a situation that is frequently found in a practical filtration plant.

• We could predict the performance of filtration plant on the basis of experimental results.

• If a test is carried out using constant pressure, collecting and measuring the filtrate at measured time intervals

Filtration Equipment

The basic requirements for filtration equipment are:

• mechanical support for the filter medium

• flow accesses to and from the filter medium

• provision for removing excess filter cake.

• In some instances, washing of the filter

cake to remove traces of the solution may be necessary.

• Pressure can be provided on the upstream side of the filter, or a vacuum can be

drawn downstream, or both can be used to

drive the wash fluid through.

Filtration equipment: (a) plate and frame press (b) rotary vacuum filter (c) centrifugal filter

•

1.Plate and frame filter press

• In the plate and frame filter press, a cloth or mesh is spread out over plates which support

the cloth along ridges but at the same time leave a free area, as large as possible, below the cloth for flow of the filtrate.

• The plates with their filter cloths may be horizontal, but they are more usually hung vertically with a number of plates operated in parallel to give sufficient area.

• In the early stages of the filtration cycle, the pressure drop across the cloth is small and filtration proceeds at more or less a constant rate.

• As the cake increases, the process becomes more and more a constant-pressure one and this is the case throughout most of the cycle.

• When the available space between successive frames is filled with cake, the press has to be dismantled and the cake scraped off and

cleaned, after which a further cycle can be initiated.

• The plate and frame filter press is cheap but it is difficult to mechanize to any great extent.

• Filtration can be done under pressure or vacuum.

– The advantage of vacuum filtration is that the pressure drop can be maintained whilst the cake is still under atmospheric pressure and so can be removed easily.

– The disadvantages are the greater costs of

maintaining a given pressure drop by applying a

vacuum and the limitation on the vacuum to about 80 kPa maximum.

– In pressure filtration, the pressure driving force is

limited only by the economics of attaining the pressure and by the mechanical strength of the equipment

BAS stainless steel plate and frame filter press

2.Rotary filters

• In rotary filters, the flow passes through a rotating cylindrical cloth from which the filter cake can be continuously scraped.

• Either pressure or vacuum can provide the driving force, but a particularly useful form is the rotary vacuum filter.

The rotary vacuum drum filter

A suitable bearing applies the vacuum at the

stage where the actual filtration commences and breaks the vacuum at the stage where the cake is being scraped off after filtration. Filtrate is

removed through trunnion bearings.

Rotary vacuum filters are expensive, but they do provide a considerable degree of mechanization and convenience.

3.Centrifugal filters

• Centrifugal force is used to provide the driving force in some filters.

• These machines are really centrifuges fitted with a perforated bowl that may also have filter cloth on it.

• Liquid is fed into the interior of the bowl and under the centrifugal forces, it passes out through the

filter material

.

Centrifugal filters

4. Clarrifying filtration

• Filtration that use gravitational force only as the driving for

• It has low energy requirement

• Simple and low cost

• Need large amount of processing time

21/02/2017

EQUATIONS:

CONSTANT

PRESSURE:

^{V t / A  2 }  ^{  Cs P}  ^{V A }  ^{  Rm  P} 

 

 



 

 

 



 

 t

V A

t Rm t

V A

P  Cs 

.

2 2

CONSTANT RATE:

21/02/2017

Excercise 1

A slurry containing 25 kg dry solids/m³ of filtrate across the filter medium area 2 m² at a constant pressure (60 Pa). The viscosity of filtrate was 0.001 Pa.s. filtrate volume was changed by the time as theese data:

Determine the specific cake resistance and filter medium resistance.

Time (s) Filtrate volume

10 25

20 29

40 34

60 56

21/02/2017

Solution

Time (min)

Filtrate volume(m3)

tA/V V/A

10 0.8 12.5 0.8

20 1.37931 14.5 1.37931

40 2.352941 17 2.352941

60 2.142857 28 2.142857

21/02/2017

Solution

y = 0.5002x + 0.4182 R² = 0.817

0 0.5 1 1.5 2 2.5 3

12.5 14.5 17 28

21/02/2017

 

m Rm

x Rm

P Rm

/ 1 _ 25092

60 4182

. 0 )

001 .

0 (

4182 .

0





 



 

kg m

x P

Cs

/ 69 . 2400

) 60 )(

2 ( 5002 .

0 )

25 ( ) 001 .

0 (

5002 .

2 0





 







21/02/2017

Excercise 2

A slurry containing 25 kg dry solids/m³ of filtrate across the filter medium area 2 m² at a constant rate of 0.01 m³/s. The viscosity of filtrate was 0.001 Pa.s.

pressure drop was changed by the time as theese data:

Determine the specific cake resistance and filter medium resistance.

Time (min) Pressure drop

10 225

20 400

40 468

60 560

21/02/2017

y = 107.3x + 145 R² = 0.9571

0 100 200 300 400 500 600 700

10 20 40 60

∆P

t

Solution

21/02/2017

m x

Rm

x Rm

t V A

Rm

/ 1 10 9

. 2

2 145

) 01 .

0 ( )

001 .

0 (

145



7



 



 



 

 

kg m

x

x t

V A

Cs

/ 10

7168 .

1

) 4 ( 3 . 107 01

. 0 ) 25 ( ) 001 .

0 (

3 . 107

8

2 2

2





 



 











THANKS FOR YOUR ATTENTION

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