FILTRATION

Nur istianah,ST,MT,M.Eng – KP2 – THP UB

Introduction

Filtration may be defined as the separation of

solids from liquids by passing a suspension through a

permeable medium which retains the particles.

Figure 1. Schematic diagram of filtration system

The fine apertures necessary for filtration are provided

by fabric filter cloths,

by meshes and screens of plastics or metals,

by beds of solid particles.

In some cases, a thin preliminary coat of cake, or of other fine particles, is put on the cloth prior to the main filtration process.

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 )

Viscosity of filtrate ( v )

Resistance of filter cake ( α )

Resistance of filter medium ( Rm )

Properties of slurry ( μ , ฯลฯ )

rate of filtration = driving force/resistance

-(P) or

Pressure

drop

Filter cake ()

Filter medium (Rm)

Viscosity ()

Filtration Equation

Flow of fluid through packed bed : Application of Carman-kozeny’s equation

โดย k1 = constant = 4.17 for particles with definite size and shape

= viscosity of filtrate (Pa.s)

v = linear velocity based on filter area (m/s)

= void fraction หรือ porosity of cake

L = thickness of cake (m)

S0 = specific surface area of particle area per volume of solid particle (m2/m3)

Pc = pressure drop in cake (N/m2)

3

2

0

2

1)1(

svk

L

Pc

Substitute v in term of volume (V)

A = filter area (m2)

V = volume of filtrate at t sec

L = thickness of filter cake

Cs = kg of solid/m3 of filtrate

P = density of solid particle in cake (kg/m3)

A

dtdVv

Substitute L in term of height of cake (L)

Obtain: p

s

A

VCL

)1(

ms R

A

VC

P

dt

dV

A

1

Specific cake resistance ()

โดย = Void fraction

S0 = specific surface area of particle (m2)

)Pressure,,(0

Sf

nP)(0

nP)(10

Specific cake resistance;

Their specific resistance change with pressure drop across the cake ∆pc. In such cases, an average specific cake resistance av should can be determine from

If the function = (∆pc) is known from pilot filtration tests, bomb filter test or from the use of a compressibility cell.

An experimental empirical relationship can be used over a limit pressure range

Where 0 : the resistance at unit applied pressure drop

n : a compressibility index obtained from experiments

(n = 0 for incompressible substance)

n

cp )(0

nPc)(10

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 Equations for

Constant – Pressure Filtration

At t = t sec At t = t sec

P

Rm

A

V

P

Cs

AV

t

2/

Determine and Rm

Filtration equation for

Constant rate Filtration

For incompresible cake: Kv and C are

constant

dV/dt = constant = V/t

P

t

Slope Y-intercept

From constant rate equation the pressure drop

required for any desired flow rate can be found.

Also, if a series of runs is carried out under different

pressures, the results can be used to determine the

resistance of the filter cake.

Ex1. Constant pressure

Filtration area = 0.01 m2 A

Solution density = 1,062 kg/m2

Solution viscosity = 1.610-3 Pa.s

Filtration pressure = 200 kPa P

Solid concentration = 3 kg/m3 Cs

Determine specific filter cake resistance and filter medium

resistance

Time (sec) Volume (cm3)

0 0

14 400

32 800

55 1200

80 1600

107 2000

Y = aX + C

Y axis = tA/V

X axis = V/A

Slope = Cs/2P

Y intercept = Rm/P

The solution

Time (sec) Volume (cm3) Volume (m3) tA/V V/A

0 0 0 0 0

14 400 0.0004 350 0.04

32 800 0.0008 400 0.08

55 1200 0.0012 458.33 0.12

80 1600 0.0016 500 0.16

107 2000 0.0020 535 0.20

y = 1175x + 307.67R2 = 0.9912

0

100

200

300

400

500

600

0 0.05 0.1 0.15 0.2 0.25V/A

tA/V

kgm

P

CsSlope

/10792.9

1175)10200(2

)3()106.1(

11752

10

3

3

mR

R

P

Rercepty

m

m

m

/110845.3

67.30710200

)106.1(

67.307int

10

3

3

Ex2. Constant rate

A slurry containing 25.7 kg dry solids/m3 of filtrate across the

filter medium area 2.15 m2 at a constant rate of 0.00118

m3/s. If the pressure drop was observed 4,000 and 8,500 Pa

after 150 and 450 seconds of filtration, respectively. The

viscosity of filtrate was 0.001 Pa.s

Determine the specific cake resistance and filter medium

resistance.

t

V

A

Rmt

t

V

A

CsP

At

RmV

tA

CsVP

t

RmA

VCs

PAV

dt

RmA

VCs

PAdV

tCons

RmA

VCs

PA

dt

dV

.

tan

2

2

2

2

The solution

P

(Pa)

t (sec)

t

V

A

Rercepty

t

V

A

CsSlope

m

int

2

2

(1)

(2)

(4)

(3)

y = 15x + 1750R2 = 1

0100020003000400050006000700080009000

0 100 200 300 400 500t (sec)

P (

Pa)

P (Pa) t (sec)

4,000 150

8,500 450

mR

R

t

V

A

Rercepty

kgm

t

V

A

CsSlope

m

m

m

/110189.3

175015.2

)00118.0()001.0(

1750int

/10938.1

1515.2

)00118.0)(7.25()001.0(

15

9

9

2

2

2

2

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. clarrification

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

THANKS FOR YOUR ATTENTION

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Nur Istianah-KP2-2016