Table of Content

1. Introduction to Filtration1.1 Theory of Filtration1.2 Basic Types of Filtration

2. Selection of Filtration Equipment2.1 Vacuum Filters2.2 Rotary Drum Vacuum Filter (RDVF)

2.2.1 Introduction2.2.2 Working of RDVF2.2.3 Components of RDVF2.2.4 Applications and Limitations

FIGURE 1: Principle of Filtration

Cake Filter

Filtrate

Filter medium

Slurry

Support for filter medium

1. Introduction to Filtration

Filtration is a mechanical separation process in which solid particles are separated from a suspension in a liquid with the help of a porous medium, on which the solid particles are retained and the liquid passes through it.

In filtration operation, the pore-size of the filter medium are generally large than the solid particles that are to be removed, and the efficiency of the filter generally increases only when an initial deposit has been trapped in the medium. A typical filtration operation is shown in figure 1.

In general, filtration is less demanding in energy than other mechanical separation processes like, evaporation and drying where the high latent heat of the liquid (which is normally water), has to be provided. [4]

In figure 1, as the cake gradually builds up on the medium; the resistance to flow gradually increases. During the period of initial filtration, solid particles deposit inside the surface layer of the cloth that forms a true filtering medium. The most important factors on which filtration depend are: [4]

a) The pressure difference above and below the filter medium.b) The available area of the filtering surface.c) The viscosity of the filtrate.d) The resistance of the filter cake.e) The resistance of the filter medium and initial layers of cake.

1.1 Theory of FiltrationIf the filtration pressure is constant, the rate of flow progressively diminishes

whereas, if the flow-rate is to be maintained constant, the pressure must be gradually increased.

In a batch operation, if the pressure is kept constant then the flow-rate diminishes, and if the flow-rate is kept constant then pressure must be gradually increased. Because the particles forming the cake are small and the flow through the bed is slow, streamline conditions are almost invariably obtained, and at any instant, the flow rate may be presented by the following form of equation.

1 dV 1 ℮ 3 -∆P (1) A dt 5 (1- ℮)2 S2μluc = =

1

1 dV - ∆P (2)

A dt r μl

1 dV A 2 (- ∆P) (3)

A dt r μ v V

t r μ v V (4) t r μ v V (5)

V A2 (- ∆P) V 2A2 (- ∆P)

Filtration at constant rate Filtration at constant pressure

= =

Where V is the volume of the filtrate which has passed in time t, A is the cross-sectional area of the filter cake, uc is the superficial velocity of the filtrate, l is the cake thickness, S is the specific surface area of the particle, ℮ is the voidage, μ is the viscosity of the filtrate, and ∆P is the applied pressure difference.

For the above equation (1), it was assumed that the cake is uniform and the voidage is constant throughout. But in filtration, voidage, ℮, depends on the nature of support, its geometry, surface structure, and on the rate of deposition. Therefore in the initial stages of cake formation, special attention must be given due to following reasons. [4]

a) For any filtration pressure, the flow rate is greatest at the start of process, due to low resistance.

b) At high initial rates of filtration, plugging of the pores of filter cloth occurs that causes high resistance to flow.

c) The orientation of the solid particles in the initial layers may considerably influence the structure of the whole filter cake.

Depending on the physical nature of the cake, it can be either compressible or in-compressible. For in-compressible cakes a term specific resistance (r), is incorporated, that depends on voidage ℮, and specific surface area of particle S.

the term ℮3 / 5 (1 - ℮)2 S2 is constant, thus

Here equation (2), represent the basic equation of filtration, and if; v = volume of cake deposited by unit volume of filtrate, than

v = lA / VSo equation (2), becomes

Here equation (3), gives basic relationship between - ∆P, V and t. In filtration two modes of operations are involved, (a): Filtration at constant rate; and, (b): Filtration at constant pressure. Normally the second mode is more frequently adopted in continuous filtration operation. The two equations can be derived from equation (3), by assuming filtration at constant rate and than at constant pressure, as given below;

=

=

2

1.2 Basic Types of Filtration

There are two basic types of filtration, though there are some cases in which the two types are seen combined to one another. In the first type that is called cake filtration, the solid particles from the suspension, which generally has a high proportion of solids, are deposited on the surface of a porous septum, which in ideal case, offers only a small resistance to the flow. As the solids starts to accumulate up on the septum, the initial layers prevents the particles from embedding themselves in the filter cloth, and ensures that the filtrate obtained is free from solid particles.

In the second type of filtration called the depth or deep-bed filtration, the particles penetrate into the pores of the filter medium, where impact forces between the particles and the surface of the medium are basically responsible for there removal and retention. This type of configuration is commonly used for the separation of fine particles from very dilute suspensions, where the recovery of solid particles is not of primary importance. Such type of filtration include; air and water filtration. [4]

2. Selection of Filtration Equipment

Most Industrial filters employ vacuum, pressure, or centrifugal force to drive the liquid (filtrate) through the deposited cake of solids. Filtration is essentially a discontinuous process. With batch filters, such as plate and frame presses, the equipment has to be shut down to discharge the cake; and even with those filters designed for continuous operation, such as rotating-drum filters, periodic stoppages are necessary to change the filter cloths. Batch filters can be coupled to continuous plant by using several units in parallel, or by providing buffer storage capacity for the feed and product. [4][5]

The principal factors to be considered when selecting filtration equipment are:

1. The nature of the slurry and the cake formed.2. The solids concentration in the feed.3. The quantity of material to be handled.4. The nature and physical properties of the liquid: viscosity, flammability, toxicity,

corrosiveness.5. Whether cake washing is required.6. The cake dryness required.7. Whether contamination of the solid by a filter aid is acceptable.8. Whether the valuable product is the solid or the liquid, or both.

2.1 Vacuum Filters

If the pressure beneath the filter is reduced below atmospheric, the unit than operates as a vacuum filter and the driving force is again substantially greater than that available with a gravity filter.

The various types of vacuum filters may be grouped as follows:

NoteAll Filters except for the Nutsche are continuous 3

2.2 Rotary Drum Vacuum Filter (RDVF)

2.2.1 Introduction

A multi-component drum type vacuum filter consists of a rotating drum connected about a horizontal axis, arranged so that the drum is partially submerged in the trough into which the material is to be filtered is fed. The periphery of the drum is divided into compartments, each of these compartments is provided with a number of drain lines. These tubes pass through the inside of the drum and terminate as ring of ports covered by a rotary valve, through which vacuum is applied. The surface of the drum is covered with a filter fabric, and the drum is arranged to rotate at low speed, usually in the range of (0.1 – 0.25 rpm) up to (3 rpm) for very free filtering materials. [1][4]

2.2.2 Working of RDVF

As the drum rotates, each compartment undergoes the same cycle of operation and the duration of each of these is determined by the drum speed, the submergence of the drum and the arrangement of the valve. The normal cycle of operation consists of filtration, drying and discharge. It is also possible, however, to introduce other operation into the basic cycle, including: [4]

a) Separation of initial dirty filtrate (which may be an advantage if a relatively open filter fabric is used).

b) Washing of the filter cake.c) Cloth cleaning.

Figure 3 (a & b) shows typical layout of a rotary drum vacuum filter.

a) The drum is partially submerged in the feed slurry, as it starts rotating.b) The suspension to be filtered is fed continuously to the filter trough.c) A special pendulum agitator prevents sedimentation of solids inside the trough.

(a)FIGURE 3: Rotary Drum Vacuum Filter

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FIGURE 4: RDVF Valve with three bridges

d) The upper surface of the filter drum is divided into cells and is covered by a filter cloth.

e) Approximately 40 percent of the filter area is submerged in the suspension and the drum rotates at 6 – 180 rph.

f) The vacuum drum filter builds up vacuum with a liquid seal pump which is connected to the drum cell via the control head and filtrate pipes.

g) Vacuum draws liquid through the filter medium (cloth) on the drum surface where the solid particles are retained.

h) The vacuum draws air (or gas) through the cake and continues to remove moisture content from the cake, as the drum rotates.

i) If required, the cake can be washed to remove impurities or to extract more product. Additional drying of the cake follows washing.

j) Finally, the cake is discharged from the drum to a conveyor to the next process step.

k) The filtrate and air pulled through the medium flow through internal filtrate pipes and pass though the rotary valve and into the filtrate receiver.

l) The liquid stream is separated from the vapor stream in a vapor separator.m) Liquid filtrate is then pumped to the next step in the process.

2.2.3 Components of RDVF

The Drum is supported by a large diameter trunion on the valve end and a bearing on the drive end. The drum face is divided into circumferential sectors each forming a separate vacuum cell.

The Valve with a bridge setting controls the sequence of the cycle so that each sector is subjected to vacuum, blow and a dead zone. As shown in Figure 4.

1. Vacuum and Blow Zone Separating Bridge. This bridge cuts off the vacuum so it is slightly wider than the internal pipe port.

2. Dead Zone Bridge. This bridge starts up vacuum again once the compartment submerges.

3. Start-up Assist Bridge. At start-up the upper vacuum zone is open to atmosphere and a cake may be formed only when closing the valve that controls this zone. Once the cake starts to emerge from the tank the valve is gradually opened and fully opened when the entire drum face is wrapped with the cake. [1]

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The drum deck is divided into separately isolated compartments each subjected to vacuum or blow while the drum is in rotation. The timing of vacuum or blow depends on the bridge setting of the main valve.

An agitator keeps on agitating the slurry in suspension and reciprocates between the drum face and tank bottom.

The tank that houses the drum and agitator, has baffled slurry feed connections; an adjustable overflow box is used to set the desired drum submergence and a drain connection.

The scrapper or knife used to peal-off the solid particles from the drum surface.

The filter cloth winds around the drum and as the drum rotates, the slurry is sucked into the cloth. Most filter cloths are made up of synthetic material such as polypropylene or polyester.

The internal piping manifold can be of two types, trail pipes, handling the mother filtrate on the ascending side of the drum. And the lead pipes handle the wash filtrate on the descending side. The trail pipes are always connected to the outer row and have a bigger diameter than the lead pipes that are connected to the inner row. The reason for this arrangement is that the trail pipes handle more liquid than the lead pipes so require a bigger cross section to avoid vacuum losses.

2.3 Experimental Design of Rotary Drum Vacuum Filter (RDVF)

In the experimental design of RDVF a lab scale separation process is conducted to separate solid calcium carbonate (CaCO3) from its slurry in water. Equipment specification is briefly described as under:

Technical Data

Operating Principle → Vacuum Filtration

Type of Operation → Continuous

Material of Construction → Stainless Steel, Carbon Steel

Length or Width of Rotary Drum = 8 inches = 0.203 mDiameter of Rotary Drum = 36 inches = 0.914 mRadius of Rotary Drum = 18 inches = 0.457 mSubmergence of Drum inside the Trough = 40% Area of Filtering Surface, (A) = (0.914π × 0.203)

= 0.186 π = 0.584 m2

Range of Particle Size → 1 – 300 μm

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Objective 1 “To Study the Effect of Constant Pressure on the Efficiency of Rotary Drum Vacuum Filter at different Drum Speed (rph)”

Objective 2 “To Study the Effect of Constant Speed (rph) of Drum on the

Efficiency of Rotary Drum Vacuum Filter at Different Pressure inside the Drum”

Objective 3 “To Study the Relationship between Cake Resistance and Applied Pressure at Constant Drum Speed (rph)”

Analysis of Solid Particles (CaCO3)

To calculate the average particle size, screening was carried out using a number of sieves.

Average particle diameter of dry solid = dp = 120 μmSurface Area of one particle = Sp = πdp

2

= 4.5 × 10-8 m2

Volume of one particle = Vp = πdp3 ÷ 6

= 9.037 × 10-13 m3

Specific surface area of particles = S = 6 ÷ dp

= 50000 m-1

Density of CaCO3 = ρp = 2703.6 kg/m3

Number of particles for 10 kg sample = N = m ÷ ρp Vp

= 4.09 × 109

Total surface Area of Particles = A = N × Sp

= 184.05 m2

Fractional voidage of particles [4] = ℮ = 0.38

Sphericity of particles = φp = (6 / dp) ÷ (Sp / Vp)= 1

(Note: Sphericity = 1, shows that solid particles are spherical in shape)

Observation and Calculations

Weight of solids in slurry = ws = 10 kgWeight of slurry prepared = w = 200 kg

Density of water = ρ = 1000 kg/m3

Viscosity of water, at 20 °C = μ = 1 × 10-3 kg/m.sSlurry concentration; mass of solidsper unit volume of water = c = 52.6 kg/m3

Efficiency of RDVF = η = weight of dry solid out × 100weight of dry solid in

Since cake of CaCO3 is compressible so its specific resistance is the function of the pressure difference across the cake. Can be calculated as [5]:

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Specific Resistance of Incompressible Cake, α = (- Δ P) A 2 g c Kc

c μHere

Kc is ratio of time per liter per liter of filtrate, and is equal to the slope of filtration curve. [5]

Specific Cake resistance at 53.3 kPa (400 mmHg) and 80 rph

The first step to calculate specific resistance of cake is to prepare plot between volume of filtrate (V) and t / V, which is shown in Table 1.

Table 1: Volume-Time Data, at 7.74 psi (400 mmHg)Volume of

Filtrate, V (L)Time, t (s) t / V (s/L)

0.51.01.52.02.5

Evaluation at constant pressure

Sr.No.

Drum Speed (rph)

Solids in

Slurry (g)

Cake Out (g)

Dry Solid Out (g)

Sample Taken in time

(s)

Vol. of Filtrate V (ml)

Rate of filtration

V/t (ml/s)

Specific Cake

resistance (m/kg)

η (%)

1

2

3

8

Evaluation at constant drum speed (rph)

Sr.No.

Pres-sure

applied (kPa)

Solids in

Slurry (g)

Cake Out (g)

Dry Solid Out (g)

Sample Taken in time

(s)

Vol. of Filtrate V (ml)

Rate of filtration

V/t (ml/s)

Specific Cake

resistance (m/kg)

η (%)

1

2

3

Conclusions

2.2.4 Application and Limitations

Applications

• In Chemicals like (Barium Sulphate, Sodium Bicarbonate, Calcium Carbonate etc)

• In Minerals like (Frothed Coal (fine), Copper Concentrates, Lead Concentrates, Zinc Concentrates etc)

• In Paper Effluents and Bleach Washer• In foodstuff like (Starch, Sugar Cane Mud, Glucose etc)• In Effluents like (Primary Sewage, Neutralized H2SO4 Pickle etc)

Limitations

• Slurries with solids that tend to settle rapidly and will remain at the bottom of suspension under gentle agitation.

• Cakes which require long drying times to reach asymptotic moisture values.• Cakes when a single washing stage is not sufficient to remove residual

contaminants from the cake.• Not designed to handle gas-solid mixtures

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References

[1] www.solidliquid-seperation.com [2] www.petersonfilters.com [3] www.komlinesanderson.com [4] Coulson & Richardson’s, Particle Technology & Separation Process, V-2,

Fifth Edition.[5] Warren L. McCabe, Unit Operations of Chemical Engineering, Seventh

Edition.

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