size reduction, fluidisation, filtration & centrifugation

7/26/2019 Size Reduction, Fluidisation, Filtration & Centrifugation

1/20

Size Reduction/Fluidisation

If crushing rolls, 1 m in diameter, are set so that the crushing surfaces are 12.5 mm apart and the angle of nip is

31, what is the maximum size of particle which should be fed to the rolls? If the actual capacity of the machine

is 12 per cent of the theoretical, calculate the throughput in kg/s when running at 2.0 Hz if the working face of

the rolls is 0.4 m long and the bulk density of the feed is 2500 kg/m3.

Power of 3 kW is supplied to a machine crushing material at the rate of 0.3 kg/s from 12.5 mm cubes to a

product having the following sizes: 80 per cent 3.175 mm, 10 per cent 2.5 mm and 10 per cent 2.25 mm. What

power should be supplied to this machine to crush 0.3 kg/s of the same material from 7.5 mm cube to 2.0 mm

cube?


2/20

The size distribution of a powder is measured by sedimentation in a vessel having the sampling point 180 mm

below the liquid surface. If the viscosity of the liquid is 1.2 mNs/m2, and the densities of the powder and liquid

are 2650 and 1000 kg/m3 respectively, determine the time which must elapse before any sample will exclude

particles larger than 20 m. If Stokes law applies when the Reynolds number is less than 0.2, what is the

approximate maximum size of particle to which Stokes Law may be applied under these conditions?

A column 0.6 m diameter and 4 m high is, packed with 25 mm ceramic Raschig rings and used in a gas

absorption process carried out at 101.3 kN/m2 and 293 K. If the liquid and gas properties approximate to those

of water and air respectively and their flowrates are 2.5 and 0.6 kg/m 2s, what is the pressure drop across the

column? In making calculations, Carmans method should be used. By how much may the liquid flow rate be

increased before the column floods?


3/20

Glass spheres are fluidised by water at a velocity equal to one half of their terminal falling velocities. Calculate:

(a) the density of the fluidised bed,

(b) the pressure gradient in the bed attributable to the presence of the particles.

The particles are 2 mm in diameter and have a density of 2500 kg/m3. The density and viscosity of water are

1000 kg/m3 and 1 mNs/m2 respectively.


4/20

Filtration

A slurry is filtered in a plate and frame press containing 12 frames, each 0.3 m square and 25 mm thick. During the first 180

s the pressure difference for filtration is slowly raised to the final value of 400 kN/m2 and, during this period, the rate of

filtration is maintained constant. After the initial period, filtration is carried out at constant pressure and the cakes arecompletely formed in a further 900 s. The cakes are then washed with a pressure difference of 275 kN/m 2 for 600 s using

thoroughwashing (See the plate and frame press in Section 7.4.4). What is the volume of filtrate collected per cycle and how

much wash water is used? A sample of the slurry had previously been tested with a leaf filter of 0 .05 m2 filtering surfaceusing a vacuum giving a pressure difference of 71 .3 kN/m2. The volume of filtrate collected in the first 300 s, was 250 cm3and, after a further 300 s, an additional 150 cm 3 was collected. It may be assumed that the cake is incompressible and that thecloth resistance is the same in the leaf as in the filter press.


5/20

The relation between flow and head for a certain slurry pump may be represented approximately by a straight line, themaximum flow at zero head being 0.0015 m2/s and the maximum head at zero flow 760 m of liquid.

Using this pump to feed a particular slurry to a pressure leaf filter:

(a) How long will it take to produce 1 m3 of filtrate?(b) What will be the pressure across the filter after this time?A sample of the slurry was filtered at a constant rate of 0 .00015 m3/s through a leaf filter covered with a similar filter cloth

but of one-tenth the area of the full-scale unit, and after 625 s the pressure across the filter was 360 m of liquid. After a

further 480 s the pressure was 600 m of liquid.


6/20

A slurry containing 40 per cent by mass solid is to be filtered on a rotary drum filter 2 m diameter and 2 m long whichnormally operates with 40 per cent of its surface immersed in the slurry and under a pressure of 17 kN/m 2. A laboratory test

on a sample of the slurry using a leaf filter of area 200 cm2 and covered with a similar cloth to that on the drum produced 300

cm3 of filtrate in the first 60 s and 140 cm3 in the next 60 s, when the leaf was under an absolute pressure of 17 kN/m 2.The bulk density of the dry cake was 1500 kg/m 3 and the density of the filtrate was 1000 kg/m3. The minimum thickness ofcake which could be readily removed from the cloth was 5 mm. At what speed should the drum rotate for maximum

throughput and what is this throughout in terms of the mass of the slurry fed to the unit per unit time?


7/20

A plate and frame press with a filtration area of 2.2 m 2 is operated with a pressure drop of 413 kN/m2 and with a downtime of21.6 ks (6 h). In a test with a small leaf filter 0.05 m2 in area, operating with a pressure difference of 70 kN/m2, 0.00025 m3

of filtrate was obtained in 300 s and a total of 0.00040 m 3 in 600 s. Estimate the optimum filtration time for maximum

throughput. If the operating cost during filtration is 10/ks and the cost of a shutdown is 100, what is theoptimum filtration time for minimum cost?


8/20

A slurry, containing 0.2 kg of solid per kilogram of water, is fed to a rotary drum filter 0.6 m long and 0.6 m diameter. Thedrum rotates at one revolution in 360 s and 20 per cent of the filtering surface is in contact with the slurry at any instant. If

filtrate is produced at the rate of 0.125 kg/s and the cake has a voidage of 0.5, what thickness of cake is produced when

filtering with a pressure difference of 65 kN/m2? The density of the solids is 3000 kg/m3. The rotary filter breaks down andthe operation has to be carried out temporarily in a plate and frame press with frames 0.3 m square. The press takes 120 s todismantle and 120 s to reassemble and, in addition, 120 s is required to remove the cake from each frame. If filtration is to be

carried out at the same overall rate as before, with an operating pressure difference of 175 kN/m2, what is the minimum

number of frames that needs to be used and what is the thickness of each? It may be assumed that the cakes areincompressible and that the resistance of the filter medium may be neglected.


9/20

A sludge is filtered in a plate and frame press fitted with 25 mm frames. For the first 600 s the slurry pump runs at maximum

capacity. During this period the pressure rises to 415 kN/m 2 and 25 per cent of the total filtrate is obtained. The filtrationtakes a further 3600 s to complete at constant pressure and 900 s is required for emptying and resetting the press. It is found

that if the cloths are precoated with filter aid to a depth of 1.6 mm, the cloth resistance is reduced to 25 per cent of its formervalue. What will be the increase in the overall throughput of the press if the precoat can be applied in 180 s?


10/20

A plate and frame press gave a total of 8 m3 of filtrate in 1800 s and 11.3 m3 in 3600 s when filtration was

stopped. Estimate the washing time if 3 m3 of wash water is used. The resistance of the cloth may be neglectedand a constant pressure is used throughout.


11/20

In the filtration of a sludge, the initial period is effected at a constant rate with the feed pump at full capacity,

until the pressure differences reaches 400 kN/m2. The pressure is then maintained at this value for a remainder

of the filtration. The constant rate operation requires 900 s and one-third of the total filtrate is obtained during

this period. Neglecting the resistance of the filter medium, determine (a) the total filtration time and (b) the

filtration cycle with the existing pump for a maximum daily capacity, if the time for removing the cake and

reassembling the press is 1200 s. The cake is not washed.


12/20

Filtration is carried out in a plate and frame filter press, with 20 frames 0.3 m square and 50 mm thick, and the

rate of filtration is maintained constant for the first 300 s. During this period, the pressure is raised to 350

kN/m2, and one-quarter of the total filtrate per cycle is obtained. At the end of the constant rate period, filtration

is continued at a constant pressure of 350 kN/m2 for a further 1800 s, after which the frames are full. The total

volume of filtrate per cycle is 0.7 m3 and dismantling and refitting of the press takes 500 s. It is decided to use a

rotary drum filter, 1.5 m long and 2.2 m in diameter, in place of the filter press. Assuming that the resistance of

the cloth is the same in the two plants and that the filter cake is incompressible, calculate the speed of rotation ofthe drum which will result in the same overall rate of filtration as was obtained with the filter press. The

filtration in the rotary filter is carried out at a constant pressure difference of 70 kN/m 2, and the filter operates

with 25 per cent of the drum submerged in the slurry at any instant.


13/20

The relation between flow and head for a slurry pump may be represented approximately by a straight line, themaximum flow at zero head being 0.0015 m3/s and the maximum head at zero flow 760 m of liquid. Using this

pump to feed a slurry to a pressure leaf filter,

(a) how long will it take to produce 1 m3 of filtrate, and

(b) what will be the pressure drop across the filter after this time?

A sample of the slurry was filtered at a constant rate of 0.00015 m3/s through a leaf filter covered with a similar

filter cloth but of one-tenth the area of the full scale unit and after 625 s the pressure drop across the filter was360 m of liquid. After a further 480 s the pressure drop was 600 m of liquid.


14/20

A continuous rotary filter is required for an industrial process for the filtration of a suspension to produce 0.002

m3/s of filtrate. A sample was tested on a small laboratory filter of area 0.023 m2 to which it was fed by means

of a slurry pump to give filtrate at a constant rate of 0.0125 m3/s. The pressure difference across the test filterincreased from 14 kN/m2 after 300 s filtration to 28 kN/m2 after 900 s, at which time the cake thickness had

reached 38 mm. What are suitable dimensions and operating conditions for the rotary filter, assuming that the

resistance of the cloth used is one-half that on the test filter, and that the vacuum system is capable of

maintaining a constant pressure difference of 70 kN/m2 across the filter?


15/20

A rotary drum filter, 1.2 m diameter and 1.2 m long, handles 6.0 kg/s of slurry containing 10 per cent of solids

when rotated at 0.005 Hz. By increasing the speed to 0.008 Hz it is found that it can then handle 7.2 kg/s. What

will be the percentage change in the amount of wash water which may be applied to each kilogram of cake

caused by the increased speed of rotation of the drum, and what is the theoretical maximum quantity of slurrywhich can be handled?


16/20

A rotary drum with a filter area of 3 m 3 operates with an internal pressure of 71.3 kN/m2below atmospheric and

with 30 per cent of its surface submerged in the slurry. Calculate the rate of production of filtrate and the

thickness of cake when it rotates at 0.0083 Hz, if the filter cake is incompressible and the filter cloth has a

resistance equal to that of 1 mmof cake. It is desired to increase the rate of filtration by raising the speed of

rotation of the drum.If the thinnest cake that can be removed from the drum has a thickness of 5 mm, what is

themaximum rate of filtration which can be achieved and what speed of rotation of the drum is required? The

voidage of the cake = 0.4, the specific resistance of cake = 2 1012

m2

the density of solids = 2000 kg/m3, thedensity of filtrate = 1000 kg/m3, the viscosity offiltrate = 103N s/m2 and the slurry concentration = 20 per cent

by mass solids.


17/20

A slurry containing 50 per cent by mass of solids of density 2600 kg/m3 is to be filtered on a rotary drum filter,

2.25 m in diameter and 2.5 m long, which operates with 35 per cent of its surface immersed in the slurry and

under a vacuum of 600 mm Hg. A laboratory test on a sample of the slurry, using a leaf filter with an area of

100 cm2 and covered with a cloth similar to that used on the drum, produced 220 cm3 of filtrate in the first

minute and 120 cm3 of filtrate in the next minute when the leaf was under a vacuum of 550 mm Hg. The bulk

density of the wet cake was 1600 kg/m3 and the density of the filtrate was 1000 kg/m3. On the assumption that

the cake is incompressible and that 5 mm of cake is left behind on the drum, determine the theoretical maximumflowrate of filtrate obtainable. What drum speed will give a filtration rate of 80 per cent of the maximum?


18/20

A rotary filter which operates at a fixed vacuum gives a desired rate of filtration of a slurry when rotating at

0.033 Hz. By suitable treatment of the filter cloth with a filter aid, its effective resistance is halved and the

required filtration rate is now achieved at a rotational speed of 0.0167 Hz (1 rpm). If, by further treatment, it is

possible to reduce the effective cloth resistance to a quarter of the original value, what rotational speed is

required? If the filter is now operated again at its original speed of 0.033 Hz, by what factor will the filtration

rate be increased?


19/20

Centrifugation

When an aqueous slurry is filtered in a plate and frame press, fitted with two 50 mm thick frames each 150 mm

square, operating with a pressure difference of 350 kN/m2, the frames are filled in 3600 s (1 h). How long will it

take to produce the same volume of filtrate as is obtained from a single cycle when using a centrifuge with a

perforated basket, 300 mm diameter and 200 mm deep? The radius of the inner surface of the slurry is

maintained constant at 75 mm and the speed of rotation is 65 Hz (3900 rpm). It may be assumed that the filtercake is incompressible, that the resistance of the cloth is equivalent to 3 mm of cake in both cases, and that the

liquid in the slurry has the same density as water.


20/20

A centrifuge with a phosphor bronze basket, 380 mm in diameter, is to be run at 67 Hz with a 75 mm layer of

liquid of density 1200 kg/m3 in the basket. What thickness of walls are required in the basket? The density of

phosphor bronze is 8900 kg/m3 and the maximum safe stress for phosphor bronze is 87.6 MN/m2.

size reduction, fluidisation, filtration & centrifugation

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