FILTRATION 1. Water is filtered through a sand bed 150 mm thick. The depth

of water on top of the bed is 120 mm. the porosity is 0.4

and the mean particle diameter is 0.25 mm. The dynamic

viscosity is 0.89 cP and the density is 998 kg/m3. Calculate

the flow rate per square meter of area.

a. 2.72 x 10-2 m3/s

b. 0.543 x 10-3 m3/s

c. 15.50 m/s

d. 0.543 x 10-3 m/s

2. (29.7) The following relation between α and ∆ p for

superlight CaCO3 has been determined to be

∝=8.8 x1010¿Where ∆ p is in pounds for per square foot. This relation is

followed over a pressure range from 0 to 1,000 lb/in2. A

slurry of this material giving 3.0 lb of cake solid per

cubic foot of filtrate is to be filtered at a constant

pressure drop of 70 lbf/in2 and a temperature of 70°F.

Experiments on this sludge and the filter cloth to be used

gave a value of Rm= 1.2 x 1010 ft-1. A pressure filter of the

tank type is to be used. How many square feet of filter

surface are needed to give 1,400 gal of filtrate in a 1-h

filtration? The viscosity is that of water at 70°F.

a. 71.71 ft2

b. 71.44 ft2

c. 71.44 ft2

d. 71.44 ft2

3. (29.8) The filter of Prob. 29.7 is washed at 70°F and 70

lbf/in2 with a volume of wash water equal to one-third that

of the filtrate. The washing rate is 85 percent of the

theoretical value. How long should it take to wash the cake?

a. 2812.40 s or 0.781 hr

b. 2808.73 s or 0.780 hr

c. 2791.12 s or 0.775 hr

d. 2798.21 s or 0.777 hr

4. (29.10) A continuous pressure filter is to yield 1,400 gal/h

of filtrate from the slurry described in Prob. 29.7. The

pressure drop is limited to a maximum of 50 lbf/in2. How

much filter area must be provded if the cycle time is 3 min

and the drum submergence is 50 percent?

a. 483 ft2

b. 34.09 ft2

c. 27.34 ft2

d. 26.73 ft2

5. (29.17) A microfiltration unit is used to concentrate a

suspension of 0.8 micrometer spherical particles has a

permeate flux of 150 L/m2-h at 30°C and a pressure drop of

2.1 atm. Under the same conditions, the pure water flux was

280 L/m2-h. How thick a layer of particles would be needed

to account for the gel-layer resistance if ε = 0.40?

a. 2.23 x10−3m∨2.24mm

b. 2.18 x10−3m∨2.28mm

c. 2.25 x10−3m∨2.25mm

d. 2.08 x10−3m∨2.08mm6. In constant pressure filtration, the rate of filtration

follows the relation (v – filtrate volume, t- time, k and c

– constants)

a. dVdt

=kV +C

b. dVdt

= 1kV +C

c. dVdt

=kV

d. dVdt

=k V 2

7. A filtration is conducted at constant pressure to recover

solids from the dilute slurry. To reduce the time of

filtration, the solids concentration in the feed slurry is

increased by evaporating half the solvent. If the

resistance of the filter medium is negligible, the

filtration time will be reduced by a factor of:

a. 1

b. 2

c. 4

d. 8

8. In constant pressure filtration,

a. resistance decreases with time

b. resistance increases with time

c. rate of filtration increases with time

d. rate of filtration decreases with time

9. At the beginning of filtration, the cake resistance is :

a. maximum

b. optimum

c. minimum and greater the zero

d. zero

10. The filter medium resistance of an old, used filter is:

a. much smaller than that of a new filter

b. much larger than that of a new filter

c. equal to that of new filter

d. infinitely large

11. A filtration is carried out for 10 min at a constant

rate in a leaf filter and thereafter it is continued at

constant pressure which is attained at the end of the

constant rate period. If one quarter of the total value of

the filtrate is collected during the constant rate period,

what is the total filtration time? Assume that the cake is

incompressible and the filter medium resistance is

negligible.

a. 55 minutes

b. 60 minutes

c. 50 minutes

d. 57 minutes

12. A vacuum leaf filter gives a total value of 10 m3 of

filtrate of 30 minutes. If the resistance of the filter

cloth is negligible, the time taken for the collection of 20

m3 of filter is __________ minutes and for the collection of

30 m3 is _______ minutes.

a. 60 minutes, 90 minutes, respectively(V2/V1 is used)

b. 600 minutes, 300 minutes, respectively (V1 used is 1

cubicmeter)

c. 15 minutes, 10 minutes, respectively ((V1/V2) squared is

used)

d. 120 minutes, 270 minutes, respectively (proper use of

solution)

13. A constant pressure filtration test gave data that can

fit an expression:dt/dv = 9.3V + 8.5 (V = liters,t=seconds)

If the resistance of the filter medium is assumed unaffected

with pressure drop and the compressibility coefficient of

the filter cake is 0.3, what will be the time taken for the

collection of 3.5 L of filtrates at a filtration pressure

twice that is used in the test?

a. 55 s

b. 58 s

c. 50 s

d. 53 s

14. The basic filtration equation is given as dtdV

=−∝CVA

+Rm

. Where V is the filtrate, A is the filtration area; α is

specific cake resistance, μ is viscosity of the filtrate,

and C is the concentration of solids in the feed slurry.

In a 20 min. Constant rate filtration, 5m3 of filtrate was

obtained. If this is followed by a constant pressure

filtration, how much more time in minutes will it take for

another 5m3 of filtrate to be produced? Neglect filter

medium resistance, Rmi assume incompressible cake.

a. 10

b. 20

c. 25

d. 30

15. A centrifugal filtration unit at a rotational speed of

ω has inner surface of the liquid (density ρL) located at a

radial distance R from the axis of rotation. The thickness

of the liquid film is δand no cake s formed. The initial

pressure drop during filtration is__________.

a. 12ω2R2 ρL

b. 12ω2δ 2ρL

c. 12ω2δ ρL (2R+δ)

d. 12ω2R ρL (2R+2δ)

16. For laminar flow conditions, the relationship between

the pressure (∆Pc) across an incompressible filter cake and

the specific surface area (So) of the particles being

filtered in given by ONE of the following:

a. (∆Pc)is proportional to (So)

b. (∆Pc)is proportional to 1/(So)

c. (∆Pc)is proportional to (So)2

d. (∆Pc)is proportional to 1/(So)2

17. Filter aids are added to the slurry prior to filtration

in order to form ________________.

a. compact cakes of low porosity

b. cakes of increased

c. crystalline cakes

d. none of the above

18. In a filter bed, the liquid fl;ow

a. is laminar

b. is turbulent

c. is between laminar and turbulent

d. cannot be predicted, information on process conditions

and filter medium is necessary

19. Which of the following statement is true about vacuum

filtration is incorrect?

a. the solids in the slurry are very fine

b. the liquid has a high vapour pressure

c. the liquid has a low viscosity

d. the liquid is a saturated solutions which will

crystallize on cooling

20. In order to achieve rapid filtration of viscous liquids

or slurries containing fine solids, which filter do you

recommend?

a. Discontinuous pressure filter

b. Continuous vacuum filter

c. Discontinuous vacuum filter

d. none of the above

21. The filter medium resistance usually is important

a. only during the early stages of filtration

b. only during the final stages of filtration

c. all through the filtration process

d. none of these.

22. The rate of filtration in a rotary-drum filter

a. increases with an increase in drum speed

b. decreases with an increase in drum speed

c. is independent of the drum speed

d. increases with an increase in drum speed up to a

certain critical value and thereafter, filtration rate no

longer increases with drum speed

23. The flow of filtrate through cake deposited on septum

is usually which equation will use to calculate pressure

drop for this situation?

a. Leva’s equation

b. Kozeny-Carman equation

c. Blake-Plummer equation

d. Bernoulli’s equation

24. A rotary drum filter is –

a. Continuous Vacuum filter

b. A Discontinuous Vacuum Filter

c. A Continuous Pressure Filter

d. None of the above

25. The speed of rotation of an industry rotary – drum

filter is

a. 0.1 to 2 RPM

b. 25 to 50 RPM

c. 50 to 100 RPM

d. 100 to 200 RPM