Separation Processes B - Revision Questions (1)

8/13/2019 Separation Processes B - Revision Questions (1)

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2009

3)

a) Sketch a typical drying curve which would be expected from the results of

a batch drying experiment, highlighting the critical features. [5 marks]

b) In batch drying, the time for drying is given by the equation:

1

2

X

X N

dX

A

Lt

where L Mass of solid, kg A Surface area for drying, m2 N Drying rate, kg/m2s X 1,2 Initial and final solid moisture

content, kg/kg

Show that the total time for drying of a solid with 0 kg/kg equilibrium moisturecontent and with X2<XC<X1 (XC is the critical moisture content) can be writtenas:

2

1 ln

X

X X X X

AN

Lt

c

cc

c

[8 marks]

c) Experimental data collected when a sample of moist solid was dried usingair at 150°C shows that after 10 minutes the moisture content fell from 0.80kg/kg to 0.30 kg/kg, with a critical moisture content of 0.60 kg/kg. If thesolid load was 4.8 kg/m2, calculate the constant rate of moisture removal.What is the time required to dry a sample of the same material from a initialmoisture content of 0.5 kg/kg to a final content of 0.1 kg/kg?

[7 marks]



2009

4)3 kg/s of air (dry basis) is used in a co-current adiabatic tunnel dryer to dry 0.4kg/s of wet sand from a moisture content of 0.07 kg water/kg dry solid to 0.005kg water/kg dry solid. The air enters the tunnel at a dry bulb temperature of

54°C and a wet bulb temperature of 25°C. The solid enters the dryer at 20°C.The diameter of dryer is 1.5 m. Calculate:

a) the humidity of the air leaving the dryer [4 marks]b) the temperature of the air leaving the dryer [3 marks]c) the number of transfer units [7 marks]d) the length of the dryer. [6 marks]

Assume that the overall mass transfer coefficient, U , is given by the relation:

D

GaU

5.0

3.0

where G is the mass velocity of dry air in kg/s,m2 and D is the diameter of thedryer in m.

(A psychrometric chart is attached)

2009

5)a) With the aid of an appropriate diagram, explain the dependence of the drag

coefficient on the Reynolds number for a sphere. Indicate the different flow

regimes and the correspondent range of Reynolds’ numbers in which theyhappen.

[3 marks]

b) Spherical particles of density 2000 kg/m3 and size range 20-100 μm are fedcontinuously into a steam of water flowing upwards in a vertical largediameter pipe.

i) What maximum water velocity is required to ensure that no particlesof diameter greater than 50μm are carried upwards with the water? [4 marks]

ii) What maximum water velocity is required if we do not want any ofthe particles to be carried upwards?

[5 marks]iii) List the assumptions made in undertaking the calculations at points

i) and ii) above [3 marks]

Suppose that the solid particles are now substituted with air bubbles:

iv) List the relevant dimensionless groups when bubbles in water areconsidered instead, and explain the additional effect that becomes

important when dealing with bubbles [5 marks]



2007

1)

From a force balance, show that the terminal velocity of an isolated spherical

particle is given by

18

gd)(U

2

p

0

assuming Stokes’ Law applies and by

dg)(74.1U p

0

for fully developed laminar flow.[5 marks]

Two powdered ores, densities 2600kg/m3

and 9400kg/m3

, are to be separatedby elutriation in water. Calculate the maximum ratio of particle sizes whichmay be separated assuming Stokes’ Law applies to both phases.

[5 marks]Repeat the assessment assuming fully developed laminar flow applies.

[4 marks]

Over what range of particle sizes would the Stokes’ Law result be useful? [6 marks]

For Stokes’ Law, the drag coefficient is given by CD=24/Re and for fully

developed laminar flow the drag coefficient is given by CD=0.44.Stokes’ Law is applicable for Re<0.2. Take the density of water to be1000 kg/m3 and its viscosity to be 0.001 Pa.s

Symbols:CD Drag coefficient d Particle diameterRe Reynolds numberU0 Terminal velocity

Viscosity

Fluid density

p Particle density



2007

5)

A triple effect evaporation plant is used to concentrate a feed liquor from 5

mass % to 50 mass % solids. The unit is backward-feed and each effect has65 m2 of heat transfer area. The overall heat transfer coefficients in each ofthe three effects are U1 = 2.5, U2 = 2.0, and U3 = 1.6 kW m-2K-1, respectively.Dry saturated steam at 205 kN m-2 (abs) is fed to the heating element of thefirst effect, and the condensate is removed at the steam temperature in eacheffect. The third effect operates at 13 kN m-2 (abs). Assuming that boilingpoint rise, sensible heat effects, and heat losses are negligible, determine thefollowing:

(a) the temperature of each effect [8].

(b) the pressure of each effect [2].

(c) the vapour stream flowrates from each effect [3].

(d) the feed liquor rate with which the unit can cope [3].

(e) the liquor stream flowrates from each effect [2].

(f) the condensate stream flowrates from each effect [2].

(g) water required to condense the final vapour stream by a water jetcondenser if the water available is at 20 0C and saturated. Assumethat water at the exit from the jet condenser is at 30 0C and

saturated [3].(h) the steam economy of the plant [2].

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Separation Processes B - Revision Questions (1)