Chemical Engineering Department
CHE 310 Unit Operations Final Exams
4-8-1435H
Time: 3 Hours Dr Mourad Boumaza
Q1 (10marks) A) 250tons /hr of a mineral rock is crushed from a feed size where 80% is less
than 12in to a product where 80% is less than half the size of the feed. Calculate, using Bond’s
law the power required for the above operation
- Calculate the power required to crush the product further where 80% is less than half the size of
the feed entering this crusher, comment on these results (Wi= 8.78KW, 1in= 2.54cm)
B) Particle A and B are mixed in a mixer. After a time= 1mins, 50 samples were taken with
n=100, Is was found=0.250. Calculate Is after a mixing time= 10mins.
-What is the effect of n on Is?.
- Q2 (10 marks) Discuss separation by screening and explain ideal screening.
-A quartz is screened through 20 mesh screen, as shown in table below.
- Derive the equations relating mass ratios of the overflow and underflow to feed.
- Calculate the mass ratios of D/F and B/F and the overall effectiveness of the screen
- If the D/F is to be increased by 25% for the same Xf , Xb , Xd, what would be the new
overall efficiency Et ?
Mesh
Dp mm
Feed
Overflow
Underflow
14
1.168
0.47
0.85
0.195
20
0.833
0.73
0.97
0.58
28
0.589
0.885
0.99
0.83
Q3 (10 marks) A) Define SFM and DSM sedimentation process.
B) Two spherical particles of equal terminal falling velocity settle in water (ρ=1000kg/m3,
μ=10-3kg/m-sec) starting from the same level (Part1:Dp=40mm, ρ= 2800kg/m3, Part2: 3000kg/m3).
i) Determine the settling regime for these particles.
ii) Calculate the terminal settling velocity and the diameter of particle 2
iii) If particle 1 settles through a centrifuge force with a spinning rotation of 1000rev/min,
corresponding to Fc= 25Fg, calculate the spinning radius, r, and Ut if Stoke's law applies.
Q4 (10 marks) A) – A centrifuge filter rotating at 1000 Rev/min (R1=0.5m R2=1.5m and
B= 4.5m) is used to filter a slurry (Mf=2kg/sec, Cs=30% ρ=1000kg/m3, μ=10-3kg/m-sec), α =
6.38x1010 m/kg, Rm= 8.53 x1010m-1)
-Derive the equation for centrifuge filter and calculate the rate of filtration q, m3/sec.
B) A particle (Dp=0.1mm, ρ= 2000kg/m3) is settling in a water (ρ=1000kg/m3, μ=10-3kg/m-sec),
if a fluidization is produced with ε =0.5 . Calculate Ut and Vmf.
Q5 (10 marks) A slurry (1.4Kg/sec, with 25% solids) is to be filtered on a rotary filter (D=2m,
L=4m). The drum is 40% submerged , the cake contains 60% moisture .The filtrate (ρ
=998kg/m3, μ= 0.985x10-3kg/m-sec), α = 2x1011 m/kg, tc= 4 mins.
i)Calculate At , Mc, the mass of the cake (Kg/sec)) and the speed , n.
ii) Calculate ΔP (Pa), the pressure drop of the filtration process.
iii) If L= 6 m, and the submerged fraction is increased to 50% what would be the cycle time, tc,
to maintain the same operating conditions as above.
1.75(Re mf ) 2
3
 s  mf

150(1   MF )(Re mf )
3
 s2 . mf

D 3p . (  p   ) g
2
0