10.8.
599
GAS-SOLID SEPARATIONS (GAS CLEANING)
8. Cost the system and optimize to make the best use of the pressure drop available
or, if a blower is required, to give the lowest operating cost.
Example 10.4
Design a cyclone to recover solids from a process gas stream. The anticipated particle
size distribution in the inlet gas is as follows. The density of the particles is 2500 kg/m3,
and the gas is essentially nitrogen at 1508C. The stream volumetric flow rate is
4000 m3/h, and the operation is at atmospheric pressure. An 80% recovery of the
solids is required.
Particle size (mm)
Percentage by weight less than
50
90
40
75
30
65
20
55
10
30
5
10
2
4
Solution
As 30% of the particles are below 10 mm, the high-efficiency design will be required to
give the specified recovery:
4000
¼ 1:11 m3 =s
3600
1:11
Area of inlet duct, at 15 m=s ¼
¼ 0:07 m2
15
From Figure 10:44a, duct area ¼ 0:5 Dc 0:2 Dc
so, Dc ¼ 0:84
Flow-rate ¼
This is clearly too large compared with the standard design diameter of 0.203 m.
Try four cyclones in parallel, Dc ¼ 0.42 m.
Flow-rate per cyclone ¼ 1000 m3 =h
28
273
Density of gas at 150 C ¼
¼ 0:81 kg=m2 ,
22:4 423
negligible compared with the solids density.
Viscosity of N2 at 150 C ¼ 0:023 cp(mNs=m)
From equation 10.8,
"
scaling factor ¼
#1=2
0:42 3 223 2000 0:023
¼ 1:42
0:203
1000 2500 0:018
The performance calculations, using this scaling factor and Figure 10.45a, are set
out in Table 10.12.
The collection efficiencies shown in column 4 of the table were read from Figure
10.45a at the scaled particle size, column 3. The overall collection efficiency satisfies
the specified solids recovery. The proposed design with dimensions in the proportions
given in Figure 10.44a is shown in Figure 10.48.
600
CHAPTER 10
>50
50–40
40–30
30–20
20–10
10–5
5–2
2–0
Percent in
Range
10
15
10
10
25
20
6
4
Mean
Particle
Size Scaling
Factor
35
32
25
18
11
5
3
1
100
Efficiency at
Scaled Size %
(Figure 10.46a)
98
97
96
95
93
86
72
10
Overall
collection
efficiency
Collected
(2)3(4)
100
9.8
14.6
9.6
9.5
23.3
17.2
4.3
0.4
88.7
Grading at
Exit (2)–(5)
0.2
0.4
0.4
0.5
1.7
2.8
1.7
3.6
11.3
80
210
210
420
1050
Particle
Size (mm)
Calculated Performance of Cyclone Design (Example 10.4)
630
Table 10.12.
EQUIPMENT SELECTION, SPECIFICATION, AND DESIGN
160
Figure 10.48.
Proposed cyclone design, all dimensions mm (Example 10.4).
Percent
at Exit
1.8
3.5
3.5
4.4
15.1
24.8
15.1
31.8
100.0
10.8.
601
GAS-SOLID SEPARATIONS (GAS CLEANING)
Pressure-Drop Calculation
Area of inlet duct, A1 , ¼ 210 80 ¼ 16,800 mm2
Cyclone surface area, As ¼ p420 (630 þ 1050)
¼ 2:218 106 mm2
fc taken as 0.005
fc , As 0:005 2:218 106
¼ 0:66
¼
A1
16,800
rt (420 (80=2) )
¼
¼ 1:81
re
210
c¼
From Figure 10.47, f ¼ 0.9.
1000
106
¼ 16:5 m=s
3600 16,800
p 2102
¼ 34,636 mm2
Area of exit pipe ¼
4
1000
106
¼ 8:0 m=s
u2 ¼
3600 34,636
u1 ¼
From equation 10.6
DP ¼
0:81
[16:52 [1 þ 2 0:92 (2 1:81 1)] þ 2 8:02 ]
203
¼ 6:4 millibar (67 mm H2 O)
This pressure drop looks reasonable.
10.8.4.
Filters
The filters used for gas cleaning separate the solid particles by a combination of
impingement and filtration; the pore sizes in the filter media used are too large simply
to filter out the particles. The separating action relies on the precoating of the filter
medium by the first particles separated, which are separated by impingement on the filter
medium fibers. Woven or felted cloths of cotton and various synthetic fibers are commonly used as the filter media. Glass-fiber mats and paper filter elements are also used.
A typical example of this type of separator is the bag filter, which consists of a
number of bags supported on a frame and housed in a large rectangular chamber, as
shown in Figure 10.49. The deposited solids are removed by mechanically vibrating
the bag or by periodically reversing the gas flow. Bag filters can be used to separate
small particles, down to around 1 mm, with a high separating efficiency. Commercial
units are available to suit most applications and should be selected in consultation
with the vendors.