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.