Equipment Design [PDF]

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CYCLONES (CENTRIFUGAL SEPARATORS) Cyclones are the principal type of gas-solids separator employing centrifugal force, and are widely used. They are basically simple constructions; can be made from a wide range of materials; and can be designed for high temperature and pressure operation. Cyclones are suitable for separating particles above about 5𝜇m diameter; smaller particles, down to about 0.5𝜇m, can be separated where agglomeration occurs. The most commonly used design is the reverse-flow cyclone; other configurations are used for special purposes. In a reverse-flow cyclone the gas enters the top chamber tangentially and spirals down to the apex of the conical section; it then moves upward in a second, smaller diameter, spiral, and exits at the top through a central vertical pipe. The solids move radially to the walls, slide down the walls, and are collected at the bottom. Design procedures for cyclones are given by Constantinescu (1984). Strauss (1975), Koch and Licht (1977) and Stairmand (1951). Cyclone Design: Stairmand developed two standard designs for gas-solid cyclones: a high-efficiency cyclone, Figure 2a, and a high throughput design, Figure 2b. The performance curves for these designs, obtained experimentally under standard test conditions, are shown in Figures 3a and 3b. These curves can be transformed to other cyclone sizes and operating conditions by use of the following scaling equation, for a given separating efficiency: 𝑫𝑪𝟐

𝑸𝟏

∆𝝆𝟏

𝝁𝟐

d2 = d1 [(𝑫𝑪𝟏)3 x 𝑸𝟐 x ∆𝝆𝟐 x 𝝁𝟏]1/2 ……………………………………(i) where d1 = mean diameter of particle separated at the standard conditions, at the chosen separating efficiency, Figures 3a or 3b, d2 = mean diameter of the particle separated in the proposed design, at the same separating efficiency, Dc1 = diameter of the standard cyclone = 8 inches (203 mm), Dc2 = diameter of proposed cyclone, mm, Q1 = standard flow rate: for high efficiency design = 223 m3/h, for high throughput design = 669 m3/h, Q2 = proposed flow rate, m3/h, ∆𝝆1 = solid-fluid density difference in standard conditions = 2000 kg/m3, ∆𝝆2 = density difference, proposed design, 𝝁1 = test fluid viscosity (air at 1 atm, 20℃) = 0.018 mN s/m2, 𝝁2 = viscosity, proposed fluid. A performance curve for the proposed design can be drawn up from Figures 3a or 3b by multiplying the grade diameter at, say, each 10 per cent increment of efficiency, by the scaling factor given by Equation (i) above; as shown in Figure 4.

Figure 1.

Figure 2.

Figure 3.

Figure 3.

Figure 4. An alternative method of using the scaling factor, that does not require redrawing the performance curve, is designing the cyclone to give a desired inlet velocity; when the optimum inlet velocity has been acquired (15m/s (50 ft/s)).

Pressure Drop: The pressure drop in a cyclone will be due to the entry and exit losses, and friction and kinetic energy losses in the cyclone. The empirical equation given by Stairmand (1949) can be used to estimate the pressure drop:

𝝆𝓯

𝟐𝒓𝒕

∆𝑷 = 𝟐𝟎𝟑 {U12 [1 + 2Ø2 ( 𝒓𝒆 – 1) ] + 2U22} ……………………………………(ii) where ΔP = cyclone pressure drop, millibars, 𝝆f = gas density, kg/m3,

Figure 5. U1 = inlet duct velocity, m/s, U2 = exit duct velocity, m/s, rt = radius of circle to which the center line of the inlet is tangential, m, re = radius of exit pipe, m, Ø= factor from Figure 5, Ψ= parameter in Figure 5, given by: 𝑨𝒔

Ψ = fc 𝑨𝒊 ............................................................................................ (iii) fc = friction factor, taken as 0.005 for gases, As = surface area of cyclone exposed to the spinning fluid, m2. For design purposes this can be taken as equal to the surface area of a cylinder with the same diameter as the cyclone and length equal to the total height of the cyclone (barrel plus cone). A1 = area of inlet duct, m2.

Stairmand’s equation is for the gas flowing alone, containing no solids. The presence of solids will normally increase the pressure drop over that calculated using Equation (ii), depending on the solids loading. Alternative design methods for cyclones, which include procedures for estimating the true pressure drop, are given by Perry et al. (1997) and Yang (1999); see also Zenz (2001).

GENERAL DESIGN PROCEDURE 1. Select either the high-efficiency or high-throughput design, depending on the performance required 2. Obtain an estimate of the particle size distribution of the solids in the stream to be treated. 3. Estimate the number of cyclones needed in parallel. 4. Calculate the cyclone diameter for an inlet velocity of 15 m/s (50 ft/s). Scale the other cyclone dimensions from Figures 2a or 2b. 5. Calculate the scale-up factor for the transposition of Figures 3a or 3b. 6. Calculate the cyclone performance and overall efficiency (recovery of solids). If unsatisfactory try a smaller diameter. 7. Calculate the cyclone pressure drop and, if required, select a suitable blower. 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.

REFRENCES Coulson & Richardson’s, Chemical Engineering, Volume 6, Fourth Edition, Chemical Engineering Design, R. K. Sinnott pp. 450 – 455. Kraus, M. N. (1979) Chem. Eng., NY 86 (April 9th) 94. Separating and collecting industrial dusts. (April 23rd) 133. Baghouses: selecting, specifying and testing of industrial dust collectors. LAVANCHY, A. C., KEITH, F. W. and BEAMS, J. W. (1964) Centrifugal separation, in KirkOthmer Encyclopediaof Chemical Technology, 2nd edn (Interscience). LINLEY, J. (1984) Chem. Engr., London No. 409 (Dec.) 28. Centrifuges, Part 1: Guidelines on selection. Svarovsky, l. and Thew, M. T. (1992) Hydrocyclones: Analysis and Applications (Kluwer).