Ex5 Cyclones 2024-2025 PDF PAST PAPER

5 - Cyclone separators Air Pollution and Control Engineering CYCLONE SEPARATORS (BRING THE REPORT OF THE SOLUTION TO THE ORAL EXAM) (No excel files are admitted to the oral exam. The results should be...

5 - Cyclone separators Air Pollution and Control Engineering CYCLONE SEPARATORS (BRING THE REPORT OF THE SOLUTION TO THE ORAL EXAM) (No excel files are admitted to the oral exam. The results should be discussed and synthesized in a report by inherent comments, tables and figures.) Introduction Cyclone separators were one of the most widely used of all industrial gas-cleaning devices. The main reasons for the widespread use of cyclones were that they are inexpensive to purchase, they have no moving parts, and they can be constructed to withstand harsh operating conditions. These devices are famous for simple geometry and construction, low maintenance cost, continuous operation, and ability to operate over wide range of temperatures. They find wide application to collect particles from gas–solid flows encountered in steel and coal processing plants, flour and marble industries, food processing plants, etc. (to name a few). Apart from its cleaning applications it is used as classifiers too. Cyclones range in size from a miniature cyclone having a body diameter of < 2 cm, for sampling submicrometer range inhalable particles in ambient and industrial hygiene applications, to a cyclone of several meters in diameter for removing particulate matter from gases in industrial plants. Typically, a particulate-laden gas enters tangentially near the top of the cyclone, as shown schematically in the left panel of Figure 1. The gas flow is forced into a downward spiral simply because of the cyclone’s shape and the tangential entry. Another type of cyclone (axial cyclone – right panel of Figure 1) employs an axial inlet with fixed turning vanes to achieve a spiraling flow. Centrifugal force and inertia cause the particles to move outward, collide with the outer wall, and then slide downward to the bottom of the device. Near the bottom of the cyclone, the gas reverses its downward spiral and moves upward in a smaller inner spiral. The cleaned gas exits from the top through a “vortex-finder” tube, and the particles exit from the bottom of the cyclone through a pipe sealed by a spring-loaded flapper valve or rotary valve. Cyclones by themselves are generally not adequate to meet stringent air pollution regulations, but they serve an important purpose. They are ideal for use as pre-cleaners for more expensive final control devices such as baghouses or electrostatic precipitators. Cyclones are often regarded as low-efficiency collectors. However, efficiency varies greatly with particle size and cyclone design. Current literature from some of the cyclone manufacturers advertises cyclone that have efficiencies greater than 98% for particles larger than 5 microns, and others that routinely achieve efficiencies of 90% for particles larger than 15 – 20 microns. In general, operating costs increase with efficiency (higher efficiency requires higher inflow pressure), and three categories of cyclones are available: high efficiency, conventional, and high throughput. Though presenting broad generalizations and not exact relationships, generalized efficiency curves for these three types of cyclones are presented in Figure 2. In summary, advantages of cyclones are: Air Pollution and Control Engineering – A.Y. 2024-2025 1  5 - Cyclone separators 1. Low capital cost 2. Ability to operate at high temperatures 3. Low maintenance requirements because of the lack of moving parts. whereas the main disadvantages are: 1. Low efficiencies (especially for very small particles) 2. High operating costs (owing to power required to overcome pressure drop). Figure 1: Examples of cyclone design Figure 2: Efficiency curves for different types of cyclones Air Pollution and Control Engineering – A.Y. 2024-2025 2  5 - Cyclone separators Cyclone Design Configurations Extensive work has been done to determine in what manner dimensions of cyclones affect performance. In some classic work that is still used today, Shepherd and Lapple (1939, 1940) determined “optimum” dimensions for cyclones. All dimensions were related to the body diameter of the cyclone so that their results could be applied generally. Subsequent investigators reported similar work, and the so-called “standard” cyclones were born. The number of dimensions required to determine the geometric configuration of a tangential flow cyclone with a rectangular inlet is as many as eight (including inlet height, a; inlet width, b, cyclone body diameter, D; cyclone body height, h; cyclone cone height, hc; cone bottom diameter, B; vortex finder length, S; outlet diameter, De). The size of the cyclone is characterized by its principal diameter D, while its shape or configuration is defined by seven geometric ratios a/D, b/D, De /D, S/D, h/D, H/D, and B/D. Several sets of these ratios have evolved from experience to result in efficient designs, and six of the most widely used ones are listed in Table 1. The above-mentioned geometrical parameters, along with the particle density and size distribution and key operating variables, such as gas flow rate, temperature, and pressure, are used as inputs to cyclone simulation models. Several different types of cyclones have been proposed, but the reverse flow with tangential inlet, schematically shown in Figure 3, is by far the one most often used in industry. This design is the principal subject of the analysis that follows. Within the cyclone walls, as the inlet gas stream spins downwards and up through a double vortex, particles are removed by centrifugal forces. As these forces are rapidly diminishing with reduced particle sizes, the collection efficiency attains low levels for particles with diameters less than about 5 μm. The particles that are collected in the outside wall, precipitate to the bottom and are led to an outside receiver. Figure 3: Geometry of conventional reverse flow, tangential-entry cyclones Table 1: Geometric ratios for “standard” cyclone design configurations (Marinos et al., 2007) Air Pollution and Control Engineering – A.Y. 2024-2025 3  5 - Cyclone separators Characterizing the cyclone performance Three important parameters can be used to characterize cyclone performance: 1- cut diameter, 2- pressure drop, 3- collection efficiency. 1. Cut diameter (d50%) The cut diameter is defined as the size (diameter) of particles collected with 50% efficiency. It provides information on the effectiveness of removal for a particle size range. It is a characteristic of the particulate matter removal device. 9  μ b d 50% = ( ) 2  π  N e  ρ p - ρ g  u in where d50% (m) is the cut diameter, μ is the gas viscosity (Pa s), b is the inlet width (m), Ne is the effective number of turns, uin is the inlet gas velocity (m s-1), and ρp and ρg are the particle and gas densities (kg m-3). The effective number of turns Ne (i.e., the gas spins through a number of revolutions Ne in the outer vortex) can be approximated by: H +h Ne = 2a where a = height of inlet duct (m), h = length of cyclone body (m), H = overall height of cyclone (m). 2. Collection Efficiency The total collection efficiency ET (0 ≤ ET ≤ 1) is calculated based on the size dependent efficiency Ed(dp) of the cyclone and the particle size distribution f(dp). 𝐸𝑇 = ∫ 𝐸𝑑 (𝑑𝑝 ) 𝑓(𝑑𝑝 ) d𝑑𝑝 If the discrete size distribution of particles is known, ET is a weighted sum of the collection efficiencies for the various size ranges (i.e., bins): mi ET =  Ed (dpi ) f i =  Ed (dpi ) i i mtot where mi is the mass of particles in the ith size bin and mtot is the total mass of particles. Remember simple frequency is equal to the difference between the cumulative frequencies (ΔF) relative respectively to the upper and lower diameter of the ith size bin. 2.1 Size dependent efficiency Ed(dp) Size dependent efficiency (named also fractional efficiency or grade efficiency) is the removal efficiency of particles with a given diameter and is computed as a function of the particle diameter. There are several theories and several mathematical models proposed by researchers regarding particle separation in cyclones using different approaches and assumptions. No efficiency model gives a description for all cyclones and they give only an estimation of the cyclone performance. Two of them are here proposed. a) Lapple model The theory of Lapple is the most widely used example of the cut theories. Lapple assumed that particles entering the cyclone are evenly distributed across the inlet opening. The particle that Air Pollution and Control Engineering – A.Y. 2024-2025 4  5 - Cyclone separators travels from the inlet half width to the wall during the time in the cyclone is collected with 50% efficiency. Lapple then developed a general curve for standard conventional cyclones to predict the collection efficiency for any particle size; Theodore and DePaola (1980) have fitted an algebraic equation to this curve. The efficiency of collection of any size of particle (dpi) is given by: 1 𝐸𝑑 (𝑑𝑝,𝑖 ) = 𝑑50% 2 1 + ( ) 𝑑𝑝,𝑖 b) Leith and Licht model Leith and Licht (1972) model is based on assumption of continual radial back-mixing, i.e., complete remixing of the uncollected dust to maintain a uniform concentration in any horizontal cross section of the cyclone. The efficiency is expressed as: 𝑁/2 𝜌𝑝 𝑄 𝐸𝑑 (𝑑𝑝 ) = 1 − 𝑒𝑥𝑝 [−2 ( 𝐺) 𝑑𝑝 𝑁 ] 18 𝜇 𝐷3 𝑁 𝑁 = 1⁄(𝑛 + 1) 𝐺 = 8 𝐾𝑐 ⁄(𝐾𝑎 2 𝐾𝑏 2 ) where Q is the flue gas volume flow rate (m3/s), D the cyclone body diameter (m), μ the gas viscosity (Pa s), G is a dimensionless parameter depending on the cyclone configuration ratios Ka, Kb, and Kc. The use of G (G values from bottom row in Table 1 at page 3) enables the calculation for all “standard” design configurations. Vortex exponent n may be calculated according to the empirical relation by Alexander (1974). ( n = 1 − 1 − 0.67 D0.14 ) (T 283) 0.3 where D is the cyclone body in m and T is the flue gas temperature in K. The cyclone body diameter D may be estimated as a first guess with the Kalen and Zenz (1974) equation and adjusted to need. The Kalen-Zenz estimation may provide an estimation of the cyclone diameter which minimizes the risk of particle re-entrainment due to excessive inlet velocities. 0.4543 Q  2 1 − Kb   g  D = 0.0262      p K a K b 2.2   Where D in m, Q in m3/s, gas density (kg/m3) and gas viscosity (Pa s) at actual flue gas conditions, particle density in kg/m3. The diameter of the cyclone D and inlet velocity uin are related, in order to obtain the optimum operation in regard to saltation (i.e., when the gas flow rate is beyond a certain value the efficiency starts declining due to excessive re-entrainment of the centrifugally separated solid particles). Some typical values for cyclone dimensions are as follows: - Conventional cyclones: D = 1.2 m – 3.6 m and ΔP = 0.1 kPa – 1 kPa - High-efficiency cyclones: D < 0.9 m and ΔP = 2 kPa – 2.5 kPa - Multi-cyclones (several hundred tubes): D = 0.15 m – 0.6 m and moderate ΔP depending on cyclone number If needed the temperature correction for the gas actual viscosity g (Pa s) may be performed with the Sutherland’s equation (T in Kelvin) (µ20°C=1.833 10-5 Pa s): Air Pollution and Control Engineering – A.Y. 2024-2025 5  5 - Cyclone separators 293 + 𝑆 𝑇 3⁄2 −5 293 + 110.4 𝑇 3⁄2 𝜇 = 𝜇20°𝐶 ( ) ( ) = 1.833 10 ( ) ( ) 𝑇+𝑆 293 𝑇 + 110.4 293 3. Pressure drop Often the limiting factor to cyclone use is the pressure drop, as increased pressure drop means greater cost for fan power to move the exhaust gas through the control device. The pressure drop can be maintained within acceptable levels keeping low removal efficiencies for small particles. ΔP may be estimated by means of equations such as the following in terms of multiples of inlet velocity head: 2 u P =  g in NH (13) 2 where ΔP is expressed in Pa, when ρg is in kg m-3, and uin in m s-1. NH is a dimensionless parameter, typical of each standard configuration and not affected by operating conditions, representing the number of kinetic heads. It is a function of the parameter K which is equal to 16 for cyclones with tangential gas inlet without inlet vane and to 7.5 for cyclones with axial inlet: a b NH = K  2  (14) D   e  On common applications, typical values for gas inlet velocity uin and pressure drop ΔP are in the following ranges: uin  15 - 30 m s-1 ΔP  ~0.8 - ~2 kPa Higher pressure drop is expected for very high efficiency cyclones (typical values up to approx. 5 kPa). Influence of some system and operational parameters on cyclone performance Gas temperature: if the temperature of a gas increases, its density decreases while its viscosity increases. Since the gas density is negligible compared to the particle density, this has no direct effect on efficiency. A higher temperature will increase the flue gas flow rate hence the inlet velocity, increasing the particle velocity towards the wall. However, the increasing viscosity decreases the particle velocity because drag force is increased. The terminal velocity of the particle toward the wall is the result of the force balance between the centrifugal and drag forces. The net effect of the temperature variation is that in the normal operating range (5°C-370°C) the collection efficiency is essentially constant. Gas flow rate: For the same cyclone dimensions, the collection efficiency is normally higher for increasing gas flow rates (throughput) because of increased inlet velocities. However, if the entering dust is agglomerated/friable, high gas velocities may cause breakup of the particles in the cyclone, so that the increase in efficiency associated with increased velocity may be more than lost due to generation of finer material that the cyclone cannot remove efficiently. Some researchers report that collection efficiency increases with increasing gas inlet velocity up to a minimum tangential velocity at which dust is either re-entrained or not deposited because of saltation. Air Pollution and Control Engineering – A.Y. 2024-2025 6  5 - Cyclone separators Dust Loading (particulate matter concentration): In general, cyclone efficiency increases with increasing solids loading. This is so because at higher loadings, very fine particles are trapped in the interstices of the larger particles, and this entrapment increases the collection efficiency of the small particles. The pressure drop on the other hand is experimentally observed to decrease as the dust loading Ci (g m-3) increases. Cyclone diameter: A primary design factor that can be utilized to control collection efficiency is the cyclone diameter. A smaller-diameter unit has a higher efficiency than a larger diameter cyclone. Theoretically a smaller-diameter cyclone should be able to collect smaller particles because it can develop a higher centrifugal force. Small-diameter cyclones in configurations such as multi-cyclones, however, will require multiple units in parallel to give the same capacity as a large cyclone. In such cases, the smaller cyclones generally discharge the dust into a common receiving hopper. However, when cyclones discharge into a common hopper, there is a tendency of the gas to produce “cross-talk”. This occurs when the gas exiting from one small cyclone passes up the exit of an adjoining cyclone, thus reducing efficiency. The final cyclone design involves a compromise between collection efficiency and the complexity of equipment. It is customary to design systems for a single cyclone for a given capacity, resorting to multiple parallel units only if the predicted performance is inadequate for a single unit. Another problem with parallel cyclones is that it is difficult to get even distribution of flow into all the cyclones. If misdistribution occurs, this can cause inefficiencies that can negate the natural advantage of the smaller cyclones. General design procedure The following general design procedure may be used. 1. Calculate the required total efficiency, 2. Choose the standard cyclone configuration and the number of cyclones in parallel or in series, 3. Determine a first guess cyclone body diameter with the Kalen-Zenz formula (alternatively you can start with a 1m diameter), 4. Compare the tangential velocity and the pressure drop with the typical ranges and if necessary, reconsider the cyclone configuration, number of cyclones or the cyclone body diameter before going on with the efficiency calculations, 5. Calculate the size dependent efficiency, 6. Calculate the total efficiency and verify whether the required efficiency is attained, if necessary, reconsider the cyclone configuration, number of cyclones or the cyclone body diameter until the required efficiency is reached. Air Pollution and Control Engineering – A.Y. 2024-2025 7  5 - Cyclone separators PROBLEMS - CYCLONE DESIGN AND VERIFICATION note: when necessary, assume or calculate any quantity that is not provided in the text based on the notions of past precepts. 1. Compare the overall collection efficiency estimated with Lapple model and Leith and Licht approach for a conventional Lapple cyclone (standard proportions as described in Table 1), with a body diameter of 1.2 m, for air with a flow rate of 150 m3 min-1 at T = 350 K and 1 atm containing particles with a density of 1600 kg m-3 and with the size distribution given below. (gas viscosity 0.075 kg m-1 h-1) Particle Size Range Mass in size range (µm) (%) 0–2 1.0 2–4 9.0 4–6 10.0 6–10 30.0 10–18 30.0 18–30 14.0 30–50 5.0 50–100 1.0 2. Design (using Leith and Licht approach for efficiency estimation) a high-efficiency cyclone (standard proportions as described in Table 1 – for example Stairmand cyclone) for treatment of a gas stream in order to comply with the emission limit of 24 mg m-3 (at NTP). (flow rate = 5 m3 s-1 at NTP; gas temperature = 130 °C; gas viscosity 2.3 10-5 kg m-1s-1; particle concentration 100 mg m-3 at NTP; particle density 1.5 g cm-3) Particle diameter (µm) Cumulative size fraction (%) 2 11 5 30 10 51 15 64 20 72 25 78 30 82 35 85 40 87 50 90 60 92 70 94 80 95 100 97 150 99 200 100 Air Pollution and Control Engineering – A.Y. 2024-2025 8  5 - Cyclone separators 3. A cyclone system with the high efficiency Swift configuration is to serve as pre-cleaner in a dry bottom pulverized coal boiler. Design a system capable of delivering an overall efficiency of 94.5% for a lognormal size distribution (Mg=33.45 µm, Sg=5.42). (particle density = 2.3 g cm-3, maximum flow rate at actual conditions = 2.5 m3 s-1, gas temperature = 160 °C, gas pressure=1 atm, gas viscosity =2.44 10-5 Pa s) 4. For a flue gas stream with given characteristics choose a cyclone configuration and discuss the effects of operational and system parameters on the size dependent efficiency. Use plots (E(dp) vs parameter of influence) for the discussion (e.g., size dependent efficiency vs gas flowrate). (lognormal size distribution Mg=40 µm, Sg=4, particle density = 2.0 g cm-3, flow rate at actual conditions = 3 m3 s-1, gas temperature = 80 °C, gas pressure=1 atm, gas viscosity at 20°C = 1.833 10-5 Pa s) Air Pollution and Control Engineering – A.Y. 2024-2025 9