Separations II: Solid-Gas Systems

Transcription

1 Micro- and Nanoparticle Technology Separations II: Solid-Gas Systems Dr. K. Wegner - Lecture April 2018

2 1. Introduction Removal of particles from a gas stream either for recovery or for gas cleaning (off-gas treatment, pollution control). mm Particle size nm kg/m 3 mg/m 3 Solids content Product Recovery Off-gas treatment, pollution control Centrifugal separators ilters Electrostatic precipitators Wet collectors scrubbers Wet surfaces Dry mechanical (fabric) filter Electrofilter Combinations Large pore size (packed bed) Small pore size (fabrics) ilter candles (metal, ceramics) ilter cloth Porous materials (paper, cardboard) Micro- and Nanoparticle Technology S18 2

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4 Efficiencies of gas-solid separators T(x), (%) Source: A. Rhodes, Introduction to Powder Technology ; 2nd ed. 2008, J. Wiley Cyclones and wet separators are typically used as primary separation devices (pre-cleaning). ilters are frequently used downstream of cyclones as they are more efficient at lower concentrations and smaller particle sizes. Similarly, wet separators and electrostatic precipitators are often combined. Micro- and Nanoparticle Technology S18 4

5 2. Cyclone Separators Gas (+ fine solids) Cost-effective separation of dry particulate material (down to 5 μm diameter) from gas streams. Low initial cost Low operating expenses No moving parts Minimal space requirements Gas Solids Helical top Vortex Solids collection (dust trap) Solids (coarse fraction) Source: H. Schubert, Handbuch der mech. Verfahrenstechnik ; 2003, Wiley-VCH Micro- and Nanoparticle Technology S18 5

6 Cyclone separator principle of operation 1) Particle-laden gas enters cyclone through a tangential inlet, which imparts a vortex motion on the stream causing the particles to concentrate along the wall. 2) The stream (called the descending vortex) spirals down the cyclone barrel toward a bottom cone which opens into an expansion chamber. 3) Inertia forces the particles to the wall of the expansion chamber, while the lighter gas stream forms a separate vortex (caused by drag) ascending through the center of the descending vortex. 4) The cleaned gas exits through a tube extending into the center of the chamber. Micro- and Nanoparticle Technology S18 6

7 Theoretical analysis for a cyclone separator Performance parameters of interest: 1) Smallest particles that can be collected with a cyclone of given geometry - x p,crit ( Grenzkorngrösse ). 2) Grade efficiency (separation function T(x)) and x 50,T 3) Total efficiency E T =g=m G /M A 4) Pressure drop Δp Problems: - Complex 3-dim. non-symmetric flow pattern - Effect of particle loading on gas flow (varies inside cyclone) - Two-phase flow in wall boundary layer important ull description of cyclone not possible, even numerically Micro- and Nanoparticle Technology S18 7

8 Theoretical analysis for a cyclone separator Geometry: Cyclone with tangential aerosol inlet. 2r i b h in Radius cylinder: r a Radius of gas outlet tube: r i Height of inlet: h in Width of inlet: b Half cone angle: ε h r a ε Solids loading at inlet: ϕ in = M M S tot in Micro- and Nanoparticle Technology S18 8

9 2.1 Minimum particle size for collection x P,crit orces acting on particle in equilibrium: a) Due to the tangential velocity u,t : Centrifugal force: C = π 6 x 3 P ( ρ ρ ) P 2, t u r u,t = c P,t since small particles follow tangential gas flow Dynamic buoyancy ( B ) considered b) Due to the radial velocity component u,r : D π = α x 4 2 P ρ 2 u 2, r u,r = v rel,r since particles are in equilibrium on orbit of radius r Micro- and Nanoparticle Technology S18 9 D u,r u,t

10 Micro- and Nanoparticle Technology S18 10 A particle of size x is not collected (not moving towards wall) when the radial forces acting on it are in equilibrium: ( ) 2, 2 Re 2, r P t P P u x r u x = ρ π α ρ ρ π : C D = The minimum (critical) particle size x p,crit is determined for the case of the highest centrifugal force on that particle at the position r with the highest tangential velocity u,t. or determining x P,crit we need to know u,t, u,r, r and α. r u u x t r P P = 2,, Re 4 3 ρ ρ ρ α

11 Tangential velocity in a cyclone In an ideal case, the flow inside the cyclone would be a potential (vortex) flow with u r = const. (a). Wall friction, however, leads to the measured real flow profile (b). The maximum tangential velocity is observed for r = r i. u(r i ) = u,t,max = u i tangential velocity u,t u i u a 0 Source: H. Schubert, Handbuch der mech. Verfahrenstechnik ; 2003, Wiley-VCH Micro- and Nanoparticle Technology S18 11

12 Based on an angular momentum balance, Meissner (1978) and Muschelknautz et al. (1994) developed the following relationship for u i : u i ra ua ri = λ AR ra 1+ ua 2 V ri 0.5 λ: wall friction coefficient A R : Surface area of the inlet helix Unknowns (for given geometry): u a, λ, A R A R b ε + r 2 ( b + h ) = a in P. Meissner, Diss. University of Karlsruhe, E. Muschelknautz, V. Greif, M. Trefz, in VDI Wärmeatlas, 7 th ed., Lja 1 Lja 11, VDI Verlag (1994) Micro- and Nanoparticle Technology S18 12

13 Wall friction coefficient according to Morweiser (1998): Wall friction coefficient λ k: height of surface roughness Re uδ : Boundary layer Re-#, see next slide M. Morweiser, Diss. TU Darmstadt, H. Schubert, Handbuch der mech. Verfahrenstechnik ; Wiley-VCH, Micro- and Nanoparticle Technology S18 13

14 Boundary layer Re-# according to Morweiser (1998): Re ua = u a 2ra ρ η Simplified approach for λ (Muschelknautz et al., 1994): λ = ( 1+ 2 ) ϕ in Micro- and Nanoparticle Technology S18 14

15 The velocity at the wall u a is determined by: u a with = u u α in r in C in r r in a V = b hin inlet velocity b = ra 2 reduced inlet radius for slit-type inlet α C contraction coefficient Rentschler: α C = 1 1+ β β 2 β 2 1 β 1 1+ ϕ in ( 2 2β β ) ; β = b r a W. Rentschler, VDI ortschrittsberichte Reihe 3, No. 242, VDI Verlag, 1991 Micro- and Nanoparticle Technology S18 15

16 Now, u i can be calculated for a cyclone separator with given geometry. or determining the cut-off diameter of the cyclone x P,crit we further need to now the radial velocity u r,i and the drag coefficient α Re. Assumptions: Homogeneous gas outflow through a cylinder of radius r i and height h. Contribution of solids content to volume flow negligible. u ri = u r = V 2π r in i h V in M = ρ Micro- and Nanoparticle Technology S18 16

17 Large particles are soon separated out of the gas stream. or the remaining fines, flow in the Stokes regime can be assumed, thus: η αre = = Re ρ d u P ri Now, we have determined all properties required for calculation of the smallest particle size (theoretically) collected by the cyclone: x P, crit = 3 4 α Re ρ P ρ ρ u r, i u i 2 r i Micro- and Nanoparticle Technology S18 17

18 Micro- and Nanoparticle Technology S Collection efficiency of a cyclone The collection efficiency of a cyclone depends on the particle loading of the gas. Up to a maximum particle loading φ crit, all the collection occurs due to flow forces inside the cyclone. Above this particle loading the excess particles are already collected near the cyclone inlet. Now, for φ e < φ crit (Muschelknautz, 1994): + π + = D D x x x T crit 2log log log 1 cos ) ( 2 < D < 4, depending on cyclone geometry; typically D = 3

19 Cyclone separator - fields of application Chemical industry: Reactors (catalyst recovery, fluidized bed reactors), HDPE, PP, PVC manufacturing Iron and steel: Pelletising, rolling mills, sand recovery Mining and raw materials: Driers, carbon black, metal powder, salt Coal: Pulverized coal collection, lime, ash ood: Coffee, flour, tobacco Others: Powder coating, saw dust, product recovery in pneumatic conveying Micro- and Nanoparticle Technology S18 19

20 Cyclone separators - examples Quad-cyclone (4 cyclones in parallel) Pictures: Mikropul GmbH Cyclone separator and bag-house filter in series Micro- and Nanoparticle Technology S18 20

21 3. iltration Removal of fine particles of μm from gas streams by flowing the aerosol through a filter medium (cloth, membrane) or a packed bed. Particles are collected on the fibers or on other particles by one of the following mechanisms: Diffusion Impaction Interception collector Electrostatic forces Micro- and Nanoparticle Technology S18 21

22 ilter collection efficiency W.C. Hinds, Aerosol Technology, 2 nd ed., J. Wiley (1999) Micro- and Nanoparticle Technology S18 22

23 In aerosol manufacturing (e.g. chloride process for TiO 2 ) and offgases with high solids content, the collection efficiency of the filter medium becomes less important since the filter cakes build up rapidly. Similar to solid-liquid filtration, the porous filter cake becomes the main filter while the filter medium acts as support. Build-up of porous filter cake follows initial single fiber deposition increase in pressure drop. See approaches for flow through packed beds A periodic removal of the filter cake becomes necessary for continuous operation. Micro- and Nanoparticle Technology S18 23

24 Bag-house filter Clean gas outlet A Aerosol inlet A: ilter bags B: Support cages C: Sealing D: Tube sheet E: Aerosol inlet : ilter casing G: Pressurized air nozzles H: Venturi tubes I: Collection cone K: Rotary valve L: Solenoid valves M: Control unit for valves N: Manometer Micro- and Nanoparticle Technology S18 24

25 Bag-house filter Back-pressure cleaning Support cage filter cloth Pressure drop Solids concentration in clean gas iltration time iltration cycle unstable stable Micro- and Nanoparticle Technology S18 25

26 Bag-house filter for nanoparticle collection Baghouse filter of SP unit at ARCI, Hyderabad, India Micro- and Nanoparticle Technology S18 26

27 4. Electrostatic Precipitators Electrostatic precipitators use electrostatic forces to collect charged particles for aerosol sampling and air cleaning. Example of a wire and tube electrostatic precipitator. The electric field is applied between the central wire (discharge electrode) and the outer tube (collecting electrode). Micro- and Nanoparticle Technology S18 27

28 Operation mechanism Top view: Particles are charged in the electric field by a corona discharge (ionization of gas surrounding the electrode). Charged particles migrate in the electric field to the counter electrode. Particles are discharged and remain adhering to the electrode surface. Micro- and Nanoparticle Technology S18 28

29 orce acting on particles (number of charges n and electric field strength E): el = n e E or low velocity laminar flow (typical): el = = 3π η D Radial velocity of particle: u r or x > 1μm: Thus, n e E = 3 π η x u r E η n e 2 x x u = konst E r 2 x (Stokes law) or very small particles: Cunningham correction Micro- and Nanoparticle Technology S18 29 u r n e E = C 3 π η x c

30 Separation efficiency T(x) Assumption: The particle concentration is homogeneous over the cross sectional area of the tube collection tube raction of particles dn removed in dt: dn N 2π r urdt N( t) 2 ur t = = exp 2 π r N r Residence time in tube: 0 t = 2 π r V raction of particles not collected: L N N out 0 = exp u r dt wire 2π ur r V r L Micro- and Nanoparticle Technology S18 30

31 Micro- and Nanoparticle Technology S18 31 raction of particles collected: ( ) = = V x u A x N x N x N x T r C L ) ( exp 1 ) ( ) ( ) ( 0 0 Deutsch-Anderson eq. With the surface area of the collector A C for a tube collector of length L and radius r: L r A C = 2π 0 ) ( 1 ) ( N x N x T out =

32 Examples of electrostatic precipitators Possibly water spray for cleaning Tube separator Gas out Wire discharge electrodes Tube collector electrodes Honeycomb separator Aerosol in ixation low homogenizer Dust or slurry discharge H. Schubert, Handbuch der mech. Verfahrenstechnik ; Wiley-VCH, Micro- and Nanoparticle Technology S18 32

33 5. Wet Separators Particles are removed from gas stream by impact on droplets. Therefore, a liquid is nebulized into the aerosol stream. Removal of fine particles down to 0.1 μm from gas streams. Collection mechanism similar to deposition on fibers: Impaction on droplet Diffusion (esp. for small particles) Inception Electrostatic effects Disadvantage: Requires solid-liquid separation for waste water. Micro- and Nanoparticle Technology S18 33

34 Examples of wet separators Scrubbing tower Suspension outlet Clean gas outlet Packing Aerosol inlet Scrubbing towers are equipped with structured packings which lead to a high interface area between the liquid sprayed from the top and the counterflowing aerosol. The suspension is collected at the bottom while the clean gas leaves at the top after a mist elimination stage. Also used to absorb gases such as SO 2, HCl, NH 3, etc. Micro- and Nanoparticle Technology S18 34

35 Venturi scrubber Aerosol Clean gas Aerosol is expanded through a Venturi nozzle and washing liquid is added at the Venturi throat. The water jet is broken into droplets leading to intense contact between gas and liquid phases. The gas-liquid mixture passes through an elbow diffusor, where droplets start to separate out. The flow then tangentially enters a separator where droplets are collected at the walls. Micro- and Nanoparticle Technology S18 35

36 Comparison of wet precipitators Micro- and Nanoparticle Technology S18 36