PNEUMATIC/HYDRAULIC CONVEYING Ron Zevenhoven ÅA Thermal and Flow Engineering

Transcription

1 6 luid and Particulate Systems /2018 LUIDISATION PNEUMATIC/HYDRAULIC CONVEYING Ron Zevenhoven ÅA Thermal and low Engineering 6.1 luidised beds (Bs) : basic features RoNz 2

2 luidised beds: basics BR98 Bubbling fluidised bed 3 luidisation Phenomenon when solid articles are exosed to an u-going gas (or liquid) flow. When the gas velocity in a acked bed is increased to a level at which the ressure loss corresonds to the bed gravity, the bed exands and the articles will draw away from each other. The bed has then turned from a acked bed into a fluidised bed. The increased void fraction enables the gas to easier ass through, and a kind of equilibrium is attained. A fluidised bed combustor RoNz 4

3 luidisation The individual articles in a fluidised bed are in constant motion, colliding with each other and with the walls of the vessel. Different flow tyes occur in fluidised beds and souting beds. souting bed increasing gas velocity RoNz 5 Liquid-like behaviour or visual uroses. RoNz 6

4 luidisation Advantages: - good mixing of the articles fairly uniform concentration and temerature - the vessel of well-mixed solids reresents a large thermal flywheel that resists temerature changes - large contact surface between articles and gas efficient heat and mass transfer, resulting in fast chemical reactions - constantly fresh article surface due to abrasion - in certain cases, easy to handle due to the liquid like behavior of the gasarticle susension Disadvantages: - may need high fan ower Simlified model: Solid hase = erfectly mixed luid hase = in lug flow - articles may sometimes crumble too fast, sometimes get lumed together to an agglomerate, which can be difficult to fluidize - difficult to realize the rincile of counter flow - erosion of the vessel and ies can be big - exensive to regain articles (and owder) - inefficient contacting between gas and articles in bubbling beds of fine articles - raid mixing cause non-uniform residence times of solids RoNz 7 luidised beds: alication Industrial examles: -drying - synthesis reactors - cracking - metallurgical rocesses - gasification of hydrocarbon and coke - BC: combustion of solid fuels (with SO 2 -caturing limestone) - olymerisation RoNz 8

5 Design considerations 1. Can the fluidised bed rocess be realized? 2. Pressure loss in order to evaluate the needed fan ower. 3. Porosity (voidage) from the measurement of the vertical ressure rofile. 4. Minimum fluidisation velocity required to transform the acked bed into a fluidised bed. 5. Terminal velocity of the articles in order to clarify when significant entrainment occurs. 6. Dimensional analysis for evaluating exerimental and industrial fluidisation Conditions: scale-u RoNz 9 Geldart s B classification The first ste in designing a fluidisation rocess is to clarify if it can be realized with the articles and gas in question. The article diameter and density, as well as the gas density, will tell what kind of fluidizing behavior can be exected. Cohesive: difficult to fluidize due to cohesion examle: cement Aeratable: bubble free velocity range exists examle: cracking catalyst Sand-like: bubbles occur almost immediately examle: construction sand Soutable: can be fluidised in a souting bed, oor mixing examle: coffee beans Geldart s classification of owders at room temerature and atmosheric ressure (derived for ambient air.) RoNz 10

6 luidisation regimes and Geldart s classification KL91 11 Vertical article concentration ( density ) rofiles for various fluidisation regimes KL91 12

7 6.2 B Pressure dro RoNz 13 Pressure dro How much fan ower will be needed? The amount can be calculated from the ress loss of the air distributor and the bed: ower flow * ressure dro. bed distributor (erforated suort late) air inlet distributor or windbox The ressure dro over the air distributor that is required for uniform fluidisation is of the order of times the ressure dro over the bed. Usually lowest ressure dro in circulating fluidised beds; highest in bubbling beds. RoNz 14

8 Pressure loss in the distributor The ressure loss in an distributor can be calculated with a similar theory as used for connections in arallel: one oening several / many d loss,distr d g w 2 2 hole Re d d w hole d g g Re d ζ d > RoNz 15 Pressure loss across the bed The ressure loss in a acked bed is roortional to w 1 2, but when the velocity is so high that the bed becomes fluidised, the ressure loss is more or less constant. The well aerated gas-solid susension can then easily deform without areciable resistance, like a liquid. The ressure required for injection of a gas at the bottom is roughly the static ressure of the gas-solid susension, and is indeendent of the gas flow rate. Useful for determining u w is the suerficial gas velocity Pressure dro in a fluidised bed. RoNz 16

9 Pressure dro vs. velocity: fixed fluidised bed 17 Pressure dro across the bed The theoretical constant ressure loss in a fluidised bed can be derived when balancing the forces that act on a non-accelerating article in equilibrium state. drag drag drag drag drag m gravity V g V g buoyancy g g V g g V ) 1 g ( V ) (1 g ) ( g A h) 1 g ( A h) (1 g ) ( cs g cs drag buoyancy drag A cs g h (1 g h ) 1 g loss g h 1 g gravity RoNz 18

10 Non-sherical articles The bed is usually made u of non-sherical articles and for comarison it is most convenient to derive an equivalent sherical diameter d, which is defined as the diameter of a equivalent shere, which have the same volume as the non-sherical article that the bed is made u of. V In emirical equations (e.g. Ergun equation) it is, however, the article surface area that is imortant for evaluating the frictional resistance to gas flow and the heterogeneous chemical reactions. The form factor or shae factor 0 Ψ 1 is then used, and it is defined as the surface area of the equivalent shere divided by the surface area of the actual article that the bed is made u of. V d RoNz 19 Pressure loss across the bed The buoyancy force buoyancy can in ractical cases be neglected and the ressure loss of the fluidised bed can be estimated if the weight of the articles is known. The acked bed orosity ε is then needed and can be derived by an Exerimental aroximate formula or from a diagram, luidisation Engineering, KL91 where the shae factor, or shericity Ψ is the surface area of an equivalent shere (equal volume as the article) divided by the surface of the actual article that the bed is made u of ( 1). RoNz 20

11 Pressure dro across the bed Be aware of difference between ressure dro (or loss) and (static) ressure difference. or gas fluidised beds: the gas flow in excess of what is needed for fluidisation forms bubbles RoNz 21 Pressure distribution (vertical) The ressure balance for the fluid can be written (neglecting changes in the kinetic energy), 0 g g z0 1 g g z1 loss and combining it with the exression for the ressure loss in fluidised beds loss g h 1 g gives the article concentration (or gas concentration, i.e. orosity ε) at different heights in the fluidised bed when the ressure is measured. Note that the fluid can in certain cases be a liquid, with a high density that affects the ressure balance 11.3 RoNz 22

12 Minimum fluidisation velocity w is the velocity when a acked bed is turned into a fluidised bed. It can be derived from the exression for ressure loss in acked beds, General: Ergun for acked bed: 2 loss,acked bed w 1 K 3 h K 2d and the exression for ressure loss in fluidised beds, loss, fluidized bed g h 1 g Re Interesting: indeendent of article size! These exressions are set equal and w is solved for. Both exressions can additionally be utilized for evaluating the bed exansion (uniform orosity ε) at w > w RoNz 23 Terminal velocity w T is the difference in velocity between the gas and the article in fully develoed flow conditions (no acceleration); the velocity of the gas when the article is ket in lace by the gas flow, or the falling velocity of a article in a stagnant fluid. If the gas velocity around a article exceeds the terminal velocity, the bed will loose the article (entrainment). It can be derived from gravity m g A rj C drag D g w 2 buoyancy 2 T m V g dw dt If the article is not accelerating, the equation can be written g 0 drag buoyancy Solving for w T results in 11.5 w T 2( m g V A rj C D g g g) gravity RoNz 24

13 Particles of different sizes The bed has seldom uniform articles of the exact same size. A mean value of the diameter (Sauter mean diameter, SMD) can be derived from, (see course material #5) 1 d from a mixture of articles having the mass fractions X i with the diameters d i. This gives a mean diameter of the articles corresonding to the right article surface, i.e. the article diameter that has the same secific surface as that of the full distribution. The area can then be estimated from the weight. X d i i Taking into account a size distribution (x): (x)dx is the chance of article size x being in size range x x+dx Examle: loss,acked bed w Δ, h K 2d RoNz 25 2 ρ Particle size distributions - general Particle size measurements give information on how fractions of sizes are distributed according to number (d 0 ), length (d), surface (d 2 ) or volume ~ mass (d 3 ). In general: or discrete: or examle D sauter = D 3,2 Volume mean diameter D VM = D 4.3 Literature: A97! See e.g. htt:// RoNz 26

14 6.3 luidisation velocity, article terminal velocity RoNz 27 Gas velocities The suerficial gas velocity is defined for the whole cross sectional area. In a bubbling bed the gas volume that exceeds the gas volume for minimum fluidisation will form bubbles The real gas velocity between the articles is higher and can be estimated if the orosity ε is known. RoNz 28

15 Minimum fluidisation velocity u /1 ressure dro, bed height H, orosity, gravity g, fluid density, dynamic viscosity, article diameter d, article density S, flow velocity u, article shericity Pressure dro across a fluidised bed (at minimum fluidisation conditons): H fb ( 1 )( ) g Pressure dro across a acked bed (Ergun): H b (1 ) u ( d ) 2 S (1 ) u d 2 29 Minimum fluidisation velocity u /2 Re Dimensionless grous: Re, Ar 150(1 3 0 for large Re 0 for small Re 2 d ) u Re 1.75 Re 3 Ar 2 d 3 Ar ( S 2 ) g Limiting cases: Re small ( fine ), Re large ( coarse ) 30

16 gravity - lift force (buoyancy) = drag force m u g V t g C 4 3 d D 1 2 u 2 1 t S ( 1) g C D mass m, gravity g, volume V fluid density, dynamic viscosity, drag coefficient c D, article diameter d, article density, terminal velocity u t, Reynolds number Re Re if d t 2 utd 2 Re t Terminal article (settling) velocity u t ; if 1000 C Re D t 2 C 24 Re t D 24 Re ( Re t t ) 31 Terminal velocity Dimensionless article size, d* and velocity, u* u * u d * S 2 Ar ( 1 3 d )g 1 3 Re Ar ( S with )g 1 3 Re ud Determining terminal velocity, u t : calculate Ar = d * igure u* calculate u t 32/74

17 Chart for the determination of article terminal settling velocity through a fluid KL91 33/74 Geldart s classification and B reactor tyes u * d * d Ar 1 3 ( S 2 )g 2 u ( S )g Re 1 Ar 3 ud with Re KL91 34/74

18 6.4 B air distributors, non-mechanical valves RoNz 35 Air (gas) distributor RoNz 36

19 Air (gas) distributor The ressure dro over the air distributor that is required for uniform fluidisation is in the order of times the ressure dro over the bed. RoNz 37 luidisation: effect of gas distributor tye Ref: BR98 38/74

20 Non-mechanical valves 39/ luidised bed modelling RoNz 40

21 The Kunii-Levensiel bubbling bed model /1 Gas flow = gas flow via emulsion + gas flow via bubbles i.e., with bed area A, and suerficial velocity u o : flow (u o -u )*A via bubbles raction of flow u *A via emulsion Rise velocity of bubbles : u u bed in bubbles : δ u b u. (gd Rise velocity of emulsion hase : u Rise velocity of solids : u u b u u u raction of bed in emulsion :1 - δ u b b b u ε Suerficial rise velocity of emulsion gas : s s,u e s, down ) u u 0 u KL91 41/74 The Kunii-Levensiel bubbling bed model /2 KL91 42/74

22 Bed height and bubble size Bed height vs. velocity : Bubble diameter : (A o ~ bottom distributor late area) Bubble rise velocity: (Davidson & Harrison) KL91 H H H d u b b (u 0 u 0.54(u u 0 0 u u b u ) g ( h4 ) 0.711( gd b ) 1 2 A 0 ) 0.8 Or: let a CD calculation determine all this based on your inut data and your model selection htt:// 43/ B heat and mass transfer RoNz 44

23 Particle fragmentation, attrition, abrasion,... attrition abrasion BR98 45/74 Emulsion-towall heat transfer a. large articles, short contact time b. small articles, long contact time GAK97 46/74

24 Emulsion-to-wall heat transfer /2 f h Heat transfer coefficient, h (W/m 2 K) : h h article, convection convection / conduction (1 f) h h radiation gas, convection / conduction where ƒ = fraction of wall covered by articles 1 h article, convection gas articlearticlec, article h radiation Refs: GAK97, ZKTLB99 roblem:article-to-wall distance, δ?? article/wall contact time,τ?? wall coverage, ƒ?? TKK (Aalto) 98/99 47/74 Heat transfer in CB combustion reactors h ~ W/m 2 K Ref: GAK97 48/74

25 Single article mass transfer in a CBC riser Nusselt number Nu min = 2 2 Comare with standard Ranz- Marshall equation ( 52): Nu = Re 0.5 Pr 0.33 Imorant asect considering heat / mass transfer analogy : inert, bed material articles are imortant from a heat transfer oint of view, not from a mass transfer oint of view. Inert articles contribute to heat transfer, not to mass transfer!! Ref: P98 49/ Exercises 11 RoNz 50

26 Exercises Calculate the mean diameter of a material with following distribution, Cumulative weight of a d smaller than [μm] 360 g samle [g] A laboratory scale fluidised bed is fed with air (20ºC) with the volumetric flow of 125 l/min. The distributor (1 mm thick) has 170 small holes, each with a diameter of 0.3 mm. Calculate the ressure loss in the distributor A fluidised bed reactor (h = 10 m, A cs = 2,5 m 2 ) is fed with 12.5 m 3 /s air ( g = 0,310 kg/m 3 and g = 44, kg/ms). The bed is made u of articles with a diameter d of 0,320 mm with a density ρ of 2600 kg/m3. The static ressure has been measured at different heights, h (m) (kpa) Calculate the orosity, which is suosed to be constant, in the area between 0 m 0.2 m and 7 m 10 m. RoNz 51 Exercises Calculate the minimum fluidisation velocity w for a bed of shar sand articles ( =160 μm, ψ=0.67, ρ =2600 kg/m 3 ) when the fluidizing gas is ambient air (ρ g =1.2 kg/m 3, η g = kg/(m s). The orosity of the acked bed is Can the articles a) be retained in the bed although the gas velocity is higher that the terminal velocity? b) be lost from the bed although the gas velocity is less than terminal velocity? 11.6 An industrial fluidised bed reactor is going to use articles with d =1.50 mm and ρ =2600 kg/m 3. The fluidizing gas (ρ g =0.45 kg/m 3, η g = kg/ms) has a lanned suerficial velocity of 5 m/s. The orosity need to be known before the actual rocess can be started, but because the articles and the gas are very exensive, air (ρ g =1.2 kg/m 3, η g = kg/ms) and articles of a cheaer test material are used instead. Calculate by means of dimensional analysis the values for the test material, d and ρ, as well as the suerficial velocity of the air in the orosity evaluation exeriment. Can the industrial fluidised bed reactor be utilized in the test? RoNz 52

27 6.8 Pneumatic conveying RoNz 53 Solids transort methods Susended articles Pneumatic (hydraulic) conveying Gravity chutes Air slides Suorted articles Belt conveyors Screw conveyors Bucket elevators Vibratory conveyors RoNz 54

28 Pneumatic conveying Negative (a) (i.e. vacuum ~ 0.4 atm) and ositive (b) ressure conveying, also combined (ush and ull) exists (a) Dust free feeding, no leakage, toxic / hazardous solids (b) Large distances, large loadings Searation by cyclone, usually RoNz 55 Pneumatic conveying Physical roerties of tyical solids for neumatic conveying RoNz 56

29 Pneumatic conveying: classification kg solids / kg gas C06 RoNz 57 Pneumatic conveying: regimes Increasing article loading Often dense transort is associated with large ressure fluctuations RoNz 58

30 Pneumatic conveying: ressure dro Imortant for ower consumtion calculation Acceleration of articles requires significant ower Acceleration length Increased solid loading RoNz 59 Pneumatic conveying low regime diagrams (log-log) for fine article neumatic conveying J = 0: only gas RoNz 60

31 Pneumatic conveying: drag reduction Tyically at kg/ kg solids loading, turbulent, d < 0.2 mm Left: Right: RoNz 61 Minimum transort velocity /1 U = mean gas stream velocity; D d = ie diameter; µ, ρ = gas viscosity, density; α = article volume fraction; ρ = article density; (taken from an and Zhu, 1998 chater 11) RoNz 62

32 Minimum transort velocity /2 (taken from an and Zhu, 1998 chater 11) RoNz 63 Pneumatic conveying Pressure dro for flow of solids in neumatic conveying (comared to air velocity) for article velocity u s, solid feed rate, article terminal velocity u o. Electrostatic charging changes u s and Δ!! Examle exerimental result for a given system at u to 35 m/s air velocity RoNz 64

33 Pneumatic conveying: air movers Rotary lobe ( roots ) blower C06 RoNz Hydraulic conveying RoNz 66

34 Hydraulic conveying Liquid (water) instead of gas (air) used for transort Much lower velocities Solids blockage easier avoided Three tyes: Homogeneous Settling, vertical Settling, horizontal horizontal PRESSURE DROP ESTIMATION: SEE C06 section 4.2 RoNz 67 Transitional velocities (horizontal) C06 RoNz 68

35 Critical velocity: Wilson s diagram (1979) or a 0.4 m ie, 0.2 mm articles start settling at U cm ~ 3 m/s if S = ρ S /ρ L = 2.65 Corrects to U cm ~ 2.3 m/s if S = ρ S /ρ L = 2.0 C06 RoNz 69 Critical velocity: Wani s correlations (1982) C06 α s = volumetric solids concentration m 3 /m 3. RoNz 70

36 Hydraulic conveying Examle: coal mining tailings transort to a storage lagoon at 1,1 km, rate 15 kg/s RoNz Exercises 12 RoNz 72

37 12.1 Exercises RoNz 73 Sources / further reading BR98: G L Bormand KW Ragland Combustion engineering McGraw-Hill (1998) Chater 17 CRBH83: Coulson, J.M., Richardson, J.., Backhurst, J.R., Harker, J.H. Chemical Engineering, Vol. 2 : Unit Oerations Pergamon Press, Oxford (1983) Z98: L-S an, C Zhu Princiles of gas-solid flows Cambridge Univ. Press (1998) GAK97: Grace, J.R., Avidan, A.A., Knowlton, T.M. (Eds.) "Circulating fluidised beds", Chaman & Hall, London (1997) IGH91: Iinoya, K., Gotoh, K., Higashitani, K. Powder technology handbook, Marcel Dekker, New York (1991) KL91: D Kunii, O Levensiel luidisation engineering 2nd ed, Butterworth-Heinemann (1991) P98: Palchonok, G.I. Heat and mass transfer to a single article in a fluidised bed Chalmers Univ. of Technol., Sweden, Ph. D. thesis (1998) PL98: Peirano, E., Leckner, B. undamentals of turbulent gas-.solid flows alied to circulating fluidised bed combustion Progr. Energy Combust. Sci. 24(1998) ZH00: R. Zevenhoven, K. Heiskanen Particle technology for thermal ower engineers art 1 & art 2, ostgraduate course ene , TKK, Esoo, Set./Oct ZKLTB99: Zevenhoven, R., Kohlmann, J., Laukkanen, T., Tuominen, M., Blomster, A.-M. Susension-to-wall heat transfer in CB combustion: near-wall article velocity and concentration measurements at low and high temeratures Proc. 6th Int. Conf. on CB, Würzburg, Germany, August 1999 (J. Werther, Ed.), rankfurt/main (1999) C06: Crowe, C.T., ed., Multihase low Handbook CRC Press, Taylor & rancis Grou (2006) Chaters 4-5 A11: Ahlskog, M. Dimensioning of Polymer Pielines for Slurries MSc thesis ÅA KT VST 2011 A97: Allen, T. Particle size measurement. Vol. 1, Powder samling and article size measurement, and Vol. 2, Surface area and ore size determination. Chaman & Hall (1997) 74