Introduction to Mechanical Process Engineering WS 2013/2014

Transcription

1 Introduction to Mechanical rocess Engineering WS 2013/2014 rof. Dr.-Ing. Rolf Gimbel - FOR ERSONAL USE ONLY! - Institut für Energieund Umwelterfahrenstechnik (EUT) Department of rocess Engineering / Water Technology Bereich Wassertechnik IWW IWW Rheinisch-Westfälisches Institut for für Wasserforschung Rhenish-Westfalian Institute Water 1

2 Systematics of Basics in rocess Engineering Reaction chemical or biological (partly with catalysts) Agglomeration physical: mechanical Separation physical: thermal or mechanical Controlled crushing physical: mechanical Heat transfer physical: thermal Storage, packing, physical: mechanical, transport electromagnetical 2

3 Basics of Mechanical rocess Engineering Knowledge of processes, which change the state of substances: Background for: lanning, design, construction and operation of: o o o o Apparatus Machines lants (production, separation, elimination of certain substances ) systems 3

4 Introduction to Mechanical rocess Engineering Subject: Mechanical effects on substances in order to change their properties and behaior. Special mechanical effects are macroscopic forces like: Momentum change Flow resistance Contact forces 4

5 Fie Basic rocesses in Mechanical rocess Engineering (by Rumpf) 1.) Controlled crushing (breaking, grinding, cutting, defibration, desagglomeration...) 2.) Agglomeration / Flocculation (Granulating, pelletizing, compacting, tabletting, briquetting...) 3.) Separation (Classification, sieing, sorting, separation, clarification, sedimentation, flotation, filtration, centrifugation, remoal of dust...) 4.) Mixing (Homogenization, stirring, solids mixing, kneading, dispersing, emulsifying, aerating, spraying...) Special topic: 5.) article measurement / analysis (incl. description of disperse systems) disperse (finely) distributed 1.) and 2.) Change in particle size or degree of dispersion 3.) and 4.) article size and degree of dispersion remain unchanged rocesses in mechanical process engineering are usually related to collecties of many particles disperse systems 5

6 articles and Disperse Systems In mechanical process engineering systems of substances exist as: 1. Granular material, packed beds 2. owders 3. Aerosols (particles or droplets in air ) 4. Gas bubbles in fluids, foams, or emulsions Disperse system collecties of particles ( disperse phase ) surrounded by homogenous medium (continuous phase ) Disperse phase as well as continuous phase could be: Solid (s) Liquid (l) Gaseous (g) Characterization of disperse systems: Size particle size distribution Shape Chemical composition Specific surface area Colour, 6

7 Density Distribution Function q r (x) as a Histogram Interal i x i x i-1 x i x particle size r indicates the type of quantity aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992) 7

8 Graphic Representation of a Cumulatie Distribution Function and corresponding Density Distribution Function fraction fraction median alue W Wendepunkt (inflection point) aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992) 8

9 Types and Measures of Quantity Index Type of quantity Measure of quantity Application r0 Number q 0, Q 0 Very frequently r1 Length q 1, Q 1 Very unusual r2 Area q 2, Q 2 Frequently r3 Volume q 3, Q 3 Frequently r3* Mass q 3*, Q 3* Very frequently 9

10 Example of a Number Density Distribution Function and corresponding Volume Density Distribution Function aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992) 10

11 Interaction of articles: Scheme of Major Adhesion Mechanisms (in a Gaseous Continuum) Distinction: with material binders without material binders (due to interaction of electrostatic, electrodynamic, (magnetic and graimetric fields) Interlocking aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992) 11

12 Adhesion Forces in Liquids (Completely Immersed System) Major difference from those in a gaseous continuum no capillary forces.d.w.- and electrostatic forces are generally weaker. otential energy of interaction between particles in fluids. aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992) 12

13 Examples of Aggregates of a Flocculation rocess in Water 13

14 hysical rinciples of Mechanical Solid-Liquid Separation rocess Driing forces Examples Sedimentation / Flotation Filtration ressing / Squeezing Graity field Magnetic field Centrifugal field Graity field Excess pressure Negatie pressure Centrifugal field Graity field Excess pressure in fluid Deformation resistance Centrifugal field Sedimentation tanks Sorting procedures Settling centrifuges Dewatering hopper ressure filter Vacuum filter Centrifugal filter Cake and sludge compression 14

15 Mechanical Waste Water Treatment Mechanical methods are used for remoal of solid waste water compounds (coarse matter, sand (grit), organic compounds with the ability to settle,...) Substance group Density in g/cm 3 article size in cm Method Coarse matter ρ or 1 > 0.5 Bar-Screens ρ or 1 < 0.5 Siees Settleable matter ρ > 2.5 > 0.01 Grit chamber, Hydrocyclons ρ > 1 > Sedimentation basin Flotable matter ρ 1 < 0.5 Flotation systems 15

16 Technology of Waste Water Treatment Mechanical Waste Water Treatment Biological Waste Water Treatment hysic. / chemical Waste Water Treatment Screen Grit Chamber rimary Sed. Basin Actiated Sludge Basin Flocculation / recipitation Final Sed. Basin Outlet Inlet Settled Sand rimary Sludge O 2 - input Surplus Sludge Sludge Recycling Sludge Treatment + Flotation! Treated waste water to receiing water 16

17 The rinciple of Sedimentation and Flotation Stationary Conditions ( constant) Drag force Drag force for laminar flow for conditions (Re < 1): Flocks consisting of: black solid particles grey metal-hydroxide white air bubbles 17

18 Basics of Sedimentation rocesses in WWT rinciple: Suspended particles in water with a density larger than the density of water will sediment (settle) due to graity. Use of sedimentation processes in wastewater treatment (WWT): Remoal of mineral solids (sands) in grit chambers in order to aoid operating trouble due to mechanical stress of pumps and to separate mineral solids from solids which can be digested (use of sewage sludge, optimal use of space in digestion tanks). Remoal of settleable (organic) matter in the primary sedimentation basin in order to improe following biological treatment. (High concentration of organic solids leads to decreasing oxygen concentration). Remoal of settleable organic matter in the final sedimentation basin (after biological actiated sludge treatment or flocculation respectiely) in order to recycle a part of settled actiated sludge into the biological treatment basin (sludge cycle) and for improement of wastewater quality prior to discharge into the receiing rier. 18

19 Basics of Sedimentation I Description of the stationary (i.e. s const.!) settling behaior of a single spherical particle in stagnant water bodies: F r A Fr W Forces acting on a particle: S const. r F r F r F A W G ρ ρ ρ Fl Fl V c V W g ( ) g Re A 2 s 2 F r G F A F W F G buoyancy (lift) force drag force, hydrodynamic resistance graity force 19

20 Basics of Sedimentation II Dependency of drag coefficient c w from Reynolds number aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992) 20

21 Basics of Sedimentation III F r A Fr W Force balance for particle : ρ Fl c W F r 0 i 2 s ( Re) A V g ( ρ ρ ) 2 ( ρ ) ρfl ( Re) ρ A 2 2 g s c w Fl Fl V F r G 21

22 Force balance for particle : Basics of Sedimentation IV ( ρ ) ρfl ( Re) ρ A 2 2 g s c w Fl V V π 6 d 3 ; A π 4 d 2 V A 2 3 d 4 d g 2 s 3 c Re ( ρ ρ ) ρ w ( ) Fl Fl In general the equation is not explicitly solable, due to c w f (Re) and Re f ( s ). 22

23 Basics of Sedimentation V Special case: Re < 1 c w 24 Re 24 ν d s 2 s 4 d g 3 ( ρ ρ ) 24 ρ Fl ν Fl s d s d 2 g 18 ( ρ ρ ) η Fl ν kinematic is cosity η dynamic is cosity 23

24 24 Basics of Sedimentation VI s s H H t H ; t L Simplified design of a settling tank V & H B L H s L H H L t t H s s H H s H B V H & L B V s & Surface Loading!

25 Centrifugal Decanter Rotor with cylindrical and conical centrifuge casing and slower rotating coneyor screw in the inner part to transport deposited solids out of the centrifuge. 25