Mechanical Process Engineering 181y54

Introduction to Mechanical Process Engineering WS 2013/2014 Prof. Dr.-Ing. Rolf Gimbel - FOR PERSONAL USE ONLY! -

Institut für Energieund Umweltverfahrenstechnik (EUT) Department of Process Engineering /

Water Technology Bereich Wassertechnik

IWW IWW

Rheinisch-Westfälisches Institut for für Wasserforschung Rhenish-Westfalian Institute Water

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Systematics of Basics in Process Engineering •

Reaction

Ö

chemical or biological (partly with catalysts)

•

Agglomeration

Ö

physical:

mechanical

•

Separation

Ö

physical:

thermal or mechanical

•

Controlled crushing

Ö

physical:

mechanical

•

Heat transfer

Ö

physical:

thermal

•

Storage, packing, transport

Ö

physical:

mechanical, electromagnetical 2

Basics of Mechanical Process Engineering Knowledge of processes, which change the state of substances: Ö Background for:

•

Planning, design, construction and operation of: o o o o

Apparatus Machines Plants (production, separation, elimination of certain substances…) systems 3

Introduction to Mechanical Process Engineering Subject:

•

Mechanical effects on substances in order to change their properties and behavior.

•

Special mechanical effects are macroscopic forces like: ƒ Momentum change ƒ Flow resistance ƒ forces

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Five Basic Processes in Mechanical Process Engineering (by Rumpf) 1.)

Controlled crushing

2.)

Agglomeration / Flocculation

(breaking, grinding, cutting, defibration, desagglomeration ...) (Granulating, pelletizing, compacting, tabletting, briquetting ...)

3.)

Separation (Classification, sieving, sorting, separation, clarification, sedimentation, flotation, filtration, centrifugation, removal of dust ...)

4.)

Mixing (Homogenization, stirring, solids mixing, kneading, dispersing, emulsifying, aerating, spraying ...)

Special topic: 5.)

Particle measurement / analysis (incl. description of disperse systems)

disperse = (finely) distributed 1.) and 2.) Ö Change in particle size or degree of dispersion 3.) and 4.) Ö Particle size and degree of dispersion remain unchanged Processes in mechanical process engineering are usually related to collectives of many particles → disperse systems

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Particles and Disperse Systems ƒ

In mechanical process engineering systems of substances exist as: 1. Granular material, packed beds 2. Powders 3. Aerosols (particles or droplets in air ) 4. Gas bubbles in fluids, foams, or emulsions Disperse system

= collectives 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) ƒ

Characterization of disperse systems: ƒ Size → particle size distribution ƒ ƒ ƒ ƒ

Shape Chemical composition Specific surface area Colour, …

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Density Distribution Function qr(x) as a Histogram Interval i

∆xi

xi - 1 x = particle size

xi

r indicates the type of quantity

aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992)

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Graphic Representation of a Cumulative Distribution Function and corresponding Density Distribution Function fraction

fraction

WP = Wendepunkt (inflection point) median value

aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992)

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Types and Measures of Quantity Index

Type of quantity

Measure of quantity

Application

r=0

Number

q0, Q0

Very frequently

r=1

Length

q1, Q1

Very unusual

r=2

Area

q2, Q2

Frequently

r=3

Volume

q3, Q3

Frequently

r=3*

Mass

q3*, Q3*

Very frequently

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Example of a Number Density Distribution Function and corresponding Volume Density Distribution Function

aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992)

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Interaction of Particles: Scheme of Major Adhesion Mechanisms (in a Gaseous Continuum)

Distinction: ƒ ƒ

ƒ

with material binders without material binders (due to interaction of electrostatic, electrodynamic, (magnetic and gravimetric fields) Interlocking

aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992)

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Adhesion Forces in Liquids (Completely Immersed System) ƒ Major difference from those in a gaseous continuum → no capillary forces → v.d.W.- and electrostatic forces are generally weaker.

Potential energy of interaction between particles in fluids. aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992)

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Examples of Aggregates of a Flocculation Process in Water

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Physical Principles of Mechanical Solid-Liquid Separation

Process

Driving forces

Examples

Sedimentation / Flotation

Gravity field Magnetic field Centrifugal field Gravity field Excess pressure Negative pressure Centrifugal field Gravity field Excess pressure in fluid Deformation resistance Centrifugal field

Sedimentation tanks Sorting procedures Settling centrifuges Dewatering hopper Pressure filter Vacuum filter Centrifugal filter

Filtration

Pressing / Squeezing

Cake and sludge compression

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Mechanical Waste Water Treatment ƒ Mechanical methods are used for removal of solid waste water compounds (coarse matter, sand (grit), organic compounds with the ability to settle,...) Density Substance group Coarse matter

Settleable matter

Flotable matter

Particle size in cm

Method

ρ ≥ or ≤ 1

∅ > 0.5

Bar-Screens

ρ ≥ or ≤ 1

∅ < 0.5

Sieves

ρ > 2.5

∅ > 0.01

Grit chamber, Hydrocyclons

ρ>1

∅ > 0.001

Sedimentation basin

ρ≤1

∅ < 0.5

Flotation systems

in g/cm3

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Technology of Waste Water Treatment

Screen

Grit Chamber

Physic. / chemical Waste Water Treatment

Biological Waste Water Treatment

Mechanical Waste Water Treatment

Primary Sed. Basin

Activated Sludge Basin

Flocculation / Precipitation Final Sed. Basin Outlet

Inlet Settled Sand

Primary Sludge

Surplus Sludge

O2 - input

Sludge Recycling

Sludge Treatment

+ Flotation!

Treated waste water to receiving water 16

The Principle of Sedimentation and Flotation Stationary Conditions ( v = constant)

Drag force

v

v Drag force

for conditions (Re < 1): for laminar flow

Flocks consisting of: black = solid particles grey = metal-hydroxide white = air bubbles 17

Basics of Sedimentation Processes in WWT Principle: ƒ Suspended particles in water with a density larger than the density of water will sediment (settle) due to gravity.

Use of sedimentation processes in wastewater treatment (WWT): ƒ

Removal of mineral solids (sands) in grit chambers in order to avoid 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).

ƒ

Removal of settleable (organic) matter in the primary sedimentation basin in order to improve following biological treatment. (High concentration of organic solids leads to decreasing oxygen concentration).

ƒ

Removal of settleable organic matter in the final sedimentation basin (after biological activated sludge treatment or flocculation respectively) in order to recycle a part of settled activated sludge into the biological treatment basin (sludge cycle) and for improvement of wastewater quality prior to discharge into the receiving river. 18

Basics of Sedimentation I Description of the stationary (i.e. vs = const.!) settling behavior of a single spherical particle in stagnant water bodies: Forces acting on a particle:

r FA

r FW

v S = const .

r FG

r FA = ρFl ⋅ VP ⋅ g r FW = ρFl ⋅ c W (Re ) ⋅ A ⋅ r FG = ρP ⋅ VP ⋅ g

v 2s 2

FA = buoyancy (lift) force FW = drag force, hydrodynamic resistance 19 FG = gravity force

Basics of Sedimentation II Dependency of drag coefficient cw from Reynolds number

aus: Mechanische Verfahrenstechnik 1, Matthias Stieß, Springer Verlag (1992)

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Basics of Sedimentation III Force balance for particle :

∑ r FA

r FW

r F =0 i

v 2s ρFl ⋅ c W (Re ) ⋅ A ⋅ = VP ⋅ g ⋅ (ρP − ρFl ) 2 v 2s

2 ⋅ g ⋅ (ρP − ρFl ) ⋅ VP = c w (Re ) ⋅ ρFl ⋅ A

r FG

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Basics of Sedimentation IV Force balance for particle :

v 2s

2 ⋅ g ⋅ (ρP − ρFl ) ⋅ VP = c w (Re ) ⋅ ρFl ⋅ A

VP 2 π 2 π 3 VP = ⋅ dP ; A = ⋅ dP ⇒ = ⋅ dP A 3 6 4 v 2s

4 ⋅ d P ⋅ g ⋅ (ρ P − ρ Fl ) = 3 ⋅ c w (Re ) ⋅ ρ Fl

In general the equation is not explicitly solvable , due to cw= f (Re) and Re = f (vs). 22

Basics of Sedimentation V Special case: Re < 1

24 24 ⋅ ν ⇒ cw = = Re v s ⋅ dP

4 ⋅ dP ⋅ g ⋅ (ρP − ρFl ) ⋅ v s ⋅ dP ⇒v = 3 ⋅ 24 ⋅ ρFl ⋅ ν 2 s

d ⋅ g ⋅ (ρP − ρFl ) ⇒ vs = 18 ⋅ η 2 P

ν = kinematic vis cos ity

η = dynamic vis cos ity 23

Basics of Sedimentation VI Simplified design of a settling tank

V&

vH

H vs B

H L vH = ; vs = tH ts L H = t s = tH ⇒ vH v s V& vH = B⋅H

⇒

L

⇒

V& vs = B⋅L

vH ⋅ H vs = L Surface Loading! 24

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

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