Powder Technology – Part II

DT275 Masters in Pharmaceutical and Chemical Process Technology

Gavin Duffy, School of Electrical Engineering Systems, DIT

Summary

We’ve looked at Gravity conveying Dilute phase pneumatic conveying

Other methods include Screw conveyors Eductors (also part of pneumatic conveying)

Screw Conveyor

Screw Conveyors can be Constant speed for constant flowrate Variable speed for controlled flowrate

A screw conveyor can be used to move material in a horizontal and/or a vertical distance

Normally used when an accurate delivery of material is required

Loss in weight feeders are used for accurate measurement of solids flowrate/delivery

LIW Feeders

Entire feeder plus screw sits on a weigh scales

Rate of weight loss is equivalent to mass flow rate

Stops when total batch weight has been delivered

Material cannot be added to the feeder while it is operating

Accuracy of the order of grammes

Hopper

Screw

Discharge

Eductor

An eductor is an alternative to a rotary valveHigh pressure motive air or nitrogen is passed into the eductorHigh velocity reduces pressure and creates suctionMaterial is conveyed in the transport stream 20m/s

Eductor

Advantage over rotary valve is that there are no moving parts

Eductor or Rotary Valve

An eductor can do the same thing as a rotary valve combined with a blower

Cyclones

Gas solid separator

No moving parts

Incoming dust laden air travels downwards in a spiral path (vortex)

Centrifugal forces throw the particles to the wall and are pushed down in the vortex

Reverse flow - Air travels up the centre and out the top

Centrifugal force (mv2/r) decreases as radius increases so smaller cyclones are better separators than large ones

Group a number of small cyclones in parallel instead of one large cyclone to increase efficiency

Not great at recovering fines less than 10m

Cyclones

Cyclone Efficiency

Total Efficiency = Mass of Coarse product

Mass of Feed

Grade efficiency = mass of solids of size x in coarse product

mass of solids of size x in feed

Cyclone – Activity

Read the handout on cyclones provided

In groups of two answer the following questions What effect do the following have on efficiency?

Particle size Cyclone diameter Gas velocity

Cyclone Design

Key design parameters are Collection efficiency Pressure drop

These are governed by the dimensions of the cyclone Small diameters give greater efficiency Cyclone height – efficiency and P increase with height; normally

height is between 2 and 6 diameters Cone apex angle is normally between 10 and 20°; smaller angle

gives better efficiency

Ref: http://www.wsu.edu:8080/~gmhyde/433_web_pages/cyclones/-CycloneOverview.html

Cyclone Pressure Drop

Energy is lost in a cyclone at the entrance to and exit from the cyclone due to

friction losses Due to the rotational flow in the vortex

This results in a pressure dropPressure drop Q2

Q is the gas flowrate

Pressure drop usually of the order of 50 to 150 mm of H2O Pressure drop is related to efficiency – It increases with efficiencyIn practice the efficiency is limited because at high P, velocities become high, and turbulence causes re entrainment and loss of particles

Efficiency, Flowrate and P

0

0

Gas Flowrate, Q

0

ΔP,

m o

f g

as

colu

mn

Effi

cien

cy

A

B

OptimumOperation

Eff

P

Theory

Practice

40

100

Cyclone efficiency and Particle Size

Efficiency increases with mass which increases with particle size

As particle size is increased, a point is reached where 50% of the particles are collected. This is the cut size. This size particle has a 50% chance of making it.

Activity – Cyclone Efficiency

Using the test data for the cyclone provided calculate: Total efficiency of the cyclone Grade efficiency for each size range Determine cut size

Removal of material from a cyclone

‘submarine hatch’ base of cyclone not open to atmosphere during discharge

operate valves on a timed basisonly allow one open at a time

Size reduction

Options for size reduction are base on the size of the particle

From Rhodes (Introduction to Particle Technology)

Down to 3 mm 3 mm to 50 μm < 50 μm

Crushers

Table mill

Edge Runner mill

Ball mill

Rod mill

Pin mill

Tube mill

Vibration mill

Ball mill

Vibration mill

Sand mill

Perl mill

Colloid mill

Fluid energy mill

Milling

Rotated or vibrated hollow cylinder partially filled with balls

Slightly tilted, material enters one end and leaves through the other

Fluid Energy Mill or Microniser

High pressure compressed air

Pulverised in a shallow cylindrical chamber

Jets arranged tangentially around chamber

Solid is thrown to the outside wall

Shear stresses, inter particle collision break particles up

Centrifugal force is stronger for large particles and they move to the outside of the chamber for more grinding

Small particles fall out of the centre for collection

Size reduction to 1 to 10 m

Microniser

Fluid inlet

Material inlet

Product outletGrinding fluid(compressed air)

Fluid outlet

Jets

Size Enlargement

Small particles are combined to form clumps of particles that appear to be a larger particleReasons include: reduce dusts increase bulk density to improve mixing, prevent segregation control surface to volume ratio

Methods include: Granulation Compaction/tabletting Extrusion

Granulation

Binding liquid sprayed in

Particles coalesce

Some attrition

Hazardous area classification

Like zone 0, 1 and 2 for fluids like organic solventsDusts and powders are given zone 20, 21 and 22 Zone 20 means a flammable atmosphere is expected

continuously during normal operations. This would happen inside a storage vessel

Zone 21 means the possibility of a flammable atmosphere existing in normal operations (e.g. around manholes to vessels containing flammable materials)

Zone 22 means the possibility of a flammable atmosphere existing only in abnormal situations (e.g. spill containment or bunds)

Temperature classification also, the surface of a motor can not exceed the ignition temperature of dust, e.g. 200 ºC (T1=450ºC, T3=200ºC, T6=85ºC)

Safe Design

Avoid sources of ignition

Electrical and mechanical equipment must be Ex rated

Avoid build up of static by earthing all objects

Containment – keep powders contained so the Zone 20 only applies inside the vessel

Rate vessels and piping for explosions – e.g. can withstand 10barg pressure even though normally operated at atmospheric

Provide house vacuum system to clean up spills

Use fume cupboards and glove boxes for opening bags

Cleanroom classificationISO classification number(N) CLASS LIMITS (particles/m3)Maximum concentration limits (particles/m3 of air) for particles equal to and larger than the considered sizes shown below

0.1 um 0.2 um 0.3 um 0.5 um 1 um 5 um

ISO Class 1 10 2        

ISO Class 2 100 24 10 4    

ISO Class 3 1000 237 102 35 8  

ISO Class 4 10000 2370 1020 352 83  

ISO Class 5 100000 23700 10200 3520 832 29

ISO Class 6 1000000237000 102000 35200 8320 293

ISO Class 7       352000 83200 2930

ISO Class 8       35E5 832000 29300

ISO Class 9       35E6 83E5 293000

Old classification

Particle Counts/ft3 Federal Standard Particle Counts/m3 New Class

(>0.5um) 209 E Class (>0.5um)

75000 Class 100000 2640000 ISO Class 8

1500 Class 10000 52800 ISO Class 7

675 Class 1000 23800 ISO Class 6

25 Class 100 880 ISO Class 5

7 Class 10 246 ISO Class 4

1 Class 1 35 ISO Class 3

Reading materialEssential Reading

Introduction to Particle Technology, Martin Rhodes, 2004, Wiley Unit Operation of Chemical Engineering, McCabe, Smith and

Harriott, 2001

Additional Reading Chemical Engineering, Volume 2, Particle Technology and

Separation Processes, Coulson and Richardson, 5th Ed., 2002 Handbook of Powder Technology, Volume 10, Handbook of

Conveying and Handling of Particulate Solids, A. Levy and H. Kalman (editors), 2001, Elsevier

Unit Operations Handbook, Volume 2, Mechanical Separations and Materials Handling, J. J. McKetta, 1993