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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

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powder technology part ii

Powder Technology – Part II

DT275 Masters in Pharmaceutical and Chemical Process Technology

Gavin Duffy, School of Electrical Engineering Systems, DIT

  • We’ve looked at
    • Gravity conveying
    • Dilute phase pneumatic conveying
  • Other methods include
    • Screw conveyors
    • Eductors (also part of pneumatic conveying)
screw conveyor
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
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



  • An eductor is an alternative to a rotary valve
  • High pressure motive air or nitrogen is passed into the eductor
  • High velocity reduces pressure and creates suction
  • Material is conveyed in the transport stream 20m/s
  • Advantage over rotary valve is that there are no moving parts
eductor or rotary valve
Eductor or Rotary Valve
  • An eductor can do the same thing as a rotary valve combined with a blower
  • 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 10m
cyclone efficiency
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
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
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
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 drop
  • Pressure 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 efficiency
  • In 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
Efficiency, Flowrate and P








ΔP, m of gas column








Gas Flowrate, Q

cyclone efficiency and particle size
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
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
Removal of material from a cyclone

‘submarine hatch’

base of cyclone not open to atmosphere during discharge

operate valves on a timed basis

only allow one open at a time

size reduction
Size reduction
  • Options for size reduction are base on the size of the particle
  • From Rhodes (Introduction to Particle Technology)
  • 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
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

Material inlet

Fluid outlet

Fluid inlet


Grinding fluid

(compressed air)

Product outlet

size enlargement
Size Enlargement
  • Small particles are combined to form clumps of particles that appear to be a larger particle
  • Reasons include:
    • reduce dusts
    • increase bulk density
    • to improve mixing, prevent segregation
    • control surface to volume ratio
  • Methods include:
    • Granulation
    • Compaction/tabletting
    • Extrusion

Binding liquid sprayed in

Particles coalesce

Some attrition

hazardous area classification
Hazardous area classification
  • Like zone 0, 1 and 2 for fluids like organic solvents
  • Dusts 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
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 classification
Cleanroom classification

ISO 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 um0.2 um0.3 um0.5 um1 um5 um

ISO Class 1102

ISO Class 210024104

ISO Class 31000237102358

ISO Class 4100002370102035283

ISO Class 51000002370010200352083229

ISO Class 61000000237000102000352008320293

ISO Class 7352000832002930

ISO Class 835E5832000 29300

ISO Class 935E6 83E5 293000

old classification
Old classification

Particle Counts/ft3Federal Standard Particle Counts/m3New Class

(>0.5um) 209 E Class (>0.5um) 75000Class 1000002640000ISO Class 8

1500Class 1000052800ISO Class 7

675Class 100023800ISO Class 6

25Class 100880ISO Class 5

7Class 10246ISO Class 4

1Class 135ISO Class 3

reading material
Reading material
  • Essential 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