Powder Technology – Part II DT275 Masters in Pharmaceutical and Chemical Process Technology Gavin Duffy, School of Electrical Engineering Systems, DIT
Dec 17, 2015
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