Transcript Powder Technology part II (ppt file 255kb)

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
Hopper
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
Discharge
Screw
Eductor
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
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 10m
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 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
B
Efficiency
A
Optimum
Operation
Practice
P
100
Eff
40
0
0
0
Gas Flowrate, Q
ΔP, m of gas column
Theory
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 basis
only allow one open at
a time
Size reduction
Options for size reduction are base on the size of the particle
Down to 3 mm
3 mm to 50 μm
Crushers
Ball mill
Table mill
Rod mill
Edge Runner mill Pin mill
Tube mill
Vibration mill
< 50 μm
Ball mill
Vibration mill
Sand mill
Perl mill
Colloid mill
Fluid energy mill
From Rhodes (Introduction to Particle Technology)
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 outlet
Material inlet
Fluid inlet
Jets
Product outlet
Grinding fluid
(compressed air)
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
Granulation
Binding liquid
sprayed in
Particles
coalesce
Some attrition
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
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
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 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
1000000 237000 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 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