Powder Technology
DT275 Masters in Pharmaceutical and Chemical
Process Technology
Gavin Duffy, School of Electrical Engineering Systems, DIT
Learning Outcomes
After this lecture you should be able to
 List the main characteristics of solid particles
 Describe how powders can be stored
 List a number of methods of conveying particles
 Design a pneumatic conveying system
 Explain some particle size reduction techniques
 Describe how powders can be mixed
A Particle
Particles have many different shapes so how do we define
size?
Simplest shape is the sphere – same from all directions
Particle shape can be characterised by comparing it to a
sphere in one of two ways:
1. Sphericity is given as
 = surface area of sphere of same volume as particle
surface area of particle
2. Equivalent diameter is the diameter of a sphere of
equivalent volume
Sphericity
What is the sphericity of a cylinder?
Sphericity given by
10mm

6v p
10mm
Dp s p
(m3)
vp= volume of packing element
Dp = characteristic length (diameter of
cylinder) of element (m2)
sp = surface area of packing element (m)

2
0
.
01
6

0.01
4

0.01 2 0.012  2 0.01 0.01
4.7 E  06

12.6 E  06
  0.37


Particle Size Analysis – Sieves
Sieves are metal dishes with woven mesh at base
100 mm to 20 m
Finer sieves are not woven but electo etched nickel
Sieves are stacked with widest aperture mesh first
Pan at base
Vibrate for a time
Empty each sieve and weigh
Each result is a size range
-600 +500 fell through 600 but
stayed on the 500 m sieve
Other methods
Microscopic analysis (1 to 100m)
Sedimentation and elutriation (> 1 m)
Permeability methods (>1 m)
Electronic particle counters
Laser diffraction analysers
For example the Focused Beam Reflectance Measurement
(see http://www.lasentec.com/)
 Particle sizes from 0.5 m to 3 mm
Bulk Properties
Compared to fluids, particulate solids are more difficult to
store and convey. Some problems are:
Voidage exists, this is the fraction of total volume made up
of free space
Agglomeration can occur, particles adhere to each other to
form clusters (e.g. due to electrostatic forces, moisture)
Angle of repose is between the horizontal and the sloping
side of a conical pile of material
 20º for free flowing solids
 60º for poor flowing solids
 Low angle of repose for large particles (>100m)
 More of a problem with fines
Categories of Cohesion
Powders can classified into six categories as follows (source =
Pneumatic Conveyors for Bulk Materials, Gericke):
1.
Product can be suspended in air and flows as freely as a liquid;
2.
Free flowing product, angle of repose, a, being such that 0 < a 
30°
3.
Product flows normally, angle of repose, a, being such that 30° <
a  45°
4.
Product is slow flowing, angle of repose, a, being such that 45° <
a  60°
5.
Compact product, angle of repose, a, being greater than 60°
6.
Non-collapsible product, entangled, susceptible to arching,
resistant to separation
In general, materials that fall into 5 and 6 are not suitable for gravity
conveying
Other properties
Hardness – measured on the Mohs’ scale. Graphite = 1 and Diamond = 10
Toughness – resistance to the propagation of cracks
A tough material resists cracking, will deform plastically instead
Opposite is brittle
Can sometimes make tough material brittle by reducing temperature
Cohesivity – degree to which particles stick together
Related to moisture content and particle size
Decrease size or increase moisture => more cohesive
Bulk density = mass of particles/volume incl. voidage
 B  1    P
ε = Voidage
ρB = bulk or bed density
ρP = particle density
Particle Terminal Velocity
Terminal velocity is determined by using a chart of drag
coefficient v particle Reynolds number
CD v Re Chart
Calculate UT
Calculate the terminal velocity for aspirin particle in
assignment
3
d
4
P  f  P   f g
2
Calculate: CD Re P 
3
2
Then assume two different values of Rep and calculate CD
Draw this line on the chart above to get Rep at a particular
sphericity
Get UT from:
 f UT d p
Re P 

d = particle diameter
ρf = fluid density
ρp = particle density
g = acceleration due to gravity
μ = fluid viscosity
UT = terminal velocity
Storage of Solids
Main types are as follows:
Hoppers
Intermediate Bulk Containers (IBC)
Big Bag (FIBC)
Hoppers
A hopper is a straight walled vessel of circular or square cross section
with a tapered (conical or tapered square section) base
It is filled at the top and must be vented for both filling and emptying
Discharge is from the base, usually controlled by a sliding knife valve,
butterfly valve or rotary valve
Bridging can occur above the discharge
Hopper can be vibrated externally or aerated to encourage flow
Angle of sloping sides  angle of repose. A more scientific approach is
given in Ch 8, Introduction to Particle Technology, M Rhodes.
Aeration can be through a sintered metal membrane on the conical end
and this is fluidised with low pressure air to assist flow (< 0.5 bar)
Hoppers are often pressure rated (e.g. 10barg) to contain explosions
Mass and Core flow from a Hopper
Core flow occurs when
only material in the centre
flows from the hopper
All the powder in the
hopper is in motion with
Mass flow
Weighing a hopper
A hopper is weighed using load cells
Normally 3 load cells per hopper to create the three legged
stool effect
Electronics condition each of the three signals into one
reading
Weight can be tared on a regular basis
All connections to hopper must be flexible to ensure
support and weight rests on the load cells alone
Flexible connections must take pressure rating if applicable
Hopper Weighing
Load Cell
Three supports
Conveying solids
Simplest is gravity feed – factory must be tall (must think
about planning permission!)
Pneumatic conveying – particles are transported in a
stream of air
 Pulled through with a fan at the far end
 Pushed through with compressed air/nitrogen
 Pulled through with compressed air (eductor/ejector)
 Removed from conveying fluid at end with a cyclone
Mechanical conveying – widely used in mineral processing
 Screw conveyors – large range of sizes, small enough
for pharmachem
 Belt conveyors etc.
Dilute Phase Pneumatic Conveying
A pneumatic conveying system typically consists of the
following:
 A blower/vacuum pump at the start or end of the line
pushes or pulls air and material to/from the unloading
station through the system
 Material is separated from air at the end by a cyclone
 Solids fall out of cyclone
 A dust collector is installed downstream of the cyclone
to collect fine particles
A Supply System has the blower at the start pushing air
An Exhaust System has the blower at the end pulling air
Activity – Pneumatic Conveying
Have a go at designing a pneumatic conveying system
 Material is unloaded from an IBC on the ground floor
and conveyed to a hopper on the top floor
 A blower is used to pull the air through the system
 A dust collector is also needed to catch any fines
missed by the cyclone
 Use straight lines for pipes
 Cross off each piece or equipment and instrumentation
as you use it
 You should not be left with anything at the end!
Conveying Velocities
Rules of thumb for design velocities for gases and dusts
are as follows:
 5 to 6 m/s for vapours and gases
 10 to 13 m/s for very fine light dusts
 20 to 23 m/s for heavy dusts
These velocities are normally sufficient to prevent settling
of dusts in pipework
Designing ducting networks
There are four methods of designing ducting networks (ref.
Fan Handbook, FP Bleier)
 Static regain
 Equal friction per unit length of duct
 Velocity reduction
 Constant velocity
Static regain
 This method uses a constant duct diameter to supply an
equal volume of air to several branches
 Mainly applicable to HVAC
Equal Friction and Velocity Reduction
Equal Friction
 Calculates duct diameters and air velocities based on
equal friction per 30m/100ft of duct length
 Suitable for systems of moderate velocity (1000 to
2000 fpm, 5 to 10 m/s)
 Symmetrical branch layout
Velocity reduction
 Assume a velocity at the start
 Calculate the reduction in velocity through the system
 Duct area and pressure drop are then calculated using
these velocities
 Suitable for asymmetric systems with branches of
different lengths
Constant Velocity Method
Suitable for pneumatic conveying where high duct
velocities are needed to keep material entrained
Minimise velocity to minimise head loss due to friction
and attrition of particles, but velocity must be sufficient to
entrain material
Duct diameters and pressure drops are calculated based on
the velocity required to prevent settling (3000 to 7000 fpm,
15 to 35 m/s)
Total pressure drop is then determined – this must be
overcome by the static head generated by the fan
See worked example taken from ‘Introduction to Particle
Technology’ by Martin Rhodes
Design Considerations
Use long radius bends to minimise pressure drop
Pipe size should be a minimum of 10 times the particle
diameter
Vertical rise should be at the start of the piperun
Pneumatic Conveying system
Pharma grade polished
Stainless steel
Gravity feed
Material is dropped from one vessel into another
Number of steps/unit operations depends on the height of
the building
At the ground floor, material is stored in an IBC and
transported by lift to the top for further processing
Amount of material discharged by gravity is controlled
based on weight loss or weight gain
Butterfly valve or Rotary valve between stages to control
the flow
Load cells on hoppers/vessels to know when they’re full or
empty
Gravity feed control
Butterfly valve with
inflatable seal on the disc
helps containment of dust
Rotary valve driven by a
motor for a fixed
time/speed
M
Case Study – Gravity Feed of Toner
Toner is a marginally free flowing powder with an angle of
repose of between 30 and 60° depending on the process
step – pre classifier is 30° and classified (fine) is 60°
It falls into category of cohesion number 2 to 4
Experience showed an angle of 70° to be the best
Successful gravity feed is achieved using
 8” pipelines
 Incline of no less than 70° to the horizontal
 For pipes (implications for equipment location)
 And for hopper conical ends
 Assist discharge from hoppers/IBCs using fluidising air
at a low pressure (<0.5 bar)
Case study cont.
For inclined lines < 70° flow is assisted by gentle aeration
Flow through rotary valves should not be throttled by
reducers, i.e. 8” line for a 8” valve
Pneumatic conveying is done by vacuum so leakage is
prevented
Tall building to allow gravity feed through process steps –
this has implications for planning permission
Dust collectors contained in a canyon to absorb noise to
keep noise at site boundary within limit
Intermediate Bulk Container
An IBC is a mobile hopper moved by pallet/fork truck
Volume usually about 1m3, stainless steel
Not pressure rated
Can store material that is WIP waiting for further
processing
Conical base can have sintered membrane for fluidising air
Filled at top and placed on an IBC discharge station for
emptying. Vented when filling
Base of IBC has one half of a split butterfly valve
Other half of valve is on the discharge station
Valve can only open when two halves are joined
Valve must be closed before separation
IBC can be weighed using a platform weighing scale
IBC - Pictures
IBC
IBC unloading station
Split Butterfly Valve
Allows contained discharge of dry powders during transfer
Big Bag
Flexible IBC (FIBC)
Large bag, 1m3
Placed in a modified IBC before filling
Large loops at each corner for lifting
Emptied through a big bag discharge station
Manual process where bag is untied through a glove box
Neck of bag connected to short length of pipe and flow
controlled by a butterfly valve
Vibrated during discharge
Big bag filling
Big bag discharge
End of Part I