Transcript Powder Technology part I (ppt file 496kb)
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
Sphericity given by 6
v p D p s p
v p = volume of packing element (m 3 ) D p = characteristic length (diameter of cylinder) of element (m 2 ) s p = surface area of packing element (m) What is the sphericity of a cylinder?
10mm 10mm 0 .
01 2 6 0 .
0 .
01 2 01 2 2 4 0 .
01 0 .
01 4 .
7
E
06 0 .
01 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.
2.
3.
4.
5.
Product can be suspended in air and flows as freely as a liquid; Free flowing product, angle of repose, a , being such that 0 < a 30° Product flows normally, angle of repose, a , being such that 30° < a 45° Product is slow flowing, angle of repose, a , being such that 45° < a 60° 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 ρ B = bulk or bed density ρ P = particle density
Particle Terminal Velocity
Terminal velocity is determined by using a chart of drag coefficient v particle Reynolds number
C
D
v Re Chart
Calculate U
T Calculate the terminal velocity for aspirin particle in assignment Calculate:
C D
Re 2
P
4 3
d P
3
f
P
2
f
g
Then assume two different values of Re p and calculate C D Draw this line on the chart above to get Re p sphericity at a particular Get U T from: d = particle diameter ρ f = fluid density ρ U p = particle density g = acceleration due to gravity μ = fluid viscosity T = terminal velocity Re
P
f U T
d p
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 1m 3 , 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, 1m 3 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