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
10mm 10mm Sphericity • Sphericity given by What is the sphericity of a cylinder? vp= volume of packing element (m3)Dp = characteristic length (diameter of cylinder) of element (m2)sp = surface area of packing element (m)
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 100m) • 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 (>100m) • More of a problem with fines
Categories of Cohesion • Powders can classified into six categories as follows (source = Pneumatic Conveyors for Bulk Materials, Gericke): • 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° • 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 cracksA tough material resists cracking, will deform plastically insteadOpposite is brittleCan sometimes make tough material brittle by reducing temperature • Cohesivity – degree to which particles stick togetherRelated to moisture content and particle sizeDecrease size or increase moisture => more cohesive • Bulk density = mass of particles/volume incl. voidage • ε = VoidageρB= bulk or bed densityρP= 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 • Calculate: • 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: 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
M Gravity feed control Rotary valve driven by a motor for a fixed time/speed • Butterfly valve with inflatable seal on the disc helps containment of dust
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 unloading station IBC
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
