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 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 (mv 2 /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: 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  Q 2 Q is the gas flowrate Pressure drop usually of the order of 50 to 150 mm of H 2 O 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 0 0 Gas Flowrate, Q 0 ΔP, m of gas column Efficiency A B Optimum Operation Eff PP Theory Practice

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 From Rhodes (Introduction to Particle Technology) Down to 3 mm3 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 outlet Grinding fluid (compressed air) Fluid outlet Jets

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/m 3 ) Maximum concentration limits (particles/m 3 of air) for particles equal to and larger than the considered sizes shown below 0.1 um0.2 um0.3 um0.5 um1 um5 um ISO Class 1102 ISO Class ISO Class ISO Class ISO Class ISO Class ISO Class ISO Class 8 35E ISO Class 9 35E683E

Old classification Particle Counts/ft 3 Federal Standard Particle Counts/m 3 New Class (>0.5um) 209 E Class (>0.5um) 75000Class ISO Class Class ISO Class 7 675Class ISO Class 6 25Class ISO Class 5 7Class ISO Class 4 1Class 135 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, 5 th 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