CE20226 - Particle Tech Ming Flashcards
What’s particle technology?
The branch of science and engineering dealing with the production, handling, modification, and use of a wide variety of particulate materials, both wet or dry, in sizes ranging from nanometers to centimeters.
What are the (4 main) different unit operations for particle technology?
Size reduction Size enlargement Separation Mixing Storage, transport and dosage Particle characterisation (Size and shape analysis)
What are examples of size reduction, size enlargement, separation
and mixing processes?
Size reduction •Crushing •Grinding •Milling •Cutting / Slicing •Defibrating •Deagglomerating
Size enlargement •Agglomeration •Briquetting •Pelletizing •Coating •Compacting
Separating •Screening •Sieving •Grading •Sedimentation •Filtration
Mixing •Homogenizing •Stirring •Emulsifying •Spraying / Dispersing •Nebulizing / Pulverizing
What’s a dispersed system?
A system where particles or droplets are dispersed in a continuous phase.
Particle technology deals with particulate materials, bulk solids or powders, and particles or droplets that are contained within a gas or a liquid. Such particle collectives are named “Dispersed systems”.
Dispersed systems usually consist of many single particles - the dispersed phase - and the surrounding medium -the continuous phase. Both dispersed phase and the continuous phase can be solid, liquid or gaseous.
How are the volume concentrations and mass concentrations of the dispersed phase in a dispersed system found?
c. v = Vd / Vt
c. m = md / Vt
Where d represents the dispersed phase value.
How does specific surface area change with particle size?
Sv ~ 1/x
What are the properties of particles as their size is decreased?
specific surface area increases: SV ≈ 1/x
adhesive forces increase vs mass forces
increased tendency to agglomerate and stick to surfaces
pellets and agglomerates become more stable
free-flowing properties (flowablity) decreases
bulk-density decreases and porosity increases
mixing becomes more difficult but less self-segregation
reactivity increases
solubility, vapour-pressure and reaction rate increase
drag force increases vs mass force under flow conditions
electrostatic interactions increase
optical properties change (scattering, diffraction, reflection, absorption)
homogeneity of single particles increases: rigidity increases and grindablity decreases
What are the most important characteristics of a particle?
Size - affects properties such as SA:V and the rate at which a particle will settle in a fluid.
Shape - regular (e.g. spherical or cubical) or irregular (grains, corn flakes)
Composition - determines properties such as density and conductivity.
In many cases, the particle is not complete uniform. Particles might be porous or may be consist of a continuous matrix in which small particles of a second material are distributed.
What must be considered when choosing a method for particle size analysis?
Nature of the material to be sized, e.g. – Estimated particle size and particle size range – Solubility – Ease of handling – Toxicity – Flowability – Intended use
Cost
– Capital
– Running
Specification requirements
What are the 4 main particle size measurement techniques?
Sieve
Microscope
Sedimentation
Laser diffraction analysis
What are the features of sieving for particle size measurement?
Considers weight distribution
Sieve analysis is performed using a nest or stack of sieves where each lower sieve has a smaller aperture size than that of the sieve above it.
Sieves can be referred to either by their aperture size = mesh size = sieve number
The mesh size is the number of wires per linear inch.
– 250 μm = No. 60
– 125 μm = No. 120
Sieving may be performed wet or dry, by machine or by hand, or a fixed time or until powder passes through the sieve at a constant low rate.
Advantages and disadvantages of sieving:
Advantages
– Easy to perform
– Wide size range
– Inexpensive
Disadvantages – Known problems of reproducibility – Wear/damage in use or cleaning – Irregular/agglomerated particles – Rod-like particles : overestimate of under-size – Labour intensive
What are the features of microscopy for particle size measurement?
2 main types: optical and electron microscopes
Considers number distribution
Being able to examine each particle individually has led to microscopy being considered as an absolute measurement of particle size.
Can distinguish aggregates from single particles
Can be coupled to image analysis computers, each field can be examined, and a distribution obtained
Most severe limitation of optical microscopy is the depth of focus being about 10μm at x100 and only 0.5μm at x1000
Advantages and disadvantages of optical microscopy:
Advantages: – Relatively inexpensive – Each particle individually examined: detect aggregates, 2D shape, colour – Permanent record e.g., photograph – Small sample sizes required
Disadvantages:
– Time consuming - high operator fatigue
– Very low throughput (amount of material passing though)
– No information on 3D shape
– Certain amount of subjectivity
Advantages and disadvantages of electron microscopy:
Advantages:
– Particles are individually examined
– Visual means to see sub-micron specimens
– Particle shape can be measured
Disadvantages: – Very expensive – Time consuming sample preparation – Materials limitation – Low throughput - not for routine use
What are the features of sedimentation for particle size measurement?
Considers wight distribution, and different between fluid and particle density
These methods depend on the fact that the terminal velocity of a particle in a fluid increases with size.
2 categories:
Incremental: changes with time in the concentration or density of the suspension at known depths are determined.
Cumulative: the rate at which the powder is settling out of suspension is determined.
What’s stokes diameter?
The diameter of the sphere that would settle at the same rate as the particle.
Advantages and disadvantages of sedimentation:
Advantages:
– Equipment required can be relatively simple and inexpensive.
– Can measure a wide range of sizes with accuracy and reproducibility.
Disadvantages:
– Sedimentation analyses must be carried out at concentrations which are sufficiently low for interactive effects between particles to be negligible so that their terminal falling velocities can be taken as equal to those of isolated particles.
– Large particles create turbulence, are slowed and are recorded undersize.
– Particle re-aggregation during extended measurements.
– Particles have to be completely insoluble in the suspending liquid.
What are features of laser light scattering particle measurement?
Considers volume distribution.
Particles pass through a laser beam and the light scattered by them is collected over a range of angles in the forward direction.
The angles of diffraction are, in the simplest case inversely related to the particle size.
Advantages and disadvantages of laser light scattering for particle measurement:
Advantages:
– Non-intrusive : uses a low power laser beam
– Fast : typically <3minutes to take a measurement and analyse.
– Precise and wide range.
– Absolute measurement, no calibration is required.
– Simple to use
– Highly versatile
Disadvantages:
– expense
– volume measurement all other outputs are numerical transformations of this basic output form, assuming spherical particles
– must be a difference in refractive indices between particles and suspending medium
What are the different statistical diameters to identify the equivalent spherical diameter?
Martins’s Diameter: The distance between opposite sides of a particle are measured on a line bisecting the projected area.
Feret’s Diameter: The distance between parallel tangents on opposite sides of the particle profile.
Both Martin’s and Feret’s diameters are generally used for particle size analysis by optical and electron microscopy.
What are equivalent diameters?
Equivalent diameter is the diameter of a circle or a sphere with the same characteristics than the considered particle. This can be both geometric or physical equivalent diameters d.
Equivalent Circle Diameter: the diameter of a circle having an area equal to the projected area of the particle in random orientation.
Equivalent Spherical Diameter: the diameter of a sphere that has the same volume as the irregular particle being examined.
What are the different equivalent diameters?
- dV: Equivalent diameter of a sphere with the same volume
- dS: Equivalent diameter of a sphere with the same surface area
- dP: Equivalent diameter of a circle with the same projected surface area
- dPe: Equivalent diameter of circle with same circumference than particle image
- dW: Equivalent diameter of a sphere with the same terminal settling velocity
- dSt: “Stokes diameter” – Terminal setting velocity in the Stokes region
Equivalent Circle Diameter: the diameter of a circle having an area equal to the projected area of the particle in random orientation.
Equivalent Spherical Diameter: the diameter of a sphere that has the same volume as the irregular particle being examined.
How can particle shape be described?
Spherical Angular Irregular Flake Spheroidal Granular Geometric Etc
How can particle size be defined, considering the particle to be a sphere?
Assumption: 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) Equivalent diameter is the diameter of a sphere of equivalent volume.
2) Sphericity is given as:
Ø = SA of sphere of same vol as particle / SA of particle
= dv/ds
= 6Vp / (de*Sp)
Where:
Vp - particle volume
de - equivalent diameter
Sp - particle SA
How is particle sphericity calculated?
Ø = SA of sphere of same vol as particle / SA of particle
= dv/ds
= 6Vp / (de*Sp)
Where:
Vp - particle volume
de - equivalent diameter
Sp - particle SA
What is a monodisperse particle distribution?
When all particles are the same size
What is a polydisperse particle distribution?
When there is a wide distribution with particles of many sizes
What’s a distribution display (considering particle size)?
A graph of volume against particle size considering histograms, differential distributions and cumulative distributions of the particles in a sample.
What are the typical particle size distributions?
Normal distribution: particles resulting from growth processes e.g. natural products such as grains, crystallised products, granules agglomeration.
Log normal distribution: particles resulting from natural size reduction processes or industrial crushing e.g. sand, crushed rocks.
Rosin-Rammler distribution: fine and ultrafine industrial size reduction processes e.g. cement, pigments.
What are the 3 main dispersed systems?
Molecular dispersions
Colloidal dispersions
Coarse dispersions
What are properties of molecular dispersions?
- Particles invisible in electron microscope
- Pass through semipermeable membranes and filter paper
- Particles do not settle down on standing
- Undergo rapid diffusion
- E.g. ordinary ions, glucose
What are properties of colloidal dispersions?
- Particles not resolved by ordinary microscope, can be detected by electron microscope.
- Pass through filter paper but not pass through semipermeable membrane.
- Particles made to settle by centrifugation
- Diffuse very slowly
- E.g. colloidal silver sols, naural and synthetic polymers
What are properties of course dispersions?
- Particles are visible under ordinary microscope
- Do not pass through filter paper or semipermeable membrane.
- Particles settle down under gravity
- Do not diffuse
- E.g. emulsions, suspensions, red blood cells
What are colloids?
A homogeneous non-crystalline substance consisting of large molecules or ultramicroscopic particles of one substance dispersed through a second substance.
Colloids include gels, sols, and emulsions; the particles do not settle, and cannot be separated out by ordinary filtering or centrifuging like those in a suspension.
What are the 3 classifications of a dispersed system?
Hydrophilic colloidal dispersion (in water)
- surfactant micelles and phospholipid vesicles, also known as association colloids.
Lyophilic colloids (lyo=solvent) - colloidal systems are proteins, rubber, gelatin and gums.
Lyophobic colloids
- gold, silver and sulfur.
What are properties of the sizes and shapes of colloids?
Particles lying in the colloidal size have large surface area when compared with the surface area of an equal volume of larger particles.
The possession of large specific surface results in:
1- platinium is effective as catalyst only when found in colloidal form due to large surface area which adsorb reactant on their surface.
2- The colour of colloidal dispersion is related to the size of the paticles
e.g. red gold sol takes a blue colour when the particles increase in size.
The shape of colloidal particles in dispersion is important:
The more extended the particle, the greater its specific surface, the greater the attractive force between the particles of the dispersed phase and the dispersion medium.
Flow, sedimentation and osmotic pressure of the colloidal system affected by the shape of colloidal particles.
• Particle shape may also influence the pharmacologic action.
How are colloidal solutions purified?
1) Dialysis:
Semipermeable cellophane membrane prevent the passage of colloidal particles, yet allow the passage of small molecules or electrolytes.
2) Electrodialysis:
In the dialysis unit, the movement of ions across the membrane can be sped up by applying an electric current through the electrodes induced in the solution.
- The most important use of dialysis is the purification of blood in artificial kidney machines.
- The dialysis membrane allows small particles (ions) to pass through but the colloidal size particles (haemoglobin) do not pass through the membrane.
What’s the Faraday-Tyndall effect?
When a strong beam of light is passed through a colloidal solution, the path of light is illuminated (a visible cone formed).
- This phenomenon resulting from the scattering of light by the colloidal particles (doesn’t occur with pure solutions)
What are the optical properties of colloids?
When a strong beam of light is passed through a colloidal sol, the path of light is illuminated (a visible cone formed)
- light scattering depends on Tyndall effect. It’s described in terms of turbidity.
Ultra-microscope has declined in recent years as it does not able to resolve lyophilic colloids. Thus electron microscopes are capable of yielding pictures of actual particles size, shape and structure of colloidal particles.
What’s turbidity?
Turbidity: the fractional decrease in intensity due to scattering as the incident light passes through 1 cm of solution.
- Turbidity is proportional to the molecular weight of lyophilic colloid
