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29 Cards in this Set

  • Front
  • Back

Powders

are solid particles (generally heterogenous systems)


They are widely used in pharmaceuticals:


1) as powders


2) as intermediates leading to other dosage forms

clarification

process aiming at removing or separating a solid from a fluid


-remove any unwanted particles from a liquid product


-to collect the solid as the product itself

Mechanisms of filtration

-straining/sieving


-impingement


-attractive forces


-auto filtration

Key parameters of filtering elements

-capacity


-efficiency


-degree of filtration


-porosity

Filtration: Buchner funnel

image

Rateof filtration: the Darcy’s equation

V/t = K A ^ P / u L


V/t - rate of filtration


K-proportionality constant


A-area


^P - pressure difference


u- viscosity


L - thickness

Waysto increase the filtration rate

• Increase the area


• Increase ΔP


• Decrease the viscosity ofthe liquid


• Decrease the thickness of thefilter cake


• Increase the permeability of thecake

Centrifugation

Particleof mass m


Spinningin a centrifuge of radius r


Ata velocity v


Thecentrifugal force F = mv2/r


C=F/G

Milling

is a mechanical method that gives you a reduction in particle size

Cutter mill

the material is cut by one or more blades


Feed -> stationary knives outside -> rotating knives inside -> screen -> product


Screen has pores that only allow certain size material through


Good for fibrous material

Compressionmethods


compression mills

A pressure is applied


e.g. roller mills, mortar pestle


1 roll is driven directly


1 roll rotates as consequence of the friction


Good for crystalline material

Impactmethods

Principle


a) Particles are hit by a moving surface


b) moving particles hit a surface

Hammer mill

Same as cutter mill but instead of blades, it has hammers


Feed -> stationary hammers outside -> rotating hammers inside -> screen -> product

Attrition methods

Principle:


Pressure and friction

Attrition methods


Roller mills

The rollers rotate at different speeds


Each cylinder is attached to anengine and rotate independently of the others, all at different speeds.


Semi solid material e.g. ointment

CombinedImpact and Attrition methods:


Ball mill



Chamber filled with balls, then addpowders that you want to reduces in size. The particles get hit by the ball andbroken by attrition,2 factors that affect theefficiency1.Amountof material – too much material the probability of the material hitting theballs decreases, it will hit itself (cushioning effect). Too little the ballswill hit themselves and premature wear of the equipment.2.Speed– the ball are subjected to gravity. As the mill pushes them up, gravity pushesthem down

Combined Impact and Attrition methods:


Ball mill (2)

Centrifugaleffect push the balls outwards


Lowspeed the balls will be at the bottom of the mill (not desirable)


Highspeed (above critical angular velocity- the speed at which the centrfugalforces over come gravity) ball on the outside of the mill (not desirable)


Thebest milling speed is a 2/3’s of the angular forces


Fluidenergy mill is a solid sample is inserted under high pressure into a liquid

Combined Impact and Attrition methods:


Fluid energy mill

Solid inlet -> fluid inlet jets -> centrifuging action throws coarser particles outwards -> classifier remove fine particles and fluid

Selectionof the appropriate type of mill


-Look at the size that you want toobtain


-Look at characteristics ofthe material

-particle size to obtain


-characteristics of the material:


Cutter mills -> elastic, fibrous materials


Attrition methods -> ointments, solid in suspensions and pastes


Impact methods -> brittle materials


-other factors (cost, time, stability of the ingredients)

Size analysis


Projected area diameter (da)


Projected perimeter diameter (dp)

We approximate that particles havea diameter equivalent to a sphere of a for a different material


We draw a circle that has an areathat is the same at the particle


Wedraw a circle that has the same perimeter as the particle and measure thediameter

Statisticaldiameters:


Feret’s (dF)


Martin’s (dM)

Feret’s –first you chose a direction and then draw to tangent that are parallel to thechosen direction, and then measure the distance between the two particles. Thenrepeated but chose a different direction. You repeat this a few time andcalculate the average


Martin’s – dived the particle intotwo identical ideas. You repeat many times but with the particle orientateddifferently and then calculate the average.

Methods to measure the particle size

Direct methods:


1) sieving 2) microscopy


Indirect methods: they determine a parameter correlated with size


examples: - sedimentation rate -permeability

Sieve methods

Sievesare classified based on the sieve aperture diameter (expressed in µm).

Sieveaperture diameter

distancebetween two consecutive wires

light microscopy

size range 1-1000 um


Adrop of a very dilutedsuspensionis placed on a microscope slide and particles are measured in relation to the equivalent diameter chosen (dp, da, dF or dM).Theimages are 2D

Electron microscopy

Size range (as low as 0.001um)


Scanningelectron microscopy (SEM):


Advantages-3D images, gives information on shape


Disadvantages– expensive, high level of operator expertise

Coulter counter

Measure the volume of particles - 0.1-1000um


Container with a small hole. Insidethe container you have a electron and outside you have another electrodeThe suspension is suspended in theelectrolyte solution. The vacuum line sucks some of the particles of the hole,it will displace an equivalent volume of the electrolyte solution and changesin the resistance between the two electrodes

Laserlight scattering methods

Principle:laser light interacts with particlesLightis diffracted by particles by an angle that is inversely proportional to thevolume of the particles.Adetector analyses the radiation diffracted by the particles


Heand Nelaser are the most widely used laser

Stokes equation

Vst = h/t =d2 0s-0f Fg / 18n


h-distance


t-time


0s-density of particle


0f-density of the fluid


n-viscosity


Fg -gravity acceleration