unit operations

What are unit operations?

Operations involved in processing materials , such as drying, centrifugation, heating, distillation and filtration

What are the two main aspects of unit operations?

• Mechanical engineering: equipment design, stresses, strains and mechanical integrity

• Process/Chemical engineering: how processes function, mathematical analysis, process design and equipment performance

What are the main categories of particle and powder operation?

• Particle production: crystallisation, comminution and drying

• Particle separation: sieving, sedimentation + filtration

• Fluidised bed processes

• Powder storage, handling+ processing: mixing, conveying, granulation + tabletting

What is a powder?

A collection of smaller sized solid particles

What are the main individual particle properties?

• Particle size and size distribution

• particle shape

• particle density + porosity

• surface properties including electrostatic charge und Von per walls forces

What are the main bulk powder properties?

• Bulk density+ bed porosity

• compressibility

• flowability

• permeability

• wettability+ dispersibility

• fire + explosion properties

What is a representative example?

A portion of the whole material selected so that it accurately represents the properties of the entire bulk.

Why is representative sampling important?

Only a small quantity is usually tested. If the sample is not representative, the results may be misleading and cannot reliably describe the whole powder

How con representative sampling be improved?

• Take samples from different locations throughout the bulk

• take as many small samples as practical

• combine and mix samples where appropriate

• Avoid sampling methods that preferentially collect certain particle sizes

Why might scooping powder from the top of a bag give a misleading result?

Particles can segregate according to size or density. For example, fine particles may accumulate towards the bottom, making the top sample unrepresentative

Why is particle size important?

It affects flowability, filtration, fluidisation, separation, drying, rehydration compressibility, toxicity and explosibility

What is the particle size conundrum?

Particles are three dimensional, but particle size is usually expressed using one- dimensional measurements. Different particle shapes make it difficult to define one meaningful size.

What is the equivalent spherical diameter concept?

The diameter of a sphere that has the same value of a selected property as the actual irregular -shaped particle.

Common types of equivalent spherical diameter

Diameter Property

Mass Mass

Volume Volume

Surface area Surface area

Sedimentation Terminal settling velocity

Sieve Sieve aperture through which it passes

Projected area Projected 2D area

What is on equivalent spherical volume diameter?

The diameter of a sphere having the same volume as the actual particle

What is an equivalent projected area diameter?

The diameter of a sphere having the same two- dimensional projected area as the particle when viewed under a microscope

Why can different particle sizing techniques give different results for the same powder?

Because different techniques measure different particle properties and therefore different equivalent spherical diameters

What are the three main types of particle size distribution?

• Number fraction PSD

• Surface area fraction PSD

• Mass (or volume) fraction PSD

How is a mass fraction psd obtained using sieve analysis

A known mass of powder is passed through a stack of sieves. The mass retained on each sieve is measured and divided by the original total mass.

What is the difference between a number psd and a mass psd

• number psd: proportion of the total number of particles within each size

• Mass psd: proportion of the total particle mass within each size range

Why does a number psd usually appear further to the left than a mass psd?

There are usually many more small particles than large particles, but larger particles contribute disproportionately more than mass.

For spherical particles:

M = np (pid³/6)

What is the relationship between mass + volume psds?

If particle density is constant across particle sizes, mass and volume fraction distributions are the same

Why must you compare like psds?

Number, mass and surface area psds can be very different for the some powder. Comparing different types can produce misleading conclusions

What is a cumulative particle size distribution?

A graph of cumulative fraction F (x) against particle size x.

It represents the fraction of particles (by number, mass or surface area) with a size less than or equal to x

What are the characteristics of a cumulative psd?

• Starts at 0

• increases towards 1

• The value at any particle size represents the fraction at or below that size

What is a continuous ( differential) psd?

A graph showing the rate of change of the cumulative distribution with particle size

f(x) = dF(x)/dx

The total area under the continuous distribution curve equals 1

What is the relationship between cumulative and continuous PSD’s?

Differentiate the cumulative distribution to obtain the cumulative curve.

Integrate the continuous distribution to obtain the cumulative distribution

F(x) = intergrsk from x to 0 f(x) dx

What is the median particle size?

The particle size that occurs most frequently. On a continuous psd, it is the size at which the curve reaches its maximum

What is the arithmetic mean particle size?

The average particle size, calculated by:

X= integral infinity to 0 xf(x) dx

It can be thought of as the centre of gravity of the distribution

What is the relationship between mean, median and made for a normal distribution?

Mean = median = mode

For an asymmetrical distribution, these values may differ

What are the common particle size D notations?

Notation. Meaning

(4,3) Arithmetic mean value of volume PSD

(3,2) Arithmetic mean value of surface area, Sauter mean

(1,0) Arithmetic mean of number PSD

(v,0.5). Median is volume PSD

What as did, D 10, D 50 and D90 mean?

D10: 10% of the distribution is at or below this time

D50: 50%of the distribution is at or below this time

D90: 90% of the distribution is at or below this time

What are variance and standard deviation?

What are the main particle size measurement techniques and what is they measure?

Microscopy: principle, advantage, disadvantage

Principle: visual measures particles using image analysis

Advantage: also provides particle shape and morphology

Limitation: requires sufficient representative particles to be measured

Sieve analysis: principle, advantage, disadvantage

Principle: separates particles according to sieve aperture

Advantage: cheap and suitable for large particles

Limitations: unsuitable for particles below approx 40 $\mu m$ and can struggle with cohesive powders

Sedimentation: principle s advantage, disadvantage

Principle: uses stokes law to calculate particle size from settling velocity. Requires a density difference and knowledge of liquid viscosity

Disadvantage: gravity sedimentation can be slow: centrifugal sedimentation is faster

Coulter counter

measures changes in electrical conductivity as particles pass through an orifice. The change is related to particle volume. Produces a number psd

Laser diffraction

Measures light scattering from particles. Smaller particle scatter light at larger angles. Produces a volume psd and is fast , reproducible and versatile

What is moisture content on a wet basis

MC wb = (Mass of water / total mass of material ) x100

What is moisture content on a dry basic?

MC Db = (Mass of water / mass of dry solids)x 100

How is moisture content commonly measured using on oven?

1. Weigh the empty dish

2. Add and weigh the sample

3. Heat an approximate 102-110 degrees

4. Allow water to evaporate

5. Reweigh the dried simple

6. Calculate the water mass from the difference

What are two potential problems with oven moisture measurement?

• Some water may remain trapped or strongly bond within the material

• excessive heat my cause decomposition or release water through chemical changes.

What is the Karl Fischer method used for?

A chemical method for accurately measuring low moisture contents, particularly below 1%

What are the three main density definitions for powders?

Density. Definition

Solid density. Mass/volume of solid material

Particle density. Mass/volume of particles

Bulk density. Mass/volume of powder bed

What is the difference between interparticle and intro particle voids?

• Interparticle voids: spaces between individual particles

• intraparticle voids: pores within individual particles.

What are interparticle, intraparticle and total porosity?

What happens to bulk density when a powder bed becomes more compact?

Bulk density increases because the same mass occupies a smaller bed volume.

What is poured bulk density?

The mass of powder divided by its volume after being poured into a graduated cylinder without tapping

What is tapped bulk density?

The mass of powder divided by its volume after the cylinder has been tapped a specific number of times

What is the hausner ratio and what does it indicate?

HR = tapped bulk density / poured bulk density

It is a crude indicator of powder flowability

Why does a cohesive powder generally have a higher hausner ratio?

Cohesive particles form lumps and trap air , resulting in a large power volume

Tapping breaks up lumps and releases air, reducing the volume and increasing tapped density

What is gas pycnometry?

A technique that measures particle volume using gas displacement. The mass divided by the displaced volume gives on apparent density.

How does pore accessibility affect gas pycnometry measurements?

Solid particles: gives solid and particle density.

Closed pores: gas cannot enter, so they contribute to measured particle volume

Open pores: gas enters, so they are excluded from displaced volume, giving solid density

Partially accessible pores: gives a density between solid and particle density.