ICAR AICE-JRF/SRF Mechanical Unit Operations: Size Reduction, Mixing & Separation: practice questions

1 · Peeling methods · medium

Compared with abrasion peeling, flash steam peeling generally gives

1. higher losses and more wet waste
2. a raw surface with no heating at all
3. the same losses but uses sodium hydroxide
4. lower losses and less effluent

2 · Sieving and particle-size analysis · hard

A sample contains 3 particles of 10 µm, 2 of 20 µm and 1 of 30 µm. Its Sauter mean diameter d32 is closest to

1. 23.0 µm
2. 16.7 µm
3. 20.0 µm
4. 25.2 µm

3 · Sieving and particle-size analysis · medium

Laser diffraction particle sizing differs from sieve analysis in that it

1. measures particle mass directly by weighing each particle on a balance
2. typically cannot be used for emulsions or for dry powders
3. always gives the same result as sieving
4. gives volume-based distributions with fine resolution below 50 µm

4 · Settling and sedimentation · medium

The capacity of a gravity settling tank to remove particles of a given size depends mainly on its

1. depth
2. colour
3. wall thickness
4. surface area

5 · Membrane processes: microfiltration, ultrafiltration, nanofiltration and reverse osmosis · hard

In reverse osmosis, transmembrane pressure is 4.0 MPa, osmotic pressure difference 2.0 MPa, permeate viscosity 0.001 Pa s and total resistance 2 × 10¹³ m⁻¹. Using J = (ΔP − Δπ)/(μRt), the flux is closest to

1. 720 L/m² h
2. 0.10 L/m² h
3. 360 L/m² h
4. 1080 L/m² h

6 · Membrane processes: microfiltration, ultrafiltration, nanofiltration and reverse osmosis · hard

Diafiltration in whey protein production means

1. adding water to the retentate and filtering again to wash out lactose and salts
2. filtering the permeate a second time to recover more of the protein lost through the membrane
3. heating the retentate to denature the proteins
4. drying the retentate in a spray dryer

7 · Membrane processes: microfiltration, ultrafiltration, nanofiltration and reverse osmosis · hard

Reverse osmosis cannot concentrate fruit juice much beyond about 25–30 % solids mainly because

1. RO membranes start to pass sugars freely once the juice exceeds 20 % solids
2. RO heats the juice and caramelises sugars
3. juice proteins block all membranes immediately
4. osmotic pressure rises with concentration and opposes the applied pressure

8 · Crystallisation · hard

100 kg of water saturated with lactose at 60 °C (solubility 59 g per 100 g water) is cooled to 20 °C (solubility 19 g per 100 g water). Ignoring water of crystallisation, the lactose that can crystallise is about

1. 19 kg
2. 59 kg
3. 78 kg
4. 40 kg

9 · Crystallisation · medium

High supersaturation during crystallisation tends to produce

1. few large crystals
2. no crystals
3. only amorphous glass
4. many small crystals

10 · Distillation and supercritical fluid extraction · medium

Small amounts of ethanol are added as a co-solvent to supercritical CO₂ mainly to

1. lower the critical pressure of CO₂ to atmospheric
2. make the extract alcoholic for sale
3. extend extraction to more polar compounds
4. prevent any extraction of lipids