Drying Theory and Dryers Questions and Answers

Answer: C

It is fitted to experimental data with expressions such as the Page or Henderson equations and becomes the building block for deep bed simulation. A deep bed is treated as a stack of thin layers, each seeing the air leaving the one below.

Answer: D

Correct answer: Cooler and more humid than the air entering

As air passes through the lower layers of a deep bed it gives up sensible heat to evaporate water and picks up that water as vapour. It therefore leaves those layers cooler and more humid than it entered - its drying potential partly spent.

This is the fundamental limit on bed depth. By the time the air reaches the top of a deep bed it may be near saturation and able to remove almost nothing, so the upper layers dry very slowly while the lower ones may be over-dried. Practical designs limit the depth, reverse the airflow, or recirculate the grain.

Answer: A

Correct answer: Achieve high, uniform transfer rates

Fluidising the bed suspends each particle in the air stream, so every surface is exposed to the drying air and the particles mix vigorously. Heat and mass transfer coefficients are therefore high and, just as importantly, uniform - there are no stagnant pockets and no steep gradient through the bed as there is in a deep static bed.

The price is a substantially higher air flow, since the velocity must exceed the minimum fluidisation velocity; it is not reduced. Air must still be heated, and although fluidisation does classify particles somewhat, that is a side effect and not the purpose.

Answer: B

Correct answer: Drying liquids to a powder in seconds

A spray dryer atomises a liquid feed into a fine mist within a chamber of hot air. Because the droplets are tiny, their surface area per unit mass is enormous and drying is complete in seconds, producing a free-flowing powder collected at the base and in a cyclone.

The short residence time is what allows heat-sensitive materials such as milk, coffee extract and egg to be dried at high air temperatures - evaporative cooling keeps the droplet near the wet bulb temperature until it is nearly dry. It cannot handle solids such as grain, fodder or fruit slices.

Answer: C

Correct answer: Spreading a film on a heated drum

A drum dryer applies a thin film of liquid or slurry to the surface of an internally steam-heated rotating drum. The film dries by conduction as the drum turns, and a doctor blade scrapes off the dried sheet, which is then broken into flakes or powder.

It suits pastes and slurries too viscous to atomise, and it is thermally efficient because heat passes directly into the film. Atomising into hot air describes a spray dryer, freezing and subliming describes a freeze dryer, and blowing air through a bed describes a fluidised or deep bed dryer.

Answer: D

The product is frozen and the pressure lowered below the triple point so the ice passes straight to vapour. Structure and flavour are preserved better than by any other method, at a cost that restricts it to high-value products.

Answer: A

Correct answer: It evens out moisture between passes

A recirculating batch dryer passes the grain repeatedly through a drying section and back into a holding column. Each kernel therefore spends roughly the same total time in the hot zone, and the moisture differences that a single pass through a deep bed would leave are progressively evened out.

The rest periods in the holding section also let moisture gradients within each kernel relax, which reduces fissuring and protects head rice yield. The arrangement still needs a fan, heated air and a plenum; what it removes is the non-uniformity of a static deep bed.

Answer: B

Correct answer: Let moisture gradients relax

Tempering is a rest period between drying passes, with the grain held without airflow. During drying the surface dries much faster than the centre, leaving a steep internal moisture gradient; resting lets moisture migrate outward until the kernel is more uniform, so the next pass can remove water from a wetter surface without over-drying it.

The benefit is a marked reduction in stress cracking, and hence a higher head rice yield after milling. Tempering does not change the average moisture - it only redistributes it - and it neither cleans nor cools the grain.

Answer: C

Correct answer: Heat used in evaporation over heat supplied

Drying efficiency is the fraction of the heat supplied that actually goes into evaporating water:

\[ \eta = \frac{m_w h_{fg}}{Q_{supplied}} \times 100 \]

The remainder is lost in the exhaust air as sensible heat, in heating the product itself, and through the structure. Typical hot air dryers achieve somewhere between 40 and 60 per cent, which is why exhaust recirculation and heat recovery are worth doing. Moisture removed over moisture present is a measure of drying extent, not of energy use.

Answer: D

Correct answer: 2.3 MJ per kg

The latent heat of vaporisation of water is about \( 2.26 \) MJ/kg at 100 degrees Celsius and a little higher at typical drying temperatures, so \( 2.3 \) MJ/kg is the working figure.

It is worth carrying in mind because it sets the theoretical floor on drying energy: removing a tonne of water needs at least \( 2.3 \) GJ however good the dryer, and real specific energy consumption of \( 3 \) to \( 5 \) MJ per kg of water reflects the losses on top of that. The value \( 4.2 \) is the specific heat of liquid water in kJ/kg per degree, a different quantity.