Drying Theory and Dryers Questions and Answers

Answer: C

Correct answer: Only three

Three pairs are right. Free moisture is the moisture above the equilibrium value; critical moisture is the value at which the constant rate period ends; and equilibrium moisture is that in balance with the surrounding air.

The fourth is wrong. Bound moisture is moisture held so firmly - chemically, or in fine capillaries and cell walls - that it exerts a vapour pressure below that of free water, so it is the hardest fraction to remove, not something any dryer can take out. In air of a given condition part of it cannot be removed at all.

Answer: D

Correct answer: Only three

Three pairs are right. Recirculating part of the exhaust air improves drying efficiency by recovering sensible heat that would otherwise be thrown away; recirculating the grain between passes evens out the final moisture; and tempering between passes raises head rice yield by letting internal gradients relax before the next pass.

The fourth is wrong. Raising the air velocity does little in the falling rate period, because the rate is limited by moisture movement inside the solid rather than by transfer across the boundary layer. It is during the constant rate period that velocity helps.

Answer: A

Correct answer: Equilibrium moisture content

Equilibrium moisture content is a property of the product - how much moisture that particular material holds when in balance with air of a given temperature and humidity. Two materials in the same air will settle at different values, and a sorption isotherm has to be measured for each.

The other three describe the air alone. Humidity ratio is the mass of vapour per unit mass of dry air, the dew point is the temperature at which that air would saturate on cooling, and the wet bulb temperature is set by the air's temperature and humidity together.

Answer: B

Correct answer: Internal diffusion coefficient

The internal diffusion coefficient governs the falling rate period, in which the surface is no longer continuously wet and the rate is set by how fast moisture can travel from the interior to the surface.

The other three all act on the external conditions and so govern the constant rate period: air velocity determines the boundary layer resistance, air temperature the sensible heat available, and air humidity the driving force for evaporation. This is the practical division worth remembering - external conditions early, internal transport late, and different remedies apply to each.

Answer: C

Correct answer: Fluidised bed dryer

A fluidised bed dryer handles solids - free-flowing granular material suspended in an upward air stream, such as grain, granules or crystals.

The other three all handle liquids or slurries. A spray dryer atomises a liquid feed into hot air, a drum dryer spreads a film of liquid or slurry on a heated surface, and a thin film evaporator concentrates a liquid as a film on a heated wall. The distinction is worth holding onto because it determines the whole architecture of the machine: a liquid feed needs atomising or spreading, a solid feed needs conveying or suspending.

Answer: D

Correct answer: Exhausting hot unsaturated air

Exhausting air that is still hot and still short of saturation throws away twice over: the sensible heat it carries, and the drying capacity it never used. This is the single largest loss in most hot air dryers.

The other three all raise efficiency. Recirculating part of the exhaust returns that sensible heat to the inlet; insulating the chamber cuts wall losses; and a heat exchanger on the exhaust recovers heat without returning the moisture. The design aim is to have the exhaust leave as cool and as near saturation as the process allows.

Answer: A

Correct answer: Drying rapidly in a single pass to the final moisture

Drying rapidly in one pass is what threatens head rice yield. The surface dries far faster than the interior, a steep moisture gradient builds within the kernel, and the resulting internal stresses fissure it. The fissures may be invisible in the whole grain but are planes of weakness, so the kernel breaks during milling.

The other three all protect yield. Tempering lets gradients relax between passes; limiting the temperature rise per pass keeps the gradient shallow in the first place; and recirculating the grain evens out moisture so no portion is over-dried.

Answer: B

Correct answer: Case hardening has sealed the surface

The pattern is characteristic of case hardening. Rapid initial drying dried the surface far faster than moisture could move out from the interior, and the shrunken outer layer formed a skin of low permeability. Drying then almost stops, because moisture can no longer reach the surface - yet the pieces remain soft inside, which is exactly what the question reports.

Saturated air or too low a velocity would have slowed drying from the beginning, not after a fast start. Reaching equilibrium moisture would leave the pieces uniformly dry, not soft inside.

Answer: C

Correct answer: The bed is in the falling rate period

Late in a batch the grain is in the falling rate period, where the rate is limited by how quickly moisture can move from inside each kernel to its surface. The air's job is only to carry away what arrives there, and doubling the fan speed does nothing to speed up internal diffusion.

Air velocity is effective earlier, during the constant rate period, when the surface is continuously wet and the boundary layer is the resistance. Neither a failed heater nor saturated air fits - both would have shown up earlier - and true equilibrium would stop drying altogether rather than merely slow it.

Answer: D

Correct answer: Reduce the bed depth or recirculate

A seven-point spread between bottom and top is the signature fault of a deep bed: air gives up heat and takes up moisture as it rises, so it reaches the upper layers cooler, more humid and nearly spent. Reducing the depth, or recirculating the grain so every kernel spends comparable time near the inlet, attacks the cause directly.

Raising the air temperature would over-dry the bottom further before helping the top. Increasing the depth makes the spread worse - it grows rapidly with depth - and reducing the air flow leaves even less drying potential for the upper layers.