Drying Theory and Dryers Questions and Answers

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. In freeze drying the product is frozen and the ice removed by sublimation under vacuum, so water passes directly from solid to vapour and no liquid phase ever exists within the product.

Because there is no liquid, there is no capillary shrinkage, no migration of dissolved solutes to the surface and no heat damage, so the porous structure, colour, flavour and volatile compounds are preserved far better than in hot air drying. The price is a long cycle and high capital and energy cost, which restricts it to high-value products.

Answer: B

Correct answer: Statement I is incorrect but Statement II is correct

Statement I is incorrect. Tempering does not lower the average moisture content - the grain is held without airflow, so there is nowhere for water to go and the average is unchanged.

Statement II is correct and states what tempering actually achieves: resting allows moisture gradients within each kernel to relax, so the surface rewets from the interior. The benefit is a more uniform kernel, less fissuring and a higher head rice yield, and a subsequent drying pass that can remove water from a wetter surface without over-drying it. The gain is in quality and in the effectiveness of the next pass, not in the average moisture.

Answer: C

Correct answer: 2, 1, 4, 3

The curve runs in this order. First an initial adjustment (2), in which the product warms - or cools - from its starting temperature until it reaches the wet bulb temperature of the air. Then the constant rate period (1), lasting as long as free water covers the surface and holding the product at the wet bulb temperature.

At the critical moisture the surface can no longer be kept wet, and the falling rate period (4) begins, now controlled by internal diffusion rather than by the air. Finally the rate tails away as the moisture approaches equilibrium with the air (3), which is the floor drying cannot pass.

Answer: D

Heating raises the air's capacity to carry moisture without changing what it already holds, and the product then supplies that moisture. Recirculating part of the exhaust recovers heat at the cost of a lower drying potential.

Answer: A

Correct answer: 2, 1, 4, 3

Residence time follows the scale of the material and the mechanism. A spray dryer (2) is the fastest, drying atomised droplets in seconds because their surface-to-mass ratio is enormous. A fluidised bed dryer (1) takes minutes, every particle being fully exposed to the air.

A deep bed grain dryer (4) takes hours, since the air must work its way through a static bed and moisture must diffuse out of whole kernels. A freeze dryer (3) is slowest of all, often a day or more, because sublimation under vacuum is inherently slow and heat must be supplied without melting the ice.

Answer: B

Correct answer: 1, 2, 3, 4

The chain runs from cause to commercial consequence. Drying too hard makes the surface dry far faster than the interior (1); that establishes a steep moisture gradient within the kernel (2).

Because the drier outer layer shrinks against the wetter core, internal stresses build until the kernel fissures (3). The fissures are often invisible in the whole grain, but they are planes of weakness, so the kernel breaks during milling and the head rice yield - the proportion recovered as whole grains, which commands the premium - falls (4). Tempering between passes interrupts the chain at step 2.

Answer: C

Heating moves the state horizontally and drying moves it along a constant wet bulb line, which is why the chart makes the whole process visible at a glance. The moisture picked up is read as the change in humidity ratio.

Answer: D

Correct answer: Only three

Three pairs are right. A spray dryer suits liquids and slurries, which it atomises into hot air; a fluidised bed dryer suits free-flowing granular material that can be suspended in an upward air stream; and a freeze dryer suits high-value heat-sensitive products, whose structure and flavour it preserves.

The fourth is wrong. A drum dryer handles liquids and slurries, spreading a thin film on a heated drum - not whole grains in deep beds, which need a through-flow air dryer. A drum could not process a granular bed at all.

Answer: A

Correct answer: Only three

Three pairs are right. The humidity ratio is the mass of water vapour per unit mass of dry air; relative humidity is the actual vapour pressure over the saturation pressure at the same temperature; and the dew point is the temperature at which saturation occurs on cooling at constant moisture content.

The fourth is wrong. The wet bulb depression is the difference between the dry bulb and wet bulb temperatures, not the dry bulb temperature itself. It is the practical measure of how far the air is from saturation and hence of its drying potential.

Answer: B

Correct answer: Only three

Three pairs are right. The constant rate period is controlled by the external conditions of the air, and during it the surface sits at the wet bulb temperature - both true, and in fact two aspects of the same situation, since the wet bulb temperature is where evaporative cooling balances convective heating. The falling rate period is controlled by internal moisture movement.

The fourth is wrong. Air velocity governs the boundary layer and so matters during the constant rate period, but in the falling rate period the bottleneck is inside the solid, and raising the velocity gives little further benefit.