Sugar and Fermentation Industries Questions and Answers

Answer: C

Correct answer: centrifuging

A centrifugal basket with a perforated screen spins the massecuite so that the mother liquor is thrown through the screen and the crystals are retained, then washed with a little water or steam. The separation is mechanical and takes minutes, which is why a factory can process the enormous throughput a cane crop demands.

Answer: D

Salts and invert sugar depress the recovery so that further boiling costs more than the sugar is worth. The molasses is sold instead for fermentation or as animal feed. Exhaustion of the molasses has an economic limit.

Answer: A

Clarified juice is an intermediate stream inside the process, not something that leaves the factory. The other three all leave as products or by-products in their own right. Juice is an intermediate on the way to syrup.

Answer: B

The first two are correct. The third is wrong, since beet has no such fibrous residue and beet factories must buy their fuel, which is one of the economic differences between the two crops. Beet pulp is sold as animal feed instead.

Answer: C

Saccharomyces converts each mole of glucose to two of ethanol and two of carbon dioxide. Oxygen would divert the yeast to respiration and give cells rather than ethanol. The fermenter is sealed against air for that reason.

Answer: D

Correct answer: about 51 per cent

From the Gay-Lussac equation \[ \mathrm{C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2}, \] the masses are 180 for glucose and \( 2\times46 = 92 \) for the ethanol, giving \[ \frac{92}{180} = 0.511 = 51\%. \] The remaining 49 per cent leaves as carbon dioxide, and a real fermentation reaches about 90 to 95 per cent of this theoretical figure because some substrate goes to cell mass and by-products.

Answer: A

Both are true and the reason gives the accounting: the carbon dioxide is a genuine co-product and is increasingly captured for beverage or industrial use. Half the substrate mass leaves as gas. Distillers increasingly sell that gas as a product.

Answer: B

Correct answer: distillation, then dehydration

Fermentation gives a broth of perhaps 10 to 15 per cent ethanol, which distillation concentrates to about 95 per cent by volume. Getting beyond that requires a further step - azeotropic or extractive distillation, or more usually molecular sieve adsorption - for the reason the next item gives.

Answer: C

Correct answer: False

Ethanol and water form a minimum-boiling azeotrope at about 95.6 per cent ethanol by mass, and at that composition the vapour has the same composition as the liquid, so no number of distillation stages will separate them further. Anhydrous ethanol therefore needs a method that does not rely on a vapour-liquid difference - adsorption on molecular sieves being the modern choice.

Answer: D

Correct answer: sulphuric acid

Sulphuric acid is made by the contact process, burning sulphur and oxidising the dioxide over a vanadium catalyst - a purely chemical route with no organism involved. Citric acid, penicillin and industrial ethanol are all fermentation products, and between them they cover the three main classes: an organic acid, an antibiotic and a solvent or fuel.