By-product Utilisation Questions and Answers

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason explains the dominance. The sugars in molasses are sucrose, glucose and fructose, all directly fermentable, so the process needs only dilution and nutrient adjustment.

Every alternative feedstock requires the sugar to be released first - saccharification for starch, pretreatment and enzymatic hydrolysis for lignocellulose - and those stages carry most of the cost. Molasses also arises concentrated at the sugar factory, so collection costs nothing, which reinforces the advantage.

Answer: B

Correct answer: 1 and 2 only

Statement 1 is correct: rice husk is 15 to 20 per cent silica, far more than any other common residue. Statement 2 is correct: the ash is pozzolanic provided the husk is burnt below about 700 degrees, so the silica stays amorphous.

Statement 3 is false. Husk is almost entirely indigestible and contains very little protein; the silica that makes it valuable to the cement industry is exactly what makes it useless and abrasive as a feed.

Answer: C

Correct answer: 1 and 2 only

Statement 1 is correct: briquetting raises the bulk density several fold. Statement 2 is correct: because transport cost per tonne is governed by volume for a low-density material, that density increase is what makes distant use economic.

Statement 3 is false. The calorific value per kilogram is essentially unchanged, since compaction alters no chemistry. What rises is the energy per cubic metre - the same energy in less space, which is a quite different claim.

Answer: D

Correct answer: 1 and 2 only

Statement 1 is correct: digestion works in water, so a wet residue is no obstacle where drying for combustion would consume more energy than the material contains. Statement 2 is correct: biogas is typically around 60 per cent methane, the balance mostly carbon dioxide.

Statement 3 is false. The digestate is a useful product, not a waste - it retains the nitrogen, phosphorus and potassium of the feedstock in a stabilised, largely deodorised form and is applied as fertiliser.

Answer: A

Correct answer: 1 and 2 only

Statement 1 is correct: removing the oil concentrates everything else, so cake typically runs to 40 per cent protein or more. Statement 2 is correct: several cakes carry antinutritional factors - gossypol in cottonseed, glucosinolates in rapeseed - that must be removed or bound.

Statement 3 is false, and inverts the effect. Over-treatment damages protein quality: excessive heat causes the Maillard reaction, which ties up lysine and reduces digestibility. The treatment must be sufficient but no more.

Answer: B

Correct answer: A-2, B-1, C-4, D-3

Each residue is matched to its principal outlet. Rice husk (A) goes to boiler fuel and pozzolanic ash (2); rice bran (B) to edible oil and defatted feed (1).

Molasses (C) goes to ethanol and feed supplement (4), and citrus peel (D) to pectin (3). Note the pattern: each outlet exploits the one constituent present in unusual concentration - silica, oil, fermentable sugar and pectin respectively - which is the general rule for by-product utilisation.

Answer: C

Correct answer: A-2, B-1, C-4, D-3

Each route is matched to its product. Anaerobic digestion (A) yields biogas and a fertiliser digestate (2); gasification (B) yields producer gas (1) by partial oxidation.

Fermentation (C) yields ethanol (4), and briquetting (D) yields a denser solid fuel (3). The first three are conversions that change the material chemically; briquetting alone is purely physical, which is why its product is still recognisably the residue it started as.

Answer: D

Correct answer: A-2, B-1, C-4, D-3

Each by-product is matched to the constituent that gives it value. Rice husk ash (A) is valued for its amorphous silica (2); oilseed cake (B) for its protein (1), concentrated by the removal of the oil.

Wheat bran (C) is valued for its dietary fibre (4), and maize germ (D) for its oil (3), which is about a third of the germ's mass. In every case the outlet follows from whichever constituent happens to be unusually concentrated in that fraction.

Answer: A

Correct answer: A-2, B-1, C-4, D-3

Each difficulty is matched to the residue it affects. Very low bulk density (A) afflicts loose rice husk (2), at around 100 kg/m\(^3\). A rapid rise in free fatty acids (B) afflicts rice bran (1), once milling releases lipase onto the oil.

Antinutritional factors (C) afflict cottonseed and rapeseed cake (4) - gossypol and glucosinolates. Very high moisture (D) afflicts fruit and vegetable peel (3), which is why those go to digestion rather than combustion.

Answer: B

Correct answer: Molasses

Molasses comes from sugar manufacture, not rice milling - it is the syrup left when no more sucrose can economically be crystallised from cane or beet juice.

The other three all arise in a rice mill: husk from hulling at about 20 per cent of the paddy, bran from whitening at 8 to 10 per cent and carrying the oil, and broken rice from grading, sold at a discount to whole grain or used for starch, flour and brewing. Each has its own outlet, which is what makes a rice mill's residue stream so well developed.