By-product Utilisation Questions and Answers

Answer: A

Correct answer: 39.6 kg

Work through the two stages in turn:

\[ \text{husk} = 220\ \mathrm{kg}, \qquad \text{silica} = 220 \times 0.18 = 39.6\ \mathrm{kg} \]

The distractor 180 applies the percentage to the wrong quantity, taking 18 per cent of the paddy rather than of the husk. Note that not all of this silica is recoverable as useful ash: it must survive combustion in amorphous form, which requires the burning temperature to be held below about 700 degrees.

Answer: B

Correct answer: 7.0 GJ per hour

Multiply the firing rate by the calorific value:

\[ Q = 500 \times 14 = 7000\ \mathrm{MJ/h} = 7.0\ \mathrm{GJ/h} \]

Roughly two megawatts of heat, which is a useful output for a mill's own drying and steam requirements. The unit conversion is the only trap: a gigajoule is a thousand megajoules, and misplacing that factor gives either 70 or 0.7 GJ, both of which appear among the options.

Answer: C

Correct answer: 6.5

For a fixed mass, volume varies inversely with density, so the reduction factor is the ratio of the two densities:

\[ \frac{650}{100} = 6.5 \]

The same mass now occupies less than a sixth of the space. Since a lorry carrying loose husk fills its volume long before its weight limit, that factor translates almost directly into transport cost per tonne - which is what turns a residue usable only at the mill into one that can be sold at a distance.

Answer: D

Correct answer: 85.9 kg

The oil-free solids are conserved through extraction, so work from them:

\[ \text{oil in} = 500 \times 0.18 = 90\ \mathrm{kg}, \qquad \text{solids} = 410\ \mathrm{kg} \]

\[ \text{meal} = \frac{410}{0.99} = 414.1\ \mathrm{kg}, \qquad \text{oil left} = 4.1\ \mathrm{kg} \]

\[ \text{recovered} = 90 - 4.1 = 85.9\ \mathrm{kg} \]

The distractor 90 assumes complete extraction, which no process achieves; the residual one per cent is a real and normal loss.

Answer: A

Correct answer: 900 tonnes per day

Apply the yield fraction to the cane crushed:

\[ 3000 \times 0.30 = 900\ \mathrm{tonnes\ per\ day} \]

Nine hundred tonnes a day is a very large quantity of fuel, and it is why a sugar mill is normally self-sufficient in steam and power - often with a surplus to export. It also explains why bagasse is burnt rather than converted to anything more elaborate: it arises in quantities the mill can use immediately, at the point where the energy is needed.

Answer: B

Correct answer: 840 MJ per day

Multiply the gas volume by its calorific value:

\[ 40 \times 21 = 840\ \mathrm{MJ\ per\ day} \]

The 60 per cent methane figure is what gives the 21 MJ/m\(^3\); pure methane would be nearer 36, and the balance of the biogas is mostly carbon dioxide, which contributes nothing. Raising the methane fraction by scrubbing out the carbon dioxide is how biogas is upgraded to a substitute for natural gas.

Answer: C

Correct answer: 230 kg

Take the fermentable sugar, apply the theoretical yield, then the efficiency:

\[ \text{sugar} = 1000 \times 0.50 = 500\ \mathrm{kg} \]

\[ \text{theoretical} = 500 \times 0.511 = 255.5\ \mathrm{kg} \]

\[ \text{actual} = 255.5 \times 0.90 = 230\ \mathrm{kg} \]

The 0.511 figure is the stoichiometric maximum from the fermentation equation, the remainder of the sugar's mass leaving as carbon dioxide. No process reaches it, since some sugar goes to yeast growth and by-products.

Answer: D

Correct answer: 64 kg

Apply the percentage to the dry peel:

\[ 2000 \times 0.032 = 64\ \mathrm{kg} \]

Pectin is present at only a few per cent, but it is worth a great deal per kilogram as a gelling agent, which is what makes the extraction viable. The dry basis matters: fresh citrus peel is around 80 per cent water, so 2000 kg of dry peel represents something like ten tonnes of wet peel arriving at the plant.

Answer: A

Correct answer: 231 kg

Work through the two stages:

\[ \text{germ} = 5000 \times 0.11 = 550\ \mathrm{kg} \]

\[ \text{oil} = 550 \times 0.42 = 231\ \mathrm{kg} \]

The commonest error is to apply the second percentage to the maize rather than to the germ, which gives a figure several times too large. Note the cascade: the oil is under 5 per cent of the maize entering the mill, yet it is valuable enough that degermination is worth doing for its own sake as well as for shelf life.

Answer: B

Correct answer: 550

The margin is simply the difference between value and cost:

\[ 950 - 400 = 550\ \mathrm{per\ tonne} \]

This is the calculation that decides whether a by-product route is pursued at all. The collection and processing figure is where most schemes fail, since residues are bulky and often scattered - and a route that is comfortably profitable for material arising at a mill may lose money on the same material gathered from fields.