Natural products: sugar, starch and pulp and paper

Why it matters

Sugar, starch and paper are large agro-based industries in India, and each is a chain of unit operations an engineer already knows: size reduction, leaching, clarification, multiple-effect evaporation, crystallisation, centrifugation, filtration and drying. Their economics depend on recovering by-products and chemicals (bagasse and molasses, corn steep liquor, kraft cooking chemicals), which is exactly where process engineering adds value.

Key ideas

Cane sugar. Sucrose (C₁₂H₂₂O₁₁) is extracted from sugarcane (about 10–14 % sucrose).

1. Milling: cane is cut, shredded and passed through a tandem of 4–6 three-roller mills. Water or thin juice is added before the last mills (imbibition) to leach remaining sugar. The fibrous residue, bagasse, is the boiler fuel and a pulp raw material.
2. Clarification: raw juice (about 13–16 °Brix) is heated and treated with lime to raise pH to about 7 and precipitate impurities. Indian plantation-white sugar mills use double sulphitation (lime + SO₂ for juice, SO₂ again for syrup, which also bleaches). Refineries use carbonation (lime + CO₂). Mud is settled in clarifiers and filtered.
3. Evaporation: multiple-effect evaporators (4–5 effects, exhaust steam to the first effect) concentrate juice to syrup of about 60–65 °Brix. Steam economy rises with the number of effects.
4. Crystallisation: in vacuum pans at about 60–70 °C to avoid inversion and colour; massecuite is cooled in crystallisers. A three-boiling (A, B, C) scheme recovers as much sucrose as possible.
5. Centrifugation and drying: crystals are separated from mother liquor; the final exhausted mother liquor is molasses, the feedstock for ethanol distilleries.

Key measures: Brix (mass % dissolved solids), Pol (apparent sucrose by polarimetry) and purity = Pol / Brix × 100. Sucrose inverts (hydrolyses) to glucose + fructose at low pH and high temperature, which is why pH and temperature are controlled throughout.

Starch (corn wet milling).

1. Steeping: maize is soaked about 30–50 h at about 50 °C in water containing 0.1–0.2 % SO₂, which softens the kernel and breaks the protein matrix. The steep water is concentrated as corn steep liquor (a fermentation nutrient).
2. Degermination: coarse grinding frees the germ, separated in hydrocyclones (it gives corn oil).
3. Fine grinding and fibre separation on screens.
4. Starch–gluten separation: centrifuges and hydrocyclones use the higher density of starch; gluten is an animal feed.
5. Washing, dewatering and flash drying of the starch. Starch is a mixture of amylose (linear α-1,4-glucose, about 20–30 %) and amylopectin (branched, with α-1,6 links). It is converted to glucose syrups by acid or, now mostly, enzymatic hydrolysis (α-amylase liquefaction, then glucoamylase saccharification). Dextrose equivalent (DE) measures the extent of hydrolysis. Each anhydroglucose unit (162.14 g/mol) gains one water to give glucose (180.16 g/mol).

Pulp and paper. Wood is about 40–50 % cellulose, 20–30 % hemicellulose and 20–30 % lignin (the binder that also causes yellowing).

• Mechanical pulping: grinding; yield over 90 % but lignin remains, giving weak, yellowing paper (newsprint).
• Chemical pulping: kraft (sulphate) process, the dominant route: chips are cooked in white liquor (NaOH + Na₂S) at about 160–170 °C for 1–2 h; yield about 45–50 %, strong pulp. The sulphite process uses bisulphite and gives a lighter, weaker pulp.
• Kraft recovery cycle: spent black liquor is concentrated in multiple-effect evaporators, burnt in a recovery boiler (organics give steam; Na₂SO₄ make-up is reduced to Na₂S), the smelt dissolves to green liquor (Na₂CO₃ + Na₂S), causticising with lime converts Na₂CO₃ to NaOH, and the lime mud is re-burnt in a lime kiln. Recovery of chemicals is above 95 %.
• Bleaching: multi-stage, now elemental-chlorine-free (ClO₂, O₂, H₂O₂). Residual lignin is measured by the kappa number.
• Paper making: stock preparation (beating/refining fibrillates fibres for bonding), then the Fourdrinier or twin-wire machine: forming on a moving wire, pressing, steam-cylinder drying, calendering.

Formulas

Purity (%) = Pol / Brix × 100

Water evaporated = F·(1 − B_F / B_P) (solids balance)

• F: juice feed (kg/h), B_F and B_P: Brix of feed and product.

(C₆H₁₀O₅)ₙ + n H₂O → n C₆H₁₂O₆ (starch hydrolysis; 162.14 g gives 180.16 g) C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose) (inversion)

Kraft liquor (all as Na₂O, g/L): Active alkali (AA) = NaOH + Na₂S Effective alkali (EA) = NaOH + ½ Na₂S Sulphidity (%) = Na₂S / (NaOH + Na₂S) × 100 Conversions: NaOH × 61.98 / 80.00 and Na₂S × 61.98 / 78.04 give Na₂O equivalents. Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃ (causticising)

Worked examples

Example 1 (standard): juice concentration. Clear juice of 15 °Brix and 13 Pol is fed at 100 t/h to an evaporator set producing 60 °Brix syrup. Find the purity and the water evaporated.

1. Purity = 13 / 15 × 100 = 86.7 %.
2. Solids = 0.15 × 100 = 15 t/h; syrup = 15 / 0.60 = 25 t/h.
3. Water evaporated = 100 − 25 = 75 t/h.

Answer: purity 86.7 %; 75 t/h of water evaporated.

Example 2 (GATE level): white liquor analysis. Kraft white liquor contains 90 g/L NaOH and 35 g/L Na₂S. Express as Na₂O and find active alkali, effective alkali and sulphidity.

1. NaOH as Na₂O = 90 × 61.98 / 80.00 = 69.73 g/L.
2. Na₂S as Na₂O = 35 × 61.98 / 78.04 = 27.80 g/L.
3. AA = 69.73 + 27.80 = 97.53 g/L as Na₂O.
4. EA = 69.73 + 27.80 / 2 = 83.63 g/L as Na₂O.
5. Sulphidity = 27.80 / 97.53 × 100 = 28.5 %.

Answer: AA 97.5 g/L, EA 83.6 g/L (as Na₂O), sulphidity 28.5 %.

Common mistakes

• Saying sucrose hydrolyses to two glucose molecules; inversion gives glucose and fructose.
• Forgetting the water added on starch hydrolysis (100 g starch gives about 111 g glucose, not 100 g).
• Mixing NaOH and Na₂O bases in kraft liquor calculations.
• Equating pulp yield with cellulose content: kraft pulp also keeps some hemicellulose and loses some cellulose.
• Treating Brix as sucrose content; Brix counts all dissolved solids.

For GATE CH

Expect process-sequence MCQs (milling → clarification → evaporation → crystallisation; steeping → degermination → separation; kraft cooking and recovery), purposes of individual steps (imbibition, sulphitation, SO₂ in steeping, causticising), and by-products (bagasse, molasses, corn steep liquor, black liquor). Numericals: Brix/purity balances, evaporator water removal, starch-to-glucose yield, and kraft liquor definitions.

Quick check

1. What is imbibition?
2. Why is SO₂ added to steep water?
3. Name the two active chemicals in kraft white liquor.
4. What is the final mother liquor of sugar crystallisation called and what is it used for?

Answers: 1. adding water to bagasse between mills to leach remaining sugar; 2. to soften the kernel, loosen the protein matrix and control microbes; 3. NaOH and Na₂S; 4. molasses, used for ethanol (and yeast, cattle feed).