Sugar Milling & Boiling-House Control Comprehensive Notes

Fundamental Mill Balance Equation

• Overall material balance for milling:
  • $\text{CANE} + \text{IMBIBITION WATER} = \text{MIXED JUICE (MJ)} + \text{BAGASSE (B)}$
  • Imbibition water (W) increases the liquid phase, improving sucrose leaching.
  • Expressed on % cane basis: $100 + W = J + B$

• Extraction definition
  • % of sucrose recovered in MJ relative to sucrose originally in cane.
  • High extraction requires: good cane preparation, proper mill setting, optimum hydraulic pressure, effective imbibition, sound equipment condition.

• Usual sources of weight (or sucrose) loss
  • Cane desiccation before milling
  • Evaporation of imbibition water and juice at high-temperature rolls
  • Sampling/handling losses of bagasse

Key Milling-Performance Indices

• Pol % Bagasse
  • $\text{Pol\%Bagasse}$ = sucrose left in bagasse; lower → better extraction.
  • Requires accurate laboratory bagasse analysis.

• Pol Extraction %
  • $\text{Pol extraction\%}= \dfrac{\text{weight of pol in MJ}}{\text{weight of pol in cane}} \times 100$
  • Alternate form after pol balance: $=\dfrac{\text{pol\% cane} - \text{pol in bagasse\% cane}}{\text{pol\% cane}} \times 100$

• Reduced Extraction (%) – normalises performance to a standard fibre basis (12.5 % fibre) so mills with different cane qualities are comparable.

  • Deerr’s postulate: juice (sucrose) loss per 100 parts fibre is constant.
  • Deerr’s formula:
  $E_{12.5}=100-\frac{(100-e)(100-f)}{7f}$
  where $e$=actual pol extraction %, $f$=fibre % cane.

  • Mittal’s formula:
  $E_{12.5}=100\times\Bigg[1-\frac{12.5(1-e)}{f}\Bigg]$
  – uses same standard fibre but simpler algebra.

• Milling Loss %
  • Measures sucrose retained per unit fibre in bagasse.
  $\text{Milling Loss\%}=\Bigg[\frac{\text{Pol\% bagasse}}{\text{Fibre\% bagasse}}\Bigg]\times100$
  • Lower → better; quick monitoring tool requiring only bagasse results.

• Mittal’s Whole Reduced Extraction (WRE)
  • $\text{WRE}=100-\text{Milling Loss}$
  • Reflects total recovery normalized to 12.5 % fibre.

• Extraction Ratio %
  • Loss indicator per unit fibre in cane:
  $\text{Extraction ratio}=\frac{100-e}{f}\times100$
  • Lower value → higher efficiency.

• Absolute Juice in Bagasse % Fibre
  • $=\frac{(100-e)(100-f)}{f}$
  • Basis for Deerr’s reduction; estimates real juice trapped in fibre.

Practical Notes on the Indices

• A good efficiency formula should:
  • be independent of cane quality,
  • rely on easily measured data,
  • quantify sucrose retained in bagasse,
  • support routine lab control.

• "Pol % Bagasse" alone is fibre-dependent → misleading when comparing varieties or seasons; reduced extraction solves this.

Sample-Problem Highlights

• Problem 3 (Factory A & B)
– Data led to $e\approx95.04\,\%$, $E<em>{12.5}\,(\text{Deerr})\approx94.42\,\%$, $E</em>{12.5}\,(\text{Mittal})\approx94.50\,\%$, $\text{WRE}\approx93.57\,\%$.
– Similar computations let managers benchmark the two factories directly.

• Problem 4
– Cane wt $=271\,439\,\text{t}$, fibre $=14.10\,\%$, bagasse wt $=82\,852\,\text{t}$.
– Key results: $e=91.85\,\%$; $E<em>{12.5}(\text{Deerr})=92.91\,\%$; $E</em>{12.5}(\text{Mittal})=92.77\,\%$; Extraction ratio $=6.34\,\%$; Milling loss $=6.34\,\%$; $\text{WRE}=93.66\,\%$.
– Mass-balance outputs: Absolute-juice wt $=233\,166\,\text{t}$; $Brix<em>{AJ}=15.52\,\%$; $Pol</em>{AJ}=12.78\,\%$; Absolute-juice extraction $=78.65\,\%$; Imbibition water $=16.58\,\%$ cane; Dilution $=7.40\,\%$ cane.

Wet-Milling Test & Cane-Preparation Index (PI)

• Wet-milling test: routine control; MJ & bagasse sampled at each mill, WRE computed mill-by-mill to spot irregularities.

• Preparation Index assesses cell rupture in shredded cane.
  • If water-to-sample ratio identical in tumbler & cold digester:
  $PI = 100\times\frac{\text{Tumbler conc.}}{\text{Digester conc.}}$
  • $PI=100\,\%$ → perfect preparation; typical well-prepared cane $75!–!90\,\%$.
  • Better PI → higher extraction, less bagasse pol, lower energy.

Boiling-House Overview (Lecture 5)

• Considered the “heart” of raw-sugar manufacture; chain of:
  • Clarification & Filtration
  • Evaporation
  • Crystallisation
  • Centrifugation / Purging

• Overall objective: remove impurities early, prevent sucrose losses, & maximise recoverable sugar.

Clarification Station

• Process flow: mixed juice → liming → heating → clarifier → clear juice (top) & muddy juice (bottom).

• Goals: coagulate impurities, adjust pH (6.8–7.2), reach Kopke clarity ≥ 24 mm, turbidity ≤ 150 NTU.

• Liming details
  • Lime types: quicklime $\text{CaO}$, hydrated lime $\text{Ca(OH)}<em>2$, milk of lime ≈ 5 °Bé. • Recommended dosage: $0.6!–!2.0\,\text{lb CaO t}^{-1}</em>{cane}$ (≈0.3–1 kg t⁻¹).
  • Quality specs: ≥ 65 % available CaO, settling ≥ 30 min, Mg + Al ≤ 2 %.
  • $P<em>2O</em>5$ in juice should be ≥ 200–300 ppm.

• Alternative purification chemistries
  • Carbonatation: excess lime then $CO<em>2$ neutralisation. • Sulphitation: excess lime then $SO</em>2$.
  • Phosphatation: phosphoric acid before liming.
  • Magnesia addition (rare; usually avoided).

• Fundamental clarification balance: total solids, pol & water must close between MJ, milk-of-lime, filter cake, filtrate, wash water etc.

Rotary Vacuum Filter (RVF) Operation

• Drum zones:
  • Low vacuum 6–12 in Hg – mud deposition.
  • High vacuum 15–20 in Hg – wash & de-watering.
  • Zero vacuum – cake discharge to conveyor.

• Mud-cake specs: moisture 60–75 %, pol 1.5–2 % (Phil. limit ≤ 2 %).

• Good practice: minimise pol in mud without over-washing (water economy).

Evaporation Station

• Objective: drive juice to 60–70°Bx (theoretical limit 72–75°Bx).

• Principle: condensing live steam (higher $T$) transfers latent heat → juice boils, releasing vapour at lower $T$.

• Sucrose-loss mechanisms: caramelisation, inversion (high $T$, low pH), entrainment of droplets.
  – High vacuum, high evaporation rate & fine droplets exacerbate entrainment.

• % Evaporation (sample calc):
  $\%\,\text{evap}=\frac{\text{Water removed}}{\text{Water in feed}}\times100 = \frac{66.77-15.18}{66.77}\times100\approx77.3\,\%$ (sample key ≈78 %).

Crystallisation & Centrifugation

• Goal: maximise sugar recovery; produce well-exhausted final molasses.

• Key concepts
  • Constant sucrose-deposition rate ∝ crystal surface → larger surface → lower mother-liquor purity.
  • Massecuite cooled to 41–45 °C; lower → viscosity ↑, purging difficult; higher → crystals re-dissolve.

• Purging losses: fine crystals through screen, dissolution during basket wash, airborne carry-over.

Final-Molasses Exhaustion

• RS/Ash ratio
  $RS/Ash = \frac{\%\,\text{reducing sugar}}{\%\,\text{ash}}$
  • If RS/Ash>1.5 → good exhaustibility; <1.5 → poor.

• Target-purity empirical formula (Hugot)
  $\text{Expected purity}=40-4\times\frac{100}{RS/Ash}$

• Exhaustion of Final Molasses (EFM)
  $EFM=\frac{\text{Pur}<em>{Mo}-\text{Pur}</em>{M1}}{\text{Pur}<em>{S}}\times100$ • $\text{Pur}</em>{Mo}$ actual molasses, $\text{Pur}<em>{M1}$ fully exhausted, $\text{Pur}</em>{S}$ sugar produced.

• Molasses Factor
  $=\frac{\text{pol lost in molasses}}{\text{non-pol entering in MJ}}$
  – Indicator of overall boiling-house recovery.

Purity & Solids Relationships (terminology)

• Apparent Purity (AP): $\text{pol}/\text{brix} \times100$.

• Refractometer Pol Purity: $\text{Pol}/\text{Ref solids} \times100$.

• True Purity: $\text{sucrose}/\text{dry substance} \times100$.

• Gravity Purity: $\text{sucrose}/\text{brix} \times100$.

Fundamental Station Balances (summary)

• Clarification, evaporation & boiling-house each obey total-mass, solids, pol & water balances; solving those gives unknown flows such as clarified-juice tonnage, syrup tonnage, % evaporation, etc.

• Example page 14 shows step-wise solution for missing data (bagasse %, fibre %, MoL tonnage, clarified-juice wt, etc.).

Practical / Ethical / Economic Considerations

• Every 0.1 % loss of sucrose can translate to major revenue loss for a mill.

• Efficient extraction reduces fuel (bagasse) consumption for boilers, lowering environmental footprint.

• Good clarification & filtration also cut downstream chemical/water use and produce cleaner effluent (mud used as fertiliser).

Quick Reference Formula List

• Mill balance: $100+W=J+B$

• Pol extraction %: $\dfrac{\text{pol in MJ}}{\text{pol in cane}}\times100$

• Pol in bagasse % cane: $\dfrac{B\,\%<em>{cane}\times Pol\%</em>{bag}}{100}$

• Deerr’s $E_{12.5}$: $100-\dfrac{(100-e)(100-f)}{7f}$

• Mittal’s $E_{12.5}$: $100\Big[1-\dfrac{12.5(1-e)}{f}\Big]$

• Milling loss %: $\dfrac{Pol\%<em>{bag}}{Fibre\%</em>{bag}}\times100$

• WRE: $100-\text{Milling Loss}$

• Extraction ratio: $\dfrac{100-e}{f}\times100$

• Absolute juice in bagasse % fibre: $\dfrac{(100-e)(100-f)}{f}$

• Preparation Index: $100\times\dfrac{Tumbler~conc.}{Digester~conc.}$

• RS/Ash ratio, Hugot target purity, EFM & molasses factor as given above.