Solid–Liquid Extraction & Expression – Comprehensive Study Notes

Overview

• The lecture covers two unit operations: Solid–Liquid Extraction and Expression.
• Flow of content:
  • Introduction to mass transfer between phases.
  • Sequential stages that govern extraction.
  • Variables that influence extraction rate.
  • Equipment designs (single-stage, multi-stage static bed, continuous moving bed).
  • Commercial food‐industry uses (sugar, oils, coffee, tea, etc.).
  • Expression as a separate but related operation (pressing liquids out of solids).
  • Expression equipment (hydraulic, cage, roller, screw, belt presses).
  • Industrial applications of expression.

Objectives

• Define and explain:
  • $\text{Solid–liquid extraction (leaching)}$
  • $\text{Expression}$
• Describe the physicochemical stages that occur during extraction.
• Discuss how surface area, concentration gradient, temperature, and flow velocity alter extraction kinetics.
• Identify construction & operating principles of extraction and expression hardware.
• Link equipment choice to food-industry cases (beet sugar, instant coffee, olive oil, etc.).

Introduction to Mass Transfer & Phase Contact

• Mass transfer = movement of a chemical species from one phase to another because of a driving force (usually a concentration gradient).
• Phases in food processes: solid, liquid, gas.
• Key property differences exploited:
  • Volatility ➔ migration to gaseous phase.
  • Solubility ➔ migration to liquid phase.
• When phases mix intimately, solutes redistribute until equilibrium is reached.
  • Separation strategies aim to interrupt contact before full equilibrium so that one stream is enriched.

Solid–Liquid Extraction: Definition & Principles

• Purpose: Separate a soluble constituent (solute) from an insoluble solid matrix by contacting with a liquid (solvent).
• Output streams per stage:
  • Overflow (miscella): clear liquid containing solvent + dissolved solute.
  • Underflow (raffinate/residue): depleted solid + adhering solution.
• A stage = one unit of equipment in which contact, hold-up, and mechanical separation occur.
• Stage efficiency: attained when solution retained on solids has the same composition as overflow; real stages are less than 100 % efficient.
  • Everyday analogy: “Mitchell’s Plain tea bag.”

Stages of the Extraction Process

1. Solvent penetrates solid, solute dissolves.
2. Dissolved solute diffuses through pore liquid to particle surface.
3. Solute migrates from particle surface to bulk solvent.

• Rate can be limited by any single step; often external film diffusion or internal pore diffusion dominates.

Factors Affecting Rate of Extraction

• Interfacial Area
  • $R \propto A$ (rate proportional to solid–liquid contact area).
  • Reducing particle size doubles rate by: (i) larger surface, (ii) shorter internal diffusion path.
  • Caution: too-fine powders hinder percolation (hydraulic resistance → channeling).
• Concentration Gradient (Driving Force)
  • Selective solvent should dissolve solute strongly while leaving matrix intact.
  • Low viscosity ➔ good circulation.
  • Counter-current arrangements sustain a high gradient even when solute concentration overall is low.
  • Counter-current concept: fresh solvent meets nearly exhausted solids; rich miscella contacts fresh solids, minimizing loss.
• Temperature
  • Higher $T$ increases solubility & diffusivity: $D \uparrow, \; S \uparrow$.
  • Limited by heat-sensitive nutrients, flavors, enzymes.
• Solvent Flow Rate / Turbulence
  • Velocity ↑ ⇒ boundary-layer thickness ↓ ⇒ mass-transfer coefficient $k_L$ ↑.
  • Achieved via pumping, sparging, or mechanical agitation.

Extraction Equipment

Single-Stage (Percolation) Extractors

• Open tank with false bottom.
• Solvent sprinkled over fixed bed; percolates by gravity.
• Overflow collected, possibly heated & recycled.
• For volatile solvents, condensers & recovery systems added.

Multi-Stage Static-Bed Extractors

• Series of single-stage cells.
• Overflow of cell $n$ feeds cell $n-1$ (counter-current path).
• Solids remain static; solvent inlet position shifts over time so every cell experiences fresh solvent periodically.
• Isolation capability ⇒ cleaning / refilling without stopping line.

Continuous Moving-Bed Extractors

• Rotating or horizontally moving baskets.
• Provide both co-current and counter-current sections.
• Example schematic (page 14): flakes enter wet-flake hopper → contact with pure solvent → miscella strengthens progressively → heavy miscella leaves for evaporation.

Food-Industry Applications of Solid–Liquid Extraction

• Sugar from sugar beet
  • Sliced beets, water solvent, multi-stage static beds (diffusion towers).
• Sugar cane
  • Primary juice via roller presses (expression).
  • Remaining soluble solids leached with water.
• Edible oils
  • Hexane / heptane extraction of soybean, peanut, sunflower, fish livers, cocoa nibs.
• Instant coffee
  • Roasted, ground beans extracted with $90–100\,^{\circ}\text{C}$ water; miscella 25–30 % solids; counter-current columns.
• Instant tea
  • Hot-water extraction of blended leaves; 2.5–5.0 % solids.
• Supercritical CO\textsubscript{2}
  • Decaffeination of coffee, hop-resin concentration.
  • SC-CO$_2$ behaves like both gas & liquid: high diffusivity, low viscosity, easy separation, non-flammable.
• Additional: perfumes (floral oils), hydrolysed yeast clarification, grape pomace sugar, oil from wheat germ/rice/coconut.

Expression: Definition & Underlying Principles

• Unit operation that separates liquids from solids via compressive forces.
• Common in fruits, vegetables, seeds.
• Often preceded by pretreatments:
  • Pulping / maceration (break cell walls, minimize bitterness or astringency).
  • Thermal conditioning (denature enzymes, soften structure, e.g., sugar cane).
• Two fundamental steps:
  1. Comminution to expose juices/oils.
  2. Mechanical pressing to expel liquid.

Expression Equipment

Hydraulic Press

• Rigid frame; plates form closed cavity.
• Feed placed between filter cloths; ram forces plate assembly; liquor exits ports into tray.
• Batch; high pressure >!20\,\text{MPa}; slow but gentle (quality juice).

Cage Press

• Perforated cylindrical cage + movable platen.
• “Cake” formed; strings aid discharge.
• Common for small oilseed lots, specialty olive oil.

Roller Press

• Two or three heavy rolls crush fibrous stalks (sugar cane).
• Juice drains into tray; bagasse discharged.

Screw (Expeller) Press

• Helical screw with decreasing pitch along perforated barrel.
• Pressure builds toward tapered outlet; liquids drain; de-oiled cake exits.
• Continuous; moderate shear heat.

Belt Press

• Continuous feed between two woven belts that pass over rollers increasing in pressure.
• Large area, gentle, but expensive and sanitation-challenging.

Food-Industry Applications of Expression

• Hydraulic presses: premium fruit juices, cider.
• Screw presses:
  • Finishers post-pectolytic enzyme treatment (clarify citrus juice).
  • Extraction of edible oils from oilseeds.
• Roller presses: first-stage cane juice extraction.
• Continuous screw presses: grape juice separation for wine (replacing basket presses).
• Belt presses: high-volume fruit–vegetable juice plants; where clarity is critical.

Comparative Summary: Extraction vs. Expression

• Driving force: concentration gradient vs. mechanical pressure.
• Preferred when solute is easily soluble but entrapped vs. when liquid already present but physically trapped within matrix.
• Often combined (e.g.
  • Sugar cane ➔ roller pressing + hot-water extraction.
  • Oilseed ➔ expeller press + solvent extraction to get residual oil).

Key Equations & Quantitative Pointers

• Mass-transfer rate (external film limited):
  $N<em>A = k</em>L A (C<em>s - C</em>b)$
  where $N<em>A$ = molar flux, $k</em>L$ = liquid-film coefficient, $A$ = interfacial area, $C<em>s$ surface concentration, $C</em>b$ bulk concentration.
• Diffusion through particle (Fick’s law):
  $J<em>A = -D</em>{eff} \frac{\partial C}{\partial x}$
• Overall extraction efficiency (single stage):
  $\eta = \frac{C<em>{overflow} - C</em>{inlet\,solvent}}{C<em>{solid\,init} - C</em>{inlet\,solvent}}$
• Mechanical expression yield (simplified):
  $Y = \frac{m<em>{liquid\,recovered}}{m</em>{feed}} \times 100\%$

Ethical, Practical & Quality Considerations

• Solvent safety: hexane is flammable & poses residue limits; SC-CO$_2$ preferred where cost permits.
• Temperature–flavor balance: high temps accelerate extraction but degrade aromatics (tea, coffee) and nutrients (vitamin C).
• Fine grind enhances yield but complicates filtration/clarification; optimisation required.
• Pressing pressure/time chosen to avoid excessive phenolic extraction (wine tannins, olive oil bitterness).
• Waste valorisation: spent solids become animal feed, biofuel, compost.

Connections to Other Lectures & Future Topics

• Builds on diffusion theory (previous mass-transfer lecture).
• Precedes upcoming module on Maize & its products (will revisit extraction for corn oil & steepwater).
• Ties into drying & evaporation because miscella or juices usually undergo downstream concentration.