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Downstream Processing. Introduction. Desired products are separated and purified after successful fermentation or enzyme reactions This step is up to 60% of total cost, not including the purchase of raw materials

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Desired products are separated and purified after successful fermentation or enzyme reactions

This step is up to 60% of total cost, not including the purchase of raw materials

Products may be dilute in aqueous medium, temperature sensitive, have multiple items to separate, and/or have properties similar to contaminants

Products may require high purification and homogeneity

solid liquid separations
Solid-Liquid Separations
  • Products may be biomass, extracellular, or intracellular
  • Purpose: to separate insolubles from fermentation broth
  • Cell sizes vary widely, from 1µm (bacteria)to 40µm (suspension plant cells)
  • The separation of particles by forcing the fluid through a filtering medium
  • Based on particle size and pressure drop
  • Filters are either based on pressure principle or vacuum
rate of filtration
Rate of Filtration

Rate of filtration: (dvf/dt) = ΔP · A/ µ(L/K)

A= area of filter

K= filter’s permeability

L= thickness of the filter cake

Filtration time (t)= M∞Pc/2ΔP(Vf/A)2

  • 2 types: tubular and disk
  • Disk used in continuous operations:
    • Has a short, wide bowl 8-20 inches in diameter which turns on a vertical axis
    • Cone-shaped disks with uniform spacing are placed inside the bowl to separate suspended solids

Terminal velocity= vt = dp2(ρs-ρ)a/18µ

  • Fermentation broth consists of dilute aqueous solution from which products have to be recovered and purified.
  • Overlaps exist between recovery and purification
  • Extraction and adsorption are exclusively classified as recovery
extraction liquid liquid separation
Extraction (liquid-liquid separation)
  • Separation of solutes from the feed by contact with another insoluble liquid (solvent)
  • The effectiveness of the solvent increased with:
    • optimum pH
    • addition of counterions (acetate or butyrate)
types of extractions
Types of Extractions
  • Single stage extraction- either in batch or continuous mode
    • Overall material balance for the mixer-settler is F + S = R + E and the material balance for the solute is FxF + Sys = Rx + Ey
  • Multistage extraction
    • Crosscurrent extraction- use continuously or in batch
    • Counter-current extraction- more efficient but cannot be operated in batch mode
adsorption physical and chemical
Adsorption (physical and chemical)
  • This is effective from separation of very dilute dissolved substances

1- Conventional adsorption

-based on intermolecular forces of attraction (Vander Waals force) between the molecules of solid and substance (activated carbon used to purify water)

- process is mixing of fruit pits and calcium chloride

adsorption continued
Adsorption (continued)

2- Ion Exchange

-components include: polymeric network, ionic functional group attached to network and counter ion

3- Affinity adsorption

involves chemical interactions between solute and ligand attached to surface of carrier particle

  • Methods include precipitation, chromatography, and electrophoresis, and membrane separation
  • Precipitation- Used for recovery of proteins and is induced by salt, organic solvent, or heat addition
  • Chromatography- include a mobile phase with solutes to be separated and a stationary phase which may be adsorbent, ion-exchange resin, gel, or porous solid packed in cylindrical column
  • Electrophoresis- separation of charged species according to migration rates in electrical field
purification continued
Purification (continued)
  • Membrane Separation
    • Microfiltration- separation of suspended material based on pore size of 0.02 to 10 µm
    • Ultrafiltration- if separation is achieved at the molecular level, most commonly 100 to 200 Å
    • Reverse osmosis- to separate suspended and dissolved material using high pressure to reverse flow across semipermeable membrane