Solvent Extraction 

Overview

  • Isolates and concentrates the products
  • Partially purifies the product by removing similar chemical species
  • Based on the solubility differences of the compound in one phase relative to the other
  • Transfer of a solute from one phase to another
    • liquid-liquid
    • solid-liquid
  • Solute is distributed or partitioned between the solvents
  • Increase in solute in one phase
    • Depletion of solute from the second phase

Ideal Solvent

  • Nontoxic
  • Selective
  • Inexpensive
  • Immiscible with feed

Equations

  • K = distribution coefficient or partition coefficient
  • y = concentration of solute or product in solvent
  • x = concentration of solute or product in raffinate
    • residual feed after extraction with solvent

Partition Coefficient K

  • based on partitioning of solute between two immiscible solvents
    • citric acid is more soluble in methyl amyl ketone than H20 at pH 4.0
    • Penicillin dissolved readily in amyl acetate than H20 at pH 5.5
    • Catalase has higher conc. in polyethylene glycol-rich solution that dextran-rich solution
  • for lowest volume of extract to be used, K must be very large
  • If K <= 1
    • large volumes of solvent and multiple extraction will be required to recover the product
  • If K = 0
    • extraction is impossible

Factors

  • molecular size of solute

  • pH

  • types of solvent

  • temperature

  • For aqueous two-phase extraction:

    • concentration of polymers

    • molecular weight of polymers

Determining best solvent

  • no reliable thermodynamic theory to predict the best choice of solvent
    • rely on solubility parameters

Examples

  • Change solute ion pairs
    • Lactic acid (hydroxypropionic acid, (CH3CHOHCOOH) production
    • Convert lactic acid to calcium lactate which is insoluble in water and therefore can be easily recovered
  • Change in solute pH
    • eg Novobiocin at pH 7
    • the K value in butylacetate is 100
    • at pH 10.5 the K value is 0.01

Batch Extraction

Equations

Steps

  • Perform solute mass balance around the extractor
  • Assume solvent and feed are immiscible
  • Equilibrium
  • Dilute solution

Continuous Extraction

Assumptions

  • The solvent and feed are immiscible
  • The solute concentration is sufficiently low that the flowrates of raffinate and extract are constant
  • The streams leaving each stage are in equilibrium
  • The second assumption is true if the concentrations of the bioproducts are low eg 10 g/l but often it is less than 1 g/l

Design of solvent extractors

  • In the chemical industry there are several types of extractors, but in \n the biotechnology industry there are only a few extractors. The two \n common extractors used in the biotech industry are: \n • Agitated extraction columns (reciprocating-plate extraction column) \n • Centrifugal extractors (used mostly for antibiotics extraction) \n • For column extraction design, the Height Equivalent to Theoretical \n stage (HETS) is commonly used or the overall stage efficiency is used. \n • HETS = (height of extractor)/(number of theoretical stages (n)) \n • n = {Ln[xf/xn(E-1)+1]/[Ln(E)]} -1