Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 195

7.7 SEPARATION
The broth at the end of the fermentation process contains the product (or mixture of
products), unconverted substrate, cells and other soluble and insoluble components. To
recover the desired product from the broth, a separation scheme is required. The separation
scheme used depends on the location of the product (intracellular or extracellular), size,
solubility, charge and concentration of the product in the broth, the product value and purity
of the product. The higher the purity of the product needed, the higher the separation cost.
Different separation methods are used to recover acids, alcohols and diols from the
fermentation broth, which is discussed in the next section.
7.7.1 Separation of acids
Organic acids, such as acetic and lactic acids, are recovered from the fermentation broth by
first removing cells and other insoluble components followed by a precipitation step. For
example, for citric acid, calcium hydroxide is used to produce calcium or sodium citrate.
The precipitate is filtered and transferred to a stirred tank to which sulfuric acid is added to
form calcium sulfate precipitate, which is then removed by filtration. The acetic acid is
further purified by evaporation followed by crystallization, centrifugation and drying
(Bailey and Ollis, 1986; Rogers et al ., 2006). The traditional separation method for suc-
cinic acid is similar to acetic acid. Zeikus and co-workers (1999) reviewed succinic acid
production and highlighted a method called simultaneous acidification and purification by
electrodialysis and crystallization as an alternative to the traditional separation method.
This method was claimed to be cost effective and easy to scale up for commercial use.
Other researchers used adsorbents to separate succinic acid from the fermentation broth
(Davison et al ., 2004). The preferred choice for acetic acid recovery from the fermentation
broth containing between 1.2 and 3.5% acid has been solvent extraction (Rogers et al .,
2006). Solvent extraction is a method to separate a selected compound from a mixture
based on its relative solubility when two different immiscible solvents are used, which
form two phases.
7.7.2 Separation of alcohols
Separation of alcohols, such as ethanol and butanol, from the fermentation broth is
traditionally done by distillation. The higher the alcohol concentration in the fermentation
broth, the lower the energy required for distillation. For ethanol fermentation, the broth
usually contains 10-15% (w/w) ethanol. After distillation, ethanol concentration in the
distillate is about 90% (w/w). The distillation process will not yield more than 93% (w/w)
ethanol, which is the azeotropic mixture of ethanol-water. Azeotropic mixtures cannot be
separated by distillation because the compositions of ethanol in the vapor and liquid
phases are the same. Therefore, azeotropic distillation with benzene or dehydration with
molecular sieves is usually used to remove the remaining water and produce fuel grade
ethanol (99.9 wt-%).
Distillation is based on the differences between the boiling points of the desired alcohol
and water. The distillation column has two distinct sections, the rectifying section above the
feed stream and the stripping section below the feed stream. In the rectifying section the
alcohol increases in purity as it travels as a vapor to the top of the column, where it is