text: The most critical aspect in biomass fermentation processes is related to its productivity. The ABE fermentation via Clostridium beijerinckii or Clostridium acetobutylicum for instance is characterized by product inhibition. This means that there is a product concentration threshold that cannot be overcome, resulting in a product stream highly diluted in water.
  For this reason, in order to have a comparable productivity and profitability with respect to the petrochemical processes, cost and energy effective solutions for the product purification sections are required to provide a significant product recovery at the desired purity.
  The main solutions adopted during the last decades have been as follows:
  * The employment of less expensive raw materials, and in particular lignocellulosic waste or algae;
  * The microorganisms  modifications or the research of new strains less sensitive to the butanol concentration poisoning to increase productivity and selectivity towards the butanol species;
  * The fermentation  reactor optimization aimed at increasing the productivity;
  * The reduction of the energy costs of the separation and purification downstream processing and, in particular, to carry out the separation in-situ in the reactor;
  * The use of side products such as hydrogen and carbon dioxide, solid wastes and discharged microorganisms and carry out less expensive process wastewater treatments.
  In the second half of the 20th century, these technologies allowed an increase in the final product concentration in the broth from 15 to 30 g/L, an increase in the final productivity from 0.46 to 4.6 g/(L*h) and an increase in the yield from 15 to 42%.
  From a compound purification perspective, the main criticalities in the ABE/W product recovery are due to the water–alcohol mixture's non-ideal interactions leading to homogeneous and heterogeneous azeotropic species, as shown by the ternary equilibrium diagram.
  This causes the separation by standard distillation to be particularly impractical but, on the other hand, allows the exploitation of the liquid–liquid demixing region both for analogous and alternative separation processes.
  Therefore, in order to enhance the ABE fermentation yield, mainly in situ product recovery systems have been developed. These include gas stripping, pervaporation, liquid–liquid extraction, distillation via Dividing Wall Column, membrane distillation, membrane separation, adsorption, and reverse osmosis. Green Biologics Ltd. implemented many of these technologies at an industrial scale.
  Moreover, differently from crude oil feedstocks, biomasses nature fluctuates over the years seasons and according to the geographical location. For this reasons, biorefinery operations need not only to be effective but also to be flexible and to be able to switch between two operating conditions rather quickly.[citation needed']