3.5.1 Ethanol Recovery from Fermented Broths

The distillation of ethanol from fermented broths remains the dominant practice in ethanol recovery in large and small ethanol production facilities.250 Other physical techniques have been designated as having lower energy requirements than simple distillation, and some (vacuum dehydration [distillation], liquid extraction, super­critical fluid extraction) can yield anhydrous ethanol for fuel purposes from a dilute aqueous alcohol feed (figure 4.12).251 Only water removal by molecular sieving* has, however, been successful on an industrial scale, and all new ethanol plants are built with molecular sieve dehydrators in place.252

Nevertheless, the economic costs of dehydration are high, especially when anhydrous ethanol is to be the commercial product (figure 4.12). In the early 1980s, the energy requirements were so high that the practical basis for fuel ethanol pro­duction was questioned because the energy required for distillation approximated the total combustion energy of the alcohol product.253,254 The investment costs of rivals to distillation were, however, so high (up to 8.5 times that of conventional distillation) that little headway was made and attention was focused on improving

Synthetic zeolite resins are crystalline lattices with pore sizes of 0.3 nm, sufficiently small to allow the penetration of water molecules (0.28 nm in diameter) but exclude ethanol (molecular diameter 0.44 nm).

process efficiencies and energy cycling with the development of low-energy hydrous ethanol distillation plants with 50% lower steam-generating requirements.252

The economics of downstream processing are markedly affected by the concen­tration of ethanol in the fermented broth; for example, the steam required to produce an ethanol from a 10% v/v solution of ethanol is only 58% of that required for a more dilute (5% v/v) starting point, and pushing the ethanol concentration in the fermenta­tion to 15% v/v reduces the required steam to approximately half that required for low conversion broth feeds.253