The incoming liquid will consist of two main components: water and ethanol. These can be separated in a standard distillation tower. As ethanol will be used as a fuel, the ethanol coming out of the distiller may only contain 7 vol-% water (Thuijl et al., 2003). Therefore, this will be the goal of our distiller. The outgoing stream of water will still contain ethanol. To keep efficiency as high as possible, this stream should be inserted in the fermentation reactor.

Recycle

The water stream coming out of the distillation unit still has valuable components in it. To preserve these, this stream is used to be the water stream in the fermentation reactor. Because the reaction of the bacteria is dependent on pH as well as on the quantity of nutrient in the flow this should be kept at highest performance

levels. This can be done by adding nutrients and controlling the pH of the recycle stream.

Figure 17.5 shows an overview of the total process design and Table 17.2 shows the mass balance for 30 000 ton per year input of wood chips to ethanol plant (Van Kasteren et al., 2005).

Apart from ethanol acetic acid is also produced. An important issue for the whole process is the energy required for the distillation of the ethanol/water mixture.

Table 17.3 shows the energy requirements. The concentration of ethanol, which can be reached is crucial to the efficiency and economics of the process.

At least 3-5 wt% ethanol in water should be reached otherwise the energy requirements to separate the ethanol from the water become too high.

Conclusion is that the feasibility of the process is determined by the ethanol concentration reached. Coskata (www. coskata. com) is trying to solve this problem via the use of a membrane.

Exhaust Ash Exhaust Water exhaust

77.5 Process scheme biomass to ethanol plant (Van Kasteren et a/., 2005).