The combination of simulation and bench-scale experiments may also be used to investigate the benefits of process integration regarding energy consumption. The integration can be performed internally within the ethanol plant or by integrating various parts of the ethanol plant with another type of plant, such as a pulp mill or a heat-generating plant. The latter requires, of course, detailed knowledge of the plant with which the ethanol plant is to be integrated. The internal integration will be exemplified by the incorporation of the distillation unit with a multiple-effect evaporation unit. Examples of integration of an ethanol plant and a pulp mill, as well as a power plant, will also be presented.

Internal Process Integration. The evaporation and distillation steps are large consumers of steam in the ethanol plant. The steam consumption in the evaporation and the distillation steps can be reduced by an increase in the dry matter content in the stillage or by increasing the ethanol concentration in the feed to the distillation step (60). This can be achieved by recirculation of various streams as described above. This might however, as shown, inhibit fermentation and/or hydrolysis depending on the degree of recirculation. Another way of increasing the ethanol content is to incorporate a stripper into the evaporation line, as shown in Figure 9. Instead of first distilling the fermentation broth and then evaporating the distillation stillage, the fermentation broth is fed to the stripper, evaporator effect 4, which is equipped with a reboiler, while the condensate from effect 5 is fed to the distillation unit.

In such case, the ethanol concentration in the feed to the distillation unit is increased from about 2-3% (wt/wt) to 18-20% (wt/wt), thus reducing the refining cost. With this configuration the steam consumption in the evaporation unit is not affected, while it is lowered by 60% in the distillation unit. For separate distillation and evaporation of a 100 tonnes/h liquid stream, the cost is 0.20 and 0.13 US$/kg product for dry matter contents in the feed of 4 and 6% (wt/wt), respectively (60). In both cases the liquid was concentrated to 65% (wt/wt) DM. With the incorporation of

Figure 7. COD and BOD of various evaporated fractions.

Figure 8. Recycling of the distillation feed. 1: Pretreatment; 2: Filtration; 3: Enzyme production; 4: Hydrolysis; 5: Fermentation; 6: Distillation.

a stripper into the evaporation unit, the cost decreases to 0.15 and 0.10 US$/kg product, as shown in Figure 10, illustrating the cost improvements when recycling is employed.

Further improvement in the economy can be achieved by integrating the energy­demanding steps in the entire process. One example is to release the high-pressure steam used in pretreatment to a back pressure of 3 bar and to use the secondary steam formed in other parts of the process, e. g. pre-steaming of the wood chips, in the distillation or in the evaporation unit (54).

External Process Integration. In an investigation by the Swedish engineering company AF-IPK (61), based on 150,000-250,000 tonnes/year raw material, the benefits of integrating an ethanol plant and existing processes handling biomass were evaluated. The plants investigated were a pulp mill, a power plant and a combined saw mill and peat-drying facility. The common infrastructure and the wood intake and storage are advantageous for all cases. The contribution of the common infrastructure is difficult to quantify in economic terms. The largest overall effects were obtained for a combination of an ethanol plant with either a pulp mill or a power plant. The saw mill has fewer possibilities for integration, since it is almost only the material handling that is common for the plants.

There are several opportunities for integrating redundant equipment or capacity in an ethanol plant and a pulp mill. The most interesting process step is that of steam generation as a pulp mill usually has an excess of low-pressure steam at 4 bar, which can be used in different process steps in the ethanol plant. The second most interesting process is effluent treatment. Due to the increased efforts in recent years to close the processes at pulp mills, the treatment plant will have surplus capacity which can be used by the ethanol plant. Furthermore, there is also the possibility of using a bark-fuelled boiler.

Assuming that the power plant is designed to receive large quantities of wood residue, the raw material handling, or part of it, could be shared with the ethanol plant. Furthermore, the boiler in the power plant can be used for steam generation in the ethanol plant. It is also possible to take advantage of the sorting of incoming wood to obtain a better raw material for ethanol production.

The potential for synergetic effects was determined to be in the range of 0.05­

0. 10 US$/kg ethanol, which should be compared with a calculated total production cost of 0.5-0.67 US$/kg ethanol for a plant with no integration. These benefits must be shared between the ethanol plant and the process with which it is integrated.