Process simulation of ethanol production from spruce using a process con­cept based on SO2-catalyzed steam pretreatment followed by SSF, as shown in Fig. 3 ([20], Wingren et al. 2007 (submitted)), has been used to illustrate the effect of various process parameters on the energy demand and on the ethanol production cost. The general conclusions are, however, also valid for most of the process configurations described in Table 1. The model input was based on experimental data obtained from a process development unit. SSF was performed at 10% WIS with 2 gL-1 yeast. In the model, the overall ethanol yield was 296 liters per metric dry ton, corresponding to 69.4% of the theoretical based on the hexosan content in the raw material. Pentose fer­mentation was not included. Regarding production cost data, the proposed ethanol plant is assumed to be located in Sweden, with a capacity of 200 000 dry tons of raw material annually.

The ethanol yield affects both the raw material and capital costs and is the single most important parameter in reducing the cost of ethanol pro­duction, as was already stated in 1988 [39]. High energy efficiency is also of great importance for the process to be economically feasible. In most techno­economic evaluations, live steam for the process is generated in a steam boiler by burning part of the solid residue. From the excess solids it is possible to generate heat and electricity or pellets that can be sold to improve the pro­cess economics. Thus, the energy demand of the process affects the amount of solid residue that may add to the income as a solid fuel co-product and, therefore, it is very important for the process to be energy-efficient.

The heat duty of the process depends to a large extent on the process con­figuration. For the process alternative described above, the heat duty of the energy-demanding process steps is shown in Fig. 5. The white bars represent the primary steam demand while the gray bars represent the amount of sec­ondary steam that is generated in each process step. The overall process heat duty, i. e. the total energy demand in the form of boiler-generated steam, is the sum of the black bars. Distillation (including preheating of the SSF broth) and evaporation account for the major part of the process energy demand. The contributions from pretreatment and drying, with the latter assumed to work as a steam dryer, are comparatively small, due to the generation of secondary steam in these process steps.

The energy demand of the distillation step, in which the ethanol in the mash from fermentation is concentrated, is highly dependent on the ethanol

Ethanol feed concentration (% [w/w])

Fig. 6 Energy demand in the distillation step, where ethanol is concentrated to 94 wt %, as a function of the ethanol feed concentration. The step was assumed to consist of two stripper columns (25 trays each) and a rectification column (35 trays) heat integrated by operating at different pressures. The inlet feed temperature was increased from 80 °C to the boiling temperature before entering each stripper column feed concentration, as shown in Fig. 6. The distillation step normally consists of a stripper column, in which the ethanol is separated from all solid and non-volatile compounds, and a rectification column, in which the ethanol is concentrated close to the azeotropic point. The implementation of heat inte­gration, for instance by using the overhead vapor from the stripper as the heat
source in the reboiler of the rectification column, significantly reduces the en­ergy demand. Nevertheless, it is of great importance to obtain a high ethanol concentration in the distillation feed. In a starch-based process the ethanol concentration in the stream entering the distillation step is normally above 8% (w/w). In a lignocellulose-based process, however, the aim has been to reach at least 4-5% (w/w) ethanol. In addition, a high ethanol concentration results in a high concentration of non-volatile compounds, which also leads to a decrease in energy demand in the evaporation step.

Recirculation of process streams is one way of reducing the overall energy demand, which results in a decrease in overall production cost, as shown by Wingren et al. [38]. Recirculation of part of the stream after distillation back to the fermentation step would result in an increased concentration of non­volatiles and thus a reduction in the energy demand in the evaporation step. Recirculation of part of the stream before distillation would also result in an increase in the ethanol concentration and thus a reduction in the energy de­mand in both the distillation and evaporation steps. This is true for both the SSF and SHF configurations. However, in the same study it was shown that it is even more beneficial to increase the substrate concentration in the SSF step. This would affect not only the costs related to distillation and evaporation, but also the cost of SSF. On the basis of this fact, one of the main objectives of several experimental studies performed during recent years has been to increase the substrate concentration in SSF [40-43]. This results in reduced water consumption, which greatly reduces the energy demand for distillation and evaporation, provided that the ethanol yield is maintained at a high level. In Fig. 7, the process heat duty (in MJ L-1) and the overall production cost

(in US$ L-1) are presented as functions of the WIS concentration in SSF. The ethanol yield and the amount of yeast (NB: not the yeast concentration), were the same as in the 10-% WIS case when varying the WIS concentration. The reduction in production cost is due to an increase in co-product credit and a reduction in the fixed capital cost.

Process simulations clearly demonstrate the potential reductions in pro­duction cost and energy demand that can be obtained by running SSF at higher substrate concentrations. However, given the large number of com­pounds involved, and due to the fact that they may act synergistically, it is impossible to predict the impact of increased concentrations on the perform­ance of the yeast and enzymes using process models. Effects on parameters such as productivity (yield, residence time), yeast and enzyme dosages have to be determined experimentally, preferably on pilot scale.

Savings in energy demand can also be accomplished by changes in the process design. Evaporation is the traditional, but energy-demanding, way to concentrate the water-soluble, non-volatile components in the stillage stream. To reduce the energy requirements for evaporation, multiple evaporation ef­fects are used. This has a significant effect on the overall process heat duty, as shown in Fig. 8. (In the simulation results presented in Figs. 6 and 7, evapo­ration was carried out with five effects.) The energy savings have, of course, to be weighed against the increase in capital cost. Also shown in Fig. 8 is a case where the use of mechanical vapor recompression (MVR) has been implemented in the evaporation unit. In a traditional multiple-effect evap­orator system, a large proportion of the energy supplied ends up as latent heat in the vapor phase leaving the last effect in the evaporator. This vapor is

§

normally condensed using cooling water. Another option is to compress the vapor, thereby raising the temperature to a level at which the latent heat can be utilized. The vapor can then be used as a heating medium to replace most of the primary steam. When compression is carried out by aid of a mechanical compressor the process is referred to as MVR. An electrical motor or a steam turbine provides power to the compressor. The overall process heat duty was reduced from 15.1 (base case configuration) to 10.3 MJ L-1 when MVR was applied to the evaporation step (Fig. 8), while the overall electric power re­quirement was estimated to increase from 2.2 (base case configuration) to

2.8 MJ L-1 (data not shown).

It has also been proposed that the entire evaporation step be replaced by an anaerobic digestion step, in which most of the organic material (unfermented sugars, acids, yeast, etc) is converted to biogas mainly consisting of methane and carbon dioxide. This was estimated to reduce the production cost by about 7%. The performance of such a system is dependent on a number of parameters such as the composition of the feed, residence time, temperature, etc. A crucial question is also how to handle the sludge from the anaerobic digestion. Further investigation is required since very limited data regarding the performance of this kind of system have been published.

3.2