Switchgrass holds great promise as a valuable fuel crop for cellulosic ethanol production with pretreatments discussed earlier such as dilute sulfuric acid, sodium hydroxide, soaking in aqueous ammonia, ammonia fiber explosion, hot water, and lime pretreatment, etc. (Yang et al. 2009; Digman et al. 2010; Xu et al. 2010; Tao et al. 2011). Based on the discussed requirements for cellulosic ethanol process integration, soaking in aqueous ammonia (SAA) pretreatment may be the most feasible for lignocellulosic feedstock such as switchgrass (Isci et al. 2008; Isci et al. 2009). However, for the SAA pretreatment, a pressure vessel is required. The design of the pressure vessel is dependent on the concentration of aqueous ammonia, operating temperature, switchgrass loading, and a ratio of switchgrass to aqueous ammonia. The vapor pressure exerted by 15% (w/w) aqueous ammonium hydroxide at 80°C is approximately 31.5 psi (absolute). After the pretreatment, ammonium hydroxide may be recovered through condensation followed by lignin separation in the pretreated solvent using a filter press. The recovered lignin may then be used for power generation or have industrial importance in making biomaterials and paints (Gargulak and Lebo 1999; Lora and Glasser 2002; Keshwani and Cheng 2009; Laser et al. 2009). After recovering the lignin, the filtered water should be recycled for use in both washing SAA-treated solids after the pretreatment or to make up the ammonium hydroxide concentration after the condensation of recovered ammonium hydroxide. Recovered ammonium hydroxide concentration could be maintained up to 35% (w/v) in the separate vessel.

After SAA pretreatment, further processes could be approached in two different ways using either SHCF or SSCF for cellulosic ethanol production. As mentioned earlier, SSCF has an advantage of requiring a minimal number of vessels compared to SHCF. However, the consideration of downstream processing and energy requirements during the process would help in economical process integration for the cellulosic ethanol production. The addition of reverse osmosis between the hydrolysis step and the fermentation step would be beneficial in the concentration of the hydrolyzed sugar slurry thus minimizing the energy requirement for both ethanol fermentation and distillation. Moreover, the reverse osmosis is often more energy efficient when compared to conventional evaporation techniques (Madaeni et al. 2004). Gul and sek (2009) have mentioned that the concentration of 15% (w/v) sugar syrup to 65% (w/v) requires 86% less energy using reverse osmosis and evaporation technique compared to evaporation technique alone. The enzyme hydrolysis yields up to 10% (w/v) sugars syrup using SAA-treated lignocellulosic biomass. The addition of a reverse osmosis step following enzyme hydrolysis would allow increasing the concentration of hydrolyzed lignocellulosic sugar syrup from 10% (w/v) to 20% (w/v). The concentrated sugar syrup would influence both the fermentation and distillation steps in terms of energy saving and increasing ethanol productivity.

Figure 6 shows the overall process scheme for ethanol fermentation of glucose and xylose illustrated in separate fermenters using S. cerevisiae and P. stipitis, respectively. The dissolved chemicals in the spent broth after the ethanol distillation could be recovered using evaporation. The evaporated water vapor could be condensed and recycled to the enzyme hydrolysis step.