In this chapter, the different configurations for fuel ethanol production employing the three most important types of raw materials (sucrose-containing, starchy, and lignocellulosic materials) are presented. In particular, such configurations involv­ing integrated processes are discussed. The role played by process systems engi­neering during the definition and development of the diverse process flowsheets is emphasized. Finally, examples of process synthesis procedures applied to ethanol production are presented.

11.1 ETHANOL PRODUCTION FROM

SUCROSE-CONTAINING MATERIALS

Average ethanol yields from sucrose-containing feedstocks based on sugarcane can reach 70 L/ton cane and 9 L/ton of C-grade molasses (in addition to about 100 kg of sugar; Moreira, 2000). The most used fermenting microorganism is Saccharomyces cerevisiae due to its ability to hydrolyze cane sucrose for conver­sion to glucose and fructose, two easily assimilable hexoses. Fermentation pH is 4 to 5 and temperature is 30° to 35°C. Ethanol in Brazil is obtained from sugar­cane, and the country is the world’s leading producer, followed by India. About 80% of ethanol in Brazil is produced from fresh sugarcane juice and the remain­ing percentage from molasses (Wilkie et al., 2000). Sugar cane molasses is the main feedstock for ethanol production in India (Cardona and Sanchez, 2007). In Colombia, different sucrose-containing streams are used to produce fuel ethanol, especially the B-grade molasses. Berg (2001) indicates that the output/input ratio of energy for ethanol production from cane is the highest among the main types of feedstocks, reaching a value of 8. This indicator expresses the ratio between the energy released during the combustion of ethanol and the energy required for its production using the whole life cycle of the product from extraction of raw materials until the transformation process producing ethanol (Sanchez and Cardona, 2005).

In general, the process for ethanol production for sugarcane includes the extrac­tion and conditioning of cane juice to make it more assimilable by yeasts during

fermentation. From the resulting culture broth, cell biomass is separated and then the concentration of ethanol and its dehydration are carried out employing differ­ent unit operations. The product is the anhydrous ethanol that is the trade form in which fuel ethanol is utilized as a gasoline oxygenate. This process can utilize not only the crushed cane but also cane molasses as a feedstock as well as other streams with high content of fermentable sugars derived from the process for cane sugar production in sugar mills, as mentioned in Chapter 3, Section 3.1.5 (for example, a fraction of the clarified syrup). In the latter case, ethanol production facilities are located next to sugar mills, as in the case of Colombian distilleries. In the former case, distilleries can operate in an independent way as in Brazil, where an important number of autonomous (stand-alone) distilleries employing cane juice are currently in operation.

The overall process for fuel ethanol production from sugarcane in autonomous distilleries is shown in Figure 11.1. Production process in a distillery co-located at a sugar mill differs from the autonomous distilleries in the first steps (cleaning, milling, clarification). After these steps, the process is almost the same, although it is necessary to condition the molasses before fermentation. Fuel ethanol pro­duction process from sugar beet is similar to the process from sugarcane. Culture broth (fermented wort) is extracted from fermenters and sent to centrifuges for cell biomass separation. The removed microorganisms can be reutilized in the fermentation step. The obtained wine is directed to the first distillation column (concentration column) where the ethanol concentration of the wine is increased. Exhaust gas from fermenters having a fraction of volatilized ethanol is collected and sent to the scrubber. In this unit, ethanol is dissolved in a water stream, where a dilute alcoholic solution is obtained that is also sent to the first distillation col­umn. The resulting ethanol-enriched stream is fed to the second distillation col­umn (rectification column) whose distillate has a high ethanol concentration (near

96% by weight). This stream is sent through the dehydration step that can be car­ried out using different technologies. In the flowsheet depicted in Figure 11.1, it is indicated that the ethanol dehydration is performed by adsorption with molec­ular sieves. The bottoms of the concentration column or stillage (vinasses) are directed to the effluent treatment step where they are generally evaporated for their concentration.

The fermentation step is central to the overall process for fuel ethanol produc­tion because it represents the transformation of sugar-containing raw materials into ethyl alcohol employing yeasts or other ethanol-producing microorganisms. Ethanolic fermentation technologies of sucrose-based media, mainly cane juice and cane or beet molasses, can be considered relatively mature, especially if they are operated batchwise. However, many research efforts are being made worldwide in order to improve the efficiency of the process. In particular, these efforts are aimed at increasing conversion of the feedstock and ethanol produc­tivity, and at reducing production costs, especially energy costs. The features of ethanolic fermentation of sucrose-containing materials are discussed in Chapter 7, Section 7.1.2.

Process simulation plays a crucial role during the analysis of the technical, economic, and environmental performance of fuel ethanol production from sucrose-containing materials. In addition, simulation tools are very significant when process synthesis procedures are being applied, particularly when the knowledge-based process synthesis approach is employed (see Chapter 2). Thus, if the hierarchical decomposition procedure is applied, process simulation can provide the necessary data on the process behavior in order to select or discard the different alternatives proposed during each hierarchical level of analysis (see, for example, the work of Sanchez, 2008).