11.2.1 Configuration Involving the Separate Hydrolysis

and Fermentation (SHF) of Corn Starch

Starch is a high yield feedstock for ethanol production. Glucose is obtained by the hydrolysis of starch. Then, the glucose solution undergoes fermentation toward ethanol. From each 100 g of starch, 111 g of glucose theoretically can be obtained, which implies a stoichiometric ratio of 9:10. The output/input ratio of energy for

corn ethanol is in the range of 1.1 to 1.2 (Prakash et al., 1998; Sanchez and Cardona, 2005). Fuel ethanol production from materials with a high content of starch needs some additional process steps compared to the process that employs sucrose-based materials as the feedstock. The process flowsheet comprises a pretreatment step of the starchy materials to make starch more susceptible to enzymatic hydrolysis. In this step, known as liquefaction, starch is partially hydrolyzed at a high tempera­ture. In the following step of saccharification, liquefied starch undergoes a deeper hydrolysis where fermentable sugars (glucose) are obtained (Figure 11.5). After glucose fermentation, the process does not significantly differ from that one that employs materials with a high content of sucrose. Nevertheless, depending on the specific type of employed starchy feedstock, certain co-products used for animal feed can be produced during the evaporation of stillage.

When cereals are used as the feedstock for producing fuel ethanol, the raw material enters the process as grains, which should undergo cleaning and milling. Either wet-milling or dry-milling processes can carry out grain milling of such cereals as corn, wheat, and barley. The wet-milling process implies that only the starch enters fuel ethanol production process, as discussed in Chapter 4, Section 4.2. In this process, all the components of the kernel should be separated prior to the cooking step. These components represent value-added products that are used for food and feed. Moreover, part of the produced starch can be deviated to the production of sweeteners, such as the high fructose corn syrup (HFCS). During the dry milling of grains, the whole kernel enters the ethanol production line, which means that all its components are processed along with starch. Nonutilized components of the kernel are built up in the bottoms of the first distillation col­umn and are concentrated to form a product utilized as animal feed. In general, the liquefaction, saccharification, and fermentation steps are the same for both types of technologies.

The overall production process of bioethanol from corn by the dry-milling technology includes the breakdown of this polysaccharide to obtain an appropriate

concentration of fermentable sugars, which are transformed into ethanol by yeasts (Figure 11.6). After washing, crushing, and milling the corn grains, the starchy material is gelatinized in order to dissolve the amylose and amylopectin (cooking step). This process is accomplished with the help of a jet cooker working at 110°C. In dissolved form, starch is accessible for enzymatic attack in the following step, the liquefaction that is carried out at 88°C. This step is considered a pretreatment process because of the partial hydrolysis of the starch chains using thermostable bacterial a-amylase that yields a starch hydrolyzate with a hydrolysis degree of approximately 10%. The obtained hydrolyzate has a reduced viscosity and con­tains oligomers, such as dextrins.

Then, this liquefied starch enters the saccharification process where it is hydro­lyzed by microbial glucoamylase to produce glucose. This process operates at 60°C. The saccharified starch is cooled and sent to the next step where it is fer­mented by the yeast S. cerevisiae and converted into ethanol at 30°C. Fermentation gases, mostly CO2, are washed in an absorption column to recover more than the 98% of the volatilized ethanol from the fermenter and sent to the first distillation column. The culture broth containing 11% (by weight) ethanol is recovered in a separation step consisting of two distillation columns. In the first (concentra­tion) column, aqueous solutions of ethanol are concentrated to 50%. In the second (rectification) column, the concentration of the ethanolic stream reaches a com­position near the azeotrope (95.6%). The dehydration of the ethanol is achieved through adsorption in a vapor phase with molecular sieves. The stream obtained during the regeneration of molecular sieves containing 70% ethanol is recycled to the rectification column.

The stillage from the concentration column is evaporated and the obtained sol­ids are separated by centrifugation. These solids are dried for producing the dis­tiller’s dried grains with solubles (DDGS), a co-product used for animal feed due to its high content of proteins and vitamins. The remaining liquid or thin stillage is evaporated in a double effect evaporator. The obtained syrup is combined with the DDGS and dried. The condensed water from the evaporators is recirculated to the liquefaction stage, while the bottoms of the rectification column and one frac­tion of the thin stillage (backset) are recycled back to the saccharification step.