Ethanol by Classical Fermentation

United States and Brazil

Keywords

Cornstarch; sugar cane; fermentation process description; ethanol blending wall; production economics

2.1 US Cornstarch

The enactment of the 2005–2007 RFS by the US Congress focused on the classical production of ethanol by fermentation of cornstarch. The selection of this technology was driven by its long commercial history in the United States, and the fact that it was a readily renewable resource that would reduce dependence on fossil fuels that were seen to be in increasingly short supply. An additional factor was that it sizably produced less greenhouse gases than fossil fuels when burned. The prevailing view that fermentation of corn would lead to energy savings was challenged almost immediately by Pimental and Patzek of Cornell University [4]. They contend that such savings are illusory and that fermentation of cornstarch to ethanol would in fact require 29% more energy than would be contained in the ethanol so produced. Their study factored in total energy costs such as labor, azeotropic distillation, equipment, and fertilizers. The validity of their conclusions is the subject of some debate by such parties as H. Shapouri and J.A. Duffield at the U.S. Department of Agriculture (USDA) and M. Wang at Argonne National Laboratory [5]. The latter argue that the critique by Pimental and Patzek failed to include the value of the substantial byproducts that are produced in cornstarch fermentation.

Any uncertainty relating to ethanol and its energy savings is countered by its efficacy in lowering greenhouse gas emissions. Blended gasoline that contains 10% ethanol produces 17% less carbon monoxide and 4% less carbon dioxide than regular gasoline. Such blended ethanol also has a higher octane number (100 vs. 87) than regular unleaded gasoline and offers about the same road mileage. A more comprehensive evaluation of greenhouse gas emissions from corn ethanol by type of energy used in its production appears in Figure 2.1. This evaluation, conducted by Argonne National Laboratory, addresses raw material extraction, processing, distribution, and disposal or recycling, etc.

The dry milling process is commonly used in the fermentation of cornstarch to ethanol (Figure 2.2). In this technique, the more profitable plants recover distillers dried grains (DDGs) as an animal feed.

In the process, cornstarch kernels are first separated from the rest of the corn plant and then ground up, The starch content of the ground up material accounts for 75% of the total corn plant. The overall process consists of four principal steps.

By dry milling the grain in roller mills, the finely divided starch is readily hydrated when dispersed in water. Typical grinding of the corn enables 76% to pass through a 20 mesh screen. Finely pulverized ground material reduces the downstream cooking time.

The gelatinization of the starch is necessary before it can be hydrolyzed, and the effectiveness of this step is governed by the species, its time–temperature relationship, its particle size, and the concentration of the mash. Cornstarch is 80–85% gelatinized at 70–75°C, and the remainder is gelatinized as the temperature increases further to 180°C. The ground corn is combined with water and recycled stillage in a slurry tank to form a mash. The latter is continuously pumped to a steam jet heater where the mixture is maintained at 180°C and then passed through a pipe reactor designed for a retention time of 5 min. The cooked mash is first cooled to 110°C and then to 63°C.

The starch is hydrolyzed to its glucose components by a fungal enzyme (usually Saccharomyces cerevisiae). Approximately 80% of the total starch is converted to maltose and the remainder to branched fragments commonly called residual or limit dextrins. The dextrins are subsequently hydrolyzed to maltose in the course of fermentation. The fungal enzyme is introduced as a solution downstream of a mash cooler, and at a temperature of 63°C, and 2 min residence time, the starch is converted to a maltose/dextrose mixture.

Fermentation is dependent on time, temperature, interfacial contact, concentration, the pH of the system, and the strain of enzyme employed. A fermentation time of 40–60 h is set by the slow conversion to maltose at a temperature <32°. Monosaccharides are faster reacting. The fermented system contains 6.5–8.5% ethanol by volume. The products are held in the fermentation tank for about 60 h during which the ethanol concentration is usually about 8.5–10%. The ethanol solution is then sent to a still from which a distillate of 50% ethanol, aldehydes, fusel oil, and waste is obtained. The condensate, frequently referred to as high wines is sent to a rectification column from which low boiling impurities, such as aldehydes, are removed. Since only about 10% ethanol is produced in fermentation, excess water is removed by azeotropic distillation alone or in combination with semipermeable membranes.

Fusel oil, a mixture of lower alcohols such as propanols and butanols, is removed during the course of distillation and sold as a byproduct credit against the process.

The wet milling process enables the manufacturer to separate the starch from the gluten. In that system, the corn kernels are soaked in hot water with sulfur dioxide to loosen the kernels. The grain is ground gently to loosen the germ from the kernel and is then separated. The oil, which is then recovered from the germ, is either expressed or extracted with a solvent and is used for animal feed. The degermed kernel is subsequently ground and washed to remove the hull. The remainder of the material is centrifuged to recover the cornstarch from the gluten.

The final recovered product is 95% aqueous ethanol. Automotive fuel requires 99.7% ethanol. During fermentation, each six-carbon glucose molecule is split into two three-carbon molecules of pyruvic acid that are subsequently metabolized to two-carbon molecules of ethanol as shown in Figure 2.3. NAD represents adenine dinucleotide and is an enzyme cofactor which successively donates and removes hydrogen in a series of cyclic redox reactions. NADH represents its reduced form in which it is bonded to the hydrogen and NAD+ is its transient oxidized version in which it has given up its hydrogen.

Production of US fuel grade ethanol soared [6] from 2.1 billion gallons in 2002 to 13.9 billion gallons in 2011, a nearly sevenfold increase. The number of plants increased from 100 to 123 in this time period. The rapid growth was nearly entirely driven by fermentation of cornstarch and the mandated RFS. It was secondarily aided by duties on imported ethanol and a tax incentive of $0.45 per gallon of nearly pure ethanol blended with gasoline. The tax incentive had first to be taken as a credit against the blender’s tax liability, and any excess over this could be claimed as a direct payment from the Internal Revenue Service. Further impetus to the growth in cornstarch ethanol was the parallel phase out of t-butylmethyl ether as an octane improver.

Since 1 acre of corn is required to produce 328 gallons of ethanol [7] production of motor fuel ethanol quickly consumed 40% of the corn produced in the United States. Consumption for fuel had been only 14% of the total crop in 2005.

As would be expected, the competing demands for food and fuel drove corn prices from $2.78 per bushel in 2006 to $7.25 per bushel in mid-2012. A significant contributing factor was the severe drought that struck the corn producing mid-western part of the United States in 2012. As a consequence, profitability in corn production fell sharply for simple plants in which corn oil was not recovered from the DDGs as a byproduct credit. Beginning in the summer of 2012, the prices for ethanol and corn reached levels where the production costs at such relatively simple ethanol plants exceeded revenue [9]. Figure 2.4...