PRODUCTION OF ETHANOL AND HIGH-PROTEIN FEED CO-PRODUCTS FROM HIGH-SOLIDS CONVERSION OF CEREAL GRAINS AND LEGUMES
This application claims the benefit of provisional application Serial No. 60/602,362, filed Sept. 12, 2003. Contractual Origin of the Invention The United States Government has rights in this invention under Contract No. DE-AC3699GO 10093 between the United States Department of Energy and the National Renewable Energy Laboratory, a division of the Midwest Research Institute, Battelle, and Bechtel. Technical Field The invention relates to a process for conversion of cereal grains and legumes to ethanol and high-protein feed co-products by affecting a high solids pretreatment that results in higher product concentration and lower energy requirements for ethanol recovery. The pretreatment causes high starch, hemicellulose and cellulose conversion at high solids before the starch saccharification and ethanol fermentation stages. Background Art At present, in corn milling ethanol plants, jet cooking of ground corn has the objective of improving the enzymatic saccharification of starch, but ignores the hemicellulose and cellulose that contribute additional fermentable sugars, and therefore higher ethanol production. Furthermore, the conversion of hemicellulose and cellulose would enrich the protein content of the residual solids after fermentation, i.e., the dry distillers grain with solubles (DDGS). Lignocellulose is ubiquitous in all wood species and all agricultural and forestry waste. In addition, municipal waste, which typically contains about half waste paper and yard waste, is also a source of lignocellulosic materials. Currently, municipal waste is buried or burned at considerable expense to the disposer or the government organization providing solid waste services. Lignocellulosic biomass is a complex structure of cellulose fibers wrapped in a lignin and hemicellulose sheath. The ratio of the three components varies depending on the type of biomass. Typical ratios are as follows:
*RDF = Refuse Derived Fuel from municipal systems waste
Cellulose is a polymer of D-glucose with β [1— >4] linkages between each of the about 500 to 10,000 glucose units. Hemicellulose is a polymer of sugars, primarily D-xylose with other pentoses and some hexoses with β [l->4] linkages. Lignin is a complex random polyphenolic polymer. Therefore, lignocellulose represents a very cheap and readily available substrate for the preparation of sugars, which may be used alone or microbially fermented to produce alcohols and other industrial chemicals. Ethanol, one of the alcohols, which can be produced from lignocellulosic biomass, has a number of industrial and fuel uses. Of particular interest is the use of ethanol as an additive to gasoline to boost octane, reduce pollution and to partially replace gasoline in the mixture. This composition is the well-known commercial product called "gasohol." It has been proposed to eliminate gasoline completely from the fuel and to burn ethanol alone. Such a fuel would produce considerably less air pollution by not forming as much carbon monoxide or hydrocarbon emissions. Furthermore, gasoline is produced from crude oil, which fluctuates in price, availability, and is the subject of unpredictable world politics. It has been estimated that about 1 x 109 tons of lignocellulosic wastes are produced every year. This amount exceeds the total amount of crude oil consumed per year. In theory, if properly managed, the lignocellulose produced by the United States is sufficient to produce all of the country's needs for liquid fuel if the cellulose and hemicellulose can be completely converted into ethanol. The amount of energy theoretically obtainable from the combustion
of cellulose or the glucose or alcohol derived there from is about 7200 BTU per pound or roughly equivalent to 0.35 pounds of gasoline. Hemicellulose has similar value when converted into sugars or ethanol. Consequently, cellulose and hemicellulose represent a readily available potential source for ethanol production. The technology for the production of ethanol from grain and fruit for beverage purposes has been well developed for centuries. However, the costs have been relatively high compared to the cost of gasoline. Accordingly, many methods have been proposed to reduce the cost and increase the efficiency of ethanol production. Among the techniques proposed for the production of fuel grade ethanol include the hydrolysis of cellulose and hemicellulose to produce sugars which can be fermented to produce ethanol. Cellulose in the form of wood, newsprint and other paper, forest, agricultural, industrial and municipal wastes is quite inexpensive compared to grain, fruit, potatoes or sugarcane which is traditionally used to prepare alcohol beverages. Hydrolysis of lignocellulosic biomass using an acid catalyst to produce sugars has been known for decades but can be costly and requires special equipment. The hydrolyzed sugars themselves are somewhat labile to the harsh hydrolysis conditions and may be degraded to unwanted or toxic byproducts. If exposed to acid for too long, the glucose derived from cellulose degrades into hydroxymethylfurfural, which can be further degraded into levulinic acid and formic acid. Xylose, a hemicellulose sugar, can be degraded into furfural and further to tars and other degradation products. In order for acid to completely hydrolyze the cellulose and hemicellulose in a lignocellulosic substrate, degradation of the desirable sugars and formation of the toxic byproducts cannot be avoided due to kinetic constraints. On the other hand, to use conditions sufficiently gentle that significant degradation of sugars will not occur does not result in complete hydrolysis of substrate. Furthermore, the acid is corrosive and requires special handling and equipment. Accordingly, attention has been focused on enzymatic hydrolysis of cellulose with cellulase followed by fermentation of the resulting sugars to produce ethanol which in turn is distilled to purify it sufficiently for fuel uses. Cellulase is an enzyme complex that includes three different types of enzymes involved in the saccharification of cellulose. The cellulase enzyme complex produced by Trichoderma reesei QM 9414 contains the enzymes named endoglucanase (E.C.
126.96.36.199), cellobiohydrolase (E.C.188.8.131.52) and β-glucosidase (E.C.184.108.40.206). Gum et al. Biochem.Biophys.Acta, 446:370-86 (1976). The combined synergistic actions of these three enzymes in the cellulase preparation completely hydrolyses cellulose to D- glucose. However, cellulase cannot completely degrade the cellulose found in native, unpretreated lignocellulose. It appears that the hemicellulose and lignin interfere with the access of the enzyme complex to the cellulose, probably due to their coating of the cellulose fibers. Furthermore, lignin itself can bind cellulase thereby rendering it inactive or less effective for digesting cellulose. For example, raw ground hardwood is only about 10 to 20% digestible into sugars using a cellulase preparation. Currently, over 50% of biomass ethanol produced in the U.S. is derived from starch source (mainly corn) via the dry grind process. This process generally involves grinding the corn, slurrying the corn flour with backset (recycled thin stillage), and heating the slurry to gelatinize the starch and render it accessible to amylase enzymes, which are added to liquefy the starch and hydrolyze to fermentable glucose. The process, because of relatively low operating temperatures, does not solubilize starch that is chemically bound with fiber and protein. As a result, not all the available starch is hydrolyzed and consequently converted to ethanol. US Patent 4,529,699 discloses a process for obtaining ethanol by continuous acid hydrolysis of cellulosic materials by providing a homogenized slurry of heated (160° to 250°C) cellulosic material continuously into a reactor, adding concentrated acid to the pressurized and heated cellulosic material to obtain hydrolysis, neutralizing and fermenting the resulting aqueous solution to obtain ethanol, and recovering resulting byproducts of methanol, furfural, acetic acid and lignin. A process for the production of sugars and optionally cellulose and lignin from lignocellulosic raw materials is disclosed in US Patent 4,520,105. The process entails subjecting vegetable materials to a chemical pretreatment with a mixture of water and lower aliphatic alcohols and/or ketones at 100° to 190°C for a period of from 4 hours to 2 minutes with control of the breakdown of the hemicellulose components followed by separation of residue and a subsequent chemical treatment with a similar solvent mixture at elevated temperatures for a period of from 6 hours to 2 minutes. US Patent 5,411,594 discloses a hydrolysis process system for continuous hydrolysis saccharification of lignocellulosics in a two-stage plug-flow-reactor system. The process utilizes dilute-acid hydrolysis and is primarily by reverse
inter-stage transfer-flow, opposite to biomass, of second-stage surplus of: process heat; dilute-acid; and ingredient and solution water, all in an alpha cellulose hydrolysate, dilute-acid solution. The primary final product is the combined hydrolysate sugars in a single solution, including pentose, hexose and glucose sugars, which are fermented into ethanol and/or Torula yeast. The secondary final solid product is an unhydrolyzed lignin solid. A method of treating biomass material using a two-stage hydrolysis of lignocellulosic material is disclosed in US Patent 5,536,325. The conditions during the first stage is such as to hydrolyze or depolymerize the hemicellulosic component without substantial degradation of resulting monosaccharides and conditions during the second stage being such as to hydrolyze the cellulose to glucose without substantial degradation of the glucose. Hydrolysis in both stages is accomplished by the use of nitric acid, and the pH, retention time, and temperature in both stages are selected to maximize production of the desired monosaccharide or monosaccharides. US Patent 6,022,419 discloses a multi-function process for hydrolysis and fractionation of lignocellulosic biomass to separate hemicellulosic sugars from other biomass components such as extractives and proteins; a portion of the solubilized lignin; cellulose; glucose derived from cellulose; and insoluble lignin from the biomass by introducing a dilute acid into a continual shrinking bed reactor containing a lignocellulosic material at 94° to 160°C for 10 to 120 minutes at a volumetric flow rate of 1 to 5 reactor volumes to solubilize extractives, lignin, and protein by keeping the solid-to-liquid ratio constant throughout the solubilization process. A process for rapid acid hydrolysis of lignocellulosic material is disclosed in US Patent 5,879,463. The process is a continuous process for acid hydrolysis of lignocellulosic material through which delignification and saccharification are carried out in a single reaction cycle employing a solubilizing organic solvent of lignin and a strong and extremely diluted inorganic acid to obtain highly concentrated recoveries of sugar. There is a need when converting cereal grains and legumes to produce ethanol (particularly in common steam jet cooking of ground corn in which the objective is principally to improve enzymatic saccharification of starch) to not ignore the hemicellulose and cellulose, but instead, to provide a process for equally achieving high starch, hemicellulose and cellulose conversion so as to enhance production of fermentable sugars, and therefore provide higher ethanol production.
Disclosure of the Invention One goal of the present invention is to provide an effective pretreatment process for cereal grains and legumes, and biomass materials in general that goes beyond principally improving enzymatic saccharification of starch, by additionally also achieving high starch, cellulose and hemicellulose conversion, so as to provide additional fermentable sugars and therefore higher ethanol yield. Another goal of the present invention is to provide an effective pretreatment process for biomass materials, inclusive of cereal grains and legumes, at high solids before the starch saccharification and ethanol fermentation stages to achieve higher ethanol concentration in the fermentation of broth. A further goal of the present invention is to provide high solids conversion of biomass materials, inclusive of cereal grains and legumes, to obtain conversion at high solids to obtain higher ethanol yield and high-protein feed co-products such as dry distillers grain with solubles (DDGS). The foregoing and other goals of the invention will be better understood by reference to the detailed description of the preferred embodiments of the invention. Detailed Description of Prepferred Embodiments The present invention is a pretreatment process that solubilizes essentially all the starch and a large portion of the hemicellulose, and thus enhances the enzymatic digestibility of the cellulose in the fiber. The chemical composition of typical yellow dent corn kernel is as follows: Component % dry weight Starch 71.7 Protein 9.5 Fat 4.3 Pentosan (as xylose) 6.2 Cellulose 3.0 Sugar 2.6 Ash 1.4
With the conversion of bound starch and fiber (cellulose and hemicellulose) the ethanol yield may be increased as much as 15%. Furthermore, with most of the carbohydrates converted to ethanol, the crude protein content of the residual solids
(or distiller's grain or DG) would increase significantly to about 45%. The new high- protein DG product would then be suitable as a feed supplement for poultry and swine. In the process of the invention cereal grains (such as corn kernel, wheat, barley, milo, rice, or a combination of these), legume (e.g., beans), or materials derived from these grains and legume (e.g., hulls) are ground (if necessary to facilitate the pretreatment) using a hammer mill or similar size-reduction equipment. The feedstock is then acid impregnated with dilute acid solution in a high-speed mixer/blender. Thin stillage (filtrate or centrate from the whole stillage liquid/solid separation equipment) could be used to make up the dilute acid solution. Low- pressure steam may be added to the mixer to improve the acid impregnation effectiveness. The acid may include one or more of the following: sulfuric, nitric, hydrochloric, sulfur dioxide or any strong acid that effects a pH of less than 4, preferably between about 1.5 and about 3, in the liquid portion of the acid- impregnated grain or feedstock. The solid content of acid-impregnated raw material is preferably in the range of about 40% to about 70% to facilitate quick heating by direct steam injection while minimizing steam consumption. The acid-impregnated feedstock is fed into a batch or continuous pretreatment reactor (such as an expander/extruder normally used in the food and feed industry), in which reactor walls could be heated externally with heat jackets (using steam, electric heater or heating fluid) to maintain the desired reaction temperatures. Steam is directly injected into the reactor to heat the acid-impregnated grain to a temperature of from about 120° to about 190°C, preferably from about 140° to about 160°C. When the temperature of the mixture reaches the desired pretreatment temperature, the mixture is held at that temperature (either inside the reactor or in a retention vessel or tube connected to the reactor) for a period from about 0.5 minutes to about 30 minutes to allow gelatinization and limited solubilization of the starch and breakdown of the non-starch carbohydrate and lignin matrix to improve the enzyme accessibility. A significant portion of the hemicellulose is solubilized by this pretreatment. The pretreated material is sent to a flash tank where a portion of the steam condensate and liquid are flashed off and the heat is recovered. The invention pretreatment method provides advantages over the conventional jet cooking. They are: (1) high solids pretreatment, which results in higher product
concentration and low energy requirements for ethanol recovery; (2) more effective pretreatment of cellulose and hemicellulose, which leads to higher ethanol yield when appropriate saccharification enzymes are added, and (3) less degradation of soluble protein as a significant portion of the recycled thin stillage bypasses the pretreatment step. Thin stillage is added to the pretreated material in the flash tank. The slurry is cooled down to the operating temperatures of the liquefying enzyme alpha-amylase. The pH of the slurry is adjusted to the desirable pH for enzymatic hydrolysis by adding an alkali (such as calcium hydroxide, sodium hydroxide or ammonia). Alpha amylase is then added to the slurry to reduce the visiosity of the slurry. The liquefied starch slurry is forwarded to the hydrolysis reactor. A saccharification enzyme cocktail that includes at least two of the following enzymes: glucoamylase, cellulase, hemicellulase (e.g., xylanase and arabinase) lipase, cutinase, esterase and protease, is added to the hydrolysis reactor, either together or in sequence to achieve up to about 50%) hydrolysis of starch and non-starch carbohydrates to their respective sugars. The hydrolysate slurry from the enzymatic hydrolysis step is cooled down to the desired fermentation temperature before being forwarded to the ethanol fermentors. Ethanol fermentation is carried out using a fermenting organism with a broad sugar utilization capability or using a mixture of fermenting organisms to achieve broad substrate utilization. If more than one fermentating organism is used, the fermentation may be carried out either in co-culture or in sequence mode. In the fermentation step, further enzymatic hydrolysis may still take place since end product inhibition is lessened as the sugar concentration decreases. For yeast fermentation, a mixed culture of C6- and C5- utilizing yeast strains is used. In this case, a small amount of air is added to achieve micro-aerophilic conditions. Alternatively, recombinant organisms capable of fermentating C6 and C5 sugars may be used. Ethanol is recovered from the fermentation broth by distillation. The residual solids are separated from the whole stillage by centrifugation pressurized membrane filtration. Part of the thin stillage i.e., the centrate or the filtrate from the solids/liquid (S/L) separation step is recycled to the enzymatic hydrolysis step. The remainder of the thin stillage is sent to the evaporator and concentrated to a syrup. The solids from the S/L separation step (or the DG cake) are sent to a low-temperature dryer. The syrup is then blended into the dried DG cake to form the high-protein DDGS product.
The foregoing description is illustrative only, since numerous modifications and changes will readily occur to those skilled in the art. Accordingly, the invention is not limited to the exact construction and operation shown and described, as suitable modifications and equivalents may be resorted to within the scope of the invention, which is defined by the claims.