IMPROVED HOMOACIDOGENIC FERMENTATION AND INDIRECT PROCESS FOR PRODUCING ALCOHOLS

FIELD OF THE INVENTION

[0001] This invention pertains to improved processes for the production of organic acids by the fermentation of carbohydrates. The processes of this invention enable the high yield of organics from the fermentation without the need to convert the acid being produced to salts. The processes are especially useful in the indirect production of ethanol by the fermentation of carbohydrates to produce acetic acid which is then converted to alcohols, especially ethanol. BACKGROUND TO THE INVENTION

[0002] Homoacidogenic fermentation processes are well known. In these processes, a carbohydrate is converted to an organic acid. In these processes, the acid, if not neutralized, will eventually cause the fermentation menstruum to become so acidic that the microorganism will be killed or inactivated. One approach for homoacidogenic fermentation is to conduct the fermentation in a dilute aqueous medium. Water must be removed to provide the sought acid. Also, adding a base, such as calcium oxide, sodium hydroxide and potassium hydroxide can avoid a build-up of acid. However, the salts must be recovered and reacidified. [0003] An interest exists in producing ethanol from biomass as fuel ethanol, either as an additive to or a replacement for liquid transportation fuels or as chemical feedstock. An important consideration in any process for the synthesis of alternative fuels is the energy ratio. The energy ratio is the ratio of the energy produced divided by the energy consumed in making the alternative fuel. Also of concern is the waste discharge from such a process. A further concern is the efficiency of conversion of the biomass to the sought product.

[0004] Ethanol can be synthesized from biomass by various processes, including through fermentation. The direct fermentation of a sugar as the biomass with yeast results in the inherent generation of carbon dioxide and results in usually less than 65, and sometimes less than 50, percent of the sugar being converted to ethanol. [0005] Another process for making ethanol is the indirect process in which the fermentation results in the production of acetic acid. The acid is not readily hydrogenated and thus is converted into an ester and then hydrogenated. The advantage is that the sugars can be converted to acetic acid in nearly 100 percent yield. This homoacidogenic

fermentation is well known. See, for instance, U.S. Patent Nos. 4,371,619; 4, 506,012; 4,935,360 and 6,509,180. U.S. Patent Nos. 6,509,180 and 7,074,603 disclose corn dry milling to provide the sugars for fermentation to produce acetic acid for conversion to ethanol. [0006] Although the yield of fermentation product per unit of fermentable biomass is substantially greater for the indirect process than for direct fermentation to ethanol, consideration must still be given to the energy consumption per unit volume of ethanol produced. The indirect process involves three key process steps. First is the fermentation. The fermentation menstruum provides a relatively dilute solution in water, often less than about 5 mass percent. And to even reach this level, the acetic acid fermentation product is neutralized. Hence not only must solids be removed, but also distillation or solvent extraction is used to remove water or esterification product to allow the esterification to proceed. Then the esterification product is hydrogenated and the product ethanol must be obtained. Not only does the indirect process pose additional steps but also the associated unit operations represent consumption of energy, thus lowering the energy ratio.

[0007] U.S. Patent Nos. 6,509,180 and 7,074,603 propose the use of a reactive distillation for esterification. In this process, the liquid from the fermentation menstruum which contains calcium acetate and about 95 percent water is contacted in the reactive distillation column with carbon dioxide and an excess of ethanol. An azeotrope of ethanol, ethyl acetate and water is taken as an overhead. The patentees state that the azeotrope boils at about 70°C. A water, ethanol and calcium carbonate mixture constitutes the bottoms stream. The azeotrope must be broken to obtain the ethyl acetate for hydrogenation to ethanol. The patentees suggest doing this by the addition of water for a phase separation. Although the process has the advantage of not having to distill the water from the ethyl acetate, which represents an energy savings, the presence of water in such large concentrations, hinders the rate of esterification.

SUMMARY OF THE INVENTION

[0008] By this invention, homoacidogenic processes are provided that facilitate the generation of acid in high yields without the need for conversion of the acid to a salt. Moreover, the processes enable recovery of the sought fermentation product from an aqueous fermentation medium without the need for distillation or the use of reverse osmosis. In processes of this invention, the organic acid generated by the fermentation is

converted to an esterification product which is in a water-immiscible, liquid phase. The water-immiscible phase can be removed from the aqueous fermentation medium and processed, e.g., to provide the organic acid, or to make an alcohol of the organic acid. These processes are most useful for the production of ethanol. [0009] In the broad aspects, processes of this invention may desirably comprise: a. subjecting an aqueous menstruum containing carbohydrate to homoacidogenic fermentation conditions to provide an organic acid, said conditions comprising the presence of nutrients and an acid-producing microorganism; b. esterifying at least a portion of said organic acid with alcohol, preferably comprising primary alcohol, to provide an ester product, said alcohol being of a type and in a concentration insufficient to be unduly adverse to said acid-producing microorganism, said esterifying occurring in the presence of said aqueous menstruum and a catalytically-effective amount of esterification catalyst, said esterifying being at a rate sufficient to maintain the pH of the aqueous menstruum above that which would unduly adversely affect the acid-producing microorganism; c. contacting said aqueous menstruum with a water-immiscible, liquid phase to provide a mixed liquid phase medium, wherein the esterification product is selectively provided in the water-immiscible, liquid phase; and d. removing from the mixed liquid phase medium at least a portion of the water-immiscible, liquid phase. [0010] Because the water-immiscible phase can be easily removed, processes of this invention are particularly suited for continuous fermentation processes, i.e., steps (a) and (b) are continuous. The homoacidogenic fermentation and esterification can occur in the same zone, or a portion of the fermentation menstruum can be withdrawn, subjected to esterification with the water-immiscible phase removed, and the menstruum recycled to the fermentation zone.

[0011] The water-immiscible, liquid phase can occur in a number of ways. For instance, a water-immiscible solvent that is substantially non-toxic to the fermentation

microorganism may be provided into which one or both of the organic acid and ester are soluble. The alcohol for the esterification may be water-immiscible and for part or all of the water-immiscible phase. And the ester may form all or part of the water-immiscible phase. [0012] The alcohol for the esterification should be relatively non-toxic to the microorganism for the homoacidogenic fermentation. Especially preferred alcohols are aliphatic, primary alcohols of at least 3 carbon atoms. Where the alcohol provides the water-immiscible phase, it preferably has at least 4, and more preferably at least 5, and usually between about 8 and 24 carbon atoms. [0013] The catalyst for the esterification can be heterogeneous or homogeneous and may be in the aqueous phase, water-immiscible phase, or both. Thus, the esterification catalyst may be solid esterification catalyst, dissolved catalyst or even an esterase capable of converting organic acid to ester. [0014] As stated above, processes of this invention are particularly useful for the indirect synthesis of ethanol. These processes may desirably comprise: a. subjecting an aqueous menstruum comprising carbohydrate-containing feed to homoacidogenic fermentation conditions to provide a fermentation product comprising acetic acid, said conditions comprising the presence of nutrients and an acid-producing microorganism; b. esterifying at least a portion of the acetic acid with alcohol, preferably comprising primary alcohol, to provide an ester product, said alcohol being of a type and in a concentration insufficient to be unduly adverse to said acid-producing microorganism, said esterifying occurring in the presence of said aqueous menstruum and a catalytically-effective amount of esterification catalyst, said esterifying being at a rate sufficient to maintain the pH of the aqueous menstruum above that which would unduly adversely affect the acid-producing microorganism; c. contacting said aqueous menstruum with a water-immiscible, liquid phase to provide a mixed liquid phase medium, wherein the

esterification product is selectively provided in the water-immiscible, liquid phase; d. removing from the mixed liquid phase at least a portion of the water-immiscible, liquid phase; e. hydrogenating at least a portion of the esterification product in the removed water-immiscible phase to provide ethanol and said alcohol; and f. recycling at least a portion of said alcohol from step (e) to step (b). Preferably step (c) is conducted under agitation conditions, most preferably high turbulence mixing, that increases the surface area of the interface between the aqueous menstruum and the water-immiscible, liquid phase. Steps (b) and (c) may be conducted simultaneously or sequentially. Where sequential, preferably at least a portion of the aqueous menstruum-containing fraction of step (c) is recycled to step (a).

[0015] Those skilled in the art and guided by the teachings herein provided will appreciate that a process or selected process steps referred to herein as being "continuous" or being conducted in a "continuous" manner may require a period of time and/or operation to, respectively, arrive at or shut down from a desired state of operation. In particular, such processing may involve a "ramping up" to arrive at a sought operation and/or a "ramping down" from such operation, such as in the event of process shut down. For instance, the concentration of acetic acid in the aqueous menstruum may be allowed to build-up during an initial operational phase prior to initiating esterification. Similarly, esterification may proceed before introducing any additional water-immiscible phase. The addition of alcohol to provide the esterification product may also change with time, yet still have a continuous operation. If desired, the addition rates of alcohol and of any water-immiscible solvent may be constant through the duration of the continuous period of the process.

[0016] Those skilled in the art and guided by the teachings herein provided will also appreciate that a continuous process or processing step in accordance with the invention may in practice generally have a duration that is limited or restricted such as by the viability of microorganisms employed in such process or processing step, for example. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic description of an apparatus for synthesizing ethanol in accordance with processes of this invention.

[0018] Figure 2 is a schematic description of another type of apparatus for synthesizing ethanol in accordance with processes of this invention.

[0019] Figure 3 is a schematic description of yet a further type of apparatus for synthesizing ethanol in accordance with processes of this invention DETAILED DESCRIPTION

[0020] The homoacidogenic fermentation processes of this invention are suitable for the production of a wide variety of organic acids, including diacids, especially those having from 1 to 5, especially one to 3, carbon atoms. The acids may or may not be substituted, e.g., with hydroxyl or lower alkoxy moieties. Exemplary acids include, but are not limited to formic acid, acetylformic acid, acetic acid, hydroxyacetic acid, methoxyacetic acid, propionic acid, hydroxypropionic acid, and butyric acid. [0021] Carbohydrates are compounds containing carbon, oxygen and hydrogen that contain a saccharose unit or its first reaction product and in which the ratio of hydrogen to oxygen is the same as in water. Any suitable carbohydrate-containing feedstock may be used in the processes of this invention that is converted to acetic acid by the chosen microorganism for the fermentation. Examples of carbohydrate-containing feedstocks are cellulosic materials such as derived from wood, grasses, cotton, corn stover, and the like, especially hemicellulosic materials; starches and sugars including, but not limited to, xylose, sucrose, dextrose, fructose, lactose, maltose, cellobiose, gum Arabic, tragacanth, and the like. The sugars may be derived from various sources such as sugar cane, sugar beet, milk, milo, grapes, sorghum, maple syrup, corn, and the like.

[0022] The carbohydrate-containing feedstocks may be used directly, but most often are pretreated to recover other useful components therefrom or to convert the carbohydrate into a form more suitable for fermentation. Examples of pretreatment include milling; extraction; fermentation to an intermediate such as hydroxypropionic acid thereof or acetylformic acid, especially where a lower molecular weight acid is sought; enzyme hydrolysis and chemical treatment such as hydrolysis. Particularly advantageous sources of carbohydrate-containing feedstocks are sugar cane, sugar beets, wheat and corn. The corn may be dry milled or wet milled to recover other useful products. If desired, the feedstock may be pretreated to remove oils, if present, e.g., glycerides, or proteins.

[0023] The fermentation is preferably an anaerobic fermentation and is conducted in an aqueous menstruum in the presence of nutrients and growth factors for the

microorganism. Numerous microorganisms are known for homoacidogenic fermentation. Representative acidogenic microorganisms are those of the Acetobacterium, Clostridium, Lactobacillus, and Peptostreptococcus species, such as Clostridium thermoaceticum, Acetogenium kivui, Acetobacterium woodii, Clostridium formicoaceticum, Lactobacillus casei.Lactobacillius delbruckii, Lactobacillus heiveticus, Lactobacillus acidophilus, Lactobacillus amylovorus, Lactobacillus leichmanii, Lactobacillus bulgaricus. Lactobacillus amylovorus, Lactobacillus pentosus, Propionibacterium shermanii, Clostridium butyricu, Clostridium tyrobutylicum, Propionibacterium acidipropionic, and Clostridium thermobutyricum. [0024] The conditions of the fermentation can fall within a broad range depending upon the microorganism used and the fermentor design. Generally, the concentration of carbohydrate to water is in the range of about 2 to 50, preferably 3 to 20, and most often between about 3 and 10, mass percent. Amino acids and trace metals and other components may need to be provided, if not contained in the feedstock, to assure a sufficient nutrient medium for the microorganisms. Buffers may also be present. The temperature of the fermentation is often within the range of about 25° to 75°C, say, about 40° to 70°C. The fermentation may be conducted in batch or continuous or semi-continuous modes. Advantageously, the fermentation vessel is agitated, e.g., by stirring, pumped recycle or vibration. The microorganism may be dispersed in the fermentation menstruum or growing on a solid support such as activated carbon, pumice stone and corn cob granules. The fermentation may occur in a single stage, or two or more sequential fermentation stages may be used.

[0025] The conversion of the carbohydrate to acetic acid or salt is usually at least about 90, preferably at least about 95, and sometimes in excess of 98, percent. Typically the fermentation liquid contains between about 2 and 7, most frequently between about 3 and 6, mass percent acid (calculated as the acid).

[0026] The pH of the fermentation menstruum is typically maintained at a suitable level for the growth of the microorganisms. In accordance with one aspect of the invention, the pH is maintained by esterifying and removing the ester at a sufficient rate to achieve the desired pH. Usually the pH is within the range of about 2 to 7, say, 3 or 4 to 7. The pH selected will depend, in part, upon the tolerance and productivity of the

microorganism for the homoacidogenic fermentation. The fermentation menstruum can have a lower pH with more acid-tolerant microorganisms.

[0027] In processes of this invention, an organic ester is formed with alcohol.

Preferred feed alcohols comprise primary alcohols and may contain secondary and tertiary alcohols. As these secondary and teriary alcohols have a slower reaction rate, the amount contained in the preferred alcohol feed should not be so great as to unduly adversely affect the process. In general, of the alcohol feedstock, at least 50, and preferably at least about 80, mole percent of the alcohol is primary alcohol. The alcohols have sufficient carbons that at least the formed ester is substantially water insoluble. Thus often the alcohol has at least 4 carbon atoms, and preferably at least about at least about 6, preferably at least about 8, and most conveniently between about 8 and 24, carbon atoms. The primary alcohol used in this invention should be relatively non-toxic to the microorganisms used for the fermentation. Hence aliphatic alcohols are generally used. Suitable such alcohols include propanol, isoproanol, butanol, isobutanol, pentanol, methylpentanol, hexanol, lauryl alcohol, cetyl alcohol, alcohol derived from biodiesel and the like.

[0028] In processes of this invention, the esterification is conducted in the presence of the aqueous fermentation menstruum and in the presence of a substantially water-immiscible, liquid phase in which the ester is soluble. The substantially water-insoluble, liquid phase is any suitable organic material that is liquid under the conditions of the esterification. The substantially water-insoluble, liquid phase comprises the alcohol for the esterification or is a liquid in which the alcohol is soluble. Suitable organic liquids are substantially non-toxic to the microorganisms for fermentation and include hydrocarbons having at least 4 carbon atoms, such as butanes, hexanes, octanes, dodecanes, petroleum fractions including kerosenes, white oils, and naphthas, biodiesel and mixtures thereof. Preferably the primary alcohol to be used for the esterification comprises at least a portion of the water-immiscible phase.

[0029] The volume ratio of the water-immiscible phase to the aqueous fermentation menstruum with which it contacts may vary widely and will depend upon the apparatus and conditions used. For instance, where the fermentation and esterification are only conducted in the same zone, the volume ratio may range from 5:100 to 50:100 or more. Where a slip stream of fermentation media is taken and contacted with the water-immiscible phase, it is feasible to have a volume ratio of 20:1 or more. The primary alcohol is provided in a

molar ratio to acid of at least 0.5:1, preferably at least about 1.5:1, and sometimes as high as 100:1 or 200:1 or more.

[0030] Esterifi cation conditions typically comprise the use of elevated temperature and the presence of esterification catalyst. The pressure for esterification is not critical but should be sufficient to maintain the acid and alcohol in the liquid state. Advantageously, the esterification is conducted under substantially the same conditions as the fermentation. However, where a slip stream is taken from the fermentation of contact with the water-immiscible phase, different conditions may be used, but preferably not such that any material damage will be done to the microorganism. Usually the pressure is between about 50 kPa absolute to about 10 MPa absolute. Temperatures for esterification are often in the range of about 50 ⁰ C to 300°C, say, about 7O ⁰ C to 250 ⁰ C.

[0031] The catalyst may be heterogeneous or homogeneous. Where an organic layer is provided or is formed in the reaction medium, catalyst is preferably contained in the organic layer or at least is such that it is active at the interface between the organic layer and the aqueous medium. Typical esterification catalysts are acidic, and hence preferably reside mostly in the water-immiscible phase to avoid deleteriously affecting the microorganism. In one embodiment of the invention, a slip stream from the fermentation zone is taken and most of the solids removed by filtration or centrifugation. The solids can be returned to the fermentation vessel and the nascent liquid, which will contain little, if any, of the microorganism, can be subjected to more acidic conditions. Catalysts for esterification include acids such as carbonic acid, hydrochloric acid, sulfuric acid, sulfonic acid, especially toluene sulfonic acid, acidic molecular sieves, ion exchange resins, especially Nafion™ resins, and esterases. Preferred catalysts are solid catalysts and those highly soluble in the water-immiscible, liquid phase, especially the alcohol, such as alkylbenzene sulfonates, e.g., toluene sulfonate (preferably p-toluene sulfonate), nonylbenzene sulfonic acid, and the like. Another preferred catalyst are esterases. Esterases are typically present in the aqueous menstruum as opposed to the water-immiscible liquid phase. One preferred mode of operation with esterases and other catalysts preferentially located in the aqueous medium, is to use a water miscible alcohol that converts to a water-immiscible ester, e.g., n-butanol. The catalyst is provided in a catalytically effective amount. The catalyst is provided in an amount of at least about 0.005, say, 0.01 to 20, mass percent based upon the mass of acid.

[0032] It is preferred that the esterifϊcation be conducted under conditions that provide high surface area interfaces such as by high turbulence mixing and ultrasonic agitation. High turbulence mixing can be by stirring or by reactor design, such as the presence of vanes, tortuous microtubes, and the like to physically disperse the phases. Desirably, the high turbulence mixing is not so vigorous that undue lysing of the fermentation microorganism occurs.

[0033] Preferably, the duration of the esterifϊcation is sufficient to convert at least about 20, and preferably at least about 30, mole percent of the acid to ester. It is not essential to convert a high percentage of the acid to ester as the unreacted acid can be recycled to the esterifi cation reactor or fermentation reactor. Rather, the rate of removal of acid needs to be sufficient to maintain the desired pH. Often, the organic acid has some solubility in the water-immiscible phase. Hence not only will the pH be controlled by conversion of the acid to an ester, but also by acid being dissolved in the water-immiscible phase. Advantageously, the processes of this invention, regardless of the conversion to ester, facilitate a low energy separation of water from the sought acid and ester product.

[0034] At least periodically, and preferably continuously, a portion of the water-immiscible phase is withdrawn for recovery of the sought organic, whether it be acid, the ester or a subsequent synthesis product such as an alcohol, amine, aldehyde or the like. Any suitable technique may be used for further processing. For instance, to generate the acid, the water-immiscible phase may be contacted with ion exchange resin and hydrolyzed to generate the acid and alcohol. A particularly useful application of the processes of this invention is to generate alcohol such as ethanol, propanol, and the like from the ester. The ester is much more readily hydrogenated to provide the alcohol than is the acid. Since the homoacidogenic fermentation is nearly 100 percent efficient to the production of acid, high conversions to alcohol can be achieved while minimizing the discharge of carbon dioxide.

[0035] The manner in which water-immiscible phase containing the esterification product is recovered will, in part, be determined by the nature of the esterification process. Often phase separation will be adequate. [0036] Where generating an alcohol, the hydrogenation may be conducted in the liquid or vapor phase. Due to the high boiling point of the esters, the hydrogenation is preferably conducted in the liquid phase or an ebulating or trickle bed where the liquid is

mixed with gaseous hydrogen. Hydrogenation conditions include the presence of hydrogen at elevated temperatures and pressures in the presence of a catalytically-effective amount of selective hydrogenation catalyst. The hydrogenation should not be so severe that neither the product alcohol such as ethanol nor the primary alcohol is converted to hydrocarbons. [0037] A number of options exist for the hydrogenation. One mode of hydrogenation is to conduct the hydrogenation to substantially consume the introduced hydrogen, albeit at a loss of conversion. In this mode, the reaction medium containing ester can be recycled. The advantages of this mode of hydrogenation is that lower hydrogen partial pressures, and hence lower reaction pressures can be used, saving costs in hydrogen compression and in the installation of a stripping column to recover unreacted hydrogen. Although the per pass conversion of ester to alcohol may be low, pumping costs for liquids are relatively inexpensive. In this mode, the hydrogenation may be conducted at between about 300 and 5000 kPa absolute with between about 0.1 and 0.9, say, 0.2 and 0.7, moles of hydrogen per mole of ester. [0038] Another mode of hydrogenation is the conventional, higher pressure hydrogenation where high conversion of ester to alcohols is achieved on a per pass basis. Typically in this mode, the hydrogenation is conducted at a pressure of at least about 3 MPa, say, 3 to 50 MPa (absolute). Typically at least about 1.1, and more frequently about 2, preferably 2 to 8, moles of hydrogen are provided per mole of ester. [0039] In either mode of hydrogenation, the temperature is often in the range of about 150 ⁰ C to 300 ⁰ C. Hydrogenation catalysts comprise a hydrogenation metal component which may be one or more metals selected from noble metals and base metals. The noble metal can desirably be a platinum-group metal is selected from platinum, palladium, rhodium, ruthenium, osmium, iridium and mixtures thereof. The base metal can desirably be selected from the group consisting of rhenium, chromium, tin, germanium, lead, cobalt, nickel, iron, indium, gallium, zinc, uranium, dysprosium, thallium, and mixtures thereof. A promoter or modifier may also be used in the catalyst formulation. Such promoters or modifiers are one or more of base metals, IUPAC groups 1, 2, 5, 6, 7, 11, 12, 13, 14, 15, 16 and 17. The catalyst may be supported or unsupported. Supports include carbonaceous supports and refractory oxides such as silicas, aluminas, silica-aluminas including molecular sieves, and the like. Raney nickel, nickel, rhenium,

mixtures of nickel and rhenium, iridium, and copper chromite are examples of hydrogenation catalysts.

[0040] The sought product alcohol, such as ethanol, can be recovered during or subsequent to the hydrogenation by distillation from the higher, primary alcohol and any unreacted ester, acetic acid and any other organic material used to form the water-immiscible phase. At least a portion of the water-immiscible phase can be recycled to the fermentation. If desired, another portion can be recycled to the hydrogenation operation. [0041] The hydrogenation may also convert glycerides present to the corresponding alcohols and glycerin. The alcohols can be recycled as alcohol for the esterification. A purge stream may be taken to maintain steady state operation in a continuous process. This purge can be used as biofuel after suitable processing to remove undesirable components, e.g., glycerin to provide a biodiesel product. [0042] The drawings are provided to facilitate an understanding of the invention but are not in limitation of the invention.

[0043] With respect to Figure 1, an apparatus 100 is provided for the indirect process to make ethanol wherein the esterification occurs in the same zone as the fermentation to make the acid. As shown, a carbohydrate feedstock is passed via line 102 to fermentation vessel 104. The feedstock, for purposes of this discussion is an aqueous solution of corn sugars. Also fed to fermentation vessel 104 are nutrients via line 106. hi fermentation vessel 104 acetic acid is generated by microorganisms. The fermentation vessel also contains a water-immiscible phase comprising higher, primary alcohol. The fermentation vessel is agitated to not only admix the components of the aqueous fermentation menstruum but also to provide contact area with the water-immiscible phase for consuming acetic acid by esterification.

[0044] The fermentation menstruum is withdrawn from fermentation vessel 104 and passed via line 110 to separator 112. A purge of the fermentation menstruum can be periodically or continuously discharged via line 108. As shown, the purge is taken at the bottom of fermentation vessel 104 and thus will contain little of the water-immiscible phase.

[0045] Separator 112 may be any convenient separation device to separate the aqueous fermentation menstruum from the water-immiscible phase. Because the apparatus

recycles water-immiscible phase to the fermentation vessel, the separator need not be highly efficient. Nevertheless, it is desired to minimize the amount of water in the water-immiscible layer. As shown, separator 112 is a phase separator and aqueous phase is withdrawn via line 114 and can be recycled to fermentation vessel 104. [0046] The organic layer in separator 112 will contain the acetic ester, unreacted higher alcohol and acetic acid. Ethyl acetate can be formed via transesterification between product alcohol and the starting ester and thus will also be contained in the organic layer. This layer is passed via line 116 to hydrogenation reactor 118 containing solid hydrogenation catalyst, e.g., Raney nickel catalyst. Hydrogen is introduced into hydrogenation reactor 118 via line 120. In hydrogenation reactor 118, ethanol and higher alcohol are formed. The amount of hydrogen provided is sufficiently low that it is essentially completely consumed. The reaction product, which is ethanol, higher alcohol, unreacted acetate ester, acetic acid and any additional organic material used to form the water-immiscible phase, is passed via line 122 to distillation column 124. [0047] In distillation column 124, ethanol is stripped and a higher boiling fraction containing acetic acid, acetate ester of the higher alcohol, the higher alcohol, ethyl acetate and any organic material used to provide the water-immiscible phase is obtained. The lower boiling fraction, which is ethanol, from distillation column is recovered via line 126 as product ethanol. To the extent that water is present, it is below that which forms an azeotrope with water.

[0048] The higher boiling fraction is recycled via line 128 to hydrogenation reactor

118. A portion of the higher boiling fraction is passed to fermentation vessel 130 via line 130. Make-up esterification catalyst, e.g., toluene sulfonic acid, can be provided via line 132. [0049] As can be seen, the processes of this invention provide for an indirect process for producing ethanol in an energy efficient manner.

[0050] In Figure 2, apparatus 200 is adapted to produce ethanol by the indirect process using two esterification stages. As shown, a carbohydrate feedstock is passed via line 202 to fermentation vessel 204. The feedstock, for purposes of this discussion is an aqueous solution of corn sugars. Also fed to fermentation vessel 204 are nutrients via line 206. In fermentation vessel 204 acetic acid is generated by microorganisms. The fermentation vessel also contains a water-immiscible phase comprising higher, primary

alcohol. The fermentation vessel is agitated to not only admix the components of the aqueous fermentation menstruum but also to provide contact area with the water-immiscible phase for consuming acetic acid by esterification.

[0051] Fermentation vessel 204 contains annular tube 208 having sparger 210 in a lower portion. Sparger 210 is adapted to introduce water-immiscible phase, containing primary alcohol and transesterifϊcation catalyst, which rises in annular section 208, carrying with it a co-current flow of aqueous fermentation menstruum. The aqueous fermentation menstruum then passes on the outside of annular tube 208 to the bottom of fermentation vessel 204 to form a cyclic pattern. Acetic acid is absorbed in the water-immiscible phase as well as reacted to form an ester. Coalescer 212 in an upper portion of fermentation vessel 204 serves to facilitate phase separation and a water-immiscible phase is withdrawn from fermentation vessel 204 via line 214 and passed to esterification reactor 216. Since the aqueous fermentation menstruum has been separated, the esterification conditions in esterification reactor 216 may be selected based upon desired conversion without regard to effect on the microorganism.

[0052] The esterification product is passed from esterification reactor 216 to hydro genation reactor 220 via line 218. Hydro genation reactor 220 is a conventional, high pressure hydrogenator designed to achieve high conversion of the ester to product alcohol, ethanol, and the higher molecular weight primary alcohol. The hydrogenation product exits via line 222 and is passed to flash stripper 224 for removal of hydrogen which is returned to hydrogenation reactor 220 via line 226. Make-up hydrogen is provided via line 228.

[0053] The liquid from flash stripper 224 is passed via line 230 to distillation column 232 where a product ethanol stream is provided via line 234 and a water-immiscible bottoms stream is recycled via line 236 to sparger 210. Make-up catalyst can be provided via line 238.

[0054] Fermentation vessel 204 is provided with line 240 for purging aqueous fermentation menstruum. [0055] Figure 3 depicts an embodiment of the invention wherein a slip stream from the fermentation vessel is subjected to esterification. In apparatus 300, a carbohydrate feedstock is passed via line 302 to fermentation vessel 304. The feedstock, for purposes of this discussion is an aqueous solution of corn sugars. Also fed to fermentation vessel 304

are nutrients via line 306. In fermentation vessel 304 acetic acid is generated by microorganisms. A slip stream is withdrawn via line 308 and contains the aqueous fermentation menstruum including acetic acid and microorganism, and is passed to solids separator 310. [0056] Solids separator 310 serves to provide a concentrated, solid-containing phase which is rich in the microorganism. Solids separator may be a filtration device, or even more conveniently, a centrifuge. The concentrated, solids-containing phase, is passed via line 312 for recycle to fermentation vessel 304. The aqueous phase having a reduced concentration of solids is passed from solids separator 310 via line 314 to esterifi cation reactor 316. Esterifϊcation reactor 316 is operated as a liquid-liquid extraction vessel for contact between aqueous fermentation menstruum containing acetic acid and water-immiscible, liquid phase containing primary alcohol. Packing 318 is provided in esterifϊcation reactor 316 to enhance contact between the phases and on the packing is supported acidic esterifi cation catalyst (Nation™ resin). An aqueous phase is withdrawn from esterification reactor 316 via line 320 for recycle to fermentation vessel 304. A purge of aqueous fermentation menstruum can be taken via line 322.

[0057] The water-immiscible phase from esterification vessel 316 contains ester and is passed via line 324 to phase separator 326. Phase separator 326 serves to remove aqueous menstruum entrained in the water-immiscible phase, and the removed aqueous menstruum is withdrawn via line 328 and may be recycled, if desired, to fermentation vessel 304. Line 330 serves to direct the water-immiscible phase provided by phase separator 326 to hydrogenation reactor 332 for hydrogenation of ester to product alcohol, ethanol, and primary alcohol. Hydrogen is provided by line 334 to hydrogenation reactor 332. As with Figure 1, the apparatus in Figure 3 operates on a total consumption mode of hydrogen. The hydrogenation product is passed via line 336 to distillation column 338. Ethanol product is stripped and is directed via line 340. The higher boiling fraction, which contains primary alcohol and ester, is passed via line 342 to esterification reactor 316 as the water-immiscible liquid. A portion of this stream can, if desired, be directed via line 344 to line 330 for recycle to hydrogenation reactor 332. The stripped ethanol in line 340 can be directed to condenser 346. Hydrogen and non-condensables are removed via line 348 and ethanol is directed to product storage via line 350.