Hydrogenation process with reduced residence time for vapor phase reactants

The present invention is directed to the separation of ethanol from a crude ethanol product obtained from the vaporization and hydrogenation of acetic acid using reduced residence time for the vapor phase reactants between exiting the vaporizer and entering the reactor. The crude ethanol product is separated in at least one column to recover an ethanol product.

FIELD OF THE INVENTION

The present invention relates generally to processes for producing alcohol by hydrogenation. The reactants, a mixture of acetic acid and ethyl acetate, are converted to a vapor phase and the residence time of the vapor phase reactants is reduced to control esterification of the acetic acid reactant.

BACKGROUND OF THE INVENTION

Ethanol for industrial use is conventionally produced from organic feed stocks, such as petroleum oil, natural gas, or coal, from feed stock intermediates, such as syngas, or from starchy materials or cellulose materials, such as corn or sugar cane. Conventional methods for producing ethanol from organic feed stocks, as well as from cellulose materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in organic feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulose materials, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol, which is suitable for fuels or human consumption. In addition, fermentation of starchy or cellulose materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.

Ethanol production via the reduction of alkanoic acids and/or other carbonyl group-containing compounds has been widely studied, and a variety of combinations of catalysts, supports, and operating conditions have been mentioned in the literature. During the reduction of alkanoic acids, e.g., acetic acid, other compounds are formed with ethanol or are formed in side reactions. These impurities limit the production and recovery of ethanol from such reaction mixtures. For example, during hydrogenation, esters are produced that together with ethanol and/or water form azeotropes, which are difficult to separate. In addition, when conversion is incomplete, acid remains in the crude ethanol product, which must be removed to recover ethanol.

EP02060553 describes a process for converting hydrocarbons to ethanol involving converting the hydrocarbons to ethanoic acid and hydrogenating the ethanoic acid to ethanol. The stream from the hydrogenation reactor is separated to obtain an ethanol stream and a stream of acetic acid and ethyl acetate, which is recycled to the hydrogenation reactor.

The need remains for improved ethanol production processes obtained by reducing alkanoic acids, such as acetic acid, and/or other carbonyl group-containing compounds.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention is directed to a process for producing ethanol, comprising hydrogenating a feed stream in a reactor in the presence of a catalyst to form a crude ethanol product, and separating at least a portion of the crude ethanol product in two or more columns to produce ethanol and a liquid recycle stream. The feed stream is produced by vaporizing an acetic acid stream and the liquid recycle stream in a vaporizer. Preferably, the residence time of the feed stream between the vaporizer and the reactor is less than 120 seconds.

In a second embodiment, the present invention is directed to a process for producing ethanol, comprising hydrogenating a feed stream in a reactor in the presence of a catalyst to form a crude ethanol product, separating at least a portion of the crude ethanol product in a first distillation column to yield a first residue comprising acetic acid and a first distillate comprising ethanol, ethyl acetate and water, separating at least a portion of the first distillate in a second distillation column to yield a second residue comprising ethanol and water and a second distillate comprising ethyl acetate, wherein the feed stream is produced by vaporizing an acetic acid stream and the second distillate in a vaporizer, and wherein a residence time of the feed stream between the vaporizer and the reactor is less than 120 seconds.

In a third embodiment, the present invention is directed to a process for producing ethanol, comprising hydrogenating a feed stream in a reactor in the presence of a catalyst to form a crude ethanol product, separating at least a portion of the crude ethanol product in a first distillation column to yield a first residue comprising acetic acid and a first distillate comprising ethanol, ethyl acetate, and water; removing water from at least a portion of the first distillate to yield an ethanol mixture stream comprising less than 10 wt. % water, and separating a portion of the ethanol mixture stream in a second distillation column to yield a second residue comprising ethanol and a second distillate comprising ethyl acetate, wherein the feed stream is produced by vaporizing an acetic acid stream and the second distillate in a vaporizer, and wherein the residence time of the feed stream between the vaporizer and the reactor is less than 120 seconds.

In a fourth embodiment, the present invention is directed to a process for producing ethanol, comprising hydrogenating a feed stream in a reactor in the presence of a catalyst to form a crude ethanol product and separating a portion of the crude ethanol product in a first distillation column to yield a first distillate comprising ethyl acetate and acetaldehyde, and a first residue comprising ethanol, acetic acid, and water. In some embodiment, the first residue may also comprise ethyl acetate. The process further comprises separating a portion of the first residue in a second distillation column to yield a second residue comprising acetic acid and water and a second distillate comprising ethanol and ethyl acetate. In one embodiment, the process may comprise separating a portion of the second distillate in a third distillation column to yield a third residue comprising ethanol and a third distillate comprising ethyl acetate. The feed stream is produced by vaporizing an acetic acid stream and at least one of the first distillate or third distillate in a vaporizer. Preferably, the residence time of the feed stream between the vaporizer and the reactor is less than 120 seconds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to processes for producing ethanol produced by vaporizing acetic acid in a vaporizer to form a feed stream and hydrogenating the feed stream in a reactor in the presence of a catalyst. The hydrogenation reaction produces a crude ethanol product that comprises ethanol, water, ethyl acetate, acetaldehyde, acetic acid, and other impurities. The processes of the present invention further comprise separating the crude ethanol product to recover ethanol in two or more distillation columns. In some embodiments, at least one distillation column may be used to separate the crude ethanol product and recover ethanol. Distillate streams comprising ethyl acetate and acetaldehyde may be recycled to the vaporizer as liquid recycle streams to reduce the amount of purges. The liquid recycle stream is combined with a fresh acetic acid stream, i.e. an acetic acid stream that has not been passed over a catalyst, and both the liquid recycle stream and acetic acid reactants are vaporized in the presence of hydrogen to yield a feed stream comprising vapor phase reactants.

The liquid recycle stream comprises at least 5 wt. % ethyl acetate, e.g., at least 10 wt. % ethyl acetate, at least 20 wt. % ethyl acetate, at least 40 wt. % ethyl acetate, at least 50 wt/% ethyl acetate or at least 60 wt. % ethyl acetate. The distillate streams may also comprise small amounts of ethanol and water.

When the liquid recycle streams are combined with the fresh acetic acid feeds prior to the vaporizer, the reactants are mixed and undesired esterification of the fresh acetic acid may occur. This may lead to lower amounts of acetic acid being passed over the hydrogenation catalyst. Although the hydrogenation catalyst may convert both acetic acid and ethyl acetate, the conversion of acetic acid is relatively larger. Thus, a reduction in acetic acid being passed over the catalyst may lead to reduced amounts of ethanol.

In one embodiment, the volumetric ratio of acetic acid to ethyl acetate in the vaporizer is greater than 1:1, e.g., greater than 1.5:1, greater than 2:1, greater than 3.5:1 or greater than 4:1. The vaporizer may be operate at a temperature from 20° C. to 250° C. and may operate at a pressure from 10 kPa to 3000 kPa. Under the conditions in the vaporizer, a non-catalyzed reaction may occur to drive the formation of additional ethyl acetate and reduce the amount of fresh acetic acid. In addition, because the vapor feed stream may be further heated prior to the reactor, the residence time of vapor feed stream may also contribute to additional undesired esterification of the fresh acetic acid. Generally, the vapor feed stream exits the vaporizer at a temperature of about 120° C. and is further heated to a temperature of about 275° C. before entering the reactor. Typically, lowering the temperature of the vapor stream feed would reduce the formation of additional ethyl acetate. However, because of the pre-heating step required in the present processes, the formation of ethyl acetate cannot be controlled by cooling the vapor stream feed.

For purposes of the present invention, the amount of time between exiting the vaporizer and entering the reactor is referred to as “residence time.” In some embodiments, the residence time is less than 120 seconds, e.g., less than 90 seconds or less than 60 seconds. In terms of ranges, the residence time may be from 0.1 seconds to 120 seconds, e.g., from 0.1 seconds to 90 seconds or from 0.1 seconds to 60 seconds. By controlling the residence time, the amount of ethyl acetate formed can be controlled. Thus, controlling the residence time allows for recycling distillate streams comprising ethyl acetate from the separation process to reduce waste and increase ethanol production.

In recovering ethanol, the processes of the present invention use two or more distillation columns. In one embodiment, a first distillation column is used to separate the feed stream into a residue stream comprising acetic acid and a distillate stream comprising ethanol, ethyl acetate, water. The distillate may be further separated in an extractive column and the ethanol subsequently recovered from the remaining water. There may be at least one liquid recycle stream that comprises ethyl acetate from this separation.

In another embodiment, a first distillation column is used to separate the feed stream into a residue stream comprising water and acetic acid from the crude ethanol product and a distillate stream comprising ethanol, acetaldehyde and ethyl acetate. The residue may comprise a substantial portion of the water. In one embodiment, 30 to 90% of the water in the crude ethanol product is removed in the residue, e.g., from 40 to 88% of the water or from 50 to 84% of the water. Water may also be removed from the distillate to form an ethanol mixture stream, preferably comprising less than 10 wt. % water, less than 6 wt. % water or less than 4 wt. % water. In terms of ranges the ethanol mixture stream comprises from 0.001 to 10 wt. % water, e.g., from 0.01 to 6 wt. % water or from 0.1 to 4 wt. % water. Water may be removed, for example, using an adsorption unit, membrane, molecular sieves, extractive column distillation, or a combination thereof. Suitable adsorption units include pressure swing adsorption (PSA) units and thermal swing adsorption (TSA) units. The adsorption units may comprise molecular sieves, such as aluminosilicate compounds. Product ethanol is then recovered from the ethanol mixture stream. There may be at least one liquid recycle stream that comprises ethyl acetate that is also recovered from the ethanol mixture stream.

In another embodiment, a first distillation column is used to separate the feed stream into a residue stream comprising ethanol, ethyl acetate, water and acetic acid from the crude ethanol product and a distillate stream comprising acetaldehyde and ethyl acetate. The distillate may be at least one liquid recycle stream. The residue stream, for example, may comprise at least 50% of the ethanol from the crude ethanol product, and more preferably at least 70%. In terms of ranges, the residue stream may comprise from 50% to 97.5% of the ethanol from the crude ethanol product, and more preferably from 70% to 97.5%. The amount of ethanol from the crude ethanol recovered in the residue may be greater than 97.5%, e.g. up to 99.9%. The residue may also comprise a substantial portion of the water and acetic acid from the crude ethanol product. The residue stream comprising ethanol, ethyl acetate, water, and acetic acid may be further separated to recover ethanol. Because these compounds may not be in equilibrium, additional ethyl acetate may be produced through esterification of ethanol and acetic acid. In one preferred embodiment, the water and acetic acid may be removed as another residue stream in a separate distillation column.

Hydrogenation of Acetic Acid

The process of the present invention may be used with any hydrogenation process for producing ethanol. The materials, catalysts, reaction conditions, and separation processes that may be used in the hydrogenation of acetic acid are described further below.

The raw materials, acetic acid and hydrogen, used in connection with the process of this invention may be derived from any suitable source including natural gas, petroleum, coal, biomass, and so forth. As examples, acetic acid may be produced via methanol carbonylation, acetaldehyde oxidation, ethylene oxidation, oxidative fermentation, and anaerobic fermentation. Methanol carbonylation processes suitable for production of acetic acid are described in U.S. Pat. Nos. 7,208,624; 7,115,772; 7,005,541; 6,657,078; 6,627,770; 6,143,930; 5,599,976; 5,144,068; 5,026,908; 5,001,259; and 4,994,608, the entire disclosures of which are incorporated herein by reference. Optionally, the production of ethanol may be integrated with such methanol carbonylation processes.

As petroleum and natural gas prices fluctuate becoming either more or less expensive, methods for producing acetic acid and intermediates such as methanol and carbon monoxide from alternate carbon sources have drawn increasing interest. In particular, when petroleum is relatively expensive, it may become advantageous to produce acetic acid from synthesis gas (“syngas”) that is derived from other available carbon source. U.S. Pat. No. 6,232,352, the entirety of which is incorporated herein by reference, for example, teaches a method of retrofitting a methanol plant for the manufacture of acetic acid. By retrofitting a methanol plant, the large capital costs associated with CO generation for a new acetic acid plant are significantly reduced or largely eliminated. All or part of the syngas is diverted from the methanol synthesis loop and supplied to a separator unit to recover CO, which is then used to produce acetic acid. In a similar manner, hydrogen for the hydrogenation step may be supplied from syngas.

In some embodiments, some or all of the raw materials for the above-described acetic acid hydrogenation process may be derived partially or entirely from syngas. For example, the acetic acid may be formed from methanol and carbon monoxide, both of which may be derived from syngas. The syngas may be formed by partial oxidation reforming or steam reforming, and the carbon monoxide may be separated from syngas. Similarly, hydrogen that is used in the step of hydrogenating the acetic acid to form the crude ethanol product may be separated from syngas. The syngas, in turn, may be derived from variety of carbon sources. The carbon source, for example, may be selected from the group consisting of natural gas, oil, petroleum, coal, biomass, and combinations thereof. Syngas or hydrogen may also be obtained from bio-derived methane gas, such as bio-derived methane gas produced by landfills or agricultural waste.

In another embodiment, the acetic acid used in the hydrogenation step may be formed from the fermentation of biomass. The fermentation process preferably utilizes an acetogenic process or a homoacetogenic microorganism to ferment sugars to acetic acid producing little, if any, carbon dioxide as a by-product. The carbon efficiency for the fermentation process preferably is greater than 70%, greater than 80% or greater than 90% as compared to conventional yeast processing, which typically has a carbon efficiency of about 67%.

Optionally, the microorganism employed in the fermentation process is of a genus selected from the group consisting ofClostridium, Lactobacillus, Moorella, Thermoanaerobacter, Propionibacterium, Propionispera, Anaerobiospirillum, andBacteriodes, and in particular, species selected from the group consisting ofClostridium formicoaceticum, Clostridium butyricum, Moorella thermoacetica, Thermoanaerobacter kivui, Lactobacillus delbrukii, Propionibacterium acidipropionici, Propionispera arboris, Anaerobiospirillum succinicproducens, Bacteriodes amylophilusandBacteriodes ruminicola. Optionally in this process, all or a portion of the unfermented residue from the biomass, e.g., lignans, may be gasified to form hydrogen that may be used in the hydrogenation step of the present invention. Exemplary fermentation processes for forming acetic acid are disclosed in U.S. Pat. Nos. 6,509,180; 6,927,048; 7,074,603; 7,507,562; 7,351,559; 7,601,865; 7,682,812; and 7,888,082, the entireties of which are incorporated herein by reference. See also US Publ. Nos. 2008/0193989 and 2009/0281354, the entireties of which are incorporated herein by reference.

Examples of biomass include, but are not limited to, agricultural wastes, forest products, grasses, and other cellulosic material, timber harvesting residues, softwood chips, hardwood chips, tree branches, tree stumps, leaves, bark, sawdust, off-spec paper pulp, corn, corn stover, wheat straw, rice straw, sugarcane bagasse, switchgrass, miscanthus, animal manure, municipal garbage, municipal sewage, commercial waste, grape pumice, almond shells, pecan shells, coconut shells, coffee grounds, grass pellets, hay pellets, wood pellets, cardboard, paper, plastic, and cloth. See, e.g., U.S. Pat. No. 7,884,253, the entirety of which is incorporated herein by reference. Another biomass source is black liquor, a thick, dark liquid that is a byproduct of the Kraft process for transforming wood into pulp, which is then dried to make paper. Black liquor is an aqueous solution of lignin residues, hemicellulose, and inorganic chemicals.

U.S. Pat. No. RE 35,377, also incorporated herein by reference, provides a method for the production of methanol by converting carbonaceous materials such as oil, coal, natural gas and biomass materials. The process includes hydrogasification of solid and/or liquid carbonaceous materials to obtain a process gas which is steam pyrolized with additional natural gas to form synthesis gas. The syngas is converted to methanol which may be carbonylated to acetic acid. The method likewise produces hydrogen which may be used in connection with this invention as noted above. U.S. Pat. No. 5,821,111, which discloses a process for converting waste biomass through gasification into synthesis gas, and U.S. Pat. No. 6,685,754, which discloses a method for the production of a hydrogen-containing gas composition, such as a synthesis gas including hydrogen and carbon monoxide, are incorporated herein by reference in their entireties.

The reactants fed to the hydrogenation reaction may comprise fresh acetic acid and ethyl acetate, as well as other carboxylic acids and anhydrides, as well as acetaldehyde and acetone. Generally ethyl acetate may be recovered from the liquid recycle streams when recovering ethanol. Preferably, a suitable acetic acid feed stream comprises one or more of the compounds selected from the group consisting of acetic acid, acetic anhydride, acetaldehyde, ethyl acetate, and mixtures thereof. These other compounds may also be hydrogenated in the processes of the present invention. In some embodiments, the presence of carboxylic acids, such as propanoic acid or its anhydride, may be beneficial in producing propanol. Water may also be present in the acetic acid feed.

Alternatively, acetic acid in vapor form may be taken directly as crude product from the flash vessel of a methanol carbonylation unit of the class described in U.S. Pat. No. 6,657,078, the entirety of which is incorporated herein by reference. The crude vapor product, for example, may be fed directly to the ethanol synthesis reaction zones of the present invention without the need for condensing the acetic acid and light ends or removing water, saving overall processing costs.

Some embodiments of the process of hydrogenating acetic acid to form ethanol may include a variety of configurations using a fixed bed reactor or a fluidized bed reactor. In many embodiments of the present invention, an “adiabatic” reactor can be used; that is, there is little or no need for internal plumbing through the reaction zone to add or remove heat. In other embodiments, a radial flow reactor or reactors may be employed, or a series of reactors may be employed with or without heat exchange, quenching, or introduction of additional feed material. Alternatively, a shell and tube reactor provided with a heat transfer medium may be used. In many cases, the reaction zone may be housed in a single vessel or in a series of vessels with heat exchangers therebetween.

In preferred embodiments, the catalyst is employed in a fixed bed reactor, e.g., in the shape of a pipe or tube, where the reactants, typically in the vapor form, are passed over or through the catalyst. Other reactors, such as fluid or ebullient bed reactors, can be employed. In some instances, the hydrogenation catalysts may be used in conjunction with an inert material to regulate the pressure drop of the reactant stream through the catalyst bed and the contact time of the reactant compounds with the catalyst particles.

The hydrogenation reaction may be carried out in either the liquid phase or vapor phase. Preferably, the reaction is carried out in the vapor phase under the following conditions. The reaction temperature may range from 125° C. to 350° C., e.g., from 200° C. to 325° C., from 225° C. to 300° C., or from 250° C. to 300° C. The pressure may range from 10 kPa to 3000 kPa, e.g., from 50 kPa to 2300 kPa, from 100 kPa to 1500 kPa or from 200 kPa to 2000 kPa. The reactants may be fed to the reactor at a gas hourly space velocity (GHSV) of greater than 500 hr−1, e.g., greater than 1000 hr−1, greater than 2500 hr−1or even greater than 5000 hr−1. In terms of ranges the GHSV may range from 50 hr−1to 50,000 hr−1, e.g., from 500 hr−1to 30,000 hr−1, from 1000 hr−1to 10,000 hr−1, or from 1000 hr−1to 6500 hr−1.

The hydrogenation optionally is carried out at a pressure just sufficient to overcome the pressure drop across the catalytic bed at the GHSV selected, although there is no bar to the use of higher pressures, it being understood that considerable pressure drop through the reactor bed may be experienced at high space velocities, e.g., 5000 hr−1or 6,500 hr−1.

Although the reaction consumes two moles of hydrogen per mole of acetic acid to produce one mole of ethanol, the actual molar ratio of hydrogen to acetic acid in the feed stream may vary from about 100:1 to 1:100, e.g., from 50:1 to 1:50, from 20:1 to 1:2, or from 12:1 to 1:1. Most preferably, the molar ratio of hydrogen to acetic acid is greater than 2:1, e.g., greater than 4:1 or greater than 8:1.

Contact or residence time can also vary widely, depending upon such variables as amount of acetic acid, catalyst, reactor, temperature, and pressure. Typical contact times range from a fraction of a second to more than several hours when a catalyst system other than a fixed bed is used, with preferred contact times, at least for vapor phase reactions, of from 0.1 to 100 seconds, e.g., from 0.3 to 80 seconds or from 0.4 to 30 seconds.

The hydrogenation of acetic acid and ethyl acetate to form ethanol is preferably conducted in the presence of a hydrogenation catalyst. Preferably, the hydrogenation catalyst is capable of converting both acetic acid and ethyl acetate. Suitable hydrogenation catalysts include catalysts comprising a first metal and optionally one or more of a second metal, a third metal or any number of additional metals, optionally on a catalyst support. The first and optional second and third metals may be selected from Group IB, IIB, IIIB, IVB, VB, VIB, VIIB, VIII transition metals, a lanthanide metal, an actinide metal or a metal selected from any of Groups IIIA, IVA, VA, and VIA. Preferred bimetallic combinations for some exemplary catalyst compositions include platinum/tin, platinum/ruthenium, platinum/rhenium, palladium/ruthenium, palladium/rhenium, cobalt/palladium, cobalt/platinum, cobalt/chromium, cobalt/ruthenium, cobalt/tin, silver/palladium, copper/palladium, copper/zinc, nickel/palladium, gold/palladium, ruthenium/rhenium, and ruthenium/iron. Additional metal combinations may include palladium/rhenium/tin, palladium/rhenium/cobalt, palladium/rhenium/nickel, platinum/tin/palladium, platinum/tin/cobalt, platinum/tin/copper, platinum/tin/chromium, platinum/tin/zinc, and platinum/tin/nickel.

Exemplary catalysts are further described in U.S. Pat. No. 7,608,744 and U.S. Pub. No. 2010/0029995, the entireties of which are incorporated herein by reference. In another embodiment, the catalyst comprises a Co/Mo/S catalyst of the type described in U.S. Pub. No. 2009/0069609, the entirety of which is incorporated herein by reference.

In one embodiment, the catalyst comprises a first metal selected from the group consisting of copper, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, titanium, zinc, chromium, rhenium, molybdenum, and tungsten. Preferably, the first metal is selected from the group consisting of platinum, palladium, cobalt, nickel, and ruthenium. More preferably, the first metal is selected from platinum and palladium. In embodiments of the invention where the first metal comprises platinum, it is preferred that the catalyst comprises platinum in an amount less than 5 wt. %, e.g., less than 3 wt. % or less than 1 wt. %, due to the high commercial demand for platinum.

As indicated above, in some embodiments, the catalyst further comprises a second metal, which typically would function as a promoter. If present, the second metal preferably is selected from the group consisting of copper, molybdenum, tin, chromium, iron, cobalt, vanadium, tungsten, palladium, platinum, lanthanum, cerium, manganese, ruthenium, rhenium, gold, and nickel. More preferably, the second metal is selected from the group consisting of copper, tin, cobalt, rhenium, and nickel. More preferably, the second metal is selected from tin and rhenium.

In certain embodiments where the catalyst includes two or more metals, e.g., a first metal and a second metal, the first metal is present in the catalyst in an amount from 0.1 to 10 wt. %, e.g., from 0.1 to 5 wt. %, or from 0.1 to 3 wt. %. The second metal preferably is present in an amount from 0.1 to 20 wt. %, e.g., from 0.1 to 10 wt. %, or from 0.1 to 5 wt. %. For catalysts comprising two or more metals, the two or more metals may be alloyed with one another or may comprise a non-alloyed metal solution or mixture.

The preferred metal ratios may vary depending on the metals used in the catalyst. In some exemplary embodiments, the mole ratio of the first metal to the second metal is from 10:1 to 1:10, e.g., from 4:1 to 1:4, from 2:1 to 1:2, from 1.5:1 to 1:1.5 or from 1.1:1 to 1:1.1.

The catalyst may also comprise a third metal selected from any of the metals listed above in connection with the first or second metal, so long as the third metal is different from the first and second metals. In preferred aspects, the third metal is selected from the group consisting of cobalt, palladium, ruthenium, copper, zinc, platinum, tin, and rhenium. More preferably, the third metal is selected from cobalt, palladium, and ruthenium. When present, the total weight of the third metal preferably is from 0.05 to 4 wt. %, e.g., from 0.1 to 3 wt. %, or from 0.1 to 2 wt. %.

In addition to one or more metals, in some embodiments of the present invention the catalysts further comprise a support or a modified support. As used herein, the term “modified support” refers to a support that includes a support material and a support modifier, which adjusts the acidity of the support material.

The total weight of the support or modified support, based on the total weight of the catalyst, preferably is from 75 to 99.9 wt. %, e.g., from 78 to 97 wt. %, or from 80 to 95 wt. %. In preferred embodiments that utilize a modified support, the support modifier is present in an amount from 0.1 to 50 wt. %, e.g., from 0.2 to 25 wt. %, from 0.5 to 15 wt. %, or from 1 to 8 wt. %, based on the total weight of the catalyst. The metals of the catalysts may be dispersed throughout the support, layered throughout the support, coated on the outer surface of the support (i.e., egg shell), or decorated on the surface of the support.

As will be appreciated by those of ordinary skill in the art, support materials are selected such that the catalyst system is suitably active, selective and robust under the process conditions employed for the formation of ethanol.

Suitable support materials may include, for example, stable metal oxide-based supports or ceramic-based supports. Preferred supports include silicaceous supports, such as silica, silica gel, silica/alumina, a Group IIA silicate such as calcium metasilicate, pyrogenic silica, high purity silica, and mixtures thereof. Other supports may include, but are not limited to, iron oxide, alumina, titania, zirconia, magnesium oxide, carbon, graphite, high surface area graphitized carbon, activated carbons, and mixtures thereof.

As indicated, the catalyst support may be modified with a support modifier. In some embodiments, the support modifier may be an acidic modifier that increases the acidity of the catalyst. Suitable acidic support modifiers may be selected from the group consisting of: oxides of Group IVB metals, oxides of Group VB metals, oxides of Group VIB metals, oxides of Group VIIB metals, oxides of Group VIIIB metals, aluminum oxides, and mixtures thereof. Acidic support modifiers include those selected from the group consisting of TiO2, ZrO2, Nb2O5, Ta2O5, Al2O3, B2O3, P2O5, and Sb2O3. Preferred acidic support modifiers include those selected from the group consisting of TiO2, ZrO2, Nb2O5, Ta2O5, and Al2O3. The acidic modifier may also include selected from the group consisting of WO3, MoO3, Fe2O3, Cr2O3, V2O5, MnO2, CuO, Co2O3, and Bi2O3.

In another embodiment, the support modifier may be a basic modifier that has a low volatility or no volatility. Such basic modifiers, for example, may be selected from the group consisting of: (i) alkaline earth metal oxides, (ii) alkali metal oxides, (iii) alkaline earth metal metasilicates, (iv) alkali metal metasilicates, (v) Group IIB metal oxides, (vi) Group IIB metal metasilicates, (vii) Group IIIB metal oxides, (viii) Group IIIB metal metasilicates, and mixtures thereof. In addition to oxides and metasilicates, other types of modifiers including nitrates, nitrites, acetates, and lactates may be used. Preferably, the support modifier is selected from the group consisting of oxides and metasilicates of any of sodium, potassium, magnesium, calcium, scandium, yttrium, and zinc, as well as mixtures of any of the foregoing. More preferably, the basic support modifier is a calcium silicate, and even more preferably calcium metasilicate (CaSiO3). The calcium metasilicate, may be crystalline or amorphous.

A preferred silica support material is SS61138 High Surface Area (HSA) Silica Catalyst Carrier from Saint Gobain NorPro. The Saint-Gobain NorPro SS61138 silica exhibits the following properties: contains approximately 95 wt. % high surface area silica; surface area of about 250 m2/g; median pore diameter of about 12 nm; average pore volume of about 1.0 cm3/g as measured by mercury intrusion porosimetry and a packing density of about 0.352 g/cm3(22 lb/ft3).

A preferred silica/alumina support material is KA-160 silica spheres from Sud Chemie having a nominal diameter of about 5 mm, a density of about 0.562 g/ml, an absorptivity of about 0.583 g H2O/g support, a surface area of about 160 to 175 m2/g, and a pore volume of about 0.68 ml/g.

The catalyst compositions suitable for use with the present invention preferably are formed through metal impregnation of the modified support, although other processes such as chemical vapor deposition may also be employed. Such impregnation techniques are described in U.S. Pat. Nos. 7,608,744 and 7,863,489 and U.S. Pub. No. 2010/0197985 referred to above, the entireties of which are incorporated herein by reference.

In particular, the hydrogenation of acetic acid may achieve favorable conversion of acetic acid and favorable selectivity and productivity to ethanol. For purposes of the present invention, the term “conversion” refers to the amount of acetic acid or ethyl acetate in the feed that is converted to a compound other than acetic acid or ethyl acetate, respectively. Conversion is expressed as a percentage based on acetic acid or ethyl acetate in the feed. The conversion of acetic acid may be at least 10%, e.g., at least 20%, at least 40%, at least 50%, at least 60%, at least 70% or at least 80%. The conversion of ethyl acetate acid preferably is greater than 0%, meaning that more ethyl acetate is consumed than produced. During the hydrogenation of acetic acid, ethyl acetate may be produced. Without consuming any ethyl acetate from the mixed vapor phase reactants, the conversion of ethyl acetate would be negative. However, for purposes of the present invention, enough of the ethyl acetate is consumed to at least offset the produced ethyl acetate. Thus, preferably conversion of ethyl may be at least 0%, e.g., at least 5%, at least 10%, at least 20%, or at least 35%. Although catalysts that have high conversions are desirable, especially acetic acid conversions that are at least 80% or at least 90%, in some embodiments a low acetic acid conversion may be acceptable at high selectivity for ethanol. It is, of course, well understood that in many cases, it is possible to compensate for low acetic acid conversion by appropriate recycle streams or use of larger reactors, but it is more difficult to compensate for poor selectivity.

Selectivity is expressed as a mole percent based on converted acetic acid and/or ethyl acetate. It should be understood that each compound converted from acetic acid and/or ethyl acetate has an independent selectivity and that selectivity is independent from conversion. For example, if 60 mole % of the converted acetic acid is converted to ethanol, we refer to the ethanol selectivity as 60%. The total selectivity is based on the combined converted acetic acid and ethyl acetate. Preferably, the catalyst total selectivity to ethanol is at least 60%, e.g., at least 70%, or at least 80%. Preferably, the total selectivity to ethanol is at least 80%, e.g., at least 85% or at least 88%. Preferred embodiments of the hydrogenation process also have low selectivity to undesirable products, such as methane, ethane, and carbon dioxide. The selectivity to these undesirable products preferably is less than 4%, e.g., less than 2% or less than 1%. More preferably, these undesirable products are present in undetectable amounts. Formation of alkanes may be low, and ideally less than 2%, less than 1%, or less than 0.5% of the acetic acid passed over the catalyst is converted to alkanes, which have little value other than as fuel.

The term “productivity,” as used herein, refers to the grams of a specified product, e.g., ethanol, formed during the hydrogenation based on the kilograms of catalyst used per hour. A productivity of at least 100 grams of ethanol per kilogram of catalyst per hour, e.g., at least 400 grams of ethanol per kilogram of catalyst per hour or at least 600 grams of ethanol per kilogram of catalyst per hour, is preferred. In terms of ranges, the productivity preferably is from 100 to 3,000 grams of ethanol per kilogram of catalyst per hour, e.g., from 400 to 2,500 grams of ethanol per kilogram of catalyst per hour or from 600 to 2,000 grams of ethanol per kilogram of catalyst per hour.

Operating under the conditions of the present invention may result in ethanol production on the order of at least 0.1 tons of ethanol per hour, e.g., at least 1 ton of ethanol per hour, at least 5 tons of ethanol per hour, or at least 10 tons of ethanol per hour. Larger scale industrial production of ethanol, depending on the scale, generally should be at least 1 ton of ethanol per hour, e.g., at least 15 tons of ethanol per hour or at least 30 tons of ethanol per hour. In terms of ranges, for large scale industrial production of ethanol, the process of the present invention may produce from 0.1 to 160 tons of ethanol per hour, e.g., from 15 to 160 tons of ethanol per hour or from 30 to 80 tons of ethanol per hour. Ethanol production from fermentation, due the economies of scale, typically does not permit the single facility ethanol production that may be achievable by employing embodiments of the present invention. In various embodiments of the present invention, the crude ethanol product produced by the hydrogenation process, before any subsequent processing, such as purification and separation, will typically comprise acetic acid, ethanol and water. Exemplary compositional ranges for the crude ethanol product are provided in Table 1. The “others” identified in Table 1 may include, for example, esters, ethers, aldehydes, ketones, alkanes, and carbon dioxide.

In one embodiment, the crude ethanol product comprises acetic acid in an amount less than 20 wt. %, e.g., less than 15 wt. %, less than 10 wt. % or less than 5 wt. %. In terms of ranges, the acetic acid concentration of Table 1 may range from 0.1 wt. % to 20 wt. %, e.g., 0.2 wt. % to 15 wt. %, from 0.5 wt. % to 10 wt. % or from 1 wt. % to 5 wt. %. In embodiments having lower amounts of acetic acid, the conversion of acetic acid is preferably greater than 75%, e.g., greater than 85% or greater than 90%. In addition, the selectivity to ethanol may also be preferably high, and is preferably greater than 75%, e.g., greater than 85% or greater than 90%.

Ethanol Separation

Ethanol produced by the reactor may be recovered using several different techniques. InFIG. 1, the separation of the crude ethanol product uses four columns. InFIG. 2, the crude ethanol product is separated in two columns with an intervening water separation. InFIG. 3, the separation of the crude ethanol product uses three columns. Other separation systems may also be used with embodiments of the present invention.

Hydrogenation system100includes a reaction zone101and separation zone102. Fresh acetic acid in line105and a liquid recycle stream from separation zone102, shown by line113inFIG. 1are mixed prior to vaporizer106to form a mixed feed in line114. As stated herein, liquid recycle stream in line113comprises ethyl acetate. Preferably, the liquid recycle stream is a distillate stream from separation zone102. Hydrogen in line104and mixed feed in line114are fed to a vaporizer106to create a vapor feed stream in line107that is directed to reactor108. Hydrogen feed line104may be preheated to a temperature from 30° C. to 150° C., e.g., from 50° C. to 125° C. or from 60° C. to 115° C. Hydrogen feed line104may be fed at a pressure from 1300 kPa to 3100 kPa, e.g., from 1500 kPa to 2800 kPa, or 1700 kPa to 2600 kPa. In one embodiment, lines104and114may be combined and jointly fed to the vaporizer106.

Vaporizer106may operate at a temperature of from 20° C. to 250° C. and at a pressure from 10 kPa to 3000 kPa. Vaporizer106produces vapor feed stream in line107by transferring the acetic acid and ethyl acetate from the liquid to gas phase below the boiling point of acetic acid in reactor108at the operating pressure of the reactor. In one embodiment, the acetic acid in the liquid state is maintained at a temperature below 160° C., e.g., below 150° C. or below 130° C. Vaporizer106may be operated at a temperature of at least 118° C.

The temperature of feed stream in line107is preferably from 100° C. to 350° C., e.g., from 120° C. to 310° C. or from 150° C. to 300° C. A preheater115may be used to further heat the feed stream in line107to the reactor temperature.

Any feed that is not vaporized is removed from vaporizer106in a blowdown stream and may be recycled or discarded thereto. The mass ratio of feed stream in line107to blowdown stream may be from 6:1 to 500:1, e.g., from 10:1 to 500:1, from 20:1 to 500:1 or from 50:1 to 500:1.

As shown inFIGS. 1 to 3, residence time is measured from point A, the exit point of feed stream107from vaporizer106, to point B, the entry point of feed stream107into reactor108. The residence time may be less than 120 seconds, e.g., less than 90 seconds or less than 60 seconds. In terms of ranges, the residence time may be from 0.1 seconds to 120 seconds, e.g., from 0.1 seconds to 90 seconds or 0.1 seconds to 60 seconds. As the residence time increases beyond 120 seconds, there is an increased reduction in fresh acetic acid in feed stream107. Preferably, there are no storage vessels located between the vaporizer and reactor. The residence may include the time for preheating the feed stream107. Control of the residence times reduces ethyl acetate formation and prevents fresh acetic acid concentrations from being reduced.

Although line107is shown as being directed to the top of reactor108, line107may be directed to the side, upper portion, or bottom of reactor108. Reactor108contains the catalyst that is used in the hydrogenation of the carboxylic acid, preferably acetic acid. In one embodiment, one or more guard beds (not shown) may be used upstream of the reactor, optionally upstream of vaporizer106, to protect the catalyst from poisons or undesirable impurities contained in the feed or return/recycle streams. Such guard beds may be employed in the vapor or liquid streams. Suitable guard bed materials may include, for example, carbon, silica, alumina, ceramic, or resins. In one aspect, the guard bed media is functionalized, e.g., silver functionalized, to trap particular species such as sulfur or halogens. During the hydrogenation process, a crude ethanol product stream is withdrawn, preferably continuously, from reactor108via line109.

The crude ethanol product stream in line109may be condensed and fed to a separator110, which, in turn, provides a vapor stream111and a liquid stream112. In some embodiments, separator110may comprise a flasher or a knockout pot. The separator110may operate at a temperature of from 20° C. to 250° C., e.g., from 30° C. to 225° C. or from 60° C. to 200° C. The pressure of separator110may be from 50 kPa to 2000 kPa, e.g., from 75 kPa to 1500 kPa or from 100 kPa to 1000 kPa. Optionally, the crude ethanol product in line109may pass through one or more membranes to separate hydrogen and/or other non-condensable gases.

The vapor stream111exiting separator110may comprise hydrogen and hydrocarbons, and may be purged and/or returned to reaction zone101. When returned to reaction zone101, vapor stream110is combined with the hydrogen feed104and co-fed to vaporizer106. In some embodiments, the returned vapor stream111may be compressed before being combined with hydrogen feed104.

InFIG. 1, the liquid stream112from separator110is withdrawn and introduced in the lower part of first column120, e.g., lower half or lower third. First column120is also referred to as an “acid separation column.” In one embodiment, the contents of liquid stream112are substantially similar to the crude ethanol product obtained from the reactor, except that the composition has been depleted of hydrogen, carbon dioxide, methane and/or ethane, which are removed by separator110. Accordingly, liquid stream112may also be referred to as a crude ethanol product. Exemplary components of liquid stream112are provided in Table 2. It should be understood that liquid stream112may contain other components, not listed in Table 2.

The amounts indicated as less than (<) in the tables throughout present specification are preferably not present and if present may be present in trace amounts or in amounts greater than 0.0001 wt. %.

The “other esters” in Table 2 may include, but are not limited to, ethyl propionate, methyl acetate, isopropyl acetate, n-propyl acetate, n-butyl acetate or mixtures thereof. The “other ethers” in Table 2 may include, but are not limited to, diethyl ether, methyl ethyl ether, isobutyl ethyl ether or mixtures thereof. The “other alcohols” in Table 2 may include, but are not limited to, methanol, isopropanol, n-propanol, n-butanol, 2-butanol or mixtures thereof. In one embodiment, the liquid stream112may comprise propanol, e.g., isopropanol and/or n-propanol, in an amount from 0.001 to 0.1 wt. %, from 0.001 to 0.05 wt. % or from 0.001 to 0.03 wt. %. In should be understood that these other components may be carried through in any of the distillate or residue streams described herein and will not be further described herein, unless indicated otherwise.

Optionally, crude ethanol product in line109or in liquid stream112may be further fed to an esterification reactor, hydrogenolysis reactor, or combination thereof. An esterification reactor may be used to consume residual acetic acid present in the crude ethanol product to further reduce the amount of acetic acid that would otherwise need to be removed. Hydrogenolysis may be used to convert ethyl acetate in the crude ethanol product to ethanol.

In the embodiment shown inFIG. 1, line112is introduced in the lower part of first column120, e.g., lower half or lower third. In first column120, unreacted acetic acid, a portion of the water, and other heavy components, if present, are removed from the composition in line121and are withdrawn, preferably continuously, as residue. Some or all of the residue may be returned and/or recycled back to reaction zone101via line121. Recycling the acetic acid in line121to the vaporizer106may reduce the amount of heavies that need to be purged from vaporizer106. Reducing the amount of heavies to be purged may improve efficiencies of the process while reducing byproducts.

First column120also forms an overhead distillate, which is withdrawn in line122, and which may be condensed and refluxed, for example, at a ratio of from 10:1 to 1:10, e.g., from 3:1 to 1:3 or from 1:2 to 2:1.

When column120is operated under standard atmospheric pressure, the temperature of the residue exiting in line121preferably is from 95° C. to 120° C., e.g., from 110° C. to 117° C. or from 111° C. to 115° C. The temperature of the distillate exiting in line122preferably is from 70° C. to 110° C., e.g., from 75° C. to 95° C. or from 80° C. to 90° C. Column120preferably operates at ambient pressure. In other embodiments, the pressure of first column120may range from 0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPa to 375 kPa.

Exemplary components of the distillate and residue compositions for first column120are provided in Table 3 below. It should also be understood that the distillate and residue may also contain other components, not listed, such as components in the feed. For convenience, the distillate and residue of the first column may also be referred to as the “first distillate” or “first residue.” The distillates or residues of the other columns may also be referred to with similar numeric modifiers (second, third, etc.) in order to distinguish them from one another, but such modifiers should not be construed as requiring any particular separation order.

As shown in Table 3, without being bound by theory, it has surprisingly and unexpectedly been discovered that when any amount of acetal is detected in the feed that is introduced to the acid separation column120, the acetal appears to decompose in the column such that less or even no detectable amounts are present in the distillate and/or residue.

The distillate in line122preferably comprises ethanol, ethyl acetate, and water, along with other impurities, which may be difficult to separate due to the formation of binary and tertiary azeotropes. To further separate the distillate, line122is introduced to the second column123, also referred to as the “light ends column,” preferably in the middle part of column123, e.g., middle half or middle third. Preferably the second column123is an extractive distillation column. In such embodiments, an extraction agent, such as water, may be added to second column123. If the extraction agent comprises water, it may be obtained from an external source or from an internal return/recycle line from one or more of the other columns.

Extractive distillation is a method of separating close boiling components, such as azeotropes, by distilling the feed in the presence of an extraction agent. The extraction agent preferably has a boiling point that is higher than the compounds being separated in the feed. In preferred embodiments, the extraction agent is comprised primarily of water. As indicated above, the first distillate in line122that is fed to the second column123comprises ethyl acetate, ethanol, and water. These compounds tend to form binary and ternary azeotropes, which decrease separation efficiency

The molar ratio of the water in the extraction agent to the ethanol in the feed to the second column is preferably at least 0.5:1, e.g., at least 1:1 or at least 3:1. In terms of ranges, preferred molar ratios may range from 0.5:1 to 8:1, e.g., from 1:1 to 7:1 or from 2:1 to 6.5:1. Higher molar ratios may be used but with diminishing returns in terms of the additional ethyl acetate in the second distillate and decreased ethanol concentrations in the second column distillate.

In one embodiment, an additional extraction agent, such as water from an external source, dimethylsulfoxide, glycerine, diethylene glycol, 1-naphthol, hydroquinone, N,N′-dimethylformamide, 1,4-butanediol; ethylene glycol-1,5-pentanediol; propylene glycol-tetraethylene glycol-polyethylene glycol; glycerine-propylene glycol-tetraethylene glycol-1,4-butanediol, ethyl ether, methyl formate, cyclohexane, N,N′-dimethyl-1,3-propanediamine, N,N′-dimethylethylenediamine, diethylene triamine, hexamethylene diamine and 1,3-diaminopentane, an alkylated thiopene, dodecane, tridecane, tetradecane and chlorinated paraffins, may be added to second column123. Some suitable extraction agents include those described in U.S. Pat. Nos. 4,379,028, 4,569,726, 5,993,610 and 6,375,807, the entire contents and disclosure of which are hereby incorporated by reference. In the embodiments of the present invention, without the use of an extractive agent, a larger portion of the ethanol would carry over into the second distillate in line127. By using an extractive agent in second column123, the separation of ethanol into the second residue in line126is facilitated thus increasing the yield of the overall ethanol product in the second residue in line126.

Second column123may be a tray or packed column. In one embodiment, second column123is a tray column having from 5 to 70 trays, e.g., from 15 to 50 trays or from 20 to 45 trays. Although the temperature and pressure of second column123may vary, when at atmospheric pressure the temperature of the second residue exiting in line126preferably is from 60° C. to 90° C., e.g., from 70° C. to 90° C. or from 80° C. to 90° C. The temperature of the second distillate exiting in line127from second column123preferably is from 50° C. to 90° C., e.g., from 60° C. to 80° C. or from 60° C. to 70° C. Column123may operate at atmospheric pressure. In other embodiments, the pressure of second column123may range from 0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPa to 375 kPa. Exemplary components for the distillate and residue compositions for second column123are provided in Table 4 below. It should be understood that the distillate and residue may also contain other components, not listed, such as components in the feed.

In preferred embodiments, the weight ratio of ethyl acetate in the second residue to second distillate preferably is less than 0.4:1, e.g., less than 0.2:1 or less than 0.1:1. In embodiments that use an extractive distillation column with water as an extraction agent as the second column123, the weight ratio of ethyl acetate in the second residue to ethyl acetate in the second distillate approaches zero. Second residue may comprise, for example, from 30% to 99.5% of the water and from 85 to 100% of the acetic acid from line122. The second distillate in line127comprises ethyl acetate and additionally comprises water, ethanol, and/or acetaldehyde. Second distillate127may be substantially free of acetic acid. In an optional embodiment, a portion of the second distillate in line127′ may be combined with line132discussed below and fed to vaporizer106.

The weight ratio of ethanol in the second residue to second distillate preferably is at least 3:1, e.g., at least 6:1, at least 8:1, at least 10:1 or at least 15:1. All or a portion of the third residue is recycled to the second column. In one embodiment, all of the third residue may be recycled until system100reaches a steady state and then a portion of the third residue is recycled with the remaining portion being purged from the system100. The composition of the second residue will tend to have lower amounts of ethanol than when the third residue is not recycled. As the third residue is recycled, the composition of the second residue, as provided in Table 4, comprises less than 30 wt. % of ethanol, e.g., less than 20 wt. % or less than 15 wt. %. The majority of the second residue preferably comprises water. Notwithstanding this effect, the extractive distillation step advantageously also reduces the amount of ethyl acetate that is sent to the third column, which is highly beneficial in ultimately forming a highly pure ethanol product.

As shown, the second residue from second column123, which comprises ethanol and water, is fed via line126to third column128, also referred to as the “product column.” More preferably, the second residue in line126is introduced in the lower part of third column128, e.g., lower half or lower third. Third column128recovers ethanol, which preferably is substantially pure with respect to organic impurities and other than the azeotropic water content, as the distillate in line129. The distillate of third column128preferably is refluxed as shown inFIG. 1, for example, at a reflux ratio of from 1:10 to 10:1, e.g., from 1:3 to 3:1 or from 1:2 to 2:1. In one embodiment (not shown), a first portion of the third residue in line130is recycled to the second column and a second portion is purged and removed from the system. In one embodiment, once the process reaches steady state, the second portion of water to be purged is substantially similar to the amount water formed in the hydrogenation of acetic acid. In one embodiment, a portion of the third residue may be used to hydrolyze any other stream, such as one or more streams comprising ethyl acetate.

Third column128is a tray column as described above and operates at atmospheric pressure or optionally at pressures above or below atmospheric pressure. The temperature of the third distillate exiting in line129preferably is from 50° C. to 110° C., e.g., from 70° C. to 100° C. or from 75° C. to 95° C. The temperature of the third residue in line130preferably is from 15° C. to 100° C., e.g., from 30° C. to 90° C. or from 50° C. to 80° C. Exemplary components of the distillate and residue compositions for third column128are provided in Table 5 below. It should be understood that the distillate and residue may also contain other components, not listed, such as components in the feed.

In one embodiment, the third residue in line130is withdrawn from third column128at a temperature higher than the operating temperature of the second column123.

Any of the compounds that are carried through the distillation process from the feed or crude reaction product generally remain in the third distillate in amounts of less 0.01 wt. %, based on the total weight of the third distillate composition, e.g., less than 0.05 wt. % or less than 0.02 wt. %. In one embodiment, one or more side streams may remove impurities from any of the columns in system100. Preferably at least one side stream is used to remove impurities from third column128. The impurities may be purged and/or retained within the system100.

The third distillate in line129may be further purified to form an anhydrous ethanol product stream, i.e., “finished anhydrous ethanol,” using one or more additional separation systems, such as, for example, distillation columns, adsorption units, membranes, or molecular sieves. Suitable adsorption units include pressure swing adsorption units and thermal swing adsorption unit.

Returning to second column123, the second distillate preferably is refluxed as shown inFIG. 1, optionally at a reflux ratio of 1:10 to 10:1, e.g., from 1:5 to 5:1 or from 1:3 to 3:1. As explained above, at least a portion of second distillate in line127may be purged or recycled to the reaction zone. In one embodiment, at least a portion of second distillate in line127is further processed in optional fourth column131, also referred to as the “acetaldehyde removal column.”

In optional fourth column131, the second distillate is separated into a fourth distillate, which comprises acetaldehyde, in line132and a fourth residue, which comprises ethyl acetate, in line133. The fourth distillate preferably is refluxed at a reflux ratio of from 1:20 to 20:1, e.g., from 1:15 to 15:1 or from 1:10 to 10:1, and at least a portion of the fourth distillate is returned to vaporizer106. Additionally, at least a portion of fourth distillate in line132may be purged.

Without being bound by theory, since acetaldehyde may be reacted, e.g., by hydrogenation, to form ethanol, the recycling of a stream that contains acetaldehyde to the reaction zone increases the yield of ethanol and decreases byproduct and waste generation. In another embodiment, the acetaldehyde may be collected and utilized, with or without further purification, to make useful products including but not limited to n-butanol, 1,3-butanediol, and/or crotonaldehyde and derivatives.

The fourth residue of optional fourth column131may be purged via line133. The fourth residue primarily comprises ethyl acetate and ethanol, which may be suitable for use as a solvent mixture or in the production of esters. In one preferred embodiment, the acetaldehyde is removed from the second distillate in optional fourth column131such that no detectable amount of acetaldehyde is present in the residue of column131.

Optional fourth column131is a tray column as described above and may operate above atmospheric pressure. In one embodiment, the pressure is from 120 kPa to 5,000 kPa, e.g., from 200 kPa to 4,500 kPa, or from 400 kPa to 3,000 kPa. In a preferred embodiment the fourth column131may operate at a pressure that is higher than the pressure of the other columns.

The temperature of the fourth distillate exiting in line132preferably is from 60° C. to 110° C., e.g., from 70° C. to 100° C. or from 75° C. to 95° C. The temperature of the residue in line133preferably is from 70° C. to 115° C., e.g., from 80° C. to 110° C. or from 85° C. to 110° C. Exemplary components of the distillate and residue compositions for optional fourth column131are provided in Table 6 below. It should be understood that the distillate and residue may also contain other components, not listed, such as components in the feed.

In one embodiment, a portion of the third residue in line130is recycled to second column123. In one embodiment, recycling the third residue further reduces the aldehyde components in the second residue and concentrates these aldehyde components in second distillate in line127and thereby sent to optional fourth column131, wherein the aldehydes may be more easily separated. The third distillate in line129may have lower concentrations of aldehydes and esters due to the recycling of third residue in line130.

FIG. 2illustrates another exemplary separation system. The reaction zone101ofFIG. 2is similar toFIG. 1and produces a liquid stream112, e.g., crude ethanol product, for further separation. In one preferred embodiment, the reaction zone101ofFIG. 2operates at above 80% acetic acid conversion, e.g., above 90% conversion or above 99% conversion. Thus, the acetic acid concentration in the liquid stream112may be low.

Liquid stream112is introduced in the middle or lower portion of a first column150, also referred to as acid-water column. For purposes of convenience, the columns in each exemplary separation process, may be referred as the first, second, third, etc., columns, but it is understood that first column150inFIG. 2operates differently than the first column120ofFIG. 1. In one embodiment, no entrainers are added to first column150. InFIG. 2, first column150, water and unreacted acetic acid, along with any other heavy components, if present, are removed from liquid stream112and are withdrawn, preferably continuously, as a first residue in line151. Preferably, a substantial portion of the water in the crude ethanol product that is fed to first column150may be removed in the first residue, for example, up to about 75% or to about 90% of the water from the crude ethanol product. First column150also forms a first distillate, which is withdrawn in line152.

When column150is operated under about 170 kPa, the temperature of the residue exiting in line151preferably is from 90° C. to 130° C., e.g., from 95° C. to 120° C. or from 100° C. to 115° C. The temperature of the distillate exiting in line152preferably is from 60° C. to 90° C., e.g., from 65° C. to 85° C. or from 70° C. to 80° C. In some embodiments, the pressure of first column150may range from 0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPa to 375 kPa.

The first distillate in line152comprises water, in addition to ethanol and other organics. In terms of ranges, the concentration of water in the first distillate in line152preferably is less than 20 wt. %, e.g., from 1 wt. % to 19 wt. % or from 5 wt. % to 15 wt. %. A portion of first distillate in line153may be condensed and refluxed, for example, at a ratio of from 10:1 to 1:10, e.g., from 3:1 to 1:3 or from 1:2 to 2:1. It is understood that reflux ratios may vary with the number of stages, feed locations, column efficiency and/or feed composition.

Operating with a reflux ratio of greater than 3:1 may be less preferred because more energy may be required to operate the first column150. The condensed portion of the first distillate may also be fed to a second column154.

The remaining portion of the first distillate in152is fed to a water separation unit156. Water separation unit156may be an adsorption unit, membrane, molecular sieves, extractive column distillation, or a combination thereof. A membrane or an array of membranes may also be employed to separate water from the distillate. The membrane or array of membranes may be selected from any suitable membrane that is capable of removing a permeate water stream from a stream that also comprises ethanol and ethyl acetate.

In a preferred embodiment, water separator156is a pressure swing adsorption (PSA) unit. The PSA unit is optionally operated at a temperature from 30° C. to 160° C., e.g., from 80° C. to 140° C., and a pressure of from 0.01 kPa to 550 kPa, e.g., from 1 kPa to 150 kPa. The PSA unit may comprise two to five beds. Water separator156may remove at least 95% of the water from the portion of first distillate in line152, and more preferably from 99% to 99.99% of the water from the first distillate, in a water stream157. All or a portion of water stream157may be returned to column150in line158, where the water preferably is ultimately recovered from column150in the first residue in line151. Additionally or alternatively, all or a portion of water stream157may be purged via line159. The remaining portion of first distillate exits the water separator156as ethanol mixture stream160. Ethanol mixture stream160may have a low concentration of water of less than 10 wt. %, e.g., less than 6 wt. % or less than 2 wt. %.

Exemplary components of ethanol mixture stream160and first residue in line151are provided in Table 7 below. It should also be understood that these streams may also contain other components, not listed, such as components derived from the feed.

Preferably, ethanol mixture stream160is not returned or refluxed to first column150. The condensed portion of the first distillate in line153may be combined with ethanol mixture stream160to control the water concentration fed to the second column154. For example, in some embodiments the first distillate may be split into equal portions, while in other embodiments, all of the first distillate may be condensed or all of the first distillate may be processed in the water separation unit. InFIG. 2, the condensed portion in line153and ethanol mixture stream160are co-fed to second column154. In other embodiments, the condensed portion in line153and ethanol mixture stream160may be separately fed to second column154. The combined distillate and ethanol mixture has a total water concentration of greater than 0.5 wt. %, e.g., greater than 2 wt. % or greater than 5 wt. %. In terms of ranges, the total water concentration of the combined distillate and ethanol mixture may be from 0.5 to 15 wt. %, e.g., from 2 to 12 wt. %, or from 5 to 10 wt. %.

The second column154inFIG. 2, also referred to as the “light ends column,” removes ethyl acetate and acetaldehyde from the first distillate in line153and/or ethanol mixture stream160. Ethyl acetate and acetaldehyde are removed as a second distillate in line161and ethanol is removed as the second residue in line162. Second column154may be a tray column or packed column. In one embodiment, second column154is a tray column having from 5 to 70 trays, e.g., from 15 to 50 trays or from 20 to 45 trays.

Second column154operates at a pressure ranging from 0.1 kPa to 510 kPa, e.g., from 10 kPa to 450 kPa or from 50 kPa to 350 kPa. Although the temperature of second column154may vary, when at about 20 kPa to 70 kPa, the temperature of the second residue exiting in line162preferably is from 30° C. to 75° C., e.g., from 35° C. to 70° C. or from 40° C. to 65° C. The temperature of the second distillate exiting in line161preferably is from 20° C. to 55° C., e.g., from 25° C. to 50° C. or from 30° C. to 45° C.

The total concentration of water fed to second column154preferably is less than 10 wt. %, as discussed above. When first distillate in line153and/or ethanol mixture stream comprises minor amounts of water, e.g., less than 1 wt. % or less than 0.5 wt. %, additional water may be fed to the second column154as an extractive agent in the upper portion of the column. A sufficient amount of water is preferably added via the extractive agent such that the total concentration of water fed to second column154is from 1 to 10 wt. % water, e.g., from 2 to 6 wt. %, based on the total weight of all components fed to second column154. If the extractive agent comprises water, the water may be obtained from an external source or from an internal return/recycle line from one or more of the other columns or water separators.

Exemplary components for the second distillate and second residue compositions for the second column154are provided in Table 8, below. It should be understood that the distillate and residue may also contain other components, not listed in Table 8.

The second residue inFIG. 2comprises one or more impurities selected from the group consisting of ethyl acetate, acetic acid, and acetaldehyde. The second residue may comprise at least 100 wppm of these impurities, e.g., at least 250 wppm or at least 500 wppm. In some embodiments, the second residue may contain substantially no ethyl acetate or acetaldehyde.

The second distillate in line161, which comprises ethyl acetate and/or acetaldehyde, preferably is refluxed as shown inFIG. 2, for example, at a reflux ratio of from 1:30 to 30:1, e.g., from 1:10 to 10:1 or from 1:3 to 3:1. In one aspect, the second distillate161or a portion thereof is returned to vaporizer106. Additionally, at least a portion of second distillate161may be purged.

FIG. 3illustrates another exemplary separation system. The reaction zone101ofFIG. 3is similar toFIG. 1and produces a liquid stream112, e.g., crude ethanol product, for further separation. In one preferred embodiment, the reaction zone101ofFIG. 3operates at above 80% acetic acid conversion, e.g., above 90% conversion or above 99% conversion. Thus, the acetic acid concentration in the liquid stream112may be low.

In the exemplary embodiment shown inFIG. 3, liquid stream112is introduced in the upper part of first column170, e.g., upper half or upper third. In one embodiment, no entrainers are added to first column170. In first column170, a weight majority of the ethanol, water, acetic acid, and other heavy components, if present, are removed from liquid stream112and are withdrawn, preferably continuously, as residue in line171. In one embodiment, the first residue in line171may also comprise ethyl acetate. First column170also forms an overhead distillate, which is withdrawn in line172, and which may be condensed and refluxed, for example, at a ratio of from 30:1 to 1:30, e.g., from 10:1 to 1:10 or from 1:5 to 5:1. The overhead distillate in stream172preferably comprises a weight majority of the ethyl acetate from liquid stream112. Overhead distillate in stream172may be combined with line178discussed below, and returned to vaporizer106.

When column170is operated under about 170 kPa, the temperature of the residue exiting in line171preferably is from 70° C. to 155° C., e.g., from 90° C. to 130° C. or from 100° C. to 110° C. The base of column170may be maintained at a relatively low temperature by withdrawing a residue stream comprising ethanol, water, and acetic acid, thereby providing an energy efficiency advantage. The temperature of the distillate exiting in line172from column170preferably at 170 kPa is from 75° C. to 100° C., e.g., from 75° C. to 83° C. or from 81° C. to 84° C. In some embodiments, the pressure of first column170may range from 0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPa to 375 kPa. Exemplary components of the distillate and residue compositions for first column170are provided in Table 9 below. It should also be understood that the distillate and residue may also contain other components, not listed in Table 9.

In one embodiment of the present invention, column170may be operated at a temperature where most of the water, ethanol, and acetic acid are removed from the residue stream and only a small amount of ethanol and water is collected in the distillate stream due to the formation of binary and tertiary azeotropes. The weight ratio of water in the residue in line171to water in the distillate in line172may be greater than 1:1, e.g., greater than 2:1. The weight ratio of ethanol in the residue to ethanol in the distillate may be greater than 1:1, e.g., greater than 2:1

The amount of acetic acid in the first residue may vary depending primarily on the conversion in reactor108. In one embodiment, when the conversion is high, e.g., greater than 90%, the amount of acetic acid in the first residue may be less than 10 wt. %, e.g., less than 5 wt. % or less than 2 wt. %. In other embodiments, when the conversion is lower, e.g., less than 90%, the amount of acetic acid in the first residue may be greater than 10 wt. %.

The distillate preferably is substantially free of acetic acid, e.g., comprising less than 1000 wppm, less than 500 wppm or less than 100 wppm acetic acid. The distillate may be combined with line178, described below, and fed to vaporizer106. In some embodiments, the distillate may be further separated, e.g., in a distillation column (not shown), into an acetaldehyde stream and an ethyl acetate stream. Either of these streams may be returned to the reactor108or separated from system100as a separate product.

Some species, such as acetals, may decompose in first column170such that very low amounts, or even no detectable amounts, of acetals remain in the distillate or residue.

To recover ethanol, the residue in line171may be further separated in a second column173, also referred to as an “acid separation column.” An acid separation column may be used when the acetic acid concentration in the first residue is greater than 1 wt. %, e.g., greater than 5 wt. %. The first residue in line171is introduced to second column173preferably in the top part of column173, e.g., top half or top third. Second column173yields a second residue in line174comprising acetic acid and water, and a second distillate in line175comprising ethanol. Second column173may be a tray column or packed column. In one embodiment, second column173is a tray column having from 5 to 150 trays, e.g., from 15 to 50 trays or from 20 to 45 trays. Although the temperature and pressure of second column173may vary, when at atmospheric pressure the temperature of the second residue exiting in line174preferably is from 95° C. to 130° C., e.g., from 100° C. to 125° C. or from 110° C. to 120° C. The temperature of the second distillate exiting in line175preferably is from 60° C. to 100° C., e.g., from 75° C. to 100° C. or from 80° C. to 100° C. The pressure of second column173may range from 0.1 kPa to 510 kPa, e.g., from 1 kPa to 475 kPa or from 1 kPa to 375 kPa.

Exemplary components for the distillate and residue compositions for second column173are provided in Table 10 below. It should be understood that the distillate and residue may also contain other components, not listed in Table 10.

The weight ratio of ethanol in the second distillate in line175to ethanol in the second residue in line174preferably is at least 35:1. In one embodiment, the weight ratio of water in the second residue174to water in the second distillate175is greater than 2:1, e.g., greater than 4:1 or greater than 6:1. In addition, the weight ratio of acetic acid in the second residue174to acetic acid in the second distillate175preferably is greater than 10:1, e.g., greater than 15:1 or greater than 20:1. Preferably, the second distillate in line175is substantially free of acetic acid and may only contain, if any, trace amounts of acetic acid.

As shown, the second distillate in line175is fed to a third column176, e.g., ethanol product column, for separating the second distillate into a third distillate (ethyl acetate distillate) in line178and a third residue (ethanol residue) in line177. Second distillate in line175may be introduced into the lower part of column176, e.g., lower half or lower third. Third distillate178is preferably refluxed, for example, at a reflux ratio greater than 2:1, e.g., greater than 5:1 or greater than 10:1. At least a portion of third distillate178is returned to vaporizer106. Additionally, at least a portion of third distillate178may be purged. In an optional embodiment, a portion of the overhead distillate in stream178may be fed to first column170via line179as shown inFIG. 3. Third column176is a tray column as described herein and preferably operates at atmospheric pressure. The temperature of the third residue exiting from third column176preferably is from 60° C. to 110° C., e.g., from 70° C. to 100° C. or from 75° C. to 95° C. The temperature of the third distillate exiting from third column176preferably is from 70° C. to 115° C., e.g., from 80° C. to 110° C. or from 85° C. to 105° C., when the column is operated at atmospheric pressure.

The remaining water from the second distillate in line175may be removed in further embodiments of the present invention. Depending on the water concentration, the ethanol product may be derived from the second distillate in line175or the third residue in line177Some applications, such as industrial ethanol applications, may tolerate water in the ethanol product, while other applications, such as fuel applications, may require an anhydrous ethanol. The amount of water in the distillate of line175or the residue of line177may be closer to the azeotropic amount of water, e.g., at least 4 wt. %, preferably less than 20 wt. %, e.g., less than 12 wt. % or less than 7.5 wt. %. Water may be removed from the second distillate in line175or the third residue in line177using several different separation techniques as described herein. Particularly preferred techniques include the use of distillation column, membranes, adsorption units, and combinations thereof.

Some of the residues withdrawn from the separation zone102comprise acetic acid and water. Depending on the amount of water and acetic acid contained in the residue of first column, e.g.,120inFIG. 1, 150inFIG. 2, or residue of second column173inFIG. 3, the residue may be treated in one or more of the following processes. The following are exemplary processes for further treating the residue and it should be understood that any of the following may be used regardless of acetic acid concentration. When the residue comprises a majority of acetic acid, e.g., greater than 70 wt. %, the residue may be recycled to the reactor without any separation of the water. In one embodiment, the residue may be separated into an acetic acid stream and a water stream when the residue comprises a majority of acetic acid, e.g., greater than 50 wt. %. Acetic acid may also be recovered in some embodiments from the residue having a lower acetic acid concentration. The residue may be separated into the acetic acid and water streams by a distillation column or one or more membranes. If a membrane or an array of membranes is employed to separate the acetic acid from the water, the membrane or array of membranes may be selected from any suitable acid resistant membrane that is capable of removing a permeate water stream. The resulting acetic acid stream optionally is returned to the reactor108. The resulting water stream may be used as an extractive agent or to hydrolyze an ester-containing stream in a hydrolysis unit.

In other embodiments, for example, where the residue comprises less than 50 wt. % acetic acid, possible options include one or more of: (i) returning a portion of the residue to reactor108, (ii) neutralizing the acetic acid, (iii) reacting the acetic acid with an alcohol, or (iv) disposing of the residue in a waste water treatment facility. It also may be possible to separate a residue comprising less than 50 wt. % acetic acid using a weak acid recovery distillation column to which a solvent (optionally acting as an azeotroping agent) may be added.

Exemplary solvents that may be suitable for this purpose include ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, vinyl acetate, diisopropyl ether, carbon disulfide, tetrahydrofuran, isopropanol, ethanol, and C3-C12alkanes. When neutralizing the acetic acid, it is preferred that the residue comprises less than 10 wt. % acetic acid. Acetic acid may be neutralized with any suitable alkali or alkaline earth metal base, such as sodium hydroxide or potassium hydroxide. When reacting acetic acid with an alcohol, it is preferred that the residue comprises less than 50 wt. % acetic acid. The alcohol may be any suitable alcohol, such as methanol, ethanol, propanol, butanol, or mixtures thereof. The reaction forms an ester that may be integrated with other systems, such as carbonylation production or an ester production process. Preferably, the alcohol comprises ethanol and the resulting ester comprises ethyl acetate. Optionally, the resulting ester may be fed to the hydrogenation reactor.

In some embodiments, when the residue comprises very minor amounts of acetic acid, e.g., less than 5 wt. % or less than 1 wt. %, the residue may be neutralized and/or diluted before being disposed of to a waste water treatment facility. The organic content, e.g., acetic acid content, of the residue beneficially may be suitable to feed microorganisms used in a waste water treatment facility.

The columns shown inFIGS. 1 to 3may comprise any distillation column capable of performing the desired separation and/or purification. Each column may comprise a tray column having from 1 to 150 trays, e.g., from 10 to 100 trays, from 20 to 95 trays or from 30 to 75 trays. The trays may be sieve trays, fixed valve trays, movable valve trays, or any other suitable design known in the art. In other embodiments, a packed column may be used. For packed columns, structured packing or random packing may be employed. The trays or packing may be arranged in one continuous column or they may be arranged in two or more columns such that the vapor from the first section enters the second section while the liquid from the second section enters the first section, etc.

The associated condensers and liquid separation vessels that may be employed with each of the distillation columns may be of any conventional design and are simplified in the figures. Heat may be supplied to the base of each column or to a circulating bottom stream through a heat exchanger or reboiler. Other types of reboilers, such as internal reboilers, may also be used. The heat that is provided to the reboilers may be derived from any heat generated during the process that is integrated with the reboilers or from an external source such as another heat generating chemical process or a boiler. Although one reactor and one flasher are shown in the figures, additional reactors, flashers, condensers, heating elements, and other components may be used in various embodiments of the present invention. As will be recognized by those skilled in the art, various condensers, pumps, compressors, reboilers, drums, valves, connectors, separation vessels, etc., normally employed in carrying out chemical processes may also be combined and employed in the processes of the present invention.

The temperatures and pressures employed in the columns may vary. As a practical matter, pressures from 10 kPa to 3000 kPa will generally be employed in these zones although in some embodiments subatmospheric pressures or superatmospheric pressures may be employed. Temperatures within the various zones will normally range between the boiling points of the composition removed as the distillate and the composition removed as the residue.

As will be recognized by those skilled in the art, the temperature at a given location in an operating distillation column is dependent on the composition of the material at that location and the pressure of column. In addition, feed rates may vary depending on the size of the production process and, if described, may be generically referred to in terms of feed weight ratios.

The ethanol product produced by the process of the present invention may be an industrial grade ethanol comprising from 75 to 96 wt. % ethanol, e.g., from 80 to 96 wt. % or from 85 to 96 wt. % ethanol, based on the total weight of the ethanol product. Exemplary finished ethanol compositional ranges are provided below in Table 11.

The finished ethanol composition of the present invention preferably contains very low amounts, e.g., less than 0.5 wt. %, of other alcohols, such as methanol, butanol, isobutanol, isoamyl alcohol and other C4-C20alcohols. In one embodiment, the amount of isopropanol in the finished ethanol composition is from 80 to 1,000 wppm, e.g., from 95 to 1,000 wppm, from 100 to 700 wppm, or from 150 to 500 wppm. In one embodiment, the finished ethanol composition is substantially free of acetaldehyde, optionally comprising less than 8 wppm acetaldehyde, e.g., less than 5 wppm or less than 1 wppm.

In some embodiments, when further water separation is used, the ethanol product may be withdrawn as a stream from the water separation unit as discussed above. In such embodiments, the ethanol concentration of the ethanol product may be higher than indicated in Table 11, and preferably is greater than 97 wt. % ethanol, e.g., greater than 98 wt. % or greater than 99.5 wt. %. The ethanol product in this aspect preferably comprises less than 3 wt. % water, e.g., less than 2 wt. % or less than 0.5 wt. %.

The finished ethanol composition produced by the embodiments of the present invention may be used in a variety of applications including applications as fuels, solvents, chemical feedstocks, pharmaceutical products, cleansers, sanitizers, hydrogenation transport or consumption. In fuel applications, the finished ethanol composition may be blended with gasoline for motor vehicles such as automobiles, boats and small piston engine aircraft. In non-fuel applications, the finished ethanol composition may be used as a solvent for toiletry and cosmetic preparations, detergents, disinfectants, coatings, inks, and pharmaceuticals. The finished ethanol composition may also be used as a processing solvent in manufacturing processes for medicinal products, food preparations, dyes, photochemicals and latex processing.

The finished ethanol composition may also be used as a chemical feedstock to make other chemicals such as vinegar, ethyl acrylate, ethyl acetate, ethylene, glycol ethers, ethylamines, aldehydes, and higher alcohols, especially butanol. In the production of ethyl acetate, the finished ethanol composition may be esterified with acetic acid. In another application, the finished ethanol composition may be dehydrated to produce ethylene. Any known dehydration catalyst can be employed to dehydrate ethanol, such as those described in copending U.S. Pub. Nos. 2010/0030002 and 2010/0030001, the entireties of which is incorporated herein by reference. A zeolite catalyst, for example, may be employed as the dehydration catalyst. Preferably, the zeolite has a pore diameter of at least about 0.6 nm, and preferred zeolites include dehydration catalysts selected from the group consisting of mordenites, ZSM-5, a zeolite X and a zeolite Y. Zeolite X is described, for example, in U.S. Pat. No. 2,882,244 and zeolite Y in U.S. Pat. No. 3,130,007, the entireties of which are hereby incorporated herein by reference.

While the invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to those of skill in the art. In view of the foregoing discussion, relevant knowledge in the art and references discussed above in connection with the Background and Detailed Description, the disclosures of which are all incorporated herein by reference. In addition, it should be understood that aspects of the invention and portions of various embodiments and various features recited herein and/or in the appended claims may be combined or interchanged either in whole or in part. In the foregoing descriptions of the various embodiments, those embodiments which refer to another embodiment may be appropriately combined with other embodiments as will be appreciated by one of skill in the art. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.