Continuous process for highly selectively converting carbohydrates that produce aldohexoses into ethylene glycol

Abstract

The present application relates to a highly selective continuous process for converting hexoses in a feed comprising hexose-producing carbohydrates into ethylene glycol, the process comprising: a. continuously or intermittently introducing the feed comprising carbohydrates into a reaction zone having an aqueous hydrogenation medium, the aqueous hydrogenation medium comprising an aldolase catalyst, hydrogen and a hydrogenation catalyst; b. maintaining the aqueous hydrogenation medium in the reaction zone under hydrogenation conditions to provide a product solution comprising ethylene glycol, propylene glycol, glycerol and hexitol.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 201780045893.3, titled "Continuous Process for the Highly Selective Conversion of Hexose-Producing Carbohydrates to Ethylene Glycol", filed on June 2, 2017.

[0002] Cross - reference to related applications

[0003] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 345,399, filed on June 3, 2016, the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0004] The present invention relates to a process for producing ethylene glycol, particularly an efficient continuous process for converting hexose-producing carbohydrates to ethylene glycol. BACKGROUND ART

[0005] Ethylene glycol is a valuable commodity chemical that has a wide range of uses as a building block for other materials such as polyethylene terephthalate (PET) and for its inherent properties such as an antifreeze agent. The demand for ethylene glycol is quite large, making it one of the most produced organic chemicals in the world. It is currently prepared by a multi-step process starting from ethylene derived from hydrocarbon feedstocks.

[0006] Proposals have been made to manufacture ethylene glycol from renewable resources such as carbohydrates. See, for example, U.S. Application No. 5,210,335; EP 2419393; U.S. Published Patent Application 2012 / 0172633; and Green Chem., 2014, 16, 695 - 707. Until recently, these proposed methods for preparing ethylene glycol from carbohydrates have suffered from the problem of extremely low selectivity to ethylene glycol. More recent proposals have focused on using two catalysts to convert carbohydrates to ethylene glycol. One catalyst effects the retro-aldol reaction and a second catalyst is used for hydrogenation. Thus, for example, hexose is converted to glycolaldehyde and erythrose, and glycolaldehyde is in turn hydrogenated to ethylene glycol. Erythrose can undergo additional retro-aldol reactions to provide more than two glycolaldehyde molecules. While these methods provide higher selectivity to ethylene glycol, there is still a need to provide a method that is commercially competitive with conventional methods using ethylene as a feedstock.

[0007] Schreck et al. disclose in U.S. Published Patent Application 2015 / 0329449 an improved continuous process for converting carbohydrates to ethylene glycol and propylene glycol. They disclose using a reactor for converting carbohydrates to diols that has a first zone containing an aldolase catalyst and a second zone containing an aldolase reduction catalyst. In the case where the feed is an aldose, glycolaldehyde from the aldolase reaction is hydrogenated to ethylene glycol in the second zone of the reactor. They also disclose using a ketose as the carbohydrate to produce propylene glycol.

[0008] However, there remain challenges to further enhance the selectivity of carbohydrate conversion to diols, particularly ethylene glycol. These challenges are not insignificant due to the many reactions that can occur under the conditions required for the aldolase reaction and hydrogenation (including but not limited to the hydrogenation of hexose to hexitol and the formation of by-products such as methane, methanol, ethanol, propanol, glycerol, 1,2-butanediol, threitol, and humins). While some of the by-products may be marketable, their recovery to meet merchant-grade specifications can be expensive. Moreover, glycolaldehyde is highly reactive. SUMMARY OF THE INVENTION

[0009] The present invention provides a continuous process for enhancing the selectivity of the aldolase and hydrogenation conversion of carbohydrates that yield hexoses to ethylene glycol.

[0010] According to a first broad aspect of the present invention, rapidly heating the carbohydrate feed to the reaction zone can reduce the production of hexitol and other by-products such as propylene glycol. The mechanism by which the rapid heating of the carbohydrate feed results in enhanced selectivity to ethylene glycol conversion is not fully understood. Without wishing to be bound by theory, it is thought in part that, in the presence of the aldolase catalyst, at temperatures above 230 °C, the rate of aldose aldolase conversion to glycolaldehyde is fast enough compared to other reactions that can produce by-products or reduce the selectivity to ethylene glycol conversion such that the aldose preferably reacts to produce glycolaldehyde and intermediates that can provide ethylene glycol.

[0011] Because of the rate at which heating occurs, it is problematic to directly measure the heating rate of a carbohydrate feedstock that contains carbohydrates that yield hexoses. The problem of temperature measurement is further confounded by the heat and mass transfer of the fluid being heated. The heat and mass transfer parameters within a given fluid will depend on many factors, including but not limited to the heating method, the temperature difference, and the physical structure of the region where heating occurs. Moreover, analytical techniques for measuring the temperature in substantially all regions of the fluid are effectively unavailable. Accordingly, determining whether the heating rate is sufficient can be accomplished only by reference to the relative formation of certain compounds that are produced in the practice of the method. However, it is believed that the heating rate is sufficient to raise the temperature of the entire carbohydrate feedstock from about 170 °C to at least 230 °C in less than about 10 seconds, and more preferably in less than about 5 seconds, and in some cases in less than about 3 seconds, and in some other cases in less than about 1 second.

[0012] The temperature range for rapidly heating the carbohydrate feedstock according to the method of the present invention is from below 170 °C to above 230 °C. In some cases, when rapid heating begins, the carbohydrate feedstock can be at a temperature below about 150 °C or even below about 100 °C. In some cases, preferably, when the carbohydrate feedstock contains an aldolase catalyst, the rapid heating of the carbohydrate feedstock begins before about 100 °C.

[0013] In this first broad aspect of the present invention, a highly selective continuous process for converting hexoses in a feedstock containing carbohydrates that yield hexoses to ethylene glycol comprises:

[0014] a. continuously or intermittently introducing the carbohydrate feedstock into a reaction zone having an aqueous hydrogenation medium that contains an aldolase catalyst, hydrogen, and a hydrogenation catalyst;

[0015] b. maintaining the aqueous hydrogenation medium in the reaction zone under hydrogenation conditions to produce a product solution that contains ethylene glycol, propylene glycol, and hexitol, the hydrogenation conditions including a temperature in the range of about 230 °C - 300 °C, a ratio of aldolase catalyst to hydrogenation catalyst, and a hydrogen partial pressure, the conditions combined being sufficient to:

[0016] i. convert at least about 95% of the carbohydrates that yield hexoses,

[0017] ii. provide a conversion efficiency of at least about 60% of the hexoses to ethylene glycol, and

[0018] iii. provide a mass ratio of ethylene glycol to hexitol in the product solution that is greater than about 10:1; and

[0019] c. continuously or intermittently withdrawing the product solution from the reaction zone.

[0020] wherein the carbohydrate feed is at least partially hydrated and at a pressure sufficient to maintain the partial hydration; wherein the carbohydrate feed is at a temperature below about 170 °C; and wherein the carbohydrate feed is heated to above 230 °C immediately before or in the reaction zone, and the heating rate of the carbohydrate feed from below 170 °C to above 230 °C is sufficient to provide a product solution having at least one of the following:

[0021] A. a mass ratio of ethylene glycol to propylene glycol from the hexose is at least about 15:1, and

[0022] B. a mass ratio of glycerol to propylene glycol from the aldose is less than about 0.5:1.

[0023] The carbohydrate feed may be mixed with an aldolase catalyst before being heated to a temperature above 230 °C or may be substantially free of any aldolase catalyst. In some aspects of the present invention, the carbohydrate feed will contain an aldolase catalyst. In these aspects, an aldolase reaction may occur during heating. Since the carbohydrate feed is introduced into the aqueous hydrogenation medium when heated, any glycolaldehyde produced is rapidly available for hydrogenation to ethylene glycol.

[0024] In another expression of this first broad aspect of the present invention, a highly selective continuous process for converting hexose in a feed containing a carbohydrate that produces hexose to ethylene glycol, comprising:

[0025] a. continuously or intermittently passing the carbohydrate feed into a reaction zone having an aqueous hydrogenation medium comprising an aldolase catalyst, hydrogen, and a hydrogenation catalyst;

[0026] b. maintaining the aqueous hydrogenation medium in the reaction zone under hydrogenation conditions to produce a product solution comprising ethylene glycol, propylene glycol, glycerol, and hexitol, the hydrogenation conditions including a temperature in the range of about 230 °C - 300 °C, a ratio of aldolase catalyst to hydrogenation catalyst, and a hydrogen partial pressure, the conditions combined being sufficient to convert at least about 95% of the carbohydrate that produces hexose; and

[0027] c. continuously or intermittently removing the product solution from the reaction zone,

[0028] wherein the carbohydrate feed is at least partially hydrated and at a pressure sufficient to maintain the partial hydration; wherein the carbohydrate feed is at a temperature below about 170 °C; and wherein the carbohydrate feed is heated to above 230 °C at least partially by direct heat exchange with a hotter fluid immediately before or in the reaction zone, and the heating rate of the carbohydrate feed from below 170 °C to above 230 °C is sufficient to provide a product solution having at least one of the following:

[0029] A. The mass ratio of glycerol to propylene glycol from the aldose is less than about 0.5:1, and

[0030] B. The mass ratio of ethylene glycol to hexitol is greater than about 20:1, and the mass ratio of ethylene glycol to propylene glycol from the aldohexose is at least about 15:1.

[0031] The hotter fluid for direct heat exchange can be any suitable fluid and typically includes water. The temperature and amount of the hotter fluid are sufficient to achieve a temperature of at least 230 °C for the carbohydrate feed in combination with any other heating source. Typically, the hotter fluid is above 230 °C and in some cases above 235 °C. The carbohydrate feed can be introduced into the reaction zone for hydrogenation or, if used, into a preceding retro-aldol reaction zone substantially free of hydrogenation catalyst, and the aqueous medium contained in that reaction zone serves as the hotter fluid. Optionally, the hotter fluid can be combined with the carbohydrate feed and then the combination introduced into the retro-aldol or combined retro-aldol and hydrogenation reaction zone.

[0032] The start of the contact between the carbohydrate feed and the retro-aldol catalyst can occur in the reaction zone containing the hydrogenation catalyst or in a separate reaction zone. In the case where the contact starts in a separate reaction zone, all or part of the aldohexose can react in the separate reaction zone. In some cases, all or part of the aldohexose undergoes retro-aldol conversion in the reaction zone containing the hydrogenation catalyst, e.g., at least about 10 mass%, sometimes at least about 20 mass% to substantially all of the aldohexose in the carbohydrate feed undergoes retro-aldol conversion in the reaction zone containing the hydrogenation catalyst. In certain embodiments of the disclosed method, the retro-aldol catalyst is a homogeneous catalyst and the hydrogenation catalyst is heterogeneous. Thus, the dispersion of the retro-aldol catalyst within the region occupied by the hydrogenation catalyst can provide glycolaldehyde and other intermediates that can provide ethylene glycol in proximity to the hydrogenation sites.

[0033] The amount of hydrogenation catalyst required for a given case will depend on the relative activity of the catalyst and the mass transfer of hydrogen and glycolaldehyde and intermediates to the catalyst. Preferred hydrogenation catalysts are supported nickel-containing hydrogenation catalysts, particularly nickel catalysts containing one or both of rhenium and iridium. The ratio of the retro-aldol catalyst to the hydrogenation catalyst is preferably sufficient to minimize, for example, the production of hexitol from the hydrogenation of aldohexose. However, it is preferred that the hydrogenation catalyst has a density in the reaction zone sufficient to hydrogenate glycolaldehyde and other intermediates before competitive reactions of glycolaldehyde, and that the other intermediates can produce products other than ethylene glycol.

[0034] A preferred carbohydrate-containing feed provides about 120 - 700 or 800 grams, preferably about 150 - 500 grams, such as about 200 - 400 grams of total carbohydrates per liter of aqueous hydrogenation medium to provide a solution product that has a favorable ratio of ethylene glycol to propylene glycol and reduced co-production of 1,2-butanediol. Without wishing to be bound by theory, it is believed that glycolaldehyde produced by retro-aldol conversion of hexose sugars can form dimers or other structures ("protected species") that have lower reactivity compared to glycolaldehyde under hydrogenation conditions. Thus, at low concentrations of carbohydrate feed in the aqueous reaction medium, there are mass transfer limitations that reduce the production of protected species. Accordingly, a greater portion of the glycolaldehyde produced can undergo competitive reactions with other components in the aqueous reaction medium. Conversely, at high concentrations of carbohydrate feed in the aqueous reaction medium, unreacted aldose sugars and other carbohydrates in the carbohydrate feed will increase the likelihood of contact with glycolaldehyde or the hydrogenation catalyst for competitive reactions.

[0035] According to certain embodiments, the proportion range of the carbohydrate feed is sufficient to provide a product solution having a mass ratio of 1,2-butanediol to ethylene glycol of less than about 1:30 and preferably less than about 1:50.

[0036] To the extent not clearly stated in other broad aspects of the present invention, each of the elements described herein for each of the above broad aspects of the method for preparing ethylene glycol of the present invention is an optional or preferred element for other broad methods of the present invention.

[0037] Another broad aspect of the present invention relates to an aqueous production composition comprising (excluding catalyst and water) ethylene glycol, propylene glycol, hexitol, glycerol, and 1,2-butanediol, wherein:

[0038] a. ethylene glycol is present in an amount of at least about 70 mass%,

[0039] b. propylene glycol is present in a mass ratio of ethylene glycol to propylene glycol between about 15:1 and 35:1,

[0040] c. hexitol is present in a mass ratio of ethylene glycol to hexitol greater than about 20:1,

[0041] d. glycerol is present in a mass ratio of glycerol to propylene glycol less than 0.5:1, and

[0042] e. 1,2-butanediol is present in a mass ratio of 1,2-butanediol to ethylene glycol less than about 1:30.

[0043] With respect to the above - broad aspects of the present invention, the carbohydrate feed comprises carbohydrates that yield aldohexoses. The carbohydrates that yield aldohexoses can themselves be aldohexoses or can be disaccharides or polysaccharides that yield aldohexoses upon hydrolysis. Within the broad scope of the present invention, the carbohydrate feed can also include carbohydrates that yield ketoses or other carbohydrates that yield aldoses. In cases where the carbohydrates that yield ketoses or other carbohydrates that yield aldoses are included in the carbohydrate feed, a lower mass ratio of ethylene glycol to propylene glycol as compared to a feed that provides essentially only aldohexoses will result in retro - aldol and hydrogenation reactions. The method of the present invention also contemplates adding propylene glycol to the reactor. However, the mass ratio of ethylene glycol to propylene glycol attributable to aldohexoses is preferably greater than 15:1.

[0044] In some cases, the carbohydrate feed can be a molten solid, in which case it should remain at least partially hydrated to avoid caramelization during heating. Preferably, the carbohydrate feed is provided as an aqueous solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a schematic diagram of a facility capable of using the method of the present invention according to certain embodiments. DETAILED DESCRIPTION

[0046] The entire contents of all patents, published patent applications, and articles cited herein are incorporated herein by reference.

[0047] DEFINITIONS

[0048] As used herein, the following terms have the meanings set forth below, unless otherwise specified or clear from the context in which they are used.

[0049] In the case of ranges used herein, only the endpoints of the range are stated to avoid having to set forth and describe every value included in the range in length. Any suitable intermediate value and range between the recited endpoints can be selected. For example, if a range between 0.1 and 1.0 is recited, all intermediate values (e.g., 0.2, 0.3, 0.6, 0.815, etc.) are included, as well as all intermediate ranges (e.g., 0.2 - 0.5, 0.54 - 0.913, etc.).

[0050] The use of the terms "a" and "an" is intended to include one or more of the elements being described.

[0051] Mixing means the formation of a physical combination of two or more elements, which two or more elements can have a homogeneous or non - homogeneous composition throughout and includes, but is not limited to, solid mixtures, solutions, and suspensions.

[0052] An aldose refers to a monosaccharide that contains only a single aldehyde group (CH=O) per molecule and has the chemical general formula C n (H2O) n . Non-limiting examples of aldoses include aldohexoses (all six-carbon aldehyde-containing sugars, including glucose, mannose, galactose, allose, altrose, idose, talose, and gulose); aldopentoses (all five-carbon aldehyde-containing sugars, including xylose, lyxose, ribose, and arabinose); aldotetroses (all four-carbon aldehyde-containing sugars, including erythrose and threose); and aldotrioses (all three-carbon aldehyde-containing sugars, including glyceraldehyde).

[0053] Carbohydrates that produce aldoses refer to aldoses or disaccharides or polysaccharides that can produce aldoses upon hydrolysis. Most sugars are in a cyclic structure under environmental conditions, and thus the aldose form appears under the conditions of the methods of the present invention. For example, sucrose is a carbohydrate that produces aldoses, although it also produces ketoses upon hydrolysis.

[0054] Aqueous and aqueous solutions refer to those in which water is present but it is not required that water be the major component. For purposes of illustration and not limitation, a solution of 90 volume percent ethylene glycol and 10 volume percent water is an aqueous solution. Aqueous solutions include liquid media containing dissolved or dispersed components, such as, but not limited to, colloidal suspensions and slurries.

[0055] Biologically sourced carbohydrate feedstocks refer to products that include carbohydrates that are wholly or mostly sourced, derived, or synthesized from biological products or renewable agricultural materials (including, but not limited to, plant, animal, and marine materials) or forestry materials.

[0056] Initial contact means that a fluid begins to contact a component, such as a medium containing a homogeneous or heterogeneous catalyst, but it is not required that all molecules of the fluid contact the catalyst.

[0057] The composition of an aqueous solution is determined using gas chromatography for lower boiling components (usually components having 3 or fewer carbons and a normal boiling point of less than about 300 °C) and high performance liquid chromatography for higher boiling components (usually 3 or more carbons).

[0058] The conversion efficiency of aldohexose to ethylene glycol is in mass percent and is calculated as the mass of ethylene glycol contained in the product solution divided by the mass of aldohexose theoretically provided by the carbohydrate feed, thus including any aldohexose present in the carbohydrate feed itself and any aldohexose theoretically produced upon hydrolysis of any disaccharides or polysaccharides contained in the carbohydrate feed.

[0059] An alditol refers to a six-carbon compound having one hydroxyl group per carbon and the empirical formula C6H 14 O6. Alditols can have different stereoconfigurations, such as sorbitol and mannitol.

[0060] High shear mixing involves providing fluids that travel at different speeds relative to adjacent regions, which can be achieved by fixed or moving mechanical means that affect shear to promote mixing. As used herein, the components subjected to high shear mixing can be immiscible, partially immiscible, or miscible.

[0061] Hydraulic distribution refers to the distribution of an aqueous solution in a vessel that includes contact with any catalyst contained therein.

[0062] Immediately prior to means that there is no intermediate unit operation that requires a residence time of more than one minute.

[0063] Intermittently means from time to time, which can be at regular or irregular time intervals.

[0064] Ketose refers to a monosaccharide that contains one keto group per molecule. Non-limiting examples of ketoses include hexuloses (all ketohexoses, including fructose, psicose, sorbose, and tagatose), pentuloses (all ketopentoses, including xylulose and ribulose), tetruloses (all ketotetroses, including erythrulose), and triuloses (all ketotrioses, including dihydroxyacetone).

[0065] The pH of an aqueous solution is determined at ambient pressure and temperature. When determining the pH of, for example, an aqueous hydrogenation medium or a product solution, the liquid is cooled and allowed to stand at ambient pressure and temperature for 2 hours before measuring the pH.

[0066] pH control agent refers to one or more buffers and an acid or a base.

[0067] Pressure sufficient to maintain at least partial hydration of a carbohydrate means pressure sufficient to maintain sufficient hydration water on the carbohydrate to retard caramelization. At temperatures above the boiling point of water, the pressure is sufficient to keep the hydration water on the carbohydrate.

[0068] Rapid dispersion mixing is a type of mixing in which at least one of two or more fluids to be mixed is subdivided to promote mass transfer and form a substantially homogeneous composition. Fractal mixing is a type of rapid dispersion mixing.

[0069] A reactor can be one or more vessels in series or parallel, and the vessels can contain one or more zones. The reactor can be of any suitable design for continuous operation, including but not limited to tanks and tubes or tubular reactors, and can have fluid mixing capabilities if desired. Types of reactors include but are not limited to laminar flow reactors, fixed bed reactors, slurry reactors, fluidized bed reactors, moving bed reactors, simulated moving bed reactors, trickle bed reactors, bubble column reactors, and loop reactors.

[0070] Soluble means capable of forming a single liquid phase or forming a colloidal suspension.

[0071] Carbohydrate feedstock

[0072] The process of the present invention uses a carbohydrate feedstock comprising carbohydrates that yield hexoses. In some cases, the carbohydrate feedstock comprises at least about 40% by mass, preferably at least about 50% by mass, of hexose-yielding carbohydrates based on the total carbohydrates in the feedstock. In cases where a product solution with a high mass ratio of ethylene glycol to propylene glycol is sought, the carbohydrates in the feedstock include at least about 90% by mass, preferably at least about 95 or 99% by mass, of hexose-yielding carbohydrates. The carbohydrate feedstock generally comprises carbohydrate polymers such as starch, cellulose, or partially hydrolyzed fractions of such polymers, or mixtures of polymers or mixtures of polymers and partially hydrolyzed fractions.

[0073] Most biogenic carbohydrate feedstocks yield glucose when hydrolyzed. The process of the present invention can be effectively used to convert glucose and glucose precursors to ethylene glycol. Glucose precursors include, but are not limited to, maltose, trehalose, cellobiose, kojibiose, pullulanose, auranthinose, isomaltose, β,β-trehalose, α,β-trehalose, sophorose, laminaribiose, gentiobiose, and mannotriose. Glucose as the major or sole reactive component of the carbohydrate feedstock is also acceptable. Of course, other aldoses can be used in the process of the present invention. Other carbohydrate polymers and oligomers can be used, such as hemicellulose, partially hydrolyzed forms of hemicellulose, disaccharides such as sucrose, lactulose, lactose, turanose, maltulose, palatinose, gentiobiulose, melibiose, and melibiulose, or combinations thereof. However, the nature of these substances can result in variable mixtures of ethylene glycol and propylene glycol.

[0074] The carbohydrate feed can be a solid or a liquid suspension or dissolved in a solvent such as water. In the case of the carbohydrate feed being in a non-aqueous environment, it is preferred that the carbohydrate is at least partially hydrated. Most preferably, the carbohydrate feed is provided in an aqueous solution. The mass ratio of water to carbohydrate in the carbohydrate feed is preferably in the range of 4:1 to 1:4. Aqueous solutions of some carbohydrates such as glucose and sucrose at 600 or more grams per liter are sometimes commercially available. In some cases, a recycled aqueous hydrogenation solution or an aliquot or a separated portion thereof can be included in the carbohydrate feed. When the carbohydrate feed contains ethylene glycol or propylene glycol, the sum of ethylene glycol and propylene glycol and the mass ratio of carbohydrate is in the range of about 10:1 to 1:20, and sometimes in the range of about 2:1 to 1:20. Within the scope of the present invention, water is added to the carbohydrate feed before introducing the aqueous hydrogenation medium. The content of carbohydrate in the carbohydrate feed is about 120 to between 700 or 800, usually about 150 to 500 grams per liter of the aqueous hydrogenation medium. Optionally, a separate reaction zone containing a retro-aldol catalyst with substantially no hydrogenation catalyst can be used. In the case of using a separate reaction zone containing a retro-aldol catalyst, it is preferred that the carbohydrate contained in the carbohydrate feed in this reaction zone is provided at about 120 to between 700 or 800, usually about 150 to 500 grams of total carbohydrate per liter of the aqueous medium.

[0075] Rapid temperature rise

[0076] According to a first broad method of the present invention, the carbohydrate feed is rapidly transformed from a temperature zone of 170 °C to 230 °C and preferably to a temperature of at least about 240 °C. The present invention has found that rapid heating to a temperature of at least 230 °C results in a higher mass ratio of ethylene glycol to propylene glycol. The carbohydrate in the carbohydrate feed that produces hexose sugars provides a product solution having a mass ratio of ethylene glycol to propylene glycol of at least about 15:1, sometimes at least about 20:1 and up to 30:1 or higher. In the case where the carbohydrate feed contains other hexose-producing carbohydrates or ketose sugars, it is expected that propylene glycol is produced due to the retro-aldol reaction and hydrogenation, and this production of ethylene glycol and propylene glycol needs to be considered when determining the selectivity of the conversion of hexose sugars to ethylene glycol.

[0077] In some cases, it has been found that rapid heating in the temperature zone of 170 °C to 230 °C provides a relatively low mass ratio of glycerol to propylene glycol. In such cases, the mass ratio of glycerol to propylene glycol is generally less than about 0.5:1. This mass ratio of glycerol to propylene glycol can be achieved regardless of whether the carbohydrate feed contains other aldose or ketose sugars in addition to the carbohydrate that produces hexose sugars. The low production of glycerol as a by-product is advantageous because the market value of glycerol is lower compared to ethylene glycol and propylene glycol.

[0078] Other chemicals may be present in the carbohydrate feed during heating. For example, hydrogen for hydrogenation can be supplied at least in part together with the carbohydrate feed. Other aids, such as pH control agents, may also be present if desired. In one embodiment, the carbohydrate feed contains an aldolase catalyst, and in this case, the carbohydrate that produces hexose sugars undergoes catalytic conversion during heating. The degree of conversion of the carbohydrate that produces hexose sugars during heating is affected by factors such as the duration of heating, the relative concentrations of the carbohydrate and the aldolase catalyst, and the activity of the aldolase catalyst.

[0079] As described above, the heating of the carbohydrate feed can be accomplished in any suitable manner and one or more types of heating can be used. All or part of the heating of the carbohydrate feed may or may not occur before the carbohydrate feed is introduced into the aqueous hydrogenation medium. For example, but not limited to, the heating of the carbohydrate feed in the temperature range of 170 °C to 230 °C can occur before being introduced into the aqueous hydrogenation medium, and the heated carbohydrate feed can be kept in contact with the aldolase catalyst to effect the conversion of the carbohydrate that produces hexose sugars to intermediates for further conversion to ethylene glycol, such as glycolaldehyde and erythrose. In embodiments where the heated carbohydrate feed is kept in contact with the aldolase catalyst before being introduced into the aqueous hydrogenation medium, the duration of such contact before being introduced into the aqueous hydrogenation medium is typically less than about 15 seconds, preferably less than about 10 seconds, and in some cases less than about 5 seconds. Generally, any holding time before the heated carbohydrate feed is introduced into the aqueous hydrogenation medium is a result of the equipment configuration, where the equipment configuration includes, for example, pipe distances and residence times in auxiliary equipment such as fluid distributors from a heat exchange zone into a hydrogenation zone. It will be understood that startup and shutdown operations will affect the inherent holding time.

[0080] The heat source for heating the carbohydrate feed in the temperature range of 170 °C to 230 °C is not critical. For example, the heating can be provided by radiative or microwave excitation, indirect heat exchange with other process streams, or direct heat exchange with process streams that also pass through the aqueous hydrogenation medium or a combination thereof. In cases where the carbohydrate feed is heated at least in part by direct heat exchange with the aqueous hydrogenation medium in the temperature range of 170 °C to 230 °C, it is generally preferred that an aldolase catalyst is already present in the aqueous hydrogenation medium. As described above, the heating rate will be affected by heat and mass transfer parameters. It is generally desirable to promote mixing of the carbohydrate feed during heating to facilitate mass and heat transfer, thereby reducing the time required for the carbohydrate feed to completely pass through this temperature range. This mixing can be carried out in any suitable manner, including but not limited to mechanical and static mixing and rapid dispersion mixing. The thoroughness of mixing can also affect the mass transfer of reactants, intermediates, catalysts, and products, thereby affecting the selectivity to ethylene glycol and the rate of by-product formation.

[0081] A particularly useful stream for direct heat exchange with the carbohydrate feed is the withdrawn product solution (recycle). If a soluble aldolase catalyst is used in the aqueous hydrogenation medium, recycle provides a significant recycle of the aldolase catalyst to the reaction system. The recycle can be at a temperature of at least about 180 °C, for example, in the temperature range of about 230 °C to 300 °C. The mass ratio of recycle to carbohydrate feed will depend on the relative temperatures of the two streams and the combined temperature sought. Generally, in cases where recycle is used, the mass ratio of recycle to carbohydrate feed is in the range of about 1:1 to 100:1. The recycle can be an aliquot of the withdrawn product solution, or unit operations can be carried out to separate one or more components from the recycle stream, including but not limited to degassing to remove hydrogen and filtration to remove, for example, any entrained heterogeneous catalyst. In cases where the product solution is degassed to recover at least a portion of the hydrogen, the recycle is often an aliquot of the degassed product solution. One or more components can be added to the recycle prior to its combination with the carbohydrate feed in the direct heat exchange in the operation. These components include, but are not limited to, the aldolase catalyst, pH control agents, and hydrogen. By using the recycle of the withdrawn product solution, the combined carbohydrate feed and recycle can contain unreacted carbohydrate that yields aldose, intermediates in the conversion to ethylene glycol, and ethylene glycol. When the carbohydrate feed is not used in an aqueous solution, such as being a solid or a melt, the recycle provides water to dissolve the carbohydrate and stabilizes the carbohydrate against caramelization.

[0082] Conversion method

[0083] The method of the present invention provides a high conversion efficiency of carbohydrates to glycols through retro-aldol and hydrogenation reactions. In this method, a carbohydrate feed is introduced into an aqueous hydrogenation medium that contains a retro-aldol catalyst, hydrogen, and a hydrogenation catalyst. Before being introduced into the aqueous hydrogenation medium, the carbohydrate feed may or may not be subjected to retro-aldol conditions, and upon contact with the aqueous hydrogenation medium, the carbohydrate feed may or may not be heated in a temperature range of 170 °C to 230 °C. Thus, in some cases, the retro-aldol reaction occurs only after the carbohydrate feed is introduced into the aqueous hydrogenation medium, and in other cases, the retro-aldol reaction may occur at least partially before the carbohydrate feed is introduced into the aqueous hydrogenation medium. It is generally preferred to rapidly disperse the carbohydrate feed in the aqueous hydrogenation medium, especially when the aqueous hydrogenation medium is used to provide direct heat exchange with the carbohydrate feed. This dispersion can be achieved by any suitable procedure, including but not limited to using mechanical and static mixers and rapid diffusion mixing.

[0084] Although the retro-aldol reaction can occur at lower temperatures, such as as low as 90 °C or 150 °C, the preferred temperature for the retro-aldol reaction is generally between about 230 °C and 300 °C, more preferably between about 240 °C and 280 °C. The pressure (gauge pressure) is generally in the range of about 15 to 200 bar (1500 to 20,000 kPa), for example, between about 25 and 150 bar (2500 and 15,000 kPa). The retro-aldol reaction conditions include the presence of a retro-aldol catalyst. A retro-aldol catalyst is a catalyst that catalyzes the retro-aldol reaction. Examples of compounds that can provide a retro-aldol catalyst include but are not limited to heterogeneous and homogeneous catalysts, including catalysts supported on a carrier, comprising tungsten and its oxides, sulfates, phosphides, nitrides, carbides, halides, etc. Also included are tungsten carbide, soluble phosphotungstic compounds, tungsten oxide supported on zirconia, alumina, and alumina-silica. Preferred catalysts are provided by soluble tungsten compounds such as ammonium metatungstate. Other forms of soluble tungstates, such as ammonium paratungstate, partially neutralized tungstic acid, and sodium metatungstate. Without wishing to be bound by theory, the substance exhibiting catalytic activity may be the same as or different from the soluble tungsten compound introduced as a catalyst. Instead, the catalytically active substance may form during the course of the retro-aldol reaction. The concentration of the retro-aldol catalyst used can vary widely and will depend on the activity of the catalyst and other conditions of the retro-aldol reaction such as acidity, temperature, and the concentration of the carbohydrate. Generally, the retro-aldol catalyst is provided in an amount to provide about 0.05 to 100 grams, such as about 0.1 to 50 grams, calculated as elemental metal tungsten per liter of aqueous hydrogenation medium. The retro-aldol catalyst can be added to the aqueous hydrogenation medium as a mixture with the carbohydrate feed or as a separate feed or both.

[0085] Where the carbohydrate feed is subjected to retro-aldol conditions prior to introduction into the aqueous hydrogenation medium, introduction into the aqueous hydrogenation medium preferably occurs in less than 1 minute, sometimes less than about 0.5 minute, and in some cases less than about 0.1 minute from the time the carbohydrate feed is subjected to retro-aldol conditions. Typically, at least about 10%, preferably at least about 20% of the aldose-producing carbohydrates in the carbohydrate feed are retained upon introduction into the aqueous hydrogenation medium. By continuing the retro-aldol conversion of the carbohydrate in the aqueous hydrogenation medium, the duration between the retro-aldol conversion of the aldose and the start of contact with the hydrogenation catalyst is reduced.

[0086] Typically, in the presence of aldose-producing carbohydrates in the aqueous hydrogenation medium, the aqueous hydrogenation medium is maintained at a temperature of at least about 230 °C until substantially all of the aldose-producing carbohydrates have reacted. Thereafter, if desired, the temperature of the aqueous hydrogenation medium can be reduced. However, hydrogenation proceeds rapidly at these higher temperatures. Thus, the temperature of the hydrogenation reaction is typically between about 230 °C and 300 °C, for example, between about 235 °C or 240 °C and 280 °C. The pressure (gauge) is typically in the range of about 15 to 200 bar (1500 to 20,000 kPa), for example, between about 25 and 150 bar (2500 and 15,000 kPa). The hydrogenation reaction requires the presence of hydrogen and a hydrogenation catalyst. Since the solubility of hydrogen in aqueous solution is low, the hydrogen concentration in the aqueous hydrogenation medium will be primarily determined by the partial pressure of hydrogen in the reactor. The pH of the aqueous hydrogenation medium is typically at least about 3, for example between about 3.5 and 8, and in some cases between about 4 and 7.5.

[0087] Hydrogenation is carried out in the presence of a hydrogenation catalyst. The hydrogenation catalyst can also be referred to as a reducing metal catalyst and is a catalyst for reducing carbonyl groups. The hydrogenation catalyst is typically a heterogeneous catalyst. It can be applied in any suitable manner, including but not limited to fixed bed, fluidized bed, trickle bed, moving bed, slurry bed, and structured bed. Nickel, palladium, and platinum are more widely used reducing metal catalysts. However, many reducing catalysts will work in the present application. The reducing catalyst can be selected from various supported transition metal catalysts. Nickel, Pt, Pd, and ruthenium are well-known for their ability to reduce carbonyl groups as the main reducing metal components. A particularly advantageous catalyst for the reducing catalyst in this method is the Ni-Re catalyst supported on silica-alumina. Similar forms of Ni / Re or Ni / Ir can be used for good selectivity in the conversion of glycoaldehyde formed to ethylene glycol. Nickel-rhenium is the preferred reducing metal catalyst and can be supported on alumina-silica, silica, or other carriers. The supported Ni-Re catalyst with B as a promoter is useful. Generally in a slurry reactor, the hydrogenation catalyst is provided in an amount of about 0.1 - 100 grams, more often about 0.5 or 1 - 50 grams per liter of aqueous hydrogenation medium, while in a packed bed reactor, the hydrogenation catalyst accounts for about 20 - 80 volume % of the reactor.

[0088] Typically, the retro-aldol reaction proceeds more rapidly than the hydrogenation reaction, and thus the residence time of the carbohydrate feed in the hydrogenation reactor is selected to reflect the degree of hydrogenation sought. In some cases, the weight hourly space velocity is about 0.01 - 20 h -1 , and is typically about 0.02 - 5 h -1 . In some cases, it is desirable to keep the aqueous hydrogenation medium well dispersed to ensure a relatively uniform concentration of the intermediate relative to the ethylene glycol therein.

[0089] The retro-aldol and hydrogenation environments can lead to undesirable reactions. See, for example, the GreenChem. article cited in the Background section. Tables 1 on page 697 and 4 on page 700 of the article report the product compositions when various aldoses are subjected to retro-aldol and hydrogenation conditions. The main by-products they report include sorbitol, erythritol, propylene glycol, and glycerol. These by-products not only represent a loss in efficiency in the conversion to ethylene glycol, but by-products such as glycerol have an economic value lower than that of ethylene glycol. The by-products can further react under these conditions, and ethylene glycol degrades as shown in their Table 3. The formation of by-products can be attributed in part to the generation of reactive species as intermediates, especially glycoaldehyde and erythrose.

[0090] In the method of the present invention, the combination of reaction conditions (e.g., temperature, hydrogen partial pressure, catalyst concentration, hydraulic distribution, and residence time) is sufficient to convert at least about 95% by weight, typically at least about 98% by weight, and sometimes substantially all of the hexose-producing carbohydrates. Determining one or more combinations of conditions that will provide the conversion of the hexose-producing carbohydrates sought is well within the skill of those in the art who benefit from the disclosure herein. The method of the present invention also uses combinations of conditions that can provide an efficiency of converting at least about 60% by weight, preferably at least about 70% by weight, more preferably at least about 75% by weight of hexose to ethylene glycol. The efficiency of conversion to ethylene glycol is determined as the percentage of the mass of ethylene glycol in the product solution relative to the mass of the hexose-producing carbohydrates in the carbohydrate feed. In cases where the carbohydrate feed contains other carbohydrates, the contribution of the other feed to the ethylene glycol content is considered.

[0091] Without wishing to be bound by theory, it is believed that the formation of intermediates by the retro-aldol reaction requires that they be hydrogenated to ethylene glycol in a short time close to those intermediates so that they are hydrogenated before a significant amount of the intermediates can be consumed in competing reactions. Thus, under the reaction conditions, for a given retro-aldol catalyst and a given hydrogenation catalyst, the balance between the retro-aldol catalyst and the hydrogenation catalyst can be determined to achieve a high efficiency of conversion to ethylene glycol. In addition, it is believed that rapid heating of the carbohydrate feed provides a feed at a temperature at which the retro-aldol rate of the reaction can more easily match the hydrogenation rate of the reaction.

[0092] It is also believed that the ratio of the retro-aldol catalyst to the hydrogenation catalyst can be used to reduce the production of hexitols by minimizing the presence of carbohydrates and providing a concentration of intermediates converted to ethylene glycol that preferentially accesses the active hydrogenation sites. One mode of operation of the method according to certain embodiments is to use a homogeneous retro-aldol catalyst and a heterogeneous hydrogenation catalyst such that the retro-aldol catalyst can be physically located near the hydrogenation catalyst. Since smaller molecule intermediates diffuse more rapidly to the catalyst sites than larger carbohydrate molecules, and in the case of the limited solubility of hydrogen in the aqueous hydrogenation medium, it is believed that the mass transfer rate of hydrogen to the hydrogenation catalyst regulates the hydrogenation reaction. Preferably, the mass ratio of ethylene glycol to hexitol in the product solution is greater than about 10:1, and in some cases greater than about 20:1 or 25:1 or even greater than about 40:1 or 50:1. As described above, providing the total carbohydrates in the carbohydrate feed in an amount of about 120 - 700 or 800, or 150 to 500 grams per liter of aqueous hydrogenation medium can be used to reduce the production rate of 1,2-butanediol.

[0093] Determining the appropriate ratio of the retro-aldol catalyst to the hydrogenation catalyst is within the skill of those in the art who benefit from the disclosure herein. Among other reasons, this ratio depends on the relative activities of the two catalysts under steady-state conditions. The relative activity is affected by the intrinsic activity of the catalyst and the physical structure of the catalyst. Accordingly, the ratio of these catalysts can vary widely within the range of the retro-aldol catalyst and the hydrogenation catalyst. However, for a given retro-aldol catalyst and hydrogenation catalyst, the desired ratio can be determined. If a retro-aldol reaction zone substantially free of a hydrogenation catalyst is used, as taught by Schreck et al. in U.S. Published Patent Application 2015 / 0329449, conditions including but not limited to hydraulic residence time and retro-aldol catalyst concentration can be adjusted to achieve the sought efficiency of conversion to ethylene glycol and the mass ratio of ethylene glycol to sorbitol. If desired, the reaction zone containing the hydrogenation catalyst can have a different ratio of retro-aldol catalyst to hydrogenation catalyst. For example, in a continuous stirred tank reactor, a homogeneous retro-aldol catalyst and a heterogeneous hydrogenation catalyst are used, and the carbohydrate feed is introduced at or just below the surface of the aqueous hydrogenation medium, and the stirring rate can be such that a density gradient of the hydrogenation catalyst exists. The lower concentration of the hydrogenation catalyst at the top of the aqueous hydrogenation medium subjects the carbohydrate to the retro-aldol reaction before significant amounts of hydrogenation occur.

[0094] Reaction workup

[0095] The product solution is withdrawn from the reaction zone continuously or intermittently. After the reactor, a portion of the withdrawn product solution can be separated and recycled back to an earlier stage of the process as described above. Preferably, at least a portion of the retro-aldol catalyst is recycled or recovered from the withdrawn product solution for recycle. The withdrawn product solution can be depressurized with the trapped gas to recover hydrogen and remove unwanted gaseous by-products such as methane and carbon dioxide.

[0096] Upon cooling, the less soluble portion of the catalyst dissolved from the bed or fed to the reactor is removed at a reduced temperature, and the remaining liquid is transferred to the recovery portion of the process. Depending on the catalyst stability and solubility, it may be possible to recover the degassed reactor effluent, where a portion of the volatile products are recovered and the heavy bottoms are treated to, for example, recover the tungsten catalyst for reuse in the reactor.

[0097] In recovery, low-boiling components such as ethanol and methanol are removed via distillation. Water is also removed via distillation, followed by recovery of propylene glycol and ethylene glycol. Multiple-effect evaporators are commonly used in the preparation of ethylene glycol to minimize energy utilization in ethylene glycol recovery.

[0098] It is possible that the separation of ethylene glycol from propylene glycol or other diols with similar boiling points requires additional, more complex separation techniques. Simulated moving bed technology is one such option that can be used. This option depends on the quality of the product required for the end use of the product.

[0099] Accompanying drawings

[0100] Reference is made to the accompanying drawings, which are provided to assist in understanding the present invention but are not intended to limit the present invention. The accompanying drawings are schematic views of an apparatus generally designated 100, which is suitable for practicing the methods disclosed herein. Minor equipment, such as pumps, compressors, valves, instruments, and other devices, the placement and operation of which are known to those skilled in the art of chemical engineering, are omitted from the accompanying drawings. Auxiliary unit operations are also omitted from the accompanying drawings.

[0101] The carbohydrate feed is provided by line 102. The carbohydrate feed can be solid or liquid, including solutions with water. For purposes of discussion, the carbohydrate feed is an aqueous glucose solution containing approximately 71 volume % carbohydrate. The retro-aldol catalyst is provided via line 104. At this point in the method, addition of the retro-aldol catalyst is optional. For purposes of discussion, the retro-aldol catalyst is ammonium metatungstate in an aqueous solution, and the ammonium metatungstate is provided in an amount sufficient to have an ammonium metatungstate concentration of approximately 10 g / L.

[0102] The carbohydrate feed is then combined with a hotter recycle stream of the withdrawn product solution, as will be described later. This combination effects direct heat exchange to increase the temperature of the carbohydrate feed and provides a combined stream. The combined stream then reaches a distributor 108 and a reactor 110 via line 106. The distributor 108 can be of any suitable design. For purposes of discussion, the distributor 108 is a spray head that distributes the combined stream as fine droplets onto the surface of an aqueous hydrogenation medium 112 in the reactor 110. The reactor 110 contains a stirrer 114 to provide mechanical mixing of the aqueous hydrogenation medium 112. This mechanical mixing helps to disperse the fine droplets of the combined stream within the aqueous hydrogenation medium to further increase the rate at which the combined stream reaches the temperature of the aqueous hydrogenation medium. It also aids in the mass transfer of intermediates from the retro-aldol reaction to the hydrogenation catalyst. The reactor 110 also contains a particulate heterogeneous hydrogenation catalyst, such as a nickel / rhenium / borohydride catalyst on a silica support, which is dispersed in the aqueous hydrogenation medium by mechanical mixing.

[0103] Hydrogen is supplied to the reactor 110 via line 116. The hydrogen can be supplied through nozzles to provide small hydrogen bubbles to facilitate mass transfer of hydrogen into the aqueous hydrogenation medium. If desired, additional retro-aldol catalyst and other additives can be supplied to the reactor through line 118.

[0104] The aqueous hydrogenation medium is withdrawn as a product solution from reactor 110 via line 120. As shown, a portion of the product solution is conveyed via line 122 to line 106 as recycle to be combined with carbohydrate feed 102. This recycle will contain the homogeneous retro-aldol catalyst. Optionally, the recycle stream in line 122 can be heated in indirect heat exchanger 124 to bring the combined stream in line 106 to a higher temperature.

[0105] The following examples are provided to further illustrate the invention and are not intended to limit the invention. Unless otherwise indicated, all parts and percentages are by weight.

[0106] Examples 1 - 20

[0107] The following general procedure was used in Examples 1 - 20.

[0108] A 300 ml Hastelloy C Parr reactor was equipped with a stirrer and one or two feed supply lines and a draw tube attached to a sample bomb. The end of the draw tube was positioned such that approximately 100 ml of solution would remain in the reactor. The reactor was charged with a heterogeneous hydrogenation catalyst and an aqueous solution of a tungsten-containing retro-aldol catalyst as shown in Table I. The charge was approximately 170 ml of aqueous solution. The reactor was then sealed and purged to remove oxygen. The reactor was pressurized to 50 psig (345 kPa gauge) with nitrogen and then vented to atmospheric pressure, for a total of three cycles, thereby completing the purge. The liquid level in the reactor was reduced to approximately 100 ml by draining through the draw tube. While stirring the aqueous solution, hydrogen was used to reduce the nitrogen concentration and then vented to atmospheric pressure, thereby performing an additional three purge cycles.

[0109] Stirring was started and carried out at a rate sufficient to keep the heterogeneous hydrogenation catalyst in a slurry dispersion. The reactor was heated to the reaction temperature as shown in Table I and pressurized to 10700 kPa gauge with hydrogen. When the reactor reached the operating temperature and pressure, the feed of glucose solution was initiated and maintained at a constant rate for the time shown in Table I. The retro-aldol catalyst was added continuously at a constant rate during the time of operation. The reactor was vented to a constant liquid level determined by the position of the draw tube at fixed intervals, thereby achieving near-continuous operation. A filter attached to the end of the draw tube ensured that all heterogeneous catalyst particles remained inside the reactor. Every 10 to 15 minutes, the reactor pressure was adjusted to 10700 kPa by adding hydrogen or venting.

[0110] During operation, a sample of the aqueous medium is collected through a draw tube and a sample cartridge and cooled to room temperature. Typically, samples are collected after about 4 hours of operation, but the sample in Example 16 was collected after about 1.6 hours, the sample in Example 20 was collected after 2 hours, and the sample in Example 3 was collected after about 2 hours. The samples are analyzed by high performance liquid chromatography (HPLC) and gas chromatography (GC). The HPLC is equipped with a refractive index detector and uses a Hi-Plex H resin column purchased from Agilent Technologies, Santa Clara, California. GC analysis is performed using an HP 5890 GC (Agilent Technologies, Santa Clara, California) with a flame ionization detector having a 25:1 split injection. A J&W DB-WAX 30 m X 0.32 mm X 0.5 micron capillary column (Agilent Technologies, Santa Clara, California) is used.

[0111] Unless otherwise stated, ammonium metatungstate is used as a precursor for the catalytically active retro-aldol catalyst material. Unless otherwise stated, the hydrogenation catalyst is a silica-alumina supported nickel, rhenium and boron catalyst prepared using the procedure described in U.S. Patent No. 6,534,441, column 8, line 62 to column 9, line 27. The silica-alumina support is a 3 mm extrudate and has a surface area of about 125 square meters per gram and a pore volume of about 0.7 - 0.9 milliliters per gram. Unless otherwise stated, the catalyst contains about 6.8 mass % nickel and the atomic mass ratio of nickel:rhenium:boron is about 5:1.3:1.

[0112] In the examples, glucose is used as the carbohydrate feed and is provided in an aqueous solution at a concentration of about 32.4 mass %, unless otherwise stated. When using a single feed supply line, the glucose-containing feed contains the retro-aldol catalyst. In the case of two feed supply lines, one supply line is used to supply the carbohydrate feed and the other is used to supply an aqueous solution of the retro-aldol catalyst. The supply lines vary in inside diameter and length and in their positioning within the reactor. The supply lines used have an outside diameter of 1 / 8 inch (3.2 mm) or 1 / 16 inch (1.6 mm). The length and diameter of the supply lines and their positioning affect the heating rate of the carbohydrate feed and also affect the flow rate through the supply lines. At a feed rate of 1 milliliter per minute, for a 1 / 16 inch supply line, the residence time per 2.5 cm length is about 0.4 seconds and the residence time per 2.5 cm length of a 1 / 8 inch supply line is about 3.7 seconds. In the case of using two supply tubes, the feed rates in Table I are the sum of the two feeds.

[0113] Comparative examples are indicated by the abbreviation "comp" in Table I.

[0114] Table II summarizes the product composition for each of the samples.

[0115] Table I

[0116]

[0117] a. The pressure is 5500 kPa gauge

[0118] b. The feed contains 31.6 mass % glucose, 37 mass % propylene glycol, and water

[0119] c. The feed contains 32.4 mass % glucose, 30 mass % ethylene glycol, and water

[0120] d. The feed contains 32.4 mass % glucose, 30 mass % ethylene glycol, and water

[0121] e. The hydrogenation catalyst is 5% loaded (product number 206180) ruthenium on carbon available from Sigma-Aldrich Chemical Company, St. Louis, USA.

[0122] f. Sodium metatungstate is used

[0123] g. The feed contains 50 mass % glucose

[0124] h. 0.5 mass % sodium metatungstate and 0.6 mass % sodium tungstate

[0125] Table II

[0126]

[0127] a. Mannitol approximate sorbitol content

[0128] b. Not calculated due to PG in the feed stream

[0129] c. No hexitol was detected

Claims

1. A highly selective continuous process for converting hexose sugars in a feed containing hexose sugar-producing carbohydrates to ethylene glycol, the process comprising: a. Continuously or intermittently introducing the feed containing hexose sugar-producing carbohydrates into a reaction zone having an aqueous hydrogenation medium, the aqueous hydrogenation medium comprising an aldolase catalyst, hydrogen, and a hydrogenation catalyst; b. Maintaining the aqueous hydrogenation medium in the reaction zone under hydrogenation conditions to provide a product solution comprising ethylene glycol, propylene glycol, glycerol, and hexitol, the hydrogenation conditions including a temperature in the range of 230°C - 300°C, a ratio of aldolase catalyst to hydrogenation catalyst, and a hydrogen partial pressure, the combination of these conditions being sufficient to: i. Convert at least 95% of the hexose sugar-producing carbohydrates; ii. Provide a conversion efficiency of at least 60% of the hexose sugars to ethylene glycol; and c. Continuously or intermittently withdrawing the product solution from the reaction zone, wherein the feed containing hexose sugar-producing carbohydrates is at least partially hydrated and under a pressure that maintains the partial hydration; wherein the feed containing hexose sugar-producing carbohydrates is at a temperature below 170°C; and wherein just before or in the reaction zone, the feed containing hexose sugar-producing carbohydrates is heated to above 230°C, and the heating rate of the feed containing hexose sugar-producing carbohydrates is sufficient to raise the temperature of the entire feed from below 170°C to above 230°C in less than 10 seconds and is sufficient to provide a product solution having a mass ratio of ethylene glycol to hexitol greater than 10:1 and having at least one of the following: A. A mass ratio of ethylene glycol to propylene glycol from the hexose sugars of at least 15:1, and B. A mass ratio of glycerol to propylene glycol less than 0.5:

1.

2. The process according to claim 1, wherein the hexose sugar-producing carbohydrates introduced into the aqueous hydrogenation medium are in an aqueous solution.

3. The process according to claim 2, wherein the aqueous solution comprises an aldolase catalyst.

4. The process according to claim 3, wherein the aqueous solution is maintained at a temperature above 170°C and below 230°C for less than 10 seconds before being introduced into the aqueous hydrogenation medium.

5. The process according to claim 4, wherein the aqueous solution comprises an aldolase catalyst at least during the heating from below 170°C to above 230°C.

6. The process according to any one of claims 1 - 5, wherein the heating of the feed containing hexose sugar-producing carbohydrates from below 170°C to above 230°C is at least partially effected by direct heat exchange by mixing the feed containing hexose sugar-producing carbohydrates with a hotter fluid.

7. The process according to claim 6, wherein the hotter fluid comprises the aqueous hydrogenation medium.

8. The process according to claim 6, wherein the mixing of the feed containing hexose sugar-producing carbohydrates with the hotter fluid involves high-shear mixing.

9. The process according to claim 6, wherein the mixing of the feed containing hexose sugar-producing carbohydrates with the hotter fluid involves rapid-diffusion mixing.

10. The method according to claim 6, wherein a portion of the withdrawn product solution is provided at a temperature of at least 180 °C and mixed with the feed comprising the hexose-producing carbohydrate before the feed comprising the hexose-producing carbohydrate is introduced into the aqueous hydrogenation zone.

11. The method according to claim 10, wherein the recycled portion of the withdrawn product solution is an aliquot portion.

12. The method according to claim 11, wherein the withdrawn product solution is degassed to remove at least part of the hydrogen, thereby providing a degassed product solution, and then an aliquot portion of the degassed product solution is recycled.

13. The method according to any one of claims 1-5, wherein the feed comprising the hexose-producing carbohydrate is heated from below 170 °C to above 230 °C at least in part by indirect heat exchange.

14. The method according to any one of claims 1-5, wherein the feed comprising the hexose-producing carbohydrate is heated from below 170 °C to above 230 °C in the absence of a hydrogenation catalyst when the carbohydrate is contacted with an aqueous retro-aldol solution comprising a retro-aldol catalyst for a certain time to provide a mass ratio of ethylene glycol to hexitol greater than 25:1 in the product solution.

15. The method according to any one of claims 1-5, wherein the feed comprising the hexose-producing carbohydrate is in a solution comprising at least one of ethylene glycol and propylene glycol, and the total mass ratio of ethylene glycol and propylene glycol to the carbohydrate is 10:1-1:

20.

16. The method according to any one of claims 1-5, wherein the feed comprising the hexose-producing carbohydrate comprises 120-800 grams of carbohydrate per liter of aqueous hydrogenation medium.

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