Production of ethylene by oxidative dehydrogenation of ethane

Abstract

A process for producing ethylene using ethane is described. A process may include oxidative dehydrogenation of ethane to produce a product stream including ethylene, ethane, and a gas having a boiling point lower than ethylene. The product stream can be cooled under mild conditions and then separated to produce a product stream comprising ethylene and ethane and a product stream comprising a gas having a boiling temperature lower than that of ethylene.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] none. Background Art A. Technical field

[0002] The present invention generally relates to the production of ethylene by the oxidative dehydrogenation of ethane. A gas stream containing ethylene and light gases produced during the oxidative dehydrogenation process can be cooled under mild conditions and then separated to produce a gas stream containing ethylene. B. Description of Related Technology

[0003] Olefins, such as ethylene, are building blocks for a wide variety of commercially valuable polymers. Because natural sources of olefins are not available in commercial quantities, polymer producers rely on methods for converting the more abundant lower alkanes into olefins. The method of choice for today's commercial-scale producers is steam cracking, a highly endothermic process in which steam-diluted alkanes are briefly subjected to temperatures of at least 800°C. The fuel required to generate the required temperatures, as well as the equipment required to withstand them, significantly increase the overall cost. Furthermore, high temperatures promote the formation of coke, which accumulates within the system, necessitating periodic, costly reactor shutdowns for maintenance and coke removal.

[0004] Oxidative dehydrogenation (ODH) is an alternative to steam cracking. It is an exothermic reaction that produces almost no coke. Examples of ODH processes include U.S. patent application publication number 2020 / 0407289 by Simazhenkov et al., entitled “Mitigating the output of oxygen, carbon dioxide and / or acetylene from an ODH process,” international application publication number WO2017046315 by Renée et al., entitled “Oxidative dehydrogenation of alkane,” and international application number WO2015113747 by Winkler et al., entitled “Dilution of alkane oxidative dehydrogenation reactants with carbon dioxide.” The disadvantage of these processes is that large amounts of methane, carbon dioxide and / or nitrogen are used as diluents, requiring more energy to be consumed in the separation process. Many prior art references describe how to remove such compounds and / or other light gases, such as WO 2010 / 115108 by Stephen et al., entitled “Process for producing ethylene by oxidative dehydrogenation of ethane (ODH).” Such processes employ extensive compression and significant low temperatures in a cooling unit to liquefy the gas stream prior to cryogenic separation to remove by-products and produce ethylene, which may require more energy to be consumed in the separation process.

[0005] In summary, while technologies exist for producing olefins (ie, ethylene), these technologies are energy inefficient and costly. Summary of the Invention

[0006] A discovery provides a solution to at least one problem associated with the oxidative dehydrogenation of ethane. In one aspect, the discovery can include a process that can cool a first product stream obtained from an oxidative dehydrogenation process (ODH) without the need for compression. The first product stream can include ethane, ethylene, and one or more gases having a boiling point temperature lower than that of ethylene. The process can include cooling the first product stream to a temperature of 40°C to -10°C and maintaining the pressure of the product stream at or near the pressure of the ODH outlet. The ethylene and ethane can then be separated from the first product stream to form a second product stream containing ethylene and ethane, which can be further processed to produce an ethylene stream. An advantage of the process of the present invention is that there is no need to compress the ethylene-containing stream from the ODH reactor to create conditions for separating the product stream before distillation. The ethylene production process of the present invention is more efficient and more economically viable than the known processes described above. One advantage of the process of the present invention is that the need to compress the product stream to create conditions for separating the first product stream is reduced or eliminated. The present process avoids compression because it has one or more of the following advantages: (1) no diluent is introduced into the ODH reactor; (2) relatively few light byproducts are formed due to operation at limited conversion and high selectivity; and (3) the ODH reactor operates at high pressure. This results in a more efficient and economical ethylene production process compared to known processes (such as the aforementioned process).

[0007] In one aspect of the invention, an ODH process for converting ethane into ethylene is described. A process may include one or more processing steps, such as steps (a), (b) and (c). In step (a), a feed stream comprising ethane may be contacted with an oxidative dehydrogenation catalyst in the presence of oxygen, the conditions being sufficient to produce a first product stream. The oxidative dehydrogenation conditions may include acceptable temperature and pressure conditions. In one aspect, the pressure may range from 1.8 MPa to 5 MPa, preferably from 2 MPa to 4 MPa, more preferably from 2 MPa to 3 MPa, or most preferably from 2 MPa to 2.5 MPa. In one aspect, the temperature may range from 200°C to 450°C, preferably from 250°C to 350°C. In certain specific aspects, at the inlet of the ODH reactor, the temperature may range from 200°C to 450°C, preferably from 250°C to 350°C. The ethane conversion may be between 20% and 40%. The first product stream may include ethane, ethylene and one or more gases having a boiling point lower than that of ethylene. The gas having a boiling point temperature lower than that of ethylene may include methane, nitrogen, argon, carbon monoxide or oxygen, or any combination or all of the above gases. The total mole fraction of these gases in the first product stream is lower than 3%. In one aspect, the first product stream can be placed under conditions suitable for carbon monoxide to be oxidized to carbon dioxide. Carbon dioxide can be separated from the first product stream downstream of the ODH device. In one aspect, methane is not added to the feed stream of step (a). In some aspects of the present invention, the first product stream may also include water and acetic acid. In some aspects, before cooling the first product stream, for example, by intermediate cooling and gas-liquid separation, part of water and acetic acid can be removed from the first product stream.

[0008] Step (b) of the process of the present invention may include lowering the temperature of the first product stream to -40°C to -10°C, preferably 30°C to -10°C. The pressure of the product stream may be equal to or slightly lower than the pressure at the outlet of the ODH unit, for example, the pressure of the last ODH reactor in the ODH unit. For example, 1.5 MPa to 4.5 MPa, preferably 2 MPa to 3 MPa, more preferably 2 MPa to 3 MPa, or most preferably 2 MPa to 2.5 MPa. Step (b) may also include feeding the first product to the inlet of a gas separation unit (such as a demethanizer) at a temperature of -40°C to -10°C, preferably 30°C to -20°C. In some aspects, the cooled first product stream is preferably in a liquid phase. Advantageously, the first product stream does not need to be compressed before entering the gas separation unit and / or the first product stream is cooled to -40°C to -10°C. This helps to improve the energy efficiency and cost-effectiveness of the entire ODH process.

[0009] Step (c) of the process of the present invention may include subjecting the cooled first product stream of step (b) to conditions sufficient (e.g., in a demethanizer) to separate one or more gases having a boiling point lower than that of ethylene from the cooled first product stream, thereby producing a second product stream comprising ethane and ethylene. Further processing of the second product stream may include separating the second product stream into an ethylene product stream and an ethane stream. In certain aspects, the ethane may be recycled to the feed stream of step (a).

[0010] A system for performing the ODH process of the present invention is also described. The system may include an oxidative dehydrogenation (ODH) unit, a cooling unit, and a gas separation unit (e.g., a demethanizer). These units may be configured to transfer product streams from one unit to another (e.g., via conduits, inlets, outlets, valves, etc.). The ODH unit may produce a first product stream at a pressure of 1.8 MPa to 5 MPa. The cooling unit may cool the first product stream to a temperature between 40°C and -10°C, preferably between -30°C and -20°C. The gas separation unit (e.g., a demethanizer) may separate one or more gases with a boiling point lower than that of ethylene from the first product stream to produce a second product stream comprising ethane and ethylene. The second product stream may contain a lower content of the one or more gases with a boiling point lower than that of ethylene than the first product stream. Advantageously, the system of the present invention does not require a compression unit before the first product stream enters the gas separation unit. The system also includes a carbon monoxide oxidation unit capable of oxidizing carbon monoxide in the first product stream to produce carbon dioxide. The carbon monoxide oxidation unit may be located within the oxidative dehydrogenation unit or downstream of the oxidative dehydrogenation unit. The system may further include a C2 separation unit capable of separating the second product stream into an ethane stream and an ethylene stream.

[0011] Other embodiments of the present invention are also discussed in this application. Any embodiment discussed with regard to one aspect of the present invention is also applicable to other aspects of the present invention, and vice versa. Each embodiment described herein is understood to be an embodiment of the present invention applicable to other aspects of the present invention. It can be believed that any embodiment or aspect discussed herein can be combined with other embodiments or aspects discussed herein and / or implemented with any method or composition of the present invention, and vice versa. In addition, composition of the present invention can be used to realize method of the present invention.

[0012] Included below are definitions of various terms and phrases used throughout this specification.

[0013] The phrase "a gas having a boiling temperature lower than that of ethylene" refers to a gas having a normal boiling temperature lower than that of ethylene (-103.7°C).

[0014] The term "about" or "approximately" is defined as close to the degree understood by one of ordinary skill in the art. In one non-limiting embodiment, the term "about" or "approximately" is defined as within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%.

[0015] The terms "wt. %," "vol. %," or "mol. %" refer to the weight percent, volume percent, or mole percent of a component, respectively, based on the total weight, total volume, or total moles of the material in which the component is contained. In a non-limiting example, 10 grams of a component in 100 grams of material is 10 wt. % of the component.

[0016] The term "substantially" and variations thereof are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0017] The terms "inhibit" or "reduce" or "prevent" or "avoid" or any variation of these terms as used in the claims and / or specification include any measurable reduction or complete inhibition to achieve the desired effect.

[0018] The term "effective" as used in the specification and / or claims means sufficient to achieve an intended, desired, or purposeful result.

[0019] When used with any of the terms "including," "comprising," "containing," or "having" in the claims or the specification, "a" or "an" may mean "one," but is also consistent with the meaning of "one or more," "at least one," and "one or more."

[0020] The words "comprise" (and any form of including, such as "include" and "comprising"), "have" (and any form of having, such as "have" and "having"), "include" (and any form of including, such as "include" and "including"), or "contain" (and any form of containing, such as "including" and "including") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0021] The processes and systems of the present invention may "comprise," "consist essentially of," or "consist of" specific ingredients, components, compositions, etc. disclosed throughout this specification. With respect to the transition phrase "consisting essentially of," in one non-limiting aspect, a fundamental and novel feature of the processes and / or systems of the present invention is their ability to cool a product stream comprising ethane, ethylene, and gases with a boiling point lower than ethylene to a temperature of -10°C to -40°C under mild conditions, and then separate the gases with a boiling point lower than ethylene from the product stream to produce a product stream comprising ethane and ethylene. In certain aspects, no compression is employed in one or both of the first and second product streams, or in the entire ODH reaction and separation process.

[0022] Other objects, features, and advantages of the present invention will become apparent from the following figures, detailed description, and examples. However, it should be understood that these figures, detailed description, and examples, while indicating specific embodiments of the present invention, are intended for illustrative purposes only and are not intended to be limiting. Moreover, variations and modifications within the spirit and scope of the present invention will be apparent to those skilled in the art. In further embodiments, features of a particular embodiment may be combined with features of other embodiments. For example, features of one embodiment may be combined with features of any other embodiment. In further embodiments, other features may be added to the specific embodiments described herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Advantages of the present invention may become apparent to those skilled in the art upon benefit of the following detailed description and with reference to the accompanying drawings.

[0024] Figure 1 Schematic diagram of an oxidative dehydrogenation system for producing ethylene from ethane.

[0025] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings. The drawings may not be drawn to scale. DETAILED DESCRIPTION

[0026] It has been found that a solution is provided for solving at least one problem associated with the oxidative dehydrogenation of alkanes (such as ethane). In one aspect, the discovery can include maintaining the pressure of the first product stream generated by the ethane ODH reaction and cooling the first product stream to a temperature between -40°C and -10°C before entering a gas separation unit (such as a demethanizer), at a pressure that is or is similar to the pressure at the outlet of the ODH unit, without compression. The advantage of doing so is that there is no need to compress the gas stream before separating one or more gases having a boiling point temperature lower than that of ethylene from the first product stream. The processes and systems of the present invention can also reduce greenhouse gas emissions in the process (e.g., emissions associated with the combustion of fuel in boilers and / or furnaces), or indirect greenhouse gas emissions associated with purchased energy (e.g., electricity or steam). These and other non-limiting aspects of the present invention will be discussed in further detail with reference to the drawings. A. Ethane Oxidative Dehydrogenation System and Process

[0027] refer to Figure 1 , describes a system for producing ethylene from ethane. System 100 may include an ODH unit 102, a cooling unit 104, and a gas separation unit (eg, a demethanizer) 106, as well as other auxiliary units.

[0028] The ethane feed stream 102 and the oxygen-containing stream 108 can enter the ODH unit 102. The ODH unit 102 can include one or more ODH reactors (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more). The ODH reactor can be a fixed bed reactor or a fluidized bed reactor. Heat can be removed from the ODH reactor or between / after the reactors. The conditions of the ODH reactor can include temperature and pressure parameters. The pressure in the ODH unit can be 1.8 MPa to 5 MPa, preferably 1.9 MPa to 4 MPa, more preferably 2 MPa to 3 MPa, and most preferably 2 MPa to 2.5 MPa. Non-limiting examples of pressure include 1.8 MPa, 1.9 MPa, 2 MPa, 2.0 MPa, 2.1 MPa, 2.2 MPa, 2.3 MPa, 2.4 MPa, 2.5 MPa, 2.6 MPa, 2.7 MPa, 2.8 MPa, 2.9 MPa, 3.0 MPa, 3.1 MPa, 3.2 MPa, 3.4 MPa, 3.5 MPa, 3.6 MPa, 3.7 MPa, 3.8 MPa, 3.9 MPa, 4.0 MPa, 4.1 MPa, 4.2 MPa, 4.3 MPa, 4.4 MPa, 4.5 MPa, 4.6 MPa, 4.7 MPa, 4.8 MPa, 5.0 MPa, and any range or value therebetween. The temperature of the ODH unit (e.g., ODH reactor) is in the range of 200° C. to 450° C., preferably 250° C. to 350° C. Non-limiting examples of temperatures include 200°C, 225°C, 250°C, 275°C, 300°C, 325°C, 350°C, 375°C, 400°C, 425°C, 450°C, and any range or value therebetween.

[0029] In the ODH unit 102, the ethane and oxygen-containing feed can be mixed and contacted with a dehydrogenation catalyst to produce an ODH product gas stream (a first product gas stream). The oxidative dehydrogenation catalyst comprises a metal or compound thereof from columns 5, 6, 7, 8, 9, 10, and / or 11 of the periodic table and / or any combination thereof. A non-limiting example of a catalyst used in the present invention is Mo. a V b Nb c X d, where X = at least one of the following: Te, Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, La, Zn, Cd, Hg, Al, Pb, As, Bi, Sb, U, Mn, and W; a = 0.05 to 0.9, b = 0.1 to 0.4, c = 0.001 to 0.2, and d = 0.001 to 1.0. The values ​​of a, b, c, and d are the relative gram-atomic numbers of Mo, V, Nb, and X, respectively. These elements are present in the form of various oxides combined with oxygen. The catalyst may be supported or unsupported. Suitable supports include silica, alumina, silicon carbide, zirconia, titania, and mixtures thereof. In principle, any catalyst that is stable under the given reaction conditions and exhibits sufficient activity and selectivity may be used. The catalyst may be in the form of a bulk catalyst, a supported catalyst particle, a supported catalyst structure (such as a catalytic foam or a monolithic catalyst), or any other form that can promote the reaction of gaseous substances on a solid catalyst.

[0030] The ODH product gas stream 112 may include hydrocarbons (eg, ethane and ethylene) and non-hydrocarbons (eg, water, acetic acid, argon, nitrogen, carbon oxides, oxygen, etc.).

[0031] In some embodiments, the ODH unit 102 may include a carbon monoxide oxidation unit that receives the ODH product gas stream. The carbon monoxide oxidation unit may include one or more carbon monoxide oxidation reactors. In the carbon monoxide oxidation unit, the carbon monoxide in the ODH product gas stream may be oxidized to carbon dioxide. Supplemental oxygen may be provided to the carbon monoxide oxidation reactor to ensure that there is sufficient oxygen when the carbon monoxide is oxidized to carbon dioxide. The carbon monoxide oxidation device may be any known device capable of oxidizing carbon monoxide to carbon dioxide (CO2). In some cases, the CO oxidation unit may be placed downstream of the ODH unit 102, for example, after the quenching unit 114.

[0032] The ODH product gas stream 112 (first product gas stream) can exit the ODH unit 102 and enter a quenching unit 114. In the quenching unit 114, the first product gas stream 112 can be cooled to a sufficiently low temperature, such as 10°C to 50°C, to condense water and / or acetic acid in the gas stream, thereby producing a quenched first product gas stream 116. A water and / or water-acetic acid mixture stream 115 can exit the quenching unit 114 and can be further processed, stored, or transported. For example, any known acetic acid / water separation method can be used to separate and recover water and acetic acid. The quenching unit 114 can include any equipment capable of performing liquid / gas separation (e.g., a quenching tower or a combination of a cooling exchanger and a gas-liquid separator). The quenched first product gas stream 116 can exit the quenching unit 114 and enter a carbon dioxide removal unit 118. The pressure of the exiting gas stream can be the same as or slightly lower than the pressure at the outlet of the ODH unit. For example, 1.5 MPa to 4.5 MPa, preferably 2 MPa to 3 MPa, more preferably 2 MPa to 3 MPa, or most preferably 1.5 MPa to 1.9 MPa.

[0033] The carbon dioxide removal device 118 can be any known device that can separate carbon dioxide from the quenched first product gas stream 116 and form a carbon dioxide stream 120 and a first product gas stream 122 having a lower carbon dioxide content than the first product gas stream 116 (e.g., the treated first product gas stream). In certain embodiments, the carbon dioxide removal device 118 can be an absorption system (e.g., using an amine or carbonate absorbent) or a membrane device. The carbon dioxide stream 120 can flow out of the carbon dioxide removal unit 118 and be stored and / or further processed. Advantageously, the carbon dioxide removal unit 118 can be located after the carbon monoxide oxidation unit so that it can remove carbon dioxide formed by the oxidation of carbon monoxide. In one embodiment, during the removal of carbon monoxide, carbon dioxide, water and / or acetic acid, the product stream is not compressed before the first product stream is fed to the inlet of the gas separation unit 106 at a temperature of -40°C to -10°C.

[0034] First product stream 122 can exit carbon dioxide removal unit 118 and enter drying unit 124. In drying unit 124, the carbon dioxide concentration of first product stream 122 can be reduced to the parts per billion or parts per million level under certain conditions (if necessary) and all or substantially all remaining water can be removed. Drying unit 124 can be any unit known in the art capable of reducing the water concentration in first product stream 122 and forming a dried first product stream 126, such as, for example, an adsorption system based on a molecular sieve desiccant. Dried first product stream 126 can exit drying unit 124 and pass through cooling unit 104 to form cooled first product stream 128. Cooling unit 104 can be a heat exchanger that rejects process heat to a refrigerant. For example, the first product stream can be cooled by heat exchange to a temperature between approximately 40°C and -10°C, between 30°C and -20°C, or to a temperature between -40°C, -35°C, -30°C, -25°C, -20°C, -15°C, -10°C, or any range or value therebetween. After the cooling unit 104, the first cooled product stream 128 is in a liquid phase, or a combination of a liquid phase and a vapor phase. It is noteworthy that when the dried first product stream enters the separation unit 106 from the ODH unit 102, it is not necessary to compress it in order to condense the first product stream and separate it in the separation unit 106 (e.g., a demethanizer). One reason why a compression step is not required is that when removing water, carbon dioxide, and one or more gases having a boiling point lower than that of ethylene, the pressure of the first product stream can be between 1.9 MPa and 5 MPa, preferably between 1.9 MPa and 2.3 MPa.

[0035] The cooled first product stream 128 is fed to the separation unit 106 without further pumping. In one aspect, if the cooled first product stream 128 is substantially in the liquid phase, it can be pumped to a higher pressure. In one aspect, if the cooled first product stream is partially in the vapor phase (e.g., with a molar vapor fraction of 1%), the vapor and liquid substreams can be separated in a vapor-liquid separator (e.g., a flash drum), with only the liquid substream directed to the separation unit 106 (with or without a pump). In this case, ethane or ethylene entrained in the vapor substream does not impact process yield, so favorable process conditions are selected to achieve a molar vapor fraction of less than 2%, less than 1%, or less than 0.5% in the cooled first product stream. The separation unit 106 (e.g., a demethanizer) can be any known separation unit capable of separating gases having a boiling point lower than ethylene from the first product stream 128. For example, the separation unit may be a demethanizer unit and / or a distillation unit operated at a temperature in the condenser of -101°C to -94°C and a pressure of 2.0 MPa to 5 MPa, preferably 2 MPa to 4 MPa, more preferably 2 MPa to 3 MPa, most preferably 2.0 MPa to 2.5 MPa. 3 MPa, 4.4 MPa, 4.5 MPa, 4.6 MPa, 4.7 MPa, 4.8 MPa, 5.0 MPa, and any range or value therebetween. In separation unit 106, one or more gases having a boiling point lower than that of ethylene are separated from first product stream 128 to produce gas stream 130 and second product stream 132, which include ethylene and ethane. The second product stream 132 can flow from the separation unit 106 and enter the C2 separation unit 134 (second gas separation unit). The C2 separation unit 134 can be any known separation unit capable of separating ethane from ethylene (e.g., a membrane system and / or a distillation system). In the C2 separation unit 134, the second product stream can be separated into an ethylene product stream 136 and an ethane stream 138. The ethane stream 138 can be recycled to the ODH reactor 102 (e.g., combined with the ethane feed stream 108 and / or directly fed to the ODH unit 102). The ethylene product stream 136 can flow out of the ethylene separation unit 134 and can be collected, stored, sold, transported, or directly provided to other processing units.

[0036] exist Figure 1In the embodiment, units 102, 104, 106, 114, 118, 124, and 134 may include one or more heating and / or cooling devices, as well as control devices (e.g., valves, flow controllers, and related instrumentation), inlets, outlets, etc., which can help control the temperature and pressure of the reaction or separation process, or the movement of fluids. It goes without saying that one or more reactors can be installed in a device. The temperature, pressure, and flow rate can vary depending on the reaction to be performed, which is within the skill of the person performing the reaction (e.g., an engineer or chemist). B. Stream composition.

[0037] The ethane gas fed to the ODH unit 102 can be obtained from any source, such as an intermediate stream from natural gas processing or other hydrocarbon processing facilities (e.g., a pyrolysis plant), provided that impurities are sufficiently removed to prevent catalyst poisoning and final product contamination, and that excessive levels of other accompanying compounds (including methane, C3+ hydrocarbons, hydrogen, carbon dioxide, nitrogen, argon, etc.) are avoided. The feed stream to the ODH unit 102 may include fresh ethane, such as ethane from the above sources, or ethane from Figure 1 Recycle of unreacted ethane recovered from the C2 separation unit 134 shown in FIG.

[0038] The oxygen-containing gas fed to the ODH unit 102 can include air, oxygen, and / or an air / oxygen mixture. The oxygen-containing gas can be pure oxygen produced by an air separation process. The oxygen-containing gas can also contain inert gases such as nitrogen, helium, or argon. In certain embodiments, the addition of an inert gas component can be beneficial in limiting the flammability of certain effluents that may contain residual oxygen. Preferably, the proportion of components other than oxygen in the oxygen-containing gas is relatively small, so that the fluid contains at least 90% by volume, at least 95% by volume, or at least 99% by volume oxygen.

[0039] The feed ratio of ethane and oxygen can be determined to obtain a desired combination of ethane and oxygen conversions, which are interconnected by material balance and reaction selectivity. The feed ingredients, ethane, oxygen / air, etc., can be gasified, preheated, and mixed before being fed to the ODH unit. This can be accomplished by processes known to those skilled in the art (such as engineers or chemists). For example, preheating techniques can include heat exchange between steam, a heat transfer fluid, reactor effluent, and a furnace. The overall conversion of ethane by the ODH unit can be achieved in a single reactor stage or in multiple reactor stages housed in a single or multiple containers. In certain embodiments, all oxygen-containing feeds are mixed with the ethane feed stream before entering the ODH unit (i.e., before contacting any catalyst). In other embodiments, the oxygen-containing feed is divided into several parts and fed into different locations before and between reactor stages. The reactor stage can be cooled or adiabatic and can be configured according to any reactor concept known in the art. The conversion of ethane through the ODH unit increases with increasing ratios of oxygen-containing gas to ethane feed gas, and can be selected to achieve economically advantageous conversion levels, for example, between 10% and 100%, 15% and 50%, or 20% and 40%. The ethane conversion rate refers to the portion of the total molar amount of ethane entering the ODH unit (e.g., fresh ethane and recycled ethane) that is chemically converted to other compounds within the ODH unit.

[0040] The various feed streams may be fed to the ODH unit via separate feed lines, combined feed lines, or a combination thereof. These feed streams may include a fresh ethane feed, a recycled ethane stream, and an oxygen feed. The stream resulting from combining all of these feed streams is referred to herein as a combined feed stream. The combined feed stream comprises ethane and oxygen and may also include low concentrations of one or more other gases with boiling points lower than ethylene, such as contaminants introduced with the ethane or oxygen feed streams. In the process of the present invention, it is preferred to limit the concentration of gases with boiling points lower than ethylene (e.g., methane, nitrogen, argon, etc.) in the combined feed stream. The combined mole fraction of gases with boiling points lower than ethylene (excluding oxygen) in the combined feed stream entering the ODH unit is less than 2%, or from 0% to 1%, or less than 0.5%, or less than 2%, 1.75%, 1.5%, 1.25%, 1.0%, 0.75%, 0.5%, 0.25%, 0%, or any value or range therebetween.

[0041] The first product stream can include ethylene, ethane, and one or more gases having a boiling point lower than that of ethylene. Non-limiting examples of gases having a boiling point lower than that of ethylene include methane, nitrogen, argon, and carbon monoxide. The total mole fraction of gases having a boiling point lower than that of ethylene in the first product stream can be less than 3%, or from 0% to 2%, or from 0.01 to less than 1.5%, or less than 0.5%, or 0%, 0.5%, 0.75%, 1%, 1.25%, 1.5%, 1.75%, 2%, 2.25%, 2.5%, 2.75%, or any value or range therebetween. The first product stream can also include carbon dioxide, acetic acid, and / or water. Example

[0042] The present invention will be described in more detail by way of specific examples. The following examples are provided for illustrative purposes only and are not intended to limit the present invention in any way. It will be readily apparent to those skilled in the art that various noncritical parameters may be varied or modified to produce substantially the same results. Example 1 (Oxidative Dehydrogenation of Ethane to Ethylene)

[0043] A computer simulation of the ethane oxidative dehydrogenation process to produce ethylene was developed using a commercial reactor design (AspenTech, Bedford, MA). Figure 1 , the oxidative dehydrogenation unit 102 includes 6 adiabatic reactors in series, operating at an inlet temperature of 372°C to 381°C and a pressure of 2.1 MPa to 2.3 MPa. The overall ethane conversion is 30% and the ethylene selectivity is about 90%. The ODH unit includes a CO oxidation unit for converting most of the CO formed in the ODH reaction into CO2. Since the pressure of the ODH reactor unit 102 is high, there is no need to further compress the product gas stream from the ODH reactor. In addition, the first product gas stream coming out of the cooling unit 104 (stream 128 in Table 1) is a liquid at -18°C and 1.9 MPa, so that the pressure of the first product stream can be further increased to 2.3 MPa by a pump before it is sent to the demethanizer unit (gas separation unit) 106. Table 1 provides Figure 1 Information for the selected stream in this example. Mass flow rate data is in kg / hr, temperature is in °C, and pressure is in MPa. Table 1

[0044] Although the embodiments of the present application and their advantages have been described in detail, it should be understood that various changes, substitutions and modifications can be made herein without departing from the spirit and scope of the embodiments defined by the appended claims. In addition, the scope of the present application is not limited to the specific embodiments of the processes, machines, manufactures, compositions of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the above disclosure, currently existing or later developed processes, machines, manufactures, compositions of matter, means, methods or steps can be used that perform substantially the same functions or achieve substantially the same results as the corresponding embodiments described herein. Therefore, the appended claims are intended to include such processes, machines, manufactures, compositions of matter, means, methods or steps within their scope.

Claims

1. A process for the oxidative dehydrogenation of ethane into ethylene, the process comprising: (a) contacting a feed stream comprising ethane with an oxidative dehydrogenation catalyst in the presence of oxygen under conditions sufficient to produce a first product stream comprising ethane, ethylene, and one or more gases having a lower boiling temperature than ethylene, wherein the conditions include a pressure of 1.8 MPa to 5 MPa; (b) feeding the first product stream comprising ethane, ethylene and one or more gases having a boiling point lower than that of ethylene to the inlet of the gas separation unit at a temperature of -40°C to -10°C, preferably -30°C to -20°C; and (c) separating one or more gases having a boiling point temperature lower than that of ethylene from the first product stream of step (b) to produce a second product stream comprising ethane and ethylene.

2. The method according to claim 1, wherein the oxidative dehydrogenation conditions comprise a pressure of 2 MPa to 4 MPa, preferably 2 MPa to 3 MPa, more preferably 2 MPa to 2.5 MPa.

3. The method of claim 2, wherein the oxidative dehydrogenation conditions comprise a temperature of 200-450°C, preferably 250-350°C.

4. The method according to any one of claims 1-3, further comprising separating the second product stream into an ethylene product stream and an ethane product stream.

5. The process of claim 4, further comprising recycling the ethane product stream to step (a).

6. The method of any one of claims 1-5, wherein the first product stream is not compressed in step (a), step (b), step (c), or a combination thereof.

7. The method of any one of claims 1-6, wherein the first product stream further comprises water and acetic acid, and the method further comprises removing at least a portion of the water and the acetic acid from the first product stream prior to step (b).

8. A process according to any one of claims 1 to 7, wherein the gas having a boiling point lower than that of ethylene comprises carbon monoxide, and the process further comprises removing the carbon monoxide from the first product stream or subjecting the first product stream to conditions suitable for oxidizing the carbon monoxide to carbon dioxide, or a combination thereof, prior to step (b).

9. The method of any one of claims 1-8, wherein the first product stream comprises carbon dioxide, and the method further comprises removing the carbon dioxide from the first product stream prior to step (b).

10. The process according to any one of claims 1 to 9, wherein the ethane conversion in step (a) is 20% to 40%.

11. The process according to any one of claims 1 to 10, wherein the first product stream in step (b) is in a liquid phase.

12. The process according to any one of claims 1 to 11, wherein the molar steam fraction of the first product stream in step (b) is less than 1%.

13. The process of any one of claims 1 to 12, wherein no methane is added to the step (a) feed stream.

14. The process of any one of claims 1-13, wherein the gas having a boiling temperature below that of ethylene comprises at least one of methane, nitrogen, argon, carbon monoxide, and oxygen, and the combined mole fraction of said gases in the first product stream is less than 3%.

15. The process according to any one of claims 1 to 14, wherein the combined mole fraction of gases having a boiling point lower than ethylene of all streams other than oxygen entering the oxydehydrogenation unit comprising the oxydehydrogenation catalyst of step (a) is lower than 2%.

Images