Ethane oxidative dehydrogenation method

Abstract

The present invention relates to a process for producing ethylene by oxidative dehydrogenation (ODH) of ethane, the process comprising: a) supplying ethane and oxygen to a first ODH zone formed by a plurality of reactor tubes containing a mixed metal oxide ODH catalyst bed; b) contacting the ethane and oxygen with the catalyst to produce a plurality of effluent streams, wherein the plurality of reactor tubes are cooled by a coolant; c) mixing at least a portion of the plurality of effluent streams from step b) to produce a mixture comprising ethylene, unreacted ethane and unreacted oxygen; d) supplying at least a portion of the mixture from step c) to a second ODH zone containing a mixed metal oxide ODH catalyst bed; e) contacting at least a portion of the mixture from step c) with the catalyst in the second ODH zone to produce a stream comprising ethylene and unreacted ethane.

Description

Technical Field

[0001] The present invention relates to a process for producing ethylene by oxidative dehydrogenation (oxydehydrogenation; ODH) of ethane. Background Art

[0002] It is known to oxidative dehydrogenate alkanes (such as alkanes containing 2 to 6 carbon atoms, for example ethane or propane) in an oxidative dehydrogenation (oxydehydrogenation; ODH) process to produce ethylene and propylene, respectively. Examples of alkane ODH processes (including catalysts and other process conditions) are disclosed, for example, in US7091377, WO2003064035, US20040147393, WO2010096909, and US20100256432. Mixed metal oxide catalysts containing molybdenum (Mo), vanadium (V), niobium (Nb), and optionally tellurium (Te) as metals can be used as such oxydehydrogenation catalysts.

[0003] In particular, in the ethane ODH process, oxygen can be used as an oxidant. In addition to ethylene and unreacted ethane, the ethane ODH effluent can contain unreacted oxygen. Generally, to avoid the risk of operating the ethane ODH process under oxygen-depleted conditions, especially near the outlet of the ODH reactor, more oxygen is fed than is needed, in which case the ethane ODH effluent contains unreacted oxygen. However, on the other hand, having unreacted oxygen in the ODH effluent increases the risk of explosion hazards because there are hydrocarbons (ethane, ethylene) and a relatively high oxygen concentration in the downstream separation section, where distillation can be applied to achieve the desired separation. Additionally, this unreacted oxygen can involve some undesirable trace chemicals, such as the formation of peroxides, which themselves can also pose an unwanted explosion hazard risk.

[0004] In view of the above, it is desirable to remove unreacted oxygen from the ODH effluent and then feed the latter effluent into the above-mentioned downstream separation section.

[0005] WO2018153831 discloses a process for producing ethylene by oxidative dehydrogenation of ethane, in which after first removing water from the ethane ODH effluent, unreacted oxygen is removed from the effluent by oxidizing carbon monoxide and acetylene with unreacted oxygen in a separate step before the downstream separation section. The acetylene can be produced by further oxidative dehydrogenation of ethylene. According to WO2018153831, a preferred oxidation catalyst for such an oxygen removal step is a catalyst containing copper and / or platinum.

[0006] Thus, in the method of WO2018153831 described above, carbon monoxide and acetylene are also removed simultaneously with the unreacted oxygen. Removing carbon monoxide and acetylene is beneficial because they may cause problems in downstream conversion processes. For example, carbon monoxide and / or acetylene may be harmful to the catalysts used in such further downstream conversion processes, resulting in a decrease in catalyst activity. An example of such a downstream conversion process is a process in which the ethylene product of the ethane ODH process is further converted. In addition, in the back-end separation section, which is also downstream of the ethane ODH step, there may not be sufficient capacity to remove carbon monoxide and acetylene, for example, by distillation.

[0007] In addition, WO2018153831 described above recommends not feeding less oxygen to the ODH reactor and / or not completely converting the oxygen in the ODH reactor. To this end, this is described in WO2018153831 as a less desirable alternative compared to the case of feeding more oxygen to the ODH reactor than required and the ethane ODH effluent containing unreacted oxygen. According to the solution of WO2018153831, instead of this less desirable alternative, it is to include the above-mentioned separate oxygen removal step downstream of the ODH step. However, this separate oxygen removal step means that there is a delicate balance between the operation of the ODH reactor and the operation of the oxygen removal reactor, because the removal of carbon monoxide and acetylene from the ODH effluent occurs jointly with the removal of the unreacted oxygen present in the same ODH effluent. That is, the unreacted oxygen from the ODH effluent is used to oxidize (burn) carbon monoxide and acetylene to carbon dioxide.

[0008] The interdependence between the removal of unreacted oxygen and the removal of carbon monoxide and acetylene described above can be troublesome. First, in the case where there is too little unreacted oxygen in the ODH effluent, not all carbon monoxide and acetylene will be removed by oxidation. In this case, additional oxygen can be fed to the oxygen removal step to completely convert all carbon monoxide and acetylene to carbon dioxide, as disclosed in WO2018153831 above. However, having to add additional oxygen is a troublesome additional step that requires precisely determining how much oxygen to add. In addition, in the case where there is too much unreacted oxygen in the ODH effluent and all carbon monoxide and acetylene will be removed by oxidation, some unreacted oxygen remains after the oxygen removal step. In this case, additional combustible components (such as hydrocarbons or hydrogen (H 2 )) can be fed to the oxygen removal step to completely convert all the oxygen. However, having to add additional combustible components is also a troublesome additional step that requires precisely determining how much of these components to add.

[0009] Furthermore, another common drawback of the deoxygenation step in the method of WO2018153831 described above is the production of carbon dioxide, which also needs to be separated from the effluent in a separate carbon dioxide removal step. This is even more disadvantageous when considering that, as an alternative to oxidizing acetylene to carbon dioxide, this acetylene can be hydrogenated to ethylene, thereby increasing the overall yield of ethylene while minimizing the formation of carbon dioxide. This hydrogenation to ethylene is a conventional method for removing acetylene. For example, acetylene can be removed from a stream containing desired ethylene and undesired acetylene by selectively hydrogenating the acetylene to ethylene. Such a stream can originate from a so-called "C2 separation column" in which ethane is separated from ethylene by means of cryogenic distillation, where relatively high pressures and relatively low (low temperature) temperatures are applied.

[0010] Accordingly, an object is to provide a method for oxidative dehydrogenation of ethane in which it is no longer necessary to jointly remove (i) carbon monoxide and acetylene from the ODH effluent and (ii) unreacted oxygen present in the same ODH effluent in a separate deoxygenation step downstream of the ODH step, thereby avoiding the above-mentioned drawbacks associated with such a separate deoxygenation step by applying such an ODH reactor configuration and operating it in such a way that the ODH effluent from such a reactor configuration does not contain or substantially does not contain unreacted oxygen.

[0011] Furthermore, an object is to provide such a method in which the ODH reaction conditions in the above-mentioned ODH reactor configuration can be optimized, which method is technically advantageous, efficient and affordable. Such a technically advantageous method will preferably result in a lower energy requirement and / or lower capital expenditure. More specifically, an object is to provide such a method in which the ODH reaction conditions in the above-mentioned ODH reactor configuration can be set such that the long-term stability of the ODH catalyst, in particular a mixed metal oxide catalyst containing molybdenum, vanadium, niobium and optionally tellurium, in the catalyst bed in the ODH reactor configuration, in particular in the downstream part of such a configuration, can be improved.

[0012] Still further, an object is to provide a different method using the above-mentioned ODH reactor configuration in which carbon monoxide and acetylene can be removed from the alkane ODH effluent in a manner different from the above-mentioned deoxygenation step in which carbon monoxide and acetylene are oxidized by unreacted oxygen from the upstream ODH step, which different method is technically advantageous, efficient and affordable. Such a technically advantageous method will preferably result in a lower energy requirement and / or lower capital expenditure. Summary of the Invention

[0013] Surprisingly, it has been found that one or more of the above objects can be achieved in a process for producing ethylene by oxidative dehydrogenation (ODH) of ethane, wherein at least two oxidative dehydrogenation zones arranged in series are used, said zones containing a mixed metal oxide catalyst, and wherein ethane and oxygen are supplied to a first oxidative dehydrogenation zone formed by a plurality of reactor tubes containing said catalyst, and at least a portion of the plurality of effluent streams from these plurality of tubes is mixed to produce a mixture comprising ethylene, unconverted ethane and unconverted oxygen, and then at least a portion of said mixture is supplied in series to a second oxidative dehydrogenation zone to further convert the unconverted ethane and unconverted oxygen.

[0014] Accordingly, the present invention relates to a process for producing ethylene by oxidative dehydrogenation of ethane, said process comprising:

[0015] a) supplying ethane and oxygen to a first oxidative dehydrogenation zone formed by a plurality of reactor tubes containing a catalyst bed, said catalyst bed containing an oxidative dehydrogenation catalyst which is a mixed metal oxide catalyst;

[0016] b) contacting said ethane and oxygen with the catalyst in said plurality of reactor tubes in said first oxidative dehydrogenation zone to produce a plurality of effluent streams, wherein said plurality of reactor tubes are cooled by a coolant;

[0017] c) mixing at least a portion of the plurality of effluent streams from step b) to produce a mixture comprising ethylene, unconverted ethane and unconverted oxygen;

[0018] d) supplying at least a portion of the mixture from step c) to a second oxidative dehydrogenation zone containing a catalyst bed, said catalyst bed containing an oxidative dehydrogenation catalyst which is a mixed metal oxide catalyst;

[0019] e) contacting at least a portion of the mixture from step c) with the catalyst in said second oxidative dehydrogenation zone to produce a stream comprising ethylene and unconverted ethane. Detailed Description

[0020] The process of the present invention comprises steps a), b), c), d) and e). The process may comprise one or more intermediate steps between step a) and b), between step b) and c), between step c) and d) and between step d) and e). In addition, the process may comprise one or more additional steps before step a) and / or after step e).

[0021] While the method of the present invention and one or more of the streams used in the method are described as "comprising", "containing" or "including" one or more different said steps or components, they may also "consist essentially of the one or more different said steps or components" or "consist of the one or more different said steps or components".

[0022] In the context of the present invention, in cases where a stream contains two or more components, these components are selected in a total amount not exceeding 100% by volume or 100% by weight.

[0023] Within this specification, "substantially free of" means that no detectable amount of the component under discussion is present.

[0024] In the present invention, the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone may be contained within the same reactor vessel or, preferably, within separate reactor vessels arranged in series, the reactor vessel or each reactor vessel comprising a reactor shell. The first oxidative dehydrogenation zone is formed by a plurality of reactor tubes containing a catalyst bed. The second oxidative dehydrogenation zone also contains a catalyst bed and may also be formed by a plurality of reactor tubes. In cases where the oxidative dehydrogenation zone is formed by a plurality of reactor tubes, these tubes are disposed inside the reactor shell of the reactor vessel. Additionally, in such cases, it is preferred that a coolant circuit is fluidly connected to the portion of the reactor vessel containing the plurality of reactor tubes such that the plurality of reactor tubes can be cooled by coolant from the coolant circuit, which coolant can be supplied to the shell space of the reactor vessel containing the plurality of reactor tubes. Alternatively, the second oxidative dehydrogenation zone is not formed by a plurality of reactor tubes and may be operated adiabatically without the use of coolant. For example, the second oxidative dehydrogenation zone may be contained within a single reactor containing a single catalyst bed, separate from the first oxidative dehydrogenation zone. Additionally, for example, the second oxidative dehydrogenation zone may be contained within the bottom of the reactor vessel containing the first oxidative dehydrogenation zone, which bottom may have a dome shape and is also separate from the first oxidative dehydrogenation zone. The catalyst beds in the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone can be any type of bed, including fixed beds and fluidized beds. Suitably, these catalyst beds are fixed beds.

[0025] In the present invention, the first oxidative dehydrogenation zone preferably contains one oxidative dehydrogenation zone, but it may also contain two or more oxidative dehydrogenation zones, where each zone is formed by a plurality of reactor tubes containing a catalyst bed, the catalyst bed containing an oxidative dehydrogenation catalyst, which oxidative dehydrogenation catalyst is a mixed metal oxide catalyst as described above. These plurality of oxidative dehydrogenation zones for the first oxidative dehydrogenation zone may be arranged in parallel or in series. If they are arranged in parallel, the plurality of effluent streams from these plurality of oxidative dehydrogenation zones may be mixed together in step c) of the method of the present invention.

[0026] Step a) of the process according to the invention comprises supplying ethane and oxygen to a first oxidative dehydrogenation zone which is formed by a plurality of reactor tubes containing a catalyst bed which contains an oxidative dehydrogenation catalyst, and step b) comprises bringing the ethane and oxygen into contact with the catalyst in the plurality of reactor tubes in the first oxidative dehydrogenation zone, giving rise to a plurality of effluent streams, where the plurality of reactor tubes are cooled by a coolant. In step b), a part of the ethane supplied to step a) is converted to ethylene. In addition, in step b), a part of the oxygen supplied to step a) is converted. Preferably, in step b), at most 99%, more preferably at most 97%, more preferably at most 95%, more preferably at most 92%, most preferably at most 90% of the oxygen supplied to step a) is converted. In addition, preferably, in step b), at least 80%, more preferably at least 85%, more preferably at least 87%, more preferably at least 89%, most preferably at least 90% of the oxygen supplied to step a) is converted.

[0027] Step c) of the process according to the invention comprises mixing at least a part, preferably all, of the plurality of effluent streams from step b), giving rise to a mixture comprising ethylene, unconverted ethane and unconverted oxygen. That is to say, in step c), the effluent streams from the plurality of reactor tubes in the first oxidative dehydrogenation zone are mixed. Thus, in step c), a part (i.e. at least two) or preferably all of the plurality of effluent streams from step b) are mixed.

[0028] Preferably, the mixture comprising ethylene, unconverted ethane and unconverted oxygen produced in step c) comprises 1,000 parts per million by volume (ppmv) of unconverted oxygen (volume-averaged oxygen concentration). More preferably, the oxygen concentration is at least 2,000 ppmv, more preferably at least 5,000 ppmv, more preferably at least 10,000 ppmv (= 1% by volume), most preferably at least 2% by volume. In addition, preferably, the oxygen concentration is at most 10% by volume, more preferably at most 5% by volume, more preferably at most 3% by volume, more preferably at most 2% by volume, most preferably at most 1% by volume.

[0029] Surprisingly, it has been found, as confirmed by the examples in Document A below, that at relatively high oxygen conversion rates, for example, small flow rate non-uniformities produce significant and adverse variations in unreacted oxygen in the effluent streams from the plurality of reactor tubes which form the oxidative dehydrogenation zone within the reactor vessel. Advantageously, in the present invention, the adverse effects of such non-uniformities are removed or significantly reduced by: (i) using at least two oxidative dehydrogenation zones arranged in series, where the first oxidative dehydrogenation zone is formed by the plurality of reactor tubes as described above, and (ii) mixing the streams before supplying at least a portion of the plurality of effluent streams from the first zone to the second zone for further conversion of unreacted ethane and unreacted oxygen.

[0030] Accordingly, preferably, the unreacted oxygen is uniformly or substantially uniformly distributed in the mixture containing ethylene, unreacted ethane and unreacted oxygen produced in step c).

[0031] Accordingly, it is preferred that the oxygen concentration in at least 95 vol%, more preferably at least 96 vol%, more preferably at least 97 vol%, more preferably at least 98 vol%, more preferably at least 99 vol%, more preferably at least 99.5 vol%, more preferably at least 99.6 vol%, more preferably at least 99.7 vol%, more preferably at least 99.8 vol%, most preferably at least 99.9 vol% of the mixture produced in step c) is equal to or deviates from the volume average oxygen concentration in the total mixture by at most 5%.

[0032] In addition, alternatively, it is preferred that the oxygen concentration in at least 90 vol%, more preferably at least 91 vol%, more preferably at least 92 vol%, more preferably at least 93 vol%, more preferably at least 94 vol%, more preferably at least 95 vol%, more preferably at least 96 vol%, more preferably at least 97 vol%, more preferably at least 98 vol%, most preferably at least 99 vol% of the mixture produced in step c) is equal to or deviates from the volume average oxygen concentration in the total mixture by at most 1%.

[0033] The mixing in step c) of the process of the present invention involves combining at least a portion, preferably all, of the plurality of effluent streams from step b). Preferably, the mixing in step c) is carried out in such a way that a uniform or substantially uniform distribution of the above-mentioned unreacted oxygen in the mixture containing ethylene, unreacted ethane and unreacted oxygen produced in step c) is achieved. Such uniform or substantially uniform mixing can be carried out in any manner.

[0034] For example, in the case where the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone are contained in separate first and second reactor vessels arranged in series, the mixing in step c) can be carried out in the following parts: (i) in the bottom of the first reactor vessel, a plurality of reactor tubes project into the bottom and the bottom may have a dome shape, and (ii) in one or more pipes connecting the first reactor vessel to the second reactor vessel, wherein the one or more pipes should have a length that enables uniform or substantially uniform mixing.

[0035] Furthermore, in the case where the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone are contained in the same reactor vessel, the mixing in step c) can be carried out in an intermediate part of the first reactor vessel located between the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone, and a plurality of reactor tubes project into the intermediate part.

[0036] Step d) of the method of the present invention comprises supplying at least a portion, preferably all, of the mixture from step c) to a second oxidative dehydrogenation zone containing a catalyst bed containing an oxidative dehydrogenation catalyst, and step e) comprises contacting at least a portion of the mixture from step c) with the catalyst in the second oxidative dehydrogenation zone to produce a stream containing ethylene and unreacted ethane. In step e), additional ethane from the ethane supplied to step a) is converted to ethylene. Furthermore, in step e), additional oxygen from the oxygen supplied to step a) is converted. Preferably, in the combination of steps b) and e), at least more than 99%, more preferably at least 99.1%, more preferably at least 99.2%, more preferably at least 99.3%, more preferably at least 99.4%, most preferably at least 99.5% of the oxygen supplied to step a) and any additional oxygen supplied to step d) is converted. Furthermore, preferably, in the combination of steps b) and e), at most 99.5%, more preferably at most 99.8%, more preferably at most 99.9%, more preferably at most 99.95%, more preferably at most 99.98%, more preferably at most 99.99%, most preferably at most 100% of the oxygen supplied to step a) and any additional oxygen supplied to step d) is converted. Most preferably, all or substantially all of the remaining oxygen is converted in step e).

[0037] Preferably, the stream containing ethylene and unreacted ethane produced in step e) does not contain or substantially does not contain unreacted oxygen. More preferably, the oxygen concentration in the stream is at most less than 1,000 ppmv (volume average oxygen concentration), more preferably at most 500 ppmv, more preferably at most 300 ppmv, more preferably at most 200 ppmv, more preferably at most 100 ppmv, more preferably at most 50 ppmv, more preferably at most 20 ppmv, more preferably at most 10 ppmv, more preferably at most 5 ppmv, and most preferably at most 1 ppmv.

[0038] The oxidative dehydrogenation catalyst in the first oxidative dehydrogenation zone is a mixed metal oxide catalyst, that is, a catalyst containing a mixed metal oxide. Preferably, the catalyst in the first oxidative dehydrogenation zone is a heterogeneous catalyst.

[0039] Preferably, the catalyst in the first oxidative dehydrogenation zone is a mixed metal oxide catalyst containing molybdenum, vanadium, optional niobium, and optional tellurium, and the catalyst may have the following formula:

[0040] Mo 1 V a Te b Nb c O n

[0041] Where:

[0042] a, b, c, and n represent the molar ratio of the elements under discussion to the molar amount of molybdenum (Mo);

[0043] a is from 0.01 to 1, preferably from 0.05 to 0.60, more preferably from 0.10 to 0.40, more preferably from 0.20 to 0.35, and most preferably from 0.25 to 0.30;

[0044] b is 0 or >0 to 1, preferably from 0.01 to 0.40, more preferably from 0.05 to 0.30, more preferably from 0.05 to 0.20, and most preferably from 0.09 to 0.15;

[0045] c is 0 or >0 to 1, preferably from 0.01 to 0.40, more preferably from 0.05 to 0.30, more preferably from 0.10 to 0.25, and most preferably from 0.14 to 0.20; and

[0046] n is a number determined by the valence and frequency of elements other than oxygen.

[0047] Preferably, the catalyst in the first oxidative dehydrogenation zone is a mixed metal oxide catalyst containing tellurium. More preferably, the catalyst is a mixed metal oxide catalyst containing molybdenum, vanadium, niobium, and tellurium. Most preferably, the catalyst is a mixed metal oxide catalyst in which the metals are composed of molybdenum, vanadium, niobium, and tellurium.

[0048] In addition, the catalyst in the first oxidative dehydrogenation zone can be a mixed metal oxide catalyst having the following formula:

[0049] Mo 1 V a X b Y c Z d O n

[0050] where:

[0051] a, b, c, d, and n represent the molar ratio of the elements under discussion to the molar amount of molybdenum (Mo);

[0052] X is at least one of Nb and Ta;

[0053] Y is at least one of Sb and Ni;

[0054] Z is at least one of Te, Ga, Pd, W, Bi, and Al;

[0055] a ranges from 0.05 to 1;

[0056] b ranges from 0.001 to 1;

[0057] c ranges from 0.001 to 1;

[0058] d ranges from 0.001 to 0.5; and

[0059] n is a number determined by the valence and frequency of elements other than oxygen.

[0060] Furthermore, the catalyst in the first oxidative dehydrogenation zone can be a mixed metal oxide catalyst having the following formula:

[0061] Mo 1 V a X b Y c Z d M e O n

[0062] where:

[0063] a, b, c, d, e, and n represent the molar ratio of the elements under discussion to the molar amount of molybdenum (Mo);

[0064] X is at least one of Nb and Ta;

[0065] Y is at least one of Sb and Ni;

[0066] Z is at least one of Te, Ga, Pd, W, Bi, and Al;

[0067] M is at least one of Fe, Co, Cu, Cr, Ti, Ce, Zr, Mn, Pb, Mg, Sn, Pt, Si, La, K, Ag and In;

[0068] a ranges from 0.05 to 1;

[0069] b ranges from 0.001 to 1;

[0070] c ranges from 0.001 to 1;

[0071] d ranges from 0.001 to 0.5;

[0072] e ranges from 0.001 to 0.3; and

[0073] n is a number determined by the valence and frequency of elements other than oxygen.

[0074] The oxidative dehydrogenation catalyst in the second oxidative dehydrogenation zone is also a mixed metal oxide catalyst, that is, a catalyst containing a mixed metal oxide. Preferably, the catalyst in the second oxidative dehydrogenation zone is a heterogeneous catalyst. The catalyst in the second oxidative dehydrogenation zone can be the same as the catalyst in the first oxidative dehydrogenation zone. Preferably, these catalysts are different. More preferably, the catalyst in the second oxidative dehydrogenation zone is not a catalyst that conforms to the above description of the catalyst in the first oxidative dehydrogenation zone.

[0075] The catalyst in the second oxidative dehydrogenation zone can be a mixed metal oxide catalyst having the following formula:

[0076] Mo 1 V a Te b Nb c A d O n

[0077] Wherein:

[0078] A is at least one metal selected from the group consisting of Pt, Pd, Cu, Ag and Fe;

[0079] a, b, c, d and n represent the molar ratio of the elements under discussion to the molar amount of molybdenum;

[0080] a ranges from 0.01 to 1, preferably from 0.05 to 0.60, more preferably from 0.10 to 0.40, more preferably from 0.20 to 0.35, most preferably from 0.25 to 0.30;

[0081] b is 0 or >0 to 1, preferably from 0.01 to 0.40, more preferably from 0.05 to 0.30, more preferably from 0.05 to 0.20, most preferably from 0.09 to 0.15;

[0082] c is 0 or >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, still more preferably 0.10 to 0.25, and most preferably 0.14 to 0.20;

[0083] d is 0 or >0 to 0.3, preferably 0.01 to 0.25, more preferably 0.02 to 0.20, still more preferably 0.03 to 0.15, and most preferably 0.05 to 0.10; and

[0084] n is a number determined by the valence and frequency of elements other than oxygen.

[0085] In addition, the catalyst in the second oxidative dehydrogenation zone may be a mixed metal oxide catalyst having the following formula:

[0086] a) Mo 1 V a Sb b Nb c A d O n ;

[0087] b) Mo 1 V a Sb b Nb c O n ;

[0088] c) Mo 1 V a Sb b A d O n ; or

[0089] d) Mo 1 V a Sb b O n ;

[0090] wherein:

[0091] A is at least one metal selected from the group consisting of Pt, Pd, Cu, Ag, and Fe;

[0092] a, b, c, d, and n represent the molar ratio of the elements under discussion to the molar amount of molybdenum;

[0093] a is 0.01 to 1, preferably 0.05 to 0.60, more preferably 0.10 to 0.40, still more preferably 0.20 to 0.35, and most preferably 0.25 to 0.30;

[0094] b is >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, still more preferably 0.05 to 0.20, and most preferably 0.09 to 0.15;

[0095] c is 0 or >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, even more preferably 0.10 to 0.25, and most preferably 0.14 to 0.20;

[0096] d is 0 or >0 to 0.3, preferably 0.01 to 0.25, more preferably 0.02 to 0.20, even more preferably 0.03 to 0.15, and most preferably 0.05 to 0.10; and

[0097] n is a number determined by the valence and frequency of elements other than oxygen.

[0098] Preferably, the catalyst in the second oxidative dehydrogenation zone is a tellurium-free mixed metal oxide catalyst. Additionally, the catalyst may contain molybdenum, vanadium, and niobium. More preferably, the catalyst in the second oxidative dehydrogenation zone is a mixed metal oxide catalyst, wherein the metal is composed of two or more metals selected from the group consisting of the metals from the above formula.

[0099] As confirmed by the examples in Section B. below, in a non-oxidizing environment where there is no oxygen or a relatively low amount of oxygen, tellurium may be lost from the mixed metal oxide catalyst containing tellurium. This tellurium loss can lead to a decrease in activity and selectivity (i.e., catalyst deactivation), and thus result in less stable catalyst performance. As confirmed by the examples in Section A. below, at a relatively high oxygen conversion rate, there will be many reactor tubes in the multiple reactor tubes, and these tubes form an oxidative dehydrogenation zone within the reactor vessel where such a non-oxidizing environment may cause tellurium to be lost in these tubes. In addition to the catalyst deactivation, this loss of tellurium may cause serious operating problems in the section downstream of the catalyst bed, as well as possible health, safety, and environmental problems.

[0100] Therefore, in the present invention, it is preferred that the mixed metal oxide catalyst in the second oxidative dehydrogenation zone (where such a non-oxidizing environment may exist due to further conversion of the remaining oxygen) does not contain tellurium, thereby preventing any tellurium loss, and thus ensuring stable catalyst performance and preventing the above-mentioned problems in the downstream section. Even though the use of tellurium in the mixed metal oxide catalyst may lead to an increase in the selectivity of the oxidative dehydrogenation of ethane to ethylene, any loss of selectivity is less important in the second oxidative dehydrogenation zone because in any case most of the oxygen (and ethane) has been converted in the first oxidative dehydrogenation zone.

[0101] On the other hand, in the present invention, it is preferred that the mixed metal oxide catalyst in the first oxidative dehydrogenation zone does contain tellurium, because the conversion rate of oxygen in this first oxidative dehydrogenation zone can be kept relatively low, since any further conversion will take place in the downstream second oxidative dehydrogenation zone anyway, such that any non-oxidizing environment and any subsequent tellurium loss can be prevented, thus ensuring stable catalyst performance of the selectively tellurium-containing catalyst and preventing the above-mentioned problems from occurring in the downstream section.

[0102] In steps b) and e) of the process according to the invention, ethylene is produced by oxidative dehydrogenation of ethane. Ethylene is initially formed. However, in the same step, ethylene can be oxidized to acetic acid. In addition, in the same step, ethylene can be dehydrogenated to acetylene (acetylene gas). Ethane can also be directly converted to acetic acid or acetylene. Further, in the same step, carbon monoxide (CO) and carbon dioxide (CO 2 ) can be produced, for example, by combustion of ethane and / or ethylene and / or acetic acid and / or acetylene.

[0103] In step a) of the process according to the invention, ethane and oxygen (O 2 ) are supplied to the first oxidative dehydrogenation zone. These components can be fed together or separately into a reactor vessel containing the first oxidative dehydrogenation zone. That is to say, one or more feed streams containing one or more of these components can be fed into the reactor vessel. For example, one feed stream containing oxygen and ethane can be fed into the reactor vessel. Alternatively, one feed stream containing oxygen and another feed stream containing ethane can be fed separately into the reactor vessel, and these feed streams can form a combined feed stream inside the reactor vessel. In step a), ethane and oxygen are suitably supplied in the gas phase.

[0104] The oxygen supplied to step a) is the oxidant, which causes the oxidative dehydrogenation of ethane. The oxygen can be sourced from any source, such as air. A suitable molar ratio of oxygen to ethane ranges from 0.01 to 1, more suitably from 0.05 to 0.5. The ratio of oxygen to ethane is the ratio before oxygen and ethane come into contact with the catalyst in the first oxidative dehydrogenation zone. In other words, the ratio of oxygen to ethane is the ratio of the fed oxygen to the fed ethane. Obviously, after contact with the catalyst, part of the oxygen and ethane are consumed. Based on the total feed in step a), the relative amount of oxygen can vary within a wide range and can be, for example, 3 vol% to 50 vol% or 8 vol% to 40 vol% or 12 vol% to 30 vol%.

[0105] In the process according to the invention, preferably between steps b) and d), more preferably in step c), additional oxygen and / or additional ethane not originating from the first oxidative dehydrogenation zone can be supplied to the second oxidative dehydrogenation zone.

[0106] In addition to ethane and oxygen, one or more diluents may be supplied to the first oxidative dehydrogenation zone in step a) of the process of the present invention. The one or more diluents may be selected from the group consisting of inert gases, nitrogen (N 2 ), steam (H 2 O), methane, and carbon dioxide (CO 2 ). Additionally or alternatively, one or more such diluents not originating from the first oxidative dehydrogenation zone may be supplied to the second oxidative dehydrogenation zone in step d) of the process of the present invention.

[0107] Preferably, in steps b) and e) of the process of the present invention, i.e., during the contact of ethane with oxygen in the presence of a catalyst, the temperature is from 300 °C to 500 °C. More preferably, the temperature is from 310 °C to 450 °C, even more preferably from 320 °C to 420 °C, and most preferably from 330 °C to 420 °C. Advantageously, the temperatures in the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone may be different. In cases where the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone are formed by a plurality of reactor tubes, the temperatures in these zones may be varied by applying different inlet coolant temperatures. Furthermore, the temperature variation between the two zones may be created by having a second oxidative dehydrogenation zone that is not formed by a plurality of reactor tubes and operates adiabatically without the use of a coolant. Thus, advantageously, in the present invention, the temperatures in the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone can be controlled independently, allowing for the separate control of oxygen conversion and by-product formation, as well as the optimization of the overall ethylene selectivity and yield. Additionally, advantageously, since in the present invention at least two oxidative dehydrogenation zones are used instead of only one zone, each of these multiple zones can be smaller than in the case of using a single oxidative dehydrogenation zone, which is beneficial from a safety perspective (higher pressure / containment design).

[0108] Furthermore, in steps b) and e) of the process of the present invention, i.e., during the contact of ethane with oxygen in the presence of a catalyst, the typical pressure is from 0.1 bara to 30 bara or from 0.1 bara to 20 bara (i.e., "absolute bar"). Additionally, preferably, the pressure is from 0.1 bara to 15 bara, more preferably from 1 bara to 10 bara, and most preferably from 3 bara to 10 bara. The pressure refers to the total pressure. The pressures in the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone may be different.

[0109] In steps b) and e) of the process according to the invention, in addition to the desired ethylene product, water is formed which ultimately remains in the product stream. In addition, as mentioned above, acetic acid, acetylene, carbon monoxide and carbon dioxide may be formed in steps b) and e). In addition, some of the ethane is not converted in steps b) and e), and not all of the oxygen may be converted in step e). That is to say, step e) gives an effluent comprising ethylene, optionally acetic acid, unconverted ethane, water, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene.

[0110] Water and optionally acetic acid can be removed from at least a portion of the above-mentioned effluent produced in step e) of the process according to the invention. This water removal step is suitably effected by condensation. The water in the effluent produced in step e) can be condensed by cooling the effluent to a lower temperature (e.g. room temperature), after which the condensed water and optionally acetic acid can be separated, giving a liquid stream comprising condensed water and optionally acetic acid. The water removal step after step e) can be carried out before or after the optional oxidation step mentioned below, preferably before. Additionally, such a water removal step can also be applied to at least a portion of the effluent produced in step b), that is to say between step b) and d) and between the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone.

[0111] In the optional oxidation step, before or after the above-mentioned water removal step, carbon monoxide and acetylene can be removed from at least a portion of the effluent produced in step e) which comprises ethylene, unconverted ethane, carbon dioxide, carbon monoxide and acetylene and may comprise unconverted oxygen, by oxidizing the carbon monoxide and acetylene to carbon dioxide with oxygen, giving an effluent comprising ethylene, unconverted ethane and carbon dioxide.

[0112] In the above-mentioned optional oxidation step, oxygen is also added, that is to say in addition to any oxygen remaining from the oxidative dehydrogenation step e). Advantageously, this oxidation step is separated from steps b) and e) of the process such that it is not critical how much unconverted oxygen (if any) remains after the oxidative dehydrogenation step, since in this optional oxidation step, exactly the additional amount of oxygen required to combust the carbon monoxide and acetylene with oxygen can be added and essentially no oxygen remains after this oxidation step.

[0113] In addition, the above-mentioned optional oxidation step can be carried out in the same manner as the deoxygenation step (c) as described in WO2018153831 (the disclosure of which is incorporated herein by reference), except that additional oxygen should be added in the optional oxidation step in this specification.

[0114] In the above optional oxidation step, the temperature can vary within a wide range and is typically from 50 °C to 500 °C, such as from 100 °C to 400 °C. Preferably, in the said oxidation step, the temperature is from 100 °C to 400 °C, more preferably from 150 °C to 300 °C, and most preferably from 200 °C to 260 °C. Further, in the said oxidation step, the typical pressure is from 0.1 bara to 30 bara or from 0.1 bara to 20 bara (i.e. "absolute bar"). In addition, preferably, the pressure is from 0.1 bara to 15 bara, more preferably from 1 bara to 8 bara, and most preferably from 2 bara to 7 bara.

[0115] Suitably, the stream produced by the above optional oxidation step does not contain oxygen or contains a residual amount of oxygen, which is at most 10,000 parts per million by volume (ppmv) or at most 1,000 ppmv or at most 500 ppmv or at most 100 ppmv or at most 50 ppmv or at most 10 ppmv or at most 2 ppmv or at most 1 ppmv, based on the total volume of the stream produced by the said oxidation step. In addition, suitably, in the said oxidation step, carbon monoxide and acetylene can be removed to such an extent that the stream produced by the said oxidation step does not contain carbon monoxide and acetylene or contains a residual amount of carbon monoxide and acetylene, which is at most 15 volume % or at most 10 volume % or at most 5 volume % or at most 1 volume % or at most 500 parts per million by volume (ppmv) or at most 100 ppmv or at most 50 ppmv or at most 10 ppmv or at most 2 ppmv or at most 1 ppmv, based on the total volume of the stream produced by the said oxidation step.

[0116] The above optional oxidation step can be carried out in the presence of an oxidation catalyst. Suitably, the said oxidation catalyst catalyzes the conversion of carbon monoxide, acetylene and oxygen to carbon dioxide by oxidizing carbon monoxide and acetylene to carbon dioxide.

[0117] Preferably, the oxidation catalyst that can be used in the above optional oxidation step contains a transition metal. More preferably, the catalyst contains one or more metals selected from the group consisting of nickel (Ni), copper (Cu), zinc (Zn), palladium (Pd), silver (Ag), platinum (Pt), gold (Au), iron (Fe), manganese (Mn), cerium (Ce), tin (Sn), ruthenium (Ru), and chromium (Cr), more preferably one or more metals selected from the group consisting of nickel, copper, zinc, silver, platinum, and ruthenium, more preferably one or more metals selected from the group consisting of nickel, copper, zinc, platinum, and ruthenium, more preferably one or more metals selected from the group consisting of nickel, copper, zinc, and silver, even more preferably one or more metals selected from the group consisting of nickel, copper, and zinc. Most preferably, the catalyst contains copper and / or platinum. Suitably, the catalyst contains copper or platinum, more suitably contains copper. For example, the catalyst may contain copper and zinc. In particular, the catalyst may be a metal oxide catalyst, and the metal oxide catalyst may be a partially reduced metal oxide catalyst in which the metal is as described above, for example, a catalyst containing copper oxide and optionally zinc oxide. The catalyst may be a supported catalyst in which one or more of the above metals are supported by a carrier, or an unsupported catalyst. In the case where the catalyst is a supported catalyst, the carrier may be any carrier, such as alumina, titanium dioxide, silica, zirconia, or silicon carbide, suitably alumina. In addition, the supported catalyst may be formed into any shape, including tablets and extrudates, or coated on a substrate.

[0118] In addition, the above oxidation catalyst that can be used in the above optional oxidation step may contain one or more metals selected from the group consisting of palladium, silver, platinum, gold, copper, and ruthenium, or one or more metals selected from the group consisting of palladium, silver, platinum, and gold, or platinum.

[0119] In addition, in the carbon dioxide removal step after the above water removal step and the optional oxidation step, carbon dioxide can be removed, for example, from a stream containing ethylene, unreacted ethane, and carbon dioxide and which may contain carbon monoxide and acetylene without performing the oxidation step, thereby producing an effluent containing ethylene, unreacted ethane, optionally carbon monoxide, and optionally acetylene.

[0120] In the above carbon dioxide removal step, carbon dioxide can be removed by any well-known method. Suitable carbon dioxide removal agents that can be fed to the carbon dioxide removal step can be aqueous solutions of bases such as sodium hydroxide and / or amines. After removing carbon dioxide in this way, the stream from which carbon dioxide has been removed can be dried to remove any residual water from the stream before feeding it to the next step. In cases where the amount of carbon dioxide is relatively high, such as in the case of an alkane ODH effluent, it is preferred to contact an aqueous solution of an amine with the stream containing carbon dioxide. In cases where the amount of carbon dioxide is relatively low, such as in the case of an alkane ODH effluent that has been treated with an aqueous solution of an amine and still contains some residual carbon dioxide, it is preferred to contact an aqueous solution of sodium hydroxide with the stream containing carbon dioxide.

[0121] At least a portion of the effluent from the carbon dioxide removal step can be further separated.

[0122] In a first case where the effluent produced by the carbon dioxide removal step contains ethylene and unreacted ethane, the latter stream can be separated into a stream containing ethylene and a stream containing unreacted ethane. The unreacted ethane from the latter stream can be recycled to step a) of the process of the present invention.

[0123] In a second case where the effluent produced by the carbon dioxide removal step contains ethylene, unreacted ethane, carbon monoxide, and acetylene, at least a portion of the effluent is subjected to an acetylene hydrogenation step in which acetylene is hydrogenated to ethylene using hydrogen. This second case can be applicable to cases where the above optional oxidation step is not carried out. Advantageously, in this step, the undesired acetylene is removed by selectively hydrogenating acetylene to the desired ethylene. In the latter case, the above optional oxidation step is not required, in which acetylene would be lost by combustion to carbon dioxide.

[0124] In one embodiment of the second case, at least a portion of the effluent produced by the carbon dioxide removal step can first be separated into a stream containing carbon monoxide and a stream containing ethylene, unreacted ethane, and acetylene. Then at least a portion of the latter stream can be subjected to an acetylene hydrogenation step, thereby producing a stream containing unreacted ethane and ethylene, and at least a portion of the latter stream can be further separated into a stream containing unreacted ethane and a stream containing ethylene. The unreacted ethane from the latter stream can be recycled to step a) of the process of the present invention.

[0125] The above separation can be carried out in any known manner, such as by distillation, absorption, or adsorption, preferably by distillation. In addition, the above acetylene hydrogenation step is carried out using hydrogen as the hydrogenating agent and can be carried out in a catalytic reactor, which is a reactor containing a catalyst. The catalyst can be any known acetylene hydrogenation catalyst.

[0126] The present invention is further illustrated by the following examples.

[0127] Examples

[0128] A. Significant changes in unreacted oxygen

[0129] Modeling experiments were conducted. The model under discussion is a non-isothermal model of a single ethane oxidative dehydrogenation (ODH) reactor tube. In Table 1, the relevant parameters and results are shown.

[0130] Table 1 shows data for 4 sets of 3 different ethane ODH cases in the model, where each case is operated under slightly different reaction conditions, representing the effect of minor tube-to-tube statistical or non-statistical variations in pressure drop and / or catalyst packing density that would occur in an industrial-scale multitubular reactor, where the reactor tubes are arranged in parallel and together form an ethane ODH zone. These 4 sets of cases are: 1) cases 1a, 1b, 1c; 2) cases 1d, 1e, 1f; 3) cases 2a, 2b, 2c; and 4) cases 2d, 2e, 2f. For each of these sets of 3 cases, there is a base case (a or d) and 2 other cases (b and c or e and f) operated under slightly different conditions, where the base cases 1a and 1d are the same. In all the said cases, the pressure is 4.5 bara, and a coolant with a certain inlet temperature is used for external cooling of these tubes.

[0131] For example, in the base cases 1a and 1d, the reaction conditions were selected such that the oxygen conversion was relatively high, i.e., 99.92%. Such an oxygen conversion corresponds to an outlet oxygen concentration of 99 ppmv. The composition of the inlet feed stream is shown in Table 1 in terms of oxygen (O 2 ) as the oxidant, ethane (C 2 H 6 ) as the reactant, and nitrogen (N 2 ) and carbon dioxide (CO 2 ) as diluents. In addition, Table 1 shows the flow rate and gas hourly space velocity (GHSV) for each case. The inlet coolant temperature for all cases 1a to 1f and 2a to 2c is 348.0 °C.

[0132]

[0133] As mentioned above, the other two cases are operated under slightly different conditions, which are different from those of the base case. For cases 1a, 1b, and 1c, the flow rates in the three cases are different. For cases 1d, 1e, and 1f, the catalyst dosages and flow rates in the three cases are different. This represents variations in the flow rate per tube and the catalyst dosage per tube in a multitubular reactor that can contain from 1,000 to 50,000 tubes. In practice, the statistical variations can typically be a few percentage points of the average value, and the non-statistical variations can even be greater, for example, if the catalyst loading is not carried out very carefully. In this model, for cases 1b and 1c, the flow rate in the tubes takes a 5% variation (see the "Relative Flow Rate" column), and for cases 1e and 1f, the flow rate and catalyst dosage in the tubes take a 1% variation (see the "Relative Flow Rate" and "Relative Catalyst Mass" columns). Different catalyst packings will result in an increase or decrease in the catalyst dosage. Tubes with a catalyst dosage, for example, 1% higher will also have a slightly higher pressure drop and thus a lower flow rate (e.g., 1%), as also shown in Table 1 for cases 1e and 1f.

[0134] As shown in Table 1, the case where the operating flow rate is 5% lower (Case 1b), thus having a lower GHSV and a longer contact time, results in an increase in the O 2 conversion rate and the O 2 in the outlet stream of this single tube is almost 0 ppmw. On the other hand, the case where the flow rate is 5% higher (Case 1c) results in a decrease in the O 2 conversion rate, i.e., the conversion rate is 98.60% instead of the target 99.92% (Case 1a).

[0135] Case 1abc in Table 1 represents a multitubular reactor in which one-third of the tubes have an average flow rate (Case 1a), one-third of the tubes have a flow rate 5% lower (Case 1b), and one-third of the tubes have a flow rate 5% higher (Case 1c), and the multiple outlet streams from all these tubes are mixed to produce a mixture containing ethylene, unreacted ethane, and unreacted oxygen. This model predicts that the O 2 concentration in the outlet of this mixture increases significantly from 99 ppmv to 672 ppmv, i.e., a 579% increase compared to a multitubular reactor in which all tubes have the same flow rate as in the base case (Case 1a).

[0136] In the base case 2a, also shown in Table 1, the reaction conditions were chosen (i.e., only the oxygen inlet concentration was increased from 14 vol% to 19 vol%) such that the oxygen conversion was lower than in base case 1a, i.e., 90.30% for case 2a as opposed to 99.92% for base case 1a. Additionally, the same + / - 5% variation with respect to the flow rate was applied. Similarly, cases 2abc in Table 1 represent a multitubular reactor in which 1 / 3 of the tubes have an average flow rate (case 2a), 1 / 3 of the tubes have a flow rate that is 5% lower (case 2b), and 1 / 3 of the tubes have a flow rate that is 5% higher (case 2c), where multiple effluent streams from all these tubes are mixed, resulting in a mixture containing ethylene, unreacted ethane, and unreacted oxygen. The model predicts that the outlet O 2 concentration increases from 16,419 ppmv to 16,499 ppmv, i.e., an increase of only 0.5% compared to a multitubular reactor in which all tubes have the same flow rate as in the base case (case 2a), this increase being significantly less than the 579% increase in case 1abc described above.

[0137] When the results of cases 1a to 1c are compared with the results of cases 2a to 2c in the model, it is surprisingly found that at relatively high oxygen conversions, a smaller flow rate non-uniformity results in a significant and adverse change in unreacted oxygen in the effluent streams from multiple reactor tubes, which tubes form the oxidative dehydrogenation zone within the reactor vessel. Advantageously, in the present invention, this adverse effect of the non-uniformity is removed or significantly reduced by: (i) using at least two oxidative dehydrogenation zones arranged in series, where the first oxidative dehydrogenation zone is formed by multiple reactor tubes as described above, and (ii) mixing at least a portion of the multiple effluent streams from the first zone before supplying them to the second zone for further conversion of unreacted ethane and unreacted oxygen.

[0138] In cases 1a to 1c, the change in unreacted oxygen in the effluent streams from multiple reactor tubes that together form a multitubular reactor was caused only by the + / - 5% variation with respect to the flow rate. In fact, the amount of catalyst can also vary, since during the loading of thousands or tens of thousands of tubes, due to the random nature of the packing, some tubes will have slightly more catalyst while other tubes will have slightly less catalyst. Tubes with a slightly higher packing density will have a slightly higher pressure drop and thus will have a slightly lower flow rate. Both the higher packing density and the lower flow rate will reduce the GHSV, resulting in a higher oxygen conversion in such tubes with a relatively larger amount of catalyst.

[0139] Cases 1d to 1f are the same as cases 1a to 1c, except that in cases 1d to 1f, the catalyst amount varies by + / -1%, and following the above principle, in combination with a + / -1% change relative to the flow rate, rather than the + / -5% change relative to the flow rate in cases 1a to 1c. It also seems that in the case where the change is only + / -1% relative to the amount of catalyst and only + / -1% relative to the flow rate, as in cases 1d to 1f, the outlet O of the mixture containing ethylene, unreacted ethane, and unreacted oxygen produced by mixing multiple effluent streams from all tubes of a multitubular reactor 2 concentration still increases significantly by 160%, from (i) the case where all tubes from such a multitubular reactor have the same flow rate of the base case (case 1d) (where the outlet O 2 concentration is only 99 ppmv) to (ii) the case where 1 / 3 of the tubes have the average flow rate (case 1d), 1 / 3 of the tubes have a flow rate 1% lower (case 1e), and 1 / 3 of the tubes have a flow rate 1% higher (case 1f) (where the outlet O 2 concentration is 258 ppmv (case 1def)).

[0140] In addition to increasing the oxygen inlet concentration as confirmed above with reference to base case 2a, there may be other means targeted at reducing the oxygen conversion rate, such as reducing the inlet coolant temperature. In base case 2d also shown in Table 1, the reaction conditions are selected (i.e., only the inlet coolant temperature is reduced from 348.0 °C to 344.5 °C) such that the oxygen conversion rate is lower than that of base case 1d, i.e., case 2d is 93.60%, rather than 99.92% of base case 1d. In addition, the same + / -1% change relative to the catalyst amount and the flow rate is applied. Similarly, case 2def in Table 1 represents a multitubular reactor where 1 / 3 of the tubes have the average flow rate (case 2d), 1 / 3 of the tubes have a flow rate 1% lower (case 2e), and 1 / 3 of the tubes have a flow rate 1% higher (case 2f), and the multiple effluent streams from all these tubes are mixed, thereby producing a mixture containing ethylene, unreacted ethane, and unreacted oxygen. The model predicts that the outlet O 2 concentration of this mixture increases from 8,236 ppmv to 8,302 ppmv, i.e., only an increase of 0.8% compared to a multitubular reactor where all tubes have the same flow rate of the base case (case 2d), and this increase is significantly less than the 160% increase in case 1def above. Therefore, when comparing the results of cases 1d to 1f with the results of cases 2d to 2f (with a + / -1% change in the catalyst amount and the flow rate), the above surprising finding obtained by comparing cases 1a to 1c with cases 2a to 2c (with a + / -5% change in the flow rate) is confirmed.

[0141] The (total) outlet O of a multitubular reactor operating at a relatively high oxygen conversion rate 2 This increase in concentration (as confirmed above for cases 1abc and 1def) is disadvantageous because the relatively high oxygen concentration generated in the backend separation section increases the risk of explosion hazards, as further discussed in the "Background Art" section of this specification. In addition, it appears that there is no oxygen or relatively low outlet O 2 concentration in some (individual) tubes (see cases 1b and 1e in Table 1). It has been found (see section B below) that such a non-oxidizing environment that may exist (especially near the outlet of some reactor tubes containing catalyst beds) may disadvantageously cause deactivation of the mixed metal oxide ethane ODH catalyst that may be present in such reactor tubes.

[0142] B. Loss of tellurium in the absence of oxygen

[0143] B.1 Preparation of the catalyst

[0144] Prepare a mixed metal oxide catalyst containing molybdenum (Mo), vanadium (V), niobium (Nb), and tellurium (Te), for which the molar ratio of the four metals is Mo 1 V 0.29 Nb 0.17 Te 0.12 .

[0145] Prepare two solutions. Solution 1 is obtained by dissolving 15.8 g of ammonium niobium oxalate and 4.0 g of oxalic acid dihydrate in 160 ml of water at room temperature. Solution 2 is prepared by dissolving 35.6 g of ammonium heptamolybdate, 6.9 g of ammonium metavanadate, and 5.8 g of telluric acid (Te(OH) 6 ) in 200 ml of water at 70 °C. Then 7.0 concentrated nitric acid is added to Solution 2. The two solutions are combined to produce an orange gel-like precipitate. The mixture is evaporated to dryness at 50 °C by means of a rotary evaporator ("rotavap").

[0146] The dried material is further dried in static air at 120 °C for 16 hours, ground into a fine powder, and then calcined in static air at a temperature of 325 °C for 2 hours. After air calcination, the material is further calcined in a nitrogen (N 2 ) stream at 600 °C for 2 hours. Then the material is treated with a 5% aqueous oxalic acid solution at 80 °C, filtered, and dried at 120 °C.

[0147] The dried catalyst powder is pressed into pellets, and then these pellets are ground. Then the ground material is sieved using a sieve with a mesh size of 40 to 80 mesh. Then the sieved material with a size of 40 to 80 mesh and consisting of porous catalyst particles is used in the following ethane oxidative dehydrogenation experiment.

[0148] B.2 Catalytic oxidative dehydrogenation of ethane

[0149] The catalyst thus prepared was used in experiments involving oxidative dehydrogenation of ethane (ethane ODH) in a small-scale test unit, which was a vertically oriented cylindrical quartz reactor with an inner diameter of 3.0 mm. 0.65 g of the catalyst was loaded into the reactor. The height of the catalyst bed was 6 cm. At the top of the catalyst bed, another bed with a height of 8 cm was placed, and the latter bed contained inert silicon carbide (SiC) particles with an average diameter of 0.8 mm.

[0150] In these experiments, a gas stream containing 63 vol% of ethane, 21 vol% of oxygen (O 2 ) and 16 vol% of nitrogen (N 2 ) was fed to the top of the reactor and then transported downward through the catalyst bed to the bottom of the reactor. The gas stream was a combined gas stream containing an ethane stream with a rate of 3.00 Nl / hr, an oxygen stream with a rate of 1.00 Nl / hr, and a nitrogen stream with a rate of 0.77 Nl / hr. "Nl" represents "normal liter" measured at standard temperature and pressure, i.e., 0 °C and 1 bara (100 kPa). The pressure in the reactor was 2.3 bara. The reactor was heated so that the temperature of the catalyst (at the end of the catalyst bed) was 370 °C.

[0151] The conversion of ethane and the product composition were measured using a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and another GC equipped with a flame ionization detector. The acetic acid by-product and water from the reaction were collected in a quench tank.

[0152] The above conditions (hereinafter referred to as "reference conditions") were maintained for 100 hours (phase A). Under these conditions, the oxygen conversion was incomplete, and based on the total volume of the gas stream, the gas stream exiting the reactor outlet contained 3.85 vol% of (unconverted) oxygen (i.e., an oxidative environment).

[0153] Then the oxygen flow rate was reduced from 1.00 Nl / hr to 0.60 Nl / hr. In addition, the nitrogen flow rate was increased from 0.77 Nl / hr to 1.17 Nl / hr so that the total flow rate remained unchanged. Under these conditions, the oxygen conversion was complete, and the gas stream exiting the reactor outlet did not contain oxygen (i.e., a non-oxidative environment).

[0154] The subsequent conditions were maintained for 60 hours (period B), and then the above reference conditions were restored and maintained for 25 hours (period C).

[0155] In Table 2 below, the experimental results (conversion of ethane and selectivity to ethylene) for the above periods A and C are shown.

[0156] Table 2

[0157] Time period Conversion rate of ethane (%) Selectivity to ethylene (%) A 41.3 92.9 C 37.1 90.9

[0158] It seems that after the period (period B above) when the gas stream exiting the reactor outlet does not contain oxygen (in which case a non-oxidizing environment is formed in the reactor, especially near the reactor outlet where oxygen is completely consumed and thus absent), compared with the period before the oxygen flow rate decreases (period A above), the conversion and selectivity significantly decrease in the subsequent period (period C above) when the gas stream exiting the reactor outlet again contains oxygen due to increasing the oxygen flow rate to its original level.

[0159] Furthermore, it is found that in the above-mentioned period B (where a non-oxidizing environment exists), tellurium is lost from the catalyst in the reactor, and the lost tellurium ultimately remains in the reactor outlet stream.

[0160] Based on the above, it can be concluded that the above-mentioned decrease in activity and selectivity (i.e., catalyst deactivation) is caused by the loss of tellurium from the catalyst in a non-oxidizing environment. In addition to the said catalyst deactivation, this loss of tellurium may cause serious operational problems in the section downstream of the catalyst bed, as well as possible health, safety, and environmental problems.

[0161] Therefore, in the present invention, it is preferred that the mixed metal oxide catalyst in the second oxidative dehydrogenation zone does not contain tellurium.

Claims

1. A method for producing ethylene by oxidative dehydrogenation of ethane, the method comprising: a) Supplying ethane and oxygen to a first oxidative dehydrogenation zone formed by a plurality of reactor tubes containing a catalyst bed, the catalyst bed containing an oxidative dehydrogenation catalyst, the oxidative dehydrogenation catalyst being a mixed metal oxide catalyst; b) Contacting the ethane and oxygen with the catalyst in the plurality of reactor tubes in the first oxidative dehydrogenation zone to produce a plurality of effluent streams, wherein the plurality of reactor tubes are cooled by a coolant; c) Mixing at least a portion of the plurality of effluent streams from step b) to produce a mixture comprising ethylene, unreacted ethane, and unreacted oxygen, wherein the oxygen concentration of at least 90 vol% of the mixture is equal to or deviates from the volume average oxygen concentration in the total mixture by at most 1%; d) Supplying at least a portion of the mixture from step c) to a second oxidative dehydrogenation zone containing a catalyst bed, the catalyst bed containing an oxidative dehydrogenation catalyst, the oxidative dehydrogenation catalyst being a mixed metal oxide catalyst; e) Contacting at least a portion of the mixture from step c) with the catalyst in the second oxidative dehydrogenation zone to produce a stream comprising ethylene and unreacted ethane, wherein the mixture produced by step e) contains at most 500 ppmv of unreacted oxygen.

2. The method according to claim 1, wherein the first oxidative dehydrogenation zone and the second oxidative dehydrogenation zone are contained within the same reactor vessel or within separate reactor vessels arranged in series.

3. The method according to claim 1 or 2, wherein in step b), 80% to 99% of the oxygen supplied to step a) is converted.

4. The method according to claim 1 or 2, wherein the mixture produced by step c) contains from 1,000 parts per million by volume (ppmv) to 10 vol% of unreacted oxygen.

5. The method according to claim 1 or 2, wherein in the combination of steps b) and e), more than 99% to 100% of the oxygen supplied to step a) and any additional oxygen supplied to step d) is converted.

6. The method according to claim 1 or 2, wherein the mixture produced by step e) does not contain unreacted oxygen.

7. The method according to claim 1 or 2, wherein the mixture produced by step e) contains at most 200 ppmv of unreacted oxygen.

8. The method according to claim 1 or 2, wherein the catalyst in the first oxidative dehydrogenation zone is a mixed metal oxide catalyst containing tellurium.

9. The method according to claim 8, wherein the catalyst in the first oxidative dehydrogenation zone contains molybdenum, vanadium, niobium, and tellurium.

10. The method according to claim 1 or 2, wherein the catalyst in the second oxidative dehydrogenation zone is a mixed metal oxide catalyst without tellurium.

11. The method according to claim 10, wherein the catalyst in the second oxidative dehydrogenation zone contains molybdenum, vanadium, and niobium.

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