OA19409A - Ethane oxidative dehydrogenation. - Google Patents

Ethane oxidative dehydrogenation. Download PDF

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Publication number
OA19409A
OA19409A OA1202000077 OA19409A OA 19409 A OA19409 A OA 19409A OA 1202000077 OA1202000077 OA 1202000077 OA 19409 A OA19409 A OA 19409A
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OAPI
Prior art keywords
stream
carbon dioxide
ethane
resulting
ethylene
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OA1202000077
Inventor
Rossum Guus Van
Laura Mariel Calvo
Cassibba Ivana Daniela Esposito
Georgios Mitkidis
Roman Macia Maria San
Peter Anton August Klusener
Ronald Jan Schoonebeek
Pejman Pajand
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Shell Internationale Research Maatschappij B.V.
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Publication of OA19409A publication Critical patent/OA19409A/en

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Abstract

The invention relates to a process for oxidative dehydrogenation of ethane, comprising the steps of: (a) subjecting a stream comprising ethane to oxidative dehydrogenation conditions; (b) removing water from at least part of the effluent resulting from step (a); (c) optionally removing unconverted oxygen and/or carbon monoxide and/or ycetylene from at least part of the stream comprising ehtylene, unconverted ethane, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene resulting from step (b); (d) removing ethylene from at least part of the stream comprising ehtylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) by a complexation separation method; (e) partially and selectively removing carbon dioxide from at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d); (f) recycling at least part of the stream comprising unconverted ethane and carbon dioxided resulting from step (e) to step (a).

Description

Surprisingly it was found that the above-mentioned object may be obtained by separating unconverted ethane and carbon dioxide diluent together, and at the same time recovering ethylene product, by means of a step which involves complexation séparation and which comprises contacting at least part of a stream comprising said ethane, carbon dioxide and ethylene with a liquid solvent comprising a complexation agent, which step results in a stream comprising ethylene and a stream comprising unconverted ethane and carbon dioxide, from which latter stream subsequently carbon dioxide is partially and selectively removed, and recycling the resulting stream having a reduced carbon dioxide content to the ethane ODH step.
Accordingly, the présent invention relates to a process for oxidative dehydrogenation of ethane, comprising the steps of :
(a) subjecting a stream comprising ethane to oxidative dehydrogenation conditions, comprising contacting the ethane with oxygen in the presence of a catalyst comprising a mixed meta! oxide, wherein a diluent comprising carbon dioxide is fed to step (a), resulting in an effluent comprising ethylene, optionally acetic acid, unconverted ethane, water,
- 3 carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene;
(b) removing water from at least part of the effluent resulting from step (a), resulting in a stream comprising ethylene, unconverted ethane, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene and a stream comprising water and optionally acetic acid;
(c) optionally removing unconverted oxygen and/or carbon monoxide and/or acetylene from at least part of the stream comprising ethylene, unconverted ethane, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene resulting from step (b), resulting in a stream comprising ethylene, unconverted ethane and carbon dioxide;
(d) removing ethylene from at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) by a complexation séparation method, which comprises contacting at least part of said stream with a liquid solvent comprising a complexation agent, resulting in a stream comprising ethylene and a stream comprising unconverted ethane and carbon dioxide;
(e) partially and selectively removing carbon dioxide from at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d), resulting in a stream comprising unconverted ethane and carbon dioxide and having a reduced carbon dioxide content;
(f) recycling at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (e) to step (a).
Brief description of the drawings
Figure 1 depicts an embodiment covering steps (a) to (f) of the process of the présent invention.
- 4 Figure 2 depicts an embodiment in relation to step (d) of the process of the présent invention.
Detailed description of the invention
The process of the présent invention comprises steps (a) to (f), wherein step (c) is an optional step. These steps and optional further steps are described in further detail hereinbelow.
Thus, the process of the présent invention comprises steps (a) and (b), optional step (c) and steps (d), (e) and (f). Said process may comprise one or more intermediate steps between steps (a) and (b), between steps (b) and (c) , between steps (c) and (d), between steps (d) and (e), and between steps (e) and (f). Further, said process may comprise one or more additional steps preceding step (a) and/or followmg step (f).
While the process of the présent invention and a composition or stream used in said process are described in terms of comprising, containing or including one or more varions described steps and components, respectively, they can also consist essentially of or consist of said one or more various described steps and components, respectively.
In the context of the présent invention, in a case where a composition or stream comprises two or more components, these components are to be selected in an overall amount not to exceed 100 vol.% or 100 wt.%.
Within the présent spécification, substantially no means that no detectible amount of the component in question is présent in the composition or stream.
Further, within the présent spécification, by fresh ethane reference is made to ethane which does not comprise unconverted ethane. Within the présent spécification, by unconverted ethane reference is made to ethane that was
- 5 subjected to oxidative dehydrogenation conditions in step (a) of the process of the présent invention, but which was not converted.
Step (a)
Step (a) of the présent process comprises subjecting a stream comprising ethane to oxidative dehydrogenation (ODH) conditions, comprising contacting the ethane with oxygen (O2) in the presence of a catalyst comprising a mixed métal oxide, wherein a diluent comprising carbon dioxide is fed to step (a), resulting in an effluent comprising ethylene, optionally acetic acid, unconverted ethane, water, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene.
In ethane ODH step (a), ethylene is produced by oxidative dehydrogenation of ethane. Ethylene is initially formed. However, in said same step, ethylene may be oxidized into acetic acid. Further, in said same step, ethylene may be dehydrogenated into acetylene (ethyne). Ethane may also be directly converted into acetic acid or acetylene. Still further, in said same step, carbon monoxide (CO) and carbon dioxide (CO2) may be produced, for example by combustion of ethane and/or ethylene and/or acetic acid and/or acetylene.
In ethane ODH step (a), ethane, oxygen (O2) and carbon dioxide (CO2) may be fed to a reactor. Said components may be fed to the reactor together or separately. That is to say, one or more feed streams, suitably gas streams, comprising one or more of said components may be fed to the reactor. For example, one feed stream comprising oxygen, ethane and carbon dioxide may be fed to the reactor. Alternatively, two or more feed streams, suitably gas streams, may be fed to the reactor, which feed streams may form a combined stream inside the reactor. For example, one feed stream comprising oxygen, another feed stream comprising ethane and still another feed
- 6 stream comprising carbon dioxide may be fed to the reactor separately. In ethane ODH step (a), ethane, oxygen and carbon dioxide are suitably fed to a reactor in the gas phase.
Preferably, in ethane ODH step (a), that is to say during contacting ethane with oxygen in the presence of a catalyst, the température is of from 300 to 500 °C. More preferably, said température is of from 310 to 450 °C, more preferably of from 320 to 420 °C, most preferably of from 330 to 420 °C.
Still further, in ethane ODH step (a), that is to say during contacting ethane with oxygen in the presence of a catalyst, typical pressures are 0.1-30 or 0.1-20 bara (i.e. bar absolute). Further, preferably, said pressure is of from 0.1 to 15 bara, more preferably of from 1 to 10 bara, most preferably of from 3 to 10 bara. Said pressure refers to total pressure.
In addition to oxygen and ethane, carbon dioxide is also fed to ethane ODH step (a), as a diluent. One or more additional diluents, selected from the group consisting of the noble gases, nitrogen (N2) , steam (H2O) and methane, suitably nitrogen and methane, may be fed to ethane ODH step (a). However, since in the présent process carbon dioxide is already fed as a diluent to ethane ODH step (a), there is no need to add any additional diluent. Therefore, suitably, no additional diluent, in particular no steam, is fed to ethane ODH step (a). Some methane may be fed to step (a) as an impurity in the ethane feed to step (a). Further, some nitrogen may be fed to step (a) as an impurity in the oxygen feed to step (a). In these cases, methane and nitrogen function as additional diluent, in addition to carbon dioxide.
Generally, the proportion of the overall feed stream to step (a) which is attributable to a diluent is in the range from 5 to 90 vol.%, preferably from 25 to 75 vol.%.
- 7 Preferably, in the case of an isothermally operated reactor, the proportion of the overall feed stream to step (a) which is attributable to a diluent is in the range from. 5 to 90 vol.%, preferably from 25 to 75 vol.% and more preferably from 40 to 60 vol.%. Further, preferably, in the case of an adiabatically operated reactor, the proportion of the overall feed stream to step (a) which is attributable to a diluent is in the range from 50 to 95 vol.%, preferably from 60 to 90 vol.% and more preferably from 70 to 85 vol.%.
Preferably, the diluent as fed to step (a) comprises from 1 to 100 vol.%, more preferably 5 to 100 vol.%, more preferably 10 to 100 vol.%, more preferably 20 to 100 vol.%, more preferably 40 to 100 vol.%, more preferably 60 to 100 vol.%, more preferably 80 to 100 vol.%, more preferably 90 to 100 vol.%, more preferably 95 to 100 vol.%, and most preferably 99 to 100 vol.% of carbon dioxide, the balance consisting of one or more other diluents, selected from the group consisting of the noble gases, nitrogen (N2) , steam (H2O) and methane, suitably nitrogen and methane. Diluents other than carbon dioxide may be used in any desired ratio relative to each other. When one or more of said additional diluents other than carbon dioxide are fed to step (a), the upper limit for the proportion of carbon dioxide in the diluent may be 20 vol.%, preferably 40 vol.%, more preferably 60 vol.%, more preferably 80 vol.%, more preferably 90 vol.%, more preferably 95 vol.%, and most preferably 99 vol.%.
The oxygen as fed to ethane ODH step (a) is an oxidizing agent, thereby resulting in oxidative dehydrogenation of ethane. Said oxygen may originate from any source, such as for example air. Suitable ranges for the molar ratio of oxygen to ethane cover ratios below, at and above the stoichiometric molar ratio (which is 0.5 for the ethane ODH reaction), suitably of from 0.01 to 1.1, more suitably of
- 8 from 0.01 to 1, more suitably of from 0.05 to 0.8, most suitably of from 0.05 to 0.7. In one embodiment, the molar ratio of oxygen to ethane is of from 0.05 to 0.5, more suitably of from 0.05 to 0.47, most suitably of from 0.1 to 0.45. Further, in another embodiment, the molar ratio of oxygen to ethane is of from 0.5 to 1.1, more suitably of from 0.53 to 1, most suitably of from 0.55 to 0.9. Said ratio of oxygen to ethane is the ratio before oxygen and ethane are contacted with the catalyst. In other words, said ratio of oxygen to ethane is the ratio of oxygen as fed to ethane as fed. Obviously, after contact with the catalyst, at least part of the oxygen and ethane gets consumed. Further, said ethane in said molar ratio of oxygen to ethane comprises both fresh ethane and recycled (unconverted) ethane.
Preferably, pure or substantially pure oxygen (O2) is used as oxidizing agent in step (a) of the process of the présent invention. Within the présent spécification, by 'pure or substantially pure oxygen reference is made to oxygen that may contain a relatively small amount of one or more contaminants, including for example nitrogen (N2) , which latter amount may be at most 1 vol.%, suitably at most 7,000 parts per million by volume (ppmv), more suitably at most 5,000 ppmv, more suitably at most 3,000 ppmv, more suitably at most 1,000 ppmv, more suitably at most 500 ppmv, more suitably at most 300 ppmv, more suitably at most 200 ppmv, more suitably at most 100 ppmv, more suitably at most 50 ppmv, more suitably at most 30 ppmv, most suitably at most 10 ppmv.
Alternatively, however, it is also possible to use air or oxygen-enriched air as oxidizing agent in step (a). Such air or oxygen-enriched air would still comprise nitrogen (N2) , in an amount exceeding 1 vol.% up to 78 vol.% (air), suitably of from 1 to 50% vol.%, more suitably 1 to 30 vol.%, more
- 9 suitably 1 to 20 vol.%, more suitably 1 to 10 vol.-s, most suitably 1 to 5 vol.%. Said nitrogen would function as additional diluent, in addition to carbon dioxide, and would end up in the stream comprising unconverted ethane and carbon dioxide resulting from complexation séparation step (d) of the présent process, part of which stream is recycled to ethane ODH step (a) of the présent process after partial and sélective removal of carbon dioxide from that stream in step (e) .
In order to prevent a build-up of nitrogen in the présent process, nitrogen may be removed before recycling in step (f), for example by means of cryogénie distillation which is cumbersome. Further, said build-up may be prevented by purging part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d) or step (e) before the recycle, as further described below. However, by purging a part of said stream, a part of unconverted ethane is lost and not recycled to step (a). Therefore, because carbon dioxide is used as a diluent which is recycled in the présent process, the above-described pure or substantially pure oxygen is preferably used as oxidizing agent in step (a) of the process of the présent invention. However, in case the oxygen feed to step (a) still comprises a relatively small amount of nitrogen, such small amount of nitrogen may still be purged, before the recycle in step (f), together with additional carbon dioxide resulting from carbon dioxide production in step (a) and possibly in optional step (c).
In step (a), the ethane ODH catalyst is a catalyst comprising a mixed métal oxide. Preferably, the ODH catalyst is a heterogeneous catalyst. Further, preferably, the ODH catalyst is a mixed métal oxide catalyst containing molybdenum, vanadium, niobium and optionally tellurium as the metals, which catalyst may hâve the following formula:
- 10 MOlV a T ©bNbcOn wherein:
a, b, c and n represent the ratio of the molar amount of êlemsnt in question to the molar amount of molybdenum (Mo) ;
a (for V) is from 0.01 to 1, preferably 0.05 to 0.60, more preferably 0.10 to 0.40, more preferably 0.20 to 0.35, most preferably 0.25 to 0.30;
b (for Te) is 0 or from >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, more preferably 0.05 to 0.20, most preferably 0.09 to 0.15;
c (for Nb) is from >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, more preferably 0.10 to 0.25, most preferably 0.14 to 0.20; and n (for 0) is a number which is determined by the valency and frequency of éléments other than oxygen.
The amount of the catalyst in ethane ODH step (a) is not essential. Preferably, a catalytically effective amount of the catalyst is used, that is to say an amount sufficient to promote the ethane oxydehydrogenation reaction.
The ODH reactor that may be used in ethane ODH step (a) may be any reactor, including fixed-bed and fluidized-bed reactors. Suitably, the reactor is a fixed-bed reactor.
Examples of oxydehydrogenation processes, including catalysts and process conditions, are for example disclosed in above-mentioned US7091377, WO2003064035, US20040147393, WO2010096909 and US20100256432, the disclosures of which are herein incorporated by reference.
In ethane ODH step (a), water is formed which ends up in the product stream in addition to the desired ethylene product. Further, as mentioned above, acetic acid, acetylene, carbon monoxide and carbon dioxide may be formed in step (a). Further, carbon dioxide is fed as a diluent to step (a).
- 11 Still further, some of the ethane is not converted in step (a) and it may be that not ail of the oxygen is converted in step (a). That is to say, ethane ODH step (a) results in an effluent comprising ethylene, optionally acetic acid, unconverted ethane, water, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene.
Step (b)
Step (b) of the présent process comprises removing water from at least part of the effluent resulting from step (a), resulting in a stream comprising ethylene, unconverted ethane, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene and a stream comprising water and optionally acetic acid.
In water removal step (b), water is suitably removed by condensation. Water in the effluent resulting from step (a) may be condensed by cooling down the latter effluent to a lower température, for example room température, after which the condensed water can be separated, resulting in a liquid stream comprising condensed water.
In water removal step (b), the température may be of from 10 to 150 °C, for example of from 20 to 80 °C. Suitably, in said step (b), the température is at least 10 °C or at least 20 °C or at least 30 °C. Further suitably, in said step (b), the température is at most 150 °C or at most 120 °C or at most 100 °C or at most 80 °C or at most 60 °C.
Still further, in water removal step (b) , typical pressures are 0.1-30 or 0.1-20 bara (i.e. bar absolute). Further, preferably, said pressure is of from 0.1 to 15 bara, more preferably of from 1 to 10 bara, most preferably of from 3 to 10 bara. Said pressure refers to total pressure.
In a case wherein the stream as fed to water removal step (b) additionally comprises acetic acid, said acetic acid may
- 12 be removed in water removal step (b) together with the water from said stream, suitably together with the water as condensed from said stream. During or after water removal step (b), additional water may be added to facilitate the removal of any acetic acid.
Thus, water removal step (b) results in a stream comprising ethylene, unconverted ethane, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene and a stream comprising water and optionally acetic acid. The latter stream may be a liquid stream comprising condensed water and optionally acetic acid.
Optional step (c)
Optional step (c) of the présent process comprises optionally removing unconverted oxygen and/or carbon monoxide and/or acetylene from at least part of the stream comprising ethylene, unconverted ethane, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene resulting from step (b), resulting in a stream comprising ethylene, unconverted ethane and carbon dioxide.
In case the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) additionally comprises unconverted oxygen and/or carbon monoxide and/or acetylene, these additional components may be removed in optional step (c) before complexation séparation step (d). Alternatively, these additional components may be removed during and/or after complexation séparation step (d) , as further decribed below. However, it is preferred to remove these additional components before complexation séparation step (d), to prevent any difficulties in removing these during and/or after complexation séparation. For example, acetylene may form a strong bond with the complexation agent in step (d). Thus, by removal of any acetylene in optional step (c), potential problems associated with the presence of
- 13 acetylene in step (d) may advantageously be prevented. Likewise, in addition to the desired ethylene product, also carbon monoxide may complex to the complexation agent in step (d). Carbon monoxide complexes strongly to Cu(I) that may be présent in the complexation agent used in step (d). Finally, oxygen may oxidize the métal, for example Cu(I), from a métal sait or métal complex that may be used as complexation agent in step (d). Therefore, the removal of any unconverted oxygen and/or carbon monoxide before complexation séparation step (d) is also preferred.
In optional step (c) of the présent process, any acetylene may be removed in any known way. For example, acetylene may be removed by sélective hydrogénation or by an absorption process that uses acetone or dimethylformamide. Hydrogen (H2) is a hydrogénation agent which may be used to hydrogenate acetylene into ethylene. Further, preferably, a sélective acetylene hydrogénation catalyst is used that favours catalyzing the hydrogénation of acetylene to ethylene over the hydrogénation of ethylene to ethane.
Further, in optional step (c) of the présent process, any unconverted oxygen and/or carbon monoxide may also be removed in any known way. For example, unconverted oxygen and carbon monoxide may be removed by catalytic oxidation of carbon monoxide into carbon dioxide, wherein suitably a platinum or palladium containing oxidation catalyst is used (see for example above-mentioned US20160326070). Suitably, in a case where both acetylene and unconverted oxygen and carbon monoxide are removed in optional step (c), this may be done by performing first the above-described sélective hydrogénation of acetylene using hydrogen as a hydrogénation agent, followed by the above-described oxidation of carbon monoxide into carbon dioxide, so that any residual hydrogen may react with oxygen into water.
-14Alternatively, in optional step (c) unconverted oxygen and carbon monoxide may first be removed by distillation, for example by cryogénie distillation, followed by the abovedescribed sélective hydrogénation of acetylene using hydrogen as a hydrogénation agent. Further, it is possible to first perform the above-described sélective hydrogénation of acetylene using hydrogen as a hydrogénation agent, followed by said distillation to remove unconverted oxygen, carbon monoxide and any residual hydrogen.
However, in the above-described cases it is cumbersome having to apply multiple steps to remove unconverted oxygen, carbon monoxide and acetylene before complexation séparation step (d). It has been found that in one embodiment of optional step (c) of the présent process, in a case where the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) additionally comprises unconverted oxygen, carbon monoxide and optionally acetylene, these additional components are preferably removed advantageously in one step by oxidation of carbon monoxide and any acetylene into carbon dioxide. Thus, in said preferred embodiment, optional step (c) comprises optionally removing unconverted oxygen, carbon monoxide and optionally acetylene from at least part of the stream comprising ethylene, unconverted ethane, carbon dioxide, unconverted oxygen, carbon monoxide and optionally acetylene resulting from step (b), wherein carbon monoxide and optionally acetylene are oxidized into carbon dioxide, resulting in a stream comprising ethylene, unconverted ethane and carbon dioxide. Like with any oxidation of hydrocarbons, like acetylene, in said preferred embodiment water is produced in case acetylene is présent.
In said preferred embodiment of optional step (c), unconverted oxygen may advantageously be used to oxidize both
- 15 carbon monoxide and acetylene into carbon dioxide. There would be no need to add additional oxidizing agent or any other Chemical, like hydrogen which can be used as a hydrogenating agent to hydrogenate acetylene, as described above. Furthermore, in said prefered embodiment, there is neither any need to apply a cumbersome (cryogénie) distillation step to remove unonverted oxygen, carbon monoxide and any hydrogen.
In said preferred embodiment of optional step (c), the température may vary within wide ranges and is generally of from 20 to 500 °C, and may be of from 50 to 500 C or of from 100 to 400 °C. Preferably, in said step (c), the température is in the range of from 100 to 400 °C, more preferably 150 to 300 °C, more preferably 170 to 260 °C, most preferably 200 to 260 °C. In said step (c), the température may be at least 20 °C or at least 50 °C or at least 100 °C or at least higher than 100 °C or at least 110 °C or at least higher than 110 °C or at least 120 °C or at least higher than 120 °C or at least 130 °C or at least higher than 130 °C or at least 140 °C or at least higher than 140 °C or at least 150 °C or at least higher than 150 °C or at least 160 °C or at least higher than 160 °C or at least 170 °C or at least higher than 170 °C or at least 180 °C or at least higher than 180 °C or at least 190 °C or at least higher than 190 °C or at least 200 °C or at least higher than 200 °C or at least 210 °C or at least 220 °C or at least 230 °C or at least 240 °C. Further, in said step (c), the température may be at most 500 °C or at most 400 °C or at most 350 °C or at most 340 °C or at most 330 °C or at most 320 °C or at most 310 °C or at most 300 °C or at most 290 °C or at most 280 °C or at most 270 °C or at most 260 °C or at most 250 °C.
Still further, in said preferred embodiment of optional step (c), typical pressures are 0.1-30 or 0.1-20 bara (i.e.
- 16 bar absolute). Further, preferably, said pressure is of from 0.1 to 15 bara, more preferably of from 1 to 8 bara, most preferably of from 2 to 7 bara. Said pressure refers to total pressure.
Further, in said preferred embodiment of optional step (c), additional oxygen may be fed to said step (c). Such additional oxygen is added in addition to the oxygen from the stream comprising ethylene, unconverted ethane, carbon dioxide, unconverted oxygen, carbon monoxide and optionally acetylene that is fed to said step (c). Such additional oxygen may be needed in a case where the latter stream does not contain sufficient unconverted oxygen to oxidize ail of the carbon monoxide and any acetylene from the same stream into carbon dioxide. Such additional oxygen may be added either directly or indirectly to said step (c), in particular at any point before and/or during said step (c).
In said preferred embodiment of optional step (c), oxygen, carbon monoxide and optionally acetylene are removed from the stream comprising ethylene, unconverted ethane, carbon dioxide, unconverted oxygen, carbon monoxide and optionally acetylene by oxidation of carbon monoxide and any acetylene into carbon dioxide. That is to say, unconverted oxygen from the latter stream is used to oxidize carbon monoxide and any acetylene into carbon dioxide. As mentioned above, additional oxygen may be fed to fully convert ail carbon monoxide and acetylene (if any) into carbon dioxide. Such oxidation may also be referred to as combustion. Thus, said step (c) results in a stream comprising ethylene, unconverted ethane and carbon dioxide.
It is also envisaged that in a case where acetylene is produced in ethane ODH step (a), such acetylene may be removed as part of said preferred embodiment of optional step (c), after water removal step (b) but before the above
- 17 described oxidation step, in particular by means of hydrogénation of acetylene into ethylene, as described above.
Suitably, in said preferred embodiment of optional step (c), oxygen may be removed to such an extent that the stream resulting from said step (c) comprises no oxygen or 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 resulting from said step (c). Further, suitably, in said preferred embodiment of optional step (c), carbon monoxide and any acetylene may be removed to such an extent that the stream resulting from said step (c) comprises no carbon monoxide and acetylene or a residual amount of carbon monoxide and acetylene which is at most 15 vol.% or at most 10 vol.% or at most 5 vol.% or at most 1 vol.% 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 resulting from said step (c).
Said preferred embodiment of optional step (c) may be carried out in the presence of a catalyst, suitably an oxidation catalyst. Suitably, said oxidation catalyst catalyzes the oxidation of carbon monoxide and any acetylene into carbon dioxide. In particular, suitably, said oxidation catalyst catalyzes the conversion of carbon monoxide and any acetylene and oxygen into carbon dioxide by means of oxidation of carbon monoxide and any acetylene into carbon dioxide.
In said preferred embodiment of optional step (c), any oxidation catalyst that catalyzes the oxidation of carbon monoxide into carbon dioxide may be used. For example, one of the carbon monoxide oxidation catalysts as described in
- 18 EP499402A1, US4956330, EP306945A1, EP421169A1, US5157204 and US5446232 may be used in said step (c), the disclosures of which are herein incorporated by reference. Preferably, said catalyst also catalyzes the oxidation of any acetylene into carbon dioxide.
Preferably, the above-mentioned oxidation catalyst that may be used in said preferred embodiment of optional step (c) comprises a transition métal. More preferably, said catalyst comprises 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), manganèse (Mn), cérium (Ce), tin (Sn), ruthénium (Ru) and chromium (Cr), more preferably one or more metals selected from the group consisting of nickel, copper, zinc, platinum and ruthénium, even more preferably one or more metals selected from the group consisting of nickel, copper and zinc. Most preferably, said catalyst comprises copper and/or platinum. Suitably, said catalyst comprises copper or platinum, more suitably copper. For example, said catalyst may comprise copper and zinc. In particular, said catalyst may be a métal oxide catalyst, which may be a partially reduced métal oxide catalyst, wherein the métal(s) is (are) as described above, for example a catalyst comprising copper oxide and optionally zinc oxide. The catalyst may be a supported catalyst, wherein one or more of said metals are carried by a support, or an unsupported catalyst. In case the catalyst is a supported catalyst, the support may be any support, for example alumina, titania, silica, zirconia or Silicon carbide, suitably alumina. Further, the supported catalyst may be shaped into any shape, including tablets and extrudates, or coated on a substrate.
In some cases, in said preferred embodiment of optional step (c), it may not be possible or desired to completely
- 19 remove oxygen, carbon monoxide and optionally acetylene by oxidation of carbon monoxide and optionally acetylene into carbon dioxide, using unconverted oxygen and any additional oxygen as described above. If that is the case and if it is desired to remove any remaining amount of oxygen and/or carbon monoxide and/or acetylene, after said oxidation, a further removal treatment may be carried out as part of said preferred embodiment of optional step (c). Such further removal treatment may comprise passing the stream though a guard bed comprising a sorbent (absorbent and/or absorbent) which is capable of selectively sorbing any remaining oxygen, carbon monoxide and acetylene.
Step (d)
Step (d) of the présent process comprises removing ethylene from at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) by a complexation séparation method, which comprises contacting at least part of said stream with a liquid solvent comprising a complexation agent, resulting in a stream comprising ethylene and a stream comprising unconverted ethane and carbon dioxide.
In step (d) of the présent process, at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) is subjected to a complexation séparation method. In such complexation séparation method olefins (ethylene) may be selectively removed from non-olefins (unconverted ethane). In the présent invention, advantageously, ethylene is not only selectively separated from unconverted ethane by the complexation séparation method, but also from carbon dioxide diluent which may be présent in a relatively large amount and which diluent also needs to be recycled, just like unconverted ethane. In the feed to step (d) of the présent process, the amount of
- 20 carbon dioxide, based on the total amount of ethylene, unconverted ethane and carbon dioxide, may be of from 1 to 99 vol.%, preferably of from 5 to 95 vol.%, more preferably of from 10 to 90 vol.%, more preferably of from 20 to 85 vol.%, more preferably of from 30 to 80 vol.%, more preferably of from 40 to 75 vol.%, most preferably of from 50 to 70 vol.%. Further, said amount of carbon dioxide may be at least 1 vol.% or at least 5 vol.% or at least 10 vol.% or at least 20 vol.% or at least 30 vol.% or at least 40 vol.% or at least 50 vol.%. Still further, said amount of carbon dioxide may be at most 99 vol.% or at most 95 vol.% or at most 90 vol.% or at most 85 vol.% or at most 80 vol.% or at most 75 vol.% or at most 70 vol.%.
In the présent invention, the above-mentioned complexation séparation method comprises contacting at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) with a liquid solvent comprising a complexation agent. The complexation agent is dissolved in said liquid solvent. That is to say, the complexation séparation method in step (d) of the présent process comprises so-called absorption complexation séparation. In such absorption complexation séparation, ethylene is preferentially complexed to the complexation agent that is dissolved in the liquid solvent.
Generally, complexation séparation of olefins uses a complexation agent to selectively form a réversible complex, preferably a π-bond complex, with the olefins:
Olefin + Complexation agent <-> Olefin-Agent Complex Reversibility of the complexation reaction allows the olefin to be captured and released by shifting the direction of the reaction equilibrium. The forward complexation reaction is favoured by higher olefin partial pressure and lower température, whereas the reverse desorption reaction is
- 21 favoured by lower olefin partial pressure and higher température. Therefore, a complexation/desorption cycle can be generated by swinging the pressure, the température, or both.
Preferably, in the présent invention, complexation séparation step (d) comprises the following cycle of substeps:
(dl) contacting at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) with the liguid solvent comprising the complexation agent, resulting in a stream comprising unconverted ethane and carbon dioxide, at least part of which stream is fed to step (e), and a liquid stream comprising solvent, complexation agent and complexed ethylene; and (d2) desorbing complexed ethylene from at least part of the liquid stream comprising solvent, complexation agent and complexed ethylene resulting from step (dl), resulting in a stream comprising desorbed ethylene and a liquid stream comprising solvent and complexation agent; and (d3) recycling at least part of the liquid stream comprising solvent and complexation agent resulting from step (d2) to step (dl).
In step (d) of the présent process, a suitable complexation agent is one which selectively and reversibly forms a complex with ethylene, and not or substantially not with unconverted ethane and carbon dioxide. The complexation agent may be in the form of a métal sait or a métal complex which is soluble in the liquid solvent. Salts or compounds of silver(I) or copper(I), either by themselves or combined with another métal, such as aluminium, may be used. The complexation agent is preferably a métal sait, which further preferably contains a silver(I) ion or a copper(I) ion, more preferably a silver(I) ion. Optionally, a mixture of
- 22 complexation agents may be employed, for example, a mixture of copper and silver salts.
Suitable silver(I) ion containing salts include silver nitrate, silver tetrafluoroborate, silver hexafluorosilicate, silver hydroxytrifluoroborate, silver trifluoroacetate, silver perchlorate, silver triflate (CF3SO2O“Ag+) , and silver hexafluoroantimonate (V) (SbF6“Ag+) . Suitable copper (I) ion containing salts include cuprous nitrate; cuprous halides such as cuprous chloride; cuprous sulfate; cuprous sulfonate; cuprous carboxylates; cuprous salts of fluorocarboxylic acids, such as cuprous trifluoroacetate and cuprous perfluoroacetate; cuprous fluorinated acetylacetonate; cuprous hexafluoroacetylacetonate; cuprous dodecylbenzenesulfonate; copper-aluminium halides, such as cuprous aluminium tetrachloride; CUAICH3CI3; CUAIC2H5CI3; and cuprous aluminium cyanotrichloride. Silver nitrate is the most preferred complexation agent.
The concentration of the complexation agent in the liquid solvent should be such that substantially ail complexation agent is dissolved in that solvent, which dépends on the (maximum) solubility of said agent in said solvent. For example, silver nitrate has a solubility (in water) of 10.9 molar (75.4 wt.%) at 35 °C. Generally, the concentration of the complexation agent may be of from 1 to 10 molar, more suitably 1 to 8 molar, more suitably 1 to 6 molar, more suitably 2 to 5 molar, most suitably 2.5 to 4 molar.
Any suitable liquid solvent or mixture of liquid solvents may be used in step (d) to dissolve the complexation agent. Within the présent spécification, by liquid solvent reference is made to a solvent which is in the liquid State at a température of 25 °C and a pressure of 1 atmosphère. Preferably, said liquid solvent is water, an organic solvent,
- 23 an ionic liquid or a mixture thereof. Water is most preferred.
Water may be used as a solvent for silver or copper salts whereas hydrocarbon solvents, such as aromatic solvents, may be used for salts that contain organic ligands. Water is the preferred solvent because ethane and other non-olefins such as nitrogen are exceedingly sparingly soluble in aqueous solutions. In contrast, ethane has a higher solubility in hydrocarbon solvents. Olefins, like ethylene, hâve sufficient solubility in water for mass transfer to the dissolved complexation agent to occur at a reasonable rate.
As mentioned above, the liquid solvent to be used for dissolving the complexation agent may be an ionic liquid. As defined by Wasserscheid and Keim in Angewandte Chemie 2000, 112, pages 3926-3945, ionic liquids are salts which melt at a relatively low température. Ionic liquids are therefore already liquid at relatively low températures. In addition, they are in general not combustible and hâve no measurable vapour pressure. Within the présent spécification, ionic liquid means a sait which has a melting point or melting range which is below 100 °C.
Ionic liquids are formed from positive ions and négative ions (cations and anions, respectively), but are overall neutral in charge. The positive and also the négative ions are prédominantly monovalent, but multivalent anions and/or cations which hâve up to five, preferably up to four, particularly preferably up to three and particularly preferably up to two electric charges are also possible. The charges within the respective ions are either localized or delocalized.
In a case where in the présent invention an ionic liquid is used to dissolve the complexation agent, said ionic liquid may comprise a cation which is an N,N'-dialkylimidazolium ion
- 24 or an N-alkylpyridinium ion, preferably an N,N'dialkylimidazolium ion.
The alkyl groups in the above-mentioned N,N'dialkylimidazolium ion and N-alkylpyridinium ion may be Ci-Cio alkyl groups, preferably C1-C4 alkyl groups. Examples of suitable C1-C10 alkyl groups are methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, 2,4,4-trimethylpentyl and decyl. Preferably, the cation for the ionic liquid is an N,N'dialkylimidazolium ion, preferably an N,N'-dialkylimidazolium ion wherein the alkyl groups are C1-C10 alkyl groups as described hereinabove, preferably C1-C4 alkyl groups as described hereinabove.
A particularly preferred N,N'-dialkylimidazolium ion is l-butyl-3-methylimidazolium ion (BMIM ion). Another particularly preferred N,N'-dialkylimidazolium ion is 1,3dimethylimidazolium ion (DMIM ion). Yet another particularly preferred N,N'-dialkylimidazolium ion is l-ethyl-3methylimidazolium ion (EMIM ion).
In a case where in the présent invention an ionic liquid is used to dissolve the complexation agent, said ionic liquid may comprise an anion which is selected from the group consisting of tetrafluoroborate ion (BF4-) , alkoxyphosphonate ions, alkylsulfonate ions, hexafluorophosphate ion (PFe“) and amide ions. More preferably, said anion is selected from the group consisting of tetrafluoroborate ion, alkoxyphosphonate ions and amide ions. Most preferably, said anion is tetrafluoroborate ion.
The above-mentioned alkoxyphosphonate ion is of the formula RO-PH(=O)O“ wherein R is an alkyl group, preferably a C1-C10 alkyl group, more preferably a C1-C4 alkyl group. Examples of suitable C1-C10 alkyl groups are methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl,
- 25 hexyl, heptyl, octyl, 2-ethylhexyl, 2,4,4-trimethylpentyl and decyl. A particularly preferred alkoxyphosphonate ion is methoxyphosphonate ion.
The above-mentioned alkylsulfonate ion is of the formula R-S(=O)2O- wherein R is an alkyl group, preferably a Ci-Cio alkyl group, more preferably a C1-C4 alkyl group. Examples of suitable Ci—Cio alkyl groups are methyl, ethyl, propyl, isopropyl, n-butyl, sec—butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, 2,4,4-trimethylpentyl and decyl.
The above-mentioned amide ion is of the formula R-N’-R' wherein R and R' may be the same or different and are preferably electron-withdrawing substituents. Electronwithdrawing substituents, in general, are substituents that draw électrons away from an électron rich place in a molécule, in this case from the électron rich nitrogen atom in said amide ion. Preferably, R and R' are selected from the group consisting of cyano and alkanesulfonyl.
A particularly preferred amide ion is dicyanamide ion, that is to say an ion of said formula R-N“-R' wherein both R and R' are cyano.
Said alkanesulfonyl substituent in said amide ion is of the formula R-S(=O)2- wherein R is an alkyl group, preferably a Ci-Ci2 alkyl group, more preferably a C1-C4 alkyl group, for example methyl, ethyl or n-butyl. Said alkyl group may be linear or branched. Further, said alkyl group may be substituted with one or more halogen atoms. Said alkanesulfonyl substituent is preferably a trihalogenmethanesulfonyl substituent which is of the formula CX3-S(=O)2- wherein X is a halogen atom selected from the group consisting of fluorine, chlorine, bromine and iodine. More preferably, said halogen atom is fluorine. Most preferably, said trihalogenmethanesulfonyl substituent is trifluoromethanesulfonyl (CF3-S(=0) 2-) .
- 26 In a case where in the présent invention an ionic liquid is used to dissolve the complexation agent, the ionic liquid preferably comprises an N,N'-dialkylimidazolium ion as described hereinabove as the cation and a tetrafluoroborate ion as the anion. Preferably, said N,N'-dialkylimidazolium ion is l-butyl-3-methylimidazolium ion or l-ethyl-3methylimidazolium ion, more preferably l-butyl-3methylimidazolium ion.
Generally, suitable ionic liquids which may be used to dissolve the complexation agent are disclosed in Potential of Silver-Based Room-Temperature Ionic Liquids for Ethylene/Ethane Séparation, Galan Sanchez et al., Ind. Eng. Chem. Res., 2009, 48, pages 10650-10656, in particular in Table 1 of said article, the disclosure of which article is herein incorporated by reference. Further suitable ionic liquids are disclosed in Olefin Paraffin Séparation Using Ionic Liquids, Goodrich, Cat. Rev., 2015, 28, pages 9-13, the disclosure of which article is herein incorporated by reference. Still further suitable ionic liquids are disclosed in WO201108664, WO200359483, WO200198239 and GB2383328, the disclosures of which are herein incorporated by reference.
Further, it is envisaged that in complexation séparation step (d) of the présent process, an ionic liquid is used as the liquid solvent comprising the complexation agent. In such a case, the ionic liquid is simultaneously both said liquid solvent and said complexation agent. Preferably, such ionic liquid comprises a silver(I) ion or a copper(I) ion, more preferably a silver(I) ion. Further, the anion in such ionic liquid may be an anion as described above. Examples of such ionic liquids which can be used in such a way are silver(I) bis(trifluoromethanesulfonyl) amide which is of formula [ (CF3-S (=0) 2-) 2N] “Ag+ (Ag[NTf2]), and silver(I) tris(perfluoroethyl) trifluoro phosphate which is of formula
- 27 [ (CF3CF2) 3F3P] ~Ag+ (Ag[FAP]). These and other suitable silver(I) ion containing ionic liquids are disclosed in Liquid silver tris(perfluoroethyl) trifluoro phosphate salts as new media for propene/propane séparation, Pliquette et al., Phys. Chem. Chem. Phys., 2016, 18, pages 28242-28253. However, for these ionic liquids it is still preferred that an additional liquid solvent, for example another ionic liquid which does not hâve a métal ion as cation, is added for liquefying and/or diluting the métal ion containing ionic liquid.
In addition to the complexation agent, the liquid solvent may comprise a modifier or mixture of modifiers, such as an acid, a sait that does not complex olefins, an oxidizing agent, or a functional organic compound. Such modifier may be used to increase the solubility and/or stability of the complexation agent in the solvent. Suitable examples of acid modifiers are nitric acid (HNO3) and fluoroboric acid (HBF4) . In addition, such acid modifier, especially nitric acid, may reduce the physical solubility of carbon dioxide in the liquid solvent, which advantageously simplifies the séparation of carbon dioxide from ethylene in complexation séparation step (d). In a case where in the présent invention an ionic liquid is used to dissolve the complexation agent, it is preferred that the anion of the acid modifier (e.g. HBF4) corresponds with that of the ionic liquid.
Silver nitrate is the most preferred complexation agent in the practice of the présent invention. Silver nitrate has high solubility and is very stable in water. Further, any elemental silver that would be formed can easily be reconverted into silver nitrate, by using a small amount of nitric acid. Thus, preferably, in step (d) of the présent invention, an aqueous solution is used which comprises silver nitrate as the complexation agent. The latter aqueous
- 28 solution further preferably comprises nitric acid as a modifier.
In above-mentioned complexation step (dl), at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) is contacted with the liquid solvent comprising the complexation agent. The ethylene partial pressure in said step (dl) may be of from 0.5 to 30 bar, more suitably of from 1 to 20 bar, most suitably of from 2 to 10 bar. The ethylene partial pressure in step (dl) may be at least about as high as the ethylene partial pressure in the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c), or higher. Preferably, the ethylene partial pressure is increased prior to step (dl), for example by compression in a compresser. Further, the température of the liquid solvent as fed to said step (dl) is preferably below 50 °C, more preferably below 40 °C, and may be of from —20 to 75 C, more suitably of from 0 to 50 °C, most suitably of from 10 to 40 °C. During step (dl) an excessive température rise may be avoided by internai cooling.
Step (dl) may be carried out in a countercurrent-flow column. Preferably, at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) is fed to the bottom of said column and liquid solvent comprising the complexation agent is fed to the top of said column. Said column may contain a packing or trays, preferably a packing. The ethylene recovery in complexation step (dl) is preferably above 95%, more preferably above 98%.
In above-mentioned desorption step (d2), complexed ethylene is desorbed from at least part of the liquid stream comprising solvent, complexation agent and complexed ethylene resulting from step (dl). In the présent invention,
- 29 desorption in step (d2) may be effected by decreasing the ethylene partial pressure or by increasing the température or by both. A combination of decreasing the ethylene partial pressure and increasing the température is preferred. The total pressure in said step (d2) may be of from 1 mbar to 5 bar, more suitably of from 1 mbar to 3 bar, most suitably of from 0.5 to 1.5 bar. Further, the température of the liquid stream as fed to said step (d2) may be of from 55 to 130 C, more suitably of from 65 to 130 °C, most suitably of from 80 to 120 °C. Preferably, the température of said liquid stream j_s increased prior to feeding to step (d2) , for example by heating. The ethylene recovery in desorption step (d2) is preferably above 95%, more preferably above 98%. The liquid stream comprising solvent and complexation agent resulting from desorption step (d2) is recycled to complexation step (dl), preferably after cooling.
In above-mentioned complexation step (dl), a part of the unconverted ethane and carbon dioxide may be physically absorbed (dissolved) in the liquid solvent, not complexed with the complexation agent (hereinafter referred to as absorbed unconverted ethane and carbon dioxide). In a case wherein the liquid stream resulting from step (dl) comprises solvent, complexation agent, complexed ethylene and absorbed unconverted ethane and carbon dioxide, it is preferred to strip away said unconverted ethane and carbon dioxide from said liquid stream before feeding it to desorption step (d2). In such case, preferably, absorbed unconverted ethane and carbon dioxide are stripped from at least part of said liquid stream by contacting with a stream comprising ethylene, resulting in a stream comprising ethylene, unconverted ethane and carbon dioxide, at least part of which stream is fed to step (dl), and a liquid stream comprising solvent, complexation agent and complexed ethylene, at least part of
- 30 which liquid stream is fed to step (d2). Preferably, in said stripping step, the ethylene partial pressure and the température are substantially not changed, so as to avoid any prématuré desorption before step (d2).
The above-mentioned stripping step may be carried out in a countercurrent-flow column. Preferably, at least part of the liquid stream resulting from step (dl) is fed to the top of said column and the stripping stream comprising ethylene is fed to the bottom of said column.
If acetylene is formed in ethane ODH step (a) and is not removed in optional step (c), acetylene may be présent in the feed to complexation séparation step (d). As mentioned above, acetylene may form a strong bond with the complexation agent in step (d). Acetylenes that contain an active hydrogen may form silver or copper acetylide compounds that hâve limited solubility in aqueous solution and do not décomposé during desorption, so they can accumulate until they precipitate. This consumes complexation agent and may interfère with flow and generate a safety hazard. These précipitâtes are susceptible to détonation, especially when dry, so précautions must be taken to deal with them effectively. One way of dealing with this is to maintain silver acetylide concentration at a safe level by using silver permanganate as an oxidant. A small sidestream may be withdrawn from the desorber and heated, for example to 75 °C, under partial vacuum. Solid silver permanganate is added to destroy the acetylide, which forms carbon dioxide and free silver ion. The resulting manganèse dioxide précipitâtes and is filtered from solution. This recovers silver without adding a foreign ion. Data and treatment of silver acetylides are given in US4174353, the disclosure of which is herein incorporated by reference.
- 31 If carbon monoxide is formed in ethane ODH step (a) and is not removed in optional step (c), carbon monoxide may be présent in the feed to complexation séparation step (d). As mentioned above, in addition to the desired ethylene product, also carbon monoxide may complex to the complexation agent in step (d). Carbon monoxide complexes strongly to Cu(I) that may be présent in the complexation agent used in step (d). In the latter case, such complexed carbon monoxide is not removed in the above-mentioned stripping step, but would be desorbed, together with the ethylene, in the above-mentioned desorption step (d2), resulting in a stream comprising both ethylene and carbon monoxide.
If not ail oxygen is converted in ethane ODH step (a) and the unconverted oxygen is not removed in optional step (c), oxygen may be présent in the feed to complexation séparation step (d). As mentioned above, oxygen may oxidize the métal, for example Cu(I), from a métal sait or métal complex that may be used as complexation agent in step (d). Any oxygen may be removed in the above-mentioned complexation step (dl) as part of the stream comprising unconverted ethane and carbon dioxide.
Further, hydrogen (H2) may be présent in the feed to complexation séparation step (d). Hydrogen can cause a graduai réduction of Ag(I) to metallic silver. It is preferred to eliminate such silver réduction to prevent silver from being lost by forming colloïdal particles and by plating out on surfaces. The addition of small amounts of hydrogen peroxide coupled with a maintenance level of nitric acid in the solution may stabilize the dissolved silver against précipitation. More information on such method is given in US4174353, the disclosure of which is herein incorporated by reference.
Step (e)
- 32 Step (e) of the présent process comprises partially and selectively removing carbon dioxide from at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d), resulting in a stream comprising unconverted ethane and carbon dioxide and having a reduced carbon dioxide content.
Advantageously, in step (e) of the présent process only part of the carbon dioxide is removed, after which the remaining carbon dioxide can be recycled in step (f) as a diluent to ethane ODH step (a). It is preferred that in step (e) only additional carbon dioxide resulting from carbon dioxide production in step (a) and possibly in optional step (c) is removed. In step (e), a part or ail of such additional carbon dioxide may be removed. An example of a case wherein only a part of such additional carbon dioxide is removed in step (e) may be a case wherein a portion of the recycle stream is purged (non-selectively) before recyling, as further described below. By removing such additional carbon dioxide before recycling, in step (e) and in any additional purge step, the level of carbon dioxide diluent in ethane ODH step (a) can be advantageously kept at a constant level.
Further, advantageously, in step (e) of the présent process carbon dioxide is selectively removed, implying that other components are substantially not removed. This has the advantage that substantially no unconverted ethane is lost in step (e), thereby making it possible to recycle as much as possible of the unconverted ethane to step (a). This is different from a case wherein additional carbon dioxide resulting from carbon dioxide production in step (a) and possibly in optional step (c) is only removed by purging a portion of the recycle stream before recyling, which purging is a non-selective removal method which would also remove valuable, unconverted ethane from the process.
- 33 Generally, the amount of additional carbon dioxide resulting from carbon dioxide production in step (a) and possibly in optional step (c), as compared to the amount of carbon dioxide that is used as diluent in ethane ODH step (a), is relatively small. Therefore, it is preferred that in step (e) of the présent process carbon dioxide is partially and selectively removed from only a part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d). In particular, it is preferred that at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d) is split into at least two substreams, wherein at least part of one split substream is fed to step (e) and at least part of one split substream is recycled to step (a). The foregoing advantageously results in that the stream to be subjected to step (e) is smaller so that a relatively small unit for performing step (e) may be used, resulting in a further réduction of capital expenditure.
In step (e) of the présent process, 1 to 15%, preferably 3 to 12%, more preferably 5 to 10%, of the amount of carbon dioxide from the stream comprising unconverted ethane and carbon dioxide resulting from step (d) may be removed, which removal is sélective, resulting in a stream having a reduced carbon dioxide content. By said amount of carbon dioxide from the stream comprising unconverted ethane and carbon dioxide resulting from step (d) reference is made to the amount of carbon dioxide in the stream comprising unconverted ethane and carbon dioxide resulting from step (d) before any step wherein the latter stream is split. In a case wherein a portion of the recycle stream is purged (removed from the process) before recyling, which purge is non-selective, as further described below, then said amount of carbon dioxide to be removed in step (e) may be lower. It is preferred that
- 34 the total amount of (i) carbon dioxide removed in step (e) and (ii) carbon dioxide removed in any step wherein a portion of the recycle stream is purged before recyling, is of from 1 to 15%, preferably 3 to 12%, more preferably 5 to 10%, of the amount of carbon dioxide from the stream comprising unconverted ethane and carbon dioxide resulting from step (d). Said total amount of carbon dioxide removed may be at least 0.5% or at least 1% or at least 2% or at least 3% or at least 5% of the carbon dioxide from the stream comprising unconverted ethane and carbon dioxide resulting from step (d). Further, said total amount of carbon dioxide removed may be at most 15% or at most 12% or at most 10% or at most 8% of the carbon dioxide from the stream comprising unconverted ethane and carbon dioxide resulting from step (d). It is preferred that said total amount of carbon dioxide removed corresponds to the total amount of additional carbon dioxide resulting from carbon dioxide production in step (a) and possibly in optional step (c) .
In step (e) of the présent process, carbon dioxide may be selectively removed from at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d) by any one of well-known methods. A suitable carbon dioxide removal agent that may be fed to said step (e) may be an aqueous solution of a base, for example sodium hydroxide and/or an amine. Further, a carbon dioxide sélective membrane may be used in said step (e). Further, removing only part of the carbon dioxide in said step (e) may for example be achieved by basing the amount of carbon dioxide removal agent on the amount of carbon dioxide that needs to be removed in step (e). Another example of how this may be achieved is to use a carbon dioxide sélective membrane, of which the carbon dioxide recovery capacity
- 35 corresponds to the amount of carbon dioxide that needs to be removed in step (e).
Step (f)
Step (f) of the présent process comprises recycling at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (e) to step (a).
Optional further steps
Optionally, any propane is removed from the ethane containing stream in a pre-separation step prior to feeding to step (a), for example by means of distillation. Thus, in a case where propane is présent in the ethane feed, it is preferred that in a step prior to step (a) of the présent process, the stream comprising ethane and propane is fed to a distillation column to obtain a stream comprising propane and a stream comprising ethane. The latter stream comprising ethane, containing no or substantially no propane, is fed to step (a) of the présent process. Since no or substantially no propane is présent in said step (a), no or substantially no propylene is formed by ODH from propane in step (a). This advantageously prevents a cumbersome post-separation step of removing propylene from ethylene as recovered in step (d), as both propylene and ethylene may be complexed to the complexation agent used in step (d). A pre-separation step removing propane from an ethane containing stream prior to feeding to an ethane ODH step is disclosed in WO2017072086, the disclosure of which is herein incorporated by reference.
Further, in a case where the stream comprising ethylene resulting from step (d) additionally comprises one or more contaminants selected from the group consisting of propylene, carbon monoxide, oxygen, carbon dioxide and water, this or these contaminant(s) may be removed in one or more further steps. However, such further purification is not always needed. In some cases, a crude ethylene stream could be sent,
- 36 without further purification, to a unit where the ethylene is further converted. In case said contaminant(s) hâve to be removed, this can be done in any way known to the skilled person. Propylene may be removed by distillation. Carbon monoxide may be removed by conversion (oxidation) into carbon dioxide, for example using copper oxide as oxidation catalyst, and subséquent removal of carbon dioxide. Oxygen may be removed by using it as an oxidation agent, for example in oxidizing metallic copper. Carbon dioxide may be removed by a caustic wash. Water may be removed by drying, for example by using molecular sieves.
Still further, (i) a portion of the stream comprising unconverted ethane and carbon dioxide resulting from step (d) and/or (ii) a portion of a stream originating from the stream comprising unconverted ethane and carbon dioxide resulting from step (d), may be purged before recyling to ODH step (a), as further described below. For example, such purge may be performed between steps (d) and (e) and/or between steps (e) and (f).
The stream comprising unconverted ethane and carbon dioxide resulting from step (d) may comprise of from 5 to 90 vol.% of carbon dioxide, more suitably of from 10 to 80 vol.%, most suitably of from 20 to 70 vol.%. Further, said recycle stream may comprise of from 10 to 95 vol.% of unconverted ethane, more suitably of from 20 to 90 vol.%, most suitably of from 30 to 80 vol.%. In addition, said recycle stream may comprise methane, nitrogen, carbon monoxide and/or oxygen. Suitably, the amount of methane is less than 20 vol.%, more suitably less than 10 vol.%, more suitably less than 5 vol.%, even more suitably less than 500 parts per million by volume (ppmv). Further, suitably, the total amount of nitrogen, carbon monoxide and/or oxygen is less than 10 vol.%, more suitably less than 5 vol.%, more
- 37 suitably less than 3 vol.%, even more suitably less than 500 ppmv. Said methane may originate from the feed of fresh ethane to ethane ODH step (a). Further, said nitrogen may originate from the feed of fresh oxygen to ethane ODH step (a) .
Before recycling ethane to ODH step (a), the abovementioned (i) stream comprising unconverted ethane and carbon dioxide resulting from step (d) and/or (ii) stream originating from the stream comprising unconverted ethane and carbon dioxide resulting from step (d), may be split into at least two substreams, wherein at least one split substream is removed from the process. Further, preferably, at least part of at least one split substream is fed to step (e) or recycled to ODH step (a). Said at least one split substream that is removed from the process (purged) may be discarded.
Said stream originating from the stream comprising unconverted ethane and carbon dioxide resulting from step (d) , as mentioned under (ii) above, may for example be: the stream comprising unconverted ethane and carbon dioxide and having a reduced carbon dioxide content resulting from step (e); and/or a stream that results from splitting at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d) into at least two substreams, wherein at least one split substream is fed to step (e) and at least one split substream is recycled to step (a). In the latter case, a portion of one or both of said split substreams is purged (removed from the process) before feeding to step (e) or recyling to ODH step (a), respectively.
In the above-mentioned case wherein a purge is performed, at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d) may be split into at least three substreams, wherein at least one split substream
- 38 is fed to step (e), at least one split substream is recycled to step (a) and at least one split substream is removed from the process (purged).
Further, in the above-mentioned case wherein a purge is performed, the proportion of (i) the split substream(s) that is (are) not removed from the process (not purged) to (ii) the total stream resulting from step (d) before any splitting is preferably of from 90 to 99.9 vol.%, more preferably of from 93 to 99 vol.%, more preferably of from 95 to 99 vol.%, most preferably of from 97 to 99 vol.%. Further, said proportion may be at least 90 vol.% or at least 93 vol.% or at least 95 vol.% or at least 97 vol.% or at least 98 vol.% or at least 99 vol.% or at least 99.5 vol.%. Further, said proportion may be at most 99.9 vol.% or at most 99.7 vol.% or at most 99.5 vol.% or at most 99.3 vol.% or at most 99 vol.%.
Advantageously, in a case where additional carbon dioxide is produced in ethane ODH step (a) and possibly in optional step (c), such additional carbon dioxide may be removed by splitting the recycle stream before recycling, so that the amount of carbon dioxide diluent fed to ethane ODH step (a) can be kept constant. However, the disadvantage of such split and purge procedure is that valuable, unconverted ethane is lost. Therefore, it is preferred in the présent invention that substantially no or only a relatively small portion of unconverted ethane is purged from the process. The présent invention makes this advantageously possible by the partial and sélective removal of carbon dioxide in step (e) thereby saving unconverted ethane.
In the above-mentioned case of splitting before recycling, including purging one of the split substreams, a build-up of certain components in the présent process may be prevented, by purging a portion of the recycle stream before recyling. Said additional components that may be purged
- 39 comprise for example methane from the fresh ethane feed and nitrogen from the oxygen feed, as mentioned above. However, since this split and purge procedure has the disadvantage of losing valuable, unconverted ethane, it is preferred that the fresh ethane feed to step (a) contains substantially no methane impurity or an amount of methane impurity, which is suitably up to 3 vol.% or up to 2 vol.% or up to 1 vol.% or up to 5,000 parts per million by volume (ppmv) or up to 2,000 ppmv or up to 1,000 ppmv or up to 500 ppmv or up to 200 ppmv. A suitable example of a relatively pure ethane containing stream is an ethane containing stream originating from an ethane or naphtha cracker, which contains ethane that was not converted in the cracking process and which contains substantially no methane or propane or any higher hydrocarbons. Using such pure ethane stream also has the additional advantage that the above-described propane preseparation step would not be needed. Alternatively, a low methane level in the fresh ethane feed to step (a) may be achieved by a methane pre-separation step, for example by using a demethanizer (distillation) upstream of step (a). Likewise, it is preferred that the oxygen feed to step (a) contains substantially no nitrogen impurity or an amount of nitrogen impurity, which is suitably up to 3 vol.% or up to 2 vol.% or up to 1 vol.% or up to 5,000 parts per million by volume (ppmv) or up to 2,000 ppmv or up to 1,000 ppmv or up to 500 ppmv or up to 200 ppmv.
Figures 1 and 2
The process of the présent invention is further illustrated by Figures 1 and 2.
Figure 1 depicts an embodiment covering steps (a) to (f) of the process of the présent invention. In Figure 1, stream 1 comprising fresh ethane and some propane is fed to distillation column 2, wherein it is separated into top
- 40 stream 3 comprising fresh ethane and bottom stream 4 comprising propane. Said stream 3, stream 6 comprising oxygen and recycle stream 25 comprising carbon dioxide (diluent) and unconverted ethane are fed to oxidative dehydrogenation (ODH) unit 5 containing an ethane ODH catalyst comprising a mixed métal oxide and operating under ODH conditions, wherein ethane is converted into ethylene in accordance with the above-described step (a) of the process of the présent invention. Product stream 7 coming from ODH unit 5 comprises water, ethane, ethylene, oxygen, carbon monoxide, acetylene, carbon dioxide and acetic acid. Said stream 7 is fed to water condensation unit 8. In water condensation unit 8, water and acetic acid are removed by condensation via stream 10 in accordance with the above-described step (b) of the process of the présent invention. Optionally, additional water is fed to water removal unit 8 via stream 9. Stream 11 coming from water condensation unit 8, which comprises ethane, ethylene, oxygen, carbon monoxide, acetylene and carbon dioxide, is fed to gas clean-up reactor 12. In gas clean-up reactor 12, oxygen, acetylene and carbon monoxide are removed in accordance with the above-described step (c) of the process of the présent invention. In particular, carbon monoxide and acetylene are oxidized into carbon dioxide, using the abovedescribed oxidation catalyst, in particular an oxidation catalyst which comprises copper. Optionally, additional oxygen is fed to gas clean-up reactor 12 via stream 13. Product stream 14 coming from gas clean-up reactor 12 comprises ethane, ethylene and carbon dioxide. Said stream 14 is fed to complexation séparation unit 15. In complexation séparation unit 15, a complexation séparation method is applied in accordance with the above-described step (d) of the process of the présent invention. Complexation séparation unit 15 is further described below with reference to Figure
- 41 2. Stream 18 coming from complexation séparation unit 15 comprises ethylene. Stream 17 coming from complexation séparation unit 15 comprises carbon dioxide and (unconverted) ethane. Said stream 17 is split into substream 17b which is fed to carbon dioxide removal unit 22 and recycle substream 17a which is fed to ODH unit 5. Carbon dioxide removal agent is fed to carbon dioxide removal unit 22 via stream 23. Said carbon dioxide removal agent is an aqueous solution of a base, for example sodium hydroxide and/or an amine. Carbon dioxide is removed via stream 24 in accordance with the above-described step (e) of the process of the présent invention. Stream 25 coming from carbon dioxide removal unit 10, which comprises unconverted ethane and carbon dioxide and which has a reduced carbon dioxide content, is fed to ODH unit 5. Streams 17a and 25 may be combined before recycling to ODH unit 5 (as shown in Figure 1).
In a case wherein stream 17 coming from complexation séparation unit 15 comprises methane, nitrogen, carbon monoxide and/or oxygen (and/or any other contaminant), in addition to carbon dioxide and (unconverted) ethane, a buildup of such contaminant(s) may be prevented by a split and purge procedure. Such a split and purge procedure involves splitting a stream into a substream which is purged (removed from the process) and a substream which is not purged. Said split and purge procedure may be applied to stream 17 (either before or simultaneously with splitting stream 17 into substreams 17a and 17b); or to stream 17b; or to stream 17a; or to stream 25; or to any stream resulting from combining streams 17a and 25 before recycling to ODH unit 5. In ail of the latter streams the ratio of such contaminant(s) to (unconverted) ethane is substantially the same because in carbon dioxide removal unit 22 carbon dioxide is removed selectively, so that the same amount of (unconverted) ethane
- 42 would be lest when applying said split and purge procedure to any one of those streams.
Figure 2 depicts an embodiment in relation to step (d) of the process comprising steps (a) to (f) as depicted in Figure 1. In Figure 2, stream 14 comprising ethane, ethylene and carbon dioxide, which cornes from gas clean-up reactor 12, is fed to the bottom of absorber 15a which is part of complexation séparation unit 15. Before feeding said stream 14 to absorber 15a, it is compressed in a compresser (not shown in Figure 1 or 2). In absorber 15a, said stream 14 is contacted with the liquid solvent comprising the complexation agent, in accordance with the above-described step (dl), which liquid solvent is fed to the top of absorber 15a via stream 16. The ethylene partial pressure in absorber 15a may be about 4 bar and the température of liquid stream 16 as fed to absorber 15a may be about 30 °C. Top stream 17 coming from absorber 15a comprises carbon dioxide and ethane. Bottom stream 19 coming from absorber 15a is a liquid stream comprising solvent, complexation agent, complexed ethylene and absorbed ethane and carbon dioxide, which stream is fed to the top of stripper 15b which is also part of complexation séparation unit 15. In stripper 15b, said absorbed ethane and carbon dioxide are stripped by contacting with stream 18b comprising ethylene, as described above, which stream 18b is fed to the bottom of stripper 15b. The ethylene partial pressure in stripper 15b may be about 4 bar and the température of liquid stream 19 as fed to stripper 15b may be about 30 °C. Top stream 20 coming from stripper 15b comprises ethylene, ethane and carbon dioxide and is compressed in a compresser (not shown in Figure 2) and then fed to absorber 15a via stream 14. Bottom stream 21 coming from stripper 15b is a liquid stream which comprises solvent, complexation agent and complexed ethylene and is fed to desorber 15c which
- 43 is also part of complexation séparation unit 15. In desorber 15c, ethylene is desorbed in accordance with the abovedescribed step (d2). The total pressure in desorber 15c may be about 500 mbar and the température of liquid stream 21 as 5 fed to desorber 15c may be about 80 °C. Top stream 18 coming from desorber 15c comprises desorbed ethylene and is split into two substreams 18a and 18b. Substream 18b is compressed in a compresser (not shown in Figure 2) and then fed to stripper 15b. Substream 18a may be further purified. Bottom 10 stream 16 coming from desorber 15c is a liquid stream which comprises solvent and complexation agent and is recycled to absorber 15a in accordance with the above-described step (d3) .

Claims (10)

1. Process for oxidative dehydrogenation of ethane, comprising the steps of:
(a) subjecting a stream comprising ethane to oxidative dehydrogenation conditions, comprising contacting the ethane with oxygen in the presence of a catalyst comprising a mixed métal oxide, wherein a diluent comprising carbon dioxide is fed to step (a), resulting in an effluent comprising ethylene, optionally acetic acid, unconverted ethane, water, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene;
(b) removing water from at least part of the effluent resulting from step (a), resulting in a stream comprising ethylene, unconverted ethane, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene and a stream comprising water and optionally acetic acid;
(c) optionally removing unconverted oxygen and/or carbon monoxide and/or acetylene from at least part of the stream comprising ethylene, unconverted ethane, carbon dioxide, optionally unconverted oxygen, optionally carbon monoxide and optionally acetylene resulting from step (b), resulting in a stream comprising ethylene, unconverted ethane and carbon dioxide;
(d) removing ethylene from at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) by a complexation séparation method, which comprises contacting at least part of said stream with a liquid solvent comprising a complexation agent, resulting in a stream comprising ethylene and a stream comprising unconverted ethane and carbon dioxide;
(e) partially and selectively removing carbon dioxide from at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d), resulting in a stream comprising unconverted ethane and carbon dioxide and having a reduced carbon dioxide content;
(f) recycling at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (e) to step (a).
2. Process according to claim 1, wherein step (d) comprises: (dl) contacting at least part of the stream comprising ethylene, unconverted ethane and carbon dioxide resulting from step (b) or (c) with the liquid solvent comprising the complexation agent, resulting in a stream comprising unconverted ethane and carbon dioxide, at least part of which stream is fed to step (e), and a liquid stream comprising solvent, complexation agent and complexed ethylene; and (d2) desorbing complexed ethylene from at least part of the liquid stream comprising solvent, complexation agent and complexed ethylene resulting from step (dl), resulting in a stream comprising desorbed ethylene and a liquid stream comprising solvent and complexation agent; and (d3) recycling at least part of the liquid stream comprising solvent and complexation agent resulting from step (d2) to step (dl).
3. Process according to claim 2, wherein the liquid stream resulting from step (dl) comprises solvent, complexation agent, complexed ethylene and absorbed unconverted ethane and carbon dioxide, wherein absorbed unconverted ethane and carbon dioxide are stripped from at least part of said liquid stream by contacting with a stream comprising ethylene, resulting in a stream comprising ethylene, unconverted ethane
- 46 and carbon dioxide, at least part of which stream is fed to step (dl), and a liquid stream comprising solvent, complexation agent and complexed ethylene, at least part of which liquid stream is fed to step (d2).
4. Process according to any one of the preceding daims, wherein in the feed to step (d) the amount of carbon dioxide, based on the total amount of ethylene, unconverted ethane and carbon dioxide, is of from 1 to 99 vol.%, preferably of from 10 5 to 95 vol.%, more preferably of from 10 to 90 vol.%, more preferably of from 20 to 85 vol.%, more preferably of from 30 to 80 vol.%, more preferably of from 40 to 75 vol.%, most preferably of from 50 to 70 vol.%.
5. Process according to any one of the preceding daims, wherein the complexation agent in step (d) is a métal sait.
6. Process according to daim 5, wherein the métal sait contains a silver(I) ion or a copper(I) ion, preferably a silver(I) ion.
7. Process according to daim 6, wherein the métal sait is silver nitrate.
8. Process according to any one of the preceding daims, 15 wherein the liquid solvent in step (d) is water, an organic solvent, an ionic liquid or a mixture thereof, preferably water.
9. Process according to any one of the preceding daims, wherein at least part of the stream comprising unconverted ethane and carbon dioxide resulting from step (d) is split into at least two substreams, wherein at least part of one
- 47 split substream is fed to step (e) and at least part of one split substream is recycled to step (a).
10. Process according to any one of the preceding daims, wherein the total amount of (i) carbon dioxide removed in step (e) and (ii) carbon dioxide removed in any step wherein a portion of the recycle stream is purged before recyling, is of from 1 to 15%, more preferably 3 to 12%, most preferably 5 to 10%, of the amount of carbon dioxide from the stream comprising unconverted ethane and carbon dioxide resulting from step (d).
OA1202000077 2017-08-16 2018-08-02 Ethane oxidative dehydrogenation. OA19409A (en)

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