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Title:
A PROCESS FOR OXIDATIVE COUPLING OF METHANE (OCM) AND A PROCESS SYSTEM FOR SUCH A PROCESS
Document Type and Number:
WIPO Patent Application WO/2018/202828
Kind Code:
A1
Abstract:
The present invention relates to a process for oxidative coupling of methane (OCM, 100) comprising the steps of feeding methane (104) and at least one oxidizing agent (103), preferably oxygen, into at least one OCM reactor (101) comprising at least one OCM catalyst, wherein C2+ alkene is generated from methane by the OCM catalyst; wherein at least part of the methane (104) is provided by at least one methanation unit (111), in which hydrogen and carbon dioxide are reacted to methane, wherein at least part of the hydrogen fed to the methanation unit (111) is provided from a hydrogen stream (211) generated in a propane dehydrogenation process (200), and/or wherein ethane (105) is fed into the at least one OCM reactor (101) comprising further at least one cracking unit, wherein C2+ alkene is generated from ethane in the cracking unit, wherein at least part of the ethane (105) is provided from ethane (213) generated in a propane dehydrogenation process (200). The invention also relates to a process system for such a process.

Inventors:
MATHIVANAN GUHAN (AT)
FAGERSTOLT KRISTER (FI)
OUNI TUOMAS (FI)
DREHER EBERHARD (AT)
Application Number:
PCT/EP2018/061454
Publication Date:
November 08, 2018
Filing Date:
May 04, 2018
Export Citation:
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Assignee:
BOREALIS AG (AT)
International Classes:
C07C1/12; C07C2/84; C07C4/06; C07C5/327; C07C5/333; C07C9/04; C07C9/06; C07C11/04; C07C11/06
Domestic Patent References:
WO2016209508A12016-12-29
WO2011119286A22011-09-29
Foreign References:
EP2080747A12009-07-22
US20080097134A12008-04-24
US2419997A1947-05-06
Other References:
CHIN ET AL., INT. J. CHEM., NUCL., METALLURGIC. AND MATERIALS ENGINEERING, vol. 5, no. 4, 2011, pages 19 - 25
Attorney, Agent or Firm:
MAIKOWSKI & NINNEMANN PATENTANWÄLTE PARTNERSCHAFT MBB (Berlin, DE)
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Claims:
Claims

A process for oxidative coupling of methane (OCM, 100), comprising the steps of

- feeding methane (104) and at least one oxidizing agent (103), preferably oxygen, into at least one OCM reactor (101 ) comprising at least one OCM catalyst , wherein C2+ alkene is generated from methane by the OCM catalyst;

- wherein at least part of the methane (104) is provided by at least one methanation unit (1 1 1 ), in which hydrogen and carbon dioxide are reacted to form methane,

- wherein at least part of the hydrogen fed to the methanation unit (1 1 1 ) is provided from a hydrogen stream (21 1 ) generated in a propane dehydrogenation process (200), and/or

- wherein ethane (105) is fed into the at least one OCM reactor (101 ) comprising further at least one cracking unit, wherein C2+ alkene is generated from ethane in the cracking unit,

- wherein at least part of the ethane (105) is provided from ethane (213) generated in a propane dehydrogenation process (200).

Process according to claim 1 , characterized in that the at least one part of the hydrogen for the at least one methanation unit (1 1 1 ) is separated from a stream leaving the propane dehydrogenation reactor (201 ) comprising light gases, such as hydrogen and methane, and a product mixture comprising propane, propene, and lighter hydrocarbons such as ethane and ethene in at least one separating unit arranged downstream of the at least one propane dehydrogenation reactor (201 ). Process according to claim 2, characterized in that the light gases are separated from the product mixture in at least one cold box unit (202) as part of the at least one separating unit arranged downstream from the propane deydrogenation reactor (201 ).

Process according to claim 2 or 3, characterized in that the hydrogen is separated from the remaining light gases leaving the cold box (202) in a pressure swing adsorption unit (203) as a further part of the at least one separating unit and subsequently fed to the at least one methanation unit (1 1 1 ) as part of the OCM process.

Process according to one of the preceding claims, characterized in that the hydrogen stream (21 1 ) fed from the propane dehydrogenation process comprises hydrogen and methane.

Process according to one of the preceding claims, characterized that the ethane fed to the OCM reactor (101 ) is provided from at least one de-ethanizer unit (204) of a dehydrogenation process.

Process according to one of the preceding claims, characterized that the ethane fed to the cracking unit of the OCM reactor (101 ) is provided from at least one de-ethanizer unit (204) of a dehydrogenation process

Process according to claim 6 or claim, 7, characterized in that the stream (213) leaving the at least one de-ethanizer unit (204) comprises ethane and ethene.

A process system for oxidative coupling of methane (OCM) comprising

- at least one OCM reactor (101 ) comprising at least one OCM catalyst unit for generating C2+ alkene from methane and at least one cracking unit for generating C2+ alkene from ethane;

- at least one methanation unit (1 1 1 ) arranged downstream of the at least one OCM reactor (101 ) for generating methane from carbon dioxide and/or carbon monoxide and hydrogen, wherein the at least one methanation unit (1 1 1 ) is in fluid connection to at least one separating unit arranged downstream of a propane dehydrogenation reactor (201 ) for separating hydrogen from a product mixture comprising propane, propene and other hydrocarbons, and/or

- at least one fluid connection between the at least one OCM reactor (101 ) and at least one de-ethanizer unit (204) arranged downstream of the propane dehydrogenation reactor (201 ), wherein ethane separated in the at least one de-ethanizer unit (204) is fed to the OCM reactor (101 ), preferably to the cracking unit of the OCM reactor.

10. Process system according to claim 9, characterized in that the at least one cracking unit is arranged downstream of the OCM catalyst unit.

1 1 . Process system according to one of claims 9 to 10, characterized in that the at least one separating unit comprises at least one cold box (202) for separating light gases from the product mixture leaving the propane dehydrogenation reactor (201 ) and at least one pressure swing adsorption unit (203) for further separating hydrogen from the light gases leaving the pressure swing unit (203).

12. Process system according to one of the claims 9 to 1 1 , characterized in that the at least one de-ethanizer unit (204) is arranged downstream of the at least separating unit in the propane dehydrogenation cycle.

Description:
A process for oxidative coupling of methane (OCM) and a process system for such a process

The present invention relates to a process for oxidative coupling and a process system for conducting the same.

Description

OCM process

The oxidative coupling of alkanes, such as the oxidative coupling of methane, converts methane to valuable products such as, for example, ethane and ethene.

In the course of the oxidative coupling process methane is oxidized according to the following equation:

2 CH 4 + 0 2 → C 2 H 4 + 2 H 2 0

This reaction is exothermic and is typically conducted at high temperatures in the range between 750°C to 950^. In the course of the reaction methane is activated on the catalytic surface forming methyl radicals, which subsequently react in the gas phase with each other forming higher, long chain hydrocarbons such as ethane or dehydrogenation products thereof such as ethene. Suitable catalysts for OCM include various forms of iron oxide, manganese oxide, magnesium oxide, molybdenium oxide, cobalt oxide and others on various supports. A number of doping elements have also been proven to be useful in combination with the above catalysts.

The methane feedstock for the OCM reactor can be provided from various sources such as natural gas or from a methanizer in which carbon dioxide and hydrogen are reacted to form methane.

The OCM process provides C 2+ compounds comprising two or more carbon atoms that can include a variety of hydrocarbons such as hydrocarbons with saturated or unsaturated carbon- carbon bonds. Saturated hydrocarbons can include alkanes such as ethane, propane, butane, pentane and hexane. Unsaturated hydrocarbons include alkenes, such as ethene and propene (or propylene). C 2+ alkanes comprise saturated hydrocarbons with two or more carbon atoms such as ethane, propane, butane, and C 2+ alkenes comprise unsaturated hydrocarbons with two or more carbon atoms such as ethene, propene, butene and others. Ethene (or ethylene) in turn is an important chemical intermediate for the production of different plastics and compounds such as polyethylene plastics, polyvinylchloride, ethylene oxide, ethylbenzene, alcohols and other alkanes and alkenes.

Low costs of methane, especially in the US and Middle Eastern region allow the process to be operated economically despite the relatively low yields and high energy costs. Low costs for ethane steam cracking for providing ethene sets ethene market prices too low such that OCM process is not competitive.

However, transportation of ethene is expensive and in locations where ethene is not accessible through pipelines but methane is accessible OCM could become borderline competitive.

In order to increase the overall competiveness of an OCM process it may be of an advantage to combine and integrate the OCM process with another industrial process providing similar products.

One possible process to consider is the dehydrogenation of hydrocarbons. Alkane dehydrogenation process Dehydrogenation of hydrocarbons or alkanes, such as aliphatic hydrocarbons, to convert them into to their respective olefins is a well-known process. For example, the hydrocarbons propane, butane, isobutane, butenes and ethyl benzene are well known catalytically dehydrogenated to produce the respective propylene, butene, isobutene, butadiene and styrene. Dehydrogenation reactions are strongly endothermic and thus, an increase of the heat supply favours the olefin conversion.

In particular, dehydrogenation of paraffinic and other hydrocarbons such as propane dehydrogenation (reaction 1 ) or butane dehydrogenation (reaction 2) or i-butane dehydrogenation (reaction 3) are well known: C 3 H 8 QH 6 + ^ (1 )

C 4 H 0 <=>QH 6 +2H 2 (2) i— 4 H l0 i '— C H + H 2 (3)

Houdry CATOFIN ® and Oleflex™ are well known dehydrogenation processes where propene is produced from propane using a dehydrogenation catalyst. The product mixture leaving the propane dehydrogenation reactor comprises besides the main product propene also hydrogen, ethane, ethene and methane.

The product mixture leaving the propane dehydrogenation reactor comprises besides the main product propene also hydrogen, methane, ethane and small amounts of ethene.

It is an object of the present invention to provide an OCM process that is coupled or connected to a propane dehydrogenation process in order to reduce feedstock costs and to increase the overall efficiency of an OCM process. This object is being solved by a process for oxidative coupling of methane and a process system for conducting such a process as defined in the claims.

Accordingly, a process for oxidative coupling of methane (OCM) is provided, wherein process comprises the steps of

- feeding methane and at least one oxidizing agent, preferably oxygen, into at least one OCM reactor comprising at least one OCM catalyst unit, wherein C 2+ alkene is generated from methane by the OCM catalyst;

- wherein at least part of the methane is provided by at least one methanation which hydrogen and carbon dioxide are reacted to form methane,

- wherein at least part of the hydrogen fed to the methanation unit is provided from a hydrogen stream generated in a propane dehydrogenation process, and/or

- wherein ethane is fed into the at least one OCM reactor further comprising at least one cracking unit, wherein C 2+ alkene is generated from ethane in the cracking unit,

- wherein at least part of the ethane is provided from an ethane stream generated in a propane dehydrogenation process. Specifically a process is provided wherein hydrogen generated in the dehydrogenation reaction is fed into the oxidative coupling methane process, preferably for synthesizing methane from carbon dioxide (as side product of the oxidative coupling process) and hydrogen. It is also possible to feed ethane generated in the dehydrogenation reaction into the oxidative coupling process, preferably into the cracking unit or post-reactor of the oxidative coupling reactor for dehydrogenation (cracking) to ethene.

According to the present invention it is also possible to combine both, i.e. feeding hydrogen and ethane from the propane dehydrogenation process into the OCM process.

Thus, a process is provided that integrates at least one endothermic dehydrogenation process, preferably a propane dehydrogenation process, and at least one process for oxidative coupling of alkanes, preferably of methane.

This integration allows to reduce costs for methane feedstock in the OCM process by using hydrogen from the propane dehydrogenation process to convert carbon dioxide to methane. Furthermore, ethane and methane that are contained in the propane dehydrogenation off-gas can be directly used as OCM feedstock. By integrating the two cost intensive processes - propane dehyrogenation and OCM - it allows savings in both processes.

Typically, in an OCM process the effluent leaving the OCM reactor comprises at least one C 2+ compound (ethane, ethene), carbon dioxide, carbon monoxide, hydrogen and methane. The OCM effluent is subsequently separated into i) one first stream comprising at least a part of the C 2+ compounds and ii) a second stream comprising carbon monoxide (CO), carbon dioxide (C0 2 ), hydrogen (H 2 ) and methane (CH 4 ). At least a portion of the first stream comprising ethane as C 2+ compound is fed to the cracking unit in the OCM reactor, wherein at least a part of the stream comprising ethene is fed to at least one pre-heating unit before entering the cracking unit of the OCM reactor

The second stream is fed into at least one methanation unit to generate a first OCM reactor feed comprising CH 4 from H 2 and C0 2 and/or CO. Besides the second stream a third stream iii) comprising CH 4 and H 2 from a demethanizer unit (De-C1 ) is fed into the methanation unit. Methane synthesized in the at least one methanation unit is fed to at least one pre-heating unit before entering the OCM reactor. Besides methane from the methanation unit further methane for the OCM reactor stems from natural gas.

Hydrogen reacts with carbon monoxide and carbon dioxide in a methanation unit (as part of the OCM process) to methane in exothermic processes. The heat generated may be used as heat input to other process units or for preheating reactants such as methane and/or an oxidizing agent prior to an OCM reaction.

The (exothermic) methanation reaction can take place in two or more reactors in series. The methanation system can include one or more methanation reactors and heat exchangers. CO, C0 2 and H 2 can be added along various stream to the one or more methanation reactors. A compressor can be used to increase the C0 2 stream pressure up to the methanation operating pressure. In an embodiment the methanation unit comprises a first reactor and the second reactor that can be operated as adiabatic reactors. The adiabatic reactor can be in one stage or multiple stages depending on the concentration of CO, C0 2 and H 2 in the feed stream to the methanation unit. It is preferred if the methanation unit is at least one fixed-bed reactor with pre-heater, economizer and product cooler. Typical operating temperature is between 300-400 °C and a typical operating pressure is between 10-30 bar. The methanation reaction requires a suitable catalyst. For example, nickel-based catalysts can be used that may include nickel supported on alumina. Such nickel-based catalyst may be those used to produce SNG (substitute natural gas or synthetic natural gas) from syngas as for example from the KATALCO series. A methanation catalyst can be tableted or extruded. The shapes of such catalysts can be for example cylindrical, spherical or ring structure. Typically not all carbon dioxide recovered in the carbon dioxide removal unit of the OCM process can be converted to methane since there is not enough hydrogen produced in the OCM reaction for converting or carbon dioxide into methane. It is therefore desirable to add additional hydrogen to the methanation unit thus yielding methane and reducing the amount of fresh methane fed into the OCM process.

Thus, according to the invention hydrogen rich gas from other sources like a propane dehydrogenation process is introduced into the methanation unit.

As mentioned the OCM reactor comprises a catalyst unit and a cracking unit for generating unsaturated alkenes, preferably ethene, from ethane.

Ethane is thermally dehydrogenated via the following reaction:

This reaction is endothermic (ΔΗ = -144 kJ/mol) and can utilize the exothermic reaction heat produced during methane conversion in the OCM catalyst unit.

The OCM reactor cracking unit (or rather section) is an empty-tube post-reactor utilizing the exotherm from the actual OCM reaction. This post reactor is usually within the same reactor shell as the actual OCM catalyst bed. Typically the effluent of OCM catalyst bed is at temperatures above 1000 'C. Mixing ethane into the effluent of the OCM catalyst bed will result in thermal cracking of ethane and consequent reduction in temperature due to endothermic nature of thermal cracking reaction.

The typical source of ethane to be fed into the cracking unit of the OCM reactor comprises natural gas.

However, the transport of ethane in form of natural gas is expensive and has to be transported often over long distances.

Replacing at least parts of the ethane from another source such as a dehydrogenation process reduces the additional amount of ethane required from natural gases. By feeding ethane as a side product from the propane dehydrogenation product to the OCM cracking unit ethane is valorized in ethene and co-products.

In a dehydrogenation process propane is reacted in an endothermic dehydrogenation reactor in the presence of a suitable catalyst such as chromium oxide or Pt-Sn based catalyst. A typical Chromium oxide dehydrogenation catalyst manufactured on an alumina support comprises from about 17wt% to about 22 wt% Cr 2 0 3 . These type of dehydrogenation catalyst are known for instance under the name Catofin® Standard catalyst (US 2008/0097134 A1 ). The product gas mixture along with unreacted propane leaving the dehydrogenation reactor goes to at least one separating unit comprising at least one cold box unit and at least one pressure swing adsorption unit (psa unit). The light gases including hydrogen are separated in the cold box unit or cold section unit. The hydrogen from the light gases is then further separated from the remaining light gases using a pressure swing adsorption unit.

The liquid products from the cold box are sent further to at least one de-ethaniser where components such as ethane and ethene are removed at the top. Propene and the other heavier components are sent to a C3 splitter tower (for separating C3 hydrocarbons) where propene is obtained at the top. The bottom stream of C3 splitter is then fed to depropaniser where the components heavier than propane are removed at the bottom. The used catalyst afterthe dehydrogenation reactor is regenerated in a regeneration step.

As mentioned previously the present process is an integrated process, wherein the side- product gases hydrogen and ethane from an alkane dehydrogenation process (such as a propane dehydrogenation process) are fed into an OCM process. By integrating both systems it is possible to reduce the amount of methane and ethane that is additionally required for the OCM process. Furthermore, the amount of C0 2 that is otherwise vented to the outside in the OCM process can be drastically reduced. In an embodiment of the present process the at least one part of the hydrogen for the at least one methanation unit in the OCM process is separated from a stream leaving the propane dehydrogenation reactor in at least one separating unit arranged downstream of the at least one propane dehydrogenation reactor. The stream leaving the propane dehydrogenation reactor comprises light gases, such as hydrogen and methane, and a product mixture comprising propane, propene, and lighter hydrocarbons such as ethane and ethene. It is preferred, if the light gases comprising hydrogen are separated from the product mixture in at least one cold box unit as part of the at least one separating unit arranged downstream from the propane deydrogenation reactor. The hydrogen is separated from the remaining light gases leaving the cold box in a pressure swing adsorption unit (psa unit) as a further part of the at least one separating unit. The separated and partially purified hydrogen is subsequently fed to the at least one methanation feed or unit as part of the OCM process. The hydrogen stream leaving the separating unit of the propane dehydrogenation process comprises predominantly hydrogen and methane. However, further minor components such as propane, propene, ethane, carbon monoxide and carbon dioxided may also be present in said stream. If the pressure of hydrogen rich stream is higher than that of methanation reaction, the hydrogen rich stream is preferably fed directly to methanation reactor feed. If the pressure is lower, it is preferably fed to the process gas compressor inlet.

In another preferred embodiment of the present process at least a part of the ethane fed to the OCM reactor, preferably to the cracking unit, is provided from at least one de-ethanizer unit of a dehydrogenation process. Specifically, in a de-ethanizer of the dehydrogenation process C 2 components such as ethane that are lighter than propene and other C 3+ compounds are removed at the top of the de-ethanizer. Thus, the overhead of the de-ethanizer is fed into the OCM process.

The overhead or the stream leaving the at least one de-ethanizer unit of the dehydrogenation process comprises as major components ethane and ethene. The ethane stream can fed into the OCM system at different feed points after the C2 splitter and before the pre-heater. The present process is conducted in a process system for oxidative coupling of methane

(OCM) wherein the process system comprises

- at least one OCM reactor comprising at least one OCM catalyst unit for generating C 2+ alkene from methane and at least one cracking unit for generating C 2+ alkene from ethane; - at least one methanation unit arranged downstream of the at least one OCM reactor for generating methane from carbon dioxide and/or carbon monoxide and hydrogen, wherein the at least one methanation unit is in fluid connection to at least one separating unit arranged downstream of a propane dehydrogenation reactor for separating hydrogen from a product mixture (comprising propane, propene and other hydrocarbons), and/or

- at least one fluid connection between the at least one OCM reactor and at least one de-ethanizer unit arranged downstream of the propane dehydrogenation reactor, wherein ethane separated in at least one de-ethanizer unit is fed to the OCM reactor, preferably to the cracking unit of the OCM reactor.

In an embodiment of the present process system at least one cracking unit is arranged downstream of the OCM catalyst unit (as post-bed cracking unit). In a further embodiment of the present process system at least one separating unit as part of the dehydrogenation process system comprises at least one cold box for separating light gases from the product mixture leaving the propane dehydrogenation reactor and at least one pressure swing unit (psa) for further separating hydrogen from the light gases leaving the psa unit. Cold box unit and psa unit are in thereby in fluid communication with each other.

In yet another embodiment of the present process system the at least one de-ethanizer unit arranged downstream of the at least separating unit in the propane dehydrogenation cycle.

The invention is further described in more detail by means of examples with reference to the figures. It shows:

Figure 1 A a scheme of a conventional stand-alone OCM process;

Figure 1 B a scheme of a conventional Oleflex stand-alone propane dehydrogenation process; Figure 1 C a scheme of a conventional Catofin stand-alone propane dehydrogenation process;

Figure 2 a scheme of a first embodiment of the present process;

Figure 3 a scheme of a second embodiment of the present process;

Figure 4 a scheme of a third embodiment of the present process; Figure 5 a scheme of a fourth embodiment of the present process;

Example 1 :

Fig. 1 A shows a schematic overview of a conventional OCM process.

The OCM process system 100 comprises an OCM reactor unit 101 with an OCM catalytic unit and the post-bed cracking unit (PBC) for generating olefins (for example ethene) from alkanes (for example ethane and/or propane). OCM catalytic unit and PBC unit can be situated in separate reactors or can be integrated into the same reactor.

Methane (stream 104) and oxygen (stream 102) as oxidizing agent are injected into the catalytic unit and ethane can be injected into the PBC unit. In the catalytic unit methane is converted to C 2+ compounds and is subsequently directed to PBC unit in which one alkanes are converted to alkenes. The OCM effluent (stream 106) leaving the OCM reactor unit 101 is directed to a TLE and Quench tower (not shown) and further to a compression unit 107.

In the quench tower the OCM effluent gases are quenched with a cooling medium and any process condensates are condensed and removed. The cooled OCM effluent is then set to the compression unit 107, which can comprise a single or multiple stages of compression. The compression unit 107 can also comprise coolers and separator vessels which wasted pressure of the OCM effluent stream and water from the OCM effluent stream.

The product stream leaving the compression unit 107 is further transported to a carbon dioxide removal unit 1 10 which can remove carbon dioxide from the OCM product stream. At least a portion of the carbon dioxide (stream 1 12) can be directed to a methanation unit 1 1 1 . The other portion of the carbon dioxide can be directed for other uses (stream 1 13). The carbon dioxide removal unit 1 10 can comprise pressure swing absorption unit (PSA) or can be based on any other membrane separation processes. The effluent from the carbon dioxide removal unit can be treated (for example in the molecular sieve dryer).

Next the OCM product stream can be directed from the carbon dioxide removal unit 1 10 to a CDC and turboexpander unit (not shown). CDC (compression, drying, chilling) train in OCM first precools C0 2 -free gas from C0 2 removal, then removes any water by molecular sieve absorbents and the cools down the process gas in order to be able to separate methane from C 2+ by distillation. The Turboexpander is used to provide cooling for chilling unit by expanding the pressurized demethaniser overhead to lower pressure and consequently to lower temperature. This low temp methane stream is then used to cool down the process gas through a heat exchanger. The OCM product stream leaving the CDC / turboexpander unit is subsequently introduced to a demethanizer unit (De-C1 ) 1 14 which can separate or recover methane (and hydrogen) from higher molecular weight hydrocarbons (such as ethane, ethene, propene). The demethanizer unit 1 14 may include one or more distillation columns. The methane (stream 1 17) separated in the demethanizer unit 1 14 (and after PSA purge stream 1 15) can then be directed to the methanation unit 1 1 1 . In the methanation unit 1 1 1 further methane is generated from carbon dioxide, carbon monoxide and hydrogen. Methane generated in the methanation unit 1 1 1 can then be directed to the OCM catalytic unit 101 . In side reactions of the OCM significant amounts of hydrogen, carbon monoxide and carbon dioxide are formed and are thus contained in the OCM effluent stream. Hydrogen content in the effluent stream can range between 5% and about 15%, the content of carbon monoxide and carbon dioxide can range between one and 5%. In some cases this effluent stream is directly recycled to the OCM reactor 101 . However, if carbon monoxide and hydrogen are recycled to the OCM reactor 101 along with methane they can react with oxygen to produce carbon dioxide and water causing negative impact to the overall process. Thus, in order to make effectively use of the side products the stream comprising carbon dioxide, carbon monoxide and hydrogen is fed (after removal from the product stream in the carbon dioxide removal unit) to a methanation unit 1 1 1 . In the methanation unit 1 1 1 carbon monoxide and carbon dioxide react with hydrogen to methane in exothermic processes. The heat generated may be used as heat input to other process units or for preheating reactants such as methane and/or an oxidizing agent prior to an OCM reaction. The methanation reaction can take place in two or more reactors in series. In an embodiment the methanation unit 1 1 1 comprises a first reactor and the second reactor that can be operated as adiabatic reactors.

The methanation reaction 1 1 1 requires a suitable catalyst. For example, nickel-based catalysts can be used that may include nickel supported on alumina.

Methane synthesized in the methanation unit 1 1 1 is subsequently mixed and replenished with fresh methane from natural gas (stream 108). The mixed methane stream 104 enters then the catalytic unit of the OCM reactor 101 .

Higher molecular weight hydrocarbons separated from methane in the demethanizer unit 1 14 can then be directed to a deethanizer unit (De-C2 unit) 1 18. In the deethanizer unit 1 18 C 2 compounds (such as ethane and ethene) are separated from C 3+ compounds (such as propane and propene).

C2 compounds are then directed from the deethanizer unit 1 18 to a C2 splitter 121 which can separate ethane from ethene. The C2 splitter 121 can be a distillation column. The C2 splitter 121 can also be coupled to an acetylene converter 1 16 where acetylene (C 2 H 2 ) is reacted with hydrogen to generate ethane and/or ethene.

Recovered ethene (stream 122) can be employed for any downstream use (like polymer production) whereas ethane (stream 105) is subsequently recycled from the C2 splitter 121 to the OCM reactor unit 101 , preferably to the cracking unit (PBC unit). C 3+ compounds (stream 120) separated in the deethanizer unit 1 18 from the C2 compounds are further directed to a depropanizer unit (De-C3 unit) in which C3 compounds are separated from C 4+ compounds. The C 3 compounds stream comprises predominantly propene and propane.

The ethane (stream 105) recycled from the C2 splitter 121 to the OCM reactor unit 101 is mixed and replenished with fresh ethane form a natural gas source (stream 102). Recycled ethane and fresh ethane enter the reactor unit as combined streams.

Table 1 depicts the flow rate of the different streams in a conventional OCM process (as described for example in WO 2015/106023 A1 ).

Table 1 : Flow table OCM unit streams (in Ib/hr) Example 2:

Fig. 1 B is schematic view of the known Oleflex propane dehydrogenation process.

Fresh propane feed is mixed with the recycle propane feed to form a combined feed and fed to a fired heater (not shown). The heated feed is then reacts in dehydrogenation reactor 201 in the presence of a catalyst.

The product gas mixture along with unreacted propane goes to a compressor 210 and subsequently to the cold section or cold box 202 where light gases are separated. The hydrogen from the light gases is then separated from the remaining light gases using a pressure swing adsorption (PSA) unit 203. Hydrogen and the other light gases exit the dehydrogenation system 200 as stream 21 1 .

The liquid from the cold box 202 is then sent via a SHP reactor 206 to a de-ethaniser 204 where components lighter than propene (such as methane and ethane) are removed at the top for export (stream 213). Propene and the other heavier components are sent to a C3 splitter tower 205 where propene is obtained (stream 212).

The bottom stream of C3 splitter 205 is then fed to depropaniser (De-C3) 207 where the components heavier then propane (such as C 4+ compounds) are removed at the bottom (stream 209). Propane together with freshly injected propane (stream 203) is recycled back to the dehydrogenation reactor 201 .

The used catalyst from the dehydrogenation reactor 201 is regenerated in a regeneration section (not shown) and recycled back to the dehydrogenation reactor 201 .

Table 2 below depicts the composition of the stream leaving the dehydrogenation reactor in a conventional propane dehydrogenation process (see Chin et al., Int. J. Chem., Nucl., Metallurgic. and Materials Engineering; 201 1 , Vol. 5; No. 4, pages 19-25).

Table 2: PDH reactor exit stream

It is assumed that the components lighter than C2 (i.e. methane and H 2 ) leave the Cold box 201 as stream 21 1 . C2 components (i.e. ethane and ethene) leave the de-ethaniser 204 as stream 213 and propane leaves the propane-propene-splitter 205 as stream 212. Fig. 1 C depicts a scheme of the known Catofin propane dehydrogenation process 200.

The CATOFIN propane dehydrogenation process is a cyclic process where during regeneration and reduction steps, heat is supplied to the catalyst bed and during dehydrogenation step catalyst bed cools down due to the endothermic dehydrogenation reaction. Propylene production is normally controlled by equilibrium at the bottom section (US 2,419,997).

According to the scheme of Fig. 1 C fresh propane is mixed with the recycle propane feed to form a combined feed and fed to a fired heater (not shown). The heated feed is then reacts in dehydrogenation reactor 201 in the presence of a catalyst.

The product gas mixture along with unreacted propane goes to a compressor 210 and subsequently to the cold section or cold box 202 where light gases are separated. The hydrogen from the light gases is then separated from the remaining light gases using a pressure swing adsorption unit 203. Hydrogen and the other light gases exit the dehydrogenation system 200 as stream 21 1 and are typically used as fuel gas.

The liquid from the cold box 202 is then sent to a de-ethaniser 204 where components lighter than propene (such as methane and ethane) are removed at the top for export (stream 213). Propene and the other heavier components are sent to a C3 splitter tower 205 where propene is obtained (stream 212).

The bottom stream of C3 splitter 205 is recycled back then together with freshly injected propane (stream 203) to the dehydrogenation reactor 201 .

Example 3:

Fig. 2 illustrates a first embodiment of the present process.

As in the conventional OCM process 100 of Fig. 1 A ethane and oxygen enter the OCM reactor 101 . The OCM reactor effluent (stream 106) is fed into the C0 2 removal unit 1 10. Part of the removed C0 2 will be guided into the methanation unit 1 1 1 , whereas the remaining C0 2 part is vented out of the system (stream 1 13). Next the OCM product stream can be directed from the carbon dioxide removal unit 1 10 to further work up units such as a demethanizer unit (De-C1 ) 1 14 which can separate and recover methane from higher molecular weight hydrocarbons (such as ethane, ethene, propene). The recovered methane (stream 1 17) is fed to the methanation unit 1 1 1 . In the methanation unit 1 1 1 further methane is generated from carbon dioxide, carbon monoxide and hydrogen. Methane generated in the methanation unit 1 1 1 is then be directed to the OCM catalytic unit 101 . Additional hydrogen is now added to the methanation unit 1 1 1 . The additional hydrogen is part of the stream 21 1 leaving the separating unit (cold box unit 202) of the propane dehydrogenation process 200.

In addition ethane (stream 213) from the de-ethanizer unit 204 of the propane dehydrogenation process is united with the ethane recycled in the OCM process system. The ethane rich overhead of the de-ethanizer 204 comprises besides the main component ethane and ethene.

The OCM process 100 and the dehydrogenation system 200 are thus combined such that hydrogen and ethane as side products in a propane dehydrogenation process are tunneled into an OCM process.

Example 4

Fig. 3 illustrates a second embodiment of the present process. Reference is made to the schemes shown in Figs. 1 A, 1 B and 1 C.

Here, only the hydrogen stream 21 1 separated from the cold box unit 202 is tunneled to the methanation unit 1 1 1 in the OCM process. As a result the amount of fresh methane (natural gas stream 108) that is required can be reduced by 407.4 kg per ton PDH propylene capacity. Moreover, the amount of C0 2 that can be recycled in the OCM process is increased thereby reducing the C0 2 stream (stream 1 13) that is vented by 1 120.4 kg per ton PDH propylene capacity. Example 5

Fig. 4 illustrates a third embodiment of the present process. Reference is made to the schemes shown in Figs. 1 A, 1 B and 1 C.

Here, only the ethane-ethene stream (213) from the de-ethanizer unit 204 in the dehydrogenation process is fed into the OCM process. Stream 213 is united with the ethane stream from the C2-splitter 1 16 in the OCM process. As a result the amount of fresh ethane (stream 102) is reduced by 83.3 kg per ton PDH propylene capacity compared to a stand-alone OCM plant. Besides the amount of ethylene (stream 122) produced in the OCM process is increased by 9.3 kg per ton PDH propylene capacity. Example 6:

Fig. 5 illustrates a fourth embodiment of the present process. Reference is made to the schemes shown in Figs. 1 A, 1 B and 1 C. Here, both hydrogen stream (stream 21 1 ) from the separating unit and ethane stream (stream 213) from the de-ethanizer unit 204 in the dehydrogenation process are fed into the OCM process.

As a result the amounts of both fresh methane and ethane (streams 108 and 102) is reduced compared to a stand-alone OCM plant, ethane by 83.3 kg per ton PDH propylene capacity and methane by 407.4 kg per ton PDH propylene capacity.

Besides, the amount of C0 2 that can be recycled in the OCM process is increased thereby reducing the C0 2 stream (stream 1 13) that is vented by 1 120.4 kg per ton PDH propylene capacity. Furthermore, the amount of ethylene (stream 122) produced in the OCM process is increased by 9.3 kg per ton PDH propylene capacity.