EP3018190A1

EP3018190A1 - Process for production of methane rich gas

Info

Publication number: EP3018190A1
Application number: EP14191663.5A
Authority: EP
Inventors: Christian Wix
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 2014-11-04
Filing date: 2014-11-04
Publication date: 2016-05-11
Also published as: WO2016071192A1

Abstract

The invention relates to process for production of a methane rich product gas comprising the steps of:
(a) providing a feed gas comprising carbon oxide such as carbon monoxide and/or carbon dioxide, and hydrogen,
(b) reacting said feed gas in the presence of a catalyst in one or more methanation reactors with methanation catalyst, thereby forming a first gas rich in methane,
(c) recycling a first part of said first gas to said feed gas to the one or more methanation reactors,
(d) optionally, reacting a second part of said first gas rich in methane in one or more further methanation reactors of the one or more methanation reactors,
(e) withdrawing water from said second part of said first gas or from the gas resulting from reacting said second part of said first gas rich in methane in the one or more further methanation reactors, thereby forming a second gas,
(f) recycling a first part of said second gas to said feed gas to a first methanation reactor of the one or more methanation reactors,
(g) providing said methane rich product gas from a second part of said second gas,
wherein an ejector causes said recycling of said first part of said first gas in step (c) and a compressor causes said recycling of said first part of said second gas in step (f), and said ejector has a steam feed as motive gas and a recycled methane rich product gas as a suction gas.

Description

FIELD OF THE INVENTION

Embodiments of the invention generally relate to a process and a reactor system for production of a methane rich product gas. In particular, embodiments of the invention relate to a process and a reactor system for the production of substitute natural gas (SNG) from carbonaceous materials. Particularly the invention relates to a process for the production of SNG from a carbonaceous material in which the carbonaceous material is converted to a synthesis gas.

BACKGROUND

The low availability of fossil liquid and gaseous fuels such as oil and natural gas has revived the interest in developing technologies capable of producing combustible gas synthetically from widely available resources such as coal, biomass as well as off-gasses from coke ovens. The produced gas goes under the name substitute natural gas or synthetic natural gas (SNG) having methane as its main constituent.
Coke is a solid fuel produced from coal by baking the coal in an airless furnace. During coke production, volatile coal constituents are driven off, purified, and an off-gas comprising i.a. one or both of carbon dioxide and carbon monoxide, as well as hydrogen and hydrocarbons is produced. This coke oven off-gas is energy rich, and may often be combusted for generation of heat, e.g. for heating the coke furnace. However, especially when coke is produced as a solid fuel in a plant without other requirements for energy, excess off-gas may be available.
In relation to gasification of biomass or waste, similar gases comprising carbon oxides, hydrogen and hydrocarbons may also be produced.
In methanation processes the formation of methane from carbon oxides and hydrogen proceeds quickly to equilibrium in the presence of a methanation catalyst and in accordance with either or both of the following reaction schemes:

CO + 3H₂ <=> CH₄ + H₂O (1)

CO₂ + 4H₂ <=> CH₄ + 2H₂0 (2)
These reactions will be coupled to an equilibrium between carbon monoxide and carbon dioxide as follows:

CO + H₂O <=> CO₂ + H₂ (3)
The net reaction of methane formation whether by reaction (1) or (2) or both will be highly exothermic.
It is known from the field of steam reforming that catalysts may form carbon depending on the operating conditions and the actual catalyst formulation. Carbon may be formed on the catalyst either from methane, carbon monoxide, or higher hydrocarbons. The formation of carbon from methane and carbon monoxide may be expressed by the following reactions:

CH₄ <=> C(s) + 2H₂ (4)

2CO <=> C(s) + CO₂ (5)

CO + H₂ <=> C(s) + H₂O (6)
The carbon formed depends on the operating conditions and the catalyst. Typically, carbon on a Ni-catalyst is in the form of carbon whiskers. Carbon whiskers are described in the literature, see e.g. "Concepts in Syngas Manufacture" of Jens Rostrup-Nielsen and Lars J. Christiansen, "Catalytic Science Series - Vol. 10", 2011, pages 233-235. As mentioned, the choice of catalyst and operating conditions will determine whether or not carbon will form. According to the so-called principle of equilibrated gas, carbon will form if thermodynamics predict carbon formation from one or more of reactions (4-6) after equilibration of reactions (1-3). See for example the above referenced book, pages 247-252. Means to avoid carbon formation in this case include reducing the temperature and increasing the steam content in the feed gas to the reactor.
It should be pointed out that carbon may form in the form of whiskers or gum even if the principle of equilibrated gas does not predict carbon formation. This possibility depends on the actual catalyst and detailed operating conditions and is typically assessed based on experimental data.
Carbon may also form from higher hydrocarbons according to a reaction similar to reaction (4) as given below (for ethane):

C₂H₆ => 2C(s) + 3H₂
The carbon formed from higher hydrocarbons may also be in the form of whisker, graphite, or gum. It is a complex task to assess the risk of carbon formation from higher hydrocarbons. The risk of carbon formation in this case also depends upon the catalyst and the selected operating conditions. Also in this case, increasing the content of steam is one way to ensure operation out of the carbon forming operating conditions. In some cases the so-called critical steam to higher hydrocarbon ratio (S/HHC) can be used as an indicator of whether or not carbon will form on the catalyst.

SUMMARY OF THE INVENTION

Embodiments of the invention generally relate to a process for production of a methane rich product gas comprising the steps of:

(a) providing a feed gas comprising carbon oxide such as carbon monoxide and/or carbon dioxide, and hydrogen,
(b) reacting the feed gas in the presence of a catalyst in one or more methanation reactors with methanation catalyst, thereby forming a first gas rich in methane,
(c) recycling a first part of the first gas to the feed gas to the one or more methanation reactors,
(d) optionally, reacting a second part of the first gas rich in methane in one or more further methanation reactors of the one or more methanation reactors,
(e) withdrawing water from the second part of the first gas or from the gas resulting from reacting the second part of the first gas rich in methane in the one or more further methanation reactors, thereby forming a second gas,
(f) recycling a first part of the second gas to the feed gas to a first methanation reactor of the one or more methanation reactors,
(g) providing the methane rich product gas from a second part of the second gas. In the process, an ejector causes the recycling of the first part of the first gas in step (c) and a compressor causes the recycling of the first part of the second gas in step (f). The ejector has a steam feed as motive gas and a recycled methane rich product gas as a suction gas.

As used herein, the term "a first methanation reactor of the one or more methanation reactors" is meant to denote the most upstream methanation reactor of the one or more methanation reactors. As used herein, the term "methanation catalyst" is meant to denote any material, in any configuration, catalytically active in methanation. The term "catalyst" may also cover more than one material, so that the "one or more methanation reactors with methanation catalyst" may contain more than one kind of methanation catalyst, e.g. catalysts with different composition and/or form. One methanation reactor may comprise more than one kind of catalyst and/or different methanation reactors may comprise different kinds of catalyst.
We have found that the use of an ejector for driving the recycle of gas in step (c), viz. an inner recycling, is beneficial in combination with the use of a compressor for driving the recycle of gas in step (f), viz. an outer recycling. This is due to the effect of increased steam addition via an ejector in combination with the recycling of step (f) taking place after removal of water from the gas to be recycled. When the recycling in step (f) takes place after water removal from the gas to be recycled, the temperature of the gas may be relatively low. Thus, the overall system may be cheaper and/or the compressor may be more reliable than compressors arranged for higher temperatures. The latter is due to the volume of the gas to be compressed being lower due to the lower temperature. The combination of steam addition via the ejector and recycle of gas after water removal has the effect of providing a water content ensuring a reduced risk of deactivation of the methanation catalyst due to carbon formation.
In an embodiment of the invention, step (b) of reacting the feed gas takes place in one or two reactors with methanation catalyst in series. Thus, the recycling of the first part of the first gas may take place after reaction in one or two methanation reactors, with optional further methanation reactors downstream the recycling. In an alternative embodiment, the first methanation reactor is a boiling water reactor. In this case, no further methanation reactors are necessary in step b) downstream the boiling water reactor, even though the embodiment may comprise one or more optional methanation reactors of step (d).
In an embodiment, the methanation catalyst comprises nickel as a catalytically active constituent.
In an embodiment, the methanation catalyst is provided on a support comprising alumina. The support may further comprise one or more constituents from the group consisting of MgAl spinel, alumina-zirconia, and calcium aluminates.
In an embodiment, step (a) is preceded by a gas purification step wherein at least sulfur is removed from the feed gas. Herein, the term "at least sulfur" is meant to cover components and compounds comprising sulfur, such as e.g. hydrogen sulfide H₂S, COS, CS₂, thiophene, or mercaptans. The gas purification step may be arranged to remove further elements from the gas, such as for example chlorine, arsenic, oxygen and/or olefins.
The term "sulfur is removed" is meant to denote that some sulfur is removed. The term is thus not meant to indicate that no sulfur is left after sulfur removal.
In an embodiment, the feed gas to the first methanation reactor is formed by combining:

a fresh synthesis gas, optionally subsequent to subjecting it to a gas purification step wherein at least sulfur is removed,
the steam added by the ejector,
the first part of the first gas from the recycling of step (c), and
the first part of the second gas from the recycling of step (f).

In an embodiment, step (d) comprises reacting the second part of said first gas rich in methane together with fresh synthesis gas in the second methanation reactor of the one or more methanation reactors. This embodiment is called a split-flow reaction, in that the fresh synthesis gas is split between the first and second methanation reactor.
In an embodiment, the fresh synthesis gas is a gas generated from a carbonaceous material selected from the group of: coke, coal, petroleum coke, biomass, oil, black liquor, waste and combinations thereof. Petroleum coke is also denoted "petcoke".
In an embodiment, the fresh synthesis gas further comprises at least 0.1 vol%, at least 0.2 vol% or at least 1 vol% C₂₊ hydrocarbons. The term "C2+ hydrocarbons" is meant to denote any hydrocarbon or hydrocarbonaceous gas comprising at least two carbon atoms, also denoted "higher hydrocarbons" (abbreviated to "HHC"). Examples of such C₂₊ hydrocarbons, viz. C₂-, C₃- or C₄-hydrocarbons, are for example ethane, propane, butane. In an embodiment, the fresh synthesis gas further comprises between 0 and 30 vol%, methane CH₄. As an example, the fresh synthesis gas could comprise between 4 and 18 vol% CH₄, such as between 10 and 17 vol% CH₄.
In an embodiment, a stream comprising carbon oxides is added to the gas downstream the first methanation reactor. The carbon oxides are e.g. added upstream the second methanation reactor or the carbon oxides are mixed with the second part of the second gas, e.g. upstream one or more final methanation reactor(s). This is advantageous in case of addition of a relatively large amount of carbon dioxide. Preferably, the stream comprising carbon oxides is a substoichiometric stream with a ratio (H₂-CO₂)/(CO-CO₂) < 3.
In an embodiment, step (g) is preceded by the step of reacting the second part of the second gas in the presence of a methanation catalyst in one or more final methanation reactors in order to provide the methane rich product gas. In an embodiment, step (g) further comprises the step of separating water from the third gas, thereby forming the methane rich product gas. In this case, a stream comprising carbon oxides could be added to the second part of the second gas upstream the final methanation reactor, in order to control the quality of the product gas.
Embodiments of the invention further comprise cooling the gas output from one or more of the methanation reactors.
In an embodiment, the feed gas entering said first methanation reactor has a temperature of between 280°C and 380°C, wherein the first gas exiting from the first methanation reactor has a temperature in the range from 500°C to 750°C. The gas exiting subsequent methanation reactors will have a temperature equal to or lower than the temperature of the first gas exiting from the first methanation reactor.
In an embodiment, at least one methanation reactor of the one or more methanation reactors used in step (b) further comprises a layer of shift catalyst directly upstream the methanation catalyst. When using a downwards direction of the gas flow within the methanation reactor, the shift catalyst thus forms a layer directly on top of the methanation catalyst. The shift catalyst may be a conventional shift catalyst. Such conventional shift catalysts typically comprise at least two of the metals Cu, Zn and Cr, optionally in the form of oxides and optionally on a carrier.
The weakly exothermic shift process (reaction (3)) will heat the feed gas a little and partly convert carbon monoxide. When the gas is hereafter passed over the methanation catalyst, any tendency to nickel carbonyl formation has been substantially removed because of the weak temperature increase and lower CO-contents.
By using a shift catalyst directly upstream the methanation catalyst, the inlet temperature to the first methanation reactor may be decreased compared to a situation without shift catalyst. Thus, the inlet temperature to the first methanation reactor may e.g. be 250°C or even lower, whilst the first gas exiting from the first methanation reactor has a temperature in the range 500-750°C.
In an embodiment, the water withdrawal in step (e) is carried out by condensation at a temperature of at least about 80°C.
Another aspect of the invention relates to a reactor system for production of a methane rich product gas from a feed gas, where the reactor system comprises:

(a) one or more methanation reactors with methanation catalyst,
(b) a feed line arranged for providing the feed gas into a first methanation reactor of the one or more methanation reactors, where the feed gas comprises carbon oxide such as carbon monoxide and/or carbon dioxide, and hydrogen, and wherein the one or more methanation reactors is/are arranged to react the feed gas in the presence of the methanation catalyst, thereby forming a first gas rich in methane,
(c) a first recycle line arranged to recycle a first part of the first gas to the one or more methanation reactors,
(d) a separator arranged to withdraw water from a second part of the first gas or from the gas resulting from reacting the second part of the first gas rich in methane in one or more further methanation reactors, thereby forming a second gas, (e) a second recycle line arranged to recycle a first part of the second gas to the first methanation reactor,

An ejector is arranged to cause the recycling in the first recycle line and a compressor is arranged to cause the recycling of the second recycle line. The ejector is configured for having a steam feed as motive gas and a recycled methane rich product gas as a suction gas.
In an embodiment, a separator is arranged to withdraw the methane rich product gas from a second part of the second gas, subsequent to a further methanation reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are explained, by way of example, and with reference to the accompanying drawings. It is to be noted that the appended drawings illustrate only examples of embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

Figure 1 illustrates a methanation process with an ejector according to prior art.
Figure 2 illustrates a methanation process with a compressor according to prior art.
Figure 3 illustrates a methanation process with an ejector and a compressor according to the invention.
Figure 4 illustrates a methanation process with an ejector and a compressor according to the invention, the process comprising a split-flow around the first methanation reactor.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention.
Furthermore, in various embodiments the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to "the invention" shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).
The described embodiments are examples only and are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
Figure 1 illustrates a methanation process 100 with an ejector according to prior art.
The methanation process shown in figure 1 relates to a methanation process 100 with four adiabatic methanation reactors 124, 134, 140, 162 and an ejector 118.
A fresh synthesis gas 102 is heated 104 and led to a sulfur guard 112 in addition with steam 108, providing a desulfurized synthesis gas 114. The desulfurized synthesis gas 114 is mixed with a mixture of steam and methane rich recycled gas 120 in order to obtain a first methanation reactor feed gas 122 to be inlet to a first methanation reactor 124.
The first methanation reactor feed gas 122 is directed to a first methanation reactor 124, providing a first methane rich gas 126, which is cooled in a heat exchanger 128. In figure 1, a part of the first methane rich gas 126 is recycled around the first methanation reactor 124, driven by an ejector 118 with steam 110 as a motive gas.
The part 132 of the first methane rich gas which is not recycled, is allowed to react further in a second methanation reactor 134 and in a third methanation reactor 140 with intermediate cooling 138 of a second stage methane rich gas 136 from the second methanation reactor 134 and intermediate cooling 144 of a third stage methane rich gas 142 from the third methanation reactor 140.
Prior to a fourth and final methanation reactor 162, water 148 is condensed, after cooling 144, in a separator 146 in order to shift the reaction equilibrium of a final methanation feed gas 150. The final methanation feed gas 150 is subsequently heated in 160 and led to a final methanation reactor 162 resulting in a final stage methane rich gas 164. The final stage methane rich gas 164 led from the final methanation reactor 162 is cooled 166 and led to a separator 168 in order to separate water 170 from the final stage methane rich gas 164. The separator 168 is arranged to separate water 170 from the final stage methane rich gas and thereby produce a synthetic natural gas 172.
Figure 2 illustrates a methanation process 200 with a compressor according to prior art.
The methanation process shown in figure 2 relates to a methanation process 200 with four adiabatic methanation reactors 224, 234, 240, 262 and a compressor 254.
A feedstock gas 202, e.g. a synthesis gas, is heated 204 and led to a sulfur guard 212 in addition with steam 208, providing a desulfurized synthesis gas 214. The desulfurized synthesis gas 214 is mixed with a mixture of steam 210 and methane rich gas recycled 256 in order to obtain a first methanation feed gas 222 to be inlet to a first methanation reactor 224.
The first methanation feed gas 222 is directed to a first methanation reactor 224, providing a first stage methane rich gas 226, which is cooled in a heat exchanger 228. Subsequently, the cooled, first methane rich gas 226 is driven to a second methanation reactor 234, providing a second stage methane rich gas 236, and thereafter to a third methanation reactor 240, with intermediate cooling the second stage methane rich gas 236.
Prior to a fourth and final methanation reactor 262, water 248 is condensed, after cooling 244, in a separator 246 in order to shift the reaction equilibrium of a final methanation feed gas 250.
In the process 200 of figure 2, a part of the final methanation feed gas 250 is recycled as recycled methane rich gas 252 to the first methanation 224, via a cold compressor 254, and heated by means of a heat exchanger 255, providing the recycle stream 256. The part 258 of the final methanation feed gas not recycled is heated 260 and allowed to react further in a fourth and final methanation reactor 262 resulting in a final stage methane rich gas 264. The final stage methane rich gas 264 is cooled 266 and led to a separator 268 in order to separate water 270 from the final stage methane rich gas 264 and thereby produce a synthetic natural gas 272.
Figure 3 illustrates a methanation process 300 with an ejector and a compressor according to the invention.
The methanation process 300 shown in Figure 3 is a methanation process with four adiabatic methanation reactors 324, 334, 340, 362, an ejector 318 and a compressor 354.
A feedstock gas 302, e.g. a synthesis gas comprising carbon monoxide and/or carbon dioxide, and hydrogen, is heated 304 and directed to a sulfur guard 312 in addition with steam 308 from a source 306 or steam, providing a desulfurized feedstock gas 314. It should be noted, however, that if the methanation catalysts in the methanation reactors 324, 334, 340, 362 are insensitive to sulfur or if sulfur is absent in the feedstock gas, the process 300 could do without the sulfur guard 312.
The desulfurized synthesis gas 314 is mixed with a mixture 320 of steam, a first recycle stream 330 of methane rich gas and a second recycle stream 356 of methane rich gas in order to obtain a feed gas 322 for the first methanation reactor. The first recycle is driven by an ejector 318 with steam 310 from the steam source 306 as a motive gas.
The feed gas 322 is directed to a first methanation reactor 324, providing a first gas 326 rich in methane; this first methane rich gas 326 is subsequently cooled in a heat exchanger 328.
As mentioned above, the first part 330 of the first methane rich gas 326 is recycled back to the first methanation reactor 324. This first part 330 of the first gas is driven together with steam 310 by the ejector 318, providing the first recycle stream 320. The part 332 of the first methane rich gas 326 which is not recycled is allowed to react further in a second methanation reactor 334 providing a second stage methane rich gas 336 and is cooled 338. Subsequently, the cooled second stage methane rich gas 336 is allowed to react further in a third methanation reactor 340 providing a third stage methane rich gas 342 which is cooled 344. Subsequently, the cooled third stage methane rich gas is led to a separator 346 in order to condense water 348, resulting in a second gas 350 rich in methane. The separation of water from the cooled third stage methane rich gas 342 ensures that the equilibrium is shifted in further methanation reactor.
A first part 352 of the second gas 350 is recycled to the feed gas to the first methanation reactor 324. The recycling of the first part 352 of the second gas 350 is driven or caused by a cold compressor 354. The first part 352 of the second gas 350 is heated by a heat exchanger 355, providing a second recycle stream 356 of methane rich gas. The second recycle stream of methane rich gas 356 is mixed with the desulfurized synthesis gas 314 and with a first recycle of methane rich gas 320 at or upstream of an inlet to the first methanation reactor 324 with methanation catalyst.
In a system without a de-sulfurisation unit 312, for example in a case where the fresh synthesis gas contains no or only very small amounts of sulfur, the fresh synthesis gas 302 may be provided directly to the first methanation reactor 324 together with steam added by the ejector 318, the first part 330 of the first methane rich gas 326 recycled and the second recycle stream of methane rich gas 356.
A second part 358 of the second gas, i.e. the part of methane rich gas 350 not recycled, is heated 360 and allowed to react further in a fourth and final methanation reactor 362. A final stage methane rich gas 364 exiting the fourth reactor 362 is cooled 366 and led to a separator 368. In the separator 368, water 370 is separated from the cooled final stage methane rich gas 364 and the remaining gas 372 is the methane rich product gas 372 in the form of a synthetic natural gas 372.
Figure 4 illustrates a methanation process 400 with an ejector 418 and a compressor 454 according to the invention, the process comprising a split-flow around the first methanation reactor 424.
The methanation process 400 shown in figure 4 is a methanation process with five adiabatic methanation reactors 424, 434, 440, 444, 462, an ejector 418 and a compressor 454.
A feedstock gas 402, e.g. a synthesis gas comprising carbon oxides and hydrogen, is heated in a heat exchanger 404 and directed to a sulfur guard 412 in addition with steam 408, providing a desulfurized synthesis gas 414. The desulfurized synthesis gas 414 is heated in a heat exchanger 413. It should be noted, however, that if the methanation catalysts in the methanation reactors 424, 434, 440, 444, 462 are insensitive to sulfur or if sulfur is absent in the feedstock gas 402, the process 400 could do without the sulfur guard 412.
The desulfurized synthesis gas 414 is split into a first part 415 and a second part 421. The first part 415 of the desulfurized synthesis gas is mixed with a second recycle stream 456 of methane rich gas in order to provide stream 416. The mixed stream 416 is mixed with a mixture 420 of steam, a first recycle stream 423 of methane rich gas in order to obtain a feed gas 422 for the first methanation reactor 424. The first recycle is driven by an ejector 418 with steam 410 from a steam source 406 as a motive gas. The second recycle is driven by a cold compressor 454.
The feed gas 422 is directed to a first methanation reactor 424, providing a first methane rich gas 426. The first methane rich gas 426 is subsequently cooled in a heat exchanger 425.
As mentioned above, the first part 423 of the first methane rich gas 426 is recycled back to the first methanation reactor 424. This first part 423 of the first gas is driven together with steam 410 by the ejector 418, providing the first recycle stream 420. The part 427 of the first methane rich gas 426, which is not recycled, is mixed with a second part 421 of the desulfurized synthesis gas 414. The mixture 428 of the non-recycled part 427 of the first methane rich gas and the second part 421 of the desulfurized synthesis gas is allowed to react further in a second methanation reactor 434, a third methanation reactor 440 and a fourth methanation reactor 444 with intermediate cooling of the second stage 437 and third stage 443 methane rich gas, by means of the heat exchangers 436, 442. The fourth stage methane rich gas 446 exiting from the fourth methanation reactor 444 is cooled by the heat exchanger 445.
Subsequently, the cooled fourth stage methane rich gas 446 is led to a separator 447 in order to condense water 448, resulting in a second gas 450 rich in methane. The separation of water from the cooled fourth stage methane rich gas 446 ensures that the equilibrium is shifted in a further methanation reactor.
A first part 452 of the second gas 450 is recycled to the feed gas to the first methanation reactor 424. The recycling of the first part 452 of the second gas 450 is driven or caused by a cold compressor 454. The first part 452 of the second gas 450 is heated by a heat exchanger 455, providing a second recycle stream 456 of methane rich gas. The second recycle stream of methane rich gas 456 is mixed with the first part 415 of the desulfurized synthesis gas 414 and with a first recycle of methane rich gas 423 at or upstream of an inlet to the first methanation reactor 324 with methanation catalyst.
In a system without a de-sulfurisation unit 412, for example in a case where the fresh synthesis gas contains no or only very small amounts of sulfur, a first part of the fresh synthesis gas 402 may be provided directly to the first methanation reactor 424 together with steam added by the ejector 418, the first part 430 of the first methane rich gas 423 recycled and the second recycle stream of methane rich gas 456, whilst a second part of the fresh synthesis gas 402 may be provided to the second methanation reactor 434.
A second part 458 of the second gas, i.e. the part of methane rich gas 450 not recycled, is heated in a heat exchanger 460 and allowed to react further in a fifth and final methanation reactor 462. A final stage methane rich gas 464 exiting the fifth reactor 462 is cooled in a heat exchanger 466 and led to a separator 468. In the separator 468, water 470 is separated from the cooled final stage methane rich gas 464 and the remaining gas 472 is the methane rich product gas 472 in the form of a synthetic natural gas 472.

Comparison between the processes in figures 1, 2, 3 and 4:

FRESH SYNTHESIS GAS:

The composition of the fresh synthesis gas used in the processes shown in of Figure 1, Figure 2 and Figure 4 corresponds to that of the fresh synthesis gas used in the first example, the example "Ex I", of figure 3. For the process shown in figure 3, another example, "Ex II", the composition of final SNG product has also been calculated for a gas with another composition, as indicated in Table 1.

Table I

Fresh synthesis gas composition	Nm³/h	Mole%	Nm³/h	Mole%
	Fig. 3 - Ex I Fig 1, 2, 4		Fig. 3 - Ex II
Ar	-	-	465	0.10
C2+	226	0.22	1395	0.3
CH₄	8372	8.23	70663	15.20
CO	20795	20.43	89165	19.18
CO₂	1889	1.86	6136	1.32
H₂	70169	68.94	296132	63.70
N ₂	322	0.32	465	0.1
O₂	-	-	465	0.1
H₂O	-	-	-	-
Total	101773	100	464886	100
Molar weight (g/mol)	9.41		9.86

It is seen from Table I, that the fresh synthesis gas used in Example II of Figure 3 is somewhat harsher, due to the higher content of C2+ gas. In Ex I the C2+ gas is substantially exclusively ethane C₂H₆, whilst the C2+ gas in Ex II of figure 3 comprises two thirds ethane C₂H₆ and one third propane C₃H₈.

PRODUCT QUALITY:

Tables 2 and 3 below show the low heating value and the composition of the final methane rich product gas from the processes shown in figure 1-4, with the fresh synthesis gas as indicated in Table 1.

Table 2

	Final SNG Product - Low Heating Value
	Figure 1	Figure 2	Figure 3 Ex I	Figure 3 Ex II	Figure 4
LHV [kcal/N m³]	8,273	8,383	8,384	8,436	8,393

Table 3

	Final SNG Product - Composition (mole%)
	Figure 1	Figure 2	Figure 3 Ex I	Figure 3 Ex II	Figure 4
CH₄	96.0	97.7	97.7	97.5	97.5
CO	80 ppm	10 ppm	9 ppm	1 ppm	9 ppm
CO₂	0.5	0.2	0.2	0.01	0.2
H₂	2.2	0.8	0.8	1.4	0.9
N₂	1.0	1.0	1.0	0.3	1.0
H₂O	0.3	0.3	0.3	0.3	0.3

Tables 2 and 3 above show that the process 100 shown in figure 1, having an ejector for recycling around the first methanation reactor 124, results in a final product gas comprising 95.5 mole% of CH₄ and having a Lower Heating Value (LHV) of 8,226 kcal/Nm³.
Moreover, the tables show that the process 200 of figure 2 comprising a cold compressor for recycling methane rich gas from a third methanation reactor 240 to the first methanation reactor 224, results in a final product gas comprising 97.4 mole% of CH₄ and having a LHV of 8,359 kcal/Nm³. Table 4

Ejector (figure 1) Compressor only (fig 2) Compressor and ejector (figure 3) Split flow (figure 4)

Shaft power 0 kW 614 kW 415 kW 277 kW

% CH4, wet 96 97.7 97.7 97.5
Table 4 indicates the shaft power necessary for driving the compressor in the examples shown in figures 1-4 as well as the methane content of the product gas. It is clear that the embodiments of the invention shown in figures 3 and 4 provides a product gas with a comparable methane content to the embodiment shown in figure 2, whilst the power consumption for driving the process is reduced considerably. Compared to the embodiment where only an ejector is used, figure 1, the power consumed for driving the processes of the invention, shown in figures 3 and 4, is higher; however, the methane content in the product gas is also higher.
Finally, it is seen that the processes 300, 400 of the invention shown in figures 3 and 4 and including both an ejector recycle around the first methanation reactor 324, 424 and a cold compressor recycle from the third or fourth methanation reactor 340, 444 to the first methanation reactor, results in a final product comprising 97.5 mole% of CH₄ and having a LHV similar to that of the example shown in figure 2, while the energy usage and thus the expenses of operation of the processes 300 and 400 are lower than that of figure 2 as seen in table 3 below

CARBON LIMITS:

It is well known that the carbon limit for a methanation reactor depends upon i.a. the catalyst used and the composition of the fresh synthesis gas. In addition to the above indicated calculation on the product quality obtained by the processes shown in figure 1-4 (for two gas compositions in relation to the process shown in figure 3), the applicant has carried out calculations on the carbon limit for the catalyst in the embodiment shown in figure 3 for the two different compositions of fresh synthesis gas indicated in table 1 above.
These calculations on the carbon limit show that the carbon limits for the first methanation reactor 324 in Figure 3 are comparable for the two different gas compositions shown in table 1 above.
Moreover, the carbon limit of the first methanation reactor 224, 424 of figures 2 and 4, respectively, is comparable to that of the first methanation reactor 324 of figure 3 (for both gas compositions of Ex I and Ex II). Even though the carbon limit of the first methanation reactor of the processes of figures 2-4 is somewhat lower than that of the process shown in figure 1, at least partly due to lower water content, the carbon limit of the first methanation reactor of the processes in figures 2-4 is sufficient. Thus, it is shown that the process of the invention whereby an ejector adding steam is used together with a cold compressor to drive two independent recycle streams is suitable for the production of methane rich gas whilst controlling the extent of carbon formation. It should be noted, that the process of figure 3 even the harsher gas of Ex II
While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.

Claims

A process for production of a methane rich product gas comprising the steps of:
(a) providing a feed gas comprising carbon oxide such as carbon monoxide and/or carbon dioxide, and hydrogen,

(b) reacting said feed gas in the presence of a catalyst in one or more methanation reactors with methanation catalyst, thereby forming a first gas rich in methane,

(c) recycling a first part of said first gas to said feed gas to the one or more methanation reactors,

(d) optionally, reacting a second part of said first gas rich in methane in one or more further methanation reactors of the one or more methanation reactors,

(e) withdrawing water from said second part of said first gas or from the gas resulting from reacting said second part of said first gas rich in methane in the one or more further methanation reactors, thereby forming a second gas,

(f) recycling a first part of said second gas to said feed gas to a first methanation reactor of the one or more methanation reactors,

(g) providing said methane rich product gas from a second part of said second gas,
wherein an ejector causes said recycling of said first part of said first gas in step (c) and a compressor causes said recycling of said first part of said second gas in step (f), and said ejector has a steam feed as motive gas and a recycled methane rich product gas as a suction gas.
A process according to claim 1, wherein the step (b) of reacting said feed gas takes place in one or two methanation reactors with methanation catalyst in series.
A process according to any of the claims 1 to 2, wherein said methanation catalyst comprises nickel as a catalytically active constituent.
A process according to claim 3, wherein said methanation catalyst is provided on a support comprising alumina.
A process according to claim 4, wherein said support further comprises one or more constituents from the group consisting of MgAl spinel, alumina-zirconia, and calcium aluminates.
A process according to any of the claims 1 to 5, wherein step (a) is preceded by a gas purification step wherein at least sulfur is removed from said feed gas.
A process according to any of the claims 1 to 6, wherein said feed gas to the first methanation reactor is formed by combining:
- a fresh synthesis gas, optionally subsequent to subjecting it to a gas purification step wherein at least sulfur is removed,

- said steam added by said ejector,

- said first part of said first gas from the recycling of step (c), and

- said first part of said second gas from the recycling of step (f).
A process according to any of the claims 1 to 7, wherein step (d) comprises reacting said second part of said first gas rich in methane together with fresh synthesis gas in said second methanation reactor of the one or more methanation reactors.
A process according to claim 7 or 8, wherein said fresh synthesis gas is a gas generated from a carbonaceous material selected from the group of: coke, coal, petroleum coke, biomass, oil, black liquor, waste and combinations thereof.
A process according to any of the claims 7 to 9, wherein said fresh synthesis gas further comprises at least 0.1 vol%, at least 0.2 vol% or at least 1 vol% C2+ hydrocarbons.
A process according to any of the claims 7 to 10, wherein said fresh synthesis gas further comprises between 0 and 30 vol% methane CH₄, such as between 4 and 18 vol% methane CH₄, such as 10 and 17 vol% methane CH₄.
A process according to any of the claims 1 to 11, wherein a stream comprising carbon oxides is added to the gas downstream the first methanation reactor.
A process according to any of the claims 1 to 12, wherein step (g) is preceded by the step of reacting said second part of said second gas in the presence of a catalyst in one or more final methanation reactor(s) with methanation catalyst in order to provide said methane rich product gas.
A process according to any of the claims 1 to 13, wherein the feed gas entering said first methanation reactor has a temperature of between 280°C and 380°C wherein the first gas exiting from the first methanation reactor has a temperature in the range 500°C to 700°C.
A process according to any of the claims 1 to 13, wherein at least one methanation reactor of the one or more methanation reactors used in step (b) further comprises a layer of shift catalyst directly upstream the layer of methanation catalyst.
A process according to any of the claims 1 to 15, wherein the water withdrawal in step (e) is carried out by condensation at a temperature of at least about 80°C.
A reactor system for production of a methane rich product gas from a feed gas, said reactor system comprising:
(a) one or more methanation reactors with methanation catalyst,

(b) a feed line arranged for providing said feed gas into a first methanation reactor of said one or more methanation reactors, where said feed gas comprises carbon oxide such as carbon monoxide and/or carbon dioxide, and hydrogen, and wherein said one or more methanation reactors is/are arranged to react said feed gas in the presence of said methanation catalyst, thereby forming a first gas rich in methane,

(c) a first recycle line arranged to recycle a first part of said first gas to said one or more methanation reactors,

(d) a separator arranged to withdraw water from a second part of said first gas or from the gas resulting from reacting said second part of said first gas rich in methane in one or more further methanation reactors, thereby forming a second gas,

(e) a second recycle line arranged to recycle a first part of the second gas to said first methanation reactor,
wherein an ejector is arranged to cause the recycling in the first recycle line and a compressor is arranged to cause the recycling of the second recycle line, and wherein said ejector is configured for having a steam feed as motive gas and a recycled methane rich product gas as a suction gas.
A reactor system according to claim 17, said reactor system further comprising a separator arranged to withdraw the methane rich product gas from a second part of said second gas, subsequent to a further methanation reactor.