TECHNICAL FIELD

The present invention relates to production of methane-rich gas from syngas, especially, to a system for producing methane -rich gas from syngas and a process for producing methane rich gas using the same. BACKGROUND

Methanation is one of the key steps to convert solid carboneous materials such as coal and biomass into synthesis natural gas (or substitute natural gas, SNG). In this process, a coal or biomass gasification product gas stream rich in carbon monoxide, carbon dioxide, and hydrogen (usually called synthesis gas or syngas) is converted into methane -rich gas as pipeline-quality product by the following reversible reactions:

CO + 3 H ₂ ^> CH ₄ + H ₂ 0 (Rxn 1)

2 CO + 2 H ₂ ^> CH ₄ + C0 ₂ (Rxn 2)

CO + H ₂ 0 ^> C0 ₂ + H ₂ (Rxn 3)

While conventional methanation processes are based on Rxn 1, requiring a H2/CO ratio of approximately 3:1, sour methanation is mainly based on Rxn 2, requiring a 1 : 1 molar ratio H ₂ /CO. Sour methanation offers benefits over conventional ones including: 1) less H ₂ is required in the feed gas and therefore less pre-treatment of raw gas; 2) some catalyst for sour methanation show high sulfur-resistance thus the pre-desulfurization could be eliminated in some specific conditions; and 3) no carbon coking on catalyst occurs as in conventional processes, so catalyst lifetime is longer.

The methanation reactions are reversible. According to thermodynamics, existence of C0 ₂ tends to shift the reaction equilibrium towards the left side, and drive the reaction in a direction not in favor of methane formation. Therefore, C0 ₂ is an inhibitor of CH ₄ production, which decreases the reaction rate as well as the maximum conversion of outcome. In conventional industrial process, as C0 ₂ accumulates during the methanation proceeds, the reaction rate gradually slows down, and the conversion of outcome is significantly reduced.

C0 ₂ formed during sour methanation doesn't only bring about thermodynamic limitation in the system. C0 ₂ produced during the methanation exits the system together with CH ₄ as side product. It must be removed with various methods known to those skilled in the art, such as Selexol, MDEA, lime sorption, etc. Such independent removal of C0 ₂ , or purification of CH ₄ , is also contributing to the significant increase of cost of the overall methanation process. Such C0 ₂ removal is a part of post-treatment of CH ₄ product rather than a part of methanation process.

Syngas produced from gasification contains sulfur component mainly in the forms of H ₂ S and COS, which poisons the methanation catalyst and have to be removed prior to the methanation process. In the industrial practice, syngas has to go through a deep clean-up unit to reduce the sulfur contents to the level of O.lppm before sent to the methanation process. Such clean up is usually acheived by one of the commercial desulfurization processes such as Rectisol, Selexol, etc. The capital cost increases significantly due to the needs of pre-cleanup. Also, such clean up processes requires low temperature (room temperature or lower) so the hot syngas produced from gasification has to be cooled down and energy efficiency is lost.

US 6610264 disclosed a process and system for removing sulfur from gas admixture, that could be applied to separate sulfide gas from the above syngas raw material. Meanwhile US7713421 disclosed a process and system for separating components of a fluid mixture, wherein sorbent structure could absorb certain gas components including above sulfide gas.

Although there exists highly sulfur resistant methanation catalyst, for example sulfur resistant methanation catalyst including molybdenum and lanthanide or actinide, as disclosed in US4,151,191, price of such a catalyst is high. In addition, as there is no regeneration mechanism in the system, poisoned catalyst accumulates in the system, resulting in reduction or loss of catalyst activity and selectivity. Also, replacement of the catalyst involves complete shutdown of the system and thus causes high increase of cost, thus there is a need to find a way to prolong on-line time for catalyst.

US 4774261 also disclosed a sulfur resistant catalyst and a process using such catalyst used in the presence of sulfur. However, under conditions of such process, excessive C0 ₂ is produced and accumulated as the methanation proceeds which drive the chemical equilibrium to the reverse of methanation, thus inhibit the further conversion of methane and limits the maximum conversion rate. Therefore in the products, large amount of unconverted syngas is left and the heating value is low, in such case, further purification of methane has to be applied to produce pipeline quality SNG.

In addition to the over accumulation of C0 ₂ and poisoning of catalyst by sulfide gas, in prior art, methanation suffers from the following problem.

Due to the exothermic nature of the reaction, a lower temperature favors the conversion to methane. As a result, to avoid the thermodynamic barrier, a low temperature around 300°C to 400°C is used to achieve an acceptable conversion. Such a temperature, however, results in a slower reaction rate and therefore a much larger reactor, and/or large amount of recycle steam to complete the reaction, thus, capital cost is significantly increased. Additionally, the sulfur-tolerance of the catalyst decreases under lower temperature, and the catalyst life is shorter

Also, the highly exothermic nature of the reactions raises high demand on heat removal in these technologies. Careful designs of heat removal equipment such as multi-tubular heat exchanger or inter-stage coolers are required, resulting in a much higher operation complexity and plant capital cost.

Also, it is a common practice that heat exchangers are installed in the system to transfer the reaction heat away in order to control the reaction temperature and also to utilize the generated steam for generating electricity or driving mechanical equipments. Such arrangement requires higher reaction temperature, which, however, as mentioned above, is not favorable to the reaction.

It is an objective of the invention to carry out sour methanation of syngas without one or more, or even all above identified problems.

SUMMARY OF THE INVENTION

The present inventors have found that, by rapidly removing C0 ₂ and sulfide gas (such as H ₂ S and COS) using a sorbent from the reaction system as the methanation reaction proceeds, and the regeneration of the sorbent, aforesaid objective of the invention can be achieved.

By removing C0 ₂ and sulfide gas simultaneously from the methanation reaction system, the equilibrium in methanation reaction is shifted to the production of methane and thereby a higher methane yield can be achieved. Such removal also purifies methane product so that methane of higher quality can be obtained and/or the cost associated with methane purification can be reduced. In addition, simultaneous removal of C0 ₂ and sulfide gas also prevents catalyst from being poisoned, so that higher catalyst activity and/or longer life time of catalyst can be achieved, as well as pretreatment of syngas to reduce sulfide can be deleted, and/or non- or less sulfur-resistant catalyst (thus lower cost) can be used in the methanation reaction.

Finally, via regeneration of the sorbent, the actual consumption of sorbent used in the system can be greatly reduced, hence lower cost can be achieved. This is particularly advantageous at industrial scale.

The simultaneous removal of C0 ₂ and sulfide gas from the methanation reaction system can be implemented by a system including a reactor and at least one sorbent regenerator, where the reactor retains methanation catalyst in it while allowing sorbent for C0 ₂ /sulfide gas passing through the reactor. Spent sorbent which is saturated by C0 ₂ /sulfide gas can be regenerated in the generator and recycled back into the system. The methanation catalyst and the fresh sorbent could be mixed together, and spent sorbent could be separated from the methanation catalyst by some specific mechanisms.

Thus, the present invention relates to a system for producing methane rich gas from syngas, comprising a reactor having a syngas inlet at an end of the reactor, an methane rich gas outlet at another end of the reactor, at least one reaction- sorption zone inside the reactor between the syngas inlet and the methane rich gas outlet; said reaction-sorption zone comprises methanation catalyst and a sorbent capable of absorbing carbon dioxide and sulfide gas; and at least one sorbent regenerator communicating with said reactor by a line for spent sorbent delivery and another line for regenerated sorbent delivery, wherein spent sorbent produced in the reactor via the line for the spent sorbent delivery enters the regenerator, and is regenerated therein, then regenerated sorbent recycles back into the reactor via the line for regenerated sorbent delivery.

In a preferred embodiment of the invention, the syngas inlet is located at the bottom of the reactor, while the methane rich gas outlet is located at the top of the reactor; the reaction- sorption zone comprises a fluidizing bed of particles of the catalyst and the sorbent, the fluidizing bed further comprises a perforated baffle at the bottom with at least one downcomer installed on it, with the upper end of the downcomer above the perforated baffle and the lower end of the downcomer below the perforated baffle, the upper end of the downcomer is covered by a mesh, with the following relationship among particles size of catalyst and sorbent and the size of hole of the mesh: the smallest particle size of C wt of catalyst > Size of the holes of the mesh > the biggest particle size of A wt of sorbent, C wt and A wt are independently over 60 wt , preferably over 75 wt , more preferably over 85wt particularly preferably over 95wt , most preferably 100wt .

In the above case, the catalyst resides in the fluidizing bed, the syngas flows upwards through the perforated baffle and then into the reaction- sorption zone, being converted to methane-rich gas under catalysis of the methanation catalyst, and continues to flow upwards to exit the reaction- sorption zone, the sorbent flows downwards into the fluidizing bed to simultaneously absorb carbon dioxide and sulfide gas therein, the spent sorbent overflows through the mesh and drops below perforated baffle and continues to flow downwards to exits the reaction- sorption zone.

In another preferred embodiment of the invention, the reaction- sorption zone comprises a fluidizing bed of particles of the methanation catalyst and the sorbent, the fluidizing bed comprises a perforated baffle at the bottom and at least two vertical baffles, one vertical baffle having upper end of at least one side cut above the perforated baffle and another vertical baffle having lower end of at least one side cut nearby the perforated baffle, the upper end of the at least side cut is covered by a mesh, with following relationship among particles size of methanation catalyst and sorbent and size of hole of the mesh: the smallest particle size of C wt of catalyst > Size of the holes of the mesh > the biggest particle size of A wt of sorbent, C wt and A wt are independently over 60 wt , preferably over 75 wt , more preferably 85 wt , particularly preferably over 95wt , most preferably 100wt .

In the above case, the catalyst resides in the fluidizing bed, the syngas flows upwards through the perforated baffle and then into the reaction- sorption zone, being converted to methane-rich gas therein under catalysis of the methanation catalyst, and continues to flow upwards to exit the reaction- sorption zone, the sorbent flows downwards into the fluidizing bed via the lower end of the side cut to simultaneously absorb carbon dioxide and sulfide gas, spent sorbent overflows through the mesh and drops below perforated baffle via the upper end of the side cut and continues to flow downwards to exits the reaction- sorption zone.

Preferably, more than one reaction- sorption zones are present in the reactor, wherein the reaction- sorption zones could be the same or different, one or more sorption zones could be inserted below, between or above the reaction-sorption zones, the downcomers and/or two vertical baffles each with end of at least one side cut are arranged in an alternating pattern, at least one heat exchanger could be installed in the reactor and /or the sorbent regenerator to transfer heat generated in the reaction away out of the reactor and /or sorbent regenerator. In the same way, at least one cyclone or filter could be installed in the reactor and /or the sorbent regenerator to separate gas from solid particles.

In the above system according to the present invention, the methanation catalyst is preferably not or low sulfur resistant methanation catalyst, and syngas raw material is preferably not subjected to be pretreated for desulfurization.

The present invention further relates to a process for producing methane rich gas by using the above system according to the present invention, comprising following steps in turn: syngas containing carbon monoxide, carbon dioxide, hydrogen, sulfide gas and optional steam etc is fed into reactor via syngas inlet; the syngas fed into reactor passes through the reaction-sorption zone and produces methane and carbon dioxide therein under the catalysis of methanation catalyst, while carbon dioxide and sulfide gas are rapidly and simultaneously absorbed by the sorbent in the reaction-sorption zone; gas rich in reacted methane separated from carbon dioxide and sulfide gas by sorption exits reactor from the methane rich gas outlet; spent sorbent via the line for spent sorbent delivery enters the sorbent regenerator, and then reacts with gas stream containing oxygen at 600 - 1200°C to be converted into regenerated sorbent; the regenerated sorbent is recycled back to the reactor via another line for the regenerated sorbent delivery.

In a preferable embodiment according to the above system and process of the present invention, the sorbent could be metal oxide selected from group consisting of the oxide of Ca, Zn, Cu, Fe, Mg and Al, alkaline-earth metal or admixture of the same. The said catalyst could be low or non sulfur resistant methanation catalyst, meanwhile the said syngas could be not subjected to be pretreated for desulfurization

The above system and process according to the present invention have following benefit: because the methanation reaction is reversible, if the reaction outcomes including methane, carbon dioxide and sulfide gas are removed rapidly from reaction system, reaction conversion rate will be greatly enhanced; generally, sulfide gas in syngas could deteriorate the activity of catalyst, if such gas could not be removed from reaction system in the short time, the function of catalyst would be greatly reduced or even lose its catalytic effect. Therefore, the inlet syngas must be desulfurized or sulfur-resistant catalyst has to be used. However, the sulfur-resistant catalyst is expensive. The system and process according to the present invention not only can use low or non- sulfur resistant catalyst, but also need not to pre-treat inlet syngas for desulfurization; by absorbing carbon dioxide and sulfide gas using sorbent, separation between methane and sulfide gas and carbon dioxide could make methane rich gas more pure. It will remarkably reduce the difficulty for post-treat for methane rich gas, and greatly lower cost for post- treat for methane rich gas. Because the consumption of sorbent is very big during methanation reaction, if the spent sorbent could not be re-used, the cost for sorbent usage is extremely high. By sorbent regeneration in the present invention, the spent sorbent is converted to fresh sorbent by heated gas stream containing oxygen. The consumption of sorbent is greatly reduced, therefore the cost for it is remarkably decreased, It is extremely beneficial in the industrialized process; through the circulation and regeneration of sorbent, it is guaranteed that sorbent in reactor always is fresh. Also, the residence time of spent sorbent in reactor is minimized, leading to a higher activity. It is very beneficial to conversion completion of methanation and protection of catalyst from sulfide gas, because C0 ₂ and sulfide gas are simultaneously and promptly removed from the reaction system by sorption. Furthermore, since there is no need to replace sorbent in reactor, the productivity of reactor is largely enhanced with great reduction of cost for operation and maintenance.

DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic illustration showing the principle of operation of the system of the present invention.

Fig. 2A to 2B shows two kinds of preferable structure of the reaction- sorption zone in the system in Fig. 1.

Fig. 2A shows a reaction- sorption zone with a fluidizing bed, with perforated baffles to retain catalyst there above, and vertical downcomer with upwards-flare-opening covered by mesh to allow spent sorbent going there through.

Fig. 2B is a reaction- sorption zone with a fluidizing bed, with a perforated baffles to retain catalyst there above, and one of two vertical baffles with upper end of the side cut covered by mesh to allow spent sorbent going there through .

Fig. 2C shows a cyclone nearby methane rich gas outlet to separate methane rich gas from tiny solid particles of spent sorbent and /or catalyst.

Fig. 3A shows a preferred embodiment of the system according to the present invention, wherein the system comprises two reaction- sorption zones as shown in Fig. 2A, three sorption zones and three heat exchangers to recover heat from the reactor and the regenerator.

Fig. 3B is the plan view showing the downcomers distribution in the reactor in Fig. 3A. BEST MODE FOR IMPLEMENTING THE INVENTION

As a general embodiment according to the present invention, methanation process according to the present invention is implemented by the system comprising a reactor 100 and at least one sorbent regenerator 200, as shown as shown in Fig. 1, the reactor 100 carries out the methanation of the feed syngas while C0 ₂ and sulfide gas are rapidly removed from the reactor 100 together with by the aid of the sorbent, while the regenerator 200 converts the spent sorbent back to regenerated sorbent and recycle the regenerated sorbent back to the reactor 100.

Syngas could be introduced as feed into volume beneath the reaction- sorption zone 105 of the reactor 100 via inlet 101, and then enters into the reaction- sorption zone 105. On the other hand, fresh/regenerated sorbent is introduced via line 104 into the reaction- sorption zone 105 where it reacts with C0 ₂ and sulfide gas to absorb them, then flows to volume beneath the reaction- sorption zone 105, and eventfully leaves the reactor 100 via line 103. The reaction- sorption zone 105 accommodates both the catalyst and sorbent so that methanation reaction and removal of C0 ₂ and sulfide gases are carried out simultaneously. In other words, in the reaction- sorption zone 105, syngas is converted into CH ₄ and C0 ₂ in the presence of the methanation catalyst while C0 ₂ and sulfide gas are rapidly sorbed by the sorbent simultaneously.

Once fresh/regenerated sorbents flows around the catalyst, C02 and sulfide gas are rapidly removed by sorption as methanation reaction proceeds under catalysis of the catalyst. In such a manner, with C0 ₂ removal from reaction site where the catalyst performs its catalytic function, the chemical equilibrium is shifted to the production of CH ₄ , the methanation process can reach near complete conversion. Also, with sulfide gas is absorbed and removed from the reaction site, the requirements to catalyst on sulfur resistance is significantly lowered, so the catalyst without sulfur tolerance (which is usually much cheaper than its sulfur-resistant counterpart) can be used in the system. In addition, with C0 ₂ and sulfide gas removal from reaction- sorption zone 105, highly pure methane rich gas can be obtained and purification of methane rich product becomes easier or even unnecessary. After going through the reaction- sorption zone 105, single-pass conversion of reaction can be up to near completion, therefore the load of downstream purification of CH ₄ is significantly reduced.

As will be discussed in detail with reference to Fig. 3A and 3B now, it is possible to have more than one reaction- sorption zone 105 in one reactor 100. In such case, each reaction- sorption zone 105 can contain the same or different catalyst and /or sorbent particles, which can perform the same or different functions. Meanwhile, it is also possible to insert one or more sorption zones of sorbent 112 above, between and/or below the reaction- sorption zones 105. Depending on the quality of syngas, the sorbent type, and the catalyst type, such the above zones can be arranged such that the desired sorption intensity is achieved.

Additional components can be installed in the reactor 100 to perform their respective functions. For example, one or more coils or multi-tubular heat exchangers can be installed with high pressure boiler feed water passing through and generating high pressure steam, to remove and utilize reaction heat generated, and cyclones or filters can be installed nearby or in exit lines (such as lines 102) to separate solid particles form gas. For example, Fig. 2C shows such a cyclone 109 nearby methane rich gas outlet to separate methane rich gas from tiny solid particles of spent sorbent and /or catalyst.

In the reaction- sorption zone 105, pretreated syngas (such as preheated and /or pressurized syngas) is converted to CH ₄ and C0 ₂ by reaction of the above Rxn 2, and sorption of C0 ₂ and sulfide gas represented by H ₂ S happens rapidly and simultaneously according to the following reactions:

MO +C0 ₂ => MC0 ₃ (Rxn 4)

MO + H ₂ S => MS + H ₂ 0 (Rxn 5)

M could be one or more suitable metals, e.g., Ca, Zn, Cu, Fe, Mg, Al, alkaline-earth metal and/or admixture of them. As a result of Rxn 4 and 5, C0 ₂ and sulfide gas originated from the raw syngas and produced during reaction are promptly reduced down, especially, the amount of sulfide gas therein is decreased to ppm level, and the sorbent finally is saturated and becomes spent sorbent.

Depending on the upstream process, the feed syngas can be obtained via gasification of coal, petcoke, biomass or other carboneous material, or by any other processes producing CO and H ₂ mixtures known to those skilled in the art. In a preferred embodiment, on dry gas basis, the syngas consists of CO 20%~70% by volume, H ₂ 10%~60% by volume, C0 ₂ up to 60% by volume, and H ₂ S 0.1% to 10% by volume etc. The raw syngas used by the system and process according to the present invention need not to be pretreated for desulfurization before feeding.

In a preferred embodiment according to the present invention, the reactor pressure can be ranged from atmosphere to 100 bar, reaction temperature could be ranged from 100°C to 900°C.

The catalysts used in the present invention can be one of any commercial methanation catalysts used in the industry. Such catalysts are known to those skilled in the art. For example, a preferable catalyst can be mixture of Mo and Ni. Preferably, the catalysts used in the present invention could be low or non-sulfur resistant methanation catalyst.

The sorbent used in present invention can be selected from those compounds that can react with C0 ₂ and sulfide gas to produce solid product, so as to reduce concentration of them in the reaction- sorption zone 105. Preferred sorbent is selected from group consisting of CaO and ZnO, CuO, or Fe ₂ 0 ₃ or admixture of them. Such sorbents are known to those skilled in that art.

The sorbent and/or the catalyst can be combined with inert material and/or formed into certain shapes, such as particle of a certain particle size. It can be seen from the description concerning Fig. 2A to 2B that properties such as particle size can be important to a good result of the implementation of the present invention. Which property is important depends on the mechanism that the sorbent is removed from the reaction- sorption zone 105.

Preferably, as shown in Fig. 1, spent sorbent exits the reactor 100 via line 103 and enters into the bottom of the riser 201 of sorbent regenerator 200 via inlet 202, and was raised by a hot gas stream containing oxygen to the regeneration zone 203, preferably in the form of a fluidizing bed. The spent sorbent is regenerated into fresh sorbent at 600-1200°C in the regeneration zone 203, and was cooled to desired temperature by internal cooling devices such as coils and multi-tubular heat exchanger with high pressure boiler feed water passing through to remove the heat and generating high pressure steam. The regenerated sorbent returns to the reactor 100 via line 104. Waste sour gas generated during the regeneration exits the sorbent regenerator 200 via line 204 and can be treated as customary to those skilled in the art.

Regeneration of spent sorbent can be achieved in any manner known to those skilled in the art. Typically, in the regeneration zone 203, regeneration reactions occur as following:

MC0 ₃ => MO + C0 ₂ (Rxn 6)

MS + 0 ₂ => MO + S0 ₂ (Rxn 7)

As the result of Rxn 6 and 7, the spent sorbent is regenerated, and becomes a metal oxide again; the C0 ₂ and S0 ₂ leave the regenerator 200 via line 204 after being optionally separated from solid particle by cyclones or filters, and proceed to subsequent known treatments, such as sulfur and/or carbon recovery and separation. The regenerated sorbent flows back to the reactor 100 as fresh sorbent via line 104.

The gas stream entering the inlet 202 should carry oxygen as needed by Rxn 7, and is heated enough to drive completion of Rxn 6 and Rxn 7. A gas stream containing oxygen of 5 ~50 , possibly air or mixture of oxygen and inert gas, can be used. In a preferred embodiment, mixture of 0 ₂ and C0 ₂ is used so that the gas exiting via line 204 contains highly pure C0 ₂ suitable for an easier downstream carbon capture. Depending on the composition of this stream and the temperature in the reactor 100, the temperature of this stream is in a range of 300 to 1000°C.

As shown in Figs. 2A and 2B, the reaction- sorption zone 105 can have different structures. For example, the reaction- sorption zone 105 can comprise a fixed bed of catalyst through which the sorbent passes. Alternatively, in a preferred embodiment of the present invention, fluidizing bed of the catalyst and sorbent particles are used in the reaction- sorption zone 105.

Fig. 2A shows a preferred embodiment of the reaction- sorption zone 105 in Fig. 1 in which the reaction- sorption zone 105 includes a fluidization bed, such as spout bed, enclosed by perforated baffles 106 at bottom or similar mechanism such as bubble-cap tray or float valve tray with one or more vertical upwards-flare-opening downcomer(s) 107. The upwards-flare-opening of downcomer 107 is covered by a mesh 108. The particles size of sorbent and catalyst and the size of hole of the mesh 108 have relationship as following: The smallest particle size of C wt of catalyst > Size of the holes of the mesh 108 > the biggest particle size of A wt of sorbent.

C wt and A wt can be independently more than 60 wt , preferably over 75 wt , more preferably over 85 wt , particularly preferably over 95 wt , most preferably 100 wt . The above-identified size of particles of catalyst and sorbent as well as the hole of the mesh 108 is referred to their diameters.

As shown Fig. 2A, the fresh sorbent goes down through a downcomer or a hollow conduit and enters into area nearby the perforated baffles 106, then is upwardly fluidized and rapidly sorbs C02 and sulfide gas therein. During upward floating movement by fluidization, fresh sorbent completes sorption and becomes spent sorbent due to saturation of sorption. As the spent sorbent accesses to upwards-flare-opening of downcomer 107, it has to pass through the mesh 108 covering the upwards-flare-opening by effect of fluidization, the mesh 108 allows small particles of spent sorbent pass through, but retains larger particles of catalyst in reaction -sorption zone 105. The spent sorbent particles passing through the mesh 108 go down through downcomer 107 and enters into volume beneath of the reaction sorption zone 105. Finally, the spent sorbent exits reactor 100 via line 103, and be fed in the sorbent regenerator 200 for regeneration.

It must be noted that the mesh 108 shown in Fig. 2A is optional. It could be deleted from the system according to the present invention. In that case, a minute quantity of catalyst particles would be brought into spent sorbent particles, and then enters into the sorbent regenerator 200 together with spent sorbent particles.

The mesh 108 should be tolerable to high temperature of about 800°C, and should possess enough strength and deformation resistance at above high temperature. A lot of material could be used to manufacture the above mesh 108, for instance, high temperature resistant alloy, based on Fe, Co and/or Ni, or porous high temperature resistant ceramic film, based on SiC and/or Si3N4 could be used to manufacture the above mesh 108.

Under the effect of fluidizing bed, the sorbent and catalyst particles are fluidized and floating above the perforated baffles 106. However, because of the fact that the size of sorbent particles is much smaller than that of catalyst particles, the weight of the spent sorbent particle is also less than that of catalyst particle, thereby the fluidizing or floating height of the spent sorbent particles, relative to the perforated baffles 106, is higher than that of the catalyst particles, and thus makes the spent sorbent approach the above upwards-flare-opening of downcomer 107, and the spent sorbent particles are easily captured by the upwards-flare-opening of downcomer 107. As a result, the separation between the catalyst particles and spent sorbent particles are realized in this way.

In the reaction- sorption zone 105, position of the top opening of overflow conduits (upwards-flare-opening downcomer 107) relative to the perforated baffle 106, which determines the height of the fluidized bed, is designed to control the rate of overflow to a desired value so as to control the saturation of the sorbents in the reaction- sorption zone 105. And thus the C0 ₂ and sulfide gas concentration inside reactor 100.

Before methanation process is carried out in the system as shown in Fig. 1 with the reaction- sorption zone 105 as shown in Fig. 2A, catalyst to facilitate the methanation reaction is loaded in the reaction- sorption zone 105 prior to the operation of the methanation process of the present invention. Upon starting of the operation, syngas (which could have been subjected to optional pretreatment such as preheating, pressurization or pre-sulfur removal, not shown in the figures) goes through the perforations in the perforated baffle 106 and enters into the fluidizing bed. At the same time, sorbent particles enter fluidizing bed and are fluidized together with the catalyst particles. Methanation is carried out, and C0 ₂ and sulfide gas including H ₂ S generated by reaction and originated from raw syngas are rapidly absorbed by sorbent simultaneously. At the same time, sulfide gas prompt removal by sorption prevents the catalyst from been poisoned by sulfide gas. If the relationship of the size of catalyst and sorbent particles and the hole of the mesh 108 is as defined above, mesh 108 allows only the sorbent particles to pass through while the catalyst particles are retained. Fluidized spent sorbent particles overflow into the downcomer 107 through the mesh 108 and drop down to the zone beneath reaction- sorption zone 105. Then it leaves the reactor 100 eventually via line 103.

In a preferred embodiment according to the present invention, the sorbent particles have a size ranging from 1 to 1000 microns, and the catalyst particles have a size ranging from 0.1mm to 1cm. The temperature and pressure of the reactor 100 lie in any range suitable for methanation reaction, such as 200~900°C, 1 atm to 100 bar.

Fig. 2B shows another preferred embodiment of the reaction-sorption zone 105 as shown in Fig. 1. The embodiment is the same as that in Fig. 2A, with the exception that two vertical baffles 107' with end of one or more side cut(s) are used instead of vertical upwards-flare-opening downcomer(s) 107 (overflow conduits). It is apparent to those skilled in the art that the principle for the operation of this reaction-sorption zone 105 is the same as that in Fig. 2A.

As shown in Fig. 2B, there is a gap or tunnel between the internal vertical wall of reactor 100 and two vertical baffles 107' . The fresh sorbent goes down through the gap or tunnel between the internal vertical wall of reactor 100 and one vertical baffles 107' with lower end of at least one side cut, and enters into area near by perforated baffle 106 via the lower side cut., Then it is upwardly fluidized and rapidly sorbs C02 and sulfide gas therein. During upwardly floating movement by fluidization, fresh sorbent completes sorption and becomes spent sorbent due to saturation of sorption. As spent sorbent accesses to upper end having at least one side cut of another vertical baffle 107', it has to pass through a mesh 108 covering the said upper end with side cut by effect of fluidization, the mesh 108 allows small particles of spent sorbent pass through, but retains larger particles of catalyst in reaction -sorption zone 105. The sorbent particles passing through mesh 108 go down through the gap or tunnel between the internal vertical wall of reactor 100 and one vertical baffles 107' with upper end of at least one side cut, and enters into volume beneath of the reaction sorption zone 105. Finally, the spent sorbent exits reactor 100 via line 103, and be fed in the sorbent regenerator 200 for regeneration.

In the above case, there is following relationship among the particles size of sorbent and catalyst and the hole size of mesh 108:

The smallest particle size of C wt of catalyst > Size of the holes of the mesh 108 > The biggest particle size of A wt of sorbent.

C wt and A wt can be independently more than 60 wt , preferably over 75 wt , more preferably over 85 wt , particularly preferably over 95 wt , most preferably 100 wt . The above-identified size of particles of catalyst and sorbent as well as the hole of the mesh 108 is referred to their diameters.

The mesh 108 shown in Fig. 2B is optional. It could be removed from the system according to the present invention. In that case, a minute quantity of catalyst particles would be brought into spent sorbent particles, and then enters into the sorbent regenerator 200 together with spent sorbent particles.

The mesh 108 should be tolerable to high temperature of about 800°C, and should possess enough strength and deformation resistance at above high temperature. A lot of material could be used to manufacture the above mesh 108, for instance, high temperature resistant alloy, based on Fe, Co and/or Ni, or porous high temperature resistant ceramic film, based on SiC and/or Si3N4 could be used to manufacture the above mesh 108.

Under the effect of fluidizing bed, the sorbent and catalyst particles are fluidized and floating above the perforated baffles 106. However, because the fact that the size of sorbent particles is much less than that of catalyst particles causes that the weight of the spent sorbent particle is also less than that of catalyst particle, thereby the fluidizing or floating height of the spent sorbent particles, relative to the perforated baffles 106, is higher than that of the catalyst particles, and thus makes the spent sorbent approach the above upper end of side cut of another vertical baffle 107', and the spent sorbent particles are easily captured by the upper end of side cut of another vertical baffle 107' . As a result, the separation between the catalyst particles and spent sorbent particles are realized in this way.

Fig. 3A shows a more preferred embodiment of the system according to the present invention, comprising the reactor 100 and regenerator 200, said reactor 100 comprises two reaction- sorption zones 105, three sorption zones 112, three heat exchangers 110, and two cyclones/cyclone cascades 111 to separate gas from solid particle before the gas exit the reactor 100 and the regenerator 200. In the reaction- sorption zones 105, both methanation and simultaneous sorption of C0 ₂ and sulfide gases happens, and in the sorption zones 112, only the sorption of C0 ₂ and sulfide gases happens to further remove C0 ₂ and sulfide gases. It is preferable that the reaction- sorption zones 105 and the sorption zones 112 and their respective downcomers are arranged in an alternating pattern so as to prompt removal of C0 ₂ and/or sulfide gas, as shown in Figs. 3 A and 3B. In such a manner, the sorbent particles are forced to travel longer distance in the fluidizing bed of catalyst/sorbent, higher efficiency of mixing and therefore better sorption result can be achieved. It is also preferred that one of the sorption zones 112 is located at the bottom of reactor 100, so that most sulfide gas are removed before it meets the catalyst in the lowest reaction- sorption zone 105 to reduce poisoning of the methanation catalyst. This means that a catalyst with lower sulfur tolerance can be used, and/or life time of a specific catalyst can be prolonged due to reduction of poisoning by sulfide gas. In addition, heat produced by sorption in sorption zone 112 can be used as a heat resource to pre-heat the syngas to an acceptable temperature for the methanation. Although the reaction- sorption zones 105 in Fig. 3A are designed to have structure as shown in Fig. 2A, it is apparent that each reaction- sorption zone 105 can have other structure, for example structure as shown in Fig. 2B, and could be designed independently and has the same or different catalyst and /or sorbent..

Although the heat exchangers 110 are in the form of coils through which heat exchange medium (preferably water) run in Fig. 3A, it is apparent that other forms of heat exchangers known to those skilled in the art can be used. When multiple heat exchangers are installed, each heat exchanger can be designed as the same or different from each other. As methanation proceeds in the reaction- sorption zone 105, reaction heat is generated and the temperature in the reactor 100 is raised. Heat exchange medium passing through the heat exchanger 110 is heated to generate super-heated medium, and transfers heat out of the reactor 100, and thereby temperature of the reaction- sorption zones 105 is controlled to the desired range. In particular, when the heat exchange medium is water, heat is removed by heat exchanger 110 to generate steam. As the methanation reaction is carried out at higher temperature, high quality steam can be generated in the heat exchanger 110.

The gas rich in methane generated in the reaction- sorption zone 105 enters into line 102 after gas-solid separation. Such separation can be carried out in any manner known to those skilled in the art, such as by a filter, a cyclone or cyclone cascade 111 as shown in Figs. 3 A and 2C.

In the more preferred embodiment according to the present invention as shown in Fig. 3 A and 3B, the inlet syngas could have the same composition as that in the embodiment according to the present invention as shown in Fig. 2A, and raw syngas has a temperature ranging from 80-120°C, and a pressure ranging from 16-24 bar, a flow rate of 80-120, preferably 100 kg/hr. The reactor 100 is controlled to be at a temperature ranging from 550-650°C and a pressure ranging from 18-22 bar. 100-140, preferably 120 kg/hr of sorbent is circulating between reactor 100 and sorbent regenerator 200, and total 80-120, preferably 100 kg/hr hot air at 900-1100°C, preferably 1000°C is blown into the bottom of the regenerator.

The goal of the invention is achieved by rapid and simultaneous removing C0 ₂ and sulfide gas from the reaction system and sorbent regeneration. As the methanation occurs in the reactor 100, C0 ₂ and sulfide gas are absorbed rapidly and simultaneously and no accumulation of them happens in reaction -sorption zone 105, thus C0 ₂ and sulfide gas as reaction inhibitors are eliminated and the conversion of reaction can continuously proceed without suffering from the thermodynamic limitation. As a result, a high conversion rate is achievable. Also, because of the removal of thermodynamic limitation, high reaction temperature up to 600°C or even to 800°C can be used, and the reaction rate is much faster than in the traditional conditions, so the equipment size can be reduced. With C0 ₂ and sulfide gas removal from the reaction system, cost associated with purification of product CH ₄ is also eliminated. It is well known that it is easier to find a non- sulfur tolerate methanation catalyst at higher temperature, Thus, the present invention enables easier selection and design of catalyst. In addition, the sorbent can also reduce sulfur contents, which lower the requirements on the catalyst tolerance significantly, and low sulfur resistant materials, such as most of current methanation catalysts in industry, can be used. Along with the higher sulfur-tolerance resulting from the high reaction temperature, the catalyst life time is prolonged and operational cost is reduced. The high reaction temperature can also provide with much higher quality of steam and therefore achieve higher energy efficiency. Finally, the fluidized bed type of system ensures a more uniform temperature distribution along the reactor and thus a much easier temperature control and heat management, which was a big challenge for the traditional fixed bed reactors, due to the strong exothermic of the reaction.

Those skilled in the art can utilize the concept of retaining catalyst while removing a reaction product and/or catalyst poison from reaction system in other applications. For example, what is removed is not limited to side product of the reaction and/or catalyst poison. As long as removal of specific components can facilitate to completion of the reaction, the present invention is applicable. For example, if the reaction has only one product (rather than two in above described embodiments, i.e., C0 ₂ and CH ₄ ), the removal of such product can also facilitate to reach completion of the reaction.

EXAMPLE

System as shown in Figs. 3A and 3B was used to perform the methanation process according to the present invention. The catalyst was a mixture of Mo-based and Ni-based with weight ratio of 1:1, with 95wt% of the particle greater than 1 mm. The sorbent was a mixture of ZnO and CaO with weight ratio of 1:10, with particle size ranging from 1 micron to 1 mm, where 95wt% was smaller than 100 microns. Particle sizes of the catalyst and sorbent were measured by method of sieving or specific surface area. The diameter of hole of mesh 108 was 1 mm. The mesh 108 was made from high temperature resistant alloy based on Ni, which shows excellent strength and deformation resistance at about 900°C.

The inlet syngas flow rate is 100 kg/hr. The above inlet syngas was not pretreated to be desulfurized. The inlet syngas has a temperature of 100°C and a pressure of 20 bar. Molar composition of the syngas is as following:

Table 1

H ₂ CO C0 ₂ H ₂ 0 H ₂ S

30% 40% 10% 18% 2% The reactor 100 operates at a temperature of 600°C and a pressure of 20 bar. 120 kg/hr of sorbent is circulating between reactor 100 and sorbent regenerator 200, and 100 kg/hr hot air at 1000°C is blown into the bottom of the sorbent regenerator 200. After the syngas passes through the bottom sorption zone 112 as shown in Fig 3A, the majority of sulfide gas originated from syngas is removed by sorption and the concentration of which decreased to approach lppm. The thickness of reaction- sorption zones 105 and the sorption zones 112 is independently 0.8-1.2 meter, depending on flowing rate of syngas passing through reaction- sorption zones 105 and the sorption zones 112,

At the exit of reactor 100, the product composition is as following:

Table 2

The total conversion rate of CO reaches 95.2%, the purity of methane in exit gas exceeds 90% on the dried basis. It is well known that under such conditions, conventional methanation processes have only the maximum conversion rate of CO of about 70%.

With an even escalated temperature of 700°C in the reactor 100 and the rest conditions unchanged, the total CO conversion rate still approached to 92%.

Although a few embodiments of the present invention have been shown and described, the present invention is not limited to the described embodiments. Instead, it would be appreciated by those skilled in the art that any changes and modification may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.