GB2073236A - Improved reactor and methanation method - Google Patents

Abstract

An improved reactor apparatus comprises at least two catalyst beds through which a reactant stream is passed until a pressure drop above a selected maximum results, whereafter at least a major portion of the reactant stream is charged to the reactor between the catalyst beds so that it flows through at least one of the catalyst beds. The apparatus is of particular interest for the conversion of a reactant stream comprising carbon monoxide and hydrogen to methane over a catalyst.

Description

SPECIFICATION Improved reactor and methanation method This invention relates to an improved reactor for reacting one or more reactants over a catalyst and to an improved methanation method for producing methane from reactant streams comprising carbon monoxide and hydrogen.

In recent years a considerable amount of effort has been directed to the development of processes for producing substitute natural gas from fuels such as coal of various grades, heavy residual oils and the like. The conversion of coals has been of particular interest and one approach has been the conversion of coal into raw synthesis gas in slagging gasifiers, conventional gasifiers and the like. One slagging gasifier process is disclosed in U.S. Patent 4,071,329 issued January 31, 1978. The use of such vessels produces a synthesis gas which is relatively high in carbon monoxide and relatively low in water.

Conventional methanation processes such as disclosed in U.S. Patent 3,854,895 issued December 17, 1974, U.S. Patent 3,890,113 issued June 17, 1975 and U.S. Patent 3,922,148 issued November 25, 1975, while effective with the synthesis gas stream produced in conventional gasifiers, have been found to be less desirable for the production of methane from such streams.

It has been found desirable to conduct the shift reaction, i.e.

CO + H,O -t H, + CO, and the methanation reaction, i.e.

CO + 3H2 e CH4 + H20 concurrently in the same reaction vessel when such feedstreams having a hydrogen to carbon monoxide ratio (H2/CO) less than about 3.0 are used. Same processes wherein such concurrent shift-methanation is practiced are U.S. Patent 3,938,968 issued February 17, 1976, U.S. Patent 3,958,956 issued May 25, 1976, U.S. Patent 4,017,274 issued April 12, 1977 and U.S. Patent 4,133,825 issued January 9, 1979. The reader is referred to the whole disclosure of these references as well as those cited previously.

U.S. Patent 3,938,968 discloses the use of a high temperature reaction zone wherein a concurrent shift-methanation reaction is accomplished over a unique catalyst as described in columns 3 and 4 of the reference.

U.S. Patent 3,958,956 discloses a process wherein a closed loop system is used for charging water to the synthesis feed gas to the process.

U.S. Patent 4,017,274 discloses a process for the methanation of scrubbed raw gases containing in excess of 3 mol % carbon monoxide and a methane concentration of less than 25 mol %. The reference process adds water or steam to the feedstream to the concurrent shift-methanation reaction zones and uses a steam reforming catalyst in the concurrent shift-methanation reaction zone. The reference process does not appear to use a product recycle.

U.S. Patent 4,133,825 relates to a process wherein a portion of the product from a concurrent shift-methanation reaction zone is recycled as a diluent to the synthesis gas feedstream passing to the inlet of a concurrent shift-methanation zone.

The reference process adds water to the synthesis gas feedstream.

U.S. Serial No. 086,861, entitled "Concurrent Shift-Methanation Process," filed October 22, 1979, by Byers, Melquist and Spangler discloses a concurrent shift-methanation process.

U.S. Patent 2,256,969 discloses a process wherein two catalyst beds are positioned in parallel for alternate use until a given temperature is obtained in the catalyst bed. There is no indication that the reactant stream should be passed through both beds in series nor is any means provided for passing the reactants through both beds.

U.S. Patent 3,870,738 discloses a process wherein a reactant gas stream comprising carbon oxides and hydrogen is passed through a first methanation catalyst bed with additional streams of dehumidified reactant gases being injected for mixture with the product stream from the first catalyst bed and subsequent passage through a second catalyst bed. A third catalyst bed is operated in a similar fashion. There is no indication that any of the catalyst beds could be by-passed.

U.S. Patents 3,730,694; 3,927,997 and 3,977,843 were also considered in the preparation of this application.

The reader is referred to the whole disclosure of the references listed above.

In view of the processes disclosed, it is clear that a continuing effort is being directed to the improvement of processes for the conversion of carbon monoxide into methane for use as a substitute natural gas. In such processes, a continuing problem has been the tendency of methanation catalysts to degrade as they are exposed to high carbon monoxide content gases.

Such degradation tends to result in the decomposition of the particulate catalyst into finely divided particles which tend to result in an increased pressure drop across the catalyst beds.

Such catalyst degradation is particularly likely when reactant mixtures of carbon oxides and hydrogen are passed through spent catalyst beds.

In the past, catalyst beds have been used by passing gases therethrough until the pressure drop became prohibitive or until the bed was spent. A plurality of reactors has been used in some processes to increase capacity and the like.

It has now been found that an improved reactor in such processes comprises a reactor vessel having a first and second end; at least two catalyst beds positioned in the reactor vessel between the first end and the second end; an outlet means for recovering a product stream from the reactor vessel; a first inlet means for charging a reactant stream to the reactor vessel so that the reactants pass through all of the catalyst beds; a second inlet means for charging a reactant stream to the reactor vessel between the two catalyst beds so that the reactants charged through the second inlet means pass through at least one of the catalyst beds and a control means for controlling the flow of reactants through the first inlet means and the second inlet means.

In another aspect our invention provides an apparatus for reacting one or more reactants over a catalyst comprising at least two catalyst beds connected in series flow, means for detecting an unsatisfactory condition in each of said beds, and means for at least partially by-passing said beds in turn other than the final bed, when said beds successively develop an unsatisfactory condition.

The unsatisfactory condition of a catalyst bed may e.g. be detected by means for detecting an excessive pressure drop across the bed, or by means for detecting an unsatisfactory state of reaction in the product emerging from said bed.

Our invention further comprises a method for producing methane from carbon monoxide and hydrogen by passing a reactant stream comprising carbon oxide and hydrogen through a catalytic reaction zone comprising at least a first and second catalyst bed until the pressure drop across the first catalyst bed increases to a value greater than a selected maximum and thereafter bypassing the first catalyst bed with at least a major portion of the reactant stream and passing the major portion of the reactant stream through the remaining catalyst bed(s). Optionally, a minor portion of the reactant stream is passed through the first catalyst bed while the major portion of the reactant stream is bypassed to the second catalyst bed.

An apparatus of known type and two embodiments of apparatus according to the present invention are illustrated diagrammatically in the accompanying drawings, wherein: FIGURE 1 is a schematic diagram of a catalytic methanation reaction vessel known to the art; FIGURE 2 is a schematic diagram of an embodiment of the apparatus of the present invention; and FIGURE 3 is a schematic diagram of a further embodiment of the apparatus of the present invention.

In Figure 1, a methanation reactor 10 is shown.

A catalyst bed 12 is positioned in reactor 10 and reactants are charged to reactor 10 through a line 14 and passed through catalyst bed 12 to produce a product stream recovered through a line 1 6. In the reaction of carbon oxides and hydrogen to produce methane, the reaction is normally relatively fast and when the catalyst bed is fresh, the reaction can be expected to be substantially complete in a very short distance. As the catalyst bed ages, the reaction zone will move downwardly through reactor 10 so that the reaction zone ultimately moves from the top of catalyst bed 12 to the bottom of catalyst bed 12.In many instances the spent catalyst, which is exposed to the reactant streams, tends to be stripped of its nickel compounds and otherwise degraded so that it begins to disintegrate into relatively fine particles which increase the pressure drop across catalyst bed 12. It will be clear that in the use of reactors such as reactor 10, there is little choice when the pressure drop across the catalyst bed 12 increases beyond a given limit but to discard the entire catalyst bed. While the discussion will be directed to methanation reactions generally, wherein methane is produced from carbon monoxide and hydrogen it is to be understood that concurrent shift-methanation reactions wherein carbon monoxide and water are reacted to form hydrogen in the same reaction zone are contemplated.

In Figure 2, the improved reactor 20 of the present invention is shown. Two catalyst beds, 21 and 22 are positioned in reactor 20. A reactant stream is charged to reactor 20 through a line 23 and passes through catalyst bed 21 and catalyst bed 22 to produce methane which is recovered through a line 24. After operation has continued for a period of time, so that catalyst bed 21 has become substantially spent or has degraded to result in an excessive pressure drop across catalyst bed 21 r a valve 26 in line 23 is closed and a valve 27 in line 25 is opened to bypass the reactant stream through line 25 to pass through catalyst bed 22 as shown.As indicated previously, the reaction zone required for the completion of the reaction is relatively short and the reaction is normally completed in catalyst bed 22 so that a product stream comprising primarily methane is recovered from line 24. It is clear that by the use of the apparatus set forth in Figure 2, an extended catalyst bed life can be obtained when catalyst degradation or the like occurs.

In Figure 2 a further embodiment of the reactor of the present invention is shown. Reactor 30 contains a catalyst bed 31 and a catalyst bed 32.

Initially a reactant stream is passed to reactor 30 through a line 33 and flows to both catalyst beds 31 and 32 to produce a methane-containing product stream which is recovered through a line 34. After reactor 30 has operated for a period of time so that catalyst bed 31 is substantially spent or the pressure drop across catalyst bed 31 has increased to an excessive level, the reactant stream is passed through a line 35 to catalyst bed 32 by closing a valve 36 in line 33 and opening a valve 37 in line 35. The operation of the reactor shown in Figure 3 is similar to the operation of the reactor in Figure 2 except that the feedstream is introduced at the top rather than at the bottom.

In the operation of the reactor of the present invention, as shown in Figure 2, a reactant stream comprising carbon oxides and hydrogen is charged to reactor 20 through a line 23. The reactant stream may also contain methane, a diluent stream such as the product stream from a methanation reactor, water and the like as known to the art. The reactant stream is passed through catalyst beds 21 and 22 initially until catalyst bed 21 is either substantially spent or an undesirable pressure drop across catalyst bed 21 occurs.

While the definition of an undesirable or excessive drop is somewhat arbitrary based upon the equipment available for the compression of the gaseous stream charged to reactor 20, pressure drops greater than about 1 psi per foot of catalyst bed are generally considered undesirable. In any event, when catalyst bed 21 is spent or presents an operating problem, valve 26 is closed and the stream is routed through line 25 by opening valve 27 so that the reactant stream is charged to catalyst bed 22 without having passed through catalyst bed 21. As a result of the short reaction time required with such gaseous streams, catalyst bed 22 is more than sufficient to provide adequate residence time for the reaction to go to substantial completion to produce the desired product stream.

Clearly, a plurality of catalyst beds can be used with the apparatus being adapted to bypass each bed sequentially so that each bed may be bypassed as it becomes spent or results in an undesirably high pressure drop. Such variations are considered to be within the scope of the invention and have not been shown for simplicity.

In the practice of the present invention, after catalyst bed 21 has been bypassed a major portion of the reactant gas stream is desirably passed through line 25 to catalyst bed 22 with a minor portion, usually less than about 25 volume percent of the reactant stream being passed through catalyst bed 21. Such a variation is desirable since in many instances while catalyst bed 21 may be substantially spent and offer an undesirable pressure resistance at normal reactor flow rates such resistance may well be within tolerable limits at lower flow rates and the reaction may proceed although at a much slower rate even though the bed is substantially spent.

Even if the reaction does not move to completion in catalyst bed 21, the partially reacted stream can be further reacted in catalyst bed 22 to produce substantially the same product stream as if the portion of the reactant stream passing through catalyst bed 21 had passed to catalyst bed 22 initially. Further, the use of spent catalyst bed 21 in this fashion increases the reactor capacity since it results in the production of a stream which functions as a recycle stream from another methanator vessel for use as a diluent for mixture with the stream flowing through line 25. As a result, the mixture charged to bed 22 has a reduced CO content (mol percent) and is charged to bed 22 at an increased temperature thus reducing the tendency of the feed mixture to degrade the catalyst in bed 22.The amount of reactant stream passed through catalyst bed 21 will vary widely dependent upon the pressure drop across catalyst bed 21 and the residual activity of the spent catalyst in bed 21. Such variations are within the skill of those in the art based upon the particular system used.

The catalysts used in reactor 20 are those commoniy used in the art for methanation and while it is not considered necessary to discuss such catalysts they generally comprise nickel or nickel oxide supported on a suitable carrier, such as alumina, kieselguhr or the like. Some such catalysts are discussed in the references incorporated by reference above.

While not shown in the Figures, a pressure monitoring means is normally used to compare the pressure of the inlet stream to reactor 20 and the outlet stream from reactor 20 is well as the pressure drop across each of the catalyst beds.

Such pressure sensing means are known to those in the art and form no part of the present invention except as a part of the apparatus described.

Clearly, the reactor of the present invention is suitable for use with a variety of reactant streams although the invention has been described by reference to methanation reactions for which the reactor is particularly suited.

It is respectfully noted that the embodiments described above while preferred, are illustrative rather than limiting in nature and many variations and modifications are possible within the scope of the present invention. Such variations and modifications may appear obvious and desirable to those skilled in the art based upon a review of the foregoing description of preferred embodiments.

Claims (19)

1. A reactor apparatus comprising: (a) a reactor vessel having a first end and a second end; (b) at least two catalyst beds positioned in said reactor vessel between said first end and said second end; (c) an outlet means for recovering a product stream from said second end of said reactor vessel; (d) a first inlet means for charging a reactant stream to said first end of said reactor vessel so that the reactants charged through said first inlet means flow through all of said catalyst beds; (e) a second inlet means for charging a reactant stream to said reactor vessel between said catalyst beds so that the reactants charged through said second inlet means pass through at least one of said catalyst beds; and (f) control means for controlling the flow of reactants through said first inlet means and said second inlet means.

2. The apparatus of claim 1 wherein said catalyst beds contain a methanation catalyst and wherein said reactant streams, in use of said apparatus, comprise carbon monoxide and hydrogen.

3. The apparatus of claim 1 or 2 comprising more than two catalyst beds, additional inlet means for charging a reactant stream to said reactor vessel between each successive pair of catalyst beds, and control means for controlling the flow of reactants through each of said inlet means.

4. The apparatus of claims 1,2 or 3 including means for detecting an unsatisfactory condition in each of said catalyst beds.

5. The apparatus of claim 4 wherein said means for detecting an unsatisfactory condition comprises means for monitoring the pressure drop across each of said catalyst beds.

6. Apparatus for reacting one or more reactants over a catalyst comprising at least two catalyst beds connected in series flow, means for detecting an unsatisfactory condition in each of said beds, and means for at least partially bypassing said beds in turn other than the final bed, when said beds successively develop an unsatisfactory condition.

7. The apparatus of claim 6 wherein said detecting means comprises means for detecting an excessive pressure drop across each of said beds.

8. The apparatus of claim 6 wherein said detecting means comprises means for detecting an unsatisfactory state of reaction in the product emerging from each of said catalyst beds.

9. The apparatus of claims 6, 7 or 8 comprising more than two of said catalyst beds.

10. The apparatus of any of claims 6 to 9 wherein said catalyst is a methanation catalyst and said reactants in use of said apparatus, comprise carbon monoxide and hydrogen.

11. The apparatus of claim 1, substantially as illustrated in Figures 2 or 3 of the accompanying drawings.

12. The apparatus of claim 6, substantially as described herein.

13. A method for reacting one or more reactants over a catalyst, which comprises passing said reactants through at least two catalyst beds connected in series flow, detecting when an unsatisfactory condition develops successively in said catalyst beds, and at least partially bypassing said beds in turn, other than the final bed, when said unsatisfactory condition develops.

14. The method of claim 13 wherein said unsatisfactory condition is detected by sensing an excessive pressure drop across the respective bed.

1 5. A method for converting carbon oxides and hydrogen into methane, said method comprising: (a) passing a reactant stream comprising carbon oxides and hydrogen through a catalytic reaction zone, said catalytic reaction zone comprising at least a first and a second catalyst bed until the pressure drop across said catalytic reaction zone increases to a value greater than a selected maximum; and (b) thereafter bypassing said first catalyst bed with at least a major portion of said reactant stream and passing said major portion of said reactant stream through the remaining catalyst bed(s).

16. The method of claim 15 wherein a minor portion of said reactant stream is passed through said first catalyst bed while said major portion of said reactant stream is bypassed to said remaining catalyst bed(s).

17. The method of claim 15 or 16 wherein more than two of said catalyst beds are used.

18. The method of any of claims 15 to 17 wherein said pressure drop is about 1.0 psi or less per foot of catalyst bed depth.

19. The method of claim 15 substantially as described herein.