Patent Application
20180086634
The invention relates to a process for the preparation of a syngas comprising hydrogen and carbon monoxide from a preheated methane comprising gas and to a cooling device for cooling hot raw syngas.
The expression “syngas” as used herein refers to synthesis gas, which is a common term to refer to gas mixtures comprising carbon monoxide and hydrogen.
Processes for the preparation of syngas from a methane comprising feed gas are well known. Typically such process comprises reacting the methane comprising gas with an oxidising gas, generally oxygen or an oxygen-containing gas such as air. The methane reacts with the oxygen to form carbon monoxide and hydrogen. This partial oxidation reaction is highly exothermic and the raw syngas formed accordingly has a high temperature and needs to be cooled before it can be further processed.
Devices for cooling hot syngas are also well known in the art and widely used in industry. Such devices typically comprise a vessel with heat exchange tubes arranged therein around which the cooling medium—typically water—can flow to absorb the heat from the hot syngas. Accordingly, when in operation water is present in and flows through the vessel around the heat exchange tubes. The outer walls of these heat exchange tubes are in direct contact with the water. The hot syngas is then typically passed through the heat exchange tubes, so that the tube walls absorb the heat from the hot syngas and release this heat to the water. In order to find an optimum balance between size of the vessel and heat exchange surface provided by the outer walls of the heat exchange tubes, heat exchange tubes are often helically coiled. For example, in a boiler the water used to absorb the heat from the hot syngas is used to generate saturated steam or even superheated steam.
A cooling device in which superheated steam is prepared is described in WO-A-2007/131975. This cooling device comprises spirally ascending conduits comprising an evaporating section located in a water bath space in the lower end of a vertically oriented vessel and a superheater section located in the upper end of the same vessel. The conduit of the superheater section is surrounded by a second conduit, thus forming an annular space between said superheater conduit and said second conduit. This annular space has an inlet and an outlet. When in operation, the hot syngas flows through the spirally ascending conduit, generating saturated steam in the evaporation section. This saturated steam is allowed to flow to the upper end of the vessel where it is passed into the inlet of the aforesaid annular space in the superheater section. The—still hot—syngas flowing through the conduits of the superheater section transfers its heat to the saturated steam flowing through the annular space, thereby generating superheated steam. Saturated steam and hot syngas can flow either co-currently or counter-currently in the superheater section. According to WO-A-2007/131975 the cooled syngas leaving the cooling device may have a temperature of up to 600° C., but suitably has a temperature of between 200 and 450° C.
When using syngas as the hot gas, the temperature of the cooled syngas leaving the cooling device should preferably not exceed 450° C. because of the corrosive nature of raw syngas to metals at elevated temperatures and the high pressure of the hot syngas in the cooling device (typically between 3 and 7 MPa). Higher temperatures may pose a serious corrosion risk to transfer lines for transferring the pressurized syngas from the cooling device to a further device for cooling or treating the syngas.
The cooled syngas leaving the cooling device as described in WO-A-2007/131975 and suitably having a temperature of up to 450° C. still contains recoverable heat. This heat could, for example, be used to preheat the natural gas feed to any partial oxidation reaction. However, the maximum attainable temperature of the natural gas when preheated via indirect heat exchange against such cooled raw syngas would be limited to about 400° C. Hence, additional preheating of the natural gas feed or supply of additional oxygen to the partial oxidation reactor would be required to effectively perform the partial oxidation reaction. However, supplying additional oxygen would result in the formation of more carbon dioxide and hence syngas of poorer quality, whilst additional preheating requires more energy input from external sources.
The present invention aims to provide a process wherein heat contained in the hot raw syngas can be more effectively recovered and wherein the natural gas feed can be effectively preheated to temperatures higher than 400° C. using such heat.
The present invention relates to a process for the preparation of a syngas, wherein a preheated methane comprising feed gas is reacted with an oxidising gas and wherein the hot raw syngas thus obtained is cooled in a cooling process which is carried out in a single cooling device and comprises at least indirect heat exchange against water to produce saturated steam and indirect heat exchange against a methane comprising gas to obtain the preheated methane comprising feed gas from which the hot raw syngas is prepared. In this way the heat contained in the hot raw syngas produced is used to effectively preheat the methane comprising feed to a temperature between 400 and 650° C. The term “indirect heat exchange” as used herein generally refers to heat exchange between two mediums by heat transfer from one medium to the other medium through a means separating both mediums (e.g. a wall) which means is capable of transferring the heat from one medium to the other.
The present invention also relates to a cooling device for cooling a hot raw syngas comprising an evaporation section for indirect heat exchange of the hot raw syngas against water and a gas heat exchange section for indirect heat exchange of the hot raw syngas against a cooling gas.
An important advantage of the present invention is that the methane comprising feed for the reaction with an oxidising gas to produce the raw syngas can be effectively preheated to temperatures up to 650° C., while at the same producing saturated steam. A further advantage is that the process and device of the present invention can be further expanded to include a superheating section to produce superheated steam or a further evaporation section to produce more saturated steam. With the process and device according to the present invention the heat generated in the reaction between the methane comprising feed and oxidising gas is very effectively recovered.
Accordingly, the present invention relates to a process for the preparation of a syngas comprising hydrogen and carbon monoxide comprising the steps of:
(a) reacting a preheated methane comprising gas with an oxidising gas to obtain a hot raw syngas comprising carbon monoxide and hydrogen;
(b) cooling the hot raw syngas resulting from step (a) to obtain the syngas by indirect heat exchange against water to produce saturated steam;
(c) further cooling the raw syngas obtained in step (b) by indirect heat exchange against a methane comprising gas to obtain a cooled raw syngas and the preheated methane comprising gas for use in step (a), wherein:
(i) steps (b) and (c) take place in a single cooling device for combined indirect heat exchange against water and against the methane comprising gas; and
(ii) the preheated methane comprising gas obtained in step (c) has a temperature between 400 and 650° C.
In step (a) of the present process a methane comprising gas is reacted with an oxidising gas to obtain a hot raw syngas. This can, for example, suitably be attained by means of partial oxidation (PDX) or autothermal reforming (ATR) of a methane comprising feed.
Examples of suitable methane comprising feeds include (coal bed) methane, natural gas, associated gas, refinery gas or a mixture of C1-C4 hydrocarbons. The methane comprising feed suitably comprises more than 90 v/v %, especially more than 94%, C1-C4 hydrocarbons and at least 60 v/v % methane, preferably at least 75 v/v %, more preferably at least 90 v/v %. Most preferably natural gas or associated gas is used.
The oxidising gas used may be oxygen or an oxygen-containing gas. Suitable gases include air (containing about 21 percent of oxygen) and oxygen enriched air, which may contain at least 60 volume percent oxygen, more suitably at least 80 volume percent and even at least 98 volume percent of oxygen. Such pure oxygen is preferably obtained in a cryogenic air separation process or by so-called ion transport membrane processes. The oxidising gas may also be steam or carbon dioxide.
The PDX process can take place in the presence of a suitable reforming catalyst or in the absence of a catalyst. The PDX reaction is highly exothermic and hence results in a hot raw syngas. Publications describing examples of PDX processes are EP-A-291111, WO-A-97/22547, WO-A-96/39354 and WO-A-96/03345.
The PDX process is typically carried out in a partial oxidation reactor. This can be a catalytic or non-catalytic PDX process. When carried out in the absence of a catalyst such partial oxidation reactor typically comprises a burner placed at the top in a reactor vessel with a refractory lining. The reactants are introduced at the top of the reactor. In the reactor a flame from the burner is maintained in which the methane comprising feed gas reacts with the oxygen or oxygen-containing gas to form a syngas. Reactors for catalytic PDX processes usually comprise a burner at the top and one or more fixed beds of suitable catalyst to react the methane in the feed with the oxygen added to the top of the reactor to form a syngas.
ATR processes are well known. In such ATR process the methane comprising gas reacts with the oxidising gas to produce syngas. The ATR process takes place in an autothermal reformer which typically comprises a burner, a combustion chamber and a catalyst bed in a refractory lined pressure vessel. The burner is placed at the top of the pressure vessel and extends into the combustion chamber which is located in the top section of the pressure vessel. The catalyst bed is arranged below the combustion chamber. Examples of ATR processes and autothermal reformers are e.g. disclosed in WO-A-2004/041716, EP-A-1403216 and US-A-2007/0004809.
Suitable reforming catalysts and arrangements for such catalysts which can be used in the autothermal reformer, are known in the art. Such catalysts typically comprise a catalytically active metal, suitably nickel, on a refractory oxide support such as ceramic pellets. Pellets, rings or other shapes of refractory oxide materials like zirconia, alumina or titania could also be used as support material. Further examples of suitable reforming catalysts are disclosed in US-A-2004/0181313 and US-A-2007/0004809.
For the purpose of the present invention step (a) may be a PDX process (preferably a non-catalytic PDX process) or an ATR process, as both processes will allow for maximum heat recovery and optimum preheating of the methane comprising feed in steps (b) and (c). Non-catalytic PDX processes are well known. The raw synthesis gas produced in such process typically has a temperature of between 1100 and 1500° C., suitably between 1200 and 1400° C. The pressure at which the syngas product is obtained may be between 3 and 10 MPa and suitably between 5 and 7 MPa. In an ATR process the raw syngas leaving the autothermal reformer typically has a temperature in the range of from 950 to 1200° C. , more suitably 970 to 1100° C. Operating pressures in an autothermal reformer are typically between 2 and 6 MPa, more suitably between 2 and 5 MPa. These temperatures and pressures of the raw syngas formed will also be the temperatures and pressures at which the raw syngas enters the cooling device in step (b). In addition to the oxygen-containing gas, steam may also be added.
In step (b) the hot raw syngas produced in step (a) is first cooled by indirect heat exchange against water to produce saturated steam followed by further cooling against a methane comprising gas in step (c) to obtain the cooled hot raw syngas. According to the present invention these steps take place in a single cooling device. It was found that this is essential for optimum heat recovery from the hot raw syngas so as to effectively preheat the methane comprising gas to temperatures as high as between 400 and 650° C. whilst at the same time effectively cool the hot raw syngas to a temperature between 200 and 450° C. The indirect heat exchange against water in step (b) is suitably carried out by passing the hot gas through a (coiled) tube immersed in water, thereby producing saturated steam. The saturated steam resulting from step (b) has a temperature between 150 and 350° C., more suitably between 220 and 310° C.
The temperature of the raw syngas after step (b) should be sufficiently high to preheat the methane comprising gas in step (c) to the desired temperature between 400 and 650° C. Preferred target temperature of the methane comprising gas obtained in step (c) is between 450 and 600° C. The indirect heat exchange against water in step (b) will, therefore, be designed such that the temperature of the raw syngas after cooling step (b) will be sufficiently high to preheat the methane comprising gas in step (c) to the desired temperature between 400 and 650° C., preferably between 450 and 600° C.
In step (c) the cooled hot raw syngas resulting from step (b) is further cooled by indirect heat exchange against a methane comprising gas in the same cooling device. The methane comprising gas is the same methane comprising gas which, after preheating in step (c), will be used as the preheated methane comprising feed in step (a) of the present process. The indirect heat exchange process step (c) is designed such that effective heat transfer takes place from the hot raw syngas to the methane comprising gas. Such single-phase heat exchange can be attained by means known in the art provided it can be combined with the two-phase heat exchange taking place in step (b).
The two-phase heat exchange step (b) and the single-phase heat exchange step (c) take place in a single cooling device. This cooling device should accordingly comprise at least one two-phase heat exchange section and at least one single-phase heat exchange section which are separated to ensure the cooling mediums water and methane comprising gas remain effectively separated and cannot get mixed, while at the same time the hot raw syngas can flow through both sections to be cooled by indirect heat exchange against both cooling mediums. A fire tube heat exchanger design with the hot raw syngas on the tube side with appropriate separation between both sections can suitably be used for both sections.
The cooled raw syngas leaving the single cooling device has a temperature of between 200 and 600° C., preferably between 250 and 450° C. and hence still contains recoverable heat. Such recoverable heat can, for instance, be used to preheat the methane comprising feed before it enters step (c). Accordingly, in a preferred embodiment of the present invention the methane comprising gas used as cooling medium and preheated in step (c) is first preheated to a temperature of up to 400° C. by indirect heat exchange against the cooled raw syngas that leaves the single cooling device to obtain a further cooled raw syngas. The temperature of such further cooled syngas will depend on the temperature of the cooled syngas leaving the single cooling device and the supply temperature of the methane comprising gas and will typically be between 150 and 350° C., more suitably between 200 and 300° C.
Such further cooled syngas may still contain recoverable heat which can, for instance, be used to preheat the water used as the cooling medium in step (b) to further optimize the heat integration. Hence, in a preferred embodiment the water used in step (b) is first preheated by indirect heat exchange against the further cooled raw syngas. The syngas resulting from this heat recovery step will typically have a temperature below 200° C., suitably between 100 and 180° C.
In one embodiment of the present invention the cooled raw syngas which leaves the single cooling device may suitably be the cooled raw syngas resulting from step (c). However, the cooled raw syngas resulting from step (c) may also first be subjected to one or more further heat recovery steps before it leaves the single cooling device in which steps (b) and (c) take place. Accordingly, in another embodiment of the present invention step (c) is followed by a step (d) as follows: (d) further cooling the cooled raw syngas obtained in step (c) by indirect heat exchange against water in the single cooling device to obtain further saturated steam and further cooled raw syngas. Step (d) takes place in the same single cooling device in which Steps (b) and (c) take place. In this embodiment the cooled raw syngas resulting from step (c) is suitably passed back in step (d) to the section in the cooling device where the indirect heat exchange against water takes place to produce further saturated steam before it leaves the single cooling device.
In yet another embodiment the saturated steam obtained in step (b) (and step (d), if present) is superheated by indirect heat exchange against the cooled raw syngas obtained in step (c) in a superheater section contained in the single cooling device to obtain superheated steam and the cooled raw syngas leaving the single cooling device. Accordingly, in this embodiment step (c) is followed by a step (d′) as follows:
(d′) further cooling the cooled raw syngas obtained in step (c) by indirect heat exchange against the saturated steam obtained in step (b) in the single cooling device to obtain superheated steam and further cooled syngas. A suitable superheater section that can be included in the single cooling device is, for example, described in WO-A-2007/131975. Such superheater section will typically be included in the top part of the single cooling device or be integrated with the single-phase heat exchange section where step (c) takes place.
The present invention also relates to a cooling device for cooling a hot raw syngas which can be used in the process of the present invention as described above. Accordingly, the present invention relates to a cooling device for cooling a hot raw syngas by indirect heat exchange against water in an evaporation section I and against a cooling gas in gas heat exchange section II, which device comprises a vertically oriented vessel 1 comprising at least one spirally ascending conduit 2, an inlet 4 for the hot gas fluidly connected to the upstream end of the conduit 2 for upward passage of the hot raw syngas through the spirally ascending conduit 2, an outlet 5 for cooled raw syngas fluidly connected to the downstream end of the conduit 2, an inlet 6 for fresh water and an outlet 7 for dry steam, a water bath space 8 in the lower part of the vessel 1, a saturated steam collection space 9 above said water bath space 8 and a dry steam collection space 23 above said saturated steam collection space 9 in the upper part of vessel 1, wherein
(i) the evaporation section I is located in the lower part of vessel 1 and the gas heat exchange section II is located immediately above the evaporation section I in vessel 1,
(ii) said spirally ascending conduit 2 comprises an evaporating section 10 located in the water bath space 8 in evaporation section I and a preheating section 11 located in gas heat exchange section II,
(iii) the conduit 2 of the preheating section 11 is surrounded by a second conduit 12 forming an annular space 13 between said conduit 2 and said second conduit 12,
(iv) the annular space 13 is provided with an inlet 14 for cooling gas fluidly connected to an inlet 15 for cooling gas and an outlet 16 for heated cooling gas located at the opposite end of said annular space 13 which outlet 16 is fluidly connected to outlet 17 for the heated cooling gas,
(v) the inlet 14 and/or outlet 16 is located in water bath space 8 below the water level 21, and
(vi) a separation means 25 is arranged inside vessel 1 between steam collection space 9 and dry steam collection space 23.
The one or more spirally ascending conduits 2 in pressure vessel 1 may be spirally ascending around the vertical axis 3 of pressure vessel 1, but could also be arranged in different bundles of spirally ascending conduits 2, which bundles are arranged around the central axis 3 of pressure vessel 1. Both configurations could be applied in the cooling device of the present invention. In a preferred embodiment, however, the conduits 2 are spirally ascending around the vertical axis 3 of the pressure vessel 1 and the drawings illustrating the invention will show this preferred embodiment. The cooling device of the present invention will be further described with reference to the drawings.
In
Also shown is an inlet 6 for fresh water. This inlet is preferably positioned such that the direction of the flow as it enters the vessel 1 enhances the circulation of water in a downward direction through a preferred downcomer 18. Alternative entry points for fresh water are, however, possible. For example, fresh water could also be added at an water inlet point in hot raw syngas inlet 4 (not shown). Downcomer 18 is preferably an open ended tubular part centrally positioned in water bath space 8 as shown. An upward direction of the water through an annular space 24 between downcomer 18 and inner wall of the vessel 1 will then result and circulation of water is created as shown by arrows in
The separation means 25 as shown in
Demister 22 can be any demister means suitable to remove liquid water droplets from the saturated steam collected in saturated steam collection space 9 and moving upward through support tube 19. For example, the demister 22 may be a demister mesh, a vane pack or a swirl tube cyclone deck.
The conduits 2 of the preheating section 11 are each surrounded by a second conduit 12 forming an annular space 13 between the conduit 2 and the second conduit 12. This annular space 13 is provided with an inlet 14 for cooling gas, which inlet 14 is fluidly connected to a vessel inlet 15 for the cooling gas. At its downstream end the annular space 13 is fluidly connected with an outlet 16 for the heated cooling gas. This outlet 16 is fluidly connected to vessel outlet 17 for the preheated cooling gas. When used in the process of the present invention, the cooling gas is a methane comprising gas.
In such configuration it is important that the inlet 14 is located in water bath space 8 below the water level 21 to provide cooling to the hot raw syngas carrying conduit 2. In this way overheating of the walls of this conduit 2 can be avoided where the methane comprising cooling gas enters the annular space 13. For the counter-current flow embodiment shown in
In the transition from the evaporation section I to the gas heat exchange section II the multiple spirally ascending conduits 2 suitably run in a vertical direction through a common header or may individually run into gas heat exchange section II. If a common header is used, this common header is in fluid communication with annular space 13 surrounding the conduits 2 via inlet openings 14 (in co-current mode as shown in
The conduits 2 can be made of materials being resistant to metal dusting. Because of the corrosive nature of the syngas such metal dusting resistance is important. Suitable materials include chromium-molybdenum steel and—the more preferred—nickel based metal alloys. Example of a suitable nickel based metal alloys are Inconel® alloy 693 as obtainable from Special Metals Corporation, USA.
The cooling device of the present invention may be combined with a superheater section positioned above gas heat exchange II for further heating the saturated steam produced in evaporation section I to superheated steam. This embodiment is further illustrated in
In order to ensure continuous cooling of the raw syngas flowing through conduit 2, the second conduit 12 surrounding the conduit 2 of the preheating section 11 is connected with second conduit 31 surrounding conduit 2 of superheating section 30. They are, however not fluidly connected: annular space 13 is separated from annular space 32 by gas-tight separation plate 37. By ensuring such continuous cooling of the hot raw syngas the wall temperature of the conduit 2 can be kept low enough to avoid metal dusting.
The device shown in
The superheating section III may also be integrated with gas heat exchange section II. For example, spirally descending conduits 2 with second conduits 31 which form the superheating section III could be arranged inside the bundles(s) of spirally ascending conduits 2 surrounded by a second conduits 12 which form gas heat exchange section II. When in operation the syngas flows ascending in the gas heat exchange section II and descending in the superheating section III.
The cooling device may also be combined with a further evaporation section in which the raw syngas leaving gas heat exchange section II is passed back to a further evaporation section positioned in water bath space 8 to produce further saturated steam. Such second evaporation section is suitably located inside evaporating section 10 of spirally ascending conduit 2 in water bath space 8, wherein such second evaporation section comprises at least one spirally descending conduit fluidly connected at its upstream end to the spirally ascending conduit 2 leaving the gas heat exchange section II and at its downstream end with vessel outlet 5 for cooled gas. Alternatively, the second evaporation section comprises one or more straight heat exchange tubes fluidly connected at their upstream end to the spirally ascending conduit 2 leaving the gas heat exchange section II and at their downstream end with vessel outlet 5 for cooled gas, wherein at least one of these straight tubes is surrounded by a sheath tube comprising closing means at its upper end and being open at its lower end as further described in co-pending European patent application No. 14174590.1. By using such sheath tubes the heat exchange capacity of the heat exchange tube can be varied.
The heat exchange tubes in the second evaporation section may also comprise a combination of spirally descending conduits and straight conduits with sheath tubes around it or may comprise heat exchange tubes consisting of a spirally descending section fluidly connected with a straight section surrounded by a sheath tube as described above.
The invention is further illustrated by the following examples. The examples are calculated examples using an integrated calculation model which includes detailed heat transfer algorithms and gas properties.
Hot raw syngas having a temperature of 1350° C. and a pressure of 6 MPa is fed into a cooling device comprising an evaporation section and a gas heat exchange section with counter-current flow of syngas and methane comprising gas.
The temperature of the cooled raw syngas leaving the cooling device is 400° C. at a pressure of 5.4 MPa, whilst the preheated methane-comprising gas has a temperature of 525° C. with the methane-comprising gas entering the cooling device at a temperature of 273° C. In the evaporation section saturated steam is produced having a temperature of 293° C.
Example 1 was repeated except that the cooling device now also contains a superheater section downstream of the gas heat exchange section and that the gas heat exchange section has a co-current flow of syngas and methane comprising gas. Saturated steam produced in the evaporation section is passed through the superheater section to produce superheated steam of 410° C. The temperature of the cooled raw syngas leaving the cooling device is 400° C. at a pressure of 5.2 MPa, whilst the preheated methane-comprising gas has a temperature of 525° C. with the methane-comprising gas entering the cooling device at a temperature of 385° C.
Example 1 was repeated except that the cooling device now also contains a second evaporation section downstream of the gas heat exchange section and that the gas heat exchange section has a co-current flow of syngas and methane comprising gas.
The temperature of the cooled raw syngas leaving the cooling device is 400° C. at a pressure of 5.8 MPa, whilst the preheated methane-comprising gas has a temperature of 480° C. with the methane-comprising gas entering the cooling device at a temperature of 385° C. The combined saturated steam from the first and second evaporation section has a temperature of 293° C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15167763.0 | May 2015 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/059994 | 5/4/2016 | WO | 00 |
