WO2004103896A1

WO2004103896A1 - Process comprising a synthesis gas formation and a hydrocarbon product formation

Info

Publication number: WO2004103896A1
Application number: PCT/GB2004/002210
Authority: WO
Inventors: John Richard Hensman
Original assignee: Davy Process Technology Limited
Priority date: 2003-05-21
Filing date: 2004-05-21
Publication date: 2004-12-02
Also published as: GB0311725D0

Abstract

A process for the formation of a hydrocarbon product is described which comprises the steps of: (a) providing a methane containing gas stream to a reactor to form synthesis gas; (b) reacting the synthesis gas to form a hydrocarbon product and water by a reforming process without the addition of oxygen containing gases; and (c) recovering the hydrocarbon product stream and recycling the water to the reactor of step (a) where at least a portion of the recycled water is consumed.

Description

PR_OCESS COMPRISING A SYNTHESIS GAS FORMATION AND A HYDROCARBON PRODUCT FORMATION

The present invention relates to a process in which light hydrocarbons such as methane are converted to heavier hydrocarbons such as those having 5 or more carbon atoms. In a preferred arrangement the present invention relates to a process in which the light hydrocarbons, for 5 example from natural gas, are converted to a synthesis gas which in turn is converted to longer- chain hydrocarbons, for example by a Fischer-Tropsch reaction. In a most preferred arrangement the present invention relates to a process in which water produced in the production of the heavier hydrocarbons is recycled and consumed by the overall process.

Natural gas is readily available in many parts of the world. This natural gas can be converted 10 to synthesis gas. Synthesis gas is a gas comprising hydrogen and carbon monoxide which is generally produced from the natural gas by a process of steam reforming. Synthesis gas can also be produced from gasified coal and other sources. A preferred means for preparing the synthesis gas is a steam reforming process in which the light hydrocarbons such as methane are reacted with steam over a catalyst. Conventional steam reforming processes may be 15 modified such that the synthesis gas is produced by autothermal reforming where a steam reforming process is combined with a partial oxidation process.

However the synthesis gas is formed, a suitable process, for example a Fischer-Tropsch process, can then be carried out to convert the synthesis gas to a hydrocarbon product particularly a liquid hydrocarbon product and by this means the natural gas can be converted 20 to usable liquid fuel.

The Fischer-Tropsch reaction can be characterised by the following general formula:

2H₂ + CO -→ — CH₂— + H₂O where — CH₂ — is a hydrocarbon having a plurality of carbon atoms.

The specific reactions which take place, and hence the composition of the end product, will 25 depend on the reaction conditions. These include the ratio of hydrogen to carbon monoxide and the catalyst selected. The desired product of the process is a liquid hydrocarbon. By-products of the reaction include gaseous hydrocarbons such as methane and ethane.

Whichever reaction conditions are used, a substantial volume of water is generated by the Fischer-Tropsch process. Where other hydrocarbon synthesis routes are used, water may also be produced. The resultant wastewater stream may contain contaminants which means that the water is unsuitable for release into the environment. For example, where a Fischer-Tropsch process is utilised, the waste water produced will generally contain small amounts of alcohol and other oxygenates which are by-products of the reaction. A water treatment facility is therefore generally required.

One example of a suitable water treatment facility is a biological treatment plant. Whilst such biological methods may be environmentally appropriate and successful, they have high capital and operating costs.

In US 5053581 a process has been suggested in which a hot gaseous mixture comprising methane and steam is contacted with the wastewater stream in an attempt to strip the contaminants from the water. The methane stream is then used in the production of synthesis gas. However, this process suffers from certain disadvantages and drawbacks. First, the process has not recognized nor addressed the relative vapour to liquid ratio required to efficiently strip the stream in order to produce high purity water. It is also noted that even after the stripping process has taken place, the water is passed to a water treatment plant before being released to the environment.

US 5891345 describes a process for the purification of an acrylonitrile plant wastewater stream in which a volatilized wastewater stream in a reactor with a catalyst at an elevated temperature so as to convert the volatile organic compounds and ammonia in the stream to a mixture comprising hydrogen, nitrogen and carbon dioxide. Whilst this process could be adapted to be suitable for use with a wastewater stream formed during, for example, a Fischer-Tropsch reaction, the process requires a separate reactor and is therefore expensive to set up and to operate in terms of the heat input required and the catalyst which will be consumed. In US 6225358 an alternative arrangement is suggested in which the process flowsheet includes a stripper subsystem which receives the aqueous by-product stream and removes the contaminants from the stream. The stripper subsystem includes a concentrator column which concentrates the contaminants in the aqueous stream and removes the contaminants therefrom. In view of the complexity of the subsystem, the costs of this process are high.

EP 0989093 and US 2001/0051662 describe processes in which water formed in the Fischer- Tropsch process is recycled to the synthesis gas generation system. However, both processes require the addition of oxygen containing gases to the synthesis reaction. Thus the water consumed in the synthesis gas generation is less than the water produced by the Fischer-Tropsch synthesis and there remains a produced water treatment requirement.

Whilst these processes go some way to successfully removing contaminants from the wastewater stream, they are costly to set up and run and they still result in substantial volumes of wastewater being released into the environment. It is therefore desirable to provide an improved process for the production of higher hydrocarbons in which large volumes of water are not released into the environment. The present invention overcomes these problems by providing a process in which the water produced in the reaction is consumed within the reaction and thus the requirement to treat contaminated process water is minimised or obviated.

There is therefore provided a process for the formation of a hydrocarbon product comprising the steps of: (a) providing a methane containing gas stream to a reactor to form synthesis gas;

(b) reacting the synthesis gas to form a hydrocarbon product and water by a reforming process without the addition of oxygen containing gases; and

(c) recovering the hydrocarbon product stream and recycling the water to the reactor of step (a) where at least a portion of the recycled water is consumed.

The portion of the recycled water consumed is preferably a maj or portion of the recycled water. The process of the present invention is particularly suitable where the reforming process is a steam reforming process. However it is equally applicable to any reaction system for the production of synthesis gas in which water is present and consumed in the reaction.

The methane containing gas stream is preferably natural gas. Any suitable arrangement may be used in the reactor, which is preferably a steam reformer, to convert the methane containing gas stream to the synthesis gas. Such processes and the associated conditions and catalysts are well known to the skilled man.

The process of the present invention optionally provides that the composition of the synthesis gas may be adjusted before it is passed to step (b). The gas may be adjusted by the removal of hydrogen or carbon dioxide. The adjustment of the gas composition may be by carbon dioxide adsorption, membrane separation or the use of molecular sieves.

Any suitable reaction may be used to convert the synthesis gas in step (b) to the hydrocarbon product and water. One suitable reaction is a Fischer-Tropsch reaction. The reaction for the conversion of synthesis gas to the hydrocarbon product may be carried out in the presence of a catalyst. Where the reaction is a Fischer-Tropsch reaction, the catalyst may be a Group VIII metal catalysts. Examples of other suitable Fischer-Tropsch catalysts can be found in US 6100304, US 6087405, US 5968991, US 5545674, US5102851, US5023277 andUS4874732 which are incorporated herein by reference. In one arrangement of the present invention cobalt or ruthenium is the main active component of the catalyst. The reaction may be carried out in any suitable reactor. One suitable steam reforming reactor is described in WO 03068379. Any suitable reaction conditions may be utilised.

The hydrocarbon product formed in step (b) may be a mixture of hydrocarbons. If the product is a mixture it may have a broad molecular weight distribution. In this connection, it is noted that a Fisher-Tropsch reaction produces a distribution of chain lengths with the molar quantity declining as the chain length increases. The product may comprise predominantly straight chain saturated hydrocarbons which typically have a chain length of 5 or more carbon atoms. The recycled water will generally contain contaminants. The contaminants may include one or more of alcohols, aldehydes, etones, acids, or carbon dioxide. The recycled water is consumed in the reactor of step (a) in the generation of the synthesis gas. By this means there will be a net consumption of water for the overall process. Where the reactor is a steam reformer, the water may be vaporized in the presence of the natural gas, it may be vaporized and then mixed with the natural gas or a combination of these processes to ensure that a fully vaporised and well mixed flow of vapour enters a reaction zone witiiin the steam reformer. The vaporisation step, where present, may be by direct heating or may preferably be by means of heat recovered from the process. In the vaporisation step the by-products of the reaction contained in the recycled water stream become vaporized.

The process may include a step in which the product stream from the steam reformer passes through a condenser to separate out excess water. This excess water is preferably recycled to the steam reformer either directly or, where the water recycled in step (c) is vaporised and mixed with the methane containing stream prior to addition to the steam reformer it may be combined with this step .

Some or all of the hydrocarbon product may be subjected to a treatment at a reaction temperature with hydrogen over a catalyst to modify the properties of the hydrocarbon product. If water is produced in this modification process, it may be recycled to the steam reformer of step (a).

It will be understood that at start up it will be necessary to provide water to the reaction step

(b). Where the reaction of step (b) is a steam reforming process the water will be provided in the form of steam. During operation, additional water may be mixed with the methane containing stream to meet the requirements of the steam reforming process. Make-up water may also be required due to losses from the system. Such losses include reactions involving water to produce, for example, alcohols, aldehydes, ketones, acids or carbon dioxide.

Additional or start-up water added to the system is preferably high quality water and will generally have been through a demineralization process. In one arrangement, a trace amount of make-up water may be purged to prevent the build up of salts introduced in the water addition. The purge will generally only be requhed mfrequently. The purge is preferably located after the water is vaporised and before the stream is added to the steam reformer.

In an alternative arrangement, any make-up water may be added as steam since this will obviate the requirement for the infrequent purge. In a further alternative arrangement, the additional water may be derived from flue gas condensate from the combustion side of the reformer. This latter arrangement will be desirable where the reformer is operated in such a manner that it does not introduce poisons into the process which would damage any catalyst present for the hydrocarbon synthesis reaction.

In one arrangement of the present invention, any synthesis gas not successfully converted to hydrocarbon product may be recycled to the steam reformer. The recycle may be direct or after treatment to adjust its composition.

The separation of the hydrocarbon product stream from the water which is to be recycled may be carried out by any suitable means. Suitable means include one or more of decanting, steam stripping, distillation and absorption. The separation of the product from the water may occur in multiple stages. Where a multi-stage process is used, the water from each stage may be recycled to the steam reformer either in combined form or separately.

Where the by-products/contaminants in the water stream, such as alcohols, aldehydes, ketones and acids may be useful products themselves, they may be substantially recovered from the water stream before recycling occurs.

The present invention will now be described, by way of example, with reference to the production of synthesis gas in a steam reformer and with reference to the accompanying drawing in which: Figure 1 is a block flow diagram of one arrangement of the present invention. It will be understood by those skilled in the art that the drawing is diagrammatic and that further items of equipment such as feedstock drums, pumps, compressors, gas recycling compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, feedstock and catalyst preparation systems, product treatment systems and the like may be required in a commercial plant. It will be understood that the apparatus will include means for feeding gases, removing product, and retaining catalyst with the reactor shell. Provision of such ancillary equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

As shown in Figure 1 a methane containing gas feed is fed in line 1 to a mixer 2 where it is mixed with vaporized water to form stream 3 which is passed to the steam reformer 4. In the steam reformer, the methane is converted to a reformed gas stream comprising synthesis gas. The synthesis gas is passed in line 5 to a system for steam condensation and separation 6. The remaining gas is passed in line 7 to the Fischer Tropsch reactor 10 either directly or via other treatment streams.

Condensed water produced in the condenser 6 is removed in line 8 where it is mixed with the water being recycled in line 15. In one alternative arrangement, it may be added directly to the vaporizer 16.

In the Fischer-Tropsch reactor 10 the synthesis gas is converted to the hydrocarbon product which is removed in line 11 and passed to a separator 12. Unreacted gas is removed in line 13. The gas may be recycled to the flowsheet if required. The hydrocarbon product is removed in stream 14. Although only one stream is illustrated it will be understood that there may be one or more product streams removed from the separation system. The or each product stream may undergo further processing and any water removed in these processing steps may be recycled to the steam reformer.

The water produced by the Fischer-Tropsch reaction is removed from the separator 12 in line

15. This stream of water, dissolved hydrocarbons and by-products is heated in vaporizer 16 and then passed in line 9 to the mixer 2. Any unvaporised water in the mixer 2 may be recovered and recycled in line 17 to the vaporiser 16.

Claims

1. A process for the formation of a hydrocarbon product comprising the steps of:

(a) providing a methane containing gas stream to a reactor to form synthesis gas;

2. A process according to Claim 1 wherein the portion of the recycled water consumed is a major portion of the recycled water.

3. A process according to Claim 1 or 2 wherein the reaction of step (b) is a steam reforming process.

4. A process according to any one of Claims 1 to 3 wherein the methane containing gas stream is natural gas.

5. A process according to any one of Claims 1 to 4 wherein the composition of the synthesis gas is adjusted before it is passed to step (b).

6. A process according to Claim 5 wherein the adjustment of the gas composition may be by carbon dioxide adsorption, membrane separation or the use of molecular sieves.

7. A process according to any one of Claims 1 to 6 wherein the reaction to form the hydrocarbon product and water is a Fischer-Tropsch reaction.

8. A process according to any one of Claims 1 to 7 wherein the recycled water contains contaminants including one or more of alcohols, aldehydes, ketones, acids, or carbon dioxide.

9. A process according to any one of Claims 1 to 8 additionally including a step in wliich the product stream from the reactor is passed through a condenser to separate out excess water.

10. A process according to Claim 9 wherein the excess water is recycled to the reactor.

11. A process according to any one of Claims 1 to 10 wherein some or all of the hydrocarbon product is subjected to a treatment at a reaction temperature with hydrogen over a catalyst to modify the properties of the hydrocarbon product and any water produced is recycled to the reactor of step (a).

12. A process according to any one of Claims 1 to 10 wherein additional water is mixed with the methane containing stream to meet the requirements of the process of step (a).

13. A process according to any one of Claims 1 to 10 wherein additional water is mixed with the methane containing stream to make up losses from the system.

14. A process according to Claim 12 or 13 wherein a trace amount of make-up water may be purged to prevent the build up of salts due to water addition.

15. A process according to Claim 12 or 13 in which the make-up water is added as steam.

16. A process according to Claim 12 or 13 in which the make-up water is derived from flue gas condensate from the combustion side of the reactor.

17. A process according to any one of Claims 1 to 16 wherein any synthesis gas not converted to hydrocarbon product is recycled to the steam reactor.

18. A process according to any one of Claims 1 to 17 wherein the separation of the hydrocarbon product stream from the water which is to be recycled is carried out by one or more of decanting, steam stripping, distillation and absorption.

19. A process according to any one of Claims 1 to 18 wherein the separation of the product from the water may occur in multiple stages.

20. A process according to Claim 19 in which the water, from each stage is recycled to the reactor.