GB2407818A - Steam reforming process - Google Patents

Abstract

A steam reforming process comprises the steps of passing a feedgas comprising a hydrocarbon feedstock and steam over a steam reforming catalyst to form a partially reformed feedgas, partially combusting the partially reformed feedgas with an oxygen-containing gas and bringing the partially combusted gas towards equilibrium over a steam reforming catalyst, cooling the resultant crude synthesis gas to below the dew point of steam and removing condensed water therefrom to give a de-watered synthesis gas, synthesising via Fischer Tropsch process, hydrocarbons from the de-watered synthesis gas and separating the hydrocarbons from co-produced water. At least part of the co-produced water is vapourised and used as at least part of the steam that is mixed with the hydrocarbon feedstock.

Description

SYN 60025 240781 8 Steam reforminQprocess This Invention relates to a

process for the steam reforming of hydrocarbons and In particular to the catalytic steam reforming of hydrocarbons to produce a synthesis gas containing hydrogen and carbon oxides suitable for the Fischer-Tropsch process.

Steam reforming Is widely practsed and Is used to produce hydrogen streams and synthesis gas comprising hydrogen and carbon oxides for a number of processes such as ammonia and methanol synthesis and the Fischer-Tropsch process.

Steam reforming may be performed over one or more stages. In prereforming or primary reforming stages, a desulphursed hydrocarbon feedstock, e.g methane, natural gas or naphtha, is mixed with steam and passed at elevated temperature and pressure over a suitable catalyst, generally a transition metal, especially nickel, on an alumina or calciumalumnate support. Methane reacts with steam to produce hydrogen and carbon oxides. Any hydrocarbons containing two or more carbon atoms that are present are converted to methane, carbon monoxide and hydrogen, and in addition, the reversible methane/steam reforming and shift reactions occur The main steam reforming reaction may be described by the following; CxHy + x H2O = x CO + (Y/2 + X) H2 The methane/steam reforming reaction Is highly endothermic and so the conversion of methane to carbon oxides and hydrogen is favoured by high temperatures. In pre-reformng, the hydrocarbon feedstock/steam mixture is heated, typically to a temperature in the range 350-650 C, and then passed adiabatically through a bed of a suitable catalyst, usually a nickel catalyst having a high nickel content, for example above 40% by weight. In contrast, primary steam reforming is usually effected at outlet temperatures above about 600 C, typically in the range 650 C to 950 C, by passing the feedstock/steam mixture over a primary steam reforming catalyst disposed In externally heated tubes. The composition of the product gas depends on, Inter alla, the proportions of the feedstock components, the pressure and temperature. The product normally contains methane, hydrogen, carbon oxides, steam and any gas, such as nitrogen, that Is present In the feed and which is inert under the conditions employed. For applications such as Fischer-Tropsch synthesis, it Is desired that the molar ratio of hydrogen to carbon monoxide Is about 2 and the amount of carbon dioxide present is small.

In order to obtain a synthesis gas better suited to a Fischer-Tropsch process, the reformed feed gas Is subjected in a separate reformer to further reforming reactions comprising (i) partial combustion of the reformed gas with a suitable oxidant, e g. air, oxygen or oxygen-ennched air using burner apparatus mounted near the top of the reformer and (i) adiabatic catalytic steam reforming of the partially combusted gas over a bed of a steam reforming catalyst, usually SYN 60025 e * e tee e ec.e. . . . : : : : : . : 2: * ...... e nickel on alumina disposed below the burner apparatus, to bring the gas composition towards equlibnum The partial combustion reactions may be described as follows, CXHy + x/2 O2 x CO + y/2 H2 CXHy + x O2 x CO2 + y/2 H2 The partial combustion reactions are exothermc and the temperature of the partially combusted reformed gas Is Increased to between 1000 and 1500 C. Thus the energy for the endothermic steam reforming reaction Is supplied by the hot, partially combusted reformed gas.

As the partially combusted reformed gas contacts the steam reforming catalyst it Is cooled by the steam reforming reaction to an exit temperature in the range 800-1100 C.

Alternatively, an un-reformed feed gas composing hydrocarbon feedstock and steam may be fed to the partial combustion stage. Where the feed gas to the partial combustionstage is an un-reformed hydrocarbon/steam mixture or a pre-reformed feed gas, the partial combuston/steam reforming process is known as autothermal reforming. Where the feed gas is a primary reformed gas, the process Is known as secondary reforming, the principal difference between the processes being the composition, e.g. the hydrogen content, and temperature of the feed gas. Typically a pre- reformed gas fed to an autothermal reformer will contain less than 10% by volume hydrogen and be at a temperature less than 650 C whereas a primary reformed gas fed to a secondary reformer will contain greater than 10% hydrogen by volume and be at a temperature greater than 650 C. Secondary reforming serves three purposes: the increased temperature resulting from the partial combustion and subsequent adiabatic reforming results in a greater amount of reforming so that the secondary reformed gas contains a decreased proportion of residual methane. Secondly the Increased temperature favours the reverse shift reaction so that the carbon monoxide to carbon dioxide ratio is Increased. Thirdly the partial combustion effectively consumes some of the hydrogen present In the reformed gas, thus decreasing the hydrogen to carbon oxides ratio. In combination, these factors render the secondary reformed gas formed from natural gas as a feedstock more suited for use as synthesis gas for applications such as Fischer-Tropsch synthesis than if the secondary reforming step was omitted. Also more hgh-grade heat can be recovered from the secondary reformed gas. in particular, the recovered heat can be used to heat the catalyst containng tubes of the primary reformer. Thus the primary reforming may be effected In a heat exchange reformer in which the catalyst-contanng reformer tubes are heated by the secondary reformed gas. Examples of such reformers and processes utlisng the same are disclosed In for example US 4 690 690 and US 4 695 442.

Fischer-Tropsch processes produce hydrocarbons from the synthesis gas stream. Watems a co-product In the reaction, which may be described as follows; SYN 60025 e tee ce as ee e e e e e e e e e e 3 eve e see ese e e nCO + 2nH2 (CH2)n + nH2O We have found that the efficiency of such synthesis gas generation processes may be further Improved, particularly at low steam ratios, by utilisng at least a portion of the co-produced water from the Fischer-Tropsch process to provide steam for the steam reforming process Moreover, the co-produced water from a F'scher-Tropsch process can contain significant quantities of oxygenated hydrocarbons such as alcohols, aldebydes, ketones and carboxylc acids These give rise to a need for subsequent waste-water treatment. By returning F'scher Tropsch co-produced water to the reforming process as steam, the present nvenbon advantageously returns the oxygenates as a source of hydrogen and carbon oxides and also reduces the need for waste-water treatment.

Accordingly the present Invention provides a steam reforming process comprising the steps of I) passing a feed gas comprising a hydrocarbon feedstock and steam over a steam reforming catalyst to form a partially reformed feed gas, Hi) partially combusting the partially reformed feed gas with an oxygen-containing gas and bonging the partially combusted gas towards equilbnum over a steam reforming catalyst, fit) cooling the resultant crude synthesis gas to below the dew point of water and removing condensed water therefrom to give a de-watered synthesis gas, v) synthessng hydrocarbons from said de-watered synthesis gas and v) separating the hydrocarbons from co-produced water, characterized In that at least part of said co-produced watems vapourised and used as at least part of the steam that Is mixed with the hydrocarbon feedstock.

In the present Invention, the steam Is generated by vaponsing at least a portion of the co produced water and not by combining water and hydrocarbon in a conventional saturator. By eliminating the need for a saturator the present invention also offers a considerable process simplification with the associated cost savings.

The crude synthesis gas may, as stated above, be provided by a number of methods; the hydrocarbon feedstock/steam mixture may be subjected to a stage of low temperature adiabatic steam reforming, also known as prereformng, and then the pre-reformed gas fed to an autothermal reformer where it Is partially combusted with an oxygen-contanng gas and the partially combusted gas passed through a bed of steam reforming catalyst to create the crude synthesis gas. Alternatively, the hydrocarbon feedstock/steam mixture may be subjected to a step of high temperature steam reforming, also known as primary reforming, optionally with a prereformng stage prior to the primary reforming stage, and the primary reformed gas fed to a SYN 60025 .ce C: : 4. c... . secondary reformer where again the reformed gas stream Is partially combusted with an oxygen containing gas and the partially combusted gas passed through a bed of steam reforming catalyst to create the crude synthesis gas.

In the process of the invention the feedstock may be any gaseous or low boiling hydrocarbon feedstock such as natural gas or naphtha. It Is preferably methane or natural gas containing a substantial proportion, e. g over 90% v/v methane. If the feedstock contains sulphur compounds, before, or preferably after compression, the feedstock Is subjected to desulphursabon, e.g. hydrodesulphunsaton and absorption of hydrogen sulphide using a suitable absorbent, e 9. a zinc oxide bed. The feedstock Is typically compressed to a pressure In the range 20-60 bar abs If desired, the feedstock may be divided Into two streams. The first stream Is mixed with steam and subjected to a step of steam reforming and the second stream fed to a secondary or autothermal reformer.

The hydrocarbon may be preheated to a suitable temperature and contacted with steam to generate the steam-hydrocarbon mixture. Steam introduction is preferably effected by injection of steam Into the hydrocarbon feedstock. In the present Invention the water used to generate the steam composes at least part of the co-produced water from a Fischer-Tropsch hydrocarbon synthesis reaction, fed by the synthesis gas generated by the reforming process.

The steam Is preferably generated by vapoursng the co-produced FischerTropsch water In a boiler by Indirect heat exchange, for example with the crude synthesis gas. Whereas only a part of the co-produced FischerTropsch water need be fed to the boiler to generate steam, preferably all the co-produced Fischer-Tropsch watems fed to the boiler where the oxygenated hydrocarbons present will often boll as low boiling azeotropes with water, thereby depleting the remaining water of oxygenates. The water that is not boiled to produce steam is thereby reduced In oxygenated hydrocarbons compared to the co-produced water from the Fischer Tropsch process. Accordingly the requirement for waste-water- treatment of such water Is reduced.

The amount of steam used is such as to give a steam ratio of 0.5 to 2, i. e. 0.5 to 2 moles of steam per gram atom of hydrocarbon carbon In the feedstock. The amount of steam is preferably mnmised as this leads to a lower cost, more efficient process It Is preferred that In the present Invention the steam ratio Is below 1.5, more preferably 0.5 to 1.0.

The hydrocarbon feedstock/steam feed gas Is then subjected to steam reforming, preferably primary reforming In a heat exchange reformer. Before it Is fed to the heat exchange reformer, the feedstock/steam feed gas may be subjected to a step of adiabatic low temperature reforming. In such a process, the hydrocarbon/steam mixture Is heated, typically to a temperature in the range 350-650 C, and then passed adiabatically through a bed of a suitable SYN 60025 . .. .: .: .. .': . . ..

5. . . ....

catalyst, usually a nickel catalyst having a high nickel content, for example above 40% by weight Dunng such an adiabatic low temperature reforming step any hydrocarbons higher than methane react with steam to give a mixture of methane, carbon oxides and hydrogen.

The use of such an adiabatic reforming step, commonly termed pre-reformng, Is desirable to ensure that the feed to the heat exchange reformer contains no hydrocarbons higher than methane and also contains a sgniflcant amount of hydrogen This is desirable In order to mnmise the risk of carbon formation on the catalyst in the heat-exchange reformer.

After any such pre-reformng step, the mixture Is further heated, If necessary, to the heat exchange reformemnlet temperature, which Is typically in the range 300-500 C. Dunng passage through the reforming catalyst, the endothermic reforming reaction takes place with the heat required for the reaction being supplied by a combusted fuel gas or preferably from the secondary reformed gas flowing past the exterior surface of the outer tubes. The temperature of the primary reformed gas Is preferably in the range 650-850 C. The primary reforming catalyst may be nickel supported on a refractory support such as rings or pellets of calcium aluminate cement, alumina, ttania, zirconia and the like. Alternatively, particularly when a steam ratio less than 1 0 Is employed, a precious metal catalyst may be used as the primary reforming catalyst. Suitable precious metal catalysts include rhodium, ruthenium and platinum between 0 01 and 2% by weight on a suitable refractory support such as those used for nickel catalysts Alternatively a combination of a nickel and precious metal catalyst may be used. For example, a portion of the nickel catalyst may be replaced with a precious metal catalyst, such as a ruthenum-based catalyst.

Whether the reformed feed gas Is a pre-reformed or primary reformed gas it forms at least part of the feed stream fed to the partial combustion stage. The feed stream may additionally comprise a tail gas from the Fscher-Tropsch hydrocarbon synthesis and/or, where a hydrocarbon feedstock bypass Is employed, the second hydrocarbon stream. In forming the feed stream, the Fischer-Tropsch tall gas and/or second hydrocarbon stream, may be combined with the reformed gas in any order. However, If a tall gas and hydrocarbon bypass are combined with the reformed feed gas, pre-mxng the tall gas and second hydrocarbon stream has the advantage that, if necessary, they may be heated together in one rather than two heat exchangers Howsoever the second hydrocarbon stream and the F'scherTropsch tall gas may be added it Is preferable, to avoid decomposition of the hydrocarbons therein, that they are not heated to temperatures in excess of 420 C prior to combination with the reformed feed gas.

The combustion stage feed stream composing the reformed feed gas Is then fed to a reformer where it Is subjected to partial combustion with a gas containing free oxygen supplied via burner apparatus. Whereas some steam may be added to the oxygen containing gas, SYN 60025 . c a a 6.. .. .: :. ....' preferably no steam is added so that the low overall steam ratio for the reforming process Is achieved. The partial combustion reactions raise the gas temperature to between 1000 and 1500 C. The hot partially combusted gas then passes though a bed of steam reforming catalyst. The steam reforming catalyst is usually nickel supported on a refractory support such as rings or pallets of calcium aluminate cement, alumina, ttana, zrcona and the like The gas containing free oxygen may be air or oxygen enriched air but is preferably substantially pure oxygen, e 9 oxygen containing less than 5% nitrogen. However where the presence of substantial amounts of Alerts Is permissible, the gas containing free oxygen may be air or enriched air Where the gas containing free oxygen is substantially pure oxygen, for metallurgical reasons it Is preferably fed to the reformer at a temperature below about 250 C.

The amount of oxygen fed to the partial combustion stage may be varied to effect the composition of the crude synthesis gas Where the partial combustion stage Is part of a secondary reforming process and the resulting crude synthesis gas is used to heat the tubes in the primary heat exchange reformer, the amount of oxygen fed to the partial combustion stage may also be used to control the heat balance of the heat exchange reformer. In general, Increasing the amount of oxygen, thereby increasing the temperature of the reformed gas leaving the secondary reformer, causes the [H2] / [CO] ratio to decrease and the proportion of carbon dioxide to decrease. Alternatively, If the conditions are arranged such that the temperature Is kept constant, increasing the temperature at which the hydrocarbon feedstock is fed to the heat exchange reformer decreases the amount of oxygen required (at a constant oxygen feed temperature) Decreasing the required amount of oxygen is advantageous as this means that a smaller, and hence cheaper, air separation plant can be employed to produce the oxygen The temperature of the feedstock can be increased by any suitable heat source, which may, If necessary, be a fired heater, which of course can use air, rather than oxygen, for the combustion.

The amount of oxyger-contaning gas Added is preferably such that 40 to 70 moles of oxygen are added per 100 Tarn atom hydrocarbon carbon in the feedstock. Preferably the amount of oxygen added is such that the crude synthesis gas leaves the reforming catalyst at a temperature in the range 800-1100 C.

As stated a6Ove, the crude synthesis gas may if desired be used to provide the heat required for a primary reforming step by using the secondary reformed gas as the hot gas flowing externally past the tubes of the heat exchange reformer. During this heat exchange the crude synthesis gas will be cooled by transferring heat to the gas undergoing primary reforming. If used In this way, the crude synthesis gas preferably leaves the heat exchange reformer at a temperature In the range 500-650 C SYN 60025 eë e.

- ë 7: . .: e e In order to remove water from the crude synthesis gas the crude synthesis gas is cooled to below the dew point of steam at which water condenses Such cooling may be effected using a stream of cold water and/or by indirect heat exchange. The water condensate Is separated from the synthesis gas using for example, a separator Heat recovered during this cooling may be employed for pre-heating duties and for boiling the Fischer-Tropsch co-produced water used to provide the steam for the steam reforming stage. However, If crude synthesis gas from a secondary reformer is used to boll the Fischer-Tropsch co-produced water, to ensure that there Is sufficient energy in the crude synthesis gas to meet this duty, the gas stream fed to the heat exchange reformer may if desired be subjected to a heating stage e.g. by utlsng a fired heater such as that used to effect start up of the heat exchange reformer. If an autothermal reformer is used to produce the crude synthesis gas there will generally be sufficient energy In the crude synthesis gas to boll the Fischer- Tropsch co-produced water. As described hereinafter, the recovered heat may be used to generate steam, e 9. from process condensate, to meet any additional sterns requirement and/or may be used In a carbon dioxide separation step.

Whereas at least a portion of the steam for the reforming process is provided by vapoursng at least a portion of the co-produced FischerTropsch water, at steam ratios greater than about 0 9, particularly 1.0, during start-up or shutdown procedures, or during significant process excursions, the steam may additionally be provided by boiling water from other sources. One particularly suitable source Is the condensate separated from the crude synthesis gas.

Typically the de-watered synthesis gas contains 5 to 15% by volume of carbon dioxide. In one embodiment of the Invention, after separation of the condensed water, carbon dioxide is separated from the de-watered synthesis gas prior to the Fischer-Tropsch synthesis stage and recycled to the synthesis gas production. Such recycle of carbon dioxide is preferred as it provides a means to control [H2]/[CO] ratio to achieve the optimal figure for FT synthesis of about 2. Furthermore, carbon dioxide Is Inert In the Fischer-Tropsch synthesis of hydrocarbons. Preferably the amount of recycled carbon dioxide Is maximsed up to the quantity which Is needed to achieve this ratio. Typically this may be at least 75%, particularly at least 90%, of the carbon dioxide in the de- watered synthesis gas. The recycled carbon dioxide stream may be added to the hydrocarbon feed gas but Is preferably added to the partial combustion stage feed stream composing the reformed feed gas. Where the recycled carbon dioxide (either as carbon dioxide separated from the synthesis gas prior to synthesis and recycled, or as the recycled Fischer- Tropsch tall gas) Is added to the partial combustion stage feed stream, rather than to the hydrocarbon feed gas, there is an advantage in that the steam reforming process can be operated at a lower steam ratio.

SYN 60025 . .: .: .. it: 8.. .. .: :. ....

The carbon dioxide may be separated by a conventional "wet" process or alternatively a pressure swing adsorption process may be employed. In a conventional "wet" process the crude synthesis gas is de-watered and is then contacted with a stream of a suitable absorbent liquid, such as an amine, particularly methyl dethanolamine (MDEA) solution so that the carbon dioxide is absorbed by the squid to give a laden absorbent 1'qud and a gas stream having a decreased content of carbon dioxide The laden absorbent liquid Is then regenerated, for example by heating, to desorb the carbon dioxide and to give a regenerated absorbent liquid, which Is then recycled to the carbon dioxide absorption stage Alternatively, or in addition to a stage of carbon dioxide separation and recycle, before the de- watered synthesis gas Is passed to the Fischer-Tropsch hydrocarbon synthesis stage it may be further subjected to a step of hydrogen separation, e.g. through a membrane, In order to provide pure hydrogen for other uses e.g. hydrocracking or hydrodesulphursation of the hydrocarbon feedstock In this situation, the tall gas recycle, if employed (in the absence of carbon dioxide separation and recycle) or the carbon dioxide recycle stream, if employed, are controlled to give a [H2] /[CO] ratio, which Is higher than the optimum for Fischer-Tropsch synthesis, so that after the required amount of hydrogen Is separated the resulting synthesis gas has an [H2]/[CO] ratio of about 2.

In the Fischer-Tropsch process, a synthesis gas containing carbon monoxide and hydrogen is reacted in the presence of a catalyst, which Is typically a cobalt- and/omron-containng composition The process may be effected using one or more fixed catalyst beds or one or more reactors using a moving catalyst, for example a slurry of the catalyst in a hydrocarbon liquid, e 9 In a slurry bubble column reactor The synthessed hydrocarbon liquid and co produced water are separated from the residual gas. The reaction may be carried out in a single pass or part of the residual gas may be combined with fresh synthesis gas and recycled to the Fischer-Tropsch reactor. Any residual gas which is not recycled to the Fischer-Tropsch reactor for further reaction Is herein termed tall gas. Since the reaction of the synthesis gas Is Incomplete, the tall gas will contain some hydrogen and carbon monoxide. In addition, the tail gas may also contain some light hydrocarbons, e.g. paraffins including methane, ethane, butane, olefins such as propylene, alcohols such as ethanol, and traces of other minor components such as organic acids It will generally also contain some carbon dioxide, which may be present In the synthesis gas fed to the Fischer-Tropsch reaction and/or Is formed by side reactions. Possibly, as a result of Incomplete separation of the synthesised hydrocarbon product, the tall gas may also contain a small proportion of higher hydrocarbons, '.e.

hydrocarbons containing 5 or more carbon atoms. These components of the tall gas represent a valuable source of carbon and hydrogen. In one embodiment, a portion of the tall gas is added to the reformed feed gas before partial combustion thereof, i.e. addition of tall gas to the partial combustion stage feed stream comprising the primary reformed gas. The amount of tail SYN 60025 ::: .. a. ': 9. . ,* . ,2. ' .: gas that may be recycled is preferably between 5 and 100% by volume of the tail gas produced In the Fscher-Tropsch synthesis stage.

The synthesised hydrocarbon product Is separated from the co-produced water This may be achieved using one or more separators and techniques know to those skilled In the art. The thus separated co-produced water usually contains oxygenated hydrocarbons, so-called oxygenates Including alcohols, aldehydes, ketones and carboxylic acids.

In the present Invention at least a portion, preferably at least 50% by volume and more preferably substantially all of the co-produced water Is fed to a boiler where a portion is converted to steam Preferably, prior to being fed to the boiler the co-produced watems pre treated to reduce fouling or corrosion In the boiler, e.g. by passing the water through filters and/or adjusting its pH. The co-produced water is boiled In the boiler to produce steam which contains oxygenated hydrocarbons This steam/oxygenate mixture Is then mixed with the hydrocarbon feedstock at the desired steam ratio, e.g. by direct injection of steam, and the resulting mixture fed to the steam reforming stage of the synthesis gas generation process.

The co-produced water fed to the boiler that is not converted to steam is depleted in oxygenated hydrocarbons and may be recovered and sent for waste-water treatment.

In a one embodiment utilisng a heat exchange primary reformer In combination with a secondary reformer, the hydrocarbon feedstock Is divided Into first and second streams and added separately to the reforming stages of the synthesis gas generation process. Hence If desired the second hydrocarbon stream may bypass the primary steam reforming step and be added to the partial combustion feed stream prior to combustion thereof In the secondary reformer. Where this is done, the second hydrocarbon stream comprises between 5 and 50% by volume, preferably between 5 and 40% by volume and most preferably between 5 and 30% by volume of the hydrocarbon feedstock. Amounts less than 5% by volume provide too small a benefit whereas amounts greater than 50% are less economically attractive due to an Increased requirement for oxygen In the partial combustion stage and in the consequential Increase In size and cost of the primary steam reformer By providing a proportion of the hydrocarbon feedstock and particularly at least part of the FischerTropsch tall gas to the partial combustion stage feed stream comprising the reformed feed gas, it is possible to operate the process at low overall steam ratios (particularly steam ratios < 1 0) with reduced risk of carbon deposition. Operation at low overall steam ratios Improves the process efficiency and economics of using a boiler to generate the steam for the reforming process from a portion of the Fischer-Tropsch coproduced water as all the steam requirement may be met from the coproduced water.

SYN 60025 e The invention Is Illustrated by reference to the accompanying drawings In which, Figure l Is a diagrammatic flowsheet of one embodiment of the Invention employing pre- reformng and autothermal reforming stages and a Fischer-Tropsch synthesis stage with separation and recycling of the co-produced Fischer-Tropsch water and Figure 2 Is a diagrammatic flowsheet of a second embodiment of the invention employing primary and secondary reforming stages in which Fischer-Tropsch tall gas, hydrocarbon feedstock and carbon dioxide separated from the secondary reformed gas are added to the primary reformed gas.

In Figure 1, pre-heated, pressursed hydrocarbon feedstock, for example desulphurzed natural gas containing over 90% v/v methane at a temperature of 200-400 C, fed via line 10 is mixed with steam fed via line 12 and the resulting mixture heated In heat exchanger 14 before being fed via line 16 to a pre-reformer 18 where part of the hydrocarbon Is adiabatically partially reformed at 400-650 C through a bed of a nickel catalyst toproduce hydrogen and carbon oxides. The resulting reformed feed gas Is fed via line 20 to an autothermal reformer 22 having a combustion zone 24 above a bed of steam reforming catalyst 26. Air, optionally mixed with steam, is fed via line 28 to burner apparatus (not shown) above the combustion zone 24 Partial combustion of the reformed feed gas takes place In the combustion zone Increasing its temperature to between 1000 and 1500 C. The resulting partially combusted feed gas then passes through the bed of steam reforming catalyst26. Endothermic steam reforming reactions take place In the bed of steam reforming catalyst that bring the gas mixture towards equilibrium and cool the resulting crude synthesis gas to temperatures in the range 800- 1100 C. The crude synthesis gas leaves the reformer 22 and is cooled by first passing it through one or more waste-heat boilers 30 before passing it through one or more FT-water boilers 32 In which co-produced Fischer- Tropsch water Is converted to steam The partially cooled crude synthesis gas Is passed from the Fischer-Tropsch water boiler 32 through further heat exchangers 34, that cool the crude synthesis gas to below the dew point of water so that water condenses from the crude synthesis gas. The cooled crude synthesis gas Is passed to a separator 36 wherein the condensate watems separated and removed via line 38. The resulting de- watered synthesis gas is then fed via line 40, optionally with pre- heating and/or compression, to the Fischer-Tropsch hydrocarbon synthesis reactor 42 In which the hydrogen and carbon monoxide are reacted over a catalyst, e g. a cobalt omron-based catalyst, to form hydrocarbons and water The unreacted gasses are removed from the reactor via line 44 and may be recycled to the Fischer-Tropsch reactor 42. The synthessed hydrocarbon / water mixture Is fed via line 46 to one or more separators 48 where the synthessed hydrocarbons are recovered as product steam 50. The co-produced watems removed for treatment and disposal via line 52. A portion of the co-produced watems taken via line 54 and fed to the FT- water boiler 32 in which it Is converted to steam. Thereafter, the steam is fed from the boiler via line SYN 60025 c:: ' ': ': .' : ' . : : : : 11.. . tee 12 to the hydrocarbon feedstock In line 10 The co-produced wafer fed to FT-water boiler 32 that Is not converted to steam Is recovered via line 56 In Figure 2 the hydrocarbon feedstock fed via line 100 Is mixed with Fscher-Tropsch water steam fed via line 102 and optionally process steam fed via line 103 The resulting mixture Is heated in heat exchanger 104 and fed at a pressure In the range 10 to 60 bar abs., via line 106 to the catalyst-contanng tubes 108 of a heat exchange reformer 110 The mixture Is typically heated to a temperature in the range 300 to 500 C prior to entry Into the tubes 108. For simplicity only one tube Is shown In the drawing: In practice there may be several tens or hundreds of such tubes. The feedstock/steam mixture undergoes primary steam reforming In the tubes 108 and the primary reformed gas leaves the heat exchange reformer 110 via line 112, typically at a temperature In the range 650 to 850 C The primary reformed gas in line 112 Is combined with the compressed and heated Fischer-Tropsch tail gas (to be descnbed) fed via line 114. The resulting secondary reformer feed stream compnsng the primary reformed gas 112 and tall gas 114 mixture is fed via line 116 to a secondary reformer 118, having a combustion zone 120 above a bed of steam reforming catalyst 122. Oxygen is fed via line 124 to burner apparatus (not shown) above the combustion zone 120. The secondary reformer feed stream Is partially combusted In the combustion zone 120 and brought towards equilbnum by passage through the secondary reforming catalyst 122 The crude synthesis gas leaves secondary reformer via line 126, typically at a temperature In the range 900 to 1050 C.

Heat Is recovered from the hot crude synthesis gas by passing it to the shell side of the heat exchange reformer 110 so that the crude synthesis gas, which may also be termed secondary reformed gas, forms the heating medium of the heat exchange reformer. The crude synthesis gas Is thus cooled by heat exchange with the gas undergoing reforming in the tubes 108 and leaves the heat exchange reformer via line 128, typically at a temperature 50 to 200 C above the temperature at which the first hydrocarbon stream/steam mixture is fed to the tubes 108.

The partially cooled crude synthesis gas is then cooled further with heat recovery by passing it through one or more FT-water boilers 130 In which co-produced F'scher-Tropsch watems converted to steam. Optionally, additional heat exchangers (not shown) may be provided before the FTwater boilers 130 to heat other process fluids in the process. The partially cooled crude synthesis gas is passed from the Fischer-Tropsch water boiler through further heat exchangers 132, which lower the gas temperature below the dew point of the water in the crude synthesis gas The cooled crude synthesis gas Is then fed via line 134 to a separator 136 wherein condensed watems separated as a liquid water stream 137. This water may be used to generate the steam fed to line 103, for example by heating it using the crude synthesis gas SYN 60025 c. c ces e c c ccche c c e c c C CCC C 1c e8 C C ' C CC C C The de-watered synthesis gas Is fed via line 138 to an optional hydrogen separation unit 140, e g a membrane unit or a pressure swing adsorption stage, to separate part of the hydrogen in the de- watered synthesis gas as a hydrogen stream 142.

The resultant synthesis gas is then fed via line 144 to a F'scher-Tropsch synthesis stage 146, wherein liquid hydrocarbons are synthesised and are separated, together with co-produced water, as a product stream 148 leaving a tall gas stream comprising unreacted gasses 150 Part of the tall gas is purged as stream 152 to avoid a build up of Alerts, e.g. nitrogen that may be present In the hydrocarbon feedstock as a contaminant and/or is often present In small amounts as an Impurity In the oxygen used for the partial combustion. The purged tall gas 152 may be used as fuel, for example In a fired heater heating the mixture of first hydrocarbon stream and steam fed to the heat exchange reformer. The remainder of the tall gas Is fed to a compressor 154 and then to a heat exchanger 156 and then fed via line 114 to be mixed with the primary reformed gas 112.

The product stream 148 is fed to a separator 158 where co-produced water 160 is separated from the synthesised liquid hydrocarbons which are recovered as stream 162. At least a portion of the co-produced water 160 is fed via line 164 to the FT-water boiler 130 where part Is vapourzed to steam in Indirect heat exchange with the reformed gas 128 The steam Is then fed via line 102 to the hydrocarbon feedstock stream 100. The coproduced water fed to FT water boiler 130 that Is not converted to steam is recovered via line 166 The Invention is further illustrated by the following calculated example. The following table contains data calculated for a 80000 barrel-per-day Fischer-Tropsch process operated in accordance with the flowsheet depicted in Figure 2, at an overall steam ratio of approximately 0 7 with 99% of the co-produced water sent to FT-water boilers (130) used to generate steam for the process. The data show that a large percentage of the oxygenates may be recycled via the steam to the reforming process.

SYN bUU;5 . . . . . . . 13 . tT; P;: 1 _ _ 1- _ _ _ _ O t _ O 1- _ N _ C\1 o O O O O1 _ o I O u) (0 N _ N O _ 0 N _ N O O N N 1 N N 1 _ O O O c') Fl N '_ _ U) | O F | N N o 0 N I O o O O I O F I I N N N N -F o N O O O O O O O O O O O O O O O - N c F | o o 0 N F F F FF o F m NN o o o F F o == o o o OIm o Olo Olo o olo o N O N N _ m O F o G u] 1 O 0 N F F F F N O O (D -F F F o 0 N N F o o o I o o o o o o o _ O N 0 _ 0 I r-0 uD 0 _ 0 0 O O 0 - I 0 _ O O O 0 1 0 _ N O O 0 -O O F N N _ O u' F o I o o o I o o I o o F o o O N r _ N | -<o O F -N I F F F F o o F Fo Fo o o o F IF F o o o o OIm o N O O O O O O O O O 0 1 O O O O O O O O O O O _ N O N N O N _ F F F N I O O FF F F F F N u, O N O O N u 0 O O O O O a' O O O O O O O O O O O O _ _ = _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - == 'Q - -I - - - - - - - - - - - - - -I - E Q Q O _ e _ = = O O N N N 1 E = I w U o o o o

Claims (1)

1. SYN 60025 e eee eceë C 14 . . ....

   e. . ... .. . . Clalms.

   1 A steam reforming process comprising the steps of i) passing a feedgas comprising a hydrocarbon feedstock and steam over a steam reforming catalyst to form a partially reformed feedgas, ) partially combustng the partially reformed feedgas with an oxygen containng gas and bringing the partially combusted gas towards equlibnum over a steam reforming catalyst, Ail) cooling the resultant crude synthesis gas to below the dew point of water and removing condensed water therefrom to give a de-watered synthesis gas, iv) synthessng hydrocarbons from said de-watered synthesis gas and v) separating the hydrocarbons from co-produced water, charactersed in that at least part of said co-produced watems vapoursed and used as at least part of the steam that Is mixed with the hydrocarbon feedstock.

   2. A process according to claim 1 wherein the crude synthesis gas Is generated by i) passing the hydrocarbon steam mixture over a catalyst disposed In heated tubes in a heat exchange reformer to form a primary reform. ed gas, i) subjecting the primary reformed gas to secondary reforming by partially combusting the primary reformed gas with an oxygencontainng gas and bnogng the resultant partially combusted gas towards equilibrium over a secondary reforming catalyst, and All) using the resultant secondary reformed gas to heat the tubes of said heat exchange reformer.

   3 A process according to claim 1 or claim 2 wherein the co-produced water is vapoursed in one or more boilers heated by crude synthesis gas.

   4. A process according to any one of claims 1 to 3 wherein carbon dioxide Is separated from the de-watered synthesis gas prior to synthesis of the hydrocarbons and combined with the reformed feed gas before the partial combustion thereof A process according to any one of claims 1 to 4 wherein the de-watered synthesis gas Is subjected to a step of hydrogen separation before it Is passed to the Fischer-Tropsch hydrocarbon synthesis stage SYN 60025 .. . ce ë e ' .

   15: .. .: :. .. ..

   6. A process according to any one of claims 1 to 5 wherein a tall gas from the synthesis of hydrocarbons is combined with the reformed feed gas before partial combustion thereof.

   7 A process according to any one of claims 2 to 6 wherein the feedstock is divided Into two streams and the first stream Is mixed with steam and the second stream Is combined with the reformed feed gas, said second hydrocarbon stream comprising between 5 and 50% by volume of the hydrocarbon feedstock.

   8 A process according to any one of claims 2 to 7 wherein the primary steam reforming catalyst comprises a nickel and/or a precious metal steam reforming catalyst 9. A process according to any one of claims 1 to 8 wherein the process is operated at overall steam ratio In the range 0 5 to 1 0