Producing liquid hydrocarbons

This invention relates to a chemical process to produce liquid hydrocarbons, and to a plant including a catalytic reactor suitable for use in performing the process .

A process is described in WO 01/51194 and WO 03/048034 (Accentus pic) in which methane is reacted with steam, to generate carbon monoxide and hydrogen in a first catalytic reactor; the resulting gas mixture is then used to perform Fischer-Tropsch synthesis in a second catalytic reactor. The overall result is to convert methane to hydrocarbons of higher molecular weight, which are usually liquids or waxes under ambient conditions. The two stages of the process, steam/methane reforming and Fisher-Tropsch synthesis, require different catalysts, and catalytic reactors are described for each stage. The catalytic reactors enable heat to be transferred to or from the reacting gases, respectively, as the reactions are respectively endothermic and exothermic; the heat required for steam/methane reforming may be provided by combustion.

If this process is carried out in such a way as to minimise the formation of methane in the Fischer-Tropsch stage, the product includes hydrocarbons ranging between about C5 and C40, about half of the product (by weight) being in the range C5 - C19 (and typically a liquid) , and just under a third in the range C20 - C29 (typically low molecular weight waxes) . Consequently the product is waxy, and is preferably stored at an elevated temperature to ensure it remains liquid; alternatively it may, in some situations, be possible to blend this Fischer- Tropsch product with crude oil.

A further problem of the process described in the above patent applications is that the synthesis gas produced in the first stage provides a higher proportion of hydrogen than is required by the Fischer-Tropsch synthesis of the second stage, so that the tail gases from the Fischer-Tropsch synthesis inevitably contain significant quantities of excess hydrogen. In small capacity offshore or remote plants this hydrogen- containing tail gas may be utilised for generating electrical power in a turbine, but with larger capacity offshore or remote plants it is more difficult to find an economic use for this excess hydrogen.

According to the present invention there is provided a process for processing natural gas to generate longer- chain hydrocarbons, the process comprising reacting the methane to generate a mixture of carbon monoxide and hydrogen, and subjecting this mixture to Fischer-Tropsch synthesis to generate longer-chain hydrocarbons, wherein the longer-chain hydrocarbons from the Fischer-Tropsch synthesis are reacted with hydrogen in a compact catalytic reactor at a substantially constant elevated temperature and an elevated pressure above 80 atmospheres so as to perform hydrocracking, the reactor comprising a stack of plates bonded together and defining flow channels for the hydrocracking reaction alternating in the stack with flow channels for heat removal, the flow channels for the hydrocracking reaction containing a removable catalyst insert with a metal substrate.

This hydrocracking step preferably uses hydrogen from the tail gas. Indeed, the tail gas may be sufficiently hydrogen-rich that it may be used for this purpose without additional processing. Preferably the hydrocracking reaction is at a pressure above 90 atmospheres, for example 100 atmospheres or above, and at

a temperature preferably in the range 400° to 450°C.

Preferably the product stream from the hydrocracking reaction is subsequently subjected to a separation process such as distillation, to produce a liquid product fraction (which would be liquid at ambient conditions), a light hydrocarbon gas fraction, and a waxy product fraction (which would be waxy at ambient conditions) .

The hydrocracking process is exothermic, and reaction conditions are preferably substantially isothermal in order to avoid thermal instability, coking, and the production of short chain (C1-C5) hydrocarbons. A compact catalytic reactor can provide sufficiently good heat transfer to ensure substantially isothermal operation. This may utilise, as a cooling medium, the waxy product fraction produced by the separation process, and/or the waxy product feed.

The compact catalytic reactor comprises a stack of plates, and the flow channels for reactants and for coolant are defined between successive plates, the plates being stacked and then bonded together. For example the flow channels may be defined by grooves in the plates. Alternatively the flow channels may be defined by thin metal sheets that are castellated and stacked alternately with flat sheets; the edges of the flow channels may be defined by sealing strips. The stack of plates forming the reactor is bonded together for example by diffusion bonding, brazing, or hot isostatic pressing. The reactor must also be provided with headers to supply the fluids to the flow channels. Each header for the reactant channels preferably comprises a chamber attached to the outside of the reactor and communicating with a plurality of channels, such that after removal of a header, the corresponding catalyst structures in the flow channels

are removable. This ensures that the catalysts can easily be replaced when they become spent.

The catalyst structure preferably incorporates a ceramic coating to carry the catalytic material.

Preferably the metal substrate for the catalyst structure is a steel alloy that forms an adherent surface coating of aluminium oxide when heated, for example an aluminium- bearing ferritic steel such as iron with 15% chromium, 4% aluminium, and 0.3% yttrium (eg Fecralloy (TM)) . When this metal is heated in air it forms an adherent oxide coating of alumina, which protects the alloy against further oxidation and against corrosion. Where the ceramic coating is of alumina, this appears to bond to the oxide coating on the surface. The substrate may be a wire mesh or a felt sheet, but the preferred substrate is a thin metal foil for example of thickness less than 100 μm, and the substrate may be corrugated, pleated or otherwise shaped so as to define a multiplicity of flow paths. The metal substrate of the catalyst structure within the flow channels enhances heat transfer within the catalyst structure, preventing hot spots or cold spots, enhances catalyst surface area, and provides mechanical strength. The metal substrate is preferably coated with a stabilised gamma-alumina support, in which is a nickel tungsten catalyst.

Where the channel depth is no more than about 3 mm, then the catalyst structure may for example be a single shaped foil. Alternatively, and particularly where the channel depth is greater than about 2 mm, the catalyst structure may comprise a plurality of such shaped foils separated by substantially flat foils; the shaped foils and flat foils may be linked to each other, for example by projecting lugs locating in corresponding slots, or alternatively may be inserted as separate items. To

ensure the required good thermal contact the reactant channels are preferably less than 20 mm deep, and more preferably less than 10 mm deep, and may be less than 5 mm deep. But the channels are preferably at least 1 mm deep, or it becomes difficult to insert the catalyst structures, and engineering tolerances become more critical. Desirably the temperature within the channels is maintained uniformly across the channel width, within about 2-4 ⁰ C, and this is more difficult to achieve the larger the channel becomes. For example the plates forming the reactor may each be of width in the range 0.1 m to 0.8 m and of length in the range 0.3 m to 1.5 m.

Reactors of this type provide short diffusion path lengths, so that the heat and mass transfer rates can be high, and so the rates of chemical reactions can be high. Such a reactor can therefore provide a high power density. In the present context it enables a comparatively small and lightweight plant to be used for hydrocracking, leading to a hydrocarbon product which is potentially more valuable and is easier to store and transport .

The invention also provides a plant incorporating one or more compact catalytic reactors for performing such a process.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows graphically the range of chain lengths produced by the Fischer-Tropsch synthesis;

Figure 2 shows a flow diagram of a chemical process of the invention; and

Figure 3 shows a diagrammatic cross-section of a reactor suitable for use in this process.

The invention relates to a chemical process for converting natural gas (primarily methane) to longer chain hydrocarbons. The natural gas typically contains higher hydrocarbons say C2 - CIl at up to 10 % v/v depending on its source. Preferably the natural gas is mixed with steam, and passed, at a temperature of about 45O ⁰ C, through an adiabatic pre-reformer containing a nickel or a platinum/rhodium based methanation catalyst. The higher hydrocarbons react with the steam to form methane and CO. The next stage involves steam reforming, that is to say the reaction:

H ₂ O + CH ₄ → CO + 3 H ₂

This reaction may be catalysed by a platinum/rhodium catalyst at about 800 ⁰ C, and the necessary heat produced by catalytic combustion of an inflammable gas in adjacent gas flow channel. The resulting mixture of carbon monoxide and hydrogen may be referred to as synthesis gas. The synthesis gas is then cooled and used to perform a Fischer-Tropsch synthesis to generate longer chain hydrocarbons, that is to say:

n CO + 2n H ₂ → (CH ₂ ) _n + n H ₂ O

which is an exothermic reaction, carried out for example at 21O ⁰ C, and an elevated pressure for example 2.0 MPa. The preferred catalyst for the Fischer-Tropsch synthesis comprises a support of gamma-alumina, with about 10-40% cobalt (by weight compared to the alumina) , and with a promoter such as ruthenium, platinum or gadolinium which is less than 10% the weight of the cobalt.

Preferably the reactors for the steam reforming and the synthesis reaction are each compact catalytic reactors as described above, with appropriate catalysts.

The hydrocarbon products produced by Fischer-Tropsch synthesis depend upon the chain growth probability factor (α) ; the larger this probability factor, the higher the proportion of the product at longer chain lengths. Referring to figure 1, this shows graphically the mass fraction for different chain lengths (carbon number) , for a range of different values of α. This distribution is known as the Anderson-Schultz-Flory distribution, and can be represented by the equation:

Mn = n (1 - α) α n-l

where Mn is the mass fraction of a carbon chain of length n. The value of α is affected by the reaction temperature, and by the pressure. For example, with Fischer-Tropsch carried out between about 204° and 22O ⁰ C and pressure about 2.0 MPa, the value of α is in the range between about 0.80 and 0.85. Operation at these conditions minimises formation of methane, while maximising the conversion of carbon monoxide to hydrocarbons, so that it provides a good yield of products. However, inevitably a significant proportion are of chain length above about C20, and are therefore waxy at ambient conditions.

Referring now to figure 2, the overall chemical process is shown as a flow diagram in which the principal components of the plant 10 are shown. The natural gas feed 5 consists primarily of methane with a small proportion of higher hydrocarbons C ₂ to Cn. It is mixed with steam, for example in a fluidic vortex mixer 14.

The gas/steam mixture is heated in a heat exchanger 16 using the hot exhaust gas from catalytic combustion so that the gas mixture is at a temperature of 500 ⁰ C. The mixture enters an adiabatic fixed bed pre-reformer 18 where it contacts a nickel or a platinum/rhodium based methanation catalyst. The higher hydrocarbons react with the steam to form methane and carbon monoxide.

The gas mixture typically leaves the pre-reformer 18 at about 45O ⁰ C, and is supplied to a reformer 20 which is a compact catalytic reactor made from a stack of plates (castellated plates alternating with flat plates) which define flow paths for endothermic and exothermic reactions which are in good thermal contact, and which contain appropriate catalysts on corrugated metal foil supports. The reformer channels in the reformer 20 contain a platinum/rhodium catalyst, and the steam and methane react to form carbon monoxide and hydrogen. The temperature in the reformer increases from 45O ⁰ C at the inlet to about 800-850 ⁰ C at the outlet.

The heat for the endothermic reactions in the reforming reactor 20 is provided by the catalytic combustion of a mixture of short chain hydrocarbons and hydrogen which is the tail gas 22 from the Fischer-

Tropsch synthesis; this tail gas 22 is combined with a flow of air. The combustion takes place over a palladium/platinum catalyst within adjacent flow channels within the reforming reactor 20. The combustion gas path is co-current relative to the reformer gas path. The catalyst may include gamma-alumina as a support, coated with a palladium/platinum mixture 3:1, which is an effective catalyst over a wide temperature range.

A mixture of carbon monoxide and hydrogen, i.e. synthesis gas, at above 800 ⁰ C emerges from the reformer

20 and is quenched to below 400 ⁰ C by passing it through a steam-raising heat exchanger 26. Water is supplied to this heat exchanger 26, and the steam is supplied to the mixer 14. The synthesis gas may be further cooled in another heat exchanger (not shown) , and any excess water separated from it. The synthesis gas is then compressed by one or more compressors 28 to about 20 atmospheres, and is then fed to a catalytic Fischer-Tropsch reactor 30a, this again being a compact catalytic reactor formed from a stack of plates as described above; the reactant mixture flows through one set of channels, while a coolant flows through the other set .

The reaction products from the Fischer-Tropsch synthesis, predominantly water and hydrocarbons such as paraffins, are cooled to about 80-90 ⁰ C by passage through a heat exchanger 32a and supplied to a separating chamber 34 in which the gases and condensed liquids separate. The gases, primarily short-chain hydrocarbons and unreacted synthesis gas, are heated back to the reaction temperature (say 200 ⁰ C) in a heat exchanger 33 and then supplied to a second Fischer-Tropsch reactor 30b, so that still more of the carbon monoxide undergoes the synthesis reaction. The outflow from the reactor 30b is passed through a second cooling heat exchanger 32b, so it is cooled to a temperature of about 80-90 ⁰ C, and supplied to a first stage separator 35. This separates the three phases: water, longer-chain hydrocarbons 42 (which are liquid at this temperature), and a gas phase. The liquid phase from the separating chamber 34 is also fed into this separator 35. The gas phase is passed through another cooling heat exchanger 36, to cool it to a temperature of about 2O ⁰ C, and supplied to a second stage separator 38. This separates the three phases: water, shorter-chain hydrocarbons, and a gas phase. These shorter-chain hydrocarbons may be treated as a product

stream. The gas phase emerging from this separator 38 consists of the non-condensed hydrocarbons and excess hydrogen gas which constitute the Fischer-Tropsch tail gases 22; these are collected and split. A proportion passes through a pressure reduction valve 39 to provide the fuel for the catalytic combustion process in the reformer 20 (as described above) . Some of the tail gases may be fed to a gas turbine (not shown) to generate electricity.

A major part of the final tail gas stream 22 is compressed to 100 atmospheres by a compressor 40. Similarly, the longer-chain hydrocarbon stream 42 from the first stage separator 35 (which consists of the waxy hydrocarbons that condense at about 8O ⁰ C) is also raised to this pressure by a pump 44. The hydrocarbon stream 42 is then passed through a heat exchanger 46 in which it is preheated by catalytic combustion in the adjacent channels, the combustion being of part of the tail gas stream 22 combined with air. The catalyst for the combustion can be the same as that in the combustion channels of the reforming reactor 20.

The hot hydrocarbon stream 42, at a temperature of about 400 ⁰ C is then mixed with the tail gas stream 22 at a mixer 48, and the mixture is then passed in succession through two hydrocracking reactors 50 and 52. In these reactors 50 and 52 the reaction channels contain a nickel tungsten catalyst on a stabilised gamma alumina support coated on a corrugated Fecralloy foil insert. The resulting hydrocarbons are supplied to a high-pressure separator vessel 54, and unreacted hydrogen emerges through a duct 56 and is returned to the mixer 48 to be recycled through the reactors 50 and 52. The liquid phase from the separator 54 is then supplied to a distillation column 58. This separates the hydrocarbons

into three fractions. A light fraction (C5 and below) consists of hydrocarbons that are gaseous under ambient conditions; this is fed back to the mixer 14 so that it is subjected to pre-reforming again. A middle fraction, whose boiling point at atmospheric pressure would be in the range about 20° to 35O ⁰ C, constituting hydrocarbons between about C6 and C18, is similar to diesel fuel, and is the desired hydrocarbon product 60 of the plant 10. The waxy fraction 62, whose boiling point at atmospheric pressure would be above 35O ⁰ C, constituting hydrocarbons above about C19, is recycled to be combined with the hydrocarbon stream 42 and subjected to hydrocracking again .

The recycled waxy fraction 62 is used as the cooling medium in the hydrocracking reactors 50 and 52, typically being provided at a temperature of about 35O ⁰ C. The intention is to maintain the reaction temperature in the range 410° to 42O ⁰ C, despite the exothermic nature of the reaction, and to heat the waxy fraction up to about 400 ⁰ C in the process. The flow rate of the waxy fraction 62 through the coolant channels of the reactors 50 and 52 can be adjusted using valves 64 in a bypass, to maintain the required reaction temperature profile.

A significant aspect of the present invention is the use of a compact catalytic reactor for the hydrocracking reactors 50 and 52. These reactors may be made of stainless steel castellated plates. Preferably the reaction channels extend straight through the reactor, aligned vertically so that the reactants flow from top to bottom. Referring now to figure 3, a reactor 70 suitable for use as the reactor 50 (or 52) is constructed from a stack of flat plates alternating with castellated plates, with the orientations of the channels defined by the castellations being orthogonal in alternate castellated

plates. The channels (not shown in figure 3) for the hydrocracking reaction contain catalyst-carrying foils, and extend straight through the reactor between appropriate headers (not shown) , the flow along these channels being indicated by the arrows F. The coolant channels are constructed from a long strip of 1 mm thick sheet formed into castellations running along its length. As shown, the castellated strip is cut into lengths 71 and these are laid side-by-side to define flow paths 72 transverse to the direction of the arrows F, three such lengths 71 of castellated strip forming a rectangle, with edge strips 74 along the edges, so as to provide paths between an inlet port 75 and an outlet port 76. The ends of the castellated strip next to the inlet port 75 and the outlet port 76 are cut square, while the other ends are cut at 45°, and triangular pieces 77 of the castellated strip provide links between the flow paths 72.

In a modification, additional sealing strips like the edge strips 74 are also provided between side-by-side edges of the lengths 71 of castellated strip.

The stack is assembled as described above, and then bonded together for example by high-temperature brazing. In use the flow of the coolant, indicated by the arrows G, follows a zigzag path which overall is countercurrent to that of the reactants (arrows F) .

Heat transfer into and across the coolant channels 72 may be enhanced by inserting corrugated foils (not shown) , similar to the foils that would be used in a reaction channels but not incorporating a catalyst, and not needing to be removable. Such inserted foils may be perforated. In a modification, the castellations defining the flow channels 72 might not follow straight paths

along the length of the strip, but might follow a sinuous or zigzag path, and might also be perforated. It will also be appreciated that the reactor 70 allows the coolant to pass three times across the width of the hydrocracking reaction channels, in passing between the inlet 75 and the outlet 76; alternatively the coolant might pass more than three times. It will be appreciated that the steel plates forming the reactor 70 must be sufficiently strong to withstand the pressures to which they are exposed. However, it will be understood that the entire reactor assembly shown in figure 3 may instead be enclosed within a conventional pressure vessel.