GB2441509A

GB2441509A - Fischer-Tropsch synthesis

Info

Publication number: GB2441509A
Application number: GB0617408A
Authority: GB
Inventors: Michael Joseph Bowe
Original assignee: CompactGTL PLC
Current assignee: CompactGTL PLC
Priority date: 2006-09-06
Filing date: 2006-09-06
Publication date: 2008-03-12
Also published as: GB0617408D0

Abstract

Fischer-Tropsch synthesis is carried out in a reactor module 10 defining a multiplicity of first and second flow channels arranged alternately, carrying a synthesis gas, and a coolant fluid, respectively; each of the first flow channels containing a gas-permeable catalyst 22 for the Fischer-Tropsch reaction. The reactor module 10 is enclosed within a pressure vessel 30, within which the pressure is arranged to be equal to that of the gas flow undergoing Fischer-Tropsch synthesis, this pressure being between 30 and 45 bar, and the reaction temperature being between 180{ and 220{C. The coolant fluid comprises water. The coolant is held at a pressure significantly less than that in the reaction channels, so there is no risk of the coolant leaking into the reaction channels.

Description

Fischer-Tropsch Synthesis This invention relates to a way of carrying

out Fischer-Tropsch synthesis, for example as part of a process to convert natural gas to longer-chain hydrocarbons, and to a plant including a catalytic reactor to perform the process.

A process is described in WO 01/51194 and WO 03/048034 (Accentus plc) 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 liquid under ambient conditions. The two stages of the process, steam/methane reforming and Fischer-Tropsch synthesis, require different catalysts, and heat to be transferred to or from the reacting gases, respectively, as the reactions are respectively endothermic and exothermic. The reactors for the two different stages must comply with somewhat different requirements: Fischer-Tropsch synthesis is usually carried out at a higher pressure but a lower temperature than steam/methane reforming; and in the heat transfer channels of the Fischer-Tropsch reactor only a coolant fluid is required.

According to the present invention there is provided a process for performing Fischer-Tropsch synthesis using a reactor module defining a multiplicity of first and second flow channels arranged alternately in the module, for carrying a gas mixture which undergoes Fischer-Tropsch synthesis, and a coolant fluid, respectively; each of the first flow channels containing a gas-permeable catalyst for the Fischer-Tropsch reaction; wherein the reactor module is enclosed within a pressure vessel, the pressure within the pressure vessel being arranged to be at a pressure substantially equal to that of the gas mixture undergoing Fischer-Tropsch synthesis; wherein the pressure of the gas mixture undergoing Fischer-Tropsch synthesis is between 30 and 45 bar, the temperature within the first flow channels is between 180 and 230 C, and wherein the coolant fluid comprises water.

Operating at this elevated pressure has an advantage in increasing both conversion and selectivity. At this temperature a water/steam mixture is in equilibrium at a pressure of around 24 bar. Hence the coolant can be held at a pressure significantly less than that in the first flow channels (where the reaction takes place)so that if a leak were to occur between the first and second flow channels it is unlikely that the leak would enable water to enter the catalytic reaction channels.

Preferably the coolant boils as it passes through the reactor module, as the latent heat thereby absorbed reduces the requisite coolant flow rate. The boil up rate in the channels is controlled so that no more than 5% of the mass tlowrate of the coolant water in the coolant channels exists in the vapour phase, preferably no more than 2%. If the vapour phase component is greater, there is a risk that insufficient liquid water is available for evaporation and the heat transfer rate may be compromised leading to a reaction hot spot. The boil up rate is controlled by the coolant recirculation rate, the coolant feed temperature, and the coolant back pressure, which is controlled by a valve downstream of the reactor coolant channels. After the pressure control valve the coolant flow enters a steam drum where the steam disengages from the liquid water. The recovered steam can then be used as process steam or to drive a steam turbine for power generation or syngas compression. The liquid water from the steam drum can then be further cooled if necessary and returned via recirculatiori pump(s) to the FT cooling channels. A significant advantage is that if there should be a small leak in the reactor module, then water does not leak into the reaction channels (where it would have a severely detrimental effect on the catalyst); rather, the gas mixture would leak into the coolant, and would then separate from the liquid phase in the steam drum. In the event of leakage the steam could be analysed for traces of syngas to provide an early warning of a potential problem.

Since the pressure within the pressure vessel is substantially that of the fluid at higher pressure, all the flow channels within the reactor module are either at the pressure of their surroundings, or are under compression. Consequently no parts of the reactor module are under tension. Preferably the gas mixture is arranged to flow through at least part of the pressure vessel either to reach the first flow channels or (after it has undergone Fischer-Tropsch synthesis) to leave the first flow channels. A benefit of the latter arrangement is that the pressure vessel can provide a first stage of separation between liquid droplets of product hydrocarbons and gaseous products.

A wide range of materials may be selected for the reactor module, for example an aluminium alloy, stainless steel, high-nickel alloys, or other steel alloys.

Preferably the catalyst is a removable insert, preferably on a metal substrate, the catalyst structure preferably being shaped so as to define a multiplicity of parallel flow sub-channels within each first flow channel. The metal substrate for the catalyst structure may be a steel alloy, preferably one that forms an adherent surface coating of 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, 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 pm, and the substrate may be corrugated, or pleated.

The reactor module may comprise a stack of plates.

For example, first and second flow channels may be defined by grooves in respective plates, the plates being stacked and then bonded together. 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 module is bonded together for example by diffusion bonding, brazing, or hot isostatic pressing. The first flow channels are preferably of height between 1 mm and 10 mm, more preferably between 4 mm and 6 mm, and the reactor module may be of length between 0. 5 m and 2.0 m, and of width between 0.15 m and 1.0 m (these dimensions corresponding to the area of each plate, in plan) The second flow channels, which carry the coolant, are of a similar height, typically no more than 10 mm and more preferably no more than 5 mm high, but preferably at least 1 mm high. With such narrow channels the coolant flow is likely to be laminar or transitional rather than fully turbulent, and there is a risk that a film of steam may form on the hot surfaces adjacent to the reaction channels. Such a film would inhibit heat transfer. It is therefore desirable to provide mixing devices in the coolant channels to ensure that the liquid and vapour are well mixed and so to avoid segregation of the phases.

This may be by use of perforated herringbone structures within the flow paths. The use of such turbulence-enhancing structures throughout the length of the coolant channels would tend to increase the pressure drop excessively, so that it is preferable to provide such turbulence-enhancing structures at spaced-apart positions along the flow paths.

The invention also provides a plant for performing the 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 a sectional view of part of a reactor block suitable for Fischer-Tropsch synthesis; Figure 2 shows a sectional view of a reactor incorporating the reactor block of figure 1; and Figure 3 shows parts of a flow diagram of a chemical plant incorporating the reactor of figure 2.

The invention is of relevance to a process for making hydrocarbons from a synthesis gas, that is to say a mixture of carbon monoxide and hydrogen. The synthesis gas may be produced in any known way, for example by steam/methane reforming. The synthesis gas is then arranged to undergo a Fischer-Tropsch synthesis to generate a longer chain hydrocarbon, that is to say: nCO+2nH2 -+ (0H;)+nHO which is an exothermic reaction, occurring at an elevated temperature, typically between 19000 and 280 C, and an elevated pressure, in the presence of a catalyst. The preferred catalyst for the Fischer-Tropsch synthesis comprises a coating of gamma-alumina of specific surface area 140-230 m/g 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, and a basicity promoter such as lanthanum oxide.

Referring now to figure 1 a reactor block 10 is shown in section and with the components separated for clarity. The reactor block 10 consists of a stack of flat plates 12 of thickness 1 mm spaced apart so as to define channels for a coolant fluid alternating with channels for the Fischer-Tropsch synthesis. The coolant fluid channels are defined by castellated plates 14 of thickness 0.75 mm. The height of the casteilations (typically in the range 1 to 4 mm) is 2 mm in this example, and 2 mm thick solid edge strips 16 are provided along the sides, and successive ligaments are 6 mm apart (the arrangement being described in more detail below) The channels for the Fischer-Tropsch synthesis are of height 5 mm, being defined by bars 18 of square cross-section, 5 mm high, spaced apart by 350 mm and so defining straight through channels.

Referring now to figure 2, a Fischer-Tropsch reactor is shown in section, with the reactor block 10 partLy broken away. As mentioned above, the reactor block 10 consists of a stack of flat plates 12 separated from each other to define flow channels. The orientations of alternate channels in the stack are generally orthogonal.

Each flat plate 12 is 1.0 mm thick and 10/0 mm square.

The channels for the Fischer-Tropsch reaction contain catalyst-carrying corrugated foils 22, and extend straight through the reactor block 10 (from top to bottom as shown) from a header 23 to which the syngas mixture is provided at elevated pressure through a pipe 24; the flat plates 12 are held apart by bars 18 that are 5 mm square in cross-section, running from top to bottom, at a spacing of 350 mm, so there are three such channels side-by-side between successive flat plates 12.

For the coolant channels the flat plates 12 are held apart by castellated sheets referred to as 14 in figure 1; these are actually constructed from a long strip of 0.25 mm thick sheet formed into 2 mm high castellations that are 6 mm wide running along its length. The castellated strip is cut into lengths 26 and these are laid side-by-side to define transverse flow paths (in horizontal directions as shown), so as to provide a path between an inlet port 2'! and an outlet port 28. The ends of the castellated strip 26 next to these ports 2'! and 28 are cut square; the other ends are cut at 45 , and triangular pieces 29 of the castellated strip are arranged to provide links between them. Hence the overall flow path for the coolant, as shown by the broken arrows, is a zig-zag path that is partially co-current relative to the flow in the Fischer-Tropsch channels. The flat plates 12, the bars 18, and the castellated strips 26 and 29 may be of aluminium alloy, for example 3003 grade (aluminium with about 1.2% manganese and 0.1% copper).

Preferably the triangular pieces 29 of the castellated strip are shaped and perforated to provide perforated herringbone structures. This ensures that liquid and vapour phases are thoroughly mixed as they flow through the triangular pieces 29, so that any tendency for laminar flow to develop through the straight sections 26 is counteracted by the turbulence created by the triangular pieces 29.

The stack is assembled as described above, and then bonded together to form the reactor block 10 for example by brazing. The corrugated metal foil catalyst carriers 22, which incorporate an appropriate catalyst, are then inserted into the channels for the Fischer-Tropsch synthesis.

The broken arrows in figure 2 indicate that the reactor block 10 allows the coolant to pass three times across the width of the Fischer-Tropsch channels, in passing between the inlet 27 and the outlet 28; alternatively the coolant might pass just twice across the width, or yet again the coolant might pass more than three times. The closely spaced castellations in the coolant channels provide rigidity to resist bending.

The reactor block 10 is mounted within a carbon steel pressure vessel 30, being supported by support bars 32. The inside surface of the pressure vessel 30 may be coated, for example with chromium, to suppress corrosion or the formation of iron carbonyl. The pressure vessel may be cylindrical with hemispherical ends. The pipe 24 for the syngas, and pipes 37 and 38 providing coolant to and from the ports 27 and 28, extend through the wall of the pressui-e vessel 30. There is an outlet port 36 for liquid products at the base of the vessel 30, and an outlet port 40 for gaseous products at the top of the vessel 30.

In use of the reactor 20 the coolant is supplied at about 2.4 MPa, and the syngas is supplied 3.8 NPa. The products of the Fischer-Tropsch synthesis, and unreacted gases, emerge into the pressure vessel 30 from the bottom of the reactor block 10, and so the pressure within the pressure vessel 30 is also about 3.8 MPa. Liquid droplets carried by the gas stream emerging from the reactor block 10 impact with the wall of the pressure vessel 30 and coalesce, and so liquid flows down to the bottom and out of the outlet port 36. The remaining gases emerge through the outlet port 40. They may be further processed, for example being cooled to condense water vapour and longer-chain hydrocarbons, and after removing the liquid phase the remaining gases may be subjected to a second Fischer-Tropsch synthesis using a substantially similar reactor 20.

To supplement the liquid/gas separation mechanisms mentioned above, demisting packings may also be provided within the pressure vessel 30. Alternatively or additionally a cyclonic separator (not shown) may be installed within the pressure vessel 30, this having a tangential inlet through which the gas stream (which may contain droplets) enters, a gas outlet connected to the outlet port 40, and a liquid outlet for de-eritrained liquid droplets, this preferably communicating through a pipe to below the level of the liquid products at the base of the vessel 30.

It will be appreciated that the coolant channels are under compression, but are held substantially rigid by the castellated sheets 14 (ie the strip lengths 26 and 29) . The pressure shell 30 hence provides a secondary containment in the event of leakage from the reactor block 10; it is of a shape that is easy to insulate, and easy to transport and install.

Operating at this elevated pressure has an advantage in increasing both conversion and selectivity. At a reaction temperature of 222 C, with a hydrogen to carbon monoxide ratio of 3 and a space velocity of 4300 /hr, with a pressure of 20 barg, the 05+ selectivity is about 73.7% and the carbon monoxide conversion is about 40.4%; both these parameters increase with the pressure, for example to about 77% and 43.6% (respectively) at 25 barg, and to about 79.1% and 51% (respectively) at 35 barg.

Since the overall yield depends on the product of these two parameters, it will be appreciated that a significant increase in yield can be obtained by operating at these markedly higher pressures.

Referring now to figure 3, a synthesis gas stream 44 is compressed by a compressor 45 to the requisite pressure of 3.8 MPa, and supplied to the inlet pipe 24; in practice there may be a number of compressor stages 45 in series to achieve this pressure, for example three compressor stages 45, between which the compressed gas stream is cooled. The liquids emerging through the outlet port 36 are supplied to a separator vessel 46 in which the hydrocarbon product separates from the water.

As regards the coolant, this is demineralised water or a mixture of demineralised water and a higher boiling point polar liquid such as tetra-ethylene glycol. Part of the coolant boils as it passes through the reactor module 10, so absorbing latent heat. The coolant liquid/vapour mixture emerging through the outlet pipe 38 is passed through a back pressure control valve 48 and fed into a steam drum 50; a recirculating pump 52 then feeds the coolant liquid 53 back to the inlet pipe 37. The steam drum 50 allows for expansion of the coolant fluid during heating, and allows steam 54 to separate from the liquid 53. Pressure in the coolant channels is maintained at about 2.4 MPa by the recirculating pump 52 and the back pressure control valve 48 downstream of the FT reactor module 10. The recovered steam 54 can be released from the drum 50 through a control valve 55, and can then be used as process steam or to drive a steam turbine (not shown) for power generation or syngas compression. The coolant liquid 53 from the steam drum 50 is then cooled through a heat exchanger 56, and returned by the pump 52 via the inlet pipe 37 to the FT cooling channels.

The coolant recirculation rate (controlled with the pump 52), the coolant feed temperature (adjusted by the heat exchanger 56) and the coolant back pressure (set by the control valve 48) are adjusted to ensure that the boil up rate is such that no more than 5% by mass of the coolant becomes a vapour in its passage through the reactor module 10, preferably no more than 2% (by mass) This limited proportion of vapour, combined with the provision of the turbulence-enhancing perforated herringbone triangular elements 29, ensures good heat transfer between the Fischer-Tropsch reactants and the coolant.

Where recovered steam 54 is used to drive a steam turbine, the steam is preferably then condensed and returned to the drum so that the coolant water is effectively in a closed circuit. In a modification to the plant shown in figure 3, the coolant liquid 53 may be cooled by introducing into it a stream of cold coolant; this may be in addition to, or in place of, the heat exchanger $6.

The heat exchanger $6 may be used to generate high-pressure steam to drive a turbine (not shown) It will be appreciated that the coolant pressure (2.4 MPa) mentioned above is that required for water to boil at 218 C. If the coolant also contains other liquids (such as tetra-ethylene glycol) that have a higher boiling point than water, then the appropriate coolant pressure would be somewhat lower to achieve boiling at the target temperature.

Claims

Claims 1. A process for performing Fischer-Tropsch synthesis using a

reactor module defining a multiplicity of first and second flow channels arranged alternately in the module, for carrying a gas mixture which undergoes Fischer-Tropsch synthesis, and a coo] ant fluid, respectively; each of the first flow channels containing a gas-permeable catalyst for the Fischer-Tropsch reaction; wherein the reactor module is enclosed within a pressure vessel, the pressure within the pressure vessel being arranged to be at a pressure substantially equal to that of the gas mixture undergoing Fischer-Tropsch synthesis; wherein the pressure of the gas mixture undergoing Fischer-Tropsch synthesis is between 35 and 45 bar, the temperature within the first flow channels is between 1800 and 230 C, and wherein the coolant fluid comprises water.
2. A process as claimed in claim 1 wherein the coolant fluid is in communication with a vessel in which there is an interface between liquid water and a gas phase.
3. A process as claimed in claim 1 or claim 2 wherein the pressure is set by controlling the coolant flowrate, coolant feed temperature and the pressure drop through a downstream pressure controlling valve.
4. A process as claimed in any one of the preceding claims wherein boil up rate in the coolant channels is controlled so that no more than 5% of the mass flowrate of the coolant in the coolant channels is in the vapour phase.
5. A Fischer-Tropsch process substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
6. A plant for performing Fischer-Tropsch synthesis by a method as claimed in any one of the preceding claims.

16013 MdR P.1. Mansfield

Chartered Patent Agent Agent for the Applicants