WO2010133621A1

WO2010133621A1 - Process to prepare a mixture of carbon monoxide and hydrogen

Info

Publication number: WO2010133621A1
Application number: PCT/EP2010/056864
Authority: WO
Inventors: Martin John Fernie
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2009-05-20
Filing date: 2010-05-19
Publication date: 2010-11-25

Abstract

Process to prepare a mixture of hydrogen and carbon monoxide from a gaseous mixture of hydrocarbons comprising methane, ethane and optional higher carbon number hydrocarbons by performing the following steps: (a) pre-reforming the hydrocarbon / steam mixture in the presence of a suitable reforming catalyst to convert ethane, and the optional higher carbon number hydrocarbons to methane, carbon dioxide and hydrogen, (b) heating the gaseous mixture obtained in step (a) to a temperature greater than 600 °C, (c) performing a partial oxidation by contacting the heated mixture of step (b) with a source of oxygen in a reactor burner yielding a reactor effluent having a temperature of between 1100 and 1500 °C, and (d) reducing the temperature of the reactor effluent by indirect heat exchange against evaporating water to a temperature between 750 and 900 °C, and wherein the heating in step (b) is effected by indirect heat exchange between the effluent of step (d) and the pre-reformed gaseous mixture obtained in step (a) using a printed circuit heat exchanger by feeding the effluent of step (d) through a first set of numerous microchannels as present in the printed circuit heat exchanger and feeding the pre-reformed gaseous mixture through a second set of numerous microchannels as present in the printed circuit heat exchanger, wherein the first and second set of microchannels are oriented such that convective heat exchange takes place between said first and second set of microchannels.

Description

PROCESS TO PREPARE A MIXTURE OF CARBON MONOXIDE AND

HYDROGEN

The invention relates to a process to prepare a mixture of carbon monoxide and hydrogen from a gaseous mixture of hydrocarbons comprising methane, ethane and propane by partial oxidation. EP-A-1858802 (published as WO-A-2006/097440) describes a process to prepare a mixture of hydrogen and carbon monoxide from a gaseous mixture of hydrocarbons comprising methane, ethane and optional higher carbon number hydrocarbons. The process consists of pre- reforming the hydrocarbon mixture in the presence of a suitable reforming catalyst to convert ethane, and the optional higher carbon number hydrocarbons to methane, carbon dioxide and hydrogen. Heating the gaseous mixture obtained in step (a) to 800 ⁰C and subjecting this heated gas to a partial oxidation. The effluent of the partial oxidation step is cooled to a temperature of 900 ⁰C and used to provide heat the pre-reformed gas.

According to EP-A-1858802 heat exchange may be performed in a shell-tube heat exchanger or in a plate- fin heat exchanger. A shell-tube heat exchanger comprises several tubesheets, refractory lining, heat exchange enhancements such as baffles and sheath-tubes on the shell-side. Moreover mixtures of hydrogen and carbon monoxide, sometimes referred to as synthesis gas or syngas, obtained in a partial oxidation process typically have low contents of water. Such mixtures are in the so-called metal dusting corrosion region and as such being very aggressive towards low alloys steels. Because of this metal-dusting corrosion special high alloy steel grades resistant to metal dusting will have to be used for the heat exchange surfaces. Such steel grades are very expensive and add to the cost of the unit. In addition it is known that the allowable pressure differential at the high temperatures of the above processes for such high alloy steels is limited. As a result full pressure designs of shell and tube exchangers are not possible for this duty and special instrument protective systems have to be installed to limit such pressure differentials within such a unit to an acceptable level when the process is in operation. This adds to the complexity of the process. The shell and tube exchanger further requires a large number of tubes of high alloy steel and as a result it is a large and expensive unit.

A plate fin exchanger is an alternative design where banks of fins are positioned between flat plates and brazed together to form a stack of finned channels. The hot and cold gases are passed through alternate channels and heat exchange takes place. The plate-fin exchanger has advantages over the shell-tube exchanger since it has narrow finned heat exchange channels so higher heat transfer coefficients and better heat transfer are obtained. This results in a smaller more compact unit which requires less heat transfer area. This results in that less expensive high alloy steel is required making the unit cheaper. The plate-fin exchanger has a number of limitations around materials of construction because there is no experience of making the units from high alloy steel to our understanding. The design pressure and allowable differential pressure are limited and the method of construction is quite complex. The present invention provides a more efficient process. The following process achieves the above- described objective. Process to prepare a mixture of hydrogen and carbon monoxide from a gaseous mixture of hydrocarbons comprising methane, ethane and optional higher carbon number hydrocarbons by performing the following steps:

(a) pre-reforming the hydrocarbon mixture in the presence of steam and a suitable reforming catalyst to convert ethane, and the optional higher carbon number hydrocarbons to methane, carbon dioxide and hydrogen,

(b) heating the gaseous mixture obtained in step (a) to a temperature greater than 600 ⁰C,

(c) performing a partial oxidation by contacting the heated mixture of step (b) with a source of oxygen in a reactor burner yielding a reactor effluent having a temperature of between 1100 and 1500 ⁰C, and

(d) reducing the temperature of the reactor effluent by indirect heat exchange against evaporating water to a temperature between 750 and 900 ⁰C, and wherein the heating in step (b) is effected by indirect heat exchange between the effluent of step (d) and the pre-reformed gaseous mixture obtained in step (a) using a printed circuit heat exchanger by feeding the effluent of step (d) through a first set of numerous microchannels as present in the printed circuit heat exchanger and feeding the pre-reformed gaseous mixture through a second set of numerous microchannels as present in the printed circuit heat exchanger, wherein the first and second set of microchannels are oriented such that convective heat exchange takes place between said first and second set of microchannels . Applicants found that the heat exchange step of the above process can be performed in more simple equipment, which mitigates several of the disadvantages of the heat exchanger of the prior art. The allowable pressure differential in such a printed circuit heat exchanger

(PCHE) is higher than in the prior art design and a full pressure design where full process pressure on one side and zero pressure on the other side is possible which makes the exchanger much more robust. Furthermore, due to the enhanced heat transfer performance of the printed circuit heat exchanger, the required heat transfer area can be much less than a shell and tube exchanger and less than a plate fin exchanger. This may result in a reduction in the content of high alloy steel in the heat exchanger of 60% as compared to the prior art shell and tube heat exchangers. The method of construction of such a microchannel exchanger is much simpler since the unit is made up of flat plates which are etched with channels and then diffusion bonded together. The diffusion bonding process results in a very strong bond between the plates and there is no gap between the plates as in a plate fin exchanger where the finned sections are inserted between plates. This is advantageous in reducing cost and apparatus size because such high alloy steel is expensive and difficult to obtain. Further advantages are the smaller plot size required for the process according to the invention. Further advantages will be described when discussing the below preferred embodiments.

The gaseous mixture of methane, ethane and optionally hydrocarbons having more than 2 carbon atoms can be obtained from various sources such as natural gas, refinery gas, associated gas or coal bed methane and the like. The gaseous mixture suitably comprises mainly, i.e. more than 90 v/v%, especially more than 94%,

C]__4 hydrocarbons, especially comprises at least 60 v/v percent methane, preferably at least 75 volume percent, more preferably at least 90 volume percent. Preferably natural gas or associated gas is used.

Preferably any sulphur in the gaseous feedstock is removed prior to performing step (a) to levels of below 10 ppm, preferably below 0.1 ppm. At high-sulphur feed levels the removal of sulphur is suitably performed by contacting the natural gas with a liquid mixture which contains a physical and a chemical absorbent. In such a process the gas mixture is treated at super-atmospheric pressure in two steps consecutively with two different liquid mixtures which contain a physical absorbent and a chemical absorbent. In the first step, H2S is selectively removed with respect to CO2, and in the second step, the remaining acid gases are virtually completely removed. An example of a suitable process is the so-called sulfolane extraction process. In addition to such removal or at low-sulphur feed levels small amounts of sulphur may also be removed by passing the gaseous feedstock through a bed of a suitable absorbent, for example zinc oxide, to absorb any hydrogen sulphide present. Often the absorbent is preceded by a hydrogenation reactor to convert organic sulphur compounds to hydrogen sulphide.

Step (a) may be performed by well known pre-reforming processes. Pre-reforming is a well-known technique and has been applied for many years in for example the manufacture of so-called city gas. Suitably the pre- reforming step is performed as a low temperature adiabatic steam reforming process. The gaseous feed to step (a) is mixed with a small amount of steam and preheated to a temperature suitably in the range 350-700 ⁰C, preferably between 350 and 530 ⁰C and passed over a low temperature steam reforming catalyst having preferably a steam reforming activity at temperatures of below 650 ⁰C, more preferably below 550 ⁰C. The pressure at which step (a) is employed is preferably between 20 and 70 bars. Preferably the pressure is about in the same range as the pressure at which step (c) is performed. The steam to carbon (as hydrocarbon and CO) molar ratio is preferably below 1 and more preferably between 0.1 and 1. Suitable catalysts for the low temperature steam pre- reforming step (a) are catalyst comprising an oxidic support material, suitably alumina, and a metals of the group consisting of Pt, Ni, Ru, Ir, Pd and Co. Examples of suitable catalysts are nickel on alumina catalyst as for example the commercially available pre-reforming catalysts from Johnson Matthey, Haldor Topsoe, BASF and Sued Chemie or the ruthenium on alumina catalyst as commercially available from Osaka Gas Engineering.

Step (a) is preferably performed adiabatically . Thus the gaseous feedstock and steam are heated to the desired inlet temperature and passed through a bed of the catalyst. Higher hydrocarbons having 2 or more carbon atoms will react with steam to give carbon oxides and hydrogen: at the same time methanation of the carbon oxides with the hydrogen takes place to form methane. The net result is that the higher hydrocarbons are converted to methane with the formation of some hydrogen and carbon oxides. Some endothermic reforming of methane may also take place, but since the equilibrium at such low temperatures lies well in favour of the formation of methane, the amount of such methane reforming is small so that the product from this stage is a methane-rich gas. The heat required for the reforming of higher hydrocarbons is provided by heat from the exothermic methanation of carbon oxides (formed by the steam reforming of methane and higher hydrocarbons) and/or from the sensible heat of the feedstock and steam fed to the catalyst bed. The exit temperature will therefore be determined by the temperature and composition of the feedstock/steam mixture and may be above or below the inlet temperature. The conditions should be selected such that the exit temperature is lower than the limit set by the de-activation of the catalyst. While some reformer catalysts commonly used are deactivated at temperatures above about 550 ⁰C, other catalysts that may be employed can tolerate temperatures up to about 700 ⁰C. Preferably the temperature of the gaseous mixture obtained in step (a) is between 320 and 500 ⁰C.

In step (b) the pre-reformed gaseous mixture obtained in step (a) is increased in temperature to above 600 ⁰C, preferably to above 700 ⁰C and more preferably to between 750 and 900 ⁰C. The heating is performed by indirect heat exchange with the effluent of step (d) as will be described in more detail below.

The partial oxidation of step (c) may be performed according to well-known principles as for example described for the Shell Gasification Process in the Oil and Gas Journal, September 6, 1971, pp 85-90.

Publications describing examples of partial oxidation processes are EP-A-291111, WO-A-9722547, WO-A-9639354 and WO-A-9603345. In step (c) according to the process of the present invention the heated pre-reformed feed as obtained in step (b) is contacted with an oxygen containing gas under partial oxidation conditions.

The partial oxidation of step (c) is performed in the absence of a catalyst as is the case in the above referred to Shell Gasification Process. In the context of the present invention a partial oxidation process is performed in the absence of a catalyst, like for example a reforming catalyst. Thus no catalytic conversion, i.e. catalytic reforming, takes place for conversion of hydrocarbons to carbon monoxide and hydrogen after the partial oxidation has taken place. Such processes are also referred to as non-catalyzed partial oxidation processes . The oxygen containing gas may be air (containing about 21 percent of oxygen) and preferably oxygen enriched air, suitably containing up to 100 percent of oxygen, preferably containing at least 60 volume percent oxygen, more preferably at least 80 volume percent, more preferably at least 98 volume percent of oxygen. Oxygen enriched air may be produced via cryogenic techniques, or alternatively by a membrane based process, e.g. the process as described in WO-A-93/06041.

Contacting the feed with the oxygen containing gas in step (c) is preferably performed in a burner placed at the top of a vertically oriented reactor vessel. The temperature of the oxygen as supplied to the burner is preferably greater than 200 ⁰C. The upper limit of this temperature is preferably 500 ⁰C. The gaseous product of the partial oxidation reaction in step (c) preferably has a temperature of between 1100 and 1500 ⁰C, more preferably between 1200 and 1400 ⁰C and an H₂/CO molar ratio of from 1.5 up to 2.6, preferably from 1.6 up to 2.2. In a preferred embodiment the partial oxidation of step (c) results in a gaseous mixture having a temperature of between 1100 and 1350 ⁰C and comprising of hydrogen, carbon monoxide, steam, carbon dioxide, methane and soot particles. In this embodiment this gaseous mixture is, before performing step (d) , passed through a ceramic foam filter or a ceramic wall-flow filter, where the soot particles are retained on the filter for a time sufficient to convert the soot to carbon monoxide. A mixture comprising of hydrogen, carbon monoxide, carbon dioxide, methane, which has a low soot content, is thus obtained. This is advantageous because a gaseous mixture with minimal soot will be less prone to foul the microchannels of the heat exchanger. By having such a soot converter it is furthermore possible to operate step (c) at a lower temperature, suitably between 1100 and 1250 ⁰C. At this temperature less oxygen is consumed while more soot is formed. Because the soot is removed from the reactor effluent this disadvantage is mitigated and the use of a microchannel exchanger becomes more feasible .

The amount of soot will be dependent on the gasification temperature, wherein the lower the temperature the more soot will be present in the gaseous mixture. At the lower end of the temperature range, as described above, the gaseous mixture may even comprise more than 5000 mg soot per actual m^, at the actual pressure of the gaseous mixture. The soot comprises of individual soot particles wherein typically more than 50 wt% of the soot particles have a size of less than 1 micron as measured by a Malvern Mastersizer.

The ceramic foam filter or a ceramic wall-flow filter suitable for use in the above embodiment may suitably be the well-known filter designs for removing particulates from exhaust gas generated by a diesel engine or alternatively may be a ceramic foam filter such as used in molten metal filtration. Suitable ceramic foams are made of a porous refractory material, preferably having a porosity of between 60 and 95% and a pore density of between 40 and 90 pores per inch (ppi) , preferably between 50 and 80 PPi- The pores have preferably a monomodalsize distribution. Applicants have found that material having a both small and large pores are more prone to clogging. The refractory material is suitably alumina or zirconia, more preferably alumina. Such ceramic foams are described in for example US-B-7258825 and available from for example Vesuvius, a division of the Cookson Group pic and AceChemPack Tower Packing Co. Ltd. Ceramic foams are available as sheets of for example about 1.5 cm thick. In order to increase the flow path length of the gas as it passes the ceramic foam two or more of these foam sheets may be combined into 1 brick by means of an impermeable alumina spray coating sprayed around the vertical sides of the stack of foam sheets. A brick like structure having dimensions of suitably between 100 and 300 cm thickness may thus be obtained. These bricks are suitably used to form the filter.

Wall-flow filters are well known from, for example, the car industry where such filters are used as filters for removing particulates from exhaust gas generated by a diesel engine. Such filters are sometimes referred to in the art as diesel particulate filters. A wall-flow filter typically has a shape of a monolithic honeycomb, the honeycomb having an inlet end and an outlet end, and a plurality of channels extending from the inlet end to the outlet end, the channels having porous walls wherein part of the total number of channels at the inlet end are plugged along a short portion of their lengths, and the remaining part of the cells that are open at the inlet end are plugged at the outlet end along a short portion of their lengths, so that a flowing exhaust gas stream passing through the channels of the honeycomb from the inlet end flows into the open channels, through the channel walls, and out of the filter through the open channels at the outlet end. Suitable wall-flow filters have a channel density of between 100 and 200 channels per square inch. In order to increase the separation efficiency more than one wall-flow filters may be arranged in series. Preferably one wall-flow filter is arranged on top of the next wall-flow filter separated by means of a spacer to allow the gas an easy entrance into the next wall flow-filter.

Suitable wall-flow filters are made of a porous refractory material, preferably having a monomodal pore size distribution. The monomodal pores preferably have a diameter of between 5 and 25 μm, and more preferably 8- 14 μm. The refractory material is suitably alumina or zirconia, more preferably alumina because it is more stable. Suitable wall-flow filter designs are described in Structured Catalysts and Reactors 2nd Edition (Moulijn & Cybulski) pages 675 - 685 and specialist ceramic manufacturing companies such as for example Ceramiques, Techniques & Industrielles s.a. can manufacture such filters.

The ceramic foam or a ceramic wall-flow filter preferably comprises a coating of an oxide of a metal selected from the group consisting of manganese, iron, copper, tin, cobalt and cerium. The presence of these oxides catalyse the conversion of the soot particles in the filter itself. The content of these oxides in the filter is preferably between 20 and 60 wt%. In step (d) the reactor effluent of step (c) is first reduced in temperature to a temperature between 750 and 900 ⁰C against evaporating water. Examples of such processes and boilers or use in such processes are described in WC-Λ-IO^ -11604 j, U,:-Λ-2^0&014 D316, EP-A-0774103, WO-A-2005015105 or US-A-4245696.

In step (b) the gaseous mixture obtained in step (a) is heated by indirect heat exchange between the effluent of step (d) and the pre-reformed gaseous mixture obtained in step (a) . The effluent of step (d) is fed through a first set of numerous microchannels and the pre-reformed gaseous mixture is fed through a second set of numerous microchannels. The first and second set of microchannels are oriented such that convective heat exchange takes place between the gases flowing through said first and second set of microchannels.

In order to achieve the most optimal heat exchange the first and second set of microchannels of the printed circuit heat exchanger are preferably arranged alternately to ensure good thermal contact between the channels. Channels in individual steel plates suitably form the microchannels. The microchannels are suitably etched into the steel plates. The steel plates are preferably stacked and diffusion bonded together. Preferably the microchannels of the first and second set are positioned such that the flow direction in the first set of microchannels is substantially counter-current with the flow direction in the second set of microchannels . The number of microchannels in such plate may vary from more than 10 to more than 10000. The microchannel in such a plate preferably has a height of 5 mm or less, more preferably 2 mm or less, and still more preferably 1 mm or less, and in some preferred embodiments height is in the range of 0.1 and 2 mm. Channel cross-sections can be, for example, rectangular, circular, triangular, or irregularly shaped. Height and width are perpendicular to length and either or both can vary along the length of a microchannel . Height and width can be arbitrarily selected; in the present invention, height is defined as the smallest dimension of a channel that is perpendicular to flow. The thickness of a steel plate is preferably such that the sufficient heat transfer is possible, while at the same time sufficient mechanical strength is provided. The thickness will thus depend on the type of material chosen for the plate and the dimensions of the microchannel. Suitably the thickness is between 0.2 and 4 mm.

The steel plates, and preferably the steel plates which come into contact with the effluent of step (d) , are preferably made of a metal alloy, which can withstand metal dusting. Such high alloy steel will preferably comprises between 0 and 20 wt% iron, between 0 and 5 wt% aluminium, between 0 and 5 wt% silicon, between 20 and 50 wt% chromium and at least 35 wt% nickel. More preferably the content of chromium in the metal alloy is more than 30 wt%. More preferably the metal alloy comprises between 1 and 5 wt% aluminium. More preferably the metal alloy comprises between 1 and 5 wt% silicon. More preferably the metal alloy comprises between 0 and 2 wt% titanium and/or REM. An example of a suitable metal alloy is Alloy 693 as obtainable from Special Metals Corporation, USA.

The synthesis gas as obtained by the above process may advantageously be used as feedstock for a Fischer- Tropsch synthesis process, methanol synthesis process, a di-methyl ether synthesis process, an acetic acid synthesis process, ammonia synthesis process or to other processes which use a synthesis gas mixture as feed such as for example processes involving carbonylation and hydroformylation reactions.

In a preferred embodiment the synthesis gas is used in a Fischer-Tropsch synthesis step (d) wherein a hydrogen- and carbon monoxide-containing gas as obtained in step (d) is converted in one or more steps at least partly into liquid hydrocarbons in the presence of a Fischer Tropsch type catalyst which preferably comprises at least one metal (compound) selected from group 8 of the Periodic Table. Preferred catalytic metals are iron and cobalt, especially cobalt. It is preferred to produce a very heavy product in step (e) . This results in a relatively low amount of light hydrocarbons, e.g. C1-C4 hydrocarbons by-product, resulting in a higher carbon efficiency. Large amounts of heavy products may be produced by catalysts which are known in the literature (e.g. vanadium or manganese promoted cobalt catalysts) under suitable conditions, i.e. relatively low temperatures and relatively low H2/CO ratios. Any hydrocarbons produced in step (e) boiling above the middle distillate boiling range may be converted into middle distillates by means of hydrocracking. Such a step will also result in the hydrogenation of the product as well as in (partial) isomerization of the product.

The Fischer Tropsch synthesis is, as indicated above, preferably carried out with a catalyst producing large amounts of unbranched paraffinic hydrocarbons boiling above the middle distillate range. Relatively small amounts of oxygen containing compounds are produced. The process is suitably carried out at a temperature of 150 to 300 ⁰C, preferably 190 to 260 ⁰C, and a pressure from 20 to 100 bar, preferably from 30 to 70 bar. In the hydrocracking process preferably at least the fraction boiling above the middle distillate boiling range is hydrocracked into middle distillate. Preferably all C5^"1", especially all C_]__Q ⁺ hydrocarbons are hydrocracked in view of the improved pour point of the middle distillates obtained in such a process. The product stream obtained in step (d) is separated into a relatively light stream and a relatively heavy stream. The relatively light stream (off gas) comprises mainly unconverted synthesis gas, inerts, carbon dioxide and the C1-C3 hydrocarbons, preferably the C1-C4 hydrocarbons. In a preferred embodiment this stream is pre-reformed in step (a) together with the hydrocarbon mixture. A small bleed stream is not recycled to step (a) in order to avoid a build-up of inerts, defined as CO2, N2, CH4, Ar, in the recirculating gas mixture. Suitably part of the CO2 is removed from the recycle gas before using said gas in step (a) . Preferably the content of this recycle gas in the total feed to step (a) is between 10 and 50 mol%.

A preferred configuration for the first and second set of microchannels is described in Figure 1. Figure 1 shows plate 1 and numerous microchannels 2 forming the second set of numerous microchannels. Microchannels 2 have an inlet end 4 fluidly connected to a common header 4b for the pre-reformed gaseous mixture obtained in step (a) and an outlet end 5 fluidly connected to a common header 5a for the heated mixture. Figure 1 also shows plate 6 and numerous microchannels 7 forming the first set of numerous microchannels. Microchannels 7 are provided with inlet openings 11 fluidly connected to a common header channel 8 through which the effluent of step (d) is supplied to the numerous microchannels 7. At the downstream end 9b of the microchannels 7 a cooled gaseous mixture is collected in a common header 9.

Figure 2 illustrates a stack of plates 1 and 6 arranged alternately to form the heat exchanger 10 which may be advantageously used in the process according to the invention. The number of plates 1 and 6 will typically be greater than shown. Figure 2 also shows header 8, header 9, header 5a and header 4b. These headers are welded around the inlet and outflow openings as present in plates 1 and 6. Header 8 is fluidly connected to inlet 11a. Header 9 is fluidly connected to outlet 9a. Header 4b is fluidly connected to inlet 4a and header 5a is fluidly connected to outlet 12a.

Figure 3 shows an embodiment wherein more than one heat exchangers 10 are positioned in a single pressure vessel 13 to form a combined heat exchanger 14. Only the inlets 4a, 11a and outlets 9a and 12a for the various streams pass the pressure vessel wall making the design of the combined heat exchanger 14 more simple. Alternatively header 4b and inlet 4a may be omitted to obtain heat exchanger 10' modules and combined heat exchange 14' as shown in Figure 3a. The gaseous mixture obtained in step (a) , being the coldest gas stream, will then be provided by a single inlet 18 directly into the interior of the pressure vessel 19 or 14'. The inner wall of vessel 19 is preferably refractory lined. Figure 4 shows how combined heat exchanger of Figure

3 or 3a is used in a process line-up according to the present invention. De-sulphurized natural gas 15 is mixed with the gaseous by-products stream 16 from a Fischer- Tropsch synthesis step 17 to form feed 18. Feed 18 is fed to a pre-reformer reactor 19 and a pre-reformed effluent 20 is obtained. This effluent 20 is increased in temperature in heat exchanger 14 or 14' of Figure 3 against the partly cooled reactor effluent 21 of a partial oxidation reactor 22. Heated pre-reformed mixture 23 together with oxygen 24 is fed to a burner 25 of partial oxidation reactor 22. Oxygen 24 is preferably heated in heat exchanger 26. The partial oxidation reactor 22 is provided with a soot filter 27 and an outlet for a reactor effluent 28. In waste heat boiler 29 the reactor effluent 28 is reduced in temperature before said gas is used in heat exchanger 14 wherein the gas is further cooled to a finally cooled reactor effluent 31. In waste heat boiler 29 steam 30 is generated. Preferably steam 30 is super heated against effluent 31 in a first heat exchanger 33 to yield super heated steam 33. The partly cooled effluent 31 thus obtained is preferably used to increase the temperature of the natural gas feed 34 prior to the sulphur removal step described above in heat exchanger 35. The partly cooled effluent 31 obtained in heat exchanger 35 is preferably used to heat up boiler feed water 36 to heated boiler feed water 37. This heated boiler feed water 37 is fed to waste heat boiler 29. The finally cooled reactor effluent 31a preferably has a temperature within the operating temperatures of the Fischer-Tropsch , comprising hydrogen and carbon monoxide, is used as feed in Fischer-Tropsch synthesis reactor 17. From the paraffinic waxy production 38 as prepared in said reactor 17 a gaseous by-product stream 16 is recovered and recycled to reactor 19.

Claims

C L A I M S

1. Process to prepare a mixture of hydrogen and carbon monoxide from a gaseous mixture of hydrocarbons comprising methane, ethane and optional higher carbon number hydrocarbons by performing the following steps: (a) pre-reforming the hydrocarbon mixture in the presence of steam and a suitable reforming catalyst to convert ethane, and the optional higher carbon number hydrocarbons to methane, carbon dioxide and hydrogen,

(c) performing a partial oxidation by contacting the heated mixture of step (b) with a source of oxygen in a reactor burner yielding a reactor effluent having a temperature of between 1100 and 1500 ⁰C, and (d) reducing the temperature of the reactor effluent by indirect heat exchange against evaporating water to a temperature between 750 and 900 ⁰C, and wherein the heating in step (b) is effected by indirect heat exchange between the effluent of step (d) and the pre-reformed gaseous mixture obtained in step (a) using a printed circuit heat exchanger by feeding the effluent of step

(d) through a first set of numerous microchannels as present in the printed circuit heat exchanger and feeding the pre-reformed gaseous mixture through a second set of numerous microchannels as present in the printed circuit heat exchanger, wherein the first and second set of microchannels are oriented such that convective heat exchange takes place between said first and second set of microchannels .

2. Process according to claim 1, wherein the first and second set of microchannels are arranged alternately to ensure good thermal contact between the channels and wherein the microchannels are formed by channels in individual steel plates which are stacked and diffusion bonded together.

3. Process according to claim 2, wherein the steel plates are made of an metal alloy comprising between 0 and 20 wt% iron, between 0 and 5 wt% aluminium, between 0 and 5 wt% silicon, between 20 and 50 wt% chromium and at least 35 wt% nickel.

4. Process according to claim 3, wherein the content of chromium in the metal alloy is more than 30 wt%.

5. Process according to any one of claims 3-4, wherein the metal alloy comprises between 1 and 5 wt% aluminium.

6. Process according to any one of claims 3-5, wherein the metal alloy comprises between 1 and 5 wt% silicon.

7. Process according to any one of claims 2-6, wherein the metal alloy comprises between 0 and 2 wt% titanium and/or REM.

8. Process according to any one of claims 1-7, wherein the steam to carbon (as hydrocarbon and CO) molar ratio of the feed to step (a) is between 0.1 and 1.

9. Process according to any one of claims 1-3, wherein the temperature of the gaseous mixture obtained in step (a) is between 320 and 500 ⁰C.

10. Process according to any one of claims 1-9, wherein the gaseous mixture obtained in step (a) is increased in temperature to between 750 and 900 ⁰C.

11. Process according to any one of claims 1-10, wherein the partial oxidation of step (c) results in a gaseous mixture having a temperature of between 1100 and 1350 ⁰C and comprising of hydrogen, carbon monoxide, steam, carbon dioxide, methane and soot particles and wherein before performing step (d) said mixture is passed through a ceramic foam filter or a ceramic wall-flow filter, where the soot particles are retained on the filter for a time sufficient to convert the soot and wherein a mixture comprising of hydrogen, carbon monoxide, carbon dioxide, methane which is poor in soot is obtained.

12. Process according to claim 11, wherein the temperature of the gaseous mixture as obtained in step (c) as it passes the filter has a temperature of between 1100 and 1250 ⁰C.

13. Process to prepare a Fischer-Tropsch synthesis product by performing a Fischer-Tropsch synthesis step (e) , wherein the hydrogen- and carbon monoxide- containing gas, as obtained in step (d) of a process according to any one of claims 1-12, is converted in one or more steps at least partly into liquid hydrocarbons in the presence of a Fischer Tropsch type catalyst which comprises at least one metal (compound) selected from group 8 of the Periodic Table.

14. Process according to claim 13, wherein the product stream obtained in step (e) is separated into a relatively light stream and a relatively heavy stream, wherein the relatively light stream comprises mainly unconverted synthesis gas, inerts, carbon dioxide and

C]_-C3 hydrocarbons and wherein part of the light stream is added to the feed of step (a) and wherein the relatively heavy stream is the Fischer-Tropsch synthesis product or an intermediate to the Fischer-Tropsch synthesis product.