IMPROVEMENTS IN THE UTILISATION OF METHANE

The present invention relates to a process for use in the production of methanol from a methane stream.

Background of the Invention

Methanol production from methane sources such as found in coal seam deposits (Coal Seam Methane - CSM) or natural gas reservoirs is becoming increasingly common as the process begins to satisfy economic and environmental considerations. However, the process using conventional gas-fired radiant reformer furnaces has been considered inappropriate for locations in or on moving platforms, such as tethered ships or off shore platforms, due to the relative fragile nature of such furnaces.

More robust reforming processes such as the ICI partial oxidation reforming process have the problem of requiring air separation for oxygen production and the presence of separated oxygen presents major hazard problems for its ship or barge installation where produced methanol and other liquid hydrocarbons would also be present.

The StarChem process overcomes these problems but the high production rating generally requiring gas flows in excess of 50 PJ/year make these units unsuitable for smaller gas flows such as 10 to 30 PJ/year and unsuitable for the conventional progressive development of CSM deposits where a progressive and constant development with ongoing sale of gas over many years would be required prior to the installation of a StarChem™ process plant.

A further problem with most methane-based methanol production units is that a significant amount of methane must be burnt as fuel in the process and consequently a significant carbon dioxide emission is generally attributed to methanol production.

A further problem with typical methanol production plants is that a significant amount of

site erection is required making such plants difficult and expensive to erect in remote locations and difficult to recover when the supply of methane diminishes.

Reformers such as the convection steam reformers produced by Haldor Topsoe™ have enabled the design of very compact and efficient reformers. However, the reforming tubes, which are under internal pressure and which are heated to high temperatures to enable the reforming reactions must use very high-grade high temperature alloys and be of sufficient thickness to resist high temperature creep due to the internal pressure and the high tube temperatures. The heat is provided by indirect contact with a hot gas which enters the steam reformer containing the reformer tubes. After contacting the outside of the reformer tubes, the hot gas leaves the convection steam reformer as a flue gas. Due to the high pressure of the feed methane and steam gas streams within the reformer tubes the practicability of being able to fire such reformers with hydrogen rich fuel gas without overheating and damaging the reformer tubes has been questioned and provides a significant safety risk.

A further problem with steam reforming-based methanol production is that the process also requires the compression of the produced synthesis gas and also, in most cases, the recycling of un-reacted synthesis gas. Such compression requires the steam turbine or gas turbine-based energy to drive the compressors and the provision of such energy requires the use of fuel such as extra methane and which, in turn, is a significant demand for process energy and a significant source of carbon dioxide emissions allied to methanol production.

Accordingly, the present invention seeks to ameliorate at least some of the inherent problems associated with current methods and processes of methanol production and methane reforming.

Summary of the Invention

According to one aspect the present invention provides a process for use in the production

of methanol from a methane gas stream wherein the process includes the following steps: a. providing a gas feed stream including methane and steam to a convection steam reforming reactor thereby producing an outlet stream of synthesis gas including carbon oxide(s) and hydrogen, wherein the gas feed stream within the convection steam reforming reactor is heated by means of indirect contact with an inlet stream of hot gas entering the convection steam reforming reactor thereby providing an outlet stream of flue gas; b. combusting the outlet stream of flue gas and using the resultant hot gas to drive a hot gas expansion turbine; and, c. compressing the outlet stream of synthesis gas to a sufficient pressure to produce methanol wherein at least part of the energy used to compress the outlet stream of synthesis gas is provided by the hot gas expansion turbine.

In one embodiment, the inlet stream of hot gas enters the convection steam reforming reactor at substantially the same pressure as the gas feed stream including methane and steam. As the conversion of methane and steam in the convection steam reforming reactor is usually under pressures greater than atmospheric pressure, the inlet stream of hot gas maybe compressed prior to entering the convection steam reforming reactor wherein at least part of the energy used to compress the inlet stream of hot gas is provided by the hot gas expansion turbine.

According to one form, a hydrogen gas stream is mixed with a gas stream including oxygen, such as air for example, and then combusted to form the inlet stream of hot gas and/or the outlet stream of flue gas is mixed with a gas stream of hydrogen prior to combustion. The hydrogen stream may be derived from the synthesis gas exiting the convection steam reforming reactor. In a preferred form, the amount of hydrogen that is derived from the synthesis gas provides the synthesis gas with an appropriate ratio of carbon oxide(s) and hydrogen for methanol production. In one embodiment, the hydrogen stream is derived from the synthesis gas by means of membrane diffusion technology.

According to one embodiment the methane gas stream is derived from a coal mining

operation such as Coal Seam Methane (CSM) or a natural gas reservoir.

Brief Description of the Drawings

The present invention will become better understood from the following detailed description of preferred but non-limiting embodiments thereof, described in connection with the accompanying figures, wherein:

Figures 1 is a process diagram of one embodiment of the present invention; and, Figure 2 is a process diagram of a further embodiment of the present invention.

Detailed Description of the Invention

The present invention provides a process for use in the production of methanol in which a recuperated gas turbine is used to drive the synthesis gas compressors downstream of the methane steam reforming-stage of a methanol plant.

In addition, the hot gas side of the steam reformer is pressurised when indirectly contacting the methane steam feed stream within the convection steam reformer such that the pressure differential between the hot gas and the process stream within the reformer tubes is greatly reduced. This provides a significant advantage by reducing the costs of reformer tubes in a typical convection steam reformer as the tubes do not have to withstand high pressure differentials enabling the use of thinner reforming tube walls and or lower cost tube materials. Also this allows that the reformer may be heated by combusting substantially pure hydrogen derived by known means, such as membrane diffusion, from the synthesis gas so as to produce optimum hydrogen to carbon oxide(s) ratio in the residual gas for methanol synthesis and to substantially eliminate carbon dioxide emissions commonly associated with previous methane-based methanol production.

A further advantage provided by the present invention is that the pressurisation of the hot gas inlet stream may also be provided by energy derived from the recuperated gas turbine.

In accordance with one embodiment, the methanol plant may be fed with a methane and carbon dioxide mixture with an optimum 1:3 carbon dioxide to methane ratio, hi this form at least part of the carbon dioxide in the feed gas is derived by known means from coal and preferably from coal-based methanol production and the hydrogen required for firing the reformers and gas turbine is also derived from coal and preferably from coal-based methanol production. In a preferred form, the methane required for the feed stream into the steam reforming reactors is Coal Seam Methane (CSM) and the production of the CSM is assisted by the injection of carbon dioxide.

In accordance with one preferred embodiment energy derived from the hot gas expansion turbine may also be used to processes waste water which is produced by the extraction of the CSM.

It is envisaged that the hot gas expansion turbine is chosen from a turbine that can provide the necessary efficiency and work output required for the size and output for a specific methanol installation and may be chosen from a hot gas expansion turbine including an inter-cooled air compressor and is recuperated such as in a Rolls Royce WR21 gas turbine.

The methane reforming may be carried out in a pressurised convective reformer similar to a Haldor Topsoe High Temperature Convective Reformer (HTCR) unit in which the effective cross section of flue gas convection zone is suitably modified to allow for the air flow required for associated gas expansion turbine and to allow for passage of pressurised inlet hot gas stream consistent with the combustor pressure in the associated gas turbine.

The reformer may be an internally insulated pressurised vessel with a tube bundle of catalyst filled reformer tubes and with the tube bundle having horizontal baffles such as in a conventional heat exchanger with the inlet hot gas stream being heated to above 800 ⁰ C and below 1,200 ⁰ C but preferably in the range of 950 to 1050 ⁰ C before the first horizontal cross flow path across the tubes before passing through the baffle with additional hydrogen being added at the transition point in the baffle so as to bring the flue gas back up to its desired temperature before passing across the tube bundle in its second pass before being

re-heated by hydrogen rich fuel injection and so on through preferably more than 4 passes and preferably between 6 and 8 passes before exiting to pass to the final combustion stage in the gas turbine.

The reformer may be of a type similar to the Haldor Topsoe HTCR Twin reformer which may be a number of parallel HTCR Twin reformers or a HTCR Twin reformer unit having convection heated tube bundles and associated combustion chambers suitably sized for the gas turbine's power and pressure rating and where the combustion chamber and reformer outer vessels are pressurised and the combustor stages designed for operation with hydrogen or hydrogen rich fuel gas.

A process of producing methanol from synthesis gas similar to that developed by Air Products may be used to minimise the need to recycle un-reacted synthesis gas such that said un-reacted synthesis gas can be shift converted and the hydrogen removed by known means for use as the dominant source of fuel for firing the methane convection reforming reactors.

The methanol may also be used to fuel other gas turbines and engines and particularly those which incorporate an exhaust gas heated methanol dissociation stage to convert exhaust gas heat energy below 600 ⁰ C to produce a carbon oxide and hydrogen fuel having a heating value in excess of 10% and up to 29% higher than the feed methanol. The dissociated methanol can be used to feed a solid oxide fuel system and where the fuel cell system in addition to supplying power is also the first stage and principal combustion system for the hot gas expansion turbine.

Due to the above mentioned advantageous properties of the present invention, the resulting methanol production unit is an easily transported, located and relocated system which also incorporates a reforming stage and which in fully assembled form can readily withstand movement associated with its transport and also its location and operation on moving platforms such as ships, barges and offshore platforms.

The methanol that is produced may be subsequently converted to liquid fuels and in particular high octane gasoline as in the Mobil zeolite catalyst-based methanol to gasoline process or a variation of that process which could produce light distillate or high grade aviation turbine fuel for use in diesel engines or aircraft turbines.

A major advantage of producing methanol by means of this invention is that the produced methanol may have a zero or very low or does not have a significant carbon dioxide emission penalty associated with its production and any overall carbon dioxide release associated with the produced methanol's use as a fuel, can be further minimised by the use of the produced methanol in fuel cell-based energy systems and in particular in Solid Oxide Fuel Cell (SOFC) based energy conversion systems.

One possible embodiment of the present invention is where the methane used as the feed gas stream is methane derived from coal seams and the produced methanol is used as the preferred fuel for other coal mining activities, such as the coal extraction and mining operations, thus reducing CO2 emissions allied to coal extraction and transport operations and also this sector's dependence on crude oil-based fuels.

The present invention will now be illustrated with a detailed description associated with the attached figures.

Referring to figure 1 there is shown an embodiment of the invention in which item 2 is a first stage air compressor, item 4 is a second stage air compressor, item 6 is a hot gas expansion turbine. Item 20 is a combustion chamber and item 10 is a connecting shaft making items 2, 4, 6 and 10 the high pressure gas producer of a gas turbine installation which drives items 8, 12, 14, 16 and 18 and in which item 8 is a second stage hot gas expander, item 14 is a synthesis gas compressor, item 16 is a gas recirculation compressor, item 18 is a generator and item 12 is a connecting shaft for items 14, 16 and 18.

Item 22 is a water cooled compressed air intercooler and item 24 is an air humidification

and de-humidification system which enables waste water to be used to partially humidify air and to be split into distilled water and a high dissolved salt content water residue and item 26 is a water pumping and recirculation system. Item 28 is a compressed air gas turbine exhaust recuperator and item 30 is a water cooled heat exchange gas cooler.

Item 32 is a membrane-based hydrogen removal system and item 34 is a methanol synthesis and separation facility incorporating a synthesis reactor, its allied heat exchange and steam raising systems, crude methanol recovery and allied distillation and purification systems.

Items 36 and 40 are burners and air pre-heating systems and items 38 and 42 are convective reformers with items 36, 40, 38 and 42 forming, in this version of the invention, a convection heated steam reformer in which the hot gas inlet stream is provided under pressure and forms part of the aforesaid gas turbine. Item 44 is a natural gas, methane or coal seam methane (CSM) gas pre-treatment system for removal of impurities such as sulphur compounds, particulates and the like and includes steam addition, pressurisation, gas heating and water saturation so as to make the gas a suitable feed for steam reforming. Item 45 is a waste heat boiler and item 48 is a boiler which acts as a synthesis gas cooler.

Air enters item 2, it is compressed and leaves via pipeline or duct 104 and passes through item 22 in which it is cooled before passing via pipeline or duct 106 to item 4 in which it is further compressed before leaving via pipeline or duct 108 to item 24 in which it is cooled and substantially saturated with water before leaving via pipeline 124 and entering item 28 in which it is heated and leaves via pipeline 126 and passes to item 36 in which hydrogen or hydrogen rich fuel is added and burnt with the hot flue gases passing via pipeline 128 to item 38 in which the hot gases provide heat by counter current indirect heat exchange to the methane reforming taking place in item 38 before leaving as cooled flue gas via pipeline 130 to item 40 in which further hydrogen or hydrogen rich fuel is added and burnt in the remaining excess air in the flue gases before passing via pipeline 132 to item 42 in which the hot gases provide heat by counter current indirect heat

exchange to the methane reforming taking place in item 42 before leaving as cooled flue gas via pipeline 134 to the gas turbine combustor, item 20 in which further hydrogen or hydrogen rich fuel is added and combusted to further heat the partially combusted air before it passes to item 6 and then to item 8 before leaving via pipeline or duct to item 28 in which it is cooled before passing via pipeline or duct 134 to atmosphere, hi this version of the invention exhaust gas in duct 138 provides waste heat to the steam raising system in item 46 before passing to atmosphere. Pressurised feedwater passes to item 46 via pipeline 174 and steam leaves via pipeline 176.

Feed gas such as Coal Seam Methane, methane, natural gas and the like enter via pipeline 140 and passes via item 44 and pipeline 142 and then via pipeline 144 to item 38 or pipeline 148 to item 42 with the synthesis gas produced by steam reforming leaving via pipelines 148 and 150 to item 48 in which the gas is cooled and steam raised and then via pipeline 152 to item 30 in which the gas is cooled before leaving via pipeline 154 to item 14. The compressed synthesis gas leaves item 14 via pipeline 156 and joins with gas in pipeline 164 and passes via pipeline 158 to item 32.

In item 32 the combined gas stream has hydrogen removed to provide hydrogen or hydrogen rich gas for fuel and the residual hydrogen depleted gas with the correct hydrogen to carbon oxides ratio for methanol synthesis passes via pipeline 160 to item 34. Hydrogen rich fuel gas extracted from item 32 passes via pipeline 186 and then pipeline 188 to fuel item 40 and via pipeline 190 to fuel item 20 and the remainder passes via pipeline 192 to fuel item 36.

In item 34 the synthesis gas is partially converted to and extracted from gas leaving the synthesis reactor with the crude methanol being distilled to remove impurities and the residual synthesis gas being recycled via pipeline 162 to item 16 and then via pipeline 164 to item 32 and then back to item 34.

Product methanol, of suitable purity, is removed via pipeline 170. Water is removed via pipeline 168 and item 168 which may be as shown, a single pipeline, or may be a multiple

of pipelines contains other by-products such as heavier alcohols, di-methyl ether and the like which may be removed from the product methanol, item 168 may also contain the pipeline for removal of inert gases such as nitrogen and unreformed methane to be removed as a purge gas which may be either further treated for recovery of valuable product gases or may be used as a portion of the process fuel gas. Not shown is the possibility of recycling purge gas from the synthesis loop back to the reforming section to process any unreformed methane.

In this version of the invention waste water co-produced with CSM production enters via pipeline 110 and is first used as cooling water in item 22 before passing via pipeline 112 to item 24 in which part of the water leaves in the air leaving via pipeline or duct 124, a major part of the waste water is recovered as a distillate quality water which leaves via pipeline 122 and a high salt content effluent leaves via pipeline 114.

A further embodiment of the invention is described in connection with Figure 2 in a possible alternative plant arrangement where instead of the Haldor Topsoe twin convection heated reformers one or more large single shell-based pressurised reformer vessels 204 are used in which the hydrogen fuel is added progressively to ensure a substantially constant safe but high flue gas temperature on the outside of the tubes and whilst progressive injection of hydrogen on one side of the vessel is shown it would be possible to inject hydrogen from both sides of the reformer vessel to ensure a more even high temperature on the outside of the reformer tubes. Another alternative would be to increase the feed flow in tubes on the re-heat side made possible by each tube having a monitored and controlled feed gas flow to ensure a substantially constant reformed gas exit temperature from all tubes.

If the turbine used for a plant designed in accordance with this invention was based on the use of known commercial means of conversion of synthesis gas to methanol a modified Solar Mercury gas turbine fitted with a pressurised version of the Haldor Topsoe HTSR Twin reformer as described in Hydrocarbon Engineering, November 2004 and where the plant included a UOP Polysep or similar process for the side-streaming of high hydrogen

content fuel gas for the HTSR process and for fuelling the Mercury gas turbine the plant would have the following consumption and production characteristics-

Fuel and feedstock gas analysis 98% methane 2% CO ₂ Fuel and Feedstock consumption 2,400 MJ/day Ambient temperature 15 ⁰ C Plant elevation 100 M Turbine air flow 62,800 Kg/hr Methanol (fuel grade) production 330 tonnes/day CO2 emissions per tonne of methanol Less than 0.05 tonnes

A nominal 1 ,000 tonne per day methanol plants could be based on a modified Solar Titan and other turbines such as the Rolls Royce WR21 gas turbine could also be modified to produce methanol and the methane to methanol energy efficiency of such a plant could be up to about 75% with CO2 emissions allied to methanol synthesis being substantially eliminated particularly with suitable treatment of inert gases in the feed methane and unreformed methane gases which are purged from the synthesis reaction system.

In Figure 2 the same feed gas preparation, gas turbine, methanol synthesis system as shown in Figure 1 is used with the replacement of the Haldor Topsoe type of reformer by a single shell pressurised reformer, item 204, which has a number of catalyst filled reformer tubes shown as items 212 and where the hot flue gas required for convective heating is shown as making 12 passes across the reformer tubes and being re-heated 5 times inside the shell by the addition of hydrogen rich fuel gas to maintain a flue gas temperature above 85O ⁰ C and up to and preferably below 1,200 ⁰ C at the exit of each combustion stage there being 7 stages of heating and re-heating including the initial combustor, item 202 and the gas turbine combustor, item 20.

Items 208 and 210 are two flue gas baffles with item 210 having a return flow orifice or multiple orifices into which hydrogen fuel gas is injected and the flue gas apertures are sized and the hydrogen injection nozzles arranged to ensure highly turbulent flow and

mixing of the hot flue gas and hydrogen rich gas so as to ensure substantially flameless and uniform combustion.

Item 202 is a pressurised combustor for combusting hydrogen rich fuel gas and item 206 is a gas/gas heat exchanger.

The mixture of methane and steam as produced in Figure- 1 which is in pipeline 142 passes though pipeline 142 to item 206 and then via pipeline 202 and is divided into separate flows for injection into the catalyst filled reforming tubes, shown as items 212. The reformed gas leaving tubes 212 passes via pipeline 204 to exchanger 206 and then via pipeline 206 to item 48, as shown in Figure 1.

The hydrogen rich fuel gas shown in pipeline 192 in Figure 1 passes to pipeline 210 and then to the combustor, item 202. Air for item 202 comes from pipeline 126, as shown in Figure 1, and the hot combustion gases, typically between 1,000 and 1,100 ⁰ C leave via pipeline or duct 212 and pass to the first flue gas pass in the pressurised reformer, item 204. The hot gases then pass across the cross section of reformer tubes created by the baffle, item 208 and then across the next lower cross section of tubes between baffles 208 and 210. At the end of this cross flow the flue gasses pass through mixing holes/orifices into which hydrogen rich gas is injected in conditions of high turbulence typically at above 900 ⁰ C such that the hydrogen is rapidly reacted and the temperature is raised by about 15O ⁰ C. The flue gases then pass through the next two cross flow passageways to a further fuel gas mixing and combustion zone and so on until the flue gases finally leave via pipeline or duct 214 to the gas turbine combustor as shown in Figure 1 where further and final combustion takes place prior to passing to the expansion stage of the gas turbine.

According to this embodiment, where a Mercury turbine-based reformer for a 330 TPD methanol plant is selected, the reformer would have-

An internally insulated reformer shell of 2.0 M internal diameter 100 nickel catalyst filled reformer tubes

124 mm OD

l lO mm ID 7.0 M long 12 baffles at 610 mm vertical spacing

Each internal fuel injection, mixing and combustion zone would comprise of 3 off 230 mm diameter orifices/mixing holes and 40 mm diameter fuel gas pipes with gas flow control valves shown as items 318, 220, 222, 224 and 226 with 3 off 25 mm diameter fuel injection tubes at the end of each 40 mm pipe would be located 100 mm above the centre of each 230 mm mixing orifices in each second baffle ensure that pressurised hydrogen is dispersed to minimise local flame or oxidation reaction hot spots resulting from high hydrogen concentrations of hydrogen prior to its rapid auto ignition and combustion.

A typical flue gas temperature entering the mixing and combustion zone would be 95O ⁰ C and the temperature after mixing and combustion would be about 1,100 ⁰ C.

Finally, it can be understood that the inventive concept in any of its aspects can be incorporated in many different constructions so that generality of the preceding description is not superseded by the particularity of the attached drawings. Various alterations, modifications and/or additions may be incorporated into the various constructions and arrangements of parts without departing from the spirit or ambit of the present invention.