PROCESS FOR PRODUCING A GASEOUS PRODUCT

Abstract: A process for producing a gaseous product is described whereby reactants (e.g. hydrocarbons and steam) are introduced into a fluidized bed operated under conditions such that a reaction takes place in the fluidized bed to produce the gaseous product (e.g synthesis gaz), the fluidized bed comprising particles having a density and size distribution such that at least a portion of the particles are entrained by the gaseous product leaving the fluidized bed, removing the gaseous product and entrained particles from the fluidized bed, substantially separating the particles entrained in the effluent gas from the gaseous product and returning the separated particles to the fluidized bed wherein at least a portion of the energy required for the reaction is supplied by heating the separated particles during their return to the fluidized bed. Multi-solid fluidized bed processes are also disclosed.

Description: PROCESS FOR PRODUCING Λ GASEOUS PRODUCT The present invention relates to processes for producing a gaseous product. In one particular application, the invention relates to a process for the steam reforming of hydrocarbons. Steam reforming of hydrocarbons such as natural gas or naphtha involves reaction with steam to produce a mixture of carbon oxides, hydrogen and water vapour. This gas mixture, commonly referred to as "syngas" in the petrochemical industry, is the starting point for numerous catalytic synthesis products such as methanol, gasoline and wax. It is also useful for metallurgical reduction processes, production of eleptrical energy from fuel cells and production of hydrogen for use in, for example, ammonia synthesis. The reforming reaction is strongly endothermic and is conventionally performed at around 800-900°C, at pressures of 10 to 30 bar in catalyst-filled, high-alloy tubes. These tubes are usually 100 to 150mm in diameter and around 10m in length; heat transfer from hot flue gases on the outside of the tubes to the reacting mixture within is a major rate-limiting step in the overall process. Thermal driving forces for heat transfer are large and tube layout is dictated, to a large degree, by the need for sufficient radiation view-factors to avoid the formation of hot and cold-spots on the tubes. This results in a need for large furnace volumes and plan areas - a constraint more difficult to accept in some applications than in others. Numerous proposals to reduce the size of the steam reformer have been developed. For example, US Patent No. 4888131 describes a process whereby a sharp reduction in furnace volume is possible by introducing oxygen into the system. Partial oxidation schemes such as this are useful when oxygen injection is feasible and does not substantially downgrade the quality of the product gas. However, in other application examples (eg ship-board gas conversion operations) it may be undesirable to use oxygen for safety and economic reasons and other means for reducing the volume of the reformer must be considered. Immersion of catalyst-filled tubes in a fluidized bed, as described in, for example, British Patent No. 2126118, is an alternative means for reducing the size of an otherwise conventional reformer. In this case heat transfer to the outside of the tubes is achieved via the hot particles that comprise the fluidized bed. This results in a more even heat distribution and a significant reduction in the space needed between tubes. However, this process does have a number of disadvantages including hot erosion of the high-alloy tubes and unsuitability for ship-board applications, where wave motion can affect the integrity of the tubes. US Patent No. 3524819 describes a process in which a mixture of hydrocarbons and steam is reformed in an internal-free fluidized bed of fine-grained catalyst. Heat for the reforming reaction is provided by continuously withdrawing a portion of the catalyst and heating this catalyst by contact with hot flue gases before returning it to the reforming bed. However, this process exposes the catalyst (in bulk) to high temperatures which may cause the catalyst to sinter, thereby losing catalytic activity and requiring rapid (and costly) replacement. Hence, an object of the present invention is to provide an alternative process utilising fluidized bed technology, which may permit a reduction in the size of the steam reforming plant. Thus, according to a first aspect, the present invention provides a process for producing a gaseous product comprising passing reactants into a fluidized bed operated under conditions such that a reaction takes place in the fluidized bed to produce the gaseous product, the fluidized bed comprising particles having a density and size distribution such that at least a portion of the particles are entrained by the gaseous product leaving the fluidized bed, removing the gaseous product and entrained particles from the fluidized bed, substantially separating the particles entrained in the gaseous product from the gaseous product and returning the separated particles to the fluidized bed wherein at least a portion of the energy required for the reaction is supplied by heating the separated particles during their return to the fluidized bed. The separated particles are preferably heated by passing the particles through a chamber where fuel gas or the like is combusted._ he separated particles may be catalytic or inert. In a more preferred embodiment of the invention, the fluidized bed is a multi-solid fluidized bed. Thus, in a second aspect, the present invention provides a process for producing a gaseous product comprising passing reactants into a fluidized bed operated under conditions such that a reaction takes place to produce the gaseous product, the fluidized bed comprising first particles having a density and size distribution such that at least a portion of the first particles are entrained by the gaseous product leaving the fluidized bed and second particles having a density and size distribution such that said second particles substantially remain in the fluidized bed, substantially removing the gaseous product and entrained first particles from the fluidized bed, separating the first particles entrained in the gaseous product from the gaseous product and returning the separated first particles to the fluidized bed wherein at least a portion of the energy required for the reaction is supplied by heating the separated first particles during their return to the fluidized bed. It may be appreciated that in the process according to the second aspect, a minor portion of second particles may escape the fluidized bed by entrainment in the gaseous product. When the second particles are catalytically active, such "escapes" will result in a loss of catalyst. To avoid this loss, it is preferable that following separation of the particles from the gaseous product and prior to the first particles being subjected to heating, any second particles are substantially separated from the first particles and returned to the fluidized bed. This arrangement also permits the fluidized bed to be operated such that an amount of a mixture of first and second particles is normally entrained in the gaseous product. Thus, in a further aspect, the present invention provides a process for producing a gaseous product comprising passing reactants into a fluidized bed operated under conditions such that a reaction takes place to produce a gaseous product, the fluidized bed comprising first and second particles having density and size distributions such that at least a portion of both first and second particles are entrained by the gaseous product leaving the fluidized bed, removing the gaseous product and entrained first and second particles from the fluidized bed, separating the first and second particles entrained in the gaseous product from the gaseous product, substantially separating the first and second particles, and thereafter separately returning the separated first particles and separated second particles to the fluidized bed wherein at least a portion of the energy required for the reaction is supplied by heating the separated first particles during their return to the fluidized bed. The surface area of the first particles is not critical. However, the first particles are preferably catalytically inert. Examples of suitable materials include but are not limited to dense alumina, zirconia, rutile, metals such as nickel or mixtures thereof. The second particles are preferably catalytically active. The first and second particles may be separated by a fluidized bed segregator. It will be appreciated that the fluidized bed reactor and associated equipment to facilitate the separation and return of particles entrained in the gaseous product comprise a circulating fluidized bed system. The process according to the invention may be suitable for a number of heat driven reactions including pyrolysis reactions, naptha cracking, ethane conversion to ethane and maleic anhydride synthesis. However, the processes are particularly suitable for steam reforming of hydrocarbons to produce a gaseous product containing hydrogen and carbon oxides. Accordingly, the invention is hereinafter described in relation to this application. In this application of the invention, the reactants are hydrocarbons and steam and the fluidized bed is operated under conditions such that a steam reforming reaction takes place. The fluidized bed reformer may be operated at a temperature from 750 to 950°C preferably in the range of 800 to 900°C and at a pressure in the range of 5 to 80 bar, preferably 10 to 30 bar. The first particles are preferably catalytically inert and have a maximum particle size of less than about 0.5mm with an average particle size preferably lying in the range of about 0.03 to 0.3mm. The second particles are preferably catalytically active and may have a function and chemical composition typical of known steam reforming catalysts (e.g. Ni or Ru supported on alumina, calcium alumnate or zirconia) . These particles may be porous and display high catalytic activity for the steam reforming reaction. Average particle size preferably lies in the range of 0.7 to 7mm with little or no material smaller in size than 0.5mm. The suspension density of the reformer vessel may be about 200 to 2000 kg/m , preferably about 1000 to 1800kg/m . The separated particles are preferably heated to a temperature of about 950 to 1200°C, preferably about 1000 to 1100 C during their return to the fluidized bed reformer. The invention is further described by way of the following non-limiting examples and with reference to Figures 1 and 2. Figures 1 and 2 provide diagrammatic representations of apparatus suitable for performing the process of the present invention. EXAMPLE 1 A mixture of hydrocarbons (natural gas or higher hydrocarbons such as ethane or naphtha) and steam 101 is pre-heated in heat exchanger 102 before being fed into the bottom of the fluidized bed reformer 103. Unit 103 contains a dense-phase fluidized bed with two types of particle present: i) a component which is catalytically active, similar in structure and purpose to conventional alumina-supported Ni catalysts (e.g. ICI 57-series catalysts). These particles may be porous and display high catalytic activity for the steam reforming reaction. Average particle size lies in the range 0.7 to 7mm, with little or no material smaller in size than 0.5mm. ii) A component (e.g. fused alumina) which has low specific surface area and is catalytically inert. This component of the population is smaller in size than the catalytically active component, all particles being smaller than 0.5mm and average size for the inert component lying in the range 0.03 to 0.3mm. Fluidized bed reformer 103 is operated such that the fine, inert component of the particle population is preferentially entrained from the top of the vessel while the coarse, catalytic component remains in the vessel. Steam reforming takes place within vessel 103 at a temperature of about 850°C and a pressure in the range 10 to 30 bar. Effluent gas 104, along with entrained fine solids, enters cyclone 105. Bulk disengagement occurs and the cyclone overhead stream enters hot gas cleaner 106. In this unit the bulk of the remaining solid is removed by, for example, a hot multicyclone system. Cleaned gas 107 leaves the cleaning unit 106 and passes to heat exchanger 102 before being scrubbed and passed on for use downstream. Solids from the underflow of cyclone 105 are fed, via pressure recovery system such as loopseal 108, to chamber 109 where fuel gas and air are combusted at a pressure similar to that in vessel 103. Solids are heated from 800-850°C to around 1000-1100°C and are subsequently removed from the hot flue gases in cyclone 110. From here the hot solids are returned to the base of the reforming vessel 103 via pressure recovery system 111 to provided the heat necessary for the reforming reaction. The bottom inlet system for hot solids is designed to ensure a large mass flux of bed material is available to rapidly dilute the hot solids stream, thereby avoiding the exposure of catalytically active particles to (destructive) high temperatures. Hot, pressurised flue gas from the overflow of cyclone 110 is used for energy recovery purposes. Hot gas cleaning is carried out in unit 112, the precise form of which is not critical. Partial cooling (to 800°C for example) followed by a micro-cyclone system may be used, or a ceramic barrier-filter system similar to those currently under development for pressurised fluidized bed combustion and gasification. The cleaned gas is subsequently expanded through turbine 113. From here it is subjected to further heat recovery and finally vented. EXAMPLE 2 A mixture of hydrocarbons (natural gas or higher hydrocarbons such as ethane or naptha) and steam 201 is pre-heated in heat exchanger 202 before being fed into the bottom of a fluidized bed reformer 203. Unit 203 contains a dense-phase fluidized bed with two types of particles as described in Example 1. Steam reforming takes place within vessel 203 again at a temperature of about 850°C and a pressure in the range 10 to 30 bar. However, in this case the fluidized bed reformer 203 is operated such that a mixture of bed solids are entrained in gas leaving the vessel. Effluent gas 204, along with entrained solids, enters cyclone 205. Bulk disengagement occurs and the cyclone overhead stream enters hot gas cleaner 206 where the bulk of the remaining solid is removed by, for example, a hot multicyclone system. Cleaned gas 207 leaves the cleaning unit 206 and passes to heat exchanger 202 before being scrubbed and passed on for use downstream. Solids from cyclone 205 are fed to fluidized bed segregator 208. Unit 208 contains a low velocity bubbling fluidized bed within which the coarse (catalytically active) particles preferentially sink to the bottom and are returned, along with some fine heat-carrier solids, to unit 203. The fluidizing gas for unit 208 may be any convenient gas, but is preferentially steam or fuel gas. Fine heat-carrier solids, substantially free of coarse (catalytically active) solids, leave the top of unit 208 and enter unit 209 where fuel gas and air are combusted at a pressure similar to that in vessel 203. Solids are heated from 800"-850 β C to around 1000°-1100°C and are subsequently removed from the hot flue gases in cyclone 210. From here the hot solids are returned to the base of reforming vessel 203 via pressure recovery system 211 to provide the heat necessary for the reforming reaction. Hot, pressurised flue gas from the overflow of cyclone 210 is used for energy recovery purposes. Hot gas cleaning is carried out in unit 212. Partial cooling (to 800°C for example) followed by a micro-cyclone system or a ceramic barrier-filter system may be used. The cleaned gas is subsequently expanded through turbine 213, after which it may be subjected to further heat recovery and finally vented. The present invention offers a compact system, that may be operated free of the need for oxygen, for application to processing in situations such as shipboard reforming. The reforming vessel itself is free of expensive, high-maintenance tubes of high-alloy metal and associated maintenance and safety issues arising from wave motion. It may be configured to avoid substantial exposure of catalyst particles to high temperatures which would otherwise promote sintering, thereby achieving good catalyst utilisation. Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is therefore to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.

Claims: CLAIMS : 1. A process for producing a gaseous product comprising passing reactants into a fluidized bed operated under conditions such that a reaction takes place in the fluidized bed to produce the gaseous product, the fluidized bed comprising particles having a density and size distribution such that at least a portion of the particles are entrained by the gaseous product leaving the fluidized bed, removing the gaseous product and entrained particles from the fluidized bed, substantially separating the particles entrained in the gaseous product from the gaseous product and returning the separated particles to the fluidized bed wherein at least a portion of the energy required for the reaction is supplied by heating the separated particles during their return to the fluidized bed. 2. A process according to claim 1 wherein the particles are solid catalyst. 3. A process according to claim 1 or 2 wherein the reactants are hydrocarbons and steam and the fluidized bed is operated under conditions such that a steam reforming reaction takes place in the fluidized bed to produce a gaseous product containing hydrogen and carbon oxides. 4. A process according to claim 3 wherein the average particle size is in the range of about 0.7 to 7.0mm. 5. A process according to claim 4 wherein the particles are alumina-supported nickel or alumina-supported ruthenium catalyst. A process according to any one of claims 3 to 5 wherein tthhee sseeppaarraatteedd ppaarrttiicclleess aarree hheeaatteedd ttoo aabbout 950 to 1200°C during their return to the fluidized bed. A process according to claim 6 wherein the separated ppaarrttiicclleess aarree hheeaatteedd ttoo aabboouut 1000 to 1100°C during their return to the fluidized bed. 8. A process for producing a gaseous product comprising passing reactants into a fluidized bed operated under conditions such that a reaction takes place to produce the gaseous product, the fluidized bed comprising first particles having a density and size distribution such that at least a portion of the first particles are entrained by the gaseous product leaving the fluidized bed and second particles having a density and size distribution such that said second particles substantially remain in the fluidized bed, removing the gaseous product and entrained first particles from the fluidized bed, substantially separating the first particles entrained in the gaseous product from the gaseous product and returning the separated first particles to the fluidized bed wherein at least a portion of the energy required for the reaction is supplied by heating the separated first particles during their return to the fluidized bed. 9. A process for producing a gaseous product comprising passing reactants into a fluidized bed operated under conditions such that a reaction takes place to produce the gaseous product, the fluidized bed comprising first and second particles having density and size distributions such that at least a portion of both first and second particles are entrained ^ by the gaseous product leaving the fluidized bed, removing the gaseous product and entrained first and second particles from the fluidized bed, substantially separating the first and second particles entrained in the gaseous product from the gaseous product, substantially separating the first and second particles, and thereafter separately returning the separated first particles and separated second particles to the fluidized bed wherein at least a portion of the energy required for the reaction is supplied by heating the separated first particles during their return to the fluidized bed. 10. A process according to any one of claims 8 to 10 wherein the first particles are catalytically inert. 11. A process according to claim 11 wherein the first particles are particles of one or more of alumina, zirconia, rutile or nickel. 12. A process according to any one of claims 8 to 11 wherein the secpnd particles are solid catalyst. 13. A process according to any one of claims 8 to 12 wherein the reactants are hydrocarbons and steam and the fluidized bed is operated under conditions such that a steam reforming reaction takes place in the fluidized bed to produce a gaseous product containing hydrogen and carbon oxides. 14. A process according to claim 13 wherein the first particles are of a size less than about 0.5mm. 15. A process according to claim 14 wherein the first particles are in the size range of about 0.03 to 0.3mm. 16. A process according to any one of claims 13 to 15 wherein the average size of the second particles is in the range of about 0.7 to 7.0mm. 17. A process according to claim 16 wherein the second particles have a function and chemical composition typical of steam reforming catalysts. 18. A process according to any one of claims 13 to 17 wherein the separated first particles are heated to about 950 to 1200°C during their return to the fluidized bed. 19. A process according to claim 18 wherein the first ppaarrttiicclleess aarree hheeaatteedd ttoo aabbooiut 1000 to 1100 C during their return to the fluidized bed. 20. A process according to any one of claims 8 to 19 wherein the first and second particles are separated by a fluidized bed segregator. 21. A process according to any one of the preceding claims wherein the fluidized bed has a suspension density of about 200 to 2000 kg/m . 22. A process according to claim 21 wherein the fluidized bed has a suspension density of about 100 to 1800 kg/m . 23. A process according to any one of the preceding claims wherein the heated separated particles are returned to the fluidized bed via an inlet located at the base of the fluidized bed. 24. An apparatus when used with a process according to any of the preceding claims. 25. A process substantially as described with reference to example 1 or 2.

Inventors: DRY RODNEY JAMES, DRY, RODNEY, JAMES
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1993
Main classification: C01B3
IPC codes: B01J8, C01B3, C10G11, C10J3