|Title of Invention||
A REACTOR FOR SUPPRESSION OF SOOT FORMATION
|Abstract||ABSTRACT A reactor for suppression of soot formation The present invention relates to a reactor for suppression of soot formation in the preparation of a gas selected from hydrogen, carbon monoxide and both, comprising within a pressure shell a refractory lining on an irmer wall of the shell, an upper portion adapted to receive a hydrocarbon feedstock and an oxygen containing atmosphere and to partially oxidise the feedstock with oxygen, and a lower portion which may be provided with a reforming catalyst adapted to receive and steam reform the partially oxidised gas from the upper portion, and a reforming catalyst arranged at least on surface of the upper portion of the reactor.|
The present invention is directed to a reactor for suppression of soot formation in the preparation of hydrogen and/or carbon monoxide rich gas. In particular, the invention relates to a process and reactor for the preparation of such gas by autothermal catalytic reforming of a hydrocarbon feedstock.
Hydrogen and carbon monoxide rich gases are mainly used as synthesis gas in the production of ammonia and methanol or other organic compounds.
The gases find further employment during steel production and as fuel or town gas. Industrial preparation methods most usually comprise autothermal catalytic reforming and non-catalytic partial oxidation of hydrocarbons.
During partial oxidation a hydrocarbon feedstock is combusted together with air, oxygen, or oxygen enriched air in a burner mounted at the top of a reaction vessel. Oxygen is, thereby, supplied in amounts, which are less than the amount required for complete combustion, and hydrogen and carbon monoxide are produced in an effluent gas mainly by flame ignition reactions:
Both reactions are strongly exothermic for all hydrocarbons.
Partial oxidation is typically employed in the gasifica¬tion of heavy oils, where the temperature in the gas raises during the combustion to 1000-1500°C, which is high enough to give a sufficient low content of unconverted hydrocar¬bons in the combustion effluent gas. Lighter feedstocks ranging from natural gas to naphtha fractions with a boil¬ing point up to 200°C are conventionally treated by auto-thermal catalytic reforming of the feedstock.
During this process, only a part of the hydrocarbon feed¬stock is oxidized with an oxygen containing atmosphere by the above flame reactions (1,2). Residual hydrocarbons in the gas stream from the combustion are then catalytic steam reformed by the endothermic reaction:
Necessary heat for the endothermic steam reforming reaction is, thereby, provided by the exothermic flame reactions (1,2).
Somewhat lower combustion temperatures are used during autothermal catalytic reforming, which is operated at a typical temperature of about 900-1400°C. Steam is added to the feed in order to moderate the flame temperature and in¬crease hydrocarbon conversion in the burner effluent gas.
Similar to the partial oxidation process, hydrocarbon feed mixed with steam is burnt with an oxygen containing atmos¬phere at the top of a reactor. Residual hydrocarbons in the combusted gas are then steam reformed in the presence of a catalyst arranged as fixed bed in a lower portion of the
reactor. Heat for the endothermic steam reforming reactions is supplied by the hot effluent gas from the combustion zone in the upper reactor portion and above the catalyst bed. As the combustion gas contacts the catalyst, the tem¬perature in the gas cools to 900-1100°C by the steam re¬forming reactions in the catalyst bed.
In operating the above processes, suitable hydrocarbon feed, if necessary after preheating, is introduced into a burner mounted at the top of a reactor and burnt with oxy¬gen containing atmosphere. In order to protect the reactor shell against the high temperatures arising during the exo¬thermic oxidation reactions, industrial reactors are pro¬vided with a temperature resistant and insulating refrac¬tory lining on the inner wall of the reactor shell.
The lining materials must be able to withstand high tem¬perature exposure and be suited to resistant erosion by hot gases. At present, refractory materials most commonly used in industrial reactors of the above types contain more than 90% alumina.
A general problem in the preparation of synthesis gas by the above processes is formation of soot in the combustion zone at critical process conditions, such as low steam/-carbon ratios in the feedstock to the processes.
A further problem is related to start-up of the burner for the partial oxidation of the feedstock, which requires pre¬heating of the feedstock and the reactor to high tempera¬tures .
EP Patent No. 0 582 211 presents the closest prior art in relation to the present invention.
This reference discloses a process for the preparation of hydrogen and/or carbon monoxide by steps of partial oxidation of a hydrocarbon feed stock in the upper part of a reactor and subsequent steam reforming of the partial oxidised feed stock in presence of a catalyst being arranged in the lower part of the reactor. The upper part of the reactor is provided with a lining on inner reactor wall, which is coated with a steam reforming catalyst in order to cool the lining by means of heat consuming steam reforming reactions when the partial oxidised feed stock comes into contact with the catalyst on the lining.
The process of the present invention is carried in similar manner. However, in con¬trast to the known process, the process is directed to reduction of soot being formed during the partial oxidation process step in the upper part of the reactor. By means of contact with the catalyst, soot formation is substantially suppressed by reaction of soot precursors on surface of the catalyst (of page 4, second paragraph in the specifi¬cation). The problem of soot formation and its solution as provided by the invention is neither mentioned nor remotely touched in EP Patent No. 0 582 211.
It has now been found that the above problems in partial oxidation and autothermal catalytic reforming processes are substantially avoided when performing steam reforming reac¬tions on the surface surrounding the combustion zone of hy¬drocarbon feedstock. Those reactions proceed in the combus¬tion effluent gas when a suitable catalyst is arranged on the surface at least in the portion of the reactor, which surrounds the hot combustion zone.
A theoretical explanation for the reduced soot formation may be that precursor molecules participating in the forma¬tion of soot are reduced or reacted by steam reforming re¬actions proceeding on the catalysed surface adjacent to the combustion zone. An increased hydrogen concentration by the steam reforming process occurring in this region results furthermore in improved ignition property of the feed oxy¬gen mixture and start-up of the process at less severe con¬ditions .
Pursuant to the above finding, this invention provides a process for suppression of soot formation in the prepara¬tion of hydrogen and/or carbon monoxide rich gas by partial oxidation of a hydrocarbon feedstock, comprising the steps of
in a reactor with an upper and a lower portion, arranging at least on surface of the reactor upper portion catalytic material being active in steam reforming of hydrocarbon;
introducing the feedstock and an oxygen-containing atmos¬phere into the upper portion of the reactor;
partially oxidising the feedstock with oxygen in the upper portion of the reactor; and
contacting a part of the partially oxidised feedstock with the reforming catalyst in the reactor upper portion.
'. 'l A reactor being useful in carrying out the process accord¬ing to the invention comprises within a pressure shell a refractory lining on an inner wall of the shell,
an upper portion adapted to receive a hydrocarbon feedstock and an oxygen containing atmosphere, and to partially oxi¬dise the feedstock with oxygen, and
a reforming catalyst arranged in the upper portion of the reactor.
In operating a specific embodiment of the inventive process and reactor, a hydrocarbon feedstock preheated to about 400-700°C is introduced into a burner mounted at the top of a refractory lined reactor. In the burner, the feedstock is mixed with steam and oxygen containing atmosphere in an amount providing a process gas with an oxygen/carbon mole ratio of preferably between 0.5 and 0.7 and a steam/carbon mole ratio of preferably between 0.5 and 1.5.
Typical hydrocarbon feedstock suited for the process will range from methane to naphtha fractions with a boiling point up to 200°C, including natural gas, LPG and primary reformed gas, when operating the process under autothermal catalytic reforming conditions. The process gas is dis¬charged from the burner into a combustion zone in the upper
reactor portion, where part of the hydrocarbons in the gas are reacted with oxygen to carbon oxides and hydrogen by flame ignition reactions (1) and (2) as mentioned herein before.
Depending on the desired composition of the final product gas, oxygen may be supplied from air or oxygen enriched air as in the preparation of ammonia synthesis gas or from oxy¬gen for the production of oxosyn-thesis gas and reducing gas, where nitrogen is unwanted in the product gas. During hydrocarbon oxidation the temperature in the combustion zone raises to 900-1500°C.
By the endothermic steam reforming reaction (3) proceeding in the gas on the surface adjacent to combustion zone, con¬centration of hydrogen in recirculated combustion gas is increased and content of soot precursor molecules de¬creased.
The actual increase of hydrogen concentration depends, thereby, on the amount of hydrocarbons and steam in the gas from the combustion zone and the activity and amount of re¬forming catalyst in the upper reactor portion.
Catalysts suited for this purpose comprise the well-known reforming catalysts of Group VIII in the Periodic Table, including nickel and/or cobalt, which for sufficient soot reduction and flame ignition improvements are loaded in an amount of between 1 g/m2 and 0.1 g/cm2 on the lining sur¬face by conventional impregnation or coating techniques.
When the process takes place at autothermal catalytic re¬forming conditions, the effluent gas from the combustion zone is further passed through a fixed bed of conventional nickel and/or cobalt reforming catalyst arranged in the lower portion of the reactor. By passage through the cata¬lyst bed, residual hydrocarbons in the gas are further steam reformed to hydrogen and carbon monoxide.
A more detailed explanation of claimed process and the reactor for use in the process will be apparent from the following:
The Fig. 1 shows in purely schematic form an autothermal reactor.
Reactor 1 contains within a shell 2 a burner 10 arranged in upper part 8 of the reactor and fixed bed of steam reforming catalyst 3 in lower part 7. Hydrocarbon feed stock and steam are introduced into burner 10 though feed inlet 5 and oxidant through inlet 4. Feed and oxidant are partially oxidised through flame reactions proceeding in combustion zone 9 in upper part 8. Hot reaction products containing soot precursors circulate in combustion zone 9 and make contact with a coating of steam reforming catalyst 12 provided on inner surface of shell 2 in upper part 8. The partial oxidised feed stock being substantially free of soot passes from upper part 8 to catalyst bed 3 in lower part 7. By passage through catalyst bed 3, remaining amounts of hydrocar¬bon being present in the partial oxidised feed are steam reformed to hydrogen and carbon monoxide. A product gas rich in hydrocarbon and carbon monoxide without a content of soot is withdrawn from the reactor through outlet 6.
The following is an Example of a specific embodiment of the invention carried out in a reactor as described above
To illustrate the effect of introducing a reforming catalyst on the refractory surface in the combustion chamber, i.e. the upper portion of the reactor of an autothermal re¬former (ATR), the Table below shows the gas composition at the exit of said com¬bustion chamber, with and without the presence of the catalytic coated surface. The hydrocarbon feed to the reactor consists of 100 Nm3/h natural gas, 2 Nm3/h of hy¬drogen and 55 Nm3/h of steam. The natural gas has the following composition:
The oxidant feed consist of 57 Nm3/h oxygen with a content of 0.50 mol% inert (ni¬trogen) and 6 Nm3/h of steam. Accordingly, both the oxygen/carbon mole ratio and the steam/carbon mole ratio in the gas to be treated is about 0.6. The hydrocarbon feed is heated to 500'C, the oxidant to 220°C. All gases are preheated to the operat¬ing temperature and compressed to the operating pressure, which is 2.46 MPa.
The results show that the gas in the reformer with catalyst coating on the refractory has significantly lower concentrations of C2-species (ethane, ethylene and acetylene), which are known to be precursors for soot, of page 6, lines 17, 18 of original appli¬cation.
1. A reactor (1) for suppression of soot formation in the preparation of a gas selected from hydrogen, carbon monoxide and both, characterized in that the reactor comprising within a pressure shell (2) a refractory lining on an inner wall of the shell (2), an upper portion (8) adapted to receive a hydrocarbon feedstock and an oxygen containing atmosphere and to partially oxidise the feedstock with oxygen, and a lower portion (7) which may be provided with a reforming catalyst adapted to receive and steam reform the partially oxidised gas from the upper portion (8), and a reforming catalyst arranged at least on surface of the upper portion of the reactor.
2. The reactor (1) for suppression of soot formation as claimed in claim 1, the upper portion (8) comprising a burner (10) mounted at the top of the reactor (1) which is adapted to receive a hydrocarbon feedstock preheated to 400 - 700°C and to mix said hydrocarbon feedstock with steam and oxygen containing atmosphere thereby providing a process gas with an oxygen/carbon mole ratio between 0.5 to 0.7 and steam/carbon mole ratio between 0.5 and 1.5.
|Indian Patent Application Number||7/MAS/2001|
|PG Journal Number||23/2008|
|Date of Filing||03-Jan-2001|
|Name of Patentee||HALDOR TOPSOE A/S|
|PCT International Classification Number||C01B3/26|
|PCT International Application Number||N/A|
|PCT International Filing date|