Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
1999-06-22
2002-03-05
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S237000, C423S576800
Reexamination Certificate
active
06352680
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the partial oxidation (partial combustion) of hydrogen sulphide and in particular to a method of and apparatus for forming sulphur vapour by partial oxidation of hydrogen sulphide.
Hydrogen sulphide containing gas streams (sometimes referred to as “acid gas streams”) are typically formed in oil refineries and natural gas processing units. Such streams cannot be vented directly to the atmosphere because hydrogen sulphide is poisonous. A conventional method of treating a hydrogen sulphide containing gas stream (which, if desired, has been pre-concentrated) is by the Claus process. In this process a part of the hydrogen sulphide content of the gas stream is subjected to combustion in a furnace so as to form sulphur dioxide. The sulphur dioxide then reacts in the furnace with residual hydrogen sulphide so as to form sulphur vapour. Thus, the hydrogen sulphide is effectively partially oxidised. The reaction between hydrogen sulphide and sulphur dioxide does not go to completion. The effluent gas stream from the furnace is cooled and sulphur is extracted, typically by condensation, from the cooled effluent gas stream. The resulting gas stream, still containing residual hydrogen sulphide and sulphur dioxide, passes through a train of stages in which catalysed reaction between the residual hydrogen sulphide and the sulphur dioxide takes place. Resulting sulphur vapor is extracted downstream of each stage. The effluent gas from the most downstream of the sulphur extractions may be incinerated or subjected to further treatment, e.g. by the SCOT or Beavon process, in order to form a gas stream which can be vented safely to the atmosphere.
Most Claus plants are equipped with right cylindrical furnaces having a length to internal diameter ratio in the range of from 2:1 to 4:1. The furnaces may be cross-fired or tangentially-fired by a burner or burners mounted at the side. Cross or tangentially fired burners achieve good mixing of the reacting chemical species. If desired, mixing can be enhanced by providing the furnace with baffles or checkerwork walls.
Air may be used to support the combustion of hydrogen sulphide in the initial part of the process. The stoichiometry of the reactions that take place is such that relatively large volumes of nitrogen (which is, of course, present in the air that supports the combustion) flow through the process and therefore place a ceiling on the rate at which the gas stream containing hydrogen sulphide can be treated in a furnace of given size. This ceiling can be raised by using commercially pure oxygen or oxygen-enriched air to support the combustion of the hydrogen sulphide.
If commercially pure oxygen or oxygen-enriched air having a mole fraction of oxygen above 0.65 is used to support the combustion of the hydrogen sulphide there is a relatively high risk of damage to the refractory lining of the furnace being created by the resulting increase in flame temperature depending on the composition of the Claus feed gas. There are a number of proposals in the art to solve this problem. Some proposals involve introduction of flame moderators such as water into the furnace; others involve recycling to the furnace gas from a downstream part of the plant so as to moderate the temperature in the furnace; and yet others employ a plurality of furnaces so as to limit the amount of combustion that is performed in each individual furnace, thereby obviating the need for an external flame moderator or to recycle gas from a downstream part of the plant. All these proposals, however, add to the complexity of the plant.
Axially or longitudinally fired burners mounted on the back wall may be used instead of cross or tangentially fired burners in Claus furnaces. Such axially or longitudinally fired burners can be designed to provide average residence times comparable with those of cross—or tangentially—fired burners at a specified throughput, and may be preferred at higher levels of oxygen-enrichment.
The use of such an axially or longitudinally fired burner is disclosed in European patent application 0 315 225 A, in which there is a central pipe for oxygen, at least one second pipe for hydrogen sulphide containing feed gas which coaxially surrounds the central pipe, and an external coaxial pipe for air. The burner is used when the hydrogen sulphide feed gas contains at least 5% by volume of carbon dioxide or hydrocarbons. The oxygen velocity at the outlet of the burner is in the range of from 50 to 250 metres per second (typically 150 metres per second) and the corresponding feed gas velocity is in the range of 10 to 30 metres per second. Temperatures in the range of 2000 to 3000° C. are generated in the core of the burner flame, and a gas mixture having a temperature in the range of 1350 to 1650° C. leaves the furnace. This gas mixture contains at least 2% by volume of carbon monoxide and at least 8% by volume of hydrogen.
During normal operation of, for example, an oil refinery the rate at which hydrogen sulphide containing gas streams are produced for treatment by the Claus process is not constant and can vary quite widely. It is therefore desirable that the furnace be capable of effective operation over a wide range of different rates of inflow of the hydrogen sulphide containing gas.
WO-A-96/26157 also discloses the use of an axially or longitudinally fired burner in the Claus process. Generally parallel flows of a first gas containing hydrogen sulphide and a second gas enriched in oxygen are supplied to the tip (i.e. mouth) of the burner. The ratio of the velocity of the first gas to the velocity of the second gas is selected so as to be in the range of 0.8:1 to 1.2:1.
Neither EP-A-0 315 225 A nor WO-A-96/26157 discusses the problem of how to handle a wide range of different rates of inflow of the hydrogen sulphide containing gas. In fact, neither discloses a method which is capable of effective operation if the rate of supply of the feed gas containing hydrogen sulphide varies considerably.
The method and apparatus according to the invention have it as aim to address this problem and to provide a solution superior to any possible when the disclosure of EP-A-0315 255 or WO-A-96/26157 is followed.
SUMMARY OF THE INVENTION
It is the primary object of the present invention to provide a method of forming sulphur by the partial oxidation of hydrogen sulphide.
According to the present invention there is provided a method of forming sulphur vapor by partial oxidation of hydrogen sulphide, comprising operating a burner so as to establish a flame having at least three stages in a furnace in or into which the burner fires, supplying to the flame from a first region of the mouth of the burner at least one flow of a first combustible gas comprising hydrogen sulphide, causing at least one second flow of a first oxidizing gas to issue from the mouth of the burner and mix in the flame with the first combustible gas, supplying to the flame from a second region of the mouth of the burner surrounding and spaced from said first region at least one third flow of a second combustible gas comprising hydrogen sulphide, causing at least one fourth flow of a second oxidizing gas to issue from a region or regions of the mouth of the burner surrounded by said second region and mix in the flame with the second combustible gas, causing at least one fifth, outermost, flow of a third oxidizing gas to mix in the flame with the second combustible gas, and withdrawing from the furnace a resultant gas mixture including sulphur vapor, water vapor, sulphur dioxide, hydrogen and residual hydrogen sulphide.
The invention also provides apparatus for forming sulphur vapor by partial oxidation of hydrogen sulphide, comprising a furnace, a port in the furnace, a burner having its mouth located in the port and operable to establish a flame having at least three stages in the furnace, and an outlet from the furnace for a resultant gas mixture including sulphur vapor, water vapor, sulphur dioxide, and residual hydrogen sulphide to exit the furnace; wherein
Graville Stephen Rhys
Wainford John Allen
Watson Richard William
Griffin Steven P.
Neida Philip H. Von
Pace Salvatore P.
The BOC Group plc
Vanoy Timothy C
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