Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2002-04-30
2004-08-24
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S648100
Reexamination Certificate
active
06780396
ABSTRACT:
The invention relates to a process and a device that make it possible to produce a hydrogen-rich gas from a gas that contains H2S.
The technological background is illustrated by U.S. Pat. Nos. 4,481,181, 4,461,755, 4,302,434, FR-A-1 461 303 and FR-A-2 238 668.
H2S is a highly toxic compound that is present in many natural gases. It is also found in refinery gases, where it is generally obtained from decomposition reactions of sulfur-containing organic compounds that are naturally present in crude oils. The H2S is produced in particular in large amounts during hydrodesulfurization operations. These operations make it possible to lower the sulfur content of petroleum fractions by a treatment with hydrogen in the presence of suitable catalysts.
It is conventional both in a refinery as well as in natural gas production to extract the H2S that is present in the gases by a scrubbing with a suitable solvent, for example an amine-based solution. This solvent is then regenerated, generally by heating, which then produces a so-called acid gas that is rich in H2S. In a refinery, such an acid gas usually contains more than 90% H2S with minor contents of CO2, steam and hydrocarbons (methane and other heavier hydrocarbons).
Taking into account the toxicity of the H2S, this gas cannot be emitted into the atmosphere. Its combustion would generate considerable amounts of SO2 and SO3, polluting compounds of which it is also sought to reduce the releases into the atmosphere. The most generally used method for treating these acid gases consists in admitting them into a so-called Claus unit that makes it possible to convert H2S into elementary sulfur, a non-polluting compound that can be easily transported and marketed, for example for the production of sulfuric acid.
The Claus units comprise a thermal stage in which about one third of the H2S that is present in SO2 is oxidized by combustion with air (reaction (1)), and even in some cases with oxygen-enriched air. During this partial combustion, a portion of the SO2 that is formed reacts with H2S to form elementary sulfur according to Claus reaction (2):
H2S+3/2 O2→SO2+H2O (1)
2 H2S+SO2→3/x Sx+2 H2O (2)
3 H2S+3/2 O2→3/x Sx+3 H2O (3)
The hot gas that is thus obtained is then cooled in a steam boiler to a temperature that allows the condensation of elementary sulfur. After the condensed elementary sulfur is separated, the gas that contains the residual SO2 and H2S is sent to one or more (generally 2 to 3) catalytic stages where the Claus reaction is continued. Each catalytic stage consists of a stage for reheating the gas followed by a reaction stage in the presence of a suitable catalyst and finally a stage for cooling the gas for separating by condensation the elementary sulfur that is formed in the reactor.
The Claus process that is usually used makes it possible to convert into elementary sulfur only about 95 to 97% of the H2S that is treated. To limit the releases of sulfur-containing gases into the atmosphere, the gas that exits from the Claus process is usually treated in a so-called Claus tail gas treatment unit. There is a large variety of units of this type, generally complex and almost as costly in investment as the Claus unit itself.
It is seen therefore that the conversion of H2S into elementary sulfur by this method, although effective, remains a complex and costly operation.
In addition, the overall material balance of the operation amounts to forming one mol of elementary sulfur and one mol of water (reaction of balance (3)) for each H2S mol.
The decomposition of H2S into elementary sulfur and hydrogen (reaction (4)) would allow not only H2S to be converted into elementary sulfur but also hydrogen, gas with high added value, to be produced, which is also in high demand for various refining operations of crude oils (hydrotreatments).
H2S→1/x Sx+H2 (4)
This is why numerous authors have attempted to develop processes that make it possible to carry out in an economical manner such a decomposition. An examination of the various methods that are explored can be found in, for example, “
H
2
S A Potential Source of Hydrogen
” that was published by Sulphur No. 244, May-June 1996, pp. 37-47. To this day, however, it seems that no process is the subject of a large-scale industrial implementation, generally due to the inadequate conversion ratios or high operating costs.
The decomposition of H2S by an electrolytic method, such as the one developed by the Idemistsu Kosan Company, makes it possible to obtain an almost total conversion of H2S into elementary sulfur and hydrogen, but the use of electricity as the sole energy source leads to a high operating cost.
It is also the use of electricity as the sole energy source that penalizes the process by plasmalysis developed by the Kurchatov Russian Institute.
The “Hysulf” process developed by the Marathon Oil Company makes it possible to obtain hydrogen by using only thermal energy, owing to the use of organic compounds that are hydrogenated by H2S in a first step, then dehydrogenated by catalytic reforming in a second step. It is generally admitted, however, that during such reduction-oxidation cycles, the organic compounds tend to deteriorate and that consequently the processes that are based on this type of reaction most often also lead to relatively high operating costs because of the supply of chemical products that are necessary for compensating the secondary degradation reactions.
The thermal cracking method developed by the Alberta Sulphur Research Laboratory (ASRL) also makes it possible to use only thermal energy for decomposing H2S, whereby this energy is supplied directly by the flame of a thermal stage of the Claus process. The outputs of conversions that are obtained are relatively low (limited to about 35%), however, primarily for two reasons:
the theoretical limitations due to the thermodynamic equilibrium between H2S, H2 and elementary sulfur; the decomposition of H2S that becomes important only at very high temperature (1100° C. and beyond),
the difficulty in cooling very quickly the high-temperature mixture that contains hydrogen. Actually, during the cooling, the hydrogen and the elementary sulfur that are formed at high temperature have a tendency to recombine very quickly into H2S.
This last phenomenon is particularly important if it is desired to obtain a high hydrogen yield by thermal method, which therefore requires the use of very high temperatures. This phenomenon was studied in particular inside the furnaces of the Claus units (see, for example, “
What Happens to Hydrogen in a Claus Plant
,” published by Sulphur No. 214, May-June 1991, pp. 53-60). It is seen that with a dwell time, such that it is typically found in the boilers that are downstream from the Claus units, of between 0.5 and 1.5 seconds, the major portion of the hydrogen that is formed in a Claus furnace is recombined into H2S. Even with a time of cooling to 600° C. of 50 ms, only 80% of the hydrogen that is formed at equilibrium at 1300° C. in a Claus furnace would be recovered. In “
Production of Hydrogen and Sulphur from Hydrogen Sulfide in Refineries and Processing Plants
,” published by P. D. Clark & al. in the Sulphur 95 Conference, the impact on the final hydrogen yield, of a time of cooling to 500° C., for a mixture of hydrogen, H2S and elementary sulfur at equilibrium at 1200° C. is indicated. It is seen that to limit the hydrogen yield losses, a cooling time of less than 10 ms is necessary, and shorter periods on the order of 1 ms are even desirable. Such short cooling times are virtually impossible to achieve, however, by external heat exchange.
The approach is then to consider a cooling by mixing with a colder fluid (quench), but there again, it is technically difficult to attain mixing times that are short enough to avoid a significant recombining of hydrogen into H2S.
This invention has as its object a process and a device that make it possible to generate hydrogen from H2S by thermal method by eli
Institut Francais du Pe'trole
Medina Maribel
Millen White Zelano & Branigan P.C.
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