Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
1997-07-02
2003-01-14
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S220000, C423S228000, C423S242100, C423S573100, C423SDIG005
Reexamination Certificate
active
06506349
ABSTRACT:
TECHNICAL FIELD
This invention relates to the treatment of gas streams, and in particular to an improved method for solvent extraction of hydrogen sulfide and other contaminants from sour gas feed stocks, tail gases, ammonia, and flue gas streams.
BACKGROUND OF THE INVENTION
Hydrocarbon fuel sources such as crude oil, natural gas and coal are often contaminated by significant amounts of sulfur compounds. When the sulfur compounds are burned, objectionable odors and pollutants are created. The extraction of sulfur, generally present in the form of hydrogen sulfide, from feedstocks and tail gases is thus a vital aspect of refinery, natural gas and coal liquification operations.
Hydrogen sulfide is usually removed by solvent extraction, with subsequent regeneration and recycle of the solvent. In many cases, carbon dioxide is also present in significant amounts. The solvent must therefore be selective for hydrogen sulfide in the presence of carbon dioxide. Suitable solvents include aqueous solutions of secondary and tertiary amine, such as diisopropylamine (DIPA), methyldiethanolamine (MDEA) and triethanolamine (TEA). The feedstock gas is contacted with the amine at relatively low temperatures in an absorber to remove the hydrogen sulfide. This step produces a rich amine stream, loaded with H
2
S and CO
2
. This rich amine is passed to a stripper/regenerator, usually a tray type column. The solvent is heated and gives off a concentrated acid gas, leaving a lean amine stream that is recycled as fresh solvent to the absorber. The H
2
S rich concentrated acid gas is routed to a sulfur recovery unit to be converted into elemental sulfur by the well known Claus process:
H
2
S+⅔O
2
→SO
2
+H
2
O 1)
2H
2
S+SO
2
⇄2H
2
O+3S 2)
The Shell Claus Off-gas Treating (SCOT) process for removing sulfur components from Claus plant tail gas was first brought on stream in 1973.Since then, the process has been widely used in the oil refining and natural gas industries, with more than 150 units constructed all over the world. In the standard SCOT process, sulfur compounds in Claus plant tail gas are catalytically converted into hydrogen sulfide. After cooling, the hydrogen sulfide is removed by solvent extraction, in the manner already discussed. The SCOT off-gas (the gas not absorbed in the absorber) is incinerated.
The advantage of the SCOT process is the use of familiar refining technologies. Application of the SCOT process for different types of gas treating units and Claus units raises special design considerations. The sulfur dioxide emissions from the Claus and tail gas treating plants make a significant contribution to the total sulfur dioxide emissions from a refinery. It is therefore important to reduce the sulfur dioxide emissions from these plants to the lowest possible levels.
The standard SCOT process is able to recover 99.9% of total sulfur, resulting in a 250 ppmv sulfur concentration in the SCOT off-gas. In recent years, the demand for higher sulfur recovery efficiencies has resulted in the development of two improved versions to the SCOT process. These are the Low-sulfur SCOT and the Super-Scot processes. The new processes lower the total sulfur content in the SCOT off-gas to less than 50 ppmv, while maintaining low operating costs.
The Low sulfur-SCOT (LS-SCOT) version is characterized by the use of an inexpensive additive to the amine solvent. This additive improves the regeneration of the solvent, and produces regenerated solvent having less hydrogen sulfide, which in turn results in a lower off-gas hydrogen sulfide concentration. Treated off-gas specifications as low as 10 ppmv hydrogen sulfide can be achieved. Because of the additive, LS-SCOT units are preferably designed as a stand-alone SCOT unit. However, the LS-SCOT version has also been tested successfully on integrated SCOT units with DIPA and MDEA solvents.
The Super-SCOT version is based on improved stripping by two-stage regeneration and improved absorption by using a lower lean solvent temperature. These two features can be applied separately or in combination.
The performance of an amine regenerator is normally limited by the equilibrium conditions in the bottom. This condition leads to a direct relationship between stripping steam and the solvent leanness. In order to produce leaner solvent (mol H
2
S/mol amine), a higher specific steam rate (kg steam/m
3
solvent) is required. As in the LS-SCOT method, a leaner regenerated solvent will result in a lower hydrogen sulfide concentration in the SCOT off-gas. However, it is not necessary to regenerate the entire solvent flow to this lower leanness level. The Super-SCOT process therefore uses two-stage regeneration, in which part of the amine solvent flow is more deeply stripped. The resulting super-lean solvent is routed to the top tray of the absorber, while the normal lean solvent enters half-way up the absorber. The use of an additive changes the equilibrium conditions; less steam is required for the same leanness, or a greater leanness can be achieved with the same steam rate.
It is well known that the solubility of hydrogen sulfide in amine solvents increases when the temperature is lowered. Thus, using a lower amine temperature results in improved extraction of hydrogen sulfide from the feed gas, which enables a lower hydrogen sulfide concentration to be achieved compared to the normal SCOT process.
The Super-SCOT method has been shown to achieve a hydrogen sulfide concentration of 10 ppmv H
2
S or a total sulfur concentration of less than 50 ppmv, and to reduce steam consumption by 30% compared to the standard SCOT unit. Cascading the solvent similar to the standard SCOT is an option to save operating costs.
Because the acid gas from the SCOT regenerator is recycled back to the Claus feed gas, it is important that inert ingredients in the concentrated acid gas be as low as possible in order to avoid build up of inerts, which can severely limit throughput in the Claus/SCOT system. As already discussed, carbon dioxide, which is not treated by the SCOT process, is often present in significant quantities. Therefore, the solvent used in the SCOT process should preferably absorb hydrogen sulfide more readily than it will absorb carbon dioxide.
Because the capacity of the sulfur recovery plant is critical to the capacity for producing finished product, there is continued interest in improving Claus plant capacities and several incremental improvements have been developed for the refining industries.
U.S. Pat. No. 4,263,270, issued to Groenendaal et al., discloses a process for handling gases containing large quantities of hydrogen sulfide and carbon dioxide. Hydrogen sulfide and CO
2
are extracted by absorption, then the absorbed gas is subjected to the Claus process. The Claus off-gas is reacted in the presence of a catalyst with hydrogen or carbon monoxide, followed by a second extraction that is CO
2
selective. Some of the feed gas is bypassed around the Claus unit in some cases.
U.S. Pat. No. 4,153,674, issued to Verloop et al., discloses a process for treating a gas stream that is high in CO
2
, low in H
2
S, and also contains significant amounts of COS or other organic sulfur compounds. The process is quite similar to that disclosed in Groenendaal et al., except that the feed gas always bypasses the Claus unit, and is combined with the Claus off-gas before entering the catalytic reactor.
U.S. Pat. No. 4,001,386, issued to Klein et al., discloses a process using cascaded absorbers with a common desorbtion column. A Claus plant and reactor like that in Groenendaal treat the first absorber off-gas before it is fed to the second absorber. The absorbent from the second absorber is cascaded through the first absorber before being desorbed, so that the desorber always processes absorbent having higher H
2
S partial pressure than that of the Claus off-gas.
U.S. Pat. No. 4,153,674, issued to Verloop et al., discloses a Claus process adapted for increasing the H
2
S concentration in a gas stream, particularly a gas
Hubbard Jeffrey T.
Kelly & Hubbard
Silverman Stanley S.
Vanoy Timothy C.
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