Patent Application
20080233025
The present invention relates generally to recovery of sulphur from oil and gas processing, and more particularly to the removal of sulphurous compounds from gaseous streams produced during industrial processes, thereby releasing “clean gas” containing minimal amounts of sulphurous compounds.
A hazard associated with the petroleum industry is the atmospheric release of the toxic gas hydrogen sulphide (H2S). H2S is found in various gas streams, such as raw sour gas streams or in gas streams (such as tail gas streams) arising from industrial operations where fuels containing sulphur and other combustible materials are burned. H2S, being extremely toxic, must in accordance with regulations be removed before the by-products from such industrial operations can be released into the atmosphere. Regulations have necessitated the development of methodologies to recover sulphur and reduce the amounts of each of H2S and SO2 released into the atmosphere.
Conventionally, the amount of sulphur released into the atmosphere is reduced by converting H2S and SO2 into elemental sulphur. The method commonly used by industry today is known as the modified Claus process, first developed by the London chemist Carl Friedrich Claus in 1883. This method is based on the Claus reaction:
2H2S+SO2⅜S8+2H2O (1)
The modified Claus process is a two step process: 1) the oxidation of H2S to SO2 in a reaction furnace according to the equation:
H2S+⅜O2→SO2+H2O (2)
and 2) the reaction of SO2 and residual H2S into elemental sulphur via the Claus reaction (1). The second step, the reaction of H2S and SO2 into elemental sulphur is typically completed using a series of catalytic reactors, because the Claus reaction is an equilibrium reaction. Consequently, it is typical to use several catalytic reactors in series, with elemental sulphur incrementally removed at each reactor, to achieve greater sulphur recovery.
Unfortunately, thermodynamically, one does not recover all the sulphur by employing only a series of Claus reactors. A small amount of H2S remains in the tail gas stream, thereby necessitating the additional step of tail gas clean up (hereinafter “TGCU”).
There are a total of 16 TGCU processes known to be in use, 9 of which are proven technologies. TGCU units are typically used together with either Claus or modified Claus sulphur recovery units (hereinafter “SRU”).
A typical SRU involves a raw gas feed stream passing through an amine treating unit that absorbs H2S and then desorbs it, thereby concentrating the H2S. This concentrated H2S then enters a reaction furnace where it is combusted in an oxygen rich environment, producing H2S and SO2 in accordance with reaction (3) below.
H2S+aO2bH2S+cSO2+dS(elemental)+e COS+fCS2+gH2O (3)
Elemental S and H2O are then removed from the partially treated gas stream by condensation that lowers the temperature of the gas stream, which is then passed through a series of catalytic converters where COS, CS2, and elemental S are removed. H2S and SO2 undergo the Claus reaction (1) above, while COS and CS2 mainly undergo different reactions (4) and (5) to produce H2O and elemental sulphur.
COS+H2O→CO2+H2S (4)
CS2+2H2O→CO2+2H2S (5)
Disadvantageously, after a series of catalytic converters progressively remove sulphur from the gas stream, the use of catalytic converters is no longer efficient, so a small portion of the original H2S and produced SO2 are released into the atmosphere with the treated exhaust.
The following known patents teach different improvements to the above conventional method of removing sulphurous compounds from industrial gas streams.
U.S. Pat. No. 4,138,473 to Gieck (the '473 patent, issued Feb. 6, 1979) teaches the use of pure oxygen to combust H2S into SO2. Further, the use of three catalytic converters in series is combined with the repressurization and reheating of the gas stream before entering the next catalytic converter in the series, each converting H2S and SO2 into H2O and elemental sulphur. SO2 is then recycled back to the start of the process as fuel for use in the Claus reaction (1). The '473 patent further teaches that the stoichiometric ratio between H2S and SO2 maintained at 2:1 offers maximum efficiency. Disadvantageously, the '473 technology depends on an oxygen rich environment for its oxidation of H2S, leading to uncontrolled combustion of H2S, resulting in an excess of SO2 needing to be reduced to elemental sulphur by the catalytic converters. This excess production of SO2 also requires a TGCU unit to scrub out the excess SO2, thereby higher cost.
U.S. Pat. No. 4,895,670 to Sartori (issued Jan. 23, 1990) and U.S. Pat. No. 4,961,873 to Ho (issued Oct. 9, 1990) each teach the use of an amine scrubber to absorb H2S and concentrate it prior to entering the reaction furnace 130 (with reference to
U.S. Pat. No. 4,071,436 to Blanton (issued Jan. 31, 1978) teaches the use of various catalysts (e.g. alumina, typically in a fluidized bed or embedded on the surface of a moving bed) in a converter to help drive the Claus reaction (1). Disadvantageously, these technologies still require the use of a TGCU before the exhaust gases can be released to atmosphere.
An oxygen rich environment has been typical of conventional sulphur recovery until recently. However, US Patent Application 2005/0158235 to Ramani, (published Jul. 25, 2005) teaches the limited use of oxygen during the oxidation of H2S to lower the SO2 introduced to subsequent stages and thereby in the exhaust. Disadvantageously, US Application 2005/0158235 necessitates the use of a TGCU unit to remove residual SO2 in the exhaust.
US Patent Application 2006/0078491 to Lynn (published Apr. 13, 2006) teaches treating a gas stream using an excess of SO2 within an organic liquid environment such as poly glycol ether (or other tertiary amine solution), according to a process in which the stoichiometric ratio between H2S and SO2 should be maintained lower than 2:1. This process eliminates the need for an amine scrubber and absorber. Disadvantageously, this also results in a higher concentration of SO2 entering the catalytic converters, which SO2 must be recycled back to the start of the process as fuel for use in the Claus reaction (1), like the process taught in '473.
It is, therefore, desirable to provide a less costly methodology for recovering sulphur from sour gas streams, which process does not necessitate the use of a TGCU unit in order to meet modern environmental standards.
It is an object of the present invention to eliminate the need for a TGCU unit when recovering sulphur from sour gas streams.
In one broad aspect of the invention, a process for removing sulphurous compounds including H2S from an industrial gas stream is provided comprising the steps of: feeding the industrial gas stream into a reaction furnace; combusting the industrial gas stream so as to oxidize H2S therefrom in said furnace under sufficiently oxygen-deficient conditions so as to maintain a stoichiometric ratio between H2S and SO2 to be greater than 2:1; condensing the combusted gas stream so as to precipitate H2O and elemental sulphur therefrom; converting the remaining products from the combustion of H2S to elemental sulphur, using a conventional modified Claus reactor; condensing the catalyzed gas stream so as to further precipitate H2O and elemental sulphur therefrom; scrubbing unconverted H2S out of the treated gaseous stream and concentrate using a secondary regenerator; and recycling any unconverted H2S to a reaction furnace. Preferably, the industrial gas stream is pre-scrubbed in a pre-existing primary amine treatment unit.
Another object of the present invention is to take advantage of an oxygen deficient environment that exists inside a typical reaction furnace. The method of present invention uses such oxygen deficient environment to control the stoichiometric ratio between the H2S and SO2 entering the catalytic converters, and then recycles residual H2S back to an amine treating unit.
Thermodynamically, the Claus reaction (1) is an equilibrium reaction the dissociation constant of which is:
Kp=[S8]3/8[H2O]2/[H2S]2[SO2] (6)
According to a method of the present invention a gas feed stream first enters an amine treating unit in order to concentrate the H2S in that raw stream. The concentrated H2S then enters a reaction furnace where it is subjected to an oxygen deficient environment, which in turn results in less SO2 leaving the furnace, such that the stoichiometric ratio between H2S and SO2 is greater than 2:1.
The concentrated H2S in the primary gas stream entering the furnace is oxidized according to combustion reaction (3) thereby producing SO2, H2S, COS and CS2 and H2O. This is a complete reaction, only dependant upon the availability of the reactants, H2S and O2. Advantageously, limiting the amount of O2 present during the combustion of H2S results in a lower production of the by-product SO2 needing to undergo catalytic conversion.
In accordance with the dissociation equation (6), a high concentration of H2S necessarily produces a low concentration of SO2, since at a constant temperature the concentration of SO2 is inversely proportional to the concentration of H2S squared. In an oxygen-deficient environment the Claus reaction (1) produces a higher concentration of H2S and a lower concentration of SO2 as compared to the modified Claus reaction, which produces H2S and SO2 in a stoichiometric ratio of 2:1.
H2O and elemental sulphur precipitate out of the gas stream by condensation. COS and CS2 continue along in the gas stream and enter a catalytic converter where they are subjected to reactions (4) and (5) to produce H2O and elemental sulphur. The H2S and SO2, (in said stoichiometric ratio greater than 2:1) also enter a catalytic converter, where the Claus reaction (1) produces H2O and elemental sulphur.
Residual H2S is removed by a secondary amine scrubber and recycled back to primary regenerator to increase the amount of H2S available for oxidation in the furnace. In an alternative embodiment, residual H2S may be removed by the secondary amine scrubber, regenerated by a secondary regenerator, and recycled to the reaction furnace. It should be noted that the primary amine scrubber and regenerator are not part of the proposed sulphur recovery unit, but part of a pre-existing amine treating unit (hereinafter “ATU”).
An embodiment of the process of this present invention for removing sulphurous compounds, from an industrial gas stream flowing through a fluidly coupled system comprises a primary scrubber (of a pre-existing ATU), a primary regenerator (of a preexisting ATU), a reaction furnace, suitable controllers and sensors, at least two condensers, at least one catalytic converter, and a secondary scrubber.
The primary scrubber and primary regenerator scrubs H2S from the industrial gaseous stream and concentrates the H2S. The concentrated H2S enters the reaction furnace under oxygen deficient conditions and is oxidized. The oxidized gas stream enters a condenser to precipitate out H2O and elemental sulphur. The remaining gases, are catalyzed in a conventional modified Claus reactor to further produce elemental sulphur and H2O. Any unconverted H2S is further scrubbed by the secondary scrubber and then recycled through the primary regenerator to re-enter the reaction furnace.
One embodiment of the system of this present invention for removing sulphurous compounds, from an industrial gaseous stream flow, comprises a primary scrubber and a primary regenerator, both of a pre-existing ATU. These are to scrub and concentrate H2S from an industrial gaseous stream.
The system further comprises a reaction furnace, to oxidize the concentrated H2S, condensers to precipitate out elemental sulphur and H2O, a conventional modified Claus reactor, suitable sensors and controllers and a secondary scrubber. The system also recycles the scrubbed H2S back to the primary regenerator.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the method and system according to the invention and, together with the description, serve to explain the principles of the invention.
The present invention, in order to be easily understood and practiced, is set out in the following non-limiting examples shown in the accompanying drawings, in which:
Referring to
As shown in
H2S is oxidized by O2 in furnace 130 to produce gaseous forms of elemental sulphur, H2O, COS, CS2, and SO2. All products then enter condenser #1140. Inside condenser #1140, the gas stream temperature is lowered sufficiently that H2O and elemental sulphur precipitate out, leaving the gaseous form of each of COS, CS2, H2S and SO2 to flow into catalytic converter 160, which is any suitable conventional catalytic converter.
SRU sensor #2162 measures the amount of H2S and SO2 entering catalytic converter 160 and also sends a feed back signal to SRU control unit 150, which combines that signal with the feed forward signal from SRU sensor #1161 in order to regulate the amount of air entering furnace 130, and thereby the results of oxidation reaction (3), by maintaining the stoichiometric ratio between H2S and SO2 at greater than 2:1, such that a controlled amount of SO2 is produced during the initial oxidative process in furnace 130.
Inside catalytic converter 160 the reactants undergo the Claus reaction (1) to produce elemental sulphur, COS, CS2, and H2O. COS and CS2 also undergo reactions (4) and (5) to further produce H2O and elemental sulphur. Any suitable catalyst may be used to facilitate the Claus reaction. Maintaining the stoichiometric ratio between H2S and SO2 at greater than 2:1 advantageously controls the amount of H2S and SO2 entering catalytic converter 160, which is achieved by SRU control unit 150 using feed back signals from SRU sensor #2162 monitoring the amount of H2S and SO2 entering catalytic converter 160.
The treated gas stream leaving catalytic converter 160 enters condenser #2170 to further precipitate out both H2O and elemental sulphur. After which, the treated gas stream leaving condenser #2170 flows into a downstream secondary scrubber 180 where excess H2S is absorbed and any unconverted H2S is recycled back to primary regenerator 120.
As illustrated in the flow chart of
With reference to
Further, secondary scrubber 180 is incorporated into sulphur recovery unit 400.
As illustrated in the flow chart of
A series of calculations were performed to determine the potential efficiency of a system based on the present invention, including the recycling of untreated H2S from secondary scrubber 180. The results of these simulations are shown in
The calculations were based on a schematic diagram representing the preferred embodiment of the present invention (See
The definitions of the variables used are as follows:
x=amount of sulphur in the primary gas inlet stream (ie. sour gas) entering furnace 140 in moles/hour;
R=amount of recycled H2S re-entering furnace 130 from secondary scrubber 180 (in reference to
P=amount of H2S leaving furnace 130 in moles/hour;
Q=amount of SO2 leaving furnace 130 in moles/hour;
S=amount of elemental sulphur that is removed from furnace 130 in moles/hour;
a=efficiency of sulphur recovery in furnace 130, typically between 40-50%;
b=efficiency of sulphur recovery in the catalytic converter, typically between 60-90%; and
c=efficiency of sulphur recovery in the amine scrubber, typically between 90-99.9%.
As shown in the table of
In accordance with
In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.
Although the disclosure describes and illustrates various embodiments of the invention, it is to be understood that the invention is not limited to these particular embodiments. Many variations and modifications will now occur to those skilled in the art of sulphur recovery. For full definition of the scope of the invention, reference is to be made to the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2568303 | Oct 2006 | CA | national |
