A METHOD AND SYSTEM FOR RECOVERING SULPHUR FROM GAS STREAMS

FIELD OF THE INVENTION 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.

BACKGROUND OF THE INVENTION A hazard associated with the petroleum industry is the atmospheric release of the toxic gas hydrogen sulphide (H ₂ S). H ₂ S 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. H ₂ S, 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 H ₂ S and SO ₂ released into the atmosphere. Conventionally, the amount of sulphur released into the atmosphere is reduced by converting H ₂ S and SO ₂ 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: 2 H ₂ S + SO ₂ <→ 3/8 S ₈ + 2 H ₂ O (1)

The modified Claus process is a two step process: 1) the oxidation of H ₂ S to SO ₂ in a reaction furnace according to the equation: H ₂ S + 3/2 O ₂ → SO ₂ + H ₂ O (2) and 2) the reaction of SO ₂ and residual H ₂ S into elemental sulphur via the Claus reaction (1). The second step, the reaction of H ₂ S and SO ₂ 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 H ₂ S 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 H ₂ S and then desorbs it, thereby concentrating the H ₂ S. This concentrated H ₂ S then enters a reaction furnace where it is combusted in an oxygen rich environment, producing H ₂ S and SO ₂ in accordance with reaction (3) below.

H ₂ S + aO ₂ → bH ₂ S + cSO ₂ + dS _(e ι _em entai) + eCOS + fCS ₂ + gH ₂ O (3) Elemental S and H ₂ O 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, CS ₂ , and elemental S are removed. H ₂ S and SO ₂ undergo the Claus reaction (1) above, while COS and CS ₂ mainly undergo different reactions (4) and (5) to produce H ₂ O and elemental sulphur. COS + H ₂ O → CO ₂ + H ₂ S (4) CS ₂ + 2 H ₂ O → CO ₂ + 2 H ₂ S (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 H ₂ S and produced SO ₂ 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. US patent 4,138,473 to Gieck (the '473 patent, issued Feb. 6, 1979) teaches the use of pure oxygen to combust H ₂ S into SO ₂ . 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 H ₂ S and SO ₂ into H ₂ O and elemental sulphur. SO ₂ 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 H ₂ S and SO ₂ maintained at 2:1 offers maximum efficiency. Disadvantageously, the '473 technology depends on an oxygen rich environment for its oxidation of H ₂ S, leading to uncontrolled combustion of H ₂ S, resulting in an excess of SO ₂ needing to be reduced to elemental sulphur by the catalytic converters. This excess

production of SO ₂ also requires a TGCU unit to scrub out the excess SO ₂ , thereby higher cost. US patent 4,895,670 to Sartori (issued Jan. 23, 1990) and US patent 4,961 ,873 to Ho (issued Oct. 9, 1990) each teach the use of an amine scrubber to absorb H ₂ S and concentrate it prior to entering the reaction furnace 130 (with reference to Figure 1). Disadvantageously, neither of these patents overcomes the necessity of using a TGCU unit. US patent 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 JuI. 25, 2005) teaches the limited use of oxygen during the oxidation of H ₂ S to lower the SO ₂ introduced to subsequent stages and thereby in the exhaust. Disadvantageously, US Application 2005/0158235 necessitates the use of a TGCU unit to remove residual SO ₂ in the exhaust. US Patent Application 2006/0078491 to Lynn (published Apr. 13, 2006) teaches treating a gas stream using an excess of SO ₂ 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 H ₂ S and SO ₂ 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 SO ₂ entering the catalytic converters, which SO ₂ 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.

SUMMARY OF THE INVENTION 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 H ₂ S 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 H ₂ S therefrom in said furnace under sufficiently oxygen-deficient conditions so as to maintain a stoichiometric ratio between H ₂ S and SO ₂ to be greater than 2:1 ; condensing the combusted gas stream so as to precipitate H ₂ O and elemental sulphur therefrom; converting the remaining products from the combustion of H ₂ S to elemental sulphur, using a catalytic converter, such as a modified Claus reactor;; condensing the catalyzed gas stream so as to further precipitate H ₂ O and elemental sulphur therefrom; scrubbing unconverted H ₂ S out of the treated gaseous stream and concentrate using a secondary regenerator; and recycling any unconverted H ₂ S 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 H ₂ S and SO ₂ entering the catalytic converters, and then recycles residual H ₂ S back to an amine treating unit. Thermodynamically, the Claus reaction (1) is an equilibrium reaction the dissociation constant of which is: Kp = [S ₈ ] ³⁷⁸ [H ₂ O] ² / [H ₂ S] ² [SO ₂ ] (6) According to a method of the present invention a gas feed stream first enters an amine treating unit in order to concentrate the H ₂ S in that raw stream. The concentrated H ₂ S then enters a reaction furnace where it is subjected to an oxygen deficient environment, which in turn results in less SO ₂ leaving the furnace, such that the stoichiometic ratio between H ₂ S and SO ₂ is greater than 2:1. The concentrated H ₂ S in the primary gas stream entering the furnace is oxidized according to combustion reaction (3) thereby producing SO ₂ , H ₂ S ₁ COS and CS ₂ and H ₂ O. This is a complete reaction, only dependant upon the availability of the reactants, H ₂ S and O ₂ . Advantageously, limiting the amount of O ₂ present during the combustion of H ₂ S results in a lower production of the by-product SO ₂ needing to undergo catalytic conversion. In accordance with the dissociation equation (6), a high concentration of H ₂ S necessarily produces a low concentration of SO ₂ , since at a constant temperature the concentration of SO ₂ is inversely proportional to the concentration of H ₂ S squared. In an oxygen-deficient environment the Claus

reaction (1) produces a higher concentration of H ₂ S and a lower concentration of SO ₂ as compared to the modified Claus reaction, which produces H ₂ S and SO ₂ in a stoichiometric ratio of 2: 1. H ₂ O and elemental sulphur precipitate out of the gas stream by condensation. COS and CS ₂ continue along in the gas stream and enter a catalytic converter where they are subjected to reactions (4) and (5) to produce H ₂ O and elemental sulphur. The H ₂ S and SO ₂ , (in said stoichiometric ratio greater than 2:1) also enter a catalytic converter, where the Claus reaction (1) produces H ₂ O and elemental sulphur. Residual H ₂ S is removed by a secondary amine scrubber and recycled back to primary regenerator to increase the amount of H ₂ S available for oxidation in the furnace. In an alternative embodiment, residual H ₂ S 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 H ₂ S from the industrial gaseous stream and concentrates the H ₂ S. The concentrated H ₂ S

enters the reaction furnace under oxygen deficient conditions and is oxidized. The oxidized gas stream enters a condenser to precipitate out H ₂ O and elemental sulphur. The remaining gases, are catalyzed in a conventional modified Claus reactor to further produce elemental sulphur and H ₂ O. Any unconverted H ₂ S 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 H ₂ S from an industrial gaseous stream. The system further comprises a reaction furnace, to oxidize the concentrated H ₂ S, condensers to precipitate out elemental sulphur and H ₂ O, a conventional modified Claus reactor, suitable sensors and controllers and a secondary scrubber. The system also recycles the scrubbed H ₂ S 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.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention, in order to be easily understood and practised, is set out in the following non-limiting examples shown in the accompanying drawings, in which:

Fig. 1 is a schematic diagram illustrating a preferred embodiment of the system of the invention; Fig. 2 is a schematic diagram illustrating an alternate embodiment of the system of the invention incorporating a stabilizer; Fig. 3 is a flow chart demonstrating the preferred embodiment of the process; Fig. 4 is a schematic diagram illustrating an alternate embodiment of the system of the invention incorporating a secondary regenerator; Fig. 5 is a flow chart demonstrating an alternate embodiment of the process incorporating a secondary regenerator; Fig. 6 is a schematic diagram of the preferred embodiment of the invention demonstrating the mathematical relationship existing between each step of the process; and Fig. 7 is a table demonstrating sulphur recovery according to Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1 , there is illustrated one embodiment of a system, the sulphur recovery unit (hereinafter "SRU") denoted generally as 400, in which a primary gas feed stream enters primary scrubber (of a pre-existing ATU) 110 where H ₂ S is absorbed from the gas stream and is thereafter concentrated in primary regenerator (of a pre-existing ATU) 120, such that purified and concentrated H ₂ S enters reaction furnace 130. The SRU sensor ^# 1 161 , monitors the amount of H ₂ S entering furnace 130 and provides a feed forward signal to

SRU control unit 150, which regulates the amount of air entering furnace 130 via O ₂ Control Valve 165, so as to maintain an oxygen-deficient environment and achieve the designed combustion of H ₂ S. As shown in Fig. 2, the purified and concentrated H ₂ S can be stabilized inside a stabilizer 125 prior to enter the reaction furnace 130. A person skilled in the art knows that the combustion of the gas stream in the reaction furnace 130 is preferable within a reaction temperature range. Further, the composition of the gas determines the achievable temperature of the reaction furnace 130 for the oxidation of H ₂ S. For example, if there are too many inert components present in the entering gas stream, the inert components may prohibit the reaction furnace 130 from reaching an optimum temperature for H ₂ S oxidation. As a result, a portion of the entering gas stream can bypass the reaction furnace 130, directly to the catalytic converter 160. H ₂ S is oxidized by O ₂ in furnace 130 to produce gaseous forms of elemental sulphur, H ₂ O, COS, CS ₂ , and SO ₂ . All products then enter condenser #1 140. Inside condenser ^# 1 140, the gas stream temperature is lowered sufficiently that H ₂ O and elemental sulphur precipitate out, leaving the gaseous form of each of COS, CS ₂ , H ₂ S and SO ₂ to flow into catalytic converter 160, which is any suitable conventional catalytic converter. SRU sensor *2 162 measures the amount of H ₂ S and SO ₂ 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 ^# 1 161 in order to regulate the amount of air entering furnace 130, and thereby the results of oxidation reaction (3), by maintaining the stoichiometic ratio between

H ₂ S and SO ₂ at greater than 2:1 , such that a controlled amount of SO ₂ 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, CS ₂ , and H ₂ O. COS and CS ₂ also undergo reactions (4) and (5) to further produce H ₂ O and elemental sulphur. Any suitable catalyst may be used to facilitate the Claus reaction. Maintaining the stoichiometic ratio between H ₂ S and SO ₂ at greater than 2:1 advantageously controls the amount of H ₂ S and SO ₂ entering catalytic converter 160, which is achieved by SRU control unit 150 using feed back signals from SRU sensor ^# 2 162 monitoring the amount of H ₂ S and SO ₂ entering catalytic converter 160. The treated gas stream leaving catalytic converter 160 enters condenser #2 170 to further precipitate out both H ₂ O and elemental sulphur. After which, the treated gas stream leaving condenser *2 170 flows into a downstream secondary scrubber 180 where excess H ₂ S is absorbed and any unconverted H ₂ S is recycled back to primary regenerator 120. As illustrated in the flow chart of Fig. 3, the process conducted in the system of Figs. 1 and 2 comprises scrubbing and concentrating H ₂ S from a gaseous feed stream at 900. The scrubbed H ₂ S then is oxidized at 910 according to the present invention. Water and elemental sulphur are precipitated at 920. H ₂ S, SO ₂ , COS and CS ₂ are reacted at 930. Water and elemental sulphur are precipitated at 940. Unconverted H ₂ S is scrubbed from the gas stream at 950. Unconverted H ₂ S is recycled back to the primary regenerator at 960. In another embodiment, recycling of the H ₂ S from the scrubber 180 may be split into two streams, one into the reaction furnace 130 and a second

stream into the catalytic converter 160. The cold ^■ recycled H ₂ S need not needlessly be recycled to the reaction furnace, where it will require more energy to oxidize it, and then require further energy to condense it as it enters the condenser #1 140. Splitting of the recycling H ₂ S stream into the reaction furnace 130 and the catalytic converter 160 reduces the energy required. With reference to Fig. 4, in the event that primary regenerator 120 is not available, then, an alternative embodiment comprises a secondary regenerator 190 after the secondary scrubber 180, and such that recycling 191a of the H ₂ S would be to the reaction furnace 130. Alternatively, in another embodiment, some recycling 191b of the H ₂ S may be to the catalytic converter 160 directly or recycling 191c to the condenser 140, bypassing the reaction furnace 130. Advantageously, secondary scrubber 180 is a smaller and less expensive component than primary scrubber 110 used in the initial stage of the inventive process. Further, secondary scrubber 180 is incorporated into sulphur recovery unit 400. As illustrated in the flow chart of Fig. 5, the process conducted in the system of Fig. 4 comprises scrubbing and concentrating H ₂ S from a gaseous feed stream at 900. The scrubbed H ₂ S then is oxidized at 910 according to the present invention. Water and elemental sulphur are precipitated at 920. H ₂ S, SO ₂ , COS and CS ₂ are reacted at 930. Water and elemental sulphur are precipitated at 940. Unconverted H ₂ S is scrubbed from the gas stream at 950. Unconverted H ₂ S can be regenerated at 955 and recycled back to the reaction

furnace at 965. In another embodiment, the recycling 965 may be to the catalytic converter directly, or also to the condenser or catalytic converter.

Example 1 A series of calculations were performed to determine the potential efficiency of a system based on the present invention, including the recycling of untreated H ₂ S from secondary scrubber 180. The results of these simulations are shown in Fig. 7. The calculations were based on a schematic diagram representing the preferred embodiment of the present invention (See Fig. 6). 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 H ₂ S re-entering either the furnace 130 or the catalytic converter 160 from secondary scrubber 180 (in reference to Fig. 1) 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 Fig. 7, assuming a recovery of sulphur efficiency of 50%, in furnace 130, as the molar ratio between H ₂ S and SO ₂

increase, the efficiency of sulphur recovery varies between 99.0% at the minimum to a maximum of 99.9% recovery. Also accompanying the increase in the stoichiometric ratio between H ₂ S and SO ₂ is the increase in the amount of H ₂ S that is required to be recycled back to primary regenerator 120. In accordance with Fig. 7, a molar ratio of 3:1 (H ₂ S:SO ₂ ), results in an efficiency of 99.9% sulphur recovery. Advantageously, this percentage recovery is far greater than those currently required by environmental regulations in many countries. According to the method of the invention, depriving reaction furnace 130 of oxygen, in any manner that maintains the stoichiometic ratio between H ₂ S and SO ₂ at greater than 2:1 , in combination with recycling residual H ₂ S back to ATU regenerator 120, as taught herein, eliminates the need for and expense of a TGCU, while still meeting or exceeding current environmental standards. 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.