Field of the Invention

The invention relates to the combustion of H₂S containing gas streams and its associated Claus process.

Background of the Invention

Large volumes of hydrogen sulfide and ammonia containing gas streams are produced in various plants including petroleum refineries and natural gas purification plants. Hydrogen sulfide and ammonia, if released to the atmosphere, are hazardous to the environment. Due to their toxicity, they pose extreme danger to people's health.

It has been known to combust these hydrogen sulfide containing gas streams, not only to inhibit the emission of pollutants, but also to produce sulfur which is useful for making, inter alia, papers, fumigants, sulfuric acid, tires, fertilizers and gunpowder. This combustion is usually carried out in the presence of air. When it is used in conjunction with the Claus sulfur process, a feed gas containing hydrogen sulfide is partially combusted with air to form sulfur dioxide in a combustion zone. The amount of air present is such that it provides a sufficient amount of oxygen to combust about one-third of the hydrogen sulfide in the incoming feed gas and to also decompose any ammonia gas to nitrogen and steam vapour. The resulting gas in the combustion zone may also contain sulfur and water due to a portion of the uncombusted hydrogen sulfide reacting with sulfur dioxide. The sulfur in the resulting gas may be recovered in liquid form through cooling. The resulting gas is then commonly passed through two or more catalyst reactor beds to produce additional sulfur by promoting the reaction between previously unreacted hydrogen sulfide and sulfur dioxide.

The combustion of H₂S containing gas streams in the presence of air, however, is found to be inefficient. Since air contains only about 21 percent oxygen, a significant amount of unneeded nitrogen is provided to the combustion zone or its associated Claus sulfur process system. As the hydrogen sulfide concentration level in the feed gas or the need for a higher feed gas processing rate increases, a greater amount of oxygen would be required, thus passing or providing an even greater amount of unneeded nitrogen inherently present in the air into the combustion zone and/or its associated system. The increased flow of nitrogen increases the pressure drops in the combustion zone and its associated system, reduces the residence time of the reactants in the combustion zone and increases the gas volume to be treated in the combustion zone and its associated system.

To reduce the flow or the presence of unproductive nitrogen in the combustion zone and its associated system, the employment of oxygen or oxygen enriched air as the oxygen source has been proposed. U.S. Patent No, 4,138,473 - Gieck, for example, discloses the use of pure oxygen for combusting a H₂S containing feed gas in the presence of recycled sulfur dioxide from the conventional Claus processing system. The recycled SO₂ is derived from a downstream portion of the conventional Claus processing after reacting most of the H₂S and SO₂ in the combusted gas over a plurality of catalytic converter beds. This technique, although reduces the amount of nitrogen which is introduced into the combustion zone and its associated system and reduces the pollutants (SO₂) resulting therefrom, fails to address a problem associated with a temperature increase in the combustion zone. The temperature of the combustion zone can exceed the temperature tolerance of the refractories in the combustion zone when the feed gas containing a large concentration of H₂S (greater than 50% H₂S) is combusted in the presence of pure oxygen or oxygen enriched air.

Upon recognizing the problem of high combustion zone temperatures, the employment of various temperature moderating additives in the combustion zone of the Claus process system has been proposed. U.S. Patent No. 4,888,162 - Brian, for example, discloses water as a temperature moderating additive in the combustion zone. On the other hand, U.S. Patent No. 4,849,203 -Palm discloses various diluents including one containing sulfur dioxide as a temperature moderating additive. These additives, which are used as an internal heat sink, absorb some of the heat released during the combustion of H₂S, thus moderating the temperature of the combustion zone. The introduction of these additives, however, would increase unneeded gas in the combustion zone and its associated system without imparting any efficiency. The amount of the additives necessary to act as an internal heat sink may be such that it may cause the very problems which were avoided by removing nitrogen from the oxygen source (air). Thus, there is a need to find ways in which the H₂S combustion and its associated Claus process can be carried out in an efficient manner without causing the problems associated with the overflow of unneeded or unproductive substances.

Accordingly, it is an object of the invention to minimize the presence of unproductive substances which can cause the increased pressure drops, the decreased residence time for reactants and the increased fluid load in a combustion zone and its associated Claus process.

It is another object of the invention to maintain the temperature of a combustion zone at or below the temperature limit set by the refractories in the combustion zone.

It is yet another object of the invention to increase overall sulfur recovery efficiency when the combustion of H₂S containing gas streams is carried out in the context of the Claus process.

It is a further object of the invention to reduce the emission of pollutants (H₂S or SO₂) to the atmosphere.

It is an additional object of the invention to reduce the equipment and energy cost associated with the Claus process.

Summary of the Invention

The above objects and other objects which will become apparent to one skilled in the art upon a reading of this disclosure is attained by the present invention which comprises:

A process for combusting a fluid stream containing at least about 20% by volume of H₂S in the presence of an oxidant having an oxygen concentration of greater than 21% in a combustion zone in which at least a portion of H₂S is combusted to SO₂, comprising introducing into said combustion zone about 0.05 kilomole to about 0.35 kilomole SO₂ per every one kilomole of H₂S in said fluid stream whereby the temperature of the combustion zone is maintained at or below the temperature limit set by refractories in the combustion zone. The amount of SO₂ introduced to achieve the necessary temperature limit in the combustion zone is obtained from a particular Claus process. The Claus process involves receiving at least a portion of the resulting gas from the combustion zone to a thermal zone where at least a portion of sulfur dioxide and uncombusted H₂S therein are reacted to form sulfur and water, cooling at least a portion of the gas derived from the thermal zone to remove sulfur therein in liquid form, reacting the sulfur removed gas in a single catalytic converter stage to promote the reaction between any unreacted sulfur dioxide and H₂S therein to form additional sulfur, cooling at least a portion of the gas formed in the catalytic stage to remove sulfur therein in liquid form, combusting the additional sulfur removed gas in the presence of oxidant to convert any H₂S therein to sulfur dioxide in an incinerator and treating the gas formed in the incinerator for the selective recovery of sulfur dioxide which is used in the combustion zone. The use of a single catalytic stage in conjunction with a particular selective SO₂ recovery increases the sulfur recovery and imparts the required amount of SO₂ to the combustion zone such that the temperature therein does not exceed the temperature tolerance of the refractories used to build the combustion zone.

As used herein, the term "fluid" includes gases, liquids or mixtures thereof.

As used herein, the term "oxidant" means an oxygen bearing and/or oxygen containing substance.

Brief Description of the Drawings

The sole Figure is a schematic flow diagram of one preferred embodiment of the process of this invention.

Detailed Description of the Invention

The invention involves utilizing a particular amount of sulfur dioxide, which is sufficient to relieve the temperature increase associated with an exothermic reaction which is caused by the conversion of H₂S to SO₂ in the presence of pure oxygen or oxygen enriched air. By providing the particular amount of SO₂ into the combustion zone, the conversion of H₂S to SO₂ can be minimized during the combustion, thus reducing any heat resulting from the exothermic reaction. The SO₂ added to the combustion zone may be generated in a particularly arranged Claus processing system such that a valuable product (sulfur) can be recovered in an efficient manner with the minimum emission of pollutants to the atmosphere. When the particularly arranged Claus process is used, the addition of sulfur dioxide to the combustion zone not only serves to inhibit the temperature increase therein but also serves to maximize the recovery of sulfur in the Claus process through providing the desired H₂S/SO₂ ratio in a catalytic stage.

Referring now to the Figure, there illustrates one preferred embodiment of the invention wherein the combustion of H₂S containing gas streams is carried out in the presence of pure oxygen or oxygen enriched air as a part of the preferred Claus process. A feed gas having a hydrogen sulfide content of about 20 to about 100% by volume, preferably about 50% to about 100% by volume, is introduced to a reactor furnace (2) having a combustion zone through line (4) at a pressure of about 2 to about 20 psig. The feed gas may be preheated by a heating means (44) to a temperature of about 40°C to about 120°C prior to the introduction in the reactor furnace (2). A recycle stream in line (32) is either introduced directly into the reactor furnace (2) and/or introduced into the line (4) at a pressure of about 2 to 20 psig, or introduced into the waste heat boiler (8) and/or into the heater exchanger (16). The temperature of the recycle stream is within the range of about 30°C to about 100°C. The recycle stream contains a sufficient amount of SO₂ to inhibit the temperature in the furnace (2) from rising above about 1500°C-1800°C, thus preventing any deleterious effect on the refractories of the furnace (2). At least a portion of SO₂ needed, however, may also be generated through combusting a portion of H₂S during the preheating of the feed gas. The amount of SO₂ in the recycle, stream and/or the feed gas is such that about 0.05 Kilomole to about 0.35 Kilomole of SO₂, preferably about 0.05 Kilomole to about 0.15 Kilomole of SO₂, can be introduced into the furnace (2) for every one K mole of H₂S provided in the feed gas. The recycled stream preferably consists essentially of SO₂ so that the introduction of unneeded gas in the furnace (2) is minimized. An oxidant having a higher concentration of oxygen than air, preferably technically pure oxygen or an oxidant having an oxygen concentration of at least about 25% by volume, e.g. about 50% to about 100% by volume, is also introduced into the furnace (2) through line (6), thereby combusting H₂S. The amount of H₂S combusted is such that a gas having the desired H₂S/SO₂ ratio of about 2:1 is formed when SO₂ introduced in the form of a recycle stream and the SO₂ formed through the combustion are combined. At least a portion of SO₂ reacts with uncombusted H₂S to form sulfur in the furnace (2). The reactions in the furnace may be characterized by the following equation:

        H₂S + 3/2 O₂ → SO₂ + H₂O

        2H₂S + SO₂ → 3/2 S₂ + 2H₂O

Subsequently, the resulting reactor furnace effluent is delivered to a heat exchanger or waste heat boiler (8). In the waste heat boiler (8), the combustion effluents are cooled against boiler feed water in line (10), thereby producing the cooled effluents having a temperature of about 300 to about 500°C and boiler steam which can be recovered from line (11). When the feed gas introduced into the furnace (2) contains a lower concentration of H₂S, at least a portion of the SO₂ containing stream from the line (32), may be directly introduced into the waste heat boiler (8) in order to reduce gas flow in the furnance (2).

The cooled effluents from the waste heat boiler (8) are then introduced into the first condenser (12) whereby the effluents are further cooled to produce liquid sulfur which is recovered from line (14). The uncondensed effluents from the condenser (12) are heated with or without the presence of SO₂ from the line (32), in a heat exchanger (16) to provide an effluent stream having a temperature of about 300 to about 400°C which is above the sulphur dewpoint of the gas stream. When the feed gas introduced into the furnance (2) contains a lower concentration of H₂S, at least a portion of the SO₂ containing gas stream can be directly introdued into the heat exchanger (16) from the line (32) in order to reduce gas flow in the furnance (2) and the waste heat boiler (8). The heat exchanger (16) may utilize heat recovered from the waste heat boiler (8) and/or the furnace (2). The uncondensed effluent stream, for example, may be indirectly heat exchanged with the combustion effluent in the furnace (2) or the wall of the furnace (2) to obtain the desired temperature.

Once the uncondensed effluent stream is heated to the desired temperature range, it is fed to a catalytic converter bed (18). In the catalytic converter bed (18), previously unreacted H₂S and SO₂ are reacted to form additional sulfur. The reaction may be characterized as follows:

        2H₂S + SO₂ → 3/8 S₈ + 2 H₂O

The reaction effluents resulting from the catalytic converted bed (18) are cooled in the second condenser (20) to recover additional sulfur in liquid form from line (22).

Any uncondensed effluent stream from the condenser (20) is delivered to an incinerator (24). The uncondensed effluent stream may contain. inter alia, hydrogen sulfide, carbon dioxide, H₂O and sulfur dioxide. Through incinerating the effluent stream in the presence of an oxidant, H₂S in the effluent stream is converted to sulfur dioxide, thereby producing a gas stream substantially free of H₂S. The preferred oxidant employed in the incinerator (24) has an oxygen concentration of greater than 21%. As the concentration of oxygen in the oxidant increases, of course, any unneeded or unproductive gas in the system is decreased correspondingly.

The gas stream from the incinerator (24) may be sent directly to an absorption unit (28) or may be cooled to remove water through a line (29) via a third condenser (26) prior to its introduction into the absorption unit (28). The absorption unit described in Union Carbide Canada Limited Brochure, "Cansolv; a Revolutionary New Flu Gas Scrubbing Technology" and/or U.S. Patent No. 3,904,735 - Atwood et al., which is herein incorporated, is preferred. In general, the absorption unit (28) comprises a Tail Gas conditioning section (including a water and incinerator gas cooler, an incinerator gas condenser, water transfer pumps, boiler feed water pumps and tanks, an incinerator and an incinerator waste heat boiler) and a sulphur recycle section (including a contactor tower, a regenerator tower, a reflux tank, a cansolv tank, reboilers and pumps, a lean rich exchanger aerial cooler, a lean amine cooler and reflux Condenser and a recycle blower).

In the absorption unit (28), the gas stream entering at a temperature of below about 100°C through an contactor tower is counter currently contacted with an absorbent comprising at least one amine compound. Through such a contact, about 90% to 100% of SO₂ in the gas stream can be recovered. Other tail gas treatment systems, however, may be used to replace the absorption unit (28) if they can recover a high percentage of SO₂ in the gas stream in a commercially feasible manner. The SO₂ recovered through selective separation is delivered through line (32) to the furnace (2), the waste heat boiler (8) and/or the heat exchanger (16) at a pressure of about 2 psia to 20 psia via a recycle blower (30) while the remaining gas, which is substantially free of SO₂, is vented or discharged to the atmosphere.

The following examples serve to further illustrate the invention. They are presented for illustrative purposes and are not intended to be limiting.

Example 1

An acid gas having H₂S was treated in the Claus process system shown in the Figure. The conditions utilized and the products recovered in each unit of the system are tabulated in Table I below:

As shown in Table I, the combustion of H₂S can be carried out in the presence of technically pure oxygen without causing deleterious effect on the refractories of the furnace when a particular amount of SO₂ is provided in the furnace. Through utilizing a gas consisting essentially of a particular amount of SO₂, the furnace and the associated units are free or substantially free of unneeded gases, thereby increasing the capacity for an acid feed gas, i.e., H₂S to be treated. These advantages are attained along with the increased sulfur recovery (99.7% +).

Example 2

An acid gas feed having about 80% of H₂S was combusted with air in the conventional Claus process system having three catalytic converter stages. After retrofitting the conventional Claus process system to provide the Claus process system of the Figure herein, the same gas was combusted in the presence of oxygen. The results of the process simulation are tabulated in Table II below:

As shown in Table II, there are many advantages derived from utilizing the invention. The advantages include, inter alia, (1) the treatment of an increased amount of acid gas containing H₂S, (2) the increased recovery of sulfur (99.7%+), (3) the reduction in the equipment and energy cost due to elimination of two catalytic stages and (4) up to a 73% reduction in flow of gases to the water condenser unit and the absorption unit. All of these benefits are attained without increasing the pressure drops along the system and without causing the temperature of the reactor furnace to exceed the temperature tolerance of the refractories of the furnace, i.e., <1500°C.

Although the invention has been described in detail with reference to certain specific embodiments, those skilled in the art will recognize that there are other embodiments within the spirit and scope of the claims.