Abstract:
Disclosed is a method for treating a feed gas stream rich in hydrogen sulfide by partially combusting the feed gas stream rich in hydrogen sulfide with an oxygen-enriched gas in a Claus reaction furnace to produce to a combustion reaction product stream containing sulfur. The combustion reaction product stream is split into a recycle stream and a treatment stream and the recycle stream directed back into the Claus reaction furnace, without first condensing sulfur out of the recycle stream, while the treatment stream is directed into a condenser to condense sulfur out of the treatment stream.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the chemical arts. More particularly, this invention relates to the recovery of sulfur from a gas stream rich in hydrogen sulfide using the Claus process. 
     2. Discussion of the Related Art 
     It is known in the prior art to recover elemental sulfur from hydrogen sulfide (H 2 S) containing acid gas streams as is set forth in the article “Fundamentals of Sulfur Recovery by the Claus Process” by B. Gene Goar, published in the 1977 Gas Conditioning Conference Report. The Claus reaction is represented by the following equation:
 
4H 2 S+2SO 2 →3S 2 +4H 2 O
 
     Claus sulfur recovery units (SRU&#39;s) are widely utilized to recover sulfur from acid gas streams produced in natural gas purification and in petroleum refineries, primarily from amine sweetening. In refineries, the H 2 S is in crude oil and is contained in hydrocarbon desulphurization unit off gases and fluidized catalytic cracker unit off gases. Oftentimes, the acid gas stream produced from the amine unit is quite rich in H2S, particularly in petroleum refineries, where it may be 80–95 mol % H 2 S. Also in many refineries, the Claus plant units are either fully loaded or subject to becoming fully loaded (capacity limited) due to the processing of heavy crude oils, which contain relatively large amounts of sulfur compounds. With the dwindling known reserves of refinable hydrocarbons and crude oils, less attractive known oil reserves are now being processed, which less attractive oil reserves typically have high sulfur contents. The trend in refining such high sulfur containing feedstocks will increase in the future. Additionally, the requirements to produce lower sulfur fuels will result in more acid gases containing H 2 S. Therefore, it is a desideratum to increase the capacity of Claus plants to process sulfur. 
     In conventional Claus sulfur recovery systems, the feed pressure of the acid gas feed stream is only about 12 psig. This low pressure level does not provide enough driving force to allow a significant increase in the amount of acid gas feed that can be passed through the many items of equipment that constitute a typical Claus SRU. As Claus SRU feed rates are increased above capacity, several problems develop. At increased flow, the pressure drop through the Claus plant and tail gas cleanup unit increases, and the back pressure increase requires H 2 S and air feed inlet pressures beyond what is available from the amine regenerator that supplies the acid gas feed and the air blower that provides feed air. The increased flow also decreases the residence times and increases the space velocity in the reaction furnace and the catalytic reactor stages, which reduces conversion to sulfur and increases emissions to the tail gas cleanup unit. The increased flow also results in overloading some or all of the heat exchangers in the SRU, which may reduce conversion of H 2 S to sulfur and also increases sulfur vapor carryover to the tail gas unit. The increased flow to the tail gas cleanup unit increases its pressure drop and further lowers tail gas sulfur recovery, which ultimately leads to increased and usually unacceptable sulfur emissions. The increased back pressures may in some Claus plants pose the risk of blowing the liquid sulfur drain seals, which would release process gas containing highly toxic H 2 S into the atmosphere. While booster blowers for the H 2 S and air feeds, and higher pressure sulfur liquid drain seals can provide some increase in capacity, these measures will not overcome problems associated with undersized heat exchange equipment, reduced sulfur conversion, or increased sulfur emissions. 
     It is also known to use oxygen enrichment in the operation of a Claus sulfur plant in order to increase the capacity of H 2 S handled as well as the total throughput of the plant as set forth in the article “Oxygen Use in Claus Sulfur Plants” by M. R. Gray and W. Y. Svrcek published in the 1981 Gas Conditioning Conference Report. In that article, it was disclosed that oxygen can be added in the air feed to the burner of a Claus reaction furnace in order to increase the amount of H 2 S which is combusted to sulfur dioxide (SO 2 ) for later catalytic conversion, with additional H 2 S to the elemental liquid sulfur product of the Claus process. The combustion reaction of H 2 S with oxygen (whether pure oxygen or air) can be represented by the following equation:
 
2H 2 S+3O 2             2SO 2 +2H 2 O  (Equation 2)

     The Gray and Svrcek article recites that the pressure drop through the plant and the reactor space velocities determine the maximum capacity increase which can be achieved with oxygen enrichment. Consequently, it is a desideratum to improve efficiency by reducing the amount of air, thereby maximizing the amount of oxygen available to react with the H 2 S. 
     However, a further limitation set forth in the Gray and Svrcek article is that for a given plant stream, temperatures and sulfur condenser capacity may limit the potential capacity increase using oxygen enrichment. Specifically, stream temperatures in the reaction furnace and in the converter beds may increase due to oxygen enrichment and, in fact, such increase from oxygen enrichment reaches the maximum tolerable temperature of the materials used in such a furnace, namely the refractory lining. Similarly, in the 1983 publication by Linde of Union Carbide entitled “Claus Plant Oxygen Enrichment,” it is noted that oxygen-enrichment limitations exist for H 2 S rich streams due to temperature limits in the furnace or waste heat boiler of a Claus plant. Therefore, it is also a desideratum to moderate the temperature in the Claus reaction furnace. 
     It is also known in the prior art to recycle effluent gases back into the Claus reaction furnace. For example, U.S. Pat. No. 3,681,024 discloses the addition of oxygen and a recycle gas to a Claus reaction furnace. Combustion gases from a reactor unit are first sent to a water scrubber to reduce the water content of the effluent, and a sufficient amount of the scrubber off gases are then recycled to dilute the oxygen feed so that furnace conditions are essentially equivalent to operation with air. 
     U.S. Pat. No. 3,822,341 describes a Claus plant using oxygen enrichment in which water is removed from the combustion gases, first in a liquid vapor contractor and then in an SO 2  stripper, before the reaction gases are recycled to a waste heat boiler. 
     U.S. Pat. No. 4,756,900 discloses a process for splitting the effluent from the waste heat boiler of a Claus reaction furnace and recycling a portion thereof using a separate sulfur condenser and a mechanical blower to moderate the high furnace temperatures induced by oxygen-enrichment. 
     U.S. Pat. No. 4,552,747 describes a process for moderating the high temperatures in a Claus reaction furnace induced by oxygen-enrichment by passing reaction gases through a sulfur condenser and then using a mechanical blower to recycle the resulting effluent stream back to the Claus reaction furnace. U.S. Pat. No. 6,508,998 describes a process for moderating the high temperatures in a Claus reaction furnace induced by oxygen-enrichment by passing reaction gases through a sulfur condenser and then using an eductor to recycle the resulting effluent stream back to the Claus reaction furnace. 
     There remains a definite need for a simple and effective system and process for recovering elemental sulfur from H 2 S containing gas streams that minimizes the amount of air required in a Claus reaction furnace. There remains a further definite need for such a system and process that allows the processing of more acid gas through the system. There remains a still further definite need for such a system and process that moderates the temperature in the Claus reaction furnace. The present invention satisfies these and other needs, and provides further related advantages. 
     SUMMARY OF THE INVENTION 
     Now in accordance with the invention there has been found a simple and effective system and process employing steam to minimize the amount of air required in a Claus reaction furnace, to maximize the feed rate of the acid gas stream, and to moderate the temperature in the furnace. The method includes partially combusting a feed gas stream rich in hydrogen sulfide with an oxygen-enriched gas in a Claus reaction furnace to produce to a combustion reaction product stream containing sulfur. In some embodiments, one or more feed gas streams rich in hydrogen sulfide, air, and supplemental oxygen are introduced into the Claus reaction furnace to form a mixture including the feed gas stream rich in hydrogen sulfide which is then partially combusted. The resulting combustion reaction product stream is split into a recycle stream and a treatment stream. The recycle stream is then directed back into the Claus reaction furnace, without first condensing sulfur out of the stream; while the treatment stream is directed into a condenser to condense out sulfur. 
     In some embodiments, the recycle stream is passed through a pressure booster, such as a blower or an eductor before the stream enters the Clause reaction furnace. And in some embodiments the pressure of the recycle stream as it leaves the pressure booster is from about 20 to about 30 psia, preferably from 23 to about 27 psia. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a flow diagram of a Claus reactor system in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Particular embodiments of the invention are described below in considerable detail for the purpose of illustrating its principles and operation. However, various modifications may be made, and the scope of the invention is not limited to the exemplary embodiments described below. For example, in the embodiments described below, there is described a reaction furnace that includes both a burner and a reaction chamber and the reactants are introduced into the burner. However, one skilled in the art will understand that the reactants can be introduced directly into a reaction chamber. Similarly, in the embodiments described below, only one acid gas feed stream is shown. One skilled in the art will understand that the acid gas can be supplied in one or more streams. 
     Shown in  FIG. 1  is a Claus reactor system in accordance with the invention. An acid gas feed stream rich in H 2 S is introduced from an acid gas source, such as the acid gas produced by a petroleum refining plant (not shown), through at least one acid gas feed line  10  into a Claus reaction furnace  21 . In the embodiment shown in  FIG. 1 , the Claus reaction furnace includes a burner  20  and a reaction chamber  22 , such that the reactants are partially combusted in burner  20  and evolved into the reaction furnace  22 . 
     Typically, the H 2 S content of such feed streams is from about 50 to about 95 mol %. The feed stream is introduced into the reaction furnace  21  at a temperature generally of from about 350° to about 650° F., preferably from about 400° to about 600° F., and more preferably from about 450° to about 550° F., and at a pressure generally of from about 20 to about 30 psia, preferably from about 22 to about 28 psia, and more preferably from about 23 to about 27 psia. An oxygen stream, such as a stream of commercially pure oxygen or oxygen-enriched air, is provided to the reaction furnace  21  through an oxygen supply line  12 . In the embodiment shown in  FIG. 1 , a stream of commercially pure oxygen is provided by the oxygen supply line, while an air stream is separately provided through air supply line  14  at an elevated pressure, typically of from about 20 to about 30 psia, preferably from about 22 to about 28 psia, and more preferably from about 23 to about 27 psia, from compressor  16 . 
     The concentration of oxygen contained in the sum of oxygen stream  12  and air stream  14  entering the Claus reaction furnace  21  is typically from about 25 mol % to about 100 mol %, preferably from about 35 mol % to about 95 mol %. It is a significant advantage in accordance with the invention that the oxygen concentration in the Claus reaction furnace can be increased above the levels of those employed in conventional Claus recovery systems. In turn, the capacity of the Claus plant is increased due to the increased oxygen concentration, which backs out air flow. 
     The resulting reactant mixture is combusted in burner  20  and evolved into reaction chamber  22 , of the reaction furnace  21 , where the reactions of the oxygen-enhanced Claus process occur. The operating temperature in the reaction chamber is preferable at a temperature greater than about 2400° F., and more preferably greater than about 2600° F. 
     As illustrated in the combustion reaction (Equation 2), oxygen contained in the air and/or oxygen streams reacts with some of the H 2 S in the acid gas feed stream to produce SO 2  and H 2 O. It can be see from the stoichiometry of the Claus reaction (Equation 1), that the combustion reaction consumes about one-third of the H 2 S initially contained in the acid gas feed stream. The SO 2  produced in accordance with the combustion reaction then reacts with some of the remaining H 2 S in accordance with Equation 1 to produce S 2  and more H 2 O. 
     The resulting combustion reaction product stream is then passed through a circuitous heat exchange zone or waste heat boiler  24  wherein the effluents are cooled against boiler feed water in line  26 , which then produces steam in line  28 . Typically, the temperature of the cooled effluents are from about 450° F. to about 750° F., preferably from about 550° F. to about 650° F. In the waste heat boiler  24 , the sulfur is converted from one form of sulfur species to other forms according to the following equations:
 
3S 2 →S 6   (Equation 3)
 
4S 2 →S 8   (Equation 4)
 
     The cooled stream is carried out of the waste heat boiler in a line  29  at a pressure of from about 20 to about 26 psia, preferably from about 22 to about 24 psia. A portion of the combustion reaction product stream is split into line  44  as a recycle stream taken immediately downstream from the waste heat boiler  24 . This portion is recycled, in some embodiments, after passing through a pressure booster, such a mechanical blower or, as shown in  FIG. 1 , as a suction fluid through eductor  46 . A motive fluid selected from high pressure steam, air, nitrogen, carbon dioxide, sulfur or other compatible gas, powers the eductor. In the embodiment shown in  FIG. 1 , the recycle stream is fed in line  18  into the acid gas feed line  10  in order to moderate the temperature in burner  20 . Alternatively, the recycle stream can be introduced into burner  20 , oxygen supply line  12  or air supply line  14 . The pressure of the recycle stream as it leaves line  18  is preferably from about 20 to about 30 psia, and more preferably from about 23 to about 27 psia. Typically, the recycle stream is from about 10 to about 50 mol % of the combustion reaction stream, preferably from about 15 to abut 40 mol % The remaining portion of the combustion reaction product stream, a treatment stream, is directed through line  30  and introduced into a first liquid sulfur condenser  32 . The treatment stream is again heat exchanged to further cool the effluents against boiler feed water in line  34 , which produces steam in line  36 . The resulting liquid sulfur is condensed out in line  38 . The elemental sulfur remaining in the treatment stream constitutes from about 40% to about 75% of the sulfur in the original acid gas feed. 
     Because the recycle stream is removed, before the treatment stream enters the first liquid sulfur condenser  32 , the first condenser is removed from the recycle loop. Thus, it is an advantage of the inventive process that the thermal and hydraulic load on the first condenser is reduced. 
     Additionally, since the pressure drop from the first condenser is not included in the recycle loop, less differential pressure is required from the pressure booster. 
     The treatment stream is removed from the first liquid sulfur condenser in line  42  at a temperature of from about 330° F. to about 390° F., preferably from about 350° F. to about 370° F. and at a pressure generally of from about 19 to about 25 psia, preferably from about 21 to about 23 psia. 
     Those effluents that still remain in the treatment stream are carried through line  42  to be reheated in a first reheater heat exchanger  48  with process steam. The treatment stream, now in line  50 , has a temperature of from about 400° F. to about 500° F., preferably from about 440° F. to about 460° F., and is then introduced into a first catalytic converter reactor  52  wherein residual H 2 S and SO 2  are reacted to produce sulfur species and water according to the following equations:
 
12H 2 S+6SO 2 →3S 6 +6H 2 O  (Equation 5)
 
16H 2 S+8SO 2 →3S 8 +16H 2 O  (Equation 6)
 
     The thus reacted treatment stream, now in line  54 , is introduced into a second sulfur condenser  56 , which again cools the effluents with boiler feed water in line  58  to produce additional steam in line  60 . This additional elemental sulfur is recovered in line  62 . The amount of elemental sulfur remaining in the treatment stream constitutes from about 18% to about 50% of the sulfur in the original acid gas feed. 
     The further condensed treatment stream is carried from the second condenser through line  64  at a temperature of from about 310° F. to about 370° F., preferably from about 330° F. to about 350° F., and at a pressure generally of from about 18 to about 24 psia, preferably from about 20 to about 22 psia, into a second reheater, heat exchanger  66 , where the treatment stream is heated with high pressure steam to a temperature of from about 400° F. to about 460° F., preferably from about 420° F. to about 440° F. The thus reheated stream is then carried through line  68  and introduced into a second catalytic converter reactor  70 , wherein the catalytic reaction between hydrogen sulfide and SO 2  represented in Equations 5 and 6, again occur. The thus reacted treatment stream, now in line  72 , goes to a third sulfur condenser  74  which is cooled with boiler feed water  76  to produce steam in line  78 . The resulting liquid sulfur is removed in line  80 . The amount of elemental sulfur in the treatment stream constitutes from about 5% to about 15% of the sulfur in the original acid gas feed. Some units only include two stages and are complete at this point. Most have the additional processing steps, as follows below. 
     As is understood in the art, in some SRU&#39;s the sulfur recovery is substantially compete after two stages, i.e., once the treatment stream passes through the second catalytic converter  70  and then the third sulfur condenser  74 . In the embodiment shown in  FIG. 1 , the treatment stream, now in line  82 , at a temperature of from about 300° F. to about 350° F., preferably from about 315° F. to about 335° F. and at a pressure of from about 17 to about 22 psia, preferably from about 18 to about 21 psia, is again reheated in a third reheater heat exchanger  84 . The treatment stream is heated with process steam to a temperature of from about 370° F. to about 420° F., preferably from about 390° F. to about 410° F. The thus reheated stream is then carried through line  86  and introduced into a third catalytic converter reactor  88 . In the third catalytic converter reactor, substantially all or most of the remaining H 2 S and SO 2  are reacted c to produce sulfur species as represented in equations 5 and 6, which are then removed in line  90 . The treatment stream is introduced into a fourth condenser  92  cooled by boiler feed water in line  94  producing steam in line  96 . Further elemental sulfur in liquid form is removed in line  98  constituting from about 1% to about 6% of the sulfur in the original acid gas feed. 
     The treatment stream now in line  100  is at a temperature of from about 255° F. to about 330° F., preferably from about 265° F. to about 320° F. and at a pressure of from about 15 to about 20 psia, preferably from about 17 to about 19 psia. 
     The resulting treatment stream comprises predominantly steam, nitrogen, carbon dioxide, and hydrogen, as well as residual H 2 S and other sulfur compounds. The stream is carried in line  100  into a tail gas coalescer  102  wherein additional residual liquid sulfur compounds are removed in line  104 . The residual stream now in line  106  is then introduced into a tail gas cleanup unit  116 , where the bulk of the residual sulfur compounds are recovered to meet sulfur emission environmental standards typically by conversion to H 2 S. The H 2 S, is recovered and returned to the acid gas feed line  10 , while the effluent is sent to an incinerator burner  112 . 
     Alternately, the tail gas in line  106  is sent to an incinerator burner  112  that is fired with natural gas in line  108  and air in line  110 . The materials are then vented in stack  114 , at an acceptable sulfur content level, as an effluent to the atmosphere. 
     The present invention has been described with regard to preferred embodiments, but those skilled in the art will be capable of contemplating other variants, which are deemed to be within the scope of the invention, which scope should be ascertained from the claims, which follow.