Abstract:
A process for removing sulfur from a gas stream is provided in which a plurality of reactor units, each comprising a condenser and reactor, are selectively operable under Claus reaction and cold bed adsorption conditions. The arrangement of reactor units within the plant is periodically changed following a front-middle-back sequencing scheme. This ensures that the final reactor unit in the series utilizes fully cooled catalyst which is most efficient for operation under cold bed adsorption conditions. In addition, the condenser of the final reactor unit in the series operates at or below the freezing point of sulfur thereby permitting even greater sulfur recovery.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    A sulfur recovery plant is provided comprising a primary Claus reactor and a plurality of downstream reactor units, each reactor unit comprises a reactor that is selectively operable under Claus reaction conditions and cold bed adsorption (CBA) reaction conditions thereby permitting the plant to achieve greater than 99.5% average sulfur removal efficiency. The high sulfur removal efficiencies are obtained through sequencing the reactor units such that the reactor unit containing the coolest catalyst is positioned in the final spot in the series of reactor units. The order of reactor units within the sequence is periodically changed so as to permit catalyst regeneration. However, the reactor unit containing the newly regenerated catalyst is shifted to a middle position, as opposed to the final position in the sequence of reactor units. This manner of shifting the order of reactor units within the plant provides additional cooling time for the catalyst that has been most recently regenerated thereby ensuring that the final reactor unit in the series is capable of highly efficient cold bed adsorption operation. 
         [0003]    2. Description of the Prior Art 
         [0004]    The processing of natural gas or petroleum products often results in the generation of acid gas streams comprising oftentimes significant quantities of sulfur, generally in the form of H 2 S. These acid gas streams are often of limited value and are commonly disposed of by incineration. However, environmental regulations restrict the amount of sulfur that can be released into the atmosphere. Therefore, a significant portion of the sulfur present in these waste product streams must be removed prior to incineration. 
         [0005]    One approach to the removal of sulfur has been through the use of an extended Claus sulfur recovery plant, such as that disclosed in U.S. Pat. Nos. 5,015,459 and 5,015,460. In these plants, one catalytic reactor is operated under high temperature Claus conditions in series with one or more catalytic reactors each periodically operated under high temperature Claus and cold bed adsorption (CBA) conditions. Each catalytic reactor that alternates between Claus conditions and CBA conditions is associated with a sulfur condenser to comprise a reactor unit. The sequencing of the reactor units is periodically changed so that the reactor having freshly-regenerated catalyst is placed in the last position in the sequence. 
         [0006]    The CBA plants have demonstrated average sulfur removal efficiencies of up to 99.2%. However, as environmental regulations become even more strict, CBA plants such as those described in the aforementioned patents have not thus far been able to achieve 99.5% or greater average sulfur reduction efficiencies. In order to achieve this level of efficiency, conventional CBA plants would need to be equipped with an additional tail gas treating unit, such as a hydrogenation/amine treating unit, thereby adding further capital and operating expense. Thus, it would be highly desirable if a CBA plant could be configured to achieve 99.5% or greater average sulfur removal efficiency without the need for additional tail gas treatment. 
       SUMMARY OF THE INVENTION 
       [0007]    One embodiment according to the present invention comprises a process for recovering sulfur from a gas stream. A process gas comprising H 2 S and SO 2  is passed through a primary Claus reactor operable to convert at least a portion of the H 2 S and SO 2  present in the process gas into elemental sulfur. Next, the gas exiting the primary Claus reactor is passed sequentially through at least first, second, and third reactor units, each reactor unit comprising a catalytic reactor and a sulfur condenser. The catalytic reactors are capable of selective operation under both Claus reaction conditions and cold bed adsorption conditions. After a First period of operation of the reactor units, the sequence of the reactor units is rearranged such that the gas exiting the primary Claus reactor first passes through the third reactor unit, followed by the first and second reactor units. 
         [0008]    Another embodiment according to the present invention comprises a process for recovering sulfur from a gas stream. A process gas comprising H 2 S and SO 2  is passed through a primary Claus reactor operable to convert at least a portion of the H 2 S and SO 2  present in the process gas into elemental sulfur. Next, the gas exiting the primary Claus reactor is passed sequentially through a series of reactor units, each reactor unit comprising a catalytic reactor and a sulfur condenser. The condenser of the final reactor unit in the series is operated so that the gas exiting the condenser is at or below the freezing point of sulfur. 
         [0009]    Still another embodiment according to the present invention comprises a process for recovering sulfur from a gas stream. A process gas comprising H 2 S and SO 2  is passed through a primary Claus reactor operable to convert at least a portion of the H 2 S and SO 2  present in the process gas into elemental sulfur. Next, the gas exiting the primary Claus reactor is passed sequentially through a series of reactor units, each reactor unit comprising a catalytic reactor and a sulfur condenser. The catalytic reactor of the final reactor unit in the series has an inlet temperature that is within 10° F. of the freezing point of sulfur. 
         [0010]    A further embodiment according to the present invention comprises a sulfur recovery unit. The sulfur recovery unit includes a primary Claus reactor and a series of reactor units located downstream from the primary Claus reactor. The primary Claus reactor is configured to receive a process gas comprising H 2 S and SO 2  and convert at least a portion of the H 2 S and SO 2  into elemental sulfur. Each of the downstream reactor units comprises a sulfur condenser and a catalytic reactor. The catalytic reactor of the final reactor unit in the series operates at the lowest average temperature of all of the catalytic reactors in the series. 
         [0011]    Still a further embodiment according to the present invention comprises a sulfur recovery unit. The sulfur recovery unit includes a primary Claus reactor and a series of reactor units located downstream from the primary Claus reactor. The primary Claus reactor is configured to receive a process gas comprising H 2 S and SO 2  and convert at least a portion of the H 2 S and SO 2  into elemental sulfur. Each of the downstream reactor units comprises a sulfur condenser and a catalytic reactor. The condenser of the final reactor unit in the series operates at a temperature that is at or below the freezing point of sulfur. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a process flow diagram of a sulfur removal plant in which process gas exiting a primary Claus reactor is directed through a series of cold bed adsorption reactor units, the process gas first flowing through a condenser of the first unit prior to being directed through the reactor of the first unit; 
           [0013]      FIG. 2  is a process flow diagram of a sulfur removal plant as depicted in  FIG. 1 , except that the process gas exiting the primary Claus reactor bypasses the condenser of the first unit and is directed immediately to the reactor of the first unit; 
           [0014]      FIG. 3  is a process flow diagram of a sulfur removal plant in which process gas exiting the primary Claus reactor is initially directed toward a third CBA reactor unit, and particularly through the condenser of the third unit prior to being directed through the reactor of the third unit, and wherein the second reactor unit from  FIGS. 1 and 2  is now in the final position in the series of reactor units; 
           [0015]      FIG. 4  is a process flow diagram of a sulfur removal plant as depicted in  FIG. 2 , except that the process gas exiting the primary Claus reactor bypasses the condenser of the third unit and is directed immediately to the reactor of the third unit; 
           [0016]      FIG. 5  is a process flow diagram of a sulfur removal plant in which process gas exiting the primary Claus reactor is initially directed toward a second CBA reactor unit, and particularly through the condenser of the second unit prior to being directed into the reactor of the second unit, and wherein the first reactor unit from  FIGS. 1 and 2  is now in the final position in the series of reactor units; and 
           [0017]      FIG. 6  is a process flow diagram of a sulfur removal plant as depicted in  FIG. 5 , except that the process gas exiting the primary Claus reactor bypasses the condenser of the second unit and is directed immediately into the reactor of the second unit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Turning to the Figures, an exemplary sulfur removal plant  10  is represented schematically. It is noted that each figure contains both solid and dashed process lines. The solid lines indicate conduit through which process gas is flowing and the dashed lines indicating conduit that is presently closed off by valves. Plant  10  comprises a primary Claus reactor  12  configured to receive a process gas stream via conduit  14 . The process gas stream may comprise the products of a thermal reaction step in which oxygen is introduced into byproducts from natural gas or petroleum processing, such as an acid gas stream, and combusted in, for example, a thermal reactor. The products of this thermal reaction step carried by conduit  14  comprise a sulfur compound, such as H 2 S, and one or more other components, such as CO 2  and water. Reactor  12  contains a Claus catalyst such as activated Al 2 O 3  or TiO 2  that catalytically converts H 2 S and SO 2  (produced by combustion of H 2 S within the reactor) into elemental sulfur. However, conversion of H 2 S to elemental sulfur in reactor  12  is often not as complete as many environmental regulations require. Therefore, additional reaction must be carried out. 
         [0019]    The reacted process gas stream exits reactor  12  through conduit  16  and is directed toward a plurality of reactor units, depicted in  FIG. 1  as units  18 ,  20 , and  22 , for further H 2 S conversion. As discussed in greater detail below, the sequencing of the reactor units is variable so as to optimize average sulfur removal efficiency to a level of at least 99.5% for the cycle. In particular, a “front-middle-back” sequence for rotation of reactor units  18 ,  20 ,  22  is employed. Each reactor unit comprises a sulfur condenser  24 ,  28 ,  32 , and a reactor  26 ,  30 ,  34 , respectively. The reactor units themselves are of similar function, as each will be cycled through the various operational positions within plant  10 . 
         [0020]    In certain embodiments, condensers  24 ,  28 ,  32  are located upstream from respective reactors  26 ,  30 ,  34  and comprise tube/shell heat exchangers employing, for example, water on the shell side as the cooling fluid for condensing the sulfur. The liquified sulfur is then recovered from the condensers. Reactors  26 ,  30 ,  34  are catalytic reactors containing similar Claus catalyst as primary reactor  12 . But unlike reactor  12 , reactors  26 ,  30 ,  34  selectively operate under both Claus reaction conditions and cold bed adsorption conditions. Under Claus reaction conditions, sulfur formed in the presence of the Claus catalyst is continuously removed from the reactor in the vapor phase due to the relatively high temperatures associated with the reaction. Under cold bed adsorption conditions, the sulfur formed is deposited and accumulated on the Claus catalyst, which must be regenerated from time to time. However, under both sets of conditions, the reactors catalyze the Claus reaction in which H 2 S and SO 2  are converted to elemental sulfur. 
         [0021]    As depicted in  FIG. 1 , the stream carried by conduit  16  is initially directed toward reactor unit  18 . Specifically, the process gas in conduit  16  is directed through a control valve  36  and into condenser  24  via conduit  38 . Within condenser  24  at least a portion of the sulfur generated by primary Claus reactor  12  is condensed and recovered from plant  10 . The process gas exits condenser  24  at about 260° F. via conduit  40  and passes through three-way valve  42 . The process gas is then directed toward reactor  26  via conduit  44 . In reactor  26 , additional H 2 S is reacted to form elemental sulfur. The process gas exits reactor  26  via conduit  46  and is directed toward reactor unit  20 . 
         [0022]    Three-way valve  48  is positioned to direct the process gas into conduit  50  and eventually through sulfur condenser  28 . The process gas exits sulfur condenser  28  via conduit  52  at about 260° F. A three-way valve  54  directs the process gas toward reactor  30  via conduit  56 . Upon exiting reactor  30 , the process gas is directed toward reactor unit  22  via conduit  58 . Three-way valve  60  is positioned to direct the process gas into conduit  62  and eventually through sulfur condenser  32 . 
         [0023]    Sulfur condenser  32  is operated at the lowest temperature of each of the three condensers in the process scheme depicted in  FIG. 1 . In certain embodiments, the temperature of the process gas exiting condenser  32  is at or below the freezing point of sulfur, approximately 240° F. or less. The cooler temperatures in condenser  32  can be achieved by depressurizing the shell side of the condenser (in certain embodiments to about 9 to 10 psig) so as to lower the temperature of the steam exiting the condenser. When plant  10  is operated in this configuration, the steam pressure for condenser  32  is less than that of condensers  24  and  28 . In other embodiments, a liquid comprising a heat transfer fluid may be used in place of steam to provide the necessary cooling for the condensers used herein. The temperature and flow of the liquid may be adjusted to provide the desired operational temperature for the condenser and outlet temperature for the process gas. 
         [0024]    Operating a condenser at such low temperatures generally defies conventional wisdom regarding Claus plant operation, as this will result in the accumulation of solidified sulfur in the tubes of condenser  32 , especially in the tubes adjacent the condenser outlet. However, the condensers utilized in this process are generally designed to accommodate high cooling duty demands. Therefore, when a reactor unit is located in the third position, as is reactor unit  22  in the configuration of  FIG. 1 , its condenser possesses sufficient surface area to make up for loss of operating efficiency resulting from the solidification of sulfur within its tubes. Thus, accumulation of sulfur in the tubes for the period in which reactor unit  22  operates in the final position in the sequence of reactor units of plant  10  will not meaningfully affect the overall performance of the condenser. 
         [0025]    The process gas stream exits condenser  32  via conduit  64  and is directed through three-way valve  66  and toward reactor  34  via conduit  68 . The much lower temperature of the process stream exiting condenser  64  also permits reactor  34  to operate at a much cooler temperature than reactors  28  and  30 . This lower operational temperature provides more efficient H 2 S conversion under cold bed adsorption conditions. In certain embodiments, reactor  34 , the final reactor in the series, has an inlet temperature that is within 10° F. of the freezing point of sulfur (e.g., between about 230° F. to about 250° F.), or even within 5° F. of the freezing point of sulfur (e.g., between about 235° F. to about 245° F.). The process gas exiting reactor  34  through conduit  70  can be directed by three-way valve  72  to conduit  74 , which feeds, for example, an incinerator or a tail gas treatment unit. Although, it is an advantage with certain embodiments of the present invention that further tail gas treatment can be avoided because the increased efficiency of plant  10  results in a sufficiently low sulfur content gas stream which would permit incineration without a need for further sulfur removal. 
         [0026]    After operation of plant  10  in the configuration of  FIG. 1  for a predetermined period of time, in some embodiments approximately 3 hours, the flow path of process gas through plant  10  is slightly altered so as to liberate sulfur that has been deposited on the catalyst present in reactor  26 . Turning to  FIG. 2 , it can be seen that the position of valve  42  has been altered so that the process gas from conduit  16  now bypasses condenser  24  via conduit  76  and flows through valve  42  and directly into reactor  26 . The increased temperature of the process gas being introduced into reactor  26  vaporizes sulfur which may have accumulated on the catalyst inside reactor  26  thereby regenerating the catalyst. The remainder of plant  10  operates as described above for  FIG. 1 . In certain embodiments, plant  10  is operated in this configuration for approximately 9 hours, at which time the flow path of process gas through plant  10  is again altered. 
         [0027]    In preparation for a re-sequencing of reactor units, the flow path of process gas through plant  10  is changed from the configuration of  FIG. 2  back to the configuration of  FIG. 1 . Essentially, this involves bringing condenser  24  back on-line so as to cool the process gas being directed toward reactor  26 . Thus, the catalyst within reactor  26  is pre-cooled in advance of the re-sequencing of reactor units. The remainder of plant  10  continues to operate as previously described for  FIG. 1 . In certain embodiments, plant  10  is operated in this configuration for approximately 3 hours. 
         [0028]    Next, the sequencing of reactor units is changed so as to permit regeneration of catalyst in reactor  30  of reactor unit  20 . Conventionally, reactor unit  18 , having the “freshest” catalyst (i.e., having most recently undergone regeneration) would be slotted in the final position of the sequence of reactor units. However, it has been discovered that the sulfur removal efficiency of plant  10  can be improved if reactor unit  18  is not moved to the final position in the sequence of reactor units, but rather a middle position, namely the second position as shown in  FIG. 3 . Because reactor  26  has not be operated (for the second time) in the configuration of  FIG. 1  for long, the catalyst contained within reactor  26  is much warmer than the catalyst contained, for instance, in reactor  30 . Thus, the catalyst in reactor  26  will not perform as effectively as the catalyst in reactor  30  under cold bed adsorption conditions. Therefore, even though the catalyst contained within reactor  30  may contain more adsorbed sulfur, it has been discovered that its lower temperature renders it more effective under cold bed adsorption conditions than the catalyst in reactor  26 . 
         [0029]    In the plant configuration depicted in  FIG. 3 , valve  36  is closed thus diverting the flow of process gas from conduit  16  into conduit  76 . A valve  78  in reactor unit  20  also remains closed so that the process gas continues to flow toward reactor unit  22 , while a valve  80  has been opened. Thus, reactor unit  22  has assumed the first position in the sequencing of reactor units downstream of primary Claus reactor  12 . Process gas is directed through valve  80  and flows through sulfur condenser  32 . Sulfur condenser  32  is no longer operated to produce an outlet temperature at or below the freezing point of sulfur. The shell side steam pressure is now increased to 15 psig, for example, thereby increasing the outlet temperature of condenser  32 . Also, condenser  32  is receiving the hot process gas directly from primary Claus reactor  12 . Therefore, any sulfur that may have solidified in the tubes of condenser  32  from the immediately preceding operating configurations is at least melted. In certain embodiments, the process gas exiting condenser  32  via conduit  64  has a temperature of about 260° F. The process gas passes through three-way valve  66  and into conduit  68  where it is directed to reactor  34 . Three-way valve  72  has been repositioned so that the process gas exiting reactor  34  and carried by conduit  70  is diverted to conduit  82  and directed toward reactor unit  18 , which has been moved to the second position in the sequence of reactor units. 
         [0030]    The process gas carried by conduit  82  is transferred to conduit  38  (due to the position of valve  42 ) and passed through condenser  24 . In certain embodiments, the process gas exiting condenser  24  has a temperature of about 260° F. The process gas is then directed through reactor  26  and toward reactor unit  20  as previously described above. However, because reactor unit  20  is now in the final position in the sequence of reactor units, condenser  28  is operated at or below the freezing point of sulfur, much like condenser  32  was operated when in the configuration shown in  FIG. 1 . Likewise, in certain embodiments, the inlet to reactor  30  is within 10° F. of the freezing point of sulfur. Upon exiting reactor  30 , the process gas is carried by conduit  58  to three-way valve  60 . Valve  60  is positioned so as to divert the process gas into conduit  84  and then to conduit  74  to be finally disposed of. 
         [0031]    After a period of operation in the configuration shown in  FIG. 3 , approximately three hours in certain embodiments, the gas flow path is slightly altered so that condenser  32  is bypassed and the process gas flows directly toward reactor  34 . This configuration is depicted in  FIG. 4 . Thus, reactor  34  is permitted to operate under higher-temperature Claus conditions while reactors  26  and  30  operate under cold bed adsorption conditions. Further, during this period of operation, the catalyst within reactor  34  is regenerated so as to vaporize sulfur that has accumulated thereon. In certain embodiments, this particular plant configuration is operated for a period of approximately 9 hours. 
         [0032]    Subsequent to operating plant  10  according to the configuration shown in  FIG. 4 , operation of plant  10  is reverted to the configuration shown in  FIG. 3 . Namely, condenser  32  is brought back on-line so that process gas from primary Claus reactor  12  now flows through condenser  32  en route to reactor  34 . This transition begins cooling of the catalyst in reactor  34  in preparation for the next re-sequencing of reactor units. In certain embodiments, the process gas exiting condenser  32  has a temperature of approximately 260° F., and plant  10  is operated in this configuration for approximately 3 hours. 
         [0033]    As shown in  FIG. 5 , the sequencing of the reactor units is changed yet again. Reactor unit  20  assumes the first position, followed by reactor units  22  and  18 . In this configuration, the catalyst in reactor  26  is the coolest of all of the three reactor unit reactors and thus capable of most efficient operation under cold bed adsorption conditions. Valve  36  and  80  are now closed, while valve  78  has been opened so that process gas exiting primary Claus reactor  12  via conduit  16  is directed toward reactor unit  20 . Three-way valve  54  is positioned to that the process gas flows through condenser  28 . Process gas exits condenser  28  and is carried via conduit  52 , through three-way valve  54  and toward reactor  30  via conduit  56 . In certain embodiments, the process gas exiting condenser  52  has a temperature of approximately 260° F. 
         [0034]    The process gas carried by conduit  58  is then directed toward reactor unit  22  through three-way valve  60 . The process gas flows through condenser  32 , exiting the condenser at approximately 260° F. in certain embodiments, and then onto reactor  34 . The process gas carried by conduit  70  is then directed toward reactor unit  18  by three-way valve  72 . The process gas flows through condenser  24  which is operated at or below the freezing point of sulfur. In certain embodiments, the process gas exiting condenser  24  via conduit  40  has a temperature of about 240° F. or less, thereby leading to a reactor  26  inlet temperature that is within 10° F. of the freezing point of sulfur. The process gas is then directed toward reactor  26  which operates under cold bed adsorption conditions. The process gas exits reactor  26  via conduit  46  and is directed toward conduit  74  by valve  48  and ultimately to the incinerator. In certain embodiments, plant  10  operates in this configuration for approximately 3 hours. 
         [0035]    Next, the configuration of plant  10  is slightly altered so that reactor  30  is operated under Claus conditions and the catalyst contained therein is regenerated. As shown in  FIG. 6 , the position of valve  54  is changed so as to cause the process gas from conduit  76  to bypass condenser  28 . In this manner, process gas from primary Claus reactor  12  is directly fed to reactor  30  without having undergone sulfur condensation. The remainder of the process remains essentially the same as depicted in  FIG. 5  and described previously. In certain embodiments, plant  10  operates in this configuration for approximately 9 hours. 
         [0036]    Following a period of operation of plant  10  in the configuration shown in  FIG. 6 , the configuration of plant  10  is reverted to that shown in  FIG. 5  in preparation of a change in reactor unit sequencing. In certain embodiments, this period of operation in the configuration shown in  FIG. 5  is for approximately 3 hours. 
         [0037]    At this stage, reactor units  18 ,  20 , and  22  have completed a full cycle with respect to their sequencing order within plant  10 . Plant  10  continues to operate in this fashion with periodic re-sequencing of reactor units. As discussed above, during a change in reactor unit sequencing, the reactor unit immediately downstream from primary Claus reactor  12  is shifted to the second position. The reactor unit furthest downstream from primary Claus reactor  12  is shifted into the first position to be immediately downstream from reactor  12 . By following this sequencing of reactor units, it is ensured that the reactor having the coolest catalyst is last in the sequence and is capable of operating most efficiently under cold bed adsorption conditions.