Abstract:
A method is provided for separating hydrogen sulfide and carbon dioxide from an offgas stream from a direct reduced iron furnace. The offgas is directed to a separation device which will separate the carbon dioxide and hydrogen sulfide using a hydrogen sulfide absorber and a carbon dioxide absorber. The hydrogen sulfide can be recycled for reuse in the furnace and the carbon dioxide recovered for other additional uses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional application 61/968,424 filed Mar. 21, 2014. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention provides for gas separation methods for use in iron reduction furnaces. More particularly, the invention provides for methods for separating carbon dioxide from hydrogen sulfide in the offgas from a Direct Reduced Iron (DRI) furnace. 
         [0003]    The Direct Reduced Iron technique produces iron in a manner different from conventional blast furnaces. Direct Reduced Iron is produced from the direct reduction of iron ore in the form of lumps, pellets or fines by a reducing gas that is produced from natural gas or coal. The reducing gas is a mixture of hydrogen and carbon monoxide which acts as a reducing agent. These reducing gases will directly reduce the iron ore in solid form. 
         [0004]    In current Direct Reduced Iron furnace technology, a process gas heater is used in the design of the furnace. A natural gas feed is directed to a partial oxidation furnace to produce the reducing gas mixture of hydrogen and carbon monoxide. A steady stream of hydrogen sulfide is fed to the natural gas stream and will upon entering the process gas heater assist in minimizing the formation of carbon metal dust. 
         [0005]    Carbon dioxide which is also a byproduct of the partial oxidation reactor and the iron reducing process is recovered from the DRI furnace and is vented to the atmosphere. However, the offgas from the DRI furnace will also contain the hydrogen sulfide used to inhibit carbon metal dust formation. The presence of the hydrogen sulfide in the gas limits the amount that can be vented due to concerns over exceeding environmental sulfur limitations. 
         [0006]    So in order for the operator to use the carbon dioxide by either venting or selling as a byproduct, the hydrogen sulfide present must be removed. This invention addresses this need for removal by separating the hydrogen sulfide from the carbon dioxide in the offgas from the DRI furnace in a separation device which will create a carbon dioxide stream free of hydrogen sulfide as well as a hydrogen sulfide stream for use in the DRI furnace process. 
       SUMMARY OF THE INVENTION 
       [0007]    In one embodiment of the invention there is disclosed a method for operating a direct reduced iron furnace wherein synthesis gas and hydrogen sulfide are fed to the direct reduced iron furnace, the improvement comprising separating carbon dioxide and hydrogen sulfide from offgas from the direct reduced iron furnace. 
         [0008]    The carbon dioxide and the hydrogen sulfide are recovered. The recovered hydrogen sulfide is fed along with natural gas into a process gas heater. 
         [0009]    The offgas is fed to a carbon dioxide separation device and a hydrogen sulfide separation device. The carbon dioxide and hydrogen sulfide are separated from the offgas by, a carbon dioxide absorber in fluid communication with a hydrogen sulfide absorber. 
         [0010]    The hydrogen sulfide absorber contains an absorbent material comprising an amine compound. Likewise the carbon dioxide absorber contains an absorbent material comprising an amine compound. 
         [0011]    A hydrogen sulfide stripper may additionally be in fluid communication with the hydrogen sulfide absorber. 
         [0012]    In a second embodiment of the invention, there is disclosed a method for separating hydrogen sulfide and carbon dioxide from offgas from a direct reduced iron furnace comprising feeding the offgas to a separation device. 
         [0013]    The separation device comprises a carbon dioxide separation device and a hydrogen sulfide separation device. The carbon dioxide separation device is a carbon dioxide absorber and the hydrogen sulfide separation device is a hydrogen sulfide absorber. 
         [0014]    The carbon dioxide and the hydrogen sulfide are recovered and the recovered hydrogen sulfide is fed along with natural gas into a process gas heater. 
         [0015]    The carbon dioxide separation device and the hydrogen sulfide separation device are in fluid communication with each other. The hydrogen sulfide absorber contains an absorbent material comprising an amine compound. Likewise the carbon dioxide absorber contains an absorbent material comprising an amine compound. 
         [0016]    A hydrogen sulfide stripper may additionally be in fluid communication with the hydrogen sulfide absorber. 
         [0017]    In another embodiment of the invention, there is disclosed a method for recycling hydrogen sulfide from a direct reduced iron furnace comprising the steps of a) recovering offgas comprising carbon dioxide and hydrogen sulfide from the direct reduced iron furnace; b) separating the carbon dioxide from the hydrogen sulfide; and c) feeding the hydrogen sulfide to the direct reduced iron furnace. 
         [0018]    The offgas is fed to a carbon dioxide separation device and a hydrogen sulfide separation device. The carbon dioxide may be a carbon dioxide absorber which contains an absorbent material comprising an amine compound. The hydrogen sulfide separation device may be a hydrogen sulfide absorber which contains an absorbent material comprising an amine compound. 
         [0019]    A hydrogen sulfide stripper may additionally be in fluid communication with the hydrogen sulfide absorber. 
         [0020]    The offgas from the DRI furnace typically contains between 60 to 80% syngas, 10 to 15% carbon dioxide, less than 10% methane and 200 to 10,000 parts per million (ppm) of hydrogen sulfide. This offgas will typically be at an elevated pressure on the order of 100 to 200 psia. 
         [0021]    In this invention, the gas separation device can comprise a carbon dioxide absorber in fluid communication with a hydrogen sulfide absorber which is in fluid communication with a hydrogen sulfide stripper. 
         [0022]    Alternatively the gas separation device can comprise an amine absorber which will absorb carbon dioxide from the syngas and methane gas mixture and directs the carbon dioxide through a series of unit operations to produce a carbon dioxide product for reuse, venting or for resale. The hydrogen sulfide in the offgas can be separated by feeding the offgas to a hydrogen sulfide absorber which is in fluid communication with a hydrogen sulfide stripper which will separate out the hydrogen sulfide and return it to the DRI furnace system where it can be injected into the natural gas upstream of the process gas heater. In this embodiment, the carbon dioxide separation and hydrogen sulfide separation are distinct processes. 
         [0023]    The benefits of the inventive separation scheme include reduced hydrogen sulfide usage due to recycle from the DRI furnace; reduced sulfur emissions to the atmosphere and/or elimination of a sulfur recovery plant and improved carbon dioxide purity for added byproduct value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic of a Direct Reduced Iron furnace with a carbon dioxide and hydrogen sulfide separation device. 
           [0025]      FIG. 2  is a schematic of a carbon dioxide and hydrogen sulfide separation system. 
           [0026]      FIG. 3  is a schematic of a carbon dioxide separation system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring to  FIG. 1 , a Direct Reduced Iron furnace is shown with a carbon dioxide and hydrogen sulfide separation system. 
         [0028]    Natural gas  1  is mixed with recycled furnace offgas  7  and is sent to process gas heater  10 . Prior to entering the process gas heater  10 , a recycled hydrogen sulfide stream  8  is fed into the line directing the natural gas into the process gas heater. The hydrogen sulfide will inhibit the formation of metal dust which can be detrimental to the process gas heater operation. 
         [0029]    The heated stream  4  enters the DRI furnace  20  along with an injected oxygen stream (not shown). In the bottom section of the DRI, a partial oxidation reaction converts the natural gas into carbon monoxide and hydrogen via the following reaction. 
         [0000]      2CH 4 +O 2 →2CO+4H 2  
 
         [0030]    The hydrogen and carbon monoxide provide the reducing atmosphere to reduce the iron ore into iron. As seen in  FIG. 1 , iron ore  9  is added to the top of the Direct Reduced Iron furnace and Direct Reduced Iron  11  is recovered from the bottom of the furnace. 
         [0031]    The offgas  5  from the furnace is fed to the combined hydrogen sulfide and carbon dioxide separation system. In this system, carbon dioxide  6  is separated from the offgas containing hydrogen sulfide and can be recovered for reuse, venting or potential resale. The hydrogen sulfide meanwhile is recovered in stream  8  and can be fed into the feed line for the natural gas entering the fired heater. Additional fresh hydrogen sulfide may need to be added to this stream to account for system losses. 
         [0032]    In the current invention, the gas separation device  30  can comprise a carbon dioxide absorber in fluid communication with a hydrogen sulfide absorber which is in fluid communication with a hydrogen sulfide stripper. As shown in  FIG. 2 , the offgas stream  5  which contains carbon dioxide and hydrogen sulfide is first fed to a hydrogen sulfide absorber  40  to produce a hydrogen sulfide depleted stream  12 . The hydrogen sulfide in the offgas stream is absorbed in a circulating liquid which will contain an absorbent material from the family of amine compounds. The hydrogen sulfide enriched liquid  14  is diverted through a heat exchanger  60  and is heated before entering stripper column  70 . The liquid passes down the stripper column  70  and exits the bottom of the stripper in stream  19 . This stream enters reboiler  72  where it is heated to create a vapor stream  20  which is introduced into the bottom of the stripper column  70 . This gas passes up the column and strip hydrogen sulfide from the amine solution. The hydrogen sulfide rich vapor stream  16  exits stripper column  70  and consists mostly of hydrogen sulfide, water, and trace amounts of amine. Stream  16  enters condenser  71  to produce a condensed liquid stream  18  which is returned to stripper column  70 . The hydrogen sulfide stream  17  that is recovered can be fed to the natural gas feed line discussed above with respect to  FIG. 1  for entry into Direct Reduced Iron furnace system. 
         [0033]    The lean amine liquid stream  21  from reboiler  72  is sent to circulation pump  73  to produce pressurized liquid stream  22  which transmits heat to stream  14  in heat exchanger  60 , Stream  23  is cooled in heat exchanger  61  against an external cooling liquid such as cooling water to produce pressurized cooled liquid stream  24 . The cooled lean amine stream is sent to the top of carbon dioxide absorber  50 . 
         [0034]    The hydrogen sulfide depleted stream  12  leaving the hydrogen sulfide absorber is fed to a carbon dioxide absorber  50 . The carbon dioxide depleted offgas will leave the top of the carbon dioxide absorber in stream  13  and be recycled for use as a feed or a fuel in the process gas heater as necessary. 
         [0035]    Like the hydrogen sulfide absorber, the carbon dioxide absorber  50  contacts the offgas with liquid amine streams  24  and  35  that will absorb carbon dioxide. Typical carbon dioxide absorbent materials are from the family of amine compounds. The carbon dioxide enriched amine solution  25  is split into two parts. The first part  26  is sent to the hydrogen sulfide absorber  40  as described previously. The second part  27  is sent to heater  51  to produce heated CO 2  rich stream  28 . Stream  28  enters flash separator  62  which produces CO 2  rich gas  31  and amine stream  36  with reduced CO 2  content. Stream  36  is reduced in pressure across valve  66  and is sent to separator  63 . Separator  63  produces CO 2  stream  37 . This stream is mixed with stream  31  to produce CO 2  stream  32  for use in other unit operations in the plant or for resale outside of the plant. Separator  63  also produces lean amine stream  33 . Stream  33  is sent to pump  64  to produce pressurized lean amine stream  34 . Stream  34  is sent to cooler  65  where it is cooled against an external cooling liquid such as cooling water to produce cooled stream  35 . Stream  35  is fed to CO 2  absorber column  50 , 
         [0036]      FIG. 3  provides a second embodiment of this technology. The offgas stream  105  containing carbon dioxide and hydrogen sulfide is fed to a hydrogen sulfide absorber column  140  in which the offgas comes in contact with an amine solution. The amine solution absorbs the hydrogen sulfide and produces hydrogen sulfide depleted stream  106 . The hydrogen sulfide enriched amine solution  109  is diverted through heat exchanger  160  to produce heated stream  110 . Stream  110  is fed to hydrogen sulfide stripper  170  where it passes down the column and exits the bottom of the stripper to form stream  114 . Stream  114  enters reboiler  172  where it is heated to create a vapor stream  115  which is introduced into the bottom of stripper column  170 . The gas passes up the column and strips hydrogen sulfide from the amine solution. The hydrogen sulfide rich vapor stream  111  exits stripper column  170  and consists mostly of hydrogen sulfide, water, and trace amounts of amine. Stream  111  enters condenser  171  to produce a condensed liquid stream  113  which is retuned to stripper column  170 . The hydrogen sulfide stream  112  that is recovered can be fed into the natural gas feed line as discussed above with respect to  FIG. 1  for entry into Direct Reduced Iron furnace system. 
         [0037]    The lean amine liquid stream  116  from reboiler  172  is sent to circulation pump  173  to produce pressurized liquid stream  117  which transmits heat to stream  109  in heat exchanger  160 . Stream  118  is cooled in heat exchanger  180  and further cooled in exchanger  190  against an external cooling liquid such as cooling water to produce pressurized cooled liquid stream  120  which is sent to the top of hydrogen sulfide absorber  140 . 
         [0038]    The non-absorbed portion of the offgas  106  which contains carbon dioxide will exit the top of the hydrogen sulfide absorber column  140  and is fed to heat exchanger  180  where it is heated up and sent to the carbon dioxide absorber  150 . 
         [0039]    The carbon dioxide absorber  150  contacts the offgas with a liquid amine stream  131  that will absorb carbon dioxide. Typical carbon dioxide absorbent materials are from the family of amine compounds and can be selected from the group consisting of monoethanolamine, diethanolamine, methyldiethanolamine, diglycolamine, and piperazine. The carbon dioxide enriched liquid amine solution  121  is fed through heat exchanger  220  to stripper column  200 . The liquid passes down the stripper column  200  and exits the bottom of the stripper in stream  126 . This stream enters reboiler  210  where it is heated to create vapor stream  127  which is introduced into the bottom of the stripper column  200 . This gas passes up the column and strips carbon dioxide from the amine solution. The carbon dioxide rich vapor stream  123  exits the stripper column  200  and consists mostly of carbon dioxide, water, and trace amounts of amine. Stream  123  enters condenser  250  to produce a condensed liquid stream  125  which is returned to stripper column  200 . The carbon dioxide stream  124  that is recovered can be sold as a byproduct. 
         [0040]    The lean amine liquid stream  128  from reboiler  210  is sent to heat exchanger  220  where it is partially cooled against stream  121  to produce partially cooled lean amine stream  129 . Stream  129  is fed to circulation pump  230  to produce pressurized lean amine stream  130 . Stream  130  is cooled in heat exchanger  240  against an external cooling liquid such as cooling water to produce pressurized cooled liquid stream  131  which is sent to the top of carbon dioxide absorber  150 . 
         [0041]    While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.