Abstract:
A method for retrofitting a system for removing mercury and water from a gas stream is disclosed. A system is provided including a first water removal unit and a second mercury removal unit in fluid communication with the first water removal unit. The first water removal unit has a fixed capacity or reservoir for containing mole sieves for removing water from a gas stream. The second mercury removal unit has a fixed capacity or reservoir for containing mole sieves for removing mercury from the gas stream. The second mercury removal unit is filled with mole sieves adapted for removing mercury from a gas stream containing mercury. The first water removal unit is retrofit by replacing a first portion of mole sieves adapted for removing water from the gas stream with a second portion of mole sieves adapted for removing mercury. The capacity of the system for removing mercury is thereby enhanced relative to a system of the same size wherein the first water removal unit is filled with water removing mole sieves and no mercury removing mole sieves.

Description:
FIELD OF THE INVENTION 
       [0001]    This application claims the benefit of U.S. provisional patent application Ser. No. 61/767,139 filed on Feb. 20, 2013 the disclosures of which are incorporated herein by reference. The present invention relates generally to apparatus and methods for removing mercury from a gas stream containing water and mercury such as a stream separated from produced hydrocarbon fluids received from an underground reservoir, and more particularly, to apparatus and methods for retrofitting systems containing dehydration and mercury removal units. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Examples of production facilities for handling hydrocarbon containing gases include liquefied natural gas (LNG) facilities where gas is liquefied at cryogenic temperatures, gas-to-liquids (GTL) plants where gases such as methane are catalytically converted to liquid hydrocarbons and compressed natural gas (CNG) where gas is compressed to high pressure for transportation. During well appraisal stages of planning for such production facilities estimates are made of the quantities and concentrations of hydrocarbon contents, such as hydrocarbon gases and liquids, produced water, acid gases such as carbon dioxide and hydrogen sulfide, and other contaminants such as mercury. 
         [0003]    These production facilities for processing the hydrocarbon containing fluids produced from underground reservoirs are often designed well in advance of actual wells being drilled and fluids being produced. As a consequence, the capacity of the designed treating facility may not be sufficient to handle certain contaminants, such as mercury)(Hg°), as originally designed. In certain cases, such as with offshore platforms or land based facilities in environmentally sensitive areas, the footprint of production facilities can be difficult to change without having to make major redesigns. The present invention addresses retrofitting a portion of a production facility wherein the footprint of a design need not be changed while still accommodating an increased capacity in Hg removal and maintaining the planned life of a dehydration and mercury removal system. 
       SUMMARY OF THE INVENTION 
       [0004]    A method for retrofitting a system for removing mercury and water from a gas stream is disclosed. An existing system includes a first water removal unit and a second mercury removal unit in fluid communication with the first water removal unit. The first water removal unit has a fixed capacity or reservoir for containing adsorbents such as mole sieves for removing water from a gas stream. The second mercury removal unit has a fixed capacity or reservoir for containing adsorbents such as activated carbon, mole sieves or metal sulfides, for removing mercury from the gas stream. For retrofitting the system to treat the gas with a higher mercury concentration than originally anticipated, the first water removal unit is provided with two types of adsorbents; one is an preferably an adsorbent for water removal and the second absorbent, such as a metal coating mole sieve, is adapted to remove mercury or both water and mercury. For example, the first water removal unit is filled with a first or upstream portion of mole sieves adapted for removing water from the gas stream and a second or downstream portion of mole sieves, such as with a metal coating adapted for removing mercury and water. The capacity of the retrofitted system for removing mercury is enhanced relative to the existing system of the same size wherein the first or original water removal unit is designed to be filled with water removing mole sieves or adsorbents and not mercury removing mole sieves or adsorbents. 
         [0005]    In one embodiment, the mercury removing adsorbents are disposed of at the end of their estimated life. In an alternative embodiment, the mercury removing adsorbents and water removing adsorbents can be regenerated concurrently and are disposed in the water removal unit. The water removing capability and bed life of the first water removal unit of the retrofitted unit will ideally be the same as that of the existing unit which is used to absorb water only. 
         [0006]    In one retrofit embodiment, the first water removal unit contains at least 10% mercury removing mole sieves by volume. In another embodiment, the first retrofit water removal unit contains at least 20% mercury removing mole sieves by volume. In a third embodiment, the first retrofit water removal unit contains at least 30% mercury removing mole sieves by volume. In a fourth embodiment, the first retrofit water removal unit contains at least 50% mercury removing mole sieves by volume. The percentage of mercury removing mole sieves required is related to the increased mercury concentration in the plant inlet gas over the earlier planned concentration or amount of mercury. For example, the percentage may be proportionally increased with the increased concentration of Hg in the gas stream. 
         [0007]    The first water removal unit may include an existing mole sieve regeneration system which can be also used to regenerate the water removing and mercury removing mole sieves for the retrofit system. For example, the regeneration unit will remove water and mercury from the first water removal unit in a regeneration step. 
         [0008]    Also disclosed is a water dehydration and Hg removal system. The first water and Hg removal unit has a first water removal reservoir filled with water removal mole sieves and a second Hg removal reservoir filled with Hg removal mole sieves. Downstream there from is a second Hg removal unit in fluid communication with the first water and Hg removal unit. The second Hg removal unit has a Hg removal reservoir filled with Hg removal mole sieves. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    These and other objects, features and advantages of the present invention will become better understood with regard to the following description, pending claims and accompanying drawings where: 
           [0010]      FIG. 1  is a schematic illustration of a conventional system used for water and mercury removal from a natural gas treatment facility; and 
           [0011]      FIG. 2  is schematic illustration of an embodiment of the present invention wherein a water dehydration unit is retrofit to include both water and mercury removal adsorbents thereby increasing the mercury removal capacity of the system relative to the system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  shows a conventional or existing dehydration and mercury removal system  100 . Sweet gas  102  is introduced to system  100  from which water and Hg is removed leaving a dried and Hg depleted gas stream  104  which exits system  100 . Sweet gas is a gas that has previously had sour gases, such as hydrogen sulfide and carbon dioxide, removed from hydrocarbon containing gas stream. Ideally, gas stream  104  is greatly depleted in water and Hg content as compared to sweet gas  102 . 
         [0013]    System  100  includes three bed vessels  106 ,  106 ′ and  106 ″ which are used to remove water and are generally similar in construction. Of course, other similar systems can be designed with more or fewer vessels and are within the scope of this invention. In this exemplary embodiment, the first and second dehydration vessels  106  and  106 ′ are to be operated in a water absorption mode while the third vessel  106 ″ is operated in regeneration mode wherein water is stripped from vessel  106 ″. Each of dehydration vessel  106 ,  106 ′ and  106 ″ includes domed upper and lower end caps  110   a  and  110   b  which are secured relative to cylinders  114 . Upper and lower end plates  116   a  and  116   b  having perforations  120   a  and  120   b  therein. Cylinders  114  and upper and lower end plates  116   a  and  116   b  define reservoirs  122  in which dehydration mole sieves or adsorbents  124  are contained or packed. 
         [0014]    Dehydration mole sieves  124  can be selected from a wide variety of types and shapes of mole sieves adapted to capture water molecules thereon. A “molecular sieve” refers to a material containing tiny pores of a precise and substantially uniform size. In the present context, such sieves are used as an adsorbent for water removal from gases. Molecular sieves often consist of solid materials and not polymeric materials. Exemplary materials include alumino-silicate minerals, clays, porous glasses, micro-porous charcoals, zeolites, active carbons, or synthetic compounds that have open structures through which small molecules, such as nitrogen and water, can diffuse. Polar molecules (such as water molecules) that are small enough to pass into the pores are adsorbed, while slightly polarizable molecules (such as methane and nitrogen), as well as larger molecules (e.g., propane and butane) flow around the particles and crystallites, and are thus passed downstream. In the present embodiment, the molecular sieves adsorb water molecules and allow light gases to pass through. 
         [0015]    System  100  also includes a mercury removal unit (MRU)  150 . MRU  150  includes domed upper and lower end caps  152   a  and  152   b  which are attached relative to an intermediate cylinder  154 . Upper and lower end plates  156   a  and  156   b  have perforations  160   a  and  160   b  therein. A reservoir  162  is formed by upper and lower end plates  156   a  and  156   b  and cylinder  154 . Hg removal adsorbents  164  are captured within reservoir  162 . While only a single vessel is shown in this embodiment, those skilled in the art will appreciate mercury removal units can be constructed one or more of such vessels and are within the scope of the present invention. 
         [0016]    Hg removal adsorbents  164  may also be selected from a wide variety of commercially available adsorbents such as activated carbon or metal sulfides for removing Hg from a gas stream. By way of example and not limitation, Hg removal adsorbents  164  may be selected from those listed in patents such as Mercury absorbent carbon—EP0271618A, Mercury adsorbent carbons and carbon molecular sieves—EP0145539B and Removal of heavy metals from hydrocarbon gases EP2346592A. 
         [0017]    In operation, a stream of sweet gas  102  is introduced to system  100 . Sweet gas  102  enters dehydration units  106  and  106 ′ through end caps  110   a  and passes through perforations  120   a  to enter reservoirs  122 . As gas stream  102  passes through reservoir  122 , water is absorbed onto dehydration mole sieves  124  producing a dried gas stream  140 . Dried gas stream  140  passes out of perforations  120   b  and end caps  110   b  with dried gas stream  140  then being routed to MRU  150 . Dried gas stream  140  enters through end cap  152   a  and perforations  160   a  in end plate  156   a  and enters into reservoir  162 . Hg removal adsorbents  164  absorbs Hg from dried gas stream  140  producing dried and Hg depleted gas stream  104 . While dehydration units  106 ,  106 ′ and MRU  150 , respectively, strip out water and Hg from sweet gas stream  102 , dehydration unit  106 ″ may be concurrently regenerated and recharged bypassing recharge stream  170  through reservoir  122 . For example, recharge stream may be a hot stream of air which carries away water from the adsorbents  124  in vessel  106 ″. Gas stream  172  carries away water vapor from mole sieves  124  to recharge the dehydration mole sieves  124  so that they may be used again for water removal when vessel  106 ″ is placed into an absorption mode. Other conventional recharge streams, well known to those skilled in the art, may also be used to recharge the adsorbents by stripping away water and/or Hg from the adsorbents. 
         [0018]    Gas stream  104  is then suitable for further gas processing such as the production of liquefied natural gas, gas-to-liquid Fischer-Tropsch products, or for production of compressed natural gas (CNG) which is suitable for transport. Alternatively, by way of example and not limitiation, gas stream  104  may be further processed and compressed for transport through pipelines. 
         [0019]    However, system  100  may be incapable of handling a Hg load in excess of what system  100  was originally designed. For example, additional produced fluid may be introduced to a production system from one or more fields that were not originally anticipated. Or else, the fields for which system  100  was originally intended to handle Hg removal may be turn out to have a much higher concentration of Hg than was originally anticipated during preliminary designs. System  200  in  FIG. 2  may then be used as a retrofit of system  100  without significantly changing the available volume for the reservoirs  122  and  162  storingdehydration mole sieves and Hg removal adsorbents, respectively. 
         [0020]      FIG. 2  shows a dehydration and mercury removal system  200  which is a retrofit of system  100 . Like components from system  100  are generally incremented in reference numeral by  100 . 
         [0021]    Sweet gas  202 , which may have a higher mercury concentration than sweet gas  102 , is introduced to system  200  from which water and Hg is removed leaving a dried and Hg depleted gas stream  204  which exits system  200 . 
         [0022]    System  200  includes the three same dehydration vessels, now designated as vessels  206 ,  206 ′ and  206 ″, as was used in system  100 . Vessels  206  and  206 ′ are used to remove water in an absorption mode while the third vessel  206 ″ is in the regeneration mode. Later the vessels can be placed alternatively in adsorption and recharge modes, as appropriate. Each of dehydration vessels  206 ,  206 ′ and  206 ″ includes domed upper and lower end caps  210   a  and  210   b  which are secured relative to cylinders  214 . Upper and lower end plates  216   a  and  216   b  having perforations  220   a  and  220   b  therein. An additional intermediate plate  230  is secured relative to cylinder  214  and has perforations  232  therein. Each of intermediate plate  230  and upper end plate  216   a  cooperate to form an upper reservoir  234 . Similarly, intermediate plate  230  cooperates with cylinder  214  and bottom end plate  216   b  to form a lower reservoir  236 . Alternative means of separating the upper dehydration mole sieves from the lower Hg removal sieves may also be employed. By way of example and not limitation, glass beads could be used to separate the dehydration and Hg removal mole sieves instead of using perforated plate  230  in a dehydration vessels  206 . Upper reservoir  234  is filled with dehydration only mole sieves  224  while lower reservoir  236  is filled with Hg removal mole sieves  264 . Alternatively, adsorbents  264  may be selected to adsorb both water and Hg under appropriate adsorbtion conditions. Dehydration and Hg removal mole sieves  264  may be selected as described above with respect to system  100 . Alternatively, mole sieves with greater carrying capacity for water and Hg, respectively, may be selected, albeit at greater absorbent cost than the original adsorbents or mole sieves of system  100 . 
         [0023]    System  200  also includes a mercury removal unit (MRU)  250  to remove mercury in stream  240 . MRU  250  includes domed upper and lower end caps  252   a  and  252   b  which are attached relative to an intermediate cylinder  254 . Upper and lower end plates  256   a  and  256   b  have perforations  260   a  and  260   b  therein. A reservoir  262  is formed by upper and lower end plates  250   a  and  250   b  and cylinder  254 . Hg removal adsorbents  264  are placed within reservoir  262 . 
         [0024]    In operation, the retrofitted system  200  has sweet gas stream  202  introduced there to. Sweet gas  202  enters dehydration vessels  206  and  206 ′, which are in absorption mode in this exemplary embodiment, through end cap  210   a  and passes through perforations  220   a  to enter upper reservoir  234 . As gas stream  202  passes through reservoir  234 , water is absorbed onto dehydration mole sieves  224  producing a dried gas stream  240 . Gas stream  240  passes through perforation  232  into lower reservoir  236  where a portion of Hg in the gas is removed and additional water removing is completed as well, if an absorbent is suitably selected that removes water as well as Hg. 
         [0025]    Dried and Hg depleted gas stream  242  exits lower reservoir through perforations  220   b  and end caps  210   b  and is routed to Hg removal units  250 . In Hg removal unit  250 , further Hg left in the gas  242  is removed to produce stream  204  which ideally meets the specification of allowable Hg in a gas stream. Stream  242  enters through end cap  252   a  and perforations  260   a  in end plate  256   a  to reach reservoir  262 . Hg removal adsorbents  264  strip Hg from dried gas stream  242  producing dried and Hg depleted gas stream  204 . While dehydration units or vessels  206  and  206 ′ strip out water and a portion of Hg from sweet gas stream  202 , MRU  250  completes mercury removal. Dehydration vessel  206 ″ can be regenerated while the other two vessels  206  and  206 ′ are absorbing water and Hg. For example, regeneration of mole sieves  224  and  264  can be achieved by passing recharge streams  270  through reservoirs  234 ,  236  in unit  206 .″ Gas stream  272  carries away water vapor and Hg from mole sieves  224  and  264  during a regeneration step. Mole sieves may be used again for water and Hg removal after regeneration is completed and vessel  206 ′ is set into adsorption mode. 
         [0026]    Not shown is valving which alternatively passes sweet gas  202  through vessels  206 ,  206 ′ and  206 ″to remove water and Hg and recharges gas  270  passes through vessels  206 ,  206 ′ and  206 ″ so that water and Hg may be alternately absorbed in removal stages and water and Hg stripped during recharge stages. By increasing the Hg reservoir capacity by placing Hg removing mole sieves or adsorbents in vessels  206 ,  206 ′and  206 ″, system  200  will have increased Hg removal capacity as compared to system  100  of  FIG. 1 . 
         [0027]    While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention.