Patent Publication Number: US-2011068046-A1

Title: Mercury removal from water

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/243,879 filed Sep. 18, 2009, entitled “MERCURY REMOVAL FROM WATER,” which is incorporated herein in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None 
     FIELD OF THE INVENTION 
     Embodiments of the invention relate to methods and systems for removing mercury from water. 
     BACKGROUND OF THE INVENTION 
     Recovered fluids from wells drilled into hydrocarbon reservoirs often include water. Separators remove the water from oil and gas products also produced. However, the water from some reservoirs contains mercury. The mercury in the water presents environmental and safety concerns and may prevent ability to discharge the water without first being treated. 
     Techniques utilizing solid absorbents for mercury removal from the produced water tend to result in fouling of mercury removal beds. Other factors limiting applicability of prior approaches to remove mercury include expense and size requirements given limited space available when used at platforms. Due to mercury solubility in the water, effectiveness problems arise with some of the prior approaches since the mercury contaminating the water tends to be part of inorganic compounds or a mixture of the inorganic compounds and elemental mercury. 
     Therefore, a need exists for improved methods and systems for removing mercury from water. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a process of removing mercury from water includes separating crude production into a gaseous hydrocarbon stream, a liquid hydrocarbon stream and an aqueous stream. Water forms a majority of the aqueous stream. Removing mercury from a contaminated gas stream including the gaseous hydrocarbon stream provides a treated gas stream. Further, contacting the treated gas stream with the aqueous stream transfers mercury from the aqueous stream to the treated gas stream such that mercury removal from the aqueous stream is independent from the liquid hydrocarbon stream. 
     According to one embodiment, a method of removing mercury from water includes adding a reducing agent to an aqueous stream such that mercury-containing compounds in the aqueous stream are converted to form elemental mercury. The method further includes transferring the elemental mercury from the aqueous stream to a methane-containing gas stream. The transferring occurs upon contacting the gas stream with the aqueous stream combined with the reducing agent. In addition, the method includes removing the elemental mercury from the gas stream. 
     For one embodiment, a process of removing mercury from water includes separating crude production into a gaseous hydrocarbon stream, a liquid hydrocarbon stream and an aqueous stream. Water forms a majority of the aqueous stream. Transferring mercury from the liquid hydrocarbon stream to a first portion of a treated gas stream occurs by contacting the first portion of the treated gas stream with the liquid hydrocarbon stream. Furthermore, transferring mercury from the aqueous stream to a second portion of the treated gas stream by contacting the second portion of the treated gas stream with the aqueous stream is independent of the first portion of the treated gas stream being contacted with the liquid hydrocarbon stream. The treated gas stream forms by removing mercury from the gaseous hydrocarbon stream mixed with the first and second portions of the treated gas stream recycled after the contacting with the liquid hydrocarbon and aqueous streams. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic of a production system for mercury removal from water, according to one embodiment of the invention. 
         FIG. 2  is a schematic of a production system having elements shown in  FIG. 1  with a subunit for removing mercury from a hydrocarbon liquid stream, according to one embodiment of the invention. 
         FIG. 3  is a schematic of a production system having elements shown in  FIG. 1  with a subunit for initial removal of mercury from a hydrocarbon and water mixture, according to one embodiment of the invention. 
         FIG. 4  is a graph showing mercury concentration in water before and after being contacted with a stream of hydrocarbon gas, according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention relate to removal of mercury from water. The removal relies on transferring mercury from an aqueous stream to a natural gas stream upon contacting the aqueous stream with the natural gas stream. Processing of the natural gas stream after used to strip the mercury from the aqueous stream removes the mercury from the natural gas stream. 
     In some embodiments, the water comes from crude production and is thus recovered from reservoirs along with hydrocarbons that may be liquid and gaseous. Mercury concentrations in the water that is produced often prevent outputting the water as waste due to environmental issues and regulations. The removal of the mercury from the water thereby enables discharge of the water separated from the hydrocarbons. As used herein, “mercury” refers to mercury within or from compounds, such as mercuric chloride, mercury oxide and combinations thereof, containing mercury and at least one other element and/or elemental mercury. Location for removing the mercury depends on application and can be performed onsite at offshore platforms with limited space and facilities. 
       FIG. 1  illustrates a system in which crude production removed from a well defines an input stream  100  introduced into a separator  102  for separation into a hydrocarbon gas stream  104 , a hydrocarbon liquid “HC(L)” stream  108 , and an aqueous stream  106  that are each individually removed from the separator  102 . Water forms a majority of the aqueous stream  106 . Mercury-containing gas, including in part at least a portion of the hydrocarbon gas stream  104 , feeds into a mercury removal unit (MRU)  118  for removal of mercury from the mercury-containing gas, thereby forming a treated gas stream  122  output from the MRU  118 . The treated gas stream  122  includes hydrocarbon gas “HC(G),” such as methane, and may provide a supply for natural gas usable in part for sales or as fuel. 
     Part of the treated gas stream  122  forms a recycle gas stream  120 , which is introduced into a water-gas contactor  112  for contact with at least a portion of the aqueous stream  106  that also enters the water-gas contactor  112 . Through such contacting, at least a portion of the mercury contained in the aqueous stream  106  transfers to the recycle gas stream  120 , thereby forming a water-passed gas stream  116  output from the water-gas contactor  112  and a treated water “H2O” stream  114  output from the water-gas contactor  112 . The water-passed gas stream  116  hence includes hydrocarbon gas and mercury “HC(G)+HG.” For some embodiments, the water-passed gas stream  116  mixes with the hydrocarbon gas stream  104  and provides a portion of the mercury-containing gas that feeds into the MRU  118 . 
     In some embodiments, an optional chemical additive stream  110  mixes with the aqueous stream  106  to introduce a reducing agent into the aqueous stream  106  upstream from passing of the recycle gas stream  120  in contact with the aqueous stream  106 . The reducing agent breaks molecular bonds between mercury atoms and other elements in mercury-containing compounds. As used herein, the reducing agent may be provided as a liquid and includes any substance that forms a compound with such released non-mercury elements to prevent recombination with elemental mercury. Examples of the reducing agent include stannous chloride (SnCl 2 ; “SNCL2”), sodium borohydride, and hydrazine. Amount of the reducing agent introduced via the additive stream depends on concentration of mercury in the aqueous stream  106  and may be sufficient to establish an excess mole ratio of the reducing agent relative to the mercury. 
     The reducing agent supplied through the additive stream  110  may facilitate effectiveness of sparging within the water-gas contactor  112  since mercury removal ability via the sparging is higher for elemental mercury relative to when not in elemental form. Reducing inorganic compounds, such as mercury oxide or mercuric chloride, in the aqueous stream  106  tends to promote the mercury removal. Even though the mercury in the aqueous stream  106  can tend to remain in elemental form while at elevated formation temperatures, the elemental mercury may convert into mixed element compounds due to cooling of the aqueous stream  106  and temperature influence on solubility of the elemental mercury. In operation, the aqueous stream  106  may cool upon coming out of the well making introduction of the additive stream  110  desirable to reduce the inorganic compounds to the elemental mercury. 
     For some embodiments, the treated water stream  114  passes through an optional filtration system  124  to remove suspended particulates from the treated water stream  114 . The filtration system  124  operates based on size exclusion to trap or retain particles above a certain size, such as about 0.2 micron or about 0.4 micron. The cooling that is inevitable after the input stream  100  comes out of the well promotes adherence of the mercury to the particulates. Generation of a filtered water stream  126  flowing out of the filtration system  124  thus results in further mercury removal .since residual mercury still within the treated water stream  114  is associated with the particulates. 
     In some embodiments, the water-gas contactor  112  includes multiple (e.g., 2, 4, 6 or more) theoretical stages of separation between vapor and liquid phases. Either trays or packing material of the water-gas contactor  112  may form the theoretical stages by being in a flow path of fluids described herein passing through the water-gas contactor  112 . For example, the packing material making up an internal part of the water-gas contactor  112  may include random oriented objects or a shaped structure and may be made of metallic, ceramic, plastic or other solid material. For some embodiments, amount of the packing material utilized depends on a desired number of the stages provided by the packing material. 
     The MRU  118  defines a fixed bed including any mercury sorbent material capable of removing mercury from gases. In some embodiments, the treated gas stream  122  includes less than about 20 weight percent (wt. %) of the mercury within the mercury-containing gas, less than about 10 wt. % of the mercury within the mercury-containing gas, or less than about 1 wt. % of the mercury within the mercury-containing gas. The treated water stream  114  or the filtered water stream  126  may contain less than about 50 wt. %, 10 wt. %, or 1 wt. % of the mercury contained in the aqueous stream  106 . The aqueous stream  106  for some embodiments contains at least about 5 parts-per-billion (ppb), 100 ppb or 500 ppb mercury. 
     For some embodiments, the recycle gas stream  120  contacts the aqueous stream  106  at ambient temperature, such as about 21° C., or from about 0° C. to about 300° C.; a pressure in the range of from about 0.1 Bars to about 15 Bars, from about 0.5 Bars to about 10 Bars, or from about 1 Bar to about 5 Bars; and a gas to liquid ratio in the range of from about 50 to about 300 standard cubic feet of gas/barrel of liquid (SCF/bbl) or from about 100 to about 200 SCF/bbl. 
       FIG. 2  illustrates a schematic of a production system that includes a subunit for removing mercury from the hydrocarbon liquid stream  108 . The system incorporates an oil-gas contactor  200  of the subunit with elements already described herein with respect to  FIG. 1  and identified by common reference numbers. A first portion  220  of the treated gas stream  122  enters the water-gas contactor  112  to generate the treated water stream  114 . A second portion  221  of the treated gas stream  122  flows into the oil-gas contactor  200  and is introduced into the hydrocarbon liquid stream  108  also input into the oil-gas contactor  200 . Such contacting transfers mercury from the hydrocarbon liquid stream  108  to the second portion  221  of the treated gas stream  122  and occurs subsequent to separation of the hydrocarbon liquid stream  108  from the aqueous stream  106 . Resulting effluent from the oil-gas contactor  200  includes hydrocarbon liquids forming a treated oil stream  208  and hydrocarbon gases contaminated with mercury forming an oil-passed gas stream  216 . The oil-passed gas stream  216  also passes through the mercury removal unit  118  and is thereby regenerated to make up part of the treated gas stream  122 . 
       FIG. 3  shows a schematic of a production system with an exemplary alternative configuration such that a subunit provides initial removal of mercury from a hydrocarbon and water liquid mixture. U.S. patent application Ser. No. 12/538,606, which is herein incorporated by reference in its entirety, further describes such exemplary techniques depicted by the subunits in  FIGS. 2 and 3  for liquid hydrocarbon processing to remove mercury. Similar to other embodiments, the water-gas contactor  112  generates the treated water stream  114  utilizing a first portion  320  of the treated gas stream  122 . In addition to elements already described herein having like reference numbers, the system further incorporates an emulsion-gas contactor  300  of the subunit. The separator  102  may only provide separation for two phases leaving water in the hydrocarbon liquid stream  108  that feeds into the emulsion-gas contactor  300 . A second portion  321  of the treated gas stream  122  flows into the emulsion-gas contactor  300  where introduced into the hydrocarbon liquid stream  108 . Such contacting transfers mercury from the hydrocarbon liquid stream  108  to the second portion  321  of the treated gas stream  122  and occurs prior to separation of the aqueous stream  106  out of the hydrocarbon liquid stream  108 . Three individual resulting effluents from the oil-gas contactor  200  include hydrocarbon liquids forming a treated oil stream  308 , hydrocarbon gases contaminated with mercury forming an emulsion-passed gas stream  316 , and the aqueous stream  106 , which then feeds to the water-gas contactor  112  for further water treatment as set forth herein. The emulsion-passed gas stream  316  passes through the mercury removal unit  118  and is thereby regenerated to make up part of the treated gas stream  122 . 
     Independence with respect to removing mercury from the aqueous stream enables mercury to be removed from the water alone or tailoring amount of mercury to be removed from each of the water and the hydrocarbon liquids as desired. Referring to  FIGS. 2 and 3 , the oil-gas contactor  200  and the emulsion-gas contactor  300  enable reducing mercury concentration in the hydrocarbon liquid stream  108  independent of utilizing the water-gas contactor  112  to remove the mercury from the aqueous stream  106 . Thresholds for mercury concentrations in the hydrocarbon liquids depend on economics and marketability to refineries. However, mercury concentration in the water may need to meet separate set requirements necessitating individual treatment of the aqueous stream  106 . Furthermore, independent processing of the aqueous stream  106  to remove mercury makes possible optional addition of the reducing agent, optional use of the filtration system  124 , and conducting treatment without having to ensure certain temperatures of the aqueous stream  106  during the contacting to remove the mercury. 
       FIG. 4  depicts a graph showing mercury concentration in water before and after being contacted with a stream of hydrocarbon gas. Prior to being sparged with the gas, the water contained 489 micrograms per liter (μg/l) of total mercury and 1.4  82  g/l of dissolved mercury. The water after being sparged with the gas for 20 minutes contained no dissolved mercury and had only 10 μg/l of the total mercury remaining. Since the 10 μg/l of the total mercury remaining was associated with suspended particles greater than 0.45 microns in size, filtering provided an option for removing residual mercury from the water following the sparging. Results thus demonstrated effectiveness of such techniques for removing mercury from actual produced water. 
     The preferred embodiment of the present invention has been disclosed and illustrated. However, the invention is intended to be as broad as defined in the claims below. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims below and the description, abstract and drawings are not to be used to limit the scope of the invention.