Abstract:
A system and process to protect chlorine-susceptible water treatment membranes from chlorine damage without the use of chemical scavengers employs a catalytic deoxygenation system located upstream of the chlorine-susceptible membranes. The system and process not only achieves the required oxygen discharge levels, via reaction of the oxygen with hydrogen, but also dechlorinates the water, via reaction of the chlorine species with hydrogen.

Description:
BACKGROUND 
       [0001]    This invention relates to systems and processes that use nanofiltration or reverse osmosis membrane systems to treat a water stream. More particularly, the invention relates to systems and processes that treat a water stream, which could be a seawater stream, for reinjection in oilfield applications. 
         [0002]    To prevent organic growth in water treatment systems, chlorine typically in the form of hypochlorite is dosed into the water being treated. While effective for preventing the organic growth, the chlorine can permanently damage membrane technologies such as nanofiltration and reverse osmosis membranes used in the treatment process, rendering the membranes inactive or ineffective. 
         [0003]    In cases where the treatment process requires the water to be free from chlorine and oxygen, scavenging chemicals such as sodium bisulfite, along with the associated chemical injection equipment, are a required component of the process system design to allow for the necessary (reduced) levels of chlorine and oxygen to be achieved. 
         [0004]    Deoxygenation of water using conventional technologies upstream or downstream of membrane systems has been used, but chemical scavenging is still required because the treated water still has residual oxygen or residual chlorine or both in it. The residual chorine can damage the membranes. The scavenging chemical is injected upstream of the membranes for chlorine removal and downstream of the membranes into the deoxygenation equipment, for removal of residual oxygen that cannot be removed by the deoxygenation equipment. 
         [0005]    Therefore, a need exists for systems and processes that can dechlorinate a water feed ahead to the membrane technologies without the need for chemical scavengers and its associated dosing equipment (which is required by other oxygen removal technologies). 
       SUMMARY OF THE INVENTION 
       [0006]    A preferred embodiment of a system and process to protect chlorine-susceptible water treatment membranes from chlorine damage without the use of chemical scavengers employs a catalytic deoxygenation system upstream of the chlorine-susceptible membranes. The system and process not only achieves the required oxygen discharge levels, via reaction of the oxygen with hydrogen, but also dechlorinates the water, via reaction of the chlorine species with hydrogen. 
         [0007]    A chlorine-dosed water feed, or a water feed having chlorine present, is mixed with hydrogen and passed through a catalyst bed-based deoxygenation unit. The deoxygenated and dechlorinated water product then passes through a filtration system having selectively permeable membrane technologies. The selectively permeable membranes provide a membrane permeate comprised of a portion of the feed from which contaminants, such as dissolved inorganic salts and organic constituents, have been removed. The filtration system may be a nanofiltration or reverse osmosis membrane system. The filtration system may have one or two stages, with each stage containing one or more membrane elements. 
         [0008]    A process for protecting chlorine-susceptible permeable membranes includes the steps of 
         [0009]    (i) mixing hydrogen with a water stream containing chlorine to produce a mixed water feed; 
         [0010]    (ii) routing the mixed water stream to a catalyst bed-based deoxygenation unit; 
         [0011]    (iii) removing a deoxygenated and dechlorinated water stream from the catalyst bed-based deoxygenation unit; and 
         [0012]    (iv) routing the deoxygenated and dechlorinated water stream to a filtration system having a plurality of selectively permeable membranes (arranged in one or more stages), the catalyst bed-based deoxygenation unit being arranged upstream of the filtration system. No chemical scavenger dosing step occurs between the steps (iii) and (iv). 
         [0013]    The objectives of this invention are to (1) protect the chorine-susceptible membrane technologies without the need for chemical scavengers and the associated dosing equipment; (2) prolong membrane life and effectiveness; (3) simplify the dechlorination process and reduce its footprint and operating cost; and (4) reduce or eliminate downtime due to in-place cleaning of the membranes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a preferred embodiment of the system and process for protect chlorine-susceptible water treatment membranes from chlorine damage without the use of chemical scavengers. The filtration system in  FIG. 1  is a two-stage nanofiltration membrane system. 
           [0015]      FIG. 2  shows another preferred embodiment of the system and process. The filtration system in  FIG. 2  is a single stage reverse osmosis membrane system. 
           [0016]      FIG. 3  shows yet another preferred embodiment of the system and process. A single stage microfiltration or ultrafiltration membrane system is placed upstream of the catalytic bed-based deoxygenation unit to filter the incoming water stream. The deoxygenation unit is then followed by a nanofiltration or reverse osmosis membrane system (or a parallel-arranged combination of the two). 
       
    
    
     ELEMENTS AND ELEMENT NUMBERING USED IN THE DRAWINGS AND THE DETAILED DESCRIPTION 
       [0000]    
       
         
           
               10  System (and process) 
               20  Two-stage nanofiltration membrane system 
               30  Catalyst bed-based deoxygenation unit 
               40  Chlorine-dosed water feed or stream or water feed containing chlorine 
               50  First stage nanofiltration membrane unit 
               60  First stage nanofiltration membrane unit 
               70  Membrane permeate 
               80  Membrane reject 
               90  Membrane permeate 
               95  Combined membrane permeate stream from first stage 
               98  Combined membrane permeate stream from first and second stages 
               100  Membrane reject 
               105  Combined membrane reject stream 
               110  Second stage nanofiltration membrane unit 
               120  Membrane permeate 
               130  Concentrated membrane reject 
               140  Hydrogen or hydrazine supply 
               150  Combined water and hydrogen (or hydrazine) stream 
               160  Deoxygenated and chlorine-reduced water product or feed 
               170  Single-stage reverse osmosis membrane system 
               180  Reverse osmosis membrane unit 
               190  Reverse osmosis membrane unit 
               200  Membrane permeate 
               210  Membrane reject 
               220  Membrane permeate 
               225  Combined membrane permeate stream 
               230  Membrane reject 
               240  Concentrated membrane reject 
               260  Microfiltration or ultrafiltration system 
               265  Filtered chlorine-dosed water feed or water feed containing chlorine 
               270  Mixing system (e.g. static mixer, mixing valve, or a combination thereof 
               275  Combined water feed and hydrogen stream 
               280  Stream of backwash water 
               285  Backwash water supply 
               290  Backwash discharge 
               295  Stream of compressed air 
               300  Air scour supply 
           
         
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0054]    A system and process made according to this invention deoxygenates and dechlorinates a water feed dosed with, or containing, chlorine prior to the feed reaching chlorine-susceptible membrane technologies. The water feed is mixed with hydrogen (or hydrazine) and enters a catalytic bed-based deoxygenation unit. The hydrogen reacts with the oxygen and chlorine species present in the feed to produce a deoxygenated and dechlorinated water product. This water product then enters a filtration system having a nanofiltration or a reverse osmosis membrane system (or parallel arranged nanofiltration and reverse osmosis membrane systems). No chemical scavenging is required between the deoxygenation unit and the membrane systems. 
         [0055]    Referring first to  FIG. 1 , a preferred embodiment of a system  10  includes catalyst bed-based deoxygenation unit  30  arranged upstream of a two-stage nanofiltration membrane system  20 . The number of membrane units in the each stage may vary with the quantity and quality of water to be processed, the amount of available space, and other factors. 
         [0056]    A water feed  40  containing chlorine is mixed with hydrogen from a hydrogen or hydrazine supply  140  to form a combined water and hydrogen (or hydrazine) stream  150 , which is fed to the catalyst bed-based deoxygenation unit  30 . The catalyst bed-based deoxygenation unit  30  removes dissolved oxygen and chlorine from the water by reacting it with hydrogen, creating a deoxygenated and dechlorinated water product or feed  160 . The feed  160  is then directed to one of two first-stage nanofiltration membrane units  50 ,  60 . 
         [0057]    Each first-stage nanofiltration membrane unit  50 ,  60  contains a plurality of selectively permeable membranes that contact the feed  160 . A portion of the feed  160  passes through the membranes  50 ,  60 , forming a membrane permeate  70 ,  90  that is substantially free of any dissolved inorganic salts and organic constituents. The streams of membrane permeate  70 ,  90  from the first-stage nanofiltration membrane units  50 ,  60  are mixed to form a combined membrane permeate stream  95 . 
         [0058]    The remaining portion of the feed  160 , which contains the dissolved inorganic salts and organic constituents that are too large to pass through the membranes  50 ,  60 , is concentrated into a stream of membrane reject  80 ,  100 . The streams of membrane reject  80 ,  100  from the first-stage nanofiltration membrane units  50 ,  60  are mixed to form a combined membrane reject stream  105  and routed to the second-stage nanofiltration membrane unit  110 . This nanofiltration membrane unit  110  also contains a plurality of selectively permeable membranes. 
         [0059]    These membranes  110  contact the combined membrane reject stream  105  and allow a portion of it to pass through the membranes  110 , forming a membrane permeate  120  that is substantially free of the dissolved inorganic salts and organic constituents. The remaining portions of combined membrane reject stream  105 , which contains the dissolved inorganic salts and organic constituents that are too large to pass through the membranes  110 , forms a stream of concentrated membrane reject  130  which may be sent to disposal. 
         [0060]    The stream of membrane permeate  120  from the second-stage nanofiltration membrane unit  110  may be mixed with the combined membrane permeate stream  95  from the first-stage nanofiltration membrane units  50 ,  60  to form a combined membrane permeate stream from the first and second stages  98 . 
         [0061]    Referring now to  FIG. 2 , another preferred embodiment of system  10  includes a catalyst bed-based deoxygenation unit  30  arranged upstream of a single-stage reverse osmosis membrane system  170 . Although two membrane units are shown in  FIG. 2 , the number of membrane units may vary with the quantity and quality of the raw seawater to be processed, the amount of available space, and other factors. Additionally, a reverse osmosis membrane system may be arranged in parallel with a nanofiltration membrane system, with one portion of the incoming feed passing thorough the nanofiltration membrane system while another portion passes through the reverse osmosis membrane system. 
         [0062]    A water feed  40  containing chlorine is mixed with hydrogen from a hydrogen or hydrazine supply  140  to form a combined water and hydrogen (or hydrazine) stream  150 , which is fed to the catalyst bed-based deoxygenation unit  30 . The catalyst bed-based deoxygenation unit  30  removes dissolved oxygen and chlorine from the water by reacting it with hydrogen, creating a deoxygenated and dechlorinated water product or feed  160 . The feed  160  is then directed to one of two reverse osmosis membrane units  180 ,  190 . 
         [0063]    Each reverse osmosis membrane unit  180 ,  190  contains a plurality of selectively permeable membranes that contact the product  160 . A portion of the feed  160  passes through the membranes, forming a membrane permeate  200 ,  220  that is substantially free of dissolved inorganic salts and organic constituents. The streams of membrane permeate  200 ,  220  from the reverse osmosis membrane units  180 ,  190  are mixed to form a combined membrane permeate stream  225 . 
         [0064]    The remaining portion of feed  160 , which contains dissolved inorganic salts and organic constituents that are too large to pass through the membranes  180 ,  190  is concentrated into a stream of membrane reject  210 ,  230 . The streams of membrane reject  210 ,  230  from the reverse osmosis membrane units  180 ,  190  are combined to form a stream of concentrated membrane reject  240  which may be sent to disposal or be combined and routed to a filtration membrane unit or units at downstream next stage. This process may be repeated until the final stage, which routes the membrane reject for disposal. 
         [0065]    Referring to  FIG. 3 , another preferred embodiment of system  10  includes a microfiltration or an ultrafiltration system  260  arranged upstream of the catalyst bed-based deoxygenation unit  30 . Microfiltration or “MF” may remove particulates that are equal to or greater than 0.1 micrometers in size, while ultrafiltration or “UF” may remove particulates that are equal to or greater than 0.01 micrometers in size. Although one filtration system  260  is shown in  FIG. 3 , the number of filtration systems may vary with the quantity and quality of the raw seawater to be processed, the amount of available space, and other factors. 
         [0066]    A water feed  40  containing chlorine passes through the filtration system  260 , forming a stream of membrane permeate  265  that is substantially free of inorganic salts and organic constituents but still containing chlorine. If a raw or untreated water feed is used, chlorine dosing and its associated dosing equipment may be arranged upstream of the filtration system  260  or between the filtration system  260  and the catalytic bed-based deoxygenation unit  30  to provide water feed  40 . 
         [0067]    The organic constituents may be removed from the microfiltration or ultrafiltration system  260  by backwashing. In backwashing, a stream of backwash water  280  from a backwash water supply  285  is passed quickly through the microfiltration or ultrafiltration system  260  in a direction opposite to the normal direction of flow. The organic constituents trapped in the filtration system  260  are thus removed from the filter media and entrained in the backwash water  280 . 
         [0068]    The backwash water  280  then exits the filtration system  260  through the backwash overboard discharge  290  and may be sent for further treatment or disposal. Air scouring, in which a stream of compressed air  295  from an air scour supply  300  is blown through the filtration system  260  in the same direction as the stream of backwash water  280 , may be used before or intermittently with backwashing to aid in the removal of organic constituents. 
         [0069]    The membrane permeate  265 , which contains chlorine, is mixed with hydrogen from a hydrogen or hydrazine supply  140  to form a combined water and hydrogen (or hydrazine) stream  150 , which is fed to the catalyst bed-based deoxygenation unit  30 . The hydrogen or hydrazine can be dispersed through feed  40  using a mixing system  270  such as a static mixer, mixing valve, or some combination thereof (the same can be done in the embodiments of  FIGS. 1 and 2 ). The catalyst bed-based deoxygenation unit  30  removes dissolved oxygen and chlorine from the water by reacting it with hydrogen, creating a deoxygenated and dechlorinated water product or feed  160 . The feed  160  is then directed to a nanofiltration or reverse osmosis filtration system  20  or  170  (see e.g.  FIGS. 2 and 3 ). The nanofiltration and reverse osmosis membrane units may also be arranged in parallel, with the feed  160  being split between the two. 
         [0070]    In the embodiments of  FIGS. 1-3 , the water feed  40  entering the catalyst bed-based deoxygenation unit  30  may have a chlorine content of about 8,000 ppb and the deoxygenated and dechlorinated water product or feed  160  exiting the deoxygenation unit  30  has no more than 50 ppb chlorine and preferably 10 ppb chlorine or less, with no chemical scavengers being used to achieve these levels. While preferred embodiments of a system and process to deoxygenate and dechlorinate a water feed dosed with, or containing, chlorine prior to the feed reaching chlorine-susceptible membrane technologies have been described in detail, a person of ordinary skill in the art understands that certain changes can be made in the arrangement of process steps and type of components used in the process without departing from the scope of the attached claims.