Abstract:
It has been discovered that a mechanical device may be used to effectively displace a first undesired gas from within a liquid with a second desired or at least inert gas. The device is a vessel that receives the liquid containing the first gas and passes the liquid through a series of gasification chambers. Each gasification chamber has at least one mechanism that ingests and mixes a second gas into the liquid thereby physically displacing at least a portion of the first gas into a vapor space at the top of each gasification chamber from which it is subsequently removed. There is an absence of communication between the vapor spaces of adjacent chambers. The ingesting and mixing mechanisms may be a dispersed air flotation mechanism, and may be a conventional depurator. The liquid now containing the second gas and very little or none of the first gas is removed from the vessel for use.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 60/281,919 filed Apr. 5, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to methods and apparatus for removing a gas from a liquid, and more particularly relates, in one embodiment, to methods and apparatus for separating oxygen from water, particularly on an offshore hydrocarbon recovery platform.  
         BACKGROUND OF THE INVENTION  
         [0003]    In many industries, including oil, paper and pulp, textile, electricity generating and food processing, there is an ever-present problem of handling water contaminated with various substances. In particular, water is often used to aid in the production of oil and gas on offshore platforms. This water is usually pumped into a formation in order to be able to pump oil out. The water pumped into the formation is typically available water, and is sea water in the case of offshore platforms. Seawater, like all naturally available water, contains small concentrations of oxygen, typically on the order of 6-10 ppm. The pumps, pipes and other structures through which the sea water is passed prior to injection into a subterranean formation typically are iron or copper alloys. The corrosion of these metals is catalyzed by the small quantities of oxygen present in the sea water, and thus it is desirable to remove as much of this oxygen as possible prior to transporting the water through the pipes, pumps, and other apparatus prior to formation injection. Because it is especially difficult to replace and repair equipment in offshore drilling operations due to much of the equipment being underwater and relatively inaccessible, it is particularly important to minimize corrosion of the equipment as much as possible.  
           [0004]    Current practices for removing oxygen from water include stripping towers employing natural gas, and/or using a vacuum to reduce the boiling pressure of the water. Such prior art techniques usually cannot remove the oxygen to trace levels and thus chemical scavengers such as sulfites are used to remove oxygen further in a separate step. Unfortunately, floating offshore platforms typically are not tolerant of excessive vessel heights that are required of conventional stripping towers for oxygen removal. It would thus be advantageous to discover a method and apparatus for removing oxygen from water in an efficient manner involving a shorter physical profile, and particularly in an apparatus that is adapted for use on an offshore floating platform.  
           [0005]    Apparatus for ingesting and mixing gas into a liquid body are known, such as those of U.S. Pat. No. 3,993,563, that includes a tank, a rotatable impeller fixed to a vertical drive shaft, and a vertically-extending conduit which surrounds the drive shaft and which extends to location in the liquid above the impeller to serve as a channel of communication between a source of gas and the impeller.  
         Summary of the Invention  
         [0006]    Accordingly, it is an object of the present invention to provide an apparatus for displacing a gas from a liquid, which apparatus is particularly suited to be used on floating offshore hydrocarbon recovery platforms.  
           [0007]    It is another object of the present invention to provide a mechanical, cylindrical gas scavenger machine having a reduced height as compared with a stripping tower with a sump for chemical scavenger treatment to reduce oxygen content in a fluid such as water.  
           [0008]    In carrying out these and other objects of the invention, there is provided, in one form, an apparatus for removing a gas from a liquid, where the apparatus includes a vessel for receiving a flow of liquid having a first gas contained therein, and where the vessel has a plurality of partitions sequentially dividing the vessel into at least a first gasification chamber and a second gasification chamber. Each adjacent chamber is in fluid or liquid communication with one another. Each chamber also has a vapor space, and there is no communication between the vapor spaces of adjacent chambers. The vessel also includes an inlet to introduce the flow of liquid into the gasification chambers. There is present a mechanism for ingesting and mixing a second gas into the liquid of each gasification chamber for creating a turbulent area and for displacing at least a portion of the first gas to the vapor space of each chamber. Finally, there is a vent in each chamber of the vessel for removing gas plus a liquid outlet from the outlet chamber.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The single FIGURE is a schematic, cross-sectional illustration of one embodiment of the mechanical oxygen scavenger device of the invention. 
     
    
       [0010]    It will be appreciated that the Figure is a schematic illustration that is not to scale or proportion to further illustrate the important parts of the invention.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    The present invention will now be described, by way of example, and not limitation, with the influent or treated liquid being water that contains oxygen that is replaced by the inert gas nitrogen. However, it will be appreciated that the invention is not limited to this particular liquid or to these particular gases. It is expected that the methods and apparatus will find utility with liquids other than water and gases other than oxygen and nitrogen. It is to be understood that the present invention has utility in numerous applications in which it is desirable to replace one gas from a liquid with another, and that the replaced gas, the liquid containing the new gas, or both may be the desired product of the process.  
         [0012]    Referring now to the Figure, the system  10  of the apparatus of a preferred embodiment of the invention includes a vessel  12  for receiving a flow of liquid  14  having a first gas mixed therewith, where the vessel  12  in a preferred embodiment has a continuous cylindrical sidewall and is capable of withstanding substantial internal pressures. Vessel  12  is divided into a feed box or inlet chamber  16 , at least a first gasification chamber  18 , a second gasification chamber  20 , and an outlet or discharge chamber  22 , where each adjacent chamber can fluidly communicate with one another, that is, that a fluid in one chamber may and should flow into an adjacent chamber. The outlet chamber  22  may optionally function in secondary oxygen chemical scavenging. Outlet chamber  22  may optionally provide an injection booster pump plus level control as typically used in the process, though these latter functions will not influence the removal of the first gas by the second gas. It should be apparent that the flow of the liquid is from the inlet  30  to the outlet  34 . The particular vessel  12  shown in the Figure also contains third and fourth gasification chambers  24  and  26 , respectively. The chambers  16 ,  18 ,  20 ,  24 ,  26 , and  22  and are divided by a plurality of generally vertical partitions  42 ,  44 ,  46 ,  48 , and  50  respectively. Partitions  42  may, in one non-limiting embodiment, may extend from the top and bottom of the interior of vessel  12  and have an aperture  52  in the middle thereof to permit the fluid to flow into first gasification chamber  18 . Partitions  44 ,  46 ,  48 , and  50  extend from the interior top of vessel  12  downward, and are spaced from the interior bottom of vessel  12  to allow fluid communication between the adjacent chambers. The flow of liquid  14  follows liquid transport path  36  through the vessel  12 , although within each chamber, some back flow  40  of liquid  14  into the impeller or rotor  38  will occur during agitation and mixing.  
         [0013]    Each gasification chamber  18 ,  20 ,  24  and  26  may be, but is not required to be, essentially identical in design. Only gasification chamber  20  is shown in detail, and it may be assumed for the purposes of this non-limiting explanation that the other gasification chambers are the same. Each gasification chamber  18 ,  20 ,  24 , and  26  will have a vapor space  54  above the liquid  14  level, but the vapor spaces of the adjacent chambers are not in communication with one another. Most preferably, there is an absence of communication between the vapor space of any gasification chamber with the vapor space of any other gasification chamber. The lengths of partitions  42 ,  44 ,  46 ,  48 , and  50  are calculated to minimize the effect of pressure differential due to difference in flow rates under each respective partition.  
         [0014]    Inlet chamber  16  has an inlet  30  to introduce the flow of liquid  14  to the inlet chamber  16 . Each gasification chamber  18 ,  20 ,  24  and  26  has at least one mechanism  32  for ingesting and mixing gas into the liquid of each respective gasification chamber  18 ,  20 ,  24 , and  26  for creating a turbulent area where the second gas displaces the first gas to an upper portion or vapor space  54  of the vessel  12  for each respective chamber  18 ,  20 ,  24 , and  26 . Gas ingesting and mixing mechanisms  32 , in one non-limiting embodiment, may be submerged rotor mechanisms, and in another non-limiting embodiment are typically dispersed air flotation mechanisms, and are preferably the devices of U.S. Pat. No. 3,993,563, incorporated by reference herein, although it will be appreciated that other devices, including but not limited to, simple aerators, may be used. Mechanisms  32  may also be depurators. Mechanisms  32 , such as described in U.S. Pat. No. 3,993,563, may each include one or more external gas circulation ports  56  to transfer gas into the rotor assembly of mechanism  32  from the vapor space  54  in the upper portion of vessel  12 . Generally, mechanisms  32  create a vortex that draws air from vapor space  54  into the liquid. It is not the intent of the apparatus or method to re-circulate gas from the vapor space when removing the first dissolved gas with a second ingested gas. The second gas will be introduced via an external gas connection  58  which will be attached to a source external to vessel  12 .  
         [0015]    The gas ingesting and mixing mechanisms  32  obtain their source of second gas, optionally an inert gas such as nitrogen, from gas connect or inlet  58  in each gasification chamber  18 ,  20 ,  24 , and  26 . Each gas inlet  58  will be located within the standpipe diameter of each aeration chamber  18 ,  20 ,  24 , and  26 . Vertical standpipe partition  56  of generally cylindrical configuration is present between impeller  38  and vapor space  54 . Communication between gas connect  58  and the gas ingesting and mixing mechanisms  32  is by means of conduits not shown in the Figure. Second gas is not injected into the vapor space  54  in each chamber  18 ,  20 ,  24  and  26 . Instead, the first gas displaced from the fluid collects in the vapor spaces  54  and is removed from vessel  12  by tank breather  60 , located in each chamber  18 ,  20 , 24 , and  26 . It is permissible for a portion of second gas that passes through the liquid in each chamber to be vented through tank breather  60 . It may be desirable or necessary for the outlet from the vapor space  54  in each gasification chamber to be equipped with a one-way gas valve to prevent backflow of the displaced first gas. That is, it is not a requirement of the apparatus or method that all of the second gas injected into vessel  12  be carried out in fluid  14  as it exits through outlet  34 .  
         [0016]    The second gas, e.g. nitrogen, is induced into the liquid, e.g. water, to be de-aerated by the mechanisms  32 . This process also provides a means of controlling the partial pressure parameters, and allows the second gas to displace the first gas, e.g. oxygen, thus scavenging oxygen from the water. The first gas is physically not chemically displaced from the fluid by the second gas. Henry&#39;s Law of partial pressures requires that the first gas be displaced as the second gas is introduced. With each succeeding chamber, more of the first gas is replaced at each point. The number of stages or chambers is not critical, but should be sufficient in number to reduce the concentration of the first gas in the fluid to the desired level. It is expected that several gasification chambers would be necessary to remove sufficient amounts of the first gas in most cases. It should be apparent that the method of the invention is a continuous process. It is desirable to predict and control the amount of second gas ingestion based on rotor submergence of  32  and speed of rotor or impeller  38  to achieve the desired removal level for the first gas, and the rate at which the second gas is ingested.  
         [0017]    Gas ingesting and mixing mechanisms  32  may also include water draft tubes (not shown) to transfer water into the rotor assemblies of mechanisms  32  exclusively from the bottom of the vessel  12 . Inclusion of the water draft tube facilitates capacity variations within the same geometry because all water that enters the rotor assembly is directed to the rotor suction from the bottom of vessel  12 , reducing fluid by-pass and short circuiting of the fluid around the turbulent areas. The treated effluent flows out of vessel  12  via outlet  34  which may have a valve therein (not shown). Flow through the vessel is maintained via pumps or innate system pressure (not shown).  
         [0018]    There may also be present in vessel  12  an internal float displacer liquid level controller  62  (or other suitable liquid level controller) to regulate the rate at which fluid  14  enters vessel  12 . The apparatus  10  may also have a control mechanism, such as a programmable logic controller (PLC) (not shown) for controlling the liquid level in the gasification chambers  18 ,  20 ,  24 ,  26  by obtaining level information from level transmitters (not shown) and regulating flow through level control valves (LCVs, not shown) which is in fluid communication with the liquid in each chamber. The exact natures of the level transmitters, PLC and LCVs are not critical and may be conventional in the art; however, their implementation in the oxygen scavenging apparatus of the invention is expected to be inventive.  
         [0019]    In one embodiment of the invention, the oxygen scavenging apparatus  10  has a dual-cell design, that is, only two gasification cells,  18  and  20 , but more may be used as illustrated in the FIGURE. An optional chemical scavenging feed unit (not shown), which is a standard feed unit for dispensing a metered amount of a first gas scavenging chemical, such as a sulfite, into fluid  14 , to additionally treat the fluid for achieving optimum separation of the first gas from the water can be provided. This optional chemical treating may occur in outlet chamber  22 . However, it may be appreciated that such an additional chemical scavenger treatment may not be necessary.  
         [0020]    Although not shown, valves may be provided for blowdown of sludge that collects in the bottom of vessel  12 . A drain  64  for cleaning out vessel  12  may also be provided. Also not shown are optional gauges to monitor the pressure of the effluent and the flow of gas.  
         [0021]    In the method of the invention, a continuous flow of liquid  14  having a first gas mixed or dissolved therewith is introduced into inlet chamber  16  through inlet  30 . Fluid  14  flows past partition  42  into the gasification chambers  18 ,  20 ,  24 , and  26  sequentially via liquid transport path  36 . In each chamber a flow of second gas is introduced into the liquid  14  by gas ingesting and mixing mechanisms  32 , creating a turbulent area in the entirety of chambers  18 ,  20 ,  24 , and  26 , and allowing the second gas to physically displace the first gas. The first gas is forced out of the liquid  14  as bubbles to the upper portion of vessel  12  where it collects in the respective vapor space  54  of each chamber. First gas is collected and removed through tank breather  60 .  
         [0022]    Fluid  14 , progressively more free of first gas, next underflows each partition  44 ,  46 ,  48 , and  50  in turn flows through liquid outlet  34 . It will be appreciated that it is not possible to predict with accuracy how much of the first gas may be removed from the liquid  14  since such removal depends upon a number of complex, interrelated factors including, but not limited to, the nature of the gases, the nature of the liquid, the concentration of the first gas in the liquid, the ability of the liquid to absorb the second gas, the temperature of the liquid, the pressures within the vessel  12 , and the like. Nevertheless, in order to give some indication of the levels of reduction that might be achieved, it is expected that oxygen in sea water may be displaced by nitrogen from initial levels of about 6-10 ppm to about 0.5-1.0 ppm, in a non-limiting embodiment.  
         [0023]    The rate at which scavenged liquid  14  may removed from vessel  12  may be regulated by a valve or valves (not shown) in response to software program commands or other control mechanism.  
         [0024]    To summarize, advantages of the invention include, but are not necessarily limited to a decreased vessel height requirements as compared with separate stripping towers and reduced capital costs. These advantages are achieved through an oxygen scavenging machine (e.g. depurator) using physical methods to displace a first undesired gas with a second, inert gas. Floating production system operations (FPSO) installations utilizing water floods could also use the apparatus of the invention. In most expected methods of using the apparatus of the invention, it is not expected that the liquid only contain very small quantities of the second gas. It may be that the liquid contains appreciable amounts of the second gas, and this is acceptable.  
         [0025]    In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been demonstrated as effective in providing a device and apparatus for removing or stripping an undesired gas from a liquid. However, it will be evident that various modifications and changes can be made thereto without departing from the broader spirit or scope of the invention as set forth in the appended claims. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, the distances between the partitions and the volumes of the various chambers may be changed or optimized from that illustrated and described, and even though they were not specifically identified or tried in a particular apparatus, would be anticipated to be within the scope of this invention. Similarly, gas ingestion and mixing mechanisms, and level transmitting and control devices different from those illustrated and described herein would be expected to find utility and be encompassed by the appended claims.