Abstract:
It has been discovered that a mechanical flotation system having only two cells can be nearly as efficient as one having more cells, yet provide an apparatus with a considerably smaller footprint, significantly reduced capital and operating costs, as well as be resistant to floating oil recovery platform wave effects. The dual-cell mechanical flotation system has, in sequential order, an inlet chamber and two gasification chambers or cells, each with at least one gas ingestion and mixing mechanism, and a discharge chamber. A common primary skim collection channel atop the partition dividing the gasification chambers efficiently channels away the bulk of the floating collected matter. At least one baffle depending from the top of the vessel near the primary skim collection channel helps dampen the action of the fluid containing the suspended matter when the vessel is affected by wave motion against the floating oil production platform.

Description:
This application claims the benefit of Provisional application Ser. No. 60/210,692, filed Jun. 9, 2000. 

   FIELD OF THE INVENTION 
   The present invention relates to methods and apparatus for removing suspended matter from liquid, and more particularly relates, in one embodiment, to methods and apparatus for separating suspended contaminants and/or oil from water, particularly on an offshore hydrocarbon recovery platform. 
   BACKGROUND OF THE INVENTION 
   In many industries, including oil, paper and pulp, textile, electricity generating and food processing, there is an ever-present problem of contaminated water as a by-product of various processes. 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. As a result, the water becomes contaminated with oil and solids encountered in the formation, and therefore cannot be disposed of simply by dumping it into the surrounding water. Accordingly, numerous methods and systems have been devised to reduce the contaminant content of this water to a level that allows discharge of the water into the sea. 
   One such system, disclosed in U.S. Pat. No. 4,255,262, comprises a device that mixes and disperses gas in the form of fine bubbles in liquid in a tank in an attempt to remove contaminants from the liquid flowing through the tank. The gas is induced from the upper section of the tank downward into the liquid in the tank via a draft tube. The gas induction occurs as a portion of the liquid contained in the vessel is recirculated back through the individual cells or compartments using a centrifugal pump. The apparatus uses an electrically-driven mechanical skimmer assembly, which serves to remove contaminant-laden froth that accumulates above the liquid level section of the tank. The tank is rectangular-shaped. 
   The above-mentioned device suffers from various drawbacks. The skimmers are moderate- to high-maintenance items, particularly when used in corrosive environments such as may be encountered in the oil-producing and chemical industries. Also the rectangular tanks, by virtue of their construction, cannot withstand pressures in excess of 2 oz. per square inch (0.8 kPa) internal. This is particularly disadvantageous especially where system pressures upstream of the oil/water separator are prevalent or where noxious or lethal gases such as hydrogen sulfide are present. Furthermore, the rectangular tanks having the skimmers are limited in volumetric capacity because full utilization of the tank is not allowed. In addition, although these tanks are described as “gas tight”, gas pressures are maintained by continuously venting to the atmosphere, which is a potentially dangerous practice if lethal or flammable gases are present. 
   U.S. Pat. No. 4,564,457, discloses another system for separating suspended matter from fluid. The device comprises a cylindrical tank having an inlet chamber, a plurality of gasification chambers, and a quiescent outlet chamber. A skim trough is disposed near the top of the tank, and extends the length of the gasification chambers into the outlet chamber. Vertical baffles that separate the individual chambers extend downwardly and are spaced from the bottom of the tank, allowing fluid to flow along the bottom of the tank from the inlet chamber to the outlet chamber. Each gasification chamber is equipped with an eductor nozzle assembly positioned centrally in the lower portion thereof. The nozzle assembly provides for recirculation of fluid pumped from the outlet chamber. 
   In operation, fluid enters the tank through the inlet chamber, passes successively through each of the gasification chambers, and into the outlet chamber, where a portion of the fluid is drawn off to be recirculated through the eductor nozzles. The balance of the processed fluid exits the outlet chamber for further treatment, discharge or storage, depending upon the application. The recirculated fluid is pumped through the nozzle assembly, each nozzle being fed by a common header supplied by a recirculation pump, and each nozzle being positioned co-centrically in an eductor throat assembly. Each eductor throat assembly is connected to a gas header, supplied by a gas volume in the upper portion of the tank common to the aeration chambers and the outlet chamber. The passage of the fluid at high velocity through the nozzles educts gas into the gasification chambers and the gas rises in the fluid in the form of small bubbles. The gas bubbles collect oil and/or suspended solid contaminants as they rise, forming a contaminant-laden froth at the top of the gasification chambers. 
   While this device presents definite advantages over that disclosed in U.S. Pat. No. 4,255,262, such as the elimination of the mechanical skimmer and problems associated therewith, the ability to operate at higher internal pressure, and better utilization of available tank volume, it suffers from some disadvantages. For example, flow out of the tank must be interrupted in order to remove the contaminant-laden froth from the tank. This can be disadvantageous when a continuous flow of fluid is desirable. Also, the skim trough extends through the gasification chambers into the outlet chamber, which permits froth to spill into the quiescent outlet chamber and contaminate the effluent. Furthermore, there is no means for removing contaminants that may accumulate at the top of the inlet chamber, and no means for venting gas, which may have been entrained in the influent, which accumulates in the inlet chamber. In addition, there is no means for retaining gas in the vessel when the skim outlet valve is opened, creating a potentially dangerous situation if noxious, lethal or flammable gases are present in the tank. 
   Another problem that is often encountered with the baffles terminating a distance above the tank bottom is that some portion of influent tends to pass under the baffles without being directed to a high turbulence area and contacted by gas bubbles. 
   Additionally, the velocity of flow in the degasification chamber is relatively high, which leads to insufficient final oil/water separation. 
   U.S. Pat. No. 4,782,789 relates to an induced static flotation (ISF) cell having an inlet chamber, a plurality of gasification chambers, and an outlet chamber. Contaminated liquid enters the inlet chamber, passes through the gasification chambers, and exits through the outlet chamber. Gas bubbles are introduced into the bottom of each gasification chamber and attract suspended contaminants and/or oil as they rise. A contaminant-laden froth forms at the top of the cell, and is removed via a first skim trough in the outlet chamber. A liquid level displacement controller maintains the level of fluid in the gasification chambers adjacent and below the top of the first skim trough, the second skim trough being vertically adjustable to account for the difference in specific gravity between the liquid in the outlet chamber and the gasification chambers. A timer pulsing device raises the level periodically to provide additional skimming. Gas is recirculated from the top of the cell for introduction into the bottom of the gasification chambers. 
   Secondary baffles in the gasification chambers prevent a bypass by the liquid and gas of the turbulent area created by the gas flow above the eductor assemblies that deliver the gas into gasification chambers. By using a pair of baffles in the outlet chamber, the fluid retention time is increased to further improve the liquid/contaminants separation. 
   An invention related to an apparatus for the removal of suspended matter from a liquid, such as used for treatment of oil-containing water is disclosed in U.S. Pat. No. 4,986,903. A cylindrical, horizontal vessel is divided into a single gasification chamber by a partition that extends through the interior chamber of the vessel and allows fluid communication between the two chambers. A liquid to be treated is introduced through distribution header(s) adjacent a bottom of the vessel and/or an alternative inlet nozzle which is combined with a gas eductor in order to achieve a more intimate mixture of the gas and liquid. The gas eductor has its outlet slightly above the outlet of the distribution headers. The released gas bubbles carry oil and suspended matter towards the upper portion of the vessel, from which the froth is collected through a primary skim collection trough which extends through the gasification chamber and from a vertically adjustable (based on specific gravity) secondary skim collection funnel in the degasification chamber. Skim collection is accomplished through control of the liquid level in the vessel. 
   U.S. Pat. Nos. 5,011,597 and 5,080,780 also relate to an apparatus for removing suspended matter from liquid. The apparatus has a single cell vertical cylindrical hydraulic flotation vessel that is provided with a separation wall to separate a lower gasification chamber from a middle degasification chamber and an upper gas chamber. A number of alternative arrangements are provided for controlling skim collection through controlling volume of liquid within the vessel and changing the volume of liquid through the use of an adjustable timer which intermittently sends signals to outlet valves of the skim collection outlet or of the treated liquid outlet. The apparatus provides for an alternative arrangement of introducing liquid into the vessel, so as to achieve more intimate mixing of gas and liquid introduced into the vessel. 
   It would be desirable if an apparatus could be devised to overcome some of the problems in the conventional systems for removing suspended matter from a liquid, particularly in systems used on floating offshore hydrocarbon recovery platforms where the action of the waves upon the apparatus tends to cause the suspended matter to contaminate the recovered water. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide an apparatus for removing suspended matter from a liquid, which apparatus is particularly suited to be used on floating offshore hydrocarbon recovery platforms. 
   It is another object of the present invention to provide a two-cell, mechanical, cylindrical gas flotation machine having a reduced footprint, reduced power requirements and reduced capital and operating costs that overcomes the adverse wave effects on floating platforms. 
   In carrying out these and other objects of the invention, there is provided, in one form, an apparatus for removing suspended matter from a liquid, which apparatus includes a vessel for receiving a flow of liquid having suspended matter therein. The vessel has a plurality of partitions sequentially dividing the vessel into an inlet chamber, at least a first gasification chamber and a second gasification chamber, and an outlet chamber, where each adjacent chamber fluidly communicates with one another. The vessel also has a discharge chamber having a fluid communication with the outlet chamber. The apparatus includes an inlet to introduce the flow of liquid into the inlet chamber and an outlet for removing clarified liquid from the discharge chamber. The apparatus includes a mechanism for ingesting and mixing gas into the liquid of each gasification chamber for creating a turbulent area and for attracting the suspended matter and for carrying the suspended matter to an upper portion of the vessel. Also included are a primary skim collection channel extending at least partially along the top of the partition between the first gasification chamber and the second gasification chamber for collecting suspended matter in the upper portion of both gasification chambers; a secondary skim collection channel, independent of the primary channel, which is located in the upper portion of the inlet chamber; and a tertiary skim collection channel, independent of the primary and secondary channels, located in the upper portion of the discharge chamber. Finally the invention optionally includes at least one baffle near the primary skim collection channel to dampen motion of the liquid caused by movement of the vessel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The single FIGURE is a schematic, cross-sectional illustration of one embodiment of the dual-cell mechanical flotation system of the invention. 
   

   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 
   The present invention will now be described, by way of example, and not limitation, with the influent being water contaminated with oil and other suspended particulates. It is to be understood that the present invention has utility in numerous applications in which it is desirable to separate suspended matter and/or oil from a liquid, and that the suspended matter, the liquid, or both may be the desired product of the process. 
   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 suspended matter mixed therewith, where the vessel  12  in a preferred embodiment has a continuous cylindrical sidewall and is capable of withstanding substantial internal pressures as may be encountered when processing produced water from an oil well. Vessel  12  is divided into an inlet chamber  16 , at least a first gasification chamber  18 , a second gasification chamber  20 , and an outlet chamber  22 , where each adjacent chamber can fluidly communicate with one another, that is, that a fluid in one chamber may flow into an adjacent chamber. Vessel  12  also has a discharge chamber  24  that is in fluid communication  26  with outlet chamber  22 , the fluid communication  26  generally being a pipe having a valve  28  therein. The chambers  16 ,  18 ,  20 ,  22  and  24  are divided by a plurality of generally vertical partitions  42 ,  44 ,  46 , and  48 , respectively. Partitions  42  and  46  extend from the top of vessel  12  downward, and are spaced from the bottom of vessel  12  to allow fluid communication between the adjacent chambers. Partition  44  dividing first and second gasification chambers  18  and  20 , respectively, besides being spaced from the bottom of vessel  12  is also spaced from the top thereof to allow gas communication between these chambers as well; partition  44  being supported by the sides of vessel  12 . Partition  48  dividing outlet chamber  22  and discharge chamber  24  extends completely from the top of vessel  12  to the bottom sealing the entire circumference at that point, except for fluid communication  26 . The lengths of partitions  42 ,  44  and  46  are calculated to minimize the effect of pressure differential due to difference in flow rates under each respective partition. Fluid communication  26  could be any other suitable passageway having a flow control device regulating flow therethrough. 
   Inlet chamber  16  has an inlet  30  to introduce the flow of liquid  14  to the inlet chamber  16 . Each gasification chamber  18  and  20  has at least one mechanism  32  for ingesting and mixing gas into the liquid of each respective gasification chamber  18 ,  20  for creating a turbulent area where the gas attracts the suspended matter and carries the suspended matter to an upper portion of the vessel  12  for each respective chamber  18 ,  20 . Gas ingesting and mixing mechanisms  32 , in one non-limiting embodiment, 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 , such as described in U.S. Pat. No. 3,993,563, may each include one or more gas draft tubes to transfer gas into the rotor assembly of mechanism  32  from the vapor space in the upper portion of vessel  12 . Gas ingesting and mixing mechanisms  32  may also include water draft tubes 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  36  therein. Flow through vessel  12  is maintained via pumps or innate system pressure (not shown). 
   At the top of partition  44  is a primary skim collection channel  40  extending at least partially along the length of the top of partition  44 , between first gasification chamber  18  and second gasification chamber  20  for collecting suspended matter in the upper portions of both gasification chambers  18 ,  20 . Skim collection channel  40 , oriented normal to the plane of the FIGURE is generally a trough, and is illustrated as having a V-shaped cross-section in the FIGURE, although other trough contours and designs (e.g. U-shaped, etc.) would be acceptable. Froth level  50  is designed to be right at upper edge  52  of channel  40 . Primary skim collection channel  40  receives contaminant-laden froth produced in the gasification chambers  18  and  20 , and such froth is removed from the channel  40  through a primary skim outlet  54 . There may be a skim outlet  54  on either or both sides of channel  40  in the vessel  12  side walls. The channel  40  may be tapered on the bottom (e.g. V- or narrow U-shaped) to permit a better removal of skimmings from the channel  40 . The use of a common manifold or channel  40  for both gasification chambers  18  and  20  increases the efficiency of the apparatus  10  and reduces the capital cost of thereof. An open space  56  is provided above primary skim collection channel  40  to allow gaseous communication between each of the gasification chambers  18  and  20 . 
   There is provided at least one baffle  60  near primary skim collection channel  40  to dampen the motion of the liquid  14  caused by movement of the vessel  12 , such as may be due to the motion of waves against the floating hydrocarbon recovery platform (not shown). Baffles  60  have a lowermost or distal edge  62 . Vessel  12  may be considered to have a horizontal plane, which may be parallel to the top and/or bottom of the vessel  12  as illustrated in the FIG.. It has been discovered that if the angle A of a line between the upper edge  52  of channel  40  and the distal edge  62  of baffle  60  with respect to the horizontal plane of the vessel  12  is between about 5 and 15° that the motion of the liquid  14  will be dampened while permitting the oil and froth to flow over into channel  40 . In a preferred, nonlimiting embodiment of the invention, this angle A is about 10°. 
   A secondary skim collection channel or bucket  64  having a closed bottom, closed sides and an open top is located in an upper portion of inlet chamber  16 , in one non-limiting embodiment on partition or wall  42 . The secondary skim collection channel  64  is positioned to collect froth at a level  66 , which may be, and is preferably, below level  50  in gasification chambers  18  and  20 . Secondary skim collection channel  64  may also be tapered on the bottom and may have a V-shaped or U-shaped cross-section, as non-limiting embodiments. Secondary skim collection channel  64  may also remove froth through a secondary skim outlet  68 . There may be at least one consolidating collection channel (a trough, pipe or the like), such as on the outside of vessel  12 , in communication with both primary skim outlet  54  and secondary skim outlet  68  for delivering the collected suspended matter away from the vessel  12 . 
   A tertiary skim collection channel  70 , independent of both primary skim collection channel  40  and secondary skim collection channel  64 , is located in the upper portion of discharge chamber  24 . Tertiary skim collection channel  70  may be a threaded pipe centrally located in chamber  24 . Such a channel may be vertically adjustable to account for the difference in specific gravity between the liquid in the discharge chamber  24  and the gasification chambers  18  and  20 . Further details on the induced static flotation collection channel may be seen in U.S. Pat. No. 4,782,789, incorporated by reference herein. Tertiary skim collection channel  70  may be in communication with the consolidating collection channel which collects froth from primary skim outlet  54  and secondary skim outlet  68 , or may have its own collection channel or pipe for delivering suspended matter away from vessel  12 . 
   There may also be present a control mechanism, such as a programmable logic controller (PLC)  72  for controlling the liquid level in the first and second gasification chambers  18 ,  20 , by obtaining level information from level transmitter (LT)  74  and regulating flow through level control valve (LCV)  28  which is in fluid communication  26  between outlet chamber  22  and discharge chamber  24 . Level transmitter  74  may, in one non-limiting embodiment, have its sensor positioned in the opening beneath partition  44 . 
   Similarly, the control of fluid level  76  in discharge chamber or box  24  is accomplished through PLC  72  by obtaining level information from LT  78  and regulating flow through LCV  36  in outlet  34 . The exact nature of the level transmitters  74  and  78 , PLC  72  and LCVs  28  and  36  is not critical and may be conventional in the art; however, their implementation in the dual-cell mechanical flotation system of the invention is expected to be inventive. 
   In one embodiment of the invention, the mechanical flotation system  10  has a dual-cell design, that is, only two gasification cells,  18  and  20  as illustrated in the FIGURE. Previously, in a system with four cells, the retention time in each cell or chamber is about 1 minute, and efficiencies of 95% may be obtained. In the present invention, using only two gasification chambers or cells and a residence time in each chamber of about 2.0 to 2.5 minutes, the efficiency achieved of about 92% is almost as good, but at much less power utilization. That is, with the inventive apparatus, while efficiency is slightly reduced, the horsepower requirements are about half that of a conventional system (since only two gas ingesting/mixing mechanisms are required instead of four), and the “footprint” or area consumed by the apparatus is reduced by nearly half—an important consideration on an offshore oil platform where space is at a premium. 
   An optional chemical feed unit (not shown), which is a standard feed unit for dispensing a metered amount of a flocculant chemical, using a polymer or a demulsifier, into fluid  14 , to initially treat the influent for achieving optimum separation of contaminants from the water can be provided. 
   Although not shown, valves may be provided for blowdown of sludge that collects in the bottom of vessel  12 . Also not shown are optional gauges to monitor the pressure of the effluent and the flow of gas. 
   In the method of the invention, a continuous flow of liquid  14  having suspended matter mixed therewith is introduced into inlet chamber  16  through inlet  30 . Some separation of the suspended matter occurs in inlet chamber  16  by floating to the upper portion of inlet chamber  16  and floating froth level  66 . This froth or suspended matter is collected in secondary skim collection chamber  64  and passed through secondary skim outlet  68  to a consolidating collection channel away from vessel  12 . 
   Fluid  14 , still containing considerable suspended matter, underflows partition  42  into first and second gasification chambers  18  and  20  where a flow of gas is introduced into the liquid  14  in the chambers by gas ingesting and mixing mechanisms  32 , creating a turbulent area in the entirety of chambers  18  and  20 , and allowing the gas to attract the suspended matter and carry it to the upper portion of vessel  12  where it floats at level  50 . This suspended matter is collected in primary skim collection channel  40  that extends at least partially along the top of partition  44  between chambers  18  and  20 . The suspended matter is delivered through primary skim outlet  54  to the collection channel, which may be the same or different consolidating collection channel serving secondary skim outlet  68 . Level  50  is generally expected to be somewhat higher than level  66 . At least one baffle  60  near the primary skim collection channel  40  dampens the motion of the fluid  14  that occurs when vessel is rocked or moved in response to waves hitting the hydrocarbon production platform. 
   Fluid  14 , largely free of suspended matter, next underflows partition  46  and flows into outlet chamber  22 . Passage of the fluid  14  from outlet chamber  22  through pipe or fluid communication  26  is regulated by level control valve  28  in response to signals from PLC  72  according to a software program therein using information from level transmitter  74 . 
   The fluid  14  in discharge chamber  24  is relieved of the remaining suspended matter by floating such matter to level  76  where it is collected by tertiary skim collection channel  70 . Channel  70  may be of an ISF-type that is vertically adjustable to account for the difference in specific gravity between the liquid  14  in discharge chamber  24  and the liquid  14  in the gasification chambers  18  and  20 . The rate at which clarified liquid is removed from discharge chamber  24  is regulated by LCV  36  in outlet  34  in response to software program commands in the PLC  72  employing data obtained from LT  78 . 
   To summarize, advantages of the invention include, but are not necessarily limited to a decreased “footprint” (decreased space requirements), reduced power requirements, reduced capital and operating costs, and improved tolerance to platform wave motion effects. These advantages are achieved through a two-cell, mechanical, cylindrical gas flotation machine with a special baffling design to minimize surface wave action. Dual level control in some embodiments will ensure proper rotor submergence of the gas ingesting mechanism and provide a stable level at the froth surface to allow controlled skimming and enhanced performance. Efficiency is further increased by using a common skimming collection manifold or channel between the two gasification chambers. Use of draft tubes in connection with the gas ingesting and mixing mechanisms minimize short-circuiting over a wide range of capacities. 
   In the foregoing specification, the invention has been described with reference to specific embodiments thereof, and has been demonstrated as effective in providing a mechanical flotation system for removing suspended matter from liquids. 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.