Abstract:
A method and apparatus for separating mixtures of fluids having different densities. According to one aspect of the invention, a lower density fluid is separated from a higher density fluid. A mixture of the two fluids is received in a tank. A centrifugation of the mixture is induced in the tank. The lower density fluid is evacuated from the tank until the level of an interface created at least in part by the centrifugation between the higher density fluid and the lower density fluid reaches a predetermined threshold.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for separating mixtures of fluids having different densities or specific gravities, such as mixtures of oil and water. 
     BACKGROUND 
     There is often a need to separate or retrieve fluids out of fluid mixtures. An important example is cleaning water contaminated by an oil spill. Typically, the fluids have different densities, and so it is common to separate the fluids by centrifugation. 
     In a centrifugal separator, the mixture is caused to swirl in a tank, referred to herein as “vortex flow.” The heavier, higher density fluid is thrust outwardly against the sides of the tank with a greater force than the lighter, lower density fluid. The higher density fluid thereby accumulates at the periphery of the tank, while the lower density fluid accumulates at the center of the tank. 
     U.S. Pat. No. 3,743,095 provides a number of design considerations for centrifugal separators. The disclosed separator is oriented horizontally. Both the water outlet and the oil outlet are centrally located on the cylindrical axis, one outlet at one end of the tank and the other outlet at the other end of the tank. It is important to maintain the oil within a narrow central region of the tank by closely controlling the flow rate of the separated oil output from the device to ensure that it is equal to the flow rate of incoming oil in the oil/water mixture input to the device. This is due to the means used to ensure that water leaving the tank is not contaminated by the separated oil. Particularly, to prevent oil from re-mixing with the out-going water, a blocking disc is provided immediately in front of the water outlet, the blocking disc having a diameter that is less than the diameter of the water outlet but greater than or equal to the diameter of the cylindrical region in which the separated oil is contained. 
     As it is difficult to ensure that the flow rates of oil entering and leaving the tank are always equal, especially since the amount of oil entering the tank is not easily measured, the reference proposes recirculating the separated water back through the separator to improve separation. The criticality of controlling the flow of oil removed from the separator, and the requirement, to improve separation, to repeat the separation process by recirculating the separated water back through the separator, are distinct disadvantages of the system. 
     U.S. Pat. No. 2,180,811 proposes a centrifugal separator that is oriented vertically to take advantage of the force of gravity in providing vertical settling of the separated fluids. A lower outlet is provided at the bottom of the tank for removing the higher density fluid and an upper outlet is provided at the top of the tank for removing the lower density fluid. A valve is provided in the path of the outflow of higher density fluid from the lower outlet. The valve automatically opens to allow removal of the higher density fluid as the interface between the higher density fluid and the lower density fluid rises above a threshold; otherwise, the valve remains closed to ensure that the level of the higher density fluid will not fall below the prescribed threshold. 
     The &#39;811 patent fails to explain why this control is believed to be advantageous. In any event, the valve cannot ensure that higher density fluid will not back up into the outlet provided for removing the lower density fluid. This could be a problem in situations where the lower density fluid must be stored. For example, when cleaning an oil spill in the open sea, the water may simply be pumped back into the sea but the oil must be stored on-board the ship where storage space and capacity is at a premium. The valve mechanism of the &#39;811 patent is also undesirably complex. 
     Therefore, as the present inventor has recognized, there is a need for a method and apparatus for separating mixtures of fluids having different densities that provides for more simple, efficient, and effective separation than has been provided in the prior art. 
     SUMMARY 
     A method and apparatus for separating mixtures of fluids having different densities is disclosed. According to one aspect of the invention, a lower density fluid is separated from a higher density fluid. A mixture of the two fluids is received in a tank. A centrifugation of the mixture is induced in the tank. The lower density fluid is evacuated from the tank until the level of an interface created at least in part by the centrifugation between the higher density fluid and the lower density fluid reaches a predetermined threshold. 
     It is to be understood that this summary is provided as a means of generally determining what follows in the drawings and detailed description and is not intended to limit the scope of the invention. Objects, features and advantages of the invention will be readily understood upon consideration of the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a pictorial schematic view of an apparatus for separating fluids having different densities according to the present invention. 
         FIG. 2  is a plan view, looking down an axis “L” shown in  FIG. 1 , of an inlet plenum according to the present invention and flow lines created thereby. 
         FIG. 3  is a plan view, looking down the axis “L” as in  FIG. 2 , of an outlet plenum according to the present invention. 
         FIG. 4  is a schematic side elevation of the tank of  FIG. 1  showing flows for an oil and water mixture. 
         FIG. 5A  is a schematic side elevation of the tank of  FIG. 1  showing a valve for removing lower density fluid according to the invention in an open configuration. 
         FIG. 5B  is a schematic side elevation of the tank of  FIG. 1  showing the valve of  FIG. 5A  in a closed configuration. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an apparatus  10  for separating fluids having different densities according to the present invention. A preferred use of the apparatus  10  is to separate liquid fluids, but the principles of operation of the apparatus may be applied to gaseous fluids as well, and fluids containing solid particles, and the apparatus may be used for separating such fluids for any purpose. However, the apparatus  10  is particularly adapted for separating oil and water, and therefore finds particular use in cleaning water into which oil has been spilled. 
     A tank  12  is provided in which a fluid mixture  14  is introduced through a mixture inlet “A.” The inlet “A” receives the mixture from an external source (not shown), such as a pump. 
     A vortex flow of the fluid mixture introduced into the tank is created as described below about a central axis “L” of the tank. Accordingly, the tank is radially symmetric about the axis “L,” such as by being cylindrical as shown. While a cylindrical form of the tank  12  is believed to be preferable, it should be understood that other radially symmetric shapes conducive to the creation and maintenance of a vortex flow may be employed, including that shown in the aforementioned &#39;811 patent. In any case, the vortex flow produces centrifugation of the mixture  14 . 
     An inlet plenum  16  is provided in communication with the inlet “A.” Referring in addition to  FIG. 2  (looking down the axis “L”) the inlet plenum includes a plurality of nozzles  18   a  that are tangentially disposed with respect to the cylindrical inner wall  12   a  of the tank  12 . The nozzles direct the mixture tangentially against the inner wall  12   a , thereby imparting a circular or swirling movement of the mixture in the tank (flow lines “f w ” in  FIG. 2 .) 
     Centrifugation occurs because the circular movement of the mixture produces a radially outwardly directed acceleration (e.g., vector “r”) therein, pressing the mixture against the inner wall  12   a . Denser and therefore more massive elements of the fluid will experience a greater such force than lighter elements. Thence, the denser elements will crowd out the lighter elements in the vicinity of the inner wall  12   a , forcing the lighter elements to accumulate in a central region  20  of the tank nearest the axis “L.” 
     It may be noted that the manner in which the fluids are acted upon by centrifugal force depends on whether the fluids are miscible. Where two fluids are immiscible, such as water and oil, the molecules of each tend to adhere together. The force acting on the fluids as a result of centrifugation acts on the fluids in their aggregate states. Water is denser in its aggregate state than oil, and is therefore the higher density fluid for purposes of centrifugation. On the other hand, where two fluids are miscible, such as water and alcohol, the force acting on the fluids as a result of centrifugation acts on the individual molecules of the fluids as though they were particles. 
     The present inventor has recognized that it is desirable to orient the nozzles  18   a  such that a counterclockwise vortex is created in the tank  12  operated in the northern hemisphere, or alternatively so that a clockwise vortex is created for use in the southern hemisphere, to take advantage of the Coriolis force that results from rotation of the earth. This is not essential, however. 
     An outlet “B” is provided at a lower elevation in the tank than that of the inlet “A” for exhausting the tank of higher density fluid  24 . With additional reference to  FIG. 3  (looking down the axis “L” in the same direction as in  FIG. 2 ), to further facilitate the creation of a vortex flow, an outlet plenum  22  is provided in communication with the outlet “B” that is a substantially mirror image of the inlet plenum  16 , having nozzles  18   b  oriented opposite to the orientation of the nozzles  18   a  of the inlet plenum. This assists to maintain the vortex induced by the inlet plenum (flow lines “f w ” in  FIG. 3 ). Experimentally, it has been determined that the flow into the nozzles  18   b  appears substantially as shown in  FIG. 3 . 
     With additional reference to  FIG. 4 , the tank includes a bottom portion or stand  29  adapted to stand the tank in a stable position on a horizontal surface. In this stable position, the axis “L” of the tank  12  is oriented vertically to permit the influence of gravity to facilitate separation. Particularly, in addition to being induced by centrifugation to accumulate in the central region  20  of the tank, the lower density fluid also tends to rise toward an outlet “C” for exhausting the lower density fluid from the tank as a result of being displaced by higher density fluid sinking to the bottom. 
     A valve  28  is provided according to the invention for controlling outflow of the lower density fluid, to ensure that only the lower density fluid and not the higher density fluid flows through the outlet “C.” Operation of the valve  28  is described below. 
     To ensure that fluid reaches the level of the valve  28 , a pressure relief valve “V” may be provided in the path of outflow from the outlet “B.” The pressure relief valve is pre-set to open when enough fluid has accumulated in the tank to reach the level of the valve  28 . Alternatively, this function could be accomplished by controlling the rate of inflow at “A” relative to the rate of outflow at “B.” 
     Experimentally, the present inventor has determined that a fluid mixture of oil and water introduced into the tank  12  through the inlet “A” appears substantially as shown in  FIG. 4  (flow lines “f w ”), with the water (referenced as “water”) exhibiting a vortex pattern represented by flows in the direction of the arrows and the oil (referenced as “oil”) accumulating at the top of the tank in a concave-curved band. At the very top of the tank  12 , proximate the central axis “L,” a pocket of air (referenced as “air”) assumes the shape shown. The air is compressible, and its presence permits adjustment of the pressure at which the valve “V” opens. 
     The valve  28  controlling outflow at “C” is preferably a hollow tube  30  having a closed end  27  and a through-going inlet hole  32  in a side-wall of the tube. Fluid can therefore enter the outlet “C” only through the hole  32 . However, it will be readily appreciated by persons of ordinary mechanical skill that the valve  28  could be constructed in many different ways to achieve the same result. 
     Coaxially and slidably disposed on the tube is a plug  34 . The plug is adapted to cover or uncover the inlet hole  32  in response to changes in the level of fluid. Particularly, the plug has a density that is less than the density of the higher density fluid and greater than the density of the lower density fluid. Accordingly, the plug will float on the higher density fluid but will sink in the lower density fluid. 
     By opening only in the presence of low density fluid, the valve  28  provides that the fluid retrieved from the outlet “C” is substantially pure lower density fluid, uncontaminated with higher density fluid. Among the advantages provided by the valve  28 , the fluid retrieved from the outlet “C” can be stored in as compact a space as is possible. 
       FIG. 5A  shows in simplified form the tank  12  with an interface IF 1  between a high density fluid  24  and a low density fluid  26 , along with an air pocket “AP” at the top of the tank. The valve  28  is in an open configuration in which the valve passes a flow “f” through the hole  32  and the outlet “C.” The plug sinks in the lower density fluid  26  toward the interface “IF 1 .”Therefore, the interface “IF 1 ” lies below a level at which the plug  34  will cover the inlet hole  32  and block the flow. Stops  36   a  are provided on the tube  30  to keep the plug  34  from falling off the tube  30 . 
     Referring to  FIG. 5B , the valve  28  is shown in a closed configuration. A new interface “IF 2 ” lies at the higher elevation of the hole, so that higher density fluid could flow into the inlet hole  32 , except that, as the plug  34  also floats at this level it moves with the interface to close off the hole  32 . Accordingly, for the interface level at “IF j ,” flow through the outlet “C” cannot occur, which prevents higher density fluid from escaping the tank through the outlet provided for the evacuation of the lower density fluid. Stops  36   b  are provided on the tube  30  to ensure that, should the level of the interface IF 2  rise above the level shown, the plug  34  will not rise above the level of the inlet hole  32 . 
     It may be noted that the interface level at which the valve  28  becomes closed can be adjusted by adjusting the length of the tube  30  and the elevation of the inlet hole  32 . 
     It is to be understood that, while a specific method and apparatus for separating fluids having different densities has been shown and described as preferred, other configurations and methods could be utilized, in addition to those already mentioned, without departing from the principles of the invention. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions to exclude equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.