Abstract:
A device for separating constituents of a fluid mixture includes an elongate vessel oriented at an acute angle to horizontal. The vessel is operable to receive the fluid mixture and direct the fluid mixture to flow in a convection cell spanning substantially a length of the vessel. The convection cell is formed by gravitational forces acting on the fluid mixture and is operable to deposit a heavy constituent of the fluid mixture about a lower end of the vessel and a light constituent of the fluid mixture about an upper end of the vessel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of pending application Ser. No. 10/881,223 filed Jun. 30, 2004, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     This disclosure relates to separating constituents of a fluid mixture, and more particularly to systems and methods for separating constituents of a fluid mixture having disparate densities. 
     In many industries, there is a need to separate a fluid mixture into one or more of its constituents. For example, in producing hydrocarbons from a well, water and particulate solids, such as sand, are produced together with the hydrocarbons. It is not desirous to have either of these byproducts present in the hydrocarbons. Therefore, well operators have implemented numerous techniques to separate the water and sand from the produced hydrocarbons. 
     One conventional technique for removing sand from the hydrocarbons is to install sand screens in the production pipe inside the well bore. A sand screen is screen including one or more layers of mesh sized to prevent passage of sand into an interior of the screen. Sand screens have been used successfully for many years; however, like any filter, they are subject to clogging and plugging, for example, as the screen&#39;s mesh fills with sand and other particulate. 
     In the past, water has been filtered from the produced hydrocarbons or separated in a free-water knockout separator. Filters, like sand screens, are prone to clogging and plugging. Free-water knockout separators are large vessels that separate the water and hydrocarbons by allowing the water to settle vertically downward and out of the hydrocarbons. The separated water is subsequently withdrawn from the bottom of the vessel. Free-water knockout separators are generally slow at separating the water from hydrocarbons, because they rely on the water settling vertically downward and out of the hydrocarbons. 
     Accordingly, there is a need for improved systems and methods of separating constituents of a fluid mixture. 
     SUMMARY 
     The present disclosure is directed to systems, devices and methods for separating constituents of a fluid mixture. 
     One illustrative implementation encompasses a device for separating constituents of a fluid mixture. The device includes an elongate vessel oriented at an acute angle to horizontal. The vessel is operable to receive the fluid mixture and direct the fluid mixture to flow in a convection cell spanning substantially a length of the vessel. The convection cell is formed by gravitational forces acting on the fluid mixture and is operable to deposit a heavy constituent of the fluid mixture about a lower end of the vessel and a light constituent of the fluid mixture about an upper end of the vessel. 
     In some implementations, the device includes a second elongate vessel oriented at an acute angle to horizontal. The second vessel is operable to receive a fluid mixture and direct the fluid mixture to flow in a convection cell spanning substantially a length of the second vessel. The convection cell is formed by gravitational forces acting on the fluid mixture and is operable to deposit a heavy constituent of the fluid mixture about a lower end of the second vessel. The fluid mixture received by the second vessel can include either or both of a fluid mixture output from the first mentioned vessel or the fluid mixture provided to the first mentioned vessel can be split between the first mentioned vessel and the second vessel. The first and second elongate vessels can be nested to reduce the space required for the device. 
     Some implementations can incorporate a filter residing at least partially between the inlet and an outlet near the upper end or between the inlet and an outlet near the lower end. More than one filter can be provided, for example one between the inlet and the outlet near the upper end and one between the inlet and the outlet near the lower end. The convection cell can operate to separate at least a portion of a constituent from the fluid mixture prior to passage of the fluid mixture through the filter. In some instances the filter can include a membrane or a prepacked screen. A bypass can be provided to selectively allow fluid to bypass the filter. 
     Another illustrative implementation encompasses a fluid separator. The fluid separator includes an elongate receptacle having an inlet operable to receive a fluid mixture. The receptacle is oriented at an angle to horizontal such that gravitational force causes a portion of the fluid mixture to settle to a lower sidewall of the receptacle. That portion of the fluid flows along the lower sidewall to a lower end wall of the receptacle and turns at the lower sidewall to flow along an upper sidewall of the receptacle toward an upper end of the receptacle. The flow along the lower sidewall has a larger amount of a heavy constituent of the fluid mixture than the flow along the upper sidewall. 
     Yet another illustrative implementation encompasses a method of separating constituents of a fluid mixture. In the method the fluid mixture is received in an elongate receptacle oriented at an acute angle to horizontal such that gravitational force causes a portion of the fluid mixture to settle to a lower sidewall of the elongate receptacle. That portion flows along the lower sidewall to a lower end wall of the elongate receptacle and turns at the lower end wall to flow along an upper sidewall of the elongate receptacle towards an upper end of the receptacle. The flow along the lower sidewall has a larger amount of heavy constituent of the fluid mixture than the flow along the upper sidewall. 
     An advantage of some implementations is that efficient separation of fluid mixture constituents can be achieved without additional energy input. Gravitational forces can be the primary driver for separation; and therefore there are no operational costs associated with energy input. However, because of the convective flow separation, the implementations perform separation more quickly than traditional separators relying primarily on constituents settling vertically out of the fluid mixture. The convective flow separation also does not require high velocity fluid flow often required by other traditional separators (like cyclonic separators) which often cause formation of inseparable emulsions. 
     Another advantage of some implementations is that multiple separation vessels can be used in parallel to increase separation capacity. Multiple separation vessels can be used in series to separate multiple constituents of a fluid mixture. The multiple separation vessels can be nested in a space efficient manner. 
     Another advantage of some implementations is that one or more separation vessels can be used in conjunction with a filter to reduce the filtering load the filter must bear. Such reduced filtering load increases the life of the filter and reduces clogging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side cross-sectional view of an illustrative separator constructed in accordance with the invention; 
         FIG. 2A  is a schematic side view of a plurality of illustrative separators constructed in accordance with the invention and arranged in a nested configuration; 
         FIG. 2B  is a schematic side view of the illustrative separators of  FIG. 2A  configured in series; 
         FIG. 2C  is a schematic side view of the illustrative separators of  FIG. 2A  configured in parallel; 
         FIG. 3A  is a schematic side view of another illustrative separator constructed in accordance with the invention; 
         FIG. 3B  is a schematic side view of the illustrative separator of  FIG. 3A  depicted in an illustrative sub-surface application in accordance with the invention; 
         FIG. 4  is a schematic side cross-sectional view of another illustrative separator constructed in accordance with the invention incorporating a filter; and 
         FIG. 5  is a schematic side cross-sectional view of another illustrative separator incorporating a filter constructed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , an illustrative separator  100  constructed in accordance with the invention includes an elongate vessel  110  oriented with its longitudinal axis at an acute angle θ relative to horizontal. As is discussed in more detail below, the angle θ may be different in different applications. In one instance, angle θ is between about 30° and about 70°. Additionally, as is discussed in more detail below, the length of the vessel  110  is greater than a transverse dimension, for example diameter, of the vessel  110 . In one instance, the length of the vessel  110  may be greater than twice or three times the transverse dimension (e.g. diameter). In one instance, the aspect ratio of the vessel  110  is 2:1 or greater. 
     The vessel  110  can include one or more inlet ports  112  through which a fluid mixture for separation is introduced. The vessel  110  can include one or more outlet ports  114  through which the separated constituent fluids and particulate can be withdrawn. The inlet port  112  and outlet port  114  can be in various different locations. For example, the separator  100  of  FIG. 1  includes a light constituent outlet port  114   a  about an upper end of the vessel  110  and a heavy constituent outlet port  114   b  about a lower end of the vessel  110 . In another instance, the inlet port  112  can be near the bottom of the vessel  110  and the one or more outlet ports  114  can be above the inlet port  112 . In yet another instance, the inlet port  112  can be near the top of the vessel  110  and the one or more outlet ports  114  below the inlet port  112 . 
     Although depicted in  FIG. 1  as exiting a lateral wall of the vessel  110 , the outlet ports  114  may exit the vessel  110  elsewhere. For example, in illustrative separator  200  of  FIG. 2A , the outlet ports  114  exit the end walls of the elongate vessel  210 . Referring back to  FIG. 1 , the inlet port  112  is located intermediate the outlet ports  114 . Although depicted substantially equidistant between the outlet ports  114 , the inlet port  112  may be positioned closer to one or the other ends of the vessel  110 . 
     The illustrative separators described herein are operable in separating one or more constituents of disparate density from a fluid mixture. The fluid mixture can be a mixture of one or more immiscible fluids, as well as a mixture of one or more fluids and solids (e.g. particulate). The constituents of disparate density are referred to herein for convenience of reference as a light constituent and a heavy constituent of the fluid mixture. In one instance, for example in an oilfield application, the separators may be used in separating a fluid mixture of oil and water, where the heavy constituent is water and the light constituent is oil. The separators may be used in separating particulate such as formation fines (e.g. sand) and fracturing proppant from one or more liquids (e.g. oil and water). In use separating particulate from oil and/or water, the heavy constituent is particulate and the light constituent is the oil and/or water. There are many other mixtures of immiscible fluids and mixtures of fluids and solids to which the concepts described herein are applicable. For example, in another instance, such as a beverage manufacturing application, the separators can be used in separating a fluid mixture including orange juice (light constituent) and orange pulp (heavy constituent). Some other examples can include milk and particulate, paint and particulate, and lubrication oil and contaminates. 
     In operation, the fluid mixture is input through the inlet port  112  into the interior of the vessel  110 . By force of gravity, the heavy constituents  115  of the fluid mixture begin to sink substantially vertically downward (substantially parallel to the gravity vector) and collect about lower sidewall  116  of the vessel  110 . This sinking or vertically downward flow of heavy constituents  115  occurs substantially throughout the length of the vessel  110 . The collecting heavy constituents  115  about the lower sidewall  116  creates a hydrostatic pressure imbalance between the fluid mixture about upper sidewall  118  of the vessel  110  and the fluid mixture about the lower sidewall  116 , because of the density differential of the fluid mixtures. As a result, the fluid mixture about the lower sidewall  116 , containing a larger portion of heavy constituents  115 , begins to travel downward along the lower sidewall  116  and substantially parallel to the longitudinal axis of the vessel  110 . The fluid mixture about the upper sidewall  118 , containing a smaller portion of heavy constituents  115 , correspondingly begins to travel upward along the upper sidewall  118  and substantially parallel to the longitudinal axis of the vessel  110 . The result is a convection cell  120  that spans between upper end  122  and lower end  124  of the vessel  110 ; the convection cell  120  defined by fluid flowing down the lower sidewall  116 , turning at the lower end  124  of the vessel  110 , flowing up the upper sidewall  118  and turning at the upper end  122  of the vessel  110 . In addition to the convection cell  120 , the substantially vertically downward flow of heavy constituents  115  continues substantially throughout the vessel  110 . 
     As the fluid mixture containing a larger portion of heavy constituents  115  turns at the lower end  124  of the vessel  110  to flow back upward along the upper sidewall  118 , it deposits a portion of the heavy constituents  115  at the lower end  124  of the vessel  110 . Therefore, the fluid flowing from the lower end  124 , back up the upper sidewall  118  has a reduced portion of heavy constituents  115 . The amount of heavy constituents  115  in the flow flowing up from the lower end  124  further decreases as the flow continues back up the upper sidewall  118 , because the heavy constituents  115  continue to sink vertically downward (vertically downward flow of heavy constituents  115 ) and join the flow along the lower sidewall  116 . The vertically downward flow of heavy constituents  115  continues, and continues to join the flow along the lower sidewall  116  as the flow continues upward to the upper end  122 . No undulations or protrusions are needed on the interior surface of the vessel  110  to turn or otherwise disturb the fluid flow to effect the constituent separation. 
     The convection cell  120  and the vertically downward flow of heavy constituent  115  operate continuously while fluid is introduced through the inlet port  112 . Therefore, the heavy constituents  115  are separated toward the lower end  124  and the light constituents toward the upper end  122 . The heavy constituents  115  can be withdrawn through the heavy constituent outlet port  114   b  near the lower end  124  of the vessel  110 . Likewise, the light constituents can be withdrawn through the light constituent outlet port  114   a  near the upper end  122  of the vessel  110 . 
     It has been found that an angle of inclination (θ) between about 40-60 degrees produces efficient operation, although other angles also work. Steeper angles are less conducive to convective action, but may still be operable. Shallower angles, likewise may still be operable, but generally need longer sidewalls  116 . Putting the increased size of the vessel  110  aside, longer sidewalls  116  also mean more friction; thus reducing effectiveness of the separation. 
     Because the fluid circulates within the convection cell  120 , the separator  100  can separate the constituents of a fluid mixture faster than the heavy constituent  115  can settle vertically downward and out of the light constituent. Furthermore, no energy needs to be input into the system to effect the separation other than the force of gravity. Conventional separators relying solely on the heavy constituents settling vertically downward and out of the light constituents are limited by the terminal velocity of the heavy constituent in the fluid mixture. Once the heavy constituent reaches its terminal downward velocity, the separation cannot occur any faster. The convection cell  120  formed by the separator  100 , however, carries the heavy constituent  115  towards the lower end  124  of the vessel  110  at a rate that is faster than the terminal velocity of the heavy constituent  115 . Therefore, the heavy constituent  115  is transported to the lower end  124  and separated from the light constituent at a higher rate. 
     A long, narrow vessel  110  is more efficient at forming a convection cell  120  than a short, wide vessel. The efficiency of a long, narrow vessel  110  stems from the pressure in the axis of the downward flow along the lower sidewall  116  being greater than the pressure in the axis of the vertically downward flow of heavy constituent  115  at the point where the flow along the lower sidewall  116  turns to flow upward. At the lower end  124  of the vessel  110 , the downward flow along the lower sidewall  116  turns and flows against the vertically downward flow of heavy constituent  115 . To form a convection cell  120 , the upward flow from the lower sidewall  116  must overpower the vertically downward flow of heavy constituents  115 . As a transverse dimension of the vessel  110  decreases, the hydrostatic pressure differential in the axis of the vertically downward flow of heavy constituents  115  is reduced. Likewise as the length of the vessel  110  increases, the hydrostatic pressure differential in axis of the downward flow along the lower sidewall  116  increases. Therefore, as the ratio of length to width increases, so does the ability of the upward flow from the lower sidewall  116  to overpower the vertically downward flow of the heavy constituent  115 . Likewise, as the length increases, the fluid velocity gets higher. This increases friction between the fluid and the walls, and also between the two opposing fluids. Therefore, increases in length, beyond a certain length may not increase the speed of separation. However, increasing the length further would increase the quality or purity of the separation as separation continues throughout the length of the vessel 
     The separator  100  can be configured to be free-standing or linked to other equipment for above-ground or on-seafloor installations. Alternately, the separator  100  can be buried below the Earth&#39;s surface. Locating the separator  100  below the Earth&#39;s surface not only preserves the surface for other uses, but protects the separator  100  from potential damage that may occur when on the surface. Additionally the separator  100  may be placed inside of a well bore, or located adjacent one or more wells for use in separating a fluid mixture associated with the wells. As an alternative to burying the separator  100 , an equivalent structure to one or more of the vessel  110 , inlet port  112 , and/or outlet ports  114  can be bored into the Earth and used as a separator. Other configurations of separators described herein may also be buried below the Earth&#39;s surface or constructed with equivalent structures bored into the Earth. 
       FIG. 2A  shows a space efficient manner of co-locating two or more separators  200 . As is shown in the figure, the separators  200  are substantially linear, and therefore can be placed closely adjacent one another in a nested arrangement. The separators  200  can be arranged to operate in series ( FIG. 2B ), where an outlet  114  of one separator  200  feeds an inlet  112  of another separator, or the separators  200  can be arranged to operate in parallel ( FIG. 2C ), where a fluid mixture to be separated is distributed among the inlets  112  of the two or more separators  200 . Configuring the separators  200  in series ( FIG. 2B ) enables further separation of one constituent of a fluid mixture into sub-constituents. For example, a first of two separators  200  in series may separate particulate and water from oil, and the second of the two separators  200  may separate the particulate from the water. 
       FIG. 3A  depicts a plurality of alternate illustrative separators  300 , each separator  300  substantially helical and configuration. In a similar manner to the substantially linear separators  200  depicted in  FIG. 2A , the substantially helical separators  300  of  FIG. 3A  can be placed closely adjacent one another and a nested arrangement. The separators  300 , each have an elongate helical vessel  310  with an inlet port  312  and one or more outlet ports  314 , for example a light constituent outlet port  314   a  and a heavy constituent outlet port  314   b . As is best seen in  FIG. 3B , the separators  300  configured in a nested arrangement are suited for placement within a cylindrical body, such as the conductor casing  316  at or near a subsea wellhead  320 . 
     Turning now to  FIG. 4 , another alternate illustrative separator  400  incorporates a filter  426 . The separator  400  is operable to separate the heavy and light constituents of a fluid mixture by establishing a convection cell  420  as is described above with reference to  FIG. 1 . However, rather than being the primary separation mechanism, as above, the convection cell  420  in the separator  400  operates to initially separate the heavy and light constituents of the fluid mixture prior to filtration of a portion of the fluid mixture by the filter  426 . By operating to initially separate the heavy constituents of the fluid mixture prior to filtration by the filter  426 , the convection cell  420  reduces the filtering load on the filter  426 . The reduced filtering load on the filter  426  reduces clogging and prolongs the life of the filter  426 . 
     The separator  400  includes an elongate vessel  410  having an inlet port  412  and one or more outlet ports  414 , for example a light constituent outlet port  414   a  and a heavy constituent outlet port  414   b . As above, the light constituent outlet port  414   a  may be positioned about an upper end  422  of the vessel  410  and the heavy constituent outlet port  414   b  may be positioned about a lower end  424  of the vessel  410 . In one illustrative implementation, the filter  426  may be a membrane that spans, at least partially, across an interior of the vessel  410 . Gaps (not specifically shown) may be provided in the filter  426  to allow passage of fluid if the filter  426  becomes blocked. In one implementation the filter  426  may be an ionically treated porous membrane that may also or alternatively be a molecularly sized porous membrane. The filter  426  may be positioned above or below the inlet port  412 . In the configuration of  FIG. 4 , the filter  426  is positioned below the inlet port  412  and oriented to span the interior of the vessel  410  at a diagonal. One instance where it may be desirable for the filter  426  to be positioned below the inlet port  412  is a configuration where the filter  426  filters the light constituent and passes the heavy constituent. For example, the filter  426  may be oil philic and hydrophobic to filter oil from water and pass the water. One instance where it may be desirable for the filter  426  to be positioned about the inlet port  412  is a configuration where the filter  426  filters the heavy constituent and passes the light constituent. For example, the filter  426  may be a fine mesh that filters particulate from water and/or oil. 
     Operation of the separator  400  is similar to the separator  100  of  FIG. 1  above in that a fluid mixture is introduced through the inlet port  412 , heavy constituent  415  sinks substantially vertically downward (vertically downward flow of heavy constituent  415 ) toward lower sidewall  416  and begins convection cell  420  of a fluid mixture containing a larger portion of heavy constituent  415  flowing downward along the lower sidewall  416  and a fluid mixture containing the remaining light constituent and a lesser portion, if any, of the heavy constituent  415  flowing upward along upper sidewall  418 . The fluid mixture containing a larger portion of heavy constituent  415  flows down the lower sidewall  416  and through the filter  426 . As the fluid mixture flowing down the lower sidewall  416  and through the filter  426  contains a lesser portion of the light constituent, the amount of the light constituent that the filter  426  must remove is less. Accordingly the filter  426  is less prone to clogging with light constituent and will last longer than if the filter  426  is used alone without the convection cell  420 . 
     In a configuration where the filter  426  is adapted to filter the heavy flow and pass the light flow, for example in a configuration where the filter  426  is positioned above the inlet port  412 , the flow entering the filter  426  has a smaller portion of the heavy constituent  415 , thereby reducing clogging with heavy constituent  415  and increasing the life of the filter  426 . 
     The concepts described herein are not limited to use of a membrane type filter  426 . Rather, numerous other types of filters can be used, including but not limited to capillary filters, centrifuges, cyclones, and others. For example,  FIG. 5  depicts another alternate illustrative separator  500  that incorporates a prepacked screen as a filter  526 . A prepacked screen is a screen that carries filter media, for example a particulate media such as sand, operable to filter a constituent from the fluid mixture. In an instance of filtering oil from water, the filter media can be sand that is treated to be hydrophobic and thereby pass water and filter oil. As above, the separator  500  is configured to form a convection cell  520  that operates to initially separate the heavy and light constituents of the fluid mixture prior to filtration of a portion of the fluid mixture by the filter  526 . The filter  526  may be positioned above or below the inlet port  512 . Additionally, the vessel  510  may include one or more outlet ports  514 , for example a light constituent outlet port  514   a  about an upper end  522  of the vessel  510  and a heavy constituent outlet port  514   b  about a lower end  524  of the vessel  510 . 
     The filter  526  is cylindrical in configuration and resides adjacent lower sidewall  516  of the vessel  510 . Because the filter  526  resides adjacent the lower sidewall  516 , the fluid mixture entering the filter  526  contains a larger portion of the heavy constituent. The fluid mixture enters through an upper end wall  528  and/or a lateral sidewall  530  of the filter  526 , passes axially through the filter  526 , and exits about a lower end  532  of the filter  526 . The filter  526  can also be used in conjunction with a bypass mechanism  534 , for example a choke or pressure limiting valve, to allow passage of the fluid mixture should be filter  526  become plugged or otherwise stopped. 
     Although several illustrative implementations of the invention have been described in detail above, those skilled in the art will readily appreciate that many other variations and modifications are possible without materially departing from the concepts described herein. Accordingly, other implementations are intended to fall within the scope of the invention as defined in the following claims.