Abstract:
A method of breaking a foam is to direct a laser beam into the foam. The laser beam(s) may be scanned across a conduit in which the foam is travelling. The foam may be formed from gas and liquid produced from a hydrocarbon reservoir.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to British Application No GB1401322.1 filed on 27 Jan. 2014, which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    A wet foam (i.e. a mass of bubbles) may be formed at the surface of a quantity of liquid so that there is a continuous liquid phase below the foam and a continuous gaseous phase above the foam. However, when foam is formed in a pipe or other conduit or is formed in an enclosed vessel, the foam may occupy so much of the flow cross-section or so much of the volume of the enclosing vessel that any continuous gas phase or any continuous liquid phase is small or absent. 
         [0003]    The occurrence of foams can be an unwanted inconvenience in a diverse range of industrial processes. Excessive foam can cause a range of problems, particularly in relation to mechanical factors, coolants and processing liquids. These problems include reductions in pump efficiency and storage tank capacity, drainage issues within filtration systems, and associated costs related to the time required to cease production and remove foam. 
         [0004]    Foam formation can occur when liquid phase contains a substance which is surface active. This may be a surfactant which has intentionally been included in the liquid phase, or it may be a material which happens to be present in the liquid phase and has some surface activity. Structurally, a wet foam consists of thin lamellae of the surfactant loaded liquid in a three-dimensional structure. 
         [0005]    Foam is sometimes controlled by deliberate addition of a chemical which inhibits foam formation. Such a chemical may be referred to as an anti-foam or a defoamer. However, this addition of an extra chemical is an added cost and may not always be possible. Industries in which foam control is required include the oil industry, one instance being at oil rigs during wellbore cleaning operations. Currently, chemical defoamers may be used to break foams at oil rigs. Chemical defoamers are also used in many other industrial processes and products, including wood pulp, paper, paint, industrial wastewater treatment and food processing. Such applications include floatation deinking, foam control in printing, aerators for sewage treatment and fermentation. 
       SUMMARY 
       [0006]    This summary is provided to introduce a selection of concepts that are further described below. This summary is not intended to be used as an aid in limiting the scope of the subject matter claimed. 
         [0007]    In contrast with prior methods involving the use of chemical defoamers, the present invention instead relies on breaking foam by directing the beam of a laser into the foam. A laser beam comprises light travelling along parallel paths within the cross section of the beam, so that there is little or no spreading of the beam, although a laser beam may be focussed by a lens, curved mirror or other optical element so as to bring it to a smaller cross section, sometimes referred to as a waist, after which it may spread. Without wishing to be bound by theory, we believe that the high energy density within a small cross section, which is characteristic of a laser beam, leads to rapid heating of a small area of a foam lamella when a laser beam intersects the lamella. This localised heating destabilises or ruptures the lamella, possibly through vaporising a very small quantity of the liquid within the lamella, and thus breaks a bubble of the foam (or merges two adjacent bubbles into one larger bubble which may then be broken by the laser beam striking another lamella). 
         [0008]    In some embodiments of the invention, foam is broken as it passes along a conduit. The conduit may have a section in which one or more laser beams are directed into the foam, and the beam or beams may be arranged to scan across the cross-section of the conduit. This may allow the foam to be broken in a continuous process without the use of chemical additives. Breaking a foam can assist the separation of gas and liquid and separated gas and liquid may leave the conduit by separate exits. 
         [0009]    One of the features of breaking a foam in this way is that there may be little or no chemical change in the overall composition which may not be the case when using chemical additives. A consequence is that the breakage of the foam can be reversible. The foam could be reformed by agitation or aeration should there be a need to do so. 
         [0010]    When foam is broken in a conduit, the foam may be made to pass through a labyrinth positioned in the conduit upstream of a section in which a laser beam or beams are directed at the foam, in order to block any leakage of laser light. The constituents of the broken foam may pass through a labyrinth positioned downstream of such a section, so as to block any leakage of laser light in this downstream direction. A labyrinth may consist of two or more parts located internally in the conduit, serving to interrupt any straight line along the conduit and causing flow in the conduit to change direction as it passes through the labyrinth. An alternative to locating a labyrinth within a conduit is to provide turns in the conduit so as to block straight lines out of the section with the laser beam or beams. 
         [0011]    Also disclosed here is apparatus comprising a flow path or enclosure for fluid, having a section equipped with one or more lasers positioned to direct a laser beam into the flow path or enclosure, to rupture foam therein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic view of the interior of a portion of a conduit including a section in which foam is broken by laser beams; 
           [0013]      FIG. 2  is a cross section on line II-II of  FIG. 1 ; 
           [0014]      FIG. 3  is a schematic illustration of a scanning laser unit; 
           [0015]      FIG. 4  is a view similar to  FIG. 1 , showing a modified arrangement; 
           [0016]      FIG. 5  is a schematic plan view of experimental apparatus; and 
           [0017]      FIG. 6  is a schematic elevational view. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIGS. 1 and 2  diagrammatically illustrate an embodiment of a system in which a continuous flow of wet foam is broken by scanning laser beams. The diagram shows foam  101  flowing along a conduit  100  in the direction indicated by arrow  102 , i.e. from left to right as illustrated. The foam  101  passes through a labyrinth formed by plates  104  within the conduit  100 . These plates  104  extend part way across the cross section of the conduit, blocking the path of any scattered laser light and so prevent leakage of laser light into the upstream pipework. The foam is of course compelled to change its direction of flow as it passes over one plate and under the next. 
         [0019]    The foam then enters a section of the conduit in which laser units  106  (each comprising a laser and optics to manipulate the laser beam) direct laser beams  107  (shown as chain lines) through windows  108  and across the conduit  100 . As illustrated by  FIG. 2 , the laser beams  107  are repeatedly scanned to and fro across an arc  128  transverse to the conduit  100 . The edges of the arcs are indicated  129 . The width of each arc  128  is such that the entire cross section of the conduit is scanned by at least one beam  107 . It is believed that the high energy density within a small cross section, which is characteristic of a laser beam, leads to rapid heating of a small area of a foam lamella when a laser beam intersects the lamella. This localised heating ruptures the lamella, possibly through vaporising a very small quantity of the liquid within the lamella, and thus breaks the bubble of foam bounded by that lamella. 
         [0020]    Arrangements for scanning a laser beam across an arc as shown by  FIG. 2  are well-known. It can be done by moving the laser itself, but it is common to keep the laser fixed and use optical elements such as a mirror or prism to deflect the beam. 
         [0021]      FIG. 3  illustrates, by way of example, a possible arrangement. Fixed laser  130  directs its beam  132  onto a 45° mirror  134 . This mirror is attached to the shaft  136  of a stepper motor  138  which is operated to twist the mirror through an arc around the axis of shaft  136 . The result is that the output beam  107  is scanned in a plane which is transverse to the plane of the diagram. A unit as shown in  FIG. 3  would be fitted to the conduit  100  in an orientation such that the scan plane of the output beam  107  is transverse to the conduit  100 . 
         [0022]    As a result of the foam being broken, gas and liquid in the conduit are able to separate. The separated gas continues to flow along the conduit  100  as indicated by arrow  112 . It passes through a labyrinth formed by two more plates  114  which prevent leakage of laser light into the downstream pipework. Separated liquid drains through an outlet  116  and passes along an outlet pipe  117  which doubles back at  118  so as to block the path of any scattered laser light. 
         [0023]    An arrangement as shown and described above may be implemented with lasers having sufficient power that the parallel beam output from the laser can rupture a lamella in the foam. However, another possibility is to focus the laser beam to a so-called waist of reduced cross-section. In this event it may be desirable to arrange for the position of this waist to move to and fro across the cross-section of the conduit  100  while scanning the laser beam. One possible arrangement for this is included in  FIG. 3 . The laser beam  132  passes through a focusing lens  140 . This lens  140  has both spherical and cylindrical curvature so that its focal length is not uniform. The lens  140  is attached to the shaft  144  of a stepper motor  142  and is rotated continuously by this motor  142  so that the focal length where beam  132  passes through the periphery of the lens  140  varies. Consequently the waist to which the beam is reduced moves between two points corresponding to the maximum and minimum focal lengths. The curvatures of the lens  140  are chosen so that these two points are near to the wall of the conduit  100  when the laser beam extends diametrically across the conduit. 
         [0024]      FIG. 4  shows a modified arrangement of conduit. Foam  101  entering in the direction indicated by arrow  301  travels along a conduit  300  which includes a section in which scanning laser units  106  direct their beams  107  into the foam in the conduit, thus breaking the foam just as with the arrangement of  FIG. 1 . Separated gas continues to flow along the conduit and separated liquid drains to outlet  116 . However, internal baffles  104  and  114  are not used. The inlet  302  for foam and the outlet  304  for gas are each constructed as a blind tee, so that scattered laser light (if any) will be blocked by striking the flat end walls  306 . The outlet pipe  308  for liquid also connects to the outlet  116  at a blind tee with end wall  310 . 
         [0025]    The above description has presumed that the flow along the conduits  100  or  300  has filled the conduit with foam. However, the same arrangement could be used when the foam occupies only part of the cross section of the conduit, or occurs intermittently rather than continuously. 
       Experimental Work 
       [0026]    Experiments were carried out to demonstrate the efficacy of a laser beam for breaking foam. These experiments were carried out using apparatus marketed as a laser cutter to direct a laser beam onto foam in an open topped Petri dish. 
         [0027]    The experimental setup is shown by  FIGS. 5 and 6 . The beam  407  from a fixed laser  406  was directed by mirror  408  onto a mirror  410  which directed the laser beam down onto the open top of the Petri dish  412  containing foam. The mirror  408  was at one end of a carriage  414  movable to and fro in the horizontal direction indicated by arrow X and the mirror  410  was movable to and fro on the carriage  414  in the in the orthogonal horizontal direction indicated by arrow Y. Computer-controlled movement of the carriage  414  and movement of the mirror  410  on the carriage  414  enabled the mirror  410  and hence the point at which the laser beam was directed downwardly to be positioned freely within a working area in a manner similar to the motion of the print head of a dot matrix printer. 
         [0028]    The laser  406  was a 35 watt carbon dioxide laser and its output could be adjusted from 10% to 100% of full power. It emitted a beam at a wavelength of 10.5 microns and the light path included a converging lens  416  to focus the laser beam down to a cross section of about 0.5 mm 2 . 
         [0029]    For each experiment a model foam was made from a solution of 0.67 wt % sodium dodecyl sulphate and 48.2 wt % glycerol in high purity water. This solution was placed in a bottle and shaken vigorously to form a foam. The presence of glycerol in the liquid phase gave a very stable foam which, if left undisturbed, remained intact for over 30 min. A quantity of this foam was scooped into a Petri dish with a spatula and levelled off so that the dish was filled to its top with foam. The Petri-dish had internal diameter 88 mm and depth 13 mm, so that it held 79 ml of foam. 
         [0030]    The dish was then placed in the laser cutter in the path of the laser beam as shown in  FIGS. 5 and 6  and this apparatus was operated to move the beam over the foam in a series of concentric circles  418  (shown dotted in  FIG. 5 ). The circles were spaced apart by 4 mm. This spacing of 4 mm was chosen because in preliminary experiments it had been found that spacing greater than 5 mm left some of the foam intact and unaffected by the laser beam. The time for moving the laser beam in this pattern of concentric circles  418  was 205 seconds in some experiments and 32.5 seconds in other experiments. 
         [0031]    In order to check that any collapse of foam was brought about by the laser beam rather than by any other cause, a preliminary experiment was carried out in accordance with the above procedure but with the laser switched off. The foam was observed to remain intact. 
         [0032]    In each experiment, after the foam had been exposed to the laser a photograph of the Petri dish was taken and the reduction in foam height was determined from the photo by means of image analysis software using the known height of the Petri dish as a calibration scale. This determination had an estimated error of 10% but was sufficient to observe variation with power of the laser beam. 
         [0033]    For the first five experiments designated A to E in the table below, the Petri dish was positioned so that the laser beam came to its narrowest focus 2 mm below the top of the Petri dish. The foam was thus exposed to a well-focussed beam. Two further experiments designated F and G were carried out with the Petri dish positioned so that the laser beam came to its narrowest focus at a point deeper in the foam, 9 mm below the top of the Petri dish. Results obtained are set out in the following table: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Laser focus 
                 Drop in 
                 Volume of 
                   
                 Joules per 
               
               
                   
                 Laser Power 
                 Laser Speed 
                 (mm below 
                 foam height 
                 foam burst 
                 Laser Energy 
                 volume burst 
               
               
                   
                 (Watt) 
                 (mm/sec) 
                 top of dish) 
                 (mm) 
                 (mL) 
                 (Joules) 
                 (J/L) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 A 
                 28 
                 67.5 
                 2 
                 9 
                 55 
                 574 
                 10486 
               
               
                 B 
                 14 
                 67.5 
                 2 
                 6.5 
                 40 
                 287 
                 7260 
               
               
                 C 
                 7 
                 67.5 
                 2 
                 5.5 
                 33 
                 144 
                 4290 
               
               
                 D 
                 7 
                 42.5 
                 2 
                 8 
                 49 
                 228 
                 4676 
               
               
                 E 
                 3.5 
                 42.5 
                 2 
                 4.5 
                 27 
                 114 
                 4156 
               
               
                 F 
                 3.5 
                 42.5 
                 9 
                 8 
                 49 
                 114 
                 2338 
               
               
                 G 
                 3.5 
                 42.5 
                 9 
                 5 
                 30 
                 114 
                 3740 
               
               
                   
               
             
          
         
       
     
         [0034]    These experimental results demonstrate that a low-powered laser beam, brought to a focus within a foam is able to rupture lamellae within the foam and thus break or reduce the foam. 
         [0035]    It will be appreciated that the example embodiments described above can be modified and varied within the scope of the concepts which they exemplify. Features referred to above or shown in individual embodiments above may be used together in any combination as well as those which have been shown and described specifically. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.