Abstract:
In one aspect of the invention there is provided a gas scrubbing system for removing contaminants from a flow of fluid, comprising a container having an interior volume, an inlet for receiving the flow of fluid and an outlet for dispensing the stream of fluid, a treatment liquid, a porous medium positioned in the interior volume, between the inlet and outlet, said porous medium providing a high surface area to facilitate chemical interactions between the fluid flow and the treatment liquid and means to apply the treatment liquid onto the porous medium. A contact cell aspect of the porous medium and a method aspect are also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a regular application of U.S. Provisional Patent Application Ser. No. 60/893,881 filed Mar. 8, 2007 and entitled “HIGH VOLUME, LOW BACK-PRESSURE GAS SCRUBBER”, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The field of present invention relates generally to gas scrubbing equipment and, more particularly, to equipment suitable for scrubbing impurities from high volume gas streams without creating a significant rise or increase in back pressure. 
       BACKGROUND OF THE INVENTION 
       [0003]    Gas scrubbers are used in many industrial processes and applications to clean, remove or “scrub” certain undesirable gaseous components from gas streams in general. One area in which a large number of developments have been made is in the scrubbing of gases produced during, or related to, oil and gas recovery and storage operations. Examples of operations where a gas scrubber is typically used include loading and transportation of sour liquids, venting storage tanks during completion operations and well testing, purging of vessels and pipelines, bleeding off wellheads, venting settling tanks for underbalanced drilling, controlling emissions and odors from industrial processing, controlling vacuum truck emissions and odor control during plant turn-around and tank cleaning operations. 
         [0004]    During such operations, poisonous hydrogen sulfide (H 2 S) present presents a health hazard to workmen in the area. To protect the workmen and the public-at-large, the permissible conditions and levels for emissions of hydrogen sulfide are regulated by various regulatory agencies. 
         [0005]    Conventional systems for the absorption or removal of unwanted contaminants from a gas source or stream often employ a liquid solvent or scavenger to “scavenge” out the H 2 S. An example of such a treatment liquid is the hydrogen sulfide scavengers HSW705 and HSW700 manufactured by Baker Petrolite of Sugar Land, Tex., U.S.A. Information supplied by Baker Petrolite notes that the HSW705 formulation is specifically designed to remove hydrogen sulfide from produced gas and that this liquid product combines with hydrogen sulfide (H 2 S) to form stable, water-soluble reaction products that may be easily removed from the system. Baker Petrolite recommends that the point of injection of the scavenging chemical be as early as conveniently possible in the producing system to maximize contact time, i.e. injection downhole or before wellhead chokes are generally the best points of application. However, this may be impractical in some of the operations noted above, such as during the loading and transportation of sour liquids, venting storage tanks during completion operations and well testing, purging of vessels and pipelines, venting settling tanks for underbalanced drilling, controlling emissions and odors from industrial processing, controlling vacuum truck emissions and odor control during plant turn-around and tank cleaning operations. 
         [0006]    Likewise, Am-Gas Scrubbing Systems (1989) Ltd. of Didsbury, Alberta, Canada distributes and markets chemical products under the trademark PARATENE, which are used as hydrogen sulfide scavengers for use in oilfield and industrial applications and, depending on the exact formulation, forms either water-soluble or oil-soluable by-products. Examples include PARATENE M310, PARATENE M311, PARATENE M315, PARATENE M316, PARATENE M320 and PARATENE M330. 
         [0007]    The prior art is replete with various examples of devices and methods for the “scrubbing” of gas streams using such treatment liquids or liquid scavengers. However, none of the prior art devices provide a relatively portable device which is capable of efficiently removing gases like hydrogen sulfide quickly from large volumes of influent gas and without creating a significant amount of back-pressure. Furthermore, prior art devices have problems with liquid scavenger chemical exiting out of the devices when back-pressures are low, problems with dealing with the high gas volumes and flow rates when they are hooked up to a vacuum truck and problems with providing sufficient contact time to allow the liquid scavenger to treat the gas and remove or “scrub” the hydrogen sulfide. The present invention addresses these problems. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect of the invention there is provided a gas scrubbing system for removing contaminants from a flow of fluid, comprising: a container having an interior volume, an inlet for receiving the flow of fluid and an outlet for dispensing the stream of fluid, a treatment liquid, a contact cell positioned in the interior volume, between the inlet and outlet, for providing a high surface area to facilitate chemical interactions between the fluid flow and the treatment liquid and means to apply the treatment liquid onto the contact cell. 
         [0009]    In another aspect of the invention there is provided a contact cell for use in a gas scrubbing system, comprising a layer of poly-propylene beads. 
         [0010]    In a method aspect, a method to purify a stream of gas is provided. The method comprises the steps of providing a treatment liquid, treating the stream of gas with a first separator to remove any liquid and solid contaminants, scrubbing the stream of gas with a liquid scavenger and treating the scrubbed stream of gas with a second separator to remove any remaining liquid scavenger. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a top view of one embodiment of the gas scrubber according to the present invention; 
           [0012]      FIG. 2  is a diagrammatic front sectional view of the gas scrubber of the embodiment of  FIG. 1 ; 
           [0013]      FIG. 3   a  is a diagrammatic side sectional view of a preferred embodiment of a contact cell; 
           [0014]      FIG. 3   b  is a diagrammatic side sectional view of a second embodiment of a contact cell; 
           [0015]      FIGS. 4-5  are front perspective views of the gas scrubber of the embodiment of  FIG. 1 ; 
           [0016]      FIGS. 6-9  are top perspective view the gas scrubber of the embodiment of  FIG. 1 , looking down into the interior volume of the scrubber; 
           [0017]      FIG. 10  is a front view of a second embodiment of the gas scrubber; 
           [0018]      FIG. 11  is a top view of the gas scrubber of the embodiment of  FIG. 10 ; and 
           [0019]      FIGS. 12   a - 12   c  are cut-out perspective and top views of another embodiment of the gas scrubber. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The following description are of preferred embodiments by way of example only and without limitation to the combination of features necessary for carrying the invention into effect. Reference is to be had to the Figures in which identical reference numbers identify similar components. The drawing figures are not necessarily to scale and certain features are shown in somewhat schematic form in the interest of clarity and conciseness. 
         [0021]    Referring to the Figures generally, one embodiment of a gas scrubbing system constructed in accordance with the present invention  10  is illustrated in  FIGS. 1-9 , a second embodiment of a gas scrubbing system constructed in accordance with the present invention is illustrated in  FIGS. 10-11  and a third embodiment of a gas scrubbing system constructed in accordance with the present invention is illustrated in  FIGS. 12   a - 12   c . The three embodiments are similar to each other, differing only in minor aspects as shown in the figures and as further described below. Operation of the three embodiments is also similar, again with the difference between them as shown in the figures and as further described below. 
         [0022]    Referring to the Figures generally, the scrubber system  10  comprises a main vessel or container  12 . The vessel  12  has an interior volume  12   v , an inlet  12   i  for receiving a predetermined mass flow rate of gas, that may be contaminated by a pollutant such as hydrogen sulfide, and an outlet  12   o  for dispensing the gas once it has been treated with a treatment liquid, scrubber solution or scavenger  22 ,  24 . The flow of the gas is from the inlet  12   i  to the outlet  12   o  and is shown generally by the arrows designated as  15 . Preferably the treatment liquid  22 ,  24  is one of the hydrogen sulfide scavengers distributed by Am-Gas Scrubbing Systems (1989) Ltd. of Didsbury, Alberta, Canada under the PARATENE trademark. 
         [0023]    A preferred material for the vessel  12  is steel. In this embodiment, the vessel  12  is conveniently in the form of a hollow box having a closed bottom  12   b  and vertical side walls  12   w  and measuring approximately 44 inches long by 29 inches wide and 67 inches high. Preferably the system  10  further comprises an open top  16  and a detachable or removable lid  18 . Alternatively, the system  10  may take any other suitable form, such as that of a drum, without departing from the spirit or scope of this invention. Preferably the vessel  12  is a pressure vessel capable of tolerating gas pressures of 1.5 pounds per square inch (psi) or higher. 
         [0024]    The system  10  further comprises at least one suitable conventional spray nozzle  20  to convert a source of liquid scavenger  22  into a spray of droplets  24 . The spray nozzle  20  is provided or mounted to the vessel  12  so as to inject or introduce the liquid scavenger  24  into the upper portion of the vessel&#39;s interior volume  12   v . Preferably a plurality of nozzles  20  are provided at various positions inside the vessel&#39;s interior  12   v . The embodiment of  FIGS. 1-9  has two nozzles  20  depending from the lid  18  (see  FIG. 2 ) and three nozzles  20  positioned as shown in  FIG. 8 . The embodiment of  FIGS. 10-11  has three nozzles depending from the lid. The embodiment of  FIGS. 12   a - 12   c , like that of the first embodiment, has as two nozzles  20  depending from the lid (not shown) and three nozzles  20  positioned as shown in  FIG. 12   a.    
         [0025]    More preferably, pumping means, in this embodiment comprising a pump  26  along with associated hosing and tubing  28 , are provided to link the source of liquid scavenger  22  to the nozzles  20  in a conventional manner. Even more preferably, the pump is capable of pumping at least 10 gallons per minute so as to ensure that the porous medium  30  remains substantially wetted with liquid scavenger  22  during operations. 
         [0026]    Yet even more preferably, the lower portion of the vessel&#39;s interior volume  12   v  functions as a retaining reservoir for the source of liquid scavenger  22 . Advantageously, the liquid scavenger  24  released from the nozzle  20 , or nozzles  20 , in the upper portion of the vessel&#39;s interior volume  12   v  descends to the lower portion and once again become part of the source  22 . 
         [0027]    The system  10  further comprises a high surface area, porous substrate or medium  30  which is placed inside the vessel  12  and in the path of the flow of the gas  15  as it moves from the inlet  12   i  to the outlet  12   o . The porous medium  30  minimizes disruption of the normal flow pattern of the flow of gas  15  through the system  10  while at the same time providing a high surface area to carry treatment liquid  22 ,  24  that coats the medium, thereby allowing the system  10  to treat high volume gas streams without creating a significant rise or increase in back pressure. 
         [0028]    In this embodiment, the porous medium  30  is in the form of a 6-inch deep bed of approximately ⅛ th  inch diameter poly-propylene beads  32 , measuring approximately 44 inches by 22 inches for a total volume of approximately 5808 cubic inches of ⅛th inch poly-propylene beads. Such poly-propylene beads  32  are distributed by Ashland Canada Corp of Richmond, B.C., Canada. The porous medium  30  is located within the interior volume  12   v  so as to be substantially “wetted” or coated by the droplets of scavenger  24  exiting the nozzles  20  while at the same time be in the path of all, or substantially all, of the flow of the gas  15  as it moves from the inlet  12   i  to the outlet  12   o . Advantageously, this substantially “wetted” high surface area medium  30  provide for numerous interaction sites for treatment liquid  22  to interact with the gas flow  15 . More advantageously, the continual circulation of treatment liquid  22  (by the pump  26 ) from the source, through the nozzle  20 , or nozzles  20 , across the porous medium  30  and back to the source results in an efficient use of said treatment liquid  22 . 
         [0029]    The inventor has observed that using a porous medium  30  with a thickness range of about 6 inches to 30 inches of beads  32  resulted in good scrubbing or treating performance by the system  10 , allowing the system  10  to treat high volumes and flow rate gas streams  15  without creating a significant rise or increase in back pressure. 
         [0030]    Preferably, the porous medium  30  is in the form of a contact cell  30   c  and of such dimensions so as to be in the path of most or all of the flow of gas  15 . More preferably the dimensions of the contact cell&#39;s periphery are such that a very close tolerance fit is obtained when the contact cell  30   c  is placed inside the vessel, thereby providing little room or space for gas to flow around the cell  30   c.    
         [0031]    More preferably the contact cell  30   c  further comprises two ½ inch thicknesses of ⅛ inch thick reticulated open-cell foam layers  33  placed directly below and an top of the porous medium (see  FIG. 3   a ). Such reticulated open-cell foam layers  33  is distributed by Norwesco Industries (1983) Ltd. of Calgary, Alberta, Canada. Even more preferably, the contact cell  30   c  is removable by surrounding or encasing the 6-inch bed of beads  32 , and the open-cell foam  33 , with a 1/16th inch screen material  30   m  at the top and bottom and enclosing the sides  30   s  with ⅛ th  inch steel (see  FIG. 3   a ). Advantageously, the thicknesses of open-cell foam  33  provides additional stability and cushioning to the contact cell  30   c  as a whole and keeps the beads  32  well packed. More advantageously, the relatively thin layers of foam  33  (only ½ inch total thickness at both top and bottom) acts as a filter material, preventing dirt and debris from lodging in the beads  32 , while allowing the flow of gas  15  through without significantly increasing the back pressures. 
         [0032]    Even more preferably, sealing means (not shown) are used to seal the periphery of the contact cell  30   c  against the interior walls of the vessel  12 , thereby ensuring that all of the flow of gas is directed through the contact cell  30   c . The inventor initially utilized a Ethylene Propylene Diene Monomer (EPDM) seal for this purpose. This worked well initially. However, after some time this seal underwent some shrinkage and needed to be replaced. It is speculated that this shrinkage was due to heat. Subsequent experimentation with a buna seal showed that this type of seal did not undergo this kind of shrinkage and therefore lasts longer. It is to be understood that a seal or sealing means is not critical to the invention. 
         [0033]    Advantageously, the contact cell  30   c  provides for easy containment of the beads  32 , thereby allowing them to be easily removed, cleaned, replaced and/or serviced when dirty. More advantageously, the contact cell  30   c  prevents shifting of the beads  32  during operations on unleveled ground or during transportation of the system  10 . 
         [0034]    The inventor estimates that providing the above-noted 6-inch deep bed of approximately ⅛ th  inch diameter poly-propylene beads  32 , measuring approximately 44 inches by 22 inches and having a cross sectional area of 968 square inches and total volume of 5808 cubic inches, results in a surface area of approximately 81,312 square inches plus-or-minus 25%. The inventor observed that using the above-noted contact cell  30   c  configuration, with the layers of open-cell foam  33 , and said cell  30   c  being substantially wetted with treatment liquid  22 ,  24  during gas scrubbing operations, resulted in back pressure of only approximately 18″ water column with a flow rate of approximately 800 standard cubic feet per minute across said bed. 
         [0035]    Another embodiment of a contact cell  30   c  (see  FIG. 3   b ) comprises two ½ inch thicknesses of mist eliminators or demister pads  34  instead of reticulated open-cell foam  33 , but is otherwise similar to the embodiment of  FIG. 3   a . Such demister pads  34  are distributed by Industrial Process Products Ltd. of Calgary, Alberta, Canada. Advantageously, the use of demister pads  34  provides for even less back pressures during operations than a similar thickness of reticulated open-cell foam. The inventor observed that using a substantially wetted (with treatment liquid  22 ) contact cell  30   c  configuration of 10½-inch deep bed of approximately ⅛ th  inch diameter poly-propylene beads  32 , measuring approximately 44 inches by 22 inches and having a cross sectional area of 968 square inches, but with the two layers of ½ inch thick demister pad  34  (instead of the layers of open-cell foam  33 ), during gas scrubbing operations resulted in very similar back pressures, again of only approximately 18″ water column with a flow rate of approximately 800 standard cubic feet per minute across said bed. However, by using a 10½-inch deep bed of beads  32 , the total surface area provided increased significantly (estimated by the inventor to be approximately 142,296 square inches, plus-or-minus 25%). 
         [0036]    During operations, the inventor observed that, when using this second embodiment of contact cell  30   c  (i.e. having demister pad material instead of open-cell foam) in the system  10  of the embodiment shown in  FIGS. 12   a - 12   c , the system  10 , using approximately 410 liters of PARATENE M320 treatment liquid  24 , was able to completely scrub a 1½ percent sour (H 2 S) flow of gas  15  (i.e. resulting in 0 ppm H 2 S concentration at the outlet) having a flow rate of 800 cubic feet per minute and only created a back pressure of approximately 18″ water column. 
         [0037]    As will be appreciated by those skilled in the art, a number of factors will determine how long a particular batch of treatment liquid  24  will last before said batch  24  becomes spent and the system  10 , during operation, will start showing signs of H 2 S breakthrough at the outlet  12   o , such as H 2 S concentrations in the range of 5-25 ppm at the outlet. One factor is the particular treatment liquid used. Another factor is the amount of treatment liquid used (for example, one would expect a 200 liter batch of treatment liquid to last roughly half as long as a 400 liter batch, assuming all other factors are equal). A third factor is the H 2 S concentration in the flow of gas  15 . A forth factor is the volumetric flow rate of gas  15  through the system. 
         [0038]    Observations: 
         [0039]    When using 410 liters of PARATENE M320 treatment liquid  24  in the system  10  of the embodiment shown in  FIGS. 12   a - 12   c  with the second embodiment of the contact cell  30   c  ( FIG. 3   b ) and a gas flow  15  rate of 1400 standard cubic feet per minute (SCFM), the inventor has observed the following: 
         [0040]    (i) light duty operation, of scrubbing a gas flow with 2,000 ppm H 2 S concentration at the inlet  12   i , resulted in the system  10  being able to operate for 50 hours or more before any signs of H 2 S breakthrough at the outlet  12   o ; and 
         [0041]    (ii) heavy duty operation, of venting two storage tanks at approximately 80,000 ppm H 2 S concentration at the inlet  12   i , resulted in the system  10  initially having a 20 ppm H 2 S concentration at the outlet  12   o , with this having increased to 80 ppm H 2 S concentration after one hour of operations. 
         [0042]    When the treatment liquid  24  starts showing signs of breakthrough, i.e. it becoming less effective at combining with hydrogen sulfide (H 2 S) to form stable end products and resulting in an unacceptable concentration of H 2 S at the outlet (such as an H 2 S concentration greater than 10 ppm), the liquid scavenger  24  can be drained from the system  10  and replaced with a fresh batch of such treatment liquid  24 . 
         [0043]    Using a contact cell  30   c  composed only of demister pad material  34  (see  FIGS. 6-7 ) proved to be cost prohibitive in that a given thickness of demister pad is much more expensive that a given thickness of poly-propylene beads  32  and the surface/contact area provided by demister pads is significantly inferior to the amount of surface area provided by a similar thickness of poly-propylene beads  32 . 
         [0044]    Similarly, the inventor has observed that using glass particles, instead of poly-propylene beads also had disadvantages. Although glass particles provide a similar amount of surface area per unit volume, as compared to the poly-propylene bead, one of the disadvantage of such glass particles is that over time they could break into even smaller pieces which may escape from the contact cell and can get caught in the pump  26 , potentially damaging the pump&#39;s internal mechanisms. 
         [0045]    Preferably, the scrubber system  10  further comprises an inlet separator  40  and catch reservoir  41  associated with the vessel&#39;s inlet  12   i  and an outlet separator  42  associated with the vessel&#39;s outlet  12   o . More preferably, the inlet and outlet separators  40 ,  42  are cyclone separators (see  FIG. 2 ). Cyclone separators as such are well-known in the art and rely on generated centrifugal and shear forces to achieve separation into two streams of different densities. 
         [0046]    Briefly and as shown in  FIG. 2 , the cyclone separators  40 ,  42  comprises a chamber  40   c ,  42   c  having a vertical axis with an upper cylindrical portion  40   u ,  42   u  and a lower, inverted frustro-conical portion  40   f ,  42   f . The mixture is introduced through a tangential inlet  12   i ,  42   i  to the cyclone separator, which causes heavier particles to be flung, under centrifugal force, against the outer wall of the chamber and flow downwardly along, as underflow, and around the wall to a lower axial outlet  401 ,  421 , while the lighter, remaining, proportion of the mixture is drawn off by an axial pipe, known as a vortex finder  40   v ,  42   v , from a point within the body of the cyclone separator  40 ,  42  as overflow and conveyed overhead through upper axial outlet  40   u ,  42   u . One form of cyclone separator is disclosed in U.S. Pat. No. 4,737,271, but other forms of cyclone separators are known in the art and may also be used. The cyclone separators are preferably used, for the reason that a cyclone separator is a simple, reliable and relatively inexpensive piece of equipment that is highly effective in separating lower and higher density materials. 
         [0047]    Other forms of inlet and outlet separators  40 ,  42  may be utilized. For example,  FIGS. 12   a - 12   c  illustrate another embodiment of the system  10  wherein the separators  40 ,  42  comprise a generally cylindrical chamber  40   c ,  42   c  having a vertical axis, an upper axial outlet  40   u ,  42   u  and an internal cylindrical member  40   m ,  42   m  positioned around the upper axial outlet  40   u ,  42  and depending partway downward into the separator  40 ,  42 . The cylindrical member  40   m  has a bottom axial opening  40   b ,  42   b . The mixture is introduced into the separator  40 ,  42  through one or more a tangential inlets  12   i ,  42   i , which, under gravitational and centrifugal forces, causes heavier particles to be flung against the outer wall of the separator and flow downwardly to the bottom of the separator  40 ,  42 , while the lighter, remaining, gaseous mixture is forced around the cylindrical member  40   m  in a generally downward rotary motion until conveyed into the cylindrical member  40   m  (through its bottom axial opening  40   b ,  42   b ) and finally out through upper axial outlet  40   u ,  42   u.    
         [0048]    Advantageously, the inlet separator  40  facilitates the removal of contaminants  41   c  such as oil, water and dirt from the inlet flow of fluid prior to entering the interior volume  12   v  (through passage  70 ) and directing said contaminants  41   c  into the catch reservoir  41 , thereby preventing such contaminants  41   c  from plugging or contaminating the contact cell  30   c . More advantageously, the outlet separator  42  facilitates separation of any liquid scavenger  24  from the gaseous flow  15  (coming via inlets  42   i ), that did not fall back into the source  22 , prior to the gaseous flow exiting of the vessel  12  through the outlet  12   o . Even more advantageously, the axial outlet  421  directs any separated out scavenger  24  back to the main source  22 , preferably via openings  65 . Alternatively, other forms of separators may be utilized. 
         [0049]    As noted above, the contact cell  30   c  provides a high surface area for the liquid scavenger  22 ,  24  to cling to, and/or coat, while still allowing for the gas flow  15  to move therethrough without producting a great deal of back pressure. The contact cell  30   c , along with the liquid scavenger  22 ,  24 , thereby creates a gas filtering means that results in an efficient absorption of the contaminants (such as hydrogen sulfide) by the scavenger  22 ,  24  without creating a large amount of back pressure compared to that in conventional absorption towers or columns (where gas is typically allowed to bubble through a volume of liquid scavenger). 
         [0050]    Preferably, the gas scrubbing system  10  further comprises one or more valved drains  50 ,  52  to allow an operator to drain away any contaminants  41   c  from the inlet separator  40  and/or liquid scavenger  24  from the system  10 . In this embodiment, drain  50  is associated with the inlet separator  40  to facilitate draining of any contaminants  41   c  and drain  52  is associated with the scavenger  24  reservoir to facilitate draining of said scavenger  24 . Even more preferably, the scrubber system  10  further comprises a burst plate  60  associated with the inlet  12   i , so as to protect the vessel  12  and/or any vacuum pump (not shown) that may be hooked up to the system  10  from damage due to excess pressures. Preferably the burst plate  60  is set to burst at 5 psi. 
         [0051]    Those of ordinary skill in the art will appreciate that various modifications to the invention as described herein will be possible without falling outside the scope of the invention.