Abstract:
The invention relates to an apparatus and method for more effectively dissolving a gas into a liquid stream. A gas is introduced into a liquid stream, at atmospheric pressure, to create a mixed/dissolved stream. The mixed/dissolved stream is then pressurized to an elevated pressure and directed into a hydrocyclone where the gas is further dissolved and more thoroughly distributed in the liquid thereby creating a more intimately mixed/dissolved stream. The intimately mixed/dissolved stream is then directed to a pressure retention vessel where the liquid is held at a predetermined pressure for a predetermined time period for effectively treating, disinfecting and/or clarifying said intimately mixed/dissolved stream. The treated intimately mixed/dissolved stream can then be directed to further processing steps.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to an apparatus and method for dissolving a gas into a liquid stream.  
       BACKGROUND OF THE INVENTION  
       [0002]     The introduction of a gas into a liquid stream is common in many different processing operations. Accordingly, the present invention finds applications in a number of fields such as the treatment of waste and water streams, and disinfecting and clarifying potable water and other fluids. The present invention also finds applications in the food and pharmaceutical industry, as well as industries requiring products that require efficient mass transfer of ozone, air, or other gas for the purpose of flotation, clarification, and/or disinfection. More specifically, the introduction of a gas, such as ozone, air or oxygen, into a liquid stream is common in many disinfecting, treatment and clarifying processes. Very often, ozone is introduced into drinking water sources, ballast water, waste water streams and/or cooling water streams so as to disinfect, treat and/or clarify such liquids due to its superior disinfecting effectiveness over other gases, such as chlorine. Many different methods and techniques have been designed to try and improve the various disinfecting, treatment and clarifying process. When dealing with ozone, additional factors must be considered namely, the unstable nature of the gas which tends to result in higher equipment and operational costs as well as less compact systems. Accordingly, there is a desire to improve the techniques and methods used for introducing, mixing, blending and dissolving a gas into a liquid stream, especially for processes that involve the use of ozone gas.  
         [0003]     Canadian Patent Application No. 2,301,583 (Separation Technologies Group PTY. LTD.) discloses a method and apparatus for mixing a first material and a second material, wherein the first material comprises a mixture of two or more dissimilar components that are to be separated. The &#39;583 application discloses the use of a hydrocyclone to mix different materials together prior to their separation. The use of a hydrocyclone in the pre-treatment of the materials to be separated was found to improve the subsequent separation of the materials. The &#39;583 application also discusses the benefits of introducing air or a gas into the mixture of materials to be separated prior to the mixture entering the hydrocyclone. More specifically, the aeration or gasification of the first material facilitates the separation of the dissimilar components in the first material as one of the dissimilar components is entrained or otherwise associated with the air or gas bubbles that are formed from mixing the first and second materials together. It is the formation of millions of tiny gas bubbles that facilitates the subsequent separation of materials as the bubbles entrain or suspend the solid particles or droplets, bringing them to the surface during the subsequent processing steps. The &#39;583 application does not disclose the complete dissolution of a gas within a liquid, as it relies on the formation of gas bubbles within the mixed stream to assist in subsequent separation processes. As well, the system is not necessarily well suited for the dissolution of large amounts of ozone in a liquid stream.  
         [0004]     U.S. Pat. No. 6,629,686 (Morse et al.) discloses a process and system for dissolving gas into a liquid at greater concentrations and saturations than previous methods known in the art. A hydrocyclone is used to introduce an intended gas into the liquid stream to be treated. The amount of gas dissolved in the liquid can be optimized by adjusting various parameters of the hydrocyclone, namely by altering the pressure of the incoming liquid, changing the aspect ratio of the inlet, and varying the diameter D and length L of the barrel. Upon exiting the hydrocyclone, the mixed liquid and gas stream enters a diffusion chamber, which converts the radial spin of energized liquid from the hydrocyclone into laminar axial flow. The diffusion chamber is disposed within a pressure chamber, which includes an upper gas region and a lower liquid region. The diffusion chamber is located in the lower liquid region of the pressure chamber so that only large bubbles of undissolved gas coalesce and rise into the gas region of the pressure chamber, while the dissolved gas and micro-size gas bubbles that are retained in the liquid flow with the liquid into the liquid region of the pressure chamber. The gas in the upper region of the pressure chamber is recycled back through the system to the hydrocyclone so that gas is not unnecessarily wasted, and the liquid and dissolved gas mixture can exit the pressure chamber and be held in a storage tank or can be passed along to the next process step in the system. While the &#39;686 patent discloses the use of a pressure chamber, the pressure chamber does not serve as the primary treatment or disinfection vessel. Furthermore, the system does not achieve complete dissolution of the gas into the liquid as it relies on the creation of micro-bubbles to distribute the gas evenly through the liquid.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention, however, provides an apparatus and method for more effectively dissolving a gas into a liquid stream. According to one aspect of the invention there is provided an apparatus for dissolving a gas into a liquid stream for the treatment, disinfection and/or clarification thereof. The apparatus comprises means for introducing a gas, at atmospheric pressure, into the liquid stream to create a mixed stream, and a pump having an inlet for receiving the mixed stream of liquid and gas, and an outlet for discharging the mixed stream at an elevated pressure. At least one hydrocyclone is connected downstream from the pump outlet for more thoroughly mixing and dissolving the gas into the mixed stream, creating a more intimately mixed stream, the at least one hydrocyclone including at least one inlet for receiving the pressurized mixed stream and having one outlet for discharging the more intimately mixed stream. A pressure retention vessel is connected downstream from the hydrocyclone for holding the intimately mixed stream at a predetermined pressure for a predetermined time period for effectively treating and/or disinfecting the intimately mixed stream, thereby creating a treated stream. The pressure retention vessel has an inlet for receiving the intimately mixed stream from the hydrocyclone, a first outlet for discharging the treated intimately mixed stream, and a second outlet for discharging any residual gas that has escaped from the intimately mixed stream. Pressure control means are provided in communication with the at least one hydrocyclone and the pressure retention vessel for adjusting the pressure of the mixed and intimately mixed streams to ensure effective dissolution of the gas within the liquid stream.  
         [0006]     According to another aspect of the invention there is provided a method for dissolving a gas into a liquid stream comprising the steps of (i) injecting a gas into a liquid stream at atmospheric pressure to create a mixed stream, (ii) pressurizing the mixed stream to a predetermined level, (iii) directing the mixed stream into a hydrocyclone to create a more intimately mixed stream, (iv) directing the intimately mixed stream from the hydrocyclone to a pressure retention vessel and holding the intimately mixed stream in the pressure retention vessel at a predetermined pressure for a predetermined time period to ensure the proper disinfection or treatment of thereof, thereby creating a treated stream.  
     
    
     BREIF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will be better understood with reference to the detailed description taken in combination with the drawings in which:  
         [0008]      FIG. 1  is a schematic view of the mass transfer apparatus according to a preferred embodiment of the invention;  
         [0009]      FIG. 2  is a partial cutaway view of the hydrocyclone of the mass transfer apparatus of the present invention;  
         [0010]      FIG. 3A  is an elevation view of a spin inducer used in conjunction with the hydrocyclone of the mass transfer apparatus according to the present invention;  
         [0011]      FIG. 3B  is another elevation view of the spin inducer as seen 90° from the view shown in  FIG. 3A ;  
         [0012]      FIGS. 3C-3D  are respective top and bottom views of the spin inducer of  FIGS. 3A-3B ;  
         [0013]      FIG. 4A  is a is a cross-sectional view of a hydrocyclone liner used in the hydrocyclone of  FIG. 2 ;  
         [0014]      FIG. 4B  is a side view of the spin inducer of  FIGS. 3A-3D  attached to the hydrocyclone liner;  
         [0015]      FIG. 4C  is a top view of the hydrocyclone liner;  
         [0016]      FIG. 4D  is a cross-sectional view of the configuration of  FIG. 4B  showing the flow characteristics inside the spin inducer and hydrocyclone liner;  
         [0017]      FIG. 5  is a side view of the pressure retention vessel of the mass transfer apparatus of the present invention; and  
         [0018]      FIG. 5A  is a top view of the pressure retention vessel of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     Referring to the drawings, there is shown in  FIG. 1 a  schematic of the mass transfer apparatus  10  according to the present invention. The apparatus includes a liquid feed line  12  located proximal to an untreated liquid source  11  and connected to the suction side of a progressive cavity or helical rotor pump  14 . Liquid entering the liquid feed line  12  is controlled by a foot valve  16  located at the inlet end of the liquid feed line  12 , the foot valve  16  being in communication with the untreated liquid source  11 . A second valve  18  located in the feed line  12  downstream from the foot valve  16  controls the amount of untreated liquid entering the pump  14 . When in operation, valve  18  is pinched, or partially closed, so as to maintain a slight vacuum at the pump inlet. A pressure indicator  20  is mounted at the inlet to the pump  14 , which is designed to show the amount of vacuum present at the pump inlet or suction. A gas feed line  22  is provided for introducing the intended gas, at atmospheric pressure, into the liquid stream. Ozone, air, oxygen or any other suitable gas, depending on the type of process, may be used. The apparatus  10  is particularly well-suited for use with ozone gas since the gas is introduced at atmospheric pressure. Due to the unstable nature of ozone gas, handling of the gas becomes more difficult when higher pressures are required; therefore the present invention avoids any such difficulties as the ozone is introduced at atmospheric pressure.  
         [0020]     The gas feed line  22  connects with the liquid feed line  12  at junction  23  to create a mixed stream  24  of liquid and gas which then enters the pump  14 . A gas flow meter (or rotameter)  25  and needle valve  26  are used to control and provide a visual reading of the amount of gas that is being introduced into the liquid stream. Progressive cavity/helical rotor pumps are able to accept the mixed stream  24  with the entrained vapours/gas without detrimental cavitation, which is what makes this type of pump ideal for use in the subject apparatus. Once the gas has been introduced into the liquid stream and the mixed stream  24  enters pump  14 , the liquid and gas are pressurized to between about 80-150 psig depending on the type of gas and liquid stream involved in the process. When ozone is the gas being used, for instance in a water treatment process, the mixed untreated water ozone stream is pressurized to about 150 psig. This pressure has been found to be optimal for ozone, as much more ozone can be dissolved into the liquid at this pressure, thereby increasing its effectiveness as a disinfectant. Conventional mass transfer systems have been unable to achieve the same level of dissolution of ozone into the liquid stream.  
         [0021]     If high-pressure gas (i.e. more than 150 psig) other than ozone is being introduced into the liquid stream, an alternate set-up can be used where a gas feed line  22 ′ connects with the liquid stream on the discharge side of the pump  14  (as opposed to the suction side of the pump  14 ) at junction  23 ′ to create mixed steam  24 ′ on the discharge side of the pump  14 . Once the mixed stream  24  ( 24 ′) has been created and is pressurized to the desired level, the mixed stream  24  ( 24 ′) enters a shearing hydrocyclone  28  where the gas is further sheared and dissolved and therefore is more completely mixed with the liquid. Once again, this system proves advantageous when using ozone as the gas, since the ozone is completely dissolved in the liquid rather than being diffused or bobbled into the liquid, as is common with many conventional mass transfer systems. Complete dissolution of the ozone gas into the gas is preferable as it provides the most complete contact with the liquid for more effective treatment/disinfection thereof.  
         [0022]     As shown in  FIG. 2 , the hydrocyclone  28  comprises an outer housing vessel  54  that is divided into two sections by mounting plate  56 . The hydrocyclone vessel  54  can have one or more tangential inlet ports  30 , which may be equipped with ramps to initially induce a rotational flow at the head of the hydrocyclone  28 . The vessel  54  contains one or more hydrocyclone liners  58 , depending on the desired flows and pressures of the system. Not only can one or more hydrocyclone liners  58  be enclosed in one vessel  54 , but more than one vessel containing a number of hydrocyclone liners can be used depending on the size and economics of the apparatus.  
         [0023]     A spin inducer  60  (see  FIGS. 3A-3D ) is also housed within the vessel  54  and is attached to the upper portion of the hydrocyclone liner  58 . The spin inducer  60  includes one or more inlet openings  62  in communication with the one or more tangential inlet ports  30  of the vessel  54 . As the mixed stream of liquid and gas enters the hydrocyclone  28  through the one or more tangential inlet ports  30 , it is directed towards the openings  62  of the spin inducer  60 , which force the mixed stream  24  ( 24 ′) of liquid and gas to travel in a circular motion. According to one embodiment, the spin inducer  60  is secured to the hydrocyclone liner  58  by means of a flexible lip  64  ( FIG. 3B ) located on the bottom rim of the spin inducer  60  which mates with a corresponding lip  65  ( FIG. 4A ) on the hydrocyclone liner  58 , when the spin inducer  60  is made of a flexible material such as polyurethane. Alternate materials for both the spin inducer  60  and the hydrocyclone liner  58  include various grades of stainless steel. If the material being used for the spin inducer  60  is of a rigid nature, such as steel or ultra high molecular weight polyethylene, the spin inducer  60  is preferably threaded to the hydrocyclone liner  58 . The hydrocyclone liner  58  with the spin inducer  60  attached thereto is shown in  FIG. 4B .  
         [0024]     From the spin inducer  60 , the liquid and gas mixed stream  24  enters the neck of the hydrocyclone liner  58 . The reducing internal diameter of the hydrocyclone liner  58  (see  FIGS. 4A and 4C ) causes the gas and liquid mixed stream  24  ( 24 ′) to accelerate to the single outlet  32  of the hydrocyclone  28 . The typical flow pattern created by the hydrocyclone liner  58  is shown in  FIG. 4D . The cyclonic action of the entire feed stream (i.e. the liquid and gas mixed stream  24 ) as it enters the hydrocyclone  28  promotes instantaneous, intimate contact between the liquid and the gas. As the mixture accelerates, any gas bubbles are sheared, then dissolved, and are dispersed evenly throughout the liquid forming a homogeneous, stable, aerated and blended product stream or more intimately mixed/dissolved stream  34 . With no other exit or outlet provided in the hydrocyclone  28  for the less dense, entrained gas to escape, the gas follows the liquid to the only outlet  32  provided which ensures the thoroughly mixed/dissolved and blended product stream  34  at the outlet  32  of the hydrocyclone  28 .  
         [0025]     Referring back to the  FIG. 1 , the more intimately mixed/dissolved stream  34  of completely dissolved gas and liquid exits the hydrocyclone  28  via outlet  32  and is directed toward a pressure retention vessel  36 . The intimately mixed/dissolved stream  34  remains in the pressure retention vessel  26  for a pre-determined period of time required for the proper disinfection or treatment of the intimately mixed/dissolved stream  34  to create a treated stream  44 . The pressure within the pressure retention vessel  36  is maintained at a predetermined level to ensure that the gas remains completely dissolved in the liquid, and is not permitted to escape. This provides for more effective disinfection and/or treatment of the intimately mixed/dissolved stream  34  as there is more complete contact between the gas and the liquid to be treated. This is particularly true in the case of ozone. As well, the gas—liquid (e.g. ozone—liquid) contact time required in the present system is significantly reduced due to the complete dissolution of the gas within the liquid which, therefore, decreases the overall “treatment time”. Furthermore, various sizes of pressure retention vessels may be used which allows for more complete usage of the gas. In the case of ozone gas, the more complete usage of the gas reduces ozone generation capacities for any given treatment or disinfection operation.  
         [0026]     The pressure across the hydrocyclone  28  and the pressure retention vessel  36  is controlled by a back pressure control valve  37  located downstream of the pressure retention vessel  36 . The back pressure control valve  37  can be hand controlled, controlled by a programmable logic controller (PLC), or controlled by a conventional pressure control loop. A pressure indicator  38  is provided at the inlet to the hydrocyclone  28 , which provides a reading of the pressure of the mixed stream  24  ( 24 ′) as it enters the hydrocyclone  28 . A second pressure indicator  40  is located downstream of both the hydrocyclone  28  and the pressure retention vessel  36  which shows the pressure at the outlet  32  of the hydrocyclone  28  as well as the pressure within the pressure retention vessel  36 .  
         [0027]     As shown in  FIGS. 5 and 5 A, the pressure retention vessel  36  includes an inlet  66  for receiving the intimately mixed/dissolved stream  34 , and has two outlets  68 ,  70 . The first outlet  68  is for the disinfected/treated intimately mixed/dissolved stream or treated stream  44  and the second outlet  70  provides a means for evacuating any residual gas that may have escaped from the liquid or accumulated in the pressure retention vessel  36 . The gas is evacuated through the second outlet  70 , and can then be recycled through a vapour-return line  42  to the inlet or suction side of the pump  14 , so that no gas is wasted. As is shown more clearly in  FIG. 5 , the first outlet  68  extends into the pressure retention vessel  36  so that it is in contact with the liquid in the vessel. This ensures that only the liquid, treated stream  44  exits through the first outlet  68 .  
         [0028]     Once the disinfection/treatment period is complete, the treated stream  44  can be directed to a storage tank or can be put through additional processing steps. It is only once the disinfection/treatment period is complete that the pressure downstream of the pressure retention vessel is reduced, thereby allowing any remaining vapours to be released in micro-bubbles, which promotes additional contact between the liquid and the gas. If the treated stream  44  is going through additional processing steps, the micro-bubbles that are released as the pressure is reduced not only serve to promote further contact between the liquid and the gas, but also serve to facilitate additional processing steps. For instance, the treated stream  44  can be directed from the pressure retention vessel  36  and fed into a dissolved air flotation system  46  (shown in dotted lines in  FIG. 1 ) for further treatment where the micro-bubbles act as a gas supply for the additional processing steps. The dissolved air flotation system  46  produces a purified stream  47 . When the gas being used is ozone, the purified stream  47  from the dissolved air flotation system then passes through a degassing vessel  48 . In the degassing vessel  48 , any residual ozone gas is separated out of the stream  47  and is directed to an ozone destruct chamber  49  for a final treatment before being released from the ozone destruct chamber as air  50 . The purified stream  47  exits the degassing vessel  48  as a disinfected, clean effluent stream  52 , in accordance with practices known in the art. Alternatively, the treated stream  44  from the pressure retention vessel can pass directly to the degassing vessel  48  and ozone destruct chamber  49 . As well, a portion of the disinfected/treated intimately mixed stream  44  can also be recycled back into the liquid feed line  12  via a liquid return line  54  as it exits pressure retention vessel  36 .  
         [0029]     While the present invention has been described with respect to certain preferred embodiments, it will be understood by persons skilled in the art that variations or modifications can be made without departing from the scope of the invention as described herein.