Patent Publication Number: US-6986507-B2

Title: Mass transfer method

Description:
This application claims priority to the provisional U.S. patent application entitled, MASS TRANSFER METHOD AND PROCESS, filed Sep. 26, 2002, having a Ser. No. 60/413,445, the disclosure of which is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to a mass transfer process and method. More particularly, the present invention relates to a more efficient process for dispersing gas or other fluids into a liquid which may have solid suspension. The invention is useful, for example, for use in waste treatment plants for introducing oxygen into water that is utilized by biological elements that digest the waste. The invention is also useful in various other bio-reaction processes. 
   BACKGROUND OF THE INVENTION 
   In mass transfer processes such as waste treatment and bio-reactions, it is common to carry out these processes in a stirred vessel in which gas, such as oxygen or an oxygen containing gas, is introduced into a liquid containing a solid phase and/or micro-organisms therein. These aforementioned processes are oftentimes utilized by municipalities and industry to treat waste wherein the object of the process is to introduce oxygen to the liquid and the micro-organisms in the liquid can then proceed to use this oxygen to digest the waste. The gas is commonly introduced by means of sparge pipes into a tank containing liquid and/or use of a surface aerator such as an impeller. 
   During the treatment process, the gas is initially introduced to the liquid, and after a period of time, the micro-organisms will have effected sufficient reaction for clear and/or treated liquid to be run off, possibly after a settling stage. 
   One disadvantage with these processes is that they are very inefficient. The length of time required to effect the reaction can be as long as 24 hours. This time period combined with the fact that these waste treatment processes are oftentimes carried out continuously year round, provide a process that is very inefficient in terms of both time consumption and energy consumption. 
   Accordingly, it is desirable to provide a mass transfer process for effectuating the energy efficient dispersement or transfer gas or other fluids into a liquid or liquid suspension in mixing systems. 
   SUMMARY OF THE INVENTION 
   The foregoing needs are met, at least in part, by the present invention where, in one aspect, a method for maximizing mass transfer efficiency in a mixer containing a liquid or liquid suspension is provided, comprising the steps of: selecting a desired mass transfer value for an impeller; selecting an impeller; selecting a Standard Aeration Efficiency; selecting a Froude Number value for the impeller that corresponds to the Standard Aeration Efficiency; Analyzing the Standard Aeration Efficiency relative the Froude number value; and determining the optimum impeller rotational speed and optimum diameter for the impeller. 
   In accordance with another aspect of the present invention, a method for maximizing mass transfer efficiency in a mixer containing a liquid or liquid suspension is provided, comprising the steps of: selecting a desired mass transfer value for an impeller; selecting an impeller; selecting a Standard Aeration Efficiency; selecting a Surface Power Density value for the impeller that corresponds to the Standard Aeration Efficiency; analyzing the Standard Aeration Efficiency relative to the surface Power Density value; and determining the optimum power utilized to operate the mixer per surface area of the liquid. 
   In accordance with still another aspect of the present invention, a method for maximizing mass transfer efficiency in a mixer containing a liquid or liquid suspension is provided, comprising the steps of: determining a mass transfer efficiency value range for an impeller; selecting a desired mass transfer value for the impeller; identifying the impeller&#39;s diameter; identifying the impeller&#39;s revolutions per minute that corresponds to the desired mass transfer value; determining a Froude Number value that corresponds to the desired mass transfer value; and utilizing the Froude Number value to obtain the desired mass transfer efficiency of the mixer. 
   In accordance with yet another aspect of the present invention, method for maximizing mass transfer efficiency in a mixer assembly having an impeller and mixing vessel containing a liquid or liquid suspension is provided, comprising the steps of: determining a mass transfer efficiency value range for the mixing assembly; selecting a desired mass transfer value for mixing assembly; determining the power level required to circulate the liquid per the surface area of the liquid at the desired mass transfer value; determining the Surface Power Density value that corresponds to the desired mass transfer value; and determining a desired liquid surface area utilizing the Surface Power Density value. 
   In accordance with another aspect of the present invention, a method for maximizing transfer efficiency in a mixer having an impeller and containing a liquid or liquid suspension is provided, comprising the steps of: selecting a desired mass transfer value for the impeller; providing a desired operational speed at which to operate the mixer; selecting a desired Froude Number value; utilizing the desired Froude Number value to determine the diameter of the impeller; and utilizing the determined impeller diameter and the desired speed to calculate a desired power level required to circulate the liquid per surface area of the liquid. 
   In accordance with yet another aspect of the present invention, a method for maximizing transfer efficiency in a mixer having an impeller and containing a liquid or liquid suspension is provided, comprising the steps of: selecting a desired mass transfer value for the impeller; providing a desired operational diameter of the impeller; selecting a desired Froude Number value; utilizing the desired Froude Number value to determine a desired operational speed; and utilizing the determined operational speed and the desired diameter to calculate a desired power level required to circulate the liquid per surface area of the liquid. 
   There has thus been outlined, rather broadly, several features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
   In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a standard pitched blade turbine utilized in a mass transfer process in accordance with the present invention. 
       FIG. 2  is a perspective view of a disc surface aerator impeller utilized in a mass transfer process in accordance with the present invention. 
       FIG. 3  is a perspective view of an impeller having a vertical extension utilized in a mass transfer process in accordance with the present invention. 
       FIG. 4  is a perspective view of an impeller having both vertical and horizontal extensions utilized in a mass transfer process in accordance with the present invention. 
       FIG. 5  is graph depicting the Standard Aeration Efficiency values for the impellers illustrated in  FIGS. 1-4  relative to the Froude Number values. 
       FIG. 6  is graph depicting the Standard Aeration Efficiency values for the impellers illustrated in  FIGS. 1-4  relative to the Surface Power Density values. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
   The present invention provides a process for efficient mass transfer of gas or other fluids into a liquid and/or liquid suspension. The process is preferably used in conjunction with waste treatment processes and/or fermentation processes that are commonly carried out in a mixing vessel. In such an arrangement, the mass transfer process is utilized to contact oxygen gas to liquid in a mixing vessel and is referred to as pounds of oxygen per hour transferred to the liquid per horsepower or the Standard Aeration Efficiency (SAE) which is equal to the Standard Oxygen Transfer Rate or value (SOTR) divided by the shaft horsepower. It should be understood, however, that the present invention is not limited in its application to waste treatment, but, for example, can be used with other processes requiring efficient gas-liquid contact. 
   It has been discovered in accordance with the present invention that there is a peak in the mass transfer efficiency for various impellers and impeller configurations utilized in mass transfer apparatuses as a function the impeller&#39;s Froude Number relative to the SAE. Further, it has been discovered in accordance with the present invention that there is a peak in the mass transfer efficiency for various impellers and impeller configurations of mixing vessels as a function of the Surface Power Density of the impellers relative to the SAE. Thus, the efficiency in waste treatment processes and other mass transfer processes can be maximized in terms of delivering pounds of oxygen per hour to the liquid per horsepower. 
   It is understood that the Froude Number as applied in this context, a mixing vessel employing an impeller to transfer oxygen to a fluid or semi-solid fluid, is the ratio of inertia forces to the buoyancy forces. It is the function of the parameter that determines the wave action of waves. In mixing applications, the Froude Number is equal to the impeller rotational speed, N squared multiplied by the diameter (D) of the impeller divided by a constant C, where C is the speed of gravity multiplied by any conversion factors required (these factors on dependent upon the units of N and D), N is revolutions per minute (RPM) and D is measured in inches then C equals 1.39*10 6 . This equation is an example of an equation that may be used to obtain a Froude Number value. Other equations, for example, functions that yield a result proportional to the Froude Number, may be used. If the Froude Number is too low, insufficient energy exists in the surface wave of the liquid to generate ample splashing to allow for significant oxygen transfer to the liquid. Alternatively, if the Froude Number is too high, there is too much kinetic energy. The excess amount of kinetic energy causes the impeller blades to skim across the surface of the liquid and not pump the fluid adequately, resulting in inefficient oxygen transfer. However, through the above-described equation, an optimum SAE for a gas to liquid transfer for a mixing assembly can be obtained. 
   As previously described, the efficiency of mass transfer may also be optimized by analyzing the Surface Power Density relative to the SAE. It is understood that Surface Power Density is equal to the horsepower (used to operate the mixer) per one thousand square feet of liquid surface area (horsepower/1000 ft. 2  surface area). The equation is an example of an equation that may be used to obtain a power per area value. Other equations, for example, functions that yield a result proportional to the power per area value, may be used. The physics behind the Surface Power Density are such that at low power there is an insufficient amount of turbulence on the surface of the liquid to maximize transfer of gas to the liquid. Alternatively, at high power densities, the transfer process can also be inefficient. This is due to the fact that the spray may be so great that it impacts the vessel walls and loses its energy on the walls verses imparting the energy to the liquid surface to increase the turbulence and optimize oxygen transfer. Furthermore, as more energy is imparted onto the liquid surface, more liquid is aerated, creating a very low density liquid, which puts a greater load on the impeller, because it is more difficult to pull low density liquid down into the mixing vessel. However, again, through the above-described Surface Power Density equation, an optimum SAE for a mixing assembly can be obtained. 
   Referring now to the figures, wherein like reference numerals indicate like elements,  FIGS. 1-7  depict a series of impellers and their corresponding SAE values relative to the Froude Number values and Surface Power Density values.  FIGS. 5 and 6  depict the peak SAE efficiency curve for the impellers illustrated in  FIGS. 1-4  as a function of the Froude Number value and Surface Power Density value, respectively. The SAE efficiency curves depicted were obtained by operating the impellers referenced in  FIGS. 1-4  according to the maximum and minimum test conditions and parameters illustrated in Table I below. The location and values of the peak SAE may vary with different installation geometries and impeller submergence. 
   
     
       
         
             
             
             
             
             
             
             
             
             
           
             
                 
               TABLE I 
             
             
                 
                 
             
             
                 
               A200 
               A200 
               A240 
               A240 
               A245 
               A245 
               R335 
               R335 
             
             
                 
               min 
               max 
               min 
               max 
               min 
               max 
               min 
               max 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
          
             
               Speed rpm 
               27 
               51 
               37 
               58 
               35 
               55 
               42 
               105 
             
             
               Diameter inch 
               96 
               148 
               83 
               113 
               95 
               118 
               60 
               103 
             
             
               Diameter Tank 
               663 
               831 
               663 
               960 
               663 
               831 
               663 
               960 
             
             
               Equivalent, inch 
             
             
               Power, shaft Hp 
               23 
               137 
               129 
               213 
               33 
               152 
               13 
               198 
             
             
               Shp/k ft{circumflex over ( )}2 
               9.8 
               36.5 
               13 
               47 
               14 
               58 
               5.5 
               82 
             
             
               Froude No. 
               0.055 
               0.180 
               0.100 
               0.230 
               0.084 
               0.210 
               0.096 
               0.480 
             
             
               Volume, gal 
               201,000 
               481,000 
               196,000 
               566,000 
               298,000 
               474,000 
               194,000 
               547,000 
             
             
               Depth, inch 
               134 
               321 
               129 
               216 
               161 
               317 
               117 
               340 
             
             
               No. of Runs 
               64 
                 
               107 
                 
               44 
                 
               115 
             
             
                 
             
          
         
       
     
   
   Referring now to  FIG. 1 , a pitched blade turbine  10  that splashes on the surface of the liquid is depicted. As can be observed on  FIGS. 5 and 6 , the impeller&#39;s  10  optimum SAE value is approximately 3.3 (delivers 3.3 pounds of oxygen per hour to the liquid per horsepower) at a Froude Number value of about 0.14 to about 0.15 and a Power Surface Density value number of about 15.0 to about 18.0. Thus, when employing a standard pitched turbine  10 , the SAE can be maximized by either using the Froude Number value and its corresponding equation or the Surface Power Density value and its corresponding equation. 
   When utilizing the Froude Number value to maximize efficiency, the optimum value is approximately 0.14, the therefore impeller rotational speed (RPM in the test conditions) and the impeller diameter can be modified accordingly, to achieve maximum SAE efficiency. Similarly, the optimum Surface Power Density value is about 15.0 to about 18.0 and therefore mixing vessel dimensions (liquid surface area) and the power level required to circulate the liquid may be adjusted accordingly to obtain maximum gas to liquid transfer efficiency. 
     FIG. 2  depicts a disc surface aerator impeller  12 . As can be observed on  FIGS. 5 and 6 , the impeller&#39;s  12  optimum SAE value is approximately 4.2 (delivers approximately 4.2 pounds of oxygen per hour to the liquid per horsepower). When using this impeller  12 , the maximum efficiency Froude Number value is about 0.16 to about 0.18 and the Power Surface Density value is about 22.0 to about 25.0. Thus, when employing a disc surface aerator impeller  12 , the SAE can be maximized by either using the Froude Number value and its corresponding equation or the Surface Power Density value and its corresponding equation. 
   When utilizing the Froude Number value, the optimum value is approximately 0.17, therefore impeller rotational speed (RPM in the test conditions) and the impeller diameter can be modified accordingly, to achieve SAE efficiency. Similarly, the optimum Surface Power Density value is about 25.0 and therefore mixing vessel dimensions (liquid surface area) and the power level required to circulate the liquid may be adjusted accordingly to obtain maximum gas to liquid transfer efficiency. 
   Referring now to  FIG. 3 , an impeller  14  having a vertical extension  15  utilized in a mass transfer process is illustrated. Again, as can be observed on  FIGS. 5 and 6 , the impeller&#39;s  14  optimum SAE value is approximately 3.67 (delivers approximately 3.67 pounds of oxygen per hour to the liquid per horsepower). In order to obtain maximum efficiency, the Froude Number value should be about 0.13 to about 0.17 and the Power Surface Density value number should be about 20.0 to about 30.0 as depicted on  FIGS. 5 and 6 . Thus, when employing an impeller  14  such as the one depicted in  FIG. 3 , the SAE can be maximized by either using the Froude Number value and its corresponding equation or the Surface Power Density value and its corresponding equation. 
   When utilizing the Froude Number value, the optimum value is approximately 0.15, therefore impeller rotational speed (RPM in the test conditions) and the impeller diameter can be modified accordingly, to achieve SAE efficiency. Similarly, the optimum Surface Power Density value is about 20.0 to about 30.0 and therefore mixing vessel dimensions (liquid surface area) and the power level required to circulate the liquid may be adjusted accordingly to obtain maximum gas to liquid transfer efficiency. 
     FIG. 4  depicts an impeller  16  having both vertical and horizontal extensions  15  and  17  respectively, utilized in a mass transfer process. As can be observed on  FIGS. 5 and 6 , the impeller&#39;s  16  optimum SAE value is approximately 4.0 (delivers approximately 4.0 pounds of oxygen per hour to the liquid per horsepower). As illustrated by the efficiency curve of  FIG. 5 , the optimum Froude Number value is about 0.12 to about 0.14. As depicted by  FIG. 6 , the optimum Power Surface Density value is about 23.0 to about 28.0. Thus, when employing an impeller  16 , such as the one depicted in  FIG. 4 , for example, the SAE can be optimized by either using the Froude Number value and its corresponding equation or the Surface Power Density value and its corresponding equation. 
   When utilizing the Froude Number value to maximize mass transfer efficiency, the optimum value is about 0.12 to about 0.14, therefore the impeller rotational speed (RPM in the test conditions) and the impeller diameter can be modified accordingly, to achieve maximum SAE efficiency. Similarly, the optimum Surface Power Density value figure is about 23.0 to about 28.0 and therefore mixing vessel dimensions (liquid surface area) and the power level required to circulate the liquid may be adjusted accordingly to obtain maximum gas to liquid transfer efficiency. 
   It should be understood that the above described impellers and corresponding test data are representative examples only, used to demonstrate how the Froude Number and the Surface Power Density values can maximize mass transfer efficiency. The Froude Number and Surface Power Density values can be utilized to maximize the mass transfer efficiency of various impeller and mixing systems. 
   For example, the optimum Froude and Surface Power Density values can be determined for virtually any impeller design, similarly to the above-referenced impeller examples. Upon this determination, a table of attributes that includes SAE values relative to both Froude and Surface Power Density values can be provided which will enable the efficiency of virtually any mass transfer assembly to be maximized. Alternatively, certain applications may be limited by vessel size, impeller design, and/or power level or certain applications may require a specific vessel size, impeller design and/or power level. Given the restraints or requirements of the mixing system, the Froude Number and Surface Power Density values can be maximized to the system&#39;s mass transfer efficiency. 
   In accordance with another embodiment of the present invention, the mass transfer efficiency of a mixer or mixer assembly can be optimized by employing multi-step processes that utilize the Froude Number value and/or the Surface Power Density value as parameters. 
   One multi-step process includes first determining a mass transfer efficiency range for an impeller. This step can be accomplished by determining a mass transfer efficiency value range for an impeller. This is preferably accomplished by performing a series of trial runs in a mixing vessel wherein the rotational speeds and power level are varied and the resulting oxygen transfer rates are plotted and recorded, providing an transfer efficiency curve. Next, the desired mass transfer value is selected from the transfer efficiency curve. Preferably, this value is the optimum oxygen transfer efficiency. Next, the impeller&#39;s diameter and the revolutions per minute (RPM) that correspond to the desired mass transfer value are recorded. Finally, the Froude Number value that corresponds to the desired mass transfer value is determined by utilizing the formula: Froude Number=N 2 (Impeller Diameter (D))/Constant (C), where C is 1.39×10 6 , N is revolutions per minute and D in measure in inches. The determined Froude Number value may now be used as a parameter to design and manufacture efficient oxygen transfer impellers. Please note the above described steps of the process are not limited to the order in which they are described and may be carried out in various combinations. 
   Another multi-step process in an accordance with an embodiment of the present invention includes first determining a mass transfer efficiency range for a mixer system. This step, like the previous described process, can be accomplished by first determining a mass transfer efficiency value range for the particular mixing system. This is preferably accomplished by performing a series of trial runs in a mixing vessel wherein the rotational speeds and power level of the mixing system are varied and the resulting oxygen transfer rates are plotted and recorded, providing an transfer efficiency curve. Next, the desired mass transfer value is selected from the transfer efficiency curve. Preferably, this value is the optimum oxygen transfer efficiency. Next, the power level required to circulate the liquid in the vessel per the surface area of the liquid at the desired mass transfer value is determined. Finally, the Surface Power Density value that corresponds to the desired mass transfer value is determined by utilizing the formula: Surface Power Density=horsepower per one thousand square feet of liquid surface area. The determined Surface Power Density value may now be used as a parameter to maximize the oxygen transfer efficiency of mixers and mixer assemblies. Please note the above described steps of the process are not limited to the order in which they are described and may be carried in various combinations. 
   Preferably, both of the above described processes are used in combination with one another wherein the determined Froude Number value and the determined Surface Power Density value are both used as parameters to design and manufacture efficient oxygen transfer impellers and mixing systems. In addition, both these values may also be utilized to optimize the oxygen transfer efficiency of impellers and mixing already in existence. 
   Furthermore, both of the aforementioned processes can be optimized in terms of SAE efficiency by adjusting the submergence level of the impeller within the liquid contained in the mixing vessel. 
   The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirits and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.