Abstract:
An apparatus and method for generating an aerosol wherein the aerosol has a known concentration of metals or other chemical components, and wherein the aerosol concentration remains constant over a long period of time, rendering the apparatus and method suitable for application as a reference standard

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation-in-Part of U.S. Provisional Application No. 60/463,799, filed Jan. 14, 2005. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    This invention relates to a Quantitative Aerosol Generator method and apparatus. 
         [0004]    2. Background of the Invention 
         [0005]    In order to monitor emissions from a stack, a continuous emissions monitoring system is often used. Emissions must be monitored to demonstrate compliance with emissions standards. It would be useful to be able to generate an aerosol with a known concentration that can be used to verify the accuracy and precision of continuous emissions monitoring systems. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    This invention is directed to an apparatus and method for producing a continuous aerosol stream having a known concentration of aerosolized metals. The apparatus and methods of this invention can be used to evaluate the bias, precision, and linearity of sampling approaches used to test the concentration of metals in stack gas emissions at hazardous waste incinerators, for example, and provides a much improved alternative to current reference methods. 
         [0007]    An object of the invention is to provide an apparatus and method for generating an aerosol wherein the aerosol has a known concentration of metals or other chemical components. 
         [0008]    Another object of the invention is to provide an apparatus and method for generating an aerosol wherein the aerosol has a desired droplet size. 
         [0009]    A still further object of the invention is to provide an apparatus and method for continuous analysis of a liquid containing one or more chemical components of interest wherein the liquid is continually passed through the apparatus. 
         [0010]    The above objects are accomplished with a quantitative aerosol generator (QAG) apparatus comprising a nebulizer, a droplet-size selector and a drying chamber. 
         [0011]    The above objects are accomplished with a method that comprises passing the liquid of interest through a nebulizer to create liquid droplets, passing the liquid droplets through a droplet-size selector and then drying the selected droplets in a drying chamber. 
         [0012]    In one embodiment, the QAG generates an aerosol with a known concentration of a desired analyte by using a solution wherein the analyte concentration is known. In this embodiment, the solution is provided to the QAG from a large reservoir. 
         [0013]    In an alternate embodiment, the QAG generates an aerosol with an unknown concentration by using a solution with an unknown concentration. The unknown concentration can then be determined by using known sampling and testing techniques. In this embodiment, the solution is also provided to the QAG from a large reservoir. 
         [0014]    In still another embodiment, the QAG generates an aerosol with an unknown concentration by using a solution with an unknown concentration, wherein the solution is continuously flowing through the QAG system, rather than being contained in a large reservoir. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The nature, principle and utility of the present invention will be clearly understood from the following detailed description when read in conjunction with the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a schematic view of the quantitative aerosol generator apparatus. 
           [0017]      FIG. 2  is a more detailed schematic view of the quantitative aerosol generator depicted in  FIG. 1 . 
           [0018]      FIG. 3A  is a schematic view of the nebulizer shown in  FIGS. 1 and 2  above. 
           [0019]      FIG. 3B  is a schematic view of the nebulizer and droplet generation chamber. 
           [0020]      FIG. 4  is a schematic view of the droplet size-selection chamber. 
       
    
    
       [0021]    The drawings are for illustrative purposes only and are not drawn to scale. In the drawings, the same numbers are used for the same part or portion throughout the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    A number of terms will be used throughout to describe the invention. Among those terms, the following are defined as follows;
       Accuracy. The agreement between an experimentally determined value and an accepted reference value   Analyte. Element of interest in the analysis, e.g. As, Cd, Cr, Pb and Hg.   Analyte Line. X-ray emission line used to quantify the analyte.   Attenuation. Reduction of X-ray intensity due to energy dissipation in filter and deposit.   Calibration. The process of comparing a sampling or instrumental response with a known parametric value for the purpose of obtaining a quantitative relationship between the response and the parametric value that can be used to determine the parametric value for an unknown sample.   Detection Limit. The smallest concentration that a particular measurement can detect.   EDXRF. Energy dispersive X-ray fluorescence   EPA Practical Limits of Quantitation. The lowest level above which quantitative results may be obtained with an acceptable degree of confidence. 16      Interference. An undesired positive or negative output caused by a substance other than the analyte.   Limit of Detection. Lowest concentration that can be detected by an instrument without correction for the effects of sample matrix or method-specific parameters such as sample preparation.   Limit of Quantitation. Lowest concentration that can be reliably achieved within specified limits of precision and accuracy during routine laboratory operating conditions.   NIST. National Institute of Standards and Technology.   Precision. The degree of mutual agreement between individual measurements of a parameter having the same value, namely repeatability and reproducibility.   Relative Percent Standard Deviation. The standard deviation of a set of measurements divided by the mean of the set of measurements times 100.   SRM. Standard Reference Materials.   Standard Deviation. The square root of the variance, or the precision of repeated measurements.   Standard. A value for a parameter that has been established by authority, custom, or agreement to serve as a model or rule in the measurement of quantity or the establishment of a practice or procedure.   Traceability to NIST. A documented procedure by which a measured response is related to a standard with an accuracy defined by and certified by the National Institute of Standards Technology (NIST)   Uncertainty. A statistically defined value associated with a single measurement or a value associated with a group of measurements that defines the range and probability of additional measurements falling within the defined range, and can include allowance for both systematic and random sources of error.   Unknown. A sample submitted for analysis whose elemental concentration is not known.   XRF. X-ray fluorescence.       
 
         [0044]    In one embodiment of the invention an aerosolized metal is produced by a Quantitative Aerosol Generator (QAG) which uses a collision nebulizer to combine cooled and saturated air with a NIST-traceable solution of a known concentration of a metal of interest. The aerosol containing the metals is then dried and transported in an entraining air stream for analysis by XRF or other analytical methods. In one embodiment this method is useful for measuring the concentration of metals in stack gas, although the invention is not limited thereto. In one preferred embodiment the method has demonstrated applicability to the measurement of metals ranging from magnesium (Mg, atomic number 12) to uranium (U, atomic number 92) on the periodic table in a concentration ranges from five to one thousand micrograms per cubic meter (5-1000 μg/m 3 ). The method&#39;s precision at concentrations of about 100 μg/m 3  is +/− 2% with an accuracy of 5%. This represents a significant advance over known methods which are normally not able to provide a precision of less than about 20%. 
         [0045]    A preferred embodiment of a quantitative aerosol generator (QAG) according to the invention is described in more detail in reference to  FIGS. 1 and 2 . The QAG generates an aerosol from a solution containing one or more analytes. A relatively large amount of the solution is provided in a solution reservoir  41 . The solution and solution reservoir  41  are placed on a balance  42 . The balance  42  is used to measure the mass of the solution flowing into the nebulizer  31 . A computer  43  records the changing mass of the solution in the reservoir  41 . A peristaltic pump  33  circulates the solution between the droplet generation chamber  30  and the reservoir  41 . The use of a relatively large solution reservoir  41  is advantageous because the relatively large amount of solution allows for only a minimal concentration change due to loss of water vapor. 
         [0046]    The droplet generation chamber  30  and the nebulizer  31  are discussed in more detail with reference to  FIG. 2 . The amount of solution flowing into the droplet generation chamber  30  via inlet  202  is roughly equal to the amount of solution flowing out of the droplet generation chamber  30  via outlet  206 . Therefore, depth of the solution  204  in the droplet generation chamber remains roughly constant. The solution of interest  204  is aerosolized by a collision nebulizer  31  located within the droplet generation chamber  30 . Nebulizer air, which is generated by a method that will be described in greater detail below, enters the nebulizer  31  at inlet  210 . The air is forced through the nebulizer  31  and out into the droplet generation chamber  30  via small holes  212  in the side of the nebulizer  31 . The total flow rate of the resulting aerosol can be changed by changing the number of small holes  212  in the side of the nebulizer  31 . For example, nebulizers with 1, 6 or 12 small holes may be used to achieve a desired total flow rate of the resulting aerosol. A bottom portion of the nebulizer  31  is submerged in the solution  204  and the force of the nebulizer air as it is pushed out of the holes  212  draws the solution  204  up through small holes  214  at the bottom of the nebulizer  31 . The liquid spray  208  exiting the nebulizer  31  collides with the side  216  of the droplet generation chamber  30 , and the force of the impact creates aerosolized liquid droplets. 
         [0047]    The method for generating nebulizer air will be described with reference to  FIG. 2 . Air generated by compressor  21  is directed through filter units  22  in order to remove any undesirable contaminants, such as oil, from the air. In one preferred embodiment he QAG requires at least 20 psi and 50 slpm (2 cfm) of instrument air. Two air compressors are used to push air through the QAG in order to aerosolize the metal-containing NIST-traceable solution. The air generated by the first compressor is directed to the collision nebulizer at a rate of approximately 13 slpm. This air, hereafter referred to as “nebulizer air,” is combined with the NIST-traceable solution to create the aerosolized metals. The air generated by the second compressor, hereafter referred to as “drying air,” is used to help dry the aerosolized metals and is directed into drying chamber at a rate of approximately 34 slpm. The drying air is actively dried with a refrigerated compressed air dryer such as those manufactured by Speedair. 
         [0048]    A pressure regulator  23  is used to control the pressure of the nebulizer air flowing into the QAG system. A solenoid valve  24  serves as a safety shut-off switch, so that the QAG can be shut down if the flow rate of the main exhaust drops below a given set point. A rotometer  25  measures the flowrate of the nebulizer air. The air saturator  26  is a bubbler containing distilled water. The nebulizer air is diffused into the water through a small filter and the air leaving the air saturator  26  is saturated at room temperature. A ball valve  27  is used as a shut-off valve for the nebulizer air. The nebulizer air is then passed through a cooler  28  containing an ice bath in order to cool the nebulizer air to 32° F. Although not depicted here, a cooling nebulizer saturator  29 , droplet generation chamber  30 , nebulizer  31 , and droplet size-selection chamber  32  are all housed within the cooler  28 . 
         [0049]    Following the cooler  28 , the nebulizer air is passed through the cooling nebulizer saturator  29 , which saturates the nebulizer air at 32° F. The cooling nebulizer saturator  29  is similar to air saturator  26 . The cold, saturated nebulizer air then flows to the nebulizer  31  through inlet  210 , as described above with reference to  FIG. 2 . It is preferable that the nebulizer air be cold and saturated in order to allow for accurate calculation of the loss of water vapor. 
         [0050]    After collision against the chamber wall  216 , the aerosolized liquid droplets pass out of the droplet generation chamber  30  and into the droplet size-selection chamber  32 . The droplet size-selection chamber  32  is shown in more detail in  FIG. 3 . In the droplet size-selection chamber  32 , small droplets of aerosol pass through the droplet size-selection plate  34  and into the drying chamber  40 . The droplet size-selection plate is designed to allow only small droplets to pass through into the drying chamber  40 . The plate consists of a PTFE gasket and PTFE funneling piece. Large droplets impact the side  302  of the chamber  32 , impact the droplet size-selection plate  34  at the top of the chamber  32 , or fall back into droplet generation chamber  30  due to a lack of force. All large droplets are recovered in the solution  204  ( FIG. 2 ) at the bottom of the droplet generation chamber  30 . The droplet size-selection chamber  32  allows control of the size of the resulting analyte particles. This prevents the generation of large particles that might be lost in the transport system and thus contribute to uncertainty in the resulting aerosol concentration. In other embodiments, the droplet size-selection chamber  32  may use cyclonic or plate impaction. 
         [0051]    Drying air, which is described is greater detail below, and nebulized liquid droplets are combined in the drying chamber  40 , which is heated to approximately 250° F. The droplets are dried in the chamber  40  and the resulting aerosol is then transported from the chamber  40  to a sampler via outlet  50 . The drying chamber  40  is heated by a tape heater  36  and a blanket heater  37  ( FIG. 1 ). A temperature controller maintains the temperature of the heaters at approximately 250° F. 
         [0052]    The method for generating the drying air will be described in more detail with reference to  FIG. 1 . The air generated by compressor  11  is directed through a drier  12 , where the air is actively dried with a refrigerated compressed air dryer. The drying air then passes through filter units  13  in order to remove any undesirable contaminants, such as oil, from the drying air. A pressure regulator  14  controls the flowrate of the drying air, which is measured by a rotometer  15 . A ball valve  16  is used as a shut-off valve for the drying air. The drying air is then split into two lines downstream of the drying air ball valve  16  and each line is passed through a tube heater  39  just before the air enters the drying chamber  40 . The tube heaters  39  are preferably maintained at 300° F. by temperature controllers. After being heated by the tube heaters  39 , the drying air enters the drying chamber  40  through the drying air ring  35 . The drying air ring  35  is located just above the droplet size-selection plate  34 , and the air enters the drying chamber  40  through a series of holes in the ring. The air increases the drying rate of atomized droplets and acts as a sheath that keeps the droplets from hitting the chamber walls. 
         [0053]    The drying air, nebulizer air, and the solution flow from their sources to the other QAG components via PFA and stainless steel tubing. All of the saturators, chambers, and the nebulizer used in the QAG are stainless steel. The saturators are lined with PFA to prevent corrosion. The drying chamber and some of the post-drying chamber transport components are insulated with 1″ thick fiber glass. Any parts that come into contact with the drying air, nebulizer air, solution, and the aerosol are corrosion resistant. 
         [0054]    In a first embodiment, the QAG described above generates an aerosol with a known concentration of a desired analyte. In this embodiment, the solution in the reservoir  41  has a known concentration. The concentration of the aerosol can then be calculated as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     N 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             W 
                             i 
                           
                           - 
                           
                             W 
                             f 
                           
                         
                         ) 
                       
                        
                       
                         C 
                         s 
                       
                        
                       E 
                     
                     V 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0000]    where 
         [0055]    C N =aerosol concentration 
         [0056]    W i =initial weight of the reference solution reservoir (before pump  33  is turned on) 
         [0057]    W f =final weight of the reference solution reservoir (after pump  33  is turned on and equilibrium is reached) 
         [0058]    C s =concentration of the analyte in the solution 
         [0059]    E=aerosol generation and transport efficiency 
         [0060]    V=volume of nebulizer air and drying air 
         [0061]    The aerosol generation and transport efficiency can be calculated as follows: 
         [0000]        E=M   t /( W   i   −W   f ) C   s   Equation 2 
         [0000]    where 
         [0062]    M t =the total mass collected when the QAG-generated and transported aerosol is sampled at the QAG outlet  50   
         [0063]    An aerosol with a known concentration is useful for several applications, including verifying the accuracy and precision of a sampling method. For example, an aerosol with a known concentration can be used to verify the accuracy and precision of a continuous emissions monitoring system. 
         [0064]    In a second embodiment, the QAG generates an aerosol with an unknown concentration of an analyte. In this embodiment, the solution in the reservoir  41  has an unknown concentration. Using a known sampling method, the concentration of the aerosol can be determined and the concentration of the solution can be calculated using the above equations. 
         [0065]    A third embodiment is similar to the second embodiment, except that the solution with an unknown concentration is not contained in a reservoir. The solution is continuously flowing through the QAG system via the inlet  202  and outlet  206  in the droplet generation chamber  30  (see  FIG. 2 ). This embodiment has several applications, including continuous monitor of species in such solutions as drinking water or process effluents. In this embodiment, the pump  33 , reservoir  41 , balance  42  and computer  43  are eliminated from the QAG system. 
         [0066]    The QAG optionally comprises an aerosol form modifier (not shown) at the outlet  50  of the QAG. The form modifier treats the resulting aerosol with conditioners to modify the resulting aerosol. For example, catalysts or combustion chambers could be introduced down stream of the QAG to impart different characteristics to the aerosol or other aerosols, gases or vapors could be blended down stream of the QAG. For example, the addition of an alternative flow pattern down stream of the QAG could direct the QAG generated aerosol through a catalyst that might convert a mercuric chloride aerosol or a mercuric nitrate aerosol from its ionic form to its elemental form to evaluate the performance of mercury measurement instruments and their response to different forms of mercury. 
         [0067]    The particle size of the aerosol generated by the QAG can be adjusted by adjusting the parameters of the droplet-size selection chamber  32 . Additionally, the particle size can be adjusted by adjusting the solution concentrations. Because of the potential to control the particle size of the aerosol, it is possible to use the QAG for particle size and transport studies as well as other research projects. 
         [0068]    The QAG may additionally be applicable in areas such as inorganic or organic analytes, aqueous or non-aqueous solutions and generation of aerosols of varying particle sizes. 
         [0069]    While the invention has been described by reference to the preferred embodiments described above those skilled in the art will recognize that the invention as described and illustrated can be modified in arrangement and detail without departing from the scope of the invention.