Abstract:
Method and apparatus for detecting the presence of or removing chlorine in a gaseous product stream from a chlorine dioxide generator by mixing gaseous ammonia or ammonia vapors with the product stream and either observing smoke generated by the reaction of ammonia with any chlorine present in the chlorine dioxide product stream or separating reaction products from the chlorine dioxide.

Description:
BACKGROUND OF THE INVENTION 
     The present invention pertains to the production of chlorine dioxide gas and in particular the detection of chlorine gas in the chlorine dioxide gas product. 
     Chlorine dioxide is known to be a useful gas for controlling microbial contamination of a myriad of products, most importantly for disinfecting drinking water. In the treatment of water with chlorine dioxide for drinking and other uses, it is beneficial to have pure chlorine dioxide that is substantially free of molecular chlorine. Molecular chlorine can react with organic compounds in the water to produce undesirable toxic chlorinated by-products, such as trihalomethanes (THM&#39;s), whereas pure chlorine dioxide does not react with the organic species to produce, to any significant degree, these undesirable chlorinated disinfection by-products (DBP&#39;s). 
     Chlorine dioxide is also used for bleaching paper pulp. The presence of molecular chlorine in the chlorine dioxide gas can react with lignin to produce undesirable environmental pollutants, such as dioxins. Pure chlorine dioxide does not produce dioxins. 
     Another use for chlorine dioxide is in the sterilization of medical products. Use of chlorine dioxide for such applications is described in U.S. Pat. Nos. 4,504,422, 4,681,739 and 4,908,188. The materials from which certain medical devices are constructed are functionally unaffected by chlorine dioxide, but are incompatable with molecular chlorine. If molecular chlorine is present in the chlorine dioxide gas, some of the products being sterilized may be spoiled or rendered unsafe for use. Because chlorine dioxide has a unique chemistry it is compatible with many products that are used in health care. 
     For some applications of chlorine dioxide, small amounts of chlorine can be tolerated without rendering the chlorine dioxide unsuitable for the particular application. 
     Because chlorine dioxide can not be shipped or stored as a compressed gas it is almost always generated at the point of use. On site production of chlorine dioxide can be achieved through several processes. For example, sodium chlorate (NaClO 3 ) can be used as the starting material and reacted with HCl according to the following reaction: 
     
       
         2NaClO 3 +4HCl&gt;2ClO 2 +Cl 2 +2NaCl+2H 2 O. 
       
     
     Sodium chlorite (NaClO 2 ) can be used as a starting material, which is reacted with chlorine gas according to the following equation to form gaseous chlorine dioxide. 
     
       
         2NaClO 2 +Cl 2 &gt;2ClO 2 +2NaCl. 
       
     
     Because chlorine is often an undesirable contaminant in chlorine dioxide, as set forth above, it is desirable to detect and measure the concentration of chlorine in the chlorine dioxide product. Because the amount of chlorine in the chlorine dioxide produced by some methods is a function of the operating parameters employed, (e.g. concentration, time, temperature, pressure), measurement of chlorine levels in the chlorine dioxide will be useful in helping to alert the operator of the generating process when chlorine in the chlorine dioxide exceeds an acceptable level. 
     U.S. Pat. Nos. 5,110,580, 5,234,678, 5,290,524 and 5,326,546 all describe gas-solid methods for producing chlorine dioxide gas. The principal chemical reaction utilized in these processes is that defined by the reaction of dry sodium chlorite and chlorine gas as the starting materials, the second equation noted above. 
     Chlorine gas, upon contact with solid sodium chlorite in a reaction cartridge, is replaced by chlorine dioxide. As the sodium chlorite is used up, the reaction front moves forward. A low, but increasing, concentration of unreacted gaseous chlorine in a high concentration of chlorine dioxide (ClO 2 ), along with inert gaseous carrier, eventually reaches the outlet of the reaction cartridge. The level of emergent chlorine initially detectable at the outlet is called the “breakthrough concentration”. Breakthrough detection at an appropriate level signals the need to switch the chlorine-bearing input gas stream to a fresh, unreacted, sodium chlorite cartridge. 
     Prior to the present invention, there appears to have been no simple, effective method for detecting low breakthrough concentrations of chlorine (e.g. 500 ppm of chlorine) in the presence of, for example, 40,000 ppm of chlorine dioxide. Methods based upon inexpensive forms of mass spectrometery, for instance, would be adversely affected by the proximity and breadth of the peaks representing chlorine and chlorine dioxide. Similarly, the two molecules have overlapping ultraviolet absorbence spectra. Therefore, ultraviolet (UV) spectrophotometry is not a practical analytical technique. This is especially true when the concentration of chlorine dioxide is high relative to the concentration of chlorine. There are various methods for dissolving the gas mixture in water and measuring the concentrations in the liquid. These, under certain circumstances, may be useful for detecting the presence of chlorine, but quantitative measurements are limited in accuracy because of the imprecision of the dissolution processes. Also, solution-based processes involve multiple steps and are often difficult to automate. Therefore, there is a need to find a method for detecting and measuring chlorine gas contamination of gaseous chlorine dioxide at various levels of chlorine gas contamination. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes the discovery that in the gas phase, chlorine reacts with ammonia, while chlorine dioxide does not so react. The reaction of molecular chlorine gas with ammonia in the gas phase produces a white smoke, which is believed to be ammonium chloride particles. It is further believed that the overall reaction proceeds according to the following: 
      2NH 3 +Cl 2 &gt;NH 2 Cl+NH 4 Cl. 
     Mixing an excess quantity of ammonia with a sample gas suspected to contain chlorine results in the density of the smoke increasing with the concentration of the chlorine gas present in the sample. According to one embodiment of the present invention the smoke can be detected by optical means, such as light absorbence, light scatter or light transmission. When a beam of light shines through a cell containing a sample of chlorine gas in chlorine dioxide gas with air and ammonia, the amount of transmitted light decreases with the amount of chlorine in the sample. According to a preferred embodiment of the present invention an effective method of detecting the presence of smoke produced by the reaction of ammonia and chlorine utilizes radiation in the red to infrared region of the spectrum (greater than 630 nanometer wavelength). It is believed that light of any wavelength that is not absorbed by chlorine, ammonia, or chlorine dioxide can be used. Other parts of the spectrum, e.g. yellow light may work. 
     According to the present invention, a sample of chlorine dioxide can be introduced into a transparent cell having means to emit and detect radiation of a certain frequency passed through this sample. The sample is premixed with an excess amount of ammonia so that any chlorine present in the sample will react with the ammonia to produce smoke. The smoke is then detected by a change in the light transmittance through the cell. The amount of decrease in transmittance is directly related to the quantity of chlorine present in the sample. 
     According to the present invention, depending upon the cycle requirements, sampling can take place periodically with the sample being drawn from the outlet of a chlorine dioxide generator. Auxiliary equipment can be used to clean the cell to avoid contaminants after each sample is removed from the cell, immediately before a new sample is introduced to the cell, or after Cl 2  is detected, given that the chlorine and ammonia reaction leaves no interfering residue. 
     Therefore, in one aspect the present invention is a method for detecting chlorine breakthrough during production of chlorine dioxide in a chlorine dioxide generator comprising the steps of: withdrawing a sample of chlorine dioxide from a product outlet of the generator; mixing the sample of chlorine dioxide with gaseous ammonia; and introducing the mixture of chlorine dioxide and gaseous ammonia into a test receptacle wherein smoke generated by reaction of chlorine in the chlorine dioxide with ammonia can be observed. 
     In another aspect the present invention is a method including the step of measuring density of any smoke observed in said test receptacle. 
     In still another aspect, the present invention is an apparatus for detecting the presence of chlorine in a product stream from a chlorine dioxide generator comprising in combination; a test receptacle having an inlet and an outlet, the test cell fabricated from optically transparent material, means to connect the inlet to a source of the product chlorine dioxide stream and a source of gaseous ammonia or ammonia vapor, and means to detect products of reaction between the ammonia and any chlorine present in the chlorine dioxide product stream. 
     In yet another aspect the present invention is a process for eliminating chlorine from a gaseous stream containing chlorine dioxide, chlorine and a diluent comprising the steps of; reacting the stream containing chlorine dioxide, chlorine and a diluent with gaseous ammonia to produce a mixture of chlorine dioxide, diluent and ammonia chloride; and separating the chlorine dioxide and diluent from the ammonia chloride. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1, is a schematic diagram of an analytical process according to the present invention. 
     FIG. 2, is a schematic representation of a test cell according to the present invention. 
     FIG. 3, is a plot of chlorine content against differential direct current voltage. 
     FIG. 4, is a plot of excess chlorine content against any NEP values both for actual values and increased values. 
     FIG. 5, is a schematic flow diagram illustrating a separation process according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention utilizes the discovery that chlorine gas reacts with ammonia (NH 3 ), while chlorine dioxide gas does not. When chlorine reacts with ammonia in the gas phase it produces a white smoke, believed to contain ammonium chloride particles. If an excess of ammonia is mixed with the gas to be measured, the density of the smoke increases with the concentration of the chlorine in the sample. According to one aspect of the invention smoke generated by the reaction can be detected by optical means, such as light absorbence, light scatter or light transmission. When a beam of light shines through a transparent cell containing a sample of chlorine in chlorine dioxide and air into which ammonia has been introduced, the amount of transmitted light decreases with the amount of chlorine in the sample. In other words, if no chlorine is present there will be no decrease in the amount of light passing through the cell, other than losses that would normally be encounted from the cell itself and/or the solution. 
     There are other techniques, which may be useful for detecting and measuring the density of the smoke. However, many of the available techniques, although they would work are not preferred because of the corrosive nature of chlorine and/or chlorine dioxide in the sample. For instance, corrosion may pose a serious problem in any system where the gas sample, especially humidified gas, comes in contact with metallic components, or with components made of many types of non-metallic materials. 
     Referring to FIG. 1, the process of the present invention is designated generally by the numeral  10 . According to the process of the present invention an inlet conduit  12  having a control valve  14  is connected to an outlet conduit of the chlorine dioxide generator  11  so that a sample of the product chlorine dioxide can be introduced into an apparatus according to the present invention. 
     Valve  14 , is in turn connected via conduit  16  to a central manifold conduit  18 . A source of water is introduced via conduit  20 , control valve  22  and conduit  24  into the manifold  18 . A source of ammonia  26  is connected via conduit  28 , control valve  30  and conduit  32  to manifold  18 . Lastly, a source of air is introduced via conduit  34 , valve  36  and conduit  38  into the manifold  18 . Manifold  18  in turn is connected to the inlet of a sample block or sample holder  40 . Inlet  42  of the sample block  40  permits the various fluids or gases to be introduced into a sample tube  44 . Sample cell  44  has an outlet conduit  46 , which is connected via conduit  48 , control valve  50 , and conduit  52  to a vacuum ejector  54 . Sample cell  44 , sometimes referred to as a test receptacle, test cell, sample tube or sample holder, is made of a transparent material, preferably glass, so that corrosion does not become a problem. The glass sample cell  44  can be mounted vertically as shown in FIG.  1 . The glass sample cell  44  has a light source  56  and a light receiver  58 , which are connected to test instrumentation via leads  60  and  62  respectively. Leads  60  are connected to a DC (direct current) power supply to power the light source. Leads  62  are connected to a digital volt-ohm-multimeter. Such instruments are well known to workers skilled in the art. 
     FIG. 2, shows one form of the sample block or apparatus  40 , which can be made from three separate blocks of material such as acrylic resins or other structural plastic materials. The first or top block  70  contains an inlet conduit  72 , which communicates with a vertical conduit  74 . Conduit  74  extends to the top surface of block  70  where it is closed by a cap or other suitable closure device  76 . Conduit  74  terminates at an entrance to the test receptacle  44 . Test receptacle  44  is contained in the middle block  78 . Middle block  78  has suitable conduits  80 ,  87  to position the light emitter  56  and the light receptacle  58 . The bottom of glass of sample tube  44  is closed by the bottom block  84 , which has a vertical conduit  86 , which extends from the bottom of the sample cell  44  to the bottom surface  87  of block  84 . Conduit  86  is closed by a cap or other suitable closure device  88 . An outlet conduit  90  extends from the vertical conduit  86  to an outer surface of bottom block  84  so that the sample can be withdrawn. As stated above the sample block  40  can be made from separate pieces of structural plastic materials which can be held together by through bolts  92 ,  94  and nuts  96 ,  98 . “O” rings  57  and  59  are used to prevent leaking of the sample from the top or bottom of the sample tube  44 . 
     The apparatus shown schematically in FIGS. 1 and 2 has been designed to work in conjunction with a production scale chlorine dioxide generator. In view of the fact that the production scale chlorine dioxide generator used with the present invention operates under a vacuum (typically 15 inches of mercury) the sample and the sample cell  44  must be under slightly higher vacuum to draw the sample from the generator into the cell  44 . 
     The analytical process according to the present invention can by cyclical in nature. According to one embodiment of the present invention the entire cycle lasts approximately 2.5 minutes, though it could be considerably longer or shorter. At the beginning of the cycle valves  14  and  50  are opened to draw a sample through the cell. The light emitter  56  shines on the receiver  58  and the output of the receiver is amplified into a signal that is used as the zero signal. Thereafter valve  30  is opened and ammonia begins to flow through the cell containing the sample. The signal from the infrared receiver is measured relative to zero and the difference is interpreted as to the concentration of the chlorine. This is shown in the graph of FIG.  3 . Thereafter, valves  14  and  30  are closed and valve  22  is opened. Water flows through the cell and tubing to dissolve any build up of ammonium chloride. Valve  22  is then closed and valve  36  is opened to vent the cell and drain the wash water from the cell. Valves  36  and  50  are then closed and the system is idle until the next cycle is initiated. The exact sequence may be varied. For example, if the idle time were long, it might be desirable to wash the cell  44  immediately prior to introduction of the chlorine dioxide sample in order to wash out any condensation left in the cell. On the other hand if the idle time is short it might be desirable to have the idle time immediately after the wash cycle to give the cell more time to drain before sampling. 
     According to the present invention the source of ammonia vapor for reaction ammonia used is an aqueous solution of ammonia with the concentration of ammonia between 10 and 30 percent. Tests have shown that there is no difference in effectiveness between reagent grade ammonia solutions and household ammonia solutions. For example, the solution of ammonia can be injected into sample cell and enough volatilizes to cause reaction. For each sample, only between one and two cubic centimeters of ammonia solution or less is necessary. Gaseous anhydrous ammonia could also be used but it is not preferred because of problems associated with handling gaseous anhydrous ammonia. 
     The light used in detecting the smoke should be of a wavelength that does not cause breakdown of the chlorine dioxide. It is well known that chlorine dioxide is subject to photolysis, e.g., ultraviolet light decomposes chlorine dioxide into chlorine and oxygen. Tests according to the present invention have shown that chlorine produced by photolytic decomposition of chlorine dioxide reacts with ammonia to generate smoke, thereby leading to a false positive signal. Tests have also shown that lower frequency light in the visible range also appears to cause such a breakdown. Samples of pure, dilute chlorine dioxide with gaseous ammonia in a glass flask remain clear yellow in the dark, but turn cloudy within a few seconds in low-intensity ambient fluorescent light. It has also been found that mixtures of pure chlorine dioxide gas and ammonia gas exposed to high intensity incandescent light in a turbidimeter deposit a white coating on the inside of the glass sample tube. The shape of the deposited spot of this white coating is the same as the shape of the light beam. It is not clear in either the case of incandescent or fluorescent light, whether the breakdown of chlorine dioxide is caused by visible light or by high frequency components of light which are otherwise in the visible range. Experiments have shown, light in the red to infrared region of the spectrum (greater than 630 nanometers wavelength) show no breakdown of chlorine dioxide. The device according to a preferred embodiment of the present invention utilizes light in the infrared spectrum. 
     In view of the fact that clouding of a glass sample cell can occur by deposits of ammonium chloride produced when chlorine is present, a preferred method and apparatus includes the cycle in which the sample cell and all the tubing that might contain ammonium and chlorine are flushed out with clean water and then drained. Since there is a possibility that the sample tube might cloud up over extended periods due to insoluble deposits or chemical reactions in the glass sample cell, the cycle of the present invention has been designed to be self-zeroing. In order to accomplish self-zeroing according to the invention, after each water/flush cycle the infrared light is passed through the sample cell containing air, chlorine dioxide and chlorine (if there is any chlorine to sample), but no ammonia. The signal is then measured from the infrared receiver. This signal intensity is used as the zero level which corresponds to the presence of zero concentration of chlorine. Changes in signal intensity from this zero level are measured to indicate the presence of chlorine when ammonia is added to the cell. It is also possible to zero the analyzer when the cell is only filled with air. However, measurements have shown no difference in light transmission, regardless of whether that cell is filled with air, chlorine, chlorine dioxide or any mixture of these gases as long as there is no ammonia present in the cell. Measurements using red to infrared light (radiation) have also shown no difference in the signal between air in the cell versus a blend of air, chlorine dioxide and ammonia, as long as there is no chlorine present in the cell with this mixture. 
     As there is a possibility of water condensation inside the sample cell one embodiment may include means for heating the sample cell or otherwise evaporating the condensate between cycles. This can be accomplished by well known techniques. 
     The process and apparatus of the present invention is intended first for use as a chlorine breakthrough detector in the product gas from a gas-solid chlorine dioxide generator. Therefore, a simple device can be constructed where the emphasis is placed upon reliability, low cost and low maintenance. Typically, devices of this general design would be used to detect the presence or absence of chlorine and can be operated unattended without maintenance for days or weeks. 
     Tests were carried out using a HACH turbidimeter. This instrument measures the side-scatter of light from a high intensity incandescent bulb in units of NEP (Nephelos). It was found necessary to equip the device with infrared filters to prevent fogging of the sample cell. When so equipped, the device was able to quantitatively and repeatedly measure the concentration of smoke and hence indicate the levels of chlorine gas and the chlorine dioxide gas, with chlorine at the 100 to 1000 ppm level. Results of these tests are plotted in FIG.  4 . 
     According to another part of the invention, the discovery that ammonia reacts with chlorine in the gas phase while it does not react with chlorine dioxide makes it possible to separate chlorine from a gaseous stream containing chlorine dioxide and a diluent such as air. Referring to FIG. 5, the contaminated gas containing chlorine dioxide, chlorine and a diluent is represented by arrow  102 . Ammonia gas is introduced into the contaminated stream as shown by arrow  104 . Contained in the reaction mixture represented by arrow  106  are gaseous chlorine dioxide, a diluent, and solid particles of ammonium chloride. The mixture of chlorine dioxide, diluent and solid ammonium chloride particles can be passed through a filter, represented by dashed line  108 , which traps and removes the solid ammonium chloride particles represented by arrow  112 . This leaves a product stream represented by arrow  110  which contains chlorine dioxide and diluent. This process can be extremely important when it becomes necessary to remove even small amounts of chlorine from a chlorine dioxide/diluent gas stream. Any excess ammonia gas present in the product stream  110  can be removed by wellknown techiques. 
     Having thus described our invention, what is desired to be secured by Letters Patent of the United States is set forth in the appended claims, which should be read without limitation.