Patent Publication Number: US-2017349436-A1

Title: Method of chlorine dioxide generation in highly acidic environments

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Pursuant to 35 U.S.C. §119(e), this application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/346,961, filed on Jun. 7, 2016, the entire contents of which are incorporated herein by this reference. 
    
    
     BACKGROUND 
     (1) Field of Endeavor 
     The present disclosure relates generally to the field of efficiency optimization of chlorine dioxide generating facilities, and more particularly, to using a deliberate and targeted method of introducing a chloride donor compound into the acid and/or reducing agent feeds of the facility to boost efficiencies of the overall facility. 
     (2) Description of Related Art 
     Many commercial chemical facilities generate chlorine dioxide (ClO 2 ) on a large scale. As one example, bleach plants in pulp mills almost exclusively use ClO 2  generators that reduce sodium chlorate with a reducing agent, such as methanol or hydrogen peroxide. The ClO 2  chemical reaction involves the reduction of sodium chlorate with the reducing agent in the presence of sulfuric acid to produce gaseous ClO 2  and other byproducts. A chloride ion (Cl—) is an intermediate species needed for the formation of chlorine dioxide in both methanol-type and hydrogen peroxide-type generating processes. 
     The ClO 2  generating process stops if acidities become too high. For example, at acidities above about 10 N, depending on the generator conditions, the chloride intermediate cycle in a methanol reaction will be stopped, and the reaction will produce chlorine gas. This is called a “Whiteout.” At this point, the production of ClO 2  ceases, chlorine is produced, and the chemical facility shuts down to stop the addition of chemicals, therefore resulting in the loss of production of the chemical facility. Whiteouts can be stopped by adding water to reduce high acidity concentrations. Another method of preventing Whiteouts is by adding a chloride donor in the chemical process to enable the continued generation of ClO 2  in highly acidic environments. Most modern ClO 2  plants use a diluted crystal sodium chlorate feed that is low in chlorides in order to minimize chlorine production. Some plants add sodium chloride in bulk in the chlorate feed to maintain a residual chloride content throughout the reaction volume to avoid Whiteouts and to run in the upper values of the acidity control range. The widespread chloride content in the reaction volume results in generator inefficiencies by the unwanted production of chlorine. 
     Inefficient mixing of acid creates localized boundaries of elevated acid concentration that result in a chlorine generating reaction, especially near the injection location. The partial creation of chlorine results in an efficiency loss in the chemical facility. The inefficient reduction of chlorate into chlorine occurs near the acid injection point where the acidity is highest. 
     Therefore, what is needed is a process by which a chloride donor is introduced into the generator or recirculation lines using a deliberate and targeted method that sustains the chloride intermediate cycle in high acidity zones, thereby boosting the overall efficiency of the chlorine dioxide producing chemical facility. 
     SUMMARY 
     The method is directed to boosting the efficiency of chlorine dioxide production in a chemical facility. A chloride donor is introduced into a feed of sulfuric acid or a reducing agent using a deliberate and targeted method. Both of the acid feed and reducing agent feed are added into the recirculation lines or directly into the generator of a chlorine dioxide producing facility. The chloride donor provides the necessary chloride ion at high local acidity in the wake zone of the injection location, or when overall acidities reach high levels in the generator, thereby enabling greater efficiency in the reduction of chlorine dioxide from sodium chlorate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows one embodiment of a large scale ClO 2  generator. 
         FIG. 2  shows a segment of a representative recirculation line where a representative injector for the acid feed or reducing agent feed introduces acid into the recirculation line. 
         FIG. 3  shows a diagram of turbulence created by a cylinder corresponding to different Reynolds numbers. 
         FIG. 4  shows one embodiment of a large scale ClO 2  generator having a ClO 2  feed drawn from a ClO 2  product line. 
         FIG. 5  shows a segment of a representative recirculation line where a representative injector for the acid feed or reducing agent feed and a representative chloride donor injector, where the chloride donor is introduced in the wake zone of the acid injector or the reducing agent injector. 
         FIG. 6  shows a side view and a top view of one embodiment of an acid injector. 
         FIG. 7  shows a side view of one embodiment of an acid injector or a reducing agent injector. 
         FIG. 8  shows a top view of one embodiment of an acid injector or a reducing agent injector. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings, the process for chlorine dioxide generation in highly acidic environments will now be described with regard for the best mode and the preferred embodiment. In general, the process is directed to optimizing the efficiency of chlorine dioxide producing chemical facilities. The embodiments disclosed herein are meant for illustration and not limitation of the invention. An ordinary practitioner will appreciate that it is possible to create many variations of the following embodiments without undue experimentation. 
     Referring to  FIG. 1 , an exemplary chlorine dioxide producing chemical facility comprises a chlorine dioxide generator  10 , a reboiler  11 , and one or more recirculation lines  12  that fluidly connect the generator  10  and the reboiler  11 . Many such systems comprise a pump  14  for circulating fluid through the recirculation lines  12 . Referring to  FIG. 2 , the recirculation lines  12  comprise one or more injection ports  15  configured to receive various injectors  20 , such as one or more acid injectors  20  that introduces acid from one or more acid feeds  21  into the recirculation lines  12 . The ports  15  can also be configured for receiving reducing agent injectors  20  for receiving a reducing agent from a reducing agent feed  32 . In one embodiment, the injectors  20  emit a plume  16  from the tip  21 , where the plume  16  develops a highly acidic zone  17 . The highly acidic zone  17  occurs downstream from the injector  20  in or near the wake zone  18 . 
     In general, there are many commercially available processes for the large-scale generation of chlorine dioxide (ClO 2 ). As one example, bleach plants in pulp mills almost exclusively use ClO 2  generators that reduce sodium chlorate with methanol or hydrogen peroxide. The ClO 2  chemical reaction involves the reduction of sodium chlorate with a reducing agent in the presence of acid, such as sulfuric acid, to produce gaseous ClO 2 , a byproduct sulfate salt, some amounts chlorine, and other minor byproducts. While there are several acids that are suitable for ClO 2  production, the present discussion is presented in terms of the exemplary acid of sulfuric acid. There are many forms of writing equations for this reaction. The following are two representative examples of this reaction with methanol acting as the reducing agent: 
       6NaClO 3 +CH 3 OH+4H 2 SO 4 →6ClO 2 +CO 2 +5H 2 O+2Na 3 H(SO 4 ) 2  
 
       or 
       6NaClO 3 +1.5CH 3 OH+4H 2 SO 4 →6ClO 2 +1.5HCOOH+4.5H 2 O+2Na 3 H(SO 4 ) 2  
 
     Chlorine dioxide can also be produced by reducing sodium chlorate with hydrogen peroxide in a manner similar to that shown above. An example of one such reaction is: 
       NaClO 3 +0.5H 2 O 2 +0.5H 2 SO 4 →ClO 2 +H 2 O+0.5O 2 +0.5Na 2 SO 4  
 
     The hydrogen peroxide reaction may be carried out in different acid concentrations producing neutral sodium sulfate or acidic sodium sesquisulfate byproduct salt. 
     Chloride Intermediate Reactions 
     A chloride ion (CF) is an intermediate species needed for the formation of chlorine dioxide in the ClO 2  generating process, whether the reducing agent is methanol or hydrogen peroxide. 
     Methanol Chemistry 
     The following reaction equations show an example of the chloride ion intermediate cycle represented by HCl sustained in the methanol-based process. The HCl intermediate is produced and consumed as the reaction proceeds: 
       HClO 3 +HCl→HClO 2 +HClO  Reaction (1)
 
       HClO 3 +HClO 2 →2ClO 2 +H 2 O  Reaction (2)
 
       HClO+CH 3 OH→HCl+HCHO+H 2 O  Reaction (3)
 
     At high acidities the following reaction, which produces chlorine, becomes an inefficient reaction: 
       HClO+HCl→Cl 2 +H 2 O(High acidity reaction)  Reaction (4)
 
     Peroxide Chemistry 
     The following equations show an example of the chloride ion intermediate cycle in the hydrogen peroxide based process: 
       2ClO 3   − +2+4H + →2ClO 2 +Cl 2 +2H 2 O  Reaction (5)
 
       Cl 2 +H 2 O 2 →2Cl − O 2 +2H +   Reaction (6)
 
     This overall reaction can be summarized as: 
       2ClO 3   − +H 2 O 2 +2H + →2ClO 2 +2H 2 O+O 2   Reaction (7)
 
     Whiteouts 
     At acidities typically above about 10 N, depending on generator conditions, the chloride intermediate cycle in Reactions (1), (2) and (3) will be stopped, and the reaction will produce chlorine gas (Reaction (4)). This phenomenon is known in the industry as a “Whiteout.” The production of ClO 2  ceases, chlorine is produced, and the chemical facility interlocks, or shuts down, to stop the addition of chemicals, therefore resulting in the loss of production of the chemical facility. Whiteouts can be stopped by adding water to reduce acidity or by adding chloride—the required intermediate—to generate ClO 2 . During normal operations, the localized high acidities adjacent to the acid feed  31  injection point can drive part of the production of chlorine via Reaction (4), generating chlorine alongside chlorine dioxide, thereby decreasing the efficiency of the plant. 
     Stoichiometric Consumption and Efficiency of Sodium Chlorate 
     The stoichiometric consumption of sodium chlorate required to produce chlorine dioxide is about 1.58 T NaClO 3 /T ClO 2 . In general, chlorine dioxide-producing chemical facilities have chlorate consumptions typically from about 1.64 to about 1.8 NaClO 3 /T ClO 2  or lower. These correspond to efficiencies between about 96% and 87% and lower, respectively. There are many non-chemical operational factors that contribute to inefficiencies in ClO 2  producing chemical facilities, such as equipment inefficiencies, measurement inaccuracies, contaminants, human error, and other similar factors. However, even during the rigors of performance tests in new chemical facilities when the equipment is in optimal condition, efficiencies close to or above about 96% are not encountered. 
     Chemical Feed Addition in a ClO 2 —Producing Chemical Facility 
     In one embodiment, an acid, such as sulfuric acid, is introduced into a chlorine dioxide producing chemical facility near the outlet of the reboiler  11  into a recirculating generator solution stream several orders of magnitude larger than the flow of the acid itself. For instance, in an 80 tons per day (tpd) ClO 2  chemical facility, the flow of acid mixed with dilution water is about 11 gallons per minute (gpm). This is introduced into generator solution stream with flow of about 12,000 gpm flowing at about 12 ft/sec. The acid is introduced through one or more acid injectors  20  that typically protrude some distance inside the inner wall of the recirculation line  12  (See  FIG. 2 ). The pipe for the recirculation line  12  increases in diameter progressively downstream from the acid injection point at the injection port  15 . The pipe is designed to promote acid mixing by the Venturi effect. Residence time of acid in this recirculation pipe is brief, typically only a about few seconds. Conversion reaction of sodium chlorate to ClO 2  or Cl 2  occurs at a relatively rapid rate. Therefore, most of the ClO 2  generating reaction is complete before the reactants leave the recirculation line  12  and enter into the generator  10 . 
     Mixing Inside Recirculation Lines 
     Flow in the recirculation lines  12  is turbulent at Reynolds numbers of about 10 5  to about 10 6 . At these Reynolds numbers, the wake zone  18  is very narrow and disorganized, as shown in  FIG. 3 . Feeding the acid into the recirculation line  12  at these Reynolds numbers creates zones of high acidities, as shown in  FIGS. 2 and 3 . The rapid nature of the reaction allows Reaction (4) to proceed in the high acidity zones after the acid is injected into the recirculation lines  12 . 
     ClO 2 -producing chemical facilities typically must dilute the acid feed  31  with water, which is referred to as acid dilution water, through a dilution water feed  33 . This water is added to reduce the acid heat of dilution and to promote mixing prior to entering the reaction stream. The sulfuric acid concentration mostly used in ClO 2  producing chemical facilities is about 92%. The acid is diluted with dilution water to a solution that ranges from about 60 weight percent (wt %) to about 70 wt % prior to entering the recirculation line  12 . After acid dilution, the resulting acid concentrations added to the recirculation line  12  are about 14 N to about 22 N. The acid feed  31  concentration is therefore well above the concentration threshold for favoring localized chlorine production according to Reaction (4).  FIG. 3  shows flow patterns through a cylindrical body, such as a pipe, for acid injectors  20  and different Reynolds numbers typically encountered in the pipes of the recirculation lines  12 . 
     Referring to  FIG. 4 , one embodiment of an acid injector  20  that introduces the diluted acid feed  31  from the tip  21  of the injector  20 . The acid introduced remains in the tip  21  vortex wake zone  18  near the injection point, as shown in  FIG. 2 . 
     Since the reduction of sodium chlorate into ClO 2  occurs very rapidly, a significant portion of the reaction occurs while the acid plume  16  is still in the vortex wake zone  18  of the tip  21 . This results in a localized highly acidic zone  17  where the chloride intermediate is depleted, resulting in the production of some chlorine via Reaction (4) and in decreased efficiency of the overall chemical facility. 
     If the flow of acid dilution water stops while ClO 2  is being generated, a plant shutdown typically follows. The shutdown is triggered ultimately by an increase in generator gas pressure due to an increase in the amount of gases inside the generator. This increase in gases can be the result of a gaseous ClO 2  decomposition, which produces chlorine gas and oxygen and/or an increase in chlorine generation via Reaction (4) in the extremely high acidity regions created after feeding undiluted acid. Thus, stopping the flow of dilution water and its subsequent result can be due to inefficient acid mixing. 
     Loss of Efficiency Due to Poor Mixing: 
     Referring to an embodiment where the acid is sulfuric acid, inefficient mixing of the sulfuric acid creates localized boundaries of elevated acid concentration in a highly acidic zone  17  (see  FIG. 2 ) that favor Reaction (4) above. The partial creation of chlorine results in efficiency loss in the chemical facility. The inefficient reduction of chlorate into chlorine occurs near the sulfuric acid injection point where the acidity is highest. The conversion to ClO 2  instead of chlorine improves as the reactants travel through the circulation line with increased mixing and reduction of localized high acidity. The same inefficient reaction can occur at any place in the generating system when chlorate is reacted with a reducing agent at high acidities. The addition of a chloride ion donor in the reducing agent will enhance efficiency wherever chlorate reduction takes place in a highly acidic zone. As an added benefit, addition of a chloride donor enables continuous operation of the generator  10  at the higher acidity concentrations in the system, thus increasing the overall efficiency of ClO 2  generation in the system. 
     Efficiency Increase by Adding Chloride Donor in Acid Feed and/or Reducing Agent Feed 
     In order to increase or maintain the efficiency of ClO 2  production in the chemical facility, the chloride intermediate cycle shown in example Reactions (1), (2), and (3) must be sustained or enhanced in high acidity zones. To avoid Reaction (4) from occurring, a chloride donor is added via a chloride donor feed  40 . The chloride donor could be one of a number of chemicals or compounds, such as alkali metal chlorides (sodium chloride, potassium chloride, lithium chloride, and the like), hydrochloric acid, sodium hypochlorite, thionyl chloride, sulfuryl chloride, phosphate-chloride compounds (phosphorous trichloride and phosphorous pentachloride), sodium chlorite, other suitable organic or inorganic chloride donors, chloride donor promoters (compounds that yield chlorides) or combinations thereof. For example, introduction of the chloride donor feed  40  into the acid feed  31  or reducing agent feed  32  avoids Reaction (4), thereby avoiding the inefficient production of chlorine gas. 
     Sodium chlorite is in the chloride donor promoter category, and it is of special interest. Sodium chlorite can react with the inefficient Reaction (4) as follows: 
       HClO+HCl→Cl 2 +H 2 O(High acidity reaction)  Reaction (4)
 
       2NaClO 2 +HClO+HCl→2ClO 2 +H 2 O+2NaCl  Reaction (8)
 
       or 
       2NaClO 2 +Cl 2 →2ClO 2 +2NaCl  Reaction (9)
 
     In this case sodium chlorite prevents the formation of chlorine through Reaction (8), or it reacts with chlorine generated from Reaction (4) to generate ClO 2  through Reaction (9). Under either Reaction (8) or (9), the sodium chlorite provides NaCl, which is a chloride donor for Reactions (1), (2) and (3). Sodium chlorite can be manufactured in the ClO 2  facility by reacting ClO 2  with NaOH or other suitable reactants, such as hydrogen peroxide, or combination thereof, and returned to be added into the ClO 2  generating process at the desired feed locations. For example, in one embodiment, shown in  FIG. 4 , a ClO 2  feed  35  is drawn from a ClO 2  product line  36 . The ClO 2  feed  35  is circulated back for introduction into the recirculation lines  12 . A NaOH feed  37  is introduced into the ClO 2  feed  35 , and the solution is mixed to form a quantity of sodium chlorite in situ. The sodium chlorite then acts as the chloride donor compound for the chloride donor feed  40 . Sodium chlorite can also be used throughout the chlorine dioxide generating system to mitigate the production and occurrence of chlorine. 
     Referring again to  FIG. 1 , the chloride donor is introduced into the system via a chloride donor feed  40 , which is preferably located upstream from the chlorine dioxide generator  10 . In one embodiment, the upstream introduction of the chloride donor is accomplished by introducing the chloride donor feed  40  directly into the acid feed  31  prior to the acid being injected into the recirculation lines  12 . The selection of the addition points into the acid depends on the nature and compatibility of the chloride donor selected. In another embodiment, the upstream introduction of the chloride donor is accomplished by introducing the chloride donor feed  40  directly into the dilution water feed  33  prior to the acid being injected into the recirculation lines  12 . Alternately, the chloride donor could be added directly to the dilution water or the acid prior to the dilution water or acid being introduced into their respective feeds  33 ,  31 . These methods enable the necessary chloride intermediates to be located inside the high acidity zones that occur in the wake zone  18  of the acid injection location where chloride intermediates are depleted. In this way, chloride ions are available as intermediate species to produce ClO 2  through Reactions (1), (2) and (3), even when localized acidities are high. 
     Some ClO 2  chemical facilities feed methanol at locations downstream or upstream of the acid injectors  20  in the recirculation line  12 . Methanol is typically diluted with water or the sodium chlorate feed  38  prior to entering the recirculation line  12 . Similar to the method of introduction into the acid feed  31  described above, chloride donors can be added to the methanol feed, methanol dilution water, or the sodium chlorate feed  38  at concentrations that promote Reactions (1), (2), and (3) above. Particularly for chemical facilities that feed methanol at a location downstream of the acid injection location, residual chloride donors enhance efficiency. Chlorides can also be added to hydrogen peroxide systems in a similar manner to favor the production of ClO 2 , as shown in equation (5) above. Alternately, the chloride donor can be added directly to the methanol, methanol dilution water, or the sodium chlorate prior to their respective introduction into their respective feeds. 
     Referring to  FIG. 5 , in one embodiment, the chloride donor is introduced in separate chloride donor feed  40  via an chloride donor injector  19  located in the wake zone  18  of one of the injectors  20  from the acid feed  31 , the reducing agent feed  32 , or the sodium chlorate feed  38 . It is preferred, but not required, that the chloride donor injector  19  is located in the wake zone  18  of the acid feed  31 . 
     With the addition of chlorides as described above, stable production of ClO 2  is possible at higher generator acidity ranges than are normally encountered in the industry. As a result, ClO 2  chemical facilities are able to operate more efficiently at higher acid concentrations, and the Whiteouts risks are lowered because of the constant chloride feed in the correct high acidity zones when needed. Similarly, in one embodiment, the chloride donor is added through a chloride donor injector  19  located in the wake zone  18  of the injector  20  of the reducing agent feed  32 . 
     Referring to  FIGS. 6-8 , the shape of the injectors  20 , and specifically the injectors  20  for the sulfuric acid, can aid in the mixing of the acid and/or reducing agent. Injectors  20  with other than cylindrical shapes can contribute to acid mixing. The acid injection orifices  22  ( FIG. 7 ) and their orientation can contribute to the mixing of acid, thereby increasing ClO 2  chemical facility efficiency. In one embodiment, shown in  FIG. 6 , the injector  20  comprises a mixing plate  23  connected to the tip  21  of the injector  20 . The mixing plate  23  is oriented substantially perpendicular to the flow through the recirculation line  12  such that the mixing plate  23  increases turbulence in the wake zone  18  of the injector  20 . The increased turbulence causes an increase in the mixing action of the acid with the flow in the recirculation line  12 . 
     Referring to  FIG. 7 , another embodiment of the injectors  20  comprises one or more orifices  22  through which the sulfuric acid is introduced into the recirculation line  12 . This embodiment preferably comprises more than one orifice  22 . The plurality of orifices  22  disperses the acid over a larger space inside the recirculation line  12 , thereby promoting an enhanced mixing effect. In another embodiment, shown in  FIG. 8 , the injector  20  further comprises one or more fins  24 . These fins  24  are oriented substantially perpendicular to the direction of flow inside the recirculation line  12 , and they promote mixing in a manner similar to that of the mixing plate  23 . 
     Other ClO 2  Systems 
     Chloride donor addition to improve efficiency can be used in all ClO 2  producing systems, regardless of the size or complexity of the system. The feed of chloride intermediate into the acid and/or reducing agent is cascaded to the flow of the acid and/or reducing agent. In one embodiment of the method, the shutdown of the chloride donor feed is programed with the existing interlocks of the chlorine dioxide chemical facility. Compatible equipment and materials capable of resisting the corrosive environment are used throughout the system. 
     The foregoing embodiments are merely representative of the method for boosting the efficiency of chlorine dioxide production and not meant for limitation of the invention. For example, persons skilled in the art would readily appreciate that there are several embodiments and configurations of acid feeds, injectors, and recirculation lines that will not substantially alter the nature of the method. Consequently, it is understood that equivalents and substitutions for certain elements and components set forth above are part of the invention described herein. 
     The foregoing embodiments are merely representative of the method for chlorine dioxide generation and not meant for limitation of the invention. For example, persons skilled in the art would readily appreciate that there are several embodiments and configurations of acid feeds, injectors, and recirculation lines that will not substantially alter the nature of the method. Likewise, elements and features of the disclosed embodiments could be substituted or interchanged with elements and features of other embodiments, as will be appreciated by an ordinary practitioner. Consequently, it is understood that equivalents and substitutions for certain elements, components, and steps set forth above are part of the invention described herein, and the true scope of the invention is set forth in the claims below.