Method for generating high concentration chlorine dioxide by means of electrolysis

Abstract

A method for generating high concentration chlorine dioxide having purity over 90% by means of an electrolytic procedure using an electrolytic material prepared from NaCl and NaClO2 subject to the control of optimal operation parameters of current 80˜110 Amp, concentration of sodium chloride to be 20˜25% sodium chloride and sodium chlorite to be 5% minimum, operation temperature 55˜65° C., and material feeding speed 30˜50 ml/min. Under the effect of ClO2 after 20 minutes, no bacteria count is detectable, and bacteria count Log value of SPA water of aerobic count 1.5×105 CFU/mL is dropped by 1.46 Log CFU/g in 120 ppm ClO2. Therefore, the ClO2 preparation method of the invention is an economical and convenient process suitable for on-line continuous disinfection or sterilization application.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for generating high concentration chlorine dioxide for sterilization and more particularly, to an economic and convenient chlorine dioxide generating method for continuously generating high concentration chlorine dioxide by means of an electrolytic procedure using an electrolytic material prepared from NaCl and NaClO₂.

2. Description of the Related Art

Chlorine dioxide is the chemical compound with the formula ClO₂ and the molar mass 67.452. Chlorine dioxide is a reddish-yellow gas that changes the color subject to its concentration, and has a stinky smell similar to chlorine gas and ozone. Chlorine dioxide has a boiling point of 11° C. and a melting point of −59° C. Chlorine dioxide (ClO₂) has an oxidation-reduction potential higher than chlorine gas (Cl₂) and oxygen gas (O₂), therefore Chlorine dioxide is a strong oxidant. When compared to chlorine disinfection, Chlorine dioxide has volatile and high-energy molecules. Chlorine dioxide can be stably dissolved in water when kept away from light. Chlorine dioxide tends to be changed into Clo₂⁻, ClO₃⁻ and Cl⁻ that have no disinfection effect but affecting chlorine dioxide purity. Unlike chlorine, chlorine dioxide does not react with ammonia nitrogen or ammonia, however it oxidizes NO₂⁻ into NO₃⁻. Further, pure chlorine dioxide does not make electrophilic substation with organics, and therefore it produces less amount of halide disinfection byproducts after oxidation reaction when compared to chlorine.

Chlorine dioxide (ClO₂) has been intensively used in drinking water sterilization to substitute for hypochlorous acid (HOCl) and hypochlorite (Ocl⁻). Because Chlorine dioxide neither reacts with humic substance to produce THMs (CHCl₃, CHBrCL₂, CHBr₂Cl, CHBr₃) nor reacts with ammonia to produce ammonia nitrogen, it is handled similar to chlorine-added disinfection systems, further; it has a wide pH application range.

Further, regular water purification plants use chlorine as an oxidant. However, because the problem of water pollution is more and more serious in many countries around the world, water purification plants may increase the amount of chlorine to enhance the effect. However, increasing the amount of chlorine will also increase the risk of carcinogen (THMs) of disinfection byproducts. Therefore, it is desirable to find a safety and effective disinfectant to substitute for chlorine.

In food disinfection, chlorine dioxide has been intensively used as a preservative to protect beverage and foods fresh. After immersion in chlorine dioxide solution, meat, seafood, poultry and etc., can prohibit multiplication of microbes. Adding chlorine dioxide to beverage (such as mineral water) can extend the validity.

Many researches indicate that washing fishes with lactate-activated stable chlorine dioxide shows a satisfactory antiseptic effect. Using lactate-activated stable chlorine dioxide in seafood can effectively improve the quality and extend the preservation time.

Further, treating fish slices with chlorine dioxide does not affect the composition of its fatty acid or the content of its protein fat, microbes and minerals.

Reports affirm that immersing strawberry, cucumber and melon with chlorine dioxide can effectively lower the count of pathogens of E. coli 0157:H17, Listeria monocytogenes and Salmonella spp. It is also confirmed that using chlorine dioxide in lettuce, cabbage (Zhang and Farber 1996), poultry cooling water, pig carcass and cow carcass that is contaminated with excrement can lower the count of Samonella spp. in poultry carcasses. The FDA (Food and Drug Administration of the USA approves the use of chlorine dioxide solution in cooling water for poultry and meat processing. According to FDA's regulations, the maximum residue limits is to be 3 ppm, and the allowable limit for residue in rinse and disinfection of uncut and unpeeled vegetables and fruits is 5 ppm. WHO (World Health Organization) lists chlorine dioxide to be an Al grade safety sterilizer.

Chlorine dioxide is volatile. Because of the instability characteristic, chlorine dioxide is normally produced for application at the fabrication place. “Traditionally, chlorine dioxide for disinfection applications has been made by one of three methods using sodium chlorite or the sodium chlorite-hypochlorite method: 2NaClO₂+2HCl+NaOCl→2ClO₂+3NaCl+H₂O or the sodium chlorite-hydrochloric acid method: 5NaClO₂+4HCl→5NaCl+4ClO₂ All three sodium chlorite chemistries can produce chlorine dioxide with high chlorite conversion yield, but the chlorite-HCl method suffers from the requirement of 25% more chlorite to produce an equivalent amount of chlorine dioxide. Catalytic chlorine dioxide generators produce extremely high conversion yields (>98.5%). With these systems sodium chlorite solution is passed through an ion exchange column. The process of ion exchange yields chlorous acid. This is then passed through a catalyst column which assists in the conversion to chlorine dioxide. The advantage of these systems is that low concentrations of chlorine dioxide can be produced directly at the point of application. Chlorine dioxide can also be produced by electrolysis of a chlorite solution: 2NaClO₂+2H₂O→2ClO₂+2NaOH+H₂ High purity chlorine dioxide gas (7.7% in air or nitrogen) can be produced by the Gas: Solid method, which reacts dilute chlorine gas with solid sodium chlorite. 2NaClO₂+Cl₂→2ClO₂+2NaCl” (referred to Wikipedia). The sodium chlorite-hypochlorite method requires a high investment in equipment, resulting in high preparation cost. The sodium chlorite-hydrochloric acid method encounters an acid fluid removal problem. Sodium chloride electrolysis process has the drawbacks of low yield and production of harmful byproducts due to contact of chloride or hypochlorite with organics.

Preferably, chlorine dioxide is prepared in-situ. Chorine dioxide will deteriorate during storage. Stabilized products of chlorine dioxide are marketed. These stabilized products of chlorine dioxide are solutions of buffered sodium chlorite of NaHCO₃ and Na₂CO₃. Before application, a weak acid must be added to the stabilized solution of buffered sodium chlorite to make chlorine dioxide in-situ. Applied weak acid destroys the buffer system, thus: H⁺+ClO₂⁻→ClO₂. Chlorine dioxide prepared by adding a weak acid to a solution of buffered sodium chlorite is applicable for environmental disinfection but, however it is not suitable for cleaning foods or other organic products due to redidual acid substances. Further, this method is not economic for mass application.

Therefore, it is desirable to provide an economical high concentration chlorine dioxide that is suitable for continuous on-line fresh products disinfection applications.

SUMMARY OF THE INVENTION

The present invention has been accomplished under the circumstances in view. The invention uses an inexpensive electrolytic material prepared from NaCl and NaClO₂ for continuously generating high concentration chlorine dioxide by means of an electrolytic procedure, and the factors of current, concentration, temperature and material feeding speed are the optimal operation parameters for controlling fabrication of high concentration chlorine dioxide (over 90%). Therefore, the high concentration chlorine dioxide fabrication method of the invention is practical for on-line continuous disinfection or sterilization process, providing a convenient and effective way to disinfect fresh agricultural and fishery products or foods.

The method for generating high concentration chlorine dioxide having purity over 90% according to the present invention comprises the step of mixing sodium chloride and sodium chlorite to provide an electrolytic material, and the step of putting the electrolytic material thus obtained into an electrolyzer for enabling the sodium chloride in the electrolytic material to be electrolyzed to produce chloride for causing the sodium chlorite in the electrolytic material to make a reduction and to produce chlorine dioxide subject to the control of optimal operation parameters of current, concentration, operation temperature and material feeding speed. The optimal operation parameters are: current 80˜110 Amp, concentration of the electrolytic material to be 20˜25% sodium chloride and at least 5% sodium chlorite, operation temperature 55˜65° C., and material feeding speed 30˜50 ml/min.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing an original model of chlorine dioxide generator according to the present invention.

FIG. 2 is a schematic sectional view of the electrolyzer of the original model of chlorine dioxide generator according to the present invention.

FIG. 3 is operation flow chart of the original model of chlorine dioxide generator according to the present invention.

FIG. 4A is a picture of a test sample of SPA water of bacterial count 1.5×10⁵ CFU/mL diluted at 1/100 according to the present invention.

FIG. 4B is a picture of a test sample of SPA water of bacterial count 1.5×10⁵ CFU/mL diluted at 1/1000 according to the present invention.

FIG. 5 is a bactericidal ability chart showing the bactericidal ability of chlorine dioxide prepared according to the present invention on bacteria count.

FIG. 6A is a picture showing the growth of bacteria count after disinfected with 5 ppm chlorine dioxide prepared according to the present invention (contact time 5 minutes).

FIG. 6B is a picture showing the growth of bacteria count after disinfected with 5 ppm chlorine dioxide prepared according to the present invention (contact time 15 minutes).

FIG. 6C is a picture showing the growth of bacteria count after disinfected with 5 ppm chlorine dioxide prepared according to the present invention (contact time 25 minutes).

FIG. 7 is a bactericidal ability chart showing the bactericidal ability of chlorine dioxide prepared according to the present invention on Coli-form.

FIG. 8 shows different Coli-form disinfection status under 5 ppm ClO₂ at contact times 5, 15 and 25 minutes (from the left to the right) respectively.

FIG. 9 is a comparison chart showing the results before and after ClO₂ treatment on fish bodies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an original model of chlorine dioxide generator is shown using a loop design with waste liquid overflow feeding control. The chlorine dioxide generator comprises a feeding unit (power supply and electrolyte solution) (a), an electrolyzer unit (b), and a finished product collector unit (c). The feeding unit (a) uses a 220V-to-DC converter to provide power supply and a 30 L tank to provide electrolyte solution. The feeding unit (a) comprises a thermostat 1 (which controls the upper temperature limit, a main power transformer 2 (which controls output voltage and current), an electrolyte material storage tank 5, and a waste liquid overflow vessel 6. The electrolyzer unit (b) comprises a cooling bath 3 and an electrolyzer 4. The finished product collector unit (c) comprises a cooling water and negative pressure trough 7, a collection tank 8, a drain trough motor 9, and a collection tank motor 10. The negative electrode of the electrolyzer 4 is made of an anti-corrosion metal coated with acid and alkaline protective coating. The electrolyzer 4 further comprises a neutral electrode 100 of high-density graphite is set in between the positive electrode and the negative electrode (see FIG. 2).

Further, the electrolyte solution used according to the present invention is prepared from the mixture of sodium chloride and sodium chlorite. The first step of the method of generating high concentration chlorine dioxide is material preparation where the mixture of sodium chloride and sodium chlorite is filtrated and then stored in the electrolyte material storage tank 5 (see FIG. 1). Before starting the chlorine dioxide generator, the prepared mixture of sodium chloride and sodium chlorite is fed into the electrolyzer 4. When the electrolyzer 4 is started, sodium chloride is electrolyzed to produce chloride, which is provided to sodium chlorite for making a reduction to produce chlorine dioxide. The negative pressure pumping mechanism (cooling water and negative pressure trough 7) causes a negative pressure in the collection tank 8 to suck in chlorine from the electrolyzer unit (b) through a delivery tube 11, enabling chlorine to be solved in distilled water in the collection tank 8. The waste liquid overflow vessel 6 collects overflowed waste liquid from the electrolyzer 4 for further cycling (see FIG. 3), or plant building sterilization.

In the aforesaid chlorine dioxide generating method, the factors of current, concentration, temperature and material feeding speed are the optimal operation parameters. Electrolyte concentration and operation temperature have a great concern with the productivity and purity of chlorine dioxide. Increasing electrolyte concentration will increase current conductivity, and will also accelerate electrochemical reaction. Increasing operation temperature will accelerate sodium chlorite reduction, thereby increasing the productivity of chlorine dioxide. The invention is to electrolyze sodium chloride in sodium chloride and sodium chlorite mixture to produce chloride for enabling sodium chlorite to make a reduction to further produce chlorine dioxide. The use of sodium chloride and sodium chlorite mixture instead of pure sodium chlorite greatly increases the conductivity of electrolyte solution, activates electric current, and accelerates electrocatalytic oxidation, The use of sodium chloride and sodium chlorite mixture shows a better effect than the use of pure sodium chlorite because saline solution increases electric current conductivity and accelerates electrocatalytic oxidation. Further, chloride obtained from electrolysis of saline solution can be used as a reducing agent for sodium chlorite to promote generation of chlorine dioxide.

Under the considerations of cost and safety and after several tests, the optimal operation range is obtained: electric current 80˜110 Amp, saline solution 20˜25%, sodium chlorite to be 5% minimum, operating temperature 55˜65° C., material feeding speed 30˜500 ml/min. Preferably, the concentration of sodium chlorite is within 5˜10%. Further, if cost is not in consideration, high concentration sodium chlorite can be used to increase the productivity and purity of chlorine dioxide.

In a test operated subject to the parameters of current 100 Amp, sodium chlorite 7%, saline solution 23%, operating temperature 65° C., waste fluid flowrate 50 ml/min, the test result analyzed subject to continuous iodometric method shows average concentration of chlorine dioxide 3423 ppm at the first hour and 4626 ppm at the second hour, and total average concentration 4024 ppm, and purity 98% at each concentration. Further, analysis of sodium chlorite in waste water once per half hour during electrolysis of 7% sodium chlorite shows total consumption of sodium chlorite 98.44% in two hours.

For understanding of the disinfection effect of chlorine dioxide, chlorine dioxide obtained from the aforesaid test was diluted into 1 ppm, 3 ppm, and 5 ppm for disinfection test. Test samples were obtained by diluting SPA water of aerobic count 1.5×10⁵ CFU/mL into 1/100 (see FIG. 4A) and 1/1000 (see FIG. 4B). 1 ml of each sample was respectively added to 9 ml each of 1 ppm, 3 ppm, and 5 ppm chlorine dioxide solution. 0.2 ml of each mixture (each mixture of 1/100 SPA water of aerobic count 1.5×10⁵ CFU/mL and 9 ml each of 1 ppm, 3 ppm, and 5 ppm chlorine dioxide solution, and each mixture of 1/1000 SPA water of aerobic count 1.5×10⁵ CFU/mL and 9 ml each of 1 ppm, 3 ppm, and 5 ppm chlorine dioxide solution) was taken at different contact time by and applied to culture media by spread method by means of a glass rod. FIG. 5 shows the test result. Five minutes after disinfection, the bacteria count was below 30 colony forming units/mL, therefore the indication was obtained by multiplying the means value of bacteria count by 5. 5 minutes after disinfection with 1 ppm chlorine dioxide showed drop of bacteria count by 99.46%, i.e., effective disinfection. No bacteria count was examined 25 minutes after disinfection with 3 ppm chlorine dioxide or 20 and 25 minutes after disinfection with 5 ppm chlorine dioxide. The test result shows effective disinfection 25 minutes after disinfection with 3 ppm chlorine dioxide. Therefore, the invention is applicable to sterilization of water or drinking water. FIGS. 6A, 6B and 6C show growth of bacteria count at different contact time (5 ppm ClO₂ at 5 minutes, 15 minutes and 25 minutes respectively).

FIG. 7 shows the result of the test made by using 1 ppm, 5 ppm and 10 ppm chlorine dioxide to disinfect test samples of Coli-form of bacteria count 4.5×10² CFU/mL supplied by PingTung County Government, Taiwan and using Petrifilm E. coli Count Plates (3M-6414) to cultivate E. Coli at 37° C. for 24 hours. The test shows effective disinfection of 10 ppm chlorine dioxide in killing E. coli after contact time 25 minutes.

For easy understanding of the disinfection effect of the product of the present invention in actual practice, we made a test in washing fish bodies with chlorine dioxide. According to this test, 10 ppm, 30 ppm, 60 ppm, 90 ppm, and 120 ppm chlorine dioxide solutions were prepared, and sample fishes were fresh Taiwan Tilapia. Fresh sample fishes were merged in prepared chlorine dioxide solutions for 3 minutes, and then disinfected sample fishes were carried to a laminar flow cabinet, and then 10 g fish meat with skin was taken from the back of each sample fish and mixed in a 90 ml blender and diluted to a suitable concentration, and then took 1 ml of the blended sample fish meat mixture for mixing in a 5 ml PCA agar for growth and culturing of bacteria. FIG. 8 shows different Coli-form disinfection status under 5 ppm ClO₂ at contact times 5, 15 and 25 minutes (from the left to the right) respectively.

FIG. 9 shows the results before and after ClO₂ treatment. After dipped in 10 ppm ClO₂, bacteria count Log value dropped from 4.74 Log CFU/g by 0.16 Log CFU/g, i.e., the bacteria count is reduced by 3%; bacteria count Log value dropped by 0.34 Log CFU/g in 30 ppm ClO₂; bacteria count Log value dropped by 0.7 Log CFU/g in 60 ppm ClO₂; bacteria count Log value dropped by 1.03 Log CFU/g in 90 ppm ClO₂; bacteria count Log value dropped by 1.46 Log CFU/g in 120 ppm ClO₂ or by 31%. The above results show that the disinfection power of ClO₂ in fish bodies is significantly enhanced when the concentration is increased.

When simply using ClO₂ without other antimicrobial agents, the cost is low. The market price of a 20 L 2000 ppm chlorine dioxide solution is about US$120 or US$6.0/L.

The chlorine dioxide generator according to the present invention can generate chlorine dioxide 4720 ppm per liter, and will consume 98.44% sodium chlorite of original concentration after a continuous operation of 2 hours. When the design cost of the chlorine dioxide generator is not taken into account, the direct manufacturing cost of the present invention is about US$1.82/L (4720 ppm) subject to market price for sodium chlorite at about US$3.03/kg. Therefore, the economical benefit of the invention is high, and the invention can be employed to sterilize drinking water with less labor cost.

Further, the manufacturing process of the present invention can be employed to electrolyze sodium chloride (salt), thereby producing liquid chlorine for inorganic application, i.e., for sterilizing plants, equipments, and the like.

As indicated above, the invention uses an inexpensive electrolytic material prepared from NaCl and NaClO₂ for continuously generating high concentration chlorine dioxide by means of an electrolytic procedure, and the factors of current, concentration, temperature and material feeding speed are the optimal operation parameters for controlling fabrication of high concentration chlorine dioxide (over 90%). Therefore, the high concentration chlorine dioxide fabrication method of the invention is practical for on-line continuous disinfection or sterilization process, providing a convenient and effective way to disinfect fresh agricultural and fishery products or foods. In general, the invention has industrial value, and meets the requirements of novelty and unobviousness/inventive step.

Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.

Claims

1. A method for generating high concentration chlorine dioxide having purity over 90%, comprising the step of mixing sodium chloride and sodium chlorite to provide an electrolytic material, and the step of putting said electrolytic material thus obtained into an electrolyzer for enabling the sodium chloride in said electrolytic material to be electrolyzed to produce chloride for causing the sodium chlorite in said electrolytic material to make a reduction and to produce chlorine dioxide subject to the control of optimal operation parameters of current, concentration, operation temperature and material feeding speed.

2. The method for generating high concentration chlorine dioxide as claimed in claim 1, wherein said optimal operation parameters are current 80˜110 Amp, concentration of said electrolytic material to be 20˜25% sodium chloride and at least 5% sodium chlorite, operation temperature 55˜65° C., and material feeding speed 30˜50 ml/min.

3. The method for generating high concentration chlorine dioxide as claimed in claim 2, wherein the concentration of sodium chlorite is most preferably within 5˜10%.