Patent Grant
8663481
1. Field of the Invention
The invention relates to a method of treating water with chlorine dioxide (ClO2).
2. Description of the Related Art
Chlorine dioxide is used in water treatment because of its high bactericidal, virucidal and algicidal activity. Due to the explosive tendency of gaseous chlorine dioxide (c>300 g/m3) and chlorine dioxide solutions (c>26 g/l), chlorine dioxide cannot be stored in compressed form or in solutions of relatively high concentration. Owing to these chemical properties, chlorine dioxide must be produced at the point of use. This is conventionally achieved by mixing basic chemicals in special reactors of chlorine dioxide generation systems. The chemical storage vessels, the metering appliances and also the reactor of the chlorine dioxide systems form a locally linked unit of apparatus which is generally erected in rooms accessed by people.
There are a plurality of methods for synthesizing ClO2 which are used commercially for water treatment, but principally three underlying methods are used. These methods use sodium chlorite (NaClO2) as one of the starting materials. The underlying chemistry of the three methods is explained below. The substances used in these methods are termed starting chemicals, or else reactants.
1. Method Using Sodium Chlorite and Strong Acid
In the first method, a strong acid is used together with sodium chlorite. The strong acid is usually hydrochloric acid or sulphuric acid. When hydrochloric acid is used the reaction stoichiometry is as follows:
5NaClO2+4HCl→4ClO2+5NaCl+2H2O
In addition, chlorine dioxide can be formed with the use of sulphuric acid in accordance with the following reaction:
10NaClO2+5H2SO4→8ClO2+5Na2SO4+2HCl+4H2O
2. Method Starting from Sodium Chlorite and Chlorine
This method uses gaseous chlorine together with sodium chlorite. The reaction proceeds in two stages, first with the formation of hydrochloric acid.
Cl2+H2O→HOCl+HCl
The intermediate, hypochloric acid (HOCl), then reacts with sodium chlorite, forming chlorine dioxide (ClO2).
HOCI+HCI+2NaClO2→2ClO2+2NaCl+H2O
The stoichiometric reaction from the two equations is
Cl2+2NaClO2→2ClO2+2NaCl
3. Method Starting from Sodium Chlorite and Sodium Hypochlorite
In the third method, sodium hypochlorite (NaOCl) is used together with sodium chlorite:
NaOCl+HCl→NaCl+HOCl
HCl+HOCl+2NaClO2→2ClO2+2NaCl+H2O
The synthesis reactions for generating chlorine dioxide are generally carried out in reactors which are operated either continuously or by the batch method.
Two explosion limits must be taken into account in the generation of chlorine dioxide:
more than 6 g of ClO2/l of solution [contact with air] and more than 26 g of ClO2/l of solution [autodecomposition of the aqueous solution]. In the case of the chlorine dioxide syntheses carried out by methods 1 to 3, when use is made of feed chemicals which are passed into the reaction chamber at a concentration of greater than approximately 26 g of ClO2/l of solution, dilution water is added to the reaction chamber in order to bring this concentration below that of spontaneous autodecomposition. The chlorine dioxide solution leaving the reaction chamber which generally contains 20 g of ClO2/l or less is diluted with a further water stream to concentrations of roughly less than 3 g of ClO2/l of solution.
In order that the prior art methods can be operated with satisfactory results with respect to plant safety, chlorine dioxide yield and time-specific production rate, a variety of processing variations are performed, inter alia,
Despite the use of these procedures, in the event of incorrect operation of the chlorine dioxide generation systems, e.g. due to loss of dilution water or by failure of pressure control, spontaneous decomposition (explosion) of chlorine dioxide can occur, or chlorine dioxide may, due to leakage or breakage of separation surfaces between the chlorine-dioxide-containing solution and the environment, lead to hazards in areas surrounding the generation systems. The use of diluted starting chemicals which leads to chlorine dioxide solutions of a concentration of less than 6 g/l, and therefore the sacrifice of relatively high time-specific generation rates of the chlorine dioxide systems, also cannot exclude hazard to the surrounding areas of the generation systems by exceeding the MAK value [maximum workplace concentration] of 0.1 ppm in the event of incorrect operation. In order to minimize these hazards, various measures are implemented at the generation systems themselves, and also at the sites where the chlorine dioxide generation systems are erected, e.g. complex servicing work on the generation systems including regular replacement of the reactors, spatially isolated erection sites for the generation systems, forced aeration and air monitoring of the atmosphere of the erection site by continuous gas analysis.
A safe method for environment and people is thus needed, with avoidance of the emission of ClO2 into the environment, in particular into the chambers in which the plant is customarily operated. At the same time, the advantages resulting from the use of concentrated starting chemicals such as, e.g. reduced material transport, higher reaction rate, higher yields, lower reactor volume, should be made utilizable.
Accordingly, one object of the present invention is therefore to design the treatment of water with chlorine dioxide so as to be safer and more efficient.
An additional object of the present invention is, at a high time-specific generation rate of the chlorine dioxide methods, to minimize the hazard potential of this type of water treatment and simultaneously to reduce the expenditure on the safety installations.
These and other objects of the invention, alone or in combinations of two or more thereof, have been satisfied by the discovery of a method of treating water with chlorine dioxide (ClO2), comprising:
surrounding a reaction chamber, in which the ClO2 is generated, with water, wherein the water surrounding the reaction chamber is simultaneously the water to be treated; and
passing a reaction solution comprising the ClO2 formed in the reaction chamber out of the reaction chamber through an outlet directly into the water, thus treating the water.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
The present invention relates to a method of treating water with chlorine dioxide (ClO2), comprising:
surrounding a reaction chamber, in which the ClO2 is generated, with water, wherein the water surrounding the reaction chamber is simultaneously the water to be treated; and
passing a reaction solution comprising the ClO2 formed in the reaction chamber out of the reaction chamber through an outlet directly into the water, thus treating the water.
Preferably, in the present method, the combination of the concentrations of the starting chemicals and dilution water used if desired is selected in such a manner that the concentration at the reactor outlet of the chlorine dioxide formed is greater than 3 g/l of solution, preferably greater than 26 g/l of solution, and particularly preferably greater than 80 g/l of solution.
More preferably, the reaction chamber in which the ClO2 is generated is a reactor without a pressure control appliance and equipped with a free outlet and the combination of the concentrations of the starting chemicals and dilution water used if desired is selected in such a manner that the concentration at the reactor outlet of the chlorine dioxide formed is greater than 3 g/l of solution, preferably greater than 26 g/l of solution, and particularly preferably greater than 80 g/l of solution.
In a particularly preferred embodiment the reaction chamber in which the ClO2 is generated is a reactor without a pressure control appliance and equipped with a free outlet, the water surrounding the reaction chamber (which is simultaneously the water to be treated) exerts a pressure on the chlorine dioxide-containing solution in the reaction chamber of a magnitude that the solubility limit of chlorine dioxide in water at a given temperature is not exceeded, and the combination of the concentrations of the starting chemicals and dilution water used if desired is selected in such a manner that the concentration at the reactor outlet of the chlorine dioxide formed is greater than 3 g/l of solution, preferably greater than 26 g/l of solution, and particularly preferably greater than 80 g/l of solution.
The method of the present invention enables safe working by avoiding the escape of ClO2 into working rooms or the surrounding areas and eliminates adverse consequences of explosive decompositions. The reaction chamber in which the ClO2 is generated is completely surrounded by water and the water which surrounds the reaction chamber is simultaneously the water which is to be treated.
Shifting the point of formation of the chlorine dioxide out of spaces accessed by people and the storage site of starting chemicals significantly increases safety. Leaks up to explosions of the reaction chamber are virtually neutralized by the large volume of water.
The resultant ClO2 is passed directly without bypasses from the reaction chamber in which the ClO2 is formed into the water which is to be treated, since the outlet is directly on the end of the reaction chamber and is therefore likewise surrounded by the water which is to be treated.
It has likewise surprisingly been found that the combination of the use of concentrated starting chemicals with dilution water used, if desired, which leads to a chlorine dioxide concentration in the reaction chamber of greater than 26 g/l of solution and particularly preferably greater than 80 g/l, no autodecomposition of chlorine dioxide occurs if the mean residence time in the reaction chamber is within a certain time range which prevents the formation of gaseous chlorine dioxide and the chlorine dioxide solution leaving the reaction chamber is abruptly diluted to concentrations in the milligram range. In addition, it has been found that the necessary superstoichiometric, yield-increasing increased amounts of acid or chlorine can be reduced by increasing the chlorine dioxide concentration in the reaction chamber.
The advantages of the present invention method will be described in more detail below.
A leak of the reaction chamber, in particular of the reactor, can be simply and safely handled in the water which is continuously flowing past its wall. The chlorine dioxide, in particular, which escapes in the event of a leak of the reaction chamber is diluted to a non-critical concentration and transported away. The same applies to the starting chemicals (reactants) which possibly exit from the reaction chamber, in particular the reactor. Since the synthesis of chlorine dioxide from concentrated starting chemicals can proceed without dilution by water, the necessary superstoichiometric, yield-increasing increased amounts of acid or chlorine can be reduced and there is in addition a significant increase of the reaction rate, resulting from a high specific generation rate of the reaction chamber. By reducing the necessary median residence time of the reactants in the reaction chamber, there is the possibility of minimizing the reaction chamber volume, as a result of which, e.g. the installation of the reaction chamber, in particular the reactor, into a pipe becomes possible, through which pipe the water which is to be treated flows. In addition, from the safety aspect, there is an improvement of the ratio between the amount of chlorine dioxide permanently present in the reaction chamber during synthesis and the amount of water which is to be treated.
The shifting of the point of production of the chlorine dioxide out of the spaces accessed by people and the storage site of the starting chemicals significantly increases safety. Reaction chamber leaks up to reaction chamber explosions are virtually neutralized by the large amount of water which is to be treated relative to the amount of chlorine dioxide which is present in the reaction chamber.
The features of the present method enable safe working, even with the use of concentrated starting chemicals without dilution water, by avoiding the escape of ClO2 into working rooms or the surroundings and eliminates adverse consequences of explosive decompositions.
The use of one reactor as reaction chamber is preferred.
According to the present method, the resultant ClO2 is passed directly into the water which is to be treated without bypasses or further additional lines directly from the reaction chamber in which the ClO2 is formed, since the outlet is situated directly at the end of the reaction chamber, preferably the reactor, and is therefore likewise surrounded by the water which is to be treated. This measure is a preferred embodiment of the method.
In addition, there is the possibility of transporting the chlorine dioxide solution exiting from the reaction chamber via an offtake line situated at the end of the reaction chamber (outlet) to one or more other sites. This can be used, inter alia, for distribution of the chlorine dioxide solution in a large total system to be treated or for apportionment over a plurality of individual systems to be treated. In such a system, a preferred embodiment of the present invention is a method of treating water with chlorine dioxide (ClO2), comprising:
surrounding a reaction chamber, in which the ClO2 is generated, with water; and
passing a reaction solution comprising the ClO2 formed in the reaction chamber out of the reaction chamber through an offtake situated at one end of the reaction chamber to one or more other sites, directly into the water at the one or more other sites, thus treating the water.
For the offtake, all appliances are suitable by which the above described effects can be achieved, such as, for example, fixed mounted lines or flexible tube systems. At the end of the offtake, units for distributing the chlorine dioxide solution, e.g. water-jet liquid pump, circulation pump, can also be placed.
The expression, passed into the water which is to be treated, means that the ClO2 from the reaction chamber is introduced into the water which is to be treated more preferably by the starting chemicals introduced directly via an offtake into the reaction chamber and if desired by the dilution water directly or via an offtake introduced into the reaction chamber.
The renewal rate at the outlet of the reaction chamber, preferably the reactor outlet, or the offtake, of the water which is to be treated can be increased by technical appliances.
The reaction chamber, preferably the reactor, is preferably operated without a pressure control appliance. A free outlet at the end of the reaction chamber, preferably the reactor outlet, or the offtake, ensures that the pressure in the reaction chamber can only increase up to the value which is exerted on the reaction chamber by the surrounding water.
The concentration of the chlorine dioxide which forms in the reaction chamber, preferably the reactor can be set in combination with pressure and temperature of the surrounding water in such a manner that the solubility limit of chlorine dioxide in water is not exceeded. As a result, the formation of a 2-phase system due to a chlorine dioxide gas phase which forms can be prevented.
The pressure which is exerted on the reaction chamber by the water which is to be treated can be altered, for example, by the immersion depth of the reaction chamber, preferably the reactor, in a water reservoir. The pressure conditions for a reactor which is used in a pipe can be influenced, for example, by shut-off elements integrated into the pipe.
If the water which is to be treated is renewed at the outlet of the reaction chamber, preferably the reactor, at an appropriate rate, the concentration of the chlorine dioxide solution leaving the reaction chamber, preferably the reactor, can be abruptly shifted to a milligram range. This can be achieved, for example, by the reaction chamber, preferably the reactor, being used in a pipe through which water to be treated flows at a rate which corresponds to the desired renewal rate.
A reaction chamber, preferably a reactor, immersed in a water reservoir can be placed locally in the vicinity of the suction side of a circulation pump in order to ensure the corresponding renewal rate at the end of the reaction chamber, preferably the reactor outlet, or the offtake, of the water to be treated. If a circulation pump is not present, a variety of conventional alternatives are available, such as, for example, water-jet liquid pumps or other circulation appliances, which induce the desired renewal rate at the end of the reaction chamber, preferably the reactor outlet or the offtake, of the water which is to be treated.
In principle, all chemical production methods of ClO2 can be employed in the reaction chamber, in particular the methods 1. to 3. which are described in the background section above, or else starting from chlorate.
Preference in this invention is given to the hydrochloric acid-chlorite method (1.). In this method the starting chemicals (reactants) of alkali metal chlorite salt, preferably sodium chlorite, can be present in aqueous solutions of from 3.5% to 40%. The acid is preferably hydrochloric acid in a concentration of 3.5% to 42%.
In a particularly preferred embodiment of the invention, concentrated starting chemicals are used and the hydrochloric acid-chlorite method (1) is employed. The concentration of hydrochloric acid is then about 33-42% and that of the sodium chlorite solution is about 25-40%. The starting chemicals are not diluted before or in the reaction chamber, preferably the reactor.
The starting chemicals (reactants), in particular acid and chlorite, are passed into the reaction chamber as aqueous solution, as described above, separately by inherent pressure of the solutions or using pumps, and brought to reaction.
In a preferred embodiment, the reactants are used as concentrated solutions, and the use of dilution water is dispensed with, so that the chlorine dioxide concentration at the end of the reactor chamber, preferably at the end of the reaction chamber, preferably at the reactor outlet or the offtake, is set to greater than 80 g/l of solution. Alternatively, dilution water can be used in order to set the chlorine dioxide concentration at the end of the reaction chamber, preferably at the reactor outlet or the offtake, between greater than 3 g/l of solution, preferably greater than 26 g/l of solution, and particularly preferably greater than 80 g/l of solution.
The device for carrying out the method according to the invention is not particularly limited, so long as it has the requisite ability to have the reaction chamber surrounded by the water to be treated and has an outlet through which the produced chlorine dioxide can be passed into the water to be treated. The device comprises essentially any suitable devices and apparatuses. The device typically includes one or more tanks for the starting chemicals (reactants), such as, in a preferred embodiment, an acid storage tank and a chlorite storage tank, an aqueous acid solution being stored in the acid storage tank and a solution of an alkali metal salt of a chlorite ion being stored in the chlorite storage tank. Apparatuses are provided which not only can feed the suitable components into the storage tanks but also can take off solutions. Preferably, these apparatuses include pumps and feed lines which are sufficient to ensure the flow rates of the starting chemicals (reactants), in a preferred embodiment, aqueous acid solutions and solutions of alkali metal salts of a chlorite ion, and also of the amount of dilution water. Those of ordinary skill in the art can readily determine suitable sizes for the relevant storage tanks, feed lines and pumps in order to achieve the required feed rates of reactant solutions (e.g. aqueous acid solutions, solutions of an alkali metal salt of a chlorite ion).
Preferably, the device has embodiments having at least two pumps for two starting chemicals (reactants), but in a preferred embodiment, one for the solution of the alkali metal salt of a chlorite ion and the other for the aqueous acid solution, or three pumps, if dilution water is added.
The device further comprises an apparatus for mixing the solution of the starting chemicals (reactants), in a preferred embodiment the solution which contains the alkali metal salt of a chlorite ion and the aqueous acid solution, in order to provide an aqueous reaction solution of the starting chemicals (reactants). Any apparatus which mixes the abovementioned solutions adequately can be used, including conventional T-pieces or other connection elements which combine two streams or three streams to form one combined throttle lines and/or a stirred tank. The aqueous reaction solution can then be fed into the reaction chamber after mixing. Preferably, the two reactants and the dilution water which is used if desired are mixed in the reaction chamber. The mixing process can be introduced by any appliance such as baffle plates, injectors or packings, for example, which ensures optimum mixing.
The reaction chamber which is used can be any reactor which is able to initiate the reaction between the starting chemicals (reactants), in a preferred embodiment the aqueous acid solution and the alkali metal salt of a chlorite ion, including continuous stirred reactors, simple tanks, mass-flow or plug-flow reactors and tubular reactors. A tubular reactor is particularly preferred. Usually, a chlorine dioxide generation unit comprises only one tubular reactor, but the generation rate of a unit can be increased by the parallel arrangement of a plurality of reactors, for example to form a tube bundle. The reactor can be not only temperature-controlled, but also comprise a good heat-conducting material in order to release liberated heat of reaction to the surrounding water. The material of which the reactor is fabricated comprises materials which exhibit good stability to the respective reaction solutions. In the generation of chlorine dioxide solutions having concentrations greater than 28 g/l, the reaction material used is, for example, titanium, alloy 31, glass or chemistry materials e.g. polymers such as e.g. PVDF or PTFE. The ClO2 is removed from the reactor by any mechanism which is able to remove an aqueous solution from a reactor. Preferably, the reaction is carried out continuously and ClO2 is removed continuously from the reactor. After the ClO2 leaves the reactor, it is metered directly into the water which is to be treated, or transported to one or more other sites via an offtake situated at the reactor outlet.
A tubular reactor is preferably used according to the present invention. Generally, the tube of the tubular reactor is constructed in such a manner that it has a sufficient length to ensure sufficient residence time in the reactor in order, with respect to flow rate of the reaction solution, its concentration of reactants and the temperature of the reaction solution, to react the components sufficiently. A particularly preferred reactor which can be used for producing a suitable generator of aqueous chlorine dioxide on site is a (temperature-controlled) tubular reactor which contains one or more tube coils. Those of ordinary skill in the art are able to vary the size and shape of the reactor as a function of the amount of aqueous chlorine dioxide to be produced, the flow rate and concentration of the reactants, the pH of the aqueous reaction solution, the pH of the ClO2 and the temperature of the reactor. Those of ordinary skill in the art are likewise able to modify the temperature of the reactor appropriately.
The reaction time in the reaction chamber can vary. With increasing concentration of the reactants in the reaction chamber, the optimum of the residence time decreases. If a solution having a chlorine dioxide concentration of 20 g/l is produced, the median reactor residence time is from about 4 minutes to 60 minutes, preferably from approximately 4 to 6 minutes, in order to achieve a yield of approximately 85%. If the chlorine dioxide concentration according to the particularly preferred embodiment increases to greater than 80 g/l, the median reactor residence time is from about 0.1 minutes to 1.5 minutes, preferably from 0.3 to 0.6 minutes, particularly preferably approximately 0.4 minutes for a 95% yield. The minimum of the median residence time can be achieved when the reactants are used as concentrated solutions, dilution water is not used and the necessary stoichiometric excess of acid or chlorine is minimized.
The conductivity value is preferably measured in the solution leaving the reaction chamber before it is released to the water which is to be treated. For example, an inductive measuring method can be used. From the conductivity values measured, information can be obtained with respect to the chemical reaction of the reactants used and also if desired with respect to the amount of dilution water used.
The chlorine dioxide solution leaving the reactor outlet, or the offtake, is diluted in such a manner that the renewal rate at the reactor outlet of the water which is to be treated is from about 0.1 m3/h to 20 m3/h per gram and hour of generated chlorine dioxide, preferably from 1 m3/h to 4 m3/h per gram and hour of generated chlorine dioxide.
The method according to the invention can be carried out, for example, by use of the devices pictured in
In
At the top (in the Figure), opposite, end of the reactor 7 there is situated the reactor outlet 8, upstream of which is connected a conductivity measurement 13.
At the transfer of the chlorine dioxide solution into the water which is to be treated, a water-jet liquid pump 14 can be arranged, as a result of which the renewal rate of the water which is to be treated at the chlorine dioxide inlet can be increased. The feed line from the reactor outlet 8 to the water-jet liquid pump 14, in this case, has a relief bore hole which ensures that the pressure of the water which is to be treated 9 acts in the reactor 7.
The reactor is completely surrounded by the water which is to be treated 9 which flows through an intermediate store. The treated water is taken off from the intermediate store by a suction line and fed by the circulation pump 10 to the site of use—treated water—11. Via a return line 12, then water which is again to be treated 9 is recycled to the intermediate store and again passes through the reaction outlet 8 or the outflow from the water-jet liquid pump 14. The reactor outlet 8 can also be placed close in position to the suction side of the circulation pump 10 in order to ensure rapid exchange of the water to be treated 9 at the reactor outlet 8. By suitable choice of the parameters “depth of immersion of reactor in the water to be treated” (pressure) and “concentration of the chlorine dioxide solution generated in the reactor”, taking into account the temperature of the water which is to be treated, as shown by way of example in
A further preferred device for the method according to the invention is shown in
In
In
The method according to the invention is illustrated by the following examples, without being restricted thereto:
The device described in
The device described in
The device described in
The present application is based on German Patent Applications DE102007061360.3, filed Dec. 19, 2007; DE102008041081.0, filed Aug. 7, 2008; and DE102008042424.2, filed Sep. 29, 2008, the entire contents of each of which are hereby incorporated by reference.
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
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This application is a Continuation of U.S. application Ser. No. 12/339,533, filed on Dec. 19, 2008, abandoned, and incorporated herein by reference.
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| Child | 13109508 | US |
