Method of treating water with chlorine dioxide

Abstract

The present invention relates to a method of preparing chlorine dioxide (ClO.sub.2) from hydrochloric acid (HCl) and sodium chlorite (NaClO.sub.2) in the presence of water (H.sub.2O). The invention has for its object to further develop the method such that it is more economical to install and operate. The object is achieved when the hydrochloric acid is used in aqueous solution at a concentration of 27 to 33 wt %, the sodium chlorite is used in aqueous solution at a concentration of 22 to 27 wt % and the molar ratio of hydrochloric acid used to sodium chlorite used is between 2.14 and 4.2.

Claims

1. A method of preparing chlorine dioxide (ClO.sub.2), comprising: conveying a first aqueous solution of hydrochloric acid comprising 27 to 33 wt % HCl at a controlled rate to a reactor immersed in water; conveying a second aqueous solution comprising 22 to 27 wt % sodium chlorite (NaClO.sub.2) at a controlled rate to the reactor immersed in water; controlling the rates of flow of the hydrochloric acid solution and the sodium chlorite solution such that a molar ratio of the HCl to the NaClO.sub.2 in the reactor is from 2.84 to 3.19 reacting the HCl with the NaClO.sub.2 in the reactor to obtain a solution of ClO.sub.2 at a concentration of 26 g/l or higher; and removing the ClO.sub.2 solution from the reactor such that a residence time from reaction of the HCl and NaClO.sub.2 to removal of the ClO.sub.2 solution from the reactor is less than 6 seconds.

2. The method according to claim 1, wherein: a) the first aqueous solution comprises 30 wt % of the HCl; b) the second aqueous solution comprises 25 wt % of the sodium chlorite; or both a) and b).

3. The method according to claim 1, wherein: the chlorine dioxide emerges from the reactor in a third aqueous solution; and the third aqueous solution is diluted with water and then mixed with the water surrounding the reactor.

Description

(1) Illustrative embodiments of the invention will now be more particularly described with reference to drawings, where

(2) FIG. 1 shows a schematic construction of a plant for conducting the method of the present invention with a reactor immersed in a basin;

(3) FIG. 2 shows a schematic construction of a plant for conducting the method of the present invention with a reactor inserted in a pipe line;

(4) FIG. 3 shows solubility limits of chlorine dioxide in water;

(5) FIG. 4 shows the chlorine dioxide yield as a function of the molar ratio of hydrochloric acid to sodium chlorite.

(6) The method according to the invention can be conducted using for example the apparatuses depicted in FIG. 1 and FIG. 2.

(7) FIG. 1 depicts an in-principle construction for conducting the method of the invention in essentially standing water. The apparatus for treating water with chlorine dioxide comprises two tanks 1 and 2 for the feed chemicals (reactants), a sodium chlorite storage tank 1 with conveyor pump 3 and a hydrochloric acid storage tank 2 with conveyor pump 4. The volume flow rates V.sub.HCl and V.sub.NaClO2 can be adjusted by adjusting the speeds of the two conveyor pumps 3 and 4. The pumps 3 and 4 are connected via individual lines to a reactant inlet on the bottom side of a reactor 5. The reactor contains state of the art appliances to ensure rapid and complete mixing of introduced components in the reaction space. The free volume V in reactor 5 is available as reaction space. By varying the concentration contents of reactant solutions or any dilution water quantity used the resultant chlorine dioxide solution can be adjusted to a concentration of above 3 g/l, preferably above 26 g/l and more preferably to above 80 g/l. The residence time t for reactants in the reactor volume and the molar ratio R of hydrochloric acid to sodium chlorite is controlled via the volume flow rates V.sub.HCL and V.sub.NaClO2.

(8) The reactor 5 is fully immersed in a water-filled basin 6 to ensure, in the event of an accident, immediate dilution of chlorine dioxide produced. The water in the basin is in this case the water 7 which is to be treated.

(9) The upper, opposite end of reactor 5 is equipped with the reactor outlet 8, which is assigned a conductivity measurement.

(10) At the point where the chlorine dioxide solution transfers into the water 7 which is to be treated, a water-jet liquid pump 9 can be arranged to increase the rate at which the water which is to be treated is renewed at the point of chlorine dioxide entry. The feed line from reactor outlet 8 to water-jet liquid pump 9 is in this case equipped with a relief drill-hole to ensure that the pressure of the water 7 that is to be treated takes effect in reactor 5.

(11) The reactor is completely surrounded by the water 7 to be treated, which is standing in basin 6. The treated water is withdrawn from basin 6 via a suction line and fed by a circulation pump 10 to the use site 11. Basin 6 can be a cooling tower basin for example. In that case, use site 11 is a heat exchanger cooled with water which is to be treated. But basin 6 can also be a tap water reservoir. In that case, use site 11 is a manufacturing unit in the biomedical or food industry for example.

(12) A return line 12 then carries the water 7 to be treated again back into the basin 6 to pass through the reactor outlet 8 again or to be fed into the water-jet liquid pump 9. The reactor outlet 8 can also be positioned close to the suction side of circulation pump 10 in order that rapid exchange at reactor outlet 8 may be ensured of water 7 to be treated. A suitable choice for the parameters “depth of reactor immersion in water to be treated” (pressure) and “concentration of chlorine dioxide solution generated in reactor” having regard to the temperature of the water to be treated, as shown in FIG. 3 by way of example, can be used to prevent the formation of a chlorine dioxide gas phase. An additional possibility is for the chlorine dioxide solution emerging from the reactor 5 to be transported via an exit line connected to reactor outlet 8 (and not depicted in FIG. 1) to one or more than one other location. Assemblies to distribute the chlorine dioxide solution, for example a water-jet liquid pump, a circulation pump, may also be placed there.

(13) A second apparatus for the method of the present invention is shown in FIG. 2. The essential aspect here is that the reactor 5 is positioned within a pipe line 13 through which water 7 to be treated flows, while water to be treated flows around the reaction space V. The water to be treated is thus not standing water but is flowing water. (It will be appreciated that flows can also occur in basin 6, caused especially by the circulation pump 10 or the water-jet pump 9, but the volume in flow is small compared with the overall volume of the basin, and therefore the reference is to standing water there.)

(14) Reactor 5 in FIG. 2 is identically connected to the same feed lines as in FIG. 1. Reactor 5 is likewise surrounded by water 7 which is to be treated, but is positioned within a pipe line 13 through which water 7 which is to be treated flows and which feeds the water 7 to be treated to use site 11 after it has passed through reactor outlet 8.

(15) The molar ratio R of reactants and the residence time t likewise determine the volume flow rates set at the pumps 3 and 4.

(16) More detailed descriptions of suitable equipment for conducting the method of the present invention appear in DE102010027840 in respect of standing water and in DE102010027908 in respect of water flowing through a pipe line.

(17) The concentration of product solution at reactor outlet 8 can rise to above 9 g/l [without water of dilution the ClO.sub.2 content rises to 9.1 g/l even on using 3.5% strength reactants], preferably above 26 g/l and more preferably to above 80 g/l of chlorine dioxide, per liter. In this preferred variant, the reactor volume is advantageously minimized. Generally, no further appliances are needed for increasing the renewal rate at reactor outlet 8 of the water 9 which is to be treated in order that the concentration of the chlorine dioxide solution, following entry into the water 7 which is to be treated, may be rapidly shifted from preferably above 80 g per liter into the milligram range. It is likewise generally not difficult to adjust the pressure in pipe line 13 of the water 7 which is to be treated such that the solubility limit of chlorine dioxide is not exceeded in the aqueous solution in reactor 7, as depicted in FIG. 3.

(18) FIG. 3 depicts the solubility limits of chlorine dioxide in an aqueous solution as a function of pressure and temperature, for example for the chlorine dioxide concentrations 70 g/l and 80 g/l.

REACTION EXAMPLES

(19) To demonstrate the effect due to the present invention, an existing chlorine dioxide production plant was run in the course of ongoing operation at two different reaction volumes while varying the reactant molar ratio, the reactant concentration and the residence time and determining the chlorine dioxide conversion achieved in the process. The reaction volume was artificially reduced by introducing glass balls into the reactor. The reactant concentration was varied by adding water to the reactant tank. Residence time and molar ratio were adjusted via the volume flow rate produced by the reactant pumps.

Example 1

(20) Apparatus shown in FIG. 1 is used. The solution in chlorite storage tank 1 contains a 25% strength aqueous sodium chlorite solution and varying amounts of this solution are pumped by conveyor pump 3 into reactor 5. At the same time, varying amounts of a 30% strength aqueous hydrochloric acid solution are likewise fed from acid storage tank 2 into reactor 5 by conveyor pump 4. The reactant temperature was 20° C. After packing with glass balls, the reactor has a free volume V of 13 milliliters and reaction mixture residence time in the reaction space is adjusted via the choice of reactant throughput such that a residence time t of 5 or somewhat less than 5 seconds is obtained at all times [equation 11].

(21) The chlorine dioxide solution emerging from the reactor outlet 8 is continuously mixed, in the water-jet liquid pump 9, with the water 7 which is to be treated, and the mixture is supplied to the disinfection process. After a steady state had become established, samples were taken at the point of exit from the water-jet pump to determine the ClO.sub.2 content therein photometrically by measuring the extinction (at 345 nm) and to compute the yield therein. Different HCl and NaClO.sub.2 throughputs were established in order that the molar ratio R of feedstocks may be varied from 1.35:1 to 3.46:1. The measured results are reported in Table 1 and plotted using the □ symbol in the diagram of FIG. 4.

(22) Operating at between R=2.84 and 3.19 actually provides complete conversion, i.e. the greatest degree of optimization for the process.

(23) TABLE-US-00001 TABLE 1 HCl 30%; NaClO.sub.2 25%; t = 5 s; V = 13 ml R Yield [—] [%] 1.35 38.1 2.14 92.5 2.19 93.5 2.44 96.8 2.51 97.7 2.84 100.0 3.19 100.0 3.46 93.8

(24) This aqueous ClO.sub.2 solution was then additionally admixed in basin 6 with the returned amount of water to be treated. The throughput of water enriched with chlorine dioxide is about 1000 m.sup.3 per hour and is pumped by circulation pump 10 out of the basin 6 and to the use site 11. The return of the water 7 to be treated carries the water depleted in chlorine dioxide back into the basin. The reactor outlet 8 is 4 meters below the water in the basin, and the temperature of the water to be treated is up to 32° C.

(25) The apparatus described concerns recooled water 7 in a cooling tower basin (basin 6) from a cooling circuit. The circuit pump 10 pumps the cooling water over heat-exchange surfaces of a chemical manufacturing plant having exothermic sources of heat (use site 11 of treated water) and then trickled over the internals of an evaporative cooling tower before ending up back in the cooling tower basin (basin 6). The cooling tower basin has a capacity of 800 m.sup.3. The water level in the intermediate store is under closed-loop level control, so the evaporated cooling water is automatically made up with makeup water.

Example 2

(26) This time the reduced, 13 ml capacity reactor in the same plant was charged with only 20% strength hydrochloric acid and 25% strength sodium chlorite solution. Reactant flow rates were varied to vary the molar ratio of hydrochloric acid to sodium chlorite in the range from 1.24:1 to 5.41:1. Residence time was in each case set to about 5 seconds. The measured results are reported in Table 2 and plotted with the ◯ symbol in the diagram of FIG. 4.

(27) TABLE-US-00002 TABLE 2 HCl 20%; NaClO.sub.2 25%; t = 5 s; V = 13 ml R Yield [—] [%] 1.24 25.5 2 57.6 2.88 86.9 3.72 87.1 4.7 88.0 5.41 90.3

Example 3

(28) The glass balls were now removed from the reactor in the same plant. The reactor, the undiminished volume V of which was now 26.67 ml, was again charged with 30% strength hydrochloric acid and 25% strength sodium chlorite solution. Reactant flow rates were varied to vary the molar ratio of hydrochloric acid to sodium chlorite in the range from 0.85:1 to 4.28:1. Residence time was in each case set to about 23 seconds. The measured results are reported in Table 3 and plotted with the Δ symbol in the diagram of FIG. 4.

(29) Complete conversion is achieved between R=2.11 and R=2.59.

(30) TABLE-US-00003 TABLE 3 HCl 30%; NaClO.sub.2 25%; t = 23 s; V = 27 ml R Yield [—] [%] 0.85 10.7 1.09 61.6 1.51 86.8 2.11 100.0 2.59 100.0 4.03 92.9 4.28 89.0

(31) Comparing the conversions obtained in Examples 1 and 2 teaches that increasing the hydrochloric acid concentration causes the conversion of chlorine dioxide to increase (conversions in Example 1 better than in Example 2). In principle, this is not surprising. Crucially, however, conversion increases with increasing hydrochloric acid excess only to then drop off again or only to further increase slowly later. The conversion maximum is located in the range claimed for the molar ratio of hydrochloric acid to sodium chlorite. The technical teaching derivable from that is if possible to exploit the first conversion maximum and to use the more concentrated acid.

(32) Comparing Examples 1 and 3 first confirms once more again the conversion maximum within the claimed range. Surprisingly, there is scarcely any decrease in conversion when residence time is shortened to 5 seconds: complete conversions are also achieved in Example 1. The reaction is accordingly so fast that it has ended after about 5 seconds and hence there is no further conversion. In Example 3, the reactants were accordingly allowed a much too generous residence time. This insight can be technically exploited by reducing the reactor volume V for the same volume flow rate V′=V.sub.HCL+V.sub.NaClO2; cf. Equation 8. The reactor accordingly becomes smaller and cheaper, but achieves almost the same reaction performance, as evidenced by Example 1.

(33) The results of the three series of tests are plotted in the diagram of FIG. 4. Intermediate values are interpolated. The curves all show a distinct conversion maximum located in the range from 2.14 to 4.2 claimed according to the present invention, preferably between 2.19 and 3.6 and most preferably between 2.4 and 3.4. Conversion here is above 90%, preferably above 93% and most preferably above 96%.

LIST OF REFERENCE NUMERALS

(34) 1 sodium chlorite storage tank 2 hydrochloric acid storage tank 3 sodium chlorite conveyor pump 4 hydrochloric acid conveyor pump 5 reactor (reaction space) 6 basin 7 water to be treated 8 reactor outlet 9 water-jet liquid pump 10 circulation pump for water to be treated 11 use site 12 return line 13 pipe line in FIG. 2