GB2156796A - Chlorine dioxide generator - Google Patents

Abstract

In the production of chlorine dioxide from concentrated solutions of sodium chlorate and hydrogen chloride, plugging of the generator is prevented by initially mixing the solutions under vigorous agitation conditions. <IMAGE>

Description

SPECIFICATION Chlorine dioxide generator This invention relates to the generation of chlorine dioxide from sodium chlorate and hydrochloric acid precursor solutions and to retarding the accumulation of precipitates which plug the reaction chamber.

Chlorine dioxide is used in water treatment as a biocide and hydrogen sulphide scavenger.

Chlorine dioxide is also stable at a higher pH than chlorine, making it more useful for water treatment purposes. However, pure liquid chlorine dioxide and its vapours are explosive.

Therefore, only dilute aqueous solutions are commercially available, adding greatly to transportation cost. Therefore, field generation of chlorine dioxide is the most practical and economical method.

Several chemical methods for generating chlorine dioxide have been tried in the past. The most useful methods for field generation use either sodium chlorite solution or sodium chlorate solution as one of the reactants, which, when combined with hydrochloric acid yield the desired product. Sodium chlorite is much more expensive than sodium chlorate. Unfortunately, however, use of the less costly chlorate has previously been troublesome because it reacts with the hydrochloric acid to produce large quantities of sodium chloride.

For large industrial applications, chlorine dioxide has been generated from the sodium chlorate process by utilising sulphuric acid in place of hydrochloric acid. The generators for this sodium chlorate/sulphuric acid process have been designed to collect the precipitated sodium sulphate, which was then recycled. For smaller on-site generator systems, the recycling of sodium sulphate is both impractical and uneconomical. Until recently, the sodium chlorite/hydrochloric acid, sodium chlorite/hydrochloric acid/sodium hypochlorite or sodium chlorite/chlorine gas processes have been utilised almost exclusively, all of which employ the relatively expensive sodium chlorite precursor solution.

The salt plugging problem can be alleviated by use of dilute solutions of sodium chlorate or hydrochloric acid. However, use of such solutions reduces efficiency of the genrating process due to considerably slow reaction rates and lower chlorate-to-chlorine dioxide conversion. In addition, transportation of the dilute solutions proves more cumbersome and less cost effective.

One earlier method involved the use of a generator that cleared the sodium chloride plug resulting from the sodium chlorate/hydrochloric acid process with an intermittent water flush.

Dilution of the chlorate/hydrochloric acid mix with this water flush greatly reduces the chlorine dioxide yield and thus the overall efficiency of the process. Because of the general recycling nature of this procedure, certain restrictions on continuous applications are imposed. This also creates another problem when running very large generators in that very large quantities of fresh water are required.

Other methods for generating chlorine dioxide from sodium chlorate utilise reducing agents, such as sulphur dioxide, nitrogen dioxide or methanol, usually in the presence of a strong acid (normally sulphuric acid). The use of the reducing agent essentially adds another reactant to the process and consequently increases the complexity and cost of generating equipment, thus reducing the utility of these processes for field generation purposes.

In the invention chlorine dioxide is produced by a process comprising initially mixing in a mixing zone a solution of sodium chlorate with a solution of hydrochloric acid, optionally in the presence of sodium chloride, wherein the concentrations of the solutions are such that precipitation of sodium chloride is liable to occur during the initial mixing but the initial mixing is conducted with agitation that is sufficiently vigorous that the precipitation is substantially prevented.

Thus in the invention the process conditions are such that, with significantly less vigorous initial agitation precipitation of sodium chloride does occur, with consequential salt plugging or other serious problems. However in the invention the initial mixing is conducted with sufficiently vigorous agitation that the precipitation is substantially prevented. Thus even though sodium chloride particles may, and usually are, formed during the initial mixing they do not precipitate to the extent necessary to cause salt plugging or other serious problems.

The mixing zone is usually provided with an impeller for providing the vigorous agitation, and most preferably the mixing zone is a centrifugal pump. The agitation is preferably such that the mixing in the initial mixing zone is turbulent mixing characterised by shear flow. The shear flow is generally in excess of the shear flow of the individual solutions prior to their introduction into the mixing zone.

The produced chlorine dioxide may be extracted from the mixing zone. Generally the mixture formed in the mixing zone is transferred to a reaction chamber to permit completion of the reaction, for instance over a period of 5 minutes.

In carrying out the invention sodium chlorate solution and hydrochloric acid are injected simultaneously into a high speed mixer. Without the mixer, sodium chloride precipitates immediately, and after a few minutes enough of the solid salt collects to plug the generator. The high speed mixer adds mechanical energy to the system at the initial mixing zone. By either forming a supersaturated solution, or by eliminating the formation of large crystals of solid masses, sodium chloride is dispersed and kept in suspension in the fluid product, thus eliminating the troublesome precipitates and the interruptions of the generation process occasioned thereby.

This invention effectively solves the sodium chloride plugging probelm by condcting the reaction under conditions which maintain the sodium chloride product as extremely small, even colloidal, particles which remain in such form until the reaction products are mixed with and diluted by the water being treated and become dissolved therein. Typically, and in one embodiment of the invention, these conditions are met by introducing reactants into a turbulent zone. The desired degree of turbulence may be obtained by introducing the reactants into a mechanical mixer designed to provide the desired turbulence.

The invention is illustrated in the accompanying drawings, in which: Figure 1 is a flow diagram illustrating the process described herein.

Figure 2 is a sectional view of a turbulent mixer employing an impeller for mixing the principal reactants.

Figure 3 is a view of an apparatus containing multiple chemical injection points upstream of the reaction chamber.

Figure 4 is a view of a modified centrifugal pump used in this invention.

The sodium chlorate/hydrochloric acid process for production of chlorine dioxide generally requires a minimum of about a five minute reaction time at room temperature for optimum yield. A static mixer depends on the linear velocity (and hence kinetic energy) of the fluid flowing through it to function. In order to satisfy the requirements of a five minute reaction time and sufficient linear velocity to produce a high Reynolds Number, a mixer of small diameter and considerable length would be required for medium and large size generators. This would be nearly impossible to achieve with very small generators. A more practical method to produce the desired turbulence is to use a motor driven impeller in the initial reaction zine. An open impeller centrifugal pump sized large enough to act only as a mixer at the reactant flow rates passing through it serves this purpose well.A further advantage of the impeller design is that it adds mechanical energy to the system rather than relying on the energy present in the flowing reactants to achieve sufficient turbulence.

The mechanical mixer is used at the point where the precursor solutions are initially combined. Initial mixing is in a turbulent zone in which the Reynolds Number is in excess of the Reynolds Number of the reactants in a vessel downstream of the initial mixing zone in which the reaction may be carried to completion. Also, the Reynolds Number in the mixing zone should preferably be greater than the Reynolds Number of the solutions as they are fed into the mixing zone.

The Reynolds Number (herein designated by the symbol RN) is a dimensionless constant representing the ratio of shear forces to viscous forces in a moving liquid. In its most general form, it is given by the relation: RN = D Vd/y Equation 1 D is the dimension of the container measured at a right angle to the principal direction of fluid movement, V is the average velocity of movement, d is the density of the liquid, and,z is its viscosity. All of the variables must be expressed in consistent units.

It has been found that when RN exceeds about 3500, the fluid has a turbulent or random motion superimposed on its viscous motion in the direction of flow. The greater the RN, the more violent is the turbulence, and the greater are the shear forces throughout the liquid.

The working formula for calculating RN may vary with the hydrodynamic condition (whether the flow is in round pipes, open rectangular channels, in the annulus between a rotating and stationary cylinder, in the chamber of a centrifugal pump, or in other conduits), but is always devised to estimate the ratio of shear to viscous forces.

For flow through round pipes, RN is readily calculated from Equation 1. This equation may be used to obtain a minimum RN for static mixers, such as lengths of baffled pipe, by using the velocity of flow across the baffled section. Actual shear forces, and RN, will be underestimated in this case because no account is taken of so-called "entrance" and "exit" head losses, which contribute to the shear.

In many kinds of rotating devices handling fluids, such as mixers, pumps, and turbines, the minimum value of RN can be represented by RN = Tr, (rl-r2) d/,u Equation 2 where T is the rotation rate of the impeller in radians per second, r, is the housing radius, r2 is the impeller radius, and d and p have their previous meanings.

Using the cgs system of units, this can be written as: RN = 9.5 Rpm r, (r,-r2)d/,u Equation 3 where Rpm represents impeller rotation rate in revolutions per minute, r, and r2 are in centimeters, density is in grams per cubic centimeter, and lb is in centipose.

Using this formula and the specifications of the centrifugal pump of Fig. 4, we calculate RN from Equation 3 to be a minimum of 114,000 for Rpm = 3,500; r, = 3.5 cm; r2 = 2.7 cm; d = 1.25 g/cc; and y= 1.02 cp.

In like manner, substituting a centrifugal pump running at 1400 Rpm would provide a minimum RN of 45,600.

Each of these cases shows very turbulent conditions with high shear forces. The desired rapid mixing of reactants is affected in such shear zones. With pumps of this type another degree of high shear (not included in the above calculations) exists between the radial edges of the rotating impeller blades and the pump housing. Entrance and exit losses also contribute to an actual RN higher than that calculated.

In the practice of this invention, mixers or pumps of various specifications may be selected for differing reactant systems and flow rates. An effort is made to prevent solids precipitation with the least expense for equipment and operation. In experimenting with various systems and equipment, it has been found that a minimum RN of 5,000 is required and that higher values, preferably above 45,600 or even above 114,000, are even more desirable in creating smaller crystals of solid reaction products.

The chemical and physical effects involved in this process of rapid mixing or reactants for the preparation of chlorine dioxide are not completely understood. As best understood, the process is at least partially dependent upon the sudden formation of a supersaturated solution which causes the precipitation of many very minute or colloidal particles of sodium chloride (which, incidentally, may be further reduced or fluidised by the shear forces in the mixing zone). Similar processes involving very rapid creation of a supersaturated solution, although generally with much less soluble precipitate than the sodium chloride formed in our process, have been described for preparation of colloidal solutions by P. P. Von Weimarn, Kolloid Zeitschrift, Vol. 2, pp. 119 et seq, (1908).

The above is offered as one possible explanation bearing on the surprising effects obtained in this method. However, postulation of this mechanism is in no way intended as limiting the scope of the method described herein.

When generating chlorine dioxide at room temperature, solutions containing from between about 15% and about 50% sodium chlorate, by weight, are usually employed in the acidchlorate process. In some cases, the addition of sodium chloride to the chlorate precursor solution aids in generating chlorine dioxide. Hydrochloric acid solution is added to the sodium chlorate solution in an amount such that the mole ratio of acid to chlorate is from between about 2.0 and about 4.0 to 1.0.Using this procedure, the sodium chloride precipitation and consequent generator plugging occurs when the sodium chlorate content exceeds about 25% of the precursor solution with no sodium chloride added; or when the sodium chlorate content exceeds about 20% if more than about 3% sodium chloride is added to the precursor solution; or when the HC1 content of the acid solution exceeds about 25%.

The precursor solution may contain from between about 15% and about 50% by weight sodium chlorate, preferably about 25%-50%, and most preferably from between about 35% and about 50% sodium chlorate. Sodium chloride may be added to the chlorate solution such that it comprises from between about 1 % to about 10% by weight of the solution, or preferably from between about 1% and about 7%. Hydrochloric acid may be used in a range of from between about 15% and about 35% by weight, preferably from between about 25% and about 35% by weight, and most preferably from between about 28% and about 35% by weight.

It can be seen from Table 1 that under conditions of the preferred formulations, the precipitation related plugging problem occurs.

TABLE 1 Effect of Precursor Composition and Acid Concentration on Precipitate Formation Experiment Chlorate Precursor Hydrochloric Acid Precipitate Number Solution Solution Formed % NaCIO3' %NaCl1 % HCI2 1 37 7.0 30 Yes 2 28 5.2 30 Yes 3 24 4.5 30 Yes 4 22 4.2 30 Yes 5 20 3.8 30 No 6 18.5 3.5 30 No 7 40 0 30 Yes 8 30 0 30 Yes 9 25 0 30 No 10 40 0 25 Yes 11 40 0 22.5 No 12 40 0 20 No ' % of solution by weight.

2 % of solution by weight. Acid was added such that moles of HCl/moles of NaClO3 = 3/1.

In practice, the high speed mixer may consist of a centrifugal pump 2 rated for very corrosive conditions and revolving, for example, at a rate of about 1400-3500 rpm, and comprising an impeller 1 mounted on drive shaft 3. The reactants are injected into the pump suction through a tee 4 with a partition 6 fixed into it. This partition serves to separate the reactants until they enter the turbulent zone in the centrifugal pump (see Fig. 2). This pump is typically sized to act only as a mixer at the chemical flowrate at which the generator is designed to operate.

It has been found that adequate mixing is obtained if the pump is sized to provide at least about 50% greater flow capacity than the maximum chemical flowrate of the generator. The optimum value is subject to change with the process variables for a particular system. The output of the "mixing pump" is piped to a reaction chamber 8 sized to give the minimum required (5 min.) reaction time and then discharged to the system to be treated. Employing a centrifugal pump in this manner produces very high tubulence and shear in the reaction zone and eliminates the formation of plugging precipitates.

Example 1 The apparatus used in the present example consisted of a modified commercially available chlorine dioxide generator which was originally designed to use the sodium chloride/hydrochloric acid process. The modification consisted primarily of an additional set of chemical injection points 4a that entered through a high speed mixer, and another set 4b was positioned as close as possible to the reaction chamber (see Fig. 3). The standard generator 8 utilises an educator 10 to draw reactants into the generator. The efficiency of the eductor was found to be somewhat dependent on the water pressure to the educator. Reactant flow rates were measured with flowmeters. The sodium chlorate solution (37% by weight) was fed at 81 ml/min. the hydrochloric acid (20"Be) was fed at 99 ml/min. In this example, the mixing pump 2 consisted of a 1/3 HP mag drive centrifugal pump made by March Manufacturing Company. As can be seen from Table 2, without the mixer the generator plugged in nine minutes or less, whereas use of the mixer allowed the generator to be run for two full hours with no plugging, at which time it was shut down.

TABLE 2 Modification of Commercial Acid-Chlorine Generator for Acid Chlorate Process Pressure to Eductor Mixer Operating Time to Plugging (psig) of Reactor 35 No 9 min 45 No 9 min 60 No 9 min 75 No 7 min 55 Yes > 2 Hrs.-did not plug The 1/3 HP pump was subsequently replaced with a specifically modified (see Fig. 4) /25 HP mag drive centrifugal pump, made by March Manufacturing Company, which was utilised as the high speed mixer for all the other work done using the eductor design to draw reagents into the generator. No plugging associated with sodium chloride precipitation was encountered during five separate runs of at least one hour duration. In one instance, the generator was operated for three hours and 40 minutes without any evidence of plugging precipitates.

Example 2 Several problems were encountered with the eductor type chemical feed. It was replaced with a diaphragm metering pump for introducing each of the reactant solutions into the mixing zone.

When the mixer was utilised along with the metering pumps, the generator was run several times for extended periods, on the order of two hours, without plugging. When the mixer was turned off the generator plugged in a six and one-half minutes.

Example 3 A new generator utilising the metering pump and high speed mixer design was constructed.

This unit was used in a field test which consisted of three-hour/day treatments over five days.

The generator was operated at approximately half capacity such that 1 3.8 L/Hr 37% sodium chlorate solution and 24.3 L/Hr 30% hydrochloric acid solution were consumed. On four of the five days, the generator worked flawlessly for two hours or longer but became plugged in less than three hours. The generator was flushed with water and the treatment completed. On the remaining day the generator ran for the full three hours without plugging. It appears that in this example plugging will be completely eliminated by using a larger centrifugal pump for the mixer and/or modifying the outlet piping from the mixer.

Example 4 A much smaller generator uses the metering pump, high speed mixer design. This particular device shown in Fig. 4 incorporates a 1/25 HP centrifugal pump 2' of the type manufactured by March Manufacturing Company and at maximum output consumes 93 ml/min. 37% sodium chlorate solution in the chlorine dioxide generation process. This unit consistently operates for two hours with no plugging.

Claims (14)

1. A process in which chlorine dioxide is produced by a process comprising initially mixing in a mixing zone a solution of sodium chlorate with a solution of hydrochloric acid, optionally in the presence of sodium chloride, wherein the concentrations of the solutions are such that precipitation of sodium chloride is liable to occur during the initial mixing, and in which the initial mixing is conducted with agitation that is sufficiently vigorous that the precipitation is substantially prevented.

2. A process according to claim 1 comprising mixing a first solution containing 1 5 to 50% by weight sodium chlorate and 0 to 10% by weight chloride with a second solution containing 1 5 to 35% by weight hydrogen chloride.

3. A process according to claim 2 in which the first solution contains 25 to 50% by weight sodium chlorate.

4. A process according to claim 2 in which the first solution contains 35 to 50% by weight sodium chlorate.

5. A process according to any of claims 2 to 4 in which the first solution contains 1 to 7% by weight sodium chloride.

6. A process according to any of claims 2 to 5 in which the second solution contains 25 to 35% by weight hydrochloric acid.

7. A process according to claim 6 in which the second solution contains 28 to 35% by weight hydrochloric acid.

8. A process according to any preceding claim in which the Reynolds Number during the intial mixing is above 5,000.

9. A process according to any preceding claim in which the Reynolds Number during the intial mixing is above 114,000.

10. A process according to any preceding claim in which the mixing zone is static and the agitation is caused by an impeller.

11. A process according to any preceding claim in which the initial mixing is effected by a centrifugal pump.

1 2. A process according to any preceding claim in which the initial mixing is effected by a pump having a flow capacity at least 50% greater than the maximum flow rate of the solutions entering and leaving the mixing zone.

13. A process according to any preceding claim in which, after the initial mixing with agitation in a mixing zone, the resultant mixture is transferred to a reaction chamber to permit further production of chlorine dioxide.

14. A process according to claim 1 3 in which the Reynolds Number in the mixing zone is greater than the Reynolds Number in the reaction chamber.

1 5. A process according to claim 1 3 or claim 14 in which the solutions are fed to the mixing zone with a Reynolds Number less than the Reynolds Number in the mixing zone.

1 6. A process according to claim 1 substantially as herein described with reference of any of the drawings.