FI72108C - FRAMSTAELLNING AV KLORDIOXID. - Google Patents

Description

1 72108

This invention relates to the preparation of a mixture of hydrochloric acid and sulfuric acid for use in the production of chlorine dioxide.

U.S. Patent No. 3,864,456 to the assignee of this patent describes a process for preparing chlorine dioxide and chlorine in which sodium chlorate is reduced by added chloride ions in an aqueous acidic reaction medium containing sulfuric acid with a total acidity of about 4 to about 4, .8 N. The reaction mixture is maintained at its boiling point at a pressure below normal pressure so that chlorine and chlorine dioxide are removed from the reaction medium as a gas mixture with water-steam. Anhydrous neutral sodium sulfate as a by-product precipitates from the reaction medium after the reaction medium becomes saturated with it after starting the process. A gaseous mixture of chlorine dioxide, chlorine and water vapor removed from the reaction zone is treated with water, usually after at least partial condensation of water vapor, to form a chlorine dioxide solution which also contains dissolved chlorine.

It has previously been proposed in U.S. Patent 3,347,628 to form an aqueous solution of chlorine dioxide from a gaseous mixture of chlorine dioxide, chlorine and water vapor which is removed from a chlorine dioxide generator and to which external water vapor is added to dilute the gases by contacting the gas mixture with water. In this previous method, the gaseous chlorine remaining in the absorption is reacted with sulfur dioxide and water to form sulfuric acid and hydrochloric acid, which are fed to a chlorine dioxide generator.

As described in detail in U.S. Patent No. 4,086,329 (assigned to the assignee of this application), the latter plan is not directly applicable to the process of U.S. Patent No. 3,384,456, since the chemical efficiency of 2,7108 chlorine dioxide production in a boiling reaction medium and a reduced pressure medium at a low acid content is less than 100% in the latter process. As described in U.S. Patent 4,086,329 5, it is critical to adjust the hydrogen and chloride ion concentrations in the acid feed, given this low efficiency or otherwise continuous operation is impractical.

It has now been found that the presence of unreacted sulfur dioxide in the mixture of hydrochloric acid and sulfuric acid obtained from the reaction between chlorine, sulfur dioxide and water and introduced into the reaction medium is detrimental to the chlorine dioxide-generating process, even at very low concentrations. Often cell fluid, i.e. the sodium chlorate solution obtained from the electrolysis of the sodium chloride solution without the diaphragm is used as a source of sodium chlorate feed to the reaction medium. Such cell fluid usually contains some dissolved sodium dichromate as a result of the use of this chemical to promote the electrolysis reaction. The presence of dissolved sulfur dioxide in the acid mixture stream has been found to reduce dichromate ions to trivalent chromium, which in turn causes the anhydrous sodium sulfate precipitate to form very small crystals that are very difficult to filter or otherwise separate from the reaction medium. In addition, it has been found that in the absence of dichromate ions, sulfur dioxide dissolved in the reaction medium in the acid mixture stream lowers the efficiency of chlorine dioxide production.

However, the process of reacting chlorine as a by-product of chlorine dioxide absorption with sulfur dioxide and water to form a mixture of sulfuric and hydrochloric acids for reuse in a chlorine dioxide generator is not commercially attractive. Since hydrochloric acid is used to meet at least part of the chloride ion requirement and part of the acid requirement in the chlorine dioxide production process, the total amount of sodium sulphate produced per mole of chlorine-35 dioxide produced is reduced compared to sodium chloride to give all necessary chloride and sulfuric acid, such as sulfuric acid. :

NaC10- + (1x) NaCl + x HCl + 2-x H_SO, -> 3 2 4 5 2 CIO- + 1 / 2Cl- + H-O + 2-x Na-SO, (1) 2 2 2 - - 2 4

NaClO. + 5 (1-x) NaCl + 5x HCl + 6-5x H.SO. -> 3 - £ - 2 4 10 3C1_ + 3H.0 + 6-5x Na ~ SO. (2) 2 2 —2 2 4 'where x is the proportion of spent hydrochloric acid that is a decimal number less than or equal to about 1.00. Equation-15 (1) shows a reaction in which chlorine dioxide is formed; the reaction according to Equation (1) is more preferable than the reaction according to Equation (2) in that chlorine dioxide is formed in the former.

It can be seen that the proportion of sodium sulfate formed decreases as the proportion of hydrochloric acid used instead of sodium chloride and sulfuric acid increases. The need for sodium sulphate in the pulp industry has decreased, while the need for chlorine dioxide has increased. It is preferred that less sodium sulfate be produced using hydrochloric acid.

25 Furthermore, since the chlorine gas from the absorption reacts to form reusable chemicals, it is hardly necessary to absorb the chlorine separately, usually in sodium hydroxide solution, into hypochlorite. As more and more efforts are made to use chlorine dioxide to replace the substantial 30 parts of chlorine previously used as a bleaching agent in the first stage of a multi-stage bleaching process, the need for chlorine has decreased, while the need for chlorine dioxide has increased.

The problem to which this invention relates is the preparation of an aqueous solution of hydrochloric acid 35 and sulfuric acid by a reaction between sulfur dioxide, chlorine and water, which solution does not contain dissolved sulfur dioxide and has a total acid content of 4,710,108, for use in the preparation of chlorine dioxide according to U.S. Pat. No. 3,864,456.

According to the invention, a mixture of hydrochloric acid and sulfuric acid is prepared by a reaction between sulfur dioxide, chlorine and water in the presence of a certain excess of chlorine in the reaction mixture.

The excess chlorine required to avoid the presence of dissolved sulfur dioxide in the acid mixture product increases approximately linearly as the concentration of the acid mixture produced increases. This result is surprising, since it has previously been believed that since chlorine and sulfur dioxide react stoichiometrically, the acid concentration should have little or no effect on the required chlorine partial pressure.

The effect of excess chlorine on the pi-15 content of dissolved sulfur dioxide in the acid alloy product is approximately linear in nature. Thus, at a certain concentration of a mixture of hydrochloric acid and sulfuric acid, the concentration of dissolved sulfur dioxide decreases to zero, and then the concentration of dissolved chlorine increases linearly as the excess gaseous chlorine increases.

The reaction between sulfur dioxide, chlorine and water is carried out according to the invention in a reaction zone which may consist of one, two or more reaction vessels having a gaseous sulfur dioxide inlet, a gaseous chlorine and air inlet, a reactor and an inlet of acid acting as a reactant and an absorption medium. a mixture outlet and an excess chlorine and air outlet. According to the invention, the partial pressure of chlorine in the effluent mixture of chlorine and air has been found to be in a certain proportion to the concentration of sulfur dioxide in the acid-mixture product stream and can be monitored and used as a source of information to determine said concentration.

When the water flow rate to the reaction zone is such that the total acid normality 35 of the acid mixture product stream is approximately constant, a chlorine flow rate that increases the partial pressure of chlorine in the reaction zone effluent linearly changes the concentration of dissolved gases in the acid mixture effluent.

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For example, at an acid mixture concentration of 6.5 N, calculated as the total normality of the mixture of hydrochloric acid and sulfuric acid with respect to acid, a partial pressure of 100 mmHg in the gas effluent results in a dissolved sulfur dioxide concentration of 1 g / l in an acid mixture, while a partial pressure of about 150 mmHg 0 g / l. When the partial pressure of chlorine still increases, the concentration of dissolved chlorine in the acid mixture increases as the partial pressure of chlorine increases.

Even a dissolved SO 2 content of less than 1 g / l has a detrimental effect on the chlorine dioxide production reaction; the efficiency decreases and the precipitation and recovery of the by-product sodium sulfate in the presence of dichromate ions becomes more difficult. According to the present invention, the concentration of dissolved sulfur dioxide is maintained at 0 g / l by appropriately adjusting the partial pressure of chlorine in the gas stream leaving the reaction zone.

The presence of dissolved chlorine in the acid mixture feed does not appear to adversely affect the chlorine dioxide production process, but nevertheless other than a very small amount is avoided, which ensures that dissolved sulfur dioxide is not present, as excess chlorine only causes a permanent load on the system as it leaves gaseous reactor effluent. and recycled back to the sulfur dioxide-chlorine-water reaction zone with the chlorine feed stream.

The partial pressure of chlorine required to keep dissolved sulfur dioxide away from the acid mixture product increases linearly as the total acid normality increases. That is, the partial pressure of 30 chlorine in the effluent varies linearly with the total acid normality of an acid mixture free of dissolved sulfur dioxide.

For example, an acid mixture stream with a total acid normality of 6.5 N and a dissolved SO 2 content of 0 g / l is generated in a chlorine partial pressure outlet gas flow of 150 mmHg, while an acid mixture stream with a total acid normality of 14 N and dissolved SO 2: n concentration 0 g / l, 6 72108 is formed when the partial pressure of gaseous chlorine is 550 germination.

In light of the above linear dependencies, the partial pressure of chlorine in the gas flow from the reaction zone is used easily, reliably, and hy-5 with repeatability to control the total acid normality of the acid mixture and the concentration of dissolved gases.

The invention is further illustrated by the accompanying drawings, in which:

Figure 1 is a flow chart of one embodiment of a process for producing chlorine dioxide according to the invention;

Figure 2 is a graph illustrating the change in dissolved gas concentration in an acid mixture as a function of chlorine partial pressure with a constant total acid normality; 15 and

Figure 3 is a graph illustrating the change in acid normality as a function of chlorine partial pressure with a dissolved sulfur dioxide concentration of zero in the acid mixture.

In Figure 1, chlorine dioxide is continuously formed by the process of U.S. Patent No. 3,864,456 in a chlorine dioxide generator 10. Chlorine dioxide, chlorine, and water vapor formed in the generator 10 as gaseous reaction products are continuously removed along line 12. Anhydrous neutral sodium sulfate is also generated in generator 10 as a solid reaction product and is removed continuously or intermittently along line 14.

Generator 10 has an aqueous acidic reaction medium containing chlorate ions which are continuously fed there as sodium chlorate along line 16. The sodium chlorate solution fed along line 16 30 may be a cell liquid, with the feed stream also containing sodium chloride. The reaction medium is maintained at its boiling point under reduced pressure and has a total acid normality of about 2 to about 4.8 N. The acid is a mixture of sulfuric acid and hydrochloric acid, which is continuously fed to the generator along line 18.

A gas mixture consisting of chlorine dioxide, chlorine, water vapor and air 721 along the flow line 19 is passed on, usually after initial cooling to condense at least most of the steam in the condenser (not shown), into a chlorine dioxide absorber 20 along to dissolve the chlorine dioxide and form a product solution stream formed from the chlorine dioxide solution in line 24. Some of the chlorine contained in the gas mixture of line 12 is also soluble in the chlorine dioxide solution.

The residual gas stream along line 26 is directed 10 to an air ejector 27 which provides a vacuum to generator 10. The chlorine and air stream, now at approximately normal pressure, is directed along line 28 to reaction zone 30 where chlorine is replenished, if necessary, with chlorine from an external source. 15 along line 32, reacts with the sulfur dioxide fed along line 34 and the water coming along line 36.

In the illustrated embodiment, the reaction zone 30 consists of a main reactor 38 and a residual gas reactor 40. A chlorine feed stream 28 and a sulfur dioxide feed stream 34 are passed directly to the main reactor 38 where they react with a dilute acid solution coming from line 42 desired concentration.

The main reactor 38 may be a falling film absorber, while the second reactor 40 may be a packing tower. Reaction zone 30 of a mixture of sulfur dioxide, chlorine and water using this combination is described in F1 application 813,645 filed by the present applicant.

Since the process used is exothermic in nature and tends to result in the loss of HCl from the reaction product at high temperatures, it is preferable to carry out the reaction at a temperature below about 70 ° C.

As described in more detail in the aforementioned prior application, water forms a downwardly flowing film in the membrane reactor and chlorine and sulfur dioxide gases are easily absorbed by the aqueous reaction phase. The cooling channels built into the membrane absorber 8 72108 allow the exothermic reaction to be controlled by passing a cool heat transfer medium, usually water, through them.

As the gases pass through the membrane absorber and the reaction with water takes place, the partial pressures of chlorine and sulfur dioxide in the gas phase decrease, which causes the mass transfer rate of the gases to the liquid phase to decrease. Thus, in order to react a larger proportion of sulfur dioxide and chlorine, a larger reactor volume must be used.

The unreacted gases, together with the air from line 28, are passed from line membrane reactor 38 via line 48 to a packed tower residual gas reactor 40, where residual sulfur dioxide reacts with chlorine and water fed along line 36 to form a dilute mixture of hydrochloric acid and sulfuric acid. to the main reactor 38. The proportion of unreacted gases transferred from the main reactor 38 to the second reactor 40 may vary widely depending on the volume balance between the membrane reactor and the residual gas reactor. Since the cooling of the residual gas reactor 40 depends on the volume of water and the size of the reactor, it is common for at least the majority of the reaction to take place in the membrane reactor 38, usually at least about 75% and typically about 80% of the reaction.

The aqueous mixture of hydrochloric acid and sulfuric acid generated in reaction zone 30 is fed along line 44 to acid supply line 18. The additional amounts of sulfuric acid and hydrochloric acid required to maintain the stoichiometry of the reactions in generator 10, such as U.S. Patent 4,086,329 describes in detail.

The total acidity of the aqueous mixture in line 44 is determined by the relative flow rates of sulfur dioxide, chlorine and water to the reaction zone 30 and is preferably from about 7 to about 9 N, typically about 8 N, due to the evaporation requirements in the reactor 10. In the generator 10, it is advantageous to maintain a substantially constant volume of reaction medium under constant conditions of use, which makes it necessary to boil off the water coming in and forming in the generator. Assuming that the amount of water entering the generator from other sources remains constant, as the concentration of the acid mixture entering line 44 increases, the volume of water to be evaporated decreases.

However, as the water volume decreases, the rate of chlorine dioxide production decreases. As the concentration of the acid mixture decreases, the amount of water to be evaporated increases and thus also the consumption of external heat.

As mentioned above, the total acid normality is preferably from about 7 to about 9. With a total acid normality of less than about 7 N, the amount of heat involved in evaporation increases dramatically and at less than about 6 N the need for heating is a significant economic burden. When the total acid normality is above about 9 N, the amount of water to be evaporated is significantly reduced, leading to a significant decrease in the production rate. If water can be fed from another source to compensate for the smaller amount of water in the acid mixture, the concentration of the acid mixture can be as high as 14 N.

The reaction mixture is usually evaporated so that the weight ratio of water vapor / chlorine dioxide in the gas product stream is about 7: 1, although, based on the concentration of the acid mixture feed and the volume of water fed from other sources, the weight ratio may be from about 4: 1 to about 10: 1.

The reactions in the reaction zone 30 are carried out according to the invention in the presence of an excess of chlorine gas, whereby the unreacted chlorine gas leaves the residual gas reactor 40 and thus the reaction zone 30 without the line 30 50 present. The chlorine in the latter stream can be removed by treating the stream with sodium hydroxide solution before removing the air with a suction fan.

According to the invention, the reaction between sulfur dioxide, chlorine and water in the reaction zone 30 is monitored 35 so that dissolved sulfur dioxide is not present in the acid mixture stream at line 44. This control is achieved by adjusting the partial pressure of chlorine at line 50, which in turn is by adjusting the chlorine feed along line 32 and / or air supply along line 19 and / or sulfur dioxide supply from line 34.

As the concentration of the acid-5 pose mixture stream free of dissolved sulfur dioxide increases with the same amount of sulfur dioxide fed per volume of water, the required excess chlorine also increases, so that the partial pressure of chlorine in line 50 also increases.

The method of Figure 1 described above makes it possible to use the process of U.S. Patent 4,086,329 without the difficulties caused by the presence of dissolved sulfur dioxide in the acid mixture stream and so as to obtain an acid mixture stream whose total acidity can be adjusted.

The following examples illustrate the invention.

Example 1 This example illustrates the detrimental effect that the presence of dissolved sulfur dioxide has on the chlorine dioxide production process.

20 (a) The chlorine dioxide generator (production 16 t / day) contained a boiling medium with a total acid normality of 3.5 N, a temperature of 70 ° C and a vacuum of 190 mmHg. A gas mixture consisting of chlorine dioxide, chlorine and water vapor was removed from the generator.

Cell fluid containing 490 g / l NaClO 2 and 110 g / l NaCl was fed to the generator at a flow rate of 38 l / min. A mixture of hydrochloric acid and sulfuric acid with a total acidity of 4 N and 2 N for HCl and 2N for E 2 SO 4 was also fed to the generator at a flow rate of 38 l / min. The reaction medium was orange in color due to the dichromate fed with the cell fluid.

After the start-up step, the reaction medium became saturated with anhydrous sodium sulfate, which was removed from the generator as a slurry in the reaction medium, separated by filtration from the reaction medium, and the reaction medium was returned to the reaction vessel by a recirculation pump.

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When the generator was operating normally in this way, the acid mixture feed stream was converted to dissolved sulfur dioxide (18 g / l) for three hours. This led to a browning of the reaction medium, an increase in the density of the reaction medium 3 3 5 from 1.5 g / cm to 1.7 g / cm, an increase in the recirculation pump current from 58 to 64 ares, a dramatic increase in the chlorine dioxide production rate from 16 to 24 t / days and serious impurities in chlorine dioxide. After the reactor was completely shut down, which was necessary due to bad impurities, the crystals were examined and it was found that when normally 90-95% of the crystals remained on the 200 mesh screen, only 40-50% of the crystals remained on this screen, indicating a significant reduction in crystal size.

(b) The experiment according to 1 (a) was repeated with the change that the dissolved SC 2 content in the acid mixture stream was 0.1 to 0.2 g / l. After only 15 minutes, the orange solution began to turn brown, the flow of the recirculation pump increased and the crystals settled more slowly.

The results of Experiments 1 (a) and 1 (b) show that the results deteriorate significantly in a very short time in the presence of dichromate ions and the acid mixture feed contains dissolved SC 2, even as in Example 1 (b) in very small amounts.

(c) A 10 L laboratory scale generator was constructed containing a boiling reaction medium having a total acid normality of 3.8 N, a temperature of 71 ° C and a vacuum of 170 mmHg. A gas mixture containing chlorine dioxide, chlorine and steam was removed from the reactor.

5.75 M sodium chlorate was fed to the reaction medium at a flow rate of 6.7 ml / min, sodium chloride solution (300 g / l) at a flow rate of 6.7 ml / min and 18 N sulfuric acid. The chemical efficiency of chlorine dioxide production was 91% based on the conversion of sodium chlorate to chlorine dioxide.

Sulfur-35 oxide was then fed to the reaction medium at a rate of 1.8 g / min, which corresponds to the amount of 200 g / l sulfur dioxide dissolved in the acid mixture feed. The efficiency of the production of chlorine dioxide was reduced to only 73.8% based on the conversion of sodium chlorate to chlorine dioxide.

Although the amount of equivalent dissolved sulfur dioxide was abnormally high in this experiment, it was used to indicate that in the presence of sulfur dioxide in the chlorine oxide generator, there is a significant decrease in production efficiency. Lower amounts of dissolved sulfur dioxide cause less detriment to efficiency, but any decrease in efficiency is a significant economic loss, 10 because the main raw material for chlorine dioxide, namely sodium chlorate, is an expensive chemical.

Example 2 This example illustrates the process of the invention.

(a) Reaction zone 30 according to Fig. 1 was constructed. The reaction between sulfur dioxide, chlorine and water was used to produce a mixture of hydrochloric acid and sulfuric acid. The following table provides flow rate information for a single use.

Table I

20 Power Line No. Feed speed

Chlorine feed 28 10 956 1 / h + 2 640 1 / h air

Sulfur dioxide supply 34 6 864 1 / h

Water supply 36 37.2 1 / min HCl / H2SO4 44 41.8 1 / min 8 N acid 25 (no dissolved SC> 2) Residual gas flow 48 1 030 1 / h SC »2, 4 488 1 / h

Cl2, 2,640 1 / h air

Dilute acid flow 42 38 1 / min 1.5 N acid

Chlorine gas removal 50 3 344 1 / h Cl2, 2 640 1 / h air 30 flow (partial pressure of Cl2 252 mmHg) (b) The experiment shown in Table I was repeated by changing the partial pressure of chlorine in line 50 to determine its effect on the concentration of dissolved gases in line 44. in the current acid mixture stream, the total acid normality of this stream being constant. The flow rate was adjusted so that the total acid normality of flow 44 was 6.5 N.

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Dissolved gases and their concentrations were determined at each chlorine partial pressure. The results were shown graphically and are shown in the accompanying drawings in Figure 2. As shown in Figure 2, the partial pressure and the concentration of dissolved gases in the constant acid content were determined to be approximately linear. As the partial pressure increased, the concentration of dissolved sulfur dioxide decreased to zero, with a partial pressure of about 150 mmHg, and as the partial pressure increased further, the concentration of dissolved chlorine in the acid mixture increased.

(C) The experiment according to Table I was repeated again by varying the partial pressure of chlorine in line 50, but this time the highest total acidity of line 44 was determined, in which the concentration of dissolved sulfur dioxide in the acid mixture was still zero.

The partial pressure of chlorine necessary for the concentrations of the various acid mixtures to maintain a dissolved sulfur dioxide content was determined for each acid concentration. The results were expressed as curves and are shown in Figure 3 of the accompanying drawings. As shown in Figure 3, the relationship between the pressure of the chlorine sub-20 and the dissolved sulfur dioxide content of the total acid normality at 0 was approximately linear. As the partial pressure increased from about 150 mmHg to about 55 mmHg, the total acid normality of the acid mixture stream that could be achieved with the sulfur dioxide content still at zero increased from 6.5 N to 14 N.

In summary of this disclosure, the invention relates to methods for controlling the characteristics of a feed stream of a mixture of hydrochloric acid and sulfuric acid used in the production of chlorine dioxide by adjusting the partial pressure of chlorine involved in the reaction of sulfur dioxide with water to form an acid mixture. The above can be modified without departing from the scope of the invention.

Claims (8)

A process for the preparation of chlorine dioxide and chlorine by reduction of sodium chlorate by means of added chloride ions in an aqueous reaction medium containing sulfuric acid, which reaction medium is halified at its boiling point under reduced pressure and from which neutral sodium sulfate precipitates, and by feeding of a mixture of hydrochloric acid and sulfuric acid, which mixture is formed by reaction between sulfur dioxide, chlorine and water, to the reaction agent, characterized in that the reaction between chlorine, sulfur dioxide and water is carried out in the reaction zone in the presence of air and an excess of chlorine, and that the concentration of sulfur dioxide in the mixture of hydrochloric acid and sulfuric acid formed in the reaction zone is halized at 0 g / l by measuring the partial pressure of chlorine in that case stream, soiti is emitted from the reaction zone, and determination of the partial pressure applicable where the concentration of sulfur dioxide is 0 g / l, adjusting the excess of chlorine in the reaction zone to or partial pressure to said level. 2. The process of claim 1, characterized in that the partial pressure of the unreacted chlorine discharged from the reaction zone is controlled so that the acid normality of the mixture of hydrochloric acid and sulfuric acid is in the range 6-14 N. Method according to claim 2, characterized in that the acid normality is in the range 7-9 N. 4. Process according to claim 2 or 3, characterized in that the partial pressure for chlorine is 150-550 mm Hg. Process according to any one of claims 1-4, characterized in that the reaction is carried out in two reaction zones, the reaction between a major proportion of the sulfur dioxide, chlorine and water being carried out in