FI69292B - FOERFARANDE FOER FRAMSTAELLNING AV KLORDIOXID - Google Patents

Description

69292

This invention relates to a process for the production of chlorine dioxide by reducing chlorate-5 ions in the reaction zone with chloride ions in an aqueous acidic reaction medium containing sulfuric acid in a concentration greater than 6 as a normal solution, while maintaining

U.S. Patent No. 4,081,520 to Erco Industries Limited describes a process for efficiently producing chlorine dioxide using sodium chlorate, sulfuric acid and methanol. Chlorine dioxide is formed by a mechanism in which chlorine produced together with chlorine dioxide is reacted with methanol to form chloride ions, which then reduce chlorate ions to form chlorine dioxide and chlorine. The total reaction can be represented by the following equation: 2NaClO3 + 2H2SO4 + CH3OH-> 2C102 + NaHCHO + 2H2O

The reaction medium, from which chlorine dioxide is formed and which contains sodium chlorate, methanol and sulfuric acid, is maintained under reduced pressure at its boiling point, usually in the range of about 50 to 85 ° C. Evaporated water acts as a diluent for chlorine dioxide when removed from the reaction zone. The total acid normality of the reaction medium is high, being more than about 9 normal, and the by-product precipitated from the reaction medium immediately after the onset of saturation is acidic sodium sulfate and may be any sodium bisulfate (NaHSO 4) or sodium sesquisulfate (Na 3 H). ) 2>.

The process of U.S. Patent No. 4,021,520,692,292 is highly effective when presented as the conversion of chlorate ions to chlorine dioxide. Nearly 100% efficiency is achieved, and chlorine dioxide is removed from the reaction zone virtually uncontaminated with chlorine, which can be advantageous in many cases for the final use of chlorine dioxide.

The commercial exploitation of this prior process has been hampered by the shape and nature of the resulting solid by-product. Because sodium sulfate precipitates in acid form, removal of this substance from the reaction zone causes a decrease in the acidity of the reaction solution. In addition, acidic sodium sulfates are difficult to physically handle and exhibit wettability.

The present invention makes it possible to overcome this problem by a unique process for the exchange of solid acidic sodium sulfate to form solid neutral sodium sulfate, and the acid values are recycled to the reaction zone.

Although the present invention is described in detail herein and in the conversion of acidic sodium sulfate formed to the neutral sodium sulfate formed in the process of the aforementioned U.S. Patent No. 4,081,520, the present invention is widely applicable to the conversion of neutral sodium sulfate from any highly acidic sulfuric acid-based chlorine dioxide process. if in the manufacturing process the reaction medium is kept under reduced pressure at its boiling point and if acid sulphate precipitates in the reaction zone from the reaction medium.

For example, the process of the present invention can be used to convert acidic sodium sulfate obtained as a by-product from a chlorine dioxide generator in which added chloride ions have been used as chlorate reducing agents in the presence of highly acidic sulfuric acid, as disclosed in CA Patent No. 825,084.

69292

Furthermore, a chlorine dioxide-producing process that produces acidic sodium sulfate may be one in which sulfur dioxide is used to reduce chlorine as a by-product to chloride ions in a manner similar to that used in methanol in U.S. Patent No. 4,081,520. The use of sulfur dioxide is described in U.S. Patent No. 3 933,988, Hooker Chemicals & Plastics Corporation.

The process according to the invention is characterized in that, after removal from the reaction zone, the acidic sodium sulfate is contacted with water in the presence of methanol, the water being used in a weight ratio to acidic sodium sulfate (calculated as Na 2 H (SO 4) 2) in the range of 0.4: 1 , 4: 1 and methanol by weight to acidic sodium sulfate (calculated as Na 2 H (SO 2) 2) in the range 0.01: 1 15 to 2: 1 to form a solid neutral sulfate and an aqueous phase containing sulfuric acid, and the sulfuric acid is recycled the reaction medium.

It has previously been proposed to use water or methanol or another water-soluble alcohol or ketone to convert acidic sodium sulfate to neutral sodium sulfate. Such a method is disclosed in U.S. Patent No. 4,104,365 to Howard and Lobley. The process of said patent is directed to the recovery of neutral sodium sulfate from the liquid phase of the process for the production of chlorine dioxide based on highly acidic sulfuric acid. Such processes do not operate at the boiling point of the reaction medium and do not crystallize the by-product sodium sulfate from the reaction medium in the reaction vessel. The starting material in this prior art process is an aqueous solution of acidic sodium sulfate as opposed to the solid starting material used in this invention.

The previous process required a pre-distillation step to remove dissolved gases and residual sodium chlorate that would otherwise inhibit the reaction.

Such a step is not required in the process of the present invention because the starting material is solid acidic sodium sulfate prepared in a chlorine dioxide-producing reaction zone.

4,69292

The process of the present invention differs from the previous process not only in that the starting materials are in a different physical form, but also in that the process of the present invention requires significantly smaller volumes to form neutral sodium sulfate, water and methanol added to the effluent than the previous process. Due to the large volumes of water and methanol-lithium, considerable evaporation has been required in previous processes, above all to recover the spent methanol and also to concentrate the sulfuric acid to a concentration suitable for reuse in the generator.

The invention will now be described in connection with the use of methanol where possible, but the invention also includes the use of other water-soluble alcohols and ketones.

The process of the present invention has a weight ratio of water to acidic sodium sulfate (calculated as ^ 2 * 1 (SO 2) 2) of about 0.4: 1 to 1.4: 1, preferably about 0.6: 1 to 0.8: 1. The ratios of water to acid sulfate in the ranges listed are critical to the processes of the present invention, as weight ratios less than 0.4: 1 only result in poor conversion of acid sulfate to neutral sulfate, while weight ratios greater than 1.4: 1 result in it. that a larger amount of sodium sulfate is soluble in the aqueous phase. When using weight ratios in this critical range, the resulting aqueous phase contains sulfuric acid of sufficient acid normality and is thus suitable for recycling to the chlorine dioxide-producing process without concentration.

As previously mentioned, the acidic sodium sulfate that precipitates from the reaction medium may exist in various forms, one of which may be Na 2 H 2 SO 4 depending on the total acidity of the reaction medium. When calculating the weight of water and methanol used to effect the acidic sodium sulfate exchange reaction, the weight of acidic sodium sulfate must be expressed on the basis of the equivalent weight of Na 2 H (SO 2) 2. If the acidic sodium sulfate is determined to be in a form other than Na 2 H 2 SO 4), such as sodium sulfate or a mixture of sodium bisulfate and sodium sesquisulfate, the weight of the acidic sodium sulfate is changed.

Na 2 H (SO 2) 2 to the equivalent weight. If the acidic sodium sulfate is determined to be in the form Na 2 HUSO 2), the weight basis of the exchange reaction is the actual weight of the acidic sodium sulfate.

In contrast to the typical process of U.S. Patent No. 4,104,365, the weight ratio of water to acidic sodium sulfate (viewed as Na 2 H (SO 2) 2 is about 1.83: 1 and the weight of water required to be removed for reuse in a suitable manner). to obtain a sulfuric acid solution, is about 3.6 kg per kilogram of neutral sodium sulfate obtained (or about 4.14 kg per kilogram of ClO 2 formed).

The weight ratio of methanol to acidic sodium sulfate, calculated as Na 2 H (SO 4) 2, is not as critical as the ratio with water and can range from 0.01 to 2: 1. The weight ratio of meta-20 to acidic sodium sulfate is preferably about 0.3: 1 to 0.8: 1 with the above-mentioned preferred weight ratio of water to acidic sodium sulfate. Due to its miscibility with water, methanol reduces the volume of water in which sodium sulfate can dissolve, thus preventing the dissolution of neutral sodium sulfate in the aqueous phase.

As the weight ratio of methanol increases, the proportion of neutral sodium sulfate dissolved in the aqueous phase decreases until the weight ratio of methanol is so high that increasing the amount of methanol does not increase the yield of neutral solid sodium sulfate, with a yield limit of about 80-85% by weight. Therefore, no advantage is obtained if the weight ratio of methanol to acidic sodium sulfate is increased to greater than about 2: 1.

11 ____— 69292

In selecting the process recommended by the invention, i.e., when chlorine dioxide is prepared by the aforementioned U.S. Pat. 4,081,520, part of the aqueous phase from the exchange reaction can be recycled directly to the chlorine dioxide-producing reaction medium to give at least a portion, preferably all, of the methanol it requires. This recycled stream also provides some of the sulfuric acid required by the chlorine dioxide manufacturing process. In this way, the methanol recycled to the reaction medium does not have to be recovered from the recyclable portion of the aqueous phase. The remaining aqueous phase is stripped of methanol to a generator to obtain a suitable sulfuric acid solution for recycling.

The amounts of methanol used in this invention differ significantly from those of the prior U.S. Pat. Of the amounts used in the process of 4,104,365, the previous process typically uses a weight ratio of methanol to acidic sodium sulfate (calculated as Na 2 H (SO 2)) of about 9.33: 1; and methanol must be recovered for reuse, and the difference is also that the sulfuric acid solution can be concentrated to an acid normality suitable for recycling to the chlorine dioxide production process.

The steam requirement of the process of the present invention for evaporating the aqueous phase is limited to the steam required to distill methanol, and in a typical preferred embodiment of the invention, steam costs $ 2.50 per tonne of chlorine dioxide produced (steam is calculated to be $ 6.61 per 1000 kg). This steam demand is substantially lower than that of U.S. Pat. 4,104,365 in a process in which steam is required to distill a large amount of methanol and concentrate the sulfuric acid-30 poly solution, and in its typical embodiment, the steam demand costs $ 35.00 per tonne of chlorine dioxide produced, i.e. nearly fifteen times the steam cost of this invention.

69292

Usually, water and methanol are added to solid acidic sodium sulfate as a solution containing the required ratios of water and methanol. However, since the sole function of methanol is to contribute to the reduction of the Liu-5 content of neutral sodium sulphate in the aqueous phase, methanol can also be added after the addition of water.

The above description of the process of the invention has been made with regard to the use of methanol because this solvent is readily available and because it is effective in the process of the invention.

However, if necessary, other water-soluble alcohols and ketones can be used, for example n-propanol, isopropanol and acetone.

The exchange reaction used in this invention can be carried out over a wide range of temperatures, usually in the range of about 10 ° to about 70 ° C. The reaction proceeds efficiently at room temperature (about 20 ° -25 ° C), although higher temperatures are usually recommended because the rate of the reaction increases as the temperature is raised. Preferably, the temperature is in the range of about 20 ° to about 50 ° C.

The exchange reaction of the present invention can be carried out in any conventional manner. Although a batch process can be used, a continuous process is recommended because the process of the invention involves a continuous chlorine dioxide producing process.

The exchange reaction can be carried out in a simple reaction vessel or in a decantation-washing column, which is described in detail in our DE patent application OLS 2856504.

The mixing of the aqueous-methanol solution and the acidic sodium sulfate 30 to effect the exchange reaction can be performed by stirring in a reaction vessel. Although agitation speeds up the mass transfer required in the exchange reaction, rapid agitation is unnecessary and only slow agitation is required, although it requires a longer period of time.

The time required to complete the exchange reaction can vary widely and is usually about 10 minutes with rapid stirring and about 60 minutes in a decanter.

8 69292 The invention will now be described in more detail with reference to the accompanying drawing, which is a flow diagram illustrating an embodiment of the invention.

Referring to the drawing, a chlorine dioxide generator 10 produces a gaseous mixture of chlorine dioxide and steam on line 12, from which chlorine dioxide is absorbed into water to obtain an aqueous solution of chlorine dioxide which can be used for wood pulp bleaching or some other purpose.

Generator 10 produces chlorine dioxide from the aforementioned U.S. Pat. 4,081,520 according to the process by feeding sodium chlorate solution to line 10 along line 14, sulfuric acid along line 16 and methanol along line 18.

An aqueous reaction medium having a total acid normality greater than about 9 is maintained at its boiling point below the temperature at which substantial decomposition of chlorine dioxide occurs, generally at a temperature in the range of about 30 ° to about 85 ° C at a boiling point pressure, usually in the range of about 20 ° C to about 20 ° C. about 400 mmHg and acidic sodium sulfate precipitates from the reaction medium as soon as the reaction medium reaches saturation after initiation.

In generator 10, the volume of the reaction medium is kept substantially constant by equilibrating the aqueous phase entering the generator 10 with the volume of volatile water forming the gaseous product stream 12 from the reaction medium, and with the water leaving the reaction mixture with solid acidic sodium sulfate.

In the generator 10, the acidic sodium sulfate precipitated from the reaction medium is directed along the line 20 to the reactor 22. The acidic sodium sulfate, usually sodium sesquisulfate, can be removed from the generator 10 as a slurry with the reaction medium and separated by filtration before the reactor 22.

In reactor 22, acidic sodium sulfate is concentrated by feeding water along line 24 and methanol along line 26. The weight ratio of water to acidic sodium sulfate (calculated as Na 2 H (SO 4) 2) in the reactor is about 0.4: 1 to 1.4: 1.9,69292 methanol and acidic sodium sulfate (calculated as Na 2 H (SO 4) 2) the weight ratio is from about 0.01 to about 2: 1, preferably from about 0.3: 1 to 0.8: 1.

In Reactor 22, the temperature of the medium combined with solid acidic sodium sulfate is preferably about 20 ° to about 50 ° C, and the exchange reaction produces anhydrous neutral sodium sulfate. The acidic sodium sulfate exchange reaction produces not only neutral sodium sulfate but also sulfuric acid. The neutral sodium sulfate and aqueous phase are separated by any suitable method such as filtration, and the neutral sodium sulfate is removed along line 28 for the desired use, typically to form sodium and sulfur values in the pulp mill to which the chlorine dioxide generator 10 is associated.

The aqueous phase, containing sulfuric acid, methanol and slightly dissolved sodium sulphate, is removed along line 30 and divided into two streams, usually about one third of the volume being recycled along line 32 to line 18 of methanol fed to the chlorine dioxide generator to give at least part, preferably all the methanol required in the generator 10, and if necessary, methanol is fed along line 33 to the methanol supply line 18. The sulfuric acid contained in the aqueous phase on line 32 provides part of the need for sulfuric acid-25 in the medium of generator 10.

Typically, two-thirds of the volume of the aqueous phase is directed along line 34 to a methanol distiller 36 where the methanol is removed from the aqueous phase. Methanol vapor is directed along line 38 to condenser 40 to form liquid methanol 30, which is further directed along line 42 to methanol feed line 26 to exchange reactor 22 such that the balance of methanol demand of reactor 22 is fed along line 44.

The desulphurized sulfuric acid solution is circulated along line 46 to the sulfuric acid feed stream of line 10 of line 10. In addition, the required sulfuric acid is fed along line 48 to the sulfuric acid supply line 16.

10 69292

The process described above with the aid of the drawing therefore produces essentially chlorine-free chlorine dioxide efficiently without catalyst additions to the reaction medium. At the same time, the by-product sodium sulphate is obtained in neutral form, preferably in anhydrous neutral form, so that sulfuric acid is not lost from the systemic by-product. The system uses a starting material, namely methanol, to convert acid sulfate to neutral sulfate.

The invention is further illustrated by the following Examples 10.

Example 1 This example illustrates the preparation of acidic sodium sulfate in U.S. Pat. 4,081,520. A chlorine dioxide generator was used to form chlorine dioxide from sodium chlorate, sulfuric acid and methanol. The reaction medium was kept at its boiling point under reduced pressure, and sodium sesquisulfate precipitated from the reaction medium. The operating parameters are shown in the following Table I: 70

Table I Operating conditions:

- temperature 74 ° C

- pressure 135 mm Hg 25 Starting material concentrations and feed rates - MeOH 33%, 3.4 ml / min.

- H 2 SO 4 9 M, 3.6 ml / min.

30 - NaClOg 6.74M, 10.5 ml / min.

Generator fluid concentrations

-H 2 SO 4 9.3N

35 -NaClO3 1.1M

Solid crystals NaoHtSO,) - J 4 λ 11 69292

Chlorine dioxide production: - rate 0.48 g / l / min - chlorate-based efficiency> 99% - gas analysis C1C> 2 * ^ 9%, Cl2 <^%

Example 2 This example illustrates the exchange reaction of sodium sesquisulfate under various conditions.

A series of experiments were performed in which solid sodium sesquisulfate was contacted with water and / or methanol at ambient temperature (20 ° C) while the slurry was stirred slowly for about 15 minutes.

15 A series of tests was performed to determine the critical limits of the exchange process and the best operating conditions. In each case, the weight of the sodium sulfate obtained and the proportion of sulfuric acid remaining in the sodium sulfate were determined. Considering the efficiency of the process, the residual sulfuric acid concentration of less than 1% indicated the conversion of substantially acidic sodium sulfate from 100% to neutral sodium sulfate.

a) Critical lower weight ratio of water to acidic sodium sulphate 25 The test results showing the criticality of the lower weight ratio of 0.4: 1 for water (HiSO 4) are shown in the following Table II:

Table II

30 Experiment Weight ratio Yield H 2 SO 4 No. H 2 O: Na 3 H (SC> 4) 2 CH 3 OH: Na 3 H (SO 4) 2% by weight in solids ____ (3) __ (%) _ 1 0 1.1 99 (1) 15 .2 2 0.125 0.7 93.3 (1) 18.4 35 3 0.25 0.6: 84.5 (1) 10.4 4 0.4 0.6 64.2 77 0.92 5 0 .5 0.4 67.3 81 0.84 69292 12

Note: (1) The yield value is insignificant due to the high oxygen content of these products.

Table II above clearly shows that it is essential to adjust the weight ratio of water to acidic sodium sulfate to at least 0.4: 1 in order to obtain complete exchange conversion to the neutral salt.

(b) Critical overall weight ratio of water to acid sulphate

The experimental results showing the criticality of the upper weight ratio limit of 1.4: 1 for water (Na 2 H 2 SO 4) ^ 10 are given in the following Table III:

Table III

15 Experiment Weight Ratio Yield H «90.

2 4 No. H 2 O: Na 3 H (SO 4) 2 CH 3 OH: Na 3 H (9O 4) 2% by weight of solids ____ <a> __ (%) 6 1.2 0.6 47.9 56 0.86 7 1.4 0 , 6 39.8 48 0.97 20 8 1.6 0.6 no precipitate i

Table III above clearly shows that it is essential that the weight ratio of water to acidic sodium sulfate is not adjusted to be greater than 1.4: 1 in order to obtain a precipitate from the reaction.

c) Criticality of the presence of methanol during the exchange reaction

The experimental results showing the criticality of the presence of methanol during the reaction are shown in the following Table IV: 13 69292

Table IV

Experiment I Weight ratio Yield H „SO.

I 2 4 n H 1 H 2 O (Na 2 H 2 SO 2) 2% by weight in the solid 5 μl (2) __ (%) δ 0.75 0 16.8 20, 2 1, 0 10 0,75 0,1 57,3 69 1,1 11 I 0,875 0,1 37,5 45 0,73 12! 0.75 I 0.2 j 52.3 63 j 0.94 10!

The results in the table show a strong increase in the yield of neutral sodium sulfate with a weight ratio of methanol to acidic sodium sulfate as low as 0.1: 1.

15 d) Criticality of the upper weight ratio limit for methanol and acid sodium sulphate

The test results showing the criticality of the upper weight limit for methanol and acid sodium sulfate are shown in Table V below: 20

Table V

Experiment Weight Ratio Yield H „S0.

2 4 No. H ^ OtNa ^ HiSO4) ^ CH2OHiNa ^ HtSOj) ^% by weight in the solid 25 _______ (3) __ (%) 9 0.75 0 16.8 20.2 1.0 10 0.75 0.1 57.3 69 1.1 11 0.875 0.1 37.5 45 0.73 12 0.75 0.2 52.3 63 0.94 30

As can be seen from Table V above, no further increase in yield is observed when the weight ratio of methanol to acidic sodium sulfate exceeds 2: 1.

14 69292 e) Results obtained in the recommended ranges The experimental results showing that the best results are obtained between the critical limits of the ratio I ^ O.-Na 2 H (SO 2) 2 and the ratio CH 2 OH: Na 2 H (SO 2) 2 are: shown in Table VI below:

Table VI

Experiment Weight Ratio Yield 10 No. I (O 2 Na 2 HiSO 3) ^ CH 2 OH 2 Na 2 HiSO 2) ^% by weight in solids ____ (2) __ (%) 16 0.645 0.34 63.3 76 0.53 17 0.75 0.4 64.5 77 0.56 18 0.75 0.6 67.3 81 0.34 15 19 0.75 | 0.8 j 70.7 85 0.13

From the results of Table VI above, it can be seen that when the weight ratio of N 2 O 2 Na 2 H (SO 2) 2 H is in the preferred range of 0.6 to 0.8: 1 and the molar ratio of CH 2 OH in the range of 0.3 to 0.8: 1, product yields greater than 75% are obtained and the acid level of the other is less than 0.6%.

Example 3 This example illustrates the use of alcohols and keto-25s other than methanol.

The procedure of Example 2 was repeated using acetone and ethanol instead of methanol twice for 20 minutes at 20 ° C with slow stirring. The results are summarized in the following Table VII: 69292 15

Table VII

Weight ratio Weight ratio Recovery% H „SO„ Water 2 4 I ^ ChNa ^ H (£ 50 ^) 2 Solvent: taken from ^ 280 ^: phase 5 Solvent in Na2H (SO2) ^ Na2SO4 normal weight _ ( g) _lisuus

Acetone 0.75: 1 0.4: 1 62.4 1.6 5.1

Ethanol 0.75: 1 0.4: 1 75.9 0.13 5.1 10

The results in Table VII above illustrate the utility of acetone and ethanol in the exchange reaction.

In summary of this disclosure, the present invention provides an efficient chlorine dioxide production process in which chlorine-free chlorine dioxide is formed and sodium sulfate as a by-product.

Claims (4)

69292 A process for producing chlorine dioxide by reducing chlorate ions in the reaction zone with chloride ions in an aqueous acidic reaction medium containing sulfuric acid in a concentration greater than 6 as a normal solution, while keeping the reaction medium at its boiling point under reduced pressure and acidic sodium sulfate then the acidic sodium sulphate is contacted with water in the presence of methanol, the water being used in a weight ratio to acidic sodium sulphate (calculated as Na 2 SO 2 (SO 4) 2) in the range of 0.01: 1-2: 1 to form a solid neutral sulfate and an aqueous phase containing sulfuric acid, and the sulfuric acid is recycled back to the reaction medium. Process according to Claim 1, characterized in that the weight ratio of water to acidic sodium sulphate is in the range from 0.6: 1 to 0.8: 1 and the weight ratio of methanol to acidic sodium sulphate is in the range from 0.3: 1 to 0, 8: 1. Process according to Claim 1 or 2, characterized in that the normality of the reaction medium is greater than 9 normal and the chloride ions in the medium 25 are formed by reduction of chlorine, which is produced simultaneously with chlorine dioxide with methanol. Process according to Claim 3, characterized in that the aqueous phase obtained after the contacting step is divided into two streams containing sulfuric acid 30 and methanol; a second stream is recycled to the reaction medium, methanol is removed from the second stream to form a methanol stream and a water stream; the methanol stream is recycled to the contact phase and the water stream is recycled to the reaction medium.