FI61458C - FOERFARANDE FOER FRAMSTAELLNING AV KLORDIOXID - Google Patents

Description

ΙΓ-ννΐ "- ·! R„, / 44 KU RELEASE PUBLICATION / 44CQ

W i11) UTLÄGCNINOSSKIIIPT 6i4oo ('♦ S) ia L. lii' i - (I d .Ί ut ^ ^ (51) Kv.lk.3 / lr * t.CI.3 c 01 B 11/02 ENGLISH I - Fl N LAN D (21) Ptt «ittlh» kMnu «- NtMtaMBknlng 2773/73 (22) Hakamtoplivi - AmMutlngadag 06.09 · 7 3 (23) Start date — Glltlfh« t »dag 06.09.73 (41) Tullut | ulklMk * l - Bllvlt offantllg 07.03.75 _ · (44) Date of issue of the publication -, „

Patent and Registration Office 'AmMcm uthgd och utUkrMkm pubticerad 30.0U.62 (32) (33) (31) fyy «Uttolkni * - Begird prlorlMt (71) Erco Industries Limited, 2 Gibbs Road, Islington 678, Ontario, Canada The present invention relates to the production of chlorine dioxide, and more particularly to the production of chlorine dioxide, and more particularly to the production of chlorine dioxide, and more particularly to the production of chlorine dioxide, and more particularly to the production of chlorine dioxide, and more particularly to the production of chlorine dioxide. economical manufacture from two or more generators.

Chlorine dioxide is used to bleach cellulosic fibrous materials such as sulphate or wood pulp from sulphite processes.

Several methods are known for the preparation of chlorine dioxide using the reduction of an alkali metal chlorate, usually sodium chlorate, in an acidic solution using sulfur dioxide, sulfuric acid, chromium sulphate, methanol, sodium chloride or hydrochloric acid as reducing agents. : 2H + + C103 "+ Cl" -> C102 + 1 / 2Cl2 + H2O

If sodium chloride or hydrochloric acid is the reducing agent, chlorine is formed together with chlorine dioxide. In the presence of other reducing agents, chlorine is reduced to chloride in the reaction solution so that the chloride oxide formed contains little chlorine.

61458

The acidic medium can be formed from sulfuric acid when the reducing agent is sodium chloride and from both sulfuric acid and hydrochloric acid when the reducing agent is hydrogen chloride. The acidic medium can be formed exclusively from hydrogen chloride, which at the same time acts as a reducing agent.

CA Patent 825,087, issued October 1, 1969 to the Electric Reduction Company of Canada, Limited, discloses a process for forming chlorine dioxide and chlorine from alkali metal chlorate, alkali metal chloride or hydrogen chloride or a mixture thereof and sulfuric acid to form chloride oxide and chlorine and alkali metal acid sulfates. precipitates in the same reaction zone. Water is evaporated out of the reaction medium to remove chlorine dioxide and chlorine from the reaction space. This can be accomplished using reduced pressure in the reaction zone and operating at the boiling point of the reaction medium.

When the alkali metal is sodium, the form of the acid sulfate depends on the temperature and the acidity of the reaction space. At high acidities of about 10-12 N and temperatures of about 75-100 ° C, the acidic sodium sulfate is in the form of sodium bisulfate (NaHSO 4). The reaction that takes place is represented by the formula:

NaClO3 + NaCl + 2 ^ 30 ^ -> 2NaHSO) j + C102 + 1/2 Clg + HgO

At lower acids, about U, 8-9 N, and at lower temperatures of about 30-70 ° C, sodium sesquisulfate (Na 2 O 5 O 2) is formed. The reaction occurs is represented by the formula: 3NaC103 + 3 NaCl + UHgSO4 -3C102 + 3/2 Clg + 3H2O = 2Na3H (SO2) 2 CA Patent 826,577, issued October 1969 to the Electric Reduction Company of Canada Limited, discloses a process to produce chlorine dioxide and chlorine by reacting sodium chlorate, sodium chloride and sulfuric acid under low acidity conditions, generally about 2 to 0.8 N. This method can be used in one reaction zone in the process of CA Patent 825 08U ^) reaction state.

The reaction that takes place is represented by the formula:

NaClO3 + NaCl + HgSO4 -> C102 + 1/2 Clg + H2O + NagSO4

In all of the above reactions, a side reaction occurs:

NaClO3 + 5NaCl + 3H2S (^ - ^ 3Cl2 + ÖNagSO4 + 3H2O to form no chlorine dioxide. This reaction becomes significant when the molar ratio of chloride to chlorate in the reactor feed substantially exceeds 1: 1 3 61458

In order to maximize the yield of chlorine dioxide from chlorate, it is preferable to use approximately equimolar or slightly higher molar ratios of chloride to chlorate in the feed in the above process.

According to the present invention, chlorine dioxide and chlorine are formed in a number of different zones, wherein chlorine dioxide and chlorine are removed from these zones by evaporating water vapor from the aqueous reaction media. According to the invention, the water vapor leaving one zone is used to generate at least a part of the heat required by the other zone.

CA Patent Application 08U 077 to Electric Reduction Company of Canada, Limited discloses a process for the production of chlorine dioxide using a plurality of sulfuric acid-based reaction zones operating at different acids. One of the zones operates with a low acidity below H, 8 N and the other with high acidities above U, 8 N. High acidity zones or a zone form acidic sodium sulfate, which is used in part to meet the acidity requirements of a low acidity reactor.

The present invention is particularly useful in this latter method and will be described below with particular reference to the latter method.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which a flow chart of an embodiment of the invention is shown.

The chlorine dioxide generator 10, for example of the type disclosed in the aforementioned CA patent 825 08U, comprises an aqueous reaction medium containing sodium chlorate and sodium chloride fed through line 12, generally as an aqueous solution containing both salts and sulfuric acid fed through line IU. The molar ratio of sodium chlorate in the sodium chloride feed stream is preferably almost equimolar. The individual concentrations of chloride and chlorate in the reaction medium can vary widely. For example, the concentration of chlorate may range from about 0.005 to 3 moles and the concentration of chloride may range from about 0.001 to 2 moles.

The generator 10 generally maintains a partial vacuum and the aqueous reaction medium is preferably at its boiling point. The temperature of the medium can vary over a wide range, such as about 30-80 ° C, although it is preferable to operate at the upper end of the range for later reasons.

Chlorine dioxide at normal atmospheric pressure decomposes spontaneously. The water vapor evaporating from the aqueous solution in the generator dilutes the chlorine dioxide, making it less susceptible to self-decomposition. The chlorine dioxide and chlorine formed in the generator 10 are removed from the generator with steam through a pipe 16.

11 61458

The reaction temperature in the generator 10 can be maintained in any suitable way, for example by means of a heat exchanger 18 to which water vapor is fed through a pipe 20.

The acidity of the aqueous solution in the generator 10 is greater than i *, 8 K, generally about 6 N to 12 N. The generator is preferably operated so as to maintain a substantially constant liquid level therein by controlling the feed rate and evaporation of aqueous sodium chloride and sodium chloride solution and water. Excess solution can be recycled through the supply lines.

Acid sodium sulfate is precipitated in the generator. The form of the acid sulfate depends on the acidity and temperature of the reaction solution. The acid sulfate is generally in the form of sodium bisulfate (NaHSO 4) because the generator 10 generally operates at a high level of acidity (about 10 N) and at a high temperature (about 75 ° C). However, the acidic sodium sulfate may be in the form of sodium sesquisulfate (Na 2 HiSO 4) if lower acidities and / or temperatures are used.

Acid sodium sulfate is passed through line 22 from generator 10 to second generator 2b. Sodium chlorate and sodium chloride, generally as a solution containing both salts, are fed to generator 2b via line 26 and an aqueous reaction intermediate is formed in the generator.

Acid sodium sulfate fed through line 22 forms the sole source of acid in the reaction space. In certain cases, it may be desirable to add sulfuric acid to this acid supply. The acidity of the reaction medium is below U, 8 N, generally 2 N to U, 8 N, typically about N.

Depending on the form of the acidic sodium sulphate, two possible reactions can occur: a) IJaC103 + NaCl + 2NaHSO4-7 · 2Na2S01 + + C102 + 1/2 Clg + H2O and h) NaClO + NaCl + ENa ^ SO ^ g -7 ^ UNagSO ^ + C1C > 2 + 1/2 Clg + H 2 O.

The generator 2b is preferably operated under reduced pressure and at the boiling point of its aqueous medium. The level of medium in the generator is kept substantially constant in the same manner as above in generator 10. Anhydrous sodium sulfate precipitates and can be removed through line 28.

According to the present invention, the operating temperature of the generator 2b is obtained by conducting water vapor containing chlorine dioxide and chlorine from the generator 10 via a pipe 16 to a heat exchanger 30 connected to the generator 2b.

The heat exchange takes place between the water vapor and the aqueous medium of the generator 2b. The water vapor condenses and an aqueous solution of chlorine dioxide, which usually contains slightly dissolved chlorine, is recovered from the heat exchanger through line 32 5 61458. The gaseous chlorine and any remaining chlorine dioxide are transferred from the heat exchanger via a pipe 3 * + to the chlorine dioxide absorber 38.

Vapor vapor from the aqueous reaction medium of generator 2k removes chlorine dioxide and chlorine from generator 2k through line 36 and the gas mixture passes to absorber 38. Water can be fed through line * + 0 to absorber 38 and aqueous chlorine dioxide may contain some dissolved chlorine. Gaseous chlorine is removed through pipe 1 + 1 +.

Alternatively, the absorption device may be in the form of a condenser in which the water vapor condenses and the heat of condensation is absorbed into the water in the heat exchanger and this water is used through the pipe 20 to generate the steam supplied to the heat exchanger IB. In this way, thermal economy can be further improved. In addition, the water supplied through pipe 1 + 0 may be a condensate of the first heat exchanger 18. The chlorine dioxide solutions in tubes 32 and 1 + 2 can be combined into a single stream 1 + 6, which can be used in a pulp mill as a cooking chemical or for bleaching cellulose pulp or paper.

In the generators 10 and 21 +, a reduced pressure can be used, provided in some suitable way, such as by means of a venturi pipe or a rotating pump connected to the combined chlorine gas pipe kk.

An aqueous solution containing sodium chlorate and sodium chloride fed through tubes 12 and 26 to generators 10 and 21 + can be obtained from a chlorate cell in which an acidic solution of sodium chloride is electrolyzed. Some of the sodium chloride is converted to sodium chlorate and hydrogen is evolved in the cell. Alternatively, solutions can be prepared from solid chemicals.

Solid chemicals can be fed directly to the generators if it is desired to reduce the amount of water to be evaporated. Part of the sodium chloride feed can be replaced with hydrogen chloride. This hydrogen chloride can be formed by burning a portion of the gaseous chlorine in the tube UU together with a portion of the hydrogen from the chlorate cell.

The sodium sulfate recovered from tube 28 can be used to prepare sodium and sulfur-containing materials for the sulfate cellulose process.

The economic advantages of the above process, in which the reaction medium in the generator 2k is heated to its reaction temperature at least in part by the gaseous products obtained from the generator 10, are considerable compared to the same process without this heating. When generator lo mixes sodium bisulfate, the corresponding reactions are:

Generator 10: NaClO3 + NaCl + 2 ^ 0 ^ -> 2NaHSO4 + C103 + 1/2 Cl2 + H2O

Generator 2k: NaClC> 3 + NaCl + PNaHSO ^ -7 * PNagSO ^ + C103 + 1/2 Cl0 + H 2 O 6 61 458 In this case, half of the total amount of ClO 2 produced is obtained from generator 10 and half from generator 2h. Therefore, if all the heat required by the generator 2b is generated by the steam from the generator 10, the total heat demand is reduced by about half.

When using low-grade residues, more water must be evaporated than at high acids to achieve the required precipitation of sodium sulfate. Generator 2b operates at a lower temperature than generator 10. However, the vacuum of generator 2b is greater than that of generator 10. It may be necessary to add more steam to the heat from the steam from the tube 16. However, it is obvious that significant thermal savings will be achieved.

When generator 10 precipitates sodium sesquisulfate, the corresponding reactions are:

Generator 10: 3NaC103 + ÖNaCl + UH ^ O ^ -> 3C102 + 3/2 Cl2 + H2O + 2Na HCSO ^ g

Generator 2b: NaClO3 + NaCl + 2Na3H (S01 |) 2- ^ ClOg + 1 / 2Cl2 + UNagSO4 + HgO

In this case, 3A of the total formed ClOg is obtained from generator 10 and iA from generator 2b. Only part of the water vapor in the pipe 16 has to pass to the heat exchanger 30. In this case, the amount of heat required is about 1A of the total heat required without heat exchange.

The present invention can be used in a number of different ways to improve thermal economy in a variety of generators.

For example, a third generator can be added to the system according to the drawing above. This third generator, which operates at a high degree of acidity and under reduced pressure at the boiling point, forms sodium sulfate. Sodium bisulfate can then be used, at least in part, through line IU, to supply acid to generator 10 instead of sulfuric acid. Generator 10, which operates at a high acid level of about 8 N, but at a lower temperature than the third generator, forms sodium sesquisulfate, which becomes the acid feed to generator 2b.

The gaseous products of the third generator, which contain steam, chlorine dioxide and chlorine, are directed at least in part through the pipe 20 of the heat exchanger 18 of the generator 10 to provide at least part of the heat required by the generator 10. An aqueous solution of chlorine dioxide containing some dissolved chlorine can be removed from the heat exchanger 18 via line 32. Gaseous chlorine and possibly the remaining chlorine dioxide taken from the art lr mr exchanger can be introduced into the pipe 3U.

The processes that take place in this three-generator operation are: 61,458 τ

First generator: 2NaC103 + 2NaCl + ^ 2C102 + Clg + UNaHSO ^ + ^ 0

Generator 10:

NaClO3 + NaCl + 1 + NaHSO4 -V C102 + 1/2 Clg + 2Na «(SO2 Jg + H2O

Generator 2b:

NaClO + NaCl + / Na3H (SO2) 2 -ψ C102 + 1/2 Cl2 +! + Na2SO4 + HgO

In this case, the yield of chlorine dioxide n from the first generator is equal to the sum of the yields of generators 10 and generators 2b, which in turn are equal.

The use of the gaseous products of the first generator to heat the generator 10 and the use of the gaseous products from the generator 10 to heat the generator 2b improve the overall heat economy.

It is still possible to use several generators which operate independently of one another, i. with their own chlorate, chloride and acid feeds, high acidity, low acidity or some high and others low acidity. The gas leaving one generator can be used according to the invention to generate at least part of the heat required by at least one of the other generators. Steam from one generator, the heat required by which is provided by steam from another generator, can be used to generate at least a portion of the heat required by at least one other generator.

Claims (3)

8 61458 A process for the preparation of chlorine dioxide by reducing sodium chlorate with chloride in a sulfuric acid-containing medium at an elevated temperature where substantial decomposition of chlorine dioxide does not yet occur, using two different reaction zones, each producing a first , in which the acidity is kept above U, 8-n, acidic sodium sulphate precipitates and in the second reaction zone, the acidity of which, below β, 8-n, is at least partially effected by the introduction of acidic sodium sulphate formed in the first zone, neutral, anhydrous sodium sulphate precipitates, characterized in that the temperature in the second reaction zone is lower than in the first reaction zone and that at least part of the heat required to maintain the temperature of the second reaction zone is obtained by conducting the gas from the first reaction zone; several for indirect heat exchange with the reaction medium of the second reaction zone. Process according to Claim 1, characterized in that the zone having an acidity of more than 1 L, Sn is divided into two parts, the latter being maintained at a temperature below the temperature of the first reaction zone but above the temperature of the third reaction zone, and at least one of the gas mixtures in the second reaction zone, is subjected to heat exchange with the third reaction zone to obtain at least a portion of the heat required to maintain the temperature of this zone. Process according to Claim 1 or 2, characterized in that at least the first reaction zone is kept at a pressure below atmospheric pressure and in that at least the first reaction medium is kept at boiling temperature. Process according to Claim 3, characterized in that each reaction zone is kept separately under reduced pressure and each reaction medium is kept at a boiling temperature.