PROCEDURE FOR PRODUCTION OF CHLORINE DIOXIDE

Abstract: The invention describes a procedure for production of chlorine dioxide where, in a primary reactor chlorate, sulphuric acid and a reducing agent with optional extra additions of chloride ions are reacted, the resulting gas mixture mainly containing chlorine dioxide and air are forced through a scrubber to an absorption tower for absorption of the product gas. Where the reactor solution is withdrawn from the reactor optionally to a secondary reactor in which reducing agent is also added, the procedure being characterized by that part of the primary reactor solution is recirculated optionally via a cooler to the upper part of a scrubber and then returned to the reactor.

Description: PROCEDURE FOR PRODUCTION OF CHLORINE DIOXIDE Chlorine dioxide is presently to a large extent used as a bleaching agent for production of fully bleached chemical pulps. In the 1940s it was dis¬ covered that chlorine dioxide had excellent abi¬ lity to remove lignin from cellulosic fibres with¬ out noticeably effecting the cellulosic fibre itself. During this period chlorine and sodium hypochlorite were used as main bleaching and delignification agents. The drawbacks with these chemicals were that during the bleaching and delignification step they also attacked and ruptu¬ red the cellulose molecule resulting in a weak fibre and ultimately weak paper products. A combination of chlorine and chlorine dioxide later turned out to give optimum pulp properties regarding strength, brightness and cleanliness. Within the bleaching technology the following expressions are used: C = chlorine (Cl 2 ) , D = chlorine dioxide (C10 2 ) , E = extraction with sodium hydroxide (NaOH) , 0 = oxygen (0 2 ) . In the main, chlorine dioxide was usually used at the end of the bleaching sequence. A typical bleaching sequence' would be: C E D E D. During the 1960s and 1970s about 5 - 15% of the chlorine in the first bleaching stage started to be substi- SUBSTITUTE SHEET tuted by chlorine dioxide calculated as available chlorine. The relationship between chlorine dioxide and chlorine within the bleaching field is expressed as follows: 1 kg of chlorine = 1 kg of available chlorine. 1 kg of chlorine dioxide = 2.63 kg as available chlorine. The primary reason for adding a small amount of chlorine dioxide in the first bleaching stage was that the fibre strength could be maintained. During the 1970s, but particularly during the 1980s, the use of chlorinating bleaching agents has more and more been questioned. The primary- reason is that when chlorine gas is used as a bleaching agent it produces large amounts of chlo¬ rinated organic compounds. Major experimental work has been done and will continue in order to decrease and perhaps elimi¬ nate the use of chlorine gas for bleaching of chemical pulps. Partially for this reason, oxygen delignification has been developed. Chlorine dioxide also produces chlorinated organic compounds when used as a bleaching agent, but are only a fraction compared to when chlorine gas is used. These compounds are also considered less toxic. For this reason chlorine dioxide is now a SUBSTITUTESHEET candidate for total substitution of chlorine in the first bleaching stage. As chlorine dioxide is a unique delignification 5 and bleaching agent for chemical pulps with bleaching sequences like 0 D (E + 0) D E D, both chemicals offer maximum utilization with these stages. When going from chlorine to chlorine di¬ oxide bleaching the pulp mill will often have to 10 increase its installed chlorine dioxide capacity substantially. This could lead to a large invest ment if the old reactor system has to be replace In the beginning chlorine dioxide was produced b 15 reduction of sodium chlorate with sulphur dioxid in sulphuric acid medium according to the so cal led Hoist-procedure. This is a batch process whe sodium chlorate solution and sulphuric acid is added to a reactor. A mixture of sulphur dioxide 20 in air was then blown through spargers in the bo tom of a reactor. The chlorine dioxide formed wa removed from the reaction medium by the sparged air stream and then absorbed in water in a speci ally designed tower. Later the so called Mathies 25 Process was developed. Here the reaction compo¬ nents, sodium chlorate, sulphuric acid and sulph dioxide in air were continuously added to the reactor as shown in Figure l. The liquor content from the primary reactor passes through a secon- 30 dary and sometimes a tertiary reactor where the concentration of sodium chlorate is depleted as SUBSTITUTE SHEET much as possible in order to minimize losses. Th liquor coming out of the tertiary reactor is cal led spent acid. According to other procedures, methanol (the Solvay Process) , and chloride (the R-2 Process, 3 Process, SVP Process, Lurgi Process etc) are b ing used as reducing agents instead of sulphur dioxide. 10 When producing chlorine dioxide according to the Mathieson procedure the following reactions are considered to take place 15 1 NaC10 3 + %H-,S0 4 —> HC10-. + ^Na 2 S0 4 2 HC10 3 + Cl " —> C10 2 + ^Cl 2 3 _C1 2 + ^S0 2 —> Cl " + ^H 2 S0 4 20 The above reactions are only used as a tentative description of the process. We do not claim that they are a full description of reality. Parti¬ cularly reaction 2 can be more complicated than 25 what is described here. This reaction is however anticipated to be the one proceeding with the slowest reaction rate and is therefore the rate limiting one. As can be seen from reaction 2 pre sence of chloride ions are necessary for chlorin 30 dioxide to be formed. Simultaneously chlorine ga is being produced which according to reaction 3 SUBSTIT can react with the reducing agent thereby return ing to the chloride state. The reducing agent for the reaction in the react 5 can also be methanol according to the Solvay pro cedure. In a Mathieson reactor the yield of chlorine di¬ oxide based on sodium chlorate added often is in 10 the range of 82 - 88%. The yield loss is primari depending on that chlorate is reduced to chlorid Thereby the reactor is continuously fed with chl ride ions to sustain the chlorine dioxide genera tion. Some chlorine is however also accompanying 15 chlorine dioxide in the gaseous phase to the ab¬ sorption tower. It is known that the yield of chlorine dioxide based on sodium chlorate added can be somewhat i 20 creased by extra addition of chloride ions to th reactor. As earlier mentioned it is often a desire both fo quality and environmental reasons that the chlo- 25 rine dioxide produced is free of chlorine. This can be achieved by extra addition of sulphur di¬ oxide which however automatically leads to a grea ter yield loss of chlorine dioxide with more ex¬ pensive production costs as a consequence. In 30 order to improve the yield it is known that a scrubber between the chlorine dioxide generator SUBSTITUTESHEET and the absorption tower can be used as shown in Figure 2. Thereby the solution of sodium chlorat is added to the upper part of the scrubber. This technique, however, has lead to an increased num 5 ber of so called puffs (low velocity detonations of chlorine dioxide) and what is more serious, f res. Attempt to circumvent these problems with extra additions of water to the scrubber leads t dilution of the reactor solution. This demands 10 added volumes of sulphuric acid which increases production costs. Below are described two known methods of produci chlorine dioxide gas which will provide a better 15 understanding of the invention later described. Reference is made to attached drawings where Fig 1 is a process scheme regarding the normal 20 Mathieson Process with extra addition of sodium chloride. Fig 2 is a process scheme regarding the modifie Mathieson Process with a scrubber between the 25 chlorine dioxide reactor and the absorption towe and Fig 3 is a process scheme regarding the new met according to the invention to produce chlorine 30 oxide gas. SUBSTITUTESHEET PROCESS A Mathieson Process with extra addition of sodium chloride 5 The principle process scheme is shown in Figure The chlorine dioxide production is mainly taking place in a primary reactor (1) but partly also i a secondary reactor (4) . The chlorate is fed to 10 the reactor as a 46% solution, the sulphuric aci as a 98% solution, the sodium chloride as a 26% solution and the sulphur dioxide as a 5 - 10% ga mixture in air. The chlorate and sulphuric acid are pumped continuously to the reactor through a 15 adequate feed control system. Filters are used t rernpve any solid impurities in, the solutions. Th addition rates are regulated in such a way that the sodium chlorate concentration in the reactor is between 22 - 35 g/1 and the H 2 S0 4 concentratio 20 is between 425 - 490 g/1. The sulphur dioxide gas is diluted with air from compressor to about 5 - 10% by volume the gas mix ture delivering a pressure of about 1 atmosphere 25 is then directed to the bottom of the reactor. Special spargers are here producing small gas bubbles. When the bubbles are rising through the liquor medium of the reactor the S0 2 gas is react ing according to equations 3 and 2 above. Whereas 30 the air bubbles are being depleted with sulphur dioxide they are picking up chlorine dioxide gas SUBSTITUTESHEET produced. The gas phase above the liquor medium the primary reactor is therefore now a mixture o air with chlorine dioxide and small amounts of sulphur dioxide and chlorine. The chlorine dioxi 5 containing air is now piped to an absorption tow (3) where the chlorine dioxide is absorbed in a down stream flow of water. The absorbed chlorine dioxide in water is removed and delivered to a storage tank and the off-gas from the absorption 10 tower fan is either vented to a scrubber or out atmosphere. A part of the reaction solution is continuously overflowing from the primary reactor to the seco 15 dary reactor where'the chlorate concentration is lowered to 2 - 10 g NaC10 3 /l. This is effected b passing part of the sulphur dioxide air mixture through a sparger in the bottom of the secondary reactor. In order to achieve this decrease of 20 chlorate concentration approximately 5 - 15% of the total amount of sulphur dioxide containing a added to the system is used in the secondary rea tor. Sodium chloride can also be added to the secondary reactor. 25 The chlorine dioxide gas formed in the secondary is led into the gas phase of the primary reactor while the liquid phase is overflowing into a stripper (5) where the solution is depleted of 30 chlorine dioxide with an air purge. The purge ai containing chlorine dioxide is brought to the pr SUBSTITUTESHEET ary reactor gas phase while the spent acid solu tion is overflowed from the reaction system. In order to remove the heat generated in the reacto both the primary and the secondary reactors are equipped with cooling devices by which the reac¬ tion temperature can be kept between 32 - 60 °C. When the reactor is not in a production mode the temperature of the liquid reaction medium should not be allowed to decrease below 25 °C. At a low temperature, salt crystallization .may occur whic can lead to plugging and other production pro¬ blems. Chlorine dioxide gas will readily decompose give the right conditions. Decomposition may occur wit light, high temperature, high gas concentration o with organic compounds in general. As a result of the decomposition, gaseous chlorine, gaseous oxy¬ gen and heat are released. The released heat will increase the temperature in the gas phase of the reactor which accelerates the decomposition rate • of the chlorine dioxide gas and may ultimately lead to an explosion. Production equipment is therefore equipped with a safety system which is triggered when the temperature of the gas phase i the reactor has risen beyond a set value. SUBSTITUTE SHEET PROCESS B Modified Mathieson Process with scrubber between reactor and absorption tower 5 Reference is now made to Figure 2 outlining a modified Mathieson Process which in the main re¬ sembles the normal Mathieson Process according t Figure 1. Reference is therefore made to the de- 10 scription of the normal Mathieson Process under above. According to the modified process the chlorate solution is now added to the reactor (1) via a 15 scrubber (2) while the sulphuric acid and the so dium chloride are still -fed directly to the pri¬ mary reactor. Sulphur dioxide gas is also added per the normal manner. 20 The gas mixture leaving the primary reactor as previously indicated, contains chlorine dioxide with small amounts of chlorine gas and sulphur dioxide. This air/gas mixture is directed throug the scrubber (2) before proceeding to the absorp 25 tion tower (3). In the scrubber the gas mixture "washed" with the chlorate solution on its way through the scrubber to the reactor. Chlorine ga and sulphur dioxide considered as impurities are now given another opportunity to react accordin 30 to equation 3 and returned back to the reactor together with the chlorate. SUBSTITUTESHEET As mentioned previously the yield can be improved by utilizing a scrubber between the chlorine di¬ oxide reactor and the absorption tower. It has been shown, however, that this process at higher 5 production rates than design capacity leads to an increased number of puffs which in certain cases are of higher detonation velocity, and therefore more dangerous. 10 I have now discovered a new way to utilize the ga scrubber principle which enhances hitherto known advantages while at the same time known dis¬ advantages are avoided. This is done by recircula ting part of the reactor solution under controlle 15 conditions to the top of a similar scrubber as, * shown by Figure 3. Through my invention 20 - fires can be eliminated the number of puffs can be reduced to a. normal level for a Mathieson Process no extra addition of water is needed 25 at the same time the yield can be substantially increased compared to a normal Mathieson Process the reactor capacity can be substantiall 30 increased a chlorine gas free chlorine dioxide ca be produced An important advantage with the invention is, th 5 it is possible with a simple modification to in¬ crease the plant output capacity, and at the sam time to substantially increase the yield of chlo rine dioxide based on sodium chlorate. 10 The invention consists of a procedure for the pr duction of chlorine dioxide through the reaction in a primary reactor (1) of chlorate, sulphuric acid and a reducing agent optionally with the addition of chloride ions. The gas mixture from 15 the top of the reactor, mainly comprising chlori dioxide and air is directed via a scrubber (2) t an absorption tower (3) where the chlorine dioxi is absorbed. The spent acid solution from the reactor is fed optionally to a secondary reactor 20 (4) where reducing agent is also added. The inve tion is characterized by that part of the primar reactor solution (1) is circulated, optionally v a cooler (6) , to the upper part of the scrubber (2) and returned to the reactor via the scrubber 25 SUBSTITUTE SHEET PROCESS C Modified Mathieson Process according to the invention 5 The procedure according to the invention is seen from the flow sheet in Figure 3. The characteris tic feature for the invention is that part of th reactor liquid is circulated from the bottom of 10 the chlorine dioxide reactor (l) via a pump and necessary a cooler . - (6) to a scrubber (2) situate above the reactor. This scrubber 'can for instanc be a packed tower, a spray tower, a venturi scru ber etc. The important thing is that the scrubbe 15 provides a big area so that unreacted sulphur di¬ oxide can react with chlorine under formation of . sulphuric acid and hydrochloric acid whiqh can be returned to the reactor. The chlorine dioxide wil after having passed the scrubber be essentially 20 free of chlorine and is fed to the absorption tower (3) . The temperature of the recirculating liquid reactor medium to the scrubber is 25 - 50 °C and can if necessary be regulated with a coole (6) . The outgoing gas from the scrubber is under 25 the detonation limit for chlorine dioxide that is maximum 70 °C, preferably however maximum 60 °C. The liquid from the scrubber being returned to th reactor has a temperature which preferably is under the temperature of the primary reactor. The 30 above mentioned temperatures are controlled by th circulating reactor solution. The volume of reac- SUBSTITUTE SHEET tor solution being recirculated can be up to two times the total volume of the liquid phase of the primary reactor per hour. If methanol is used as a reducing agent in the reactor (1) the right amount of " sulphur dioxide will have to be added in the lower part of the scrubber so that reaction 3 as mentioned earlier can take place. 10 The invention is shown by the following examples. EXAMPLES 1 - 3 15 The tests were made in a Mathieson reactor having a nominal capacity of 7, 5 tons of chlorine dioxid /day. In example 1 a normal Mathieson Process as described according to process A above was used. Example 2 describes a procedure according to the 20 process B above (modified Mathieson Process with scrubber between reactor and absorption tower; Figure 2) . Example 3 describes a procedure accord ing to the invention that is process C as descri¬ bed above (Figure 3) . 25 The following results were obtained as regards mean production, yield and number of puffs. EXAMPLE 1 2 3 PROCESS B C FIGURE 1 2 3 5 Mean .production of chlorine dioxide /24 hours 11 11 16 Yield calculated 10 on sodium chlorate ' 84 86 92 Number of puffs/month <0,5 >5 <0,5 ET

Claims: [received by the International Bureau on 26 February 1990 (26.02.90), original claim 1 amended? other claims unchanged (1 page)] •A procedure.for production of chlorine dioxide by reacting in a primary reactor (1) sodium chlorate, sulphuric acid and reducing agent and optionally addition o chloride ions whereby the product gas mixture mainly consisting of chlorine di oxide and air is led through a scrubber (2) to an absorption tower (3) for ab¬ sorption of the chlorine dioxide and tha the reactor solution (1) is withdrawn ■optionally via a secondary reactor (4) t which reducing agent is also added / cha¬ racterized by that part of the reactor solution of the primary reactor is recir culated, optionally via a cooler (6) , to the upper part of a scrubber (2) and returned to the reactor (1), and that the sulphuric acid concentration in the react solution corresponds to 425-490g H-SO^/L. Procedure according to claim 1 wherein the reducing agent in the reactor is sulphur dioxide. Procedure according to claim l wherein the reducing agent in the reactor is methanol and that sulphur dioxide is added to the gas stream entering the scrubber. STATEMENT UNDER ARTICLE 19 In order to restrict the claimed invention over the prior art in the form of ΞE 331272 cited in the International Search Report of 1990-02- 01 claim 1 has been amended by the addition of the statement "and that the sulphuric acid concentration in the reactor solution corresponds to 425-490g HaSO-ι/1.." in the characterizing part of the claim. This added statement has an explicit counterpart in the original description on page 7, lines 19-21. The purpose of the claimed invention is to increase the yield of chlori dioxide as well as increasing the capacity, i.e. the average production at a given reactor volume. This cannot be achieved at the low aciditie used according to ΞE 331272 since in the claimed process you have to operate at a considerably higher acidity around about 9 N. This is now evident from the new claim 1 containing the above-mentioned added statement about the sulphuric acid concentration as a critical feature. The sulphuric concentration recited in the new claim corresponds to an acidity range which is considerably higher than the acidity range from 4,8 N to 6 N required in the process of SE 331272.

Inventors: WICKSTROEM PETER, WICKSTROEM, PETER
Applicants: ALBY KEMI AB
Application year: 1989
Main classification: C01B11
IPC codes: C01B11