Using Hydrogen Peroxide in a Methanol-Based Chlorine Dioxide

called ECF technology, has become more and more dominant in the production of bleached chemical pulps.1. Presently, the most widely used chlorine diox...
0 downloads 0 Views 73KB Size
Download PDF
Ind. Eng. Chem. Res. 1999, 38, 3319-3323

3319

Using Hydrogen Peroxide in a Methanol-Based Chlorine Dioxide Generation Process G. Yin and Y. Ni* Dr. Jack McKenzie Limerick Pulp and Paper Research and Education Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 6C2

The effect of adding hydrogen peroxide on a methanol-based chlorine dioxide generation process was studied. We found that the rate of chlorine dioxide generation is increased and that chlorine, usually present as a byproduct in the chlorine dioxide solution, can be largely eliminated when hydrogen peroxide is present as a coreducing agent. A possible explanation to account for these observations was given. An implication of these experimental findings for the operation of commercial chlorine dioxide generators was suggested. Introduction The chlorine dioxide-based bleaching process, the socalled ECF technology, has become more and more dominant in the production of bleached chemical pulps.1 Presently, the most widely used chlorine dioxide generation process is the methanol-based process,2 whereby sodium chlorate is reduced by methanol in a strong sulfuric acid solution. In this process, methanol is oxidized stepwise by chlorate to form formaldehyde, then formic acid, and finally carbon dioxide if a complete oxidation is achieved.3 Some of the methanol and its oxidation products may leave the generator along with chlorine dioxide and end up in the chlorine dioxide solution.4 Operation at an acidity of 4-5 mol/L is desired for efficient use of methanol and good runnability. A low concentration of chloride ion must be kept in the generator5 to ensure smooth operation. Chlorine is usually present in a chlorine dioxide solution because of the necessary presence of chloride in the generator. Operation at lower acidities of 2.5-3.5 mol/L helps prevent loss of chloride ion but at the expense of inefficient methanol use.6 Hydrogen peroxide is another reducing agent which is viable to generate chlorine dioxide from chlorate. The stoichiometry of the hydrogen peroxide-based process is represented as7

2NaClO3 + H2O2 + H2SO4 f 2ClO2 + O2 + Na2SO4 + 2H2O (1) In this process, chlorine is not produced. Also, under otherwise the same operation conditions, such as temperature, acidity, and chlorate concentration, the hydrogen peroxide-based process has faster reaction kinetics than the methanol-based process.8 Consequently, the hydrogen peroxide-based process can be operated at a lower acidity of 1.75-2 mol/L H2SO4 as opposed to that of 4.5-5 mol/L H2SO4 for the methanol-based process. However, hydrogen peroxide is more expensive than any other conventional reducing agent used in chlorine dioxide manufacture. On the basis of above discussion, one may speculate that it could be advantageous to use both hydrogen peroxide and methanol as a mixture of reducing reagents to generate chlorine dioxide. Recently, a patent * To whom correspondence should be addressed. Telephone: (506) 453-4547. Fax: (506) 453-4767. E-mail: [email protected].

application9 was made with respect to the use of hydrogen peroxide in a methanol-based chlorine dioxide generation process. It was claimed that combining hydrogen peroxide and methanol causes an unexpectedly strong enhancement in the rate of chlorine dioxide generation. For example, in a continuous generator at 6 mol/L H2SO4 acidity under atmospheric pressure, by substituting 10% of the methanol with hydrogen peroxide of an equivalent reducing strength, the generation rate of chlorine dioxide is doubled. However, the detailed results, particularly with respect to the fundamentals of such processes, have not been available. These then form the basis of our present investigation. Experimental Section We used two types of systems, a UV cuvette and a batch reactor, to follow the development of chlorine dioxide generation in this study. UV Cuvette. The experiments were performed in a 3 mL cuvette of a UV spectrophotometer (Model Milton Roy 1001 Plus). Water from a temperature bath was circulated through the cuvette chamber to maintain a constant temperature. Sulfuric acid and sodium chlorate solutions were first injected into the UV cuvette; subsequently, the reaction was started by the addition of methanol or a mixture of hydrogen peroxide and methanol solution. The concentrations of chlorine dioxide and chlorine during the reaction were followed by using a UV multiple-wavelength program, as detailed in the procedure established before.10 The chloride was monitored with a chloride ion selective electrode (ISE) in a separated reaction flask. The chloride concentration was calculated by following a calibration curve which was established at the temperature and acidity identical to those of the reaction system. Batch Reactor. The setup is shown in Figure 1. The reaction flask containing the required amounts of sodium chlorate and sulfuric acid was submerged in a constant-temperature bath. The reducing agent(s) to be added was highly concentrated, so that the overall reaction volume was only changed slightly after the addition. The reducing agent was injected with a graduated syringe (Hamilton 100 µL) into a Teflon tube which extended to the bottom of the reaction flask. The total volume of the reaction solution was about 20 mL. Some runs were performed under atmospheric pressure, and nitrogen was the carrier gas to purge the generated chlorine dioxide. The others were conducted

10.1021/ie9901473 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/23/1999

3320 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999

Figure 1. Batch reactor experimental setup.

under subatmospheric conditions of about 175 mmHg vacuum. The chlorine dioxide generated was dissolved in a 2-L flask equipped with a gas dispersion tube. The absorption flask contained potassium iodide solution buffered at pH 7 with sodium phosphate. Iodine was released as the result of the rapid reaction between chlorine dioxide and/or chlorine and potassium iodide. At each specified reaction time, 5 mL of the iodine solution was withdrawn and subjected to the iodometric titration. The chlorine dioxide and chlorine concentrations were determined and calculated accordingly on the basis of the titration results at neutral (pH 7) and acidic conditions, respectively. A second gas absorption flask was placed in the setup to ensure complete absorption of chlorine dioxide. However, it turned out that the presence of chlorine dioxide is very small in the second absorption flask. The ice-water bath was used for condensing water and methanol, so that they returned to the reaction flask and the reaction volume was unchanged during the reaction. We also verified the chlorine mass balance, that is, by comparing the sum of residual chlorate, chlorine dioxide, and chlorine with the initial chlorate charged, and found that the mass balance was satisfactory. Results and Discussion First we followed the UV spectroscopic method to determine the effect of adding hydrogen peroxide on the development of chlorine dioxide generation in a methanol-chlorate system, shown in Figure 2. The reaction conditions were 70 °C, 3 mol/L H2SO4, 0.97 mol/L NaClO3, and 0.01 mol/L total reducing agent. It is evident that, in the absence of hydrogen peroxide, the generation of chlorine dioxide is much slower, and its development profile is very much similar to that reported in an earlier study.10 With the addition of hydrogen peroxide while maintaining the same total reducing agent concentration, the rate of chlorine dioxide generation increases considerably. Note that the induction period for the chlorine dioxide generation process is significantly reduced.

Figure 2. Effect of adding hydrogen peroxide on the chlorine dioxide generation (3 mol/L H2SO4, 70 °C, 0.97 mol/L NaClO3, total reducing agent 0.01 mol/L).

These conclusions are further supported by the results obtained under other conditions, shown in Figure 3. The chlorine dioxide generation rate increases with increasing hydrogen peroxide replacement ratio. It is known8 that the hydrogen peroxide-based chlorine dioxide generation process has a higher rate than the methanolbased process at a sulfuric acid concentration of 1.5-3 mol/L. A question arises as to whether the higher chlorine dioxide generation rate observed in Figures 2 and 3 in the presence of hydrogen peroxide is the result of a simple additive effect of both methanol and hydrogen peroxide as the reducing agents. To address this question, further experiments were carried out. Figure 4 shows the development of chlorine dioxide generation at 0.007 mol/L methanol (curve a) and 0.003 mol/L hydrogen peroxide (curve b). Also included in the figure are curve c, which was obtained by direct mathematic addition of curves a and b, and curve d, which is the development of chlorine dioxide generation with 0.007 mol/L methanol and 0.003 mol/L hydrogen peroxide as the mixed reducing agents (from Figure 2). Other

Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3321

Figure 3. Decrease of induction period with increase of hydrogen peroxide substitution (3 mol/L H2SO4, 45 °C, 0.167 mol/L NaClO3, total reducing agent 0.083 mol/L).

Figure 4. Comparison of chlorine dioxide generation from mixed reducing agent and from individual ones at 3 mol/L H2SO4 (70 °C, 0.97 mol/L NaClO3, total reducing agent 0.01 mol/L).

conditions were same as those in Figure 2. Evidently, the chlorine dioxide generation for the mixed reducing agent process (curve d) is much faster than that by a simple mathematic addition of two individual runs (curve c). We also studied the effect of adding both methanol and hydrogen peroxide simultaneously on the chlorine dioxide generation at a lower acidity. Figure 5 shows the development of chlorine dioxide generation obtained, following the UV analysis. The reaction conditions were 45 °C, 2 mol/L sulfuric acid, and 0.167 mol/L sodium chlorate. Under such conditions, when only methanol is used as the reducing agent, the generation of chlorine dioxide is almost negligible after 2 h of reaction. In contrast, when a small amount of hydrogen peroxide is added (about 6% of methanol), chlorine dioxide is generated immediately, and its rate is substantially higher than the sum of the two individual runs obtained with either methanol or hydrogen peroxide under otherwise the same conditions. In comparison with the reactions shown in Figures 2 and 3, which were carried out at a higher acidity (3 mol/L sulfuric acid solution), one can observe that the rate enhancement of chlorine dioxide generation caused by adding a small amount of hydrogen peroxide at lower acidity was even more pronounced.

Figure 5. Chlorine dioxide generation from mixed reducing agent and individual ones at 2 mol/L H2SO4 (45 °C, 0.167 mol/L NaClO3).

Figure 6. Chlorine and chloride profiles during the chlorine dioxide generation processes (3 mol/L H2SO4, 45 °C, 0.167 mol/L NaClO3, methanol 0.083 mol/L).

We made further efforts to understand why the addition of hydrogen peroxide changes a chlorine dioxide generation process. We followed the development of chlorine and chloride formation, in the presence and absence of 0.0048 mol/L hydrogen peroxide. The results are demonstrated in Figure 6. Evidently, in the absence of hydrogen peroxide, chloride is gradually formed. In an earlier study,10 we hypothesized that chloride is a catalyst during the methanol-based chlorine dioxide generation. This was confirmed in a later study.11 Therefore, the long induction period in the absence of hydrogen peroxide is due to an insufficient concentration of chloride, which is necessary for the chlorine dioxide generation. Also the development of the chlorine profile in the absence of hydrogen peroxide shows a small maximum in the initiation phase, which was characterized in an earlier paper.10 In contrast, the addition of hydrogen peroxide eliminates the formation of chlorine. Therefore, one can conclude that the addition of hydrogen peroxide to the methanol-based chlorine dioxide generation process results in the following changes: the chlorine formation now is not detectable, while the chloride concentration increases more rapidly. On the basis of the results in Figure 6, we propose that the following reaction

H2O2 + Cl2 f 2Cl- + 2H+ + O2

(2)

3322 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999

is responsible for these observations in the presence of hydrogen peroxide. Reaction 2 has been known to be very fast. Makower and Bray12 studied this reaction in a hydrochloric acid solution, and later, Connick13 extended the investigation to other conditions. The following reactions were proposed: ka

H2O2 + Cl2 y\ z H+ + Cl- + HOOCl k b

kc

HOOCl 98 H+ + Cl- + O2

(3) (4)

The resulting rate law is

-

d[H2O2] kcka[H2O2][Cl2] ) dt k [H+][Cl-] + k b

(5)

c

If the product of [H+][Cl-] is substantially larger than kc, eq 5 becomes

-

[H2O2][Cl2] d[H2O2] )k dt [H+][Cl-]

(6)

where k is about 5 × 103 (mol/L)‚min-1 at 25 °C. On the other hand, if the product of [H+][Cl-] is sufficiently smaller than kc, the rate expression approaches

-

d[H2O2] ) ka[H2O2][Cl2] dt

(7)

The second-order rate constant ka was found to have a value of 1.1 × 104 (mol/L)-1‚min-1 at 25 °C. We further confirmed that reaction 2 is extremely fast under conditions similar to those for the commercial chlorine dioxide generation. When 0.02 mol/L hydrogen peroxide was added to a 2 mol/L sulfuric acid solution containing 6 × 10-3 mol/L chlorine, immediately, we found that the UV absorbance at 322 nm, which is characteristic of chlorine, completely disappeared. Chloride is generated from chlorine via reaction 2. Therefore, we can conclude that the higher chlorine dioxide generation rate when hydrogen peroxide is added is due to the faster formation of chloride and a higher chloride concentration in the chlorine dioxide generator. Recently, Crump et al.14 studied the influence of adding H2O2 on chlorine dioxide formation from the chloridechlorate reaction; similarly, they observed an enhancement of chlorine dioxide generation rate with the addition of hydrogen peroxide. It was concluded that hydrogen peroxide increases the rate not only by reacting with chlorine (reaction 2) but also by rapidly reducing dichlorine dioxide (Cl2O2), the well-identified intermediate15,16 for the ClO2 generation

2Cl2O2 + H2O2 f 2ClO2 + O2 + 2HCl

(8)

where H2O2 is stepwise oxidized to O2 via a perhydroxyl radical mechanism:14

Cl2O2 + H2O2 f ClO2 + HO2• + HCl

(9)

Cl2O2 + HO2• f ClO2 + O2 + HCl

(10)

Another important implication of reaction 2 is that the addition of a small amount of hydrogen peroxide can prohibit the occurrence of the so-called “white-out” and

Figure 7. Effect of adding hydrogen peroxide on chlorine dioxide generation in a batch reactor operated at atomospheric pressure (2.25 mol/L H2SO4, 60 °C, 1 mol/L NaClO3, total reducing agent 1.06 mol/L).

eliminate the formation of chlorine in the methanolbased chlorine dioxide generation process. It is wellknown that chlorine is not totally eliminated from the methanol-based chlorine dioxide processes due to the necessary presence of chloride in the generator. The white-out occurs when the process stops producing chlorine dioxide; instead, a white gas consisting of chlorine and water vapor is produced. In commercial operations, usually a small amount of sodium chloride is fed to the generator to prevent white-out. However, the addition of sodium chloride may result in the formation of more chlorine in the chlorine dioxide solution. In contrast, if a small amount of hydrogen peroxide is added, the necessary chloride profile, which is essential for the generation of chlorine dioxide, is maintained while the undesirable byproduct can be eliminated. This will be further supported by results in the subsequent sections. The enhancement in the methanol-based chlorine dioxide generation process by the addition of a small amount of hydrogen peroxide is further confirmed in our batch reactor setup. Figure 7 shows the chlorine dioxide yield which was calculated based on the initial chlorate concentration. The detailed procedures were described in the Experimental Section. The conditions were 60 °C, 1 mol/L NaClO3, and 2.25 mol/L H2SO4. The system was operated under atmospheric conditions. The amount of reducing agent(s) required was added at the very beginning of the reaction. The generated chlorine dioxide was continuously carried by a stream of nitrogen from the reaction flask into an absorption flask. It is evident from Figure 7 that the addition of a small amount of hydrogen peroxide results in an enhancement in chlorine dioxide generation. The chlorine contamination with or without the addition of hydrogen peroxide was about 2% and 8%, respectively. It is worth noting that the chlorine dioxide yield in Figure 7 is substantially lower than 100%. This is most likely due to the following reasons. First, the acidity in Figure 7 was only 2.25 mol/L, which is substantially lower than that in a commercial chlorine dioxide generator. Second, chlorine dioxide may react with methanol or formic acid.17,18 The kinetics of the reaction between methanol and chlorine dioxide has been studied by Sharma,19 and a first-order dependence on hydrogen ion concentration was reported. Such a reaction was

Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3323

In addition, the rapid reaction between H2O2 and Cl2O2 (reaction 8) may also partly contribute to the enhancement of the ClO2 generation rate

H2O2 + 2Cl2O2 f 2ClO2 + 2HCl + O2

(8)

Reaction 2 leads to a faster accumulation of the necessary chloride, which is a catalyst in the methanol-based chlorine dioxide generation process. Also, the addition of hydrogen peroxide is an effective approach to combat the so-called “white-out”, which is due to the depletion of chloride in an industrial chlorine dioxide generation processes. Literature Cited Figure 8. Continuous feeding of reducing agent in a batch reactor operated at subatmospheric pressure (3 mol/L H2SO4, 70 °C, 3 mol/L NaClO3, reducing agent fed at 0.04 mol/(L 5 min)).

cited by Indu18 to explain why methanol must be fed to the chlorine dioxide generator gradually to avoid a high concentration of methanol. Third, some of the initially charged methanol was evaporated from the reaction flask into the absorption flask, resulting in a lower stoichiometry between methanol and chlorate in the reactor than that initially charged. This was verified by the following experiment: without a condenser between the reaction flask and the absorption flask, we found that the chlorine dioxide yield is decreased by about 4%. We performed further experiments by continuously adding reducing agent(s). The required chlorate and sulfuric acid, however, were all added at the beginning of the reaction. The generated chlorine dioxide was removed from the reactor using nitrogen as the carrier gas. We compared the chlorine dioxide yield and its purity between two cases, one with methanol as the only reducing agent and another with both methanol and hydrogen peroxide as the reducing agents in a ratio of 70% methanol to 30% hydrogen peroxide. In both cases, the reducing agent(s) was injected at a rate of 0.04 mol/ (L 5 min) with a syringe into a reaction flask containing 3 mol/L sodium chlorate and 3 mol/L sulfuric acid. The system was operated at 70 °C under subatmospheric conditions. At 30 min intervals, the released iodine by chlorine dioxide and/or chlorine in the gas absorption tank was taken for the iodometric titration. Figure 8 shows that, with 30% hydrogen peroxide replacement, the chlorine dioxide generation rate was significantly increased. The chlorine contamination in the chlorine dioxide solution after 150 min of reaction time was decreased from about 7% to 1% with the addition of hydrogen peroxide. Conclusions The addition of hydrogen peroxide to a methanolbased chlorine dioxide generation process results in an increase in the reaction rate. Furthermore, it largely eliminates chlorine as a byproduct in such a process. The above two observations can be explained by the rapid reaction between hydrogen peroxide and chlorine (reaction 2) under the chlorine dioxide generation conditions as

H2O2 + Cl2 f 2Cl- + 2H+ + O2

(2)

(1) Pryke, D. C.; Reeve, D. W. A Survey of ClO2 Delignification Practices in Canada. Tappi J. 1997, 80 (5), 153. (2) Stockburger, P. What you need to know before buying your next Chlorine Dioxide Plant. Tappi J. 1993, 76 (3), 99. (3) Hoq, M. F.; Indu, B.; Ernst, W. R.; Gelbaum, L. T. Oxidation Products of Methanol in Chlorine Dioxide Production. Ind. Eng. Chem. Res. 1992, 31 (7), 1807. (4) Hoq, M. F.; Ernst, W. R.; Gelbaum, L. T. NMR Procedure for Determining Concentrations of Methanol and Formic Acid in Solutions from a Chlorine Dioxide Generating Plant. Tappi J. 1991, 74, 217. (5) Hollingsworth, C. A. Chlorine Dioxide Plant Operation at Bowater Carolina. Proceedings of TAPPI Pulping Conference, Atlanta, GA, 1986; p 403. (6) Reeve, D. W.; Dence, C. W. Pulp BleachingsPrinciple and Practice; Tappi Press: Atlanta, GA, 1996. (7) Sokol, J. C.; Conkle, J. Peroxide-based ClO2 Process. CPPA Annual Meeting Proceedings, Technical Section, Canadian Pulp and Paper Association, Montreal, Quebec, 1990; p B47. (8) Burke, M.; Tenney, J.; Indu, B.; Hoq, M. F.; Carr, S.; Ernst, W. R. Kinetics of Hydrogen Peroxide-Chlorate Reaction in the Formation of Chlorine Dioxide. Ind. Eng. Chem. Res. 1993, 32, 1449. (9) Nonni, A. J.; Graff, R, J.; Liu, R.; Voss, J. N.; Hammond, T. R.; Pan, G. Y.; Renard, J. J. Method for Producing Chlorine Dioxide Using Methanol and Hydrogen Peroxide as Reducing Agent. PCT/ US97/17758, 1998. (10) Ni, Y.; Wang, X.; Mechanism of the Methanol Based ClO2 Generation Process. J. Pulp Paper Sci. 1997, 23 (7), J 346. (11) Yin, G.; Ni, Y. The Effect of Chloride on the HClO2-HOCl Reaction in a 4.5 mol/L Sulfuric Acid Solution. Can. J. Chem. Eng. 1998, 76 (2), 248. (12) Makower, B.; Bray, W. C. The Rate of Oxidation of Hydrogen Peroxide by Chlorine in the Presence of Hydrochloric Acid. J. Am. Chem. Soc. 1933, 55, 4765. (13) Connick, R. E. The interaction of Hydrogen Peroxide and Hypochlorous Acid in Acidic Solutions Containing Chloride Ion. J. Am. Chem. Soc. 1947, 69, 1509. (14) Crump, B.; Ernst, W. R.; Neumann, H. M. Influence of H2O2 on a Chloride-Dependent Reaction Path to Chlorine Dioxide. AIChE J. 1998, 44 (11), 2494. (15) Taube, H.; Dodgen, H. Applications of the Mechanisms of the Reaction Involving Changes in the Oxidation States of Chlorine. J. Am. Chem. Soc. 1949, 71 (10), 3330. (16) Emmenegger, F.; Gordon, G. The Rapid Interaction between Sodium Chlorite and Dissolved Chlorine. Inorg. Chem. 1967, 6 (3), 633. (17) Owen, D. Operation and Maintenance of Chlorine Dioxide Generations. Tappi Bleach Plant Operations Seminar Notes; Tappi Press: Atlanta, GA, 1989; p 157. (18) Indu, B. Kinetics and Mechanism of Methanol-chlorate in the Formation of Chlorine Dioxide. Ph.D. Thesis, Georgia Institute of Technology, 1993. (19) Sharma C. B. Kinetics of Oxidation of Methanol by Chlorine Dioxide. React. Kinet. Catal. Lett. 1982, 19 (1-2), 167.

Received for review February 26, 1999 Revised manuscript received May 10, 1999 Accepted May 21, 1999 IE9901473