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Development of an Ultra-rapid Reactor for Superabsorbent Polymer Cedric Briens,*,† Jean-Marc Le Ble´ vec,‡ Andre´ Lermite,‡ and Shu Rong Rebre§ Chemical and Biochemical Engineering Department, University of Western Ontario, Ontario N6A 5B9, Canada; Groupement de Recherches de Lacq, ATO FINA, BP 34, 64170 Lacq, France; and Centre d’Applications de Levallois, ATO FINA, Levallois, France
Superabsorbent polymers are obtained by the polymerization of acrylic acid and its salts. Industrial processes for the production of superabsorbent polymers require many labor- and capital-intensive steps in several vessels. This paper presents a new process that synthesizes superabsorbent polymer in a single vessel, a modified spray dryer. Very fast polymerization is obtained with appropriate reactants and initiators, selected from 150 different combinations. Preheating and using the heat of neutralization of the acrylic acid greatly accelerate polymerization. A new, ultra-rapid reactor quickly mixes two streams of reactants and initiators to provide fine droplets of polymerizing liquid. A two-stage mixer provides the best results: two flat jets of liquids are first impacted at high velocity, and a high velocity gas jet is applied to the resulting liquid stream to form a spray of fine droplets and enhance drying. The compact reactor is located within the inlet region of a standard spray dryer that can deliver dry particles of superabsorbent. Preliminary experiments in a modified pilot-plant spray dryer gave superabsorbent polymer with a conversion rate higher than 65%. Introduction The market for superabsorbent polymers (SAPs) has increased by a factor of 5 over the past 10 years. SAPs can absorb up to 1000 times their weight in water, and their main application is for diapers and similar products. They are also used as additives in paper, cosmetics, paints, and soils. For many of these applications, the polymer should be in the form of dry particles with a diameter of about 400 µm. SAPs can be synthesized by two main processes. The inverse suspension polymerization process is reliable but requires many processing steps. The gel process suffers from poor heat transfer that limits the production rate because SAP polymerization is highly exothermic. A Japanese patent1 proposes that SAP polymerization, atomization, and drying be performed in one step by contacting two liquid streams in a spray nozzle to form droplets. Each stream contains the same proportions of acrylic acid and sodium acrylate and complementary initiators. When the initiators contained in the two streams come into contact, polymerization starts. Each droplet then acts as a microscopic polymerization reactor. The main objective of the present paper is to provide data for a preliminary feasibility study of a SAP synthesis process such as described in the Japanese patent.1 A variant in which one stream is acidic and the other alkaline is be investigated to take advantage of the heat of neutralization to accelerate the reaction and reduce the heat required for drying. The study proceeds in three steps: (1) A kinetic study helps to select the compositions of the two streams to achieve a fast reaction. The main emphasis is on the selection of effective polymerization initiators. (2) A mixing study * Corresponding author. E-mail:
[email protected]. † University of Western Ontario. ‡ Groupement de Recherches de Lacq, ATO FINA. § Centre d’Applications de Levallois, ATO FINA
develops an appropriate mixing and atomization nozzle. Mixing should be very rapid, and each droplet should have equal proportions of the two streams. The nozzle should be resistant to fouling because liquid projections could polymerize on any exposed nozzle surface. (3) Preliminary polymerization tests are performed in a modified pilot-plant dryer. Kinetic Study A literature review identified many possible initiator combinations. For example, an oxidant such as hydrogen peroxide, tert-butyl perbenzoate, sodium peracetate, or tert-butyl hydroperoxyde can be associated with a reducing agent such as a sulfite, bisulfite, ascorbic acid, or ferrous salt.2 A reducing agent such as a sulfite or bisulfite of an alkali metal can be associated with an oxidant such ammonium persulfate or the persulfate of an alkali metal.3 Ascorbic acid or erythorbic acid can be associated with hydrogen peroxide or a sodium, ammonium, or potassium persulfate.1,4 Azo compounds have also been proposed.2-4 Finally, hydrogen peroxide and iron can be combined.5 About 150 different such combinations were tested with a simple procedure that was specially developed to obtain quick and accurate results. Equipment and Experimental Procedure. The two solutions (A and B) to be mixed are stored in temperature-regulated water baths. A predetermined amount of solution A is transferred with an automatic pipet into a small beaker. The beaker is thermally insulated, and its contents are mixed with a magnetic agitator. The temperature of the beaker contents is measured with a fast thermocouple, with a response time of 0.05 s, which is connected to a data acquisition system. A predetermined amount of solution B is transferred with an automatic pipet into a second beaker. The contents of this beaker are then dumped quickly into the first beaker. Solution B is always kept at a temperature that is at least 5 °C below that of solution A so that the time at which it is dumped into
10.1021/ie0011472 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/08/2001
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Figure 1. Temperature evolution during a typical experiment.
Figure 3. Effect of initiator concentration on the reaction conversion as a function of time. T0 ) 25 °C. The “normal” initiator concentrations are 0.84 wt % hydrogen peroxide and 1.32 wt % ascorbic acid.
Figure 2. Reaction conversion as a function of time. Solution A contains 0.84 wt % hydrogen peroxide, and solution B contains 1.32 wt % of ascorbic acid. T0 ) 25 °C.
solution A can be determined from the initial drop in the temperature of the beaker contents. Preliminary experiments were conducted by mixing solutions A and B with a 5 °C initial temperature difference and in the absence of initiator to prevent polymerization. The temperature response was that of a first-order system with a response time of about 0.20 s. Mixing therefore took place in 0.15 s. Figure 1 shows an example of the evolution of the temperature during a typical experiment. The time t ) 0 at which solution B is introduced can be easily determined from the small, initial temperature drop. After an initial induction lag, the reaction begins, and the temperature increases from 60 to 100 °C in less than 1 s. Although the reaction continues, the temperature then stabilizes at about 104 °C as the solution starts to boil. The reaction conversion can be obtained from an enthalpy balance. The heat losses were evaluated as a function of the temperature of the beaker contents with preliminary experiments in the absence of initiator, to prevent polymerization. Figure 2 presents an example of the results that were obtained with an initial temperature of 25 °C. After an induction time of about 0.8 s, about 4.5 s is required to reach a conversion of 50%. Although no information can be obtained for higher conversions (the solution is boiling), the information that can be derived from the temperature variations is sufficient to identify the most promising initiators and reaction conditions. Results. All tested solutions were such that they gave a neutralization level of 75% of the acrylic acid, as
Figure 4. Effect of initial reactant temperature on the reaction conversion as a function of time. The initiator concentrations are 0.84 wt % hydrogen peroxide and 1.32 wt % ascorbic acid.
required for many of the SAP applications (this gives a pH of about 6.2 for the final product). In all cases, the mass fraction of acrylic acid and equivalent was about 40% (for this calculation, the mass of sodium acrylate is converted to an equivalent mass of acrylic acid). Following the industry standard, the initiator concentrations are reported as the ratio of the mass of initiator to the mass of acrylic acid equivalent in the mixture of solutions A and B. A preliminary study of 150 initiator combinations indicated that the only adequate initiators were combinations of reducing and oxidizing agents. Other initiators such as azo compounds (e.g., azodiisobutyronitrile or AZDN) were not effective. Tests were thus conducted with various concentrations of oxidizing and reducing compounds. The oxidizing compounds were sorbitol, tertbutyl hydroperoxide, hydrogen peroxide, potassium persulfate, sodium persulfate, and ammonium persulfate. The reducing compounds were diethanolamine, ascorbic acid, erythorbic acid, sodium metabisulfite, sodium hyposulfite, and ferrous sulfate combined with dextrose. Mixtures of several reducing and oxidizing compounds were also tested. The combination of hydrogen peroxide and ascorbic acid gave the fastest polymerization at room temperature. Several experiments were thus conducted with these initiators to accelerate the polymerization reaction. Acrylic acid contains a hydroquinone polymerization
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Figure 5. Effect of initiator concentration on the reaction conversion as a function of time. T0 ) 50 °C. The normal initiator concentrations are 0.84 wt % hydrogen peroxide and 1.32 wt % ascorbic acid.
reach a given conversion decreases linearly with increasing initiator concentration (Figure 3). Figure 4 shows that preheating the solution prior to mixing is very effective. A maximum temperature of 60 °C is imposed to avoid polymerization prior to mixing. Increasing the initial temperature from 25 to 50 °C reduces by 65% the time required to reach 20% conversion (Figure 4). The improvements observed by increasing the initial temperature and the initiator concentration are not, however, additive, as shown by Figure 5. The best improvements are obtained by mixing nonneutralized solutions (Figure 6). Instead of using two solutions with the same concentrations of acrylic acid and sodium acrylate that differ only in their initiator, we used one solution containing acrylic acid and the other sodium hydroxide. The exothermic neutralization reaction starts as soon as the two solutions are mixed. Figure 6 shows that the induction period is eliminated and that only 0.8 s is required to reach 20% conversion, starting from solutions at room temperature. Implementing this method in an industrial reactor, however, requires very good and fast mixing of the two reactant streams. Mixing Study
Figure 6. Effect of neutralization on the reaction conversion as a function of time. T0 ) 25 °C. The normal initiator concentrations are 0.84 wt % hydrogen peroxide and 1.32 wt % ascorbic acid.
inhibitor to ensure safe storage and transportation. This inhibitor is active only in the presence of dissolved oxygen. Several methods were therefore used to remove dissolved oxygen from the solutions prior to their mixing. Bubbling nitrogen or adding sulfite or bisulfite did not significantly accelerate the polymerization. Figure 3 shows that increasing the initiator concentration can have a beneficial effect on the polymerization kinetics. Increasing the initiator concentration by a factor of 8 reduces the induction time by about 30% (Figure 3). It also decreases by about 50% the time required to reach 20% conversion. The time required to
Ultra-rapid synthesis of SAPs requires effective mixing of two liquid streams in a few tenths of a second, followed by atomization. Standard spray nozzles cannot be used because atomization would occur before mixing is completed. Any mechanical premixing, e.g., with a static mixer, cannot be used because solid deposits would form on the mixer surfaces. Mixing by jet impact, far from any solid surface, is preferable. Various studies of jet impact mixing have been published.6-9 The most interesting study for SAP synthesis seems to be the impact of flat jets.8,9 Two other jet impact geometries were investigated: a similar geometry with cylindrical jets and the mixer proposed by the Japanese patent.1 Equipment and Experimental Procedure. Figure 7 shows a schematic diagram of the mixer that uses the impact of flat jets. Each solution is pumped to the impact zone through a 10-mm-long tube with a rectangular cross section, 3 mm × 0.3 mm. The half-angle β between the two jets must be less than 30° to ensure a nonelastic collision and degradation of energy toward mixing.8,9 For the present study, the angle β was 15°. Providing proper mixing and atomization simultaneously was very difficult. The impact jet mixer was
Figure 7. Schematic diagram of the mixer by impact of flat liquid jets.
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directly in a spectrophotometer to determine the concentrations CAi and CBi of methylene blue and methyl orange, respectively. From the measurements performed with 16 containers, one can calculate the following mixing index:
MI ) 100 Figure 8. Schematic diagram of the mixer proposed by the Japanese patent.1
thus operated under conditions that favored mixing, and opposed nitrogen jets were applied to the resulting liquid jet to achieve atomization. The nitrogen jets were normally located 100 mm below the tip of the liquid tubes. A second impact jet mixer was also used, as shown in Figure 7. The only difference from the first mixer was that the tubes through which each solution was pumped to the impact zone had a 3-mm-diameter circular cross section. A third impact jet mixer was the mixer proposed by the Japanese patent.1 It is shown in Figure 8. The standard procedure for testing ultra-rapid liquid mixers uses parallel reactions.10-12 For the present study, however, a different approach was used. It proceeded in two stages: model colored solutions were used to make a first selection, and SAP synthesis was then carried out. Solution A was simulated by an aqueous solution of methylene blue, and solution B was simulated by an aqueous solution of methyl orange. In some experiments, sugar was added to these solutions to match the viscosities of the actual reactants. A first, rough optimization was performed by visual observation of the mixed liquid. For more accurate and quantitative estimates, the mixed liquid droplets were recovered in small 45-mm-high containers with a 12 mm × 12 mm cross section. The mass mi of liquid recovered in each container i was measured. Each container was then used
Figure 9. Schematic diagram of the pilot-plant reactor.
[
[( [ (
∑i
mi 1 - 2
1
-
2
CAi
CAi + CBi
∑i mi
]
)])]
2
The dye concentrations CA0 and CB0 in each of the original solutions were adjusted to account for the flow rates QA and QB of these solutions. They had to satisfy the relationship
QACA0 ) QBCB0 A mixing index of 0% indicates very poor mixing as each container contains only one of the two dyes. An index of 100% indicates perfect mixing, i.e., each container contains the same proportion of A and B. Results. The mixing nozzle from the Japanese patent1 (see Figure 8) was first tested. Each solution had a viscosity of 1 cp, and the flow rate of solution B was set equal to that of solution A. The best mixing was obtained for a liquid flow rate of solution A (QA ) QB) equal to 6.4 L/h and a nitrogen flow rate of 1.7 kg/h. The mixing index was then 63%. The jet nozzle with the cylindrical tubes (see Figure 7) gave its best results with solution flow rates of QA ) QB ) 10 L/h and a nitrogen flow rate of 1.7 kg/h, using 1 cp solutions. The mixing index was then 90%. This nozzle is thus far superior to the previous nozzle. The flat jet nozzle (see Figure 7) gave even better results. With 1 cp solutions, the best results were obtained with solution flow rates of QA ) QB ) 4.8 L/h and a nitrogen flow rate of 1.7 kg/h. The mixing index was then 98%, i.e., mixing was nearly perfect. Reducing the flow rates to 3.4 L/h reduced the mixing index to 93%; this indicates that good mixing can be achieved
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over a reasonable flow rate range. In the absence of gas, the mixing index dropped to 24%; thus, the gas jets greatly enhance mixing, in addition to providing atomization. Experiments were also conducted with the flat jet nozzle and solutions whose flow rate ratio and viscosities matched the process requirements. Solution A had a viscosity of 17 cp and solution B 16.5 cp. The ratio of the volumetric flow rate of solution A to that of solution B was 1.3. The best mixing index, 91%, was obtained for 7.1 L/h of solution A, 5.4 L/h of solution B, and 2.1 kg/h of nitrogen. Moving the gas jets from 100 to 25 mm below the tip of the liquid tubes reduced the mixing index to 55%. It is therefore important to provide some time for the liquid jets to mix before applying the gas jets. Preliminary Polymerization Tests Equipment and Experimental Procedure. Figure 9 shows how a spray dryer was adapted to test the new SAP synthesis process. The new flat jet mixer was located at the top of the spray dryer. A recovery basin was located at the bottom of the cylindrical part of the spray dryer, about 1 m below the tip of the liquid tubes of the jet mixer. The recovery basin was filled with 5 L of a 2 wt % cold aqueous solution of copper sulfate, which is a terminator for the polymerization reaction. Polymerization inside the droplets therefore stopped as they reached the recovery basin. Polymer particles were retrieved from the recovery basin by filtration, followed by a methanol wash. Each solution was supplied from a pressurized blow tank. Its flow rate was measured with a calibrated rotameter, and its temperature was set with a temperature-controlled bath (Figure 9). The flow rate of the nitrogen atomization gas was measured with a calibrated rotameter, and its temperature could be increased with an electric heater. Results. The pilot-plant reactor shown in Figure 9 failed to achieve significant polymerization. The residence time of the droplets was too short. The terminal velocity of 400-µm droplets is about 1.5m/s, giving a residence time of 0.7 s. Even under the best conditions, with neutralization, the kinetic study indicated that the conversion would be only 15%. To increase the droplet residence time, the spray dryer height was extended by about 1 m, and the mixer was moved up so that the tip of its tubes were about 2 m above the recovery basin. The solutions were preheated to 50 °C. Significant polymerization was then obtained with neutralized solutions. The initiator concentrations were 8.3% hydrogen peroxide and 13.2% ascorbic acid. The conversion rate was about 40%. Preheating the atomization gas to 100 °C increased the conversion rate to 50%. Higher conversions were obtained with nonneutralized solutions, as suggested by the kinetic study (see Figure 6). A conversion rate of 65% was obtained without preheating the atomization gas and with initiator concentrations of only 0.84% hydrogen peroxide and 1.37% ascorbic acid. Conclusions Ultra-rapid polymerization, atomization, and drying of superabsorbents can be performed in a single unit.
The best initiator combination is hydrogen peroxide and ascorbic acid. Using alkali and acidic solutions instead of neutralized solutions accelerates the polymerization by making use of the heat of neutralization. It also further simplifies the process. Rapid and effective mixing is required. A two-stage mixer gives the best results: two flat jets of liquids are first impacted at high velocity, and a high velocity gas jet is applied to the resulting liquid stream to improve mixing, to form a spray of fine droplets, and to enhance drying. A conversion rate of 65% was obtained in a 2-m reactor. Higher conversions and product quality evaluation will require a reactor that is at least 5 m high. Acknowledgment The authors thank ATOFINA for its financial support. Help from Ms. Ce´line Guiraudie, Mr. Eric Barthel, Mr. Manuel Hidlago, and Ms. Christelle Plant is gratefully acknowledged. Notation CA0 ) concentration of tracer A in initial solution A, mol/ m3 CAi ) concentration of tracer A in mixed sample i, mol/m3 CB0 ) concentration of tracer B in initial solution B, mol/ m3 CBi ) concentration of tracer B in mixed sample i, mol/m3 mi ) mass of sample i, kg QA ) volumetric flow rate of solution A, m3/s QB ) volumetric flow rate of solution B, m3/s T0 ) initial temperature of the solutions prior to mixing, °C
Literature Cited (1) JGC Corporation. Polymerization by atomization and suitable nozzles. Japanese Patent 5-132503, 1993. (2) Freeman, C. A polymerization process, apparatus and polymer. International Patent WO 96/40 427, 1996. (3) American Colloid Company. Production process for highly water absorbable polymer. U.S. Patent 4,525,527, 1985. (4) The Dow Chemical Company. Erythorbate as part of a redox intiatior system for polymerization of water swellable polymers. European patent 0 409 136 A2, 1990. (5) Blackley, D. C. Emulsion PolymerizationsTheory and Practice; Applied Science Publishers: London, 1975. (6) Ryan, J. M. Atomization characteristics of impinging liquid jets. J. Propul. Power 1995, 11 (1), 16. (7) Yamanguchi T. A novel polymerization process by means of impinging jets. Angew. Makromol. Chem. 1980, 85, 197. (8) Demyanovich, R. J.; Bourne, J. R. Rapid Micromixing by the impingement of thin liquid sheets. 2. Mixing study. Ind. Eng. Chem. Res. 1989, 28, 830. (9) Demyanovich, R. J.; Bourne, J. R. Impingement-sheet mixing of liquids at unequal flow rates. Chem. Eng. Process. 1992, 31, 229. (10) Bourne, J. R.; Kozicki, F.; Rys, P. Mixing and fast chemical reaction. I. Test reactions to determine segregation. Chem. Eng. Sci. 1981, 10, 1643. (11) Bourne, J. R.; Hilber, C.; Tovstiga, G. Kinetics of the azo coupling reactions between 1-naphthol and diazotised sulphanilic acid. Chem. Eng. Commun. 1985, 37, 293. (12) Villermaux, J.; Falk, L.; Fournier, M. C. Potential use of a new parallel re´action system to characterize micromixing in stirred reactors. AIChE Symp. Ser. 1994, 50, 253.
Received for review December 30, 2000 Revised manuscript received April 3, 2001 Accepted April 11, 2001 IE0011472
