Ind. Eng. Chem. Res. 2004, 43, 5021-5026
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Reaction Pathways in Pentachlorophenol Synthesis. 1. Temperature-Programmed Reaction Jianli Yu and Phillip E. Savage* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
Pentachlorophenol, a wood preservative for nonresidential applications, contains parts per million levels of dibenzodioxins and dibenzofurans with six or more chlorine atoms. There is interest in reducing the levels of these microcontaminants in pentachlorophenol. We conducted pentachlorophenol synthesis reactions in the laboratory to demonstrate the ability to mimic the commercially practiced temperature-programmed synthesis and to identify an operating region where mass-transfer limitations were effectively eliminated. The laboratory system was operated in a temperature-programmed fashion and produced pentachlorophenol with a yield and microcontaminant content similar to those of the commercially produced material. The experiments also revealed a stirring rate, number of gas spargers, and chlorine flow rate that enabled the laboratory-scale reaction to proceed in the kinetics-controlled regime. Results indicated that microcontaminant formation was low until near the end of the run where the microcontaminant levels increased greatly with modest increases in temperature and pentachlorophenol yield. Additionally, the presence or absence of mass-transfer limitations had little influence on the effect of temperature on the pentachlorophenol yield and micrcontaminant level. Introduction Pentachlorophenol is a widely used wood preservative. It is manufactured by the direct chlorination of phenol.1 The reaction is done batchwise in the liquid phase over the course of several hours, and a catalyst (AlCl3) is needed to produce the completely chlorinated product. Chlorine gas bubbles continuously through the molten phenol/chlorophenol mixture. As the chlorination reaction proceeds, the melting point of the mixture increases from 41 °C (melting point of phenol) to about 191 °C (melting point of pentachlorophenol). Plant operators monitor and gradually increase the reactor temperature during synthesis to ensure that it is always a few degrees above the melting point of the mixture. Thus, the commercial-scale synthesis of pentachlorophenol involves a temperature-programmed reaction. The commercial synthesis of pentachlorophenol is accompanied by the production of small (ppm) amounts of microcontaminants (e.g., hexachloro and more highly chlorinated dibenzodioxins and dibenzofurans).2 Domestic pentachlorophenol manufacturers have invested considerable effort and resources into reducing the level of microcontaminants in the pentachlorophenol product. There is interest in reducing the levels even further, and one potential route to additional reductions is a better understanding of the reaction pathways and kinetics that control pentachlorophenol production and microcontaminant formation. We initiated a research project to obtain a better understanding of the chemical reactions occurring during pentachlorophenol synthesis, with the goal of using that understanding to formulate and test strategies for reducing the microcontaminant content in technical-grade pentachlorophenol. In this paper we describe results from the initial portion of this project. The goal for this portion was to mimic plant operation (temperature-programmed reaction) in the laboratory and determine the time evolution of reactant * To whom correspondence should be addressed. Tel.: (734) 764-3386. Fax: (734) 763-0459. E-mail:
[email protected].
Figure 1. Block diagram of the chlorination reactor system.
disappearance, product formation, and microcontaminant production in a kinetics-controlled regime. We are not aware of any previous reports of work of this type in the archival chemical literature. Experimental Section Reactor Design and Operation. Figure 1 shows a block diagram of the experimental system. A four-neck 500 mL round-bottom flask served as the chlorination reactor. The reactor rested in a heating mantle. Air flowing through a small-diameter Teflon tube placed between the bottom of the flask and the heating mantle provided a means to cool the reactor exterior. The reactor flask was equipped with a stirrer, a condenser tube, and a chlorine inlet port. The system also included a thermocouple (E-type) with a programmable temperature controller (Barnant) connected with a computer. Dry nitrogen is used to purge the reaction system before and after reaction. A typical run began by charging 90 mL of 2,4,6trichlorophenol (98% pure from Aldrich) to the reactor. The trichlorophenol had been heated above its melting
10.1021/ie030827c CCC: $27.50 © 2004 American Chemical Society Published on Web 07/03/2004
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point so it could be added as a liquid. The heating mantle was then switched on so that the trichlorophenol remained in the liquid phase. The desired amount of catalyst (AlCl3) was next added to the reactor, the reactor contents were heated further to the desired initial temperature (about 105 °C), and the chlorine flow was initiated. The chlorine flow rates were recorded on the basis of calibrated rotameter readings. The unreacted chlorine, the HCl byproduct from the reaction, and some chlorophenols leave the reactor in the vapor stream that passes through the condenser. The gaseous components pass through, whereas the chlorophenols freeze on the room-temperature walls. This frozen material was removed and returned to the reactor periodically by warming the condenser walls with a heating gun. After leaving the reactor, the gaseous components pass through a gas scrubber (packed with borosilicate glass raschig rings) wherein they are contacted with an aqueous NaOH stream, which removes the chlorine-containing compounds. A Masterflex pump (Cole-Parmer Instrument Co.) circulates a 10% NaOH solution, which is mixed with a water stream to adjust the concentration of NaOH so that the pH of the scrubber effluent is kept around 7. Samples of the reactor contents are withdrawn periodically and retained for analysis. These samples solidified very quickly after removal from the reactor. During the course of a synthesis experiment, we increased the reactor temperature from about 70 to 188 °C. The precise temperature program needed is determined moment by moment by monitoring the reaction progress. The goal is to keep the reactor temperature just a few degrees above the freezing point of the reaction mixture. In a representative run, 0.75 g of AlCl3 catalyst was added to the reactor when the temperature had been increased to approximately 90 °C. The solution was heated further to 105 °C before chlorine was introduced. The reaction is exothermic, and the solution temperature was increased further to about 126 °C and held there until there was evidence that the reaction mixture was near its freezing point. This evidence is the appearance of a ring of solids on the wall of the reaction flask. At this point, we initiated a temperature ramp from 126 to 188 °C within 1-2 h depending on the stirring rate and chlorine flow rate. During this temperature ramp, no cooling air needs to be used. Analytical Methods. The information desired from this reaction study was quantitative data for the different chlorophenols and for the microcontaminants. In this section we describe the methods used to identify and quantify the yields of these products. (1) Chlorophenol Analysis. Around 0.075 g of reactor material was carefully weighed into a sample vial, and then around 70 µL of monochlorobenzene was added as an internal standard. The masses of both were recorded. A 0.5 mL portion of N,O-bis(trimethylsilyl)acetamide served as the solvent. The vial was then capped for chromatographic analysis. A computer-controlled Agilent 6890 series gas chromatograph equipped with a DB-624 column (30 m × 0.32 mm × 1.8 µm) separated the sample constituents. A 1 µL sample was injected. The gas chromatograph operated in the split mode (split ratio of 35:1) with an injection port temperature of 250 °C. The gas chromatograph oven temperature was held at 70 °C for 2 min, then increased to 250 °C at 20 °C/min, and then held at 250 °C for 10 min. Chemstation software used the
signals produced by the thermal conductivity detector to produce chromatograms and perform the quantitative analysis. Molar yields of tri-, tetra-, and pentachlorophenol were calculated as moles of the component of interest present per mole of trichlorophenol present in the reactor initially. (2) Microcontaminant Analysis. The method for analysis of microcontaminants consists of two steps: extraction and gas chromatography (GC) analysis. The solvent extraction step involves placing about 0.26 g of reactor material, 10 mL of 0.3 N NaOH, and 3.4 mL of isooctane into a 20 mL vial. The vial is shaken until all of the solid material dissolves. After the organic and aqueous phases separate, the top layer (isooctane with extractable microcontaminants) is placed in a gas chromatograph autosampler vial and capped. The quantitative analysis step involves GC analysis of the isooctane extract on an HP-5 column (30 m × 0.32 mm × 0.25 µm) with a flame ionization detector. We used an Agilent 6890 gas chromatograph with automatic sampling and injection, splitless injection of a 1 µL sample, and injection port and detector temperatures of 250 and 325 °C, respectively. The gas chromatograph oven temperature was initially at 90 °C for 2 min and then increased to 240 °C at 10 °C/min. The temperature remained at 240 °C for 5 min and then increased to 310 °C at 5 °C/min. This final temperature was held for an additional 3 min. Qualitative analysis for product identification was also done by GC with the same operating conditions but with mass spectrometric detection. We used an Agilent 6890N Network GC system with a 7973 Network MSD and an HP-5MS column (30 m × 0.25 mm × 0.25 µm) to analyze the microcontaminants. We recognize, of course, that some compounds might react in the gas chromatograph injection port, in which case the compound detected by the mass spectrometer will not be the actual compound that existed in the sample. Compound identities were based on comparisons with authentic mass spectra stored in the computer library and on manual inspection of the mass fragmentation patterns. When performing analyses for a pentachlorophenol product made in our laboratory, we always analyzed a control sample of a commercial flaked pentachlorophenol product (provided by Vulcan). This sample was determined by Vulcan to have a pentachlorophenol content of about 86%. The results presented in the next section will include analytical results for this Vulcan sample, which we used as a check of our analytical method for pentachlorophenol analysis and as a reference point for the microcontaminant levels in the pentachlorophenol produced in our experiments. Results and Discussion Our focus is on the chemical reactions occurring during pentachlorophenol synthesis. We seek the reaction paths that lead to the formation of microcontaminants and eventually to quantify their rates as a function of temperature and the concentrations of relevant chemical species. With reaction kinetics as one of the objectives, it is imperative that the laboratory reactor operates in a regime wherein the rates of mass transfer and heat transfer do not affect the observed rates of change. Therefore, the first component of the experimental plan is to identify the region where we can access the intrinsic reaction kinetics. Variables to examine are the stirrer speed, the chlorine gas flow rate,
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Figure 2. Temporal variation of pentachlorophenol yields at different stirring rates.
Figure 3. Temporal variation of the microcontaminant amount (total) at different stirring rates.
and the manner in which the chlorine is introduced. When the results become independent of these variables, then the reactor is likely operating in the regime where the intrinsic reaction kinetics controls the rates of the chemical transformations. Stirring Rate. The first variable examined was the stirring rate. We performed one set of experiments with different stirring rates but with the same reactor temperature profile (from 126 to 188 °C in 1 h and 20 min) and chlorine flow rate (1.5 mol/h). This temperature program was needed to maintain a liquid phase inside the reactor throughout the reaction. Figure 2 shows the temporal variation of the pentachlorophenol yields for the different stirring rates. The pentachlorophenol yield in the Vulcan sample was measured to be 96% (about 10% higher than it should be). Therefore, the absolute values of the pentachlorophenol yields in Figure 2 may also be too high, but the trends, which are more important here, should be reliable. The data show that the pentachlorophenol yields at each stirring rate go through a maximum, and that the maximum is reached more quickly at the faster stirring speeds. This behavior is consistent with mass transfer influencing the overall reaction rate at the lower stirring rates, and it shows the importance of mixing to the reaction progress. These results also show that pentachlorophenol is subject to over-reaction at these conditions. Therefore, the synthesis must be closely monitored to avoid loss of the desired product. Analysis of the extractable products (microcontaminants) led to 11 peaks in the chromatogram for the Vulcan sample and 17-30 peaks in samples from the reactions. Figure 3 presents the temporal variation of the total GC peak area of extractable products at the three different stirring rates. The total peak area in these samples was always higher than that in the Vulcan sample (294 for this particular analysis). The total peak area also increased with reaction time for all three stirring rates, and the total peak area was highest at the slowest stirring rate. This observation does not imply that slow stirring leads to higher microcontaminant levels. The higher levels present at the slowest stirring rate are more likely attributable to the samples
Figure 4. Temporal variation of the amount of hexachlorobenzene at different stirring rates (Vulcan sample, 19).
corresponding to a region where the pentachlorophenol yields were decreasing with time because the product was being overchlorinated. Figures 4-6 provide more detailed information about the influence of the stirring rate on the behavior of selected individual extractable compounds. The peak areas present in the Vulcan standard sample appear in the figure captions for comparison. Figure 4 shows that the amount of hexachlorobenzene increases during the experiment. Figure 5 shows that the amount of this heptachlorodibenzodioxin decreased with time at the faster stirring rates but increased with time at the slowest rate, perhaps because overchlorination occurred during this run. Figure 6 shows that the amount of octachlorodibenzodioxin increased steadily with time. This microcontaminant was typically the one with the largest peak area. It is likely, though, that any octachlorodibenzofuran present also appears in this peak. For all of these microcontaminants, the peak areas were comparable at the medium and fast mixing speeds.
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Figure 5. Temporal variation of the amount of heptachlorodibenzodioxin at different stirring rates (Vulcan sample, 11.5).
Figure 6. Temporal variation of the amount of octachlorodibenzodioxin at different stirring rates (Vulcan sample, 54).
To summarize this section, we have shown that the stirring rate affects the rate of the chlorination reaction and the rate of microcontaminant formation. Since our objective is to elucidate the reaction paths and delve into the reaction kinetics, it is important that we eliminate (or at least minimize) the mass-transfer limitations in the laboratory reactor. Therefore, all subsequent experiments were done at the fastest possible stirring rates. In addition to reducing (and hopefully eliminating) mass-transfer limitations, this strategy also reduces the amount of time required to do an experiment. Number of Spargers. The previous section showed that the stirring rate influenced the rate of chlorination. We desired to determine whether the manner in which chlorine was added to the reactor also influenced the results. Gas-liquid mass transfer could be facilitated by using more than one sparger. To investigate the effect of the number of spargers on the reaction rate, we modified the four-neck flask reactor to become a fiveneck flask reactor so that two ports were available for introducing chlorine streams. The main chlorine feed
Figure 7. Effect of the number of spargers on pentachlorophenol yields at a fixed total chlorine flow rate.
line was connected to a T-valve, and then split into two streams so that each can be introduced into the reactor at different points. This experiment assessed whether using two spargers for chlorine introduction influenced the reaction progress. Figure 7 compares the pentachlorophenol yields obtained using one sparger and using two spargers at the same total chlorine flow rate (1.5 mol/h). The reaction was clearly faster when using two spargers. In fact, using two spargers caused the rate to increase more than we had anticipated. We began sampling sooner than in the single sparger run, anticipating a faster reaction, but not soon enough to capture the increase in pentachlorophenol yield with time that occurred before 160 min. Chlorine Flow Rate. We next conducted chlorination experiments with two spargers at different chlorine flow rates (2.3, 2.7, and 3.3 mol/h) to determine whether the reaction rate was limited by the supply and dissolution of chlorine in the reaction mixture. Sampling was started early in the run to record lower pentachlorophenol yields and the corresponding amount of extractable microcontaminants that existed there. The temperature ramp for these experiments was an increase from 126 to 188 °C within 1 h. Figures 8 and 9 display the variation of the yields of pentachlorophenol and tetrachlorophenol with reaction time at different chlorine rates. The pentachlorophenol yields increased with time to a maximum yield, and then decreased. The tetrachlorophenol yields decreased with time to essentially zero. The reaction was slowest at the lowest flow rate, but the results at the two higher flow rates are very similar, suggesting that mass-transfer limitations have been largely eliminated at these conditions. Figure 10 shows the temporal variation of the total GC peak area of extractable products at different chlorine flow rates. The microcontaminant amount (corresponding to total peak area) increases with time and in several instances is below the value obtained for the Vulcan reference sample (237). Figure 11 presents the total peak area as a function of the pentachlorophenol yield at the three different chlorine flow rates. The microcontaminant levels remain below those in the Vulcan sample until a pentachlorophenol yield of over
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Figure 8. Temporal variation of pentachlorophenol yields at different Cl2 flow rates.
Figure 9. Temporal variation of tetrachlorophenol yields at different Cl2 flow rates.
80% is reached. The microcontaminant level then increases sharply with the subsequent small changes in product yield. Figure 12 shows the total peak area as a function of the reactor temperature. The microcontaminant level is low and comparable to that in the Vulcan sample until a temperature of about 170 °C is exceeded. At these higher temperatures, the microcontaminant level increases sharply. Finally, note that the total peak area for a given temperature (Figure 12) or a given pentachlorophenol yield (Figure 11) is nearly independent of the chlorine flow rate. This result suggests that even when mass-transfer rates might limit the observed chlorination kinetics, the relationship between microcontaminant levels and temperature or pentachlorophenol yields remains unaffected. Figure 13 presents the variation of the pentachlorophenol yield with temperature for the three different chlorine flow rates. The yield increases nearly linearly with temperature until the maximum yield is attained. Interestingly, there is remarkably little variation for the
Figure 10. Temporal variation of the total microcontaminant amount at different Cl2 flow rates.
Figure 11. Variation of the total microcontaminant amount at different Cl2 flow rates with pentachlorophenol yield.
three different data sets. This result indicates that the pentachlorophenol yield at a given temperature was largely independent of the chlorine flow rate. Thus, the effect of higher chlorine flow rates was to speed up the reaction rate (higher pentachlorophenol yields at a given time) and not to alter the yield-temperature trajectory. Conclusions This study shows that a laboratory-scale reactor system mimicking plant-scale reactor operation can produce pentachlorophenol with yields and microcontaminant levels comparable to those of a commercial, technical-grade product. Using two chlorine spargers and a total chlorine flow rate of 2.7 mol/h or higher may be sufficient to eliminate gas-liquid mass-transfer limitations from this chlorination reaction in the experimental system used. Operating at these flow rates permits a complete experimental run to be accomplished in 2-3 h (starting from trichlorophenol). Having vali-
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Figure 12. Variation of the total microcontaminant amount at different Cl2 flow rates with reaction temperature.
Figure 13. Variation of the pentachlorophenol yield at different Cl2 flow rates with reaction temperature.
dated the experimental system and identified conditions that minimize the importance of mass-transfer limitations, we can now use this system with confidence to probe the reaction kinetics and reaction pathways. Isothermal reaction experiments are useful to this end, and this topic is the subject of the second paper in this series. Complete elimination of mass-transfer limitations does not appear to be essential for examining relationships between microcontaminant levels and pentachlorophenol yields.
Acknowledgment
The commercial-scale chlorination reactor is very much larger than the laboratory-scale reactor used in this study. It is likely that mixing is imperfect in this larger scale reactor. Consequently, the reactor contents might not be uniform at all times and there might exist local variations in both temperature and chlorine concentration. These variables might be highest at the point where chlorine gas enters the reactor. Given the positive correlation we observed between microcontaminant amount and temperature, these local hot spots might be responsible for a disproportionate amount of the microcontaminant formation in the commercial reactor.
Literature Cited
We thank Buffy Branam of Vulcan Chemicals for assistance with the design and operation of the laboratory chlorination reactor and Dr. David Hildebrand for helpful discussions and review of this manuscript. We are grateful to Dr. Terry Nestrick for assistance with the microcontaminant analysis protocol. This work was supported under a research contract with the Pentachlorophenol Task Force.
(1) Muller, F.; Caillard, L. Chlorophenols. Ullmann’s Encyclopedia of Industrial Chemistry [Online]; DOI 10.1002/14356007/ a07_001, posted June 15, 2000. (2) Buser, H. R.; Bosshardt, H. P. Determination of polychlorinated dibenzo-p-dioxins and dibenzofurans in commercial pentachlorophenols by combined gas chromatography-mass spectrometry. J.sAssoc. Off. Anal. Chem. 1976, 59 (3), 562-569.
Received for review November 11, 2003 Revised manuscript received April 9, 2004 Accepted April 28, 2004 IE030827C
