Ind. Eng. Chem. Res. 1995,34, 3761-3765
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Application of a Droplet Column Type Two-Phase Reactor for the Epoxidation of Cyclooctene in Water as an Alternative Solvent Hung-Yee Shu,+Howard D. Perlmutter, and Henry Shaw* Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102
Droplet columns are used for their ability to greatly enhance liquid-liquid interfacial areas. The use of a droplet column for the two-phase epoxidation of cyclooctene by oxone in aqueous solution was studied as a n application of pollution prevention, i.e., the replacement of hazardous solvents with water. The dispersion of alkene droplets in aqueous oxone solution was generated by pumping the organic phase through a sparger at the bottom of the column. Then, organic droplets rise to the top of the aqueous phase. As the alkene droplets rise, they are oxidized by the oxone solution t o form epoxide. The study of aqueous epoxidation in a droplet column shows that the epoxidation of alkenes can be represented as a first-order reaction in alkene and a first-order reaction in oxone under mass transfer limiting conditions. By recycling the cyclooctene, over 60%yield of cyclooctene epoxide can be achieved in 3 h. However, due to epoxide crystals formation, a second reactor is needed to remove the solid and to bring the yield up to 80%. We found that a stirred tank reactor, which avoids the need to put the crystallized mixture through the small holes of a sparger, performed well in this application as a second reactor.
Introduction Much attention is being focused on the development of novel, environmentally benign manufacturing processes for chemical synthesis. In particular, water is being considered as an environmentally desirable solvent to replace hazardous organic solvents. Epoxides are important industrial organic intermediates due to their highly reactive moiety. Generally, epoxides are formed in an initial step and react further to provide industrially important products, such as pharmaceuticals, surfactants, detergents, antistatic agents, corrosion protection agents, additives for laundry detergents, lubricating oils, textiles, and cosmetics (Gerhartz, 1985). Epoxides are often produced from the reaction of alkenes with organic peroxy acids such as percarboxylic acids. A solvent is often employed to moderate the reaction and t o facilitate subsequent recovery of the epoxide in high yield (Kou and Chou, 1987; Roy et al., 1991;Woods and Beak, 1991). Few epoxidation processes are carried out in aqueous solution, and most of them include the use of catalysts and phase transfer agents (Csanyi and Jaky, 1991; Venture110 et al., 1983) or alternative electrochemical processes (Alkire and Kohler, 1988). A method using aqueous potassium peroxymonosulfate (oxone) solution to oxidize alkenes into epoxides and diols without phase transfer catalyst was investigated by Zhu and Ford (1991). Their research screened the epoxidation of several alkenes. They found that cyclohexene, cyclooctene, and P-methylstyrene can be oxidized into epoxides in high yield. However, no kinetic data were provided for this two-phase epoxidation reaction. To scale up laboratory results for potential industrial applications, information about reaction kinetics, mass transfer and heat transfer phenomena, and economics of the process had to be obtained. The purpose of this study is to investigate the scale up parameters for the epoxidation reaction in the absence
of hazardous solvents. The epoxidation of cyclooctene with an aqueous oxone solution was studied in our laboratory. However, the rate of this reaction is slow. To improve the reaction efficiency, longer residence times and large interfacial areas are needed (Wang and Chang, 1991). Considering that intense mixing and high interfacial areas are needed t o enhance reactor performance, a droplet column reactor was developed t o study this two-phase system. At the bottom of the reactor, a sparger was used to disperse the alkene fluid into fine droplets. A tubing pump was used t o pump cyclooctene from the top of the reactor through a sparger. The advantages of a droplet column reactor are low investment cost, large mass transfer area, and high mass transfer coefficient in both phases. The objective of the present investigation was to identify and evaluate the usefulness of the cyclooctene epoxidation reaction in a droplet column reactor as a model system for the study of two-phase epoxidation reaction. A major advantage of two-phase processes is the elimination of the need for hazardous organic solvents by using water as an environmentally benign solvent. An added advantage is improved ease for the separation of product from reagents. The study of aqueous epoxidation in a droplet column showed that the epoxidation of alkenes can be represented as a first-order reaction in alkene and a first-order reaction in oxone under mass transfer limiting conditions. Furthermore, the effects of oxone concentration in aqueous phase, temperature, alkeneoxone ratio, and flow rate of organic phase were all evaluated. The reuse of the aqueous oxone solution after regeneration is being investigated as an approach for preventing water pollution. Regeneration of spent oxone solution was investigated by reacting with 35% hydrogen peroxide, oxygen bubbling while under W irradiation, and hydrogen peroxide addition while under W irradiation and ozonation. None of these regeneration attempts were successful.
* Corresponding author e-mail address: shaw.admin.njit. edu. Current address: Department of Industrial Safety and Hygiene, Hung Kuang Institute of Nursing and Medical Technology, Taichung, Taiwan, Republic of China.
Experimental Section Materials. ACS grade cyclooctene was used as the model alkene, and oxone (commercial potassium peroxymonosulfate salt) solution was used as the oxidation
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0 1995 American Chemical Society
3762 Ind. Eng. Chem. Res., Vol. 34, No. 11,1995 Top plug for dding cydoDncnc
Table 1. Matrix of Experimental Parameters "X""P -......
parameter temperature effect 1 temperature effect 2 temperature effect 3 temperature effect 4 temperature effect 5 flow rate effect 1 flow rate effect 2 flow rate effect 3 flow rate effect 4 flow rate effect 5 oxone concn effect 1 oxone concn effect 2 oxone concn effect 3 oxone concn effect 4 oxone concn effect 5 oxone mncn effect 6 oxone concn effect 7 cyclooetene vol 1 cyclooetene vol2 cyclooctene vol3
addedin cyelooctene temp flow rate 550 cm3 (gP vol (cm3P W ) (cm3Imin) 172 100 15 5 172 100 15 23 172 100 15 34 172 100 15 44 172 100 15 55 32 23 100 15 107 23 100 15 100 15 23 172 100 15 23 273 100 15 23 357 23 172 10 15 20 15 23 172 23 172 40 15 60 15 23 172 80 15 23 172 100 15 23 172 120 15 23 172 100 15 23 172 100 25 23 172 100 40 23 172
Concentration of 100 g of oxone in 550 em3 solution is 0.296 M. * 15 m3of cyclooetene is equal to 0.110 mol of cyclooctene.
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Figure 1. &hematic sketch of droplet column experimental setup to pruduce cyclooctene oxide.
reagent in this study. Cyclooctene and oxone were obtained from Aldrich Chemical Co. and used without further purification. The material safety data sheet warns that oxone forms explosive mixtures with as little as 1%organic matter and decomposes on contact with heavy metal salts with the evolution of oxygen. Reagents for iodometric measurement of oxone concentration are all ACS reagent grade. Stock solutions were stored in a refrigerator and kept in the dark. Analysis. A temperature-programmed HP 5890 gas chromatograph with FID (flameionization detector) was used for cyclooctene and cyclooctene oxide analysis. A 1/8-in. diameter x 6 ft in length stainless steel column packed with 80/100 mesh TENAX TA was used. A 5;uL aliquot of organic solution was diluted into 1 cm3 with ethyl ether for GC analysis. Oxone concentration was measured by iodometric titration. &r reaction, a 30 cm3 volume of ethyl ether was added t o extract cyclooctene and epoxide. The organic and water solution phases were separated in a separatory funnel. The organic portion was then diluted into exactly 50 em3 in volumetric flask and used for GC analysis. A 5 cm3 volume of water phase portion was then extracted with 5 cm3 of ethyl ether three times and injected into the GC for the analysis of cycloodene and epoxide dissolved in the water phase. Procedure. (A) Epoxidation Experiments. A schematic of the experimental system used in this study is shown in Figure 1. The glass column reador (Ace Glass Inc.) used in this study was 50 mm in diameter and 450 mm in length. A 0.505-in. diameter glass fritted disk mounted on a Teflon adapter with 25-50 pm porosity was used as the sparger. All the adapters connected to the column are made of Teflon. A tubing pump (Chemflux Cole Parmer) was used to recirculate the cyclooctene liquid from the top of the aqueous solution through the sparger. The matrix of experi-
mental parameters investigated in this study is given in Table 1. Various quantities of oxone (10-120 g) were used in 550 cm3 of deionized water as the continuous phase to obtain various concentrations. From 15 to 40 em3 of cyclooctene was added to the column (floating on the aqueous oxone phase) a t the start of an experiment. Cyclooctene was then pumped through the sparger at the bottom of the column to form organic droplets. The droplets rose to the top of the oxone solution and were oxidized to epoxide. Temperature was monitored continuously with a calibrated k-type thermocouple. Experiments were performed a t 5, 23, 34,44, and 55 "C to determine reaction kinetics. The rate of organic phase pumping was studied over the range of 32-357 cm3/min. The reaction time was varied from 4 to 6 h; runs were stopped before epoxide crystals formed. (B)Regeneration of Spent Oxone Solution. In order to prepare a representative spent oxone solution, three 6-h experiments were conducted with the same solution and fresh cyclooctene charges. Four approaches were then investigated to regenerate the oxone, i.e., reaction with 35%hydrogen peroxide, oxygen bubbling under UV irradiation, mixing hydrogen peroxide under UV irradiation, and ozonation. The epoxidation ability of each regenerated oxone solution was then tested by reacting it with fresh cyclooctene. Theory. The overall kinetics of this type of twophase epoxidation reaction can be described as follows:
Let the rate of disappearance of the limiting reactant A, cyclooctene in this case, be represented by eq 1:
where k = Ae-E'RT (mo12/dm6.s; CAand CB= concentration (mol/dm3), r] = contact efficiency, then
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3763
409 oxone/550 cm' soiutm, o 118 M SO9 oxonel550 cm' solution. 0 237 M
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Figure 2. Effect of oxone concentration on the epoxidation of cyclooctene. The starting volume of the reagent was 15 cm3 of cyclooctene in 550 cm3 of oxone solution. The cyclooctene pumping rate was 172 cm3/min, temperature was maintained at 23 "C,and oxone concentration was varied from 0.0296 to 0.355 M.
Figure 3. Observed pseudo-first-order rate constants as a function of oxone concentration. Reaction condition are the same as those given in Figure 2. 15.
. . .
where kobs = Ae-E'RT?&b (i.e., CB>> CAI,eq 2 is solved to In kobs = ln(ACBq) - E/RT
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From eq 3, we know that In kobs is linear with respect to 1/T, i.e., the Arrhenius curve. From this relationship, the activation energy for epoxidation can be obtained. The slope is EIR, and the intercept is In (ACB~;~). Here the contact efficiency 7 is a function of pumping rate and oxone concentration (CB). From the effect of pumping rate, the kobs increases linearly with increasing pumping rate. It is suggested that contact efficiency 11 increases linearly with increasing pumping rate. The effect of concentration of oxone gives an optimum value for highest rate. This implies that CBnot only affects the chemical reaction but also affects the contact efficiency. The higher the oxone concentration, the lower the contact efficiency. By adding the two contributions of oxone to epoxide formation, i.e., chemical reaction and contact efficiency, an optimum value of oxone concentration is obtained.
Results and Discussion Effect of Oxone Concentration in Aqueous Phase. To study the effect of oxone concentration on overall reaction rate at a definite reaction temperature, the oxone concentration was varied from 10 g of oxone/550 cm3 (0.0296 M) to saturation (120 g of oxone/550 cm3, 0.355 M) at a temperature of 23 "C. It was found that there exists an optimum oxone concentration for the highest reaction rate. The data shown in Figure 2 illustrate the time dependence of epoxidation reaction rate for different aqueous oxone concentrations. All other parameters were held constant, and the pseudofirst-order rate constant for various oxone concentrations is given in Figure 3. It can be seen that higher epoxidation rates were obtained with increasing oxone concentrations up to 80 g of oxone added in 550 cm3 of water. When oxone concentration increased t o over 80 g/550 cm3 of water (0.237 MI, the epoxidation rate decreased. This phenomena was also observed by other researchers (Zhu and Ford, 1991). The fall in overall rate constant value may be attributed to the fact that, at higher oxone concentrations, the interfacial area decreases as the ionic effect predominates mass transfer of cyclooctene t o the interface. The reason for this optimum effect is that cyclooctene mass transfer at the
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Figure 4. Decrease of cyclooctene feed as a function of time for various initial volumes of cyclooctene reacted with 550 cm3of 0.296 M oxone solution at 23 "C. The cyclooctene volume varied from 15 to 40 cm3. The pumping rate was maintained at 172 cm3/min.
interface of dispersed cyclooctene droplets is balanced between decreasing aqueous phase solubility and increasing oxone concentration. Also as a consequence of the increase in ionic activity in the aqueous solution, the solubility of organic compounds in the aqueous phase decreases. This effect is known as "salting out". By considering both chemical reaction and mass transfer effects of oxone concentration, an optimum oxone concentration for the highest reaction rate can be obtained. Effect of the Volume Ratio of Cyclooctene to Water Phase. From the experimental data given in Figure 4, the volume ratio of cyclooctene and water affect the reaction rate. It was observed that the higher the ratio, the lower the reaction rate. Since the cyclooctene holdup volume in the droplet column is constant when pumping at the same flow rate, the higher ratio gives a larger unreactive volume, decreasing the reaction rate. However, a calculation of the total production of epoxide for each run, given in Figure 5, shows that higher rates are obtained for reactions run at a higher volume ratio of cyclooctene to water. These results imply that when there is enough oxone in the water solution, the total production rate at a constant reaction volume increases by increasing of cyclooctene to water volume ratio. This is a key factor for considering scaleup of this process. Effect of Cyclooctene Pumping Rate. Since intense mixing and high interfacial areas are needed to
3764 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 1'
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Figure 5. Total production of cyclooctene oxide in a nominal 500 cm3 reactor volume. Reaction conditions are the same as those given in Figure 4.
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Figure 7. Observed pseudo-first-order rate constants as a function of cyclooctene pumping rate. Reaction conditions are the same as those given in Figure 6.
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Figure 6. Decrease of cyclooctene feed as a function of time for pumping rates from 32 to 357 cm3/min. Reaction conditions other than pumping rate are the same as those given in Figure 4.
enhance droplet column reactor performance, the pumping rate of cyclooctene is a very important parameter. The interfacial area can be increased by pumping cyclooctene at higher flow rates, which will give higher epoxidation rates. From the results given in Figure 6, one can see that the reaction rate increases with increasing pumping rate. By plotting observed pseudofirst-order rate constants of epoxidation versus pumping rate of upper layer fluid (as shown in Figure 7), a linear relationship between rate constant and pumping rate is obtained. Effect of Temperature. The epoxidation experiments were run at different temperatures. Figure 8 shows conversion of cyclooctene as a function of time for five different reaction temperatures. It can be seen that reaction rate increases with increasing temperature. A yield of '70% epoxide can be reached in 2.5 h at 44 "C. An Arrhenius plot showing the linear relationship for the overall reaction rate constant for epoxidation is shown in Figure 9. From this figure, we can calculate the activation energy for cyclooctene epoxidation in oxone solution. The slope of the Arrhenius plot is -3639, resulting in an activation energy, E,of 30.3 kJ/mol. The reaction is in the mass transfer controlled region. Epoxidation Reaction in Spent Oxone. The oxone solution, after each reaction run, was collected and reused three times. We observe in Figure 10 that the oxidation ability of oxone decreases after each epoxida-
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Figure 8. Decrease of cyclooctene volume with time as a function of temperature from 5 to 44 "C. Reaction conditions are the same as those given in Figure 6. Insert shows the increase in yield of cyclooctene oxide as a function of time over the range of temperature studied.
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Figure 9. Arrhenius plot of epoxidation of cyclooctene for the operating conditions given in Figure 8.
tion run. The epoxidation of cyclooctene with an oxone solution that had been used three times can reach only about 22% of the yield from a fresh oxone solution. Some regeneration methods such as reacting with 35% hydrogen peroxide, oxygen bubbling while under W irradiation, hydrogen peroxide addition while under W irradiation, and ozonation were investigated. None of these regeneration methods were successful.
Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995 3766 tion systems. Reasonably high reaction rates and yields can be achieved at optimized operating conditions. The reaction is very specific; only trace amounts of organic by-products were detected. The selectivity is nearly 99%. Chemical and transport processes occurring in the vicinity of the liquid-liquid interfaces are very important, since these evidently control the overall reaction rate and thus determine scaleup strategies.
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Acknowledgment The authors are grateful to the Emissions Reduction Research Center, New Jersey Institute of Technology, for support of this research. 0
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Figure 10. Decrease in oxidation ability of fresh and spent oxone solutions measured by decrease in yield as a function of time at the operating condition given in Figure 8.
Second Reactor and Separation. To test the reaction in a second reactor (mixer stirred reactor), the same volume of fresh oxone solution was added to the alkene and epoxide mixture. It took less than 1 h t o bring the epoxide yield to more than SO%, but epoxide crystals fused into a block after 30 min of settling. The separation of product cyclooctene epoxide from oxone solution was also studied. The reaction is considered complete when cyclooctene epoxide yield is about 80%. The product can be easily removed by lowering the temperature t o 5 "C; then, cyclooctene epoxide crystallizes into a solid block. Since the density of the epoxide is less than that of the aqueous solution, the epoxide is collected from the surface of oxone solution. Conclusion The synthesis of cyclooctene oxide in water as an environmental benign method was investigated in a droplet column reactor in conjunction with a stirred tank reactor. The system appears to be a wellcharacterized model system for the study of mass transfer, reaction kinetics, and scaleup of a two-phase reactor system. The effects of parameters such as oxone concentration, alkene-oxone solution volume ratio, alkene flow rate through a sparger, and temperature on reaction rate and yield were studied experimentally. The reaction rate and yield of cyclooctene oxide were comparable to those reported by other authors, who studied the epoxidation of cyclooctene in different reac-
Literature Cited Alkire, R.; Kohler, J. Indirect Electrochemical Epoxidation of Hexene in a Liquid-liquid Electrolyte. J . Appl. Electrochem. 1988,18,405-409. Csanyi, L.; Jaky, K. Some Features of Epoxidation of Cyclohexene Catalyzed by Oxoperoxometallates under Phase-Transfer Condition. J . Catal. 1991, 127, 42-50. Kuo, M. C.; Chou, T. C. Kinetics and Mechanism of the Catalyzed Epoxidation of Oleic Acid with Oxygen in the Presence of Benzaldehyde. Ind. Eng. Chem. Res. 1987,26,277-284. Roy, S.; Gupta, B. R.; Maiti, B. R. Effect of Acid Concentration and Other Reaction Parameters on Epoxidation of Nature Rubber Latex. Ind. Eng. Chem. Res. 1991,30,2573-2576. Sienel, G.; Rieth R.; Rowbottom, K. T. Epoxides. In Ullmann's Encyclopedia of Industrial Chemistry; Gerhartz, W., Ed.; VCH Publishers: New York, 1985; Vol. A9, pp 531-545. Venturello, C.; Alneri, E.; Ricci M. A New, Effective Catalytic System for Epoxidation of Olefins by Hydrogen Peroxide under Phase-Transfer Conditions. J. Org. Chem. 1983,48,3831-3833. Wang, M.-L.; Chang, K.-R. Kinetics of the Allyation of Phenoxide by Polyethylene Glycol in a Two Phase Reaction. Can. J . Chem. Eng. 1991,69, 340-345. Woods, K. W.; Beak, P. The Endocyclic Restriction Test: An Experimental Evaluation of the Geometry at Oxygen in the Transition Structure for Epoxidation of an Alkene by a Peroxy Acid. J . Am. Chem. SOC.1991,113,6281-9283. Zhu, W.; Ford, W. T. Oxidation ofAlkenes with Aqueous Potassium Peroxymonosulfate and no Organic Solvent. J . Org. Chem. 1991, 56, 7022-7026.
Received for review December 12, 1994 Revised manuscript received J u n e 20, 1995 Accepted July 6, 1995@ IE9407341
Abstract published in Advance A C S Abstracts, October 1, 1995. @
