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Ind. Eng. Chem. Res. 2008, 47, 3870–3875
Intensification of Catalytic Oxidation with a T-junction Microchannel Reactor for Deep Desulfurization D. Huang, Y. C. Lu, Y. J. Wang, L. Yang, and G. S. Luo* The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing, 100084, China
In this paper, a microreactor system is developed to enhance the oxidation of dibenzothiophene (DBT) and 4,6-DMDBT for deep desulfurization with the oxidant of hydrogen peroxide. A T-junction microchannel was applied to form the aqueous slug flow and a long PTFE (polytetrafluoroethylene) capillary with an inner diameter of 1 mm was connected directly downstream to maintain the two-phase dispersion condition. Surfactant of octadecyltrimethyl ammonium bromide (STAB) and phosphotungstic acid (TPA) were mixed in the microchannel to form the combined amphiphilic catalyst directly. The parameters affecting slug formation and DBT oxidation were investigated, including two-phase flow rates, temperature, surfactant type, and catalyst concentrations. DBT conversion of 97% was achieved with a residence time of 1.3 min at 60 °C. Furthermore, 4,6-DMDBT could also be effectively oxidized, and increasing the reaction temperature from 25 to 70 °C led to a substantial increase in 4,6-DMDBT conversions, from 57% at 25 °C to 97% at 70 °C. This T-junction microchannel reactor is far superior to the conventional equipment in terms of providing more interfacial area with much less power input. 1. Introduction Ultradeep desulfurization of fuels has received great attention in recent years because of the worldwide increasingly stringent environmental concern and legal requirements. Since 1997, the maximum sulfur content of fuels has been limited to 500 ppm in most developed countries. New specifications have been issued or proposed by the U.S. Environmental Protection Agency (EPA) and European Commission (EC) to further reduce the sulfur content to the so-called ultralow-sulfur diesel (ULSD) level of 10-15 ppm.1,2 To meet these more stringent specifications, highly efficient removal of sulfur compounds from fuels is, therefore, of vital importance and necessity. Among them, the oxidative desulfurization (ODS) appears as particularly promising. This process is carried out in liquid phase under very mild conditionssaround room temperature and atmospheric pressure.3,4 Furthermore, oxidative desulfurization process is very efficient in removing benzothiophene, dibenzothiophene, and their corresponding derivatives from oils.5,6 It offers a great possibility to meet the more stringent sulfur level in liquid fuels all over the world. Oxidizing agents are used such as organic peroxyacids, peroxides, hydroperoxydes, and hydrogen peroxide. The aqueous hydrogen peroxide is an ideal oxidant owing to its high effectiveoxygen content and cleanliness. It produces only water as byproduct; when water accumulation is not desired, it can be removed by extraction or other process easily.7 In our previous investigations, an amphiphilic catalyst was used for deep desulfurization of dibenzothiophene (DBT) under mild conditions. The combined amphiphilic catalyst of octadecyltrimethyl ammonium bromide (STAB) and phosphotungstic acid (TPA) was formed directly in the reaction system. A complete DBT conversion with a DBT concentration of 3000 ppm could be shortened to 15 min as the temperature was elevated to 50 °C.8 Besides, the composition of the catalyst was optimized and the optimal reaction conditions were obtained. The conceptual model in DBT oxidation process was established * Corresponding author. E-mail:
[email protected].
as well. It was found that the oxidized TPA is extracted by quaternary ammonium salt from aqueous phase to the interface. DBT oxidation takes place at the surface of the oxidized TPA. The amphiphilic catalyst at the interface not only affects the interface properties but also directly affects the resistance in DBT mass transfer step and, therefore, the rate of the overall catalysis sequence. We also found that mass transfer of DBT from the organic media toward interface is rate-limiting and the reaction rate is still low.9 However, hydrogen peroxide always brings about the concerns of safety in storage and operation when adopted at industrial scale. In recent years, microreactors and microreaction technology play a significant role in chemical and biochemical kinetic investigations. Miniaturization of chemical processes will greatly increase process safety, efficiency, and productivity. Microreactors have the following advantages in comparison with normal-scale reactors: high surface-to-volume ratio, improved heat and mass transfer properties, small size, negligible chemical waste, and increased safety.10–13 It is reported that the liquid–liquid slug-flow capillary microreactor is a useful instrument for the elucidation and enhancement of fast heat and mass transfer limited reactions.14 The stable well-defined flow patterns and uniform interfacial areas permit a precise tuning of the mass transfer processes. This regime of slug flow can enhance the interfacial area and increase intensification of internal circulations. Therefore, it will accelerate the reaction a lot. In our group, we have developed a dynamic method to prepare O/W or W/O emulsions by changing surfactants with a microchannel device.15 Therefore, a microchannel reactor has high potential to overcome the mass transfer limitations for enhancing the oxidation of DBT and 4,6DMDBT, because this is an emulsion system. In our present work, a T-junction microchannel was applied to form the slug flow and a long pipe with an inner diameter of 1 mm was connected directly downstream of the T-junction channel. The parameters affecting slug formation and DBT oxidation were investigated, including two-phase flow rates, temperature, surfactant type, and catalyst concentrations.
10.1021/ie701781r CCC: $40.75 2008 American Chemical Society Published on Web 04/22/2008
Ind. Eng. Chem. Res., Vol. 47, No. 11, 2008 3871 Scheme 1. Device Used in DBT Oxidation Process
2. Experimental Section DBT was dissolved into 50 mL of n-octane to make a stock solution with an initial concentration of about 300 ppm. Some certain amount of octadecyltrimethyl ammonium bromide (STAB) was mixed with the DBT solution. Phosphotungstic acid (TPA) was added to hydrogen peroxide (30 wt %). The oil mixture was introduced into the microchannel reactor cocurrently with a stream of hydrogen peroxide. The experiments were performed in polymethyl methacrylate (PMMA) T-junction microchannels with an angle of 90°. Both the oilphase and aqueous-phase flow-channel dimensions are approximately 0.8 mm wide × 0.8 mm high. Thus, the cross section normal to the flow direction is square. The experimental setup used in the DBT oxidation process is shown in Scheme 1. Two microsyringe pumps were used to pump the two phases into the microfluidic device. The DBT solution was injected into the bottom channel, whereas H2O2 with TPA was injected into the perpendicular one. When two immiscible liquids were introduced into the T-junction (0.8 mm wide × 0.8 mm high), the aqueous phase was broken into slugs at the junction of two microfluidic channels and then DBT oxidation took place. In order to increase the channel length to allow the residence time, a long pipe (polytetrafluoroethylene, PTFE, 1 m in length) with an inner diameter of 1 mm was connected directly downstream of the T-junction channels. Variation of the reaction temperature was performed by heating the PTFE pipe with a water bath. At the end of the pipe, samples were cooled in an ice-water bath and DBT concentrations were analyzed by HP6890 gas chromatography (GC) with a flame ionization detector. Benzophenone was used as the internal standard. The sample (1 mL) and 1 mL of internal standard (octane as solvent) was diluted to 5 mL with octane. Samples were injected into the injection port with a microsyringe. Abel Bonded AB-FFAP capillary column (30 m × 0.32 mm i.d. × 0.25 µm) from Abel Industries made in Delaware, U.S.A., was used. GC conditions were as follows: inlet temperature 300 °C, detector temperature 250 °C, and oven temperature 235 °C. 3. Results and Discussions 3.1. Two-Phase Flow Properties. In order to get the twophase flow properties in the microchannel reactor, a high-speed charge-coupled device (CCD) video camera was connected to the microscope and the images of the flow regimes were recorded accordingly. Depending on the oil and water flow rates, different spatial distributions of the two phases were obtained and the results are shown in Figure 1. Results indicate that the aqueous phase forms a uniformly convex shaped slug successfully while octane exhibits a concave geometry for the working system with STAB as the surfactant, as would be expected with the hydrophobic PMMA wall material. The H2O2 flow rate affects the slug length greatly. Shorter slugs could be formed when H2O2 flow rate is lower, while longer slugs formed when
the H2O2 stream is of higher flow rate. Slug length of 1.66 mm was obtained at the H2O2 flow rate of 50 µL/min. The length was further increased to 2.0 and 2.42 mm when elevating the H2O2 flow rate to 100 and 150 µL/min, respectively. Furthermore, well-defined slug-flow regime was detected over a wide operating window, which guarantees a well-defined interfacial area for mass transfer. Figure 2 shows the process of the slug formation at the intersection of the T-junction channels when the oil-phase flow rate is 200 µL/min and the H2O2 flow rate is 50 µL/min. First, the H2O2 thread penetrates into the outlet channel, and then the horizontal oil flow causes the development of a clear thread head. Finally, the H2O2 stream collapses, induced into isolate slugs, and then the whole process starts again. The aqueous phase is broken into slugs at the junction of two microfluidic channels, and then DBT oxidation takes place. 3.2. Effect of Surfactant on DBT Oxidation. The effect of surfactant type on the catalytic oxidation of DBT was investigated. DBT oxidation was tested with a series of different amphiphilic quaternary ammonium salts, including octadecyltrimethyl ammonium bromide (STAB), cethyltrimethyl ammonium bromide (CTAB), and (1-tetradecyl)trimethyl ammonium bromide (TTAB). The results are shown in Figure 3. Reaction rate varies a lot when the quaternary ammonium salt is changed. The order of catalytic activity is as follows: STAB > CTAB > TTAB. On the one hand, a rise in the reaction temperature from 25 to 60 °C leads to a slow increase in the reaction rate of all the cases. On the other hand, the size of the quaternary ammonium cation plays a very important role in DBT oxidation. TTAB shows the least reactivity toward DBT oxidation, and the conversion is TTAB. The effectiveness of large lipophilic ammonium ions in DBT oxidation is in accordance with the mechanism as reported before. (2) Increasing H2O2 flow rate facilitates slight enhancement in DBT oxidation. On the one hand, the flow rate is inversely proportionally related to the residence time in the reactor. Higher total flow rates result in shorter residence time. On the other hand, an increase in the H2O2 flow rate will facilitate a larger specific surface area. The increased mass transfer area is crucial in reducing resistance in the DBT mass transfer step and, therefore, the rate of the overall catalytic reaction. The above two factors affect the reaction simultaneously. (3) A rise in the reaction temperature from 25 to 60 °C led to a remarkable increase in the reaction rate. DBT conversion was substantially increased, from 66% at 25 °C to 97% at 50 °C, and then further to 100% at 60 °C. DBT conversion of 97% is achieved with a residence time of 1.3 min at 60 °C, indicating DBT oxidation is performed with a quite fast reaction rate in the microreactor. (4) 4,6-DMDBT could also be effectively oxidized in this microchannel reactor, and increasing the reaction temperature from 25 to 70 °C led to a substantial increase in 4,6-DMDBT conversions, from 57% at 25 °C to 97% at 70 °C. (5) The well-defined regime of slug flow can enhance the interfacial area and increase intensification of internal circulations. It accelerates mass transfer greatly and, therefore, accelerates the reaction a lot. The slug-flow microreactor is far superior to the conventional equipment in term of providing more interfacial area with much less power input. Although we have found an efficient way to enhance the reaction, it is still required to develop the mathematical models to simulate the process for engineering applications. The job will be carried on in our further study. Acknowledgment We acknowledge the financial support of the National Natural Science Foundation of China (No. 20476050, 20525622) and National Basic Research Program of China (2007CB714302) on this work gratefully. Literature Cited (1) Song, T.; Zhang, Z. S.; Chen, J. W.; Ring, Z.; Yang, H.; Zheng, Y. Effect of Aromatics on Deep Hydrodesulfurization of Dibenzothiophene and 4,6-Dimethyldibenzothiophene over NiMo/Al2O3 Catalyst. Energy Fuels 2006, 20, 2344. (2) Song, C.; Ma, X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl. Catal., B 2003, 41, 207. (3) Nehlsen, J.; Benziger, J.; Kevrekidis, I. Oxidation of aliphatic and aromatic sulfides using sulfuric acid. Ind. Eng. Chem. Res. 2006, 45, 518.
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ReceiVed for reView December 29, 2007 ReVised manuscript receiVed February 22, 2008 Accepted February 26, 2008 IE701781R
