59th Annual Report on Research 2014

Overview

Supercritical water is a highly energetic partial oxidation medium that has been investigated in a wide variety of chemical processes ranging from jet fuel reformation to destruction of polyaromatic hydrocarbons. Supercritical water can also be used to partially oxidize benzene with several studies evaluating the reaction kinetics and mechanisms of benzene partial oxidation that ultimately lead to gasification.   Notable intermediates of the partial oxidation of benzene include commodity chemicals such as phenol.  This study focuses on optimizing the conversion of the supercritical water partial oxidation of benzene to phenol and other non-gaseous species non-catalytically.  During the first year of this project, the experimental procedures were developed and refined for the partial oxidation reaction using an agitated 500 mL Hastelloy C276 batch reactor with 30 wt-% hydrogen peroxide as the oxidant.  Prior to heating the reactor about 12 mL of water was placed into the reactor to facilitate heating, then the reactants are feed at reaction temperature.  At the conclusion of the experiment, the reactor was rapidly cooled using a cooling coil.  Analytical techniques were also developed for analyzing and quantifying the hydrocarbon reaction products which includes extraction of the products using methyl isobutyl ketone and quantification of the products using a calibrated gas chromatograph with flame ionization detector.  An internal standard is used in the GC analysis.  As part of the calibration, partition coefficients between water and MIBK for benzene and major constituent products were quantified.

The partial oxidation of benzene was evaluated initially by a 2³ factorial experimental design.  Subsequent experiments have been performed to further evaluate the main effects.  Future experiments will further evaluate reaction kinetics.

Year 1  Results

In year one, to efficiently determine a range of conditions for the evaluation of the reaction kinetics of the partial oxidation of benzene, a 2³ factorial experimental design was used.  This followed a set of range finding experiments.  The experimental design’s main effects were temperature varied between 390 and 420 °C, reaction time from 15 to 23 minutes, and benzene mole fraction between 1.5 and 3%.  The reaction temperature is above the critical point of water, but below temperatures reported to be used for the complete gasification of benzene, furthermore the proportion of hydrogen peroxide to benzene was less than what is required for complete gasification.  In addition to unreacted benzene, the principal non-gaseous hydrocarbon products were phenol, biphenyl, and benzoic acid, with lesser amounts of acetophenone, dibenzofuran, and xanthone.  The main effects were all found to be statistically significant with p≤0.05 for phenol hydrocarbon mole fraction.  The reported hydrocarbon mole fraction is the mole fraction of dissolved hydrocarbons extracted from the reaction products, as well as unreacted benzene, all recovered at room temperature.   The two way interaction between temperature and benzene was also statistically significant for phenol mole fraction.  It was found that conversion to phenol is favored at shorter reaction times, lower mole fractions of benzene, and higher temperature, Figure 1.  These conditions also favor the combination of benzene to form biphenyl, however more phenol is produced than biphenyl at all conditions considered.

Higher temperatures and benzene mole-fractions favor production of benzoic acid, xanthone, acetophenone, and dibenzofuran, Figure 2.  For this portion of the study, the maximum amount of phenol, the desired product, was produced at 420 °C, with a benzene mole fraction of 1.5 mol-% and reaction time of 15 minutes.  The hydrocarbon mole fractions of the products at this condition were 6.77 mol-% phenol, 4.48 mol-% biphenyl, 1.16 mol-% benzoic acid, 0.36 mol-% acetophenone, 0.34 mol-% dibenzofuran, trace amounts of xanthone.  The balance is unreacted benzene. 

Figure 1:  Contour plots of phenol mole fraction from a 2³ factorial design with replicates, temperature varied from 390 and 420 °C, reaction time from 15 to 23 minutes, and volume of benzene from 7.5 mL to 11.5 mL (5 to 10 wt-% or 1 to 3 mol-%).

Figure 2:  Contour plots of benzoic acid mole fraction from a 2³ Factorial design with replicates, temperature varied from 390 and 420 °C, reaction time from 15 to 23 minutes, and volume of benzene from 7.5 mL to 11.5 mL (5 to 10 wt-% or 1 to 3 mol-%).

Student Impact

Exploratory experimental work was performed by an undergraduate chemical engineering student for this project as part of a research elective class.  Subsequent experimental work and method development was performed by a Kettering University undergraduate chemical engineering cooperative education student.  This project provides support for a student thesis and five quarters of coop employment, which can satisfy Kettering University degree requirements for one student.  The coop student was responsible for performing the benzene partial oxidation reactions under the supervision of the PI and performing extractions and GC analysis under the direction of a chemistry faculty member.  The student has learned how to operate and maintain high pressure reactors, maintain laboratory detailed records, calibrate and operate a GC, and has gained enough proficiency to train other graduate students in the operation of the reactor. 

Presentation of Results

Initial proposal data was presented by the Principal Investigator as an oral presentation at the 2012 American Institute of Chemical Engineers National Meeting.  Exploratory work was presented by an undergraduate research student as a poster at the 2013 Michigan Academy of Science, Arts, and Letters.  Parts of year one work performed by the cooperative education student was presented as an oral presentation at the 2014 Michigan Academy of Science, Arts, and Letters and as a poster at Kettering University’s homecoming celebration. 

Future Plans

A second study is being concluded that evaluates reaction time from 5 to 30 minutes and reaction temperature from 380 to 430 °C.  These results will be presented in the Fall at the 2014 American Institute of Chemical Engineers National Meeting and will be published.  Subsequent studies will evaluate changing benzene mole fraction and reaction pressure or overall mass density.  The objective is to regress rate laws for the global chemical reactions which will guide in maximizing phenol yield, phenol selectivity, and decreasing competing reactions.