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KINETICS of HYDROLYSIS of ACETIC ANHYDRIDE by IN-SITU FTIR SPECTROSCOPY an Experiment for the Undergraduate Laboratory

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KINETICS of HYDROLYSIS of ACETIC ANHYDRIDE by IN-SITU FTIR SPECTROSCOPY an Experiment for the Undergraduate Laboratory .tA... 5-4._l_a_b _o_r._a_t_o_r.:.y________ ) KINETICS OF HYDROLYSIS OF ACETIC ANHYDRIDE BY IN-SITU FTIR SPECTROSCOPY An Experiment for the Undergraduate Laboratory SHAKER HA.JI, CAN ERKEY University of Connecticut • Storrs, CT 06269-3222 he senior-level chemical engineering undergraduate laboratory course at the University of Connecticut con­ 2 T sists of two four-hour labs per week, during which groups of three to four students typically perform five ex­ Quite a few studies have been reported in the literature on the 2 periments during the course of the semester. Each experi­ kinetics of hydrolysis of acetic anhydride. [J, A,5l Eldridge and ment is studied for either one or two weeks, depending on its Piretl41 obtained the pseudo-first-order reaction rate constant complexity and the scale of the equipment. The students are using a batch reactor. To determine the acetic anhydride con­ given only the general goals for each experiment and are re­ centration, samples from the reactor were withdrawn into tared quired to define their own objectives, to develop an experi­ flasks containing 15-20 times the quantity of saturated aniline­ mental plan, to prepare a pre-lab report (including a discus­ water required to react with the sample. Since the anhydride sion of safety measures), to perform the experiments and ana­ rapidly acetylates the aniline, producing acetanilide and ace­ lyze the data, and to prepare group or individual written and/ tic acid, the samples were then titrated to determine the con­ or oral reports. centration of acetic acid. In another study, Shatyski and One or two of the experiments in this course involve reac­ Hanesianrsi determined the kinetics of the above reaction by tion kinetics. Over the years, we have encountered some chal­ using temperature-vs-time data obtained under adiabatic con­ lenges with reaction kinetics experiments, including inaccu­ ditions in a batch reactor. rate, tedious, and/or outdated methods for measuring con­ centrations of reactants or products, and very long or very Shaker Haji received his BSc in chemical en­ short reaction times that make it difficult to monitor concen­ gineering from King Abdul Aziz University in trations with current conventional methods. Jeddah (Saudi Arabia) in 1999. He is currently a full-time PhD student in the Department of We developed a reaction engineering experiment that em­ Chemical Engineering at the University of Con­ necticut. His research focuses on removal of ploys in-situ Fourier Transfer Infrared (FTIR) spectroscopy organosu/fur compounds from diesel for fuel­ for monitoring concentrations. The FTIR is a nondestructive cell applications. technique that is increasingly employed by chemists and chemical engineers to obtain real-time data by in-situ moni­ toring. Since no sampling is required, this analytical tech­ Can Erkey received his BS degree from nique allows the reaction kinetics to be observed under ex­ Bogazici University (Turkey), his MS from Uni­ perimental conditions without disturbing the reaction mix­ versity of Bradford (England), and his PhD from Texas A&M University. He is currently an asso­ ture. The FTIR provides an effective but expensive analyti­ ciate professor in the Chemical Engineering cal capability. Department at the University of Connecticut and teaches chemical reaction engineering and ca­ The hydrolysis of acetic anhydride (Acp) to acetic acid talysis courses, both at the graduate and un­ (AcOH) was selected as the model reaction. dergraduate levels. His main research interests are in catalysis and nanostructured materials. © Copyright ChE Division of ASEE 2005 56 Chemical Engineering Education For this laboratory, the reaction is carried out in a batch ber or neoprene gloves should be used when handling con­ reactor at a minimum of three different temperatures. The centrated solutions of acetic acid. Contact with eyes or skin concentration of acetic anhydride is measured as a function should be avoided. Furthermore, both the reactant (acetic of time using in-situ FTIR spectroscopy. The data are then anhydride) and the product (acetic acid) can be monitored. analyzed to determine the reaction order and the rate con­ The IR spectra of the reactants and the product (see Figure 1) stant for this reaction. The resulting rate equation is used to indicate that each species has its own distinctive absorption predict the performance of a semi batch reactor, which is then peaks that are not obscured by those of the other two species. compared to experimental data. This experiment requires three In addition, the rate of the reaction is such that a few experi­ laboratory periods if the students construct the calibration ments can be carried out in a four-hour laboratory period. curve themselves-otherwise, the teaching assistant can do the calibrations and it requires only two laboratory periods. THE EXPERIMENT SETUP The hydrolysis of acetic anhydride reaction is a suitable A schematic diagram of the laboratory apparatus is shown reaction for many reasons. The final reaction product is a in Figure 2. The reactor used in the experiment is a three­ harmless acetic acid solution with concentrations in water necked, 500-mL jacketed flask equipped with a magnetic stir­ ranging from 8 to 20 vol %. As with most chemicals, how­ rer. Water is circulated through the jacket to keep the reac­ ever, acetic anhydride and acetic acid should be handled in tion mixture at a constant temperature. A thermometer is fit­ the hood. Safety glasses are needed when handling concen­ ted into one of the side necks of the flask and immersed in trated or moderately concentrated acid solutions. Butyl rub- the reaction mixture. A mid-IR probe consisting of a zinc selenide ATR (attenuated total reflectance) crystal is fitted into the middle neck of the flask and submerged into the re­ action mixture. The probe is connected to a Remspec mid-IR fiber-optic system comprising a bundle of 19 optical fibers, ---- Water 1 which transmits in the mid-IR range, 5000-900 cm- • Seven -- Acetic Anhydride · · · ··· ·· · Acetic Acid of these fibers are attached to a signal-launch module attached to the collimated external beam port of a Bruker Vector 22 FTIR spectrometer; twelve fibers are attached to an external liquid-nitrogen-cooled MCT detector fitted with specialized optics to optimize capture of the mid-IR signal from the end of the fiber bundle. The data are processed using computer software to obtain the IR absorption spectra. For batch operation, the reactant (acetic anhydride) is in­ 2000 1800 1600 1400 1200 1000 troduced at initial time through a glass stopper that is fitted into the open neck of the reaction flask. For semibatch op­ Wavenumber, cm- 1 eration, a graduated addition funnel is fitted to the neck that Figure 1. IR spectra of pure water, acetic anhydride, and can be calibrated to add acetic anhydride to the flask at a acetic acid. desired rate. The reactor is initially filled with a known quan­ tity of water at the beginning of each experiment. PROCEDURE Funnel Thermometer Before acquiring IR spectra during the reaction, a back­ ground spectrum of the empty reactor with optical fibers at­ Remspec tached is acquired. For each experiment, 150 ml of distilled Detector water is placed in the reactor. After heating or cooling the reactor to the desired temperature, a background spectrum is acquired again with the probe immersed in water. FTIR For the batch mode, 10 ml of acetic anhydride is added to 500 mL Flask the stirred reactor, and the reactor is sealed with a glass stop­ per. The spectra are acquired using the repeated measure­ Computer ments option enabled by the software controlling the FTIR. The settings are adjusted for the acq uisition of a spectrum every 50 seconds for 35 minutes. Spectral scans are taken at Figure 2. Schematic diagram of the experiment setup. 4 cm·1 resolution and signal averaged over 32 scans. The ab- Winter 2005 57 sorbances of the selected peaks are measured from the indi­ For the first-order case where -rA = kC A, integration of Eq. vidual spectra. It takes around 30 seconds for the 32 scans to (1) yields be acquired. £n CAO = kt (7) Care should be exercised in selecting the operating condi­ C A tions or the scan number to make sure that the concentration does not change significantly during the acquisition of each For the case where -rA= kC/, integration ofEq. (1) yields spectrum. For example, at the beginning of the reaction at 35°C, I I ----=kt (8) the concentration of acetic anhydride changes by 11 % during C A C AO the acquisition of the second spectrum (50-80 s). The differential method can also be used to analyze the For the sernibatch reactor operation, an addition funnel is rate data.l31 In this method, the reaction rate at each concen­ placed in the unoccupied neck. The acetic anhydride addi­ tration is determined by differentiating concentration-versus­ tion rate can be set between 2 to 5 ml/min. Once the first time data. By combining the mole balance (Eq. 1) with the drop hits the water, the repeated measurements function is rate law (Eq. 3), we obtain activated so that a spectrum is acquired every 50 seconds for one hour. The level in the addition flask is read at various - dCA = kCn (9) times to determine the rate of addition as a function of time. dt A The reaction mixture's temperature is recorded manually with Taking the natural logarithm of both sides of Eq. (9) gives each IR spectra acquired or as needed. THEORY 5 ~---------------~ 0.6 For a constant-volume batch reactor, the rate of appear­ ::E 0.5 g ance of reactant A (acetic anhydride), r A' is given by [Ac20J = 2.93 1•x ·.c R2 = 0.9979 g 0.4 ~ dC A u rA=dt (1) C: [AcOH] =1 3.4 14*X 0 2 0.3 ~ R = 0.9997 -0 where r Acan be expressed as :§ 0.2 £ (2) § u " Acetic Acid 0.
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