CHEM E7115 Experimental Assignments in Chemical Engineering Internal mass transfer in a heterogeneously catalyzed reaction

Transcription

1 CHEM AXXX Kurssin nimi CHEM E7115 Experimental Assignments in Chemical Engineering Internal mass transfer in a heterogeneously catalyzed reaction

2 1. Before you come to the lab Before you come to do the labwork, you have to do the following: Read the material safety data sheets (MSDS) of the components which are used in the work (available in MyCourses workspace) Read the instructions of the work Take the lab coat and safety goggles with you when you come to work in the lab The meeting point for the lab is the lobby of Industrial Chemistry and Polymer technology, 4 th floor, corridor E. The work is done in lab D Introduction In this work, activation energy of the formation reaction of ethyl acetate from acetic acid and ethanol is studied. Ethyl acetate is produced on a large scale for use as a solvent. The reaction scheme is presented in Figure 1. Figure 1: Reaction scheme for the formation of ethyl acetate (RCOOR ) from acetic acid (RCOOH) and ethanol (R OH). 3. Theory The internal effectiveness factor is used to describe the effect of mass transfer resistances inside the catalyst particle. It can be defined to present ratio of the rate of diffusion to the rate of reaction without any diffusional limitations. Thus the value of varies between 0 and 1. can be calculated for the nth reaction order as (1) 1

3 where n is the Thiele modulus which is defined as (2) where k n is the reaction rate constant, R is the radius of the catalyst particle, C As is the concentration of component A on the surface of the catalyst and D e is the effective diffusivity When the Thiele modulus is small, the surface reaction is the rate determining step, and when internal diffusion is limiting the reaction the Thiele modulus increases. Further, Weisz Prater criterion C WP can be used in determining the internal diffusion. It is defined as a function of the Thiele modulus and the internal effectiveness factor according to (3) A detailed introduction of the calculation of these parameters can be found e.g. in the in the book Elements of Chemical Reaction Engineering by H. S. Fogler, chapter 12 [1]. 4. Working in the lab The internal diffusion of ethyl acetate formation reaction is studied in a 80 ml batch reactor. The system is catalysed with Amberlyst 15 ion exchange resin. The system is heated in a water bath. Each groups studies the reaction with two catalyst particle sizes at given temperature. Two reactors are operated simultaneously, thus the work in lab takes half a day. You have to keep a lab diary all the time in the lab. Write down all the results and procedures you have done. The instructor checks the lab diary when you leave the lab to make sure that you have enough information for writing the report. 2

4 4.1 Operating the reactor Packing the reactor Start the work by turning on the water bath and set the reaction temperature in the control panel by pressing buttons with arrows. Make sure that there is enough water in the bath. If you have to add water, use distilled water, not tap water! Do not place the reactor into the water bath until it is heated to the desired temperature. Learn to use the reactor. There is a test reactor available to practise the use of reactor. The test reactor is filled with water and pressurised with air. A detailed instructions how to use the reactor are available in the lab next to the reactor. Pack the reactor. Put a magnetic stirring rod into the reactor. Attach the catalyst basket into the mixing baffle so that the basket is hanging on top of the mixing baffle. Make sure that the baffle is not touching the bottom of the reactor and the stirrer can move freely. Weigh the reactant bottle before and after filling the reactor to find out the amount of liquid in the reactor. To minimize the evaporation of the reaction mixture keep the bottle in the ice box before opening it. Fill the reactor so that appr cm of void space is left in the top of the reactor. Close the reactor quickly to avoid the evaporation of the reactants. Tighten the screws evenly first with your fingers (clockwise to close, counterclockwise to open) and then check the tightness with hexagon wrench. Do not use too much force in tightening to avoid damaging the seals in the reactor! Put the reactor into the water bath and turn on the timer to measure the reaction time. Turn on the stirrer (under the water bath) and set the stirring speed to 1000 rpm The stirring should not make any clicking sound in that case move the reactor a bit to get rid of the sound. The reactor should be placed inside the circle which is drawn in the bottom of the water bath. Put the sampling line in the ice box. Pressurise the reactor to 10 bar pressure. Make sure that the nitrogen bottle and valves in the pressure line are open. Turn the valves slowly to pressurise the system. Detailed instructions on using the valves are available next to the reactor. Sampling The zero sample is taken from the reactant bottle with a pasteur pipette. All the other samples are taken from the reactor via the sampling line. Mark the sample bottles carefully! Detailed instructions on the sampling are available next to the reactor. 3

5 Unpacking the reactor After the reaction time is complete, turn off the water bath and stirring. Move the hot reactor carefully to the ice box and let it cool down for about 10 minutes. After it is cool, remove it from the ice box and dry it with the paper towels. Release the pressure via the sampling line and collect the reaction mixture to a beaker. Open the screws of the reactor, remove the mixing baffle, catalyst and stirring rod and leave them to dry in the fume hood. Make sure that the sampling line is empty, too. Put the used reaction mixture to the waste bottle and clean the table and surroundings of the reactor. 5. Calculation and reporting of the results The analysis results from chromatographic analysis will be available in MyCourses workspace named according to the group members. The calculation of the results from the analysis data is presented in the Instructions for the gas chromatography analysis and for the calculation of the results and it is available in MyCourses. 1. Calculate the composition of each sample from the chromatographic data 2. Calculate conversion of the reactive components in each sample and plot the conversions as a function of reaction time 3. Calculate the reaction rate at each sampling time. You can assume that volume of the samples is small compared to the volume of the reaction mixture is the system, i.e. the liquid volume remains constant (V = 75 ml) in the reactor although samples are withdrawn from the system. Note that you need molar composition for the calculation but you get the composition in mass basis when using the given response factors 4. Estimate the effect of internal diffusion by using e.g. the Thiele modulus. Estimate that the catalyst particles are spherical. Useful reference for this calculation is the chapter 12 in the book Elements of Chemical Reaction Engineering by H. S. Fogler [1]. 5. Calculate the reaction rate coefficient in case of both particle sizes. Assume that the kinetics of the reaction applies the kinetics of an elementary reaction. Useful reference for this calculation is the chapter 5.2 in the book Elements of Chemical Reaction Engineering by H. S. Fogler [1]. 6. Comment your results. 7. Determine the amount of water in reactor for all sampling times. Present how the amounts of water were calculated. 4

6 8. Evaluate your results using mass balances for all sampling times. Present the calculations in your report. Each group member makes an own report which has to be returned in MyCourses return box called Internal diffusion within two weeks from the completion of the lab work. 6. Feedback on the work Include some feedback on the work in the report. This gives valuable information how to develop this work in the future. Give also an estimate how many hours of work you used for this work. 7. References 1. H. S. Fogler, Elements of Chemical Reaction Engineering, 4 th edn, Pearson, Westford,