Problem Statement - Stirred Tank Reactor-1
A client is running his CSTR with baffles, no stirring and bottom feeding location for both reactants, one at the center of the tank below the impeller and the other off to the side at the level of the impeller. A third tube hangs into the tank corresponding to a thermo-well allowing the temperature of the tank to be measured. Both of these changes were done at the same time and now his reactor conversion is much too low. His engineer told him thatthe thermo- well, feed tubes and baffles would provide sufficient mixing in this reactor so a mixerwould not be needed. He wants to know which change is responsible for the low conversions being reported. The other operating conditions for the reactor are a total reactant flow rate of 100 mL/min, and a reactor volume of 1.3 L. The reaction being performed in the reactor is the the saponification of ethyl acetate with the reactants being fed at equimolar flow rates.
Uniform mixing of reactants is critical to the conversion in a CSTR. You are to develop a series of data and calculations to show the effect of residence time on reactor conversion for presentation to the client. Since the laboratory hoods are not functioning, no chemicals with toxic vapors can be used so only residence time distributions and model calculations can be used to prove your point.
Please run a CSTR with baffles and with bottomfeed locatiosn to establish the degree of micro/macro-segregation that is observed as a function of stirring rate (rpm) for a Rushton impeller using the residence time distribution as your point of comparison. Please determine the residence time distribution for each experimental condition (rpm). Compare the residence time distributions and the micro/macro-segregation models given in Chapt 14. of Fogler’s “Elements of Reaction Engineering” 2nd edition.
The client uses the saponification of ethyl acetate
Et-Ac + NaOH ↔NaAc + Et-OH
for her reaction. The kinetics of this reaction is reported in Hovarka, R.B. and Kendall, ;H.B. "Tubular reactor at low flow rates" CEP56(8),58-62(1960), copy provided. In equimolar experiments they found this reaction to be second order overall, i.e. first order in each reactant. Use the data in this paper to determine the rate constant for this reaction. It will be needed to predict the conversion of the reaction with various mixing models.
In your final report, use reactor-mixing models to fit the results you have obtained from measurements of the residence time distribution. Use your best mixing model and the reaction kinetics to predict the real reactor conversion and compare it to the ideal CSTR reactor conversion. So that we can show the client we can predict the effects of poor mixing in his CSTR. Clearly identify which of the two changes, removal of baffles and top feeding, is responsible for the low conversion he is experiencing in his saponification reactor.
Please include this assignment in your report as an appendix but do not cite it in the body of your report.
Experimental Program
The following Experiments were performed on the glass-lined reactor.
The 1.3L stirred tank reactor was assembled with 4 baffles. The tank was filled with cold water under acidic conditions, the outlet pump turned on at maximum flow rate, the stirrer started at a given rmp and the data acquisition computer system started. Some time later, the feed to the reactor, a hot solution of high pH, was started at a flow rate of 100 ml/min. The feed solution is feed to the reactor with half at the bottom of the impeller, see picture and figure 1 below and the other half from the nearest tube in Figure 1. The product of the continuous stirred tank reactor is removed at the top of the reactor at the liquid interface. The level of the liquid in the tank is fixed at 1.3 L by the vertical position of the product removal pipe and the outlet pump being faster than the inlet pump thereby fixing the liquid level. Temperatures were measured at the inlet line, tank bottom using a thermo well inside the reactor ( tube furthest away in Figure 1), tank top which corresponds to the outlet line as well as the jacket (cold water temp.) The cooling water jacket on the reactor was empty for all of these residence time distribution experiments, providing an air insulation layer around the contents of the tank so that no heat was lost. The data acquisition system measured the date, time, mixer speed in % of full scale, bottom tank temperature (at thermo-well), Input temperature, cold water temp (jacket temperature), top tank (corresponding to the outlet temperature) and Tank pH value. The pH meter probe was located at the top of the tank immersed to 2 cm into the liquid. Files for the raw measurements are attached to this email. The file names correspond to the rpm value of the stirrer when the measurements were made. Other information on the equipment and the errors in the measured data are given in the appendices. Please note that the different thermocouples may have different calibrations/offsets and may not read exactly the same temperature as the others.
APPENDIX C
EQUIPMENT LIST
table C-1.
Specifications of Major Items of Equipment
1. Thermocouple:
Omega Engineering, Inc. Type T Thermocouple.
2. Reactor:
Jacketed Glass reactor, Applikon.
3. Pumps:
Masterflex Peristaltic Pump from Cole Parmer, Model 77200-60 Pump Head
4. Data Acquisition System:
Opto22 Data Acquisition system..
Appendix d
ERROR ANALYSIS
The following limits of error were assumed for the experimental measurements:
temp=±0.2 °C
flowrate=±0.025 ml/min
reactor volume=±0.02 L
rpm=±1 rpm