Lars-Eric Lindfors Abo Akodemi A ~ O 2, Finland
An Undergraduate Experiment in Chemical Engineering Reactor Kinetics
T h e aim of this experiment is to show the possibility of using one single apparatus to demonstrate in one experiment the isothermal performance characteristics of a batch reactor, a plug flow reactor, and a backmix reactor by determining the rate constant for the irreversible second-order reaction between ethyl acetate and sodium hydroxide in dilute aqueous solution. The reaction used is well investigated (1-3) and, therefore, suitable for laboratory experiments with chemical reactors (4, 5). As we also want to demonstrate modern instrumentation, the course of the reaction is conductometrically followed and continuously recorded.
mix reactor as well as the flow cell perform as batch reactors. Results and Discussion
The accompanying plot, recorded in a typical experiment, reveals that the steady state is reached after about 50 min. This can also be calculated theoretically by observing that the Na ions contribute to the electrolytic conductance without taking part in the chemical reaction. When the hackmix reactor is working at the steady state, the reaction rate constant k can be calculated according to the following equation
Experimental
The reaction mixture contains the ions OH-, Naf, and CHaCOO- (the latter subsequently denoted Ac-). As any increase in the Ac- concentration is equivalent to a corresponding decrease in the OH- concentration, the following expression can be derived for the acetate ion concentration
Here 1 denotes the ionic conductance, C& is the initial concentration of the sodium hydroxide, and x is the specific conductance of the solution. According to Eucken and Suhrmann (6) the following expressions are approximately valid at the concentre tion level used (initial concentrations 0.05 mole/l)
It is thus possible to follow the reaction by measuring the specific conductance of the solution. With the aid of a peristaltic pump, supplied with two channels, the reactants are fed a t identical volumetric flow rates to the hackmix reactor in which a vigorous mixing is obtained by a magnetic stirrer. The specific conductance of the reaction mixture, which leaves the backmix reactor through a conductivity cell of the flow type, is continuously recorded. Thermostatic control of the feeds, the reactor, and the flow cell provides isothermal conditions for the reaction. At the beginning of each experiment the reactor system is filled with deionized water, which, during the experiment, is replaced by the reaction mixture. When the steady state is reached the reactant flow to the system is suddenly stopped. From this moment the back472
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Journal of Chemical Education
where i denotes the mean residence time. As the flow cell can he considered a small tubular reactor (in the present worlc 2.4 ml), \re have recommended the students to modify the value of k, calculated according to eqn. (4), by assuming that the cell functions as a plug flow reactor. It is seen from the figure that the feed of reactants was stopped at about 50 min, the curve subsequently reflecting the change of concentration typical for a second-order reaction performed in a hatch reactor. In this case the value of k can be evaluated by methods known from the classic kinetics. The values of k listed below are calculated for the three different cases mentioned and based uuon ex~eriments made a t 25°C.
Plot of d l ) for the reaction between NaOH and CHaCOOCsHj when performed continuously (0-50 min, V = 80 ml, i = 7.65 min) and batchwise 150-80 min) in the same opparatu,.
k (backmix) k (backmix
=
k (batch)
=
(6.7 f 0.5)1 mole-'min-I
+ plug flow) = (6.1 (5.7
z!=
0.5)l ~ n o l e - ~ m i n - ~ ,
+ 0.5)l mole-Imin-'
With regard to the aim of the expkriment these values are good enough as the value of k given in the literature (1-3) is about 6.5 1 mole-' min-'. According to our experience this comparatively flexible experiment offers a convenient means of illustrating the basic chemical reactor types. Extensions
concerning, e.g., the reactor dynamics can be made by analog or digital simulation. Literature Cited (1) TERRY. E. M., A N D STIEBLI=TZ, J., J . Amer. Chcm. Soc.. 49, 2216 (1027). (2) Hovonn*, R. B., AND ICENDALL, H. B., Cliem. Eng. Plogr., 56, 58, No. 8 (1960). (3) DANIELS, F.. A N D ALBERTY. R. A,, " P h y ~ i dChemistry" (3rd ed.), John Wiley & Sona. Ino.. New York, 1967, p. 332. (4) PATAT.F., A N D KIRCHNER. K., "Pr&ktikum der Technisohen Chemie" (2nd ed.). Walter de Gruyter & Co., Berlin, 1968, PO. 169-78. c5) KBNDALL, H. B.,Chem. Erie. P7001.S y m p . Ser.. 63, No. 70, (1967). (8) EUCYEN. A,. A N D SUXRMANN. R . , ~ * ~ h y ~ i k ~ ~ i ~ c h - c h e m~ irsaol thtei kumshufghben" (5th ed.), Geest & Portig K.-G., Leipuig, 1960, pp. 217-8.
Volume 48, Number 7, July 1971
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