Chemical Engineering Laboratory Manual
Reaction
EXPERIMENT 461L_2
BATCH and CSTR REACTOR
OBJECTIVES:
1. Measure reaction rate constants for a pseudo-first order reversible reaction in an
isothermal batch reactor.
2. Use the same reaction to study the start-up (unsteady-state) behavior of a CSTR system.
[optional]
3. Compare the observed CSTR start-up behavior with that predicted for an ideal (perfectly
mixed) CSTR. [optional]
4. Use the measured rate constants to size a plug flow reactor. [optional]
INTRODUCTION:
The alkaline hydrolysis of phenolphthalein is an interesting pair of reactions that involve a
color change as the reaction takes place. This color change allows the use of a
spectrophotometer to indirectly monitor the reaction progress, or rate. The first reaction is the
rapid (nearly instantaneous) irreversible conversion of the colorless phenolphthalein to the
quinoid form, which has a pink color. The quinoid form (A) then slowly undergoes a rate
determining reversible reaction with a hydroxyl ion to form the carbinol form (B) which is
colorless. A light absorbance peak is observed for the quinoid form for light with a wave length
of 550 nm. Since the light absorbance is linear with concentration, i.e. it follows the BeerLambert Law, then the concentration of the quinoid form is directly proportional to the
absorbance.
EXPERIMENTAL:
This experiment consists of two parts: the batch reaction and the CSTR reaction.
a) Batch Reaction
Prepare 6 liters of 0.1 N NaOH solution in distilled water, and a solution of 0.4 g
phenolphthalein in 100 mL of methyl alcohol. Allow 30 minutes for the spectrophotometer to
warm up. Place 0.1 N NaOH in the spectrophotometer sample cell and zero the instrument at
550 nm. At an arbitrary time (called "time zero") add 1 mL of the phenolphthalein solution to
500 mL of the hydroxide solution. Mix thoroughly. Then pour about 3 mL of the mixture (now
a bright pink color) into the glass spectrophotometer sample cell and place it in the
spectrophotometer set at 550 nm wavelength. Keep all fingerprints off the clear sides of the
glass cells. Measure the solution absorbance versus time every 2 minutes (starting at t = 1
minute) until no further change occurs. Ensure the sample cell is not in the beam path (reading
location on the carousel) to prevent the light beam from heating the sample above room
temperature. Note that at all times the sodium hydroxide level is essentially constant
throughout the experiment because it is present in great excess over the phenolphthalein. Your
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results should verify that the reaction is reversible, pseudo-first order in quinoid form (A) in
the forward direction and first order in carbinol form (B) in the reversible direction.
b) CSTR Reaction
To be added after CSTR is constructed and tested.
REPORT:
a) Batch Reaction
For a reaction:
AB
(1)
in a batch system with an initial A concentration of CAo and a final A concentration (at
equilibrium) of CAe, the following equation applies (see below derivation):
C A  C Ae
 exp  k1  k 2 t 
C Ao  C Ae
(2)
For CA and CAe we can use absorbance values measured in the spectrophotometer, since the
concentrations of A are directly proportional to the absorbances measured.
From a plot of ln(CA – CAe) versus time, obtain -(k1 + k2) from the slope and ln(CAo- CAe) and
hence CAo from the intercept. The reactor asymptotically approaches equilibrium, so use an
extrapolated value for CAe that gives the best straight line fit. Since K = k1/k2 = CBe/CAe = (CAo
– CAe)/CAe we can find K (the equilibrium constant). Now that we know K and (k1 + k2) we can
find k1 and k2 individually. Obtain CAo, K, k1, and k2 in this way from your data. Also
calculate the uncertainty in your experimental k values.
b) CSTR Reaction
To be added after CSTR is constructed and tested.
c) Plug Flow Reactor problem
To be added after CSTR is constructed and tested.
SAFETY:
All must wear safety glasses at a minimum, and safety goggles are recommended. Read
appropriate MSDSs for chemicals involved in the reactions. Chemical resistant gloves must be
worn when handling MeOH and NaOH solutions. For hot solutions, chemical and heat
resistant gloves should be worn. Ensure waste chemicals are disposed of properly.
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REFERENCES:
Fogler, H. S., Elements of Chemical Reaction Engineering, 3rd edition. Prentice-Hall, New
Jersey, p. 187-200 (1999).
Hill, C. G., An Introduction to Chemical Engineering Kinetics and Reactor Design, John
Wiley, New York, p. 388-424 (1977).
Levenspiel, O., Chemical Reaction Engineering, 3rd ed. John Wiley, New York, p. 83- 9
(1999).
Barnes, M.D. and V.K. LaMer, "Kinetics and Equilibria of the Carbinol Formation of
Phenolphthalein," J. Amer. Chem. Soc., 64, 2312 (1942).
Chen, D.T.Y. and K.J. Laidler, Can. J. Chem., 37, 599 (1959).
ADDITIONAL INFORMATION:
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